A solution-based approach to composite dielectric films of surface functionalized CaCu3Ti4O12 and P(VDF-HFP)

Abstract

High permittivity CaCu₃Ti₄O₁₂/poly(vinylidenefluoride-co-hexafluoropropylene) P(VDF-HFP) nanocomposites were investigated as dielectrics for film capacitors. CaCu₃Ti₄O₁₂ was synthesized by two different soft-chemistry methods, namely, decomposition of a citrate precursor and a recently developed lactate precursor to identify a preferable route to nanometer scale spherical particles with an increased interfacial area. A ball-milling step was applied to break particle agglomerates and to enhance particle distribution in the composite films. To improve the wetting of the CaCu₃Ti₄O₁₂ oxide particles and the polymer, a variety of surfactants, e.g. carbonic acid, silane, sulfonic acid and phosphonic acid were investigated. A successful oxide surface functionalization was achieved by 2,3,4,5,6-pentafluorobenzyl phosphonic acid, leading to stable bonds and good structural compatibility between the surfactant and the highly fluorinated polymer matrix. The films were prepared from composite dispersions by a spin-coating technique and can be formed out of powders from both precursors, but the citrate method is preferable due to milder synthesis conditions and improved film homogeneity. The use of ball-milled powders as the oxide component results in homogeneous particle distributions even near to the percolation threshold. In addition, such fine-ground particles lead to homogeneous film thickness and decreased film roughness. Dielectric measurements at different frequencies revealed an enhancement in the relative permittivity by a factor of 5 compared to the pure polymer while the dielectric losses remained very low.