Radiative heat transfer in transient hot-wire measurements of thermal conductivity

New measurements of the thermal conductivity of liquid toluene between 300 and 550 K have been used to study the importance of radiative heat transfer when using the transient hot-wire technique. The experimental data were used to obtain the radiation correction to the hot-wire temperature rises. Radiationcorrected values of thermal conductivity are reported. This study shows that the transient hot-wire method is much less affected by radiation than steady-state techniques.     

Determination of thermal conductivity from specific heat and thermal diffusivity measurements of plasma-sprayed cermets

The thermal conductivities of three plasma-sprayed cermets have been determined over the temperature range 23–630°C from the measurement of the specific heat, thermal diffusivity, and density. These cermets are mixtures of Al and SiC prepared by plasma spray deposition and are being considered for various applications in magnetic confinement fusion devices. The samples consisted of three compositions: 61 vol% Al/39 vol% SiC, 74vol% Al/26vol% SiC, and 83 vol% Al/17 vol% SiC. The specific heat was determined by differential scanning calorimetry through the Al melt transition up to 720°C, while the thermal diffusivity was determined using the laser flash technique up to 630°C. The linear thermal expansion was measured and used to correct the diffusivity and density values. The thermal diffusivity showed a significant increase after thermal cycling due to a reduction in the intergrain contact resistance, increasing from 0.4 to 0.6 cm²·^–1 at 160°C. However, effective medium theory calculations indicated that the thermal conductivities of both the Al and the SiC were below the ideal defect-free limit even after high-temperature cycling. The specific heat measurements showed suppressed melting points in the plasmasprayed cermets. The 39 vol% SiC began a melt endotherm at 577°C, which peaked in the 640–650°C range depending on the sample thermal history. Chemical and X-ray diffraction analysis indicated the presence of free silicon in the cermet and in the SiC powder, which resulted in a eutectic Al/Si alloy.Paper presented at the Ninth Symposium on Thermophysical Properties, June 24–27, 1985, Boulder, Colorado, U.S.A.     

Ultralow thermal conductivity of isotope-doped silicon nanowires

The thermal conductivity of silicon nanowires (SiNWs) is investigated by molecular dynamics (MD) simulation. It is found that the thermal conductivity of SiNWs can be reduced exponentially by isotopic defects at room temperature. The thermal conductivity reaches the minimum, which is about 27% of that of pure 28Si NW, when doped with 50% isotope atoms. The thermal conductivity of isotopic-superlattice structured SiNWs depends clearly on the period of superlattice. At a critical period of 1.09 nm, the thermal conductivity is only 25% of the value of pure Si NW. An anomalous enhancement of thermal conductivity is observed when the superlattice period is smaller than this critical length. The ultralow thermal conductivity of superlattice structured SiNWs is explained with phonon spectrum theory.     

Electrical conductivity of random silver-potassium chloride composites

The complex alternating current (a.c.) impedance of random silver-potassium chloride (Ag-KCl) composite specimens near the percolation threshold has been measured in the frequency range 5 Hz to 13 MHz. The impedance of these composites as a function of metal volume fraction and direct current (d.c.) potential field are presented. This a.c. response, which is correlated with structural and compositional characterization of these composites, is used to examine the applicability of several different theoretical models as well as to determine the important factors in the electrical conductivity of such materials.     

Molecular dynamics calculations of InSb nanowires thermal conductivity

Non-equilibrium molecular dynamics calculations were performed in order to obtain the thermal conductivity coefficients of InSb nanowires in comparison with the bulk system value. For bulk, the value obtained was 16 ± 2 Wm⁻¹ K⁻¹ which is in very good agreement with experimental value of 15 Wm⁻¹ K⁻¹, and the thermal conductivity of the nanowires were 0.54 ± 0.01 Wm⁻¹ K⁻¹ and 0.50 ± 0.01 Wm⁻¹ K⁻¹ for a 5 and 4 unit cells square cross-sectional nanowires, respectively, which is almost two orders of magnitude smaller. This result is in agreement with other simulations performed for other materials.     

Effective thermal conductivity and electrical conductivity of anisotropic solids of low porosity

Equations are derived to determine the effective thermal and electrical conductivities of anisotropic media of low porosity. The influence of porosity on the thermal and electrical conductivities of anisotropic ternary alloys is established.Translated from Inzhenerno-Fizicheskii Zhurnal, Vol. 30, No. 4, pp. 686–692, April, 1976.     

Fabrication and effective thermal conductivity of multi-walled carbon nanotubes reinforced Cu matrix composites for heat sink applications

A novel particles-compositing method was used for the first time to disperse different contents of multi-walled carbon nanotubes (CNTs) in micron sized copper powders, which were subsequently consolidated into CNT/Cu composites by spark plasma sintering (SPS). Microstructural observations showed that the homogeneous distribution of CNTs and dense composites could be obtained for 0–10 vol.% CNT contents. The CNT clusters were appeared in the powder mixture with 15 vol.% CNTs, which resulted in an insufficient densification of the composites. The effective thermal conductivity of the composites was analyzed both theoretically and experimentally. The addition of CNTs showed no enhancement in overall thermal conductivity of the composites due to the interface thermal resistance associated with the low phase contrast of CNT to copper and the random tube orientation. Besides, the composite containing 15 vol.% CNTs led to a rather low thermal conductivity due possiblely to the combined effect of unfavorable factors induced by the presence of CNT clusters, i.e. large porosity, lower effective conductivity of CNT clusters themselves and reduction of SPS cleaning effect. The CNT/Cu composites may be a promising thermal management material for heat sink applications.     

Specific heat of superconductor-normal metal composites

The specific heat of Th-Nb composites has been measured to determine the extent to which a dense array of Nb rods will enhance the superconducting energy gap of the surrounding Th matrix at temperatures well belowT _c of the Th. A directional solidification technique has been used to prepare an acicular composite having a triangular array of 500-nm-diameter Nb rods in a Th matrix with a center-to-center spacing of 1500 nm. At low temperatures, where both metals are superconducting, the specific heat indicates that the Th matrix has an energy gap about 10% larger than bulk Th, so there is some enhancement by the Nb rods. BetweenT _c of Nb andT _c of Th there is clear evidence of an induced energy gap in the Th, but as yet there is no theory for the specific heat in this regime. A field as small as 20 mT will quench all evidence of proximity-induced superconductivity in the Th, indicating a strong field dependence of the Cooper pair decay lengthK _N ^–1 .This work has been authored by a contractor of the U.S. Government under contract No. W-7405-eng-82. Accordingly, the U.S. Government retains a nonexclusive, royalty-free license to publish or reproduce the published form of this contribution, or allow others to do so for U.S. Government purposes.Operated for the U.S. Department of Energy by Iowa State University under contract No. W-7405-Eng-82. This research was supported by the Director for Energy Research, Office of Basic Energy Sciences, WPAS-KC-02-02-02.     

Estimation of the thermal conductivity of composites

In this article we introduce the concept of homogenization for the approximation of the effective thermal conductivity of composites. A simple algebraic approximation method is proposed and shown to yield an upper bound for the effective conductivity. Numerical results are given for uni-directional carbon-carbon composites which demonstrate the validity of the approach.     

The thermal conductivity of carbon fibre-reinforced composites

Measurements of the thermal conductivity between approximately 80 and 270 K of a series of unidirectional and bidirectional specimens of epoxy resin DX210/BF₃400 reinforced with Morganite high modulus (HMS) and high strength (HTS) carbon fibres are reported for in-plane and out-of-plane directions. The main features of the results conform with expectations based upon known structural properties of the fibres and predictions based upon current theoretical models. Employing the results for the composites in association with results for the pure resin, the account concludes with an assessment of some of the heat transmission characteristics of the fibres.     

The thermal conductivity of Kevlar fibre-reinforced composites

The thermal conductivities of a series of blocks consisting of Shell DX210/BF₃400 resin reinforced with Kevlar 49 fibre are reported in the approximate temperature range 180–270 K. The results are used to calculate the thermal conductivities of the fibres in directions parallel and perpendicular to their length. Varying the angle between the principal fibre directions of bidirectional laminates produced in-plane results that varied in a manner which was quantitatively consistent with expectation. The out-of-plane results proved to be independent of fibre orientation, as expected. In-plane and out-of-plane results for a Kevlar 49 fabric reinforced laminate proved to be essentially similar to results for a laminate reinforced with unwoven fibres of the same type, arranged in a 90° cross-plied disposition at the same fibre volume density.     

Low-temperature thermal conductivity of two fibre-epoxy composites

Thermal conductivity data are presented for two fibre-epoxy composites. The fibre in one composite is glass and an aramid-class polymer in the other. Measurements were conducted on a glass-epoxy specimen in the direction perpendicular to the fibres at temperatures from 14 to 100 K. The aramid-epoxy specimen was measured parallel to the fibres at 5, 76, 196, and 276 K. Data are presented both in graphical and tabular form.     

陶瓷热障涂层的热导率和热扩散率测量

提出了应用3ω法进行等离子喷涂热障涂层材料的热导率和热扩散率测量的方法。测试了室温下2种典型的热障涂层材料Y2SiO5和La2Zr2O7的热导率和热扩散率,测试结果与文献中的结果吻合良好。实验中对不同孔隙率的样品的热导率在室温附近的温度区间内进行测试,结果表明,孔隙率的变化对热导率有明显的影响。另外,孔隙率对热扩散率有双向的影响,即存在某一孔隙率值使得涂层样品的热扩散率最大。     

Thermoelastic properties and conductivity of composites reinforced by spherically anisotropic particles

The present paper is concerned with the overall thermoelastic properties and conductivity of composites reinforced by spherically anisotropic particles. Based on the concept of the replacement particle an equivalent bulk modulus, thermal expansion coefficient and thermal conductivity are derived for the spherically anisotropic particle. Such equivalent properties can be employed in the micromechanical models to predict the overall behavior of the composite. In addition to these, the shear loading is considered. The effective shear modulus is evaluated on the basis of Mori and Tanaka's approximations and the dilute phase concentration factors are derived from exact solutions of an auxiliary boundary value problem.     

Impacts of atomistic coating on thermal conductivity of germanium nanowires

By using nonequilibrium molecular dynamics simulations, we demonstrated that thermal conductivity of germanium nanowires can be reduced more than 25% at room temperature by atomistic coating. There is a critical coating thickness beyond which thermal conductivity of the coated nanowire is larger than that of the host nanowire. The diameter-dependent critical coating thickness and minimum thermal conductivity are explored. Moreover, we found that interface roughness can induce further reduction of thermal conductivity in coated nanowires. From the vibrational eigenmode analysis, it is found that coating induces localization for low-frequency phonons, while interface roughness localizes the high-frequency phonons. Our results provide an available approach to tune thermal conductivity of nanowires by atomic layer coating.     

Heat capacity and thermal conductivity measurements down to 25 mk on samples of poor thermal diffusivity and small specific heat

A heat pulse technique is described with a permanent heat link to the cold sink used for the measurement of small heat capacities, such as vitreous dielectrics, down to 25 mK. Total heat capacities of the order of 5μJ K^?1 are measured below 50 mK with an accuracy of 5%. Two carbon resistor thermometers are used to avoid any systematic error.Thermal conductivity measurements are also performed in the same temperature range by the steady flow method. The accuracy of the carbon resistor thermometer calibration enables the measurement of 2 or 3 mK temperature gradients at 25 mK.     

Establishment of accuracy limits and standards for comparative thermal conductivity measurements

New techniques have been developed for reducing thermal conductivity data from thermal comparative measurements. The first of these techniques is based on making a Taylor-series expansion of the stack centerline temperature profile. The result is an expression giving the ratio of sample to reference conductivities at any temperature as a function of measured quantities, the stack thermocouple readings and stack element thicknesses. The conventional formula presently used to reduce comparative conductivity data is shown to be a special result of the general analysis. A second technique involves the use of linear least-squares (LS) techniques to derive both the sample and the reference conductivities from the measured data. The LS technique provides the coefficients for a polynomial temperature expansion of the reference and sample conductivities directly. Use of the new techniques is illustrated in a reduction of some comparative data on the conductivities of Pyrex 7740 and Pyroceram 9606. It is shown that a highly self-consistent pair of conductivity functions can be derived for these two commonly used reference materials if the conductivity vs temperature relation for Pyrex is modified slightly from its recommended value. The Pyroceram conductivity results from the comparative measurements are in good agreement with a conductivity derived from pulse diffusivity and differential scanning calorimetry measurements and also in good agreement with the recommended Pyroceram conductivity function.Paper presented at the Ninth Symposium on Thermophysical Properties, June 24–27, 1985, Boulder, Colorado, U.S.A.