Effects of Alumina Particle Characteristics on Thermal and Electrical Conductivity of Alumina Packed Layers for Immersion Heaters Used in Aluminum Melting

Immersion heaters are increasingly used for heating aluminum alloys, but conventional heaters struggle to achieve the high-power density required for aluminum melting. For high-power operation, filler must provide chemical stability, high thermal conductivity, and low electrical conductivity at over 1273 K in oxidization atmosphere. Owing to their low cost, Al2O3 particles are promising candidates. In this study, molded specimens were fabricated using Al2O3 particles with different particle-size distributions, aspect ratios, and circularities, and their thermal and electrical conductivity were evaluated. Specimens composed solely of fine particles (<50 µm) exhibited relative density of 65% and the thermal conductivity of 2.5 W/m·K at 867 K (25% of bulk Al2O3) due to thermal resistance from porosity. In contrast, their electrical conductivity reached 6.8 µS/cm at 1473 K, (370% of bulk Al2O3) attributed to discharge phenomena within pores. When coarse particles (<1.7 mm) and fine particles were mixed at 7:3 ratio, the relative density varied from 65% to 83% depending on coarse particle morphology. Thermal conductivity increased with relative density, reaching 6.0 W/m/K at 867 K (60% of bulk Al2O3). Electrical conductivity decreased with increasing coarse particle mass fraction. LN2D analysis revealed that random dispersion caused by broad size distribution, low circularity, and low aspect ratio disrupted conductive paths, reducing conductivity to 1.7 µS/cm at 1473 K (94% of bulk Al2O3). Conversely, ordering structures formed by high circularity, and high aspect ratio caused linear electrical conductivity and limited reduction effect. Based on these insights, a filler consisting of coarse particles with broad size distribution, low circularity, and low aspect ratio mixed with fine particles at 7.5:2.5 was designed. The prototype immersion heater demonstrated 20% higher power density than conventional heaters, confirming that high-power heaters can be fabricated using Al2O3 fillers that are over 50% cheaper than nitride-based materials.