Abstract
<title>Abstract</title> <p>This study examines microwave heating of cold-pressed aluminum nanoparticle compacts as a function of relative density relative to the theoretical maximum density (TMD). Spherical aluminum nanoparticles (18 nm average diameter) were compressed into cylindrical compacts at 20%, 30%, 40%, and 50% TMD and exposed to 2.45 GHz microwave radiation for five minutes under identical field conditions. Two-dimensional transient surface temperatures were measured in situ using infrared thermography, enabling spatially resolved thermal analysis during exposure. Increasing relative density from 20% to 50% TMD produced higher heating rates and elevated steady-state temperatures, indicating enhanced microwave–thermal energy conversion with reduced interparticle spacing. To interpret these trends, electromagnetic simulations were performed using ANSYS HFSS, modeling periodic arrays of core–shell aluminum nanoparticles with varying interparticle air-layer thickness. The simulations demonstrated increased power dissipation with decreasing air spacing, consistent with experimental observations. These trends were independently corroborated using a Bruggeman effective-medium model, which showed that the dielectric loss factor of the compacts increases monotonically with relative density, while normal-incidence surface reflectance also rises, representing a competing loss mechanism that is expected to dominate at higher densities. The combined experimental and numerical results establish relative density as a governing parameter in microwave heating of porous aluminum nanoparticle compacts and identify an optimal intermediate density range for efficient microwave coupling in metallic nanoparticle systems.</p>