Experiments confirm predicted high thermal conductivity of boron arsenide
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Experiments confirm predicted high thermal conductivity of boron arsenide
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eat undissipated from high-power electronics reduces the lifetime and efficiency of the devices. Materials with high thermal conductivities, such as diamond and graphite, could help shed heat. However, the cost of synthetic diamond and the low thermal conductivity of graphite normal to the layer plane of the material limit their utility, so alternative materials are needed. A trio of independent thermal-conductivity measurements on high-quality crystals of boron arsenide (BAs) now confirms predictions of the semiconductor’s high thermal conductivity, κ, as recently reported in Science (doi:10.1126/science.aat5522, 10.1126/science.aat7932, 10.1126/science. aat8982). BAs was first predicted to have a thermal conductivity as high as that of diamond or graphite—about 2000 W/ m∙K—in 2013. This was surprising at the time because BAs does not contain light atoms packed in a simple crystal, features classically thought to be important for materials with high thermal conductivity. But the calculations of the fundamental vibrational properties in BAs suggested several reasons for its high thermal conductivity: bunching of optical phonons, the quasi-particles that carry heat as vibrations through a crystal lattice, and a large frequency gap between acoustic and optical phonons suppresses scattering. But when researchers grew BAs crystals, they measured thermal conductivities roughly 10 times lower than predicted. It was thought that point defects and grain boundaries in the crystals scattered phonons and reduced the material’s actual thermal conductivity. Over the past few years, researchers refined their crystal-growing methods for BAs, overcoming challenges with working with solids that have high melting
points and the tendency of BAs to form a polycrystalline phase around 920°C. Three teams independently synthesized very pure BAs crystals using chemical vapor transport. They sealed solid boron and arsenide, along with a vapor-transport agent such as iodine, inside a quartz tube and heated one end of the tube to vaporize the solids. The gaseous atoms traveled to the other side of the tube, held at a cooler temperature. Pure BAs crystals, 500 µm to 4 mm wide, slowly grew from a single nucleation Scanning electron microscope image of a single-crystal boron site over weeks to months. arsenide. Scale bar is 5 µm. Credit: Science. The researchers characterized the purity of their crystals using various x-ray and electron modeling and experiments for new madiffraction techniques, along with electerials discovery,” Hu says. tron microscopy methods. They measured With measured BAs thermal conducthermal conductivity at various locations tivity verifying the prediction, “now we in the crystals using time-domain therknow that the theory works,” says Bing moreflectance measurements, a technique Lv, at The University of Texas at Dallas. that can measure thermal conductivity Along with David Cahill, at the University with 10 µm resolution. of Illinois, Urbana-Ch
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