Unconventional thermal transport
Department of Physics
According to conventional theories of heat conduction in semiconductors and insulators, only crystals composed of strongly-bonded light elements can have high lattice thermal conductivity, kL, and intrinsic thermal resistance comes only from lowest-order anharmonic three-phonon interactions. In this talk, I will discuss aspects of a new paradigm for achieving high kL that we proposed, in which the vibrational properties are tailored to reduce the phase space for three-phonon scattering . Our ab initio calculations predicted that one candidate material, cubic Boron Arsenide (BAs), indeed had ultrahigh three-phonon limited kL comparable to that of the best heat conductor, diamond, and significantly higher than any other semiconductor . In BAs, three-phonon scattering can become so weak that four-phonon scattering also plays an important role in limiting kL [2, 3]. Such unconventional transport behavior has been confirmed in recent experiments [3-5]. It gives rise to anomalous non-monotonic pressure dependence of kL . I will review the challenging material constraints, which must be overcome in order to achieve the unconventional high kL. An interesting case is that of group V transition metal carbides, NbC, TaC and VC. These metals have ideal vibrational properties for the desired weak phonon-phonon scattering. But, their nested Fermi surfaces give rise to strong scattering between phonons and electrons, which results in an orders-of-magnitude lower kL that is nearly temperature independent, contrary to the typical behavior.
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