In Situ Mullite Formation in 80 wt% Alumina Ceramics via Kaolin/Ca3(PO4)2 Ratio Modulation: Densification, Microstructural Evolution, and Dielectric Breakdown Behavior

Mullite–alumina composite ceramics were fabricated via in situ reaction sintering by modulating the kaolin-to-calcium phosphate (K/C) mass ratio (3, 6, 9, and 12) within an 80 wt% alumina matrix. The effects of liquid-phase sintering aid content on densification, phase evolution, microstructural development, and dielectric breakdown reliability were systematically investigated. X-ray diffraction confirmed corundum and mullite as the predominant phases across all compositions, with the mullite yield exhibiting a non-monotonic dependence on the K/C ratio. The optimal K/C ratio of 9 yielded the highest mullite content (9.15 wt%), the greatest bulk density (3.58 g·cm−3), and an apparent porosity (4.3%), attributable to a balanced liquid-phase volume and viscosity that promoted dissolution–precipitation transport while suppressing gas entrapment. Quantitative microstructural analysis revealed an average alumina grain size of 5.17 μm and uniform distribution of acicular mullite grains within the corundum matrix. Weibull statistical analysis of the dielectric breakdown strength for the optimal composition (K/C = 9) gave a characteristic breakdown strength E0 of 14.31 kV/mm and a Weibull modulus m > 20, indicating exceptional data reliability. The high breakdown resistance is ascribed to the synergistic effects of reduced porosity, increased grain boundary density, and residual compressive stresses at mullite–corundum interfaces. These findings establish a quantitative correlation among liquid-phase content, microstructural optimization, and dielectric reliability, providing a framework for the compositional design of cost-effective alumina-based energy storage ceramics.