Strontium Oxide-Based Functional Ceramics: Microstructure, Dielectric Properties, and Suitability for 5G/6G and Millimeter-wave Applications

The influence of Strontium Oxide (SrO) doping on the structural, microstructural, and terahertz (THz) dielectric properties of Al₂O₃ ceramics is comprehensively investigated. Pure and SrO-doped Al₂O₃ samples are fabricated and characterized using X-ray diffraction (XRD), scanning electron microscopy (SEM), and THz time-domain spectroscopy (THz-TDS). XRD analysis reveales the formation of SrAl₁₂O₁₉ 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 Al₂O₃, 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 SrAl₁₂O₁₉ phase formation. Overall, the results demonstrate that SrO dopants play dual role in tailoring the THz dielectric response of Al₂O₃ ceramics. These findings highlight the potential of SrO-doped Al₂O₃ ceramics as promising materials for 5G/6G and millimeter-wave device applications, and sensing systems where low-loss and structurally stable components are essential.