A novel Na_1−xK_xTaO₃ (x = 0, 0.025, 0.05, 0.075, 0.1, and 0.15) ceramic with high permittivity and high positive temperature coefficient was synthesized via the conventional solid-state method. All samples were determined to be pure phase orthorhombic NaTaO₃ structure of space group Pmcn, and larger grain and lower porosity were observed after adding an appropriate amount of K⁺ ions. The Q × f value is majored by the packing fraction and grain size, while the value of τ_f is influenced by Ta–O bond valence. The Na_0.95K_0.05TaO₃ ceramic possesses excellent dielectric properties of ε_r = 164.29, Q × f = 9091 GHz (f = 3.15 GHz), tanδ = 3.46×10^–4, τ_f = +809.52 ppm/℃, sintered at 1550 ℃. Compared with NaTaO₃ ceramics, the Na_1−xK_xTaO₃ ceramics prepared in this study demonstrate higher dielectric constants and higher positive temperature coefficients, which are promising for device miniaturization and τ_f compensators.

In this study, tri-rutile type Mg_0.5Ti_0.5TaO₄ ceramics were synthesized, where the structure–property relationship, especially the structural configuration and intrinsic dielectric origin of Mg_0.5Ti_0.5TaO₄ ceramics, and the low-firing characteristics were studied. It is found that the tri-rutile structural type is unambiguously identified through the Rietveld refinement analysis, the selected area electron diffraction (SAED), and the high-resolution transmission electron microscopy (HRTEM) along the [110] zone axis. With the increase in sintering temperature, the densification and uniformity of crystal growth play important roles in regulating the microwave dielectric properties of Mg_0.5Ti_0.5TaO₄ ceramics. Intrinsically, theoretical dielectric properties calculated by the far-infrared reflective spectra approached the experimental values, indicating the importance of structural features to dielectric properties. Furthermore, a glass additive with high matching relevance with ceramics has been developed to decrease the high sintering temperature of Mg_0.5Ti_0.5TaO₄ ceramics, where 2–4 wt% Li₂O–MgO–ZnO–B₂O₃–SiO₂ (LMZBS) glass frit was adopted to reduce the suitable temperature from 1275 to 1050 ℃ without significantly deteriorating the microwave dielectric characteristics. Specifically, Mg_0.5Ti_0.5TaO₄ ceramics containing 2 wt% glass addition sintered at 1050 ℃ for 4 h possess excellent microwave dielectric properties: dielectric constant (ε_r) = 44.3, quality factor multiplied by resonant frequency (Q×f) = 23,820 GHz (f = 6.2 GHz), and the temperature coefficient of resonant frequency (τ_f) = 123.2 ppm/℃.

The explosive process of 5G communication evokes the urgent demand of miniaturized and integrated dielectric ceramics filter. It is a pressing need to advance the development of dielectric ceramics utilization of emerging technology to design new materials and understand the polarization mechanism. This review provides the summary of the study of microwave dielectric ceramics (MWDCs) sintered higher than 1000 ℃ from 2010 up to now, with the purpose of taking a broad and historical view of these ceramics and illustrating research directions. To date, researchers endeavor to explain the structure-property relationship of ceramics with multitude of approaches and design a new formula or strategy to obtain excellent microwave dielectric properties. There are variety of factors that impact the permittivity, dielectric loss, and temperature stability of dielectric materials, covering intrinsic and extrinsic factors. Many of these factors are often intertwined, which can complicate new dielectric material discovery and the mechanism investigation. Because of the various ceramics systems, pseudo phase diagram was used to classify the dielectric materials based on the composition. In this review, the ceramics were firstly divided into ternary systems, and then brief description of the experimental probes and complementary theoretical methods that have been used to discern the intrinsic polarization mechanisms and the origin of intrinsic loss was mentioned. Finally, some perspectives on the future outlook for high-temperature MWDCs were offered based on the synthesis method, characterization techniques, and significant theory developments.

This article presents low-firing, low-loss and temperature stable ZnO-TiO₂-Nb₂O₅ microwave dielectric composite ceramics with the assistance of lithium borosilicate (LBS) and zinc borosilicate (ZBS) glass frits. There is a liquid phase (eutectic mixture) generated by LBS (ZBS) glass, and solid particles could be wetted and dissolved. Therefore, the migrations and rearrangements of particles could be performed. Besides, compared with ceramics undoped with glass frits, lower activation energies (E_a) of ceramics doped with LBS and ZBS glass suggest that the low-temperature sintering behavior is easier to carry out. The results indicated that LBS and ZBS glass both are effective sintering aids to accelerate the sintering process and improve the microwave dielectric properties of composite ceramics by controlling the phase compositions under low temperature. Combination great properties of ZnO-TiO₂-Nb₂O₅ ceramics were obtained when sintered at 900 ℃ for 4 h: ε_r = 36.7, Q × f = 20,000 GHz, τ_f = 7 ppm/^oC.