Synthesis Of Bismuth Silicate Powders By Molten Salt Method

Bismuth silicate is a very important functional material with uniqueproperties such as piezoelectric, electro-optic, magneto-optic, acousto-optic,pyroelectric and dielectric properties. Therefore bismuth silicate is a potentialmaterial for piezoelectric semiconductor, real-time optical informationprocessing, optical signal processing, optical computing, optical spatial lightmodulator, catalyst, and radiation detection. As we know three compounds,Bi¹²SiO₂₀, Bi₄Si₃O₁₂, Bi₂SiO₅have been discovered in the binary system ofBi₂O₃-SiO₂. Preparation of pure bismuth silicate single crystal usually needspure bismuth silicate powders as raw materials.In this study, Bi¹²SiO₂₀and Bi₂SiO₅powders are synthesized by molten saltmethod. The samples obtained are characterized by means of X-ray diffraction,scanning electron microscope and UV-Vis spectra, and the effects of theprocessing parameters, such as types of salts, relative amount of salt, reactiontemperature, holding time and cooling rate on the phase composition andmorphology are investigated, and the synthesis mechanisms are discussed basedon the experimental results.Pure Bi¹²SiO₂₀powders are successfully synthesized in KCl-K₂CO₃fluxusing Bi₂O₃and SiO₂powders as raw materials at635°C for1h. The XRD andSEM results show that reaction temperature, holding time, relative amount ofsalt, cooling rate and ratio of Bi₂O₃/SiO₂have no significant influence on theirphase composition but have significant influence on their morphology. TheKCl-K₂CO₃molten salt system is not only used as the flux and reaction medium,but also inhibits the formation of other bismuth silicates in the synthesis ofBi¹²SiO₂₀powders. UV-Vis spectra reveals the absorption for the sample locatesat ca.440nm. Compared with Bi¹²SiO₂₀single crystal, the absorption edge isshifted to the longer wavelengths, which lies at390nm.The Bi₂SiO₅powders are synthesized in the study using the same rawmaterials of Bi₂O₃and SiO₂powders by molten salt method in KCl-NaCl flux. The results of the XRD examinations indicate that the products obtained inKCl-NaCl flux contain Bi₂SiO₅as the main phase and Bi₄Si₃O₁₂and Bi¹²SiO₂₀as the impurities. With increasing synthesis temperature or holding time,metastable state Bi₂SiO₅transformed into stable state Bi₄Si₃O₁₂gradually. Atlow reaction temperature （650°C）, the content of Bi₂SiO₅increased withincreasing relative amount of salt. However, at high reaction temperature（690°C）, too lower or too higher relative amount of salt is not conducive toprecipitation of metastable state Bi₂SiO₅. In excess of Bi₂O₃or SiO₂is also notconducive to precipitation of metastable phase Bi₂SiO₅. The results of the SEMexaminations indicate that reaction temperature, holding time and relativeamount of salt affect the morphology of the products significantly.Pure Bi₂SiO₅powders are successfully synthesized in NaCl-Na2SO4flux at625°C for1h. The SEM images disclose that the Bi₂SiO₅samples showed nestlike morphology, which is soft aggregation of Bi₂SiO₅nanosheets. The results ofthe XRD examinations indicate metastable state Bi₂SiO₅transformed into stablestate Bi₄Si₃O₁₂gradually with increasing reaction temperature or holding time.Too lower or too higher relative amount of salt is not conducive to precipitationof metastable state Bi₂SiO₅at660°C. The SEM results show that reactiontemperature, holding time and relative amount of salt have significant effects onthe morphology of the products.The results of the study show that the growth process of Bi¹²SiO₂₀andBi₂SiO₅powders are controlled by dissolution-precipitation mechanism withmolten salt method. Molten salt method is a promising method to preparebismuth silicate powders. Pure Bi¹²SiO₂₀and Bi₂SiO₅powders are obtained nearthe eutectic of the chosen molten salt systems.