Growth characteristics of single-crystal rods and fibers of Bi12SiO20 by the floating zone method

Abstract

Single-crystal rods and fibers of Bi12SiO20 were grown directly from a rod of mixed Bi2O3 and SiO2 powders in a floating zone device heated by an infrared source with a special light shutter. The source rod was pressed from the mixed powders at room temperature; to avoid possible contamination, hot pressing and melting with die or crucible were not employed. The following are the main results: (a) The length of the stable molten zone is an increasing function of the diameter of the grown crystal rod. (b) Non-transparency is a major defect in a small diameter crystal rod or fiber. The growth velocity should be less than the critical transparent velocity (the critical value below which the grown crystal is transparent throughout). For growth of a large diameter crystal rod (diameter ≥3 mm), the growth velocity should be less than both the critical transparent velocity and the critical cracking velocity (the critical value below which the grown crystal is free of cracks). In general, both the critical transparent and critical cracking velocities are decreasing functions of the diameter of the grown crystal rod. (c) Use of a single crystal with high thermal conductivity (e.g., Al2O3) as a seed can dramatically increase both the critical transparent and critical cracking velocities for growth of a single crystal of diameter larger than 3 mm. (d) The pulling down floating zone method is more suitable than the pedestal growth method because of the low surface tension and high density of the Bi12SiO20 melt. (e) The grown Bi12SiO20 crystal is a pure body-centered cubic γ phase and has a good infrared transmission spectrum.

The growth and characterization of single crystals using a floating-zone method

“…A normal FZ device was employed to grow a smalldiameter crystal of diameter ≤ 2 mm and an improved one of diameter ≥ 2 mm. The moving and rotating devices for each are the same as that described in [16].…”

Reaction Pathways in the Synthesis of Photorefractive gamma‐Bi12MO20 (M = Si, Ge, or Ti)

“…7(b), because the density of the source rod is only 50% of that of its crystal. 16 This may result in a sudden increase in temperature in the area of reaction, if the reaction rate is sufficiently high. On the other hand, the solid-state reaction is diffusion controlled and has a large dependence on temperature.…”

“…On the other hand, a floating zone method does not produce such deficiencies and it offers the possibility of growing singlecrystal fibers. 16 It is usual in this method to sinter a raw source rod that has been pressed directly from commercially available powders (not ultrafine powders 17 ) without any chemical binders. The reaction rate of the reaction 6Bi 2 O 3 + SiO 2 → ␥-Bi 12 SiO 20 (SR1) seems to have a great effect on the quality of the sintered source rod, which ultimately affects the property of the grown BSO crystal.…”

“…An increase in the concentration of ␥-Bi 12 TiO 20 may be expected from the increase in the sintering time. For example, the concentrations of ␥-Bi 12 TiO 20 in the source rods that were sintered at 618°C for 53, 150, 300, and 800 h were 53.01, 65.82, 78.66, and 83 16.…”

Effect of the shouldering angle on the shape of the solid–liquid interface and temperature fields in sillenite-type crystals growth

“…During shoulder formation, there exists strong convective fluxes directed to the crystal center, which gives an intense convex interface. As it has been indicated, a convex SLI is unfailingly accompanied by the occurrence of the ''dark core'' in Cz growing of sillenite-type crystals [9]. Lin and Motakeff [10] show numerically that the large convexity of the SLI at the initial stage of growth is related to the specular (Fresnel) reflection at the shoulder surface, which leads to strongly non-uniform radiant heat flux distribution over the SLI.…”