If it could be made to exist, boron arsenide with a cubic crystal structure was calculated to have a high thermal conductivity – and now it has been made, by a team from the University of Illinois at Urbana-Champaign and University of Texas, Dallas.
“The boron arsenide crystals were synthesised using a technique called chemical vapour transport,” said Illinois researcher Qiye Zheng. “Elemental boron and arsenic are combined while in the vapour phase and then cool and condense into small crystals. We combined materials characterisation and trial-and-error synthesis to find the conditions that produce crystals of high enough quality.”
The result, published in Science as ‘High thermal conductivity in cubic boron arsenide crystals‘, is a material with a thermal conductivity of 1,000W/m/K (±90W/m.K) at room temperature – above that of silicon carbide (490W/m.K* in 6H), although beaten by diamond (2,200W/m·K*).
The Illinois team used electron microscopy and time-domain thermo-reflectance to determine if the synthetic crystals were free of the defects that increase thermal resistance, which they were, and to to reveal something else: “We studied the structural defects and measured the thermal conductivity of the boron arsenide crystals produced at UT Dallas,” said Illinois scientist Professor David Cahill (pictured). “Our experiments also show that the original theory is incomplete and will need to be refined to fully understand the high thermal conductivity.”
The team is considering its material for use in heat-spreaders – blocks or slivers of super-high thermal conductivity material that can be used between a tiny hot object – an LED die or power semiconductor, for example – and a copper or aluminium bulk heatsink. Without the heat-spreader, the small contact area between die and aluminium would limit the amount of heat that could be extracted.
With its high thermal conductivity and electrically-insulating nature, Diamond gets used in high-performance heat-spreading.
“Although diamond has been incorporated occasionally in demanding heat-dissipation applications, the cost of natural diamonds and structural defects in manmade diamond films make the material impractical for widespread use in electronics,” said UT Dallas Professor Bing Lv.
The next step in the work will be to try other processes to improve the growth and properties of this material for large-scale applications, the researchers said.