How do you convert Zview CPE (CPE-T and CPE-P) values to equivalent capacitance?

[Marc Dumerac]

I conducted EIS on MgAl2O4 samples (500-900C) and the spectra indicate two depressed, well-resolved arcs, corresponding to the bulk and grain boundaries.  I simulated and fitted the EIS spectra with Zview using 2 parallel RC-circuits with capacitors replaced with CPE's for a much better fit (CPE-P approximately 0.7-0.9).  However, I would like to extract capacitance values (for example to calculate the permittivity and other variables for a paper).  Should the equivalent capacitance be calculated from the frequency maxima of the depressed semi-circles (C = 1/wmaxR)?  What capacitance values are appropriate when CPE's are used? 

Thank you,

Marc

 

[Colin Gore]

You can calculate capacitance from the psuedocapacitance, resistance, and n given by the CPE fitting.  See the following paper for details.

C. M. Gore, J. O. White, E. D. Wachsman and V. Thangadurai, J. Mater. Chem. A, 2014, 2, 2363-2373.
http://dx.doi.org/10.1039/C3TA12668D


The calculation for capacitance is simple using Z-plot fits:
C = R^([1-n]/n)*Q^(1/n)

using your fitting parameters as follows:
n = CPE-P
psuedocapacitance, Q = CPE-T
R = Rp

Let me know if you have any trouble.

[Marc Dumerac]

Thanks, your reply is much appreciated!!!

[Colin Gore]

Glad I could be helpful.  You'll find this calculation to be incredibly useful when attributing various semicircles to physical processes.  A great paper for understanding their interpretation using capacitance values is

Irvine, J.T., D.C. Sinclair, and A.R. West, Electroceramics: characterization by impedance spectroscopy. Advanced Materials, 1990. 2(3): p. 132-138.

[Colin Gore]

The paper I mentioned is attached.  You should find that your bulk arc has a capacitance ~1 pF and the grain boundary arc is ~ 1 nF.  Anything > ~1µF is probably an electrode effect.

Cheers,
Colin  

On Monday, June 30, 2014 11:51:41 AM UTC-4, Marc Dumerac wrote: