CH7_2014_EngMater_Lecture1_532903513
J.Mater.Chem.A,2014,2,16403-Graphene-wrapped ZnO hollow spheres with electromagnetic wave absorption
Published on 13 August 2014. Downloaded on 15/03/2016 14:34:26.
Received 16th June 2014 Accepted 10th August 2014 DOI: 10.1039/c4ta03033h /MaterialsA
the composite exhibits a maximum absorption of À45.05 dB at 9.7 GHz with a sample thickness of only 2.2 mm. The fundamental mechanism based on electrical conductivity and the polarization between the graphene sheets and ZnO nanoparticles is discussed. The hierarchical structure of graphene-wrapped ZnO hollow spheres exhibits a promising designable approach to lightweight electromagnetic wave absorbing materials.
Volume 2 Number 39 21 October 2014 Pages 16301–16794
Materials Chemistry A
Materials for energy and sustainability
CH7WOANS
Chapter 7. Nucleic Acid1. Definition(1). Cyclic nucleotides(2). Chargaff’s Rule(3). Double Helix(4). B- form DNA and Z-form DNA(5). 5’-Cap of mRNA(6). Denaturation and renaturation(7). Tm2. Mono-choice questions(1) The compound that consists of ribose linked by an N-glycosidic bond to N-9 of adenine is:A. a deoxyribonucleoside.B. a purine nucleotide.C. a pyrimidine nucleotide.D.adenosine monophosphate.E.adenosine.(2) A major component of RNA but NOT of DNA is:A. adenine.B.cytosine.C.guanine.D.thymine.E.uracil.(3) The difference between a ribonucleotide and a deoxyribonucleotide is:A. a deoxyribonucleotide has an —H instead of an —OH at C-2.B. a deoxyribonucleotide has α configuration; ribonucleotide has the β configuration atC-1.C. a ribonucleotide has an extra —OH at C-4.D. a ribonucleotide has more structural flexibility than deoxyribonucleotide.E. a ribonucleotide is a pyranose, deoxyribonucleotide is a furanose.(4) The phosphodiester bonds that link adjacent nucleotides in both RNA and DNA:A.always link A with T and G with C.B.are susceptible to alkaline hydrolysis.C.are uncharged at neutral pH.D.form between the planar rings of adjacent bases.E.join the 3' hydroxyl of one nucleotide to the 5' hydroxyl of the next.(5) The DNA oligonucleotide abbreviated pATCGAC:A.has 7 phosphate groups.B.has a hydroxyl at its 3' end.C.has a phosphate on its 3' end.D.has an A at its 3' end.E.violates Chargaff's rules.(6) The experiment of Avery in which nonvirulent bacteria were made virulent by transformation was significant because it showed that:A.bacteria can undergo transformation.B.genes are composed of DNA only.C.mice are more susceptible to pneumonia than are humans.D.pneumonia can be cured by transformation.E.virulence is determined genetically.(7) Chargaff's rules state that in typical DNA:A. A = G.B. A =C.C. A = U.D. A + T = G + C.E. A + G = T + C.(8) Based on Chargaff's rules, which of the following are possible base compositions for double-stranded DNA?%A %G %C%T %UA. 5 45 45 5 0B. 20 20 20 20 20C. 35 15 35 15 0D.All of the above.E.None of the above.(9) In the Watson-Crick model of DNA structure:A.both strands run in the same direction, 3' 5'; they are parallel.B.phosphate groups project toward the middle of the helix, where they are protectedfrom interaction with water.C.T can form three hydrogen bonds with either G or C in the opposite strand.D.the distance between the sugar backbone of the two strands is just large enough toaccommodate either two purines or two pyrimidines.E.the distance between two adjacent bases in one strand is about 3.4 Å.(10) Which of the following is NOT true of all naturally occurring DNA?A.Deoxyribose units are connected by 3',5'-phosphodiester bonds.B.The amount of A always equals the amount of T.C.The ratio A+T/G+C is constant for all natural DNAs.D.The two complementary strands are antiparallel.E.Two hydrogen bonds form between A and T.(11) In the Watson-Crick model of DNA structure (now called B-form DNA):A. a purine in one strand always hydrogen bonds with a purine in the other strand.B.A–T pairs share three hydrogen bonds.C.G–C pairs share two hydrogen bonds.D.the 5' ends of both strands are at one end of the helix.E.the bases occupy the interior of the helix.(12) The double helix of DNA in the B-form is stabilized by:A.covalent bonds between the 3' end of one strand and the 5' end of the other.B.hydrogen bonding between the phosphate groups of two side-by-side strands.C.hydrogen bonds between the riboses of each strand.D.nonspecific base-stacking interaction between two adjacent bases in the same strand.E.ribose interactions with the planar base pairs.(13) B-form DNA in vivo is a ________-handed helix, _____ Å in diameter, with a rise of ____ Å per base pair.A.left; 20; 3.9B.right; 18; 3.4C.right; 18; 3.6D.right; 20; 3.4E.right; 23; 2.6(14) In double-stranded DNA:A.only a right-handed helix is possible.B.sequences rich in A–T base pairs are denatured less readily than those rich in G–Cpairs.C.the sequence of bases has no effect on the overall structure.D.the two strands are parallel.E.the two strands have complementary sequences.(15) Which of the following is a palindromic sequence?A.AGGTCCTCCAGGTTCCGCAAGGC.GAATCCCTTAGGD.GGATCCCCTAGGE.GTA TCCCATAGG(16) Which of the following are possible base compositions for single-stranded RNA?%A %G %C%T%UA. 5 45 45 0 5B. 25 25 25 0 25C. 35 10 30 0 25D.All of the above.E.None of the above.(17) Double-stranded regions of RNA:A.are less stable than double-stranded regions of DNA.B.can be observed in the laboratory, but probably have no biological relevance.C.can form between two self-complementary regions of the same single strand of RNA.D.do not occur.E.have the two strands arranged in parallel (unlike those of DNA, which areantiparallel).(18) When double-stranded DNA is heated at neutral pH, which change does not occur?A.The absorption of ultraviolet (260 nm) light increases.B.The covalent N-glycosidic bond between the base and the pentose breaks.C.The helical structure unwinds.D.The hydrogen bonds between A and T break.E.The viscosity of the solution decreases.(19) Which of the following deoxyoligonucleotides will hybridize with a DNA containing the sequence 5'AGACTGGTC3' ?A.5'CTCA TTGAG3'B.5'GACCAGTCT3'C.5'GAGTCAACT3'D.5'TCTGACCAG3'E.5'TCTGGA TCT3'(20) The ribonucleotide polymer 5'GTGATCAAGC3' could only form a double-stranded structure with:A.5'CACTAGTTCG3'.B.5'CACUAGUUCG3'.C.5'CACUTTCGCCC3'.D.5'GCTTGA TCAC3'.E.5'GCCTAGTTUG3'.(21) In the laboratory, several factors are known to cause alteration of the chemical structure of DNA. The factor(s) likely to be important in a living cell is (are):A.heat.B.low pH.C.oxygen.D.UV light.E.both C and D.(22) In living cells, nucleotides and their derivatives can serve as:A.carriers of metabolic energy.B.enzyme cofactors.C.intracellular signals.D.precursors for nucleic acid synthesis.E.all of the above.(23) ATP is NOT a nucleoside because it ________.A. has phosphate groupsB. has three phosphates instead of just oneC. lacks the deoxyribosyl groupD. is not connect to a carbohydrate group(24) According to Chargaff’s observations of nucleotide composition of DNA samplesA.% of (G + C) + % of (A + T) = 100%.B.A = T.C.G = C.D.%A + %G + %C + %T = 100%.E.All of the above(25) The rise and pitch of B-DNA are 0.33 nm and 3.40 nm, respectively. About how many helical urns are there in a fragment 1 mm in length?A. 3030B. 294C. 330D. 0.0034E. Cannot calculate from the information given.(26) Regions of DNA that are most easily unwound haveA. about half G and half C.B. alternating A and G.C. greater G:C content.D. greater A:T content.(27) Which is NOT true of the different conformations of DNA?A. Z-DNA is a left-handed spiral.B. A-DNA and B-DNA are right-handed spirals.C. A-DNA and Z-DNA segments are limited to short regions of DNA.D. Both A-DNA and B-DNA are dehydrated.(28) In addition to knowing the chemical structures of the nucleotides, Watson and Crick used________ of Franklin and Wilkins and the chemical equivalencies of Chargaff in order to propose their model of DNA structure.A. sequence informationB. UV spectraC. % (G + C) and % (A + T)D. X-ray diffraction data(29) In proteins, amino acids are linked by peptide bonds; in polynucleotides, nucleotides are linked byA. phosphoanhydride bonds.B. 3’-5’phosphodiester bonds.C. 5’-3’phosphodiester bonds.D. B and CE. All of the above(30) It is easier to melt DNA richer in AT than GC becauseA. it is more heat sensitive.B. there is one less hydrogen bond.C. the helix pitch is longer in AT rich regions.D. All of the above(31) As B-DNA is gradually heated, the absorbance at 260 nmA. increases.B. decreases.C. stays the same.D. is half way between that of poly (AT and poly (GC).(32) Which of the following is mismatched?A. rRNA: 80% of cellular RNAB. tRNA: carry amino acids during protein synthesisC. mRNA: stable RNA carrying the coded information from DNAD. small RNA: catalytic with or without proteins(33) Which type of RNA is the most abundant in living cells (by percent)?A. ribosomalB. messengerC. smallD. transfer(34) Which is NOT a difference between RNA and DNA?A. The sugar ring of RNA is more oxidized than that in DNA.B. RNA contains uracil; DNA usually does not.C. RNA cannot form helices.D. RNA is single-stranded; DNA is double-stranded.(35) How does a nucleotide differ from a nucleoside?A. Nucleosides are found in DNA, whereas nucleotides are found in RNA.B. Purines are only found in nucleotides.C. Nucleosides contain only deoxyribose sugars.D. A nucleotide is a nucleoside with a phosphate ester linked to the sugar .E. None of the above.(36) The feature(s) of DNA deduced by Watson and Crick includedA. two antiparallel polynucleotide chains coiled in a helix around a common axis.B. the pyrimidine and purine bases lie on the inside of the helix.C. the bases are nearly perpendicular to the axis.D. All of the above.E. None of the above.(37) The chemical forces that contribute to the stability of the DNA due to the base stacking present in the DNA helix areA. hydrogen bonds.B. van der Waals.C. disulfide bonds.D. B and C.E. None of the above.3. Short answer questions(1). A viral DNA is analyzed and found to have the following base composition, in mole percent: A = 32, G = 16, T = 40, C = 12.A. What can you immediately conclude about this DNA?B. What kind of secondary structure do you think it would have?(2). Give the following sequence for one strand of a double-strand oligonucleotide:5’ ACCGTAAGGCTTTAG 3’A. Write the sequence for the complementary DNA strand.B. Write the sequence of the RNA complementary to the strand shown above.(3). A stretch of double-stranded DNA contains 1000 bp, and its base composition is 58%(G+C). How many thymine residues are in this region of DNA?(4). Do the two complementary strands of a segment of DNA have the same base composition?Does (A+G) equal (C+T)?(5). In samples of DNA isolated from two unidentified species of bacteria, X and Y, adenine makes up 32% and 17%, respectively, of the total bases. What relative proportions of adenine, guanine, thymine, and cytosine would you expect to find in the two DNA samples? What assumptions have you made? One of these species was isolated from a hot spring (64℃). Suggest which species is the thermophilic bacterium. What is the basis for your answer?(6). Calculate the weight in grams of a double-helical DNA molecule stretching from the earth to the moon (~320,000 km). The DNA double helix weighs about 1 X 1018 g per 1,000 nucleotide pairs; each base pair extends 3.4 Å. For an interesting comparison, your body contains about 0.5 g of DNA!(7). Compare hydrogen bonding in the αhelix of proteins and in the double helix of DNA. Include the answer the role of hydrogen bonding in stabilizing these two structures.(8). Describe qualitatively how the t m for a double-stranded DNA depends upon its nucleotide composition.(9). Write the structure of cAMP and cGMP molecules.。
Beam Scannable 4×4 Patch Array Antenna Using Tunable Metamaterial Phase Shifters
Beam Scannable 4×4 Patch Array Antenna Using Tunable Metamaterial Phase ShiftersY.-K. Jung, and B. LeeKyung Hee University, Department of Electronics and Radio Engineering, Deogyeong-daero, 446-701, Yongin-si, Koreamama731@khu.ac.krAbstract – A beam scannable 4×4 patch array antenna using the tunable metamaterial (MTM) phase shifter has been designed at S-band. The phase shifter consists of four MTM unit cell composed of a series variable capacitor and a shunt tunable inductor. The inductor is realized by a variable capacitor and quarter wave impedance transformer. The phase shifting range of the phase shifter is 280° (from -87° to 193°) with the insertion loss of 0.9 dB. The 10 dB bandwidth and maximum gain of the antenna are about 94 MHz and 16 dBi, respectively. The beam scanning ranges in the E- and H-planes are 43° and 75°, respectively.I. INTRODUCTIONArray antenna technology has been widely adopted in the areas of defense, satellite communication, vehicle collision detection, and so on. Still, beam scanning of an array antenna is usually achieved through the mechanical phase shifters, which are bulky, heavy, slow, and expensive. For this reason, an electrically controlled phased array antenna may be preferred and more widely applied [1]. The electrical phase shifters have long been developed using various methods such as the multi-bit switched line method, loaded line method, hybrid coupled method, and so on. More recently, the planar MTM-based transmission line (TL) has drawn much attention [2] since it shows unusual characteristics such as positive/negative-refractive-index with a broad passband and can also be easily constructed on PCB’s with low price, low profile and light weight. In addition, the output phase through the MTM-TL can be controlled by using tunable elements. A fully scannable leaky wave antenna has been considered to be one of the important applications in the MTM technology [3]. A disadvantage of the MTM leaky wave antenna (MLWA) is that it needs too many unit cells (typically 20~30 units) to radiate most of an input power into the air due to small radiation from a unit cell. A MTM tunable phase shifter has been adopted for a scannable microstrip patch array antenna [4]. In it, the unit cell of the phase shifter consists of a series chip varactor and a shunt active inductor. However, the circuit to realize the active inductor is complicated and may possibly be avoided. In this work, the active inductor is replaced by a simple λ/4 impedance inverter and a chip tunable capacitor. Then, a simple and inexpensive phase shifter is designed using [5], [6]. Finally, a beam scannable 4x4 microstrip patch array antenna using the phase shifter is presented together with evaluations of return loss, beam scannable range, etc.II. DESIGN OF 4X4 ARRAY ANTENNA USING METAMATERIAL PHASE SHIFTER Fig. 1 show the schematics of the MTM phase shifter and the feed line using the phase shifters. They are constructed using a microstrip line on FR-4 substrate with relative permittivity of 4.6 and height of 1 mm. The design frequency is 2.2 GHz (S-band). The size of the feed line is approximately 230 mm × 70 mm. The diameter of the shorting vias in this design is 0.3 mm. The other dimensions are as shown in Fig. 1. The phase shifter has 4 MTM unit cells. The unit cell usually consists of a conventional host transmission line with electrical length kd, a series lumped capacitor with C0, and the shunt inductor with L0. kd of the unit cell is 28.8 °at 2.2 GHz. To realize the inductance L0, we use a λ/4 impedance convertor and a chip variable capacitor with C1 [7]. The characteristic impedance (Z0) of the host line is 50 Ω. The overall biasing to the variable capacitors is realized using only one DC voltage source with automatic matching [8]. Fig. 2 shows the designed 4×4 scannable microstrip patch array antenna. The main feed line is shown to have four branch lines. The sixteen patch antennas are fed by the same power. The array antenna has been designed such that with the phase shift of 0 ° for all patch radiation elements, the beam points to the broadside. In Fig. 1(b), the characteristic impedancesZ A, Z T1, Z T3, Z T5, and Z T7 are 100 Ω, 70.7 Ω, 50 Ω, 40.8 Ω, and 50 Ω, respectively. The distances between two antennas in the x and y direction respectively are 56.2 mm(d = 0.41 λ0) and 94.7 mm(d = 0.69 λ0).The size of the patch antenna is 32.2 mm × 38 mm. The beam directions are controlled by changing the bias voltages of V1 (for H-plane) and V2 (for E-plane).(a) MTM phase shifter (b) Main feed lineFig. 1. Schematics of MTM phase shifter and main feed line.Fig. 2. 4×4 microstrip patch array antenna.III. PerformanceFig. 3 (a) shows the EM-simulated ∠S21 of the MTM phase shifter with varied capacitance of the varactors from 1.5 pF to 14.2pF as a function of frequency. The SMV1283-011LF varactor diode (Skyworks Solution Inc.) has been selected to control the C0 values. The recommended capacitance range of the varactor is from 14.228 pF to 0.517 pF with the necessary reverse bias voltage from 0 V to 26 V. We restrict the use of the C0 values from 14.228 pF (0V) to 1.5 pF (7V). Using this range, the phase shift of the MTM unit cell is from -21 ° to 48 °. The range of the total phase shift is from -87 ° to 193 °. The insertion loss of the phase shifter is approximately 0.9 dB. The EM-simulated S-parameters of the main feed line in Fig. 1(b) are shown in Fig. 3 (b) and (c).(a) ∠S21 of MTM phase shifter (b) Magnitude (c) PhaseFig. 3. EM-simulated S-parameters of MTM phase shifter and power splitter.The magnitude of S11 is about -14 dB at 2.2 GHz. The magnitudes of S k1 for k = 2 to 5 are about -7 dB. When the phase shift of the phase shifter is 0°, the phase differences between port 2 and 3, port 3 and 4, and port 4 and 5, are shown to be about 180° at 2.2 GHz. The EM-simulated reflection coefficient of the antenna as a function of the frequency is shown in Fig. 4 (a) when the main beam angle is 0°. The 10 dB bandwidth of the antenna is about 94 MHz. Fig. 4 (b) and (c) show the EM-simulated gain patterns of the proposed antenna. The half power beam widths (HPBW) in the E- and H-plane are 37 ° and 21.8 °, respectively, when the main beam peak is at 0°. When the bias voltage of V1 is changed from 0 to 7 V with V2 fixed at 4.5V, the beam peak in the H-plane shifts from 30° to -45°. On the other hand, when the bias voltage of V2 is changed in the same fashion, the beam in the E-plane is changed from 19 ° to -24 °.(a) S-parameter (b) gain pattern in H-plane (c) gain pattern in E-plane Fig. 4. EM-simulated reflection coefficient and gain patterns of the proposed 4×4 microstrip patch array antenna.IV. ConclusionsA scannable 4×4 patch array antenna has been designed using the tunable simple metamaterial phase shifters. The phase of the proposed MTM unit cell can be controlled conveniently with one voltage source. The phase shifting range of the proposed MTM phase shifter is from -87° to 193°, with the insertion loss of 0.9 dB. The maximum antenna gain is about 16 dBi. The scanning ranges in the H- and E-plane of the antenna are 75° and 43°, respectively.A CKNOWLEDGEMENTThis work was supported by the National Research Foundation of Korea (NRF) grant funded by the Korea government (MEST) (No. 2013015202).R EFERENCES[1] E. Ojefors, S. Cheng, K. From, I. Skarin, P. Hallbjorner, and A. Rydberg, “Electrically steerable single-layer microstriptraveling wave antenna with varactor diode based phase shifters,” IEEE Trans. Antennas Propag., vol. 55, no. 9, p.2451–2460, Sep. 2007.[2]G. V. Eleftheriades, A. K. Iyer, and P. C Kremer, “Planar negative refractive index media using periodically L-C loadedtransmission lines,” IEEE Trans. Microw. Theory Tech., vol. 50, p.2702–2712, 2002.[3]S. Lim, C. Caloz, and T. Itoh, “Metamaterial-based electronically controlled transmission-line structure as a novelleaky-wave antenna with tunable radiation angle and beamwidth,” IEEE Trans. Microw. Theory Tech., vol. 53, no. 1, p.161-173, Jan. 2005.[4]M. A. Y. Abdalla, K. Phang, and G. V. Eleftheriades, “A Planar Electronically Steerable Patch Array Using TunablePRI/NRI Phase Shifters,” IEEE Trans. Microw. Theory and Tech., vol. 57, no. 3, p. 531-541, 2009.[5]S. Jang and B. Lee, “Investigation of I-D Meta-structured Leaky Wave Antennas Using Transmission Line TheoryConsidering Radiation Effects,” IEEE antennas and Propag. Society Int. Symp., Charlston, U.S.A., p. 1-4, Jun. 2009. [6]T. Kim and B. Lee, “Modelling and analysis of radiation effects for one-dimensional metamaterial-based transmissionlines,” IET Microw. Antennas Propag., vol. 4, no. 3, p. 278-295, Mar. 2010.[7]Y.-K. Jung and B. Lee, “Compact Metamaterial-based Tunable Phase Shifter at 2.4 GHz,” KIEES J. Electrom. Eng.Science, vol. 13, no. 2, p.137-139, 2013.[8]Y.-K. Jung and B. Lee, “Beam Scannable Patch Array Antenna Employing Tunable Metamaterial Phase shifter,” IEEEantennas and Propag. Society Int. Symp., Chicago, U.S.A., p. 1-4, Jun. 2012.。
剑桥雅思7 test 1 passage 2 word版本
Question 14-20Reading Passage 2 has seven paragraphs, A–H.Choose the correct heading for paragraphs A and B–H from the list of headings below.14. Paragraph A15. Paragraph C16. Paragraph D17. Paragraph E18. Paragraph F19. Paragraph G20. Paragraph HA.The history of human civilisation is entwined with the history of the ways wehave learned to manipulate water resources. As towns gradually expanded, water was brought from increasingly remote sources, leading to sophisticated engineering efforts such as dams and aqueducts. At the height of the Roman Empire, nine major systems, with an innovative layout of pipes and well-built sewers, supplied the occupants of Rome with as much water per person as is provided in many parts of the industrial world today.B.During the industrial revolution and population explosion of the 19th and 20thcenturies, the demand for water rose dramatically. Unprecedented construction of tens of thousands of monumental engineering projects designed to control floods, protect clean water supplies, and provide water for irrigation and hydropower brought great benefits to hundreds of millions of people. Food production has kept pace with soaring populations mainly because of theexpansion of artificial irrigation systems that make possible the growth of 40 % of the world's food. Nearly one fifth of all the electricity generated worldwide is produced by turbines spun by the power of falling water.C.Yet there is a dark side to this picture: despite our progress, half of theworld's population still suffers, with water services inferior to thoseavailable to the ancient Greeks and Romans. As the United Nations report on access to water reiterated in November 2001, more than one billion people lack access to clean drinking water; some two and a half billion do not have adequate sanitation services. Preventable water-related diseases kill an estimated 10,000 to 20,000 children every day, and the latest evidence suggests that we are falling behind in efforts to solve these problems.D.The consequences of our water policies extend beyond jeopardising human health.Tens of millions of people have been forced to move from their homes - often with little warning or compensation - to make way for the reservoirs behind dams.More than 20% of all fresh water fish species are now threatened or endangered because dams and water withdrawals have destroyed the free-flowing river ecosystems where they thrive. Certain irrigation practices degrade soil quality and reduce agricultural productivity. Groundwater aquifers*are being pumped down faster than they are naturally replenished in parts of India, China, the USA and elsewhere. And disputes over shared water resources have led to violence and continue to raise local, national and even international tensions.E.At the outset of the new millennium, however, the way resource planners thinkabout water is beginning to change. The focus is slowly shifting back to the provision of basic human and environmental needs as top priority -ensuring 'some for all, 'instead of 'more for some'. Some water experts are now demanding that existing infrastructure be used in smarter ways rather than building new facilities, which is increasingly considered the option of last, not first, resort. This shift in philosophy has not been universally accepted, and it comes with strong opposition from some established water organisations. Nevertheless, it may be the only way to address successfully the pressing problems of providing everyone with clean water to drink, adequate water to grow food and a life free from preventable water-related illness.F.Fortunately -and unexpectedly - the demand for water is not rising as rapidlyas some predicted. As a result, the pressure to build new water infrastructures has diminished over the past two decades. Although population, industrial output and economic productivity have continued to soar in developed nations, the rate at which people withdraw water from aquifers, rivers and lakes has slowed. And in a few parts of the world, demand has actually fallen.G.What explains this remarkable turn of events? Two factors: people have figuredout how to use water more efficiently, and communities are rethinking their priorities for water use. Throughout the first three-quarters of the 20th century, the quantity of freshwater consumed per person doubled on average; in the USA, water withdrawals increased tenfold while the population quadrupled.But since 1980, the amount of water consumed per person has actually decreased, thanks to a range of new technologies that help to conserve water in homes andindustry In 1965, for instance, Japan used approximately 13 million gallons* of water to produce $1 million of commercial output; by 1989 this had dropped to million gallons (even accounting for inflation) - almost a quadrupling of water productivity. In the USA, water withdrawals have fallen by more than 20 % from their peak in 1980.H.On the other hand, dams, aqueducts and other kinds of infrastructure will stillhave to be built, particularly in developing countries where basic human needs have not been met. But such projects must be built to higher specifications and with more accountability to local people and their environment than in the past.And even in regions where new projects seem warranted, we must find ways to meet demands with fewer resources, respecting ecological criteria and to a smaller budget.Question 21-26Do the following statements agree with the claims of the writer in Reading Passage 2?In boxes 21-26 on your answer sheet, writeTRUE if the statement agrees with the informationFALSE if the statement contradicts the informationNOT GIVEN if there is no information on this in the passage21? Water use per person is higher in the industrial world than it was in Ancient Rome.22? Feeding increasing populations is possible due primarily to improved irrigation systems.23? Modern water systems imitate those of the ancient Greeks and Romans.24? Industrial growth is increasing the overall demand for water.25? Modern technologies have led to a reduction in domestic water consumption. 26? In the future, governments should maintain ownership of water infrastructures.。
先进制造技术英文版第二版课程设计
先进制造技术英文版第二版课程设计1. IntroductionAdvanced manufacturing has emerged as one of the key drivers of economic growth and development in many countries around the world. In order to meet the challenges of the modern industrial landscape, it is essential for students of engineering and related disciplines to have a strong understanding of the latest manufacturing techniques and technologies. This course, based on the second edition of the Advanced Manufacturing Technology textbook, is designed to provide students with an in-depth understanding of the key principles and practices of advanced manufacturing.2. Course ObjectivesThe objectives of this course are as follows:1.To familiarize students with the latest manufacturingtechnologies and techniques2.To provide students with an understanding of the keyprinciples and practices of advanced manufacturing3.To develop students’ critical thinking and problem-solvingskills in the context of advanced manufacturing4.To enhance students’ ability to communicate effectivel y inan academic and professional context3. Course OutlineThe course will be divided into nine modules, each of which will cover a specific aspect of advanced manufacturing. The topics to be covered are as follows:1.Introduction to advanced manufacturing2.Manufacturing processes for advanced materials3.Advanced metrology and quality control4.Robotics and automation in manufacturing5.Sustnable manufacturing practices6.Additive manufacturing and 3D printing7.Digital manufacturing and Industry 4.08.Case studies in advanced manufacturing9.Future trends in advanced manufacturing4. Course Format and AssessmentThe course will be delivered through a combination of lectures, tutorials, and group assignments. Students will be assessed through a combination of individual and group assignments, presentations, and a final exam. The assignments and presentations will be designed to develop students’ critical thinking and problem-solving skills, as well as their ability to communicate effectively in an academic and professional context.5. Course MaterialsThe required textbook for this course is the second edition of Advanced Manufacturing Technology by T. K. Kuo and J. S. Yan. Inaddition to the textbook, supplementary readings and case studies will be provided to students throughout the course.6. ConclusionIn conclusion, this course is designed to equip students with the skills and knowledge necessary to succeed in the rapidly evolving field of advanced manufacturing. By the end of the course, students should be able to apply the key principles and practices of advanced manufacturing to real-world manufacturing challenges and communicate effectively about advanced manufacturing in an academic and professional context.。
organic chemistry有机化学 第五版 LGWade JR 答案
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underwriter reputation,earnings management and the long run performence of IPO
Accounting and Finance50(2010)53–78Underwriter reputation,earnings management and the long-run performance of initial public offerings Shao-Chi Chang a,Tsai-Yen Chung b,Wen-Chun Lin aa Institute of International Business,Cheng Kung University,Tainan,Taiwanb Department of Accounting,Yuan Ze University,Taoyuan,TaiwanAbstractThis study contributes to the extant literature on the nature of earnings manage-ment surrounding initial public offerings(IPOs)by investigating the role of underwriter reputation.We argue that prestigious underwriters will protect their reputation by carefully monitoring and certifyingfinancial information on IPO firms,thereby limiting any potential earnings manipulation.As a result,those IPOfirms that are associated with more prestigious underwriters are likely to exhibit substantially less-aggressive earnings management.Conversely,wefind the existence of a negative relationship between earnings management and the post-offer performance of an IPOfirm’s stocks only for thosefirms associated with less-prestigious underwriters.Key words:Earnings management;Initial public offerings;Long-run performance; Underwriter reputationJEL classification:M41,G14,G24,G32doi:10.1111/j.1467-629X.2009.00329.x1.IntroductionIt is already well documented within the extant literature that a characteristic of‘initial public offerings’(IPOs)is the high degree of information asymmetry between issuers and investors(Leland and Pyle,1977;Carter and Manaster, 1990).Given that investors have limited access tofinancial information on the We thank Chief Editor Robert Faff,Professor Ian Zimmer,and one anonymous reviewer for their challenging and constructive comments on earlier versions of this article.We also greatly appreciate the helpful comments provided by an anonymous reviewer as well as the support of the participants at the2008Financial Management Association Conference.Received18November2008;accepted5August2009by Ian Zimmer(Deputy Editor).54S.-C.Chang et al./Accounting and Finance50(2010)53–78issuingfirm prior to the IPO,they are heavily reliant upon the prospectus, despite the fact that this provides only limited information(Rao,1993);asfirms go public essentially so that they can gain access to the capital markets,present-ing the issuingfirms in the best possible light raises the likelihood of a successful offering.Thus,managers have considerable incentives to exercise their discretion, which can lead to the inflation of reported earnings.Hughes(1986)and Titman and Trueman(1986)provide evidence of a positive relationship between the information provided infinancial statements and the offer price of IPOs,while Friedlan(1994)and Teoh et al.(1998a,b)point to a significant increase in discretionary accruals prior to IPO announcements.In those cases where earnings are found to have been over-inflated prior to the announcement of IPOs,this is likely to have detrimental effects on the post-IPO performance of thefirms involved through the reversal of these accruals.Con-sistent with this argument,Teoh et al.(1998a,b)provide evidence of a negative correlation between discretionary accruals and long-run stock performance. Although numerous studies report patterns of abnormal accruals surrounding IPO issues(Friedlan,1994;Teoh et al.,1998a,b),the explanations provided for such patterns are rather inconclusive.Beneish(2001)argues that although equity offerings create situations in which managers have strong incentives to manipulate their earnings,any conclusion of intentional earnings manipulation based on changes in discretionary accruals at the time of security issuance may be premature. Ball and Shivakumar(2008)further question the hypothesis of artificial earn-ings inflation being used as a means of influencing IPO prices,as documented in the prior studies.They argue that as opposed to intentional earnings manipula-tion,the increase in discretionary accruals surrounding IPOs could actually be attributable to thefirms’use of their IPO proceeds simply to make appropriate changes to their working capital;thus,as many cash-constrained privatefirms are likely to exhibit under-investment in both receivables and inventory prior to their listing,any significant increase in discretionary accruals may simply repre-sent the relief of such resource constraints.Furthermore,managers should actually have far less incentive to manipulate their earnings around the listing date,essentially because this is likely to lead to an increase in the litigation and regulation risks for the managers engaging in such action.Thus,Ball and Shivakumar(2008)argue that much of the prior evi-dence of significant earnings management surrounding around IPO events could be misleading.Although quite convincing,the argument of Ball and Shivakumar(2008)fails to explain the significant negative relationship between earning management and post-issue stock performance documented in the extant literature on both IPOs (Teoh et al.,1998b)and‘seasoned equity offerings’(SEOs)(Teoh et al.,1998c). If an increase in discretionary accruals is mainly attributable to the alleviation offinancial constraints or signalling information,then we would not expect to see earnings management having strong,negative impacts on the subsequent performance of the stocks following such offerings.S.-C.Chang et al./Accounting and Finance50(2010)53–7855 We therefore set out in this study to determine the actual nature of earnings management by investigating the effects of underwriter reputation.As a result of the limited information available on IPOfirms,it is often difficult for investors to distinguish between changes in abnormal accruals attributable to managers exercising their discretion,and those that are due to exogenous influences from the operational decisions taken byfirms(Kaplan,1985;McNichols and Wilson, 1988;Beneish,1998).The services of underwriters are engaged by issuingfirms to purchase,market and distribute their securities to investors.It is expected that the due diligence investigations carried out by such underwriters will reflect the actualfinancial condition of the IPOfirms,thereby assisting investors in their assessment of the value of the issuingfirm.We therefore argue that underwriter reputation plays an important role in the nature of earnings management surrounding IPOs. Firstly,when the capital markets are unable to make any clear distinction between the changing nature of afirm’s accruals,prestigious underwriters can provide some added credibility.Investors in the IPO markets are faced with a serious problem of asymmetric information,while the underwriters marketing a firm’s equity have strong incentives to show that thefirm’s issued security is wor-thy of investment,regardless of whether or not they have expended sufficient resources to investigate thefirm.Some underwriters may produce a careful, detailed evaluation of thefirm’s projects,while also demonstrating good quality accounting disclosure practices,whereas others may compromise their due dili-gence responsibilities for the sake of attracting business.Chemmanur and Fulghieri(1994)argue that the more prestigious underwriters will invariably attempt to reduce the probability of marketing‘lemons’,since such business could ultimately damage the reputation of the company.Such prestigious underwriters will therefore tend to set much stricter standards in their evaluation offirms,and although this may prove costly in the short-run,their primary aim is clearly the pursuit of greater long-term benefits.Thus,in the face of noisy discretionary accruals,underwriter reputation may well be found to play an important role in certifying the actual quality of changes in accruals. Secondly,underwriters also have a role to play in restricting the extent of earn-ings manipulation.Prestigious underwriters will closely monitor the quality of thefinancial information which is being provided by engaging the services of rep-utable auditors and evaluating the viability of business models to reduce the agency costs between issuers and investors;as a result,they can provide reliable certification of their clients(Jo et al.,2007).Since the act of underwriting involves repeat business with afinite number of competitors,investors can read-ily engage in the ex-post evaluation of the quality of the underwriters’services.It would clearly be very costly and difficult for underwriters to market future issues if the investors involved had already been misled by the same underwriters in their prior IPO investments.Furthermore,the more reputable underwriters must protect the enhanced ‘reputation capital’that they already possess;they therefore have much stronger56S.-C.Chang et al./Accounting and Finance50(2010)53–78incentives to provide quality monitoring in order to enhance the transparency of earnings and prevent any aggressive earnings management.In contrast,low-quality underwriters may compromise their responsibilities towards information monitoring in order to acquire more underwriting business.Hansen and Torregrosa(1992)show that underwriter monitoring improves a firm’s performance by reducing its agency costs,while Jo et al.(2007)point to a strong negative relationship between earnings management and underwriter rep-utation.In the present study,we examine the association between underwriter reputation and the nature of earnings management.Specifically,we argue that an increase in accruals within IPOfirms that are underwritten by high-quality underwriters is more likely to represent changes in thefirms operations and overall performance.Conversely,we suggest that an increase in accruals for IPO firms associated with low-quality underwriters may be the result of opportunistic earnings inflation.Our hypothesis is that discretionary accruals may show signif-icant increases for both high-and low-quality underwriters prior to IPOs,but the nature of such increases may differ markedly.When changes in accruals are due to intentional earnings inflation,the subse-quent reversal in earnings is likely to result in a significant negative relationship between earnings management and post-IPO stock performance.In contrast, when changes in accruals are due to changes in business operations and working capital,we would not expect tofind any significant relationship.We therefore hypothesize that the negative association between earnings management and subsequent stock performance will be significant only for IPOs underwritten by low-quality underwriters,and not for those underwritten by high-quality under-writers.We collect a sample of the US IPOfirms covering the period from1989 to2003to test our hypotheses.In this study,we adopt discretionary current accruals as our measure of earnings management,while providing controls for the changes in discretionary accruals attributable to changes in performance.Our analysis also uses perfor-mance-adjusted discretionary current accruals as proposed in Kothari et al. (2005).Our results show that significant increases in discretionary current accru-als are experienced byfirms with both high-and low-quality underwriters during their IPO years;however,when using performance-adjusted discretionary cur-rent accruals,the discretionary accruals for the high-quality underwriter group become statistically insignificant,while those for the low-quality group remain significant.We alsofind that underwriter reputation has a significantly negative correla-tion with earnings management,a result which is in line with the evidence on SEOs provided by Jo et al.(2007).Consistent with the prior studies,our results indicate that earnings management has a negative correlation with post-IPO stock performance for the full sample of IPOfirms.However,when the interac-tion effect between earnings management and underwriter reputation is included within the regression model,the effect of earnings management becomes statisti-cally insignificant,whereas the interaction effect is highly significant.TheseS.-C.Chang et al./Accounting and Finance50(2010)53–7857 results highlight the importance of considering the effect of underwriter reputa-tion when assessing the relationship between earnings management and post-IPO performance.Our paper contributes to the literature on earnings management surrounding major corporate events.Although Teoh et al.(1998a–c)show that earnings man-agement has a negative correlation with post-issue performance,Shivakumar (2000)and Fan(2007)couldfind no strong evidence of such a relationship.The findings of the present study could provide one possible explanation for such inconclusive evidence in the prior studies,thereby indicating the importance of considering underwriter quality in addressing the relationship between earnings management and long-run performance.Our study is also closely related to the literature on earnings management. McNichols and Wilson(1988)and Beneish(1998)both argue that abnormal accruals could be caused by managers intentionally overstating their reporting figures or by performance changes infirms going public.The present study sug-gests that these abnormal accruals may have a strong correlation with the choice of underwriter.Managers with an incentive to manipulate earnings could engage low-quality underwriters,essentially because such underwriters are likely to pro-vide a less rigorous monitoring function.Conversely,since high-quality under-writers provide stricter monitoring and inspection of afirm’s information,the abnormal accruals offirms with high-quality underwriters could simply be the result of changes in performance.The remainder of this paper is organized as follows.The data and sample selection are presented in Section2,followed in Section3by an introduction to our methodology and presentation of the empirical results.Section4presents the robustness tests on our results,with concluding remarks subsequently being presented in Section5.2.SampleThe initial full sample of the US common stock offerings covering the period from1989to2003is obtained from the Global New Issues database of the Secu-rities Data Company(SDC).We exclude all SEOs,unit offerings and ADRs, withfirms in thefinancial and utilities industries also being excluded from the sample since they are subject to stricter government regulations and unique dis-closure requirements(Teoh et al.,1998c;Shivakumar,2000;Teoh and Wong, 2002;Fan,2007;Lee and Masulis,2008).To qualify for inclusion in the sample,companies must have made an offer price in excess of US$1.00with market capitalization of at least US$20million (in December1997purchasing power,following Teoh et al.,1998b);this selec-tion procedure resulted in a sample of3899offers,of which241were unavailable on the annual COMPUSTAT database and/or the Center for Research in Secu-rity Prices(CRSP)file.Furthermore,for inclusion in the sample,the offer must have been made by a company:(i)whose industry has more than10firms with58S.-C.Chang et al./Accounting and Finance50(2010)53–78the same two-digit SIC code to facilitate the estimation of the expected accruals; and(ii)with sufficient accounting data for the estimation of abnormal accruals amongst the sample companies and for cross-sectional analyses.These restric-tions eliminated a further1002offers,leaving afinal sample of2053offers.The sample selection process is presented in Panel A of Table1,with Panels B and C of Table1presenting the respective sample distribution,by year and indus-try.Panel B shows that the IPOs were concentrated mainly in the years 1994–1997(accounting for50.27per cent of the samplefirms).Panel C reveals that IPOs in the manufacturing industries(two-digit SIC code20–39)accounted for approximately44.57per cent of the total sample,while those in the service industries(two-digit SIC code70–89)accounted for approximately33.80per cent. Table1Sample selection and sample distributionSelection Items Sub-totals Total Panel A:sample selection processTotal no.of IPOs(January1989–December2003)a6821 Less unit offerings and ADRs1505Lessfirms in the utilities orfinancial industries1133Lessfirms whose offer price<$1or capitalization<$20million b284Sample IPOs available3899 Less IPOs not available within the annual compustat and CRSP241databases1002Lessfirms missing accounting data for computation of abnormalaccrualsLessfirms missing data included in the regression analyses603Final sample2053 Panel B:sample distribution by offer yearYear19891990199119921993199419951996 Total no.5361135124169218241359% 2.58 2.97 6.58 6.048.2310.6211.7417.49 Year1997199819992000200120022003Total Total no.2141211351464419142053%10.42 5.89 6.587.11 2.140.930.68100.00 Panel C:sample distribution by industrySIC codes Industry No.of observations%1$9Agriculture,forestry andfishing30.15 10$14Mining50 2.44 15$17Construction23 1.12 20$39Manufacturing91544.57 40$48Transportation and communications104 5.07 50$51Wholesale trade77 3.75 52$59Retail trade1818.82 70$89Services69433.80 91$99Public administration60.29 Totals2053100.00 a Obtained from the SDC database.b Based upon1997purchasing power.S.-C.Chang et al./Accounting and Finance50(2010)53–7859 Table2Descriptive statistics of the IPO samplefirmsVariables Mean SD Q1Median Q3 Proceeds(US$millions)55.14105.4220.0033.6056.10 Offer price(US$)12.51 4.459.0012.0015.00 Shares offered(millions) 3.83 5.28 2.00 2.70 4.00 Venture backed0.460.500.000.00 1.00 Underprice0.210.370.020.100.26 Underwriter7.50 1.907.008.009.00 Age(years)15.3018.84 5.009.0017.00 BV(US$millions)81261193770 MV(US$millions)351106469130314B/M0.360.380.150.270.45 TA(US$millions)1837413260126 The sample comprises of a total of2053IPOs which took place between1989and2003and which met the data requirements described in section2.The proceeds,offer price,shares offered and venture backed variables are obtained from the SDC database;Underprice is defined as the ratio of the offer price to the closing price minus the offer price on the post-issue date;Underwriter refers to the rank-ing of thefirm’s underwriter using the‘reputation rankings for IPO underwriters’obtained from the Jay Ritter website;Age is the number of years between the founding year or incorporation date (whichever is earlier)and the IPO year;BV is the book value of common equity at the end offiscal year0;MV is the market value of common equity at the end offiscal year0;B/M is the book to mar-ket ratio of common equity at the end offiscal year0;and TA is the book value of total assets at the end offiscal year0.The summary statistics of our samplefirms are presented in Table2,which shows that the IPOfirms issued an average of3.83million shares,and raised average proceeds of US$55.14million,with about46per cent of the IPOs in our sample involving venture capital.The mean(median)level of under-pricing in the offer price is found to be21per cent(10per cent).The underwriter ranking used in this study is the Loughran and Ritter(2004)measure of underwriter rep-utation;these underwriter prestige measures are ranked on a0–9scale,and are based on the pecking order seen in‘tombstone’advertisements.1For our empirical analysis,if there is more than one leading underwriter,then we use the rank of the‘bookrunner’or the highest joint-ranking‘bookrunners’. The mean(median)age for our sample of IPOfirms is15.31(9.00),while the mean(median)underwriter reputation ranking is7.50(8.00).2The samplefirms 1We are extremely grateful for the underwriter reputation rankings provided by the Jay Ritter website(http://bear.cba.ufl.edu/ritter/ipodata.htm).2According to Loughran and Ritter(2004),underwriters with a ranking of8or9are con-sidered to be prestigious national underwriters,while those ranked between5and7are considered to be quality,regional or niche underwriters.The underwriter ranking found in our sample indicates that the IPOfirms generally sought the services of relatively high-quality underwriters.60S.-C.Chang et al./Accounting and Finance50(2010)53–78had a mean(median)common equity book value of US$81million(US$37 million)in the IPO year,and a mean(median)book-to-market(B/M)ratio of 0.36(0.27).33.Methodology and empirical resultsFollowing the timing convention of Teoh et al.(1998b),we define year0as thefiscal year in which the IPO occurred.Although Teoh et al.(1998b)found evidence of earnings management for equity-issuingfirms in both year0,securi-ties prices may react negatively with the reversal of managed accruals in the post-offering period.We therefore examine post-offering returns over the756-trading-day period starting from the day immediately after the earnings announcement in year0.If announcement date offinancial report is unavailable from the COMPUSTATfile immediately,then the post-offering returns are cal-culated starting from thefirst trading day of the fourth month after the end of fiscal year0.3.1.Earnings managementThe accounting of earnings involves consideration of both operating cashflows and accruals;however,given that managers have considerable discretion in their accounting of accruals,such accruals are susceptible to earnings management. An apparent increase in earnings can be accomplished by the early recognition of revenues,the delayed recognition of expenses,or a combination of both;how-ever,not all accrual items are equally susceptible to managerial manipulation. Short-term accruals,which are accounting adjustments to short-term assets and liabilities(such as bad debt allowances for accounts receivable and warranty liabilities)are easier to manipulate.We therefore adopt discretionary current accruals(DCA)and performance-adjusted discretionary current accruals (ADCA)variables to measure the extent of earnings management.3.1.1.Discretionary current accrualsWe follow the methodology of Teoh et al.(1998b,c)to measure the expected current accruals from a modified Jones(1991)model.As in Teoh et al. (1998b,c),total current accruals are broken down into their two constituent components,non-discretionary current accruals and discretionary current 3The book-to-market(B/M)ratio forfirms listed within the broader markets(including the NYSE,the AMEX and the NASDAQ)during our sample period,was found to have a mean(median)of0.70(0.57);thus,our IPOfirms have smaller B/M ratios than other listedfirms,which thereby suggests that IPOfirms may have better growth opportunities. The B/M ratio of0.4for our IPO sample is similar to that documented in Teoh et al. (1998b).S.-C.Chang et al./Accounting and Finance50(2010)53–7861 accruals.The former are the asset-scaled proxies for unmanaged accruals, while the latter are the asset-scaled proxies for earnings that are managed at the discretion of managers.3.1.2.Performance-adjusted discretionary current accrualsKothari et al.(2005)argue that the models of discretionary accruals may be specified in a severely incorrect manner when being applied to samples offirms demonstrating abnormal performance.We also employ the Kothari et al.(2005) ADCA approach in the present study,subtracting the discretionary accruals of a set of industry and return-on-assets(ROA)matched controlfirms prior to the IPO;thus each IPO issuer is matched with a non-IPOfirm from the same indus-try based upon the nearest ROA for the previousfiscal year and the same SIC code.4The ADCA for an IPO issuer is therefore calculated as the DCA of the IPO issuer minus the DCA of the non-IPO matchedfirm.Details on the time-series distribution of DCA for the full sample,from year0 to year3,are presented in Table3,showing an average increase of4.8per cent in DCA in the IPO year,followed by a decline of1.1per cent from the baseline assets from year2to year3.The time series pattern for ADCA also exhibits a similar peak in the issue year,with a discernible steady decline thereafter.If,as claimed by Ball and Shivakumar(2008),any increase in DCA is thought to be completely attributable to changes in business operations and working capital,then we would not expect to observe any clear reversing trend in accruals in subsequent years. The evidence in Table3suggests that,for at least some proportion of the sam-plefirms,the increase in DCA is due to opportunistic earnings management, with managers advancing accruals to report higher operational performance in the issuing period,and a gradual reversal in DCA in subsequent years.Since the sample composition changes each year as a result of data availability,Table3 also presents the results from a common sample offirms for which accruals data is available from year0to year3to test whether thefindings are affected by the changing sample size.Wefind that the common sample results are the same as those for the full sample.53.2.The underwriter reputation and earnings management relationshipTo examine whether the more prestigious underwriters succeed in suppressing potential earnings management by IPOfirms,we divide thefirms into four 4The matchingfirms are selected based upon the four-digit SIC codes;where these were unavailable,the three-digit SIC codes were used instead.5We test thefindings from Tables4to9based upon the common sample,andfind that the conclusions from each table remain unchanged;thus,for space saving purposes,the common sample results are not reported here.sub-groups based upon the underwriter reputation ranking;the G1group com-prises of firms whose underwriters have an index of 9,those with an index of 8form the G2group,those ranked 6$8form the G3group,and those ranked <6form the G4group.As shown in Panel A of Table 4,the mean DCA in year 0for all sub-groups is positive and significant.Furthermore,with a decline in underwriter reputation,the results reveal a strong pattern of a monotonic increase in DCA.The mean DCA is 2.2per cent for the G1group and 13.8per cent for the G4group,with this difference between the G1and G4groups()11.6per cent)being statistically significant at the 1per cent level.6Table 3Time-series profile of discretionary current accruals (DCA)and performance-adjusted discretionary current accruals (ADCA),1989–2003aTime horizonsNo.ofobservations Mean t -test c Median c DCA bInitial offer year20530.0489.46***0.0210***Initial offer year +119830.0247.57***0.0110***Initial offer year +217930.010 3.37***0.0050***Initial offer year +31600)0.001)0.32)0.0003Common sampleInitial offer year16000.0478.18***0.023***Initial offer year +116000.0257.16***0.014***Initial offer year +216000.011 3.61***0.005***Initial offer year +31600)0.001)0.32)0.0003ADCA bInitial offer year20530.031 4.95***0.020***Initial offer year +119830.021 4.96***0.018***Initial offer year +217930.005 1.130.009Initial offer year +31600)0.010)2.26**0.003**Common sampleInitial offer year16000.028 3.92***0.020***Initial offer year +116000.020 4.11***0.018***Initial offer year +216000.007 1.510.009**Initial offer year +31600)0.010)2.26**0.003a The sample comprises of a total of 2053IPOs which took place between 1989and 2003.Discretion-ary current accruals (DCA )and performance-adjusted discretionary current accruals (ADCA )are shown for firms offering initial equity,from the initial offer year to 1,2and 3-year periods after the offer.b DCA are extracted from current accruals using a two-digit SIC code cross-sectional,modified Jones (1991)model;ADCA are constructed by matching each treatment firm with a control firm based on industry and return on assets in period t )1.c The p -value refers to the t-statistic and the Wilcoxon z -statistic is adopted to test the median value.***indicates significance at the 1per cent level;and **indicates significance at the 5per cent level.6We have also calculated median DCA.The conclusions remain unchanged.62S.-C.Chang et al./Accounting and Finance 50(2010)53–78S.-C.Chang et al./Accounting and Finance50(2010)53–7863 Table4Time-series profile of discretionary current accruals(DCA)and performance-adjusted discretionary current accruals(ADCA),by underwriter reputation(UR)Variables Year0Year0$Year1Year0$Year2Year0$Year3 Panel A:DCAG1:UR=9.0No.of observations776748667591Mean0.022)0.006)0.013)0.013t-test 3.03***)0.69)1.64)1.47G2:UR=8.0No.of observations613592537486Mean0.024)0.012)0.011)0.018t-test 2.51**)1.13)0.94)1.57G3:6.0£UR<8.0No.of observations365358321290Mean0.071)0.021)0.057)0.061t-test 6.13***)1.57)3.95***)4.17***G4:UR<6.0No.of observations299285258233Mean0.138)0.087)0.116)0.139t-test8.85***)4.38***)5.73***)6.96***DifferenceMean)0.1160.0820.1020.126t-test)6.75*** 3.77*** 4.69*** 5.81*** Panel B:ADCAG1:UR=9.0No.of observations776748667591Mean0.0100.008)0.012)0.019t-test 1.060.73)1.07)1.54G2:UR=8.0No.of observations613592537486Mean0.0000.0010.004)0.011t-test0.000.090.48)0.65G3:6.0£UR<8.0No.of observations365358321290Mean0.057)0.014)0.058)0.053t-test 3.91***)0.87)3.16***)2.92***G4:UR<6.0No.of observations299285258233Mean0.120)0.064)0.088)0.125t-test 6.52***)2.72***)3.76***)5.09***DifferenceMean)0.1110.0720.0760.106t-test)5.36*** 2.77*** 2.89*** 3.86*** DCA are extracted from current accruals using a two-digit SIC code cross-sectional,modified Jones (1991)model;ADCA are constructed by matching each treatmentfirm with a controlfirm based on industry and return on assets in period t)1.UR is the ranking of the IPOfirm’s underwriter.IPOs are divided into(i)UR=9.0;(ii)UR=8.0;(iii)6.0£UR<8.0;and(iv)UR<6.0.The p-value refers to the t-statistic.***indicates significance at the1per cent level;**indicates significance at the 5per cent level;and*indicates significance at the10per cent level.ÓThe AuthorsJournal compilationÓ2009AFAANZ。
学术英语教师版Unit7课文翻译
Unit 7 Text A艰难抉择还是由医生来做吧！在接受完外科手术培训不久，我便和一位年轻的医生一起工作。
他是那种有感染力的人，不仅是因为他的临床经验，更是因为他对病人的专注。
他身躯健硕有力，但他似乎从来不给病人居高临下的感觉，每当和病人说话时，他会出人意料地缩身下蹲，与病人平视。
他会认真地倾听病人叙述病痛的每一个细节，每次问诊结束之前，总会询问病人是否还有他没解释到的问题。
有一天下午，看到他在一个护士站，用健硕的胳膊比划着向一名护士抱怨一位病人的家属，我很惊奇。
这是位临终的病人，这位年轻的医生安排了和病人家属的病情通报会，商谈撤掉生命支持系统和药物治疗并着手开始临终关怀。
在整个过程中家属一直喋喋不休地问问题，但最终拒绝做任何决定，也不撤除任何治疗手段。
“我一直在和他们说，我们要么继续给他们的亲人造成痛苦，要么可以让他走的舒服一些，”他说的同时手仍然在比划。
“我告诉他们要么受罪，要么舒服，都要由他们做决定。
但最终他们没有做决定就径直出了房间。
”医生和病人做医疗决定的方式在过去的 50 年里发生了巨大的变化。
多少代人以来，这些决定都是医生的专属范围；病人即便参与，在做最终选择时往往也没有什么话语权。
但是这种家长式的决策模式在上世纪 60 年代末和 70 年代开始发生改变。
要求给病人授权的呼声在增长；医学伦理学家也开始清楚地阐述有关病人伦理关怀的原则。
临床医生们尤为拥护的一条宗旨是对患者个体的尊重。
这一伦理原则已被应用于病房、诊所和手术室的工作中，也导致了一种新的临床理念的出现,即以病人为中心的医护。
但是，第二条伦理原则,即尊重每个人的自主权,与第一条密切关联，在医患关系上也起到越来越重要的作用。
随着时间的推移，自主权将意味着让病人自己做决定。
而且，这一诠释也渐渐进入医学院校的课程，也被写入联邦法律中，因为法律规定在治疗方式的选择上医生必须与病人进行讨论。
在接下来的 40 年里，要培养年轻的医生(我也包括在内)，除非是急诊或者一些无足轻重的决定之外，我们要约束自己，不要越俎代庖，代替病人做任何决定。
CH4_2014_EngMater_Lecture1
In bcc -Fe, max. octahedral void radius=0.036 nm.
Interstitial solid solution (e.g., C dissolved in Fe, forming Fe(C) solid solution)
Crystalline Imperfections in Metals: Linear Defects - General Dislocations:
One-dimensional defects around which atoms are misaligned; Lattice distortions are centered around a line.
Three-dimensional defects are actually forming a noncrystalline structure.
Crystalline Imperfections in Metals: Point Defects – Vacancy, Self-interstitial
Crystalline Imperfections in Metals: Linear Defects – Screw Dislocation Screw dislocation: left screw; right screw
A spiral planar ramp resulting from shear deformation.
Crystalline Imperfections in Metals: Point s–Substitutional Atoms
Lattice distortion (Tension or Compression)
CH8_2014_EngMater_Lecture2
Hardenability improves with increasing carbon content (<0.76wt%C), due to the decreased critical cooling rate.
Heat Treatment of Steels: Heat Treating Processes – Anneal, Normalizing • Annealing: furnace cooling (very slow) after heating and holding.
Quench
Medium Oil (no agitation) Oil (agitation) H2O (no agitation) H2O (agitation) Brine (no agitation) Brine (agitation) Cooling rate at the center of a 1-in. Bar (°C/s) 18 45 45 190 90 230
Eutectoid steel (1080)
Air cool A→P
Furnace cool
Critical cooling rate
140°C/s
Comparison of IT and CCT diagrams for eutectoid steel (1080)
Heat Treatment of Steels: Continuous-Cooling Transformations (CCT)
The highest hardness of M is mainly determined by its carbon content. The highest hardness of M is mainly determined by its carbon content.
人教版高中英语选修七Module7Unit1Thefourthperiod-Listeningandspeaking
Barry:Ofthe15sightedclimbers,only7madeit.
Joan:Oh,___w_e_l_ld_o_n_e____!
Barry:Yes,itjustshowsyou _______h_o_w__d_e_te_r_m__in_e_d_t_h_e_b_li_n_d_c_li_m_b_e_r…swere.
2.Inwhatwayscansightedclimbershelpthem? Thesightedpeoplecanactastheireyes,telling themtherightdirection.
Tickthestatementthataretrue. ()T1.Barryisblind. ()F2.Barryisbetween25-45yearsold. ()T3.Theblindclimbersgotassistancefromsighted companions. ()F4.Moreofthesightedgroupreachedthetopof themountainthanthosewhowereblindor weak-sighted. ()F5.NextyearBarryisgoingtoclimbMount Qomolangma.
enoughto…,too…to
3.Thelittleboyisnottall____e_n_o_u_g_hrteoachthe apple. 4.Weare___to_o__willing___t_ohelpthedisabled. 5.Hedidn’trunfast___e_n_o_u_g_h_teoscapebeing buriedbythelava.
Joan:Well,Barry.I’mfullofadmirationforyouandyourc ompanions.Youhaveachievedsomethingquiteremar kable.
Energy Distribution of a Charged Regular Black Hole
Energy Distribution of a Charged Regular Black Hole
I. Radinschi∗ Department of Physics, “Gh. Asachi” Technical University,
Iasi, 6600, Romania
Abstract
We calculate the energy distribution of a charged regular black hole by using the energy-momentum complexes of Einstein and Møller.
complexes of Einstein and Møller. From (13) it results that in the Einstein
prescription the first two terms in the expression of the energy correspond to
2
The Einstein energy-momentum complex [1] is given by
Θi k
=
1 16π
Hik,ll,
(5)
where
Hikl
=
−Hilk
=
gin √−g
[−g[gknglm
−
glngkm)],l.
(6)
Θ00 and Θα0 are the energy and, respectively, the momentum components. The Einstein energy-momentum complex satisfies the local conservation
AD 2000-Merkblatt W 7-2014
ICS 23.020.30
AD 2000-Merkblatt
November 2014 edition
Materials for
pressure vessels
Bolts and nuts made from ferritic steels
AD 2000-Merkblatt
W7
The AD 2000-Merkblätter are prepared by the seven associations listed below who together form the “Arbeitsgemeinschaft Druckbehälter” (AD). The structure and the application of the AD 2000 Code and the procedural guidelines are covered by AD 2000-Merkblatt G 1. The AD 2000-Merkblätter contain safety requirements to be met under normal operating conditions. If above-normal loadings are to be expected during the operation of the pressure vessel, this shall be taken into account by meeting special requirements. If there are any divergences from the requirements of this AD 2000-Merkblatt, it shall be possible to prove that the standard of safety of this Code has been maintained by other means, e.g. by materials testing, tests, stress analysis, operating experience.
2014职称英语理工类a级考试真题及答案
2014职称英语理工类a级考试真题及答案2014 Job Title English science and engineering Class A Exam Questions and AnswersTitle: 2014 Job Title English science and engineering Class A Exam Questions and AnswersIntroduction:The Job Title English science and engineering Class A exam is an important assessment for professionals in the science and engineering field. In 2014, the exam featured a variety of challenging questions that tested candidates' knowledge and skills in English language. This document provides a comprehensive analysis of the exam questions and detailed answers to help candidates prepare for future exams.Exam Questions:1. Which of the following sentences is correct?A) He have gone to school yesterday.B) She were watching TV last night.C) They was studying for the exam.2. Choose the correct word to complete the sentence:The ___________ of the experiment were inconclusive.A) resultB) resultsC) resulted3. Fill in the blank with the appropriate preposition:He is interested _________ studying abroad.A) inB) onC) at4. Choose the correct tense for the following sentence:She ___________ to Paris last month.A) goB) goesC) went5. Which of the following words is the correct synonym for 'determined'?A) decidedB) confusedC) lazyAnswers:1. Correct answer: B) She were watching TV last night.2. Correct answer: B) results3. Correct answer: A) in4. Correct answer: C) went5. Correct answer: A) decidedConclusion:The 2014 Job Title English science and engineering Class A exam featured a range of challenging questions that tested candidates' proficiency in English language. By studying the questions and answers provided in this document, candidates can better prepare for future exams and improve their chances of success. Good luck to all the candidates preparing for the Job Title English science and engineering Class A exam!。
2014春季英语人教版新课标选修7U4(Language Study)公开课导学案(1)
2014春季英语人教版新课标选修7U4(Language Study）公开课导学案（1）Learning plan：Book7 Unit 4 Sharing Period 2 Language Study PS: （FOR TEACHERS）选修7 Unit 4 Sharing- ReadingA LETTER HOMEDear Rosemary,Thanks for your letter, which took a fortnight to arrive. It was wonderful to hear from you。
I know you're dying to hear all about my life here，so I’ve included some photos which will help you picture the places I talk about。
You asked about my high school。
Well, it’s a bush school – the classrooms are made of bamboo and the roofs of grass。
It takes me only a few minutes to walk to school down a muddy track. When I reach the school grounds there are lots of "good mornings” for me from the boys. Many of them have walked a long way, sometimes up to two hours，to get to school。
There’s no electricity or water and even no textbooks either! I'm still trying to adapt to these conditions。
Photoelectron angular distributions of excited atoms in intense laser fields
PHYSICAL REVIEW A90,013417(2014)Photoelectron angular distributions of excited atoms in intense laserfieldsC.H.Raymond Ooi,1WaiLoon Ho,1and A.D.Bandrauk21Department of Physics,University of Malaya,50603Kuala Lumpur,Malaysia2Laboratoire de Chimie Th´e orique,Facult´e des Sciences,Universit´e de Sherbrooke,Sherbrooke,Qu´e bec,J1K2R1,Canada(Received9March2014;published17July2014)Angular distributions of photoionization differential rates for an atom in arbitrary excited states ionized by intense laserfields with arbitrary polarization are reported.Relativistic effects are incorporated into the Keldysh theory,yielding semi-analytical expressions of ionization rates for hydrogenic initial states in intense linear, circular,and elliptical laser polarizations.Angular distributions are compared for different angular momentum quantum numbers,magnetic quantum numbers,and Keldysh parametersγ.The angular distributions are shown to depend strongly onγ,thus also reflecting the influence of relativistic effects.The sign of the magnetic quantum number,corresponding to different electron rotations,is shown to have a significant effect on photoelectron angular distributions in circularly polarized laserfields.DOI:10.1103/PhysRevA.90.013417PACS number(s):32.80.Rm,33.80.Rv,03.30.+pI.INTRODUCTIONThe study of intense laser-matter interactions and attosec-ond[1,2]physics over the past two decades has attracted signif-icant attention.Advancement in the study of electron dynamics[3]in an intense laser is based on nonperturbative theoryand involves understanding of the physical processes suchas tunneling ionization(TI),multiphoton ionization(MPI)[4],above threshold ionization(ATI),and high harmonicsgeneration(HHG)[5–9],etc.The tunneling ionization concept by Corkum[4]providesclassical description of the electron dynamics that formsthe basis for understanding the high harmonic generation.The model was used to describe an electron in the strongelectromagneticfields,whichfirst tunnels to the continuumand recollides with the parent ion,emitting a photon thathas a maximum energy,N m ω0=I p+3.17U p,where N m is the number of incident photons.The ATI spectra dependson polarization,with circularly polarized pulses giving2U p ofmaximum energy[10].Further extension by Lein incorporatesrecolliding electrons for molecular imaging[11,12]andspectroscopy[13],thus providing important applications forthe development of intense light-matter interactions[14,15].In the study of recollision processes with circular pulses,itis shown that there are a few rules of thumb which apply tosystems more complex than atoms[16].The analytical formulaobtained for HHG[17]enhances the understanding of thenonperturbative strong-field processes.It is found that a staticelectricfield can induce new effects such as dichroicism andellipticity in HHG[18].A scheme using a combination of HHGand a terahertzfield to generate a single circularly polarizedattosecond pulse wasfirst proposed by Yuan and Bandraukrecently[19].There have been efforts to generalize the Keldysh theory[20–22]to arbitrary internal states,such as the well-knownAmmosov-Delone-Krainov(ADK)theory[23]that expressedthe probability of tunneling ionization in an alternatingfield,of a complex atom and of an atomic ion.It is based on theresult of Perelomov et al.[24].Their works use the generalizedasymptotic wave function to obtain thefinal photoionizationrate for arbitrary values of n,l,and m in an electricfieldof arbitrary ellipticity,where n,l,and m are the principle,the orbital angular momentum,and azimuthal or magnetic quantum numbers,respectively.It is found that the ionization rate depends on the sign of m[25].However,the calculation of the atomic ionization rate,which involves averaging the laser fields over one period of oscillation,is valid only in the low-frequency limit of the electromagneticfields or TI regime, namely,ω ωt,whereωt=1/τt andτt is the tunneling time, as is mentioned in the work of Keldysh.The theory of Keldysh,despite being perturbative,has its appealing features,such as providing analytical expressions for certain cases of study.His adiabaticity parameter,γ=I p/2U p,determines whether the photoionization is in the TI regime or the MPI regime,where I p is the ionization potential of the atom and U p=e2E2/4mω2is the ponderomotive energy.Typically,for high frequencies and weakfields or when the ponderomotive energy is lower than the ionization potential,the MPI process becomes dominant,where k number of photons are absorbed simultaneously during the transition from ground level to the continuum level.The tunneling ionization regime corresponds to the opposite case of high fields and low frequencies.The Keldysh formalism[26]is valid only for small momenta,i.e.,the terms higher than p2/2mI p are neglected. Our recent work provides an exact evaluation of the photoion-ization rate[27]for photoelectrons with arbitrary momentum using the Keldysh-type formalism that is perturbative and approximate.However,the theory is valid for hydrogenic atoms in the ground state only,and it is limited to the nonrelativistic regime.In view of the photoelectric effect, this means that when the photon energy far exceeds the ionization threshold,the photoelectron would be relativistic. The relativistic theory of photoionization has been used along with the R-matrix approach[28]but has not been adopted along with the Keldysh theory.Our work may be relevant to Milosevic et al.’s analytic tunnel ionization rates for hydrogenlike ions obtained using the semiclassical solution of the three-dimensional Dirac equation[29].Here,we focus on the simple relativistic corrections to the Keldysh and strong-field approximation(SFA)theories,which neglects the intermediate levels involved in resonant or near-resonant transitions that may create coherent superpositions of bound states.This is justified for the following reasons.First, states above our initial excited states are completely ionized due to their smaller ionization potentials and therefore areC.H.RAYMOND OOI,W AILOON HO,AND A.D.BANDRAUK PHYSICAL REVIEW A90,013417(2014) not contributing to Stark effects,etc.Second,states lower inenergy,such as the ground state,will also be unimportant,asthey are nonresonant in the dynamics because excited stateshave large ionization rates and small lifetimes.Third,we areconsidering ionizing laser with frequencies that are of resonantfrom any bound-bound transition or any excited state.Thusthe main dynamics is described mainly by ionization,eitherby tunneling or above barrier ionization.An example of workthat neglects the resonant transitions is that of Liu and Nisoli[30],where they considered the2p excited state of He+treatedin SFA approximation to study polarization of its harmonics instrongfields without concern for coupling to higher states norground states.Similarly,Frolov and co-workers consideredan initial excited atom,namely,the p state,in obtaining theanalytical formula for HHG with elliptically polarized laserfields[13].In this paper,we present the generalization of the perturba-tive formalism to arbitrary initial excited states of a hydrogenicatom with relativistic kinetic energy and vectorial momentumof the ing semianalytical expressions,wecompute and compare the angular distributions of the pho-toionization rate for linear and circular laser polarizations witharbitrary excited atomic states of different quantum numbers,n,l,m.This enables us to systematically study the effects ofrelativistic photoelectron and atomic polarization in excitedstates on photoionization.It is helpful not to underestimateour choice of hydrogenic atom,since the wave functions forthe quantum states n,l,m are known analytically,the resultscan be interpreted more easily to provide clearer insights thanusing more complicated atoms or molecules.Likewise,actualenergy levels with spin-orbit coupling and other relativisticinteractions[31]can be included when precision is required.The relativistic photoionization rate is presented in Sec.II,andthe matrix elements for the arbitrary excited state are calculatedin Sec.III.The results are shown and discussed in Sec.IV.II.PHOTOIONIZATION RATE FOR AN EXCITED ATOMThe photoionization is performed on an excited atom usinga(linearelliptical )polarized intense laserfield E=E(ˆz cosωt(αcosωt,βsinωt,0))and E·r=(Er cosθcosωtEr sinθC(t,φ)),where C(t,φ)=αcosωt cosφ+βsinωt sinφ,with the coefficientsαandβdeterminingthe ellipticity =α/βof the laserfield.The generalphotoionization rate(as shown in our previous result)isdefined asw=m(2π 2)22ππ∞k=k0|L(p k)|2p k sin d d ,(1)with the threshold index k0=1 (I n+U p)andL(p k)=12πT/2−T/2V0( (t))e iS(p,t)e−i( −kω)tωdt=12πV0( k(u))√1−u2e iS(p k,u)du=12ππ−πV0( (s))exp iS(p k,s)ds,(2)with u=sin s and s=ωt; n(p)=1(K0+I n+U p);I n=I0/n2is the ionization energy from level n;K0=cp2+m2c2−mc2is the relativistic kinetic energy;andU p=c(eA(t))2+(mc)2 −mc2=c12(eA)2+(mc)2−mc2,with A=E/ωthe relativistic ponderomotive energy[32],taken as the time average .. of the term involving thevector potential A(t)=−tE(t )dt .Here,S(p,s)represents the action phase during the pho-toionization,S(p,t)=1t[I n+K(τ)]dτ= n(p)t+1t[K(τ)−K0−U p]dτ,(3)which is computed by numerical integration,since it is notpossible to obtain an analytical expression.For the nonrelativistic case,Eq.(3)reduces toS(p,u)→n(p)sin−1uω+1⎛⎝−eEmωp z(√1−u2)−U p u√1−u2−eEmω(α+α∗)2p x(√2)+(β+β∗)2p y u−U p(|α|2−|β|2)u√2⎞⎠,(4)where the arrow signifies the nonrelativistic limit.Although we are studying the relativistic effects on the the photoionization,we may use the nonrelativistic Schr¨o dinger equation and neglect corrections to the dipole approximation based on two recent reports.Tofirst order the electron velocity v only depends on the electricfield. According to Ref.[33],the second-order correction leads to a magneticfield B=ˆe×E/c and a ponderomotive gradi-ent dU p/dz.Thefirst scales as1/c,whereas the gradient of U p will be negligible for long-wavelength(adiabatic) pulses.Klaiber et al.[34]showed that the magnetic correction is responsible for a momentum shift I p/3c.Thus for excited states with small I p this is negligible.Therefore in comparison to excited I p,the ponderomotive energies will be much larger, thus necessitating a more accurate treatment.We may use the nonrelativistic Schr¨o dinger equation since the relativistic corrections are mainly causing the energy shift of bound levels such as the spin-orbit interactions, which introduce only a negligible quantitative effect on the photoionization process.The relativistic corrections to the ponderomotive potential, despite being small,are more important than other relativistic corrections,because the ponderomotive potential appears in the phase factor or the action part S through the V olkov wave function that is sensitive to the photoionization time scale andPHOTOELECTRON ANGULAR DISTRIBUTIONS OF ...PHYSICAL REVIEW A 90,013417(2014)therefore would have significant impact on the behavior of photoionization.The atom-light interaction is taken in the nonrelativistic dipole form E ·r within dipole approximation.The spatial dependence of the electric field is neglected and contains only the time dependence sin(ωt )or cos(ωt ),as we assume the wavelength of interest is not too short but longer than a hard x ray or γray and much larger than the atomic dimension of 0.1nm.The transition matrix element V 0( (x ))corresponds to the transition of the photoelectron from the initial stateψs (r )to the continuum V olkov state ψp (r ,t )=exp {i[ (t )·r − t0K (τ)dτ]},with (t )=m v =p +e A (t ),and K (τ)=c +m −mc 2→ (τ)22m ,i.e.,V 0(t )=eEψs (r )r cos θcos ωt F (t,φ)sin θ e −i r 2drd ,(5)where d =sin θdθdφandF (t,φ)=(αcos ωt cos φ+βsin ωt sin φ),(6)(r,θ,φ,t )=1(t )·r =B (θ,φ,t )r,(7)B (θ,φ,t )=1[Q (φ,t )sin θ+P (t )cos θ],(8)with the integration over the radial part that consists ofP = p z −e Eωsin ωt p z,(9)Q =p x cos φ+p y sin φ(p x −eE ωαsin ωt )cos φ+(p y +eEωβcos ωt )sin φ .(10)However,the angular dependence of the photoionizationrate is obtained by differentiating Eq.(1)with respect to the polar angle and the azimuthal angle :dw d a =m (2π 2)2∞k =k|L (p k )|2p k .(11)III.TRANSITION MATRIX ELEMENT FORARBITRARY INITIAL STATEHere,we focus the study on how the angular distribution depends on an individual initial state (a particular nlm )first before exploring the more elaborate situations of atomic gas in many excited states.Atomic gas in a single excited state can be realized by preparing the atoms predominantly in a single chosen excited state by the standard optical pumping technique by narrow-band lasers with certain polarizations,as used for atomic spectroscopy and laser cooling [35].We illustrate this realization in Fig.1.Photoionization with atoms in initial excited state has been studied by Bauer [36],who found additional peaks in the photoelectron spectra for the initial state in n =2but neglected the magnetic quantum number m .The general hydrogen wave function is consid-ered,ψs (r )=ψn,l,m (r )=R nl (r )Y m l (θ,φ),with normaliza-tion |ψ0(r )|2d 3r =1.The radial wave functiondefinedFIG.1.(Color online)Atomic scheme showing atoms initially prepared in several internal states (top panel)and a particular state (bottom)before photoionization.as R nl =(2ρ)3(n −l −1)!2n [(n +l )!]3e −ρr (2ρr )l [L 2l +1n −l −1(2ρr )]with ρ=4π2μe 2Z 2=Z0,and the angular wave function is defined as Y m l (θ,φ)=σ(2l +1)4π(l −|m |)!(l +|m |)!e imφP m l (cos θ),where L (a )n (x )= nj =0(−1)j (n +a n −j )x jj !is the associated Laguerre polynomi-als,σ={(−1)m if m 01if m<0}is the piecewise function,P m l (x )=(−1)m 2l l !(1−x 2)m/2d l +mdx l +m (x 2−1)l is the associated Legendre polynomials.Hence the transition matrix element of the initial state of the arbitrary energy level to the continuum V olkov state is redefined asV 0(p )=ψs (r )e E ·r exp −ip ·r d 3r =e E ·i ∇p ˜ψs (p ).(12)The goal of evaluating the matrix element V 0( (t ))can be done by direct integration:V 0( )= ∞0π02πe −i e E ·r ψn,l,m (r )r 2drd=A nlmπ02π0Z nlm (t,θ,φ)dφdθ,(13)Z nlm (t,θ,φ)=W nl (t,θ,φ)C lm (t,θ,φ),(14)with the coefficient factor varying with the different nlm ,A nlm =na 02 5/2eEσ(n −l −1)!2n [(n +l )!]3(2l +1)4π(l −|m |)!(l +|m |)!,(15)and the function that depends on time and angles θ,φ,C lm (t,θ,φ)= cos ωt cos θF (t,φ)sin θP m l (cos θ)e imφsin θ.(16)C.H.RAYMOND OOI,W AILOON HO,AND A.D.BANDRAUK PHYSICAL REVIEW A 90,013417(2014)FIG.2.(Color online)Angular distribution of photoionization rate from relativistic (left)and nonrelativistic (right)results for linear and circular polarizations on atom in state n,l,m =3,0,0.The plots are shown for large and small combinations of E 0and ω.The integration over r can be rewritten in dimensionless quantities x =2r/na 0and q =(iBa 0n +1)/2asW nl (t,θ,φ)= ∞x l +3L 2l +1n −l −1(x )exp[−qx ]dx.(17)Using the identity ∞0e −sx x βL αm (x )dx = (β+1) (α+m +1)m ! (α+1)s −(β+1)F (−m,β+1;α+1;1s ),we have the exact expressionW nl (t,θ,φ)=W nl q −(l +4)F−(n −l −1),l +4;2l +2;1q,(18)where W nl = (l +4) (l +1+n )(n −l −1)! (2l +2).For linear polarization the integration over φhas the form 2π0exp[−i 1(p x cos φ+p y sin φ)sin θr ]e imφdφ,whilefor elliptical polarization, 2π0(sin φor cos φ)exp[A cos φ+B sin φ]e imφdφ.If we neglect p x and p y for linear polarization,the matrix element is finite only for m =0since 2π0e imφdφ=2πδm,0.This givesV 0( (t ))=2πA nl 0cos ωt πW nl (t,θ)C l 0(t,θ)dθ,(19)where A nl 0=(na 02)4eE(2na 0)3(n −l −1)!2n [(n +l )!]3(2l +1)4π.For nlm =n 00the integration over r becomesW (t,θ,φ)=(4) (n +1)(n −1)! (2)q −4F1−n,4;2;1q=32n [3n 2(iBa 0−1)2+1−n 2]n −3− n +3+,(20)PHOTOELECTRON ANGULAR DISTRIBUTIONS OF ...PHYSICAL REVIEW A 90,013417(2014)FIG.3.(Color online)Angular distribution of photoionization rate for linear polarization with different initial excited atomic states in level n =3.where ±=(iBa 0n ±1)/2,with P 00(cos θ)=1and A n 00=(na 02)4eE 0 14π(2na 0)3(n −1)!2n [(n +l )!]3.For state |n 00 the results ofEq.(18)correspond exactly with Eq.(20).For n =1and linear polarization,B =1[(p x cos φ+p y sin φ)sin θ+(p z +e Eωsin ωt )cos θ].So far in many exist-ing works and our previous work,p x and p y were neglected,so there was no dependency on φand we recover the known result V 0( (t ))=64πA nlm 3cos ωt π0cosθsin θdθ(iBa +1)4=−i 2π(2a 0)4eE1πa 30a 0Pzcos ωt [1+(a 0P z )2]3.Thus our present formalismis generalized to include the transverse momentum p x andp y .Hence the φintegration is present and has to be done numerically.The matrix element for general nlm is evaluated from a semianalytical expression through numerical integrations over θand φ:V 0( (t ))=−B nlm2ππq −(l +4)Fl +1−n,l +4;2l +2;1qcos ωt cos θsin θF (t,φ)sin 2θP m l (cos θ)e imφdθdφ,(21)where B nlm =A nlm W nl .The integration over φhas to be done numerically due to the dependence on φin B or finite values of p x and p y .Equation (21)is a general semianalytical formula,convenient for computing the photoionization rate versus angle of observation for different initial states.It is valid for both linear and elliptical polarizations.IV .RESULTS AND DISCUSSIONWe have plotted the angular distributions of the differential photoionization rate dw/d d =m (2π 2)2∞k =k 0|L (p k )|2p k sin using Eq.(1)at =0for linear and circular polarizations with relativistic and nonrelativistic results in Fig.2.The L (p k )is calculated using Eq.(2),which contains the action S given by Eq.(3)and V 0given by Eq.(21).(Typically,figures are reported in V /cm for electric fields E and s −1for frequencies for which E (1a .u .)=5.14×109V /cm,ω(1a .u .)=2πf =4×1016s −1)[37].The four scenarios of high or low E and ωfor n,l,m =3,0,0show that the relativistic and nonrelativistic results agree very well only for a sufficiently small relativistic Keldysh parameterC.H.RAYMOND OOI,W AILOON HO,AND A.D.BANDRAUK PHYSICAL REVIEW A 90,013417(2014)FIG.4.(Color online)Angular distribution of photoionization rate for circular polarization with different initial excited atomic states in level n =3.γ= I p /2U p ≈ωE 2mI p 2,as in the case Fig.2(c),where the fields are strong at low frequencies.At high frequencies and even with low fields,the relativistic effect is significant,as clearly shown in Fig.2(b).This also provides good results on the relativistic photoelectric effect,where larger photon energy translates to a photoelectron with higher speed.Thus in the case of larger γ,the photoelectron emission probability is much smaller than in the nonrelativistic case and the case of small γ.Let us analyze the angular distributions of photoelectrons from different orbitals in the excited states.For linear polariza-tion (Fig.3),the angular distributions do not depend on the sign of the magnetic quantum number m .For circular polarization (Fig.4),the emission profiles are different for +|m |and −|m |,although the shape looks identical,for γ 0.4(<1).For γ>1,however,the shapes are different for positive and negative m ,such as additional more rounded lobes for positive m ,connected to recent results [25].In general,the lobes for linear polarization are almost complementary to the lobes for circular polarization,i.e.,the minimum in the linear case corresponds to the maximum in the circular case,and vice versa.For linear polarization with m =0,±2the photoelectron emission rate is the highest mainly at around =π/2and it reduces with l .For m =±1,there is zeroemission towards =π/2.This result is counterintuitive,as one would expect that a higher excited state would be more likely to be ionized and the electron is ejected predominantly along =0.For circular polarization (Fig.4)with m =0,the emission is highly directional with twin peaks close to =π/2.Now we look at the angular distributions in excited states for different values of the Keldysh parameter γ.We plotted the an-gular distributions of spherically symmetric states |n 00 with n =1–4in Figs.5and 6for linear and circular polarizations,respectively.For γ 1,the multiphoton ionization regime,the photoelectron emission basically follows the direction of the linear polarized electric field and close to the field direction of the circularly polarized light,especially for higher excited levels where the ionization energies are smaller.For γ∼1the photoelectron can be emitted into several other directions,especially for lower levels.For γ 1,the tunnel ionization regime,the photoelectron is emitted into multiple discrete directions as in Ref.[38],and it becomes hard to distinguish the angular distributions between linear and circular polarized lights.The increased isotropy in the emission reflects the nature of the tunneling process,which is probabilistic.The general trend shown in Figs.5and 6is that the emission rates are typically much larger for linear polarization and the rates increase with the electric field.But the angularPHOTOELECTRON ANGULAR DISTRIBUTIONS OF...PHYSICAL REVIEW A90,013417(2014)FIG.5.(Color online)Angular distribution of photoionization rate for linear polarization of thefirst four states|nlm =|n00 (n=1,2...4) with:(a)γ∼1(b)γ 1(c)γ 1.distributions do not change significantly with the electricfield strength.The photoionization rate increases with the initial state n up to n=3and then reduces for larger n.The shape is cos -like for the linear case and sin -like for the circular polarization corresponding to unidirectional,and bidirectional close to =π/2± ,where is a small positive value.C.H.RAYMOND OOI,W AILOON HO,AND A.D.BANDRAUK PHYSICAL REVIEW A90,013417(2014)FIG.6.(Color online)Angular distribution of photoionization rate for circular polarization of thefirst four states|nlm =|n00 (n= 1,2...4)with:(a)γ∼1(b)γ 1(c)γ 1.In regular atomic gas the atoms are in more than one initial state.We may consider the initial superposition of states instead of a single quantum state,coupled by the intense laser with a power broadening effect or a broad bandwidth of the laser pulse.Then the initial atomic state becomes a superposition of all the angular momenta l and magneticPHOTOELECTRON ANGULAR DISTRIBUTIONS OF ...PHYSICAL REVIEW A 90,013417(2014)FIG.7.(Color online)Angular distribution of photoionization rate for linear (left)and circular(right)polarizations for initial excited atom in n =2,with superposition of states in l =0,m =0and l =1,m =−1,0,1.substates m ,i.e.,ψn,l,m (r )would be replaced byψn (r )=lm =−l n −1 l =1c nlm ψn,l,m (r ),(22)where l m =−l n −1l =1|c nlm |2=1.The matrix element Eq.(13)becomesV 0( )=lm =−l n −1 =1c nlm A nlm π0 2π0Z nlm (t,θ,φ)dφdθ.(23)The eigenenergy that includes the effects of fine-structureinteraction is given by E nj =−|I n |[1+(Zα2n )2(4nj +1/2−3)],where I n =−Z 2n 2I 0,I 0=hcR ∞=α2m e c 22=13.6eV ,and |l −12|≤j ≤l +12.Except for spectroscopic purposes,the spin-orbit splitting can be neglected and we just use E nj −Z 2n213.6eV for all states within the same n .To show the effects of superposition of states and inde-pendent sum of probabilities of those states,we performsimulations for n =2with l =0,m =0and l =1,m =−1,0,1.Figure 7shows that the angular distribution assumes a more rounded shape,combining the features of all the magnetic substates.For circular polarization,the X-shape distribution in Fig.7(b)from m =±1is so strong that it stands out of the superposition with the m =0.However,typically information from the substates cannot be extracted effectively when the initial state is composed of many states.Therefore more useful information can be obtained if the atoms are prepared in a single m state,corresponding to left or right rotation,as shown in [25],which enables the study of how photoionization depends on the angular momentum and its magnetic substates.V .CONCLUSIONSWe have shown from simulations using the semiana-lytical Keldysh theory,including relativistic corrections to the ponderomotive energy [21],that relativistic corrections become more significant for a larger Keldysh parameter γ,characteristic of the MPI regime.In the case of large γand n ,photoelectrons can be emitted into many discrete directions,with no simple angular distribution.The results also show that photoelectron angular distribution is sensitive to the magnetic quantum number m ,which enables us to distinguish the state of a degenerate atom in different internal magnetic states.This could be a useful tool to identify the polarization of the atom by the angular distribution of the photoelectron in the absence of magnetic fields,since the different magnetic states cannot be distinguished by spectroscopic data.Circular and elliptic polarizations are new tools in studying the tunneling and MPI regimes in intense laser fields,but these have neglected relativistic effects which we show can be important since they modify the ponderomotive energies.Our results show that relativistic corrections to ponderomo-tive energies,which cause the large energy shifts of excited states,leads to important physical effects in strong-field physics of atoms.This shows that corrections to the strong-field approximation (SFA)models are already necessary for excited states.Our work points to a new direction in strong-field physics which needs to be verified next by exact calculations as well as experiments.ACKNOWLEDGMENTThis work is supported by High Impact Research,Grant No.UM.C/625/1/HIR/MOHE/CHAN/04,from the Ministry of Education of Malaysia.[1]F.Krausz and M.Ivanov,Rev.Mod.Phys.81,163(2009).[2]P.B.Corkum and F.Krausz,Nat.Phys.3,381(2007).[3]S.Baker et al.,Science 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T.Uzer,Phys.Rev.Lett.110,253002(2013).[17]M.V.Frolov,N.L.Manakov,T.S.Sarantseva,M.Yu.Emelin,M.Yu.Ryabikin,and A.F.Starace,Phys.Rev.Lett.102,243901 (2009).[18]B.Borca,A.V.Flegel,M.V.Frolov,N.L.Manakov,D.oˇs evi´c,and A.F.Starace,Phys.Rev.Lett.85,732(2000).[19]K.-J.Yuan and A.D.Bandrauk,Phys.Rev.Lett.110,023003(2013).[20]K.Mishima,M.Hayashi,J.Yi,S.H.Lin,H.L.Selzle,and E.W.Schlag,Phys.Rev.A66,033401(2002).[21]V.N.Ostrovsky,T.K.Kjeldsen,and L.B.Madsen,Phys.Rev.A75,027401(2007).[22]H.Mineo,S.D.Chao,K.Mishima,K.Nagaya,M.Hayashi,andS.H.Lin,Phys.Rev.A75,027402(2007).[23]M.V.Ammosov,N.B.Delone,and V.P.Krainov,Sov.Phys.JETP64,1191(1986).[24]A.M.Perelomov,V.S.Popov,and M.V.Terent’ev,Sov.Phys.JETP23,924(1966).[25]T.Herath,L.Yan,S.K.Lee,and W.Li,Phys.Rev.Lett.109,043004(2012).[26]L.V.Keldysh,Sov.Phys.JETP20,1307(1965),[Zh.Eksp.Teor.Fiz.47,1945(1964)].[27]C.H.Raymond Ooi,W.L.Ho,and A.D.Bandrauk,Phys.Rev.A86,023410(2012).[28]M.Gruchowski and R.Szmytkowski,Radiat.Phys.Chem.68,143(2003).[29]osevic,V.P.Krainov,and T.Brabec,Phys.Rev.Lett.89,193001(2002).[30]C.Liu and M.Nisoli,Phys.Rev.A85,013418(2012).[31]S.T.Manson and A.F.Starace,Rev.Mod.Phys.54,389(1982).[32]G.A.Mourou,T.Tajima,and S.V.Bulanov,Rev.Mod.Phys.78,309(2006).[33]C.T.L.Smeenk,L.Arissian,B.Zhou,A.Mysyrowicz,D.M.Villeneuve,A.Staudte,and P.B.Corkum,Phys.Rev.Lett.106, 193002(2011).[34]M.Klaiber,E.Yakaboylu,H.Bauke,K.Z.Hatsagortsyan,andC.H.Keitel,Phys.Rev.Lett.110,153004(2013).[35]C.E.Wieman,D.E.Pritchard,and D.J.Wineland,Rev.Mod.Phys.71,S253(1999).[36]J.H.Bauer,Phys.Rev.A85,063417(2012).[37]In one atomic units(1a.u.),the energy is27.2eV=2I p(1s)corresponding to wavelengthλ=45nm,frequency f=6.58×1015s−1,and angular frequencyω=2πf=4×1016s−1.The electricfield is E0(1s)=5.14×109V/cm,corresponding to intensity I0=3.5×1016W/cm2.[38]Z.Zhou and Shih-I.Chu,Phys.Rev.A83,033406(2011).。
刘启洋,外文翻译原文
刘启洋,外⽂翻译原⽂Metal grid/conducting polymer hybrid transparent electrode for inverted polymer solar cellsJingyu Zou,1Hin-Lap Yip,1,2Steven K.Hau,1and Alex K.-Y.Jen1,2,a?1Department of Materials Science and Engineering,University of Washington,Seattle,Washington98195,USA2Institute of Advanced Materials and Technology,University of Washington,Seattle,Washington98195,USAReceived19January2010;accepted23March2010;published online17May2010A simple method was developed using metal grid/conducting polymer hybrid transparent electrodeto replace indium tin oxide?ITO?for the fabrication of inverted structure polymer solar cells.Theperformance of the devices could be tuned easily by varying the width and separation of the metalgrids.By combining the appropriate metal grid geometry with a thin conductive polymer layer,substrates with comparable transparency and sheet resistance to those of ITO could be achieved.Polymer solar cells fabricated using this hybrid electrode show ef?ciencies as high as?3.2%.Thismethod provides a feasible way for fabricating low-cost,large-area organic solar cells.?2010American Institute of Physics.?doi:10.1063/1.3394679?Polymer solar cells?PSCs?are becoming as a viabletechnology for low-cost power production.1Indium tin oxide ?ITO?is the most commonly used transparent electrode for PSCs because it offers good transparency in the visible rangeof the solar spectrum as well as good electrical conductivity.However,there are several de?ciencies that exist for usingITO such as poor mechanical properties of ITO-coated plas-tic substrates,2limited conductivity for fabricating large-areasolar cells,limited availability of indium,and complicated vacuum sputtering process tend to increase the cost for ITO. These limitations set a potential barrier for the commercial-ization of low-cost PSCs.To alleviate this problem,alterna-tive materials for transparent conducting electrodes are needed to replace ITO.There has been some research on exploring conductive polymers,2carbon nanotubes,3 graphenes,4and silver nanowires5as potential candidates to replace ITO.However,lower transparency and higher sheet resistance compared to ITO strongly hinder their use for transparent electrode.Metal grids have also been investi-gated as a promising alternative for transparent electrode.6,7 Utilizing micro?uidic deposition and nanoimprinting meth-ods,metal grids coated substrates have been used for fabri-cating conventional PSCs with PCE as high as2%.Previously,inverted architecture PSC using ITO as cath-ode and evaporated silver?Ag?as anode has been proved to be more stable in ambient than the conventional devices us-ing sensitive metal as cathode.8Moreover,poly?3,4-ethylenedioxythiophene?:poly?styrenesulfonate??PEDOT-:PSS?has also been demonstrated by Hau et al.9as a potential replacement of ITO for fabricating inverted PSCs. However,the relatively high sheet resistance of PEDOT:PSS compared to ITO may limit the performance of PSCs The combination of conductive metal grids with PEDOT:PSS provides a good solution to solve this problem and obtain ITO-free and ambient stable PSCs.Here,we report a simple method to fabricate high-ef?ciency ITO-free inverted structure PSCs using a metal grid/conducting polymer hybrid transparent electrode.By us-ing soft lithography and chemical etching,the metal gridscan be easily fabricated on substrates.The inverted devicearchitecture is used to fabricate PSCs with Ag as anode tocollect holes and zinc oxide?ZnO?as an electron selectivelayer at the metal grid/conducting polymer interface to helpcollect electrons.To prepare the metal grids,a1nm alumi-num?Al??lm was deposited?rst followed by evaporating a30nm thick silver?lm onto the glass substrates.It was foundthat the very thin Al layer improves adhesion between thesubstrate and the Ag?lm.A micropatterned photoresist?SU-8,MicroChem??lm,fabricated by standard photolithography,was used as a mas-ter to replicate stamps for microcontact printing??CP?.A typical stamp was made by casting a10:1?v/v?mixture ofpolydimethyl siloxane?PDMS?and curing agent against asilanized master.The PDMS stamp was?rst soaked with an“ink”containing1mM of mecaptoundecanoic acid?MUA?in ethanol,then brought into contact with the surface of sil-ver for60s.After the removal of the stamp,the patterned Aglm was developed by wet etching with aqueousFe3+/thiourea using the patterned SAM as resist.10MUA was chosen for both generating better wettability for the process-ing of upper layer?lm.and facilitating better chargecollection.11Three types of designed grids patterns were utilizedas width and separation with?1?5?m and50?m ?5?m/50?m?,?2?10?m and100?m ?10?m/100?m?,?3?20?m and200?m ? 20?m/200?m?,respectively.The optical transmittance of as-fabricated Ag grids was measured by UV-Vis spectros-copy? Fig.1?.The sheet resistance for metal grids with dif-ferent geometries was also measured.The transmittance of ITO at550nm is85.7%.When thetransmittance of glass??93%?is taken into account,the maximum expected transmittance of the metal grids coated glass substrate is calculated to be?83–84%.It was found that the average transmittance of metal grid coated glass sub-strates in the range of250–1200nm is78%,80%,and82%, respectively,for grids of5?m/50?m,10?m/100?m, and20?m/200?m.The lower transmission may be due toa?Electronic mail:ajen@/doc/1d16844512.html.APPLIED PHYSICS LETTERS96,203301?2010?0003-6951/2010/96?20?/203301/3/$30.00?2010American Institute of Physics96,203301-1some diffusion of MUA during contact printing on the metal surface.As the result,the actual Ag grid patterns expand 1–2?m in width.Figure 2?a ?shows the optical microscope images of an as-fabricated metal grid electrode on a glass substrate.Another important parameter for transparent conducting electrodes is the sheet resistance.The sheet resistance of commercial available ITO substrates is 151,??while the Ag grid electrodes exhibited sheet resistances of9.11,146.31,and 254.11,for 5?m /50?m,10?m /100?m,and 20?m /200?m,re-spectively.Lower sheet resistances willminimize the loss of photocurrent during charge transport due to the lowered lat-eral resistance of the electrode.In general,the transmittance and sheet resistance for thin conductive ?lms are related by the equation ofT =?1+188.5R s ?OpDC2,where ?Op is the optical conductivity ?here we quoted at ?=550nm ?and ?DC is the conductivity of the ?lm.?DC /?Op is a commonly used term to describe transparent conductors.12For ITO with R s of 151and T ?550nm ?of 85.7%,the ?DC /? Op is 156.7.The best results that have been achieved for the graphene-based ?lms 13and carbon nanotubes 14are 0.5and 25,respectively.Based on the best metal grid geometry used in this work ?5?m /50?m ?,R s =9.1?/?,and T ?550nm ?=79.0%,a ?DC /?Op ratio as high as 165.6could be achieved.To fabricate solar cells,the ITO substrate and the metal grids coated substrate were cleaned using standard cleaning procedures.A thin layer of ZnO nanoparticles ?ZnO NPs ?was spin-coated onto these substrates.A C 60-based SAM ?C 60-SAM ?was deposited onto the ZnO surface using a spin-coating process as reported previously.15A 200nm bulk-heterojunction ?lm comprising of poly ?3-hexylthiophene ??P3HT Rieke Metals ?and ?6,6?phenyl C 61butyric acid methyl ester ?PCBM American Dye Source ?was then spin-coated in an argon-?lled glove box.After depositing a 50nm of PEDOT:PSS ?lm ?H.C.Starck,CLEVIOS?P VP 4083?.8A layer of Ag was vacuum deposited on top of PEDOT:PSS as anode.The solar cells were tested under ambient using a Keithley 2400SMU and an Oriel Xenon lamp ?450W ?with an AM 1.5?lter.The light intensity was calibrated to 100mW /cm 2.The device architectures are shown in Fig.2?b ?.The J -V characteristics under illumination and the solar cells perfor-mance are summarized in Fig.3?a ?and Table I ,respectively.The device with the 5?m /50?m Ag grid has the best performance with PCE of 2.97%.The lower ef?ciency of the metal grid substrate derived device is mainly due to lower J sc and ?ll factor.An important parameter that needs to be considered for the design of metal grids is that the charges generated from the voids between the grid lines need to be ef?ciently col-lected.The inverted device structure utilizes a ZnO NP layer as an electron selective layer between the active layer and the metal grids to collect electrons.The inhomogeneous and poor charge collection in the voids due to high sheet resis-tance of ZnO decrease both J sc and ?ll factor.Inaddition,FIG.1.?Color online ?Transparency vs wavelength of different geometry Ag grids on glass as compared to transparency of ITO and glass,as refer-enced againstair.FIG.2.?Color online ??a ?Optical microscope image of silver grid with 5?m width separated by a distance of 50?m.?b ? Device con?guration of the polymer solar cell using Ag grid as the transparent electrode with or without conductive PEDOT:PSSlayer.FIG.3.?Color online ?The current density-voltage ?J ?V ?characteristics of polymer solar cells with ?a ?different Ag grid geometries ?b ?different Ag grid geometries combining 40nm PEDOT:PSS PH500?lm measures under AM1.5illumination from a calibrated solar simulator with a light intensity of 100mW cm ?2.the lower transmittance of the5?m/50?m Ag grids ??78%?compared to ITO??85%?also contributes to the decrease in J sc.Increasing the width and separation while maintaining the same aspect ratio dramatically reduces the J sc and?ll-factor.To alleviate the problem for poor charge collection,a PEDOT:PSS?H.C.Starck,CLEVIOS?PH500?conduct-ing polymer was inserted between the silver grids and the ZnO layer to form the hybrid electrode.To demonstrate the function of the hybrid electrode,an inverted photovoltaic device with220nm thick PEDOT:PSS PH500?lm without metal grids as the bottom electrode was fabricated.The J-V characteristics under illumination and the solar cells performance are summarized in Fig.3and Table I, respectively.For the smaller separation Ag grids ?5?m/50?m?,the addition of the conducting PEDOT:PSS polymer layer improved the device performance to3.21% due to the reduced lateral resistance.For devices using larger separation grid lines?10?m/100?m and 20?m/200?m?,the addition of the PEDOT:PSS layer sig-ni?cantly improved the performance of the devices.For grids with separation distance of over10?m,additional layer of PEDOT:PSS is necessary to reduce the lateral resistance.All three silver grids electrodes can achieve near3%PCE using the additional PEDOT:PSS layer.The potential bene?t of using larger size grid patterns is the ease for device fabrica-tion especially for cost ef?cient industrial roll-to-roll pro-cessing.Without Ag grids,PEDOT:PSS PH500bottom elec-trode devices can only have ef?ciency of?2.2%which is due to the high sheet resistance.In conclusion,we have demonstrated that silver metal grid electrodes fabricated by microcontact printing and wet chemical etching can replace conventional ITO electrodes for fabricating organic solar cells.The patterned metal elec-trodes on glass show high optical transmittance as well as good electrical /doc/1d16844512.htmlanic solar cells with opti-mized grid geometry show encouraging device performance. It was also found that silver grid electrodes with smaller width and separation with the same aspect ratio facilitated better charge collection from the ZnO NP layers leading to increased FF,J sc,and PCE.By adding a PEDOT:PSS PH500 conducting polymer between Ag grid and ZnO,even devices with larger Ag grid spacing can achieve good performance. The use of inexpensive Ag grids compared to ITO allows the possibility of employing roll-to-roll process to realize low-cost,large-area organic solar cells.This work is supported by the National Science Founda-tion’s NSF-STC Program under Grant No.DMR-0120967, the Department of Energy’s“Future Generation Photovoltaic Devices and Process”Program under Grant No.DE-FC36-08GO18024/A000,and the Of?ce of Naval Research’s Pro-gram under Grant No.N00014-08-1-1129.A.K.-Y.J.thanks the Boeing-Johnson Foundation for?nancial support.1G.Dennler,M.C.Scharber,and C.J.Brabec,Adv.Mater.?Weinheim, Ger.?21,1323?2009?.2S.I.Na,S.S.Kim,J.Jo,and D.Y.Kim,Adv.Mater.?Weinheim,Ger.?20,4061?2008?.3M.W.Rowell,M.A.Topinka,M.D.McGehee,H.J.Prall,G.Dennler,N. S.Sariciftci,L.Hu,and G.Gruner,Appl.Phys.Lett.88,233506? 2006?. 4G.Eda,Y.Y.Lin,/doc/1d16844512.htmller,C.W.Chen,W.F.Su,and M.Chhowalla, Appl.Phys.Lett.92,233305?2008?.5J.Y.Lee,S.T.Connor,Y.Cui,and P.Peumans,Nano Lett.8,689?2008?. /doc/1d16844512.html ingstedt and O.Inganas,Adv.Mater.?Weinheim,Ger.?19,2893?2007?.7M.Kang,M.Kim,J.Kim,and L.J.Guo,Adv.Mater.?Weinheim,Ger.?20,4408?2008?.8S.K.Hau,H.L.Yip,N.S.Baek,J.Zou,K.O’Malley,and A.K.-Y.Jen, Appl.Phys.Lett.92,253301?2008?.9S.K.Hau,H.L.Yip,J.Zou,and A.K.-Y.Jen,Org.Electron.10,1401?2009?.10J.M.McLellan,M.Geissler,and Y.Xia,J.Am.Chem.Soc.126,10830?2004?.11H.L.Yip,S.K.Hau,N.S.Baek,and A.K.-Y.Jen,Appl.Phys.Lett.92, 193313?2008?.12S.De,T.M.Higgins,P.E.Lyons,E.M.Doherty,P.N.Nirmalraj,W.J. Blau,J.J.Boland,and J.N.Coleman,ACS Nano3,1767?2009?.13X.Wang,L.Zhi,and K.Mullen,Nano Lett.8,323?2008?.14H.Z.Geng,D.S.Lee,K.K.Kim,G.H.Han,H.K.Park,and Y.H.Lee, Chem.Phys.Lett.455,275?2008?.15S.K.Hau,H.L.Yip,H.Ma,and A.K.-Y.Jen,Appl.Phys.Lett.93, 233304?2008?.TABLE I.Summary of PSCs performance with different Ag grids width and separation.V oc ?V?J scmA/cm2FFPCE%R scm2?R pcm2?ITO0.6110.720.66 4.350.9780.2 Ag Grid5?m/50?m0.609.570.52 2.97 2.81341.6 Ag Grid10?m/100?m0.59 6.620.42 1.65 3.0327.4 Ag Grid20?m/200?m0.58 4.330.49 1.00 3.1322.1 Ag Grid5?m/50?m-PEDOT0.609.390.57 3.21 2.81118.0 Ag Grid10?m/100?m-PEDOT0.609.140.58 2.93 2.91213.2 Ag Grid20?m/200?m-PEDOT0.608.950.53 2.85 3.0956.3 PEDOT0.628.910.40 2.2030.7365.1。
Chapter+7+of+College+Physics+in+English
Summary
Angular momentum is a measure of an object's rotation and is equal to the product of its mass, velocity, and distance from the axis of rotation.
Visibility and Turbulence
01
Causes of Turbulence
02 Sudden changes in flow direction or velocity.
03
Rapid variations in pressure or density within the fluid.
Fluid Dynamics
01
Flow Rate and Velocity
02
Flow rate: The volume of fluid passing a point in a unit of
time.
Velocity: The speed and direction of fluid motion.
02
Mechanics
Newton's Laws of Motion
First Law
An object will remain at rest or in uniform motion in a straight line unless acted upon by an external force.
04
Electrostatics
Charges and Fields
电荷与电场的基本概念
电荷是物质的基本属性，存在于所有物质中。电荷有正负之分，同种电荷相互排 斥，异种电荷相互吸引。电场是由电荷产生的，对放入其中的电荷有力的作用。
NanoScale-剑桥大学 R. Vasant Kumar-石墨烯3D结构做成binder free(无粘结剂)
Introduction
There is an increasing demand for lightweight rechargeable batteries to meet the special needs for the next-generation highperformance electronics.1 Sulphur can react with metallic lithium to form Li2S with a large negative free energy change, which can be harnessed in a battery with a two-electron reaction with lithium as the anode and sulphur as the cathode. Therefore, a lithium–sulphur (Li–S) battery has a high theoretical specic capacity (1675 mA h gÀ1 of elemental sulphur) and a high nominal energy density (2500 W h kgÀ1 of cell weight),2–11 which offers the prospect of a signicant energy density improvement compared to the mainstream lithium-ion batteries (150 W h kgÀ1).12–15 Furthermore, elemental sulphur is readily available and poses less risk to the environment than the
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Phase:
A region in a material that differs in structure and/or composition from other regions. Phases: solid solutions, compounds, or pure metals and nonmetals. Materials: single-phase materials, multiphase materials.
Eutectoid steel (Fe-0.76wt%C): ferrite, a solid solution -Fe(C ), plus cementite, a compound Fe3C
Which structure is stronger?
Equilibrium microstructure: the microstructure obtained under equilibrium conditions (extremely slow cooling or heating). Equilibrium conditions can be approached but never fully attained. Therefore, most practical microstructures are metastable.
Solid-solution strengthening
Homogenization annealing: Cast ingots are heated to elevated temperature (below the solidus point for the alloy composition) to eliminate cored structure by accelerated solid-state diffusion.
Commercially pure iron (Fe-<0.022wt%C) One microconstituent: equiaxed ferrite (F). Single phase: ferrite (F)
Hypoeutectoid steel (Fe-0.40wt%C)
Two microconstituents: F, lamellar P (F+Fe3C). Two phases: F, Fe3C.
Microconstituent: composed of one or more phases.
A region having an identifiable and characteristic structure/morphology. Ex.: pearlite (P: F+Fe3C), proeutectoid ferrite (F) or Fe3CII in steels.
Binary - Alloy Phase Diagrams: Isomorphous Alloy System Isomorphous alloy system:
two components completely soluble in each other in both liquid and solid states. Ex.: Cu-Ni alloy system.
Eutectoid steel (Fe-0.76wt%C):
Hypereutectoid steel (Fe-1.0wt%C)
One microconstituent: Two microconstituents: Lamellar pearlite (P: F+Fe3C) P + network Fe3CII. Two phases: F, Fe3C. Two phases: F, Fe3C.
The dashed solidus line represents the average –phase composition.
Binary-Alloy Phase Diagrams: Eutectic Alloy System
Eutectic reactions: three-phase reactions.
Chapter 7 – Phase Diagram: The Equilibrium Microstructural Development
Under equilibrium conditions, what microstructures could exist at a given temperature for a given material composition?
R C 0 - CL W R S C - CL
Binary - Alloy Phase Diagrams: Isomorphous Alloy System Slow rate of cooling: producing an equilibrium microstructure. Fast rate of cooling: insufficient diffusion, producing cored structure.
Basic Concepts: Microstructure, Microconstituent, and Phase Microstructure: composed of one or more microconstituents.
Can be characterized by: the number of phases, their proportions (mass fractions), and the morphology in which they are distributed.
PF CN
where P = the number of phases that coexist in a system; F = the number of degrees of freedom, or the number of externally controlled variables (e.g., temperature, pressure, composition) which can be changed independently without changing the state of the phase or phases in equilibrium in the chosen system; C = the number of components in the system; N = the number of non-compositional variables (i.e., temperature and pressure).
For most routine materials processing: the pressure can be considered to be fixed at 1 atm. So, N = 1 (temperature), there is: F= C-P+1
Basic Concepts: Phase Diagram
Constructed (experimentally or computationally) under equilibrium conditions (extremely slow cooling or heating). From the phase diagram, we can determine: Phases and microconstituents at a given temp. for a given composition. Mass fractions of phases or microconstituents in any two-phase region.
phase A phase B; phase A phase B + phase C; phase A + phase B phase C; ……
Phase equilibrium:
more than one phase may exist in a system and enable the free energy of the system to be minimum. Gibbs phase rule:
Binary-Alloy Phase Diagrams
- Isomorphous Alloys - Eutectic Alloys - Peritectic Alloys - Euepts: Alloys, Components
Alloy and components:
Microstructure Microconstituent (s)
(Region (s) of identifiable morphology)
Phase (s)
(Region (s) of unique structure/composition)
Basic Concepts: Phase Transformation, Phase Equilibrium Phase transformation or phase change: when temperature changes.
Microstructure: the microscopic-scale (m-mm) structure of materials that can be observed under optical and electron microscopes.
Commercially pure iron (Fe-<0.022wt%C): ferrite, a solid solution -Fe (C).
liquid phase L
An alloy: a mixture of metals or of a metal (or metals) and a nonmetal (or nonmetals). Components: the elements or compounds present in the mixture.