Preparation of Crystalline Ceramic Materials by Hydrothermal Electrolytic Oxidation

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A new technique : hydrothermal electrolytic (anodic) oxidation, has been developped to fabricate films or fine powders of oxides by combining hydrothermal oxidation and anodic (electrolytic) oxidation techniques for metals and alloys. The results and summary are as follows:1. An autoclave, max 300゜C, 20 MPa, 980 ml with 3 inner electrodes has been fabricated.2. In the anodic oxidation of Zr under hydrothermal conditions, continuous anodized film was formed on Zr plate below 200゜C in <H_2SO_4> or Y <(NO_3)_3> solutions.3. Above 200゜C,Zr plate showed pitting and/or edge dissolution under high potential above 3 V in conc. Y <(NO_3)_3> solutions.4. The surface and the precipitates of the Zr plate were monoclinic <ZrO_2> polycrystals with the crystallite size of submicron.5. A small amount of Y was detected in the products by ESCA.The anodic oxidation of Nb plate in hydrothermal LiOH, <Li_2CO_3> and <LiNO_3> solutions gave th e results as follows:6. Passivation of Nb was established by the formation of the anodic oxide film in any conditions studied.7. The anodized films were amorphous by X-ray diffraction.8. Potentiostatic analysis at 4.5 V indicated the leakage current increased with temperature from 100゜ to 200^C but decreased at 250゜C because of the densification of the film at higher temperatures.9. In the galvanostatic anodic oxidation at 250゜C in 0.1 N- <LiNO_3> , oxide film formed in relation to the applied current in innitial stages but almost stopped after a certain period when a low current density was applied.By the hydrothermal oxidation method, the metals of Zn, Ti and Nb yielded ZrO, <TiO_2> (rutile) and monoclinic <Nb_2O_5> as the final products.