Senlin Fu

Solid-phase reaction in synthesis of Bi12SiO20 source-rods for single-crystal growth in a floating zone

Abstract The solid-phase reaction and the concomitant phase transitions in the synthesis of the source rod for Bi 12 SiO 20 single-crystal growth in a floating zone were studied by X-ray diffraction, differential scanning calorimeter (DSC), differential thermal analysis (DTA), and thermogravimetry (TG). The following experimental phenomena were observed. (1) The solid phase reaction (SRI), i.e., 6α-Bi 2 O 3 (monoclinic) + SiO 2 (hexagonal) → γ-Bi 12 SiO 20 (b.c.c.), appears to be a diffusion-controlled process, and the reaction is affected by both temperature and time. (2) A metastable phase, Bi 2 SiO 5 (tetragonal), may be formed in the synthesis of γ-Bi 12 SiO 20 source rod if the reaction temperature is over 500 °C. The Bi 2 SiO 5 can be caused to react with the remaining Bi 2 O 3 to form pure γ-Bi 12 SiO 20 by increasing the aging temperature or through crystal growth of Bi 12 SiO 20 by a floating zone method. However, the Bi 2 SiO 5 intermediate greatly accelerates the solid-phase reaction SR1. Thus, different reaction mechanisms are involved depending on whether Bi 2 SiO 5 is formed. At below 500 °C (without Bi 2 SiO 5 ), the reaction order of the solid-phase reaction SR1 is 3.5; and at above 500 °C (with Bi 2 SiO 5 ) it is 3.0. (3) The quality of the sintered source rod is directly dependent on the sintering method used, i.e., the reaction rate. The suitable multi-sintering process proposed in this paper proved to be useful for controlling the reaction rate and avoiding cracking of the sintered source rod.

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

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 transp...

Growth of Bi12GeO20 crystal rods and fibers by the improved floating zone method

Single-crystal rods and fibers of Bi12GeO20 (BGO) of various sizes were successfully grown from a rod of mixed Bi2O3 and GeO2 by the improved floating zone device. The grown stoichiometric BGO crystal has a lattice constant of 10.14388±0.00001 Å, a uniform composition along the axial position. For a nonstoichiometric crystal, its lattice constant decreases and its transmittance increase with the concentration of GeO2 up to the stoichiometry.

Enhancement of growth rate for BSO crystals by improving thermal conditions

Effects of various thermal conditions on growth of single crystal of Bi{sub 12}SiO{sub 20} (BSO) with a floating zone method were systematically investigated. The following are the main results: (1) Change of heating source from an infrared heater to a CO{sub 2}-laser system could have increased the temperature gradient in the grown crystal near the solid-liquid interface about 3 to 4 times, which gives rise to a dramatic increase of the critical transparent growth rate more than 3 times. (2) A single crystal seed with high thermal conductivity (e.g., Al{sub 2}O{sub 3}) can dramatically increase the critical transparent growth rates. (3) For a source rod of the multi-sintered powder rod, an increase of growth rate from 18.8 mm/h to 100 mm/h can decrease the diameter fluctuation of the grown crystal from 2--3%/cm to less than 0.5%/cm.

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

Single-crystal rods and fibres of of various sizes oriented along [100] direction were successfully produced by a floating-zone method. Each of these crystals has a purely orthorhombic structure and a clear Curie transition at (with a peak at ). Investigation of the growth characteristics revealed that the vaporization of is dependent on the quality of the source rod, the growth rate and the length of the molten zone. For a source rod of pure , a growth rate of and a sufficiently short molten zone (for example 1.7 - 2.0 mm for growth of a 2.8 - 3.2 mm diameter crystal), a good (transparent, without cracks or inclusions) single crystal can be produced directly from the stoichiometric source rod. Otherwise, a source rod with excess is necessary to compensate for the vaporization of during the growth process. The excess amount of required is dependent on the sintered state of the source rod, the growth rate and the phase structures of the original powders from which the source rod had been pressed. In general, an increase in growth rate or in sintering temperature can decrease the required excess amount of in the source rod. However, the excess amount of required can be dramatically decreased by making the source rod from and .

Solidification characteristics of metastable and stable

Solidification characteristics of metastable δ-Bi 12 SiO 20 and stable γ-Bi 12 SiO 20 from Bi 12 SiO 20 melts were systematically investigated by x-ray diffraction, differential thermal analysis and thermogravimetry. The experimental results show that the solidification either of metastable δ-Bi 12 SiO 20 or of stable γ-Bi 12 SiO 20 directly from Bi 12 SiO 20 melts appears to be dependent both on the melt temperature and on the cooling rate. In general, high melt temperatures and high cooling rates tend to solidify metastable δ-Bi 12 SiO 20 . Otherwise, low melt temperatures and low cooling rates tend to solidify stable γ-Bi 12 SiO 20 . To produce stable single-crystal γ-Bi 12 SiO 20 , the melt temperature should be controlled to be less than 935°C and the cooling rate near the solidifying temperature should be less than 30°C s -1 .

Growth crystallography of Bi12SiO20 single crystals solidified by a floating zone method

Relations between the growth directions of Bi12SiO20 (BSO) single-crystal rods solidified by a floating zone method and the pulling parameters were studied using an X-ray back-reflection Laue technique. It was found that when a platinum wire is used as a seed, the growth direction of the produced BSO single-crystal rod is related to the pulling rate. Statistically, the probability of the growth direction approaching 〈0 1 1〉, 〈1 1 2〉 or 〈0 0 1〉 appears to increase in this order with increase of the pulling rate. In addition when a BSO crystal is used as a seed, the growth direction of the produced BSO crystal rod has the same orientation as the seed crystal if the pulling rate is less than 30 mm h-1. If the pulling rate is higher than 30 mm h-1, the growth appears to incline mostly to 〈1 1 2〉 if the seed orientation is near 〈0 1 1〉 or 〈1 1 1〉 but far from 〈0 0 1〉, or to 〈0 0 1〉 if the seed orientation is near 〈0 0 1〉 but far from both 〈0 1 1〉 and 〈1 1 1〉. The angle of inclination increases with the pulling rate, and also with the difference in angle between the orientation of seed crystal and 〈1 1 2〉 or 〈0 0 1〉. The facet morphology of the BSO single-crystal rod is related to its growth direction. The cross-section of the BSO single-crystal rod grown along 〈0 0 1〉 is an octagon with tetrad-rotational symmetry, that grown along 〈0 1 1〉 is an ellipsoid with diad-rotational symmetry, and that along 〈1 1 1〉 is a hexagon with triad-rotational symmetry. The cross-sections of the BSO single-crystal rods grown in other directions are not regular, because there is no clear symmetry.