Thermal diffusion and heat conductivity of BiFeO3 and Bi0.95La0.05FeO3 multiferroics at high temperatures

The thermal diffusion and heat conductivity of BiFeO₃ and Bi_0.95La_0.05FeO₃ multiferroics at high temperatures (300–1200 K) have been examined. The dominant mechanisms of phonon heat transfer in the region of ferroelectric and antiferromagnetic phase transitions have been revealed. The temperature dependence of the mean free path of phonons has been determined.     

Thermal transport in heavily deformed and γ -irradiated LiF at temperatures near 1 k

The thermal conductivity κ of heavily deformed LiF crystals has been measured at temperatures T ? 0.5 K following exposure of the samples to γ irradiation. The results are in agreement with recent measurements of ballistic phonon propagation in similar samples at an equivalent temperature of ≈ 4 K. A fraction of the phonons have a mean free path of order 1 cm in the heavily deformed crystal, and γ-irradiation increases the fraction having a long mean free path. The measurements support a dynamic (as opposed to static) model of phonon-dislocation interaction.     

Specific heat and velocity of sound in a moderate heavy-fermion compound YbZnCu4

The specific heat at a constant pressure (C _p) and the velocity of sound (v) are measured for a moderate heavy-fermion compound YbZnCu₄ in the temperature range 3.5–250 K and at 77 K, respectively. The experimental values of C _p and v obtained in this study and the phonon thermal conductivity previously measured in the temperature range 5–300 K are used to calculate the phonon mean free path l for this compound. The temperature dependence of the phonon mean free path l thus determined is characteristic of classical amorphous materials.     

Effect of C60 fullerene additions on the thermal conductivity of low-density polyethylene films

The thermal conductivity of LDPE + C₆₀ nanocomposites with a fullerene concentration up to 10 wt % is studied in the temperature range 20–80°C. This conductivity is found to nonlinearly decrease with increasing fullerene concentration. The decrease in the thermal conductivity of the composites is considered to be caused by a decrease in the phonon mean free path as a result of an increase in the number of scattering centers. The temperature dependence of the thermal conductivity is found to have a maximum.     

Thermal conductivity of UC1 − xNx from 100 to 1000 °K

The thermal diffusivities of UC_{1 ? x}N_x of several compositions were measured from 100 to 1000 °K by a laser flash method. The thermal conductivity was separated into electronic and phonon components by assuming the constant Lorenz number. The phonon conductivity showed an anomalous behaviour against composition at low temperatures. The total thermal conductivity of UC_{1 ? x}N_x showed a minimum above 300 °K at an intermediate composition which moved to higher carbon content with increasing temperature. This behaviour was explained by the temperature dependence of the lattice and electronic components.     

Thermal conductivity of polycrystalline CVD diamond: Experiment and theory

The temperature dependences of thermal conductivity κ of polycrystalline CVD diamond are measured in the temperature range from 5 to 410 K. The diamond sample is annealed at temperatures sequentially increasing from 1550 to 1690°C to modify the properties of the intercrystallite contacts in it. As a result of annealing, the thermal conductivity decreases strongly at temperatures below 45 K, and its temperature dependence changes from approximately quadratic to cubic. At T > 45 K, the thermal conductivity remains almost unchanged upon annealing at temperatures up to 1650°C and decreases substantially at higher annealing temperatures. The experimental data are analyzed in terms of the Callaway theory of thermal conductivity [9], which takes into account the specific role of normal phonon-phonon scattering processes. The thermal conductivity is calculated with allowance for three-phonon scattering processes, the diffuse scattering by sample boundaries, the scattering by point and extended defects, the specular scattering by crystallite boundaries, and the scattering by intercrystallite contacts. A model that reproduces the main specific features of the thermal conductivity of CVD diamond is proposed. The phonon scattering by intercrystallite contacts plays a key role in this model.     

Bestimmung von Transportkoeffizienten des Stickstoffs bis 13000 °K

In a cascade arc chamber a stationary nitrogen plasma was produced at a pressure of 1 atm. In this plasma the integral material function, the E-I-characteristic, could be measured very exactly. From this characteristic the transport coefficients of nitrogen — the electrical conductivity σ and the heat flux potentialS — were evaluated in dependence on the arc radiusr with the arc currentI as parameter. With these and by the help of the temperature distributions measured by Schade the transport coefficients dependent on temperatureT only were obtained. The thermal conductivity κ was found by differentiation of theS(T)-curve. The evaluation was only performed up to 13,000 °K because the radiation has been neglected in the calculation what is no longer allowed above 13,000 °K. At low currents the influence of the high field strength causes non-LTE increasing with rising field strength. This effect of non-LTE could be regarded qualitatively and quantitatively. As result the thermal and electrical conductivity ofN ₂ from 5,000 °K to 13,000 °K were obtained. The consistence with former measurements and with theoretical calculations is very satisfying.     

Thermal conductivity of a moderate heavy-fermion compound YbZnCu4

The thermal conductivity κ and electrical resistivity ρ of a cast polycrystalline sample of YbZnCu₄, which belongs to the class of moderately heavy-fermion compounds, are measured and studied in the temperature range 5–300 K. It is shown that the phonon thermal conductivity of the sample follows an amorphous-like pattern throughout the temperature range under investigation, which should be assigned to the presence of Yb ions with a homogeneous mixed valence in this compound. The temperature dependence ρ(T) has two specific portions: a high-temperature portion (T > 220 K) characteristic of conventional metals and a moderate-temperature portion (14–35 K) typical of Kondo compounds.     

Heat capacity of a white-eucalyptus biocarbon template for SiC/Si ecoceramics

The heat capacity C _p of a biocarbon template based on white eucalyptus wood is measured at a constant pressure in the temperature range T = 3.5–300 K. The phonon mean free path l for a white-eucalyptus biocarbon template is calculated from the measured dependence C _p (T) and data available in the literature on the phonon thermal conductivity and velocity of sound. It is established that, in the range 100–300 K, the phonon mean free path l is nearly constant and equal to ～13 Å. This value is close to the smallest size of graphite-like crystallites (～12 Å), which was derived earlier from x-ray diffraction data for a quasi-amorphous biocarbon template.     

Thermal conductivity of a moderate heavy-fermion compound YbIn0.2Ag0.8Cu4

The thermal conductivity κ and electrical resistivity ρ of a cast polycrystalline sample of YbIn_0.2Ag_0.8Cu₄, which belongs to the class of moderate heavy-fermion compounds, are measured in the temperature range 5–300 K. It is shown that the phonon thermal conductivity of the sample follows an amorphous-like pattern throughout the temperature range covered, which should be assigned to the presence of Yb ions with a homogeneous mixed valence in this compound. The temperature dependence ρ(T) is divided into three portions: a high-temperature portion characteristic of conventional metals, a medium-temperature portion typical of Kondo compounds, and a low-temperature portion corresponding to a coherent Kondo lattice (the heavy-fermion regime). The Kondo temperature is estimated.     

Thermal conductivity and electrical resistivity of bulk indium and indium embedded in 7-nm channels of porous borosilicate glass

A “porous glass + indium” nanocomposite has been prepared. The thermal conductivity κ(T) and electrical resistivity ρ(T) of the nanocomposite have been measured in the temperature range 5–300 K, and their fractions accounted for by nanoindium embedded in 7-nm channels of the porous glass have been determined. For comparison, κ and ρ of the bulk polycrystalline indium sample have been measured in the same temperature range. The electronic and phonon components of the thermal conductivity have been calculated for the nanoindium and bulk indium. It has been demonstrated that, as the result of the emergence of boundary electron and phonon scattering in the nanoindium, the electrical resistivity of this material becomes larger, and the phonon thermal conductivity, smaller than those of the bulk indium.     

Unusual thermal conductivity of the negative thermal expansion material, ZrW2O8

In order to investigate the relationship between negative thermal expansion and other thermal properties, the thermal conductivity of the α-phase of ZrW₂O₈ has been determined from 1.9 to 390 K. In addition, the heat capacity was measured from 1.9 to 300 K. The thermal conductivity of ZrW₂O₈ is low, glass-like and close to its theoretical minimum value. The phonon-phonon coupling of the highly anharmonic low-frequency modes which are responsible for negative thermal expansion in ZrW₂O₈ appears to be highly efficient, leading to short phonon mean free paths and exceptionally low thermal conductivity.     

Transport properties of silicon

Electrical conductivity, thermal conductivity, and thermoelectric power of single-crystalline silicon are investigated at temperatures between 2 and 300 K. From the measured data we calculate the mean free path of electrons and phonons and separate diffusion part and phonon-drag part of the thermoelectric power. Using a new method, we evaluate the mean free path of those phonons which are responsible for the phonon drag effect.Dedicated to H.-J. Queisser on the occasion of his 60th birthday     

Thermal conductivity of crystalline fullerite C60 in the simple cubic phase

The behavior of the thermal conductivity k(T) of bulk faceted fullerite C₆₀ crystals is investigated at temperatures T=8–220 K. The samples are prepared by the gas-transport method from pure C₆₀, containing less than 0.01% impurities. It is found that as the temperature decreases, the thermal conductivity of the crystal increases, reaches a maximum at T=15–20 K, and drops by a factor of ∼2, proportional to the change in the specific heat, on cooling to 8 K. The effective phonon mean free path λ _p, estimated from the thermal conductivity and known from the published values of the specific heat of fullerite, is comparable to the lattice constant of the crystal λ _p∼d=1.4 nm at temperatures T>200 K and reaches values λ_p∼50d at T<15 K, i.e., the maximum phonon ranges are limited by scattering on defects in the volume of the sample in the simple cubic phase. In the range T=25−75 K the observed temperature dependence k(T) can be described by the expression k(T)∼exp(Θ/bT), characteristic for the behavior of the thermal conductivity of perfect nonconducting crystals at temperatures below the Debye temperature Θ (Θ=80 K in fullerite), where umklapp phonon-phonon scattering processes predominate in the volume of the sample. Pis’ma Zh. éksp. Teor. Fiz. 65, No. 8, 651–656 (25 April 1997)     

Magnetic field dependence of the thermal conductivity in EuxSr1-xS

The thermal conductivity κ of two Eu_xSr_1-x single crystals (x = 0.25 and 0.54) was measured between 1.5 and 25 K. In magnetic fields of ≈ 7 T κ is enhanced for temperatures up to 20 K with respect to κ(B = 0). At 1.5 K, where the relative increase κ(B)/κ(0) is largest, this ratio is 1.5 for x = 0.54 and ≈ 4 for x = 0.25. Two possible mechanisms for this effect, i.e. freezing of phonon scattering by magnetic excitations in a magnetic field, and heat transport by field-induced magnons, are discussed.     

The thermal conductivity of silver iodide

We have measured the thermal conductivity of pressed pellets of 99.999% AgI from 120 K to 500 K using a transient hot wire method. The temperature dependence changes from T^?1.3±0.1 at the lowest temperatures to T^?1.8±0.1 below the phase transition at 420 K. Above this phase transition where AgI is a superionic conductor we see a weak temperature dependence T^+0.5±0.1. These results indicate shortening of the phonon mean free path at high temperatures due to the mobile Ag⁺ ions.     

Specific features of low-temperature phonon scattering in materials with tilted disclination loops

Phonon scattering by static stress fields of circular wedge disclination loops is investigated in the framework of the deformation potential approach. Numerical calculations of the mean free path l and thermal conductivity κ demonstrate that the temperature dependence of κ exhibits a minimum at a certain temperature T* in the low-temperature range. The thermal conductivity κ sharply increases as T ^?3 with a decrease in temperature (T<T*) and exhibits a dislocation behavior (κ ～ T ²) with an increase in temperature (T>T*). The results obtained for the wedge disclination loop are compared with the available data for uniaxial disclination dipoles. It is shown that the properties of uniaxial disclination dipoles serving as sources of phonon scattering are similar to those of wedge disclination loops.     

Phonon propagation through photonic crystals—media with spatially modulated acoustic properties

The thermal conductivity κ of photonic crystals differing in degree of optical homogeneity (single crystals of synthetic opals) was measured in the 4.2–300 K temperature range. The thermal conductivity revealed, in addition to the conventional decrease in comparison with solid amorphous SiO₂ characteristic of porous solids, a noticeable decrease for T<20 K, the range wherein the phonon wavelength in amorphous SiO₂ approaches the diameters of the contact areas between the opal spheres. This effect is enhanced in the case of phonon propagation along the SiO₂ sphere chains (six directions in the cubic opal lattice). The propagation of light waves (photons) through a medium with spatially modulated optical properties (photonic crystals) is presently well studied. The propagation of acoustic waves through a medium with spatially modulated acoustic properties (phononic crystals) may also reveal specific effects, which are discussed in this paper; among them are, e.g., the ballistic mode of phonon propagation and waveguide effects.     

Unusually high thermal conductivity of carbon nanotubes

Combining equilibrium and nonequilibrium molecular dynamics simulations with accurate carbon potentials, we determine the thermal conductivity lambda of carbon nanotubes and its dependence on temperature. Our results suggest an unusually high value, lambda approximately 6600 W/m K, for an isolated (10,10) nanotube at room temperature, comparable to the thermal conductivity of a hypothetical isolated graphene monolayer or diamond. Our results suggest that these high values of lambda are associated with the large phonon mean free paths in these systems; substantially lower values are predicted and observed for the basal plane of bulk graphite.     

Lattice thermal conductivity of skutterudite compounds

A generalized expression is used on the basis of relaxation time approximation to facilitate calculation of lattice thermal conductivity of dielectric materials as well as skutterudite family consists of compounds of the form AB₃. It is assumed that phonon scattering processes are independent and is represented by frequency dependent relaxation times. The contributions of normal three phonon scattering processes are included explicitly as redistribution of phonon momentum between two oscillation branches is considered. Magnitudes of relaxation times are estimated from the experimental data. The result for CoSb₃ is in reasonably good agreement with the experimental result in the temperature range 1–1000°K. It is observed that redistribution of phonon momentum between two oscillation branches leads to a significant suppression of thermal conductivity maximum and it is observed that for unfilled skutterudite the main dominant mechanism at the thermal conductivity maximum is three phonon normal scattering process.