The characterization techniques of the three powders through TEM showed an increase of the particle diameter, dp, with the amount of lemon juice added in the synthesis process. The values of the dp were 27 nm, 31 nm and 37 nm corresponding to the ratio’s L/M = 0.33, L/M = 1 and L/M = 3, respectively. The L/M = 0.33 TEM images exhibited a more disaggregated powder and the EDX showed fewer K and P atoms as impurities.
Ultrasonication time was effective in particle size reduction until an optimal value of 60 min beyond which no significant dp reduction was observed. At 20 ◦C, DLS measurements of the three powders dispersed in EG/W (60:40) after 1h of sonication with SDS 1.5 wt% showed particle diameters of 80.6 ± 0.2 nm, 140 ± 5 nm and 196 ± 7 nm for the three L/M ratios (0.33, 1 and 3), respectively, which supposed a reduction ratio of 3.5, 3.1 and 2.2. While the zeta potential yielded optimal values of 61.2 ± 0.9 mV, 60.8 ± 0.7 mV and 59.6 ± 0.4 mV, which showed a significant improvement of the stability and quality of the nanofluids.
The thermal conductivity studies of the three powders revealed an enhancement of the keff compared to the base fluid. It was also verified an increase in TC when rising temperature. The L/M = 0.33 nanofluids demonstrated the highest TC improvement of 7.5 % vs. 5.4 % for L/M = 1 and 4.4 % for L/M = 3 nanofluids, all of them at 20 ◦C, 0.02 vol%and1.5 wt% of SDS. In the three samples, the TC raised linearly with temperature. The experimental data of keff were compared with two thermal conductivity mathematical models, which mainly examine the influence of cluster formation and liquid layering organization around particles. In the two models the DLS measurements of the aggregate diameter was applied. In all cases, the deviation with respect to the experimental values was under 5 % at 20 ◦C.
