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, porcela ...

s are a specialist class of high-temperature

refractory In materials science, a refractory (or refractory material) is a material that is resistant to decomposition by heat or chemical attack and that retains its strength and rigidity at high temperatures. They are inorganic, non-metallic compound ...

materials, with high strength at ambient and high temperatures, good

thermal shock Thermal shock is a phenomenon characterized by a rapid change in temperature that results in a transient mechanical load on an object. The load is caused by the differential expansion of different parts of the object due to the temperature chang ...

resistance and exceptional resistance to wetting or corrosion by molten

non-ferrous metal In metallurgy, non-ferrous metals are metals or alloys that do not contain iron ( allotropes of iron, ferrite, and so on) in appreciable amounts. Generally more costly than ferrous metals, non-ferrous metals are used because of desirable pro ...

s, compared to other refractory materials such as, for example,

alumina Aluminium oxide (or aluminium(III) oxide) is a chemical compound of aluminium and oxygen with the chemical formula . It is the most commonly occurring of several aluminium oxides, and specifically identified as aluminium oxide. It is commonly ...

. 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; it has symbol Si and atomic number 14. It is a hard, brittle crystalline solid with a blue-grey metallic lustre, and is a tetravalent metalloid (sometimes considered a non-metal) and semiconductor. It is a membe ...

(Si),

aluminium Aluminium (or aluminum in North American English) is a chemical element; it has chemical symbol, symbol Al and atomic number 13. It has a density lower than that of other common metals, about one-third that of steel. Aluminium has ...

(Al),

oxygen Oxygen is a chemical element; it has chemical symbol, symbol O and atomic number 8. It is a member of the chalcogen group (periodic table), group in the periodic table, a highly reactivity (chemistry), reactive nonmetal (chemistry), non ...

(O) and

nitrogen Nitrogen is a chemical element; it has Symbol (chemistry), symbol N and atomic number 7. Nitrogen is a Nonmetal (chemistry), nonmetal and the lightest member of pnictogen, group 15 of the periodic table, often called the Pnictogen, pnictogens. ...

(N). They are solid solutions of silicon nitride (Si₃N₄), 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

cation An ion () is an atom or molecule with a net electrical charge. The charge of an electron is considered to be negative by convention and this charge is equal and opposite to the charge of a proton, which is considered to be positive by convent ...

s such as Li⁺, Mg²⁺, Ca²⁺, Y³⁺ and Ln³⁺, where Ln stands for

lanthanide The lanthanide () or lanthanoid () series of chemical elements comprises at least the 14 metallic chemical elements with atomic numbers 57–70, from lanthanum through ytterbium. In the periodic table, they fill the 4f orbitals. Lutetium (el ...

. 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; 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, alumina,

aluminium nitride Aluminium nitride ( Al N) is a solid nitride of aluminium. It has a high thermal conductivity of up to 321 W/(m·K) and is an electrical insulator. Its wurtzite phase (w-AlN) has a band gap of ~6 eV at room temperature and has a potent ...

silica Silicon dioxide, also known as silica, is an oxide of silicon with the chemical formula , commonly found in nature as quartz. In many parts of the world, silica is the major constituent of sand. Silica is one of the most complex and abundant f ...

and the

oxide An oxide () is a chemical compound containing at least one oxygen atom and one other element in its chemical formula. "Oxide" itself is the dianion (anion bearing a net charge of −2) of oxygen, an O2− ion with oxygen in the oxidation st ...

of a

rare-earth element The rare-earth elements (REE), also called the rare-earth metals or rare earths, and sometimes the lanthanides or lanthanoids (although scandium and yttrium, which do not belong to this series, are usually included as rare earths), are a set o ...

such as

yttrium Yttrium is a chemical element; it has Symbol (chemistry), 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 a ...

. The powder mix is fabricated into a "green" compact by isostatic powder compaction or slipcasting, for example. Then the shaped form is densified, typically by pressureless sintering or hot isostatic pressing. Abnormal grain growth 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) Grinding is a type of abrasive machining process which uses a grinding wheel as cutting tool. A wide variety of machines are used for grinding, best classified as portable or stationary: * Portable power tools such as angle grinders, die gri ...

. 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 Cutting is the separation or opening of a physical object, into two or more portions, through the application of an acutely directed force. Implements commonly used for cutting are the knife and saw, or in medicine and science the scalpel an ...

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 A fossil fuel is a flammable carbon compound- or hydrocarbon-containing material formed naturally in the Earth's crust from the buried remains of prehistoric organisms (animals, plants or microplanktons), a process that occurs within geologi ...

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 A phosphor is a substance that exhibits the phenomenon of luminescence; it emits light when exposed to some type of radiant energy. The term is used both for fluorescent or phosphorescent substances which glow on exposure to ultraviolet or ...

Europium Europium is a chemical element; it has symbol Eu and atomic number 63. It is a silvery-white metal of the lanthanide series that reacts readily with air to form a dark oxide coating. Europium is the most chemically reactive, least dense, and soft ...

(II)- doped β-SiAlON absorbs in

ultraviolet Ultraviolet radiation, also known as simply UV, is electromagnetic radiation of wavelengths of 10–400 nanometers, shorter than that of visible light, but longer than X-rays. UV radiation is present in sunlight and constitutes about 10% of ...

and

visible light Light, visible light, or visible radiation is electromagnetic radiation that can be perceived by the human eye. Visible light spans the visible spectrum and is usually defined as having wavelengths in the range of 400–700 nanometres (nm ...

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 A light-emitting diode (LED) is a semiconductor device that emits light when current flows through it. Electrons in the semiconductor recombine with electron holes, releasing energy in the form of photons. The color of the light (corresp ...

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.

References

{{reflist Ceramic materials Nitrides Superhard materials Phosphors and scintillators