12 research outputs found
Phonon scattering in ortho-para hydrogen solid solutions (role of configurational relaxation)
The experimental data on the thermal conductivity of ortho-parahydrogen
solutions are analyzed on the basis of a relaxation-time model taking account
of configurational relaxation of the ortho subsystem. The influence of
configurational relaxation on the thermal conductivity is analyzed using
resonance scattering of phonons by pair clusters of orthomolecules taking
account of their rotational spectrum.Comment: 7 pages, 4 figure
Thermal Conductivity of Methane-Hydrate
The thermal conductivity of the methane hydrate CH4 (5.75 H2O) 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.Comment: 13 pages, 3 figure
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
Diffusion model of the thermal conductivity plateau of weak solid solutions of neon in parahydrogen
The diffusion kinetic of classic impurity particles has been investigated in the frame of one-dimensional two-level model and applied for the explanation of solid hydrogen thermal conductivity data with extremely low con-centrations of neon impurity in samples growth at different crystallization rates in which the plateau effect was observed. The main idea is that heavy isotopic impurities could segregate into thin long chains near dislocation cores if the growth rate is slow. Neon impurity chains can persist for a long time. Such rigid linear objects ensure inelastic scattering of phonons. The diffusion coefficient of neon atoms in (p-H₂)₁–cNec mixtures was estimated for the experimental conditions with с = 0.0001 аt. % and с = 0.0002 аt. %
Thermal conductivity of Xe clathrate hydrate at low temperatures
The effective thermal conductivity of the powder samples of xenon hydrate was measured in the interval 2-170 K using the steady-state method. The thermal conductivity of the homogeneous Xe clathrate hydrate was estimated from the effective thermal conductivity using an empirical expression. The applicability of the formula was checked by comparing two powder samples with different grain size and porosity. The temperature dependence of the thermal conductivity kappa(T)similar to T (n) of Xe clathrate hydrate is divided into four distinct temperature regimes (I-IV) with different n. In the interval 55-97 K (III) the behavior of kappa(T) shows an anomaly, where the thermal conductivity decreases by almost 50% as the temperature increases. This observation is attributed to the resonant scattering where the coupling of the lattice with "rattling" motions of Xe atom dominates the thermal resistivity at high temperature. Since the observed vibrational energy of Xe in the small cages is similar to 4 meV (or approximate to 46 K) the resonant scattering contribution to the thermal resistivity is expected to decrease in an interval of comparable temperature. The thermal conductivity in the low temperature regime (regimes I and II) is found to follow the prediction of the soft-potential model. The data on thermal conductivities of several gas clathrate hydrates are compared
Properties of solid hydrogen doped by heavy atomic and molecular impurities
Using powder x-ray diffraction we studied the structural characteristics of normal and para hydrogen crystals doped with Ar, Kr, N₂, and O₂ impurities over the range from 5 K to the melting point of the hydrogen matrix. It has been established that in spite of very low solubility of the dopants in solid hydrogen, these impurities appreciably affect the structural characteristics. In particular, only nitrogen impurities do not change the molar volume of the matrix, the other three make the matrix expand. The Ar and Kr impurities also change the c/a ratio of the hcp matrix. The fact that both Ar and O₂ have smaller molar volumes than hydrogen may be treated as evidence that these impurities form van der Waals complexes with the hydrogen lattice environment
Quantum effects in the thermal conductivity of solid krypton–methane solutions
The dynamic interaction of a quantum rotor with its crystalline environment
has been studied by measurement of the thermal conductivity of solid
Kr1_c(CH4)_c solutions at c = 0.05-0.75 in the temperature region from 2 up to
40K. The thermal resistance of the solutions was mainly determined by the
resonance scattering of phonons by CH4 molecules with the nuclear spin I=1 (the
nuclear spin of T-species). The influence of the nuclear spin conversion on the
temperature dependence of the thermal conductivity k(T) was found: a clearly
defined minimum on k(T), its temperature position depending on the CH4
concentration. It was shown that the anisotropy molecular field not increase
monotonously with the CH4 concentration. A compensation effect in the mutual
orientation arrangement of the neighboring rotors is observed at c > 0.5. The
temperature dependence of Kr1_c(CH4)_c is described within the Debye model of
thermal conductivity taking into account the lower limit of the phonon mean
free path. The anomalous temperature dependence of the thermal resistance shows
the evolution of the phonon-rotation coupling at varying temperature. It
increases strongly when the character of CH4 rotation changes from the quantum
at low temperatures to classical at high temperatures. Also, a jump of thermal
conductivity (a sharp increase in k(T) within a narrow temperature range) was
observed, whose position varies from 9.7 K to 8.4 K when the CH4 concentration
changes from 0.25 to 0.45.Comment: 13 pages, 4 figure