lishi

• Bowers et al. obtained ZT > 1 based on cross-plane and in-plane S and measurement data. • It is easier to measure cross-plane , in-plane S and than in-plane , cross-plane S and .
Disordered WSe2 Film SWCNT
Disordered films k// k 1.5 0.05
Single Crystals 9.7 2.07 4.5
k// / k 30
• Although more difficult than manipulating phonons, it is still possible to control electron transport in nanostructures so as to increase the ZT.
20 0.5 1.0 1.5 2.0 0 2.5
Time (min)
• The sensor temperature was increased to 500oC by the MEMS heater • High sensitivity: 10-100 ppb • Full recovery upon purging the sensor with air, no sensor poisoning.
T1
• L < mfp: Ballistic Transport
T1
L
T2
L
T2
d
T T1 T1 T
T2
T2
x
Diffuse surface roughness scattering reduces thermal conductivity
x
Scattering at a reflection-less contact Contact resistance Ballistic resistance
Thermal Conductivity Limits Q (heat flow rate)
Hot Th
Q A
Insulation; Thermoelectrics Air 0.01
Cold Tc
L Th Tc
L
Thermal conductivity
SiO2 1 SiGe Si Cu Electronic cooling Diamond 1000 10000 W/m-K
Th’ Ts’
RNW = (Th’-Ts’)/Q
• The NW- Pt differential thermocouples are used to measure temperature drops at the two contacts.
Pt
RC2
Ts
Pt serpentine SiNx beam
• Bi2Te3 nanowires • Carbon nanotubes
• Few defects & atomically smooth surface Long mfp ~ microns • d ~l Quantized phonon spectrum
• L > mfp: Diffusive Transport
Bi2Te3/Sb2Te3 Superlattices
Majumdar, Science 303, 777(2004)
•Nanostructured bulk thermoelectric materials
AgSb rich
The ZT improvements were attributed to reduction instead of S2 increase.
Polymer 0.1
10
100
Heat Carriers
Solids Metals: Insulators: Dominant heat carriers Electrons Phonons (lattice waves) Hot
L
Cold d
Bulk mean free path (mfp) Electron wavelength (le) Phonon wavelength (lp)
Electrical conductivity
COP
1 0
ZT
3 4 5 Insulator
S
2

T Thermal
conductivity Metal
0
1
2
ZT
Semiconductor
S
S2


e
T. Tritt (2006)
p
Carrier concentration (n)
Nanostructured TE Materials
Recovering waste heat from engine exhaust, DOE Freedomcar Project
TE Figure of Merit: ZT
• Coefficient of Performance (COP)
2
CFC unit Bi2Te3
Seebeck coefficient
TEM of a CNT
2 nm
Pt Nanofilm NW 2 mm CNT
Thermal Resistance Measurement
8 6 4 2 0 0
Th-Ts (K)
I
Pt serpentine
Th Th
Q RC1
Baidu Nhomakorabea
RNW+ RC1+RC2
10 20 30 40 50 60 Q (nW)
Ts
RBeams
T0
Q = (Ts- T0)/RBeams
Macroscopic Thermal Conductivity of a SWCNT
Tgrowth = 815 oC
Thermoelectric (TE) Effects
Thermocouple Hot n p Power Generation Hot n p Refrigeration Cooled n p
•Quantum well superlattices
•Venkatasubramanian et al. Nature 413, 597 (2001) 2.5-25nm
•Quantum dot superlattices
Harman et al., Science 297, 2229 (2002)
AgPb18SbTe20 ZT 1.8 @ 800K
Hsu et al., Science 303, 818 (2004)
InGaAlAs Films with Embedded ErAs Nanoparticles
ErAs:InGaAlAs nanocomposite films (Shakouri et al.)
Commercial Peltier Coolers: - Clean - Quite
TE Applications
Cooling and temperature control Power generation
Optoelectronics (Melcor)
PCR
Climate Controlled Seat (Amerigon / BSST)
300
350
400 T (K )
450
500
Summary
• Engineering nanostructure interfaces to manipulate phonon transport can extend the current limits in thermal properties.
Integration of Metal Oxide Nanowires with Microsystems for Nerve Agent Detection
2.35 100
Current Concentration
Current (nA)
2.30 2.25
80 60 40
2.20 2.15 0.0
Difficult to measure the intrinsic thermal properties of nanotubes and nanowires
Thermal Measurements of Individual Nanostructures
Suspended measurement device with Pt resistance thermometers & electrodes
(W /m K )
1 .8 1 .6 1 .4 1 .2 1 .0 0 .8 0 .6 0 .4 0 .2 0 .0
T h e rm a l C o n d u c tiv ity (2 p ro b e )
• Our measured in-plane is close to the crossplane , and much lower than those without embedded ErAs nanoparticles.
C. Yu, Q. Hao, L. Shi, X. Kong, Z. L. Wang, Appl. Phys. Lett. 86, 063101 (2005)
DMMP Concentration (ppb)
100 nm 1000 nm • L, d ~ bulk mfp or l classical or quantum size confinement effects
Thin film superlattice
1 nm
10 nm
Phonon Transport in Nanowires and Nanotubes
I I
-
+
I
-
+
-
+
I
Cold
I
Cold
I I
Heated
I
V V = (Sp-Sn)DT Seebeck coefficient, S = voltage generated per DT
P ST
Peltier Coefficient, P P = Heat removed per charge P/T = S = Entropy per charge