Densification and Crystallization Of Thin Transition Metal Oxide Coatings From Metal Alkoxides
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DENSIFICATION AND CRYSTALLIZATION OF THIN TRANSITION METAL OXIDE COATINGS FROM METAL ALKOXIDES H. HIRASHIMA, R. MURATAKE, T. YAMASHITA, and T. CHIBA Department of Applied Chemistry, Faculty of Science and Technology, University, 3-14-1, Hiyoshi, Kohoku-ku, Yokohama 223 Japan
Keio
ABSTRACT Monolithic gels of V 205 , Ta 2 O5 , and Nb O2 have been prepared from VO(C 2 H5 0) , Ta(C HO) , and Nb(C 2 H5 O) througg hydrolysis in ethanol Solutions. TLe conditions for gel formation have been determined. Thip coatings of these gels, less than 100 nm in thickness, have been prepared by dip coating. Changes in the thickness, d, and refractive indices, n, have been measured by ellipsometry during drying. In the first stage of drying, d decreases and n increases rapidly. Densification of the coatings takes place due to evaporation of solvent. In the following stage, increase in d and decrease in n have been observed for thin V2 05 gel coatings, about 30 nm or less in thickness. These results suggest that coarsening of pores and/or increase in surface roughness take place due to release of the capillary pressure during drying. Both d and n have increased with increasing number of dipping. After heat treating, decrease in d and increase in n have been observed.
INTRODUCTION Thin coatings of transition metal oxides have interesing physical properties, such as electronic and optical properties. They can be applied for microelectronics, micro-optics, protective coating applications, etc.. Thin oxide coatings of good homogeneity can be prepared through the sol-gel technology [1]. The sol-gel process is considered to be suitable for preparation of thin coatings. In this work, thin coatings of V 05 , Ta 2 05 , and Nb2 0 have been prepared from metal alkoxides through hygrolysis and dip coating. Densification and crystallization of these coatings during drying and heat treating have been investigated. Hydrous V2 0 gel is known to be highly conductive and anisotropic [2]. Ta 0 and Nb 0 gave high dielectric constant. Preparation of these oxide geis from meial alkoxides have been reported [3-6].
EXPERIMENTAL PROCEDURE Metal alkoxides, VO(C2H0 ) 3 , Ta(C 2 H O) 5 ", and Nb(CH 5 0)5 , were dissolved into anhydrous ethanol, ang gydrolyzed aý room temperaiure with and without catalysts. HCI or other inorganic acids have been used as catalyst. The conditions for gel formation, concentrations of H2 0 and metal alkoxides, and pH have been determined. pH of alcoholic solutions has been measured by glass electrode pH meter using an electrode with a variable sleeve. Crystallization behaviours of the gels have been investigated by TGA-DTA and X-ray diffraction. Gel coatings have been prepared by dip coating. The sols or ultrasonically peptized gels have been used as dipping solutions. Soda-limesilicate or silica glass plates, whose surface roughness was about 6 nm, have been used as substrates. The withdrawal rate has been 5 to 20 cm/mmn. Dipping has been repeated after drying at room temperature or after heat treating. The gels and the surface of
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