Heat capacity of Bio-SiC and SiC/Si ecoceramics prepared from white eucalyptus, beech, and sapele tree wood

This paper reports on measurement of the heat capacity at constant pressure C _p of silicon bio-carbide prepared within the 5–300 K temperature interval from beech tree wood (bio-SiC(BE)), and within 80–300 K, from tree wood of sapele (bio-SiC(SA)), as well as SiC/Si ecoceramics of beech, sapele, and white eucalyptus wood. It has been shown that in bio-SiC(BE) the measured heat capacity contains a significant contribution of surface heat capacity, whose magnitude decreases with increasing temperature. Of the ecoceramics, only SiC/Si(SA) characterized by a high enough porosity has revealed a small contribution to the heat capacity coming from its surface component. The experimental results obtained are discussed.     

Heat capacity of the white pine biocarbon preform and the related biocarbon/copper composite

This paper reports on measurements in the 80–300-K temperature interval of the heat capacity at constant pressure C _p (T) of high-porosity amorphous white pine carbon preforms (biocarbon) prepared by pyrolysis (carbonization) at T _carb = 1000 and 2400°C in an argon flow. The dependences C _p (T) for biocarbon/copper composites based on the carbon preforms obtained have also been determined. It is shown that the mixture rule holds for the composites, i.e., that C _p (T) of the composite is a sum of the heat capacities of the constituent materials taken in the corresponding ratios. Phonon mean free paths for the white pine carbon preforms prepared at T _carb = 1000 and 2400°C have been calculated and used to estimate the size of the nanocrystallites contributing to formation of the carbon frameworks of these preforms.     

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.     

Thermopower of Bio-SiC and SiC/Si ecoceramics prepared from sapele tree wood

The thermopower coefficients of bio-SiC and SiC/Si ecoceramics prepared from sapele tree wood have been measured in the temperature interval 5–300 K. The measurements have been performed both along and perpendicular to empty (bio-SiC), as well as empty and partially silicon-filled (SiC/Si) channels in the samples. In bio-SiC, a contribution to thermopower associated with electron drag by phonons has been shown to exist within the temperature interval 5–200 (250) K. No such effect is realized in SiC/Si. This is assumed to derive from the presence in this material of heavily doped silicon embedded in SiC channels and the dominant part it plays in the behavior of the thermopower of this ceramics. The results obtained for the thermopower are compared with the available data for bio-SiC prepared from white eucalyptus tree wood and heavily doped bismuth.     

Thermal conductivity of high-porosity heavily doped biomorphic silicon carbide prepared from sapele wood biocarbon

The electrical resistivity and thermal conductivity of high-porosity (～52 vol %, channel-type pores) bio-SiC samples prepared from sapele wood biocarbon templates have been measured in the temperature range 5–300 K. An analysis has been made of the obtained results in comparison with the data for bio-SiC samples based on beech and eucalyptus, as well as for polycrystalline β-SiC. The conclusion has been drawn that the electrical resistivity and thermal conductivity of bio-SiC samples based on natural wood are typical of heavily doped polycrystalline β-SiC.     

Heat capacity, root-mean-square displacements of atoms and thermal expansion coefficient of europium hexaboride

The temperature dependences of the specific heat capacity c _p(T), thermal expansion coefficient α(T) of europium hexaboride, and root-mean-square displacements of Eu and B atoms are determined in the temperature range from helium to room temperature (5–300 K).     

Heat capacity of the SBN relaxor ferroelectric

The temperature dependence of the heat capacity of the single-crystal relaxor ferroelectric SBN was studied. Hysteresis and a “two-level” effect were observed near the temperature of the dielectric permittivity maximum.     

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.     

On the thermopower of superionic conductors

We investigate the thermoelectric power of a superionic conductor within the Brownian particle model. The dependence on temperature and friction is calculated numerically for the one-dimensional case. The results are compared to those of other models and approaches.     

Heat capacity of isolated clusters

The character of interaction between thermal (vibrational) and configurational cluster excitations is considered under adiabatic conditions when a cluster is a member of a microcanonical ensemble. The hierarchy of equilibration times determines the character of atomic equilibrium in the cluster. The behavior of atoms in the cluster can be characterized by two effective (mean) temperatures, corresponding to the solid and liquid aggregate states, because the typical time for equilibration of atomic motion is less than the transition time between aggregate states. If the cluster is considered for a time much longer than the typical dwell time in either phase, then it is convenient to characterize the system by only one temperature, which is determined from the statistical-thermodynamic long-time average. These three temperatures are not far apart, nor are the cluster heat capacities evaluated on the basis of these definitions of temperature. The heat capacity of a microcanonical ensemble may be negative for two coexisting phases if the mean temperature is defined in terms of the mean kinetic energy, rather than as the derivative of energy with respect to microcanonical entropy. However, if the configurational excitation energy is smaller than the total excitation energy separating the phases, then the two-state model predicts a positive heat capacity under either definition of temperature. Moreover, if the cluster is sufficiently large, then the maximum values of the microcanonical and canonical heat capacities are equal.     

Heat capacity of disordered and crystalline superconductors

Low temperature heat capacity measurements on disordered and crystalline Nb_0.26Zr_0.74 alloys show that the former has a much larger heat capacity jump (ΔC/γT_c ≈ 2.4) than the latter and that the disorder decreases the superconducting transition temperature (T_c) rather than enhances it.     

Heat capacity of high-pressure ice polymorphs

The isobaric heat capacity C_P of high-pressure water ice polymorphs (ices III, V, VI, and VII) is calculated using P-V-T equations of state of ices, jump condition for C_P on the lines of phase transition, and the equations of the lines of phase transition on the P-T diagram of water substance.     

Heat capacity and magnetic susceptibility of ReO3

The specific heat and magnetic susceptibility of the transition metal oxide ReO₃ have been measured. The specific heat results give a Debye temperature Θ_D = 460 ± 10 K and an electronic specific heat coefficient γ = 6.45 ± 0.07 cal/mole K² which are in good agreement with similar measurements on the cubic sodium tungsten bronzes. The magnetic susceptibility and the electronic contribution to the specific heat are within a few percent of the corresponding parameters calculated from the free electron model with one electron per unit cell. Our results show that ReO₃ behaves much like a simple metal. No experimental evidence for narrow d-band effects was observed.     

On the correlation between thermal expansion coefficient and heat capacity of argon cryocrystals

A detailed analysis of the correlation between the volumetric thermal expansion coefficient o(T) and heat capacity C(T) of a rare-gas (Ar) cryocrystal has been performed. It has been shown that there is a clear correlation dependence o(C) not only in the low-temperature range, where it is linear and known as the Grüneisen law, but also in a significantly wider temperature range (up to the melting point of argon). The dependence o(C) substantially deviates from the low-temperature linear behavior when the heat capacity reaches the classical Dulong-Petit limit of 3R.     

Ratio of the lateral diffusion coefficient to the mobility for electrons in carbon monoxide and carbon dioxide

Measurements of the ratio of the lateral diffusion coefficient to the mobility D/μ in carbon monoxide and carbon dioxide have been carried out at ambient temperature for values of the reduced electric field E/N in the range 0.24 ? E/N ? 288.7 Td and 0.31 ? E/N ? 183.9 Td, respectively.