Preparation And Characterization Of Needle-like TiO₂ And Bi₄Ti₃O₁₂ Platelets Synthesized By Molten Salt Method

Molten salt method is a recent developed method to prepare inorganic material powders, and it has specific characters compared to solid-phase, co-precipitation and sol-gel method of powders synthesis methods. As chemical reaction mediums, molten salt provides a fine chemical reaction environment where grains have anisotropic growth velocity. So, molten salt method is widely used for the synthesis of anisotropic growth powders, and is an important method for synthesizing special morphology powders.In this paper, taking the preparation of Bi₄Ti₃O₁₂ platelets and needle-like TiO₂ as the main research work, mixed powders of Al₂O₃ platelets / needle-like TiO₂ and Bi₄Ti₃O₁₂ platelets were prepared respectively. The growth mechanism of grains in the molten salt was discussed.Mixed powders of Al₂O₃ platelets / needle-like TiO₂ were prepared at 1200℃for 5h in NaCl-KCl salt system. The crystalline morphology and the compositions of the needle-like TiO₂ grains were investigated by means ofXRD, SEM, EDS and TEM. The results indicate that there are two kinds of forms for synthesized needle-like crystals; one is solid needle-like, and the other tube-like. In the presence of liquid phase of molten salt, TiO₂ grains grow preferably along z-axis direction and form solid needle-like TiO₂ single crystals finally. Tube-like crystals are formed by the crimp mechanism of board-like grains. This result provides a new application for preparing anisotropic growth grains.Well crystallized tetragonal phase Bi₄Ti₃O₁₂ platelets with the size of about 1-3μm were prepared by using the new CaCl₂-NaCl molten salt system at 650℃for 2h. The phase compositions and crystalline morphology of Bi₄Ti₃O₁₂ platelets were investigated by means of XRD and SEM. The results indicate that the size of platelets increase obviously along with the elevation of treatment temperature, and Bi₄Ti₃O₁₂ platelets is also transformed into orthorhombic phase from tetragonal phase. All Bi₄Ti₃O₁₂ platelets are converted to orthorhombic phase at 750℃, and the maximum platelet can grow up to 10μm. With the increment of the treatment time at 650℃, the phase transition fiom tetragonal phase to orthorhombic phase occurs persistently and Bi₄Ti₃O₁₂ platelets grow up gradually. When the treatment time is 8h, Bi₄Ti₃O₁₂ platelets become the orthorhombic phase and the size of Bi₄Ti₃O₁₂ platelets can reach to 5μm. This synthesis temperature by CaCl₂-NaCl molten salt reduces 200-300℃compared to normal Bi₄Ti₃O₁₂ platelets synthesis temperature. At the same time, this work provides a new low-temperature molten salt system for preparing platelets by molten salt methods.