Properties of Zirconium dioxide

Zirconium dioxide (ZrO2), sometimes known as zirconia (not to be confused with zircon), is a white crystalline oxide of zirconium. Its most naturally occurring form, with a monoclinic crystalline structure, is the rare mineral baddeleyite. The high temperature cubic crystalline form is rarely found in nature as mineral tazheranite (Zr,Ti,Ca)O2 (and a doubtful mineral arkelite). This form, also called cubic zirconia, is synthesized in various colours for use as a gemstone and a diamond simulant.
Uses
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. 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). As of 2004, a great deal of research is ongoing to improve the quality and durability of these coatings.
It is used as a refractory material, in insulation, abrasives, enamels and ceramic glazes. Stabilized zirconia is used in oxygen sensors and fuel cell membranes because it has the ability to allow oxygen ions to move freely through the crystal structure at high temperatures. This high ionic conductivity (and a low electronic conductivity) makes it one of the most useful electroceramics.
This material is also used in the manufacture of subframes for the construction of dental restorations such as crowns and bridges, which are then veneered with a conventional feldspathic porcelain.
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.
Zirconia is also an important high-k dielectric material that is being investigated for potential applications as an insulator in transistors in future nanoelectronic devices and as electrolyte films in solid oxide fuel cells. For such applications Zirconia can be applied from suspension using a method of electrophoretic deposition.
Properties
Zirconium dioxide 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(III) oxide (Ce2O3), amongst others.
Zirconia is often more useful in its phase 'stabilized' state. Pure Zirconia when heated goes through disruptive phase changes. By adding small percentages of yttria, these phase changes are eliminated, and the material has better thermal, mechanical and electrical properties. 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.
The ZrO2 band gap is dependent on the phase (cubic, tetragonal, monoclinic, or amorphous) and preparation methods, with typical estimates from 5–7 eV
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