A. Jezowski

Thermal conductivity of GaN crystals in 4.2- 300 K range

Results of measurements of thermal conductivity of bulk GaN crystals in the temperature interval 4.2 – 300 K are reported. Experiments were performed on two types of single GaN crystals grown under high-pressure: highly conducting n-type sample and on a highly resistive sample compensated by magnesium doping. For n-GaN crystals, the highest thermal conductivity kmax is equal to 1600 W/m K at Tmax ¼ 45 K; and k . 220 W/m K at 300 K. Our analysis indicates that for the best n-GaN crystal and for T

Heat capacity and phonon mean free path of wurtzite GaN

Tmax; the contribution of Umklapp phonon scattering processes dominate whereas for other samples scattering of phonons by point mass defects represents the main contribution. The dependence of kðTÞ is used to reveal possible mechanisms of thermal resistance of GaN crystals at temperatures Tmax: Our thermal conductivity measurements yields Debye’s temperature uD < 400 K: q 2003 Elsevier Ltd. All rights reserved.

Thermal conductivity of high-porosity biocarbon preforms of beech wood

The authors report lattice specific heat of bulk hexagonal GaN measured by the heat flow method in the temperature range of 20–300K and by the adiabatic method in the range of 5–70K. The best fit with the accuracy of 3% was obtained for the temperature-independent Debye temperature ΘD=365K and Einstein temperature ΘE=880K. The authors relate these temperatures to the function of density of states. Using their results for heat conduction coefficient, they established in the temperature range of 10–100K the explicit dependence of the phonon mean free path on temperature lph∝T−2. Above 100K, there is an evidence of contribution of the Umklapp processes, which limits phonon free path at high temperatures.

Transition and rare earth element dodecaborides

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

Abstract Original results on the technology of melting and single-crystal growth of Sc and rare earth element (from Dy) dodecaborides by the floating zone method are presented. Investigations of electrical resistance and thermal conductivity for all received materials between 4.2 and 300 K were conducted. A joint analysis of the obtained results was performed. The magnetic ordering temperatures of dodecaborides were refined.

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

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.

Influence of crystallite size on the thermal conductivity in BaTiO3 nanoceramics

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 grain size effect on the thermal conductivity in BaTiO3 nanoceramics was investigated. The nanoceramics were produced by the low temperature high pressure technique. The averaged sizes of grain varied from 30to100nm. It was found that the thermal conductivity of BaTiO3 nanoceramics decreased with decreasing the grain sizes. For the smallest grains the thermal conductivity was similar to the glasslike materials, whereas for larger grains it behaved as in crystalline dielectrics.

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

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 (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.