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Nanostructures control heat transfer is expected to significantly improve equipment energy saving
In recent years, groundbreaking research into the thermal conduction properties of superlattice structures has opened up exciting possibilities for improving thermoelectric devices. These devices harness temperature differences to generate electricity, and the new findings suggest they could become significantly more efficient. The study revealed an unexpected phenomenon: thermal energy moves in waves rather than particles through a nanostructured material just a few billionths of a meter thick. This discovery challenges previous assumptions about how heat is transferred at the nanoscale.
Thermal energy originates from the vibrations of atoms and molecules within matter. Typically, heat transfer follows a "random walk," making it difficult to control. However, the latest observations indicate that heat energy can propagate in a novel manner, termed "coherent flow." This process resembles the orderly ripples observed in a pond, suggesting a highly organized transfer mechanism.
These insights hold immense potential for tailoring heat flow in advanced materials. For instance, this research could pave the way for innovative methods to dissipate heat from power devices or semiconductor lasers. Such heat often compromises device performance, and managing it effectively could extend their operational lifespan and efficiency.
The findings were published in this week's edition of *Science* and represent collaborative efforts from researchers at MIT, Boston University, the California Institute of Technology, and Boston College. Among the contributors were MIT graduate student Luckyanova, postdoctoral fellow Garg, and Professor Chen Gang. Their work focused on nanostructured materials known as superlattices, which consist of alternating layers of gallium arsenide and aluminum arsenide. These materials are fabricated using a metal-organic chemical vapor deposition process, allowing precise control over their thickness—each layer measuring just 12 nanometers, roughly equivalent to the width of a DNA strand.
Prior to this study, scientists assumed that even perfectly aligned superlattices would scatter heat-carrying quasiparticles at their interfaces. Professor Chen Gang of MIT’s Department of Electrical Engineering noted that earlier theories suggested scattering would accumulate across multiple layers, disrupting phonon wave propagation. Yet, this hypothesis lacked empirical validation, prompting him and his team to revisit the phenomenon.
Experiments led by Luckyanova and computational models developed by Garg demonstrated that while high-frequency phonons experienced significant scattering, lower-frequency phonons retained their wave-like behavior. Professor Chen expressed surprise upon seeing the initial experimental results, which clearly indicated coherent heat conduction. He explained that mastering these coherence factors could lead to improved strategies for disrupting coherence and reducing heat transfer, making it practical to recover waste heat from thermoelectric systems in power plants and electronic devices.
This research also holds promise for advancing heat dissipation technologies, such as those used in cooling computer chips. By gaining better control over heat flow, engineers could enhance the thermal management of these systems. Although the exact mechanisms remain unclear, the deeper understanding provided by this study opens new avenues for manipulating heat transfer.
Luckyanova highlighted that the materials used in the experiments exhibited excellent electrical conductivity despite their similar properties. By carefully adjusting the thickness and density of the layers, she believes it is feasible to balance thermal conductivity with the insulating requirements necessary for thermoelectric applications.
Garg emphasized the importance of understanding interfacial effects. Previous simulations overlooked the impact of surface roughness on phonon paths, but he devised methods to incorporate these variables into computational models. His approach allowed for a more accurate representation of heat transport dynamics.
The study’s significance extends beyond thermoelectrics, offering insights into controlling sound wave propagation through long-wavelength phonons. As Professor Chen pointed out, this represents a fundamental breakthrough with broad implications. Much of the credit goes to the interdisciplinary collaboration fostered by the U.S. Department of Energy’s Solid-State Solar Photothermal Energy Conversion Center, which regularly hosts conferences at MIT. These gatherings encourage sustained dialogue among experts from diverse fields, greatly enriching the research process.
Luckyanova remarked that the interdisciplinary nature of the team inspired fresh perspectives, enabling them to tackle the problem from multiple angles. This collaborative spirit, combined with cutting-edge techniques, underscores the transformative potential of modern scientific inquiry.