【作者】 刘冬；

【导师】 李庆芬； 乔英杰；

【作者基本信息】 哈尔滨工程大学 ， 材料学， 2006， 博士

【摘要】 采用改进的溶胶—凝胶法和优化的工艺条件，分别制备La、Ca、Mg、Fe钛酸铅纳米陶瓷。根据TG-DTA实验结果确定了钛酸铅干凝胶的热处理条件和烧结温度。确定并研究了影响体系的各种因素和相应的凝胶制备方法。研究发现：在适合成胶的pH值范围内，pH值越低，成胶时间越长，胶体的质量越好。而且掺杂量对体系也有一定的影响，掺杂量越大，成胶时间越长，体系越不稳定。利用XRD研究了掺杂钛酸铅纳米晶的晶体结构，以此为基础讨论了掺杂量与晶格常数变化的关系以及相变的尺寸效应，并研究了掺杂纳米晶的相组成和晶胞参数。研究发现掺杂钛酸铅纳米晶的晶胞参数均发生了不同程度的改变。此外，在镁和二茂铁掺杂钛酸铅纳米晶中，可以看到掺杂相衍射峰，在镧及钙掺杂钛酸铅纳米晶中可以看到纯相的钛酸铅衍射峰。SEM观察发现，掺杂钛酸铅纳米晶的晶粒结晶良好完整，晶粒均匀，半径在0.5～3μm。对镁、铁掺杂钛酸铅陶瓷的电性能进行了初步探索。镁、铁掺杂可以提高钛酸铅纳米晶的导电性，使钛酸铅纳米晶的电阻率降低6～7个数量级。采用密度泛函理论方法，计算了钛酸铅及其掺杂体系的电子结构，得到钛酸铅晶体的电荷分布、键级、晶体能量、态密度以及自发极化强度，并讨论了掺杂对钛酸铅晶体电子结构及性质的影响。原子位移势能面的计算结果为：Ti原子和Pb原子的位移曲线有双井特征，表明当Ti原子和Pb原子沿着软模本征矢振动时体系能量降低，这对钛酸铅铁电稳定性具有重要意义。同时电子结构计算的结果也表明：Ti-O_Ⅰ键和Pb-O_Ⅱ键在相变后成键程度得到加强，这也是钛酸铅四方相稳定的一个重要原因。对钛酸铅振动性质的计算结果没有发现钛酸铅存在低温相变的证据。振动频率随四方应变的变化关系为：随着四方应变的增大，软模的频率增大，在某一临界点，不稳定的软模转变为稳定的振模。钛酸铅的四方应变大于此临界点，这是其四方相能够成为稳定基态的一个重要原因。通过计算Mg，Fe掺杂钛酸铅的原子布居、键布居和键长等对其铁电稳定性进行了探讨。结果表明，Mg原子的掺入使得其所在的微区成键情况发生了较大的变化，电荷从Mg原子向最近邻的O原子迁移，Mg-O_Ⅱ键成键加强，而Ti-O_Ⅰ键成键减弱，体系的铁电稳定性取决于它们之间综合作用的结果。而Fe掺杂钛酸铅的情况则与掺入Mg原子相反，Fe原子是得到电荷，电荷从附近的O原子向Fe原子转移，Fe原子与周围O原子的成键作用减弱，对体系的铁电稳定性不利。更多还原

【Abstract】 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.更多还原