UCLA researchers and their colleagues have discovered a new physical principle that governs how heat is conducted through materials, and the result contradicts the conventional wisdom that heat moves faster as pressure increases.
So far, the common belief has been confirmed in recorded observations and scientific experiments involving various materials such as gases, liquids, and solids.
The researchers detailed their discovery in a study published by last week Nature. They found that boron arsenide, already considered a promising material for thermal management and advanced electronics, also has a unique property. After reaching an extremely high pressure, hundreds of times higher than the pressure at the sea floor, the thermal conductivity of boron arsenide actually begins to decrease.
The results suggest that there may be other materials that experience the same phenomenon under extreme conditions. The advance could also lead to novel materials that could be developed for smart energy systems with built-in “pressure windows,” allowing the system to turn on only within a certain pressure range before automatically turning off after reaching a maximum pressure point.
“This fundamental research finding shows that the general rule of pressure dependence begins to break down under extreme conditions,” said study leader Yongjie Hu, associate professor of mechanical and aerospace engineering at UCLA’s Samueli School of Engineering. “We anticipate that this study will not only provide a benchmark for potentially revising the current understanding of thermal motion, but could also influence established model predictions for extreme conditions, such as those found in the Earth’s interior, where direct measurements are not possible.” .”
According to Hu, the research breakthrough may also lead to a retooling of standard techniques used in shock wave studies.
Much like a sound wave travels through a ringing bell, heat travels through most materials by atomic vibrations. Because pressure squeezes atoms within a material closer together, it allows heat to move faster through the material, atom by atom, until its structure collapses or transforms into a different phase.
However, this is not the case with boron arsenide. The research team observed that heat moved more slowly under extreme pressure, suggesting possible interference caused by different ways the heat vibrates through the structure as the pressure increases, similar to overlapping waves canceling each other out. Such interferences are higher-order interactions that cannot be explained with textbook physics.
The results also indicate that the thermal conductivity of minerals can reach a maximum above a certain pressure range. “If applicable to planetary interiors, this could suggest a mechanism for an internal ‘thermal window’ – an inner layer within the planet in which the mechanisms of heat flow differ from those below and above,” says co-author Abby Kavner. Professor of Earth, Planetary and Space Sciences at UCLA. “A layer like this can produce interesting dynamic behavior inside large planets.”
To achieve the extremely high pressure environment for their heat transfer demonstrations, the researchers placed and compressed a boron arsenide crystal between two diamonds in a controlled chamber. They then used quantum theory and several advanced imaging techniques, including ultrafast optics and inelastic X-ray scattering measurements, to observe and validate the previously unknown phenomenon.
Mechanical engineering students Suixuan Li, Zihao Qin, Huan Wu and Man Li from Hu’s research group are the co-lead authors of the study. Other authors include Kavner, Martin Kunz of Lawrence Berkeley National Laboratory, and Ahmet Alatas of Argonne National Laboratory.
The study was funded by the National Science Foundation, the Alfred P. Sloan Foundation, and a VM Watanabe Excellence in Research Award from UCLA Samueli. Some experiments were conducted at two U.S. Department of Energy facilities – the Advanced Photon Source at Argonne National Laboratory and the Advanced Light Source at Lawrence Berkeley National Laboratory. Computer services were provided by the UCLA Institute for Digital Research and Education and the National Science Foundation. The authors also received support from the Nanoelectronics Research Facility and the California NanoSystems Institute (CNSI) at UCLA. Both Hu and Kavner are members of the CNSI.