![ASiAlON](https://upload.wikimedia.org/wikipedia/commons/e/e3/ASiAlON.jpg)
SiAlON
ceramic
A ceramic is any of the various hard, brittle, heat-resistant and corrosion-resistant materials made by shaping and then firing an inorganic, nonmetallic material, such as clay, at a high temperature. Common examples are earthenware, porcelain ...
s are a specialist class of high-temperature refractory materials, with high strength at ambient and high temperatures, good thermal shock resistance and exceptional resistance to wetting or corrosion by molten non-ferrous metals, compared to other refractory materials such as, for example, alumina. A typical use is with handling of molten aluminium. They also are exceptionally corrosion resistant and hence are also used in the chemical industry. SiAlONs also have high wear resistance, low thermal expansion and good oxidation resistance up to above ~1000 °C. They were first reported around 1971.
Forms
*m and n are the numbers of Al–N and Al–O bonds substituting for Si–N bonds
SiAlONs are ceramics based on the elements
silicon
Silicon is a chemical element with the symbol Si and atomic number 14. It is a hard, brittle crystalline solid with a blue-grey metallic luster, and is a tetravalent metalloid and semiconductor. It is a member of group 14 in the periodic ta ...
(Si),
aluminium
Aluminium (aluminum in American and Canadian English) is a chemical element with the symbol Al and atomic number 13. Aluminium has a density lower than those of other common metals, at approximately one third that of steel. I ...
(Al),
oxygen
Oxygen is the chemical element with the symbol O and atomic number 8. It is a member of the chalcogen group in the periodic table, a highly reactive nonmetal, and an oxidizing agent that readily forms oxides with most elements as ...
(O) and
nitrogen
Nitrogen is the chemical element with the symbol N and atomic number 7. Nitrogen is a nonmetal and the lightest member of group 15 of the periodic table, often called the pnictogens. It is a common element in the universe, estimated at se ...
(N). They are solid solutions of
silicon nitride
Silicon nitride is a chemical compound of the elements silicon and nitrogen. is the most thermodynamically stable and commercially important of the silicon nitrides, and the term "silicon nitride" commonly refers to this specific composition. It ...
(Si
3N
4), where Si–N bonds are partly replaced with Al–N and Al–O bonds. The substitution degrees can be estimated from the lattice parameters.
[ The charge discrepancy caused by the substitution can be compensated by adding metal cations such as Li+, Mg2+, Ca2+, Y3+ and Ln3+, where Ln stands for lanthanide. SiAlONs exist in three basic forms, which are iso-structural with one of the two common forms of silicon nitride, alpha and beta, and with orthorhombic ]silicon oxynitride
Silicon oxynitride is a ceramic material with the chemical formula SiOxNy. While in amorphous forms its composition can continuously vary between SiO2 (silica) and Si3N4 (silicon nitride), the only known intermediate crystalline phase is Si2N2O. It ...
; they are hence named as α, β and O'-SiAlONs.[
]
Production
SiAlONs are produced by first combining a mixture of raw materials including silicon nitride
Silicon nitride is a chemical compound of the elements silicon and nitrogen. is the most thermodynamically stable and commercially important of the silicon nitrides, and the term "silicon nitride" commonly refers to this specific composition. It ...
, alumina, aluminium nitride, silica
Silicon dioxide, also known as silica, is an oxide of silicon with the chemical formula , most commonly found in nature as quartz and in various living organisms. In many parts of the world, silica is the major constituent of sand. Silica is ...
and the oxide of a rare-earth element
The rare-earth elements (REE), also called the rare-earth metals or (in context) rare-earth oxides or sometimes the lanthanides ( yttrium and scandium are usually included as rare earths), are a set of 17 nearly-indistinguishable lustrous silv ...
such as yttrium
Yttrium is a chemical element with the symbol Y and atomic number 39. It is a silvery-metallic transition metal chemically similar to the lanthanides and has often been classified as a " rare-earth element". Yttrium is almost always found in co ...
. The powder mix is fabricated into a "green" compact by isostatic powder compaction or slipcasting
Slip casting, or slipcasting, is a ceramic forming technique for pottery and other ceramics, especially for shapes not easily made on a wheel. In this method, a liquid clay body slip (usually mixed in a blunger) is poured into plaster mo ...
, for example. Then the shaped form is densified, typically by pressureless sintering or hot isostatic pressing
Hot isostatic pressing (HIP) is a manufacturing process, used to reduce the porosity of metals and increase the density of many ceramic materials. This improves the material's mechanical properties and workability.
The process can be used to pro ...
. Abnormal grain growth
Abnormal or discontinuous grain growth, also referred to as exaggerated or secondary recrystallisation grain growth, is a grain growth phenomenon through which certain energetically favorable grains ( crystallites) grow rapidly in a matrix of fin ...
has been extensively reported for SiAlON ceramics, and results in a bimodal grain size distribution of the sintered material. The sintered part may then need to be machined by diamond grinding (abrasive cutting).[ Alternatively, they can be forged into various shapes at a temperature of ca. 1200 °C.]
Applications
SiAlON ceramics have found extensive use in non-ferrous molten metal handling, particularly aluminium and its alloys, including metal feed tubes for aluminum die casting, burner and immersion heater tubes, injector and degassing for nonferrous metals, thermocouple protection tubes, crucibles and ladles.
In metal forming, SiAlON is used as a cutting tool
In the context of machining, a cutting tool or cutter is typically a hardened metal tool that is used to cut, shape, and remove material from a workpiece by means of machining tools as well as abrasive tools by way of shear deformation. The major ...
for machining chill cast iron and as brazing and welding fixtures and pins, particularly for resistance welding
Electric resistance welding (ERW) is a welding process where metal parts in contact are permanently joined by heating them with an electric current, melting the metal at the joint. Electric resistance welding is widely used, for example, in manufa ...
.
Other applications include in the chemical and process industries and the oil and gas
A fossil fuel is a hydrocarbon-containing material formed naturally in the Earth's crust from the remains of dead plants and animals that is extracted and combustion, burned as a fuel. The main fossil fuels are coal, petroleum, oil, and natura ...
industries, due to sialons excellent chemical stability and corrosion resistance
Corrosion is a natural process that converts a refined metal into a more chemically stable oxide. It is the gradual deterioration of materials (usually a metal) by chemical or electrochemical reaction with their environment. Corrosion engin ...
and wear resistance
Wear is the damaging, gradual removal or deformation of material at solid surfaces. Causes of wear can be mechanical (e.g., erosion) or chemical (e.g., corrosion). The study of wear and related processes is referred to as tribology.
Wear in ...
properties.
Some rare-earth
The rare-earth elements (REE), also called the rare-earth metals or (in context) rare-earth oxides or sometimes the lanthanides (yttrium and scandium are usually included as rare earths), are a set of 17 nearly-indistinguishable lustrous silve ...
activated
"Activated" is a song by English singer Cher Lloyd. It was released on 22 July 2016 through Vixen Records. The song was made available to stream exclusively on ''Rolling Stone'' a day before to release (on 21 July 2016).
Background
In an inter ...
SiAlONs are photoluminescent and can serve as phosphors. Europium(II)- doped β-SiAlON absorbs in ultraviolet
Ultraviolet (UV) is a form of electromagnetic radiation with wavelength from 10 nm (with a corresponding frequency around 30 PHz) to 400 nm (750 THz), shorter than that of visible light, but longer than X-rays. UV radiation ...
and visible light
Light or visible light is electromagnetic radiation that can be perceived by the human eye. Visible light is usually defined as having wavelengths in the range of 400–700 nanometres (nm), corresponding to frequencies of 750–420 te ...
spectrum and emits intense broadband visible emission. Its luminance and color does not change significantly with temperature, due to the temperature-stable crystal structure. It has a great potential as a green down-conversion phosphor for white LEDs; a yellow variant also exists. For white LEDs, a blue LED is used with a yellow phosphor, or with a green and yellow SiAlON phosphor and a red CaAlSiN3-based (CASN) phosphor.
References
{{reflist
Ceramic materials
Nitrides
Superhard materials
Phosphors and scintillators