Structure and dielectric properties of novel series of 3CaO–RE₂O₃–2WO₃ (RE = La, Nd and Sm) microwave ceramics and the adjustment of τ_f value

Using the solid-state reaction method, novel series of 3CaO–RE2O3–2WO3 (RE = La, Nd and Sm) dielectric ceramics materials were fabricated, and the crystal structure and microstructure were studied through X-ray diffraction (XRD) and scanning electron microscope (SEM), respectively. The Ca3La2W2O12 (CLW) ceramic exhibited a single pure phase with a rhombohedral structure, and Ca3Nd2W2O12 (CNW) and Ca3Sm2W2O12 (CSW) ceramics had similar crystal structures with CLW. 3CaO–RE2O3–2WO3 (RE = La, Nd and Sm) ceramics are packed tightly, and the grains exhibited clear boundaries and uniform size. The CLW, CNW, and CSW ceramics sintered at 1375 °C for 4 h possessed excellent microwave dielectric properties: Ɛr = 19.1, Q × f = 34370 GHz, τf = − 80 ppm/°C; Ɛr = 19.3, Q × f = 36610 GHz, τf = − 74 ppm/°C and Ɛr = 18.8, Q × f = 23240 GHz, and τf = − 68 ppm/°C, respectively. Moreover, the (1 − x)CNW–xTiO2 (0.3 ≤ x ≤ 0.6) and (1 − y)CNW–yCaTiO3 (0.4 ≤ y ≤ 0.7) ceramics were designed and prepared to get a thermally stable material. The addition of TiO2 and CaTiO3 had a significant impact on the performances of ceramics. With increasing the TiO2 and CaTiO3 content, the relative permittivity (Ɛr) and temperature coefficient of resonance frequency (τf) values of ceramics increased, but the quality factor (Q × f) values of ceramics decreased. Especially, 0.55CNW–0.45TiO2 and 0.45CNW–0.55CaTiO3 ceramics sintered at 1375 °C for 4 h had microwave dielectric properties of Ɛr = 22.4, Q × f = 32,490 GHz, and τf = − 0.5 ppm/°C and Ɛr = 23.4, Q × f = 6440 GHz, and τf = − 1.5 ppm/°C, respectively.