【作者】 赵飞；

【导师】 李龙士；

【作者基本信息】 清华大学 ， 材料科学与工程， 2009， 博士

【摘要】 新型钙钛矿微波介质陶瓷是近年来功能陶瓷领域研究的热点,本论文针对包括有序双钙钛矿、正交钙钛矿和六方钙钛矿在内的几种新型钙钛矿类微波介质陶瓷的结构与性能关系开展了系统研究,并重点研究了移位型和孪晶型两种六方钙钛矿的频率温度系数与晶体结构的关系、结构稳定性及微波介电性能。首先研究了双钙钛矿微波介质陶瓷A₂MeWO₆（A=Sr,Ba;Me=Co,Ni,Zn）中结构与介电性能的关系,在双钙钛矿结构中引入“B化离子有序度”的概念,并揭示出材料的品质因数（Q×f）与B位离子的有序度之间存在依赖关系。研究了A_nB_nO_3n+2型正交钙钛矿材料CaLa₄Ti₅O₁₇,发现了zn²⁺取代对化合物中氧八面体结构基元的调制规律,建立了微波介电性能与氧八面体间连接方式的关联,并在该体系中获得了一系列具有优异微波介电性能的新材料。随后,重点研究了移位型A_nB_n1O_3n六方钙钛矿型微波介质陶瓷的频率温度系数与晶体结构的关系,在总结了现有结构数据的基础上,提出了六方结构层间畸变的概念,并建立了一个描述六方结构中填满与空位氧八面体层畸变程度的物理参量,发现六方钙钛矿类材料的频率温度系数与结构层间畸变程度密切相关,并揭示出这一相关性的内在物理机制。将该概念进一步拓展,成功地解释了六方钙钛矿形成的共生物的频率温度系数的变化规律,这一工作从晶体结构的设计与调制角度,提出了一种通过形成共生结构而改善温度稳定性的新途径和新机制。最后,系统研究了（1-x）Ba(Ni_1/2W_1/2)O₃-xBaTiO₃体系的相关系与介电性能,在该体系中发现了一类室温下稳定存在的填满型六方钙钛矿微波介质新材料（x=0.4-0.6）,该材料具有高的Q×f值和良好的温度稳定性。借助晶体结构精修、XPS和HRTEM等技术,研究了该类填满型六方钙钛矿化合物的形成机制和结构稳定性,提出了离子选择性占位的稳定机制。在该系统中还发现某些组成具有巨介电常数特性,并借助阻抗谱等技术,对其介电极化机制进行了探讨,该发现对丰富巨介电材料新体系和揭示其微观机制有重要科学意义。更多还原

【Abstract】 Microwave dielectric ceramics with new perovskite-related structures have received much attention in recent years. This dissertation summarizes a series of investigation on the structure-property relationships for such materials, including double perovskites, octahedral perovskites and hexagonal perovskites. Two type of hexagonal perovskites are systematical investigated, focusing on the correlations between their temperature coefficient of resonant frequency and structure variations.Firstly, the relationships between structure and microwave dielectric properties of A₂MeWO₆ （A = Sr, Ba; Me = Co, Ni, Zn） double perovskite ceramics are investigated. In this system, the concept of B-site cation ordering is introduced, on which the Q×f values show strong dependence. Meanwhile, From the application point of view, the modification of octahedral perovskite structure with general formula A_nB_nO_3n+2 is performed. It is found that the octahedra in this structure are markedly affected by A-site ion size, and the microwave dielectric properties are closely related with the connection of the octahedra. In this system, a series of new candidate materials with excellent microwave dielectric properties are obtained.Secondly, the relationship between the temperature dependence of resonant frequency and the hexagonal perovskite structure with general formula A_nB_n-1O_3n is thoroughly discussed. It is found that key difference between every single hexagonal perovskite compound is that, the thickness of its filled octahedral layers and that of its vacant octahedral layer is much of diversity. Based on this, a new concept named octahedral-layer aberrance in the hexagonal perovskite is raised, which can then be characterized by the parameter of octahedral thickness variance. The relationship between the temperature dependence of resonant frequency and the octahedral thicknessvariance, the degree of octahedral-layer aberrance in the hexagonal perovskite, is finally extracted. Its physical mechanism is also discussed. Furthermore, this concept is applied on the intergrowth compounds between the hexagonal perovskites, explain- ing the variation of their temperature dependence of resonant frequency. It suggests that, from the crystal structure point of view, the temperature stability of certain ma-terials can be improved by forming intergrowth structure, due to the reduction of their octahedral thickness variance.Finally, the h-BaTiO₃-type filled hexagonal perovskites as microwave dielectric ceramics are explored in （1-x）Ba(Ni_1/2W_1/2)O₃-xBaTiO₃ system. A series of temperature-stable and high Q materials are found. By using Rietveld refinement, XPS and HRTEM etc., the stability of this type of hexagonal perovskites is proved to be mainly due to the selective occupation of B-site ions. Besides, a new dielectric mate-rial with giant dielectric constant is found in this system and the physical mechanism is discussed on the basis of the dielectric and impedance measurement results. This finding is of significance due to its high dielectric constant and interesting physical mechanism.更多还原