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.     

Low-temperature deformation of nanocrystalline niobium

The strain characteristics of nanocrystalline niobium are measured in the temperature range 4.2–300 K. It is shown that the development of a strong local deformation with clearly delineated macroscopic slip bands occurs at 4.2 K and 10 K. The thermal effects at a stress jump observed upon transition of the sample (or a niobium strip placed close to the sample) from the superconducting state to the normal state are estimated. It is demonstrated that the temperature dependence of the yield point σ_s(T) can be divided into three portions: two portions (T<10 K and T>70 K) with a slight change in σ_s and the third portion with a strong dependence σ_s(T). The strain characteristics of polycrystals with nano-and larger-sized grains are compared with those of single crystals.     

Effect of pressure on the temperature dependence of the thermal conductivity of InSb

The dependence of the thermal conductivity of indium antimonide on temperature (in the range 300–450 K) and hydrostatic pressure (up to 0.4 GPa) has been investigated. It is shown that the phonon thermal conductivity λ_ph obeys the law T ^?n (n ≥ 1). Hydrostatic pressure affects the magnitude and temperature dependence of the thermal conductivity of InSb: with an increase in pressure, the thermal conductivity increases, while the parameter n in the dependence λ_ph ≈ T ^?n decreases.     

Lattice thermal conductivity of compounds with inhomogeneous intermediate rare-earth ion valence

The thermal conductivity κ (within the range 4–300 K) and electrical conductivity σ (from 80 to 300 K) of polycrystalline Sm₃S₄ with the lattice parameter a=8.505 Å (with a slight off-stoichiometry toward Sm₂S₃) are measured. For T>95 K, charge transfer is shown to occur, as in stoichiometric Sm₃S₄ samples, by the hopping mechanism (σ ～ exp(?ΔE/kT) with ΔE ～ 0.13 eV). At low temperatures [up to the maximum in the lattice thermal conductivity κ_ph(T)], κ_ph ～ T ^2.6; in the range 20–50 K, κ_ph ～ T ^?1.2; and for T>95 K, where the hopping charge-transfer mechanism sets in, κ_ph ～ T ^?0.3 and a noticeable residual thermal resistivity is observed. It is concluded that in compounds with inhomogeneous intermediate rare-earthion valence, to which Sm₃S₄ belongs, electron hopping from Sm²⁺ (ion with a larger radius) to Sm³⁺ (ion with a smaller radius) and back generates local stresses in the crystal lattice which bring about a change in the thermal conductivity scaling of κ_ph from T ^?1.2 to T ^?0.3 and the formation of an appreciable residual thermal resistivity.     

Thermal conductivity of partially graphitized biocarbon obtained by carbonization of medium-density fiberboard in the presence of a Ni-based catalyst

The thermal conductivity k and resistivity ρ of biocarbon matrices, prepared by carbonizing medium-density fiberboard at T _carb = 850 and 1500°C in the presence of a Ni-based catalyst (samples MDF-C( Ni)) and without a catalyst (samples MDF-C), have been measured for the first time in the temperature range of 5–300 K. X-ray diffraction analysis has revealed that the bulk graphite phase arises only at T _carb = 1500°C. It has been shown that the temperature dependences of the thermal conductivity of samples MDFC- 850 and MDF-C-850(Ni) in the range of 80–300 K are to each other and follow the law of k(T) ～ T ^1.65, but the use of the Ni-catalyst leads to an increase in the thermal conductivity by a factor of approximately 1.5, due to the formation of a greater fraction of the nanocrystalline phase in the presence of the Ni-catalyst at T _carb = 850°C. In biocarbon MDF-C-1500 prepared without a catalyst, the dependence is k(T) ～ T ^1.65, and it is controlled by the nanocrystalline phase. In MDF-C-1500(Ni), the bulk graphite phase formed increases the thermal conductivity by a factor of 1.5–2 compared to the thermal conductivity of MDF-C-1500 in the entire temperature range of 5–300 K; k(T = 300 K) reaches the values of ～10 W m^–1 K^–1, characteristic of biocarbon obtained without a catalyst only at high temperatures of T _carb = 2400°C. It has been shown that MDF-C-1500(Ni) in the temperature range of 40?300 K is characterized by the dependence, k(T) ～ T ^1.3, which can be described in terms of the model of partially graphitized biocarbon as a composite of an amorphous matrix with spherical inclusions of the graphite phase.     

Characteristics of the thermodynamic and kinetic properties of NbH0.83 in the phase transition region

Measurements were made of the specific heat (in the temperature range 80–400 K), electrical resistivity (4.2–300 K), thermo-emf (4.2–300 K), thermal conductivity (7–300 K), magnetic susceptibility (4.2–400 K), and lattice parameters (30–300 K) of the alloy NbH_0.83. The λ′→λβ phase transitions were studied. It was established that the structure of the niobium matrix of the hydride remains unchanged as a result of these transitions. It is shown that the β→λ phase transition (one-dimensional hydrogen ordering) is a three-stage one and is accompanied by a substantial change in the shear modulus of the hydride. The λ′ phase existing at T<110 K was determined. The electron thermal conductivity κ_el calculated for the λ′ phase in the range T<23 K is 25% higher than the measured thermal conductivity. In order to explain this fact and also the discontinuity in the concentration dependence of the coefficient of thermal expansion of NbHx for x?0.83–0.84 and the approximately 1.5% compression of the NbH_0.84 volume it is assumed that in the region of x _c in the λ′ phase the topology of the Fermi surface of the NbH_x interstitial alloy changes substantially.     

Thermal conductivity of high-porosity cellular-pore biocarbon prepared from sapele wood

This paper reports on measurements (in the temperature range T = 5–300 K) of the thermal conductivity κ(T) and electrical conductivity σ(T) of the high-porosity (～63 vol %) amorphous biocarbon preform with cellular pores, prepared by pyrolysis of sapele wood at the carbonization temperature 1000°C. The preform at 300 K was characterized using X-ray diffraction analysis. Nanocrystallites 11–30 Å in ize were shown to participate in the formation of the carbon network of sapele wood preforms. The dependences κ(T) and σ(T) were measured for the samples cut across and along empty cellular pore channels, which are aligned with the tree growth direction. Thermal conductivity measurements performed on the biocarbon sapele wood preform revealed a temperature dependence of the phonon thermal conductivity that is not typical of amorphous (and X-ray amorphous) materials. The electrical conductivity σ was found to increase with the temperature increasing from 5 to 300 K. The results obtained were analyzed.     

Thermal conductivity of the YbMgCu<Subscript>4</Subscript> “light” heavy-fermion system

The thermal conductivity and electrical resistivity of a sample of YbMgCu₄ belonging to “light” heavy-fermion compounds have been measured in the temperature range 5–300 K. The sample studied was in the region of homogeneity of this compound. It is shown that, throughout the temperature range studied, the phonon thermal conductivity of the sample has an amorphous-like character, which should be assigned to the homogeneous mixed valence of the Yb ion in YbMgCu₄.     

Thermal conductivity and electrical resistivity of the high-Tc superconductor YBa2Cu3O9−Δ

Thermal conductivity and electrical resistivity of single-phase YBa₂Cu₂O_9−Δ compound were measured in the superconducting (T_c=89.5 K) and normal states, in the temperature range 5–320 K. The electronic component of the total thermal conductivity was estimated to be 20%. The electrical resistivity changed linearly in the normal state up to highest measured temperature.     

Low-temperature thermal conductivity of an opal + epoxy-resin nanocomposite

The thermal conductivity of an opal + epoxy-resin nanocomposite under 100% filling of first-order opal voids by epoxy resin was measured in the range 5–100 K. For T < T₀ (T₀ is the temperature at which the thermal conductivity of epoxy resin becomes equal to that of amorphous SiO₂ opal spheres, with inclusion of their porosity associated with second-and third-order voids), the thermal conductivity of the opal + epoxy-resin nanocomposite undergoes a sharp decrease, which is qualitatively accounted for by the appearance of Kapitsa heat resistance at the contacts between the amorphous opal spheres and epoxy resin.     

Thermal conductivity of the amorphous and nanocrystalline phases of the beech wood biocarbon nanocomposite

Natural composites (biocarbons) obtained by carbonization of beech wood at different carbonization temperatures T _carb in the range of 800–2400°C have been studied using X-ray diffraction. The composites consist of an amorphous matrix and nanocrystallites of graphite and graphene. The volume fractions of the amorphous and nanocrystalline phases as functions of T _carb have been determined. Temperature dependences of the phonon thermal conductivity κ(T) of the biocarbons with different temperatures T _carb (1000 and 2400°C) have been analyzed in the range of 5–300 K. It has been shown that the behavior of κ(T) of the biocarbon with T _carb = 1000°C is controlled by the amorphous phase in the range of 5–50 K and by the nanocrystalline phase in the range of 100–300 K. The character of κ(T) of the biocarbon with T _carb = 2400°C is determined by the heat transfer (scattering) in the nanocrystalline phase over the entire temperature range of 5–300 K.     

Thermal conductivity of high-porosity biocarbon precursors of white pine wood

This paper reports on measurements of the thermal conductivity κ and the electrical conductivity σ of high-porosity (cellular pores) biocarbon precursors of white pine tree wood in the temperature range 5–300 K, which were prepared by pyrolysis of the wood at carbonization temperatures (T _carb) of 1000 and 2400°C. The x-ray structural analysis has permitted the determination of the sizes of the nanocrystallites contained in the carbon framework of the biocarbon precursors. The sizes of the nanocrystallites revealed in the samples prepared at T _carb = 1000 and 2400°C are within the ranges 12–35 and 25–70 Å, respectively. The dependences κ(T) and σ(T) are obtained for samples cut along the tree growth direction. As follows from σ(T) measurements, the biocarbon precursors studied are semiconducting. The values of κ and σ increase with increasing carbonization temperature of the samples. Thermal conductivity measurements have revealed that samples of both types exhibit a temperature dependence of the phonon thermal conductivity κ_ph, which is not typical of amorphous (and amorphous to x-rays) materials. As the temperature increases, κ_ph first varies proportional to T, to scale subsequently as ～T ^1.7. The results obtained are analyzed.     

Thermal conductivity at the amorphous-nanocrystalline phase transition in beech wood biocarbon

High-porosity samples of beech wood biocarbon (BE-C) were prepared by pyrolysis at carbonization temperatures T _carb = 650, 1300, and 1600°C, and their resistivity ρ and thermal conductivity κ were studied in the 5–300 and 80–300 K temperature intervals. The experimental results obtained were evaluated by invoking X-ray diffraction data and information on the temperature dependences ρ(T) and κ(T) for BE-C samples prepared at T _carb = 800, 1000, and 2400°C, which were collected by the authors earlier. An analysis of the κ(T _carb) behavior led to the conclusion that the samples under study undergo an amorphous-nanocrystalline phase transition in the interval 800°C < T _carb < 1000°C. Evaluation of the electronic component of the thermal conductivity revealed that the Lorentz number of the sample prepared at T _carb = 2400°C exceeds by far the classical Sommerfeld value, which is characteristic of metals and highly degenerate semiconductors.     

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.     

Poiseuille flow of phonons in solid hydrogen

The thermal conductivity of solid parahydrogen is investigated using the stationary method with a plane sample in the temperature range 1.5–6.0 K in order to reveal a Poiseuille flow in solid hydrogen. It is established that the thermal conductivity at temperatures below the low-temperature maximum decreases very rapidly in accordance with the law K ～ T ⁿ (3 < n < 8). This finding is a direct indication that the possibility exists of observing a Poiseuille flow in solid hydrogen. The results obtained are compared with those for solid helium, in which the Poiseuille flow was observed for the first time in dielectric solids. According to the estimates, the mean free path of phonons at a temperature of approximately 3 K exceeds the radius of a cylindrical sample (3 mm). The thermal conductivity in the vicinity of the low-temperature maximum is found to be two times higher than the value available in the literature.     

Behavior of the Lorenz number in the light heavy-fermion system YbInCu4

The electrical resistivity and thermal conductivity of two polycrystalline YbInCu₄ samples prepared by different techniques at the Ioffe Physicotechnical Institute, RAS (St. Petersburg, Russia), and the Goethe University (Frankfurt-am-Main, Germany) are studied within the temperature range 4.2–300 K. At T _v～75–78 K, these samples exhibited an isostructural phase transition from a state with an integer valence (at T>T _v) to a state with an intermediate valence (at T<T _v) of the Yb ions. It is shown that at T<T _v; i.e., in the temperature range where YbInCu₄ is assumed to be a light heavy-fermion compound, the Lorenz number behaves as it should in a classical heavy-fermion system. At T>T _v, where YbInCu₄ is a semimetal, the Lorenz number has a value characteristic of standard metals.     

Thermal conductivity and specific heat of SrCu2(BO3)2: A quasi-two-dimensional metal oxide compound with a spin gap

The thermal conductivity and specific heat of SrCu₂(BO₃)₂, a quasi-two-dimensional metal oxide compound with a spin gap, were studied at low temperatures. In the temperature interval 0.4<T<3.2 K, the thermal conductivity of a single crystal sample in the ab plane varies according to the power law κ∝T ^2.73. As the temperature increases further, a deep minimum is observed in the region of T _min≈9.8 K. This behavior is explained by the scattering of phonons—the major heat carriers—on the fluctuations of the spin subsystem.     

Study of thermal conductivity and specific heat of amorphous and partially crystalline poly(ethylene terephthalate) in relation to its structure

The thermal conductivity k(T) and the specific heat of amorphous and partially crystalline polyethylene terephthalate) were measured in the intervals 1.2–40K and 1.2–10K, respectively. For a quantitative study of the relation between the thermal conductivity and the structure and degree of crystallinity of the samples their small angle X-ray scattering was measured. For T > 20 K, k(T) increases with increasing degree of crystallinity φ, whereas for T < 10 K, k(T) decreases when φ increases. Amorphous PET shows a temperature dependence of k(T) which is typical for all amorphous materials. These results are compared with curves which were computed from experimental small angle structure functions using a model for phonon scattering in vitreous systems obtained by Klemens. It is shown that for T < 10 K the change in conductivity in the partially crystalline samples relative to that of the purely amorphous sample can quantitatively be explained by additional scattering of phonons from static long-range order fluctuations of the sound velocity which are due to the microscopic structure of the polymer. From a measurement of the optical extinction of the samples relative values of their thermal conductivity at 50 mK are estimated. The specific heat obeys a T³-law between 1.2 K and about 7 K and decreases linearly with φ. The Debye specific heat of the amorphous sample was computed from the sound velocities. It is only 85% of the measured value.     

Low-temperature thermal conductivity of terbium-gallium garnet

Thermal conductivity of paramagnetic Tb₃Ga₅O₁₂ (TbGG) terbium-gallium garnet single crystals is investigated at temperatures from 0.4 to 300 K in magnetic fields up to 3.25 T. A minimum is observed in the temperature dependence κ(T) of thermal conductivity at T _min = 0.52 K. This and other singularities on the κ(T) dependence are associated with scattering of phonons from terbium ions. The thermal conductivity at T = 5.1 K strongly depends on the magnetic field direction relative to the crystallographic axes of the crystal. Experimental data are considered using the Debye theory of thermal conductivity taking into account resonance scattering of phonons from Tb³⁺ ions. Analysis of the temperature and field dependences of the thermal conductivity indicates the existence of a strong spin-phonon interaction in TbGG. The low-temperature behavior of the thermal conductivity (field and angular dependences) is mainly determined by resonance scattering of phonons at the first quasi-doublet of the electron spectrum of Tb³⁺ ion.