Thermal conductivity is the core physical property that measures the thermal conductivity of a material. The thermal conductivity of all known materials at room temperature is distributed in the range of about 0.01-1000 Wm-1K-1. For example, the thermal conductivity of silicon and copper is in the order of 100 Wm-1K-1, which is relatively high, which can effectively help computers and mobile phones to maintain a low operating temperature. However, as the heat flux density inside advanced microelectronic chips is getting higher and higher, in order to ensure effective heat dissipation, the requirements for materials with ultra-high thermal conductivity are becoming more and more urgent.
The thermal conductivity of diamonds at room temperature is about 2000 Wm-1K-1. From 1953 to the present, it has been recognized as the highest thermal conductivity block. However, high-quality diamonds are scarce and expensive, and are not suitable for widespread heat dissipation. Graphite is an allotrope of diamond, and its thermal conductivity is close to that of diamond, and its price is much cheaper. However, the thermal conductivity of vertical orientation is only 1/300 of that of diamond. People have explored super thermal conductive materials with a room temperature thermal conductivity of more than 1000 Wm-1K-1 for decades, but there has been no substantial breakthrough; until 2013, semiconductor arsenic was predicted based on first-principles calculations The thermal conductivity of boron crystals may be comparable to diamonds. This prediction is unexpected, because based on some basic laws that have been tested in experiments, at least since 1973, it is generally believed that the thermal conductivity of boron arsenide is only about 200 Wm-1K-1. Immediately afterwards, three independent research groups reported the growth of high-quality boron arsenide crystals and the experimental measurement of their thermal conductivity in the top international journal Science in 2018. Up to about 1200 Wm-1K-1 makes boron arsenide the non-carbon material with the highest thermal conductivity, second only to diamond in all isotropic materials.
During his postdoctoral research at the Massachusetts Institute of Technology, Song Bai, a specially-appointed researcher at Peking University Institute of Technology, participated in leading one of the three experimental work on boron arsenide crystals in 2018. Since January 2019, Dr. Song Bai joined Peking University. On January 9, 2020, Song Bai and collaborators once again reported the latest discoveries about the new super thermal conductive material in Science magazine. This time the ultra-high thermal conductivity material is semiconductor cubic boron nitride crystal. Although the thermal conductivity of cubic boron nitride crystals with natural isotope abundance at room temperature is only about 850 Wm-1K-1, however, after enrichment of boron isotopes, cubic nitridation containing about 99% boron-10 or boron-11 In boron crystals, a thermal conductivity exceeding 1600 Wm-1K-1 was observed. This value greatly exceeds boron arsenide, which means that boron isotope-enriched cubic boron nitride crystals have replaced boron arsenide and become the best non-carbon and isotropic thermally conductive materials. It is also worth noting that the thermal conductivity has been increased by about 90% through isotope enrichment in experiments, which is also the largest isotope thermal effect observed so far.
The reason why Song Bai and his collaborators can obtain ultra-high thermal conductivity is to eliminate the resistance to heat flow caused by the mixing of the two isotopes of boron-10 and boron-11 in natural abundance cubic boron nitride crystals. First-principles calculations revealed that this huge isotope effect in cubic boron nitride is mainly due to the large difference in relative mass between the two isotopes boron-10 and boron-11. The same as the Group III and V semiconductors, the two crystals of boron arsenide and boron phosphide are very similar to cubic boron nitride. However, experimental and theoretical studies on boron arsenide and boron phosphide have only found very small isotopic effects. It turns out that with the gradual increase of the atomic mass matching with boron atoms (from nitrogen to phosphorus to arsenic), the degree of mass disorder due to the mixing of two boron isotopes becomes less and less important; for heat flow, almost It is no longer visible.
Behind this research work is an international team of 24 physicists, material scientists and mechanical engineers. These include Professor Gang Chen's research group at MIT, Professor David Broido's research group at Boston University, Professor David Cahill's research group at the University of Illinois at Urbana-Champaign, and Li Shi (Shi Li) at Texas State University Austin Professor Research Group, Professor Zhifeng Ren (Ren Zhifeng) Research Group of the University of Houston, Professor Bing Lv (Lv Bing) Research Group of Texas State University Dallas, Professor Takashi Taniguchi Research Group of the National Institute of Materials Research, and now Beijing University Song Bai special researcher. Former MIT postdoctoral researcher Dr. Chen Ke and former Boston University postdoctoral researcher Dr. Navaneetha K. Ravichandran and researcher Song Bai are co-first authors. Researcher Song Bai and Professor Gang Chen (陈刚) and Professor David Broido are co-corresponding authors.
Cubic boron nitride crystals have ultra-high hardness and chemical resistance, are used for machining, and can handle many cutting-edge manufacturing environments (such as high temperatures) where diamond tools are difficult to work. Cubic boron nitride also has a very wide band gap, which is a good material for manufacturing ultraviolet optoelectronic devices. With such excellent mechanical, chemical, electrical, and optical properties, coupled with such rare ultra-high thermal conductivity, cubic boron nitride crystals have broad prospects in many key thermal management applications involving high power, high temperature, and high photon energy.