Oxygen vacancies are spontaneously produced in non-stoechiometric oxides, and their concentration depend on the partial pressure of O2 in the atmosphere. Most books of materials chemistry address this old and wel known problem. As a consequence of the presence of such vacancies, it is easy to show that the eletrical conductivity of the oxide varies as a power law of the O2 pressure at a fxed temperature. When I was assistant professor, I was giving a practical chemistry class where the students were measuring the O2 concentration in air through the measurement of the electrical conductivity of a cobalt oxide wire at 900°C. It was working very well.
Alain
Well and thank you.electrical conductivity high means waht about the o2 pressure here?
The way the conductivity changes with oxygen pressure depends on the kind of non-stoichiometry of the oxide (having a deficit of either metal or oxygen), and on the oxygen pressure, promoting or not associations of vacancies in the oxide structure. In the case of cobalt oxide, increasing the oxygen pressure produces the formation of additional Co+++ cations, which can be seen as the association of Co++ and h., where h. is the postive carrier (electronic hole). The conductivity thus increases with the pressure of oxygen: in the low pressure range, it varies as P^(1/6), but it changes as P^(1/4) in the high pressure range. In other words, plotting log (electrical resistance) as a function of log (P(O2)) slighlty below atmospheric pressure gives a straight line of slope -0.25. It takes some lines for demonstrating it, but for sure you can find the demonstration in books of solid-state chemistry.
Alain
If the pressure of oxygen is directly propotional to electrict conductivity of the material. thank you Alain Celzard
No, sorry, only the log of these quantites are proportional, as they are related to each other by a power law. Sorry if I was unclear.
Alain
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