Isotope effect for the thermal expansion coefficient of germanium

The first experimental and theoretical investigation of the difference in the temperature behavior of the linear expansion coefficients of single crystals grown from isotopically highly enriched and natural germanium is reported. A comparison of the data for ⁷⁰Ge and ^natGe crystals reveals the significant influence of isotopic composition over a wide range of temperatures 30–230 K. Zh. éksp. Teor. Fiz. 115, 243–248 (January 1999)     

Molecular-dynamics calculation of the thermal conductivity coefficient of the germanium single crystal

The thermal conductivity coefficient of the germanium crystal lattice has been calculated by molecular dynamics simulation. Calculations have been performed for both the perfect crystal lattice and the crystal lattice with defects such as monovacancies. For the perfect germanium single crystal, the dependence of the thermal conductivity coefficient on the lattice temperature has been obtained in the temperature range of 150–1000 K. The thermal conductivity coefficient of the germanium lattice has been calculated as a function of the monovacancy concentration.     

Isotope effect on the thermal conductivity of boron nitride nanotubes

We have measured the temperature-dependent thermal conductivity kappa(T) of individual multiwall boron nitride nanotubes using a microfabricated test fixture that allows direct transmission electron microscopy characterization of the tube being measured. kappa(T) is exceptionally sensitive to isotopic substitution, with a 50% enhancement in kappa(T) resulting for boron nitride nanotubes with 99.5% 11B. For isotopically pure boron nitride nanotubes, kappa rivals that of carbon nanotubes of similar diameter.     

Normal phonon-phonon scattering processes and the thermal conductivity of germanium crystals with isotope disorder

The influence of the normal phonon-phonon scattering processes on the thermal conductivity was theoretically studied for germanium crystals with various degrees of the isotope disorder. The theory takes into account redistribution of the phonon momentum in the normal scattering processes both inside each oscillation branch (Simons mechanism) and between various phonon oscillation branches (Herring mechanism). Contributions to the thermal conductivity due to the drift mobility of the longitudinal and transverse phonons are analyzed. It is shown that the momentum redistribution between longitudinal and transverse phonons according to the Herring relaxation mechanism leads to a significant suppression of the drift motions (and to the corresponding drop in contribution to the thermal conductivity) of the longitudinal phonons in isotopically pure germanium crystals. The results of the thermal conductivity calculations involving the Herring relaxation mechanism agree well with the experimental data available for germanium crystals with various degrees of the isotope disorder.     

Structural defects in germanium single crystals

The origin and distribution of structural defects (dislocations, small angle grain boundaries, slip lines, polycrystalline inclusions) is studied in large single crystals of germanium applied in manufacturing optical elements.     

Nuclear spin-lattice relaxation in germanium single crystals

The temperature dependence of the spin-lattice relaxation time corresponding to the inelastic scattering of phonons by the⁷³Ge quadrupole moment in Ge single crystals is calculated in the framework of the adiabatic bond charge model. The results obtained agree with the experimental data.     

Surface structure of large germanium single crystals

The dependence between structural-defect generation and the growth kinetics of germanium single crystals pulled from melts is investigated via selective chemical etching and the optical, atomic-force and scanning electron microscopy techniques. It is ascertained that the surface microrelief of germanium crystals grown from melts by means of directional crystallization contains protrusions and cavities with spatial periodicities of 5 and 50 μm. The values of the kinetic coefficients are estimated. It is demonstrated that the main part of the crystals is formed according to the normal mechanism with the kinetic coefficients β_k = 2 × 10^?5 m s^?1 K^?1.     

Low temperature thermal conductivity of pure and doped single crystals of semiconducting SmS

We measured the thermal conductivity of pure SmS and of SmS doped with P, As and Se between 1.5 and 350 K. The lattice thermal conductivity of pure samples and of SmSSe obeys a T^{?$\text{3}\text{2}$} law for temperatures T greater than 150 K, and is very sensitive to point defects in the lattice. The highest values are measured on samples close to the stoichiometric composition. P and As dopants act as phonon scattering centers and considerably reduce the low temperature lattice conductivity.     

Multiphysical simulation analysis of the dislocation structure in germanium single crystals

To grow high-quality germanium crystals is one of the most important problems of growth industry. The dislocation density is an important parameter of the quality of single crystals. The dislocation densities in germanium crystals 100 mm in diameter, which have various shapes of the side surface and are grown by the Czochralski technique, are experimentally measured. The crystal growth is numerically simulated using heat-transfer and hydrodynamics models and the Alexander–Haasen dislocation model in terms of the CGSim software package. A comparison of the experimental and calculated dislocation densities shows that the dislocation model can be applied to study lattice defects in germanium crystals and to improve their quality.     

Phonon thermal conductivity of amorphous selenium doped by germanium

The thermal conductivity of the amorphous semiconducting system Se-Ge in the temperature range of 100 to 300°K was measured. The Debye model was used for the analysis of the experimental values and the calculated mean free path of phonons was related to the size of the basic structure units of the selenium glass. The shift of the thermal conductivity of amorphous selenium doped with germanium was explained by means of the increase of the velocity of sound observed together with the occurrence of the covalent bond Se-Ge between the basic structure units of the studied amorphous system.     

Low-temperature thermal conductivity of highly enriched and natural germanium

Experimental data on the thermal conductivity K(T) of crystals of natural and highly enriched germanium (99.99%) ⁷⁰Ge with lapped and polished surfaces are analyzed in the temperature range ∼1.5–8 K. In all the samples in the temperature range ∼1.5–4 K the standard boundary mechanism of scattering dominates. As the temperature is raised, an isotopic scattering mechanism is observed in the natural samples. In the highly enriched samples the theoretical values of K(T) turn out to be much smaller than the experimental ones. It is conjectured that a Poiseuille viscous flow regime of the phonon gas emerges in this case. Zh. éksp. Teor. Fiz. 114, 1757–1764 (November 1998)     

Nonmetallic crystals with high thermal conductivity

Nonmetallic crystals transport heat primarily by phonons at room temperature and below. There are only a few nonmetallic crystals which can be classed as high thermal conductivity solids, in the sense of having a thermal conductivity of > 1 W/cmK at 300K. Thermal conductivity measurements on natural and synthetic diamond, cubic BN, BP and AIN confirm that all of them are high thermal conductivity solids. Studies have been made of the effect on the thermal conductivity of nitrogen impurities in diamond, and oxygen impurities in AIN. The nitrogen impurities scatter phonons mostly from the strain field, the oxygen impurities scatter phonons mostly from the mass defects caused by aluminum vacancies. Pure A1N as well as pure SiC, BeO, BP and BeS conduct heat almost as well as does copper at room temperature, while pure natural and synthetic diamonds conduct heat five times better than copper.All of the nonmetallic solids that are known to possess high thermal conductivity have either the diamond-like, boron carbide, or graphite crystal structure. There are twelve different diamond-like crystals, a few boron carbide-type crystals, and two graphite structure crystals that have high thermal conductivity. Analyses of the rock-salt, fluorite, quartz, corundum and other structures show no candidates for this class. The four rules for finding crystals with high thermal conductivity are that the crystal should have (1) low atomic mass, (2) strong bonding, (3) simple crystal structure, and (4) low anharmonicity. The prime example of such a solid is diamond, which has the highest known thermal conductivity at 300K.     

蓝宝石Al_2O_3晶体的高压热传导系数研究

对蓝宝石Al2O3晶体在高压下的热传导系数这一在的理论和实验仍处于探索中的问题,本文从较严格的固体理论出发,考虑到晶格声子的最小平均自由程、Debye温度、Gruneisen参数以及冲击温度在冲击压缩状态下的变化,进而对在高压实验中最常用的窗口材料蓝宝石Al2O3晶体的导热系数进行了理论分析,计算结果较好解释了实验结果.     

Isotope effect of protonic conductivity in LiNbO3

Abstract

The electrical DC conductivity [sgrave] of LiNbO₃ :H, D was studied as a function of temperature, electric field and proton/deuteron dopant concentration. The conductivity follows linearly the hydrogen content over two orders of magnitude. Independent holographic measurements on the same samples confirm these results in the temperature range 75 – 150 °C. The activation energy for protonic and deuteronic migration turns out to be equal, E_{act, H} = E_{act, D} = 1.23 ± 0.04eV. The isotope effect of the pre-exponential factor reflects the mass dependence of the attempt frequency and shows the protons to be the migrating species rather than hydroxyl ions.     

Anisotropy of the hopping conductivity in TlGaSe2 single crystals

The temperature dependences of the conductivities parallel and perpendicular to the layers in layered TlGaSe₂ single crystals are investigated in the temperature range from 10 K to 293 K. It is shown that hopping conduction with a variable hopping length among localized states near the Fermi level takes place in TlGaSe₂ single crystals in the low-temperature range, both along and across the layers. Hopping conduction along the layers begins to prevail over conduction in an allowed band only at very low temperatures (10–30 K), whereas hopping conduction across the layers is observed at fairly high temperatures (T?210 K) and spans a broader temperature range. The density of states near the Fermi level is determined, N _F=1.3×10¹⁹eV·cm³)^?1, along with the energy scatter of these states J=0.011 eV and the hopping lengths at various temperatures. The hopping length R along the layers of TlGaSe₂ single crystals increases from 130 Å to 170 Å as the temperature is lowered from 30 K to 10 K. The temperature dependence of the degree of anisotropy of the conductivity of TlGaSe₂ single crystals is investigated.     

Frequency dependance of the conductivity in trigonal selenium single crystals

An analysis of the frequency dependences of a barrier-limited conductivity model is presented. Theoretical results are in good accordance with previous experimental results. At low temperature the model predicts the same variation σ (ω) ∝ ω^s with s < 1 as a hopping conduction process.