Study Of Preparation And Properties Of Modified Lead Titanate Ceramics

In this paper, the optimum technological parameters being adopted, lead titanate and modified lead titanate nanocrystals, i.e., PbTiO₃ nanocrystals doped with La, Ca, Mg, Fe have been prepared by using a modified hydrolytic sol-gel method. Heating treated condition and sintering temperature of PbTiO₃ xerogel was determined by the result of TG-DTA. Different factors that affecting the mixing were determined and the optimum gel preparation mothod were investigated. It was found that lower pH value in the pH arrangement and the longer time of the gel information are responsible for the quality of the gel system. With the increasing of the doped concentration and the gel time, the range of pH value of the gel formation become narrow and system was unstable.All crystalline structrue has been investigated by XRD, on the basis of which the relationship between dopant dose and lattice constant and the size effect of phase transformation were discussed. Morever, phase composition and cell parameters was also studied by XRD analysis. The results showed that cell parameters would change for doping. In addition, it was found that there were diffraction peaks of doped phase in PT nanocrystals doped with Mg or (C₅H₅)₂Fe and pure PT diffraction peaks in PT nanocrystals doped with La or Ca.It was found excellent crystal grain of doped PbTiO₃ with free gas pore, high density, small size with 0.5～3μm and no defects on the basis of the SEM photo.Electric properties of PbTiO₃ nanocrystals doped Mg, Fe, La and Ca was also investigated. It is evident that the resistance ratio of PbTiO₃ can be decreased efficiently with Mg or Fe dopant, about 6～7 orders of magnitude.Density functinal theory was applied to calculate the electronic structure of PT and relative doping materials, and charge distribution, bond order, crystalline energy, density of state and spontaneous polarization were obtained, according to which the doping effect on electronic sturcture were also discussed.The result of atomic potential energy surface shows that the character of Ti and Pb atomic displacement is double well, that is to say, the energy of system decreases when the Ti and Pb atoms move along the eigenvector of the soft mode, which is important to the ferroelectric stability of PbTiO₃. The Ti-O_I bond and Pb-O_II bond are enhanced after phase transition, which can be seen from the electronic calculation results, and this is another reason for the stability of tetragonal PbTiO₃.Moreover, the lattice vibrational modes of different craytalline phase in lead titanate and barium titanate were calculated, and the variety of lowest frequency mode with tetragonal strain were calculated, which is helpful to explain the reason why tetragonal lead titanate is in a stable ground state but barium titanate still undergoes a series ferroelectric phase transformation in low temperature. No evidence of low-temperature phase transition is found in PbTiO₃ according to the calculation results of vibration properties. The variety of vibration frequency with tetragonal strain shows that the soft mode frequency increases with the tetragonal strain increasing followed by the transition of unstable soft mode to stable vibration mode in a certain critical point. The value of tetragonal strain in PbTiO₃ is larger than that of the critical point, which results in the stable ground state of tetragonal PbTiO₃.Finally, the ferroelectric stability of Mg, Fe modified PT was discussed from the calculated atomci population, bond population and bond length etc. According to the electronic calculation results, the bonding situation has changed greatly with the doping of Mg atom, and the charge migrates from Mg atom to the nearest O atoms. Mg-O_II bond is enhanced while Ti-O_I bond is weakened, and the effects of all bonds decide the ferroelectric stability. To the doping of Fe atom, the situation opposites to that of Mg atom, and the Fe atom gain charge. The charge migrates form the nearest 0 atom to Fe atom, and the bonding effect of Fe atom and surrounding atoms is weakened, which is unfavorable to ferroelectric stability of system.