A basic tenet of college physics is that as pressure increases, thermal conductivity—a material's ability to conduct heat—increases, too, because atoms that are squeezed together interact more.
More than a century of research has confirmed this rule. But engineers have now found an exception: when they applied intense pressure to boron arsenide, a recently discovered semiconductor material, thermal conductivity decreased. The finding, described in Nature, challenges established theory and potentially upends current models of how substances behave under extreme conditions.
“Now that we've made this first discovery, we think this can't be the only material with abnormal behavior,” says study senior author Yongjie Hu, a chemist and mechanical engineer at the University of California, Los Angeles. If other substances show this property, “the established understanding of thermal conductivity might not be correct.”
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In prior studies, Hu and other researchers identified boron arsenide as having exceptionally high thermal conductivity. The scientists also calculated that conventional thermal conductivity rules might not apply to it in certain circumstances.
To test those predictions, Hu and his colleagues placed a tiny piece of boron arsenide less than 100 microns thick in the gap between two diamonds. They applied pressure to the diamond sandwich to create a force on the boron arsenide hundreds of times greater than that at the bottom of the ocean. The researchers used ultrafast optics, spectroscopy and x-rays to document how boron arsenide's thermal conductivity begins to decrease as heat propagates across the sample and it is subjected to intense pressure. They observed that the decrease comes from similar types of heat waves overlapping and canceling one another out—a phenomenon predicted by quantum mechanics.
If Hu and his colleagues can show this behavior generalizes to other materials, he says, physicists may have to revise established models for environments such as outer space or planetary interiors, including Earth's. The latter could alter predictions about climate change because terrain temperatures are affected by what happens inside the planet.
The new study provides “the first and best experimental evidence that I know of to show that thermal conductivity can be tuned,” says University of California, Berkeley, geophysicist Raymond Jeanloz, who was not involved in the research. The finding, he adds, “opens up the possibility” of advanced technologies that save energy and cool electronics by controlling thermal conductivity.
