Study On Processing And Electrical Properties Of PZT-PFW-PMN Ceramics For High Power Piezoelectric Transformers

According to the required properties of monolayer piezoelectric transformers, the possibility of improving mechanical quality factor (Q_m), electromechanical coupling factor (K_p), piezoelectric constant (d₃₃) and reducing dielectric loss (tan δ), resonant (r) was analyzed and discussed from the aspects of composition selection, processing optimization, 'hard' dopants and 'soft' dopants. The new system of PbZrO₃-PbTiO₃-Pb(Fe_2/3W_1/3)O₃-Pb(Mn_1/3Nb_2/3)O₃ (PZT-PFW-PMN) was designed based on those facets. The main contents of this paper were listed as follows:Firstly, PZT-PFW-PMN ceramics were prepared by conventional mixed-oxide method (CMO). Effects of Pb(Fe_2/3W_1/3)O₃ content on phase structure, microstructure, dielectric and piezoelectric properties of the system were studied in detail. It indicated that substitution of Pb(Fe_2/3W_1/3)O₃ was effective on forming the solid solution and inhibiting the formation of pyrochlore phase. The dielectric and piezoelectric properties were also enhanced. The optimized properties may be attibute to a high density of the ceramics and Fe³⁺ on the B-site. When Pb(Fe_2/3 W_1/3)O₃ content is in the range from 1.5mol% to 5.0mol%, the better piezoelectric properties of ceramics were obtained. Especially at 3.0mol% Pb(Fe_2/3W_1/3)O₃, the ceramic shows preferable properties. Effect of Zr/Ti ratio on the properties of 0.90PZT-0.03PFW-0.07PMN was investigated. The system transformed from the tetragonal phase to rhombohedral phase with increasing Zr/Ti ratio. K_p, d₃₃ and εr increased at first and then decreased and Q_m showed the opposite tendency with increasing Zr/Ti ratio. The optimized properties of this system were obtained in tetragonal region near MPB with Zr/Ti=52/48, which were Q_m=1655, d33=358pC/N, K_p=0.63, tanδ=0.0053, r=1.027Ω, εr=1745 and T_c=273℃. Therefore, 0.90Pb(Zr_0.52Ti_0.48)O₃-0.03Pb(Fe_2/3W_1/3)O₃-0.07Pb(Mn_1/3Nb_2/3)O₃ was selected as the basic system in our study.Secondly, at the basis of the above results, processing parameters was investigated. The results indicated that the formation of Pb(Zr,Ti)O₃ solid solution was promoted by increasing calcining temperature. The dielectric and piezoelectric properties showedslight shifts with increasing calcining temperature. With increasing the sintering temperature the bulk density of ceramic increased at first and then decreased. Besides, the dielectric and piezoelectric properties of the system were improved as the sintering temperature increasing from 1050 to 1200°C, which can be contributed to a high density of the ceramics. In order to get good piezoelectric properties, the effect of rich lead on the electrical properties was studied. The phase of all ceramics was pure perovskite phase within the lead-rich content. Both density and electrical properties of ceramic were enhanced by adding appropriate excess lead. And at an excess of 2.0wt.% Pb3C>4, the electrical properties parameters were Qm—\1A7>, Kp — 0.64, t/33 = 365pC/N, r -0.997Q, tan ï¿¡=0.0056, respectively.In order to further improving the electrical properties for the demands of the high power piezoelectric transformer materials, the dopants of MnCC>3, M^Os, Fe2C>3 and WO3 were added to the system. It indicated that neither rich manganese and rich niobium nor poor manganese and poor niobium could improve the dielectric and piezoelectric properties. Therefore, it was appropriate to keep their stoichiometric proportion. The proper addition of Fe2C>3 could improve Qm, Kp and J33. The composition containing 0.05wt.% Fe2C>3 had the desirable piezoelectric and dielectric properties, which were Qm=\S39, ATp=0.61, c/33=382pC/N, tan <5=0.0054 and r= 1 .OO6Q0 It was believed that PbO-WO3 additive could result in liquid-phase formation during the sintering process with increasing WO3 content, which densified the ceramics. The system had better microstructure, dielectric and piezoelectric properties by adding proper addition of WO3. 0.90PZT-0.03PFW-0.07PMN doped with 0.15wt.% WO3 showed optimum properties: Qm= 1882, A:p=0.64, c/33 = 351pC/N, eT—1798, tan S— 0.0052, r=0.866Q, Pr=19.94u€/cm2 andï¿¡c=11.98kV/cm.Lastly, the systems of 0.90Pb(Zr0.52Tio.48)03-0.03PFW-0.07PMN+2.0wt.%Pb304 +0.05wL%Fe2O3 and 0.90Pb(Zro.52rio.48X>3-0.03PFW-0.07PMN+2.0wt.%Pb304+ 0.15wt.%WO3 were selected out as superior compositions, which were excellent candidates for high power monolayer piezoelectric transformer applications.