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N. Kikuchi, T. Sei, T. Tsuchiya, S. Hayashi and K. Hayamizu
[J. Ceramic Soc. Jpn., Vol.101, No.7, pp.824-829, 1993]
Cordierite ceramics were prepared by sol-gel process using metal salts and a metal alkoxide as the starting materials, and their electrical and thermal properties were studied. Best starting materials were aluminium nitrate, magnesium acetate and tetraethoxysilane. Any other salts did not give a homogeneous solution and deposited after gelation. The solvent effect was very drastic. A highly homogeneous solution was obtained by using dihydric alcohols as a solvent, such as ethylene glycol. DTA and XRD were measured to identify the crystal phases of sintered specimens. IR spectra and NMR spectra were measured to study the solvent effects on the amorphous structure. DTA and XRD revealed that m-cordierite single phase was obtained at 900C° and α-cordierite single phase at 1100°C using ethylene glycol as a solvent. However other phases, such as spinel and cristobalite, were obtained using ethanol as a solvent. IR and NMR data suggested that aluminum ions were mainly six-coordinated ([Al(H_{2}O)_{6}]^{3+} etc.) in wet gels, but the amount of four-coodinated aluminium ions ([AlO_{4}]) increased at the expense of six-coordinated aluminum ions with increasing heating temperature. It was considered that almost all aluminum ions were present as hydrated ions in wet gels and incorporated into four-coordinated silica network by heating. It was found from NMR measurements that five-coordinated aluminum ions exist in amorphous samples. Dense α-cordierite ceramics were obtained by calcining the gel powder at 700°C for 3h and by sintering at 1300°C. The thermal expansion coefficient and relative dielectric constants were evaluated as 1,5×10^{-6} K^{-1} (r.t. ~800°C and 5.0 ( ļ1 MHz), respectively (Fig.1).
