Production of ceramic layers and their use

Details Production of ceramic layers and their use Production of ceramic layers and their use

1996-12-03

1999-03-23

Cameron, Erma

Coating processes

With post-treatment of coating or coating material

Heating or drying

427377, B05D 302

Patent

active

058856579

DESCRIPTION:

BRIEF SUMMARY
BACKGROUND OF THE INVENTION

The process of the invention describes the production of thin ceramic layers consisting of pure titanium dioxide or of titanium dioxide combined with other metal oxides at a temperature of 100.degree. C. within 5 minutes on ceramic and metal.
The production of thin ceramic layers from titanium dioxide is known. The best known processes, like the process of the invention, start with a hydrolyzable titanium compound, then hydrolyze this compound and dissolve the precipitate generated by the hydrolysis first with an acid, thereby creating a sol. Up to this point, the inventive process is identical with the known art.
In the processes known in the art, the sol is now transformed into a gel. This is, for instance, achieved by concentrating the sol. When the gel is concentrated further, it crystallizes and forms cracks. For this reason, a surfactant is added to the gel for obtaining a crack-free layer. At a temperature of approximately 400.degree. C. which, of course, has to be maintained over an extended period of time, the titanium dioxide particles fuse together and the surfactant evaporates or decomposes. A nanocrystalline titanium dioxide layer remains.
This process is energy intensive and time consuming and can be rather disadvantageous for practical applications, especially since the resulting strength of the titanium dioxide layers is not very high.


SUMMARY OF THE INVENTION

It is an object of the present invention to produce ceramic titanium dioxide layers which provide titanium dioxide layer with


DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

For a practical application, the inventive process is initially intended to be used for improving the production of solar cells, as described in PCT WO 91/16719, PCT WO 93/20569 and others. The processes described in the referenced literature have in common that collector electrodes from titanium dioxide can only be produced by employing surfactants. The disadvantageous production conditions described above are essential for achieving an acceptable strength of the titanium dioxide layer to be produced.
The stated object of the invention as described above is achieved by introducing a novel step after the formation of the sol following the hydrolysis of the titanium compound. The sol together with the nanocrystalline titanium dioxide is blended into a dispersion. This dispersion is applied in the form of a thin layer onto the substrate to be treated. The hydrochloric acid and the water contained therein evaporates upon heating to 100.degree. C. The titanium dioxide which was formed as a result of the hydrolysis and which is present in the sol in colloidal form, is consequently no longer in the stable equilibrium of the solution. At the same time, van der Waals binding forces are building up at the grain boundaries of the additional nanocrystalline titanium dioxide particles, thereby providing an exceptionally strong titanium dioxide layer after only a few minutes.
The stated object was also achieved by employing considerably shorter heating times and considerably lower temperatures for obtaining an improved strength of the titanium dioxide layers on the respective substrate.
Another area of use where the known method for forming titanium dioxide layers is employed, is the photocatalytic oxidation of waste water. Here, too, the inventive process overcomes the disadvantage of weak adhesion of the titanium dioxide layer to a glass substrate.
Additional experiments have shown that adding aluminum oxide in crystalline form such as ALCOA 3000 CT of a particular size exceeding 300 nm as well as in nanocrystalline form results in an equally good adhesion under the same conditions.
It has furthermore become apparent that rather advantageous results were obtained by adding other metal oxides. However, chemical reactions and the dissolution in a strongly acidic environment lead to mixed oxides, oxychlorides and the like. This outcome, however, does not prevent their utilization for special applications. With several metal oxides, a homogeneous

REFERENCES:
patent: 4271210 (1981-06-01), Yoldas
patent: 4286024 (1981-08-01), Yoldas
patent: 4614673 (1986-09-01), Bendig
Proceedings of the international society for optical engineering, vol. 1758, 20-22 Jul. 1992 San Diego, USA, pp. 67-76 H. Hirashima, T. Kusaka.
Thin solid films, vol. 219, No. 1/2, 30 Oct. 1992 Lausanne CH pp. 18-23, XP 000334896 L. Hu et al. "Effects of solvent om properties of sol-gel derived Ti02 coating films" see 2.1 and 2.2.
Yogyo-Kyokai-Shi, vol. 95, No. 2, 1987 Japan, pp. 150-155, T. Yoko et al. "Photoelectric properties of Ti02 films prepared by the sol-gel method".
Journal of materials science, vol. 29, No. 21, 1994 London GB, pp. 5773-5778, R. Mizutani et al. "Coating of polymethylmethacrylate with transparent Si02 thin films by a sol-gel method".

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