Alumina flake pigments

Thin hexagonal monocrystaUine AI2O3 flakes can be produced by hydrothermal processes [28-30]. The thickness of the resulting flakes is not uniform. Therefore, the resulting coated pigments merely assume a dull pearlescence. 

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The dominant class of lustre pigments is based upon oxide coatings of mica platelets, but newer materials have appeared e.g. silica and alumina flakes from Merck, with better performance characteristics than traditional mica." " There has also been developments in the technology for the coating of reflective metal platelets, e.g. aluminium, with oxides using chemical vapour deposition that has led to exciting new products. ... 

The first interferential pigments were created on the basis of natural muscovite mica as a substrate and coated with titanium or iron oxide. Other substrates have subsequently been used in this type of pigment, notably the silica flake and the alumina flake. The substrate provides the necessary platelet shape, while the interference colour is determined by the thickness of the metal oxide layer and its refractive index. 

In practice, platelet crystals are synthesized with a layer thickness d calculated to produce the desired interference colors (iridescence). Most pearlescent pigments now consist of at least three layers of two materials with different refractive indices. Thin flakes (thickness ca. 500 nm) of a material with a low refractive index (mica, silica, alumina, glass) are coated with a highly refractive metal oxide (Ti 2, FejOj, layer thickness ca. 50-150 nm). This results in particles with four... 

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