Abstract
The physics of heat conduction puts practical limits on many technological fields such as energy production, storage, and conversion. It is now widely appreciated that the phonon-gas model does not describe the full vibrational spectrum in amorphous materials, since this picture likely breaks down at higher frequencies. A two-channel heat conduction model, which uses harmonic vibrational states and lattice dynamics as a basis, has recently been shown to capture both crystal-like (phonon-gas channel) and amorphous-like (diffuson channel) heat conduction. While materials design principles for the phonon-gas channel are well established, similar understanding and control of the diffuson channel is lacking. In this work, in order to uncover design principles for the diffuson channel, we study structurally-complex crystalline Yb14 (Mn,Mg)Sb11, a champion thermoelectric material above 800 K, experimentally using inelastic neutron scattering and computationally using the two-channel lattice dynamical approach. Our results show that the diffuson channel indeed dominates in Yb14MnSb11 above 300 K. More importantly, we demonstrate a method for the rational design of amorphous-like heat conduction by considering the energetic proximity phonon modes and modifying them through chemical means. We show that increasing (decreasing) the mass on the Sb-site decreases (increases) the energy of these modes such that there is greater (smaller) overlap with Yb-dominated modes resulting in a higher (lower) thermal conductivity. This design strategy is exactly opposite of what is expected when the phonon-gas channel and/or common analytical models for the diffuson channel are considered, since in both cases an increase in atomic mass commonly leads to a decrease in thermal conductivity. This work demonstrates how two-channel lattice dynamics can not only quantitatively predict the relative importance of the phonon-gas and diffuson channels, but also lead to rational design strategies in materials where the diffuson channel is important.
Highlights
The flow of heat through solids is a topic of wide-spread technological importance
To build intuition regarding amorphous-like heat conduction we study the vibrational and thermal properties of crystalline Yb14MSb11 (M = Mn or Mg) which has a very complex crystal structure containing 104 atoms in its primitive unit cell (Figure 2a)
From this case study emerges a simple physical picture for heat conduction which enables the design of materials that exhibit crystalline-like conduction, amorphous-like conduction, as well as materials transitioning between the two extremes
Summary
The flow of heat through solids is a topic of wide-spread technological importance. In simple crystals the thermal conductivity (κ) can be well understood within the phonon-gas model (PGM)[1]. There is far less understanding of the detailed physics governing heat conduction in amorphous-like heat conductors which tend to occupy the bottom two to three orders of magnitude. To build intuition regarding amorphous-like heat conduction we study the vibrational and thermal properties of crystalline Yb14MSb11 (M = Mn or Mg) which has a very complex crystal structure containing 104 atoms in its primitive unit cell (Figure 2a). It is a champion p-type material system for high temperature thermoelectric energy conversion and is well studied experimentally and computationally[10,11,12,13,14]. From this case study emerges a simple physical picture for heat conduction which enables the design of materials that exhibit crystalline-like conduction, amorphous-like conduction, as well as materials transitioning between the two extremes
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