Preparation And Dielectric Properties Of Temperature Stable Relaxor Ferroelectric Ceramics

PMW-PT, PNW-PT and PMW-PNN-PT ceramics with pure perovskite structure were prepared by the Semichemical Method (SCM) or the Hydrothemal-Derived Precursor Method(HDPM). The formation mechanism of perovskite PMW-PT and PMW-PNN-PT powders were discussed. The effects of technological parameters, excess of some reactants and additives on the phase structure, micro-structure and dielectric properties of these ceramics were investigated systematically.In the preparation of PMW-PT based ceramics by SCM, the Mg(Ac)2 solution instead of MgO improved the reactivity and distribution of PbO and MgO effectively, and decreased the formation temperature of PbWO₄ and Pb₂WO₅. And this resulted the formation temperature of the perovskite PMW-PT phase decreased significantly. The best technical conditions of PMW-PT based ceramics are: calcined at 850℃ for 2h, and sintered at 1050℃ for 2h. Doping with Mn decreased the dielectric loss of PMW-PT ceramics and the proper Mn content is 0.25mol%.The best technical conditions of PNW-PT based ceramics prepared by SCM are: calcination at 850℃ for 2h and sinteration at 1050℃. The Curie temperature and maximum dielectric constant of PNW-PT ceramics were increased with the increasing of PT content. Both the density and the grain size of PNW-PT ceramics increased with the doping of Mn. Doping with Mn decreased the Curie temperature and the dielectric loss, and improved the dielectric constant. The proper Mn content of the Mn-doped PNW-PT ceramics was 0.3mol%.In the preparation of PMW-PT based ceramics by SCM, the Mg(Ac)2 and Ni(Ac)2 solution instead of MgO and NiO improved the reactivity and distribution of MgO, NiO and PbO effectively. Unstable intermediate PbWO₄ and pyrochlore phase Pb₃Nb₂O₈ were formed in the process of calcining, and this facilitated the formation of pervoskite PMW-PNN-PT. The addition of excess WO₃ improved the dielectric temperature characteristic of PMW-PNN-PT ceramics, but the dielectric constant decreased. The dielectric loss of the PMW-PNN-PT ceramics decreased and thedielectric properties improved obviously after doping with proper content of Mn.The proper composition of X7R type PMW-PNN-PT ceramics was 0.29PMW-0.41PNN -0.30PT+5mol%WO3+0.25mol%MnO2, and the proper calcining and sintering condition are 850°C/2h and 1150°C/2h, the dielectric constant at room temperature exceed 5400, the dielectric loss no more than 0.04.While the PMW-PT ceramics prepared by HDPM, the phase of Pb2TiWO7 was formed during the process of hydrothermal reaction. And Pb2TiWO7 reacted with MgO, PbO and PbTiC>3 to form perovskite phase PMW-PT at low temperature. This decreased the calcining temperature and time. The best technical conditions of 0.5PMW-0.5PT ceramics are: calcination at 800°C for lh and sinteration at 1000°C for 2h, and its Curie temperature is 55"C, the maximum dielectric constant is 12067.