A.M. Stewart

Convecto-diffusive growth of lead bromide crystals: a test of theories

Abstract A comparative study of thermosolutal convection was carried out by growing lead bromide by silver bromide and studied the transparent Bridgeman furnace. We doped the high purity lead bromide by silver bromide and studied the microsegregation of silver in the matrix of the crystal. Theories based on two extremes: complete diffusive or convective transport did not agree with the experimental data. X-ray rocking curves and contour scans showed that increasing solutal convection deteriorated the crystal quality.

Solute partitioning during silicon dendritic web growth

Abstract Measurements of the impurity content of dendritic webs grown from melts containing aluminium, gallium and indium indicate effective solute partition coefficients essentially the same as the interface partition coefficients reported in the literature. This is consistent with the predictions of a simplified model of the partitioning process which indicates that the two segregation coefficients may be of similar magnitude. Web growth is thus almost as effective as Czochralski pulling in its ability to reject impurities to the liquid from the growing crystal.

Growth and characterization of mercurous halide crystals: mercurous bromide system

Abstract Mercurous bromide single crystals were grown by the physical vapor transport method in a sealed quartz tube. Growth rates were measured for crystals growing in different orientations. Experimental data followed the equation l t = l ∞ − A exp(− t / τ ), where l t is the length of the crystal at any time t , l ∞ is the final statio nary length, and A and τ are constants. Crystals growing in 〈001〉 orientation had much higher velocity than crystals growing in 〈110〉 orientation. Large crystals were grown in 〈110〉 orientation by using preoriented seeds, an orientation needed for acousto-optic devices. The optical quality of mercurous bromide crystals was evaluated by fabricating a 5 cm long crystals. The acoustic velocity measurement data showed that the value for the slow shear velocity propagating along 〈110〉 is 2.73 × 10 4 cm/s, consistent with the values reported earlier. This translates with a high value of photelastic coefficient, into a high acousto-optic diffraction efficiency; 2600 with respect to quartz. These results showed that mercurous bromide has potential as an efficient Bragg cell with a delay time greater than 200 μs in a compact package.

Effect of thermal convection on the quality of lead bromide crystals

Abstract Lead bromide crystals were grown from the melt by the Bridgman method. Thermal Rayleigh number was varied three orders of magnitude to study the effect of convection on the quality of crystals. Crystals were characterized by studying X-ray rocking curves, X-ray contour scans, and chemical etchpits. The crystal grown at the lowest thermal Rayleigh number showed highest quality and the crystal grown at the highest thermal Rayleigh number showed worst quality.

Investigation of growth conditions for the liquid phase epitaxy of hexaferrite films using a Bi2O3-BaO-B2O3 flux

Abstract Conditions favoring the liquid phase epitaxy (LPE) growth of barium hexaferrite films have been investigated using the Bi 2 O 3 -BaO-B 2 O 3 flux system. The barium hexaferrite-saturated solutions based on this flux system exhibit relatively large degrees of supercooling (up to 65°C). However, the BaFe 12 O 19 phase field occupies a limited region of the BaO/Fe 2 O 3 /Bi 2 O 3 pseudo ternary phase diagram. LPE films of pure barium hexaferrite and aluminum-substituted hexaferrite were successfully grown on Sr(Ga, Mg, Zr) 12 O 19 substrates using this flux system. Microstructural evaluation of the films by X-ray topography and optical microscopy indicates that smooth, uncracked barium hexaferrite films can be grown on strontium hexagallate substrates from selected compositions in the BaO/Fe 2 O 3 /Bi 2 O 3 ternary system under conditions which minimize lattice mismatch between film and substrate.

The growth of CoGa2O4 single crystals as substrates for the epitaxial growth of ferrites

We report here initial studies to develop a lattice-matched substrate material for epitaxial ferrite growth. Single crystals of CoGa2O4 have succesfully been grown by Czochralski pulling and characterized with respect to lattice dimension, thermal expansion, dielectric constant, and loss tangent. Epitaxial lithium ferrite films have been grown to demonstrate the potential for epitaxy.

Degradation of czochralski silicate oxyapatite crystals by the thermomigration of liquid drops

Abstract The morphology, distribution and composition of the extraneous dielectric phases found in CaLa 4 (SiO 4 ) 3 O laser crystals are consistent with the hypothesis that the inclusions enter the crystals by the thermomigration of silica-enriched liquid drops, not via freezing front breakdown. Inclusions similar in morphology to those found in the crystals were synthesized by heating CaLa 4 (SiO 4 ) 3 O-SiO 2 assemblages. The effective liquid diffusion coefficient D ∗ calculated from the migration velociti es of the synthesized inclusions is in fair agreement with D Si for silicate liquids. By reducing the axial thermal gradient in the growing crystals, inclusions could be minimized, a fact consistent with the proposed model for inclusion formation.

Growth of pure and Ta-substituted Ba3(VO4)2 single crystals

Abstract A novel substrate material for the epitaxial growth of ferrite layers is presented. Single crystals of barium vanadate (Ba 3 (VO 4 ) 2 ) and tantalum-substituted have been grown by the Czochralski growth technique. The crystals were characterized with respect to lattice constant, thermal expansion, hardness, dielectric constant, and loss tangent.

Crystal growth of YBa2Cu3O7 by directional freezing

Abstract The growth of continuous single-crystal filaments of YBa 2 Cu 3 O 7- x within a multiphase, polycrystalline matrix has been achieved by directional solidification of copper-oxide-rich melts. The resulting filaments are oriented with the growth direction in the ab plane. Volume fractions of 30% have been achieved. The electrical and magnetic properties as well as the results of structural analysis are described.

Anomalous growth behavior of GaAs dendritic web

Abstract We have grown single-crystal GaAs ribbons by a liquid-encapsulated dendritic web technique. Undoped GaAs webs exhibited multiple dendrites and heavy surface facets despite careful optimization of the growth system. GaAs ribbons of more conventional web morphology were obtained from Ge-doped melts. The doping results indicate that atypical GaAs web morphology arises from fundamental attachment kinetics which are modified by incorporation of Ge. The observed relationships between facetting and growth behavior of both doped and undoped GaAs webs is consistent with the facetted interface model of web growth.