Chemical Properties
light grey powder
General Description
Yellow to green to bluish-black, iridescent crystals. Sublimes with decomposition at 2700°C. Density 3.21 g cm-3. Insoluble in water. Soluble in molten alkalis (NaOH, KOH) and molten iron.
Reactivity Profile
Silicon carbide is non-combustible. Generally unreactive. Soluble in molten alkalis (NaOH, KOH) and in molten iron.
Hazard
Upper respiratory tract irritant. Probable
carcinogen.
Potential Exposure
A potential danger to those involved in the manufacture of silicon carbide abrasives, refractories, and semiconductors. Silicon carbide fibers are also produced in fibrous form as reinforcing fibers for composite materials.
First aid
If this chemical gets into the eyes, remove any contact lenses at once and irrigate immediately for at least 15 minutes, occasionally lifting upper and lower lids. Seek medical attention immediately. If this chemical contacts the skin, remove contaminated clothing and wash immediately with soap and water. Seek medical attention immediately. If this chemical has been inhaled, remove from exposure. Transfer promptly to a medical facility.
Incompatibilities
Dust may form explosive mixture with air. Sublimes with decomposition @ 2700C.
Physical properties
the properties of silicon carbide are that it is a refractory material (high melting point), it has excellent thermal conductivity and low thermal expansion, consequently it displays good thermal shock resistance. In addition, the high hardness, corrosion resistance and stiffness lead to a wide range of applications where wear and corrosion resistance are primary performance requirements. Silicon carbide possesses interesting electrical properties due to its semiconductor characteristics, the resistance of different compositions varying by as much as seven orders of magnitude.
Characteristics
Silicon carbide is a premium-priced unit which is employed in lining work for its uniformity, abrasion resistance and dimensional stability. It is resistant to most organics, inorganic acids, alkalis and salts in a variety of concentrations except to hydrofluoric acid and acid fluorides. The permeable units have the lowest resistance.
Preparation
Silicon carbide is prepared by fusing a mixture of silica (sand) and carbon (coke) with some salt and saw dust in an electric arc furnace at 3000°C.
SiO2(Sand)+3C(Coke)--(3000℃)--Sic+2CO
Salt and saw dust is added to infuse air into the product so that it can be broken into pieces easily. The product obtained is first washed with strong acid followed by strong base to remove basic and acidic impurities respectively. Finally, it is washed with water.
Production Methods
Silicon carbide, also known by the trade name Carborundum,
has been manufactured and used as an abrasive material for
more than a century. It combines desirable properties of
hardness and thermal resistance. It is produced by heating
high-grade silica sand with finely ground carbon at 2400°C in
an electric furnace. In its powdered or granular form, it
has been used as the abrasive material in “paper and wheels.”
It is used as an abrasive in sandblasting and engraving. It has
been incorporated into ceramics and glass and especially into
refractory ceramic materials.
Reactions
SiC is formed by the reaction with C at high temperature. It is not etched by acid other than the mixture of hydrofluoric acid and nitric acid. It reacts with caustic solution generating H2 to produce alkali silicate. Good alloys can be formed at the composition ratio of 0%–100% with Ge. (Refer to Ge.)
Health Hazard
Silicon carbide, in certain forms,
may be a cause of pneumoconiosis in exposed
workers.
Silicon carbide has generally been considered
to be an inert dust with little adverse effect
on the lungs.
Flammability and Explosibility
Nonflammable
Industrial uses
Silicon carbide is one of the very few totally man-made minerals used in refractory work. These are:
Oxide-bonded-(S102, A1201, Si02 or silicate glass), silicon oxynitride (Si2 ON2), silicon nitride (S13N4)
The first three of these four bonding systems result in a permeable product, and when failure occurs in such masonry systems due to chemical degradation, it is usually due to attack on the bond. Thus, permeable units (where the corrodent penetrates the mass) are far more rapidly damaged.
Self-bonded”—(silicon carbide to silicon carbide) impermeable ones, where the attack is limited to the surface.
The self-bonded product can be manufactured by either of two methods: reaction bonded or sintered. Both will produce an impermeable unit, and they have roughly comparable chemical resistances, but they do not have identical physical properties.
