The ceramic material aluminium oxide, also known as alumina, is completely opaque, translucent or transparent – depending on the structure, and therefore on the manufacturing process. Light scattering is the keyword here.
Most alumina products are made out of pure aluminium oxide powder with a well defined grain size. These powder particles are getting close(r) to each other in the first shaping step (for example pressing), with empty space between the grains that is filled up with air. During the final step called sintering they are fused together at high temperature – but below the melting temperature – to form the final product. The higher the sintering temperature, the better the grains will ‘bake’ to each other and the less porosity between the grains will remain. Most alumina products do not reach 100% density after sintering. Boundaries between the grains and remaining pores – both in the same order of magnitude as the wavelength of visible light – are sources for light scattering inside these products, making them opaque and white or pale yellow in colour.
Translucent aluminium oxide is an important part of metal halide lamps and high pressure sodium lamps. You can come across the latter lamps with their typical orange colour along the highway. Aluminium oxide is one of the very few materials that can withstand the corrosive properties of sodium and the high pressure and high temperature in the lamp, and that is translucent at the same time. By using a small amount of magnesium oxide as sintering aid, nearly all pores between the grains will disappear and the aluminium oxide will be sintered up to its theoretical density at the right sinter temperature. Since some grain boundaries remain between the aluminium oxide grains, the material is not completely transparent but translucent. You could make that material even more transparent by taking aluminium oxide grains of several tens of nanometers in size (‘nano grains’) as starting material. Here the size of the grain boundaries is much smaller than the wavelength of visible light, and scattering of this light will play a much smaller role.
Synthetic sapphire is single crystalline aluminium oxide. Since the material is one large crystal, there are no grain boundaries or pores that can scatter light, and the material is transparent. Such a large single crystal is manufactured by immersing a small seed crystal of sapphire into a bath of molten aluminium oxide, and by pulling up this seed crystal carefully. The attached molten aluminium oxide cools down and crystallises into the same crystal structure as the seed crystal. Sapphire is used as camera lens in smartphones.
Transparent alumina is a ‘clear’ inspiration for the motion-picture industry. ‘Transparent aluminium’ occurs in the science fiction movie ‘Star Trek IV: The Voyage Home’ as a non-existing material from which an enormous aquarium was manufactured, to transport whales on board of a spacecraft …