CARBIDES, THEIR TYPES, SYNTHESIS AND INDUSTRIAL APPLICATIONS

1. Carbides

The compounds which are made of carbon and more electropositive elements are called carbides. In carbides, carbon is usually combined with metallic and semi-metallic  elements such as calcium carbide, tungsten carbide, iron carbide etc.

2. Types of Carbides:

Carbides are classified on the basis of chemical composition, chemical bonds, physical composition, method of manufacture and on the basis of their applications.

2.1. Saline Carbides

The saline word refers to salt-like. These carbides are composed of high electropositive element such as alkali matels, alkaline earth metals have mixed with carbon. Isolated carbon centers are present in these carbides such as “C^4-” in the metanides, “C₃^4-” in the sesquicarbide and “C₂^2-” in the acetylides

i) Methanid

Methanides get their name from the property of formation of methane gas when treated with water. Aluminium carbide (Al₄C₃) and beryllium carbide (Be₂C) are important methanides.                            

Al₄C₃ + 12H₂O → 3CH₄ + 4Al(OH)₃

ii) Acetylides

Acetylides are salt like carbides with C₂^2- anion. There is triple bond between two carbon atoms in C₂^2- anion. The elements which form acetylides include alkali metals, alkaline earth metals and lanthanoids having formula M₂C₃. Actinides form acetylides with the stoichiometry MC₂ and M₂C₃. Acetylides get their from the property of acetylene formation when treated with water.

CaC₂ + 2H₂O → C₂H₂ + Ca(OH)₂

iii) Sesquicarbides

Sesquicarbides contain C₃^4- anion , which produces methylacetylene

(propyne) on hydrolysis:

2.2. Covalent Carbides

Covalent carbides are those carbides which have less electronegativity difference between carbon and the combining atom. The elements of group IVA form covalent carbides such as boron and silicon. Silicon carbide exists in two crystalline forms having resemblance with the structure of diamond. Whereas the structure of boron carbide (B₄C) is very unusual and forms icosahedral boron units connected by carbon atom. Both boron carbide and silicon carbide also called caborundum are refractory and hard material and have important industrial applications.

2.3. Interstitial Carbides

The transition metals having atomic radius nearly greater than 0.135nm form interstitial carbides. The elements of group IVB, VB and VI and 4^th, 5^th, and 6^th fall in this category. These transition metals have interstices which act as host lattice for small atoms like carbon. According to arrangement of metal atom, interstitial carbides may have 1:1 or 2:1 stoichiometry. In 1:1 stoichiometrty, there is cubic closed packing having octahedral interstices filled with carbon  eg. rock salt structure. While in 2:1 stoichiometry, there is hexagonal close packing in which octahedral interstices are directly opposite to the layers of metal atom.

2.4. Intermediate Carbides

The intermediate carbides are formed by transition metals having size smaller than 0.135 nm and carbon atoms are not accommodated in interstices without distortion of crystal lattice. The elements of group VIIB and VIIIB form intermediate carbides.They have multiple stoichiometry and are more complex than interstitial carbides.

3. Synthesis of Carbides

Carbides can be prepared by various synthetic routes. Some of them are given below:

•  Solid combustion synthesis which involves direct combination of the elements at elevated temperature.
• Carbo-thermal reduction of metal oxide.
•  Vapour phase synthesis
• Pyrolyses of metal-organic compounds
• Sol-gel route
•  Laser-induced reactions
• Plasma-chemical synthesis

Only one of them will be discussed here.

3.1. Vapor-Phase Synthesis

In vapour phase synthesis, metal chlorides are combined with carbon containing gases eg. methane or benzene.

Ti + CH₄ → TiC + 2H₂

ZrCl₄ + CH₄ → ZrC + 4HCl

With chlorides, the reaction is carried out in the gas phase above .600°C. Nanopowders can be obtained by using this rout because transition metal halogenides are highly volatile and can also be used to deposit the solid substrate by carbide layer.

Gas phase reactions are also developed to prepare SiC:

CH₄ + SiH₄ → SiC + 4H₂

SiCl₄ + CH₄ → SiC + 4HCl (26)

7SiCl₄ + C₇H₈ + 10H₂ → 7SiC + 28HCl

3SiH₄ + C₃H ₈→ 3SiC + 10H2

The advantage of this method is the synthesis of high purity nanopowders having potential control over shape, size and crystal structure in addition to control of reaction rates. However the yield obtained by this method is not appropriate.

4. Important Industrial Applications of Carbides:

Carbides have special physical and structural properties due to which they have special industrial applications. These are

• High resistance to abrasion and have hard steel grades.
• Resistance towards deflection: Carbides have nearly three times more elasticity modulus comparing to steel bars of same grade and thus resistance towards deflection

These are properties which make these carbide chemicals very effective in extreme temperature and pressure conditions such as in drilling and machine tool bits. Carbide balls are also used as milling agents due to their hardness in Attritor mills. Some Specific carbide chemicals include;

Due to these properties, carbide chemicals are very effective in extreme pressure and temperature conditions and are used as:

·         As milling agents in Attritor mills due to hardness of carbide balls

·         In drilling and machine tool bits such as tantalum carbide which has extreme degree of stability and resistance.

·         As heated spray to protect vulnerable metal surfaces such as chromium carbide powder.

·         To produce hard coatings eg. plasma spraying and to require high surface area in solar panels and fuel cells such as hafnium carbide powder.

·         In the manufacture of grinding wheels, sand papers, cutting tools and as refractory lining such as silicon carbide

·         For decorative purposes due to resemblance with gold and as non-toxic covering of medical equipments such as carbon titanium nitride powder.

·         For electrical and radio purposes.

·         To manufacture special tools and hard gears for mining and oil exploration sectors such as tungsten carbide powder.