Researchers upend conventional wisdom on thermal conductivity

Scientists have reported growing a crystal created from boron and arsenic, with a thermal conductivity more than twice that of copper. Credit: University of Houston

Scientists have long known that diamond is the best material for conducting heat, but it has drawbacks: It is costly and is an electrical insulator; when paired with a semiconductor device, diamond expands at a different rate than the device does when it is heated.

Now a group of researchers from around the United States has reported that a crystal grown from two relatively common mineral elements—boron and arsenic—demonstrates far higher thermal conductivity than any other semiconductors and metals currently in use, including silicon, silicon carbide, copper, and silver.

The discovery has the potential to address a range of technological challenges, including cooling electronic devices and nanodevices, said physicist Zhifeng Ren, a researcher with the Texas Center for Superconductivity at the University of Houston and one of the corresponding authors on the paper announcing the discovery, published Thursday, July 5, in the journal Science.

Thermal conductivity is measured in the unit of Wm-1K-1, used to denote the amount of heat that can pass through a material that is one meter long when the temperature difference from one side to the other is 1 degree Kelvin. The boron-arsenide crystal has a conductivity in excess of 1,000 at room temperature, the researchers reported.

Copper, by comparison, has a conductivity of about 400; diamond has a reported thermal conductivity of 2,000.

Previously reported efforts to synthesize boron-arsenide have yielded crystals measuring less than 500 micrometers—too small for the useful application.

But the researchers now have reported growing crystals larger than 4 millimeters by 2 millimeters by 1 millimeter. A larger crystal could be produced by extending the growing time beyond the 14 days used for the experiment, they said.

Working with Tom Reinecke at the Naval Research Lab and Lucas Lindsay at Oak Ridge National Laboratory, David Broido, a theoretical physicist at Boston College and one of the authors of the paper, first proposed that the combination could yield a high thermal conductivity crystal, defying the conventional theory that ultrahigh lattice thermal conductivity could only occur in crystals composed of strongly bonded light elements, limited by anharmonic three-phonon processes.

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