High electric field-induced ferroelectric loss of polymer/paraelectric barium titanate particle nanocomposites

Abstract

Polymer nanodielectrics have manifested high energy densities for potential applications in film capacitors. However, ferroelectric-like large hysteresis is often observed in the bipolar polarization–electric field (P-E) loops, even though neither the polymer matrix nor the ceramic nanoparticle exhibits any intrinsic ferroelectricity. To understand this abnormal ferroelectric behavior, poly(vinyl pyrrolidone) (PVP)/60 nm BaTiO3 (BTO60) nanocomposite films were fabricated with uniform BTO60 nanoparticle dispersion. PVP was a linear dielectric polymer with an apparent dielectric constant of 6.8 and BTO60 was found to be paraelectric. When the volume fraction of BTO60 was above 30 vol%, broad ferroelectric P-E loops were yielded. Using a theoretical deconvolution of the broad P-E loops based on the Langevin-type function, three contributions were identified. First, deformational polarization generated the linear dielectric constant of nanocomposites, from which the linear dielectric constant of BTO60 nanoparticles was extracted using the Bruggeman mixing rule. Second, slim paraelectric P-E loops were obtained for the BTO60 nanoparticles. From the saturated polarization, the electric field-induced permanent dipole moment per unit cell was estimated to be 0.07–0.2 D at a poling field of 20–100 MV/m, which was significantly smaller than the dipole moment (5.3 D) of the tetragonal unit cell of BaTiO3, indicating the paraelectric nature of BTO60 nanoparticles. Third, strong particle–particle dipolar interactions induced transient ferroelectric domains among the high dielectric constant BTO60 nanoparticles, particularly when the poling electric field was high. The third contribution dominated the overall polarization process and thus accounted for the abnormal ferroelectric behavior for the linear dielectric polymer/paraelectric ceramic nanoparticle composites. The knowledge gained from this study will pave the way towards developing better strategies for polymer nanodielectrics in the applications of capacitive energy storage.