Structural, dielectric, piezoelectric, ferroelectric and electro-caloric properties of Ba1−xCaxTi0.975(Nb0.5Yb0.5)0.025O3 lead-free ceramics

Polycrystalline samples of Ba1−xCaxTi0.975(Nb0.5Yb0.5)0.025O3 (where x = 0.15, 0.2 and 0.3, abbreviated as BCTYN) were prepared by the conventional solid state reaction method. The effect of calcium (Ca) substitution in BaTi0.975(Nb0.5Yb0.5)0.025O3 (abbreviated as BTYN25) on the structural, dielectric, piezoelectric and ferroelectric properties and electro-caloric effects (ECE) was investigated. X-ray diffraction (XRD) results at room temperature showed that the BCTYN samples in the composition x < 0.3 exhibited a pure tetragonal perovskite structure. Dielectric measurements showed a classical ferroelectric behavior for all samples. With the increase of the Ca content, the Curie temperature (TC) was still maintained with a small shift towards low temperature. The evolution of the Raman spectra was studied as a function of compositions and temperatures. The Raman bands confirmed the structure and the phase transition of the BCTYN ceramics. By adding Ca, the piezoelectric properties and the remanent polarization (Pr) are relatively maintained for the compositions x = 0.15 and x = 0.2. A piezoelectric coefficient of d33 = 130 pC/N and a planar electromechanical coupling factor of kp = 28% were obtained for these compositions. Two different methods were used to calculate the electro-caloric coefficients of the BCTYN ceramics. The incorporation of Ca was found to enhance the electro-caloric strength (ξ = ΔT/ΔE) within a broad temperature range with a best value of ξ = 0.2 Kmm/kV for x = 0.2.