Researchers at the Georgia Tech Research Institute (GTRI) are developing a solid composite material to help cool small, powerful microelectronics used in defense systems. The material, composed of silver and diamond, promises an exceptional degree of thermal conductivity compared to materials currently used for this application.
The research is focused on producing a silver-diamond thermal shim of unprecedented thinness – 250 microns or less. The ratio of silver to diamond in the material can be tailored to allow the shim to be bonded with low thermal-expansion stress to the high-power wide-bandgap semiconductors planned for next generation phased-array radars.
Thermal shims are needed to pull heat from these high-power semiconductors and transfer it to heat-dissipating devices such as fins, fans or heat pipes. Since the semiconductors work in very confined operating spaces, it is necessary that the shims be made from a material that packs high thermal conductivity into a tiny structure.
Diamonds provide the bulk of thermal conductivity, while silver suspends the diamond particles within the composite and contributes to high thermal conductivity that is 25 percent better than copper. To date, tests indicate that the silver-diamond composite performs extremely well in two key areas -- thermal conductivity and thermal expansion.
'We have already observed clear performance benefits -- an estimated temperature decrease from 285 degrees Celsius to 181 degrees Celsius -- using a material of 50 percent diamond in a 250-micron shim,' said Jason Nadler, a GTRI research engineer who is leading the project.
The researchers are approaching diamond percentages that can be as high as 85 percent, in a shim less than 250 microns in thickness. These increased percentages of diamond are yielding even better performance results in prototype testing.
Natural Thermal Conductors
Diamond is the most thermally conductive natural material, with a rating of approximately 2,000 watts per meter Kelvin, which is a measure of thermal efficiency. Silver, which is among the most thermally conductive metals, has a significantly lower rating -- 400 watts per meter K.
Nadler explained that adding silver is necessary to:
- bond the loose diamond particles into a stable matrix;
- allow precise cutting of the material to form components of exact sizes;
- match thermal expansion to that of the semiconductor device being cooled;
- create a more thermally effective interface between the diamonds.
Nadler and his team use diamond particles, resembling grains of sand, that can be molded into a planar form.
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