Substantial recoverable energy storage in percolative metallic aluminum-polypropylene nanocomposites

Chemisorption of the activated metallocene polymerization catalyst derived from [rac-ethylenebisindenyl]zirconium dichlororide (EBIZrCl₂) on the native Al₂O₃ surfaces of metallic aluminum nanoparticles, followed by exposure to propylene, affords 0-3 metal-isotactic polypropylene nanocomposites. The microstructures of these nanocomposites are characterized by X-ray diffraction, transmission electron microscopy, scanning electron microscopy, and atomic force microscopy. Electrical measurements show that increasing the concentration of the filler nanoparticles increases the effective permittivity of the nanocomposites to Ïμ_r values as high as 15.4. Because of the high contrast in the complex permittivities and conductivities between the metallic aluminum nanoparticles and the polymeric polypropylene matrix, these composites obey the percolation law for two-phase composites, reaching maximum permittivities just before the percolation threshold volume fraction, v_f ≈ 0.16. This unique method of in situ polymerization from the surface of metallic Al particles produces a new class of materials that perform as superior pulse-power capacitors, with low leakage current densities of ≈10^-7-10^-9 A/cm ² at an applied field of 10⁵ V/cm, low dielectric loss in the 100 Hz-1 MHz frequency range, and recoverable energy storage as high as 14.4 J/cm³.