Thermal and Electrical Effects of Anodizing

Here is a nice article on Aluminium and its oxide coating (which is a combination of corundum, sapphire and ruby). Don't get any ideas thou, the surface barrier layer that forms almost immediately on aluminium is only a few microns thick.

http://www.popsci.com/popsci/how20/09550e0796b84010vgnvcm1000004eecbccdrcrd.html

As hard anodizing can get up to 0.002" thick, ... scraped sapphire mining?...

But I digress. That was an interesting article, and since sapphires are non-conductive, I can (sorta) understand the electrical insulation aspect. As far as thermal insulation, I suppose it would insulate better than the aluminum (or I should say conduct worse), but since it is so thin, it doesn't really have an effect. Sound plausible? Any thoughts?

Well I've answered the rest of my questions. I didn't find an actual value to compare anodized aluminum to copper and untreated aluminum, but I'm satisfied with its minimal thickness, anodized aluminum is likely very similar to untreated aluminum in its thermal characteristics.

CR4 Admin: removed broken link and edited to accommodate change

Okay, I'm off to bed after I write this! I've found the thermal conductivity of sapphire. I looked up other metals to compare it to and put all of it in the handy chart below.

Material Thermal Conductivity

(cal/sec)/(cm C)

Silver 1.01

Copper 0.99

Aluminum 0.5

Iron 0.163

Sapphire 0.065

Lead 0.083

I was not aware you could get untreated aluminium. All the figures and data on aluminium will be for its natural (usable) form with its oxidisation layer on top. The only possible application where I see an issue that may require something different would be where the barrier layer cannot form (in which case you should be using another material as Aluminium is very reactive).

That's what I meant with the exception that anodized has a thicker oxidation layer and it is different (non-conductive) than naturally oxidized aluminum.

Keep in mind that most material properties are given for ambient temperature (300 K), and sometimes for the temperature that the material is normally used at if different from ambient.

Many properties, particularly thermal characteristics can change drastically with changing temperature.

Specific heat drops to zero at 0K, most materials CTE decreases with decreasing temperature (some increase), thermal conductivity is not a constant and crystal structure can change at various temperatures (iron can be a body centered or face centered cubic at different temperatures).

Hi,

natural aluminum surface is corroded in our atmosphere within hours to a depth of only a few nanometer.

This is not at all pure oxide but mixture with hydroxide and a lot of water built into and absorbed to the surface.

If you put an additional anodising on the surface this is growing beneath the natural layer as the oxidation takes place on the surface of the aluminum.

This anodised layer is very much dependent on the electrolyte that is used.

The layer that forms has a lower nonporous part that has a thickness of 1nanometer per Volt . (So normally between 10 and 200nm).

The upper layer is porous - the higher the temperature in the anodising bath the more

If the layer grows thicker the porosity will be worse.

The pores are normally filled (by steam treatment or others including wax, oil or epoxi also teflon).

These pores - if significant- lower the thermal conductivity up to 30 %.

Insulating quality is only existant if this sealing is done properly else there will be a diode behaviour: in one direction conducting above the voltage of anodising in the opposite direction conducting at much lower voltages and slowly destroying the anodised surface.

Hard anodising has the lowest in porosity (near 3%), anodising for electrolytic capacitors is done in acids that don't form pores but solid nonporous layers but requires very high voltages (1KV for 1µm thickness).

As there are so many different formulations of bath constituents there are many different surface properties: mechanical, electrical, corrosion, wear, optical, color.

So what are your necessities?

RHABE