Research On Powder Preparation And Melt Properties Of Bismuth Silicate

Bismuth silicate （BSO） and Bi-contained oxide have many uniqueproperties such as acousto-optic, photoelectric, dielectric, and piezoelectric,which are widely used as luminescent, scintillant and dielectric materials indifferent kinds of industrial fields. In recent years, the aspects such as crystalgrowth, property improvement and powder preparation are mainly studied.However the structures and properties for melt during the process of melting andcooling are less researched.The structures and properties of bismuth silicate melt are important factorsinfluencing the crystal growth, which is significant to understand the mechanismof crystal growth and to improve the growth rate and quality of crystals. Bismuthsilicate powders are prerequisite of preparation for appropriate melt. Therefore, itis very important to study synthesis of bismuth silicate powders for commandingstructures and properties of melt.In this paper the powder preparation and melt properties of bismuth silicatewere mainly studied. By solid phase method and molten salt method, Bi₄Si₃O₁₂powders and Bi₂SiO₅powders were separately prepared. Relevant processparameters were researched. The formation mechanisms were discussed. Themelt properties such as densities and surface tensions and melt structures werestudied. The innovative achievements were obtained:（1） By solid phase method, high purity Bi₄Si₃O₁₂powders were synthesized.Bi₄Si₃O₁₂powders were produced at830℃for3h using Bi₂O₃and SiO₂as rawmaterials after the raw materials were milled for5h in ethanol. Throughanalyzing the relation among primary and secondary factors in uniformexperiment, the calcination temperature had significant effects on the number ofimpure phase, and the rest of the factors were not significant.（2） High purity Bi₄Si₃O₁₂powders were produced in NaCl-KCl molten salts.The microstructures of powders were mainly grains and sheets. And pure Bi₄Si₃O₁₂powders were prepared using NaCl-Na₂SO₄molten salts. The shapesof powders were polyhedrons. High purity Bi₄Si₃O₁₂powders were obtained at780℃for4h by reaction in NaCl-KCl molten salts using Bi₂O₃and SiO₂as rawmaterials with40wt%salts and5wt%excessive content of Bi₂O₃. Excitationspectra and emission spectra of Bi₄Si₃O₁₂powders respectively lay at266nm and457.6nm. Compared to crystalline materials, excitation spectra and emissionspectra showed blue shift. Pure Bi₄Si₃O₁₂powders were synthesized at850℃for3h under40wt%NaCl-Na₂SO₄molten salts, and the best combination of rawmaterials was bismuth oxide and silicon dioxide. The forbidden band width ofBi₄Si₃O₁₂was2.44eV. Excitation spectra and emission spectra of Bi₄Si₃O₁₂powders were separately at270nm and462nm. Compared to crystallinematerials, excitation spectra and emission spectra indicated blue shift. Bycomparing apparent activation energy for synthesizing Bi₄Si₃O₁₂powders underthe two kinds of molten salt systems, Bi₄Si₃O₁₂powders were more easilyformed using NaCl-Na₂SO₄molten salt system. By making a comparisonbetween the purity and microstructures of Bi₄Si₃O₁₂powders by solid phasemethod and by molten salt method. Pure Bi₄Si₃O₁₂powders were more easilyprepared by reaction in molten salts. The shapes and sizes of grains werefurthermore easily controlled.（3） High purity Bi₂SiO₅powders were produced under molten salts（NaCl-KCl and NaCl-Na₂SO₄）. The micrographs of powders were sheets. InNaCl-KCl molten salt system, high purity Bi₂SiO₅powders were obtained at690℃for0.5h. Bi₂O₃and SiO₂with30wt%salts were acted as raw materials.SEM images indicated that powders had preferable dispersity and the sizes ofsheets were about1-4μm. High purity Bi₂SiO₅powders were prepared byreaction at625℃for1h in40wt%NaCl-Na₂SO₄molten salts, and Bi₂O₃andSiO₂were selected as the best combination of raw materials. By comparingapparent activation energy for synthesizing Bi₂SiO₅powders under the two sortsof molten salt systems, Bi₂SiO₅powders were more easily produced byNaCl-Na₂SO₄molten salt system.（4） The melting processes of Bi₁₂SiO₂0polycrystalline powders were insitu observed by hot-stage combined with polarizing microscope. The meltingprocess could divide into three stages: powder melting, phase splitting （metastable phase splitting and unstable phase splitting）, and melt homogenizing.During metastable phase splitting, numerous black punctate structures werewidely distributed in the melt. As the melt temperature rose, the structures wouldbe more and more obvious. During unstable phase splitting, with the change oftemperature, two different types of structures mainly existed in melt. One wasannular interlocking structure. The other was vermiform structure. And duringthe process of phase splitting obvious dividing line between two phases in themelt appeared with temperature movement.