Microwave dielectric properties of composites consisting of MgAl2O4 filler synthesized by molten-salt method and isotactic polypropylene polymer matrix

MgAl2O4 particles were synthesized through the solid-state reaction method (MAO-S) or molten-salt method (MAO-M). The crystallinity, particle size, and crystal structure of spinel-structured MAO-S and MAO-M particles were characterized and these particles used as dielectric fillers were filled into an isotactic polypropylene matrix, up to 30 vol % filler concentration. Significant differences in the degree of inversion (λ), which represents the cation distribution in tetrahedral and octahedral sites, were obtained for MAO-S and MAO-M by solid-state NMR measurements and the λ value of MAO-S fired for 10 h was 0.39, while that of MAO-M fired for 10 h was 0.64. The dielectric constant of MAO-S- or MAO-M-filled composites increased from 2.4 to 3.7 with increasing filler concentration and was consistent with the Bruggeman model. The dielectric loss and thermal conductivity of the composites were remarkably improved by the addition of the MAO-M filler, depending on the increase in the duration of firing, and were 1.74 × 10−4 and 0.62 W/(m·K), respectively. The coefficient of thermal expansion and the temperature coefficient of the dielectric constant of composites also depended on the filler concentration.