William R. Wilcox

Surface tension and contact angle of molten semiconductor compounds: I. cadmium telluride

Abstract The surface tension and contact angle of molten CdTe were measured by the sessile drop technique. The dependence of surface tension and contact angle on temperature and on the deviations from stoichiometry was determined. A computer program was used to calculate the properties from the sessile drop profile. The wetting behaviour of the melt was studied on the following surfaces: quartz, HF-etched quartz, sandblasted quartz, carbon-coated quartz and pyrolytic boron nitride. The surface tension of molten CdTe decreased with increasing temperature, but increased slightly with excess Cd. The degree of wetting increased in the following order: pyrolytic boron nitride, carbon-coated quartz, sandblasted quartz, HF-etched quartz and plain quartz. The contact angle decreased with increasing temperature and seemed to increase slightly with increasing Cd.

Effect of convection on the microstructure of the MnBi/Bi eutectic

Abstract At freezing rates of 9 mm/h and above the MnBi formed quasi-regular fibers with a spacing λ that obeyed the equation λV 0.5 = 6.26 + 0.000112( RN 1.5 / V ) 1.1 , where λ is in μm, V is freezing rate in cm/h, R is radial position in mm, and N is the rotation rate of the ampoule in RPM with spin-up/spin-down (accelerated crucible rotation). This agrees well with theoretical predictions (Chandrasekhar, Eisa and Wilcox, J. Crystal Growth, submitted). At freezing rates of 4.8 mm/h and below the MnBi formed irregular blades. Their spacing increased with convection somewhat more than predicted by theory. The volume fraction of MnBi also depended on ampoule rotation and on radial position, even in the absence of ampoule rotation. Elimination of buoyancy-driven natural convection is insufficient to explain the two-fold finer microstructure observed in space-processed material.

Convection in the vertical Bridgman-Stockbarger technique

The influence of operating parameters on convection in the Bridgman-Stockbarger technique was studied with a transparent furnace and melt. Convection was nearly nonexistent near the interface in the vertically stabilized configuration (temperature increasing with height). With the addition of a short booster heater between the main heater and the cooler, a destabilizing temperature gradient (temperature decreasing with height) was produced in the melt near the interface, and significant convection was produced throughout the entire melt. Convective velocities increased and flow symmetry improved as power to the booster heater was increased.

Influence of convection on lamellar spacing of eutectics

Abstract A previous paper [J. Crystal Growth 67 (1984) 343] has been extended to more intence convection and to eutectic compositions of 10% and 30%. The trends observed previously were confirmed, i.e. vigorous convection is predicted to increase the lamellar spacing, especially at low freezing rates. A new expression was found for the interfacial undercooling, which is predicted to decrease linearly with convective stirring, especially at low freezing rates.

Surface tension and contact angle of molten semiconductor compounds: II. gallium arsenide

Abstract The surface tension and contact angle of molten GaAs were measured by the sessile drop technique. The dependence of surface tension and contact angle on temperature and on the deviations from stoichiometry was determined. A computer program was used to calculate the properties from the sessile drop profile. The wetting behavior of the melt was studied on the following surfaces: quartz, sandblasted quartz, carbon-coated quartz and pyrolytic boron nitride. The surface tension of molten GaAs decreased with increasing temperature, and decreased slightly with excess As. The degree of wetting increased in the following order: pyrolytic boron nitride, sandblasted quartz, carbon-coated quartz and plain quartz. The contact angle decreased with increasing temperature and seemed to decrease slightly with increasing As.

Modelling the growth of triglycine sulphate crystals in Spacelab 3

Two triglycine sulphate crystals were grown from an aqueous solution in Spacelab 3 aboard a Space Shuttle. Using a diffusion coefficient of 2×10-5 cm2/s, a computer simulation gave reasonable agreement between experimental and theoretical crystal sizes and interferometric lines in the solution near the growing crystal. This diffusion coefficient is larger than most measured values, possibly due to fluctuating accelerations on the order of 10-3 g (Earths gravity). The average acceleration was estimated to be less than 10-6 g. At this level buoyancy-driven convection is predicted to add approximately 20% to the steady-state growth rate. Only very slight distortion of the interferometric lines was observed at the end of a 33 h run. It is suggested that the time to reach steady state convective transport may be inversely proportional to g, so that the full effect of convection was not realized in these experiments.

Diffusional decay of striations

During crystal growth from melts and solutions, compositional striations in the bulk crystal parallel to the growth surface usually result from fluctuations in the growth rate and/or the flow of the growth fluid. This paper presents a one-dimensional analysis of the effect of solid-state diffusion on growth striations. A sinusoidal variation in the solid composition at the interface of a semi-infinite ingot growing at constant velocity is assumed and the damping behavior of this compositional fluctuation is followed as it moves away from the solid-liquid interface. Numerical results that include the effect of a temperature gradient in the solid are presented as well as an analytical solution that assumes that the solid is isothermal. The only parameter in the analytical solution is the dimensionless frequency ω=DV2tc. The dimensionless distance xd for 99.9% decay decreases with increasing ω. The relationship between xd and ω has two distinct regions. For ω 200, xd is proportional to √Dtc. The rate of crystal growth is negligible over this range of ω and the striations are spread into the solid by diffusion. An increase in the diffusivity causes the striations to penetrate deeper into the solid and, consequently, xd to increase. In general, the application of the ACRT to the vertical Bridgman-Stockbarger growth of concentrated solid solutions is not likely to lead to striations being frozen into the grown ingot. Analytical results are also given for the decay of striations in an isothermal ingot induced by a sinusoidal fluctuation in crystal growth rate. These striations decay by the same mechanisms as the sinusoidal striations discussed above. With the proper choice of dimensionless concentration, they decay in a dimensionless distance which is a maximum of 5% less than sinusoidal striations for the same value of ω.

Kinetics of silicate reaction with gibbsite

The reaction of silicate with gibbsite in sodium aluminate solution was studied. Sorption kinetics indicated that sorption of silicate was surface-induced. Silicate is believed to have adsorbed and precipitated as sodium aluminum silicate (SAS) on gibbsite by a three-step reaction; a first fast step, a lag period step, and a slow step. The fast and slow steps conformed to first-order rate laws with respect to silicate concentration. The extent of sorption depended on silicate concentration, suspension concentration and temperature. Further, silicate increased the equilibrium solubility of gibbsite in sodium aluminate solution. For a silicate concentration of 500 μgSiO 2 /cm 3 , adsorption/precipitation accounted for 86% of the reduction in gibbsite crystallization, while the remaining 14% was due to increased solubility of gibbsite. Thus, our adsorption—precipitation observations confirm the postulate that the reduction in gibbsite crystallization in the Bayer process is due mainly to the adsorption of silicate on gibbsite seed crystals.

Sorption and desorption isotherms for silicate on gibbsite

Sorption and desorption of silicate on gibbsite were investigated. The sorption isotherm was complex, and consisted of Langmuir behavior at low silicate concentrations and Freundlich behavior at high silicate concentrations. The transition region was characterized by interaction between adsorbed silicate ions and multilayer adsorption or cluster formation; this corresponds to the onset of formation of a sodium aluminum silicate phase. The equilibrium sorption isotherm data reported here corroborate the scheme proposed to explain the sorption kinetics results reported elsewhere. The desorption data conformed to a Freundlich isotherm. In addition, desorption of silicate decreased with increasing silicate concentration in the solution, and reached a constant value at a solution concentration of 80 μgSiO2/cm3 and beyond. Desorption was larger at lower temperatures.

Influence of convection on spiral structures in lead-tin eutectic

Convection was generated during directional solidification by the accelerated crucible rotation technique (spin up/spin down). This stirring increased the rotation rate of the microstructure and reduced the length solidifying cooperatively, but had no influence on the lamellar spacing.