【作者】 徐静；

【导师】 刘韩星；

【作者基本信息】 武汉理工大学 ， 材料物理与化学， 2012， 博士

【摘要】 多层陶瓷电容器(MLCC)是重要的电子元件,MLCC的微型化、集成化、低成本化和多功能化依赖于高性能陶瓷的制备,包括瓷料制备、瓷料的性能控制等。Ba1-xSrxTiO3是主要的体系之一。本论文选择Ba0.67Sr0.33TiO3(BST)基陶瓷体系作为研究对象,从粉体的制备方法、陶瓷的制备工艺、Ba0.67Sr0.33TiO3基陶瓷组成改变对介电常数及温度稳定性等性能的影响进行系统研究,探索影响性能的本征与非本征因素,为获取高质量的陶瓷提供论证。论文主要研究工作和结论如下：获得高活性粉体是BST高性能细晶陶瓷制备的基础。本文采用固相法研究了制备工艺对BST粉体、陶瓷结构和性能的影响,以及晶粒尺寸对BST陶瓷性能的影响。实验证明：950℃C合成的BST粉体纯度高、单分散性好、粒径小(约110nm)且分布均匀。1300-C烧结所得的BST陶瓷相对密度最高(98.6%),晶粒细小(约250nm),具有良好的介电性能和铁电性能。室温下具有较高的介电常数和较低的介电损耗εr=7676,tanS=0.01,较高的剩余极化Pr=9.21aC/cm2,较低的矫顽场Ec=1.58kV/cm。考虑CBT(CaBi4Ti4O15)低熔点以及包含具有弥散特性Ca, Bi离子,为有效降低BST陶瓷烧结温度以及改善介电温度稳定性,本文研究了CBT不同掺杂量对BST陶瓷结构与性能的影响。研究发现CBT使BST陶瓷烧结温度显著降为1100-C,且有效抑制了晶粒的生长。CBT掺杂明显改善了BST陶瓷温度稳定性,当CBT含量x≥6%时,介电常数温度稳定范围(△C/C<±15%)已无显著改善。掺杂CBT使BST陶瓷呈现显著的铁电弛豫行为,且随着掺杂量的增加,介电峰弥散程度和弛豫强度逐渐增强。添加稀土元素是提高BST陶瓷材料介电性能的一种最常用有效的方法。本文研究了La掺杂对BST陶瓷结构与性能的影响,分析掺杂造成的不同缺陷补偿机制对BST陶瓷电性能影响机理。研究表明,适量的稀土La掺杂(0.2<x≤0.7)使BST陶瓷介电常数显著提高,同时显著改善了介电温度稳定性。根据电流-电压I-V(J-V)特性可知,适量La掺杂(0.2<x≤0.7)可使BST陶瓷在最佳致密化条件下达到最佳半导化,掺量太少或太多时陶瓷具有良好的绝缘性。另外,对于绝缘性样品,电流-电压基本符合欧姆定律；而对于半导化的样品,在不同的电压范围内电传导机制不同,且在较高电压范围内,电流主要受Schottky电流发射机制控制。通过肖特基(Schottky)势垒模型及电学微观结构模型分析La掺杂诱发BST高介电常数以及介电弛豫现象机制。研究表明,表面或晶界势垒高度和耗尽层厚度是影响材料介电性能的主要因素；表面势垒对BST陶瓷高介电效应有很大的贡献。采用CuO对BST样品进行改性,使介电常数得到明显提高,损耗明显降低以及使破坏电压得到明显改善。BST陶瓷在测试频率范围内出现三种不同的介电弛豫现象。结合阻抗谱分析,进一步确定在半导化BST陶瓷中,存在表面势垒和晶界势垒。这些势垒的形成导致了样品的电学非线性特征,导致不同的界面极化使样品呈现出很高的表观介电常数。更多还原

【Abstract】 Multilayer ceramic capacitors (MLCC) are important electric components, the micromation, the integration, the low cost and multifunctionality of MLCC depends on preparing ceramics with high properties, including the preparation of porcelain raw material, the properties control of ceramics and so on. The Ba1-xSrxTiO3is one of the main systems. In the thesis, Bao.67Sro.33Ti03-based ceramic was selected as candidate material, the preparation methods of powder, the preparation technology of ceramics, the effect of Bao.67Sro.33Ti03-based ceramic composition change on permittivity as well as temperature stability and so on were investigated systematically, and we explore the intrinsic or extrinsic factors impacting properties for offering proof of obtaining ceramics with high quality.The main works and conclusions are summarized as follows:For the preparation of BST ceramics with optimum properties, it is necessary to obtain highly active powder first. We study the effect of preparation processes on the structure and properties of BST powder as well as ceramics using the solid method. And we futher investigate the influence of grain size on the properties of BST ceramics. The experiments show that when calcined at950℃, the BST powder present high purity, the better single dispersivity, small particle diameter (about110nm) and homogeneous size distribution. High relative density(98.6%), fine grains (about250nm) of BST ceramic sintered at1300℃show better dielectric properties and ferroelectric properties. The high room dielectric constant and low dielectric loss: εr=7676, tanδ=0.01; the large remanent polarization and low coercive field are Pr=9.2μC/cm2, Ec=l.58kV/cm, respectively.Considering the low melting point of CBT(CaBi4Ti4O15) as well as containing Ca, Bi ions with diffusion, in order to decrease sintering temperature and improve the dielectric temperature stability of BST ceramics effectively, the structures and properties of BST ceramics doped with various CBT content were investigated. The results show that CBT doping decrease the sintering temperature of BST ceramics to 1100℃, moreover restrain grain growing significantly. In addition, CBT improve the temperature characteristics of BST ceramics obviously, and the range of dielectric temperature stability(ΔC/C≤±15%) cann’t be broaden obviously when the CBT content x>6%. The BST ceramics exhibit obvious ferroelectric relaxation behaviors by doping CBT, and the BST ceramics show an increased diffuseness of the dielectric peak and an increased degree of relaxor characteristics with increasing CBT content.It is a common and effective method to enhance the dielectric properties of BST ceramics by adding rare-earth elements. The structure and dielectric properties of La-doped BST ceramics were investigated as well as the relation between the properties of BST ceramics and the different defect compensation mechanisms resulting from La doping. The results show that proper rare earth La dopant (0.2≤x≤0.7) may greatly increase the dielectric constant of BST ceramics, also flat the dielectric peak and improve the temperature stability evidently. According to the current-voltage I-V(J-V) characteristics, the proper La doped BST ceramics may reach the better semiconductivity at the optimal densification condition, when the less and more La doping, the ceramics are insulators. In addition, the current-voltage basically follows the ohm’s law for insulating samples; while for the semiconductive samples, the conduction mechanism is different at different voltage range, and the current is mainly controlled by the Schottky emission at comparatively high voltage range.By the Schottky barrier model and electric microstructure model to analyze the cause of the high dielectric constant behavior and the dielectric relaxation of La doped BST ceramics. The results show that the surface or grain boundary potential barrier height and the width of depletion layer do play an important role in impacting dielectric properties; and the surface potential barrier have a great contribution to the high permittivity of BST ceramics. CuO has been used to modify the electrical properties of BST, and it can be observed that the permittivity was further increased and the dielectric loss was decreased obviously as well as the broken voltage is improved evidently. Three various dielectric relaxations of BST ceramics are observed in the measured frequency range. On the complex impedance spectroscopy analysis, the surface barriers and grain boundary barriers have been further confirmed in the semiconductive BST ceramic. The formation of such electrical potential barriers leads to the electrical nonlinearity in the I-V characteristics and different interface polarizations resulting in very large apparent dielectric constant.更多还原