"We wanted to determine whether high quality c-BN can in fact be made to observe the large thermal conductivity magnitudes in c-BN, and whether the huge increase in thermal conductivity with isotopic purification predicted from theoretical calculations is measured in the real material."David Broido, Boston College
An international team of physicists, materials scientists and mechanical engineers has confirmed experimentally the high thermal conductivity predicted for isotopically enriched cubic boron nitride (c-BN). The team reports its findings in a paper in Science.
The thermal conductivity of a material determines how much heat can pass through it when its ends are at different temperatures. Materials with very high thermal conductivity have important technological applications, such as cooling microelectronics. But very few of them have been discovered.
Theoreticians had predicted that isotopically pure c-BN should have extremely high thermal conductivity – second only to crystals made out of carbon, such as diamond.
"We wanted to determine whether high quality c-BN can in fact be made to observe the large thermal conductivity magnitudes in c-BN, and whether the huge increase in thermal conductivity with isotopic purification predicted from theoretical calculations is measured in the real material," said David Broido, professor of physics at Boston College and a co-author of the paper.
What made this difficult is that c-BN is particularly challenging to synthesize. Also, accurately measuring a material’s thermal conductivity can be tricky when the value is high. But the team managed to overcome these challenges to find that the measured thermal conductivity values for the c-BN samples were quite close to the ones they had calculated.
"The study confirms c-BN as one of only a handful of ultrahigh thermal conductivity materials, and shows it to have the largest increase in its thermal conductivity upon isotopic enrichment ever observed," Broido said.
The team also studied the related compounds boron phosphide (BP) and boron arsenide (BAs). Most elements in nature comprise mixtures of isotopes, Broido explained. For example, naturally occurring boron has two isotopes: boron-10, which accounts for 20%, and boron-11, which accounts for 80%. These different isotopes produce disorder throughout the material that adds to the thermal resistance. By making the boron materials with just one isotope (either just boron-10 or boron-11) through isotopic enrichment, Broido and his team were able to reduce this resistance and thus increase the thermal conductivity.
By a remarkable coincidence of nature, the elements nitrogen, phosphorus and arsenic, which naturally bond with boron to make c-BN, BP and BAs, have only a single isotope. So, for all three of these materials, isotopic disorder only affects the boron atoms, Broido said. Yet, while isotopic enrichment of the boron atoms gave a doubling of thermal conductivity for c-BN, it only produced much smaller increases for BP and BAs.
The reason for this turned out to be that boron and nitrogen atoms have roughly the same mass, while arsenic and phosphorous are heavier. "We showed that the larger arsenic and phosphorous masses compared with boron caused the isotopic disorder in BAs and BP to give only small resistance to heat flow," said Broido. "It is as if the isotopic disorder becomes invisible to the heat flowing through the BAs and BP samples."
In contrast, removing the same amount of disorder through isotopic enrichment in c-BN results in a huge increase in thermal conductivity.
"It was amazing to see the measured data and theoretical calculations consistently agreeing so closely with each other. The theory has no parameters in it that can be adjusted to fit the measurements. It either agrees with the measurements or it doesn't," Broido said. "The excellent agreement highlights the accuracy of the theory, the precision of the measurements, and the high purity of the samples."
He added that further investigation will be needed to better understand the types of defects that occur in c-BN to reduce its heat conductivity. Because such ultrahigh thermal conductivity materials are so rare, he hopes that theoretical and computational searches can identify new candidates and unravel the mysteries surrounding their usual properties.
This story is adapted from material from Boston College, with editorial changes made by Materials Today. The views expressed in this article do not necessarily represent those of Elsevier. Link to original source.