Silicon carbide

SiC

1. Silicon carbide

2. Introduction Silicon carbide also known as  carborundum. It  is a  compound  of  silicon and  carbon with chemical formula Sic. It occurs in nature as the extremely rare mineral  moissanite.  Silicon carbide powder has been mass-produced since 1893 for use as an  abrasive.  Grains of silicon carbide can be bonded together by sintering to form very hard  ceramics that are widely used in applications 

3. Discovery In 1891 Edward G Acheson produced a small amount of Silicon Carbide while conducting experiments with the aim of obtaining a hard material from the reaction of clay and carbon. He passed a strong electric current from a carbon electrode through a mixture of clay and coke contained in an iron bowl, which served as the second electrode. Acheson recognized the abrasive value of the crystals obtained, had them Analyzed , found the formula to be Sic, incorporated The Carborundum Company in September 1891, and filed application for a patent on May 10, 1892.

4. Structure Silicon carbide exists in about 250 crystalline forms .  The polymorphism of Sic is characterized by a large family of similar crystalline structures called polytypes . Alpha silicon carbide ( α-Sic) is the most commonly encountered  polymorph.  it is formed at temperatures greater than 1700 °C and has a hexagonal crystal structure.  The beta modification ( β-Sic), with a zinc blende crystal structure  (similar to diamond), is formed at temperatures below 1700 ° C.

5. Properties Pure Sic is colourless, . The crystals vary in colour from very pale green to black depending on the amount of included impurities. Black and green silicon carbide both with a shape of hex crystal. The material is extremely hard and sharp, with excellent chemical properties. The hardness is between diamond and fused alumina, but the mechanism hardness is higher than fused alumina. The micro hardness is in the range of2840-3320kg/mm ². Silicon Carbide is sharp but fragile with good heat-resistance, heat-conductibility, can be antacid and antalkali, lower dilatability and electrical conductivity.

6. Sic is quite stable chemically. It is stable to acids, not reacting with fuming nitric acid, nor with boiling sulphuric hydrochloric or hydrofluoric acid. Silicon Carbide dissociates in molten iron and the silicon reacts with the metal oxides in the melt . This reaction is of use in the metallurgy of iron and steel. Sic has: high hardness high thermal consistency very good resistance at high temperatures low thermal expansion electrical conductivity is a semiconductor non linear electrical resistance Si and C as alloying additive.

7. production Silicon carbide is made today in much the same way as it was when invented in 1891: High purity quartz is mixed with a high quality coke or anthracite in large electric resistance furnaces at temperatures of over 2 000°C according to the following reaction:   SiO 2 +3C=Sic +2CO The process is an endothermic reaction requiring between 8000 – 10 000kWh per tonne of product. The simplest manufacturing process is to combine silica sand and carbon in an Acheson graphite electric resistance furnace at a high temperature, between 1,600 °C and 2,500 °C  

8. Applications Silicon carbide forms natural crystals, which are very hard, very abrasive and dissociate or sublimate at high temperatures. It is for these reasons that silicon carbide is used in the following applications: Abrasive industry: With a good hardness, silicon carbide is the first choice raw material for manufacturing abrasive pipe , pumping chamber etc. Its abrasiveness is 5-20 times than that of cast iron and rubber.

9. Automobiles: Silicon-infiltrated carbon-carbon composite is used for high performance "ceramic" brake discs, as it is able to withstand extreme temperatures. The silicon reacts with the graphite in the carbon-carbon composite to become carbon- fiber -reinforced silicon carbide ( C/Sic). These discs are used on some road-going sports cars, supercars, as well as other performance cars including the Porsche Carrera GT ,  Bugatti Veyron,  Bentleys,  Ferraris, Lamborghinis , some specific high performance  Audis etc.  

10. Electronics: Silicon carbide is a semiconductor in research and early mass-production providing advantages for fast, high-temperature and/or high-voltage devices. First devices available were  Schottky diodes, followed by Junction-gate FETs and MOSFETs for high-power switching. Bipolar transistors and  thyristors are currently developed . It is also used in making LED’s. Structural material: Like other hard ceramics, silicon carbide is used in  composite armour and in ceramic plates in bulletproof vests . ] Silicon carbide is used as a support and shelving material in high temperature kilns such as for firing ceramics, glass fusing, or glass casting. Sic kiln shelves are considerably lighter and more durable than traditional alumina shelves .

11. Typical Silicon Carbide uses: Suction box covers Fixed and moving turbine components Seals , bearings Ball valve parts Hot gas flow liners Heat exchangers Semiconductor process equipment Jewellery

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