In Situ Dynamics of Metal-Oxides Nanofluids for Solar Thermal Applications

Metal-oxides nanoparticles (NPs) have gained widespread attention owing to their high thermal conductivity, broad absorption in the visible region, cost-effectiveness, and low toxicity which can be utilized as nanofluids (NFs) for heat transfer, cooling, and solar thermal collector applications [1−3]. Water, oil, and ethylene glycol (EG) are typically used as working fluids, but these exhibit poor absorption, particularly in the visible region. To enhance the absorption and thermal conductivity of the working fluids, NPs are dispersed in these fluid mediums [1−3]. The suspension of NPs in base fluids is termed as “Nanofluids”. Due to the higher thermal conductivities of metal-oxides NPs, the heat transfer coefficient of the NFs can be greatly enhanced by increasing the density of the fluid medium and the surface area of NPs (due to their smaller size) allowing more interaction with the surrounding fluids [3].<br/>The resulting thermal performance of NFs is complex and depends on several factors, for instance, morphology, agglomeration and concentration of NPs, and viscosity of the working fluids [1−4]. One of the main challenges limiting the use of NFs is the agglomeration of NPs which deteriorates the thermal performance and stability of NFs by reducing the surface area between the fluid and NPs [1−4]. Usually, dynamic light scattering and viscosity measurements are the two widely utilized techniques to investigate the effects of NPs agglomeration and fluid dynamics [4]. Based on this, several models have been proposed for explaining the thermal conductivities of various NFs [5]. However, no conclusive picture has been drawn yet, as these methods do not mimic the exact conditions of particles in the working fluids. The techniques available to image real-time particle dynamics and agglomeration in fluids are limited and challenging.<br/>Recent advancements within <i>in situ</i> transmission electron microscopy (TEM) using MEMS-based (micro-electromechanical systems) technologies allow direct visualization of fluid kinetics (particle interactions and agglomeration) down to the atomic scale [6], offering a unique insight into fundamental processes of NFs. However, the visualization of NFs has several challenges, mainly, the viscosity of fluids, NPs concentration, imaging conditions, and electron doses need to be optimized for ensuring high-quality data.<br/>In this work, surfactant-free CuOx nanostructure (Nns) were synthesized using the plasma-induced non-equilibrium electrochemistry (PiNE) method for their feasibility as NFs [8]. CuOx Nns were obtained in powder form and annealed in air at 400 °C for 6 h. <i>Ex situ</i> TEM methods were applied to investigate the morphology and crystal structure of CuOx Nns before and after the post-synthesis annealing in order to establish their correlation to the observed optical properties. <i>In situ</i> liquid cell experiments were performed to reveal the time and temperature-dependent kinetics of Nns agglomeration in working fluids. CuO Nns were mixed in EG and drop casted on the electron transparent SiNx window of the liquid cell. These experiments were conducted to monitor how CuO Nns agglomerate as a function of time and temperature. These are the first results on this class of materials highlighting fine details of CuO Nns agglomeration. We emphasise the need for further investigations of NFs to assess the temperature-dependent kinetics, particle agglomeration, and fluid behavior. In summary, the fundamental physical mechanisms such as agglomeration as a function of time, the temperature effects, and the dynamics involved between the fluids and NPs, are key towards unlocking and developing highly stable NFs systems for their application on more efficient solar thermal applications.<br/>1. <i>Int. J. Photoenergy</i>, 17, 8039129, 2019<br/>2. <i>Appl. Therm. Eng.</i>108, 720, 2016<br/>3. <i>Sci. Rep.,</i> 11, 1882, 2021<br/>4. <i>J. Phys. Chem</i>. C, 114, 18825, 2010<br/>5. <i>J. Appl. Phys.</i>, 106, 094312, 2009<br/>6. <i>R. Soc. open sci.,</i> 7, 191204, 2020<br/>7. <i>Sol. Energy</i>, 203, 37, 2020