Department of Materials Science and Engineering, Nagoya Institute of Technology^* Materials Research & Development Div. General R & D Laboratories. Taiyo Yuden Co., Ltd., Japan^**
○Mio Terada^* Keizou Kawamura^** Isao Kagomiya^* Ken-ichi Kakimoto^* Hitoshi Ohsato^*

Cordierite (Mg₂Al₄Si₅O₁₈) is characterized by a lower dielectric constant (ε_r) and lower temperature coefficient of resonance frequency (τ_f) among the systems of MgO-Al₂O₃-SiO₂. ε_r of cordierite ceramics prepared by standard solid state reaction method showed 6.19 and its τ_f exhibited -24 ppm/^oC, which was more close to zero compared to other isotropic silicates. However, quality factor (Qf ) value is relatively lower compared to more isotropic silicate such as forsterite, diopside, etc. This study investigated an effect of Ni substitution to improve the Qf value. In addition, we discuss relationship between the crystal structure of the Ni substituted cordierite and microwave dielectric properties, particularly Qf from the viewpoint of crystal structure refined by Rietveld analysis. (Mg_1-xNi_x)₂Al₄Si₅O₁₈ with no secondly phase was obtained in the compositions range of x = 0 - 0.1. Furthermore substitution of Ni caused presence of the secondary phase. ε_r of (Mg_1-xNi_x)₂Al₄Si₅O₁₈ solid solution slightly increased with increasing the composition x. The τ_f were almost comparable in the range from x = 0.05 to 0.2. On the other hand, the τ_f abruptly shifted toward negative value with increasing x from 0.3 to 0.5. The Qf value increased with substituting a small amount of Ni and the highest Qf value of 99,110 GHz was obtained at the composition x = 0.1. According to Rietveld analysis, hexagonal rings composed of corner sharing (Si,Al)O₄ tetrahedron in the (Mg_0.9Ni_0.1)₂Al₄Si₅O₁₈ are more isotropic than that of the Mg₂Al₄Si₅O₁₈, suggesting that the Qf value is improved with the slight variation of the hexagonal rings.