Hierarchical grain structure enhanced dielectric breakdown strength of Ca9Al(PO4)7 glass-ceramics

Dielectric glass-ceramics with high breakdown strength are critical for high energy density capacitors and pulsed power systems. However, in phosphate systems, the conventional trade-off between high crystallinity (to achieve high dielectric constant) and fine grain size (to achieve high breakdown strength) limits performance optimization. This study develops a rapid quenching-short duration heat treatment strategy for fabricating Ca 9 Al(PO 4 ) 7 (CAP) glass-ceramics with a distinctive nano-micro hierarchical architecture. By controlling the competitive growth of inherited crystals in the quenched melt and secondary crystallization of the glass matrix, a hierarchical structure spanning from 60 nm to 3.8 μm (span ratio R ≈ 63) was achieved. The optimally heat-treated glass-ceramic (1050 °C/3 h) exhibits an exceptional DC breakdown strength of 131.1 kV/mm, representing a 19% improvement over the control sample and a 28% enhancement compared to the 950 °C-treated specimen. This strengthening can be attributed to the following synergistic effects: (i) reduced porosity (<3.5%), (ii) increased CAP crystallinity (71.5 wt%) with elimination of anorthite impurities, and (iii) significantly elevated grain boundary activation energy (E a,gb ) from 1.27 eV to 1.39 eV. The enhanced E a,gb increases the barrier for long-range charge migration, effectively delaying breakdown propagation. The resulting glass-ceramic also demonstrates excellent dielectric stability (ε r  ≈ 9.1, tan δ < 0.004 at 1 kHz and room temperature, with losses remaining below 0.012 up to 200 °C) across broad frequency (100 Hz–10 MHz) and temperature (20–200 °C) ranges, substantially broadening the material's application scenarios.