Abstract:
A promising energy harvesting technique involves the use of thermal nanofluids, capable of collecting solar UV/visible radiation and storing it as latent heat in phase change materials (PCM), i.e. molten salts. Carbon-based materials are very interesting candidates for UV/visible light absorption and heat transfer in the nanofluid. The crucial point of the development is the stabilisation of dispersed carbon in a suspension. This works is an investigation on the preparation of stabilized suspensions of carbon black, mesophase carbon beads or exfoliated graphite, in aqueous solutions. To achieve this goal, chemical modifications of the surface functional groups of the carbon grains have been attempted.
Carbon has many allotropes in which it can exist such as diamond, graphite and amorphous carbon. None of these forms a stable suspension in water without a proper surface treatment. The first priority of this study was to identify possible surface treatments that would modify the surface complex on the carbon materials. Once the treatments had been identified, the focus of the study moved to selecting the most effective treatment based on its dispersion properties. The temperature ranges under which the carbon material remained in a suspension were measured. Finally, the effect of the surface area of the treated carbon material on the stability of the carbon material suspension was investigated. The characterisation techniques that contributed to achieving these objectives were observations with the naked eye, mass spectrometer measurements, thermogravimetric analysis, BET and scanning electron microscope images. The mass spectrometer and scanning electron microscope provided information on the modification of the surface complexes to gain an understanding of the effect of the treatment on the surface of carbon materials. The stability of the carbon and water suspension was measured by using the mass spectrometer and doing a thermogravimetric analysis. The BET results indicated the size of the specific surface area. The size of the surface area could assist in understanding of the stability of the carbon and water suspension.
Following an analysis of the results, the researcher reached the following conclusions: First, acid treatments with a concentration of 1 molar for the duration of a minimum of 4 days modify the surface complexes of the carbon black. Second, the treatment with the most stable dispersion properties is the potassium permanganate and nitric acid treatment, which is the only treatment that formed a stable suspension. Carbon materials with a smaller surface area than the treated carbon black does not form a stable suspension, even though they have similar surface complexes. The stable dispersion remains stable only up to 250 ºC. Higher temperatures cause the carbon material to start decomposing, when the carbon surface complexes are broken and released.
