2.3.4. Electrical Conductivity

The electrical conductivity was tested by using a 4-probe DC setup, using perforated steel sheet electrodes. A DC of 10 V was supplied to the outer two electrodes, and the inner two electrodes were used to measure the voltage (Figure 1), which was recorded with a datalogger every second and illustrated in the LABView software.

**Figure 1.** Four-probe setup for electrical resistivity measurements. Outer electrodes supplied a DC = 10 V, and inner electrodes measured the change in voltage.

Electrical impedance spectroscopy (EIS) was used to characterize the electrical parameters of the materials and to investigate the frequency-dependent response and material–electrode interface. By applying an AC current with a set amplitude over a range of frequencies, the response of the specimen could be measured in terms of magnitude and phase angle. A 2-probe setup with embedded electrodes was used with specimens of 20 <sup>×</sup> 20 <sup>×</sup> 80 mm<sup>3</sup> size. Two dosages were selected for each graphite type, one at 10 wt % and one at the start of the percolation threshold (either 20 or 40 wt % for the coarse graphite), and triplicate tests were run at 7 and 28 days. The results of the EIS testing are shown in the form of Nyquist plots, with the real part of the impedance shown in the *x*-axis (*Z* (Ω)), and the imaginary part in the *y*-axis (−*Z*"(Ω)); the average of three samples is represented with each

Nyquist plot. The Nyquist plot contains semicircles or arcs, and their diameters correspond to the resistances of the different components in the composite microstructure [25].
