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[ Instrument Network Instrument R & D ] Thermal conductivity is the core physical property of a material's thermal conductivity. The thermal conductivity of all known materials at room temperature is in the range of about 0.01-1000 Wm-1K-1. For example, the thermal conductivity of silicon and copper is relatively high on the order of 100Wm-1K-1, which can effectively help computers and mobile phones maintain low operating temperatures. However, as the heat flow density inside advanced microelectronic chips becomes 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 diamond at room temperature is about 2000Wm-1K-1, and since 1953, it has been recognized as a block with higher thermal conductivity. However, high-quality diamonds are scarce and expensive, and are not suitable for widespread heat dissipation. Graphite is an isomer of diamond. Its thermal conductivity is close to that of diamond, and the price is much cheaper. However, the thermal conductivity of vertical direction is only 1/300 of that.
People have been exploring superconducting materials with room-temperature thermal conductivity exceeding 1000 Wm-1K-1 for decades. However, there has been no substantial breakthrough. Until 2013, first-principles calculations predicted semiconductor arsenization The thermal conductivity of boron crystals may be comparable to that of diamonds. This prediction was unexpected because, based on some experimentally-tested basic laws, at least since 1973, it has been generally believed that the thermal conductivity of boron arsenide is only about 200 Wm-1K-1.
Immediately afterwards, three independent research groups simultaneously reported experimental growth of high-quality boron arsenide crystals and their thermal conductivity in the international journal Science in 2018. Up to about 1200Wm-1K-1 makes boron arsenide a non-carbon material with high thermal conductivity, second only to diamond in all isotropic materials.
Figure A is an optical photo of two high-quality natural isotopic abundance cubic boron nitride crystals; Figure B is the thermal conductivity of cubic thermal conductivity materials such as cubic boron nitride, boron arsenide, and diamond at different temperatures
Song Baite, a researcher at Peking University School of Engineering, participated in leading one of the three experimental work on boron arsenide crystals in 2018 during his postdoctoral research at MIT. Since January 2019, Song Bai joined Peking University. On January 9, 2020, Song Bai and his collaborators once again reported the discovery of a new type of super thermally conductive material in Science Magazine.
The ultra-high thermal conductivity material this time is a semiconductor cubic boron nitride crystal. Although the thermal conductivity of cubic boron nitride crystals with natural isotopic abundance at room temperature is only about 850 Wm-1K-1, after the enrichment of boron isotopes, cubic boron nitride containing about 99% of boron-10 or boron-11 In the crystal, 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 a better non-carbon and isotropic thermally conductive material. It is also worth noting that the thermal conductivity is increased by about 90% experimentally through isotope enrichment, which is also the largest isotope thermal effect observed to date.
The reason why Song Bai and his collaborators can obtain ultra-high thermal conductivity is mainly to eliminate the resistance to heat flow in natural abundance cubic boron nitride crystals due to the mixing of two isotopes of boron-10 and boron-11. First-principles calculations revealed that this huge isotope effect in cubic boron nitride is mainly due to the large difference in the relative masses of the two isotopes, boron-10 and boron-11, which are both Group III and Five semiconductors. Boron and boron phosphide are very similar to cubic boron nitride.
However, experimental and theoretical studies of boron arsenide and boron phosphide have found only small isotopic effects. It turns out that as the atomic mass with the boron atom gradually increases (from nitrogen to phosphorus to arsenic), the mass disorder due to the mixing of two boron isotopes becomes less and less important; for heat flow, It is no longer visible.
Cubic boron nitride crystals have extremely high hardness and chemical resistance. They are used for machining and can be used in cutting-edge manufacturing environments (such as high temperatures) where many diamond tools are difficult to work. Cubic boron nitride also has a very wide band gap, which is a good material for manufacturing ultraviolet photovoltaic devices. With such excellent mechanical, chemical, electrical, and optical properties, coupled with such rare 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.