B. I. Smirnov

Thermal conductivity of high-porosity biocarbon preforms of beech wood

This paper reports on measurements performed in the temperature range 5–300 K for the thermal conductivity κ and electrical resistivity ρ of high-porosity (cellular pores) biocarbon preforms prepared by pyrolysis (carbonization) of beech wood in an argon flow at carbonization temperatures of 1000 and 2400°C. X-ray structure analysis of the samples has been performed at 300 K. The samples have revealed the presence of nanocrystallites making up the carbon matrices of these biocarbon preforms. Their size has been determined. For samples prepared at Tcarb = 1000 and 2400°C, the nanocrystallite sizes are found to be in the ranges 12–25 and 28–60 κ(T) are determined for the samples cut along and across the tree growth direction. The thermal conductivity κ increases with increasing carbonization temperature and nanocrystallite size in the carbon matrix of the sample. Thermal conductivity measurements conducted on samples of both types have revealed an unusual temperature dependence of the phonon thermal conductivity for amorphous materials. As the temperature increases from 5 to 300 K, it first increases in proportion to T, to transfer subsequently to ∼T1.5 scaling. The 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 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.

Determination of elastic moduli of GaN epitaxial layers by microindentation technique

It is demonstrated that Young’s modulus of epitaxial gallium nitride layers can be determined by the microindentation of their growth surface. The technique is based on the solution of the Hertz problem for the elastic indentation of a steel sphere into the studied surface. It is established that the isotropic approximation applied in this case is justified and leads to the satisfactory results. The microhardness measurements of epitaxial layers are carried out.

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 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 (Tcarb) 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 Tcarb = 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 κph, which is not typical of amorphous (and amorphous to x-rays) materials. As the temperature increases, κph first varies proportional to T, to scale subsequently as ∼T1.7. The results obtained are analyzed.

Structure, electrical resistivity, and thermal conductivity of beech wood biocarbon produced at carbonization temperatures below 1000°C

This paper reports on measurements of the thermal conductivity κ and the electrical resistivity ρ in the temperature range 5–300 K, and, at 300 K, on X-ray diffraction studies of high-porosity (with a channel pore volume fraction of ∼47 vol %) of the beech wood biocarbon prepared by pyrolysis (carbonization) of tree wood in an argon flow at the carbonization temperature Tcarb = 800°C. It has been shown that the biocarbon template of the samples studied represents essentially a nanocomposite made up of amorphous carbon and nanocrystallites—“graphite fragments” and graphene layers. The sizes of the nanocrystallites forming these nanocomposites have been determined. The dependences ρ(T) and κ(T) have been measured for the samples cut along and perpendicular to the tree growth direction, thus permitting determination of the magnitude of the anisotropy of these parameters. The dependences ρ(T) and κ(T), which have been obtained for beech biocarbon samples prepared at Tcarb = 800°C, are compared with the data amassed by us earlier for samples fabricated at Tcarb = 1000 and 2400°C. The magnitude and temperature dependence of the phonon thermal conductivity of the nanocomposite making up the beech biocarbon template at Tcarb = 800°C have been found.

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.

Elasticity and inelasticity of silicon nitride/boron nitride fibrous monoliths.

A study is reported on the effect of temperature and elastic vibration amplitude on Young’s modulus E and internal friction in Si3N4 and BN ceramic samples and Si3N4/BN monoliths obtained by hot pressing of BN-coated Si3N4 fibers. The fibers were arranged along, across, or both along and across the specimen axis. The E measurements were carried out under thermal cycling within the 20–600°C range. It was found that high-modulus silicon-nitride specimens possess a high thermal stability; the E(T) dependences obtained under heating and cooling coincide well with one another. The low-modulus BN ceramic exhibits a considerable hysteresis, thus indicating evolution of the defect structure under the action of thermoelastic (internal) stresses. Monoliths demonstrate a qualitatively similar behavior (with hysteresis). This behavior of the elastic modulus is possible under microplastic deformation initiated by internal stresses. The presence of microplastic shear in all the materials studied is supported by the character of the amplitude dependences of internal friction and the Young’s modulus. The experimental data obtained are discussed in terms of a model in which the temperature dependences of the elastic modulus and their features are accounted for by both microplastic deformation and nonlinear lattice-atom vibrations, which depend on internal stresses.

Influence of the deformation type and medium on the mechanodynamic penetration of nitrogen molecules into surface layers of armco iron

The extraction of nitrogen molecules from deformed samples of armco iron with different initial structures (annealed and subjected to equal-channel angular pressing) and different deformation prehistories (deformation in liquid nitrogen at 77 K, rolling in air at room temperature, and their combination) has been studied. It has been shown that the preliminary deformation in liquid nitrogen increases its concentration in the surface layer of the material and shifts the principal peak of its release toward low temperatures during heating. The results are associated with the existence of different types of nitrogen traps in annealed and nanostructured armco iron and with their changes during subsequent deformation.