Oxygen partial pressure measurement, solid electrolytes

An early use of oxygen-ion conducting solid electrolytes was in potentiometric devices for the measurement of oxygen partial pressure. In Section 2 it was shown that the e.m.f. of the cell relates the oxygen partial pressures on both sides through the Nemst equation,... 

NiOi + y could be measured precisely by use of the solid electrolyte, without chemical analysis. This result is shown in Fig. 1.51, as log Po. versus T curves, in which the dotted lines indicate the iso-y values. This figure shows that in order to get stoichiometric NiO, the sample has to be prepared at low temperature and low oxygen partial pressure in the NiO phase region. 

Fig. 4.36 Schematic diagram of a solid electrolyte probe for the measurement of oxygen partial pressures (or chemical activities).

Then, for example, if the oxygen partial pressure decreases in the measuring enviroiunent, the solid electrolyte loses part of the oxygen atoms in correspondence with the Equations (1.17) and (1.18) for the electrode reactions. 

When zirconia-based electrolytes are exposed to the high temperatures (T > 1100°C) and low oxygen partial pressures P02 < 10 ° Pa), usually encountered in metal melts, they exhibit mixed ionic and n-type electronic conductivities. Under these conditions, the solid electrolyte sensor generates cm/that is influenced by the electrical properties of the solid electrolyte. Schmalzried [25] has analyzed the contribution of electronic conductivity in the zirconia electrolytes to the measured emf of an electrochemical cell in the P02 region less than 10 Pa and has shown that, in the presence of n-type electronic conductivity, the emf of the sensor can be expressed as... 

The basic principle of the potentiometric ZrOz oxygen concentration cell is explained in Section 5.63.1, Fig. 5.6.2. The sensor consists of a Zr02 solid electrolyte body coated with two Pt electrodes, a measuring electrode exposed to the exhaust gas with an oxygen partial pressure of p 02, and a reference electrode exposed to a reference atmosphere with a constant oxygen partial pressure of p f2, typically ambient air. 

Already in the 70s the successful introduction of solid-electrolyte probes into the heat-treatment technique of metallic materials started. Among others, carburizing (900 to 1000 °C), car-bonitriding (900 C) and nitriding (500 to 600 °C) gas phases are used. Measurements of the oxygen partial pressure serve to keep constant conditions in the heat treatment. Only some new results of the development of solid-electrolyte probes for the nitriding technique shall be discussed here. 

The principle of the electrochemical measurement of oxygen diffusion in a metal consists in bringing the metal from a well-defined state into another well-defined state and following the diffusion-controlled relaxation process electrochemically. For example, the metal sample is placed on one side of the solid electrolyte ZrOa and functions as one electrode of a galvanic cell. On the other side of the electrolyte there is a practically unpolarizable electrode such as porous platinum in contact with air, or an Fe/FeO electrode, which has a fixed oxygen partial pressure of about 10 atm at 800°C. The following cell may be used ... 

Galvanic cells with solid electrolytes can be used for direct measurement of partial pressures in gases and concentrations in liquids and melts. An important example is cell I, which contains doped zirconium dioxide as solid electrolyte. By using cells of this type a wide range of oxygen partial pressures in gases (down to 10 atm) can be determined. The zirconium dioxide probe for such work is used at temperatures between about 500 and 1000°C. 

Thus, the electronic conductivity is calculated using the permeation flux TIq which can be measured by means of a potentiometric determination of oxygen concentration with a solid electrolyte cell. With the temperature and partial pressure dependence ( Defects in Solids), the following expression is yielded ... 

For such measurements the oxygen partial pressure of the reference electrode should be low in order to avoid the oxygen permeation through the solid electrolyte. 

Oxygen sensors with stabilized zirconia electrolytes can reliably measure oxygen pressure. However, at very low oxygen pressures, in the zirconia electrolyte is reduced, which results in an increased electronic conduction. An n-type electronic conduction appears in the electrolyte body and the sensor s output becomes unreliable. In the oxide ionic conductors, thorium oxide holds pure ionic conducting characteristics at lower oxygen partial pressures, compared to stabilized zirconia. However, as mentioned above, thorium oxide is radioactive and its commercial application is quite limited. For the purpose of measuring low oxygen partial pressures, a more suitable solid electrolyte is required. Perovskite oxides have been examined for this use, since they are based on oxides and are also very stable. 

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