Investigations of Crystal Structure, Dielectric, Ferroelectric and Piezoelectric Properties of (1-x) Ba(Zr0.2Ti0.8)O3–x (Ba0.7Ca0.3)TiO3 Ferroelectric Ceramics and its Composites with Ferroelectric Polymer

Abstract

Perovskite-based ferroelectric oxides have strong potentials to be utilized in piezoelectric sensors, actuators, transducers, non-volatile memory, energy harvesting, and energy storage devices. The development of lead-free ferroelectric materials have drawn a significant attention due to the regulations enacted by different countries on the use of hazardous substance such as Pb in electronic devices. Among the different lead-free ferroelectrics, BaTiO3 (BT) is one of the mostly studied lead-free ferroelectric materials. However, the poor piezoelectric coefficient limits its practical applications for various devices. Fabrication of solid solutions on BT based materials that lies near the morphotropic phase boundary (MPB) region is one of the suitable approaches to enhance the piezoelectric coefficients, dielectric and ferroelectric properties. The BT based ferroelectric solid solutions i.e., (1-x) Ba(Zr0.2Ti0.8)O3 – x (Ba0.7Ca0.3)TiO3 (BZT-xBCT) exhibits a giant piezoelectric property around the MPB composition for x = 0.5. Despite of extensive research on BZT-xBCT system, a clear picture has not been established that explains the coexistence of the crystal structures and enhanced piezoelectricity around the MPB region. In order to understand the structure-property relationship, a wide compositional range must be required. In the present investigations, BZT-xBCT ferroelectric solid solutions for a wide range of compositions (0.3 ≤ x ≤ 1.0) were prepared and characterized using wide variety of experimental techniques. The compositional driven structural phase transition, dielectric, ferroelectric and piezoelectric properties were studied in detailed. The MPB composition i.e., 0.5BZT-0.5BCT (BZT-BCT) shows an enhanced dielectric, ferroelectric and piezoelectric properties which is further well correlated with the crystal structure. The highest piezoelectric response observed for the critical MPB composition at x = 0.5 can be attributed to the combined and cooperative contribution from easy polarization rotation, maximum domain wall motion due to the structural heterogeneity and emergence of polar nano region due to relaxor behaviour. Recently the developments of miniaturization of electronic devices require a flexible ferroelectric ceramic polymer composite with optimum physical properties. Mixing of a ferroelectric ceramic having high dielectric permittivity with flexible polymer matrix having high breakdown strength results into a ferroelectric ceramic polymer composite with high energy storage performance. Solution casting technique is employed to prepare flexible ferroelectric ceramic polymer composite having general formula PVDF-HFP (Poly(Vinylidene fluoride-cohexafluoropropylene)) + ϕ wt.% 0.5BZT-0.5BCT (BZT-BCT) with 0 ≤ ϕ ≤ 40. This BZT-BCT (MPB composition with x = 0.5) has been chosen as the filler for the preparation of composite films as it exhibits highest dielectric, ferroelectric and piezoelectric properties among the entire compositions of BZT-xBCT system. The electroactive β phase fraction increases with the increase of filler concentration up to 20 wt.% and above that it decreases. The change in β phase fraction is well correlated with the enhancement of physical properties such as dielectric, ferroelectric and energy storage density in the composite films. Due to the difference in polarity and surface energy between filler and polymer matrix, it causes certain interfacial defects (pore or void) and agglomeration of fillers which leads to the degradation of physical properties of composite films. Hence, to overcome such problems, surface modification of filler particles using an organic modifier is one of the efficient strategies. In the present study, the surface of BZT-BCT has been modified/hydroxylated using H2O2, (denoted as h-(BZT-BCT)) and prepared PVDF-HFP + ϕ wt.% h-(BZT-BCT) (0 ≤ ϕ ≤ 40) composite films. The effect of hydroxylated filler content on structural, microstructural, vibrational, dielectric and ferroelectric properties of composite films are studied. The dielectric and ferroelectric properties of the composite increases with the increase in filler concentration up to 15 wt.% of h-(BZT-BCT) and after that it decreases. The variation of electroactive β phase fraction shows a similar trend as that observed in dielectric and ferroelectric properties for these polymer composites. The highest dielectric, ferroelectric properties with a low loss and high energy storage density (728 mJ/cm3) is observed for the composite loaded with 15 wt.% h-(BZT-BCT) among all the prepared composite films