Abstract:The anharmonicity of thermal transport in amorphous solids is underappreciated and inadequately understood, although considerable attention is paid to that of crystals. Herein, the anharmonic effects on the heat conduction of amorphous silica are investigated by combining the normal mode decomposition method with the Wigner transport equation. The existence of significant anharmonicity in amorphous silica, including temperature‐dependent linewidths and frequency shifts of the vibrational modes, is demonstrated… Show more
“…Figure 13 also shows that first-principles and GAP calculations yield more precise estimates of the specific heat compared to the BKS and Tersoff potentials (the latter has been recently employed in ref. 114 to study the thermal properties of v -SiO 2 ). Fig.…”
Predicting the thermal conductivity of glasses from first principles has hitherto been a very complex problem. The established Allen-Feldman and Green-Kubo approaches employ approximations with limited validity—the former neglects anharmonicity, the latter misses the quantum Bose-Einstein statistics of vibrations—and require atomistic models that are very challenging for first-principles methods. Here, we present a protocol to determine from first principles the thermal conductivity κ(T) of glasses above the plateau (i.e., above the temperature-independent region appearing almost without exceptions in the κ(T) of all glasses at cryogenic temperatures). The protocol combines the Wigner formulation of thermal transport with convergence-acceleration techniques, and accounts comprehensively for the effects of structural disorder, anharmonicity, and Bose-Einstein statistics. We validate this approach in vitreous silica, showing that models containing less than 200 atoms can already reproduce κ(T) in the macroscopic limit. We discuss the effects of anharmonicity and the mechanisms determining the trend of κ(T) at high temperature, reproducing experiments at temperatures where radiative effects remain negligible.
“…Figure 13 also shows that first-principles and GAP calculations yield more precise estimates of the specific heat compared to the BKS and Tersoff potentials (the latter has been recently employed in ref. 114 to study the thermal properties of v -SiO 2 ). Fig.…”
Predicting the thermal conductivity of glasses from first principles has hitherto been a very complex problem. The established Allen-Feldman and Green-Kubo approaches employ approximations with limited validity—the former neglects anharmonicity, the latter misses the quantum Bose-Einstein statistics of vibrations—and require atomistic models that are very challenging for first-principles methods. Here, we present a protocol to determine from first principles the thermal conductivity κ(T) of glasses above the plateau (i.e., above the temperature-independent region appearing almost without exceptions in the κ(T) of all glasses at cryogenic temperatures). The protocol combines the Wigner formulation of thermal transport with convergence-acceleration techniques, and accounts comprehensively for the effects of structural disorder, anharmonicity, and Bose-Einstein statistics. We validate this approach in vitreous silica, showing that models containing less than 200 atoms can already reproduce κ(T) in the macroscopic limit. We discuss the effects of anharmonicity and the mechanisms determining the trend of κ(T) at high temperature, reproducing experiments at temperatures where radiative effects remain negligible.
“…It should be kept in mind that phase 2 features a temperature transient characterized by a very rapid increase (decay) from T 1 (T 2 ) to the target T (300 K in our case). For this reason, within phase 2, T 1 and T 2 are by no means equilibrium temperatures, thereby ruling out any undesirable effect due to a dependence on T of the thermal conductivity [35,36].…”
Section: B Thermal Conductivity: the Aemd Methodsmentioning
First-principles molecular dynamics combined with the approach-to-equilibrium molecular dynamics methodology is employed to calculate the thermal conductivity of non-stoichiometric amorphous SiN. This is achieved by implementing thermal transients in five distinct models of different sizes along the direction of the heat transport. Such models have identical structural features and are representative of the same material, thereby allowing for a reliable analysis of thermal conductivity trends as a function of the relevant cell dimension. In line with the known physical law of heat propagation at short scale, the thermal conductivity increases in size with the direction of heat transport. The observed behavior is rationalized accounting for previous results on crystalline and amorphous materials, thus providing a unified description holding for a large class of materials and spanning a wide range of heat propagation lengths.
Thermal transport properties of amorphous materials are crucial for their emerging applications in energy and electronic devices. However, understanding and controlling thermal transport in disordered materials remains an outstanding challenge, owing to the intrinsic limitations of computational techniques and the lack of physically intuitive descriptors for complex atomistic structures. Here, it is shown how combining machine‐learning‐based models and experimental observations can help to accurately describe realistic structures, thermal transport properties, and structure–property maps for disordered materials, which is illustrated by a practical application on gallium oxide. First, the experimental evidence is reported to demonstrate that machine‐learning interatomic potentials, generated in a self‐guided fashion with minimum quantum‐mechanical computations, enable the accurate modeling of amorphous gallium oxide and its thermal transport properties. The atomistic simulations then reveal the microscopic changes in the short‐range and medium‐range order with density and elucidate how these changes can reduce localization modes and enhance coherences’ contribution to heat transport. Finally, a physics‐inspired structural descriptor for disordered phases is proposed, with which the underlying relationship between structures and thermal conductivities is predicted in a linear form. This work may shed light on the future accelerated exploration of thermal transport properties and mechanisms in disordered functional materials.
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