Preparation And Texture Techology Of Lead-free Na_0.5Bi_0.5TiO₃ Piezoelectric Ceramics

Sodium bismuth titanate based lead-free piezoceramics are regarded as the most promising candidates in place of lead based piezoceramics so far, however, doping modification of existing materials or developing new piezoelectric material system seems not to make any further progress for the improvement of lead-free pieoceramics. As known, the physic characteristic of piezoelectric material determines that the single crystal has the best piezoelectric performance, however, it is hard to prepare a big scale single piezoelectric crystal and also the synthetic cost is very high, thus the application of piezoelectric crystal is limited. Texturing gains in ceramic in purpose of making like-crystal ceramic in structure as well as making like-crystal performance has been attracting interests in material science and technology, also a potential way to improve the lead-free piezoceramic performances. Therefore textured sodium bismuth titanate based lead-free piezoceramics needs to be extensively investigated and thereof the theoretical and application values are significant.In this paper, the current status of lead-free piezoceramics and research progresses are reviewed in the preface. Prepare NBT based ceramic powder using conventional solid state reaction and optimize process parameters for excellent piezoelectric properties. Therein, the NBT ceramic structure, dielectric relax characteristic and phase transfer are analyzed. The ferroelectric properties and piezoelectric properties are also investigated;Based the foundation of conventional solid state reaction, excellent NBBT6 based material system was studied and the piezoelectric properties between those of single crystal and normal ceramic which textured Na_0.5Bi_0.5TiO₃-BaTiO₃-SrTiO₃ (abbreviated as NBT-BT-ST) ceramic was prepared using reacted template gain growth technology with doctor blade casting process. The plate-like SrTiO₃ crystal template with 10-12μm inlength and 2-4μm in thickness can. be fabricated by molten salt method at 1200℃dwelling for 2h. The oriental rate increases with the concentration of SrTiO₃ template and piezoelectric properties enhance accordingly as well. The optimized piezoelectric properties as d₃₃ = 134pC/N, k_t=43%,ε₃₃^T /ε₀=785, tgδ=3.4%,f=0.64 can be obtainedwith 5mol% SrTiO₃ template added but further addition of the SrTiO₃ leads to degrade the performance of the ceramic.By applying new textured technique - high magnetic pulse and gelcasting process, the textured NBBT6 ceramic is prepared successfully. The stable ceramic suspension with high solid content (50vol%) and low viscosity (<1Pa·s) can be prepared when 0.5wt% (PMAA-NH₄) dispersant is added in the slurry. By applying 5T magnetic pulse, the green body has oriental Logtering factor 0.6 was attained and the sintered ceramic has 0.36 accordingly duing to the limit of its essential qualities structure. The dielectric andpiezoelectric properties are as the following, d₃₃ = 151pC/N, k_p=43%,ε₃₃^T/ε₀=763, tgδ= 1.7%。The mean size of ceramic grain is around 2-3μm and the grain has square shape.For enhancing the oriental Logtering factor of textured, the NBT based was fabricated using the high magnetic pulse plus RTGG technology with the plate-like SrTiO₃ (abbreviated as ST) and magnetic material BiFeO₃. The oriental Logtering factor of NBT-BT-ST can reach 0.7 with the plate-like SrTiO₃ template. The anisotropy characteristic applying magnetic field is intensified significantly. The dielectric constant in the direction of⊥applying magnetic field is 4 times of that of // direction. Thecorrespondent properties are as the following,ε₃₃^T /ε₀=900, tgδ=3.9%, d₃₃=123pC/N. As for the textured ceramic with BiFeO₃, the oriental Logtering factor is around 0.59,ε₃₃^T /ε₀ =1200, tgδ=5.3%, d₃₃=108pC/N, the relative density of the ceramic is around 96%.The mechanism of magnetic orientation to NBT based ceramic is analyzed theoretically and the dynamic modeling for the magnetic texture is also proposed. The equation regarding magnetic applying time is gained. showing that the orientation duration is proportional to the viscosity of the ceramic suspension and inversely proportional to the deference of⊥and // susceptibility. The orientation duration also increases with increasing length/diameter ratio. The higher magnetic field help facilitate orientation leading to shorten orientation operation time and enhance the orientation Logtering factor.