Temperature‐stable BaLiF ₃ microwave ceramics for LTCC applications with the addition of SrTiO ₃ and LiF

Abstract Although BaLiF 3 ceramic has a high quality‐factor at microwave frequencies ( Q × f value = ∼83000 GHz) coupled with a low sintering temperature (750°C/2 h), its high negative temperature coefficient of resonant frequency ( τ f = ∼ −70 ppm/°C) prevents practical applications. In the present paper, we employed SrTiO 3 as a second phase (7 mol%) to tune the negative τ f value to near zero (∼2.5 ppm/°C) and added a small amount of LiF (4 mol%) to reduce the sintering temperature to ∼800°C. Sintering in the N 2 atmosphere has a beneficial effect on the density, while it has a detrimental impact on the Q × f value of the sintered body due to the partial reduction of Ti 4+ in the SrTiO 3 (SrTi 4+ 1‐ x Ti 3+ x O 3‐ x F x ) and F‐vacancies in the BaLiF 3 (BaLiF 3‐x ) . The 0.93BaLiF 3 +0.07SrTiO 3 composite with 4 mol% LiF addition exhibits excellent combined microwave dielectric properties ( ε r = 14, Q × f = 55 000 GHz, and τ f = −3 ppm/°C), coupled with low sintering temperature (785°C/3 h) and good cofiring chemical compatibility with silver, which is a promising candidate for LTCC applications.