Study unveils obscure heat transfer behaviors

Discovery shows heat movement slowing down under extreme pressure instead of speeding up.

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High pressure represents extreme environments and provides opportunities for materials discovery. Thermal transport under high hydrostatic pressure has been investigated for more than 100 years, and all measurements of crystals so far have indicated a monotonically increasing lattice thermal conductivity.

A new physics principle guiding how heat transfers through materials have been found by UCLA researchers and their peers. This discovery defies the belief that heat always goes faster as pressure rises.

Boron Arsenide is a promising material for heat management and advanced electronics. Scientists in this study found that the material also has a unique property. After reaching an extremely high pressure that is hundreds of times greater than the pressure found at the bottom of the ocean, boron arsenide’s thermal conductivity begins to decrease.

The findings imply that similar phenomena may occur in other materials under harsh circumstances. The development may also create innovative materials for smart energy systems with built-in “pressure windows,” enabling the system to only turn on within a specific pressure range before automatically turning off when the pressure reaches a maximum point.

Study leader Yongjie Hu, an associate professor of mechanical and aerospace engineering at the UCLA Samueli School of Engineering, said, “This fundamental research finding shows that the general rule of pressure dependence starts to fail under extreme conditions. We expect that this study will not only provide a benchmark for potentially revising the current understanding of heat movement but could also impact established modeling predictions for extreme conditions, such as those found in the Earth’s interior, where direct measurements are not possible.”

“The research breakthrough may also lead to retooling of standard techniques used in shock wave studies.”

Atomic vibrations are the main way that heat moves through most materials. Heat can pass through a material more quickly, atom by atom, as pressure forces atoms inside it closer together. This process continues until the material’s structure disintegrates or changes into a different phase.

However, this doesn’t applies on boron arsenide. According to scientists, heat started to move slower under extreme pressure, suggesting a possible interference caused by different ways the heat vibrates through the structure as pressure mounts, similar to overlapping waves canceling out each other. Such interference involves higher-order interactions that cannot be explained by textbook physics.

Co-author Abby Kavner, a professor of Earth, planetary, and space sciences at UCLA, said“If applicable to planetary interiors, this 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. A layer like this may generate interesting dynamic behavior in the interiors of large planets.”

The scientists squeezed a boron arsenide crystal between two diamonds in a controlled chamber to provide the extraordinarily high-pressure condition needed for their heat transfer experiments. They then used advanced imaging methods, such as ultrafast optics and inelastic X-ray scattering measurements, along with quantum theory to see and confirm the previously undiscovered phenomenon.

Journal Reference:

  1. Li, S., Qin, Z., Wu, H. et al. Anomalous thermal transport under high pressure in boron arsenide. Nature (2022). DOI: 10.1038/s41586-022-05381-x
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