I am working on rare earth perovskites. When I measure the electrical conductivity (dc) of my samples at low temperatures (below room temperature) it is linearly increases (upto 1.0 S.cm-1@280K) with respect to temperature. After a particular temperature (Nearly RT), the material shows negative value of conductivity (i.e. -1.3 S.cm-1@RT) again it follows the same trend. Actually I don't know how to interpret this behavior. I have used Broad band Dielectric Spectrometer (BDS) Make: NOVOCONTROL Technologies GmbH & Co. Model : Concept 80. The temperature range is from 120 K to 473 K and the frequency range up to 5MHz. So can anyone explain why I am suddenly getting negative values in conductivity at RT and above? I am awaiting your valuable suggestions.
Thanks
An electrical conductivity can't be measured directly, it should be calculated from the resistance. So if the conductivity is negative, it means that the resistance is negative, right ? This may occur if you have created a generator in your experiment, for example through a non-desired oxydoreduction phenomenon between two metals. If a current is produced in your experiment due to such reaction between 2 metals of different oxidation potentials, and if you material acts as en electrolyte, your apparatus may interpret it as a negative resistance.
You should check that.
Alain
Dear Dr. Sathayamoorthi
Before interpreting any such unexpected result, you have to make sure that whatever you are getting is not a spurious result. Since, you are working at low temperatures, you have to make sure that the vacuum in your sample holder is very good otherwise there may be condensation of moisture or some other vapours at low temperatures which might give you spurious results when they get evaporated at a particular temperature.You first check your system by measuring conductivity of some other known material having no such anomalies. If your are not getting such an anomaly, then you can say that whatever results you are getting are not spurious.You are using an instrument which measures a.c. conductivity also. Check whether you get the similar behavior at other frequencies. You can measure d.c conductivity by using some other method for example four prove method and see that you are getting the same behaviour or not. In conclusion, you have to make sure that whatever you are measuring is not spurious as negative conductivity with temperature is not a common phenomenon. You have to be careful before searching an explanation.
Thank you Professors for your kind suggestions. I have measured my samples at both low temperature and above room temperature. In low temperature, the sample shows gradual increases with respect to temperature. But at room temperature, the electrical conductivity suddenly goes down to negative value. So Why this sudden change in conductivity? I have checked 3 times both low temperature and above RT measurements.
Dear Satyamuthi,
What is the status at room temperatute before cooling. Is it negative conductivity in the beginning or you get only after cooling? This is very important to understand your sign reversal at room temperature.
Direction of observed DC current is opposite to (assumed) applied voltage.
Try to measure cell OCV. It is looking like to problems with electrode equalibrium
Dear Prof. A. Kumar
I have measured the conductivity at room temperature before cooling and after cooling and the values are not same. Room temperature before cooling shows +ve value and after cooling it shows -ve value. So there will be some reversal taking place.
Dear Dr. Satyamurthi,
I suggest you to run some other dummy sample and check whether you get the same type of sign reversal. If not, your set up is o.k. If yes, then you have to worry about the vacuum, electrical contacts and measuring set up as I pointed out in my previous answer. Good luck.
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