Carbon Bonding Controls the Electron and Phonon Thermal Conductivity in High Entropy Carbides

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Due to their diverse bonding character and corresponding property repertoire, carbides are an important class of materials regularly used in modern technologies, including aerospace applications and extreme environments, catalysis, fuel cells, power electronics, and solar cells. The recent push for novel materials has increased interest in high entropy carbides (HECs) for such applications. The extreme level of tunability alone makes HECs a significant materials platform for a variety of fundamental studies and functional applications. We investigate the thermal conductivity of high entropy carbide thin films as carbon stoichiometry is varied. The thermal conductivity of the HEC decreases with an increase in carbon stoichiometry, while the respective phonon contribution scales with elastic modulus changes as the excess carbon content increases. Based on the carbon content, the HECs transition from an electrically conducting metal-like material with primarily metallic bonding to a primarily covalently bonded crystal with thermal conductivities largely dominated by the phononic sub-system. When the carbon stoichiometry is increased above this critical transition threshold dictating bonding character, the electronic contribution to thermal conductivity is minimized and remains at a constant, and a combination of changes in film morphology and phase precipitation systematically lower the phononic contribution to thermal conductivity. This trend in thermal conductivity with carbon content in the HECs is opposite to those observed in binary metal carbide systems. Our results demonstrate the ability to tune the thermal functionality of high entropy materials through stoichiometries that dictate the type of bonding environment.