Combeite-type Na2Ca2Si3O9-based Ceramics With Low εr and Excellent Temperature Stability for X-band Antenna Applications

Combeite-type Na 2 Ca 2 Si 3 O 9 ceramics were fabricated via the conventional solid-state reaction route, and their phase compositions, microstructural features, and microwave dielectric properties were systematically investigated for the first time. The results reveal that single-phase Na 2 Ca 2 Si 3 O 9 ceramics with a hexagonal structure ( P 3 1 2 1 space group) can be obtained after sintering at 1150°C–1300°C. The specimen sintered at 1285°C delivers optimal microwave dielectric properties: a relative permittivity ( ε r ) of 6.68, a high-quality factor ( Q × f ) of 11 110 GHz, and a temperature coefficient of resonant frequency (τ f ) of −42.38 ppm/°C. To compensate the negative τ f and enhance the temperature stability of Na 2 Ca 2 Si 3 O 9 ceramic, CaSnSiO 5 was adopted as a τ f regulator to prepare (1– x ) Na 2 Ca 2 Si 3 O 9 - x CaSnSiO 5 (0.36 ≤ x ≤ 0.65) ceramics. Rietveld refinement combined with SEM/EDS characterizations verifies the two-phase coexistence of Na 2 Ca 2 Si 3 O 9 and CaSnSiO 5 with uniform phase distribution and dense microstructure. As x value increases, the τ f of (1– x ) Na 2 Ca 2 Si 3 O 9 - x CaSnSiO 5 composite ceramics gradually shifts from −12.65 ppm/°C ( x = 0.36) to +4.89 ppm/°C ( x = 0.65). Optimal comprehensive microwave dielectric properties are achieved at x = 0.61, with ε r = 7.48, Q × f = 14 046 GHz, and τ f = -1.2 ppm/°C. A microstrip patch antenna using this optimized composite as the dielectric substrate operates at 9.08 GHz (X-band), attaining a maximum radiation efficiency of 85.92% and a peak realized gain of 8.16 dBi. This work demonstrates that Na 2 Ca 2 Si 3 O 9 -based composite ceramics hold great promise as temperature-stable dielectric substrate for millimeter-wave communication devices.