Preparation Of Alkali Niobate Powder And Ceramics By Molten Salt Synthesis

Alkaline metal niobate lead-free piezoelectric ceramics have been considered as a potential alternatives to traditional PZT ceramics due to their excellent properties such as high polarization strength,high piezoelectric property,high frequency constant and high electromechanical coupling coefficient,etc.In this paper,perovskite and tungsten bronze compounds in alkaline metal niobate system were selected as research objects.Alkaline metal niobate powder were synthesized by molten salt method.Then,obtained powder were used as raw materials to prepare lead-free piezoelectric ceramics.The effects of process parameters on the sintering behavior,microstructure and electrical properties of the prepared ceramics were investigated.Firstly,needle-like KSr₂Nb₅O₁₅microcrystals with tetragonal tungsten bronze structure were synthesized by a modified molten salt synthesis method,the effects of pressure and calcination temperature on the chemical composition and morphology of obtained KSr₂Nb₅O₁₅microcrystals were analyzed.It was found that with the pressure and temperature increased,more Sr²⁺tended to occupy vacant A sites and the particle size become uneven.Increasing pressure benefited to the formation of slender particles,while raising calcination temperature could promote the particles growth along diameter direction.The obtained particles showed high aspect ratio,and their chemical composition was close to the KSN stoichiometric ratio in case of the pressure was 4MPa and the calcination temperature was 1300℃.Secondly,the columnar Na₂Nb₈O₂₁ microcrystalline of orthogonal tungsten bronze structure with good dispersion were synthesized by molten salt synthesis method,the effects of salt content on the crystal structure and morphology of obtained powder were investigated.The results showed that when the molten salt content was 6 times that of other raw materials（Sr CO₃+Nb₂O₅）,the number of long particles in the system was the largest.Then,Na₂Nb₈O₂₁ceramics with high transparency were prepared using obtained powder as raw materials.All specimens showed well thermal stability of dielectric constant over a broad temperature range（85℃～260℃）.Finally,column Na NbO₃-based particles with perovskite structure were synthesized through topochemical microcrystal conversion using needle-like crystallites as precursors.The synthesized particles exhibited uniform size distribution and high aspect ratio（10～15）.The K_0.5Na_0.5NbO₃ powder synthesized by coventional mixed oxide method.Then,alkaline metal niobate ceramics with perovskite structure were prepared by a brush tape-casting technique using K_0.5Na_0.5NbO₃ powder as the matrix,and column Na NbO₃ crystallites as the template.The effect of template content on sintering behavior,microstructure and electrical properties of prepared ceramics were investigated.It was found that with the increase of template content,the sintering temperature of ceramics increases gradually.The addition of template promoted the densification of ceramics.When the template content was 40%,the relative density of the ceramics reached 91.09%,the piezoelectric constant d₃₃ reached 101p C/N,and the Curie temperature T_c was 343.8℃.