AbstractsEngineering

In this PhD thesis, stable carbon nanotube (CNT) colloidal suspensions, known as CNT nanofluids, were synthesized and studied for use in solar thermal energy harvesting applications. The production of a CNT nanofluid that is capable of remaining stable over long periods of time and at the high solar heating temperatures involves a multi-step process, which was developed, characterized and optimized in this thesis. An inexpensive and scalable thermal chemical vapour deposition (t-CVD) process was used to produce CNTs directly from stainless steel mesh. It was found that the acetylene concentration within the t-CVD reactor during growth could be used as a way of controlling the average CNT diameter, with smaller diameter nanotubes being easier to break and remove from the substrate for nanofluid production. In addition this growth method generates an open porous network of CNTs that can then be easily surface-treated using a glow discharge plasma. Functionalization using an argon/oxygen/ethane gas mixture was found to graft oxygen-containing moieties (primarily carboxylic groups) to the surface of the CNTs, mostly likely through a sidewall defect-bonding process.Nanofluids produced with the functionalized-CNTs were quantitatively shown to remain stable in a number of polar base fluids, including water, alcohols and glycols. In particular, the CNT nanofluids using denatured alcohol and glycols as base fluids are stable over extended periods of time (currently tested up to 10 months at 20 °C), at high temperatures (up to 170 °C in glycols for 1 hour), and after repeated evaporation/condensation cycling (using denatured alcohol as base fluid). Optical absorbance measurements of the CNT nanofluids showed that for short collection depths (cm) only small amounts of CNTs (on the order of mg L-1) are required to obtain close to 100 % absorption of solar radiation (assuming no scattering). The unmatched stability of the CNT nanofluids, coupled with their strong broadband electromagnetic absorption properties make them ideal candidates for volumetric solar energy harvesting, and open the door to other interesting potential applications. Cette thèse de doctorat porte sur la synthèse et la caractérisation approfondie de suspensions colloïdales stables contenant des nanotubes de carbone (NTC) – des nanofluides de NTC – et de leur usage potentiel comme fluides absorbeurs et caloporteurs de l'énergie solaire. La synthèse d'un nanofluide de NTC qui reste stable sous les conditions opératoires typiques des panneaux solaires thermiques, c.-à.-d. sur de longues périodes et à hautes températures, nécessitait le développement d'un nouveau procédé. Ce procédé intégré est décrit, caractérisé et optimizé dans cette thèse. Un procédé de dépôt chimique en phase vapeur a été développé pour la synthèse directe de NTC sur des grillages fins d'acier inoxydable. Ce procédé s'avère économique et peut être mis à l'échelle. Il a été observé que la concentration du gaz d'acétylène présent dans le réacteur lors de la croissance des NTC peut être…