Preparation And Grain Size Effect Of （1-x）BiScO₃-xPbTiO₃ Piezoelectric Ceramics

In this project,（1-x） BiScO_3-x PbTiO₃（BSPT）,a new piezoelectric functional material was investigated.The BSPT solid solutions with rhombohedra phase（R） and morphotropic phase boundary（MPB） of pure perovskite were successfully synthesized via traditional solid method and a novel Sol-Gel method.Based on a series of rhombohedra phase specimen with different grain sizes,the grain size effect was discovered.The research showed the Sol-Gel method had its great advantages for high performance BSPT ceramics,since the synthesized BSPT powder was nanosized,homogeneous and activated.The high performance MPB-BSPT ceramics,with piezoelectric coefficient d₃₃=700pC/N,planer electromechanical coupling factor Kp=70%,the Curie temperature Tc=432℃,was prepared with Sol-Gel method and two-step sintering,after a refinement of collosol drying and gel-calcining condition.The results exhibited that the high performace was due to the coexistence of rhombohedra phase and tetragonal phase,meaning an increase of poling directions,leading that domains switched more easily.What’s more,the monoclinic phase formed during polarization process played a very important role as a transition phase between rhombohedra phase and tetragonal phase,contributing to an enhancement of electrical properties.In a word,this field-induced phase transition brought an abnormal behavior in MPB-BSPT specimen.In addition,we found that the phases of specimen decided their grain size distribution to a certain degree.The research showed for MPB-BSPT solid solutions,when the rhombohedra phase accounted for 50-75%of the total,the specimen exhibited high performance;meanwhile its grain size was round about 1.5μm.In this work,the grain size effect of rhombohedra phase BSPT solid solutions was studied preliminarily.The BSPT powder was synthesized by Sol-Gel method,and then treated with different sintering technics,including normal sintering,two-step sintering and spark plasma sintering.Samples whose grain sizes ranging from 0.02μm to 4μm were abtained. Experimental evidence indicated that the piezoelectric coefficient d₃₃ was decreasing with an increasing grain size from 0.02μm to 0.07μm.As the grain size was keeping increasing,the d₃₃ value increased obviously to maximum then decreased slightly.High performance specimen concentrated while the grain sizes were closed to 1.5μm,and their d₃₃ values could achieve to 600pC/N easily.Combined with the characterization of scanning probe microscopy, the relation between grain size,domain size and piezoelectric property was analyzed.Then we discovered that the grain size effect was ascribed to a co-action between domain size and domain density.The relative weight superposition of the two factors decided the piezoelectric performance.It was thought for rhombohedra phase BSPT solid solutions,when the grain size was near to 1.5μm,its domain size and domain density fitted best,leading to the best performance.