Preparation And Properties Of Bismuth Layer Structured Hightemperature Piezoelectric Ceramics

Bismuth Layer Structured Ferroelectrics（BLSFs）piezoelectric ceramics have the characteristics of high Curie temperature（T_C>400°C）,excellent fatigue resistance and high temperature stability.It is the core component of high-temperature piezoelectric vibration sensors.The first choice of ceramics has important application prospects in aerospace,nuclear power generation and other fields.Among them,bismuth titanate(Bi₄Ti₃O₁₂,BIT)is a typical BLSFs type piezoelectric ceramic material.Its Curie temperature T_C is as high as 675°C,and it has good fatigue resistance.It is one of the important candidate materials for piezoelectric applications at high temperatures.However,BIT piezoelectric ceramics have some shortcomings.For example,the piezoelectric coefficient d₃₃（<8 p C/N）is low,and the dielectric loss is high;at the same time,Bi vaporizes during the high-temperature sintering process to cause oxygen vacancies,which leads to BIT piezoelectric ceramics.The conductivityσis high(σ～10^-8 S/cm at 200°C).These shortcomings greatly limit its practical application in the high temperature field.At the same time,due to its special crystal structure,the unit cell parameter c/a reaches about 6,and it is easy to form obvious flaky crystal grains,so it has obvious anisotropy,and the spontaneous polarization of BIT is mainly located in the direction of the a-b plane,While the current conventional solid phase preparation of BIT piezoelectric ceramics,its crystal grain orientation is randomly arranged.Aiming at the above-mentioned problems,this thesis systematically studied the important influence of flaky grains on ceramic orientation and the influence of donor ion doping on the electrical properties of ceramics from the aspects of preparation process and composition design.The bit mechanism has been studied.The main research progress is as follows:（1）Using molten salt method under different temperature synthesis conditions,a powder with obvious flaky crystal grain morphology was obtained,and the growth process of flaky crystal grains in molten salt environment was clarified;further,by molten salt method A new texture process of powder combined with pressureless sintering was prepared,and a ceramic sample with a preferred orientation with an orientation degree f as high as 0.83 was prepared.The system reveals the source of the molten salt method combined with pressureless sintering to obtain a high degree of orientation:○1 Uniaxial pressure during the molding process is the primary driving force for the preferred orientation of grains;○2 During pressureless sintering,the growth of grains under high temperature environments It is an important factor to further increase the degree of orientation.According to this theory,based on preparing powder by the molten salt method,BIT piezoelectric ceramic materials with different orientation degrees can be obtained by adjusting the molding pressure.（2）Using the above-mentioned molten salt method combined with the new texture process of pressureless sintering,W⁶⁺-BIT piezoelectric ceramics with high orientation and high electrical properties are prepared.Among them,Bi₄Ti_2.94W_0.06O₁₂ ceramic has an orientation degree f of 0.83,and its vertical piezoelectric coefficient d₃₃ has reached30 p C/N,which is 3 times that of its parallel direction and 4 times that of pure BIT piezoelectric ceramics;Ceramics have obvious anisotropy in electrical properties.（3）The W⁶⁺-BIT series ceramic samples were prepared by solid-phase method,and the occupation mechanism of W⁶⁺ion doping in W⁶⁺-BIT and the piezoelectric activity enhancement mechanism were studied.The replacement of Ti⁴⁺ions by W⁶⁺ions can effectively reduce oxygen vacancies and increase electrical resistance.At the same time,its piezoelectric properties have been significantly improved.Among them,the Curie temperature of Bi₄Ti_2.98W_0.02O₁₂ is above 650°C,and the piezoelectric coefficient d₃₃ reaches 28 p C/N.Exploring the crystal structure of BIT found that there are two different occupancy Ti atoms in the structure,labeled Ti₁ and Ti₂.It is believed that the replacement of Ti₁ by W⁶⁺ions cause the tilt of the oxygen octahedron,which is the main reason for the enhancement of piezoelectric activity.