Heat transfer fluidsfrom fundamental aspects of graphene nanofluids at room temperature to molten salts formulations for solar-thermal conversion

Resumo

Heat transfer fluids and nanofluids constitute an important element in the industry and their performance is key to the successful application in technologies that go from heat management and cooling to heat exchangers in thermal-solar energy and electricity generation. These industries demand heat transfer fluids with a wider liquid temperature range and better thermal performance than the conventional fluids. From low-temperature fluids to high-temperature molten salts, these fluids seem to benefit from the dispersion of solid nanoparticles, leading to nanofluids which frequently feature improved thermal conductivities and/or specific heats as compared with the bare fluids. However, there are some exceptions. Contradictory reports make it necessary to study these materials in greater depth than has been usual. Yet, the liquid nature of these materials poses a real challenge, both from the experimental point of view and from the conceptual framework. The work reported in this thesis has tackled two different challenges related to heat transfer fluids and nanofluids. In the first place, a careful and systematic study of thermal, morphological, rheological, stability, acoustic and vibrational properties of graphene-based nanofluids was carried out. We observed a huge increase of up to 48% in thermal conductivity and 18% in heat capacity of graphene-N,N-dimethylacetamide (DMAc) nanofluids. A significant enhancement was also observed in graphene-N,N-dimethylformamide (DMF) nanofluids of approximately 25% and 12% for thermal conductivity and heat capacity, respectively. The blue shift of several Raman bands (max. ~ 4 cm-1) with increasing graphene concentration in DMF and DMAc nanofluids suggested that graphene has the ability to affect solvent molecules at long-range, in terms of vibrational energy. In parallel, numerical simulations based on density functional theory (DFT) and molecular dynamics (MD) showed a parallel orientation of DMF towards graphene, favoring π–π stacking and contributing to the modification of the Raman spectra. Furthermore, a local order of DMF molecules around graphene was observed suggesting that both this special kind of interaction and the induced local order may contribute to the enhancement of the thermal properties of the fluid. Similar studies were also performed in graphene-N-methyl-2-pyrrolidinone nanofluids, however, no modification of the thermal conductivity or the Raman spectra was observed. All these observations together suggest that there is a correlation between the modification of the vibrational spectra and the increase in the thermal conductivity of the nanofluids. In light of these results, the mechanisms suggested in the literature to explain the enhancement of thermal conductivity in nanofluids were discussed and some of them were discarded. The second line of research focused on the development and characterization of novel molten salts formulations with low-melting temperature and high thermal stability. In this regard, two novel formulations of six components based on nitrates with a melting temperature of 60-75 °C and a thermal stability up to ~ 500 °C were synthesized. Moreover, the complexity of the samples led to establish a series of experimental methods which are proposed for the melting temperature detection of these materials as an alternative to conventional calorimetry. These methods are Raman spectroscopy, three-omega technique, and optical transmission.