Structural Evolution, Piezoelectric and Ferroelectric Properties of (1−x)Bi4Ti3O12-xCaBi2Nb2O9 High-Temperature Composite Ceramics

High leakage current density and relatively low piezoelectric activity have become one of the main obstacles to expanding the practical application of Bi₄Ti₃O₁₂ (BIT) high-temperature piezoelectric ceramics. Although ion doping can improve electrical resistivity and piezoelectric response, it often lowers the Curie temperature. In this work, by introducing CaBi₂Nb₂O₉ (CBN) with higher Curie temperature to BIT, a composite ceramic (1−x)BIT-xCBN was designed, and the effect of CBN content on the structure and electrical properties of the ceramics was investigated. With the increase in the x value, the intensity of the highest peak (117) gradually decreased until disappearing, while the intensity of peak (115) gradually increased, and the X-ray diffraction (XRD) refinement results showed that some non-stoichiometric compounds, Bi_1.74Ti₂O_6.624 and Ca_0.5Bi_2.5Ti_0.5Nb_1.5O₉, were produced. The sheet-like grains were effectively suppressed, while the granular grains became prominent with high CBN doping content, which should contribute to improving the electrical resistivity of ceramics. The optimal electrical properties were obtained in 0.8BIT-0.2CBN composite ceramics as follows: the piezoelectric coefficient d₃₃ = 13.8 pC/N, the Curie temperature T_C = 765°C, and the electrical resistivity ρ_dc = 8.52 × 10⁵ Ω·cm (@ 500°C). In addition, the d₃₃ maintained 89.1% of the initial value after annealing at 550°C, showing good thermal stability for high-temperature sensing applications.