Microwave dielectric ceramics should be improved to advance mobile communication technologies further. In this study, we prepared Sr_1+xY₂O_4+x (x = 0–0.04) ceramics with nonstoichiometric Sr²⁺ ratios based on our previously reported SrY₂O₄ microwave dielectric ceramic, which has a low dielectric constant and an ultrahigh quality factor (Q value). The ceramic exhibited a 33.6% higher Q-by-frequency (Q×f) value (Q ≈ 12,500) at x = 0.02 than SrY₂O₄. All Sr_1+xY₂O_4+x (x = 0–0.04) ceramics exhibited pure phase structures, although variations in crystal-plane spacings were observed. The ceramics are mainly composed of Sr–O, Y1–O, and Y2–O octahedra, with the temperature coefficient of the resonant frequency (τ_f) of the ceramic increasing with Y2–O octahedral distortion. The ceramic comprises uniform grains with a homogeneous elemental distribution, clear grain boundaries, and no obvious cavities at x = 0.02. The Sr_1+xY₂O_4+x (x = 0–0.04) ceramics exhibited good microwave dielectric properties, with optimal performance observed at x = 0.02 (dielectric constant (ε_r) = 15.41, Q×f = 112,375 GHz, and τ_f = −17.44 ppm/℃). The τ_f value was reduced to meet the temperature-stability requirements of 5G/6G communication systems by adding CaTiO₃, with Sr_1.02Y₂O_4.02+2wt%CaTiO₃ exhibiting ε_r = 16.14, Q×f = 51,004 GHz, and τ_f = 0 ppm/℃. A dielectric resonator antenna prepared using Sr_1.02Y₂O_4.02+2wt%CaTiO₃ exhibited a central frequency of 26.6 GHz, with a corresponding gain and efficiency of 3.66 dBi and 83.14%, respectively. Consequently, Sr_1.02Y₂O_4.02-based dielectric resonator antennas are suitable for use in 5G millimeter-wave band (24.5–27.5 GHz) applications.

This study investigates the bulk density, sintering behaviour, and microwave dielectric properties of the MgO-2B₂O₃ series ceramics synthesised by solid-state reaction. According to the X-ray diffraction and microstructural analyses, the as-prepared MgO-2B₂O₃ ceramics possess a single-phase structure with a rod-like morphology. The effects of different quantities of H₃BO₃ and BaCu(B₂O₅) (BCB) on the bulk density, sintering behaviour, and microwave dielectric properties of the MgO-2B₂O₃ ceramics were investigated. Accordingly, the optimal sintering temperature was obtained by adding 30 wt% H₃BO₃ and 8 wt% BCB. We also reduced the sintering temperature to 825 ℃. Furthermore, the addition of 40 wt% H₃BO₃ and 4 wt% BCB increased the quality factor, permittivity, and temperature coefficient of resonance frequency of MgO-2B₂O₃ to 44,306 GHz (at 15 GHz), 5.1, and -32 ppm/℃, respectively. These properties make MgO-2B₂O₃ a viable low- temperature co-fired ceramic with broad applications in microwave dielectrics.

Lead-free ceramic capacitors have the application prospect in the dielectric pulse power system due to the advantages of large dielectric constant, lower dielectric loss and good temperature stability. Nevertheless, most reported dielectric ceramics have limitation of realizing large energy storage density (W_rec) and high energy storage efficiency (η) simultaneously due to the low breakdown electric field (E_b), low maximum polarization and large remanent polarization (P_r). These issues above can be settled by raising the bulk resistivity of dielectric ceramics and optimizing domain structure. Therefore, we designed a new system by doping (Bi_0.5Na_0.5)_0.7Sr_0.3TiO₃ into 0.9NaNbO₃-0.1Bi(Ni_0.5Zr_0.5)O₃ ceramics, which simultaneously obtained a higher bulk resistivity by decreasing the grain size and achieved a smaller P_r by optimizing domain structure, thus the better E_b of 530 kV/cm and W_rec of 6.43 J/cm³ were achieved, η was improved from 34% to 82%. Besides, the 0.4BNST ceramics show excellent temperature, frequency and fatigue stability under the conditions of 20–180 ℃, 1–100 Hz and 10⁴ cycles, respectively. Meanwhile, superior power density (P_D = 107 MW/cm³), large current density (C_D = 1070 A/cm²) and discharge speed (1.025 μs) were achieved in 0.4BNST ceramic. Finally, the charge-discharge performance displayed good temperature stability in the temperature range of 30 ℃–180 ℃. The above results indicated that the ceramics have potential practical value in the field of energy storage capacitor.