Cold-sintered temperature stable Na0.5Bi0.5MoO4−Li2MoO4 microwave composite ceramics

A cold sintering process (150 °C, 30 min and 200 MPa) was employed to fabricate Na0.5Bi0.5MoO4−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 (εr) 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 εr ∼ 17.4 and Q × f ∼7470 GHz. Functionally graded ceramics were also fabricated with 5 ≤ εr ≤ 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 εr facilitated by the NBMO-LMO system, which suggested a 78% aperture efficiency at 34 GHz.