In this work a single mode resonant microwave cavity (2.45 GHz) is loaded with spherical particles. A weakly coupled Electromagnetic-Thermal (EM-thermal) solver is used iteratively to determine how the microscopic geometry (local curvatures between the particles, grain size and neck size) modifies the electromagnetic field, and in turn the thermal field in the particles. The modelling is performed with the conformai Finite Element (FE) solver COMSOL Multiphysics. The dielectric and the thermal properties of cerium oxide (ceria) particles up to 1000°C have been determined in our previous study. The main variables in this study are: 1-The radius of the spherical ceria particle [R, 3*5*R] 2- The size of the neck between two particles [X/R= 0.1, 0.15, 0.2].