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Silver, diamond composite cools defence electronics

Posted: 09 Mar 2011     Print Version  Bookmark and Share

Keywords:cooling  diamond  composite  material  radar 

Georgia Tech Research Institute researchers have developed a composite of silver and diamond with the aim to produce a thermal shim of 250µm or less for cooling microelectronics used in defence systems such as phased array radars. Thermal shims pull heat away from high-power semiconductors and transfer it to heat dissipating devices such as fins, fans or heat pipes.

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 pull heat away from high-power semiconductors and transfer it to heat dissipating devices such as fins, fans or heat pipes. These semiconductors are usually in very confined operating spaces, making it necessary for the shims be made from a material that packs high thermal conductivity into a tiny, very thin, 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 per cent 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°C to 181°C—using a material of 50 per cent diamond in a 250µm shim," said Jason Nadler, research engineer, GTRI, who is leading the project.

The researchers are approaching diamond percentages that can be as high as 85 per cent, in a shim less than 250µm in thickness. These increased percentages of diamond are yielding even better performance results in prototype testing.

Nadler added that this novel approach to silver-diamond composites holds definite technology transfer promise. No material currently available offers this combination of performance and thinness.

Diamond is the most thermally conductive natural material, with a rating of approximately 2,000W/m-K, which is a measure of thermal efficiency. Silver, which is among the most thermally conductive metals, has a significantly lower rating of only 400W/m-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 moulded into a planar form.

The problem is, a sand-like material doesn't hold together well. A matrix of soft, ductile and sticky silver is needed to keep the diamond particles together and achieve a robust composite material.

Thermal expansion
Also, because the malleable silver matrix completely surrounds the diamond particles, it supports cutting the composite to the precise dimensions needed to form components like thermal shims. And silver allows those components to bond readily to other surfaces, such as semiconductors.

As any material heats up, it expands at its own individual rate, a behaviour known as its coefficient of thermal expansion (CTE).

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