title: The electrical conductivity of doped thoria in the temperature range 500°-1450° centigrade
Creators: name: McGinley, Charles E.
Publisher: name: University of Glasgow
Year: 1969
Subjects: subject: Applied sciences, metallurgy.

description: In recent years a great deal of interest has been aroused in the electrical conductivity of solid oxides at high temperatures. One of the main reasons is that it is believed high temperature fuel cells with solid oxygen ion conducting electrolytes of the type 0.85 ZrO2-0.15 CaO* (1) may be economically worth developing. The possibility of fuel cell applications has stimulated the Westinghouse Electric Corporation, in particular, to carry out a great deal of research on the electrical properties of oxides possessing considerable oxygen ion conductivity. Another possible commercial application is in magnetohydrodynamic (M.H.D.) power generation. Oxides may be used as insulators, and even electrodes, in M.H.D. generators at temperatures up to 2000°C (2). However, little work seems to have been done so far on the electrical conductivity side, as the more serious problems of corrosion, erosion and thermal shock have not yet been overcome. The metallurgical applications of a knowledge of the electrical properties are also very important. Prom electromotive force measurements on high temperature galvanic cells (3) of the type A, A(O)/oxide electrolyte/s, B(O) where A(O) and B(O) are oxides of metals A and B, it is possible to calculate the free energy of formation of one of the metal oxides, say B(O), provided the free energy of formation of A(O) is known and the ionic transport number in the oxide electrolyte is unity. A small electronic contribution to the conductivity of the electrolyte may lead to erroneous results due to polarization of the electrodes. So it is important to ensure that the electronic transport number is close to zero for the conditions of temperature and oxygen partial pressure under which the thermodynamic studies are being carried out. Probably the most important metallurgical application of electrical conductivity studies is in the field of metallic corrosion. According to Wagner's theory (4.5) of high temperature parabolic oxidation the rate of growth of a compact protective oxide scale on a metal surface is determined by the rates of transport of captions, anions and electrons through this scale. The transport rates of the carriers are of course directly related to their partial conductivities. It follows that a knowledge of these partial conductivities will help greatly in elucidating the oxidation mechanism. Other rate processes, as well as oxidation, are determined by ion mobilities and hence conductivities. Examples of these are sintering, solid state reactions and creep deformation. From what has been said above it is clear that the study of the transport properties of oxides is of great commercial interest. The more detailed the knowledge we have of conductivity in oxides the greater will be our understanding of the many related properties and processes, such as those mentioned. For this reason and also for the intrinsic scientific interest involved the ultimate aim of electrical conductivity studies is not simply to tabulate electronic and ionic conductivity values but also to determine completely the mechanism by which the charges are transported i.e. we want to understand perfectly, on the atomic level, the behaviour of solid oxides in an applied electric field. Much previous work on the conductivity of oxides has dealt with those in which the conductivity is predominantly electronic. The present research however is concerned with oxides possessing either mixed (electronic and ionic) or ionic conductivity. Since most of the previous research on this type of conductor has involved compounds other than oxides, for example, sulphides and alkali halides, in the next chapter the theory of the electrical conductivity of ionic solids in general. rather than oxides in particular will be reviewed. After this, the particular system chosen for the present investigation will be considered, the equipment used and experiments carried out will be described, the results obtained presented and their significance discussed. The numbers in a formula like 0.85ZrO2-0.15CaO give the mole fractions of the constituent oxides.