The thermal conductivity of materials usually increases when they are subject to very high pressures. But researchers at the University of California, Los Angeles (UCLA) have found that the opposite is true for boron arsenide – a newly discovered semiconductor that shows much promise for heat management applications and advanced electronics devices. The finding could change the way we think about heat transport under extreme conditions, such as those found in the Earth’s interior, where direct measurements are impossible.
The researchers, led by Yongjie Hu, applied hydrostatic pressure to boron arsenide samples placed between two diamonds in an anvil cell. They then examined how the atomic vibrations of the crystal lattice (phonons, the main way by which heat is carried through materials) changed with increasing pressures of up to 32 GPa. To do this, they employed a variety of ultrafast optics measurements, including Raman spectroscopy and inelastic X-ray scattering. The team found that at extremely high pressure – hundreds of times higher than that found at the bottom of the ocean – boron arsenide’s thermal conductivity begins to decrease.
Hu and colleagues, who report their work in Nature, attribute the anomalous high-pressure behaviour they observed to a possible interference caused by the competing ways in which heat travels through the boron arsenide crystal as the pressure mounts. In this case, the competition is between three-phonon and four-phonon scattering processes. In most common materials the opposite effect is observed: as pressure squeezes atoms closer together, heat moves through the structure faster, atom by atom.
A mechanism for an internal thermal window
The results also suggest that the thermal conductivity of materials can reach a maximum after a threshold pressure range. “We are very excited to see this finding breaking the general rule of heat transfer under extreme conditions and it points to new fundamental possibilities,” Hu tells Physics World, “The study could also impact our established understanding of dynamic behaviours such as for the interiors of planets. There may even be implications for outer space explorations and climate change.”
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Hu’s colleague, co-author Abby Kavner adds, “If applicable to planetary interiors, our findings may suggest a mechanism for an internal ‘thermal window’ – an internal layer within the planet where the mechanisms of heat flow are different from those below and above it.”
There might be other materials experiencing the same phenomenon under extreme conditions that break the classical rules, Hu says. Indeed, the new findings might help in the development of novel materials for smart energy systems with built-in “pressure windows” so that the system only switches on within a certain pressure range before shutting off automatically after reaching a maximum pressure point.