Zirconium dioxide, or zirconia, is seemingly an ideal refractory oxide, having a high melting point (2680°C), low thermal expansion and also a considerable resistance to most forms of chemical attack. Early researchers soon found that zirconia, in the pure form, was actually of very limited use as a refractory because of sudden and disruptive volume changes accompanying a change in crystal structure which occurs at a temperature of about 1100 o C. It was discovered that this crystal structure change could be suppressed by the addition of one of a number of alkaline earth or rare earth oxides. These additive oxides have been found to form solid solutions having a stable crystal structure for this purpose over a wide temperature range. Zirconia containing additives is termed stabilized zirconia.
Stabilized zirconia may possess rather unusual electrical conduction characteristics, depending upon the particular additive oxide. In many cases, including that of yttria stabilized zirconia, a large almost purely ionic conductivity exists over a wide temperature range.
An early application for this material was in the Nernst glower, which was popular as an incandescent light source around 1900, and which survives today as a source in some infrared spectrephotometers.
The Nernst glower was simply a filament of yttria stabilized zirconia which could be heated to incandescence in air by passing a current, either ac or dc, through the filament. The glower was at once recognized to be an unusual ionic conductor in that there was apparently little or no electrolysis, even after extended periods of operation on dc.
The chemical stability of the conducting glower material was first satisfactorily explained by Wagner (1) who found the charge carriers to be primarily oxygen ions. For operation in air the supply of oxygen at the cathode was found to be continuously replenished from the atmosphere .leaving the composition of the ceramic essentially unchanged.
Several investigators have found that when a conducting sample of yttria stabilized zirconia is deprived of atmospheric oxygen, either by flushing with an inert gas or by evacuating the test chamber, immediate changes in the material are apparent (2, 3, 4). The color of the ceramic changes from white to black, with the color change proceeding from the negative electrode to the positive electrode. The color change is accompanied by the appearance of a multiplicity of cracks in the sample which suggests a volume change, perhaps as the result of a change in crystal structure. The character of the conductivity changes rapidly from ionic to metallic as evidenced by the disappearance of the large negative temperature coefficient of resistivity associated with ionic conductivity. The original color, although not the mechanical integrity, of a "converted" sample can be restored by firing in an oxidizing atmosphere. This behavior, in conjunction with the knowledge of the conduction mechanism, is a good indication that electrolytic conversion in inert atmospheres is basically a reduction process.
It is .the purpose of this project to investigate the mechanism of electrolytic reduction in a vacuum, and to determine, if possible, the composition and structure of yttria stabilized zirconia which has been so reduced.
| Identifer | oai:union.ndltd.org:WKU/oai:digitalcommons.wku.edu:theses-3700 |
| Date | 01 June 1968 |
| Creators | Newman, John |
| Publisher | TopSCHOLAR® |
| Source Sets | Western Kentucky University Theses |
| Detected Language | English |
| Type | text |
| Format | application/pdf |
| Source | Masters Theses & Specialist Projects |
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