Engineering near-zero τf and high Qf in Nb-substituted NaTaO3–MgTiO3 composite microwave dielectrics for 5G FR1 front-end applications

To meet the stringent requirements of 5G FR1 (<6 GHz) RF front-end components for dielectric substrates with low loss (high Qf), near-zero temperature coefficient of resonant frequency (τf ≈ 0 ppm/°C), and a practical sintering process, a composition-compensation strategy is proposed by combining B–site Nb-substituted perovskite NaTa1-xNbxO3 (NTN, positive τf) with traditional MgTiO3 (MT, negative τf). After sintering at 1450 °C for 4 h, NaTa1-xNbxO3 ceramics with Nb content x = 0.10–0.30 form a single perovskite phase and exhibit a Vegard-like lattice evolution. Raman spectroscopy reveals that the BO6 octahedral stretching mode at ∼610 cm−1 correlates positively with εr, while its full width at half maximum is inversely correlated with Qf, leading to the optimized dielectric performance at x = 0.20 (εr ≈ 175, Qf ≈ 9300 GHz, τf ≈ +556 ppm/°C). Furthermore, (1-x)MgTiO3–xNTN composites (x = 0.04–0.12) sintered at 1350 °C for 4 h retain a stable physical biphasic structure without detectable reaction products or systematic peak shifts, enabling a continuous τf compensation governed by the phase fraction. At x = 0.08, a near-zero τf is achieved while maintaining a high Qf (∼98000 GHz). Device verification using a 3.5 GHz microstrip bandpass filter fabricated on the 0.92MT–0.08NTN substrate demonstrates the lowest insertion loss among the compared substrates and reduces the circuit area to ∼28.8% of that on Rogers 4003C, with excellent agreement between simulations and measurements. These results demonstrate a practical route to simultaneously realize low loss, temperature stability, and circuit miniaturization for dielectric substrates targeting RF front-end applications in the FR1 band.