# Silicon carbide

Crystal structure
General
Surname Silicon carbide
other names
• carborundum
• Carborundum
• Silicon carbide
• Silicon carbide
• SILICON CARBIDE ( INCI )
Ratio formula SiC
Brief description

In its purest state, colorless, hexagonal and rhombohedral, mostly leafy crystal tablets

External identifiers / databases
 CAS number 409-21-2 EC number 206-991-8 ECHA InfoCard 100.006.357 PubChem 9863 ChemSpider 9479 Wikidata Q412356
properties
Molar mass 40.10 g mol −1
Physical state

firmly

density

3.21 g cm −3

Melting point

Decomposition> 2300 ° C

solubility

practically insoluble in water

safety instructions
GHS labeling of hazardous substances

Caution

H and P phrases H: 315-319-335
P: 261-305 + 351 + 338
MAK

Switzerland: 3 mg m −3 (measured as respirable dust )

As far as possible and customary, SI units are used. Unless otherwise noted, the data given apply to standard conditions .

Silicon carbide (common name : carborundum ; other spellings: silicon carbide and silicon carbide ) is a chemical compound of silicon and carbon belonging to the group of carbides . The chemical formula is SiC.

## properties

Silicon carbide

### Physical Properties

Highly pure silicon carbide is colorless. Technical silicon carbide is black to green (due to Al 2 O 3 contamination) and, with increasing purity, takes on color shades up to bottle green (this quality is achieved through the selection of the raw materials, sand + petroleum coke , especially for SiC green, contamination with aluminum oxide must be avoided become). Its density is 3.217 g · cm −3 . SiC green is also “softer” than dark SiC and is only produced for special applications, also because of its much higher price.

Silicon carbide is also at temperatures above 800 ° C against oxygen relatively resistant to oxidation by the formation of a passivating layer of silicon dioxide (SiO 2 , "passive oxidation"). At temperatures above approx. 1600 ° C. and a simultaneous lack of oxygen ( partial pressure below approx. 50  mbar ), it is not the glassy SiO 2 that forms, but the gaseous SiO; there is then no longer any protective effect and the SiC is quickly burned off (“active oxidation”).

It shows a high hardness of 9.6 (Mohs) and 2600 (Vickers, Knoop), good thermal conductivity (pure SiC approx. 350 W / (m · K) technical SiC approx. 100–140 W / (m · K), depending on the manufacturing process) and semiconductor properties. The band gap lies 2.39 eV ( 3C -SiC) to 3.33 eV ( 2H -SiC) between that of silicon (1.1 eV), and that of diamond (5.5 eV).

It cannot be melted in a protective gas or vacuum , but decomposes: according to older data at approx. 2700 ° C (1986) or 2830 ° C (1988), according to more recent data (1998), however, only at 3070 ° C.

### Phases

The substance is similar in structure and properties to diamond , since silicon and carbon are in the same main group and adjacent periods of the periodic table and the atomic diameter of silicon is only slightly larger. A special feature of SiC is its polytype : It exists in many different phases that differ in their atomic structure. In all previously known polytypes of SiC, each silicon atom is linked by covalent bonds with four carbon atoms and vice versa, they therefore have a tetrahedral structure.

The so-called cubic phase β-SiC ( also called 3C due to its abc layer sequence ) crystallizes in a zinc blende structure, which is related to that of diamond . Very rare, naturally occurring silicon carbide is called moissanite and is confusingly similar to diamonds in many ways. The other polytypes have a hexagonal or rhombohedral (15R-SiC, 21R-SiC etc.) structure, the hexagonal types appearing most frequently overall. The simplest hexagonal structure (also called α-SiC) is wurtzite -like and is also referred to as 2H due to the ab layer sequence . The polytypes 4H and 6H (layer sequence abcb and abcacb ), which are a mixture of the purely hexagonal 2H polytype and the purely cubic polytype 3C and are often also referred to as α-SiC, are more common and technologically most important . One ( 4H ) or two ( 6H ) cubic layers are embedded between two hexagonal layers.

## Application in technology

### mechanics

In technology, silicon carbide is used as an abrasive (carborundum, e.g. for optical mirrors and lenses ) and as a component for refractory materials due to its hardness and high melting point . Silicon carbide grains are used in cutting discs for angle grinders . Large amounts of less pure SiC are used as metallurgical SiC to alloy cast iron with silicon and carbon. It is also used as an insulator for fuel elements in high-temperature reactors . It is also used in combination with other materials as a hard concrete aggregate to make industrial floors wear-resistant and vaults resistant. Rings on high-quality fishing rods are also mostly made of SiC. Here, too, the advantage over other materials is the hardness, which prevents the fishing line from cutting a notch in the ring under high loads and ultimately tearing it through abrasion.

SiC is a frequently used engineering ceramic due to its hardness combined with low weight and high availability. In addition, thanks to its low thermal expansion , SiC is also used in space telescope mirrors . The largest single piece that has ever been assembled is the 3.5 m mirror of the Herschel space telescope , made of 12 segments soldered together . The focus was on saving weight. Compared to a weight of 1.5 tons when manufactured using standard technology, this mirror weighed only 350 kg. The largest single piece is the 1.5 m main mirror of the GREGOR telescope made of the siliconized composite Cesic from the manufacturer ECM .

Brake discs are made from carbon fiber reinforced SiC ceramic.

### Heating elements

Heating elements made of silicon carbide are better suited for higher temperatures than those made of metal and were manufactured by Siemens in Lichtenberg (later EKL ) from 1904 .

### Semiconductor material

Replica of the HJ Rounds experiment . A negatively charged needle tip on silicon carbide creates a green, glowing light-emitting diode transition

Silicon carbide is a polytype material, but some polytypes have a band gap of up to 3.33 eV ( 2H -SiC), making SiC a wide-bandgap semiconductor . Semiconductors of this type are of interest, among other things, for the production of blue light-emitting diodes (460–470 nm, corresponds to around 2.65 eV). As early as 1907, the English scientist Henry Joseph Round discovered that when a voltage was applied to a silicon carbide crystal, it was excited to glow cold - this round effect , named after him, is the basis of the light-emitting diode. In addition to this historical role, SiC is one of the most important indirect semiconductors with a wide band gap, alongside diamond , although, despite ongoing efforts to improve the properties of SiC-based LEDs, the emission efficiency of these LEDs is still around two orders of magnitude lower than that of nitride semiconductors.

Similarly, due to the large band gap, SiC is suitable for photodiodes that are sensitive to ultraviolet radiation . The maximum sensitivity is around 300 nm. In contrast, they are almost insensitive to visible light. In the case of extremely short-wave ultraviolet radiation of around 10 nm wavelength, SiC photodiodes show a further maximum in sensitivity.

In addition to its application as an LED and photodiode, SiC is used for varistors , ultra-fast Schottky diodes , insulating layer and junction field effect transistors as well as electronic circuits and sensors based on them that have to withstand high temperatures or high doses of ionizing radiation. SiC-based semiconductor circuits can be used under laboratory conditions at temperatures of up to 600 ° C. For comparison: Semiconductor electronics based on silicon have a physically determined upper operating temperature limit in the range of 150 ° C. Above this temperature there is a rapid increase in leakage currents (more charge carriers in the conduction band due to thermal excitation). This limit can be shifted to higher temperatures by using semiconductors with a wide band gap such as SiC. The Fraunhofer Institute for Solar Energy Systems (ISE) has implemented inverters based on silicon carbide JFETs ( HERIC topology ).

The production is currently still significantly more expensive than that of silicon semiconductors (small quantities, comparatively new technology, higher intrinsic defect density, harder material).

The advantage of SiC lies on the one hand in the two orders of magnitude lower power losses compared to silicon-based MOSFETs, and on the other hand in the higher switching speeds. In addition, 4H-SiC has a breakdown voltage 10 times higher for pn junctions than silicon , which enables more compact components or higher usable voltages. The main competitor in this area is currently the compound semiconductor gallium nitride , which shows similarly good or better properties.

Due to its good thermal conductivity, SiC is also used as a substrate for other semiconductor materials.

### optics

SiC is used as an abrasive in the manufacture of optical elements. The material is also used as a material for the manufacture of mirrors. One example is the Gaia astrometry space telescope , in which a further SiC-CVD layer was applied and polished to a light, stable and pre-ground body made of sintered SiC in order to obtain the desired optical quality. The mirrors were then provided with a reflective silver layer.

## Use in biotechnology

Silicon carbide crystal needles are used in the creation of transgenic plants. Compared to the biolistic transformation, the method is characterized by significantly lower costs. Compared to the transformation by agrobacteria , the lower effort is a plus point of the method. In contrast, there is a significantly lower transformation efficiency in both cases.

## Manufacturing

### Technical silicon carbide / SiC ceramics

The typical properties have different effects in the material variants. Depending on the manufacturing technique, a distinction must be made between non-species-bound and non-species-bound ceramics for silicon carbide ceramics, as well as between open-pore and dense ceramics:

• Open-pored silicon carbide ceramics
• Silicatically bound silicon carbide
• Recrystallized silicon carbide ( RSiC )
• Nitride- or oxynitride-bonded silicon carbide (NSiC)
• Dense silicon carbide ceramics
• Reaction-bonded, silicon-infiltrated silicon carbide (SiSiC)
• Sintered silicon carbide (SSiC)
• Hot (isostatically) pressed silicon carbide (HpSiC, HipSiC)
• Liquid phase sintered silicon carbide (LPSSiC)

The type and proportion of the types of bond are decisive for the respective characteristic properties of the silicon carbide ceramics.

### Acheson method

In the Acheson process (after Edward Goodrich Acheson ), long carbon moldings, embedded in powdered coke and covered with sand, are heated to 2200-2400 ° C by an electric current in large basins. In an endothermic reaction, hexagonal α-silicon carbide is created.

${\ displaystyle \ mathrm {SiO_ {2} +3 \, C + 625 {,} 1kJ \ longrightarrow \ alpha \! - \! SiC + 2 \, CO}}$

### CVD process

With the chemical vapor deposition (engl. Chemical Vapor Deposition , CVD), a coating method, can also represent SiC. Chlorine-containing carbosilanes with the basic chemical formula are used as starting materials :

${\ displaystyle \ mathrm {C_ {n} H_ {2n + 1} Si_ {n} Cl_ {2n + 1} \,}}$

used. It makes sense that these are also substances that are gaseous at room or slightly elevated temperatures, such as methyl trichlorosilane (MTS, CH 3 SiCl 3 ) with a boiling point of 70 ° C.

During the deposition at high temperatures and with hydrogen as the catalyst gas, beta-SiC and HCl are formed on the hot surfaces and must be disposed of as waste gas.

Monocrystalline SiC is produced by CVD epitaxy or by sublimation of polycrystalline SiC in a temperature gradient (PVT process, modified Lely method).

### Silicatically bonded silicon carbide

Silicatically bonded silicon carbide is made from coarse and medium-fine SiC powders and fired with approx. 5–15% aluminosilicate bonding matrix in an air atmosphere. The strength, corrosion resistance and above all the high-temperature properties are determined by the silicate binding matrix and are therefore below the non-oxidically bound SiC ceramics. At very high operating temperatures, the silicate binding matrix begins to soften and the material deforms under load at high temperatures. The advantage is its comparatively low manufacturing cost.

This material is typically used wherever quantities and low-cost production are crucial, e.g. B. as a plate capsule in a porcelain fire .

### Recrystallized silicon carbide (RSiC)

RSiC is a pure silicon carbide material with approx. 11–15% open porosity. This ceramic is fired at very high temperatures of 2300 to 2500 ° C, whereby a mixture of the finest and coarse powder is transformed into a compact SiC matrix without shrinkage. Due to its open porosity, the RSiC has lower strengths than the dense silicon carbide ceramics.

RSiC is characterized by its excellent resistance to temperature changes due to its porosity. The shrinkage-free firing technology, analogous to SiSiC, allows the production of large-format components, which are primarily used as heavy-duty kiln furniture (beams, rolls, plates, etc.) e.g. B. be used in porcelain fire. Due to its open porosity, this ceramic is not permanently resistant to oxidation and is subject to a certain degree of corrosion as a kiln furniture or as a heating element. The maximum application temperature is around 1600 ° C.

### Nitride-bonded silicon carbide (NSiC)

NSiC is a porous material with 10–15% porosity and 1–5% of it open porosity, which is produced without shrinkage by nitriding a molded body made of SiC granulate and Si metal powder in a nitrogen atmosphere at approx. 1400 ° C. The initially metallic silicon converts to silicon nitride and thus forms a bond between the SiC grains. The material is then exposed to an oxidizing atmosphere above 1200 ° C. This creates a thin anti-oxidation layer in the form of a glass layer on the surface.

The silicon nitride matrix causes workpieces NSiC by non-ferrous metal melt are poorly wettable. Because of its smaller pore size compared to RSiC, NSiC has a significantly higher flexural strength as well as better oxidation resistance and, due to its better surface resistance, is not subject to any deformation over the period of use. This material is ideal as a heavy-duty kiln furniture up to 1500 ° C.

### Reaction-bonded silicon-infiltrated silicon carbide (SiSiC)

SiSiC consists of 85–94% SiC and correspondingly 15–6% metallic silicon. SiSiC has practically no residual porosity . This is achieved by infiltrating a molded body made of silicon carbide and carbon with metallic silicon. The reaction between liquid silicon and the carbon leads to a SiC bond matrix, the remaining pore space is filled with metallic silicon. The advantage of this manufacturing technique is that, in contrast to powder sintering techniques, the components do not experience any shrinkage during the siliconization process . Therefore, extremely large components with precise dimensions can be manufactured. The application range of SiSiC is limited to approx. 1400 ° C due to the melting point of the metallic silicon. Up to this temperature range, SiSiC shows high strength and corrosion resistance, combined with good thermal shock resistance and wear resistance. SiSiC is therefore predestined as a material for highly stressed kiln furniture (beams, rollers, supports, etc.) and various burner components for direct and indirect combustion (flame tubes, recuperators and radiant tubes).

However, it is also used in mechanical engineering for highly wear-resistant and corrosion-resistant components (mechanical seals).

In basic media, however, the free silicon is chemically attacked corrosively, which leads to notches on the component surface. Because of the notch sensitivity and low fracture toughness of this ceramic, the strength of the component is weakened.

### Pressureless sintered silicon carbide (SSiC)

SSiC is made from ground SiC fine powder, which is mixed with sintering additives, processed in the usual ceramic shaping variants and sintered at 2000 to 2200 ° C under protective gas . In addition to fine-grained versions in the micrometer range, coarse-grained ones with grain sizes up to 1.5 mm are also available. SSiC is characterized by its high strength, which remains almost constant up to very high temperatures (approx. 1600 ° C).

This material has an extremely high corrosion resistance to acidic and basic media, which it can also withstand up to very high temperatures. These properties are supplemented by high thermal shock resistance , high thermal conductivity, high wear resistance and a diamond-like hardness.

The SSiC is therefore predestined for applications with extreme demands, e.g. As for sliding ring seals in chemical pumps , bearings , high temperature burner nozzles or kiln furniture for very high application temperatures. The use of SSiC with graphite inclusions increases the performance of tribological systems .

### Hot-pressed silicon carbide (HPSiC)

Hot pressed silicon carbide (HPSiC) as well as hot isostatically pressed silicon carbide (HIPSiC) have even higher mechanical parameters compared to the pressureless sintered SSiC, since the components become almost pore-free during the sintering process through the additional application of mechanical pressures of up to approx. 2000 bar. The axial (HP) or the isostatic (HIP) press technology limits the components to be manufactured to relatively simple or small geometries and means additional effort compared to pressureless sintering. HPSiC and HIPSiC are therefore only used in areas of extreme stress.

### Liquid phase sintered silicon carbide (LPSSiC)

LPSSiC is a dense material that contains SiC and an oxynitridic SiC mixed phase as well as an oxidic secondary phase. The material is made from silicon carbide powder and varying mixtures of oxide-ceramic powders, often based on aluminum oxide and yttrium oxide . The oxidic components are responsible for the slightly higher density compared to SSiC. The components are compacted in a pressure sintering process at a pressure of 5–30 MPa and a temperature of over 1950 ° C.

The material is characterized by the fine SiC crystallite size and the fact that it is practically pore-free, by very high strength and a somewhat higher fracture toughness (compared to the other silicon carbide variants) . In terms of mechanical properties, LPSiC is thus between SSiC and silicon nitride .

### SiC fibers

SiC fibers ( Nicalon ) are made from dichlorodimethylsilane . This polymerizes to polydimethylsilane , which rearranges to polycarbosilane when heated with condensation and elimination of chlorine . Fibers are drawn from this, which are later pyrolyzed into silicon oxycarbide fibers .

### Composite materials

The development of a special carbon fiber reinforced silicon carbide composite ceramic (often also referred to as ceramic matrix composites , CMC) by the German Aerospace Center (DLR) in Stuttgart has led to new types of heat protection tiles for space vehicles. The last major practical test for this material and other fiber-reinforced ceramics took place in the European project SHEFEX ( Sharp Edge Flight Experiment ) in 2005 in Norway. The same material is now also used as a brake disc material in high-priced sports cars. Silicon carbide composite ceramics reinforced with silicon carbide fibers from MT Aerospace AG have been used since 1994 as shaft protection sleeves in water-lubricated plain bearings in large pumps from various pump manufacturers. The embedding with fibers gives the material a significantly higher fracture toughness, which is in the range of metals such as gray cast iron.

The brand name Cesic denotes an isotropic SiSiC material. Short carbon fibers are pressed with a phenolic resin to form molded bodies and pyrolyzed. The green body is porous and can be machined to size. The shaped body then reacts in a vacuum above 1600 ° C. via silicon liquid phase infiltration to form SiC in an almost dimensionally stable manner. At room temperature, the coefficient of linear thermal expansion is less than 3 · 10 −6  K −1 , about a tenth of that of aluminum .

## Trivia

The original brand name Carborundum has been used for the pseudo-Latin motto Illegitimi non carborundum since the Second World War .

Commons : Silicon Carbide  - Collection of Pictures, Videos and Audio Files

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