Effect of thermal coarsening on the thermal conductivity of nanoporous gold

The thermal conductivities of nanoporous gold (NPG) microwires annealed at different temperatures have been measured in the temperature range from 100 to 320 K. Considering the electron-surface scattering, the thermal conductivity is expected to increase with the increase of ligament diameter. However, the thermal conductivity of NPG microwire is found to decrease after thermal coarsening, and has a maximum value at around 250 K for the as-dealloyed sample. We suggest that the defects accumulating at a relatively high temperature and the reduction in defect spacing may cause these temperature behaviors of thermal conductivity. Taking into account the electron scattering on ligament surfaces and defects, a modified theoretical model for the thermal conductivity of nanoporous metal is proposed to agree with our experimental results.     

Low temperature thermal conductivity of aluminum alloy 5056

The thermal conductivity of 5056 aluminum alloy was determined from 4.2 K to 120 K using a differential steady-state method. This method has been implemented in a low temperature cryostat using a Gifford–McMahon cryocooler as heat sink. The thermal conductivity of the 5056 H39 aluminum alloy was determined since it was under consideration as a part of a thermal link for the Planck research satellite. As expected, below 10 K the thermal conductivity is exclusively given by the electron-defect scattering term. At higher temperature, the other terms from the electronic and the lattice contributions come into play but the electronic thermal conductivity term is still dominant. A workable fit, based on theory, is presented and can be used up to 300 K. Our measurements are compared with data at lower temperature and available fits from the literature.     

Low temperature physical properties of a high Mn austenitic steel JK2LB

High manganese austenitic stainless steel JK2LB is developed by the Japan Atomic Energy Agency for applications as a conduit material for superconducting cable-in-conduit conductors for the magnets of international thermonuclear experimental reactor (ITER). The low temperature physical property data of this material are very important to ITER magnet design. Therefore in this paper, our measurements of the physical properties including room temperature Young’s modulus and thermal expansion, magnetization, thermal conductivity, specific heat and resistivity at temperatures from room temperature down to 2 K are reported. We found that JK2LB is antiferromagnetic at low temperatures with a Néel temperature of 240 K. This is consistent with a prediction based on the chemical composition of the austenite stainless steel. The antiferromagnetic phase transition is also evident in the resistivity vs. T curve. Nevertheless, no anomalies are observable in its specific heat and thermal conductivity from 2 K to 300 K. The thermal expansion of this steel between 10 K and 300 K is about 0.22%. Its Young’s modulus, specific heat and thermal conductivity are comparable to that of 316LN stainless steel.     

Thermal conductivity of a CuCrZr alloy from 5 K to room temperatures

Thermal conductivity of a CuCrZr alloy containing of 0.71% of Cr and 0.23% of Zr was measured in the temperature interval from 5 K to 300 K. A method utilizing the measurement of thermal conductivity integral of a sample was verified and applied. Measurements of thermal conductivity, electrical resistivity at 4.2 K and 295 K and of Brinell hardness were performed on solution annealed, precipitation hardened and “as received” materials. The CuCrZr alloy was found to be applicable where high mechanical properties together with high and stable thermal conductivity are required. The possibility to predict the thermal conductivity of precipitation hardened copper alloys from the electrical properties even if the RRR values are lower than 10 was verified at RRR = 5.     

Low temperature thermal conductivity of aluminum alloy 1200

The thermal conductivity of aluminum alloy 1200 was determined from 4.2 K to 160 K using a thermal conductivity integral method. This steady state method has been implemented in a cryostat having a cold finger cooled with liquid helium and nitrogen. These materials were considered to create thermal link for the Planck research satellite. Two samples are studied; the “as fabricated” 1200 alloy and the 1200 H19 (cold-drawn). As expected, the evolution of the thermal conductivity with temperature of both alloys follows the electronic thermal conductivity theory with a good accuracy below 60 K. At higher temperature, the thermal conductivity reaches a maximum then decreases as T^?n and finally remains constant due to the electron–phonon scattering. As expected, the thermal conductivity of the cold-drawn alloy, 1200 H19, is reduced compared to that of the 1200 alloy due to a higher concentration of defects in the metal.     

Microstructures and thermal properties of A356/SiCp composites fabricated by liquid pressing method

A356/45vol.%SiCp composites with a uniform distribution of SiC particles have been fabricated by a liquid pressing method. Increasing the melt temperature, holding time and pre-treatment of SiCp by thermal oxidation improves the soundness of composites for the liquid pressing method. The sound composites exhibited low coefficient of thermal expansion (8 ppm/K) and high thermal conductivity (155 W/m K). The measured values for coefficient of thermal expansion agree well with the predicted values based on Turner’s model irrespective of porosity. The measured values for thermal conductivity decrease with porosity, and the effect of pore on the thermal conductivity has been evaluated based on the modified Hasselman–Johnson model.     

Thermal Conductivity of AlN–Ethanol Nanofluids

Aluminum nitride (AlN) particles of 20 nm diameter were dispersed into ethanol by a two-step process, first magnetic striation and then ultrasonic agitation. Castor oil was added as a dispersant to improve the stability of the AlN suspension. The thermal conductivities of AlN–ethanol nanofluids were measured by a hot-disk method from 0.5 vol% to 4.0 vol% at temperatures of 273.15 K and 297.15 K. Results show about 20% increase in the thermal conductivity of ethanol with the addition of 4.0 vol% at 273.15 K, and a strong temperature dependence of the thermal conductivity.     

Modified Dynamic Plane Source Method for Measuring Thermophysical Parameters of Solids

The authors proposed and experimentally verified a new method of measurements of thermal conductivity and thermal diffusivity of homogeneous and isotropic materials with thermal conductivities of <2 W ·m^-1 ·K^-1{2\,{\rm W} \cdot{\rm m}^{-1} \cdot{\rm K}^{-1}} . The theoretical model and the experimental arrangement of the method, referred to as the modified dynamic plane source method, are described. The influence of the floating temperature of the heat sink was analyzed and included into the evaluation. The method was both theoretically and experimentally compared with other transient methods, namely, transient plane source and extended dynamic plane source methods. Two different polymethylmethacrylate materials were measured at laboratory temperature using all three methods, obtaining results with coefficients of variation of <2 % and 5 % for the thermal conductivity and thermal diffusivity, respectively. In addition, the sensor calibration and the sources of measurement uncertainty are outlined.     

Absolute measurements of the thermal conductivity of alcohols by the transient hot-wire technique

New absolute measurements of the thermal conductivity of methanol, ethanol, propanol, butanol, pentanol, and hexanol at atmospheric pressure and in the temperature range 290–350 K are reported. The overall uncertainty in the reported thermal conductivity data is estimated to be better than ±0.5%, an estimate confirmed by the measurement of the thermal conductivity of water. The measurements presented in this paper have been used to develop a consistent theoretically based correlation for the prediction of the thermal conductivity of alcohols. The proposed scheme, based on an extention of the rigid-sphere model, permits the density dependence of the thermal conductivity of alcohols, for temperatures between 290 and 350 K and atmospheric pressure, to be represented successfully by an equation containing just one parameter characteristic of the fluid at each temperature.Paper presented at the Tenth Symposium on Thermophysical Properties, June 20–23, 1988, Gaithersburg, Maryland, U.S.A.     

Nanoscale size dependence parameters on lattice thermal conductivity of Wurtzite GaN nanowires

A detailed calculation of lattice thermal conductivity of freestanding Wurtzite GaN nanowires with diameter ranging from 97 to 160 nm in the temperature range 2–300 K, was performed using a modified Callaway model. Both longitudinal and transverse modes are taken into account explicitly in the model. A method is used to calculate the Debye and phonon group velocities for different nanowire diameters from their related melting points. Effect of Gruneisen parameter, surface roughness, and dislocations as structure dependent parameters are successfully used to correlate the calculated values of lattice thermal conductivity to that of the experimentally measured curves. It was observed that Gruneisen parameter will decrease with decreasing nanowire diameters. Scattering of phonons is assumed to be by nanowire boundaries, imperfections, dislocations, electrons, and other phonons via both normal and Umklapp processes. Phonon confinement and size effects as well as the role of dislocation in limiting thermal conductivity are investigated. At high temperatures and for dislocation densities greater than 10¹⁴ m^?2 the lattice thermal conductivity would be limited by dislocation density, but for dislocation densities less than 10¹⁴ m^?2, lattice thermal conductivity would be independent of that.     

Measurements of Hydrogen Thermal Conductivity at High Pressure and High Temperature

The thermal conductivity for normal hydrogen gas was measured in the range of temperatures from 323 K to 773 K at pressures up to 99 MPa using the transient short hot-wire method. The single-wire platinum probes had wire lengths of 10 mm to 15 mm with a nominal diameter of 10 μm. The volume-averaged transient temperature rise of the wire was calculated using a two-dimensional numerical solution to the unsteady heat conduction equation. A non-linear least-squares fitting procedure was employed to obtain the values of the thermal conductivity required for agreement between the measured temperature rise and the calculation. The experimental uncertainty in the thermal-conductivity measurements was estimated to be 2.2 % (k = 2). An existing thermal-conductivity equation of state was modified to include the expanded range of conditions covered in the present study. The new correlation is applicable from 78 K to 773 K with pressures to 100 MPa and is in agreement with the majority of the present thermal-conductivity measurements within ±2 %.     

Measurement of the Thermal Conductivity of Liquid Dimethoxymethane from 240 to 362 K

The thermal conductivity of liquid dimethoxymethane was measured by the transient hot-wire method using a bare platinum wire in a temperature range from 240 to 360 K. The experimental data were fitted by a function of temperature. The average absolute deviation of experimental data from those calculated by the equation was 0.18%, and the maximum deviation was 0.41%. The uncertainty of the thermal conductivity was less than ± 2% with a coverage factor of k = 2. The uncertainty of the temperature was within ± 10 mK (k = 2).     

Absolute measurements of the thermal conductivity of mixtures of alkene-glycols with water

New absolute measurements of the thermal conductivity of ethylene and propylene glycol and their mixtures with water are presented. The measurements were performed in a tantalum-type transient hot-wire instrument at atmospheric pressure, in the temperature range 295–360 K. The overall uncertainty of the reported values is estimated to be less than ±0.5%, an estimate confirmed by measurements of the thermal conductivity of water. The mixtures with water studied have compositions of 25, 50, and 75%, by weight. A recently proposed semiempirical scheme for the prediction of the thermal conductivity of pure liquids is extended to allow the prediction of the thermal conductivity of these mixtures from the pure components, as a function of both composition and temperature.     

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.     

The transport properties of ethane. II. Thermal conductivity

A new representation of the thermal conductivity of ethane is presented. The representative equations are based upon a body of experimental data that have been critically assessed for internal consistency and for agreement with theory in the zero-density limit and in the critical region. The representation extends over the temperature range from 100 K to the critical temperature in the liquid phase and from 225 K to the critical temperature in the vapor phase. In the supercritical region the temperature range extends to 1000 K for pressures up to 1 MPa and to 625 K for pressures up to 70 MPa. The ascribed accuracy of the representation varies according to the thermodynamic state from ±2% for the thermal conductivity of the dilute gas near room temperature to ±5% for the thermal conductivity at high pressures and temperatures. Tables of the thermal conductivity, generated by the relevant equations, at selected temperatures and pressures and along the saturation line are also provided.     

Electrical properties of NiO films obtained by high-temperature oxidation of nickel

Nickel oxide thin films are formed by high-temperature oxidation of nickel foils at 973 K, and are characterized using X-ray diffraction and scanning electron microscopy indicating the formation of a single NiO phase whose thickness grows following a parabolic law. The electrical properties of the formed films are examined by impedance spectroscopy at room temperature; and by measuring direct current (DC) and alternating current (AC) conductivities and dielectric properties at different temperatures. At room temperature, the conductivity is about 4 orders of magnitude higher than that of NiO single crystals. Below 200 K, DC conductivity displays a slight increase with increasing temperature indicating conduction by thermal activation hopping of small polarons. Above 250 K, large polaron conduction associated with holes in the 2p band of O²⁻ with activation energy of about 0.4 eV is observed. Frequency as well as temperature dependencies of the AC conductivity and dielectric constant exhibit trends usually observed in carrier dominated dielectrics.     

Dependence of electrical and thermal conductivity on temperature in directionally solidified Sn–3.5 wt% Ag eutectic alloy

Sn–3.5 wt% Ag alloy was directionally solidified upward with a constant growth rate (V = 16.5 μm/s) and a temperature gradient (G = 3.3 K/mm) in a Bridgman-type growth apparatus. The variations of electrical resistivity (ρ) with temperature in the range of 293–476 K for the directionally solidified Sn–3.5 wt% Ag eutectic alloy was measured. The measurements indicate that the electrical resistivity of the directionally solidified Sn–Ag eutectic solder increases with increasing temperature. The variations of thermal conductivity of solid phases versus temperature for the same alloy was determined from the Wiedemann-Franz and Smith-Palmer equations by using the measured values of electrical conductivity. From the graphs of electrical resistivity and thermal conductivity versus temperature, the temperature coefficient of electrical resistivity (α _TCR ) and the temperature coefficient of thermal conductivity (α _TCT ) for the same alloy were obtained. According to experimental results, the electrical and thermal conductivity of Sn–Ag eutectic solder linearly decrease with increasing the temperature. The enthalpy of fusion (ΔH) and the change of specific heat (ΔC _P ) during the transformation at the studied alloy were determined from heating curve during the transformation from eutectic solid to eutectic liquid by means of differential scanning calorimeter (DSC).     

Alumina shunt for precooling a cryogen-free 4He or 3He refrigerator

In this technical report a cryogen-free 1 K cryostat is described where the pot of the ⁴He refrigeration unit is precooled by the 2nd stage of a pulse tube cryocooler (PTC) from room temperature to T ∼ 3 K via a shunt made from sintered alumina (SA); the total mass of the 1 K stage is 3.5 kg. SA has high thermal conductivity at high temperatures; but below ∼50 K the thermal conductivity drops rapidly, almost following a T³-law. This makes SA an interesting candidate for the construction of a thermal shunt, especially as the heat capacity of metals drops by several orders of magnitude in the temperature range from 300 K to 3 K. At the base temperature of the PTC, the heat conduction of the shunt is so small that the heat leak into the 1 K stage is negligible.     

Interfacial microstructure and its effect on thermal conductivity of SiCp/Cu composites

Thermal conductivity of SiCp/Cu composites was usually far below the expectation, which is usually attributed to the low real thermal conductivity of matrix. In the present work, highly pure Cu matrix composites reinforced with acid washed SiC particles were prepared by the pressure infiltration method. The interfacial microstructure of SiCp/Cu composites was characterized by layered interfacial products, including un-reacted SiC particles, a Cu–Si layer, a polycrystalline C layer and Cu–Si matrix. However, no Cu₃Si was found in the present work, which is evidence for the hypothesis that the formation of Cu₃Si phase in SiC/Cu system might be related to the alloying elements in Cu matrix and residual Si in SiC particles. The thermal conductivity of SiCp/Cu composites was slightly increased with the particle size from 69.9 to 78.6 W/(m K). Due to high density defects, the real thermal conductivity of Cu matrix calculated by H–J model was only about 70 W/(m K). The significant decrease in thermal conductivity of Cu matrix is an important factor for the low thermal conductivity of SiCp/Cu composites. However, even considered the significant decrease of thermal conductivity of Cu matrix, theoretical values of SiCp/Cu composites calculated by H–J model were still higher than the experimental results. Therefore, an ideal particle was introduced in the present work to evaluate the effect of interfacial thermal resistance. The reverse-deduced effective thermal conductivities of ideal particles according to H–J model was about 80 W/(m K). Therefore, severe interfacial reaction in SiCp/Cu composites also leads to the low thermal conductivity of SiCp/Cu composites.     

Thermal Conductivity of Liquid 1, 2-Dimethoxyethane from 243K to 353K at Pressures up to 30MPa

The thermal conductivity of liquid 1, 2-dimethoxyethane was measured from 243K to 353K at pressures from 0 to 30 MPa by the transient hot-wire technique employing two anodized tantalum hot wires. The experimental data were correlated as a function of pressure and temperature. The average absolute deviation of experimental data from those calculated by the equation was 0.24 %, and the maximum absolute deviation was 0.80 %. The uncertainty of the thermal conductivity was 2.0 % with a coverage factor of k = 2.