I. A. 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 the Néel temperature from measurements of the thermal conductivity of the Co3O4 antiferromagnet nanostructured in porous glass channels

The Néel temperature TN(n) of the Co3O4 antiferromagnet nanostructured in channels of porous borosilicate glass with channel cross sections of ∼7 nm has been determined from thermal conductivity measurements. It has been shown that the Néel temperature TN(n) of this nanomaterial is approximately equal to 20 K, which is considerably lower than TN = (30–40) K for the bulk Co3O4 sample.

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.

Thermal conductivity of ultrathin InSb semiconductor nanowires with properties of the Luttinger liquid

The thermal conductivity of ultrathin (∼5 nm in diameter) and long (10 mm) InSb semiconductor quantum nanowires embedded in nanochannels of a dielectric chrysotile asbestos matrix is measured in the temperature range 5–300 K. The possible manifestation of the spin-charge separation of current carriers in these nanowires is discussed, which would provide an additional argument for the InSb nanowires possessing properties of the Luttinger liquid.

Heat transport over nonmagnetic lithium chains in LiCuVO4, a new one-dimensional superionic conductor

The thermal conductivity of three single-crystal samples of the quasi-one-dimensional spin system of LiCuVO4 with different concentrations of defects (primarily, vacancies on the lithium sublattice) was measured along the crystallographic a axis (along the nonmagnetic lithium chains) in the temperature interval 5–300 K. An increase in thermal conductivity from that of the crystal lattice was revealed for T>150–200 K. This increase can be accounted for only by assuming LiCuVO4 to be a superionic conductor. This assumption was confirmed by measuring its electrical conductivity in the temperature interval 300–500 K. Li+ ions move over vacancies on the lithium sublattice (conducting channels) and act as charge carriers in LiCuVO4. It is shown that LiCuVO4 is a fairly good superionic conductor with application potential.

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 Cp 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 Cp(T) of the composite, temperatures of the ferroelectric transition TF for KDP in nanochannels of chrysotile asbestos were determined. It turned out to be equal to ∼250 K at TF ∼ 122 K for massive KDP samples.