Thermal Conductivity Lambda Research Articles

Thermal Conductance of InAs Nanowire Composites

The ability to measure and understand heat flow in nanowire composites is crucial for applications ranging from high-speed electronics to thermoelectrics. Here we demonstrate the measurement of the thermal conductance of nanowire composites consisting of regular arrays of InAs nanowires embedded in PMMA using time-domain thermoreflectance (TDTR). On the basis of a proposed model for heat flow in the composite, we can, as a consistency check, extract the thermal conductivity Lambda of the InAs nanowires and find Lambda(NW) = 5.3 +/- 1.5 W m(-1) K(-1), in good agreement with theory and previous measurements of individual nanowires.

The influence of potential softness on the transport coefficients of simple fluids

This study explores the effects of interaction softness on the transport properties of simple fluids. The transport coefficients of soft-sphere fluids in which the particles interact via the potential, phi(r)=epsilon(rsigma)(-n), with n in the range from 6 to 1152, have been calculated by molecular-dynamics computer simulation. The self-diffusion coefficient D shear viscosity eta(s), bulk viscosity eta(b), and thermal conductivity lambda were computed over a wide packing fraction range. It was found that the Batschinski-Hildebrand expressions, in which D, eta(s) (-1), eta(b) (-1), and lambda(-1) are assumed to have a linear dependence on the molar volume, represent the data quite well for all n, although least well for the thermal conductivity. The density for which, on extrapolation, each of these quantities is zero, increases with the softness of the interaction (or approximately n(-1)), suggesting that the effective hard-sphere diameter decreases with increasing softness in the small n limit. This treatment leads to simple empirical formulas for the effect of density and n on the effective hard-sphere diameter and packing fraction (in an intermediate range) and the four transport coefficients of these fluids.

Thermal conductivity of solid argon at high pressure and high temperature: A molecular dynamics study

The thermal conductivity of solid argon at high-pressure (up to 50 GPa) and high-temperature (up to 2000 K) has been calculated by equilibrium molecular dynamics simulations using the Green-Kubo formalism and an exponential-6 interatomic potential. A simple empirical expression is given for its pressure and temperature dependence. The results are compared with predictions based on kinetic theory. The relative change of the thermal conductivity lambda with density rho is found to be consistent with a partial differential ln lambda/ partial differential ln rho slope of approximately 6 in a wide range of pressures and temperatures, in good agreement with predictions based on kinetic theory.

AXISYMMETRICAL THERMOELASTIC BEHAVIOR FOR A NONHOMOGENEOUS MEDIUM WITH A PENNY-SHAPED CRACK

Axisymmetrical thermoelastic singular stress problems for a nonhomogeneous infinite body and thick plate are considered, theoretically. It is assumed that the nonhomoge neous material properties of shear modulus of elasticity G, the coefficient of linear thermal expansion alpha, and the thermal conductivity lambda vary with the axial coordinate z according to the power product form. As an analytical model, a nonhomogeneous infinite body and thick plate with a penny-shaped crack subject to an arbitrarily distributed axisymmetrical heat supply on its crack surface are considered. The distributions of the displacement component and the stress component are analyzed using the fundamental equation system proposed in our previous paper, and the thermal stress intensity factor at a crack tip is evaluated theoretically. Numerical calculations are carried out for several cases taking into account the variety of nonhomogeneity of G, alpha, and lambda. Thereafter, the distributions of the displacements, stresses, and thermal stress intensity factor at the crack tip are shown graphically, and the influence of these nonhomogeneous material properties on the thermoelastic field and the thermal stress intensity factor is discussed.

DERIVATION OF SYSTEMS OF FUNDAMENTAL EQUATIONS FOR A THREE-DIMENSIONAL THERMOELASTIC FIELD WITH NONHOMOGENEOUS MATERIAL PROPERTIES AND ITS APPLICATION TO A SEMI-INFINITE BODY

A method of analytical development of three-dimensional thermoelastic problems for a medium with nonhomogeneous material properties is developed in this article. Assuming that the shear modulus elasticity G, the thermal conductivity lambda, and the coefficient of linear thermal expansion alpha vary with the power product form of axial coordinate variable z and introducing two kinds of displacement functions and the thermoelastic displacement function, the system of fundamental differential equations for such a three-dimensional field is established. As an illustrative example, we consider the thermoelastic problem of a semi-infinite body. The three-dimensional temperature solution in a steady state is obtained and the associated components of thermal displacement and stress are evaluated theoretically. Numerical calculations are carried out for several cases taking into account the variety of the nonhomogeneous material properties of G, lambda, and alpha, and these results are shown graphically.

Antiferromagnetism in CeAu2Si2- and CeAg2Si2-based Kondo-lattice systems

We report the electrical resistivity rho , thermopower S and thermal conductivity lambda of Ce1-xLaxAu2Si2, Ce1-yYyAu2Si2, Ce1-xLaxAg2Si2 and Ce1-yYyAg2Si2 (0<or=x, y<or=1) systems. The rho curves show a sudden change in slope at the antiferromagnetic ordering temperature TNrho . In the CeAu2Si2-based alloys, the TN depression, due to the dilution of Ce atoms by La or Y, is accounted for solely by the weakening of the inter-site interaction temperature scale TRKKY. On the other hand, in the CeAg2Si2-based alloys, apart from TRKKY, the single-ion Kondo temperature TK also plays an important role in determining the nature of the TN variation. The rho and lambda behaviours, at both low and high temperatures, show clear evidence for the Kondo effect in the presence of crystal fields.

Electrical resistivity, thermopower and thermal conductivity studies of (Sm1-xYx)Cu2 and RCu2 (R identical to Gd, Pr or Tb) systems

The authors report the electrical resistivity rho , thermopower S and thermal conductivity lambda studies of (Sm1-xYx)Cu2 and RCu2 (R identical to Gd, Pr or Tb) systems between 4.2 and 300 K. The value of the antiferromagnetic ordering temperature TN for the ((Sm1-xYx)Cu2 systems is found to become depressed, almost linearly, with Y substitution. The value of TN extrapolates to zero as x to 1. Such a behaviour is attributed solely to the dilution effects of Sm by Y atoms. Chemical pressure effects on the value of TN due to addition of Y atoms are found to be negligible. The lambda curves of SmCu2, GdCu2 and TbCu2 exhibit a peak for T < TN. This low-temperature peak at Tmaxlambda approximately= 10 K, in the lambda data of these compounds arises from the combined influence of the electronic and the magnetic contribution to the total lambda . This low-temperature peak becomes suppressed for x >or= 0.10. The S curves exhibit several extremum features, at T <or= TN, which is ascribed to the variation in the density of states at the Fermi level.

Thermal Conductivity of Monolithic Organic Aerogels

The total thermal conductivity lambda of resorcinol-formaldehyde aerogel monoliths has been measured as a function of density rho in the range from rho = 80 to 300 kilograms per cubic meter. A record-low conductivity value in air at 300 K of lambda approximately 0.012 watt per meter per kelvin was found for rho approximately 157 kilograms per cubic meter. Caloric measurements under variation of gas pressure as well as spectral infrared transmission measurements allowed the determination of solid conductivity, gaseous conductivity, and radiative conductivity as a function of density. The development of such low conductivity materials is of great interest with respect to the substitution of environmentally harmful insulating foams made from chlorofluorocarbons.

A molecular dynamics study of the thermal conductivity of CaF2 and UO2

Previous experimental work has revealed an anomalous increase of thermal conductivity lambda in solid UO2 above about 2000 K, which has been attributed to the effect of electronic excitations. The present molecular dynamics simulations of UO2 and the closely related material CaF2 are designed to test this explanation by providing quantitative results for the temperature dependence of lambda in the absence of electronic excitations. The simulation results for lambda , obtained by the Green-Kubo technique, are in fair agreement with the (limited) experimental results for CaF2. For UO2, close agreement with experiment exists for T<2000 K, but there is a large discrepancy for higher temperatures. This confirms the existence of an additional heat conduction mechanism in UO2 at high temperatures.

Thermal conductivity of high Tc YBaCuO bulk prepared by melt growth technique

A YBa2Cu3O7 sample with Tc of about 80 K and Jc of about 3000 Acm-2 at 50 K was prepared by a melt powdering melt growth technique. Thermal conductivity lambda of the sample was about 10-2 Wcm-1K-1 at 15 K. Addition of about 10 wt.%Ag increased a value of lambda about two times as large. Temperature dependence of lambda suggested the sample includes many defects. The sample is a hopeful material for superconducting current leads.

The electronic Lorenz number in some transition metals at high temperatures

The usual method of plotting the high-temperature electrical resistivity rho and thermal conductivity lambda data on transition metals in the form of lambda T against ST2/ rho plots gives a negative lattice thermal conductivity lambda g, when applied to V, Rh, Pd and Co. It is shown here that lambda T for these metals, plotted against T2, gives positive lambda g and electronic Lorenz numbers L*e, which depend on T in a particular manner, namely in proportion to rho . These values of L*e broadly conform to Kolomoets' theory. When applied to the data on titanium, zirconium and hafnium, the values obtained for L*e tend, at the highest temperatures, to the temperature-independent values of Le, obtained from the lambda T against T2/ rho plots. In the case of polycrystalline and wire-form niobium, the values of L*e conform to Kolomoets' theory while the Le obtained from the lambda T against T2/ rho plots do not.

Thermal conductivity and heat capacity of solid LiBr and RbF under pressure

The thermal conductivity lambda and the heat capacity rho cp per unit volume of solid LiBr and RbF have been measured over the temperature T range 100-400 K and at pressures p up to 2 Gpa, using the transient hot-wire method. The results are compared with recent theoretical calculations. A predicted drop of lambda (T) below T-1 dependence is observed for LiBr.

Thermal conductivity of the Y-Ba-Cu-O tetragonal structure: phase transitions and hysteresis behaviour

The authors present results of precise measurements of the thermal conductivity lambda of a non-superconducting sample of Y-Ba-Cu-O in the temperature range 1-300 K. Heating and cooling curves of lambda (T) exhibited clear hysteresis effects between 80 and 240 K, not previously reported. The temperatures at which the thermal conductivity anomalies appear were the same or close to those at which other anomalies were observed in superconducting samples. The results can be explained qualitatively if one assumes that the 1:2:3 phase is a spinodal decomposition mixture of two phases with different stoichiometry with respect to oxygen content and that a change in lattice dimensions takes place at about 80 K.

The thermal conductivity and heat capacity of single-crystal Si under hydrostatic pressure

The thermal conductivity lambda and heat capacity rho cP per unit volume of single-crystal Si have been measured under hydrostatic pressure up to 1 GPa at room temperature. The thermal conductivity was found to increase slightly with increasing pressure. For the volume dependence of lambda expressed in terms of the Bridgman parameter g=-( delta (ln lambda )/ delta (ln V))T, the authors obtained g=0.4. The specific heat capacity cP, calculated from their measured rho cP, was found to be constant against pressure.

Thermal conductivity and heat capacity of ammonium halides under pressure: NH4Br, NH4I and a comparison with NH4Cl

The thermal conductivity lambda and heat capacity per unit volume rho cp have been measured for solid phases of NH4Br and NH4I using the transient hot-wire method in the temperature T range 100-400 K and at pressures P up to 2 GPa. Comparison of present results with those of previous work for NH4Cl showed similar heat conduction behaviour amongst these ammonium halides. Continuous or near-continuous phase transitions were indicated by an abrupt change in the derivative of lambda with respect to either T or P as well as by a maximum in rho cp. lambda and rho cp changed discontinuously at the first-order I-II phase transition in NH4I. A decrease in NH4 orientational order was concluded to be mainly responsible for the observed increase of thermal resistivity W(= lambda -1) in both ferro- and antiferro-ordered phases during the isobaric temperature increase leading up to an order-disorder transition temperature. Negative (or near-negative) values of ( delta lambda / delta P)T observed for the antiferro-ordered phases were consistent with corresponding negative values of thermal expansivity reported by other workers.

Non-linear heat conduction in dense fluids

In a dense fluid with very large heat flow J. the thermal conductivity lambda = lambda 0+ lambda 2J2. An estimate of lambda 2/ lambda 0 is made for hard spheres with mass and diameter appropriate to Ar and density just below the gas-solid transition. This estimate indicates that Fourier's law will hold in the form appropriate to low J if J<or approximately=109 W m-2 and that the J dependence of pressure and internal energy is of the same order as that of lambda . The calculation utilises an earlier derivation from the classical Liouville equation which expresses lambda at high density in terms of the thermodynamic pressure. The latter can be calculated with the aid of reciprocity and integrability conditions of extended non-equilibrium thermodynamics which treats J as a state variable.

Thermophysical properties of the sodium halides under pressure

A pulsed plane-wave technique was used to measure the thermophysical properties of the sodium halides under pressure. The thermal conductivity lambda was found to increase with pressure in the 0-4 GPa. The experimental results are compared with the values estimated from the Leibfried-Schlomann model.

Thermal conductivity of REAl2compounds (RE=rare earth)

The temperature dependence of the thermal conductivity lambda (T) of the REAl2 compounds (RE=Y, La, Ce, Pr, Nd, Sm, Gd, Tb, Dy, Ho, Er, Tm, Lu) is shown in the range from 4.2K to room temperature. Generally it is assumed that lambda (total thermal conductivity) is given by the sum of two contributions ( lambda = lambda e+ lambda g), where lambda e and lambda g are the electronic and the lattice thermal conductivities respectively. It could be shown that lambda g for the REA12 compounds is negligible at least for low temperatures (T<or=20K) as well as for temperatures above about 200K. In the group of the non-magnetic REAl2 compounds (RE=Y, La, Lu) LaAl2 shows a considerably different lambda (T) behaviour which the authors have related to an enhanced electron-phonon coupling in this compound. The spin-dependent scattering contribution to the electronic thermal resistivity (Wspd) has been obtained by using Matthiessen's rule (We=We,0+We,ph+Wspd). Experimentally they found that Wspd=DT-1 for most of the heavy REAl2 compounds, which is in good agreement with a theoretical calculation of Wspd(T). From both the experiment and the theory they could show that D is proportional to the de Gennes factor. The analysis of the thermal resistivity of CeAl2 (known as a Kondo lattice system) demonstrates that Delta WT varies as (-ln T) in two temperature regions ( Delta W=W(CeAl2)-W(YAl2)). The same behaviour has been observed for rho spd(T) in CeAl2.

Transport in the Frenkel-Kontorova model. III. Thermal conductivity

For pt.II see ibid., vol.18, p.4903 (1985). The authors use molecular dynamics simulation to study the thermal conductivity lambda of the undamped Frenkel-Kontorova model. The aims of the work are to provide evidence for the existence of lambda in this one-dimensional system, to investigate a variety of simulation techniques for calculating it, and to obtain numerical results for lambda as a function of temperature and winding number. They have examined three techniques: (i) direct determination of the energy flux caused by a temperature gradient; (ii) use of the Kubo formula to obtain lambda from the autocorrelation function of the energy flux; (iii) the calculation of lambda from the response of the energy flux to a suitable chosen uniform external perturbation. They show that the results given by the three approaches support the existence of lambda and are in satisfactory agreement with each other. The dependence of lambda on temperature and winding number can be explained by simple physical arguments.

Soliton-phonon scattering in TMMC and DMMC

The field dependence of the thermal conductivity lambda of two well known quasi-one-dimensional antiferromagnetic Heisenberg systems TMMC and DMMC is measured between 1.5 and 7K and in magnetic fields up to 90 kOe. The substantial reduction of lambda as a function of field in the paramagnetic state can excellently be explained by assuming solitons as the dominant magnetic excitations and a resonant soliton-phonon scattering model.