I am testing steel embedded in mortar with a lollipop type specimen. When I test it from 1MHz to 1Hz, with ac amplitude 10 mV in solartron FRA 1260 coupled with 1287A, I am getting negative impedance in the real axis in the high frequency region of the nyquist plot. The same specimen when I test it in PARSTAT 4000, my curve is in positive side of the real axis of the nyquist plot. Which spectra is correct?
I am using standard cables from the manufacturer in both the cases. So inductive loops due to non-standardized length of wires do not arise. I tried two solutions to fix the problem in solartron.
1) I reduced the start frequency from 1MHz to 100 kHz and Solartron is giving postive results in nyquist plot. I am not sure of selecting the frequency range. I chose 1MHz to understand the solution resistance. Should I reduce the frequency range?
2) I increased the ac amplitude from 10 mV to 50 mV. This also helped to remove the negative impedance in HF region. However, I am not sure of the linearity of the spectra.
Which approach is correct? please help.
It would be helpful to my perception, if you could share a view of (2-)photos[1] with (the each case of) the electrodes on the sample, for each of the two different machines.
Also, your specimen is dehydrated ?
1. Showing your testing cell, steel embedded in mortar with a lollipop type specimen.
Hi Sripriya, several things:
- check the impedance accuracy plot of each instrument. Maybe you are trying to measure an impedance in a frequency range where both instruments are not very accurate. If so then yes, reducing the maximum frequency is a good idea.
- try to build a dummy cell with known elements (a simply resistor, capacitor or combination would be fine) with an impedance close to your sample. Then compare the measured impedance with the expected one.
- increasing the amplitude will only help with the noise, it will normally not change a high frequency inductive part into a capacitive one. Most probably in the high frequency range you won't have linearity problems. At low frequencies 50 mV seems too much, you may drive your system in a non-linear region.
- share the Nyquist diagrams if possible.
Good luck !
Dear Sir,
Thank you so much for the reply.
I have tested the dummy cell that comes with the solartron. There it gives proper semicircle for 10 mV with some noise in the initial three points. I checked with the personnel of Solartron. They said, it is in accepted tolerance range.
However, when I test my samples, the ac amplitude change has shifted the curve in the positive side of the real axis. I have attached the EIS files generated with 10 mV and 50 mV AC amplitude. My sample was in saturated condition. These files were generated with solartron. I sweeped from 1MHz in the HF region.
Sripriya , Your measurements look quite equivalent, given the fact that, according to the time-stamp in your files, you did a run in November and the other in February. The sample is probably not the same. The differences you see may come from a slightly different electrochemistry. Also there is a difference in the two OCV voltages. Impedance may be different at different steady-state points. Try to run successively two impedance measurements on a stable sample , without changing the cell setup, one at 10 mV and the another at 50 mV. I think these are better conditions to understand the influence of the amplitude only. Best regards
Dear Sir,
Actually I did run with two ac amplitudes consecutively. But I discarded the negative impedance files. The files I attached is for illustrative purpose only to show that the amplitude is influencing the EIS spectra of my samples.
Certainly AC Amplitude does have some effect and higher the amplitude the polarizing the specimen and driving to non-linear effects. I could not open your files.If you could share the files, I can throw some light on your query. You may send the ZPlot,Zview files -- Basically , Amplitude of the controlled signal (Va or I a ) should be: - high enough to induce a significant amplitude of the response - Small enough to keep the linear behavior of the cell. ...
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