Zirconium dioxide

Zirconium dioxide
IUPAC name Zirconium dioxide Zirconium(IV) oxide
Other names Zirconia Baddeleyite
Identifiers
CAS number	1314-23-4 Y
Properties
Molecular formula	ZrO 2
Molar mass	123.218 g/mol
Appearance	white powder
Density	5.68 g/cm3
Melting point	2715 °C
Boiling point	4300 °C
Solubility in water	negligible
Solubility	soluble in HF, and hot H2SO4
Refractive index (nD)	2.13
Thermochemistry
Std enthalpy of formation ΔfHo298	–1080 kJ/mol
Standard molar entropy So298	50.3 J K−1 mol−1
Hazards
MSDS	MSDS
EU Index	Not listed
Flash point	Non-flammable
LD50	> 8.8 g/kg (oral, rat)
Related compounds
Other anions	Zirconium disulfide
Other cations	Titanium dioxide Hafnium dioxide
Y (verify) (what is: Y/N?) Except where noted otherwise, data are given for materials in their standard state (at 25 °C, 100 kPa)
Infobox references

Bearing ball

Zirconium dioxide (ZrO
2), 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 mineral baddeleyite. A dopant stabilized cubic structured zirconia, cubic zirconia, is synthesized in various colours for use as a gemstone and a diamond simulant.

Production, chemical properties, occurrence

Zirconia is produced by calcining zirconium compounds, exploiting its high thermal stability.

Structure

Three phases are known: monoclinic < 1170 °C, tetragonal 1170°C-2370 °C, and cubic >2370 °C. The trend is for higher symmetry at higher temperatures, as is usually the case. A few percentage of the oxides of calcium or yttrium stabilize the cubic phase. The very rare mineral tazheranite (Zr,Ti,Ca)O₂ is cubic. Unlike TiO₂, which features six-coordinate Ti in all phases, monoclinic zirconia consists of seven-coordinate zirconium centres. This difference is attributed to the larger size of Zr atom relative to the Ti atom.

Chemical reactions

Zirconia is chemically unreactive. It is slowly attacked by concentrated hydrofluoric acid and sulfuric acid. When heated with carbon, it converts to zirconium carbide. When heated with carbon in the presence of chlorine, it converts to zirconium tetrachloride. This conversion is the basis for the purification of zirconium metal and is analogous to the Kroll process.

Engineering properties

Zirconium dioxide is one of the most studied ceramic materials. ZrO₂ adopts a monoclinic crystal structure at room temperature and transitions to tetragonal and cubic at higher temperatures. The volume expansion caused by the cubic to tetragonal to monoclinic transformation induces large stresses, and these stresses cause ZrO₂ to crack upon cooling from high temperatures. When the zirconia is blended with some other oxides, the tetragonal and/or cubic phases are stabilized. Effective dopants include magnesium oxide (MgO), yttrium oxide (Y₂O₃, yttria), calcium oxide (CaO), and cerium(III) oxide (Ce₂O₃).

Zirconia is often more useful in its phase 'stabilized' state. Upon heating, zirconia undergoes disruptive phase changes. By adding small percentages of yttria, these phase changes are eliminated, and the resulting material has superior 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.

A special case of zirconia is that of tetragonal zirconia polycrystal, or TZP, which is indicative of polycrystalline zirconia composed of only the metastable tetragonal phase.

Uses

The main use of zirconia is in the production of ceramics, with other uses including as a protective coating on particles of titanium dioxide pigments, as a refractory material, in insulation, abrasives and enamels. 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 a precursor to the electroceramic lead zirconate titanate ("PZT"), which is a high-K dielectric, which is found in myriad components.

Niche uses

The very low thermal conductivity of cubic phase of zirconia also has led to its use as a thermal barrier coating, or TBC, in jet and diesel engines to allow operation at higher temperatures.^{[citation needed]} Thermodynamically the higher the operation temperature of an engine, the greater the possible efficiency (see Carnot heat engine). Another low thermal conductivity use is a ceramic fiber insulation for crystal growth furnaces, fuel cell stack insulation and infrared heating systems.

This material is also used in dentistry in the manufacture of 1)subframes for the construction of dental restorations such as crowns and bridges, which are then veneered with a conventional feldspathic porcelain for aesthetic reasons, or of 2)strong, extremely durable dental prostheses constructed entirely from monolithic zirconia, with limited but constantly improving aesthetics.

Zirconia is used to make ceramic knives. Because of its hardness, Zirconia based cutlery stays sharp longer than a stainless steel equivalent.

On account of its infusibility and brilliant luminosity when incandescent, it was used as an ingredient of sticks for limelight.^{[citation needed]}

Zirconia is also a potential high-k dielectric material with potential applications as an insulator in transistors.

Zirconia is also employed in the deposition of optical coatings; it is a high-index material usable from the near-UV to the mid-IR, due to its low absorption in this spectral region. In such applications, it is typically deposited by PVD.

Diamond simulant

Main article: Cubic zirconia

Brilliant-cut cubic zirconia

Single crystals of the cubic phase of zirconia are commonly used as diamond simulant in jewellery. Like diamond, cubic zirconia has a cubic crystal structure and a high index of refraction. Visually discerning a good quality cubic zirconia gem from a diamond is difficult, and most jewellers will have a thermal conductivity tester to identify cubic zirconia 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 the last name is not chemically accurate. Zircon is actually the mineral name for naturally occurring zirconium silicate (ZrSiO₄).

See also

• Quenching
• Sintering
• Yttria-stabilized zirconia

Further reading

• Green, D.J.; Hannink, R.; Swain, M.V. (1989). Transformation Toughening of Ceramics. Boca Raton: CRC Press. ISBN 0-8493-6594-5. 
• Heuer, A.H., Hobbs, L.W., Eds., Science and Technology of Zirconia, Adv. Ceram., Vol. 3, p. 475 (ACerS, Columbus, OH 1981)
• Claussen, N., Rühle, M., Heuer, A.H., Proc. 2nd Int'l Conf. on Science and Technology of Zirconia, Adv. Ceram., Vol. 11 (ACerS, Columbus, OH 1984)

External links

• NIOSH Pocket Guide to Chemical Hazards