Importance

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Abstract

The invention provides porous anodic aluminium oxide film having a major face with an area of at least 50 cm.sup.2, the film material containing phosphorus derived from the anodizing electrolyte, preferably at least 5 microns thick and preferably having pores spaced 0.1-0.5 microns apart. The film may be made by anodizing one or more aluminium metal anodes of suitable area in an electrolyte based on phosphoric acid at a concentration of 5-150 g/l maintained at a temperature in the range 5.degree.-50.degree. C. using an anodizing voltage of 40-200 V. After separation from the metal substrate, the films have intersecting properties for use in filters.

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First Claim