Melt growth of BSO (Bi₁₂SiO₂₀) : critical issues for growth in a micro-g environment

Abstract

Bismuth Silicate, Bi12Si020, is a body-centered cubic oxide exhibiting non-linear optical preperties which make it a target for many applications including optical information processing. This project is to investigate and resolve some critical issues associated with the Bridgman growth of BSO under the microgravity conditions, which can be divided into three phases according to the subject in each phase. (1) The wetting behavior between BSO and the confinement material, platinum, is critical in a Bridgman type of growth (Chap 3). The sessile drop method combined with a drop shape analysis was used to measure and compute the three-phase contact angles and the surface tensions ofBSO melts under varying partial pressure of oxygen (P02). The contact angle changed from 45° to less than 15° as P02 increased from 104 atm to 10-2 atm, however, the surface tension of BSO melts (- 200 mJ/m2) showed no dependence on P02. Based on the partial wetting conditions, platinum confinements for the growth of BSO with and with out gravity were designed. (2) Thermal characteristics in a Bridgman system, specifically the distribution and the stability of the axial and radial thermal profile within the charge, are critical for the establishment of desirable growth environment (Chap 4 ). In this phase of study, the thermal characterization of a modified vertical Bridgman system for growth of BSO were conducted by measuring the axial and radial temperature distribution in a semitransparent dummy charge made of fused silica which has comparable thern1a] and optical properties as BSO. A modified control method, in which two control thermocouples are placed in the gradient zone, was proved effective in stabilizing the axial thermal gradient. Thermal radiation was found to have a significant role in determining the temperature distribution inside the semitransparent charge. Use of a metallic confmement around the charge was able to reduce the influence of the external thermal radiation, and generated the typical transition of isothermal profile from convex to planar, then to concave along the axial direction in the charge. The effect of the remaining thermal radiation through the metallic shield caused the asymmetry of the axial thermal profile and resulted iri a high position ( close to the hot zone) of the planar isotherm. (3) Three ground-based growth experiments were conducted in this phase using the proposed ampoule design and growth conditions (Chap 5). Single crystals with desirable interface morphology were obtained, which confirmed the effectiveness of the thermal characterization. The position of the interface shifted during the growth due to the end effects of a short charge. The solution to this problem is to use a longer charge and/or to place the two control thermocouples closer to each other. Embrittlement of platinum happened when the ampoule was sealed during the growth. To avoid this problem, a dynamic airflow system should be constructed in the VB furnace, and the ampoule should be left open during the growth process. Interesting solid phase inclusions with unknown identities were found in one VBgrown crystal. The origin of such inclusions, as well as their correlation to the growth conditions, is remained as the subjects for the future work. The support of the National Aeronautics and Space Administration is gratefully acknowledged.