Nanocomposites of Ferroelectric Polymers with TiO 2 Nanoparticles Exhibiting Significantly Enhanced Electrical Energy Density (original) (raw)

Electrical energy storage plays a key role in mobile electronic devices, stationary power systems, and hybrid electric vehicles. There is a great need for development of new materials with superior electrical energy density since current ceramics and polymers fall significantly short of rising demands in advanced applications. The introduction of inorganic nanoparticles into polymer matrices to form dielectric polymer nanocomposites represents one of the most promising and exciting avenues to this end. This approach is motivated by the idea that the combination of ceramic materials of large permittivity with polymers of high breakdown strength could lead to a large energy storage capacity, as energy density is proportional to the product of permittivity and the square of the applied electric field. Moreover, large interfacial areas in the composites containing nanometer scale fillers promote the exchange coupling effect through a dipolar interface layer and result in higher polarization levels and dielectric responses. Compared to conventional ceramic materials, polymer-based dielectric materials also offer processing advantages including mechanical flexibility and the ability to be molded into intricate configurations for electronic and electric devices with reduced volume and weight. While most of the current studies on dielectric nanocomposites are focused on the enhancement of dielectric permittivity, few examples have investigated dielectric properties and associated energy densities at high electric fields. Ferroelectric metal oxides such as Pb(Zr,Ti)O 3 (PZT), Pb(Mg 1/3 Nb 2/3 )O 3 -PbTiO 3 (PMNT), and BaTiO 3 have been popular choices as filler materials in dielectric nanocomposites because of their high permittivities. However, from the energy storage point of view, inclusion of nanoparticles with permittivities on the order of hundreds and even thousands into polymers, which generally possess a permittivity less than 10, might not be desirable for an appreciable increase in energy density. As the filler has a much greater permittivity than the polymer matrix, most of the increase in effective dielectric permittivity comes though an increase in the average field in the polymer matrix with very little of the energy being stored in the high permittivity filler phase. Furthermore, the presence of a large contrast in permittivity between two phases gives rise to a highly inhomogeneous electric field and thus a significantly reduced effective breakdown strength of the composite. In this communication, we report high-energy-density polymer nanocomposites based on surface-functionalized TiO 2 nanocrystals as dopants in a ferroelectric poly(vinylidene fluoride-tertrifluoroethylene-ter-chlorotrifluoroethylene) (P(VDF-TrFE-CTFE)). In this approach, the polymer matrix and TiO 2 filler possess comparable dielectric permittivities of 42 and 47, respectively, measured using an inductance, capacitance, resistance (LCR) meter at room temperature and 1 kHz. High dielectric performance in the nanocomposites is realized via the large enhancement in polarization response at high electric fields and changes in polymer microstructure induced by the nanofillers.