# A High Thermal Conductivity of MgO-H\({}_{2}\)O Nanofluid Prepared by Two-Step Technique
###### Abstract
Various physical and chemical methods have been used for preparation of MgO nanopowder, involving vapor deposition [38], co-precipitation [39; 40], sol-gel [41; 42], molecular beam epitaxial [43], spray pyrolysis [44], ultrasonic irradiation [45], microwave irradiation [46], hydrothermal [39], plasma irradiation [47], surfactant methods [48], combustion method [49], solvothermal [50], and thermal evaporation [51], in addition to biological sterilization methods [52], such as fungal strain _Aspergillus terreus_[53], and so-called green synthesis [54; 55]. Although MgO nanopowder can be prepared via the different previously mentioned approaches, the co-precipitation method has several advantages due to its simplicity, effectiveness, high surface area of the nano product, and economic route [56]. There are two major techniques that can be used to synthesize nanofluid, i.e., single-step and two-step techniques. Nevertheless, in terms of a two-step procedure, any of the aforementioned methods can synthesize MgO nanofluid. In both techniques, applying the ultrasonication treatment is the pivotal operator for breaking down the agglomerated nanoparticles to attain a high stable nanofluid [4]. Thermal conductivity and viscosity of the MgO/propylene glycol nanofluid were extensively improved by [PERSON] and [PERSON] [57] at various MgO nanopowder concentrations (0-2 vol.%), temperatures (20-60 \({}^{\circ}\)C), and ultrasonication times (0-30 h). Firstly, MgO nanopowder with an average particle size of 35 nm was prepared by a simple chemical reaction route. Then, a two-step technique was devoted to synthesizing the nanofluid using probe ultrasonication for 25 h. The average thermal conductivity enhancement was 20% for 2 vol.% MgO/propylene glycol nanofluid and was detected to be independent of temperature. The relative viscosity of the nanofluid was decreased with nanoparticle concentration at a temperature of 30 \({}^{\circ}\)C. At a higher ultrasonication time, both thermal conductivity and viscosity were clearly improved with ultrasonication time up to 0.243 (W/M.\({}^{\circ}\)K) and 25 (mPa\(\cdot\)sec), respectively. [PERSON] et al. [58] prepared MgO nanofluid by suspending nanoparticles (40 nm) in a binary base fluid of water and ethylene glycol (60:40) using a two-step technique. In this technique, an ultrasonication treatment time of up to 4 h was applied. The impact of different solid concentrations and different temperatures were investigated. The results showed that the thermal conductivity of MgO/H\({}_{2}\)O-EG nanofluid increased with an increase in nano MgO volume concentration and temperature. Using a two-step technique, a hybrid nanofluid of Ag-MgO/water was also conducted by the [PERSON] research group [5]. The nanofluid was produced by mixing 25 nm nano Ag (50%), 40 nm nano MgO (50%), and water. Ultrasonic vibration for 3 h and CTAB surfactant were employed to ensure better stability. Thermal conductivity and dynamic viscosity were measured as a function of nanoparticle volume fraction range between 0% and 2%. The obtained results revealed that both thermal conductivity and dynamic viscosity were increased with increases in the volume fraction of (Ag + MgO) nanoparticles. The effects of particle size, solid concentration, and temperature on the dynamic viscosity of ethylene glycol-based nanofluid containing MgO nanoparticles have been studied by [PERSON] et al. [59]. They conducted experiments at concentrations ranging from 5 to 20 wt.% of MgO nanopowder with particle sizes of 20 and 40 nm. MgO/EG nanofluid was agitated in an ultrasonic bath for 6 h by using a two-step technique. Nonlinear results revealed that the 20 nm and 40 nm MgO nanoparticles exhibited non-Newtonian behavior at weight concentrations higher than 5%. [PERSON] et al. [60] used molten salt LiNO\({}_{3}\)-NaNO\({}_{3}\)-KNO\({}_{3}\) with MgO nanoparticles to prepare nanofluid via a two-step technique.