微波介质陶瓷具有极低的介电损耗，是一种可用于低频、微波、毫米波—太赫兹等各个频段的介质陶瓷材料。近年来高频通信、高温电容、无线无源传感、太赫兹超材料等新的应用场景不断涌现，对微波介质陶瓷的性能提出了更苛刻的要求。在厘清微波介质陶瓷的结构、缺陷与介电性能之间关系的基础上，对其介电性能进行调控，尤其是进一步降低介电损耗，是当前微波介质陶瓷研究的重点。本文选取钨青铜结构、钙钛矿结构、金红石结构TiO2三类中高介电常数微波介质陶瓷体系，利用热激励去极化电流（TSDC）、透射电子显微镜（TEM）等方法对其中的缺陷及产生机制进行了表征与分析，借助太赫兹时域谱（THz-TDS）等方法讨论了其宽频介电响应与结构、缺陷的关系，并通过组成和工艺的调节对其性能进行了优化。以不同的离子取代/掺杂方法对钨青铜结构Ba4(Sm,Nd)9.33Ti18O54陶瓷进行了改性。结果表明，Zr4+和Hf4+对Ti4+的取代导致氧空位浓度增大、陶瓷Q×f值减小，而Al3+对Ti4+的取代则使氧空位浓度减小、Q×f值增大，这与离子间结合力和晶格振动行为的变化有关。其中，Al3+取代的陶瓷获得了εr = 67.3，Q×f = 16530 GHz，τf = +0.9 ppm/℃的优异性能。进一步设计了A位和B位离子共同掺杂的改性方式，陶瓷性能得到优化：εr = 72.2，Q×f = 16480 GHz，τf = +14.3 ppm/℃。以Sm3+和Al3+取代的方法对CaTiO3陶瓷进行改性，通过对离子间结合力和缺陷浓度的调控，优化了其宽频介电性能：取代后的陶瓷在200 ℃以下的储能和充放电特性表现出优异的温度稳定性，有望满足高温应用需求；微波和太赫兹频段的介电损耗大幅降低。而Al3+/Nb5+复合离子的取代则使Na1/2Sm1/2TiO3陶瓷在微波和太赫兹频段的介电损耗呈现不同的变化趋势，揭示了缺陷对不同频段介电响应的作用机制。通过钨青铜结构与钙钛矿结构陶瓷的叠层复合，获得了中介电常数（εr ~ 60）、高Q×f值（> 20000 GHz）的温度稳定陶瓷。对金红石结构TiO2陶瓷的研究发现，其中主要含有层错与氧空位两类缺陷，分别由烧结温度和烧结气氛决定，通过工艺调节可实现陶瓷中缺陷与性能的调控。减少缺陷可显著降低介电损耗，例如，氧气气氛中1100 ℃烧结的陶瓷可获得10–3量级的低频介电损耗和微波频段49900 GHz的高Q×f值，空气中1050 ℃烧结的陶瓷在0.5 THz下获得了低至0.012的介电损耗。用适宜的烧结温度在陶瓷中构筑层错区，则可使陶瓷表现出巨介电效应，其低频下的相对介电常数约为3000。
Microwave dielectric ceramics, benefiting from their extremely low dielectric loss, can be used in a multitude of bands like low-frequency, microwave, millimeter wave, and terahertz. Recently, new applications including high-frequency communications, high-temperature capacitors, wireless passive sensing, and terahertz metamaterials have emerged, imposing stricter requirements on the properties of microwave dielectric ceramics. Based on a thorough understanding of the relationship between the structure, defects, and dielectric properties of microwave dielectric ceramics, current studies focus on the improvement of dielectric properties, especially on the further decrease in dielectric loss. In this paper, three microwave dielectric ceramic systems including tungsten bronze, perovskite, and rutile TiO2 with medium/high permittivity are selected under study. Thermally stimulated depolarization currents (TSDC) and transmission electron microscopy (TEM) are used to analyze the defects and their generation mechanism systematically. The relationship between their broadband dielectric response, structure and defects is investigated using terahertz time-domain spectroscopy (THz-TDS). The dielectric properties of the ceramics are optimized through the adjustments to composition and process.The Ba4(Sm,Nd)9.33Ti18O54 ceramics with tungsten bronze structure were modified with different ion-substitution/doping methods. The Zr4+ and Hf4+ substitution for Ti4+ leads to an increase in oxygen vacancy concentration and a decrease in Q×f value, while the Al3+ substitution for Ti4+ lead to a decrease in oxygen vacancy concentration and an increase in Q×f value. Those are related to ionic binding forces and lattice vibration behavior. The Al3+-substituted ceramics obtained excellent properties with εr = 67.3, Q×f = 16530 GHz, and τf = +0.9 ppm/°C. Further modifications are designed by A- and B-sites co-doping, with the optimized properties of εr = 72.2, Q×f = 16480 GHz, and τf = +14.3 ppm/℃.The modification of CaTiO3 ceramics with Sm3+ and Al3+ substitution optimizes the broadband dielectric properties through the modulation of ionic binding forces and defect concentration. The substituted ceramics show excellent temperature stability in energy storage and charge-discharge characteristics below 200 ℃, which can meet the demand for high-temperature applications. The dielectric losses in the microwave and terahertz bands are significantly reduced. The Al3+/Nb5+ substitution in Na1/2Sm1/2TiO3 ceramics leads to different trends of dielectric loss in the microwave and terahertz bands, revealing the different mechanisms of defects on the dielectric responses in different frequency bands. The temperature-stable ceramics with medium-permittivity (εr ~ 60) and high Q×f values (> 20000 GHz) are obtained in tungsten bronze/perovskite layered ceramics.The study of rutile TiO2 ceramics shows that there are two main types of defects, including stacking faults and oxygen vacancies, which are determined by the sintering temperature and sintering atmosphere, respectively. The modification of defects and properties can be achieved by process adjustment. Reducing defects can significantly decrease the dielectric loss. For example, ceramics sintered at 1100 ℃ in oxygen show low-frequency dielectric loss around 10–3, and high Q×f values of 49900 GHz in the microwave band. Ceramics sintered at 1050 ℃ in air show a low dielectric loss of 0.012 at 0.5 THz. Constructing stacking fault domains with suitable sintering temperatures leads to giant dielectric response, with relative permittivity of around 3000 at low frequencies.
