We report observation of deposition of SiO2 thin films on a variety of surfaces in nitric acid and hydrochloric acid solutions. Scanning electron microscopy shows that the films are composed of closely packed spheres 1000 Å in diameter. Chemical identification is made with Auger electron spectroscopy. The films are generally between 1000 and 2000 Å thick and are possibly hydrated. Deposition occurs in either acid for molarities in the 1-8 M range and at temperatures between 80 and 100 °C. The effects for molarities greater than 8 M and temperatures greater than 100 °C have not been investigated. Materials on which the films form in nitric acid are tantalum, gold, silicon, niobium and Al2O3; in hydrochloric acid, films will form on tantalum, gold, silicon and Al2O3. Materials which are insoluble in the acids but upon which films have not been observed to form are graphite, platinum, chromium (nitric acid) and niobium (hydrochloric acid). Deposition occurs inside a sealed vessel made entirely of Pyrex-type glass. Vessels are partially filled with acid and are heated so that the upper glass surfaces (not in contact with the acid reservoir) are about 10-30 °C cooler than the reservoir. Acid vapors condense on the cooler glass surfaces forming droplets that run down the vessel walls to the reservoir. This process of acid flow on the walls leaches SiO2 from the glass and increases the SiO2 concentration in the reservoir until the SiO2 films form. It is believed that a supersaturated condition is present during SiO2 deposition. Experiments show that cessation of acid condensation halts film formation. Attack of the vessel provides the sole source of SiO2. Preliminary kinetic measurements indicate that vessels must be heated for periods longer than 150 h before deposition occurs. Iron can be incorporated into the SiO2 films by the dissolution of iron metal in the acid before sealing of the vessel. © 1982.



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10 Sep 1982

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