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Thin polycrystalline films of VO2 and V2O3 were deposited on a variety of substrates using a sol-gel process. The orientation, microstructure, optical constants, and optical and electrical switching behavior are presented. These films exhibited sharp optical switching behavior even on an amorphous substrate such as fused silica. The method yields reproducible results and is amenable to the coating of large substrates and curved surfaces such as mirrors and lenses.

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were observed, depending on the oxidizing conditions during aging. Second-phase ALTiO,, located primarily at three-grain intersections, was observed in all samples containing more than 0.45% Ti after they were aged at 1500°C. This phase was colorless in equilibrium with air and dark blue in both moist and dry Hz and was the first second-phase material to appear in polycrystalline titania-doped alumina under all conditions studied. Three types of intragranular precipitates were also observed, depending on the oxidizing conditions. The classical oriented needles reported previously were observed in the samples containing more than 0.45% Ti after they were aged in air. These precipitates were not identified, except that one subsidiary X-ray peak was found adjacent the (012) peak in sapphire, with a d spacing 0.02 .A larger than that of the (012) peak. This observation agrees qualitatively with the results of White." Similar needles less perfectly oriented with the matrix were present after samples were annealed at 1.6X lo-' atm oxygen partial pressure. A sample containing 1.65% Ti was aged for 2 weeks in air at 1500°C; the needles were much less concentrated and were shorter and wider. At still lower 0, partial pressures, the needles became less and less oriented, in that the angles between them tended to be random: occasionally, they were kinked or bent. In all cases the needles appeared to be on a plane, presumably the basal plane.Samples containing more than 0.91% Ti aged for 1 day or more in moist Hz with an 0, partial pressure of lo-" atm or less contained some blade-shaped precipitates about 30 to 50 pm long and 10 to 20 pm wide which were apparently oriented on a crystallographic plane, perhaps the basal plane. These precipitates appeared brown; the grains in which they were oriented parallel to the plane of polish appeared brown also, whereas their neighbors were blue under low magnification.Samples aged for a few hours at 1500°C in air show the entire range of second phase and precipitates described. A white layer approximately 0.03 in. thick was observed after 4 h aging at 1500°C in air. This layer contained oriented needles approximately 50 pm long and a colorless grain boundary precipitate of ALTiO,. Deeper into the sample, the matrix became blue and the intergranular second phase dark blue. The oriented needles overlap this region of the sample. In the core of the sample, which is generally brown, the dark blue ALTiO, grain boundary second phase occurs with or without some of the kinked needle-shaped or blade-shaped precipitates.Two comments are appropriate concerning the oxidation of these samples: (1) The process occurred relatively rapidly, probably by diffusion down the grain boundaries; (2) samples containing 1.4% or more Ti disintegrated to powder after they were aged in air at 1500°C for 24 h. This behavior apparently resulted from the weakening effect of A12TiQ5 in the grain boundaries. Cracks between ALTiQ, and the matrix were frequently observed, indicating a greater thermal expansion coeffici...

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