B. Ya. Gorodilov

Thermal Conductivity of Methane-Hydrate

The thermal conductivity of the methane hydrate CH4 (5.75H2O) was measured in the interval 2–140 K using the steady-state technique. The thermal conductivity corresponding to a homogeneous substance was calculated from the measured effective thermal conductivity obtained in the experiment. The temperature dependence of the thermal conductivity is typical for the thermal conductivity of amorphous solids. It is shown that after separation of the hydrate into ice and methane, at 240 K, the thermal conductivity of the ice exhibits a dependence typical of heavily deformed fine-grain polycrystal. The reason for the glass-like behavior in the thermal conductivity of clathrate compounds has been discussed. The experimental results can be interpreted within the phenomenological soft-potential model with two fitting parameters.

Locally heterogeneous dynamics in miscible blends of poly(methyl methacrylate) and poly(vinylidene fluoride)

Comparative measurements of specific heat capacities (temperature interval between 2 and 500 K), and of low frequency mechanical spectroscopy (temperature interval between 120 and 400 K) in poly(methyl methacrylate) (PMMA) and poly(vinylidene fluoride) (PVDF) amorphous blends show the existence of single calorimetric and mechanical glass transition temperatures, as a clear indication of the existence of homogeneous single-state structures. Below T g , the experimental data reveal distinct local relaxation processes within the backbone of the individual components, while the heat capacities below 15 K can be explained in terms of a two-phase model (i.e., a simple linear overlap of the contributions from wholly amorphous PMMA and PVDF, weighted by their proportions). These findings are associated with locally heterogeneous relaxation and vibrational motions, and are regarded as experimental evidence for the existence of a nanoscopic length scale where the dynamics of a blend exhibits a heterogeneous regime.

Thermal conductivity of solid krypton with methane admixture

The thermal conductivity of CH4–Kr solid solutions is investigated at CH4 concentrations 0.2–5.0% in the temperature range 1.8–40 K. It is found that the temperature dependence of the thermal conductivity has features typical of resonance phonon scattering. The analysis of the experimental results shows that the main contribution to the impurity-caused scattering of phonons is made by the scattering on rotational excitations of the nuclear spin T-species of CH4 molecules. The phonon–rotation interaction parameter is estimated.

Thermal conductivity of tetrahydrofuran hydrate

The thermal conductivity of tetrahydrofuran hydrate has been measured in the temperature region 2-220 K by the steady-state potentiometric method. The temperature dependence of the thermal conductivity exhibits behavior typical of amorphous substances. It is shown that above 100 K the mean free path of the phonons is considerably smaller than the lattice parameter and is no longer dependent on temperature.

Anisotropy of the Thermal Conductivity of Parahydrogen Crystals

The anisotropy of thermal conductivity of parahydrogen crystals has been observed for the first time. The thermal conductivity measurements have been made on samples of different diameters at the temperature range from 2 to 8 K.

Heat Transfer in Solid Solutions Hydrogen-Deuterium

The thermal conductivity of parahydrogen-orthodeuterium solid solutions with the orthodeuterium concentration of 0.01 to 100% has been investigated in the temperature range from 1.8 K to the melting point. The experimental data have been analyzed in terms of the Callaway model. It has been found that the intensity of phonon scattering by isolated orthodeuterium impurities in solid hydrogen is much higher than that in classical crystals. The impurity additional scattering of phonons has been supposed to be due to variations in force constants and lattice distortions in the vicinity of impurity molecules in quantum crystals. The above effects have been quantitatively estimated. The concentration dependences of the thermal conductivity and phonon scattering intensity have been considered.

Structure, sound velocity, and thermal conductivity of the perovskite NdGaO3

X-ray (300 K) and ultrasonic (77–270 K) studies and measurements of the thermal conductivity (30–300 K) are carried out on single-crystal samples of NdGaO3 in different crystallographic directions. The values of the lattice parameters of NdGaO3 are refined. The sound velocities in the principal crystallographic directions are measured, and the elastic constants and Debye temperature are calculated. The observed anisotropy of the thermal conductivity is described in the framework of a gaskinetic model and is linked to the anisotropy of the interaction parameters of the acoustical and optical phonons.

The role of normal processes in the thermal conductivity of solid deuterium

The thermal conductivity of orthodeuterium crystals containing a neon impurity is investigated in the temperature interval 1.8–17 K. The results of the measurements are described in the framework of the relaxation-time model with allowance for phonon–phonon scattering processes. The intensity of the normal scattering processes for deuterium are determined. The existing theoretical models are used to estimate the intensity of the phonon scattering processes for a number of cryocrystals. The calculated intensity of the normal processes is compared to the experimental result.

Phonon scattering by structural defects in solid p-H2 and in p-H2–o-D2 solutions

A study is made of the influence of structural defects on the thermal conductivity in parahydrogen crystals and in parahydrogen–orthodeuterium solutions. The defects in the crystals are generated by means of a thermal shock. The temperature dependence of the thermal conductivity is analyzed in the framework of the Callaway relaxation model in the Debye approximation for the phonon spectrum with allowance for phonon–phonon scattering processes and Rayleigh scattering on isotopic defects and structural defects such as dislocations and low-angle boundaries. The effect produced in the sample by the thermal shock is found to depend on the deuterium concentration. In pure parahydrogen an increase in the dislocation density is observed, and in parahydrogen–orthodeuterium solutions an increase in the density of low-angle boundaries. The change in the density of low-angle boundaries in the solutions after the thermal shock depends linearly on the concentration of o-D2.

Influence of Structure Defects on Thermal Conductivity in Solid p-H2 and p-H2 - o-D2 Solid Solutions

The formation of structure defects induced by thermal stress in pure H2 and H2-D2 solutions has been investigated. The thermal conductivity of p-H2 crystals and p-H2 - o-D2 solutions is measured by the steady-state method from 1.5 K to the melting point. Crystals with different numbers of structure defects were prepared by varying the growth rate and parameters of subsequent annealing, and thermal shock. The value of thermal conductivity and the character of the temperature dependence are observed to change, depending on the number of defects present. The experimental results are analyzed within the Callaway model taking into account the phonon scattering processes such as phononphonon scattering, boundary scattering, scattering on D2 impurities, and scattering on structure defects (dislocations and low-angle boundaries). The contribution of isolated dislocations into the total relaxation rate is distinct only for the pure parahydrogen sample subjected to thermal shock. In the p-H2 - o-D2 solutions this contribution is not, detectable against the background of the strong frequency-independent scattering by low-angle boundaries. It is shown that the density of the dislocations that form low-angle grain boundaries is proportional to the concentration of impurity molecules.