Structure and dielectric properties of novel series of 3CaO–RE2O3–2WO3 (RE = La, Nd and Sm) microwave ceramics and the adjustment of τf 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: Ɛ_r = 19.1, Q × f = 34370 GHz, τ_f =  − 80 ppm/°C; Ɛ_r = 19.3, Q × f = 36610 GHz, τ_f =  − 74 ppm/°C and Ɛ_r = 18.8, Q × f = 23240 GHz, and τ_f = − 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 (Ɛ_r) and temperature coefficient of resonance frequency (τ_f) 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 Ɛ_r = 22.4, Q × f = 32,490 GHz, and τ_f =  − 0.5 ppm/°C and Ɛ_r = 23.4, Q × f = 6440 GHz, and τ_f = − 1.5 ppm/°C, respectively.