Diethanolamine (DEA) suppressed the precipitation of the hydroxide on the hydrolysis of magnesium aluminium isopropoxide MgAl
2(O-iPr)
8 (MAI), and a clear solution was obtained even after the addition of water to the isopropanol solution of MAI and DEA, resulting in gellation on addition of excess water. The conditions for preparing the clear solution, MAI concentration, DEA/MAI molar ratio (R), and H
2O/MAI molar ratio (W), were determined. When a suitable plate was dipped into the resulting clear solution, pulled up, dried and heated (600°-650°C) in air, amorphous spinel thin film was obtained. The maximum thickness of the uniform and transparent films was about 100nm, and thicker films could be prepared by repeating the procedure. The refractive indices of the films were 1.60-1.72. They decreased with increasing film thickness. A crystalline film could be formed on a Si-wafer by the heat-treatment around 800°C, but the many cracks were observed in the resulting film. The film on a quartz glass plate remained amorphous at 800°C due to the reaction with the plate, and was converted to cordierite above 1200°C. The film on a glass plate also seemed to react with the glass plate at temperatures as low as 600°C.
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