Thermal Conductivity of Graphite Impregnated Paint

What % ? No way to know.

Cost effective, no. For one thing Al is a homogeneous elemental metal. It's thermal radiation/conduction are well known.

What is a graphite impregnate? What matrix surrounds the graphite? Whatever holds it together has thermal properties, too. This is all a mystery at this point. However, more heat would be radiated(lost) with any non-metallic material because it cannot conduct heat away from the source fast enough as fast as metal. So, the heat has no incentive to follow the graphite. (Path of least resistance)

I'd also argue the point that graphite is used as a "heat sink".

And finally, Al is probably cheaper, too.

The thermal conductivity of graphite is better than aluminum. There are several commercial sites that advertise thermally conductive paints and coatings using graphite as a medium. There just doesn't seem to be any data to explain how effective the coating would be at conducting heat compared to aluminum or copper.

Or maybe it is there and I don't know enough elemental physics to recognize it. So I come to the source of wisdom!

Hi beriberi,

The thermal conductivity of graphite is better than aluminum.

Well, yes and no. It can be higher, but probably depends on the orientation and how it is processed or manufactured. Graphite is used to make heat exchangers for certain chemical environments, so I know it can be a decent conductor. One source gave a range of 25 to 470 W/m-K for graphite and 250 W/m-K for Al.

As LynLynch says, there is no way of calculating what you are trying to find out. It would need to be obtained experimentally.

One of the drawbacks to using graphite is its very low tensile strength. For a heat sink application, it would almost certainly need to be mixed with a strengthening binder which tend to have low thermal conductivities.

I am thinking also that the higher thermal conductivities for graphite are from solid, unbroken blocks. Once you powder it to mix in with a binder, you've brought the thermal conductivity down to the lower end of the range.

Mike

"it would almost certainly need to be mixed with a strengthening binder which tend to have low thermal conductivities."

the fly in the ointment!

Clear thinking prevails. Thanks!

Hi Beri,

I have questions about the efficiency of heat conductivity of coating containing graphite. In case of aluminum sheet each particle of Al metal is in contact with each other. In case of coating, the binder or resin separate each particle of graphite. So, there is less heat conductivity because of uncontinuity. I have no prove about but it is illogical to compare the two systems, Gil.

I don't' have any figures on this, but have added large amounts of graphite to epoxy to create a low-friction surface. Even at large graphite loadings, the thermal conductivity is much lower than that of aluminum -- much closer to that of ordinary paint.

I have used carbon fiber pultrusions, which have the highest carbon content of any reasonable composite. I believe you can find thermal data on line for these, and I believe is is lower than that for aluminum -- and the cost is far higher.

Carbon fiber disc brakes could be researched. They would have about the highest conductivity of a structural graphite material, I'd guess.

This is not specifically on topic about paints but it is relevant to the discussion of graphite so I have ventured to add my research.

There was a study done September 1, 2000, in Garching, Germany by Messrs, Mehling, Hiebler and Ziegler.

In this study they specifically tested graphite as a supplement to two PCM's, more specifically, paraffin RT50, and a eutectic salt PCM 72 (MgNO3/LiNO3 6H2O) (sorry I don't have scientific type available). The object was to determine the effectiveness of graphite to act as a heat transfer material for each of the PCM's. The issue of the PCM's is that although they hold a great deal of heat, they do not transfer it well away from the phase front.

The results were, in short, that the eutectic salts with the graphite ( 75% salts, 8% graphite, the rest is air) were 50 times more conductive than the salts alone. With the graphite added to the paraffin( (85% paraffin, 9% graphite, balance is air) the result was that the mixture was 100 time more conductive than the paraffin alone.

The paper is titled " Latent heat storage using a PCM-graphite composite material" if you want to look it up on google. Percentages are by volume.

Thanks for the feedback! What I am looking for, and realize that I asked the wrong question, is if there is such a thing as a non metallic heat transfer medium that can compete with aluminum? I am getting the impression that if there is it isn't a commercially viable option.

IMO, the main point is that aluminium, especially polished aluminium, has very poor emissivity, and thus, radiated power will be very low. that said, a sheet of aluminium covered with almost any kind of paint wil be difficult to beat in terms of radiated power, and even more if you take in account the price of the solution.

How about conduction?

If you mean conduction through the layer of paint, its effect will be neglectible vs emissivityand conduction in the length of the aluminium foil. If you mean conduction in the length of the foil, aluminium will be more conductive since homogeneous, where a paint impregnated with carbon will look like small (very) conductive bubbles in an insulating matrix

Aluminum is still better. When I worked in aerospace we built electronic enclosures that went into orbit out of aluminum and painted them black. The aluminum conducted the heat away from the hot spots and the black paint helped radiate the heat away from the outer skin.

There's no air up there, so there is no convection to remove heat.

As I recall we used Beryllium oxide in silicone or epoxy matrix for thermal conductivity.