Silver-Diamond mix offers cool capabilities for electronics

Posted by Chris Nova

Georgia Tech Research Institute researchers are working on a new material to help cool the microelectronics in defense systems.

The material is made up of silver and diamond and it offers much better thermal conductivity than materials that are currently used.

According to a press release, the Georgia Tech research is centered on making a silver-diamond thermal shim of unheard of thinness, around 250 microns or less.

The amount of silver to diamond in the material can be tweaked 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 draw heat from these high-power semiconductors and transfer it to heat-dissipating instruments such as fins, fans or heat pipes. Seeing as how the semiconductors work in very tight operating spaces, it is necessary that the shims be made from material that is able to pack high thermal conductivity into a small space.

Diamonds supply the bulk of thermal conductivity, while silver floats the diamond particles within the mixture and contributes to high thermal conductivity that is 25 percent better than copper. Currently, tests indicate that the silver-diamond mix performs very 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 leads the project.

The researchers are nearing diamond percentages that are as high as 85 percent, in a shim less than 250 microns in thickness. These higher percentages of diamond are resulting in even better performances in prototype testing.

Nadler also added that this approach to silver-diamond mixtures holds definite technology-transfer promise. Materials that are currently available do not offer this much performance and thinness.

Fun Science Georgia Tech science fact: Diamond is the most thermally conductive material that occurs naturally, with a rating of approximately 2,000 watts per meter Kelvin, which is a measure of thermal efficiency. Silver, which is one of the most thermally conductive metals, has a significantly lower rating -- 400 watts per meter K.

Nadler and his research team use diamond particles, that resemble grains of sand, they can be molded into a planar form. This is problematic because a sand-like material doesn't stay together very well. A matrix of silver -- soft, ductile and sticky -- is necessary to keep the sand-like diamond particles together, and achieve a potent composite material.

Additionally, because the shapeable silver matrix completely covers the diamond particles, it supports cutting the mixture to the precise dimensions needed to make components like thermal shims. And silver gives those components the ability to bond readily to other surfaces, such as semiconductors.

Metals conduct heat by moving electrons, unlike metals, diamond conducts heat by way of phonons, which are vibrational wave bunches that travel through crystalline and other materials. Placing silver between the diamond-particle setup helps phonons move from particle to particle and promotes thermal efficiency.

"It's a challenge to use diamond particles to fill space in a plane with high efficiency and stability," Nadler said. "In recent years we've built image-analysis and other tools that let us perform structural morphological analyses on the material we've created. That data helps us understand what's actually happening within the composite -- including how the diamond-particle sizes are distributed and how the silver actually surrounds the diamonds."

The last difficulty involves the need to move beyond performance testing to a deep analysis of the silver-diamond material's functional ability. Nadler's goal is to explain the thermal conductivity of the mixture from a fundamental materials standpoint, instead of relying solely on performance results.

The extremely small size of the thermal shims makes in-depth testing hard, because current testing methods need larger amounts of material. Fortunately, Nadler and his research team are looking at several testbed technologies that look promising for detailed thermal-conductivity analysis.