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Heat conduction & electrical insulation properties of 20 micron black anodised aluminium

I am looking at using black anodised extruded aluminium heatsinks for cooling power transistors. I want to use the 10 to 20 micron anodising layer as an electrical insulator and can't find out what sort of electrical insulation quality a 10 micron layer would have.The applied voltage between the device and the anodised surface would not exceed 150 volts. Would this insulation quality deteriorate with time and does the anodised layer affect the heat transfer properties? if anybody can help me with this I would be very pleased. Everybody that I have asked over here are not able to give me an answer.

David O [surname deleted for privacy by Editor]
- Durban, South Africa
1999

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Generally speaking, the dielectric strength of aluminum anodic coatings produced in a sulfuric acid electrolyte should be above 10 V/micron. The heat emissivity of black anodized aluminum should be above 0.8. A 15 micron (or above) anodic coating with good quality should be able to meet your needs. It is quite common to use black hard anodized aluminum plates as heatsinks in electronics industry. Of course you need to do some experiments first.

Ling

Ling Hao
- Grand Rapids, Michigan
1999

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The Electroplating Engineering Handbook [on AbeBooks or eBay or Amazon affil links] by Lawrence Durney gives an average breakdown voltage of 40V/uM or 1000V/.001", but because of imperfections in the coating values can range from 250-1750V/.001". It would seem that you would be OK at 150V but I would suggest keeping the thickness closer to the 20 µm range.

As L. Hao points out aluminum heat sinks that have been anodized are very common and are probably being used in the rectifier that anodizes the aluminum.

Rick Richardson CEF
- Dayton, Ohio
1999