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John O | March 2019

Researchers describe unusual ways that material conducts heat when compressed

By Josh Perry, Editor


Researchers from Boston College (Mass.) revealed that, unlike typical materials, when cubic boron arsenide is compressed it does not become a better conductor of heat, but instead thermal conductivity initially improves before deteriorating.


Cubic boron arsenide has interesting properties when compressed. (Wikimedia Commons)


According to a report from the school published by Science Daily, cubic boron arsenide contradicts a century of materials science based on newly observed behavior. At first, compression improves thermal conductivity in the material, but as compression continues its thermal conductivity reduces.


Heat is transported in the boron arsenide crystals by phonons and thermal resistance is created by the phonons colliding with each other. Previous research into quantum physics demonstrated that these collisions occurred between at least three phonons at a time, but BC researchers showed that collisions between triplets rarely happens in boron arsenide.


“As a result of such rare collision processes among phonon triplets, cubic boron arsenide has turned out to be an excellent thermal conductor, as confirmed by recent measurements,” the report explained.


Compressing the material with hydrostatic pressure increased the amount of triplet collisions, while four-phonon collisions became less frequent. This caused thermal conductivity to increase at first but then reduce. This is the first time that three- and four-phonon collisions have competed in a material.


The research was recently published in Nature Communications. The abstract read:


“Recent experiments demonstrate that boron arsenide (BAs) is a showcase material to study the role of higher-order four-phonon interactions in affecting heat conduction in semiconductors.


“Here we use first-principles calculations to identify a phenomenon in BAs and a related material - boron antimonide, that has never been predicted or experimentally observed for any other material: competing responses of three-phonon and four-phonon interactions to pressure rise cause a non-monotonic pressure dependence of thermal conductivity, κ, which first increases similar to most materials and then decreases.


“The resulting peak in κ shows a strong temperature dependence from rapid strengthening of four-phonon interactions relative to three-phonon processes with temperature.


“Our results reveal pressure as a knob to tune the interplay between the competing phonon scattering mechanisms in BAs and similar compounds, and provide clear experimental guidelines for observation in a readily accessible measurement regime.”

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