Zirconia heating elements

Fig. 3 Typical variation with temperature of resistance current tension and power of a stabilised zirconia heating element.

Since doped zirconia allows one to extend the oxide electrochemistry up to very high temperatures and since it can serve as a fuel cell electrolyte and even as a heating element in high temperature furnaces, we will briefly formalize the structure element transport in zirconia, which is the basis for all of this. Let us introduce the SE fluxes in their usual form. We know that only oxygen ions and electronic defects contribute to the electrical transport (/ = 02, e, h )... 

The peculiarity of stabilized zirconia is its relatively high electrical conductivity, especially in combination with Y2O3, so that the material is suitable for the manufacture of ceramic heating elements for high temperatures (the so-called Nernst mass). Above 1000 °C, the mains voltage is sufficient to ensure suitable passage of current. The elements have to be heated to this temperature in another way. 

Development of ionic conductors based on stabilized zirconia has reached a level of maturity, where most of the research on such materials concentrates mainly on obtaining incremental empirical improvements in conductivity by better processing control and refinement of the microstructure of the solid electrolyte and SE. Further increases in the conductivity are important in terms of enhancing the efficiency of systems such as O2 sensors, zirconia-based mixed-potential gas sensors, electrochemical oxygen pumps, heating elements, and fuel cells [4-7]. The systematic errors, as have been considered before, are errors with a known determined functional connection with the source of their cause, and the conformity of their appearance can be definitely described. 

The use of oxygen conducting stabilised zirconia as heating elements in high temperatiure furnaces is of interest both because of its high melting temperature (> 2500 C) and of its stability in oxidising atmosphere (fig. 2). 

The element is still best known, however, in the form of its oxide. Zirconia linings for metallurgical furnaces are very permanent, and, because of their low heat conductivity, may be made very thin. Zirconia refractories, such as crucibles, are very resistant to the action of heat, slags, and most acids, and may safely be plunged into water while red-hot (42). 

Professor von Hevesy and Thai Jantzen separated hafnia from zirconia by repeated recrystallization of the double ammonium or potassium fluorides (20, 26). Metallic hafnium has been isolated and found to have the same crystalline structure as zirconium. A small specimen of the first metallic hafnium ever made is on permanent display at the American Museum of Natural History in New York City. Dr. von Hevesy, who prepared it, presented it to the Museum for the collection of chemical elements (29). A. E. van Arkel and J. H. de Boer prepared hafnium by passing the vapor of the tetraiodide over a heated tungsten filament (26, 30). 

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