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MSO4 (M = Ca, Sr, Ba) ceramics with relative densities exceeding 96% were prepared by solid‐state sintering at low sintering temperatures of 625–875°C, and their structures and microwave dielectric properties at 10–19 GHz were characterized. MSO4 ceramics crystallized in orthorhombic symmetry with the space groups of Amma for CaSO4 and Pnma for SrSO4 and BaSO4. The optimal microwave dielectric properties were obtained with εr = 5.85, Qf = 57 000 GHz, τf,W = −98.8 ppm/°C for CaSO4, εr = 10.95, Qf = 15 500 GHz, τf,W = 101.6 ppm/°C for SrSO4, and εr = 9.42, Qf = 38 200 GHz, τf,W = −4.7 ppm/°C for BaSO4. The increased εr and τf,W while decreased Qf value in the order of CaSO4, BaSO4, and SrSO4 were attributed to the enhanced rattling effect of M2+. Besides, the temperature dependence of τf was weak for CaSO4 and BaSO4, whereas much stronger for SrSO4. As most low‐εr microwave dielectric ceramics are of large negative τf, the near‐zero τf of BaSO4 and positive τf of SrSO4 are rare, indicating they are potential candidates for millimeter‐wave communication as a temperature‐stable dielectric ceramic and a compensator for tuning negative τf to near‐zero, respectively. [ABSTRACT FROM AUTHOR]

The temperature coefficient of resonant frequency (τf) is a key parameter for microwave dielectric ceramics, and it is usually regarded as a temperature-independent constant over a wide temperature range. However, the present theoretical prediction shows that the resonant frequency (f0) first increases then decreases with temperature, and τf decreases monotonously in Al2O3–TiO2 composite with small τf, which fits the experimental data very well. The nonlinear variation of f0 with temperature and temperature-dependent τf are attributed to the temperature-sensitive permittivity (εr) and τf of the constituting phases and also dependent on the dielectric mixing rule that the composite obeys. Such temperature-dependent dielectric behaviors are predicted to be common in microwave dielectric composites with small τf, and the negligence of them probably conceals the real large variation of f0 with temperature and seriously misleads the practical applications. Therefore, it is strongly suggested to understand the real temperature dependence of f0 and τf in microwave dielectric composites by measuring f0 at more temperatures with a smaller interval, so that the central frequency shift can be correctly evaluated for the resonator units and devices where the microwave dielectric composites are utilized. Furthermore, the nonlinear variation of f0 with temperature and temperature dependence of τf are expected to be suppressed by microstructural engineering and adopting more suitable constituting phases. [ABSTRACT FROM AUTHOR]

BPO4 ceramics with a cristobalite-like structure were prepared via solid-state sintering (SSS) and spark plasma sintering (SPS). Significant shrinkage and grain growth were observed with increasing the sintering temperature up to 1200 °C for SSS samples, while the highest relative density is only 74.6% because of the serious sublimation of BPO4, and the microwave dielectric properties of εr = 3.38, Qf = 18,200 GHz and τf = −42.8 ppm/°C were achieved at 14.6 GHz for SSS samples sintered at 1000 °C for 3 h. A much higher relative density of 92.7% was obtained in SPS sample (1000 °C/10 min) with εr = 4.20, Qf = 4,000 GHz, and τf = −20.7 ppm/°C. The Qf value increases by a factor of 6.9 and 5.9 with increasing frequency from 4.6 GHz to 14.6 GHz for SSS and SPS samples, respectively, which implies that the Qf value is dominated by extrinsic factors and has a huge improvement space. The corrected dielectric constant by porosity is only 4.65, indicating that BPO4 ceramic is a promising candidate as the microwave substrate material if the relative density and microstructure can be further improved. [ABSTRACT FROM AUTHOR]

Ba5LaTi3Ta7O30 tungsten bronze ceramics is a typical linear dielectric with high dielectric constant and low loss, which is expected as a promising candidate for energy‐storage application. In the present work, energy‐storage properties of Ba5LaTi3Ta7O30 tungsten bronze ceramics were investigated, and the dielectric breakdown mechanism was also discussed based on the morphology of the fracture surface. Dense ceramics were obtained by sintering at temperatures from 1500 to 1575°C. Both the dielectric constant and dielectric strength show strong dependences on the sample density. Optimal dielectric constant of 159, dielectric strength of 639 kV/cm, and energy storage density of 2.9 J/cm3 were obtained for ceramics sintered at 1550°C, that have the highest density and fine grains. The dielectric strength and energy storage density were also highly dependent on the sample thickness. For ceramics sintered at 1525°C, the dielectric strength increases from 283 to 585 kV/cm while the energy storage density increases from 0.5 to 2.3 J/cm3 when the thickness changes from 0.5 to 0.2 mm. A thermal breakdown mechanism was adopted to understand the breakdown process in the present ceramics. [ABSTRACT FROM AUTHOR]

Abstract: The structures and dielectric properties of La1.75Sr0.25Ni1−x Al x O4 (x =0, 0.3) ceramics were presented. A single tetragonal phase was found in La1.75Sr0.25NiO4 ceramics, while minor secondary phases were presented in La1.75Sr0.25Ni0.7Al0.3O4 ceramics. Giant dielectric response was found in these ceramics. The dielectric constant was enhanced while the dielectric loss was suppressed by partially substituting nickel with aluminum ions. After comparing the activation energies of dielectric relaxation and electrical resistivity, we concluded that the overlapped low frequency dielectric relaxation was attributed to grain boundaries, while the normal low-temperature relaxation was mainly attributed to the bulk factor, that was, thermally activated small polaronic hopping in La1.75Sr0.25NiO4 ceramics. For La1.75Sr0.25Ni0.7Al0.3O4 ceramics, the low-temperature dielectric relaxation was also mainly attributed to the bulk factor. The enhanced dielectric response should be benefited from the strengthened grain boundary layer capacitor effect in La1.75Sr0.25Ni0.7Al0.3O4 ceramics. [Copyright &y& Elsevier]