Dielectric and thermal performance of a C60-based nanofluid and a C60-loaded ferrofluid

Liquids in electrical devices often act as electrical insulators and cooling media. To enhance both dielectric and thermal properties of liquids, various nanoparticles can be dispersed in the liquids resulting in effective nanofluids. In this research, a new generation transformer oil prepared by a gas-to-liquid technology has been used to prepare a mono-nanofluid with fullerene C60 nanoparticles (0.01%w/V) and a hybrid nanofluid with C60 (0.01%w/V) and iron oxide nanoparticles (0.01%w/V), so-called C60-loaded ferrofluid. Both nanofluids and the oil were subjected to experimental investigation of frequency-dependent dielectric response, dielectric breakdown, and thermal conductivity at various temperatures. Finally, the three liquids were applied in a single-phase transformer, and temperature rise tests of the loaded transformer were conducted. The dielectric spectroscopy revealed three orders of magnitude higher dielectric losses in C60-loaded ferrofluid than in the oil and C60 nanofluid, where the losses are of conducting nature. In C60-loaded ferrofluid, an interfacial relaxation process is considered in addition. C60 particles in the oil increased its breakdown voltage by 17%, while the mixture of C60 and magnetic nanoparticles resulted in a 12.5% reduction of the breakdown voltage. The enhancement has been ascribed to the strong capacity of C60 to absorb electrons and their ability to weaken the photoionization in the head of the streamer. The thermal conductivity of both nanofluids decreases with temperature, and the effective medium theory can well predict it. A significant decrease in the transformer temperature rise up to 8 K has been found for C60 nanofluid, as compared with the temperature rise achieved with the transformer oil. The temperature rise was also reduced with C60-loaded ferrofluid (up to 5.6 K). The lower cooling efficiency of the hybrid nanofluid was attributed to the high dielectric losses generating undesirable heat with a counter-productive effect on the cooling process.