Second-Phase Grain Boundary Engineering in Al2O3-MgAl2O4 Composites: Microstructure and Dielectric Breakdown Reliability

Abstract

Highlights A dual-pathway mechanism governs in situ MgAl2O4 spinel formation in alumina matrix. The optimal kaolin/MgO ratio of 4:1 yields a 17.74 kV/mm characteristic breakdown strength. Excess MgO incorporates into glassy phase, inducing ionic conduction channels. A microstructure–defect–breakdown correlation model is established and provides a theoretical basis for optimizing low-cost alumina energy storage ceramics. Abstract The escalating demands of pulsed power technology and high-energy-density capacitors necessitate dielectric ceramics with simultaneously enhanced breakdown reliability and microstructural homogeneity. Herein, Al2O3-MgAl2O4 composites were fabricated via a solid-state reaction using 80 wt% α-Al2O3 with kaolin/MgO mass ratios of 7:1, 4:1, and 1:1. XRD presents a dual-pathway spinel formation mechanism. SEM reveals 2–5 μm columnar spinel grains uniformly dispersed at grain boundaries. Porosity decreased from 4.29% to 2.69% while grain size increased from 3.35 to 5.64 μm as MgO content rose. Weibull analysis (n ≥ 10) showed that the 4:1 group achieved optimal dielectric reliability with a characteristic breakdown strength of 17.74 kV/mm and a Weibull modulus of 16.07, representing 8.76% and 20.93% improvements over the 7:1 (16.31 kV/mm, m = 24.58) and 1:1 (14.67 kV/mm, m = 15.37) groups, respectively. The 4:1 composition balanced grain boundary pinning (4.61 μm) and residual porosity (3.59%), maximizing interfacial charge scattering. Excess MgO (1:1) introduced residual Mg2+ into the glassy phase, inducing ionic conduction and premature breakdown. This study establishes a quantitative “microstructural composition–defect size distribution–dielectric breakdown” correlation, providing a theoretical foundation for microstructural optimization of low-cost alumina-based energy storage ceramics.