ZnO-assisted sintering and near-zero τf tuning in BaZr(PO4)2 microwave dielectric ceramics

The poor sinterability and intrinsically positive τ f of BaZr(PO 4 ) 2 ceramics were addressed by introducing ZnO as a sintering aid and property modifier. In a conventional solid-state reaction route, ZnO promoted liquid-phase sintering through the in situ formation of BaZn 2 (PO 4 ) 2 , enabling near-full densification (>99% relative density) at 1250 °C. The ZnO content strongly affected phase evolution, microstructure, and microwave dielectric properties. The Q × f value reached a maximum at ∼4 wt% ZnO and then decreased because of losses associated with the secondary phase and increased grain-boundary density. Notably, τ f could be systematically tuned from positive to negative values through compensation by the negative- τ f BaZn 2 (PO 4 ) 2 phase. The optimized composition (4 wt% ZnO) exhibited ε r ≈ 10.1, Q × f ≈ 36,300 GHz, and τ f ≈ +0.4 ppm/°C. These results identify a promising microwave dielectric material that can be densified at a lower temperature and establish a practical strategy for tailoring τ f in phosphate ceramics through controlled secondary-phase formation.