High Energy Density Nanocomposites Based on Surface-Modified BaTiO3 and a Ferroelectric Polymer

The dielectric permittivity and electric breakdown strength of nanocomposites comprising poly(vinylidene fluoride-co-hexafluoro propylene) and phosphonic acid surface-modified BaTiO3 nanoparticles have been investigated as a function of the volume fraction of nanoparticles. The mode of binding of pentafluorobenzylphosphonic acid on the BaTiO3 particles was investigated using infrared and 31P solid-state nuclear magnetic resonance spectroscopy, and the phosphonic acid was found to form well ordered, tightly bound monolayers. The effective permittivity of nanocomposites with low volume fractions (<50%) was in good agreement with standard theoretical models, with a maximum relative permittivity of 35. However, for nanoparticle volume fractions of greater than 50%, the effective permittivity was observed to decrease with increasing nanoparticle volume fraction, and this was correlated with an increase in porosity of the spin-coated nanocomposite films. The dielectric breakdown strength was also found to decrease with increasing volume fraction of the BaTiO3 nanoparticles, with an abrupt decrease observed around 10% and a gradual decrease for volume fractions of 20−50%. Comparison of these results with model calculations, using statistical particle packing simulations and effective medium theory for the permittivity and breakdown strength, indicates the important roles of nanoparticle percolation and porosity of the nanocomposites on the dielectric properties. The measured energy density at a field strength of 164 V/μm, well below the breakdown strength, increased to a value of 3.2 J/cm3 as the nanoparticle volume fraction is increased to 50%, roughly in line with the trend of the permittivity. The calculated maximum energy densities indicate maximal extractable energy (7−8 J/cm3 at 1 kHz) for two different particle volume fractions, as a result of the interplay of the dependencies of permittivity and breakdown strength on volume fraction.