Showing papers on "Thermal contact conductance published in 2006"

Thermal Conductance of an Individual Single-Wall Carbon Nanotube above Room Temperature

Abstract: The thermal properties of a suspended metallic single-wall carbon nanotube (SWNT) are extracted from its high-bias (I−V) electrical characteristics over the 300−800 K temperature range, achieved by Joule self-heating. The thermal conductance is approximately 2.4 nW/K, and the thermal conductivity is nearly 3500 Wm-1K-1 at room temperature for a SWNT of length 2.6 μm and diameter 1.7 nm. A subtle decrease in thermal conductivity steeper than 1/T is observed at the upper end of the temperature range, which is attributed to second-order three-phonon scattering between two acoustic modes and one optical mode. We discuss sources of uncertainty and propose a simple analytical model for the SWNT thermal conductivity including length and temperature dependence.

Negative differential thermal resistance and thermal transistor

Abstract: We report on the first model of a thermal transistor to control heat flow. Like its electronic counterpart, our thermal transistor is a three-terminal device with the important feature that the current through the two terminals can be controlled by small changes in the temperature or in the current through the third terminal. This control feature allows us to switch the device between “off” (insulating) and “on” (conducting) states or to amplify a small current. The thermal transistor model is possible because of the negative differential thermal resistance.

Thermal conductance of interfaces between highly dissimilar materials

Abstract: The thermal conductance of interfaces between materials with low Debye temperatures (Pb or Bi) and dielectrics or semiconductors with high Debye temperatures (hydrogen-terminated Si, $\mathrm{Si}{\mathrm{O}}_{2}$, the native oxide of Be, sapphire, or hydrogen-terminated diamond) is measured using time-domain thermoreflectance. The interface thermal conductance $G$ for these combinations of materials falls within a relatively narrow range, $8lGl30\phantom{\rule{0.3em}{0ex}}\mathrm{MW}\phantom{\rule{0.2em}{0ex}}{\mathrm{m}}^{\ensuremath{-}2}\phantom{\rule{0.2em}{0ex}}{\mathrm{K}}^{\ensuremath{-}1}$, at room temperature. Because the thermal conductance of interfaces with Bi, a semimetal, and interfaces with Pb, a metal, are similar, we conclude that the coupling of electrons in a metal to phonons in a dielectric substrate does not contribute significantly to the thermal transport at interfaces. For Pb or Bi on hydrogen-terminated diamond, the measured conductance greatly exceeds the radiation limit and decreases approximately linearly with decreasing temperature, suggesting that anharmonic processes dominate the transfer of thermal energy across interfaces between materials with highly dissimilar spectra of lattice vibrations.

Thermal conductivity of phase-change material Ge2Sb2Te5

Abstract: The thermal conductivity of thin films of the phase-change material Ge2Sb2Te5 is measured in the temperature range of 27°C

Thermal Conductance of metal-metal interfaces

Abstract: The thermal conductance of interfaces between Al and Cu is measured in the temperature range 78 T 298 K using time-domain thermoreflectance. The samples are prepared by magnetron sputter deposition of a 100 nm thick film of Al on top of layers of Cu on sapphire substrates. The chemical abruptness of the Al-Cu interface is systematically varied by ion-beam mixing using 1 MeV Kr ions. The thermal conductance of the as-deposited Al-Cu interface is 4 GW m−2 K−1 at room temperature, an order-of-magnitude larger than the phonon-mediated thermal conductance of typical metal-dielectric interfaces. The magnitude and the linear temperature dependence of the conductance are described well by a diffuse-mismatch model for electron transport at interfaces.

Single-mode heat conduction by photons

Abstract: A clever experiment that makes use of two metallic islands connected by superconducting leads now confirms that thermal conduction by photons is limited by the same quantum value. Although the result is mainly of fundamental importance, there are implications for the design of bolometers, detectors of far-infrared light that are used in astrophysical studies, and electronic micro-refrigerators. The thermal conductance of a single channel is limited by its unique quantum value GQ, as was shown theoretically1 in 1983. This result closely resembles the well-known quantization of electrical conductance in ballistic one-dimensional conductors2,3. Interestingly, all particles—irrespective of whether they are bosons or fermions—have the same quantized thermal conductance4,5 when they are confined within dimensions that are small compared to their characteristic wavelength. The single-mode heat conductance is particularly relevant in nanostructures. Quantized heat transport through submicrometre dielectric wires by phonons has been observed6, and it has been predicted to influence cooling of electrons in metals at very low temperatures due to electromagnetic radiation7. Here we report experimental results showing that at low temperatures heat is transferred by photon radiation, when electron–phonon8 as well as normal electronic heat conduction is frozen out. We study heat exchange between two small pieces of normal metal, connected to each other only via superconducting leads, which are ideal insulators against conventional thermal conduction. Each superconducting lead is interrupted by a switch of electromagnetic (photon) radiation in the form of a DC-SQUID (a superconducting loop with two Josephson tunnel junctions). We find that the thermal conductance between the two metal islands mediated by photons indeed approaches the expected quantum limit of GQ at low temperatures. Our observation has practical implications—for example, for the performance and design of ultra-sensitive bolometers (detectors of far-infrared light) and electronic micro-refrigerators9, whose operation is largely dependent on weak thermal coupling between the device and its environment.

Effective thermal conductivity and thermal diffusivity of nanofluids containing spherical and cylindrical nanoparticles

Abstract: This paper reports on measurements of the effective thermal conductivity and thermal diffusivity of various nanofluids using the transient short-hot-wire technique. To remove the influences of the static charge and electrical conductance of the nanoparticles on measurement accuracy, the short-hot-wire probes are carefully coated with a pure Al2O3 thin film and only those probes that are coated well are used for measurements. In the present study, the effective thermal conductivities and thermal diffusivities of Au/toluene, Al2O3/water, and carbon nanofiber (CNF)/water nanofluids are measured and the effects of the volume fraction and thermal conductivity of the nanoparticles and temperature are clarified. The average diameters of Au and Al2O3 spherical particles are 1.65 and 20nm, respectively. The average length and diameter of CNFs are 10μm and 150nm, respectively. The uncertainty of the present measurements is estimated to be within 1% for the thermal conductivity and 5% for the thermal diffusivity. The measured results demonstrate that the effective thermal conductivities of the nanofluids show no anomalous enhancements and can be predicted accurately by the model equation of Hamilton and Crosser [Ind. Eng. Chem. Fundam. 1, 187 (1962)] for the spherical nanoparticles and by the unit-cell model equation of Yamada and Ota [Waerme-Stoffuebertrag. 13, 27 (1980)] for carbon nanofibers.

Experimental Study on the Effective Thermal Conductivity and Thermal Diffusivity of Nanofluids

Abstract: This paper reports measurements of the effective thermal conductivity and thermal diffusivity of various nanofluids using the transient short-hot-wire technique. To remove the influences of the static charge and electrical conductance of the nanoparticles on measurement accuracy, the short-hot-wire probes are carefully coated with a pure Al2O3 thin film. Using distilled water and toluene as standard liquids of known thermal conductivity and thermal diffusivity, the length and radius of the hot wire and the thickness of the Al2O3 film are calibrated before and after application of the coating. The electrical leakage of the short-hot-wire probes is frequently checked, and only those probes that are coated well are used for measurements. In the present study, the effective thermal conductivities and thermal diffusivities of Al2O3/water, ZrO2/water, TiO2/water, and CuO/water nanofluids are measured and the effects of the volume fractions and thermal conductivities of nanoparticles and temperature are clarified. The average diameters of Al2O3, ZrO2, TiO2, and CuO particles are 20, 20, 40, and 33 nm, respectively. The uncertainty of the present measurements is estimated to be within 1% for the thermal conductivity and 5% for the thermal diffusivity. The measured results demonstrate that the effective thermal conductivities of the nanofluids show no anomalous enhancement and can be predicted accurately by the model equation of Hamilton and Crosser, when the spherical nanoparticles are dispersed into fluids.

Nonequilibrium Green’s function approach to mesoscopic thermal transport

Abstract: We present a formulation of a nonequilibrium Green's function method for thermal current in nanojunction atomic systems with nonlinear interactions. This first-principles approach is applied to the calculation of the thermal conductance in a one-dimensional chain and carbon nanotube junctions. It is shown that nonlinearity already becomes important at low temperatures. Nonlinear interactions greatly suppress phonon transmission at room temperature. The peak of thermal conductance is found to be around $400\phantom{\rule{0.3em}{0ex}}\mathrm{K}$, in good agreement with experiments. High-order phonon scattering processes are important for diffusive heat transport.

Thermal Contact Resistance and Thermal Conductivity of a Carbon Nanofiber

Abstract: It has been suggested that CNTs and carbon nanofibers CNFs can be used as thermal interface materials to enhance contact thermal conductance for electronic packaging applications. Several groups have reported mixed experimental results from no improvements to large improvements in the thermal contact conductance due to the CNTs and CNFs 8‐12. These mixed results can be caused by the difference in surface coverage and perpendicular alignment of the CNTs or CNFs. Moreover, the results can be affected by two other factors. First, the CNTs and CNFs grown using different methods possess different defect densities and different intrinsic thermal conductivities. Secondly, the contact thermal resistance of the nanometer scale point and line contacts between a CNT or CNF and a planar surface can be high due to enhanced phonon-boundary scattering at the nanocontacts. We have used a microfabricated device to measure the thermal resistance of an individual CNF from a vertically aligned CNF film for applications as thermal interface materials. The measurement was conducted before and after a platinum Pt layer was deposited on the contacts between the CNF and the microdevice so as to investigate the thermal contact resistance between the CNF and a planar surface. The contact resistance was reduced by the platinum coating for about 9‐13% of the total thermal resistance of the nanofiber sample before the Pt coating. At temperature 300 K, the obtained axial thermal conductivity of the carbon nanofibers was about three times smaller than that of graphite fibers grown by pyrolysis of natural gas prior to high-temperature heat treatment.

Model for the effective thermal conductivity of carbon nanotube composites.

Abstract: We present a novel model of the effective thermal conductivity for carbon nanotube composites by incorporating the interface thermal resistance with an average polarization theory. The dependence of the effective thermal conductivity on nanotube length, diameter, concentration, and interface thermal resistance has been taken care of simultaneously in our treatment. The model predicts that the large length of the carbon nanotubes embedded plays a key role in the thermal conductivity enhancement, while the large interface thermal resistance across the nanotube-matrix interface causes a significant degradation. Interestingly, the model predicts that the nanotube diameter has a very small effect on the thermal conductivity enhancement of the nanotube composites. In addition, the model predicts that the thermal conductivity enhancement of nanotube composites increases rapidly with decreasing the thermal conductivity of the matrix and increases with increasing the thermal conductivity of the carbon nanotube. Predictions from the novel model are in excellent agreement with the experimentally observed values of the effective thermal conductivity of carbon nanotube nanofluids which the classical models have not been able to explain.

Heat flow in nonlinear molecular junctions: Master equation analysis

Abstract: We investigate the heat conduction properties of molecular junctions comprising nonlinear interactions. We find that these interactions can lead to phenomena such as negative differential thermal conductance and heat rectification. Based on analytically solvable models we derive an expression for the heat current that clearly reflects the interplay between internal molecular anharmonic interactions, the strength of molecular coupling to the thermal reservoirs, and junction asymmetry. This expression indicates that negative differential thermal conductance shows up when the molecule is strongly coupled to the thermal baths, even in the absence of internal molecular nonlinearities. In contrast, diodelike behavior is expected for a highly anharmonic molecule with an inherent structural asymmetry.

Influence of grain boundary scattering on the electrical and thermal conductivities of polycrystalline gold nanofilms

Abstract: The electrical and thermal conductivities of polycrystalline gold nanofilms have been measured simultaneously by a direct current heating method, and the measured results are compared with the Mayadas and Shatzkes theory. It is found that the reduced electrical and thermal conductivities of gold nanofilms are strongly dominated by grain boundary scattering. The reflection coefficient of electrons striking the grain boundaries for charge transport is 0.7, which agrees well with a previous scanning tunneling potentiometry study. The reflection coefficient for thermal transport, however, is only 0.25. The Lorenz numbers for the polycrystalline gold nanofilms, which are calculated from the measured electrical and thermal conductivities, are much greater than the value predicted by the Wiedemann-Franz law for the bulk material. The results indicate that the electron scatterings on the grain boundaries impose different influences on the charge and heat transport in the polycrystalline gold nanofilms. A model of effective density of conduction electrons has been utilized to interpret the violation of the Wiedemann-Franz law in polycrystalline gold nanofilms.

Reexamining the 3-omega technique for thin film thermal characterization

Abstract: The 3-omega method is widely used to measure thermal properties of thin films and interfaces. Generally, one-dimensional heat conduction across the film is assumed and the film capacitance is neglected. The change in the in-phase (real part) temperature response for the film-on-substrate case relative to the substrate-only case is, therefore, attributed to the sum of the bulk thermal resistance of the film and the thermal boundary resistance between the film and the substrate. Based on a rigorous and intuitive mathematical derivation, it is shown that this approach represents a limiting case, and that its use can cause significant errors in rather realistic situations when the underlying assumptions are not met. This article quantifies the error by introducing a new parameter called the ratio function R, which modifies the film thermal resistance and mathematically shows that it depends only on three dimensionless parameters that combine thermal properties and geometries of the film and the heated linewidth. A new data reduction scheme is suggested accordingly to determine the film thermal conductivity (cross-plane), anisotropic thermal conductivity ratio between the in-plane direction and the cross-plane direction, and the interface thermal conductance.

Thermal properties of carbon nanotube array used for integrated circuit cooling

Abstract: Carbon nanotubes (CNTs), owing to their exceptionally high thermal conductivity, have a potential to be employed in micro- and optoelectronic devices for integrated circuit (IC) cooling. In this study we describe a photothermal metrology intended to evaluate the thermal conductivity of a vertically aligned CNT array (VCNTA) grown on a silicon (Si) substrate. Plasma-enhanced chemical vapor deposition, with nickel (Ni) as a catalyst, was used to grow CNT. The experimentally evaluated thermal conductivity of the VCNTA and the thermal contact resistance at the interface between the VCNTA and the “hot” surface was found to be in a satisfactory agreement with theoretical predictions. The measured effective thermal resistance is measured to be 0.12∼0.16cm2∙K∕W. This resistance was compared to the measured resistance of commercially available thermal grease. Based on this comparison, we conclude that, although the thermal resistance of CNTs might not be as low as it might be desirable, there exists a definite inc...

Enhanced thermal contact conductance using carbon nanotube array interfaces

Abstract: Heat-conduction interfaces that employ carbon nanotube (CNT) arrays have been fabricated and studied experimentally using a reference calorimeter testing rig in a vacuum environment with infrared temperature measurements. Arrays of multiwalled CNTs are grown directly on silicon substrates with microwave plasma-enhanced chemical vapor deposition. Iron and nickel were used as CNT catalysts. CNT arrays grown under different synthesis conditions exhibit different pressure-contact conductance characteristics. The thermal contact resistance of CNTs with a copper interface exhibits promising results with a minimum value of 19.8mm2K/W at a pressure of 0.445MPa

Short hot wire technique for measuring thermal conductivity and thermal diffusivity of various materials

Abstract: A transient short hot wire technique (SHWT) is developed for simultaneous determination of the thermal conductivity and thermal diffusivity of various materials such as liquids, gases or powders. A metal wire with (or without) insulation coating serves both as a heating unit and as an electrical resistance thermometer and the wire is calibrated using water and toluene with known thermophysical properties. This SHWT includes correlation of the experimental data with numerically simulated values based on a two-dimensional heat-conduction model. For the measurements with proportional relation between temperature rise and logarithmic heating time interval, the thermal conductivity and thermal diffusivity are obtained from the slope and the intercept of the measured temperature rise and those of calculated non-dimensional temperature rise by including the heat flux and the properties of the wire. For the measurements with nonlinear relation between temperature rise and logarithmic heating time interval, the thermal conductivity and thermal diffusivity are extracted from a curve fitting method by using the downhill simplex method to match the experimental data and the numerical values. This technique is applied here using air as a testing sample. The effect of natural convection is investigated and the accuracy of this measurement is estimated to be 2% for thermal conductivity and 7% for thermal diffusivity.

Influence of stainless steel substrate preheating on surface topography and on millimeter- and micrometer-sized splat formation

Abstract: Many properties (thermal, electrical, mechanical…) of thermal sprayed coatings are strongly linked to the real contacts between the “piled-up” splats. The quality of this contact depends on droplet parameters at impact (size, temperature, velocity,…) and substrate parameters (temperature, topography…). Two different techniques have been developed in order to study the plasma sprayed particle behavior at impact. The first one allows direct studying under direct current (dc) plasma spray conditions, while the latter one, based on the millimeter-sized free-falling drop, enables the visualization of flattening phenomena, but at larger (about three orders of magnitude) time and size scales. These two techniques applied to zirconia and nickel droplets or drops bring complementary approaches and results. With millimeter-sized nickel drops impacting on stainless steel substrates, the flattening time and cooling rate of the lamellae are improved when the substrate surface is modified at the nanoscale, corresponding to a positive skewness S k parameter, by preheating it over the transition temperature. Resulting splats are disk shaped. Static wettability experiments show that the presence of nanopeaks increases the contact angle of the liquid on the substrates thus reducing the thermal contact resistance at the interface. With sprayed ZrO 2 particles the same phenomena (better wettability, and increased cooling rate) are observed on stainless steel substrate with S k > 0. It has also been shown that, when adsorbates and condensates are not eliminated from the surface, by preheating for zirconia droplets either on zirconia or stainless steel substrate (even with a positive skewness for the latter), the thermal contact resistance is increased and a fingered splat morphology is obtained.

Anisotropic Heat Transfer of Single-Walled Carbon Nanotubes *

Abstract: Heat transfer of single-walled carbon nanotubes (SWNTs) in practical situations is investigated using molecular dynamics (MD) simulations. Attenuation of the expected high thermal conductivity was simulated by mixing 13 C isotope impurities to SWNTs or binding two SWNTs with different chirality with a junction structure in between. The heat transfer through the junction can be expressed with the thermal boundary conductance by considering a virtual boundary at the junction. The lateral heat conduction was compared with the thermal boundary conductance at the interfaces between an SWNT and surrounding materials. By applying the lumped capacity method on the non-stationary molecular dynamics simulations, the thermal boundary conductance of an SWNT bundle and an SWNT confining water were calculated. Finally, some conventional properties were estimated to characterize the anisotropic heat conduction.

Ultralow thermal conductivity of a packed bed of crystalline nanoparticles: A theoretical study

Abstract: Theoretical thermal conductivity of a packed bed of crystalline spherical nanoparticles is reported. Thermal conductivity is dominated by surface and constriction thermal resistances and surface energy of the nanoparticles. Depending on the surface energy and size of the nanoparticles, thermal conductivity of the solid phase can be smaller than the minimum thermal conductivity given by the Einstein limit. It is also shown that depending on the surface energy and size of the nanoparticles, thermal conductivity of the nanoparticle bed can be smaller than the thermal conductivity of air. The range of surface energies under which these conditions are achievable for silicon-based nanoparticle beds is reported. Finally, it is shown that nanoconstrictions are more efficient in reducing thermal conductivity than superlattices and nanowires.