How to Increase Heat Transfer/Reduce Contact Resistance

Use copper....?

Are these surfaces metal?

Thermal grease.

Copper, or better yet silver, powder filled grease.

Solder.

Thermal grease would be my suggestion.

Here's some:

http://www.novagard.com/compounds/Thermally-Conductive-Compounds.html

OK Gold dust

Wrong, buffalo breath!

Ag has higher thermal conductivity than Au.

Diamond dust!

Au k≈315

Ag k≈420

Diamond k≈2200

If you wish to go for the absurd, Graphene k≈4840

It really depends on the surface materials and what you are trying to accomplish. You have asked for two very different things by the way.

What will work with plated microchip pins won't necessarily work with a 220kV aluminium overhead power joints (for example).

Please provide much more information in greater detail.

They are relatively flat only when both surfaces are in thermal equilibrium with each other. Bolting these two surfaces together will further distort the surfaces. Adding a thin amorphous media (thermal grease) between the two surfaces will allow the voids created by these deformations to be filled with a more thermally conductive media than vapors.

And both surfaces are the same material, or have the same coefficient of thermal expansion.

How much heat are you starting with?

How much transfer are you starting with?

What is the desired increase in heat transfer?

What are the surfaces made of?

How much contact resistance are you starting with?

How much contact resistance do you want?

What is the required contact area?

What type of bolts are required?

How much torque is required on the bolts?

Are there any restrictions on materials to be used?

etc. etc. etc.

Another solution looking for a problem to solve?

The question is too general for us to provide a specific answer. We need details on the materials involved. the bolting pattern, and the environmental conditions.

You could go esoteric and put a Peltier device between the surfaces.

I also have immersed my heat sink into liquid nitrogen. The heat transfer rate is tremendous.

Silver plate both mating surfaces and use thermal heat sink grease between them.

There are water soluable silver plating powder compositions on the market that are easy to apply,cost effective, and work well.

The compositins are easily applied by hand using a sponge, scuffing pad, or a rag.

Both mating surfaces must be very clean with no oily contaminants present.

Be sure to read and follow the application directions carefully.

The first question is: why do you need to increase? In general heat transfer across surfaces bolted together and with a not too bad roughness is about same as base material. Thus the question. If you have a too low heat transfer which means that you obtain a high temperature difference between the 2 flanges then they are not in contact as they should: check roughness and planeity of both. It could be possible that the surface is too small and then you should increase the flanges as diameter. The use of a cooper layer has to be considered with care since you loose assembly stiffness and the preload can decrease too much in use and too fast. Grease can be good but it depends on the temperature domain.

There are many things one could do to reduce contact resistance, but these are solutions looking for a problem that has not yet been defined. One could improve the surface finish, but if the heat transfer surfaces are in a vacuum, that solution won't be effective. The guys at NASA can explain this. You could use thermal grease between the surfaces, but you will run into temperature limitations. You say there is no temperature restriction? Well, you might have informed us! Same is true for corrosion. I could tell you that we use pure woven silver screen sandwiched and compressed between silver plated rigid copper bus bars, but you would probably find a reason why that won't work for you, perhaps cost.So, as I suggested previously, you define the problem and we will try to help you solve it.

I take exception to, "heat transfer surfaces are in a vacuum, that solution won't be effective."

Vacuum only negates convection. Conduction and radiation still obey the laws of physics.

I didn't work directly for NASA, but I'm one of those "guys".

Sorry, change that "to may not be as effective". My point remains.

Actually, two surfaces in intimate contact rely very little on convection.

In space, conduction only works when there is a conductive (probably metallic) path for the heat. To get heat out of the system, into space, we had to rely on radiation alone.

There is some truth in what he wrote although the error was to consider convection. In a contact situation the air (gas) layer is too thin to allow a true convection so that heat will be transferred by "conduction". In a vacuum this little part of conduction will be zero and only direct metal to metal conduction will work so that there is a heat transfer reduction even if VERY small.

I'd call the non-contact movement of heat between the two surfaces radiation, not conduction.

But, the amount is hardly worth debating.

Radiation being proportional to (T1^4-T2^4) if differences are small is neglectable.

Ok we have all made plenty of guesses, what exactly are you after?

What is the exact application.