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According to a new physics principle established by UCLA scientists and their peers, the heat never flows more quickly when pressure rises. This finding challenges the traditional knowledge that heat always moves more quickly through materials.
Conventional wisdom has up until now been supported by scientific investigations using a variety of materials, including gases, liquids, and solids. In a study that Nature released this week, the researchers went into detail about their discovery. They discovered that boron arsenide, which is already known to be a very promising material for advanced electronics and heat management, also has a special quality. The thermal conductivity of boron arsenide actually starts to decrease after reaching an extremely high pressure that is hundreds of times greater than the pressure at the bottom of the ocean. The results suggest that there might be other materials experiencing the same phenomenon under extreme conditions. The advance may also lead to novel materials that could be developed 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. "This fundamental research finding shows that the general rule of pressure dependence starts to fail under extreme conditions," said study leader Yongjie Hu, an associate professor of mechanical and aerospace engineering at the UCLA Samueli School of Engineering. "We expect that this study will not only provide a benchmark for potentially revising the current understanding of heat movement, but it could also impact established modelling 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 the retooling of standard techniques used in shock wave studies. Heat moves through most materials by atomic vibrations, just like a sound wave moves through a rung bell. 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 it changes into a different phase. With boron arsenide, however, such is not the case. The research team saw that heat began to flow more slowly under high pressure, indicating a potential interference from the heat vibrating through the structure in various ways as pressure increases, much like overlapping waves cancelling each other out. Such interference entails higher-order interactions that conventional physics cannot account for. The results also suggest that the thermal conductivity of minerals can reach a maximum after a certain pressure range. "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," says co-author Abby Kavner, a professor of earth, planetary and space sciences at UCLA. "A layer like this may generate interesting dynamic behaviour 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 next saw and verified the previously undiscovered event using quantum theory and a number of cutting-edge imaging techniques, such as ultrafast optics and inelastic X-ray scattering studies. (ANI)
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