This post originally appeared as a news story in the September 2013 issue of Materials Today – can be accessed here (free for a limited time): http://digital.materialstoday.com/
A team of theoretical physicists from the US Navy Research Laboratory (NRL) have identified cubic boron arsenide as a potential replacement for diamond in passive cooling applications.
Diamond’s remarkably high thermal conductivity (~2000 W/m.K) has kept it a material of interest for thermal engineers for decades. But it now seems that diamond’s place at the top of the thermal tree is under threat from an unlikely competitor – cubic boron arsenide (BAs).
The work, published in Physical Review Letters, results from a collaboration between theoretical physicists at NRL and Boston College. The team have used a new theoretical approach, based on predictive first principles, to demonstrate that boron arsenide can display a room-temperature thermal conductivity surpassing that of diamond.
The high thermal conductivity of diamond is well understood, resulting from the size of the carbon atoms that comprise its lattice, along with the stiffness of the bonds between them. BAs displays none of these characteristics, and while it has never been experimentally determined, its thermal conductivity had been previously calculated to be 10 times smaller than that of diamond.
The team, led by David Broido from Boston, used a newly-developed theoretical approach for calculating thermal conductivities, validated using known materials. This new approach led the team to conclude that BAs should display a thermal conductivity greater than 2000 W/m.K, orders of magnitude larger than previous models suggested. The team believe that their observation can be explained by an interplay of certain basic vibrational properties within the material.
In metals, electrons carry heat. But diamond and boron arsenide are electrical insulators, meaning that heat is carried by phonons – vibrational waves of the lattice atoms. Where these phonons interact, an intrinsic resistance to heat flow exists. But the NRL-Boston team found that in BAs, these ‘collisions’ are far less likely to occur in a specific range of frequencies. So at these frequencies, large amounts heat can be conducted, leading to the value of high thermal conductivity.
This work aims to improve understanding of the physics of heat transport in materials, but the next stage is to experimentally validate these observations. If this happens, the NRL team believe it will open the door to new passive cooling applications. Diamond is currently widely used to remove heat from computer chips, but as components continue to shrink, the need for ultra-high thermal conductivity materials to remove heat will increase, with boron arsenide as frontrunner.
Physical Review Letters (2013) DOI:10.1103/PhysRevLett.111.025901