# A High Thermal Conductivity of MgO-H\({}_{2}\)O Nanofluid Prepared by Two-Step Technique
## 3 Results and Discussion
### Thermophysical Properties Measurements  
#### 3.3.1 Thermal Conductivity  
The thermal conductivity of MgO-DW nanofluid is one of the most important operators that has a direct effect on heat transfer rate. Variations in thermal conductivity and relative thermal conductivity with respect to the nanoparticle concentration ratio ranging from 0.05 to 0.25 vol.% was studied at room temperature 27 \({}^{\circ}\)C \(\pm\) 2, as shown in Figure 4a,b. It is apparent from Figure 4 that the thermal conductivity and relative thermal conductivity increased with the solid content of dispersed MgO nanostructures. The thermal conductivity of MgO is approximately 48.4 (W/m.\({}^{\circ}\)K) [57] which is significantly greater than water's value of 0.598 (W/M.\({}^{\circ}\)K) [78]. As a result, raising the concentration ratio of MgO nanopowder would undoubtedly improve and increase the thermal conductivity of the prepared nanofluid. Notably, the utilized MgO nanoparticles have a high specific surface area that comes from a nanoscale of 33 nm. Hence, there is a significant rate of heat transfer between dispersed nanoparticles and deionized water molecules, which increases as their volume concentrations increase. In other words, an improvement in relative thermal conductivity means increased interactions between nanoparticles. The interactions are ascribed to the evolution of intense networks of dispersed nanoparticles inside the base fluid and the stir action caused by the Brownian motion of these nanoparticles. Furthermore, the Brownian motion becomes faster in the case of a nanostructure. Figure 4: (**a**) The thermal conductivity and (**b**) relative thermal conductivity (K\({}_{\text{eff}}\)/K\({}_{\text{bd}}\)), as a function of solid volume concentration of MgO-DW nanofluid at room temperature. The greatest thermal conductivity enhancement (by using Relation (3)) was found to be 25.08% at the highest concentration of 0.25 vol.%. In addition, the lowest improvement was 5.68% at a volume concentration of 0.05 vol.%, as observed in Table 6. Generally, increasing the volume concentrations of MgO nanoparticles in the fluid minimizes the distance between moving particles, resulting in more molecular collisions. An increase in the accelerated nanoparticles in the base fluid enhances the thermal conductivity of the nanofluid. Furthermore, the development of clusters and aggregates can take the shape of chain-like structures which provide a transformation path of the thermal energy among the nanoparticles. This may explain the high improvement ratio of thermal conductivity in the case of 0.25 vol.% MgO-DW nanofluid, despite the low zeta potential value of this sample. The results agree very well with the researchers' observations in [79, 80, 81]. The major aim of ultrasonication treatment is to ensure proper dispersion and break up of any residual agglomerations in the dispersed phase (MgO nanopowder) within the base fluid. Figure 5 reveals the changes in thermal conductivity with respect to the ultrasonication treatment time of MgO-DW nanofluid. The evident trend in Figure 5 is that the values of thermal conductivity increase with ultrasonication time. The increase in thermal conductivity with an increase in ultrasonication time may be attributed to enhancing dispersion stability, which was confirmed via zeta potential analysis (see Table 5). Therefore, the majority of the nanoparticles in the MgO nanosuspension will participate in the heat transfer process, increasing with ultrasonication agitation time, thereby, increasing its thermal conductivity. The differences in thermal conductivity \begin{table}
\begin{tabular}{c c} \hline \hline
**Concentration Ratios** & **Thermal Conductivity Enhancement (TCE\%)** \\ \(\Phi\) **(vol.\%)** & \\ \hline
0.05 & 5.685 \\
0.1 & 8.361 \\
0.15 & 15.217 \\
0.2 & 20.401 \\
0.25 & 25.084 \\ \hline \hline \end{tabular}
\end{table} 
Table 6: The improvement ratios of the thermal conductivity with solid volume concentrations of MgO/DW nanofluid at room temperature.