Preparation And Dielectric Properties Of Lead-based Relaxor Ferroelectric Ceramics

Semichemical Method(SCM),which have characteristics of both Chemical Method (CM) and Conventional Milled Oxides(CMO),as a new preparation method of Lead based relaxor ferroelectrics was proposed in this paper. Some ceramics(i.e. PMN,PNN,PFW-PT,PMN-PNN-PT) were prepared by the Method. The effects of technological parameters (such as calcining temperature,sintering temperature and soaking time),PbTiO3,excess of some reactants and additives on the phase content,microstructure and dielectric properties were investigated systematically. The difference of Semichemical Method with CMO and CPM was compared. The formation mechanism of PMN-PT and PFW calcined powders prepared by SCM was investigated by means of XRD and DTA DSC techniques. Combined Core-Shell method with SCM,PMW-PNN-PT ceramics with core-shell structure,which have excellent dielectric temperature stability,were successfully prepared. The new mechanism of improving temperature stability of dielectric properties was put forward. The main contents are listed as bellows:In the systems of PMN-PT,the effects of the chemical reactivities of the starting materials such as soluble Mg-salts instead of MgO,Pb-salts instead of PbO and Ti(OC4H9)4 instead of TiO2 respectively on the formation of perovskite phase were investigated in detail. The results show that Mg-salts are effective for the formation of perovskite phase,especially Mg(NO3)2 and/or Mg(AC)2 instead of MgO in a molecular grade. Almost pure perovskite phase were obtained by using Mg-salts and Ti(C4H9O)4 instead of MgO and TiO2 respectively. The optimum preparation parameters are:the calcining condition of 800C-85(rC,2h-4h and the sintering condition of 950-1200,lh-2h.In the systems of PNN-PT,the content of perovskite phase increased rapidly from 800 C to 850 C. The formation of perovskite phase for PNN-PT was more effective than that of PMN-PT system prepared by SCM. Almost pure perovskite phase were prepared without excess NiO or adding PT. The optimum technological parameters are listed:the calcining condition of 850C,2h-3h and the sintering condition of 1100C - 1200C,lh-2h. With the increment of MnO2 content,dielectric loss decreased drastically,but the dielectric constant decreased slightly. When the excess content of MnO2 was 0.5(mol)%,the above properties are optimum.In the systems of PFW-PT,the effects of excess PbO,and Fe2O3,the content PT on the phase structure,microstructure and dielectric properties of PFW ceramics were investigated respectively in detail. The results revealed that by adding excess PbO and Fe2O3,not only the content of perovskite phase in calcined powders and sintered ceramics increased,but also the dielectric constant increased,especially. However,the dielectric loss decreased clearly by adding excess Fe2O3. The dopped MnO2 were effective for stabilization of perovskite phase. With increasing of MnO2 content,the dielectric constant increased gradually at first,then decreased with more MnO2 content,and the dielectric loss greatly decreased and resistivity increased clearly. When the excess content of MnO2 was 1.0(mol)%,the above properties are optimum. With increasing of PT content,grain size,density,Curie temperature and increased slightly,and the diffuse phase transition 6 decreased.Compared with CMO and CPM,SCM had many advantages not only simpling preparation process,lowing calcining and sintering temperatures,shorting reacting time,but also preparing ceramics with excellent dielectric properties. With identical calcining conditions,the content of perovskite phase in calcined powders prepared by SCM was much higher than that by CMO,what's more,by SCM,the sintered ceramics had clear crystal boundaries,large-sized crystal grains and high density. Without PT as stabling perovskite phase,pure perovskite phase of PMN,PNN and PFW ceramics were successfully prepared by SCM.By analyzing the XRD and DTA or DSC data,the reaction mechanism of SCM in the systems of PMN-PT and PFW ceramics are described as follows. As for PMN-PT systems,unstable and B-site vacant pyroch...