Editor’s note: This question was asked in response to Electronics Cooling’s recent webinar by Carl Zwebin. To view the webinar, click here.
Question: Are these graphite based materials compressible?
Answer: In general, they are not compressible. However, exfoliated natural graphite is compressed during the manufacturing process used to make thermal interface materials and heat spreaders. It is first expanded using an exfoliation process.
Question: What is the thickest PGS we can get?
Answer: Maximum thickness varies with thermal conductivity. According to the latest website posting, 0.1 mm for K = 700 W/m-K. Maximum thickness decreases with increasing thermal conductivity.
Question: Does aluminum readily bond to flexible synthetic and natural graphite?
Answer: Some forms of graphite are available with pre-applied adhesives that should bond to aluminum. However, watch out for thermal stresses arising from CTE differences.
Question: Does diamond cobalt material come in bulk form, and is it machinable?
Answer: At this time, as far as I know, diamond/cobalt is not being promoted for thermal management. If you look at the slide I showed of drill bits, it clearly is machinable. The cobalt, which is electrically conductive, makes it possible to use EDM. I am not sure about what forms are available. I suggest you contact suppliers.
Question: Electrical conductivity is very important for some electronic applications. Are there any material with high thermal conductivity and low electrical conductivity?
Answer: There is a clear need for materials with high thermal conductivity and low electrical conductivity. At this time, as far as I know, the only choices are various monolithic ceramics like BeO and AlN. Another approach is to use electrically insulating coatings. For example, on one project, we coated diamond/SiC composites with CVD diamond.
Question: Have you ever used any of the carbon nanotube “velvet” TIMs?
Answer: No. I consider these materials to be developmental, but promising.
Question: Here at Applied Nanotech, Inc we have a carbon and carbon-aluminum material. CarbAl(TM). How can we get this new material included in your webinar? It has very comparible properties.
Answer: I cover your material in my short courses. Regretfully, because of time limitations, I can’t cover all materials of interest in a one-hour webinar.
Question: How do carbon materials compare with respect to thermal resistance or thermal impedence when interfaced with other components in an overall thermal managment design?
Answer: You can treat them just like any other material. However, it is necessary to consider that some are anisotropic.
Question: In a Super Latice the thermal management is drived by phonons?
Answer: It is my understanding that heat transfer in carbonaceous materials is primarily by phonon transport. However, I am not a material scientist.
Question: Could Carbon Foams or Aluminum Foams replace traditional Al Heat Sinks?
Answer: There is rarely only one perfect material for any application. I think there may be roles for both. The approach I use is to conduct design/cost trade studies, and pick the cheapest material that meets all the requirements.
Question: Isn’t contact resistnace between the flexible graphite and the heat source still a problem?
Answer: Contact resistance is a concern for all materials, not only flexible graphite.
Question: Polyonics is developing thermally conductive pressure sensitive adhesives to attach heat sinks to electrical components. What is the best way to measure thermal conductivity of very thin adhesives( 50 micron to 300 micron)?
Answer: I have not done much work with thin films.
Question: Powder metallurgical techniques for silver diamond composite?
Answer: The key processes for diamond/silver at this time involve various forms of liquid metal infiltration under pressure. A key issue with powder metallurgy is the maximum particle volume fraction that can be achieved. We saw this with Al/SiC. However, sintering is being used. I cover processes in my regular short courses.
Question: What is the mounting option recommended for these thermal interface materials?
Answer: The materials I covered in the webinar are all used as heat spreaders, heat sinks, etc., not thermal interface materials. However, there is a lot of research and development going on with advanced carbonaceous materials for TIMs.
