Dislocation Density In Crystals Research Articles

Effect of impurity atom size on grown-in dislocation density of crystals of III–V compounds

Abstract A study of the literature on reduction of grown-in dislocation density in crystals of III-V compounds by heavy doping with various impurities has uncovered errors and probable misinterpretation. On re-evaluation of the available data from the viewpoint of atomic size, a rather consistent picture has been obtained. Impurity atoms smaller than the host atoms they displace bring about a major reduction in etch pit density while those larger than the host atoms also have a beneficial effect, though of a lesser magnitude. The new interpretation leads to a suggested method for preparation of dislocation-free and precipitate-free crystals with a lower impurity concentration.

Thermal Properties of Dislocated Lithium Fluoride Crystals

Experimental results previously obtained by the authors on low-temperature thermal properties of lithium fluoride crystals containing dislocations have been reanalyzed. Specific heat increase by the introduced dislocations was well explained when Debye-type extra quasilocal phonons were anticipated. The phonons spatially localized in a rather extended region around each dislocation line. The number of the quasilocal modes increased with the density of dislocations in crystals. Thermal diffusivity data were analyzed by taking into account both the scattering of conducting thermal phonons by dynamical effect of dislocations and the scattering by quasilocal phonons around dislocations. The quasilocal phonon scattering increased in proportion to the dislocation density, while the dynamical scattering increased at a lower rate. Analysis of thermal conductivity data showed that the apparent Debye temperature of crystal was decreased, namely, specific heat increased, by the introduction of dislocations. The resu...

Kinetic effects of particle-size and crystal dislocation density on the dichromate leaching of chalcopyrite

Relatively pure, polycrystalline samples of Golden, New Mexico and Transvaal, South Africa chalcopyrite were ground and polished to fit into a density-compatible titanium alloy sandwich assembly and shock loaded (the Golden at 1.2 GPa, the Transvaal at 18 GPa peak pressure). The recovered samples were examined in the transmission electron microscope and observed to contain roughly 103 and 104 times more dislocations respectively than the natural, unshocked materials which contained roughly 107 cm−2. These materials were ground to three size fractions and leached in an acid-dichromate lixiviant at 50 and 70 °C (323 and 343 K). Corresponding size fractions were prepared from the unshocked material and leached for comparison; and these experiments were extended to a range of temperatures up to 90 °C (363 K) using a Bingham Canyon, Utah chalcopyrite. An unambiguous effect of dislocations was observed at the largest size fraction leached at 50 °C (323 K): the reaction rate constant increased from 1.44 × 10−3 to 1.62 × 10−3 to 2.73 × 10−3 min−1 for dislocation densities increasing from roughly 103 cm−2 to 1011 cm−2. A linear relationship in the leaching rate was observed between 25 and 60 °C (298 and 333 K) and the activation energy was calculated to be approximately 12 kcal/mol (50 kJ/mol). Above 60 °C (333 K) a conspicuous rate decrease was observed. This was related to a densification of the sulfur product layer, which at 50 °C (323 K) was a loose, porous precipitate while at 90 °C (363 K) it was a continuous, tenacious, glassy coating on the chalcopyrite particles. In addition, and more importantly, the high-temperature decline in leaching rate was inferred to be associated with adsorption of Cr(VI) ion which decreases with increasing temperature. The anomalous and unpredictable behavior of chalcopyrite leached in acid-dichromate lixiviant was therefore observed to be related to variations in a surface chemisorption mechanism as well as variations in the nature of the sulfur reaction product layer. These effects also mask the influence of crystal defects and other surface-related crystallographic effects in leaching.

Structure perfection of some molecular crystals grown from the melt

AbstractSingle crystals of anthracene, naphthalene, phenazine and anthracene sym. trinitrobenzene (TNB) were examined using X‐ray topography. Low‐angle boundaries were observed and misorientation vectors of adjacent mosaic blocks have been estimated. Using an X‐ray camera with oscillating film dislocation density in crystals has been evaluated. Anthracene‐TNB crystals showed a much higher structure perfection than the other crystals.

Defect characterization of mixed SrCa tartrates

AbstractA few defect characteristics which are typical of gel‐grown mixed single crystals of SrCa tartrates (Sr1−XCaxC4H4O6 · 4H2O) having different concentration, x, of the cations have been described. The temperature of decomposition has been found to be related, to some extent, to dislocation concentration. The magnitude of configurational entropy, which is a manifestation of lattice disorder, has been computed. The Vickers microhardness and dislocation density of crystals have been found to bear good correlation. An empirical formula has been suggested to explain the observed variation in microhardness with the concentration of ions in the mixed lattice.

Dependence of subgrain dimensions and disorientation on dislocation density in crystals grown by the bridgman, kyropoulos, czochralski or stepanov method

AbstractThe semiempirical relations earlier obtained for estimation of the dimensions of nucleating subgrains and their disorientation, and the equation for evaluation of disorientation of more coarse subgrains (micro‐ and macrosubgrains) have been tested experimentally on various materials (Zn, Pb, Sb, Bi, Ge, KCl, KBr, NaCl, LiF). If has been shown that the above parameters can be calculated in terms of the dislocation density in a crystal with an error of a factor of 1–4.

Shear Stresses in a Notched Circular Disk

In the dicing of a silicon disk into separate integrated circuit chips, an alignment notch is commonly used to maintain the disk in a precise and stable position throughout the process. This notch is formed in a grinding operation which causes mechanical damage to the crystal structure in the vicinity of the notch. The increase in crystal dislocation density will degrade the performance of any circuit chips located in the damaged area. Hence, it is important to learn the extent of damage in the disk so that the affected region can be avoided during the production of chips.

Dislocation densities in slowly cooled aluminium single crystals

Abstract The dislocation density of slowly cooled, zone-refined aluminium single crystals, made by the strain-anneal method, has been studied, using Lang's x-ray diffraction topography technique. Three supplies of aluminium were used. The dislocation density in crystals from two of the supplies was found to be approximately linear to the cooling rate, and it was 2 × 102 lines per cm2 for the slowest cooling rate, 2°c per hour. Three crystals were re-annealed, heated and cooled at a rate of 4°C per hour with maximum temperature 510°c, and topographs of these crystals reveal both increases and decreases in dislocation density.