Frederick Seitz Materials Research Lab and Department of Materials
Science
University of Illinois, Urbana
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Low thermal conductivity in nanostructured materials
improved thermoelectric energy conversion
improved thermal barriers
High thermal conductivity composites and suspensions
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High power density devices
solid state lighting
high speed electronics
nanoscale sensors
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Thermal conductivity L is a property of the continuum
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Observations (2001) span a very limited range
Al/sapphire à Pb/diamond
no data for hard/soft
lattice dynamics (LD) theory by Stoner and Maris (1993)
Diffuse mismatch (DMM) theory by Swartz and Pohl (1987)
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Acoustic mismatch (AMM)
perfect interface: average transmission coefficient <t> given by
differences in acoustic impedance, Z=rv
lattice dynamics (LD) incorporates microscopics
Diffuse mismatch (DMM)
disordered interface: <t> given by differences in densities of
vibrational states
Predicted large range of G not observed (2001)
For similar materials, scattering decreases G
For dissimilar materials, scattering increases G
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Optical constants and reflectivity depend on strain and temperature
Strain echoes give acoustic properties or film thickness
Thermoreflectance gives thermal properties
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four times scales:
pulse duration, 0.3 ps
pulse spacing, 12.5 ns
modulation period, 100 ns
time-delay, t
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frequency domain solution for heat flow in cylindrical coordinates using
gaussian beams.
G(k) given by iterative solution (transfer matrix)
In-phase and out-of-phase signals by series of sum and difference over
sidebands
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thermal conductivity of bulk samples and thermal conductance of
interfaces
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...with 3 micron resolution
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Pb and Bi show similar behavior.Electron-phonon coupling is not an important channel.
Weak dependence on Debye velocity of the substrate.
Pb/diamond 50% smaller than Stoner and Maris but still far too large for
a purely elastic process.
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Excess conductance has a linear temperature dependence (not observed by
Stoner and Maris).
Suggests inelastic (3-phonon?) channel for heat transport
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If the small thermal conductance of Bi/diamond could be reproduced in a
multi-layered film, then placing interfaces every 10 nm would give an
incredibly low thermal conductivity of 0.1 W/m-K (factor of 2 smaller
than a polymer).
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room temperature data
sputtered in pure Ar
atomic-layer deposition at 177 and 300 °C, S. George (U. Colorado)
G = 220 MW m-2 K-1
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At t=100 ps,
in-phase signal is mostly
determined by the heat capacity of the Al film
out-of-phase signal is mostly determined by the effusivity (LC)1/2
of the substrate
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after 500 thermal cycles (1 h)
25 °C —>1135 °C—>25 °C
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after 500 thermal cycles (1 h)
25 °C —>1135 °C—>25 °C
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Transient absorption measurements of nanoparticles and nanotubes in
liquid suspensions.
Measure the thermal relaxation time of a suddenly heat particle.If the particle is small enough,
then we have sensitivity to the interface
limited to interfaces that give good stability of the suspension
Thin planar Al and Au films.Same as before but heat flows both directions: into the fluid and
into the solid substrate.
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Optical absorption depends on temperature of the nanotube
Cooling rate gives interface conductance
G = 12 MW m-2
K-1
MD suggests channel is low frequency squeezing and bending modes
strongly coupled to the fluid.
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Isotropic fiber composite with high conductivity fibers (and infinite
interface conductance)
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Interface conductance and thermal conductivity of the fluid determine a
critical particle radius
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hydrophobic
37 MW/m2-K
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Much to learn about transport of heat across interfaces but we now have
good tools.
Pb/diamond, Bi/diamond interfaces show a temperature dependent
conductance far above the radiation limit.What is the correct description
of this inelastic channel?
Can circumvent the "minimum thermal conductivity" with high densities of
interfaces.
Conductance of hydrophilic nanoparticle/surfactant/water interfaces is
essentially independent of the surfactant layer.
Heat transfer is reduced by a factor of 4 at hydrophobic interfaces with
water.
