Strontium oxide-doped alumina-based functional ceramics: microstructure, dielelectric properties, and suitablility for 5G/6G and millimeter-wave applications

The influence of strontium oxide (SrO) doping on the structural, microstructural, and terahertz (THz) dielectric properties of Al 2 O 3 ceramics is comprehensively investigated. Pure and SrO-doped Al 2 O 3 samples are fabricated and characterized using X-ray diffraction (XRD), scanning electron microscopy (SEM), and THz time-domain spectroscopy (THz-TDS). XRD analysis reveals the formation of SrAl 12 O 19 secondary phases for doping levels of 0.3 vol% and above, while SEM observations indicate significant densification and grain refinement, particularly for the 0.3-vol% SrO composition, which exhibit the highest relative density and the most uniform microstructure. THz-TDS measurements demonstrate that SrO doping strongly affects the refractive index, absorption coefficient, dielectric permittivity, and loss tangent in a non-linear manner. All doped samples exhibit an increase in refractive index and permittivity compared to the pure Al 2 O 3 , whereas the absorption and dielectric losses are highly dependent on the microstructural quality and secondary-phase content. The 0.3-vol% SrO-doped ceramic shows the most favorable THz response, characterized by reduced absorption coefficient (< 20 cm− 1 ), and the lowest loss tangent (< 0.03), indicating minimal extrinsic scattering and optimized densification. In contrast, the 0.1-vol% composition exhibited higher losses due to insufficient microstructural refinement, while the 0.5-vol% samples display moderate losses associated with increased SrAl 12 O 19 phase formation. Overall, the results demonstrate that SrO dopants play dual role in tailoring the THz dielectric response of Al 2 O 3 ceramics. These findings highlight the potential of SrO-doped Al 2 O 3 ceramics as promising materials for 5G/6G and millimeter-wave device applications, and sensing systems where low-loss and structurally stable components are essential.