Dual Properties of Microwave Absorption and Thermal Protection in La_0.5Sr_0.5FeO₃–Al₂O₃ Pseudobinary Composites

As multifunctional material for microwave absorption and thermal protection, the dielectric and thermophysical properties of equilibrium phases in LSF(La_0.5Sr_0.5FeO₃)–Al₂O₃ pseudobinary phase diagram are investigated herein. Four pseudobinary composites are obtained by sintering at 1400 °C for 10 h with different raw components (x wt% LSF-(100−x) wt% Al₂O₃, x = 20, 40, 60 and 80), labeled as LSFA20, LSFA40, LSFA60, and LSFA80, respectively. After sintering, the main equilibrium phase in LSFA20 and LSFA40 composites is SrAl₁₂O₁₉-based solid solution. For LSFA60 and LSFA80, however, LSF-based and SrAl₁₂O₁₉-based solid solutions are the main equilibrium phases. The increase of conductive LSF content results in the highest dielectric loss tangent and the best microwave absorbing property in LSFA80, in which the bandwidths for reflection loss less than −10 and −5 dB are about 2.37 and 7.7 GHz, respectively, when the thickness is only 1.5 mm. The thermophysical properties reveal that the thermal conductivity in LSF-Al₂O₃ pseudobinary composites shows weak temperature dependence because of the opposite contribution between phonons and electrons. It is lower than that of the traditional thermal barrier coating material of 8 wt% Y₂O₃-stabilized zirconia (8YSZ).