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.

With trend of the miniaturization and the high-functionalizing of mobile communication system, low-loss microwave dielectric materials are widely used for high frequency communication components. These dielectric materials should be co-sintered with highly electric-conducting metal such as silver or copper for high-frequency and thick film process application. Sintering temperature of Ca[(Li_1/3Nb_2/3)_1-xTi_x]O_3-δ, which has excellent dielectric properties such as ε_r above 40, quality factor (Q·f₀) above 16,000 GHz, and TCF (temperature coefficient of resonant frequency) of -20~-10 ppm/℃, is reported as high as 1,175℃, so it could not be co-sintered with silver or copper. Therefore in this study, low-temperature melting glasses of Zn-B-O and Zn-B-Si-O systems were added to Ca[(Li_1/3Nb_2/3)_1-xTi_x]O_3-δ to lower its sintering temperature under 900℃ without losing excellency of dielectric properties. With 15 weight % of Zn-B-Si-O glass and sintered at 875℃, specimen showed density of 4.11 g/cm³,ε _r of 40.1, Q·f₀ of 4,869 GHz, and TCF of -5.9 ppm/℃. With 15 weight % of Zn-B-O glass and sintered at 87 5℃, specimen showed density of 4.14 g/cm³, ε_r of 40.4, Q·f₀ of 7,059 GHz, and TCF of -0.92 ppm/℃.

We investigated the physical properties of stoichiometric and non-stoichiometric oxide doped complex perovskite, Ba(Zn1/3Ta2/3)O3 ceramics and their impacts on the microwave dielectric performances using various characterization techniques such as X-ray diffraction, scanning electron microscopy, transmission electron microscopy, and network analyzer. According to the measurement of lattice constant changes, anomalous lattice volume contraction of ZrO2 doped Ba(Zn1/3Ta2/3)O3 sample only showed the dielectric quality factor enhancements, which was due to the lattice volume contraction as well as the 1:2 B-site cation ordering. In addition, NiO doping was useful to the stabilization of temperature coefficient of resonance frequency.