Phosphonic Acid-Modified Barium Titanate Polymer Nanocomposites with High Permittivity and Dielectric Strength

Materials with high dielectric permittivity are important in electronic components such as capacitors, gate dielectrics, memories, and power-storage devices. Conventional highpermittivity materials such as barium titanate (BT) can be processed into thin films by using chemical solution deposition yielding a relative permittivity (er) of about 2500 and relatively low dielectric loss but require high-temperature sintering, which is not compatible with many substrate materials. Polymer-based dielectrics, such as biaxially oriented polypropylene (BOPP), have good processability with high dielectric strengths (∼ 640 V lm) suitable for high-energy-density capacitors, but the storage capacity (ca. 1–1.2 J cm) is limited by the low er (ca. 2.2) of these materials. [6] Various approaches to high-er materials based on nanocomposites containing metal particles or other conductive materials have been pursued. Such nanocomposites have afforded huge er values but the resulting materials are limited by the high-temperature processing required, high dielectric loss, or low dielectric strength. Polymer/ceramic nanocomposites in which high-er metal oxide nanoparticles such as BT and lead magnesium niobate–lead titanate (PMN–PT) are incorporated into a polymer host are of significant current interest. The combination of high-er nanoparticles with high-dielectric-strength polymer hosts offers the potential to obtain processable highperformance dielectric materials. Simple solution processing of BT particles in a polymer host generally results in poor film quality and inhomogeneities, which are mainly caused by agglomeration of the nanoparticles. Addition of surfactants, such as phosphate esters and oligomers thereof, can improve the dispersion of BT nanoparticles in host polymers and consequently the overall nanocomposite film quality. However, in such systems, residual free surfactant can lead to high leakage current and dielectric loss. Thus, approaches to bind surface modifiers to BT nanoparticles via robust chemical bonds are highly desirable. Ramesh et al. have reported on the use of trialkoxysilane surface modifiers for the dispersion of BT nanoparticles in epoxy polymer hosts resulting in nanocomposites with reasonably high er, up to 45. [12] With the objective of identifying ligands that can form stable bonds to a BT surface through coordination or condensation, we have investigated a series of different ligand functionalities. In this Communication, we report that phosphonic acid ligands effect robust surface modification of BT and related nanoparticles and that the use of particles modified with suitable phosphonic acid ligands leads to well-dispersed BT nanocomposite films with high er and high dielectric strength. We have investigated the binding of a variety of ligands to the surface of BT nanoparticles, as the stability of the binding on the surface is vital to effective surface modification. We examined the following set of ligands, each bearing an aliphatic octyl chain with a different terminal binding group: C8H17-X, where X = PO(OH)2 (OPA), SO2ONa (OSA), Si(OCH3)3 (OTMOS), and CO2H (OCA). Trialkoxysilanes are widely used surface modifiers for silicate, indium tin oxide, and other metal oxide surfaces. Phosphonic acids have been reported to modify TiO2, ZrO2, and indium tin oxide surfaces and are thought to couple to the surface of metal oxides either by heterocondensation with surface hydroxyl groups or coordination to metal ions on the surface. Carboxylic acid and sulfonic acid groups may also bind to the surface in a similar manner. A sample of each ligand was mixed with BT nanoparticles (30–50 nm, 0.5 mmol ligand/ g BT) in an ethanol/water solution and stirred at 80 °C, followed by extensive washing with ethanol or water and centrifugation to remove excess and/or physisorbed ligand. The treated BT nanoparticles were dried and characterized by using Fourier transform infrared (FTIR) spectroscopy and thermogravimetric analysis (TGA). Figure 1a shows a comparison of FTIR spectra in the C–H stretching region for the BT nanoparticles treated with the ligands described above, followed by washing. These results C O M M U N IC A IO N