Low-Loss Energy Harvesting Materials from Rubber-Nanodiamond Composites

Abstract

Sustainable energy harvesting opportunities have attracted much interest in the past decades. Among the energy harvesters, dielectric elastomer generators (DEGs) offer a comparably new and very eco-efficient approach for harvesting energy from the ocean waves, human motion, vibrations and pressure. In such applications, dielectric elastomers are used instead of piezoelectrics, resulting in high energy generating potential combined with relatively low cost and weight. However, the technology related to DEGs is still at the early stage and the existing DEG prototypes cannot achieve economic profitability yet. One major reason for that is the lack of efficient low-loss elastomers. Most elastomeric materials currently used in DEGs have high dielectric and viscous losses, and others have poor mechanical properties and high cost. Moreover, none of these materials is designed for an energy harvesting application. These drawbacks significantly reduce the attractiveness and efficiency of energy harvesting. However, a significant improvement in the efficiency of DEGs can be possible by developing high performance elastomers with low dielectric and viscoelastic losses – e.g., silicone, natural rubber, or others.

The present research is focusing on material-related losses of elastomer films in order to understand the contribution of elastomer type, as well as the presence of impurities and fillers, to the potential performance of such materials in energy harvesting. The major focus of the research is set on dielectric and mechanical hysteresis losses at low frequencies and ambient temperatures. Moreover, the dynamic and mechanical properties of the materials are assessed. Furthermore, the addition of small amounts of nanodiamonds (NDs) to the elastomer is viewed as one of the opportunities of achieving the low-loss membranes, as NDs are expected to minimize dielectric and viscous losses in certain conditions. This effect is most probably related to the high structural activity of NDs and its active surface. Due to the active surface, NDs can be chemically modified to adjust their interaction with elastomer. Thus, finally, the effect of selected filler surface treatments on loss properties of the films is investigated, as a filler-matrix interface is a typical origin of high losses. Indeed, the results of this study showed that the higher amount of natural proteins caused higher dielectric losses in the latex-based compounds, but resulted in lower dynamic loss tangent. Moreover, potassium hydroxide stabilizer used in latex compounding was found to increase the material-related losses of the materials. The described dielectric and mechanical losses of the films were reduced by the post-treatment with water or acetone. Finally, the addition of NDs containing the chemical groups able to participate in the crosslinking reactions showed a pronounced reduction of mechanical losses, especially in the PDMS composites.

The outcomes of the research help to reduce losses and gain more understanding of the key factors affecting the elastomer performance in DEGs. This knowledge can contribute to the development of energy harvesters, which are more economically preferable and easier to mass-produce than currently available prototypes. Moreover, such improvements significantly enhance economic feasibility of wave energy harvesters, and other types of dielectric energy harvesters, including wearable generators. Low-loss elastomeric films can be implemented in other industrial areas. For example, elastomers with reduced dielectric and viscous losses may enhances the performance of variable capacitors, which are used as stretchable sensors applied in sports garments, biomedical field and robotics. Finally, tire industry can benefit from the reduced dynamic mechanical losses in rubbers, which indicate the reduction of tire rolling resistance and, thus, lead to less fuel consumption.