New Research Reveals Obscure and Conflicting Heat Transfer Behaviors

UCLA researchers and their colleagues have discovered a new physical principle governing how heat is transferred through materials, and the discovery contradicts the conventional wisdom that heat always moves faster as pressure increases.

So far, the common belief has been proven true in recorded observations and scientific experiments involving different materials such as gases, liquids and solids.

The researchers detailed their finding in a study published last week by Nature. They discovered that boron arsenide, which was once considered a very promising material for heat management and advanced electronics, also has a unique property. After reaching an extremely high pressure that is hundreds of times higher than the pressure found at the bottom of the ocean, the thermal conductivity of boron arsenide actually begins to decrease.

The results suggest that there may be other materials experiencing the same phenomenon under extreme conditions. The advance may also lead to new materials that could be developed for smart energy systems with built-in “pressure windows” so that the system only turns on within a certain pressure range before automatically turning off after reaches a maximum pressure point.

“This basic research finding shows that the general rule of pressure dependence begins to fail under extreme conditions,” said study leader Yongjie Hu, associate professor of mechanical and aerospace engineering at UCLA Samueli School of Science. Engineering. “We expect that this study will not only provide a benchmark for potential revision of the current understanding of heat motion, but it could also impact modeling predictions made for extreme conditions, such as those found inside the Earth, where direct measurements are not possible.”

According to Hu, the research breakthrough could also lead to a retooling of standard techniques used in shock wave studies.

Similar to how a sound wave travels through a bell, heat travels through most materials through atomic vibrations. When pressure brings the atoms inside a material together, it allows heat to move faster through the material, atom by atom, until its structure breaks down or changes to another phase. .

This is not the case, however, with boron arsenide. The research team observed that heat begins to move more slowly under extreme pressure, suggesting possible interference caused by different ways in which heat vibrates through the structure as pressure rises, similar to overlapping waves and cancel each other out. Such interference involves higher-order interactions that cannot be explained by textbook physics.

The results also suggest that the thermal conductivity of minerals can reach a maximum after a certain pressure range. “If this applies to planetary interiors, this may suggest a mechanism for an internal ‘thermal window’ – an inner layer within the planet’s interior where heat flow mechanisms are different from those below and above. “, explains co-author Abby Kavner, a professor of earth, planetary and space sciences at UCLA. “A layer like this can generate interesting dynamical behavior inside large planets.”

To achieve the very high pressure environment for their heat transfer demonstrations, the researchers placed and compressed a crystal of boron arsenide between two diamonds in a controlled chamber. They then used quantum theory and several advanced imaging techniques, including ultrafast optics and X-ray inelastic scattering measurements, to observe and validate the previously unknown phenomenon.

Mechanical engineering graduate students Suixuan Li, Zihao Qin, Huan Wu and Man Li from Hu’s research group are co-lead authors of the study. Other authors are Kavner, Martin Kunz of Lawrence Berkeley National Laboratory and Ahmet Alatas of Argonne National Laboratory.