Zirconia
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Zirconia (ZrO2) is a white crystalline oxide of zirconium. Its commonest naturally occurring form, with a monoclinic crystalline structure, is called Baddeleyite. Its cubic crystalline form, called 'cubic zirconia', is rarely, if ever, found in nature, but is made in various colours for sale as a gemstone.
It is used as a refractory, in insulation, abrasives, enamels and glazes, in high-temperature fuel cells and oxygen sensors.
Zirconia is one of the most studied ceramic materials. Pure ZrO2 has a monoclinic crystal structure at room temperature and transitions to tetragonal and cubic at increasing temperatures. The volume expansion caused by the cubic to tetragonal to monoclinic transformation induces very large stresses, and will cause pure ZrO2 to crack upon cooling from high temperatures. Several different oxides are added to zirconia to stabilize the tetragonal and/or cubic phases. magnesium oxide (MgO), yttrium oxide, (Y2O3), calcium oxide (CaO), and cerium oxide (CeO), amongst others.
Zirconia is very useful in its 'stabilized' state. In some cases, the tetragonal phase can be metastable. If sufficient quantities of the metastable tetragonal phase is present, then an applied stress, magnified by the stress concentration at a crack tip, can cause the tetragonal phase to convert to monoclinic, with the associated volume expansion. This phase transformation can then put the crack into compression, retarding its growth, and enhancing the fracture toughness. This mechanism is known as transformation toughening, and significantly extends the reliability and lifetime of products made with stabilized zirconia. A special case of zirconia is that of tetragonal zirconia polycrystaline or TZP, which is indicative of polycrystalline zirconia composed of only the metastable tetragonal phase.
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 turbine and diesel engines to allow operation at higher temperatures. The laws of thermodynamics tell us that the higher temperature you operate an engine at, the greater the possible efficiency (see Carnot heat engine). As of 2004, a great deal of research is ongoing to improve the quality and durability of these coatings.
Stabilized zirconia is used in oxygen sensors and fuel cell membranes because it has a unique ability to allow oxygen ions to move freely through the crystal structure at high temperatures.
Single crystals of the cubic phase of zirconia are commonly used as a substitute for diamond in jewelry. Like diamond, cubic zirconia has a cubic crystal structure and a high index of refraction. Discerning a good quality cubic zirconia gem from a diamond is difficult, and most jewelers will have a thermal conductivity tester to identify cubic zircona by its low thermal conductivity (diamond is a very good thermal conductor). This state of zirconia is commonly called "cubic zirconium" or "zircon" by jewellers, but these names are not chemically accurate.
Zirconia also occurs as a white powder and 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.
Zirconium silicate (ZrSiO4), a natural mixture of zirconia and silica, is called zircon. Its transparent form is used as a gemstone, and its opaque form as a refractory. There is a red variety called hyacinth and a yellow or brown variety from Sri Lanka called jargon.ja:ジルコニア de:Zirkoniumdioxid