Thermal Conduction Pathways within Complex Polycrystalline Microstructures

The effective thermal conductivity (TC) of a material is determined by various microstructural factors. Different phases in a material often have distinct thermal conductivities and their geometrical configurations dictate the thermal pathway. In addition, the thermal resistance of the crystalline surface disturbs the thermal transport, while it can be enhanced by the crystalline surface coated by a high conductive material. Due to these various and complicated factors, investigations for effective TCs within polycrystalline microstructures have been limited to the controlled microstructures under selected TC regimes. In this study, we use the numerical method to estimate effective TCs within complex multi-phase systems, which are validated using experimental measurements. For simulations, our comprehensive mesoscale model [1, 2] is used to investigate microstructure-aware effective TC variations within porous materials. The model can compute effective TC within realistic digital microstructures with diffuse interfaces using the FFT method, and the simulation results show quantitative agreement with the experiments [1]. In order to characterize the TC behaviors for porous microstructures, we carried out simulations with wide TC ranges of both the gas and the solid phases. The simulations show two different regimes for the TC variations due to (i) the porosity of a microstructure and (ii) the TC difference between the two phases. We suggest the semi-analytical model by introducing structure and intensification factors to capture the effective TC variations [2]. Additionally, we simulated the effective TCs for a ternary system (Fe, Fe₂O₃, and Air) within porous microstructures, which show reasonable agreement with experiments. Moreover, our simulation method can be extended to evaluate the effective TCs by varying additives and their topological features. We expect our combined experimental, analytical, and numerical approach can shed light on how the effective TC of a complex microstructures can be tailored, allowing for better designing materials microstructures for improved thermal conductivities under the wide range of operation conditions.<br/><br/><i>This work was performed under the auspices of the U.S. Department of Energy by Lawrence Livermore National Laboratory under Contract DE-AC52-07NA27344.</i><br/><br/>[1] D.-M. Kim et al., <i>Enhancement of effective thermal conductivity of rGO/Mg nanocomposite packed beds</i>, Intern. J. Heat Mass Transf. 192 (2022) 122891.<br/>[2] Y. Song and T.W. Heo, <i>Mesoscale modeling and semi-analytical approach for the microstructure-aware effective thermal conductivity of porous polygranular materials</i> (Submitted, http://dx.doi.org/10.2139/ssrn.4574964).