Boron arsenide: potential competitor to diamond as best conductive cooling material

Chestnut Hill, MA--An unlikely material, cubic boron arsenide (BAs), could deliver an extraordinarily high thermal conductivity, on par with the industry standard set by diamond, researchers at Boston College (BC) and the Naval Research Laboratory (NRL; Washington, DC) report. Although costly, diamond is sometimes used as heat spreaders and heat sinks for high-power laser diodes and other optical sources. Cubic BAs is cheaper than diamond.

Isotopic purification
One key is isotopic purification, which raises the thermal conductivity of BAs as well as boron antimonide, another material with potentially very high thermal conductivity.

The team used a recently developed theoretical approach for calculating thermal conductivities that they had previously tested with many other well-studied materials. Confident in their theoretical approach, the team took a closer look at BAs, whose thermal conductivity has never been measured.

The discovery surprised the team of theoretical physicists; a computer simulation allowed the researchers to find out why BAs has a potentially extraordinary ability to conduct heat. The results were published in Physical Review Letters.

Diamond is known as the best thermal conductor at around room temperature, having thermal conductivity of more than 2000 watts per meter per Kelvin (W/m-K) -- five times higher than the best metals such as copper. But natural diamond is rare and expensive, and high quality synthetic diamond is difficult and costly to produce (although its costs are coming down). This has spurred a search for new materials with ultrahigh thermal conductivities.

The high thermal conductivity of diamond is well understood, resulting from the lightness of the constituent carbon atoms and the stiff chemical bonds between them, according to co-author David Broido, a professor of physics at BC. On the other hand, BAs was not expected to be a particularly good thermal conductor and in fact had been estimated, using conventional evaluation criteria, to have a thermal conductivity ten times smaller than diamond.

The team found the calculated thermal conductivity of cubic BAs is remarkably high, more than 2000 W/m-K at room temperature and exceeding that of diamond at higher temperatures, according to Broido and co-authors Tom Reinecke, senior scientist at NRL, and Lucas Lindsay, a postdoctoral researcher at NRL who earned his doctorate at BC.

Vibrational waves at certain frequencies are muted
Unlike metals, where electrons carry heat, BAs and diamond are electrical insulators. For them, heat is carried by vibrational waves of the constituent atoms, and the collision of these waves with each other creates an intrinsic resistance to heat flow. The team was surprised to find an unusual interplay of certain vibrational properties in BAs that lie outside of the guidelines commonly used to estimate the thermal conductivity of electrical insulators. The simulations show that the expected collisions between vibrational waves are far less likely to occur in a certain range of frequencies; thus, at these frequencies, large amounts of heat can be conducted in BAs.

"This work gives important new insight into the physics of heat transport in materials, and it illustrates the power of modern computational techniques in making quantitative predictions for materials whose thermal conductivities have yet to be measured," says Broido. "We are excited to see if our unexpected finding for BAs can be verified by measurement. If so, it may open new opportunities for passive cooling applications using BAs, and it would further demonstrate the important role that such theoretical work can play in providing useful guidance to identify new high-thermal-conductivity materials."

The research was supported by the Thermal Transport Processes Program of the National Science Foundation, the U.S. Office of Naval Research, and the U.S. Department of Energy Office of Science.