Thermal Conductivity of Isotopically Enriched 28Si Revisited

The thermal conductivity of isotopically enriched 28Si (enrichment better than 99.9%) was redetermined independently in three laboratories by high precision experiments on a total of 4 samples of different shape and degree of isotope enrichment in the range from 5 to 300 K with particular emphasis on the range near room temperature. The results obtained in the different laboratories are in good agreement with each other. They indicate that at room temperature the thermal conductivity of isotopically enriched 28Si exceeds the thermal conductivity of Si with a natural, unmodified isotope mixture by 102 %. This finding is in disagreement with an earlier report by Ruf et al. At 26 K the thermal conductivity of 28Si reaches a maximum. The maximum value depends on sample shape and the degree of isotope enrichment and exceeds the thermal conductivity of natural Si by a factor of 8 for a 99.982% 28Si enriched sample. The thermal conductivity of Si with natural isotope composition is consistently found to be 3% lower than the values recommended in the literature

On the theory of magnetic field dependence of heat conductivity in dielectric in isotropic model

Phonon polarization in a magnetic field is analyzed in isotropic model. It is shown, that at presence of spin-phonon interaction phonon possess circular polari-zation which causes the appearance of heat flux component perpendicular both to temperature gradient and magnetic field.Comment: 5 pages, 0 figure

Phonon Hall effect in ionic crystals in the presence of static magnetic field

We study phonon Hall effect (PHE) for ionic crystals in the presence of static magnetic field. Using Green-Kubo formula, we present an exact calculation of thermal conductivity tensor by considering both positive and negative frequency phonons. Numerical results are shown for some lattices such as hexagonal lattices, triangular lattices, and square lattices. We find that the PHE occurs on the nonmagnetic ionic crystal NaCl, although the magnitude is very small which is due to the tiny charge-to-mass ratio of the ions. The off-diagonal thermal conductivity is finite for nonzero magnetic field and changes sign for high value of magnetic field at high temperature. We also found that the off-diagonal thermal conductivity diverges as $\pm{1/T}$ at low temperature

Gallium arsenide thermal conductivity and optical phonon relaxation times from first-principles calculations

In this paper, thermal conductivity of crystalline GaAs is calculated using first-principles lattice dynamics. The harmonic and cubic force constants are obtained by fitting them to the force-displacement data from density functional theory calculations. Phonon dispersion is calculated from a dynamical matrix constructed using the harmonic force constants and phonon relaxation times are calculated using Fermi's Golden rule. The calculated GaAs thermal conductivity agrees well with experimental data. Thermal conductivity accumulations as a function of the phonon mean free path and as a function of the wavelength are obtained. Our results predict a significant size effect on the GaAs thermal conductivity in the nanoscale. Relaxation times of optical phonons and their contributions from different scattering channels are also studied. Such information will help the understanding of hot phonon effects in GaAs-based devices.United States. Dept. of Energy. Office of Science (Award DE-SC0001299

Magnetothermal Conductivity of Highly Oriented Pyrolytic Graphite in the Quantum Limit

We report on the magnetic field (0T$\le B \le 9$T) dependence of the longitudinal thermal conductivity $\kappa(T,B)$ of highly oriented pyrolytic graphite in the temperature range 5 K $\le T\le$ 20 K for fields parallel to the $c-$axis. We show that $\kappa(T,B)$ shows large oscillations in the high-field region (B > 2 T) where clear signs of the Quantum-Hall effect are observed in the Hall resistance. With the measured longitudinal electrical resistivity we show that the Wiedemann-Franz law is violated in the high-field regime.Comment: 4 Figures, to be published in Physical Review B (2003

Metal-insulator transition in manganites: mixture of oxygen isotopes versus magnetic field

We have investigated the effect of oxygen isotope substitution on the metal-insulator transition temperature and the resistivity of the narrow band manganite (La0.25Pr0.75)0.7Ca0.3MnO3 in a constant magnetic field. A set of 16 samples having different mixtures of 16O, 17O and 18O isotopes with average mass varying from 16.0 to 17.8 a.m.u. was studied. We have found that the magnetoresistance and the isotope effect can be linked together with a single parameter - effective magnetic field, which decreases linearly with an increase of average oxygen mass with a slope of -2 T/a.m.u. The applicability of the small polaron model is discussed.Comment: Submitted to Moscow International Symposium on Magnetism MISM'0

Расчет гидродинамики потока в электроциклоне

To analyze the elektrocyclone flow hydrodynamic computer calculation using the finite element method (FEM) is applied. The geometry of the model corresponds to the laboratory  elektrocyclone. k-ε-turbulence model is used for the computation. The system of equations is solved by SIMPLE algorithm. The calculation results give a pattern of the flow velocity distribution and flow lines in different sections. There is conclusion based on the results about elektrocyclone flow hydrodynamic.Для анализа гидродинамики потока в электроциклоне применен компьютерный расчет с использованием метода конечных элементов (МКЭ). Геометрия модели соответствует лабораторному электроциклону. Для расчетов использована k-ε-модель турбулентности. Система уравнений решается с помощью алгоритма SIMPLE. Результаты расчета дают картину распределения скоростей потока и линий тока в различных сечениях. На основании результатов делается вывод о гидродинамике электроциклона. Выявлен факт, что в бункере электроциклона отсутствует вихревое движение, также нет развитого течения в области стенок, а ниже выхлопного отверстия скорость потока близка к 0. Это благоприятно сказывается на эффективности очистки, т. к. выходящий чистый газ не увлекает с собой осевшие частицы. Выводы: 1) гидродинамика электроциклона может быть описана с помощью математической модели и рассчитана с помощью МКЭ; 2) поток в электроциклоне, как и ожидалось, имеет закрученную структуру, угол закрутки зависит от длины активной зоны; 3) конструкция бункера обеспечивает выход очищенного газа без вовлечения в него уловленных частиц