Thermal conductivity of the SiC/Si biomorphic composite, a new cellular ecoceramic

The thermal conductivity κ and electrical resistivity ρ of a SiC/Si biomorphic composite were measured at temperatures T = 5–300 K. The composite is a cellular ecoceramic fabricated by infiltrating molten Si into the channels of a cellular carbon matrix prepared via pyrolysis of wood (white eucalyptus) in an argon ambient. The κ(T) and ρ(T) relations were measured on a sample cut along the direction of tree growth. The experimental results obtained are analyzed.     

Thermal and electrical properties of a white-eucalyptus carbon preform for SiC/Si ecoceramics

The thermal conductivity κ and electrical resistivity ρ of a white-eucalyptus cellular carbon preform used to fabricate silicon-carbide-based (SiC/Si) biomorphic ceramics have been measured in the 5-to 300-K temperature interval. The carbon preform was obtained by pyrolysis (carbonization) of white-eucalyptus wood at 1000°C in an argon ambient. The κ(T) and ρ(T) relations were measured on samples cut along the tree growth direction. The experimental data obtained were processed.     

Thermal conductivity of bio-SiC and the Si embedded in cellular pores of the SiC/Si biomorphic composite

The thermal conductivity of bio-SiC, a heavily defected material with specific cellular pores (channels), was studied in the temperature range 5–300 K. The bio-SiC sample was prepared from the SiC/Si biomorphic composite through the chemical removal of silicon. The thermal conductivity of silicon embedded in cellular pores of the SiC/Si biomorphic composite was determined.     

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 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.     

Linear expansion coefficient of the SiC/Si biomorphic composite

In the temperature range 100–650 K, the linear expansion coefficient β was measured for the SiC/Si biomorphic composite, a new cellular ecoceramic fabricated from a porous cellular carbon matrix prepared through pyrolysis of wood (white eucalyptus) in an argon ambient with subsequent infiltration of molten Si into the channels of the matrix and the formation of 3C-SiC. The SiC/Si samples studied had an “excess” ～30% volume concentration of Si and a porosity of ～13–15%. The measurements were conducted on samples cut along (β_∥) and across (β_⊥) the tree growth direction. The measured values of β(T) of SiC/Si are compared with literature data available for the linear expansion coefficients of Si and 3C-SiC.     

Anisotropy of electric resistivity of Sapele-based biomorphic SiC/Si composites

The electrical resistivity of Sapele-based biomorphic SiC/Si materials was measured in a wide temperature range from 10 K to room temperature. The samples were fabricated by the reactive infiltration of molten silicon into a carbonized Sapele (African Entandrophragma Cylindricum) wood preform. All the samples studied contained residual Si (10–35 wt %). It was found that the resistivity-temperature (ρ(T)) dependences have semimetallic behavior which becomes very close to linear metallic behavior at 100 < T < 300 K. The obtained values of resistivity were quite low (ρ ≈ 0.002–0.02 Ω cm) and showed strong anisotropy: the resistivity along the wood growth axis was several times lower than that in the perpendicular direction. The extent of this anisotropy was in correlation with the amount of residual Si (and, hence, with the amount of residual porosity) in a sample. The resistivity perpendicular to the wood growth axis drastically increased with the Si content, whereas the resistivity parallel to it was practically independent of the Si content. It is suggested that the presence of residual carbon in the samples and carrier scattering at SiC/Si interphases could determine the observed character of ρ(T) dependences.     

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 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.     

Elastic and inelastic properties of SiC/Si biomorphic composites and biomorphic SiC based on oak and eucalyptus

This paper reports on the results of a comparative investigation into the elastic and microplastic properties of biomorphic SiC/Si composites and biomorphic SiC prepared by pyrolysis of oak and eucalyptus with subsequent infiltration of molten silicon into a carbon matrix and additional chemical treatment to remove excess silicon. The acoustic studies were performed by the composite oscillator technique using resonant longitudinal vibrations at frequencies of about 100 kHz. It is shown that, in biomorphic SiC (as in biomorphic SiC/Si) at small-amplitude strains ε, adsorption and desorption of the environmental (air) molecules determine to a considerable extent the Young’s modulus E and the internal friction (decrement of acoustic vibrations δ) and that the changes in E and δ at these amplitudes are irreversible. The stress-microplastic strain curves are constructed from the acoustic data for the materials under study at temperatures of 100 and 290 K.     

Electrical properties of bio-SiC and Si components of the SiC/Si biomorphic composite

The electrical resistivity ρ of bio-SiC, a highly porous cellular material prepared from a biomorphic composite SiC/Si based on white eucalyptus wood through the chemical removal of silicon, was measured in the temperature range 5–100 K. The electrical resistivity of bio-SiC was found to be anisotropic along and across the cellular pores. The activation energy of charge transfer in bio-SiC was estimated. The measured values of ρ for the SiC/Si biomorphic composite and bio-SiC were used to determine the electrical resistivity ρ and the carrier concentration in silicon, which is one of the constituents of the composite.     

Heat capacity of silicon carbide at low temperatures

The heat capacity of biomorphic silicon carbide, a high-porosity material with specific cellular pores, is measured in the temperature range 3.5–60 K. Biomorphic silicon carbide is prepared by the chemical removal of excess silicon from the SiC/Si biomorphic composite, a product of eucalyptus wood. It is shown that the major contribution to the heat capacity of biomorphic SiC comes from surface vibrational modes.     

Electrical and thermoelectric properties of the SiC/Si biomorphic composite at high temperatures

The electrical resistivity ρ and the thermopower coefficient α of a SiC/Si biomorphic composite fabricated from a porous carbon matrix [prepared through pyrolysis of wood (white eucalyptus)] by infiltrating molten Si into the empty channels of the matrix were measured in the temperature ranges 100–950 and 100–750 K, respectively. Silicon reacts chemically with the carbon of the matrix to produce 3C-SiC, which, in combination with the excess Si unreacted with carbon, forms the SiC/Si biomorphic composite. The SiC/Si samples studied had a concentration of “excess” Si of ～30 vol % and a porosity of ～13–15 vol %. Measurements of ρ were carried out on samples cut either along (ρ_∥) or across (ρ_⊥) the tree growth direction, and α was measured on a sample cut along the tree growth direction.     

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.     

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.     

Current-conducting properties of paper consisting of multiwall carbon nanotubes

Electrical conductivity σ(T) of the paper consisting of multiwalled carbon nanotubes (MWCNTs) is studied in the temperature range 4.2-295 K, and its magnetoresistivity ρ(B) at various temperatures in magnetic fields up to 9 T is analyzed. The temperature dependence of the paper electrical conductivity σ(T) exhibits two-dimensional quantum corrections to the conductivity below 10 K. The dependences of negative magnetoresistivity ρ(B) measured at various temperatures are used to estimate the wavefunction phase breakdown length L _φ of conduction electrons and to obtain the temperature dependence L _φ = constT ^?p/2, where p ≈ 1/3. Similar dependences of electrical conductivity σ(T), magnetoresistivity ρ(B), and phase breakdown length L _φ(T) are detected for the initial MWCNTs used to prepare the paper.     

Capacity and thermal conductivity of a nanocomposite chrysolite asbestos-KDP (KH2PO4)

A nanocomposite chrysotile-KDP (KH₂PO₄) was prepared. KDP was introduced into empty nanochannels of chrysotile asbestos with diameters of ～5 nm. Thermal conductivity κ and heat capacity at a constant pressure C _p of the samples of chrysotile asbestos and nanocomposite chrysotile asbestos-KDP were measured in a temperature range of 80–300 K. Based on the analysis of the behavior of temperature dependences κ(T) and C _p (T) of the composite, temperatures of the ferroelectric transition T _F for KDP in nanochannels of chrysotile asbestos were determined. It turned out to be equal to ～250 K at T _F ～ 122 K for massive KDP samples.     

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 HgSe loaded in the pore lattice of a synthetic opal single crystal

Samples of the opal + HgSe nanocomposite with 100% filling of the first-order opal pores by mercury selenide were prepared. The effective thermal conductivity κ_eff and electrical resistivity ρ_eff were measured in the temperature range T=5–200 K, and the thermopower coefficient α was measured in the interval 80–300 K. The coefficient α of HgSe in opal was shown to remain the same as that in bulk mercury selenide samples with similar carrier concentrations. The mechanism of carrier scattering in the HgSe loaded in opal also did not change. The total thermal conductivity κ _tot ⁰ and electrical resistivity ρ⁰ were isolated from κ_eff and ρ_eff, and the electronic (κ _e ⁰ ) and lattice (κ _ph ⁰ ) components of thermal conductivity of HgSe in opal were determined. The magnitude of κ _ph ⁰ was found to be considerably smaller than κ_ph of bulk HgSe with the same carrier concentration throughout the temperature interval studied (5–200 K). For T>20 K, this behavior of κ _ph ⁰ (T) is accounted for by the presence of specific impurities and defects forming in HgSe, and for T<20 K, by the onset of boundary scattering of phonons in the bottlenecks of the horn-shaped channels connecting first-order octahedral and tetrahedral opal pores loaded by mercury selenide.     

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.