Studies On Doping And Low-Temperature Firing Of PZT-Based Piezoelectric Ceramics

PZT-based piezoceramics have been extensively used in electronic components due to their stable function, the excellent electro-mechanical properties, the available production and the low cost. The 0.2PZN-0.8PZT and 0.02PNW-0.07PMN-0.91 PZT ceramics possess excellent piezoelectric properties as well as satisfied sintering properties for the application high power piezoelectric transformers. In order to improve the piezoelectric properties and reduce the sintering temperature in a further, the testing of addition of the doping oxides or sintering reagent, respectively, in these systems have been carried out. The doping micromechanism have been revealed and the solubilities of Cr₂O₃ and Fe₂O₃ in the perovskite structure of the 0.2PZN-0.8PZT ceramics have been determined. Meanwhile the characteristics of the piezoelectric properties exhibiting a maximum or minimum value at the solubilities have been interpreted scientifically and thus the paper has some theoretic and applied virtues. Raman scattering, an effective instrument was provided in the paper to investigate the microstructural changes induced by doping of the piezoelectric ceramics with the rhombohedral and tetragonal structures. The obtained piezoelectric ceramics with excellent piezoelectric properties sintered at 950oC are expected as optional materials for high power piezotransformers and are helpful to decrease the cost of multilayer piezoelectric devices. The electric properties of the 0.2PZN-0.8PZT ceramics could be modified in a further through Cr₂O₃ and Fe₂O₃ addition. The solubilities both Cr₂O₃ and Fe₂O₃ in the perovskite structure locate at about 0.3wt%. Within the solubility limit, Cr₂O₃ or Fe₂O₃ can effectively incorporate into the crystal structure and result in the phase transition from rhombohedral to tetragonal as well as the tetragonal distortion. Meanwhile the grain growth and the densification can be improved by the doping. The most optimum piezoelectric properties could be obtained at the solubility limit, which should be attributed to the effects of the dopants on the phase structures, the domain walls movements and the grain boundaries motions. Raman scattering spectroscopy is used to investigate the microstructural changes of the 0.2PZN-0.8PZT ceramics caused by the doping. On the basis of Raman scattering spectroscopes in PZT ceramics, the Raman vibration modes in the 0.2PZN-0.8PZT ceramics have been labeled. Through the analysis of the Raman shifts and the relative intensities corresponding to the different Raman modes, the phase transition trend induced by Cr2O3 or Fe2O3 doping has been determined and the changes of the tetragonallity and the Curie temperature have been interpreted. The above conclusions are in good consistence with the XRD analysis. To realize the low firing of PNW-PMN-PZT system, BiFeO3, a typical perovskite compound with a low melting point, is selected as the sintering agent. The sintering temperature of the ceramics with 10mol% BiFeO3 additions could be lowed from 1200°C to 950°C while the electric properties could be kept. The results indicate that the ceramics with 10mol% BiFeO3 additions sintered at 950°C present uniform and compact grains, and the following excellent properties were obtained: ρ=7.4 g/cm3, εr=1952, tanδ=0.006, d33=312 pC/N, kp=0.498, Qm= 617, TC=260°C.