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#096 Other Types of Functional Alumite(Anodic oxide coatings)

Category : Anodizing
September30, 2011

There are other types of functional alumite in addition to the ones explained earlier:

(1) Heat-absorbing alumite

The internal and external surfaces of the inner pot of an electric rice cooker are coated with fluororesin and treated with alumite, respectively. The inner pot of a rice cooker with the external surface blackened by secondary electrolysis of tin or nickel heats 10-percent faster and hence is more energy-efficient than rice cookers coated with silver-white alumite.
This is because an electrolytically blackened alumite film absorbs heat faster than a silver-white alumite film. The same principle applies to kettles consisting of a body fastened to a bottom plate electrolytically blackened for improvement of thermal efficiency.

(2) Thermal radiating

Aluminum heat sinks dissipate heat from electronic parts, such as power transistors. Conventionally, aluminum heat sinks are manufactured using aluminum extrusions with the sulfuric acid anodic oxide film dyeing blackend. For sufficient blackening, the sulfuric acid anodic oxide film needs to be 10 μm thick or thicker, but at the expense of heat radiation.
Therefore, secondary electrolytic coloring is used to blacken sulfuric acid anodic oxide films that are several micrometers thick. Metals such as nickel and tin are used for the blackening.

(3) Selective heat-absorbing alumite

Solar collectors and water heaters use inexhaustible solar energy to heat water and must be able to absorb solar heat efficiently while minimizing heat loss from the heated water.
This kind of thermal function is called selective heat absorption. Electrolytically blackened alumite films meet the requirement; they efficiently absorb solar thermal radiation with a wavelength of 0.2 to 1 μm while emitting minimal thermal radiation with a wavelength of 1 μm or more from the heated water.

(4) Insulating alumite

An alumite film is a good electric insulating material with a resistivity of 1012 to 1014 Ω. As such, alumite films are widely used in electronic parts.
The electrical properties of an alumite film, however, vary significantly depending on the type of electrolytic bath used, the electrolytic conditions applied, the type of the alumite film formed, and the finishing treatments such as sealing. Therefore, extreme care is necessary in the use of insulating alumite.

(5) Conductive alumite

A sulfuric acid anodic oxide film shows a significant decrease in electrical resistance when the pores are filled from the bottom to the top with highly conductive metal such as silver by secondary electrolysis. While a conventional sulfuric acid anodic oxide film has an electrical resistance of 1 × 108 Ωcm, a silver deposited film has an electrical resistance as low as 1 × 103 Ωcm.
Conductive alumite films are useful in preventing static electricity or electromagnetic interference shielding.

(6) Other

Alumite is also used for antivacterial, deodorant, catalytic, and other effects.

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