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John O | June 2019

Researchers present unified theory on heat transfer that could enhance thermoelectric materials


By Josh Perry, Editor
jperry@coolingzone.com

 

Researchers from the NRCC MARVEL, a center for computational design and discovery of novel materials created by the Swiss National Science Foundation, derived a more general formulation for heat transfer that fits both ordered (crystals) and disordered (glasses) materials.

 


Heat conduction originates from both particle-like diffusion of phonon wave-packets (blurred spheres, following the realistic 3D phonon dispersion of CsPbBr3) and wave-like tunneling (blue waves). (Michele Simoncelli, EPFL)

 

According to a report from MARVEL, this is the first heat transfer equation that accounts for both classes of material and also demonstrates that heat can “tunnel, quantum-mechanically, rather than diffuse away, like an atomic vibration.” Researchers also believe the equation accurately predicts the performance of thermoelectric materials for the first time.

 

“Having thermoelectric materials that are more efficient (around three times the current standard) would change all our refrigeration and air-conditioning technologies completely because thermoelectric materials can be used in reverse and exploit electricity to cool us down, rather than producing electricity out of heat,” the article explained.

 

Because thermoelectric materials need to be both electrical conductors and thermal insulators, previous attempts at understanding heat transport in those materials has been limited. The new model encompasses both the crystalline and glass-like properties of thermoelectric materials.

 

“The new understanding outlined in the paper and more accurate estimates of thermal conductivity, along with data on the electrical conductivity, will allow scientists to calculate the ‘figure of merit’ of thermoelectrics, and provide an estimate of their efficiency,” the article continued. “Armed with this key piece of information, researchers will be able to screen potential materials first with computational techniques, accelerating the development path for these new technologies.” 

 

The research was recently published in Nature Physics. The abstract stated:

 

“Crystals and glasses exhibit fundamentally different heat conduction mechanisms: the periodicity of crystals allows for the excitation of propagating vibrational waves that carry heat, as first discussed by Peierls, while in glasses the lack of periodicity breaks Peierls’s picture and heat is mainly carried by the coupling of vibrational modes, often described by a harmonic theory introduced by Allen and Feldman.

 

“Anharmonicity or disorder are thus the limiting factors for thermal conductivity in crystals or glasses. Hitherto, no transport equation has been able to account for both.

 

“Here, we derive such an equation, resulting in a thermal conductivity that reduces to the Peierls and Allen–Feldman limits, respectively, in anharmonic crystals or harmonic glasses, while also covering the intermediate regimes where both effects are relevant.

 

“This approach also solves the long-standing problem of accurately predicting the thermal properties of crystals with ultralow or glass-like thermal conductivity, as we show with an application to a thermoelectric material representative of this class.”

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Heat Transfer Calculators