11 J. Appl. Phys. 108, 102806 2010 纳米银墨水 PVP 堆积密度 双模堆积密度

A simulation and experimental study on packing of nanoinks to attain better conductivityAnthony K. Amert, Dong-Hoon Oh, and Nam-Soo KimCitation: J. Appl. Phys. 108, 102806 (2010); doi: 10.1063/1.3511687View online: /10.1063/1.3511687View Table of Contents: /resource/1/JAPIAU/v108/i10Published by the American Institute of Physics.Related ArticlesLiquid flow retardation in nanospaces due to electroviscosity: Electrical double layer overlap, hydrodynamic slippage, and ambient atmospheric CO2 dissolutionPhys. Fluids 24, 072001 (2012)Thermocoalescence of microdroplets in a microfluidic chamberAppl. Phys. Lett. 100, 254105 (2012)Direct simulation Monte Carlo-based expressions for the gas mass flow rate and pressure profile in a microscale tubePhys. Fluids 24, 012005 (2012)Hydrodynamic cavitation in microsystems. I. Experiments with deionized water and nanofluidsPhys. Fluids 23, 127103 (2011)Shift of isoelectric point in extended nanospace investigated by streaming current measurementAppl. Phys. Lett. 99, 123115 (2011)Additional information on J. Appl. Phys.Journal Homepage: /Journal Information: /about/about_the_journalTop downloads: /features/most_downloadedInformation for Authors: /authorsA simulation and experimental study on packing of nanoinks to attain better conductivityAnthony K.Amert,1Dong-Hoon Oh,2and Nam-Soo Kim3,a͒1Department of Electrical and Computer Engineering,South Dakota School of Mines and Technology,Rapid City,South Dakota57701,USA2KEN Research Center,Seokyeong University,Seoul,Republic of Korea3Department of Metallurgical and Materials Engineering,The University of Texas at El Paso,El Paso,Texas79968,USA͑Received18November2009;accepted23July2010;published online24November2010͒A simulation and experimental study on the packing of nanoinks has been carried out.Simulationpredicted that the packing of two different sized monodispersed colloids would results in a moredensely packed structure than a single sized monodispered colloid alone.The simulated results wereconfirmed by conducting experiments with two different sized-monodispersed silver colloids.Abinary sized ink was printed in a rectangular pattern using a direct write method system and then thefinal pattern was sintered.The pattern was confirmed to be highly conductive and thefinalconductivity obtained from this pattern gave47%of the bulk conductivity of silver.©2010American Institute of Physics.͓doi:10.1063/1.3511687͔I.INTRODUCTIONIn the21st century,printable electronics has garnered attention in a broad range offields including solar cell,1 RFID,2PCB,3andflexible display.4Direct write has contrib-uted in this area by making head way in the miniaturization of electronic products by printing conductive inks.Important factors governing a successful ink are conductivity,viscosity, and packing density.5–17Conductive ink is typically afluid loaded with metals such as silver nanoparticles that have been fully suspended to prevent agglomeration of the metal powders.Most of the work in this area has been focused on creating nanoparticles that are very small͑Ͻ20nm͒and monodispersed.Numer-ous media such as water and organics have been used to suspend metal nanoparticles.It is desirable that nanoinks should be arranged such that the end product should give a good conductivity after heat-treatment at a relatively low temperature,say less than200C.Metals with high conduc-tivity are usually chosen as the base metal for nanoparticles in the ink but their sintering temperature is relatively high. Solutions of organometallic compounds have also been used to produce both copper,18and silver3metallization.In the case of nanoparticles,once afluid is deposited onto the target substrate the solvent is evaporated with mod-erate heating and yields a thinfilm of nanoparticles.The driedfilm is then heated to a higher temperature whichfirst removes organic materials and then ensures that the particles sinter into a dense metallicfilm.The conductivity of such films typically ranges from10%to20%of bulk silver.19For low frequency electronic applications,these materials may provide acceptable performance but are not suitable for high frequency use because of large Ohmic losses.Further,lower conductivity materials require multiple applications to reachdesirable surface resistances,which increases printing timeand cost.Most nanoparticle inks are based on monodispersed par-ticles.This means that at best,the theoretical maximumpacking fraction attainable for driedfilms would be74%ifplaced into a perfect face centered cubic lattice.But since theparticles can only be randomly packed,this fraction is fur-ther reduced to62.5%.Multidispersed particle distributionsare capable of producing much larger packing ratios.1,5,15–26McGeary found that if two different volumes of monodis-persed particles were mixed and allowed to randomly pack,the optimal volume ratio of large to small colloid was2.7:1.Further,it was found that the packing fraction increased asthe size of the large to small particles increased and attaineda maximum of86%.In this study,to increase the conductivity of thesefilms,we have chosen to focus on enhancing the sintering behaviorof thefilms rather than developing new silver colloid gen-eration techniques.It was our interpretation that the key tocreating highly conductive thinfilms using nanoparticles wasto maximize the packing density of the dried nanoparticlefilms after printing but before thermal sintering as we havepreviously shown.1Here for thefirst time,we have shownthat particles with optimized particle size distribution couldbe used to dramatically enhance the electrical conductivity ofthinfilms.II.EXPERIMENTAL PROCEDUREA.Calculation of two-dimensional packing dataA SIMULATION program was used in this experiment andthe size was500ϫ500͑unit-less͒.First the SIMULATION pro-gram carried out small sized colloid only.After that,the SIMULATION program executed only large sized colloid. Lastly,the SIMULATION program put in practice combinationsof large and small sized collides.a͒Electronic mail:nsking21@.JOURNAL OF APPLIED PHYSICS108,102806͑2010͒0021-8979/2010/108͑10͒/102806/5/$30.00©2010American Institute of Physics108,102806-1B.Synthesis of nanosized particlesTwo colloids were synthesized using the well known polyol process with average particle sizes of 300nm and 55nm,respectively.The 300nm colloid was produced by first dissolving 50g of 10000molecular weight polyvinyl pyr-rolidone ͑PVP ͒͑Sigma-Aldrich No.PVP10͒into 200ml of ethylene glycol ͑Aldrich No.102446͒and the solution was heated to 120°C in a 500ml beaker.After all of the PVP had been dissolved,the solution took on a dark,translucent yellow color.In a second 250ml beaker,25g of silver nitrate ͑Sigma-Aldrich No.S6506͒was dissolved into 100ml of ethylene glycol and the solution was heated to 120°C.Once both solutions had reached 120°C,the silver nitrate solution was dispensed into the PVP solution at 25ml per minute.Immediately,the solution changed from dark,translucent yellow to black.The beaker was then capped after combining the two solutions and its temperature was maintained at 120°C for four hours.While the temperature was main-tained,the colloid changed from black ͑1min ͒,to dark green ͑30min ͒,and finally took on a manila appearance ͑2h ͒,which was maintained until the solution was cooled ͑4h ͒.After 4h of heating,the temperature was reduced from 120to 25°C at a rate of 5°C /min.Once the colloid was generated,200ml of denatured ethanol ͑Aldrich No.362808͒was added and,which was centrifuged at 25000relative centrifugal force for 2h.Cen-trifuging formed a shiny metallic pellet of particles and left only a dark yellow,translucent solution which was removed.The resulting pellet was then turned into a powdered by dry-ing in a vacuum chamber.To generate the 55nm colloid,the same procedure was repeated with two changes.First,55000molecular weight PVP ͑Aldrich No.856568͒was used instead of 10000mo-lecular weight.Second,the combined silver nitrate and PVP solutions were maintained at 120°C for only 2h.Each powder of 100mg was dispersed in 1ml of ethanol using sonication and 10␮l of each resulting colloid was dispensed onto a brass grid and dried.Scanning electron mi-croscope ͑SEM ͒images of the dried films were taken using a Zeiss Suppra-25SEM.Particle size distribution and circular-ity distribution were gathered by using IMAGEJ to analyze multiple top down and cross sectional SEM images of dried films.C.Sintering processThree colloids were prepared as described in the earlier section and used in this analysis.The first colloid was com-posed of 55nm particles and the second was composed of300nm particles.The third colloid was a combination of both 55nm and 300nm particles at 30%and 70%by weight respectively.Each colloid was dispersed onto a brass grid and heated to 180°C at 5°C /min.The temperature was then held at 180°C for 20min and then ramped down to 25°C at 5°C /min.D.Electrical characterization of sintered inksCombinations of 55and 300nm particles were dispersed in ethanol.The percent by weight of each colloid was varied from 0to 100%in increments of 5%.The total colloid per-centage by weight dispersed in ethanol was held constant at 33%.Each colloid was then printed into a rectangular pattern measuring 10ϫ0.1mm 2using a microdispensing direct writing system.Once the films were sintered,the point to point resistance of each film was measured using a Cascade MicroTech four point probe system.III.RESULTS AND DISCUSSIONA.Calculation of two-dimensional packing dataTwo-dimensional packing data was calculated by using a SIMULATION program.According to the simulation,the pre-dicted space occupied by both the 55and 300nm colloids were found to be 45%of the total area.In contrast,the simu-lated area occupied by the combined 55and 300nm colloids was 66%.Three different particles’packing densities are shown in Fig.1.The monodispersed particles’packing density were found to be identical regardless of the size of the particles,meaning the packing fraction is size independent so long as the colloid is monodispersed.However,the simulations of the binary sized particles gave a better packing density re-gardless of the size of the nanoparticles used in theSIMULA-3035404550556065707512345678910Colloid Particle Size RatioP a c k i n g F r a c t i o n (%)FIG.2.Simulated packing fraction for a bimodal colloid vs the colloid particle sizeratio.c)Combination of 5.5and 30sized circlesa) 5.5sizedcircle b)30sized circleFIG. 1.͑Color online ͒Three different size particle packing in SIMULATION program.TIONprogram.This means monodispersed particle have a more limited maximum packing fraction than binary-dispersed particles.In theory,progressively larger packing fractions can be achieved by simply increasing the particle size ratio of a bimodal colloid until it approaches an asymptotic limit.To study the effect of the particle size ratio,simulations of the packing fraction for bimodal colloids were conducted.In each simulation the ratio of the two particle sizes in the bi-modal colloid was varied.As seen in Fig.2as the colloid particle size ratio—defined as the average particle size of the larger particles divided by the smaller—increases so does the packing fraction.However,the majority of the increase in packing fraction occurs before the colloid particle size ratio increases above e of 300and 55nm particles equates to a colloid particle size ratio of 5.45which confers the major-ity of the potential packing fraction enhancement.B.Characterization of nanosized particlesThe particle size and circularity distributions of the 300nm and 55nm colloid are shown in Figs.3and 4,respec-tively.Both particles exhibit some polydispersity and a broad range of circularity.A total of 5000particles of each colloid were measured.Printable inks were formed by dispersing ratios of each col-loidal powder in ethanol.The total particle content of the ink was held constant at 33%by weight.C.Sintering processThe first sintered colloid was composed exclusively of 55nm colloid particles as seen in Fig.5.As seen in Fig.3,the dried colloid had an average particle size of 55nm but had a significant amount of polydispersity with particles as large as 150nm visible in the image.After the colloid was sintered as seen in Fig.5͑right ͒significant void space was observed.The large voids not present in the dried,but unsin-tered film was caused by grain coarsening during the sinter-ing process.Figure 6shows dried and sintered films of the second colloid which was composed of only 300nm colloid par-ticles.The dried film showed that in addition to spherical particles a number of rodlike morphologies had formed.The circularity analysis in Fig.3also showed this trait of the 300nm particle colloid.The sintered film showed significant,but less void space than seen in the 55nm sintered film.We attribute this to the much more limited capability of larger particles to migrate.The last colloid was a bimodal composition of 30%by weight 55nm colloid and 70%by weight 300nm colloid.The dried and sintered films are shown in Fig.7.The sin-tered bimodal film was far denser than either the 55nm or 300nm colloid alone.Additionally,the necking area between particles was dramatically enhanced.As seen in both of the sintered films in Figs.5and 6,bottlenecks occurred between collections of well sintered particles.Particle Diameter (nm)0100200300400500600P e r c e n t a g e o f T o t a l P a r t i c l e D i s t r i b u t i o n0%5%10%15%20%25%Circularity0.00.10.20.30.40.50.60.70.80.9 1.0P e r c e n t a g e o f T o t a l0%2%4%6%8%10%12%14%16%18%FIG.3.͑Color online ͒Particle size distribution ͑left ͒and circularity distribution ͑right ͒of 300nm colloid.Particle Diameter (nm)020406080100120140160180200P e r c e n t a g e o f T o t a l P a r t i c l e D i s t r i b u t i o n0%2%4%6%8%10%12%14%16%18%Circularity0.00.10.20.30.40.50.60.70.80.9 1.0P e r c e n t a g e o f T o t a l0%2%4%6%8%10%12%FIG.4.͑Color online ͒Particle size distribution ͑left ͒and circularity distribution ͑right ͒of 55nm colloid.D.Electrical characterization of sintered inksOnce the films were sintered,the point to point resis-tance of each film was measured using a Cascade MicroTech four point probe system and an SEM was used to measure the cross sections of the sintered films.From these two measurements,the bulk electrical con-ductivity of the printed film was extracted and is shown in Fig.8.For each weight percent ratio,12different specimens were printed and characterized.The average and upper and lower standard of deviation for each weight percent is shown.Also,the measured conductivity of the printed films at 100%and 0%by weight 300nm colloid is approximately equal.This is further evidence that the packing fraction,re-gardless of particle size,plays a major role on the electrical properties of the printed films ͑Fig.8͒.The small difference in electrical conductivity ͑18.5%versus 20.2%͒is most likely caused by slightly different packing factors of the 55and 300nm particles because of the differences is polydis-persity and circularity as shown in Figs.3and 4.Lastly,as seen from the measured electrical conductivity,the standard of deviation drops off rapidly as the electrical conductivity of the film increases.This is most likely the result of the large degree of variability of properties when grains preferentially coarsen rather than undergo densifica-tion.In contrast,the films formed from optimized bimodal distributions of particles do not exhibit large variations in electrical properties and appear denser.IV.CONCLUSIONA simple method for increasing the conductivity of films produced using printed particles has been developed.By al-tering the particle size distribution,an over 2ϫimprovement in electrical properties has been observed using a bimodal particle size distribution.Further increases in conductivity are expected for films produced using greater degrees of dis-persity.ACKNOWLEDGMENTSThe authors are grateful for the financial supports from Seoul R&BD Program ͑Program No.10890͒,Korea,the Na-tional Science Foundation/EPSCOR through Grant No.EPS-0554609,and the state of South Dakota.1N.S.Kim,A.K.Amert,S.M.Woessner,S.Decker,S.M.Kan,and K.N.Han,J.Nanosci.Nanotechnol.7,3902͑2007͒.2H.H.Lee,K.S.Chou,and Z.W.Shih,Int.J.Adhes.Adhes.25,437͑2005͒.3D.Kim and J.Moon,Electrochem.Solid-State Lett.8,J30͑2005͒.4A.Sileikaite,I.Prosycevas,J.Puiso,A.Juraitis,and A.Guobiene,Mater.Sci.12,1392͑2006͒.5S.Nakano,T.Sekitani,T.Yokota,and T.Someya,Appl.Phys.Lett.92,053302͑2008͒.6K.F.Teng and R.W.Vest,Manuf.Technol.11,291͑1988͒.7J.A.Rogers,Z.Bao,K.Baldwin,A.Dodabarapur,B.Crone,V .R.Raju,V .Kuck,H.Katz,K.Amundson,J.Ewing,and P.Drzaic,Proc.Natl.Acad.Sci.U.S.A.98,4835͑2001͒.8K.W.Whites,T.Amert,K.Kirschenmann,and S.M.Woessner,Proceed-ings of Metamaterials 2007,Rome,Italy,22–26October 2007,pp.66–69.9K.W.Whites,T.Amert,N.-S.Kim,K.Han,J.Kellar,and S.Decker,USNC/URSI National Radio Science Meeting,Albuquerque,NM,9–14May 2006,p.414.FIG.5.55nm colloid dried and sintered at 180°C for 20min.Significant void space of the sintered colloid isobserved.FIG.6.300nm colloid dried and sintered at 180°C for 20min.A similar amount of void space in the sintered 300nm colloid is seen when compared to the 55nmcolloid.bined particles ͑30%by weight 55nm colloid ͒dried and sin-tered at 180°C for 20min.Much less void space is observed in comparison to the monodispersedcolloid.0%10%20%30%40%50%0%20%40%60%80%100%P e r c e n t o f B u l k S i l v e r C o n d u c t i v i t t yPercent of 300nm ColloidFIG.8.͑Color online ͒Measured thin film conductivity as the ratio of each colloid is varied from 100%300nm to 100%55nm.The average and upper and lower standard of deviation is shown by triangles.10D.H.Kwon,T.E.Jeong,N.S.Kirn,and K.N.Han,Transactions of the Korean Society of Machine Tool 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