Sialon
Encyclopedia
Sialon ceramic
s are a specialist class of high temperature refractory materials, with high strength (including at high temperature), 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.
s based on the elements silicon
(Si), aluminium
(Al), oxygen
(O) and nitrogen
(N). They are alloys of silicon nitride
(Si3N4) and exist in three basic forms. Each form is iso-structural with one of the two common forms of silicon nitride, beta and alpha and with silicon oxynitride. The relationship between that of sialon and silicon nitride is similar to that between brass and pure copper. The later case, copper atoms are replaced by zinc to give a better stronger alloy than the mother metal. In the case of silaon, there is substitution of silicon by aluminium with corresponding atomic replacement of nitrogen by oxygen, to satisfy valency requirements. The resulting 'solution' (sialon) has superior properties to the original pure solvent (silicon nitride).
, silica and the oxide
of a rare earth element
such as yttrium
. The powder mix is fabricated in to a 'green' compact by isostatic powder compaction or slipcasting
, for example. Then the shaped form is then densified, typically by pressureless sintering or hot isostatic pressing
. The sintered part may then need to by machined by diamond grinding (abrasive cutting)
.
In metal forming, sialon is used as a cutting tool
for machining chill cast iron and as brazing and welding fixtures and pins, particularly for resistance welding
.
Other applications include in the chemical and process industries and the oil and gas
industries, due to sialons excellent chemical stability and corrosion resistance and wear resistance properties.
Some rare-earth activated
Sialons are photoluminescent and can serve as phosphor
s. Europium
(II)-doped β-SiAlON absorbs in ultraviolet
and visible light 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 LED
s; 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. http://www.science24.com/paper/15977http://techon.nikkeibp.co.jp/english/NEWS_EN/20090915/175305/http://sciencelinks.jp/j-east/article/200602/000020060205A1031052.php
Ceramic
A ceramic is an inorganic, nonmetallic solid prepared by the action of heat and subsequent cooling. Ceramic materials may have a crystalline or partly crystalline structure, or may be amorphous...
s are a specialist class of high temperature refractory materials, with high strength (including at high temperature), 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.
Forms
Sialons are ceramic alloyAlloy
An alloy is a mixture or metallic solid solution composed of two or more elements. Complete solid solution alloys give single solid phase microstructure, while partial solutions give two or more phases that may or may not be homogeneous in distribution, depending on thermal history...
s based on the elements silicon
Silicon
Silicon is a chemical element with the symbol Si and atomic number 14. A tetravalent metalloid, it is less reactive than its chemical analog carbon, the nonmetal directly above it in the periodic table, but more reactive than germanium, the metalloid directly below it in the table...
(Si), aluminium
Aluminium
Aluminium or aluminum is a silvery white member of the boron group of chemical elements. It has the symbol Al, and its atomic number is 13. It is not soluble in water under normal circumstances....
(Al), oxygen
Oxygen
Oxygen is the element with atomic number 8 and represented by the symbol O. Its name derives from the Greek roots ὀξύς and -γενής , because at the time of naming, it was mistakenly thought that all acids required oxygen in their composition...
(O) and nitrogen
Nitrogen
Nitrogen is a chemical element that has the symbol N, atomic number of 7 and atomic mass 14.00674 u. Elemental nitrogen is a colorless, odorless, tasteless, and mostly inert diatomic gas at standard conditions, constituting 78.08% by volume of Earth's atmosphere...
(N). They are alloys of silicon nitride
Silicon nitride
Silicon nitride is a chemical compound of silicon and nitrogen. If powdered silicon is heated between 1300° and 1400°C in an atmosphere of nitrogen, trisilicon tetranitride, Si3N4, is formed. The silicon sample weight increases progressively due to the chemical combination of silicon and nitrogen...
(Si3N4) and exist in three basic forms. Each form is iso-structural with one of the two common forms of silicon nitride, beta and alpha and with silicon oxynitride. The relationship between that of sialon and silicon nitride is similar to that between brass and pure copper. The later case, copper atoms are replaced by zinc to give a better stronger alloy than the mother metal. In the case of silaon, there is substitution of silicon by aluminium with corresponding atomic replacement of nitrogen by oxygen, to satisfy valency requirements. The resulting 'solution' (sialon) has superior properties to the original pure solvent (silicon nitride).
Production
Sialons are produced by first combining a mixture of raw materials including silicon nitride, alumina, aluminium nitrideAluminium nitride
Aluminium nitride is a nitride of aluminium. Its wurtzite phase is a wide band gap semiconductor material, giving it potential application for deep ultraviolet optoelectronics.-History:...
, silica and the oxide
Oxide
An oxide is a chemical compound that contains at least one oxygen atom in its chemical formula. Metal oxides typically contain an anion of oxygen in the oxidation state of −2....
of a rare earth element
Rare earth element
As defined by IUPAC, rare earth elements or rare earth metals are a set of seventeen chemical elements in the periodic table, specifically the fifteen lanthanides plus scandium and yttrium...
such as yttrium
Yttrium
Yttrium is a chemical element with symbol Y and atomic number 39. It is a silvery-metallic transition metal chemically similar to the lanthanides and it has often been classified as a "rare earth element". Yttrium is almost always found combined with the lanthanides in rare earth minerals and is...
. The powder mix is fabricated in to a 'green' compact by isostatic powder compaction or slipcasting
Slipcasting
Slipcasting is a technique for the mass-production of pottery, especially for shapes not easily made on a wheel. A liquid clay body slip is poured into plaster moulds and allowed to form a layer, the cast, on the inside cavity of the mould...
, for example. Then the shaped form is then densified, typically by pressureless sintering or hot isostatic pressing
Hot isostatic pressing
Hot isostatic pressing is a manufacturing process used to reduce the porosity of metals and influence the density of many ceramic materials. This improves the material's mechanical properties and workability....
. The sintered part may then need to by machined by diamond grinding (abrasive cutting)
Grinding (abrasive cutting)
Grinding is an abrasive machining process that uses a grinding wheel as the cutting tool.A wide variety of machines are used for grinding:* Hand-cranked knife-sharpening stones * Handheld power tools such as angle grinders and die grinders...
.
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
Cutting tool
In the context of machining, a cutting tool is any tool that is used to remove material from the workpiece by means of shear deformation. Cutting may be accomplished by single-point or multipoint tools. Single-point tools are used in turning, shaping, plaining and similar operations, and remove...
for machining chill cast iron and as brazing and welding fixtures and pins, particularly for resistance welding
Resistance welding
Electric resistance welding refers to a group of welding processes such as spot and seam welding that produce coalescence of faying surfaces where heat to form the weld is generated by the electical reistance of material vs the time and the force used to hold the materials together during welding...
.
Other applications include in the chemical and process industries and the oil and gas
Petroleum industry
The petroleum industry includes the global processes of exploration, extraction, refining, transporting , and marketing petroleum products. The largest volume products of the industry are fuel oil and gasoline...
industries, due to sialons excellent chemical stability and corrosion resistance and wear resistance properties.
Some rare-earth activated
Activator (phosphor)
In phosphors and scintillators, the activator is the element added as dopant to the crystal of the material to create desired type of nonhomogeneities....
Sialons are photoluminescent and can serve as phosphor
Phosphor
A phosphor, most generally, is a substance that exhibits the phenomenon of luminescence. Somewhat confusingly, this includes both phosphorescent materials, which show a slow decay in brightness , and fluorescent materials, where the emission decay takes place over tens of nanoseconds...
s. Europium
Europium
Europium is a chemical element with the symbol Eu and atomic number 63. It is named after the continent of Europe. It is a moderately hard silvery metal which readily oxidizes in air and water...
(II)-doped β-SiAlON absorbs in ultraviolet
Ultraviolet
Ultraviolet light is electromagnetic radiation with a wavelength shorter than that of visible light, but longer than X-rays, in the range 10 nm to 400 nm, and energies from 3 eV to 124 eV...
and visible light 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 LED
LEd
LEd is a TeX/LaTeX editing software working under Microsoft Windows. It is a freeware product....
s; 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. http://www.science24.com/paper/15977http://techon.nikkeibp.co.jp/english/NEWS_EN/20090915/175305/http://sciencelinks.jp/j-east/article/200602/000020060205A1031052.php