Synthesis And Properties Of PbNb₂O₆ Based High Curie Temperature Piezoelectric Ceramics

Lead metaniobate(Pb Nb2O6), which is the earliest discovered tungsten bronze structured piezoelectric ceramic, have the characteristics of high Curie temperature and low mechanical quality factor. So it can meet the requirements for transducers under high temperatures. However, lead metaniobate is difficult to prepare and its sintering temperature is very high(about 1250℃). And only in the case of rapid cooling, we can get orthogonal ferroelectric phase. The research object of this thesis is to prepare Pb Nb2O6 based high Curie temperature piezoelectric ceramic. Two different sintering methods(normal sintering and two-step sintering), and Sr2+and Ta5+ substitutions, were used to prepared ceramics. Phase structure, microstructure,and dielectric, ferroelectric and piezoelectric properties were then studied.First of all, we studied the influence of sintering temperature. We prepared Pb0.92Sr0.08Nb2O6(PSN) and Pb0.92Sr0.08(Nb0.95Ta0.05)2O6(PSNT). We found that PSN ceramics of orthogonal ferroelectric phase were formed at 1240℃ or higher because that Sr2+ doping reduced the sintering temperature. While PSNT ceramics of orthogonal ferroelectric phase were formed at 1220℃ because of Ta5+ substitution further reduced the sintering temperature. With the increase of sintering temperature,the density of ceramics increased at first and then decreased.Secondly, we prepared Pb1-x Srx Nb2O6(0 ≤ x ≤ 0.1) ceramics. We used two different sintering methods, normal sintering and two-step sintering, to prepare Pb1-x Srx Nb2O6 ceramics. Through the research, we found that via normal sintering the ceramics showed the rhombohedral phase, which was the ferroelectric phase at x=0and 0.02; while the ceramic was of orthogonal ferroelectric phase at x ≥ 0.04. With two-step sintering, except pure lead metaniobate components, all the components possessed orthogonal ferroelectric phase. Microstructure suggested that the right amount of Sr2+ions doped was beneficial to the refinement of the ceramic grains, but too much would lead to the number of pores increase. The Curie temperature was higher than 550℃ with each of the two different sintering methods. And from room temperature to 400℃, the dielectric constant had good temperature stability. So the ceramics are suitable for use at high temperatures. The ceramic had the excellent performance at x=0.08, with the highest piezoelectric constant d33=43 p C/N,electromechanical coupling coefficient kt=34.85%, and mechanical quality factor Qm=11.48.Thirdly, we used the two-step sintering process to prepare Pb0.92Sr0.08(Nb1-y Tay)2O6(0 ≤ y ≤ 0.09) ceramics. we found that all the samples formed orthogonal tungsten bronze structure of ferroelectric phase. After doping Ta5+,the ceramic grains obviously formed a closer packed structure. With the increase of Ta5+ content, grain size decreased. Although the addition of Ta5+ ions reduced the Curie temperature, it is still higher than 530℃. Component y=0.01 possessed excellent electrical performance, with the piezoelectric constant d33=79 p C/N, the curie temperature Tc=562 ℃, and the electromechanical coupling coefficient kt=35.33 %, which is suitable for use at high temperatures.