Growth kinetics evaluation of hydrothermally synthesised Co2(OH)3Cl nanoparticles for application in solar thermal heat transfer fluids

Abstract

Nanofluids are advanced heat transfer fluids utilized to enhance the properties of common fluids used in solar thermal collectors. Prior experimental studies demonstrated the dependency of the thermal conductivity and viscosity of different nanofluids on their particle size, shape, concentration, material, hosting fluid properties, acidity (pH value), additives and fluid temperature. Co3O4 nanoparticles of higher aspect ratio have been reported to enhance the thermal conductivity of their water based nanofluids. Moreover, it has been reported that the synthesis of cobalt oxide nanoparticles in the presence of different propanol concentrations can significantly alter their size, thus improving the properties of the resulting nanofluids. Despite numerous studies investigating its applications in the energy storage industry, cobalt hydroxychloride, a precursor to cobalt oxide, has never been used as nano-heat transfer fluid. The impact of propanol concentration on the size of cobalt oxide nanoparticles has been studied, but no similar study has been conducted for cobalt hydroxychloride. Although the relationship between the thermal conductivity of cobalt oxide nanoparticle-based nanofluids and the impact of their shape has been previously documented, there has been no other study exploring the utilization of cobalt hydroxychloride nanoparticles in nanofluid applications. Conflicting trends about the impact of particle size and shape on the viscosity of various aqueous nanofluids show the need to investigate the influence of cobalt hydroxychloride particle size and shape on the viscosity of their nanofluids. A simultaneous Ostwald ripening (OR) and oriented attachment (OA) based growth of cobalt hydroxychloride micro/nanostructures in the presence of triethanolamine/water solvent has been reported. Therefore, time course experiments have been conducted to understand the growth of cobalt hydroxychloride in the presence of propanol and water. In the presence of cobalt chloride hexahydrate, cobalt hydroxychloride nanopowders were synthesised in pure water and solutions of pure propanol at concentrations ranging from 10% to 100%. A synthesis temperature and time of 105 ±5 °C and 6 h were maintained. The resulting powders were characterised using HR-TEM, XRD, ATR-FTIR and UV-VIS-NIR after their dispersion in pure water then, their thermal conductivity and viscosity were measured. In the presence of cobalt chloride hexahydrate, cobalt hydroxychloride nanopowders were synthesised in 70% propanol concentration solvent and pure water. Synthesis times were varied from 60 min to 960 min for both solvents, but at a constant synthesis temperature of 105 ±5 °C. The resulting powders were characterised using XRD and HR-TEM. Both pink and lavender β-cobalt hydroxychloride nanoparticles were produced from propanol concentrations ranging from 70% to 100%. Green nanopowders of mixed phases were encountered for the rest of propanol concentrations. A conversion of the shape of cobalt hydroxychloride particles from hexagonal nano-plates to spherical-shaped nanoparticles occurred at 70% propanol concentration. Contrary to a prior trend, increasing propanol concentration from 0% to 100% led to the size reduction of cobalt hydroxychloride nanoparticles. Time dependent experiments revealed that changing the solvent medium from water to 70% propanol while retaining the same processing time modify the morphology of cobalt hydroxychloride from hexagonal nanoplate to spherical. However, the morphology of cobalt hydroxychloride was not altered despite the change in solvent medium from pure water to water:propanol solution (30:70)% at aging times ranging from 9h to 16h. The production of smaller spherical cobalt hydroxychloride particles in the presence of 70% propanol was achieved at reaction times ranging from 1 h to 6 h. In water, cobalt hydroxychloride nanoplates grew first via OR kinetics then OA kinetics while a simultaneous OR and OA growth kinetics occurred for nanospheres. In propanol, the growth of cobalt hydroxychloride nanospheres were controlled first by OR kinetics, then by OA kinetics. An increase in temperature led to reducing the viscosity of cobalt hydroxychloride nanofluids while increasing their thermal conductivity. All cobalt hydroxychloride nanofluids displayed absorption peaks in the near infrared region and a considerable increase in light absorption in the visible range compared to water, demonstrating their potential solar absorption ability. At 303 K, the cobalt hydroxychloride nanofluid made of cobalt hydroxychloride nanoplates at ~58 nm displayed the highest increase in viscosity of ~11%. The thermal conductivity of the nanofluid made of cobalt hydroxychloride nanospheres at ~10 nm exhibited the highest augmentation in thermal conductivity at all temperatures with ~14% at 308 K. The thermal conductivity enhancement was due to the Brownian motion-induced convection mechanism. The thermal conductivity enhancement of the aqueous nanofluids was affected by the cobalt hydroxychloride particle size while their viscosity increase was rather affected by the shape of cobalt hydroxychloride.