hepia / HES-SO University of Applied Sciences of Western Switzerland,
Keywords: energy storage, nanocomposite, sol-gel, dielectric strength
Summary:For energy storage applications requiring quick charging and discharging cycles, capacitors based on hybrid nanocomposites (ceramic fillers in a polymer matrix) have the potential to become a very good choice. Indeed, as the stored energy density (J/cm3) in a plane capacitor is proportional to both the dieclectric constant and the square of the disruptive voltage, a nanocomposite made of polymeric matrix filled with ceramic nanoparticles appears to be a good solution : in volume, most of the materials is made of ceramics, but the disruptive discharge is largely improved with the polymer which prevent the discharge. This occurs provided the polymer –ceramic interface can sustain large charge polarization. Following this approach, several nanocomposites have already been tested, such as alpha-hematite or ferroelectric embedded in PVDF or PVDF-based polymer Although those choices certainly make sense in terms of the final electrical performances of the material, we choose to target the nanocomposite with the lowest possible cost production, and which can easily be scaled up for production. In this perspective, we then used sol-gel nanoparticles of silica sol gel embedded in PMMA (poly-methylmethacrylate). Solgel SiO2 nanoparticles have been synthesis by the standard Stöber procedure with TEOS (tetraethyl ortholicate) as a precursor. We choose the synthesis parameters is order to have a mean diameter of 150nm, confirmed by DLS (dynamic light scattering) and AFM imaging. Some of particles have been grafted with TPM (also called TPS), a molecule which one side has a silane group which can covalently bind the SiO2 surface and the other group is a methacrylate, similar to the one found in PMMA. This covalent adhesion of the hybrid interface is expected to improve the electric performance. PMMA with molecular weight of 120’000 g/mol (Aldrich) has been dissolved in cyclohexanone, a solvent more convenient for spin coating preposition than THF or chloroform, whose high volatility induce very rough and structures PMMA thin films. The films were then deposited by spin coating on polished steel substrate and the optical thickness were measured by analyzing the white-light optical reflectance. The thickness was typically in the 4-7um range. Broadband dielectric spectroscopy (BDS) at temperature ranging from 20°C up to 120°C have been performed in the 100Hz-100kHz range, and the spectra were analyzed with the Havriliask-Negami model. The dielectric strength have been measured with a home-made system. The data were analyzed with the help of Weibull distribution. The results were coherent and show the expected high electric strength, in the range of 500 kV/m. We discuss the possible contribution of trapped water in the sol-gel nanoparticles for this high value. Apart from this, our results did not show the expected improvement for TPM-grafted silica nanoparticles, the energy storage being larger for non-grafted particles.