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)(0.025)O-3 (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)(0.025)O-3 (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 (T-C) 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 (P-r) are relatively maintained for the compositions x = 0.15 and x = 0.2. A piezoelectric coefficient of d(33) = 130 pC/N and a planar electromechanical coupling factor of k(p) = 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 (xi = Delta T/Delta E) within a broad temperature range with a best value of xi = 0.2 Kmm/kV for x = 0.2.