Due to recent research in the thermal conduction of superlattice structures, thermoelectric devices that generate electricity using temperature differences may be more efficient. The new study found that, surprisingly, thermal energy is transmitted in waves rather than particles through a nanostructured material that is only a few billionths of a meter thick.

Thermal energy is generated by the reciprocal vibration of atoms and molecules in matter. Heat transfer is usually in the form of “random walk” and is difficult to control. The latest observations found that heat energy is transmitted in a completely new mode, called "coherent flow", like the orderly moving ripples in the river pond.

In the important fields of application of new materials, the above findings provide the possibility to precisely customize the flow of heat energy. For example, this research may lead to a new way to reject the heat generated by power devices or semiconductor lasers, and such heat may hinder or even destroy the performance of the device.

The work reported this week by Science magazine was co-authored by students and faculty of the Massachusetts Institute of Technology's graduate student Luckyanova, postdoctoral fellow Garg and professor Chen Gang, and other MIT students, Boston University, California Institute of Technology, and Boston College. get on.

Research involves nanostructured materials called superlattices—stacked alternately gallium arsenide and gallium arsenide wafers, each of which is electroplated in the so-called metal-organic chemical vapor deposition process.

Compounds containing this element are vacuum evaporated and further surface electroplated, the thickness can be precisely controlled by the deposition process time. The synthetic flakes are only 12 nanometers thick - approximately equivalent to the thickness of the DNA molecule, and the thickness of the entire structure ranges from 24 to 216 nanometers.

Researchers had previously believed that even when such flakes reach the perfect level of atomic level, there is still enough roughness at the thin-film interface of the scattering heat transfer quasi-particles when passing through the superlattice.

Previously, it was believed that scattering effects would accumulate in materials with multiple lamellae, thereby breaking the wave effect of phonons, said Chen Gang, a professor of electrical engineering at MIT Carl Richard Soderberg. But this guess has not been confirmed, so he and his colleagues decided to re-test this process.

In fact, the experiments conducted by Luckyanova and the computer simulations developed by Garg indicate that when such random phase scattering occurs at high frequency phonons, the wave effect will be protected at low frequency phonons. Chen Gang expressed that he was very surprised that the first experimental data reported by Luckyanova showed that "coherent heat conduction did occur."

Clearly controlling these coherence factors in turn can lead to better measures to break through the coherence and reduce heat transfer, Chen Gang revealed. This makes it feasible to use the waste heat of all thermoelectric devices from power plants to electrical equipment. The application of the above facilities and equipment requires materials with very good electrical conductivity and poor thermal conductivity.

This study also promotes the development of heat dissipation, such as cooling technology for computer chips. Having the ability to gather and indicate heat flow enables better thermal management of such devices. Chen Gang said that researchers are not yet clear on how to exert precise control, but new in-depth understanding helps. Understanding the wave-based mechanism can "provide more measures to manipulate heat transfer."

Luckyanova said that the two materials used in the experiment have very similar characteristics and very good electrical conductivity. However, by controlling the thickness and density of the sheet, I firmly believe that it is possible to control the thermal conduction while producing the insulation required by the thermoelectric device.

Grag said that the role of the interface between the material sheets has not been truly understood. The previous computer simulations did not cover the effects of indicating texture changes, but I realized that there are ways to simulate the effect of roughness in the phonon travel path through the thin layer stack.

The study not only provides the possibility of controlling the heat flow (mostly in the transmission of short-wave phonons), but also controls the movement of sound waves (mainly with long-wave phonons). Chen Gang said that this is a major basic research discovery.

The major findings of the research work are largely due to the mutual cooperation among researchers of different disciplines and the promotion of Solid State Solar Photothermal Energy Conversion Center, an energy frontier funded by the US Department of Energy, which is regularly held at MIT. Interdisciplinary conferences.

"The conference provides long-term and fruitful discussions that really enrich the research content," Luckyanova said. The members of different areas in the group "inspired us to break this issue from all angles."

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