THE INFLUENCE OF TEMPERATURE-GRADIENTS ON STRUCTURAL PERFECTION OF SINGLE-CRYSTAL SILLENITE FIBERS GROWN BY THE LHPG METHOD

Single-crystal fibers of Bi12SiO20 and Bi12TiO20 up to 90 mm in length with diameters of 200-1200 mu m were pulled along the [001] and [011] crystallographic axis by the LHPG method. The powder X-ray diffractio n and the electron-probe microanalysis were used for characterization of the grown fibers. The microstructure of sillenite fibers was studie d by the chemical etching method. The dislocations, inclusions and str iations are the main imperfections that were found-in the fibers. The dislocation density equal to 1 X 10(5) pits/cm(2) was estimated on the ground of the etching pits using optical and electronic microscopy an d was compared to that evaluated by the Tsivinskii's relation based on the axial temperature distribution in the fiber and on the elastic pr operties of the material. The conditions of the formation of microdefe cts are discussed in relation with the growth conditions, morphology a nd chemical composition of the grown fibers.