PSI - Issue 28

Angeliki-Eirini Dimou et al. / Procedia Structural Integrity 28 (2020) 1679–1685 Author name / Structural Integrity Procedia 00 (2019) 000–000

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3. Results and discussion The measurements from the EIS experiments are presented with the use of Bode Plots, in which the electrical impedance |Z| is plotted versus frequency. The Bode Plot of water without rGO is presented in Fig. 3, where the impedance for spectra are plotted at different sonication energies. As expected, the electrical impedance of water remains unchanged with the application of ultrasound energy.

Fig. 3. Electrical impedance of water without nanoparticles and for different sonication energies Zhang et al. (2010) showed that ultrasound is efficient to assist preparation of the high-quality graphene dispersion, because ultrasound energy breaks the agglomeration of graphene particles and decreases the aggregation of graphene layers. The same observation can be made for the rGO dispersions with low nanoparticles concentration as shown in Fig. 4a to 4c. The electrical impedance decreases as sonication energy increases, indicating a better and more uniform dispersion, leading to the formation of conductive networks. The initial electrical impedance of rGO_0.05 dispersion at 10 Hz is about 250 kOhm, but as the sonication energy reaches 90 kJ, the electrical impedance decreases by 36 %. The equivalent impedance decrease is about 25 % for the rGO_0.075 and rGO_0.10 dispersions, respectively. The electrical impedance values at low frequencies were approximately 200 kOhm which is considered high and therefore the dispersions are not dense enough to form a conductive network. A different trend for the rGO_0.15 and rGO_0.20 dispersions (Fig. 4d and 4e, respectively) is noticed. An increase in sonication energy up to 30 kJ leads to an increase of the electrical impedance, remaining relatively constant for sonication energies up to 60 kJ, but a further increase of the applied energy leads to lower values of the impedance. The first trend can be attributed to the denser initial concentration of the dispersions. The application of ultrasonic energy leads to an increase of the distance among nanoparticles, thus preventing the current to flow, because the ultrasonic vibrations have broken down the larger agglomerates, separating the layers and overcoming the binding energy among the sheets, e.g. Zhang et al. (2010). However, as the sonication energy increases, the dispersion becomes more uniform, allowing the electrical current to flow again through evenly-distributed networks. This happens due to the purpose served by the application of sonication energy; the form of cavitation micro-bubbles and their collapse because of the energy excess leads to the creation of micro-jets and shockwaves, thus facilitating de-bundling to achieve better dispersed higher content of nanoparticles, e.g. Muthoosamy & Manickam (2017). Similar impedance values were recorded for both concentrations and specifically after the application of considerable amount of sonication energy (~ 60 kJ). To this end, both concentrations achieved good dispersion and the lower concentration was selected to minimise the possible consumption of nanomaterials used for the field application. Special attention should be drawn to the rGO_0.25 dispersion, which corresponds to 0.25 wt% rGO of the binder. For low sonication energies, the electrical impedance drops significantly, exhibiting values around 10 to 20 kOhm (Fig. 4f). This is an indication of a tunneling percolation network, improving the conductive properties of the dispersion. Wang et al. (2013) observed a similar pattern with rGO/polypropylene composites, at which the conductivity increased significantly after a specific concentration, attributed to the percolation threshold. At larger sonication energies, the main dispersion parameters are affected and harm the structure, e.g. in Kiamahalleh et al. (2020).