Zirconium dioxide

Zirconia redirects here. For the Sailor Moon character, see Dead Moon Circus.

Zirconium dioxide (ZrO₂), sometimes known as zirconia, 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, called 'cubic zirconia', is rarely, if ever, found in nature, but is synthesized in various colours for use as a gemstone. The cubic crystal structured variety cubic zirconia is the best-known diamond simulant.

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Ceramic and engineering 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 oxide (Ce₂O₃), 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. 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.

Zirconia is sometimes classified as a wide bandgap semiconductor and tends to become more conductive at higher temperatures. Zirconia starts out with a very high resistance at room temperature, greater than 1 trillion ohm-cm. As the temperature increases it has less than 20,000 ohm-cm at 500 degrees Celsius, to having less than 1,000 ohm-cm of resistance at 1,000 degrees Celsius. It loses nearly all of its resistance around 2,000 degrees Celsius, and becomes a very good conductor. The ZrO₂ bandgap is dependent on the phase (cubic, tetragonal, monoclinic, or amorphous) and preparation methods, with typical estimates from 5-7 eV.^[1]

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 dielectric material that is being investigated for potential applications as an insulator in transistors in future nanoelectronic devices.

Diamond Substitute

Single crystals of the cubic phase of zirconia are commonly used as a substitute for diamond in jewellery. 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 jewellers 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 zirconia," "CZ," or "zircon" by jewellers, but these names are not chemically accurate. Zircon is actually the mineral name for naturally occurring Zirconium silicate (ZrSiO₄). Its transparent form is also used as a gemstone, and its opaque form as a refractory.

Patents

On August 7, 2006, Apple Computer filed a patent for using zirconia as casing for mobile devices. Future iPods likely will contain radios for Bluetooth and wide area wireless. Using zirconia rather than steel or aluminum for its radio transparency characteristics enables antennas to be hidden internally in the device. Additionally, some iPods with plastic fronts have been known to scratch easily.^[2]

References