2, CNRS, Chatenay-Malabry, , France
Compared to commonly used conventional energy storage devices such as batteries and fuel cells, polymer-based capacitors can have high energy densities, which are due to the fast charging and discharging capability. This high energy density arises from concerns of integration, especially in electrical and electronic systems that require pulses of energy in their operation mode. Another field of application is the development of soft generators for the harvesting of wearable electronics. In such a way, composite-elastomers (dielectric elastomer generators: DEGs) constitute a solution to convert ambient mechanical energy into electricity. One common factor is mandatory to develop polymer-based capacitors and DEGs: the increase in the dielectric constant of the polymer as this value is generally low for polymers dedicated to industrial applications. The solution proposed consists in benefiting both the insulating nature of polymeric materials thus ensuring a fairly high electric field and the addition of micro/nano particles (ceramics, oxides, metallic, carbon nanotubes…) inside the polymer matrix in order to obtain high dielectric constant. There is a profusion of scientific publications on the dielectric properties of micro/nano composites polymers showing a strong increase in the dielectric constant. Unfortunately, concrete applications have been slow to materialize. The major drawbacks are linked to the fact that the increase in the dielectric constant (necessary to enhance the value of the capacitance) is accompanied by a drastic increase in the dielectric losses and a collapse of the dielectric strength. One reason of that is the neighboring of particles inside the polymer matrix (without considering the percolation) which promotes conducting paths and reinforcing fields at the interfaces. The main goal of our study was to develop polymer composite dielectrics generators for wave energy converters (WECs) and for energy harvesting devices to exploit the human energy kinetic while walking. For that, carbon nanotubes (CNT) were mixed inside a polymer matrix (Dow Corning silicone Sylgard 184) to realize a 100 µm thick active layer. In order to reduce the drawbacks evoked above, a coating of CNT was made with a specific polymer. Investigations in terms of dielectric constant; loss factor, dissipation factor, ac-conductivity and dielectric strength were compared for uncoated-NTC and coated-NTC polymer composites with various weight contents (0.2%, 0.5%, 1%, 2% and 4%). The evidence is unequivocal: for example, for 1 wt%, uncoated-NTC polymer composites present a dielectric strength of 40 kV/mm against 85 kV/mm with coated-NTC. In same time, the dielectric constant in the range 0.1Hz-1MHz was around 6.5 and 8 for the uncoated-NTC and coated-NTC respectively. Lastly, the percolation threshold was strongly increased with coated-NTC polymers which is also benefit to increase more the dielectric constant for applications like polymer-based capacitors.