Aluminium carbide
Encyclopedia
Aluminium carbide, chemical formula Al
Aluminium
Aluminium or aluminum is a silvery white member of the boron group of chemical elements. It has the symbol Al, and its atomic number is 13. It is not soluble in water under normal circumstances....

4C
Carbon
Carbon is the chemical element with symbol C and atomic number 6. As a member of group 14 on the periodic table, it is nonmetallic and tetravalent—making four electrons available to form covalent chemical bonds...

3, is a carbide
Carbide
In chemistry, a carbide is a compound composed of carbon and a less electronegative element. Carbides can be generally classified by chemical bonding type as follows: salt-like, covalent compounds, interstitial compounds, and "intermediate" transition metal carbides...

 of aluminium
Aluminium
Aluminium or aluminum is a silvery white member of the boron group of chemical elements. It has the symbol Al, and its atomic number is 13. It is not soluble in water under normal circumstances....

. It has the appearance of pale yellow to brown crystals. It is stable up to 1400 °C. It decomposes in water with the production of methane.

Structure

Aluminium carbide has an unusual crystal structure that consists of two types of layers. It is based on AlC4 tetrahedra of two types and thus two types of carbon atoms. One is surrounded by a deformed octahedron of 6 Al atoms at a distance of 217 pm. The other is surrounded by 4 Al atoms at 190–194 pm and a fifth Al atom at 221 pm. Other carbides (IUPAC nomenclature: methides) also exhibit complex structures.

Reactions

Aluminium carbide hydrolyses with evolution of methane
Methane
Methane is a chemical compound with the chemical formula . It is the simplest alkane, the principal component of natural gas, and probably the most abundant organic compound on earth. The relative abundance of methane makes it an attractive fuel...

. The reaction proceeds at room temperature but is rapidly accelerated by heating.
Al4C3 + 12 H2O → 4 Al(OH)3 + 3 CH4


Similar reactions occur with other protic reagents:
Al4C3 + 12 HCl → 4 AlCl3 + 3 CH4

Preparation

Aluminium carbide is prepared by direct reaction of aluminium and carbon in an electric furnace
Electric arc furnace
An electric arc furnace is a furnace that heats charged material by means of an electric arc.Arc furnaces range in size from small units of approximately one ton capacity up to about 400 ton units used for secondary steelmaking...

.
4 Al + 3 C → Al4C3


An alternative reaction begins with alumina, but it is less favorable because of generation of carbon monoxide
Carbon monoxide
Carbon monoxide , also called carbonous oxide, is a colorless, odorless, and tasteless gas that is slightly lighter than air. It is highly toxic to humans and animals in higher quantities, although it is also produced in normal animal metabolism in low quantities, and is thought to have some normal...

.
2 Al2O3 + 9 C → Al4C3 + 6 CO


Silicon carbide
Silicon carbide
Silicon carbide , also known as carborundum, 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...

 also reacts with aluminium to yield Al4C3. This conversion limits the mechanical applications of SiC, because Al4C3 is more brittle than SiC.
4 Al + 3 SiC → Al4C3 + 3 Si

In aluminium-matrix composites reinforced with silicon carbide, the chemical reactions between silicon carbide and molten aluminium generate a layer of aluminium carbide on the silicon carbide particles, which decreases the strength of the material, although it increases the wettability of the SiC particles. This tendency can be decreased by coating the silicon carbide particles with a suitable oxide or nitride, preoxidation of the particles to form a silica coating, or using a layer of sacrificial metal
Sacrificial metal
A sacrificial metal is a metal used as a sacrificial anode in cathodic protection that corrodes to prevent a primary metal from corrosion, galvanization or rusting.-Equation:...

.

An aluminium-aluminium carbide composite material can be made by mechanical alloying, by milling aluminium powder with graphite
Graphite
The mineral graphite is one of the allotropes of carbon. It was named by Abraham Gottlob Werner in 1789 from the Ancient Greek γράφω , "to draw/write", for its use in pencils, where it is commonly called lead . Unlike diamond , graphite is an electrical conductor, a semimetal...

 particles.

Occurrence

Small amounts of aluminium carbide are a common impurity of technical calcium carbide
Calcium carbide
thumb|right|Calcium carbide.Calcium carbide is a chemical compound with the chemical formula of CaC2. The pure material is colorless, however pieces of technical grade calcium carbide are grey or brown and consist of only 80-85% of CaC2 . Because of presence of PH3, NH3, and H2S it has a...

. In electrolytic manufacturing of aluminium, aluminium carbide forms as a corrosion product of the graphite electrodes.

In metal matrix composite
Metal matrix composite
A metal matrix composite is composite material with at least two constituent parts, one being a metal. The other material may be a different metal or another material, such as a ceramic or organic compound. When at least three materials are present, it is called a hybrid composite...

s based on aluminium matrix reinforced with metal carbides (silicon carbide
Silicon carbide
Silicon carbide , also known as carborundum, 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...

, boron carbide
Boron carbide
Boron carbide is an extremely hard boron–carbon ceramic material used in tank armor, bulletproof vests, and numerous industrial applications...

, etc.) or carbon fiber
Carbon fiber
Carbon fiber, alternatively graphite fiber, carbon graphite or CF, is a material consisting of fibers about 5–10 μm in diameter and composed mostly of carbon atoms. The carbon atoms are bonded together in crystals that are more or less aligned parallel to the long axis of the fiber...

s, aluminium carbide often forms as an unwanted product. In case of carbon fiber, it reacts with the aluminium matrix at temperatures above 500 °C; better wetting of the fiber and inhibition of chemical reaction can be achieved by coating it with e.g. titanium boride
Titanium boride
Titanium diboride is an extremely hard ceramic compound composed of titanium and boron which has excellent resistance to mechanical erosion...

.

Applications

Aluminium carbide particles finely dispersed in aluminium matrix lower the tendency of the material to creep
Creep (deformation)
In materials science, creep is the tendency of a solid material to slowly move or deform permanently under the influence of stresses. It occurs as a result of long term exposure to high levels of stress that are below the yield strength of the material....

, especially in combination with silicon carbide
Silicon carbide
Silicon carbide , also known as carborundum, 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...

 particles.

Aluminium carbide can be used as an abrasive
Abrasive
An abrasive is a material, often a mineral, that is used to shape or finish a workpiece through rubbing which leads to part of the workpiece being worn away...

 in high-speed cutting tool
Cutting tool
In the context of machining, a cutting tool is any tool that is used to remove material from the workpiece by means of shear deformation. Cutting may be accomplished by single-point or multipoint tools. Single-point tools are used in turning, shaping, plaining and similar operations, and remove...

s. It has approximately the same hardness as topaz
Topaz
Topaz is a silicate mineral of aluminium and fluorine with the chemical formula Al2SiO42. Topaz crystallizes in the orthorhombic system and its crystals are mostly prismatic terminated by pyramidal and other faces.-Color and varieties:...

.
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