Solid electrolytes, applications oxygen partial pressure

A key factor in the possible applications of oxide ion conductors is that, for use as an electrolyte, their electronic transport number should be as low as possible. While the stabilised zirconias have an oxide ion transport number of unity in a wide range of atmospheres and oxygen partial pressures, the BijOj-based materials are easily reduced at low oxygen partial pressures. This leads to the generation of electrons, from the reaction 20 Oj + 4e, and hence to a significant electronic transport number. Thus, although BijOj-based materials are the best oxide ion conductors, they cannot be used as the solid electrolyte in, for example, fuel cell or sensor applications. Similar, but less marked, effects occur with ceria-based materials, due to the tendency of Ce ions to become reduced to Ce +. 

Many different types of sensors have also been fabricated by tape casting. One of the most widely used sensors is the oxygen sensor used in automotive and other applications. These sensors are based upon solid-state conductivity of the ceramic elements at elevated temperatures. Most are fabricated from stabilized zirconia, which is the electrolyte in the device. The zirconia is a solid ionic conductor that transforms oxygen partial pressure or activity gradients into an electrical signal, which can then be further processed to perform control activities such as carburation or ignition modification. The... 

In Fig. 3 the partial conductivities of ions and electronic carriers are plotted logarithmically vs. oxygen partial pressure. For the application as a solid electrolyte, the difference between ionic and hole conductivity should be at least 2-3 orders of magnitude. The transference number for electronic conductivity is between 0.01 and 0.001. 

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. 

Alumina is one of the best electrical insulators, hence its dielectric applications. Electric conduction depends primarily on impurities which act as acceptors (for example, Mg, Fe, Co, V or Ni) or as donors (for example, H, Ti, Si, Zr or Y), the balance between electronic and ionic contributions depending on the temperature and the partial pressure of oxygen [KRO 84]. Among the impurities, sodium deserves a particular mention because the synthesis of alumina by the Bayer process brings into play sodic mediums sodium aluminate (Na2011Al203), also called beta alumina, is an ionic conductor with very high conductivity, which is why this compound is envisaged for solid electrolyte applications [WES 90]. 

Solid electrolytes in solid state electrochemical applications (SOFC, SOEC, etc.) are exposed to high gradients of chemical potential and similar conditions are imposed to mixed conducting membranes used for partial oxidation of hydrocarbons, etc. The resulting stresses across CGO electrolytes were analysed by Atkinson [60] for typical gradients of oxygen partial pressures. On assuming flat constrained conditions and without additional lateral constraint, total strain will combine chemical expansion across the membrane and a stress-related contribution as follows ... 

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