Phase evolution, thermal and microwave dielectric properties of cordierite-Al₂O₃ composite were investigated. As the content of Al₂O₃ increased, mullite, sapphirine, and spinel were formed as secondary phases, implying that cordierite may be decomposed by the reaction with Al₂O₃. All sintered specimens exhibited dense microstructures. The densification occurred through liquid phase sintering. As the content of Al₂O₃ increased, the thermal expansion coefficient and the dielectric constant increased, whereas the quality factor decreased. The thermal expansion coefficient, the dielectric constant, and the quality factor of the 90 wt% cordierite 10 wt% Al₂O₃ composite sintered at 1,425℃ were 2.9×10^-6 K^-1, 5.1, and 34,844 GHz, respectively.

We investigated the phase evolution, microstructure, and microwave dielectric properties of Na- and Zr-doped Ba(Mg_0.5W_0.5)O₃ [i.e., (Ba_1-2xNa_2x)(Mg_0.5-xZr_xW_0.5)O₃] ceramics. BaWO₄ as a secondary phase was observed in all compositions, and it increased as the dopant concentration increased. All specimens revealed a dense microstructure. For the composition of x=0.01, polyhedral grains were observed. As the dopant concentration increased, the densification and the grain growth were promoted by a liquid phase. The quality factor(Q×f₀) decreased remarkably, whereas the dielectric constant (ε_r) tended to decrease as the dopant concentration increased. The dielectric constant, quality factor, and temperature coefficient of the resonant frequency of the composition of x=0.01 sintered at 1,700℃ for 1 h were 18.6, 216,275 GHz, and -22.0 ppm/℃, respectively.

Sintering, microstructure, thermal conductivity and microwave dielectric properties of xLiF-(1-x)MgO ceramics (x=0.03-0.10 mol) were investigated. The high density was obtained in the specimens of x≥0.06, i.e., 0.04 LiF-0.96 MgO in mol, whereas the amount of 0.03 mol LiF was insufficient to densify. From the result that the contact flattening in the sintered specimen was observed, the densification occurred through the liquid-phase sintering. The specimen of x=0.06 showed the highest room-temperature thermal conductivity. Relative density, thermal conductivity, dielectric constant, and quality factor (Q x f) of the specimen for x=0.06 sintered at 900°C for 4 h were 97.8%, 39.2 Wm-1K-1, 9.45, and 14,671 GHz, respectively.

Sintering and microwave dielectric properties of Zn2-2xSil+xO4 (x=O-0.10) ceramics were investigated. The secondary phase of ZnO was observed in the specimen for x=O whereas SiO2 was detected in that for x=0.05. The composition of Zn2SiO4 might be close to x=0.02, i.e., Zn1.96Si1.02O4; the ratio of Zn/Si is 1.922. The insufficient grain growth was observed in the specimen of x=O. For the specimens of x≥0.05 , the grain growth sufficiently occurred through the liquid phase sintering. The value of quality factor of all specimens was dependent on the x value, i.e., the ratio of Zn/Si, whereas that of dielectric constant was independent. Relative density, dielectric constant, and quality factor (Q×f) of the specimen for x=0.05, i.e., Znl.9Si1.05O4, sintered at 1,400℃ were 96.5%, 6.43, and 115,166 GHz, respectively.

(Na,K)NbO3-based piezoelectric ceramics were synthesized by a liquid phase sintering method with a selected glass frit. The effects of the content of the glass frit and the sintering temperature on the microstructure and the electrical properties of the samples were investigated. With the 0.1 wt% of glass frit content, (Na0.52K0.44Li0.06)(Nb0.84Ta0.10Sb0.06)O3 (NKL-NTS) ceramics showed the maximum values of the relative density (99.1%) and the electro-mechanical coupling factor (kp: 0.32) at the sintering temperature of 1,050℃. It might mean that a liquid phase sintering with a suitable glass frit having the lower flow temperature could improve the relative density and the piezoelectric properties.