In order to achieve smaller and more efficient heat transfer systems, different methods have been proposed to increase the heat transfer of fluids. Nanofluids are a new type of fluid that consists of nanoparticles dispersed in a base fluid. Compared to conventional suspensions, nanofluids have higher stability and thermal conductivity, fewer erosion effects, and require less pump power. dispersed nanoparticles in base fluids may be metallic or non-metallic nanoparticles, nanofibers, nanoflakes, or nanotubes. There are two main single-step and two-step methods for the preparation of nanofluids. In the single-step method, nanostructures (commonly nanoparticles) are made simultaneously with dispersion in the base fluid. The important problem with this method is the tendency of particles to accumulate and is not a suitable method for large-scale nanofluid production. In the two-step method, which is widely used in the preparation of nanofluids, first nanoparticles (nanostructures) are prepared, then dispersed in the base fluids [1]. The base fluids are generally water, oil, ethylene glycol, and oil. Oxide nanomaterials, especially metal oxides, due to their large surface area and high absorption efficiency have exceptional optical, electrical, and chemical properties and are good candidates as additives to base the fluids. Magnesium oxide has received special attention due to its unique properties such as wide band gap, which is an important property to increase its insulating, catalytic, and mechanical properties [2, 3, 4]. Low heat capacity, high melting point [5], and high dielectric properties [6] have made MgO nanoparticles’ applications in the production of refractories, insulation materials, good CO2 absorbent, dyes, and recycling of toxic waste [7, 8]. In addition, magnesium oxide nanoparticles as an antibacterial and anti-cancer agent in surgical environments due to their high thermal and bioactive properties have become a suitable choice in the pharmaceutical industry [9–12]. The MgO production method and reaction parameters play an important role in determining its properties. Methods such as chemical vapor deposition, laser abrasion, hydrothermal, sol-gel, and combustion have been studied for the preparation of MgO nanoparticles. Sol-gel is one of the most successful methods due to its simplicity, cost-effectiveness, ability to produce nanoparticles with purity, and the high surface-to-volume ratio [13]. Compared to other chemical methods, this method has excellent control over the metal oxide composition and facilitates the production of nanoparticles with the size of a controlled chemical structure [14, 15]. Also, the hydrothermal method is a suitable method for the preparation of nanoparticles because, in addition to being low-cost in some materials, it does not need a calcination step. Solvothermal and hydrothermal processes in the production of powder materials with nanometer dimensions have received much attention. In these methods, using an autoclave, one can increase the pressure and temperature on the reaction vessel higher than the boiling point of the solvent. On the other hand, it has been proven that the thermophysical properties of nanofluids depend on the morphology and size of the added nanoparticles [16, 17]
In this work first, the synthesizing conditions for MgO nanoparticles were optimized using Taguchi’s robust design method. In our Taguchi optimizing method, solution pH, sintering temperature, and sintering time which can affect the size and morphology of the produced particles were the controlling parameters for to design of the desired products [18]. Then two types of MgO nanostructures and their related nanofluids with different wt.% of MgO nanostructures were prepared and compared. The present work describes the relationship between synthesizing conditions, size, and morphology of MgO nanostructures in oil-based nanofluids.