3.1. Resistance Measurements
Resistance measurements in the CWRCT are mainly focused on detecting changes caused by oxidation due to exposure to the test fluids. This is the most likely scenario that would significantly alter the resistance of the copper wire during the test. While other factors can affect resistance, the controlled conditions of the CWRCT minimize their impact. Temperature is maintained at a constant throughout the test, eliminating thermal effects [
20] (excluding the temperature variation during the day and night in the heating plates tests). The use of direct current avoids frequency-related resistance changes [
21]. The isolated wire configuration and low current level (1 mA) make proximity effects [
22] and induced magnetic fields negligible. Although mechanical stress can affect resistance, the wires are carefully placed to minimize tension, and any additional stress would likely cause rupture before significantly affecting resistance due to the area- and length-dependent nature of the relationship (Equation (
1)). Finally, the high purity of the copper wire (99.9%) ensures that impurities are not a contributing factor to resistance changes [
23], leaving oxidation as the primary focus of the CWRCT.
As a reference, the resistance of an additional clean wire exposed at room temperature (23 °C) was measured simultaneously through an additional CWRCT, as shown in
Figure 4. The comparison of the resistances, given for a wire diameter of 60 µm, are shown in
Table 4. Throughout the whole test, the average resistance of the control copper wire did not substantially change, staying at an average of 6.101
with no more than 2% standard deviation due to room-temperature changes; therefore, no significant corrosion was produced.
Initial resistance values in each test might differ due to different factors, such as temperature, length, and uncertainties in the DDM connections. Additionally, due to the configuration of the test, where one wire is submerged in the fluid while the other one is placed above, the temperature of the oil phase will be higher than that of the vapor phase since temperature control is achieved with the oil phase; this temperature difference displays a resistance difference in all tests due to the relationship between them [
20]:
where
R(
T) is the resistance at temperature
T,
is the resistance of the wire at a reference temperature,
(typically room temperature), and
is the temperature coefficient of resistance.
Figure 5 and
Figure 6 show the results obtained via the tests performed with heating plates as the heating method. In
Figure 5, the first resistance measurement data using the heating plates showed that the wire in the PAO vapor phase failed after 140 h, while the rest increased their resistance by less than 1.5%. The second test did not exhibit the same failure of the PAO_VAP or any evidence of severe damage, since the resistance measurements did not increase by more than 1.5% in all wires.
Oscillations observed in the resistance measurements appeared as a consequence of the high fluctuations in ambient temperature between day and night, taking into account that heating plates were used to control temperature; therefore, there was a higher difference in resistance between the vapor and oil liquid phases.
Figure 7 shows the comparison of resistance changes in both repeated tests (and difference between them) using heating plates.
The resistance results obtained via the measurements conducted using a thermal bath for heating are shown in
Figure 8 and
Figure 9. Each figure shows the results of one test repeat. Overall, all the results show more stable resistance behavior than those obtained by using the hot plates (See
Figure 5 and
Figure 6). This is because using the thermal bath ensured uniform temperature control in the oil phase and prevented higher heat losses compared to the heating plates’ arrangement. Therefore higher resistance values were measured in this setup and no oscillations were observed.
In an initial test (
Figure 8), the wire exposed to the vapor phase of the diester showed a steady linear increase in resistance after 270 h, while the other wires did not show more than a 1% increase (
Figure 9).
The second test performed with the thermal bath showed that the wire exposed to the PAO in the vapor phase increased by 2.67% at the end of 336 h, with a linear increase starting at around 100 h, as shown in
Figure 9.
Figure 10 shows the resistance changes in both tests (and the difference between them) in the thermal bath setup.
Although most wires did not exhibit a substantial increase in resistance over the course of the experiments, the most affected ones were those in the vapor phase. When a wire failure occurred, the presence of green droplets around the wire was observed (
Figure 11). This phenomenon is often indicative of the generation of copper (II) hydroxide Cu(OH)
2 as a corrosion byproduct from the reaction with oxygen and moisture present in the air [
24,
25,
26,
27].
In the reaction, copper needs to form Cu
2+ ions, which will combine with hydroxide (OH
−) ions from the reduction of oxygen to form copper(II) hydroxide as follows:
When combined, the overall balanced chemical equation is as follows:
3.3. Copper Debris Quantification
It was found that all the fluids, except the dielectric coolant, had copper debris after the tests. A summary of the copper concentrations detected via the ICP measurements for all the tested fluids is presented in
Table 6.
Although in some cases the concentration of copper is higher than in others, it cannot be only attributed to copper corrosion because there are also connections of the DDM that may be in contact with the fluids. However, given that copper could not be detected below the detection limit in the dielectric coolant before or after the CWRCT, it can be inferred that it has a minimally corrosive effect on copper while immersed in the oil phase.
Nonetheless, notable variances in corrosion behavior among the fluids were found. Specifically, the vapor phase of PAO and the diester exhibited significant corrosion, as demonstrated by the increased resistance, the presence of corrosion byproducts after the immersion, and the elemental analysis showing copper oxide debris. Contrastingly, the dielectric coolant showed minimal corrosive effects on copper, as expected, implying superior compatibility as an e-thermal fluid for BEVs.
Finally, the advantages of this experimental approach include its sensitivity to detect slight changes in wire resistance and its provision of real-time insights into the corrosion process. However, the method’s susceptibility to environmental factors like moisture and the need for precise control over experimental conditions presents challenges. Further research must be conducted to determine the viability of fluids as e-thermal fluids. Other factors such as thermal and electrical performance, costs, biodegradability, and sustainability have yet to be considered, and they could greatly influence its suitability.