In recent years, the nanofluids (NFs) have become the main candidates for improving or even replacing traditional heat transfer fluids. The possibility of NFs to be used in various technological applications, from renewable energies to nanomedicine, has made NFs and their thermal conductivity one of the most studied topics nowadays. Hence, this review presents an overview of the most important advances and controversial results related to the NFs thermal conductivity. The different techniques used to measure the thermal conductivity of NFs are discussed. Moreover, the fundamental parameters that affect the NFs thermal conductivity are analyzed, and possible improvements are addressed, such as the increase of long-term stability of the nanoparticles (NPs).The most representative prediction classical models based on fluid mechanics, thermodynamics, and experimental fittings are presented. Also, the recent statistical machine learning-based prediction models are comprehensively addressed, and the comparison with the classical empirical ones is made, whenever possible.

The present work of review is focused on the recent advancements regarding the exploration of the ionic liquids, ionic liquids with the incorporation of nanoparticles of several materials, and ionic liquid-grafted nanoparticles operating as liquid electrodes in electrochemical cells and capacitors. The ionic liquids are generally synthesized at room temperature and by adding a solution, which can be an acid, a base, or a salt in water, and are composed of organic cations and a great number of charge-delocalized organic/inorganic anions. The electrochemical features such as the electrical conductivity and capacitance of the promising ionic liquids and their hybrids are addressed thoroughly, together with their influencing factors like the nature, concentration, and functionalization of the nanoparticles, type of base fluids, working temperature, and addition of surfactants. Moreover, this overview identifies and discusses the main applications of ionic liquids and their hybrids with nanoparticles in various possible electrochemical device configurations, along with a brief evaluation of the associated feasibility issues. Additionally, this survey of the published scientific papers on the subject enabled the listing and evaluation of the beneficial features related to the usage of these fluids including enhanced electrical conductivity and improved capacitance in comparison with the commonly employed solvents and electrolytes. Finally, it addresses the main problems associated with such types of fluids and outlines the primary prospects for further research and use of ionic liquids and their nanocomposites in different electrochemical technological applications.

The issue of thermal control for space missions has been critical since the early space missions in the late 1950s. The demands in such environments are heightened, characterized by significant temperature variations and the need to manage substantial densities of heat. The current work offers a comprehensive survey of the innovative materials and thermal fluids employed in the aerospace technological area. In this scope, the materials should exhibit enhanced reliability for facing maintenance and raw materials scarcity. The improved thermophysical properties of the nanofluids increase the efficiency of the systems, allowing the mass/volume reduction in satellites, rovers, and spacecraft. Herein are summarized the main findings from a literature review of more than one hundred works on aerospace thermal management. In this sense, relevant issues in aerospace convection cooling were reported and discussed, using heat pipes and heat exchangers, and with heat transfer ability at high velocity, low pressure, and microgravity. Among the main findings, it could be highlighted the fact that these novel materials and fluids provide enhanced thermal conductivity, stability, and insulation, enhancing the heat transfer capability and preventing the malfunctioning, overheating, and degradation over time of the systems. The resulting indicators will contribute to strategic mapping knowledge and further competence. Also, this work will identify the main scientific and technological gaps and possible challenges for integrating the materials and fluids into existing systems and for maturation and large-scale feasibility for aerospace valorization and technology transfer enhancement.