Cold-Sintered Temperature Stable Na_0.5Bi_0.5MoO₄-Li₂MoO₄ Microwave Composite Ceramics

A cold sintering process (150 °C, 30 min and 200 MPa) was employed to fabricate Na_0.5Bi_0.5MoO₄-Li₂MoO₄ (NBMO-LMO) composites with up to 96.4% relative density. X-ray diffraction traces, backscattered electron images and Raman spectra indicated the coexistence of NBMO and LMO phases in all composites with no detectable secondary phases. The pemittivity (ϵ_r) and temperature coefficient of resonant frequency (TCF) decreased, whereas microwave quality factor (Q × f) increased, with increasing weight % LMO. Near-zero TCF was obtained for NBMO-20 wt %LMO with ϵ_r ∼ 17.4 and Q × f ∼ 7470 GHz. Functionally graded ceramics were also fabricated with 5 ≤ ϵ_r ≤ 24. To illustrate the potential of these cold sintered composites to create new substrates and device architecture, a dielectric graded radial index lens was designed and simulated based on the range of ϵ_r facilitated by the NBMO-LMO system, which suggested a 78% aperture efficiency at 34 GHz.