Processing–microstructure–property relationships in cordierite electroceramics: Influence of mechanical activation on microwave dielectric characteristics

Silicate-based microwave dielectric ceramics are increasingly important for next-generation communication systems owing to their low permittivity, high quality factor, and thermal stability. In this study, the influence of mechanical activation on the sintering behavior, structural evolution, and microwave dielectric performance of cordierite ceramics was systematically investigated, with particular emphasis on the minor and major activation conditions represented by 2 h and 32 h milling durations. The powders were subsequently sintered at 1400–1430 °C. Prolonged mechanical activation significantly accelerated densification, enabling near-full density (≥99% TD) to be achieved at 1410–1420 °C, whereas the 2 h milled powders required higher sintering temperatures (1420–1430 °C). Raman spectroscopy revealed subtle local structural variations after sintering, whereas X-ray diffraction and Williamson–Hall analyses confirmed largely comparable long-range crystallographic characteristics. Quantitative SEM-based grain-size analysis involving a statistically significant population of grains demonstrated that prolonged milling primarily reduced grain-size variability and improved microstructural uniformity, while producing only minor changes in the final average grain size. Microwave dielectric measurements showed that the dielectric response evolved from a porosity-controlled regime at lower temperatures toward a microstructure-sensitive regime near full densification. While ε r steadily increased with increasing density for all milling conditions, Q×f exhibited non-monotonic behavior. The highest Q×f value of 50,582 GHz was obtained at 1420 °C for the specimen prepared from the 2 h milled powder, despite its slightly lower density compared with the 32 h counterpart. This finding demonstrates that maximum densification and improved structural homogeneity alone are insufficient to optimize microwave dielectric performance. The optimum dielectric properties corresponded to ε r  ≈ 5.35–5.57 and τ f values near −23 ppm/°C. The stability of τ f together with the preserved structural characteristics indicates that intrinsic lattice contributions remained largely unchanged, whereas the variations in Q×f were governed predominantly by processing-induced microstructural evolution, particularly the grain-boundary-related effects associated with the finer grain structure.