·
Linear Formula ZrO2
·
Molecular Weight 123.22
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.
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.
Detailed
description
Production, chemical properties, occurrence
Structure
Three phases are known: monoclinic
<1,170 °C, tetragonal 1,170–2,370 °C, and cubic >2,370 °C.[2] 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.[1] 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 change of volume caused by the structure transitions from to
tetragonal to monoclinic to cubic induces large stresses, causing it to crack
upon cooling from high temperatures.[4] 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).[5]
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 (0.80–1.12 aJ).[7]
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,[1] 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.[1] Zirconium dioxide is
also used as the solid electrolyte in electrochromic
devices.
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. 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.
Transformation toughened zirconia is used to make
ceramic knives. Because of the hardness, ceramic-edged cutlery stays sharp
longer than steel edged products.
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 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.[12]
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.[13]
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 (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).
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