Zirconium dioxide (ZrO 2), sometimes known as zirconia, 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 <1,170 °C, tetragonal 1,170–2,370 °C, and cubic >2,370 °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
tazheraniteO2 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, yttrium oxide, calcium oxide, and cerium(III) oxide.
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 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 makes it one
of the most useful electroceramics. Zirconium dioxide is also used as the solid electrolyte
in electrochromic devices. Zirconia is a precursor to the electroceramic
lead zirconate titanate, 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. Thermodynamically, the higher
the operation temperature of an engine, the greater the possible efficiency. 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.
Due to its infusibility and brilliant luminosity when incandescent, it was used as an ingredient
of sticks for limelight. Zirconia has been proposed to electrolyze
carbon monoxide and oxygen from the atmosphere of Mars to provide both fuel and oxidizer
that could be used as a store of chemical energy for use with surface transportation
on Mars. Carbon monoxide/oxygen engines engines have been suggested for early surface transportation
use as both carbon monoxide and oxygen can be straightforwardly produced by Zirconia
electrolysis without requiring use of any of the Martian water resources to obtain Hydrogen,
which would be needed for the production of methane or any hydrogen-based fuels.
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
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. 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.
See also Quenching
Sintering S-type star, emitting zirconium oxide spectral
lines Yttria-stabilized zirconia
References
Further reading Green, D.J.; Hannink, R.; Swain, M.V.. 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 Claussen, N., Rühle, M., Heuer, A.H., Proc.
2nd Int'l Conf. on Science and Technology of Zirconia, Adv. Ceram., Vol. 11
External links NIOSH Pocket Guide to Chemical Hazards