Thermal conductivity (k) measures how fast a material conducts heat. With k on the order of 100 Wm-1K-1 at room temperature, silicon and copper help to keep personal computers and cell phones cool. However, as the heat density in advanced microelectronic devices becomes ever higher, materials with substantially higher k are demanded to dissipate the overwhelming heat. With a room-temperature k of ~2000 Wm-1K-1, diamond has been recognized since 1953 as the most thermally conductive bulk material. However, high-quality diamond is scarce and expensive. Although graphite is much cheaper and have a similar high k along the carbon sheets, the k in the perpendicular direction is about 300 times smaller. The decades-long search for more bulk materials with an ultrahigh room-temperature k over 1000 Wm-1K-1 remained fruitless until 2013, when ab initio simulations predicted that boron arsenide (BAs) should have a k rivaling that of diamond. This prediction came as quite a surprise because for about forty years, the k of BAs was thought to be only ~200 Wm-1K-1. Subsequently, three experiments in 2018 reported in the journal Science a k of ~1200 Wm-1K-1, making BAs the most thermally conductive non-carbon materials and second only to diamond among isotropic materials.
Co-leading one of the three 2018 BAs experiments was Dr. Bai Song, who was then a postdoctoral associate at the Massachusetts Institute of Technology (MIT) and started in January 2019 as an assistant professor in the College of Engineering at Peking University. On Jan. 9, 2020, Song and colleagues again reported in Science their discovery of yet another material of ultrahigh thermal conductivity—the cubic, zincblende polymorph of boron nitride (cBN). While the room-temperature k of cBN crystals with natural isotope abundance (roughly 20 percent boron-10 and 80 percent boron-11) is about 850 Wm-1K-1, those of cBN crystals with enriched (~99%) boron-10 or boron-11 were found to be greater than 1600 Wm-1K-1, much higher than that of BAs. Consequently, isotope-enriched cBN now supersedes BAs as the best non-carbon, isotropic heat conductor. Moreover, the ~90% enhancement of k upon boron isotope enrichment represents the largest isotope effect on heat transport ever experimentally demonstrated.
Song and colleagues achieved the ultrahigh thermal conductivity by removing the strong resistance to heat flow which originates from the mixture of boron-10 and boron-11 isotopes in natural cBN. Ab initio calculations revealed that the large isotope effect was mainly due to the large relative mass difference between boron-10 and boron-11. In comparison to cBN, measurements and computations of the closely-related BAs and boron phosphide (BP) crystals were also performed, which showed much smaller isotope effect. It turned out that the heavier phosphorous and arsenic atoms made the isotopic mass disorder in boron rather insignificant for BP and BAs, as if the isotopic disorder became increasingly invisible to the heat flow.
An international team of physicists, materials scientists, and mechanical engineers contributed to the project, including 24 researchers led by Gang Chen at MIT, David Broido at Boston College, David Cahill at the University of Illinois, Urbana-Champaign, Li Shi at the University of Texas in Austin, Zhifeng Ren at the University of Houston, Bing Lv at University of Texas in Dallas, Takashi Taniguchi at Japan's National Institute for Materials Science, and Bai Song now at Peking University. Ke Chen and Navaneetha K. Ravichandran contributed equally with Bai Song, who was also a co-corresponding author together with Gang Chen and David Broido.
Cubic-BN has high hardness and chemical resistance and is important for machining under conditions (such as high temperature) in which diamond tools may fail. Cubic-BN also has a very wide bandgap (6.2 eV), which makes it attractive for ultraviolet optoelectronics. These excellent mechanical, chemical, electrical, and optical properties, together with the rare ultrahigh thermal conductivity, make cBN uniquely promising for critical thermal management applications involving high power, high temperature, and high photon energy.
 Ultrahigh thermal conductivity in isotope-enriched cubic boron nitride. Science. 367, 555–559 (2020).
 Unusual high thermal conductivity in boron arsenide bulk crystals. Science. 361, 582–585 (2018).