Influence of structural defects on the thermal conductivity of polycrystalline and single-crystal PbTe

We have measured the thermal conductivity of unannealed and annealed (800 K, 120 h) polycrystalline and single-crystal PbTe samples at temperatures from 80 to 303 K, evaluated the electronic and lattice components of their thermal conductivity, and determined the thermal resistivity due to structural defects, whose concentration in the unannealed single-crystal samples reaches ∼10¹⁷ cm⁻³. The results demonstrate that the thermal resistivity of the unannealed polycrystalline and single-crystal samples is 9.4 and 1.7 cm K/W, respectively. Annealing eliminates the defects, thereby increasing the lattice thermal conductivity of the material.     

Simulations of a Thermal Conductivity Cell Filled with Normal 4He and Dilute Mixtures of 3He in Superfluid 4He

We have simulated the thermal response of a cylindrical thermal conductivity cell filled with liquid helium to AC and DC heat fluxes. The conductivity cell in these simulations is realistic in that it includes sidewalls and gaps, which cannot be treated analytically or in a one-dimensional simulation. Our simulations are to able to account quantitatively for the apparent departure of the effective thermal conductivity, _eff , of dilute mixtures of ³ He in superfluid ⁴ He from theoretical predictions. We have recently demonstrated experimentally that this departure is due to the presence of gaps in previous thermal conductivity cells. These simulations also show that the additional phase lag in the response of normal ⁴ He to an AC heat flux, measured by Olafsen and Behringer, is due to gaps in the heated plate.     

The importance of water migration in the measurement of the thermal conductivity of unsaturated frozen soils

The transport of heat in frozen soil may occur by conduction and by the convective transport of sensible and latent heat arising from the flow of water in the vapor, liquid and solid states. Theory describing the coupled flow of heat and of water in the liquid and vapor states is used to derive a definition of apparent thermal conductivity (the convective transport of heat in the movement of ice in unstaturated soils is assumed to be negligible). Calculations suggest that, at temperatures close to 0°C, the transport of latent heat may exceed the contribution of heat flow by conduction. Under these conditions, the apparent thermal conductivity will be much greater than the thermal conductivity calculated from the thermal conductivities and volume fractions of the components.Insufficient published data prevent a rigorous evaluation of the theory. However the functional dependence on temperature of both thermal conductivity and the apparent thermal conductivity are calculated for a Tomakomai soil at different subzero temperatures. These values are compared to the apparent thermal conductivities of this soil which were measured at a water content in the unfrozen state of 0.48 cm³ cm^?3 and at temperatures ranging from ?0.7°C to ?10°C using the line heat source technique. The dependence of apparent thermal conductivity on subzero temperature, as calculated from theory, compares favourably to the dependence which was observed for this soil.     

Thermal Conductivity and Viscosity in Dilute 3He-4He Mixtures Below 0.6 K

The effects of three-phonon processes on the phonon thermal conductivity and phonon viscosity in dilute ³He-⁴He mixtures below 0.6 K are investigated. The characteristic scattering times for phonon-phonon interaction-limited thermal conductivity and viscosity are calculated from a collision matrix which contains information on the three-phonon processes. We find that due to phonon-³He interactions the phonon momentum range in which three-phonon processes occur is reduced in comparison with that of the pure ⁴He case. Results for thermal conductivity and viscosity show good agreement with experimental data without having to adjust the rates of phonon absorption by ³He and the rates of elastic scattering of phonons with ³He.     

Effect of Magnetic Plane Defects on Thermal Conductivity of Solid 3He in U2D2 Phase

We calculate the thermal conductivity of solid ³ He in U2D2 phase. In this system, magnons are the dominant heat carriers. The experiment by Feng et al. shows that scattering magnon by some kind of magnetic defects becomes important at low temperature. We suppose the defects are the magnetic plane defects (MPD) which are originated by lattice dislocation and calculate the thermal conductivity with Kapitza Resistance Model (KRM).     

Temperature,pressure, and concentration dependence of the thermal conductivity of very dilute solutions of3He in superfluid4He

The thermal conductivity of³He–⁴He solutions at saturated vapor pressure has been measured for the concentration range 0.011–1.3 mole %³He and the temperature range 84–650 mK. Measurements were made at 10 and 24 atm for several of the concentrations. The thermal conductivity of solutions at 24 atm does not differ greatly from the thermal conductivity of pure⁴He at this pressure. Qualitative agreement with the Baym and Ebner theory is achieved only if the boundary scattering of phonons is treated in a different manner than suggested by them.     

An Investigation of the Thermal Conductivity of Zr–2.5%Nb Reactor Alloy

Experimental data are obtained for the thermal conductivity coefficient of zirconium containing 2.5% niobium. The investigated temperature range of 400 to 1600 K covers the range of existence of hexagonal (alpha-phase) and cubic (beta-phase) structural modifications of the alloy. The low-temperature and high-temperature structures differ by the value of the temperature derivative of the thermal conductivity coefficient. The thermal cycling of sample under vacuum of 10^?3 Pa leads to a gradual decrease in thermal conductivity, which is especially pronounced at low temperatures. The available data on electrical resistance for the alpha-phase region are used to estimate the Lorentz function. The obtained values of Lorentz number are indicative of the predominating part played by the electron mechanism of thermal conductivity in the alloy. The values of thermal conductivity measured for the beta-phase are used to determine the electrical conductivity of the alloy.     

Effective Heat Conductivity and Relaxation Processes in Superfluid Mixtures of 3He-4He

The effective thermal conductivity coefficient'ceff in superfluid ³He-⁴He mixtures with concentration of 9.8% ³He has been studied experimentally between 100 and 500 mK, where the main contribution to the kinetic processes is made only by phonons and ³He impurity excitations. In this case the effective thermal conductivity is a combination of diffusivity, thermal conductivity and thermal diffusion. The κ _eff value was found from stationary measurements of the temperature gradients caused by the thermal flow and from the temperature relaxation kinetics. Both the methods provide consistent resugts which also agree with those on effective thermal conductivity calculated in terms of the kinetic theory of phonon-impuriton system.     

Measurement of the Intrinsic Thermal Conductivity of a Multiwalled Carbon Nanotube and Its Contact Thermal Resistance with the Substrate

The intrinsic thermal conductivity of an individual carbon nanotube and its contact thermal resistance with the heat source/sink can be extracted simultaneously through multiple measurements with different lengths of the tube between the heat source and the heat sink. Experimental results on a 66‐nm‐diameter multiwalled carbon nanotube show that above 100 K, contact thermal resistance can contribute up to 50% of the total measured thermal resistance; therefore, the intrinsic thermal conductivity of the nanotube can be significantly higher than the effective thermal conductivity derived from a single measurement without eliminating the contact thermal resistance. At 300 K, the contact thermal resistance between the tube and the substrate for a unit area is 2.2 × 10^?8 m² K W^?1, which is on the lower end among several published data. Results also indicate that for nanotubes of relatively high thermal conductance, electron‐beam‐induced gold deposition at the tube–substrate contacts may not reduce the contact thermal resistance to a negligible level. These results provide insights into the long‐lasting issue of the contact thermal resistance in nanotube/nanowire thermal conductity measurements and have important implications for further understanding thermal transport through carbon nanotubes and using carbon nanotube arrays as thermal interface materials.     

Measurement of the thermal conductivity of molten InSb under microgravity

Thermal conductivity of molten InSb was measured on board the TEXUS-24 sounding rocket by the transient hot-wire method using the originally designed thermal conductivity measurement facility (TCMF). Measurements made through this facility were affected by natural convection on the ground. This natural convection was confirmed to be sufficiently suppressed during a microgravity environment. The thermal conductivity of molten InSb was 15.8 and 18.2 W·m^–1·K^–1 at 830 and 890 K, respectively.     

Measurement of the thermal conductivity of molten semiconductors

The thermal conductivity of molten InSb in the temperature range between 800 and 870 K was measured by the transient hot-wire method using a ceramic probe. The probe was fabricated from a tungsten wire printed on an alumina substrate and coated with a thin alumina layer. The thermal conductivity was found to be about 18 W· m^–·K^–at the melting point and increased moderately with increasing temperature. The thermal conductivity of alumina used as the substrate for the probe was also measured in the same temperature range.Paper presented at the Tenth Symposium on Thermophysical Properties, June 20–23, 1988, Gaithersburg, Maryland, U.S.A.On leave from NEC Corporation.     

Superfluid-Normal Interface of 4He Near a Wall

We observed a profile of nonequilibrium superfluid-normal (SN) interface of ⁴He near a vertical wall. A glass, brass or copper wall was used. The SN interface was produced by cooling liquid ⁴He in a bath from the bottom, where liquid ⁴He was pumped through a flow impedance in order to cool down the liquid. Superfluid (Normal fluid) occupied the lower (upper) part of the bath. The SN interface was visualized by three methods: simple visualization, shadowgraphy and schlieren method. The interface touched a vertical glass wall at almost 90°. A large hollow was observed near a brass wall which had intermediate thermal conductivity. Downward flow was observed on a copper wall due to the very good thermal conductivity of the wall. Various types of interface profile were observed depending on the thermal conductivity of the wall used.      

Anomalous Defect Dependence of Thermal Conductivity in Epitaxial WO3 Thin Films

Lattice defects typically reduce lattice thermal conductivity, which has been widely exploited in applications such as thermoelectric energy conversion. Here, an anomalous dependence of the lattice thermal conductivity on point defects is demonstrated in epitaxial WO₃ thin films. Depending on the substrate, the lattice of epitaxial WO₃ expands or contracts as protons are intercalated by electrolyte gating or oxygen vacancies are introduced by adjusting growth conditions. Surprisingly, the observed lattice volume, instead of the defect concentration, plays the dominant role in determining the thermal conductivity. In particular, the thermal conductivity increases significantly with proton intercalation, which is contrary to the expectation that point defects typically lower the lattice thermal conductivity. The thermal conductivity can be dynamically varied by a factor of ≈ 1.7 via electrolyte gating, and tuned over a larger range, from 7.8 to 1.1 W m^?1 K^?1, by adjusting the oxygen pressure during film growth. The electrolyte‐gating‐induced changes in thermal conductivity and lattice dimensions are reversible through multiple cycles. These findings not only expand the basic understanding of thermal transport in complex oxides, but also provide a path to dynamically control the thermal conductivity.     

Effect of titanium carbide coating on the microstructure and thermal conductivity of short graphite fiber/copper composites

Graphite fiber–Cu composites have drawn much attention in electronic packaging due to its excellent machinability and thermal properties. However, the weak interface bonding between graphite fiber and copper resulted in low thermo-mechanical properties of composites. In this work, a titanium carbide coating with thickness of 0.1 μm or 1 μm was synthesized on the surface of graphite fiber through molten salts method to strengthen interfacial bonding. The enhanced composites present 24–43 % increase in thermal conductivity and achieve the thermal conductivity of 330–365 W m^?1 K^?1 as well as the coefficient of thermal expansion of 6.5 × 10^?6–14 × 10^?6 K^?1. A Maxwell–Garnett effective medium approach on the anisotropic short fiber reinforcement with interfacial thermal resistance was established. The obtained enhancement was in good agreement with the estimates. The results suggest that the major factor that influences the thermal conductivities is not the interfacial thermal resistance but the low thermal conductivity of fiber in transversal direction when a well interfacial bonding is obtained.     

Thermal conductivity of heavy-oxygen water H2O18

The ratio of thermal conductivity coefficients of heavy-oxygen water H₂O¹⁸ with different percentages of enrichment to the thermal conductivity coefficient of ordinary water is measured in the range of 0–40°C. Differences in the thermal conductivities and the temperature coefficients of the thermal conductivity of heavy-hydrogen water D₂O and and heavy-oxygen water H₂O¹⁸ are indicated.     

Composites based on thermally hyperconductive carbon fibres

Recent measurements on the thermal conductivity of carbon fibres have shown that a new class of thermal hyperconductive materials based on continuous carbon fibres could be developed. Unidirectional composites have been fabricated and their thermal conductivities parallel to the fibres have been measured. The best composite measured presents a thermal conductivity value as high as 245 Wm⁻¹K⁻¹ with specific thermal conductivity higher than that of aluminium or copper.     

The influence of microstructure on the thermal conductivity of aluminium nitride

Starting from three different commercial powders, AIN materials were densified by pressureless sintering under various temperature and time values in order to investigate the influence of microstructure on thermal conductivity. The influence of the sintering aids (3 wt% Y₂O₃ and 2 wt% CaC₂) and of the forming processes (cold isostatic pressing and thermocompression of tape cast pieces) were also been evaluated. Thermal conductivity increased with the purity level of the starting powder and with an increasing the sintering temperature and soaking time. The highest thermal conductivity values (196 Wm^–1 K^–1) were obtained with the purest powder and high temperature (1800 °C) sintering over long periods (6 h). No influence on thermal conductivity was detected from the forming technique.     

Thermal conductivity and electrical resistivity of cadmium arsenide (Cd3As2) in the temperature range 4.2–40 K

Results on electrical resistivity and thermal conductivity measured in the temperature range 4.2–40 K are presented for single-crystal and polycrystalline samples of Cd₃As₂. Hall effect has been studied at temperatures of 4.2, 77, and 300 K. The calculated value of the conduction electron concentration was in the range 1.87–1.95 10²⁴m^–3. Electrical resistivity of all investigated samples was independent of temperature up to about 10K and increased slowsly at higher temperatures. The thermal conductivity shows a maximum in the region in which the lattice component of thermal conductivity dominates. The strong anisotropy of the lattice component determines the anisotropy of the total thermal conductivity. The electronic component of thermal conductivity does not exhibit any anisotropy and shows a maximum at a temperature of about 300 K.Paper submitted to the Ninth Symposium on Thermophysical Properties, June 24–27, 1985, Boulder, Colorado, U.S.A.     

Thermal Conductivity of Opacified Powder Filler Materials for Vacuum Insulations1

The thermal conductivity of powder fillings for load-bearing vacuum insulations is investigated. Different opacifiers have been tested in mixtures with perlite powder, precipitated silica, and fumed silica. Using temperature-dependent thermal conductivity measurements, the radiative thermal conductivity and the solid conductivity of the powder samples are separated. Additionally, the influence of the pressure load on the solid conductivity is studied. The thermal conductivities of silica powders with added opacifier powders (carbon black, magnetite, silicon carbide, titanium dioxide) can be as low as 0.003 W·m^–1·K^–1 if the powder boards are pressed with moderate loads. The use of microporous silica powders as filler materials allows internal gas pressures even beyond 10 hPa with only a moderate increase of the overall conductivity.     

Analysis of Possibilities of Application of Nanofabricated Thermal Probes to Quantitative Thermal Measurements

A steady-state thermal model of the nanofabricated thermal probe was proposed. The resistive type probe working in the active mode was considered. The model is based on finite element analysis of the temperature field in the probe-sample system. Determination of the temperature distribution in this system allows calculations of relative changes in the probe electrical resistance. It is shown that the modeled probe can be used for measurements of the local thermal conductivity with the spatial resolution determined by the probe apex dimensions. The probe exhibits the maximum sensitivity to the changes in the thermal conductivity of the sample between 2 W·m⁻¹ ·K⁻¹ and 200 W·m⁻¹ ·K⁻¹. The influence of the thermal conductivity of the probe substrate on metrological characteristics of the probe as well as the thermal resistance of the probe-sample contact on the determination of the sample thermal conductivity were also analyzed. The selected results of numerical analysis were compared with data of preliminary experiments.