【作者】 方焯；

【导师】 龚树萍； 周东祥；

【作者基本信息】 华中科技大学 ， 微电子学与固体电子学， 2010， 博士

【摘要】 近年来,随着微电子和通讯技术的发展,电子元器件的小型化和集成化成为当今电子技术发展的主流。应用于正温度系数热敏电阻(PTCR)的施主掺杂的BaTiO3半导体陶瓷材料和应用于多层陶瓷电容器(MLCC)的BaTiO3铁电陶瓷材料都向着小型化方向发展。BaTiO3陶瓷的电学特性随着晶粒尺寸减小到纳米尺寸将发生巨大的变化,这种变化直接影响到器件的性能。本文以BaTiO3陶瓷作为研究对象,基于电动力学、半导体物理及Landau热力学等理论,从理论上定量的研究了BaTiO3铁电陶瓷材料的结构、相变、介电性质及其施主掺杂的半导体陶瓷材料导电性质的晶粒尺寸效应,主要内容和结论如下：考虑到表面效应,在Landau热力学理论的基础上建立了一个定量的球状纳米BaTiO3铁电陶瓷的多壳层结构模型,研究了其结构、相变及介电常数的尺寸效应。进一步,引入不均匀极化能,采用二相模型分析了各种晶粒尺寸、温度及外电压下的核壳结构、BaTiO3晶粒极化强度的径向分布、相变的尺寸效应、晶体结构c/a、热电性能和介电性能的尺寸效应等。随着晶粒尺寸减小,表面能增大,铁电-顺电相界向晶粒中心移动。随着晶粒尺寸进一步减小,最后变成稳定的立方相结构。表面效应导致了由表面顺电层和内部铁电核组成的BaTiO3纳米粒子的多壳层结构,使铁电顺电相变弥散化。介电常数的计算结果发现随着晶粒尺寸的减小,室温BaTiO3陶瓷介电常数减小；两个较低的介电峰向高温移动,而较高的介电峰向低温移动；三个介电峰值减小且变宽。随晶粒尺寸减小,表面顺电层厚度增大是介电常数减小的原因之一。研究了一唯电畴模型,计算了单畴临界尺寸、铁电临界尺寸,分析了偶极相互作用对居里温度的影响。进一步研究了三维电畴模型,通过引入一参数η来表示晶粒表面电荷补偿系数,结合畴壁能得出BaTiO3纳米陶瓷的铁电性随晶粒尺寸的变化关系,并讨论了畴壁运动对介电常数的贡献。结果指出铁电性能的消失主要由远程库仑相互作用的减弱导致。考虑电畴的影响后,能够解释BaTiO3颗粒的室温介电常数在临界尺寸以上达到最大值。从泊松方程出发,考虑了施主、受主、电子及空穴的浓度分布对晶界势垒的影响,并将晶界模型扩展到三维,首次建立了一个全面的、定量的模型来解释施主掺杂的BaTiO3半导体陶瓷的导电特性。通过建立了一个关于电子能级的微分方程,按照四价标准龙格-库塔公式求解该方程,从数值上得到晶界势垒的能级图。进一步计算得到BaTiO3半导瓷的阻温关系。结果指出室温电阻率随施主浓度的增大形成U形曲线。随着晶粒尺寸减小到受主扩散长度的2倍附近时,室温电阻急剧上升。材料的升阻比在指定的受主浓度掺杂和指定的晶粒尺寸下取得极大值。分别用倾斜能带的方法和连续介质方法研究了BaTiO3半导体陶瓷的非线性特性及晶粒尺寸效应,并分析了晶界特征。计算得到的升阻比随着外电压的增大而降低,在居里温度以下的电流电压特性很弱,而在居里温度以上很强。非线性系数在电阻达到最大值的温度、接近受主扩散长度2倍的晶粒尺寸达到最大值。非线性电流电压特性的模拟结果指出电流电压曲线分为三个区域：线性区、非线性区和回升区。随着外电压增大,晶界势垒在非线性区急剧下降。随着晶粒尺寸减小,材料的击穿电压和非线性系数都增大。总之,本文对BaTiO3陶瓷晶粒结构及尺寸效应的研究结果与实验数据基本吻合,合理的解释了BaTiO3陶瓷材料的尺寸效应,并对更小的晶粒尺寸的BaTiO3材料结构及性能作出了理论预测。本文的理论结果为铁电BaTiO3陶瓷材料的小型化、集成化的研究提供了理论依据,对制备高性能的热敏材料及电容器材料的进一步研究具有指导意义。更多还原

【Abstract】 In recent years, with the development of microelectronics and communications, electronic component miniaturization and integration have become the main research direction of today’s electronic technology. The donor-doped BaTiO3 semiconductor ceramic materials applied to positive temperature coefficient thermistor (PTCR) and the BaTiO3 Ferroelectric materials applied to multi-layer ceramic capacitors (MLCC) require miniaturization. The electrical properties of BaTiO3-based ceramics will change tremendously with the grain size decreasing to the nanometer size, which directly affects the device performance. In this dissertation, the structure, phase transition, dielectric properties and conductive properties of micro-nano ferroelectric BaTiO3-based ceramics was studied theoretically and quantitatively, based on electrodynamics theory and Landau thermodynamic theory. The main work and results are as follows:Considering the surface effects, a quantitative multishell structure model of the grain size effect of nano-BaTiO3 ceramics is proposed on the basis of Ginsburg-Landau-Devonshire thermodynamic theory. Then the size effect of the structure, the phase transition and the dielectric constant were studied. Further, introducing anisotropy energy, the size effects on the spontaneous polarization, Curie temperature, c/a ratio, electrocaloric, and dielectric properties of BaTiO3 ferroelectric nanoceramics were studied using the two-phase model. It is found that the surface energy increases and the ferroelectric-paraelectric phase boundary moves to the center of the grain with decreasing grain size. With further reducing grain size, ferroelectric phase disappears, leaving only one stable cubic structure. The surface effects lead to the multishell structure in BaTiO3 nanoparticles consisting of a surface paraelectric layer and inner ferroelectric core. The dielectric constant of BaTiO3 ceramics is reduced at room temperature with decreasing grain size. The three temperature dependent dielectric peaks are depressed and diffuse in the nano-BaTiO3 ceramics. The two low dielectric peaks move to higher temperatures, while the higher dielectric peak moves to lower temperature with decreasing grain size. The increased thickness of the surface cubic layer is one of the reasons for the decrease in dielectric constant with decreasing grain size.We have studied a one-dimensional ferroelectric domain model, calculated single-domain critical size and ferroelectric critical size, and analysised the dipole-dipole interaction. Further the three-dimensional ferroelectric domain models have been studied. By introducing a parameterηthe surface charge compensation factor, the long-range Coulomb interaction energy and the domain wall energy incorporated into the Landau-Ginzburg free-energy density, the relationship between ferroelectricity and grain size in nano-BaTiO3 ceramics can be calculated. Then the domain wall motion contribution to the dielectric constant is discussed. The results show that when the grain size reduces to the critical size, long-range Coulomb interaction weaking dramatically results in the instability of the ferroelectric phase. Taking into account the effects of domains, the room temperature dielectric constant of BaTiO3 particles shows a maximum value at some grain size above the ferroelectric critical size.On the basis of Poisson’s equation, a three-dimension comprehensive and quantitative model for semiconductive BaTiO3 ceramics was first established, after the migration of the donors, acceptor, electrons and holes has important influence on the grain boundary effect were considered. By setting a differential equation of electron level, Energy band chart was obtained by solving the equation with Runge-Kutta method. The positive temperature coefficient effect of semiconductive BaTiO3 ceramics with different grain sizes and different donors and acceptors doping were computed quantitatively. It was found that the resistivity forms a U-shaped curve with the donor concentration increasing. The resistance jumping shows a maximum at a certain acceptor doping concentration and grain size.The non-linear Current-voltage characteristics of BaTiO3 semiconductor ceramics were studied by the method of tilted energy band and continuous medium seperately at one-dimensional case. The characteristics of the grain boundary were researched. The resistance jumping is reduced with increasing electric field applied; current and voltage relation follows Ohm’s law below Curie temperature, and exhibits strong non-linear above Curie temperature; the non-linear coefficient shows a maximum value at temperature the resistivity reaches maximum and with grain size closed to depletion region width. Non-linear current-voltage characteristics indicate that the current-voltage curve is divided into three regions:The results pointed out that current and voltage characteristics divide into there regions:linear region before breakdown field, nonlinear region near breakdown field, and upturn region after breakdown field. As the applied voltage increases, the grain boundary barrier in the nonlinear zone drops drastically. As the grain size decreases, the breakdown field and nonlinear are both inhancedIn short, in this dissertation, our theoretical results about size effects of BaTiO3 ceramics are basically consistent with the experimental data, and explain reasonably the causes of size effect, predict structure and phase transition of nano BaTiO3 ceramics with a smaller grain size. Our theoretical results provide a theoretical basis for the ferroelectric BaTiO3 ceramics miniaturization and integration, and have guiding significance for further study on preparation of high-performance heat-sensitive materials and capacitor materials.更多还原