Zirconium dioxide (ZrO
2), sometimes known as
zirconia, is a white crystalline
oxideAn oxide is a chemical compound containing at least one oxygen atom as well as at least one other element. Most of the Earth's crust consists of oxides. Oxides result when elements are oxidized by oxygen in air. Combustion of hydrocarbons affords the two principal oxides of carbon, carbon...
of
zirconiumZirconium is a chemical element with the symbol Zr and atomic number 40. It is a lustrous, gray-white, strong transition metal that resembles titanium. Zirconium is used as an alloying agent due to its high resistance to corrosion. It is never found as a native metal; it is obtained mainly from...
. Its most naturally occurring form, with a monoclinic crystalline structure, is the rare
mineralA mineral is a naturally occurring solid formed through geological processes that has a characteristic chemical composition, a highly ordered atomic structure, and specific physical properties. A rock, by comparison, is an aggregate of minerals and/or mineraloids, and need not have a specific...
,
baddeleyiteBaddeleyite is a rare zirconium oxide mineral , occurring in a variety of monoclinic prismatic crystal forms. It is transparent to translucent, has high indices of refraction , and ranges from colorless to yellow, green, and dark brown...
. The high temperature
cubicThe cubic crystal system is a crystal system where the unit cell is in the shape of a cube. This is one of the most common and simplest shapes found in crystals and minerals....
crystalline form, called '
cubic zirconiaCubic zirconia , is the cubic crystalline form of zirconium dioxide . The synthesized material is hard, optically flawless and usually colorless, but may be made in a variety of different colors...
', is rarely found in nature as mineral tazheranite (Zr,Ti,Ca)O
2 (and a doubtful mineral arkelite), but is synthesized in various colours for use as a
gemstoneA gemstone or gem is a piece of attractive mineral, which—when cut and polished—is used to make jewelry or other adornments...
.
Zirconium dioxide (ZrO
2), sometimes known as
zirconia, is a white crystalline
oxideAn oxide is a chemical compound containing at least one oxygen atom as well as at least one other element. Most of the Earth's crust consists of oxides. Oxides result when elements are oxidized by oxygen in air. Combustion of hydrocarbons affords the two principal oxides of carbon, carbon...
of
zirconiumZirconium is a chemical element with the symbol Zr and atomic number 40. It is a lustrous, gray-white, strong transition metal that resembles titanium. Zirconium is used as an alloying agent due to its high resistance to corrosion. It is never found as a native metal; it is obtained mainly from...
. Its most naturally occurring form, with a monoclinic crystalline structure, is the rare
mineralA mineral is a naturally occurring solid formed through geological processes that has a characteristic chemical composition, a highly ordered atomic structure, and specific physical properties. A rock, by comparison, is an aggregate of minerals and/or mineraloids, and need not have a specific...
,
baddeleyiteBaddeleyite is a rare zirconium oxide mineral , occurring in a variety of monoclinic prismatic crystal forms. It is transparent to translucent, has high indices of refraction , and ranges from colorless to yellow, green, and dark brown...
. The high temperature
cubicThe cubic crystal system is a crystal system where the unit cell is in the shape of a cube. This is one of the most common and simplest shapes found in crystals and minerals....
crystalline form, called '
cubic zirconiaCubic zirconia , is the cubic crystalline form of zirconium dioxide . The synthesized material is hard, optically flawless and usually colorless, but may be made in a variety of different colors...
', is rarely found in nature as mineral tazheranite (Zr,Ti,Ca)O
2 (and a doubtful mineral arkelite), but is synthesized in various colours for use as a
gemstoneA gemstone or gem is a piece of attractive mineral, which—when cut and polished—is used to make jewelry or other adornments...
. The cubic crystal structured variety cubic zirconia is the best-known
diamond simulantThe high price of gem-grade diamonds has created a large demand for materials with similar gemological characteristics, known as diamond simulants or imitations. Simulants are distinct from synthetic diamond, which unlike simulants is actual diamond, and therefore has the same material properties...
.
Engineering properties
Zirconium dioxide is one of the most studied
ceramicA ceramic is an inorganic, non-metallic solid prepared by the action of heat and subsequent cooling. Ceramic materials may have a crystalline or partly crystalline structure, or may be amorphous...
materials. Pure ZrO
2 has a monoclinic
crystal structureIn mineralogy and crystallography, a crystal structure is a unique arrangement of atoms or molecules in a crystalline liquid or solid. A crystal structure is composed of a motif, a set of atoms arranged in a particular way, and a lattice exhibiting long-range order and symmetry...
at room temperature and transitions to tetragonal and
cubicThe cubic crystal system is a crystal system where the unit cell is in the shape of a cube. This is one of the most common and simplest shapes found in crystals and minerals....
at increasing temperatures. The volume expansion caused by the cubic to tetragonal to monoclinic transformation induces very large stresses, and will cause pure ZrO
2 to crack upon cooling from high temperatures. Several different oxides are added to zirconia to stabilize the tetragonal and/or cubic phases:
magnesium oxideMagnesium oxide, or magnesia, is a white solid mineral that occurs naturally as periclase and is a source of magnesium . It has an empirical formula of . It is formed by an ionic bond between one magnesium and one oxygen atom. Magnesium oxide is hygroscopic in nature and care must be taken to...
(MgO), yttrium oxide, (Y
2O
3),
calcium oxideCalcium oxide , commonly known as burnt lime, lime or quicklime, is a widely used chemical compound. It is a white, caustic and alkaline crystalline solid at room temperature. As a commercial product, lime often also contains magnesium oxide, silicon oxide and smaller amounts of aluminium oxide and...
(CaO), and
cerium(III) oxideCerium oxide is an oxide of the rare earth metal cerium. It has chemical formula Ce2O3, and is gold-yellow in color.-Exhaust catalysts:...
(Ce
2O
3), 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 concentrationA stress concentration is a location in an object where stress is concentrated. An object is strongest when force is evenly distributed over its area, so a reduction in area, e.g. caused by a crack, results in a localized increase in stress...
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 toughnessIn materials science, fracture toughness is a property which describes the ability of a material containing a crack to resist fracture, and is one of the most important properties of any material for virtually all design applications....
. This mechanism is known as
transformation tougheningIn the field of mechanical properties, strength and toughness are related as follows. Brittle materials may exhibit significant tensile strength by supporting a static load. Toughness, however, indicates how much energy a material can absorb before mechanical failure...
, 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 zirconiaCubic zirconia , is the cubic crystalline form of zirconium dioxide . The synthesized material is hard, optically flawless and usually colorless, but may be made in a variety of different colors...
also has a very low thermal conductivity, which has led to its use as a thermal barrier coating or TBC in
jetA jet engine is a reaction engine that discharges a fast moving jet of fluid to generate thrust in accordance with Newton's laws of motion. This broad definition of jet engines includes turbojets, turbofans, rockets, ramjets, pulse jets and pump-jets...
and
diesel engineA diesel engine is an internal combustion engine that uses the heat of compression to initiate ignition to burn the fuel, which is injected into the combustion chamber during the final stage of compression...
s to allow operation at higher temperatures.
ThermodynamicallyIn physics, thermodynamics is the study of the conversion of energy into work and heat and its relation to macroscopic variables such as temperature, volume and pressure...
the higher the operation temperature of an engine, the greater the possible efficiency (see
Carnot heat engineA Carnot heat engine is a hypothetical engine that operates on the reversible Carnot cycle. The basic model for this engine was developed by Nicolas Léonard Sadi Carnot in 1824...
). As of 2004, a great deal of research is ongoing to improve the quality and durability of these coatings.
It is used as a
refractoryA refractory material is one that retains its strength at high temperatures. ASTM C71 defines refractories as "non-metallic materials having those chemical and physical properties that made them applicable for structures, or as components of systems, that are exposed to environments above...
material, in
insulationThe term thermal insulation can refer to materials used to reduce the rate of heat transfer, or the methods and processes used to reduce heat transfer....
,
abrasiveAn abrasive is a material, often a mineral, that is used to shape or finish a workpiece through rubbing which leads to part of the workpiece being worn away...
s,
enamelsIn a discussion of material science, enamel is the colorful result of fusing powdered glass to a substrate by firing, usually between 750 and 850 degrees Celsius. The powder melts and flows and hardens to a smooth, durable vitreous coating on metal, glass or ceramic...
and
ceramic glazeGlaze is a layer or coating of a vitreous substance which has been fired to fuse to a ceramic object to color, decorate, strengthen or waterproof it.-Use:...
s. Stabilized zirconia is used in
oxygen sensorAn oxygen sensor, or lambda sensor, is an electronic device that measures the proportion of oxygen in the gas or liquid being analyzed. It was developed by Robert Bosch GmbH during the late 1960s under supervision by Dr. Günter Bauman...
s and
fuel cellA fuel cell is an electrochemical cell that produces electricity from a replenishable fuel tank. The electricity is generated through the reaction, triggered in the presence of an electrolyte, between the fuel and an oxidant . The reactants flow into the cell, and the reaction products flow out of...
membranes because it has the ability to allow
oxygenOxygen Oxygen Oxygen (acid, literally "sharp", from the taste of acids) and -γενής (-genēs) (producer, literally begetter) is the element with atomic number 8 and represented by the symbol O...
ionAn ion is an atom or molecule where the total number of electrons is not equal to the total number of protons, giving it a net positive or negative electrical charge...
s to move freely through the crystal structure at high temperatures. This high
ionic conductivityIonic conduction in solids has been a subject of interest as early as the beginning of the 19th century. It was established by Michael Faraday that the laws of electrolysis are also obeyed in ionic solids like PbF2 and Ag2S. There were, however, several important discoveries...
(and a low electronic conductivity) makes it one of the most useful
electroceramicsElectroceramics is a class of ceramic materials used primarily for their electrical properties.While ceramics have traditionally been admired and used for their mechanical, thermal and chemical stability, their unique electrical, optical and magnetic properties have become of increasing importance...
.
The ZrO
2 band gapIn solid state physics, a band gap, also called an energy gap or bandgap, is an energy range in a solid where no electron states exist...
is dependent on the phase (cubic, tetragonal, monoclinic, or amorphous) and preparation methods, with typical estimates from 5-7 eV.
This material is also used in the manufacture of subframes for the construction of
dental restorationA dental restoration or dental filling is a dental restorative material used to restore the function, integrity and morphology of missing tooth structure. The structural loss typically results from caries or external trauma. It is also lost intentionally during tooth preparation to improve the...
s such as
crownsA Crown is a type of dental restoration which completely caps or encircles a tooth or dental implant and is typically bonded to the tooth using a dental cement. Crowns can be made from many materials, which are usually fabricated using indirect methods...
and
bridgesA bridge, also known as a fixed partial denture, is a dental restoration used to replace a missing tooth by joining permanently to adjacent teeth or dental implants....
, which are then veneered with a conventional
feldspathicFeldspars are a group of rock-forming tectosilicate minerals which make up as much as 60% of the Earth's crust....
porcelainPorcelain is a ceramic material made by heating raw materials, generally including clay in the form of kaolin, in a kiln to temperatures between and...
.
Zirconium dioxide can occur as a white powder which possesses both
acidAn acid is traditionally considered any chemical compound that, when dissolved in water, gives a solution with a hydrogen ion activity greater than in pure water, i.e. a pH less than 7.0...
ic and
basicIn chemistry, a base is most commonly thought of as an aqueous substance that can accept hydrogen ions. Bases are also the oxides or hydroxides of metals. A soluble base is also often referred to as an alkali if hydroxide ions are involved. This refers to the Brønsted-Lowry theory of acids and bases...
properties. On account of its infusibility and brilliant luminosity when
incandescentIncandescence is the emission of light from a hot body due to its temperature. The term derives from the verb incandesce, to glow white....
, it was used as an ingredient of sticks for
limelightLimelight is a type of stage lighting once used in theatres and music halls. An intense illumination is created when an oxyhydrogen flame is directed at a cylinder of lime , which can be raised to 2572°C before melting. The light is produced by a combination of incandescence and candoluminescence...
.
Zirconia is also an important
high-k dielectricThe term high-κ dielectric refers to a material with a high dielectric constant used in semiconductor manufacturing processes which replaces the silicon dioxide gate dielectric...
material that is being investigated for potential applications as an insulator in
transistorA transistor is a semiconductor device commonly used to amplify or switch electronic signals. A transistor is made of a solid piece of a semiconductor material, with at least three terminals for connection to an external circuit. A voltage or current applied to one pair of the transistor's...
s in future
nanoelectronicNanoelectronics refer to the use of nanotechnology on electronic components, especially transistors. Although the term nanotechnology is generally defined as utilizing technology less than 100 nm in size, nanoelectronics often refer to transistor devices that are so small that inter-atomic...
devices.
Diamond simulant
Single crystals of the cubic phase of zirconia are commonly used as
diamond simulantThe high price of gem-grade diamonds has created a large demand for materials with similar gemological characteristics, known as diamond simulants or imitations. Simulants are distinct from synthetic diamond, which unlike simulants is actual diamond, and therefore has the same material properties...
in jewelery. 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 conductivityIn physics, thermal conductivity, , is the property of a material that indicates its ability to conduct heat. It appears primarily in Fourier's Law for heat conduction.-Measurement:...
(diamond is a very good thermal conductor). This state of zirconia is commonly called "cubic zirconia," "CZ," or "zircon" by
jewellersJewellery or jewelry is an item of personal adornment, such as a necklace, ring, brooch or bracelet, that is worn by a person. It may be made from gemstones or precious metals, but may be from any other material, and may be appreciated because of geometric or other patterns, or meaningful symbols...
, but the last name is not chemically accurate.
ZirconZircon is a mineral belonging to the group of nesosilicates. Its chemical name is zirconium silicate and its corresponding chemical formula is ZrSiO4. A common empirical formula showing some of the range of substitution in zircon is 1-x4x-y...
is actually the mineral name for naturally occurring zirconium silicate (ZrSiO
4). Its transparent form is also used as a
gemstoneA gemstone or gem is a piece of attractive mineral, which—when cut and polished—is used to make jewelry or other adornments...
, and its
opaqueOpacity is the measure of impenetrability to electromagnetic or other kinds of radiation, especially visible light. In radiative transfer, it describes the absorption and scattering of radiation in a medium, such as a plasma, dielectric, shielding material, glass, etc...
form as a
refractoryA refractory material is one that retains its strength at high temperatures. ASTM C71 defines refractories as "non-metallic materials having those chemical and physical properties that made them applicable for structures, or as components of systems, that are exposed to environments above...
.
Patents
On August 7, 2006, Apple Inc. filed a
patentA patent is a set of exclusive rights granted by a state to an inventor or their assignee for a limited period of time in exchange for a public disclosure of an invention....
for using zirconia oxide ceramics as casing for mobile devices. Some of the current
iPodiPod is a brand of portable media players designed and marketed by Apple and launched on . The product line-up includes the hard drive-based iPod Classic, the touchscreen iPod Touch, the video-capable iPod Nano, and the compact iPod Shuffle. The iPhone can function as an iPod but is generally...
s contain radios for built-in
Nike+The Nike+iPod Sports Kit is a device which measures and records the distance and pace of a walk or run. The Nike+iPod consists of a small accelerometer attached to or embedded in a shoe, which communicates with either the Nike+ Sportband, a receiver plugged into an iPod Nano, or directly with an...
,
BluetoothBluetooth is an open wireless protocol for exchanging data over short distances from fixed and mobile devices, creating personal area networks . It was originally conceived as a wireless alternative to RS232 data cables...
, or
wireless networkingWi-Fi is a trademark of the Wi-Fi Alliance for certified products based on the IEEE 802.11 standards. This certification warrants interoperability between different wireless devices....
. Using zirconia-based ceramics rather than
steelSteel is an alloy consisting mostly of iron, with a carbon content between 0.2% and 2.1% by weight, depending on the grade. Carbon is the most cost-effective alloying material for iron, but various other alloying elements are used such as manganese, chromium, vanadium, and tungsten...
or
aluminiumAluminium or aluminum is a silvery white and ductile member of the boron group of chemical elements. It has the symbol Al; its atomic number is 13. It is not soluble in water under normal circumstances....
for its radio transparency characteristics would enable such internal
antennaeAn antenna is a transducer designed to transmit or receive electromagnetic waves. In other words, antennas convert electromagnetic waves into electrical currents and vice versa. Antennas are used in systems such as radio and television broadcasting, point-to-point radio communication, wireless...
to be within the device, without the need for changes in thickness or a
windowA Faraday cage or Faraday shield is an enclosure formed by conducting material, or by a mesh of such material. Such an enclosure blocks out external static electrical fields....
, like the
iPod touchThe iPod Touch is a portable media player, personal digital assistant, and Wi-Fi mobile platform designed and marketed by Apple Inc. The product was launched on September 5, 2007, at an event called The Beat Goes On. The iPod Touch adds the multi-touch graphical user interface to the iPod line...
.
See also
- Quenching
- Sintering
Sintering is a method for making objects from powder, by heating the material in a sintering furnace below its melting point until its particles adhere to each other...
- Yttria-stabilized zirconia
Yttria-stabilized zirconia is a zirconium-oxide based ceramic, in which the particular crystal structure of zirconium oxide is made stable at room temperature by an addition of yttrium oxide...
- Zirconia fiber insulation
- Transformation toughening
In the field of mechanical properties, strength and toughness are related as follows. Brittle materials may exhibit significant tensile strength by supporting a static load. Toughness, however, indicates how much energy a material can absorb before mechanical failure...
Further reading
- 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