N. F. Kartenko

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.

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 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.

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.

Electroluminescent three-dimensional photonic crystals based on opal–phosphor composites

The composites opal–Zn2SiO4:Mn and opal–GaN–ZnS:Mn were synthesized by chemical bath deposition. These materials are perfect three-dimensional photonic crystals which produce effective fluorescence at room temperature when excited by an alternating current electric field. The electroluminescence spectrum is considerably modified by the photonic band gap to become anisotropic in accordance with the photonic band gap angular dispersion.

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 Tcarb 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 Tcarb have been determined. Temperature dependences of the phonon thermal conductivity κ(T) of the biocarbons with different temperatures Tcarb (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 Tcarb = 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 Tcarb = 2400°C is determined by the heat transfer (scattering) in the nanocrystalline phase over the entire temperature range of 5–300 K.

Thermopower of the light heavy-fermion compound YbMgCu4 in the region of its homogeneity

The thermopower coefficient S of the light heavy-fermion system YbMgCu4 and, for comparison, the thermopower coefficient of metallic LuMgCu4 are measured in the temperature range 5–300 K. It is shown that the YbMgCu4 compound has a fairly broad region of homogeneity. The obtained data on the temperature dependence of the thermopower coefficient S for the YbMgCu4 compound confirm that this compound belongs to heavy-fermion systems. The Kondo temperature of YbMgCu4 is shown to depend on the unit cell parameter in the region of its homogeneity.

Acoustical investigations of a La0.75Sr0.25MnO3 single crystal

The acoustical, resistive, and magnetic properties of a La0.75Sr0.25MnO3 lanthanum manganite single crystal are investigated in the temperature range involving the second-order magnetic phase transition. The acoustical measurements are performed by the pulse-echo method in the frequency range 14–90 MHz. It is found that, as the temperature decreases, the velocity of a longitudinal acoustic wave propagating along the [111] axis in the single crystal drastically increases at temperatures below the critical point of the magnetic phase transition. No dispersion of the acoustic velocity is revealed. A sharp increase in the acoustic velocity is accompanied by the appearance of an acoustical absorption peak. The observed effects are discussed with due regard for the interaction of acoustic waves with the magnetic moments of the manganese ions.

Thermal conductivity and heat capacity of Si3N4/BN fiber monoliths

This paper reports on measurements within the 5–300-K temperature interval of the thermal conductivity of Si3N4 and BN polycrystalline ceramic samples and Si3N4/BN fiber monoliths (FM) with different fiber arrangement architecture, [0], [90], and [0/90], with fibers arranged, accordingly, along and across the sample axis and the [0] and [90] layers stacked alternately. In the 3.5–300-K interval, the heat capacity at constant pressure, and at 77 K, the sound velocity have been measured in polycrystalline Si3N4 and BN samples and in Si3N4/BN [0] fiber monoliths. Our studies suggest that, with a high enough degree of confidence, but for some compositions—with minor assumptions, it can be maintained that, in the case of the Si3N4/BN fiber monoliths, one can use for calculation of their thermal conductivities and heat capacities within certain temperature intervals simple models considering mixtures of the Si3N4 and BN components with due account of their contributions to formation of the Si3N4/BN FM. It has been established that in the low-temperature domain (5–25 K), phonons in Si3N4/BN [0], [90], and [0/90] fiber monoliths scatter primarily from dislocations. This effect is not observed in ceramic Si3N4 and BN samples. The experimental data obtained on the thermal conductivity, heat capacity, and sound velocity have been used to calculate phonon mean free path lengths in polycrystalline Si3N4 and BN samples and the effective mean free path length in the Si3N4/BN [0] FM.