Study On Co-doping Modification Of CaBi₄Ti₄O₁₅-based High Temperature Piezoelectric Ceramics

Piezoelectric ceramics are important functional materials which can convert electrical energy and mechanical energy and have been extensively used for communication,computer and other electronic fields.At present,lead zirconate titanate（PZT）plays a critical role in the piezoelectric ceramic.However,one major drawback of PZT ceramic is that they contain lead oxide,which cause irreparable damage to the environment and humans.Moreover,one reason why PZT can not be used in rocket engine,oil drill exploration,nuclear energy and other environments is that the Curie temperature of PZT is low.Therefore,the aim of research is to explore a lead-free piezoelectric material with high Curie temperature in order to promote the usability of the device.In the reported material system,CaBi₄Ti₄O₁₅ has been thought of as a key material in high temperature piezoelectric ceramics owing to the 790 °C Curie temperature.However,its piezoelectric coefficient d33 is very low（generally not more than 10 p C/N）.The purpose of this thesis is to obtain the high piezoelectric activity ceramics via ion doping.In this thesis,CaBi₄Ti₄O₁₅ based piezoelectric ceramics are prepared by solid state reaction.The effects of co-doping of rare earth ion at Bi site and high-low-valent cation co-doping at Ti site on the microstructure,piezoelectric,dielectric and ferroelectric properties of CaBi₄Ti₄O₁₅ based ceramics are investigated systematically.The related mechanisms were discussed.1.The rare earth double doping at Bi site（La/Ce,Pr/Ce,Nd/Ce,Sm/Ce）.（1）.The Aurivillius type ceramics doped by rare earth at Bi-site have a significantly enhanced piezoelectric coefficient（d33）.The CBT-based ceramic（Pr/Ce doping）possess the excellent property with d33 of 20 p C/N.（2）.This study confirms that the increase of piezoelectric activity is associated with resistivity.Compared with the pure CBT ceramics,the introduction of rare earth increases the resistivity by more than two orders of magnitude and the resistivity of Sm/Ce is 4.18 * 108 Ω · cm.（3）.The CBT-based ceramics exhibits a high piezoelectricity with a not deteriorated Curie temperature and the Tc of Pr/Ce,Sm/Ce are 797 °C.2.The effect of different high-valent cations Nb,Ta and Sb on the electrical properties of piezoelectric ceramics are investigated by high and low valent cations doping（Nb/Mg,Ta/Mg,Sb/Mg）at Ti-site.The results:（1）.The doping at Ti-site can significantly improve the piezoelectric activity of CBT and Nb/Mg doping increase the piezoelectricity to 24 p C/N.（2）.The disadvantage of poor temperature stability existed in doping at Bi-site is overcome by Ti-site.After the poled sample is annealed at 600 °C,its piezoelectric activity is still above 90%.（3）.While piezoelectricity by Ti-site doping is increased,their Curie temperature is not gravely deteriorated,and the Tc of the Nb/Mg sample is decreased by 4 °C.3.Piezoelectric ceramics are prepared by solid phase reaction with high and low valent cations（Nb/Mg,Nb/Zn,Nb/Cu）doping at Ti-site.The effect of different low valent cations Mg,Zn and Cu doping on the electrical properties of CBT ceramics is studied.Results: high and low cationic co-doping effectively improve the piezoelectric performance,and it reaches 22 p C/N;the specimens show the excellent temperature stability.The piezoelectric active is more than 95% of the room temperature after the high temperature of 600 °C annealing for 1 h.