Zirconium dioxide (ZrO2), sometimes known as zirconia (not to be confused with zircon), is a white crystalline oxide of zirconium. Zirconia as a pure oxide does not occur in nature but it is found in baddeleyite and zircon (ZrSiO4) which form the main sources for the material. Of the two of these, zircon is by far the most widespread but it is less pure and requires a significant amount of processing to yield zirconia.
The processing of zirconia involves the separation and removal of undesirable materials and impurities - in the case of zircon - silica, and for baddeleyite, iron and titanium oxides. Pure zirconia exists in the monoclinic form at room temperature. Cubic and tetragonal phases are also stable at higher temperatures. Another low thermal conductivity use is a ceramic fiber insulation for crystal growth furnaces, fuel cell stack insulation and infrared heating systems. Thermodynamically the higher the operation temperature of an engine, the greater the possible efficiency (see Carnot heat engine).
Zirconium dioxide can occur as a white powder which possesses both acidic and basic properties.
On account of its infusibility and brilliant luminosity when incandescent, it was used as an ingredient of sticks for limelight. The cubic phase of zirconia also has a very low thermal conductivity, which has led to its use as a thermal barrier coating or TBC in jet and diesel engines to allow operation at higher temperatures. The transformation of monoclinic to cubic zirconia occurs at 800-1000°C and is accompanied by a large change in lattice size. A consequence of this phase change is a large volume expansion on cooling which make the fabrication of pure zirconia ceramics impossible.
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