Binary semiconductor realizes high thermal conductivity
When a laptop or cell phone heats up from overuse, it’s not just uncomfortable for the user. That excess heat also inhibits the device’s performance. Materials with high thermal conductivity help dissipate heat and improve device performance and reliability, but finding a passive cooling option that is both cost-effective and reliable has been challenging, particularly for high-power electronics. With a thermal conductivity of 2000 W m–1 K–1, diamond is the most developed material for passive cooling. But it suffers from high cost, slow synthesis rates, and inconsistent quality. Now three research groups have synthesized crystals of a semiconductor, boron arsenide, whose thermal conductivity at room temperature exceeds the record for all materials except diamond.
Half a century ago, theorist Glen Slack of General Electric proposed that high thermal conductivity in a solid requires a simple crystal structure of tightly bonded, lightweight elements. Among compound crystals, silicon carbide is the industry standard. But at 350 W m–1 K–1, its thermal conductivity falls far short of diamond’s. Theoretical predictions in 2013 suggested that high thermal conductivity could also be possible in crystals with one heavy and one light atom; follow-up work in 2017 suggested that in BAs it could be close to 1300 W m–1 K–1. But synthesizing high-quality BAs crystals in dense bulk form is difficult. Arsenic’s high volatility tends to introduce defects that reduce thermal conductivity and can lead to unwanted stoichiometric ratios. For scientists to observe the predicted high thermal conductivity, the crystals must have minimal defects.
The three teams fabricated BAs crystals with high thermal conductivity by vaporizing and depositing solids in the form of crystals using chemical vapor transport. Each team measured the thermal reflectance of a laser shined at the sample and determined the thermal conductivity within micrometer-scale surface regions. David Cahill at the University of Illinois at Urbana-Champaign and Bing Lv at the University of Texas at Dallas measured room-temperature thermal conductivity of 1000 W m–1 K–1 on their 1 mm3 crystals. Zhifeng Ren and colleagues at the University of Houston grew larger crystals, up to 4 mm x 2 mm x 1 mm. They measured 900 W m–1 K–1 across the bulk, with local surface values as high as 1200 W m–1 K–1. Yongjie Hu’s team at UCLA recorded the highest thermal conductivity value, 1300 W m–1 K–1, which is consistent with the theoretical prediction. An electron microscope image (bottom) and electron diffraction patterns of the UCLA team’s sample are shown below.
If scientists and engineers can develop the ability to grow BAs crystals in bulk, the semiconductor could find widespread application in thermal management systems. (J. S. Kang et al., Science, in press, doi:10.1126/science.aat5522 ; F. Tian et al., Science, in press, doi:10.1126/science.aat7932 ; S. Li et al., Science, in press, doi:10.1126/science.aat8982 ; image courtesy of Yongjie Hu.)
