类层状Mgn+1TinO3n+1系微波陶瓷结构优化及其超低损耗机理研究

Based on the previous research work, a very high quality factor (Q×f ≥ 380,000 GHz) can be obtained for the Mgn+1TinO3n+1 systems, coexisting with a small amount of the special layered phase and MgTiO3 phase. It means that, for a given range of the relative dielectric constant (εr ~ 15-25), the Mgn+1TinO3n+1 microwave ceramics are expected to be a new material that can exceed the ultra-low dielectric loss limit (Q×f ≥ 450,000GHz) in the current theoretical value. Based on that, a concept of the “analogous layered structure” phase has been proposed in this project. Wherein, the proportion (≥90%) of this phase will be improved by analyzing the structure and regulation mechanisms, and then, the Mgn+1TinO3n+1 ceramics with higher content of the analogous layer-type phase are regarded as the objective studies. Additionally, the parametric models of the unit cell structures with different composition n value have been predicted and created by Transmission Electron Microscope, X-ray Diffraction Refinement and Pauling's Rules et al., and also, the first-principles calculations are used to modify and validate the accuracy of these considered models. The vibration modes of the dispersion parameters are systematically elucidated by the conjunction of the first-principles calculations and the fitting data of Infrared-Raman Spectrum, and it is used to confirm the vibration modes which are most associated with the dielectric loss in the lattice. These will make a definite origination mechanism in ultra-low dielectric loss for the analogous layered-type ceramics, mainly through the analyzing of the crystal structure characteristics, and the intrinsic loss caused by the types of bond vibrations and lattice vibrations. In addition, in the process of the ions introduction to regulate and control the temperature coefficient of resonant frequency (τf) value for the analogous layered-type systems, the research on the structure-properties relations between the contribution of the different electrical microstructures and dielectric loss are also discussed systematically, so as to study the effect and mechanism of microstructural characteristics on the dielectric properties in the Mgn+1TinO3n+1 systems, which will further make a theoretical and experimental foundation on the definite origination mechanism in ultra-low dielectric loss for these kinds of ceramic materials.

基于前期研究，当少量Mgn+1TinO3n+1特殊层状相与MgTiO3相共存时，体系品质因数（Q×f）即可超过380,000GHz。这意味着在介电常数（εr=15~25）范围内，该微波陶瓷体系将有望成为可超越现阶段理论低损耗极限(Q×f≥450,000GHz)的新型材料。基于此，本项目提出一种“类层状”相概念，通过分析体系结构调控机制以优化该类层状相的占比率(≥90%)；进而以高含量类层状相体系为研究对象，结合透射电镜测试、X射线衍射精修和鲍林规则等，构建不同组分类层状相原胞模型，并通过第一性原理计算对构建模型进行修正。同时，基于计算所得振动模色散参数和红外-拉曼光谱拟合数据，确认晶格内部与介质损耗最为关联的振动模式，试图从晶体结构特征以及相关键振动、晶格振动类型所引起的本征损耗，来阐释类层状相超低损耗的来源和机理。另外，通过在离子引入调控体系谐振频率温度系数（τf）的过程中，建立不同微区电学贡献与介质损耗构效关系，从而综合探究Mgn+1TinO3n+1陶瓷的微结构特征对介电性能的影响及作用机制，进一步为明确该类材料的超低损机理奠定理论与实验基础。

在相对介电常数系列化区间，追求具有超低损耗极限的微波介质陶瓷，并分析其本征因素对材料基体介质损耗程度的影响机制一直被视为该领域的研究热点和难点。本项目提出一种具有超低介质损耗的新型Mgn+1TinO3n+1系微波陶瓷，以结构调控后类层状相含量较高的体系为研究主体，结合综合物理性能测试等方法，试图获得可实用化的类层状系微波陶瓷材料及其结构特征与超低介质损耗构效关系。主要研究内容为：以MgTiO3为原胞，结合Srn+1TinO3n+1晶体结构，构建Mgn+1TinO3n+1类层状晶体结构，当n值在2-5之间时，微波频段内该类层状体系具有超低的介质损耗（Q×f≥300,000GHz），但其谐振频率温度系数未能处于-10~+10ppm/°C之间，继而采用不同的烧结方式和制备工艺，如埋烧法、微波烧结、球磨原料和非化学计量比等方法，致力于在品质因数得以稳定时，获得具有一定超低介质损耗体系重复性的工艺，在此基础上，通过复合Srn+1TinO3n+1以及引入Zr、Ce、Sn 调控类层状体系B位，Cu、Zn、Ni调控A位，以降低体系温度系数，但结果表明均未能在维持较低介电损耗的同时，获得处于-10~+10ppm/°C之间的温度系数。复合体系未能形成两相共存体系，且A、B位离子调控在引入添加含量较少时也未能形成固溶体，分析其原因主要有两方面：（1）Mgn+1TinO3n+1（n=2-5）类层状体系在只有Mg、Ti、O三种元素时，经过精细的含量成分调控，可形成一部分的层状相，较高的相纯度处于20~30%，若再进一步提高含量，通过本工作的物理制备方法未能达成，类层状相可能类似于Ca2TiO4的结构，对化学计量比要求较高，否则易形成不稳定相。（2）当采用固相法进行A、B位离子替代改性体系介电性能时，Mg离子本身具有一定挥发性，且Mg离子半径适中，不一定能始终保持在A位，置换改性时一部分Mg进入B位或者和其它元素形成新的化合物，致使预期的共存物相和单相化合物未能形成。另外，本工作还对超高介电常数体系Bi0.5Na0.5TiO3基和超低介电常数体系硼酸高分子基材料的微波介电性能及其微结构特征等进行了系统的分析。其中，0.7H3BO3-0.3PE在具有防潮特性的同时，介电性能较优：εr≈2.38，Q×f≈46,000GHz和τƒ≈+9ppm/°C。