Microwave-terahertz response of H3BO3-added CaAlB2O5.5 ceramics sintered at low temperature for patch antenna application

The H 3 BO 3 added CaAlB 2 O 5.5 ceramics were prepared by solid-state method. Their thermal behavior, phase composition, microstructures, and dielectric properties across both microwave and terahertz frequency ranges were systematically characterized, with a particular focus on exploring their terahertz response. Ceramic samples with the composition CaAlB 2 O 5.5 + x wt% H 3 BO 3 (where x = 0, 2, 4, and 6) were synthesized using solid-state reactions and sintered at temperatures between 940 °C and 1060 °C. With 4 wt% of H 3 BO 3 , the sample shows the lowest secondary phase content and the highest relative density. Whereas, the sample sintered at 980 °C exhibited the best microwave dielectric properties: ε r = 5.8 ± 0.05, Q × f = 41372 ± 1100 GHz (@11.45 GHz), and τ f = -28 ± 2 ppm/°C, as well as an excellent terahertz dielectric property with Q × f = 4.726 × 10 8 GHz (@ 3.14 THz). By careful calculation with DFT, the Debye-Lorentz fitting reveals that lattice polarization and phonon damping dominantly govern the dielectric response from microwave to terahertz frequencies. Finally, the microstrip patch antenna fabricated on the optimized CaAlB 2 O 5.5 ceramic substrate exhibited a peak gain of 6.4 dB and a low return loss (S 11 = -33.72 dB) at a center frequency of 4.86 GHz. CaAlB 2 O 5.5 ceramics deliver high-quality microwave to terahertz dielectric performance and are promising candidates for microwave and terahertz applications.