Aluminium carbide

Aluminium carbide, chemical formula Al4C3, is a carbide of aluminium. 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 alternating layers of Al2C and Al2C2. Each aluminium atom is coordinated to 4 carbon atoms to give a tetrahedral arrangement. Carbon atoms exist in 2 different binding environments; one is a deformed octahedron of 6 Al atoms at a distance of 217 pm. The other is a distorted trigonal bipyramidal structure of 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. 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

Reactive hot isostatic pressing (hipping) at ≈40 MPa of the appropriate mixtures of Ti, Al4C3 graphite, for 15 hours at 1300 °C yields predominantly single-phase samples of Ti2AlC0.5N0.5, 30 hours at 1300 °C yields predominantly single-phase samples of Ti2AlC (Titanium aluminium carbide).

Preparation
Aluminium carbide is prepared by direct reaction of aluminium and carbon in an electric arc furnace.
 * 4 Al + 3 C → Al4C3

An alternative reaction begins with alumina, but it is less favorable because of generation of carbon monoxide.
 * 2 Al2O3 + 9 C → Al4C3 + 6 CO

Silicon carbide 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.

An aluminium-aluminium carbide composite material can be made by mechanical alloying, by mixing aluminium powder with graphite particles.

Occurrence
Small amounts of aluminium carbide are a common impurity of technical calcium carbide. In electrolytic manufacturing of aluminium, aluminium carbide forms as a corrosion product of the graphite electrodes.

In metal matrix composites based on aluminium matrix reinforced with non-metal carbides (silicon carbide, boron carbide, etc.) or carbon fibres, aluminium carbide often forms as an unwanted product. In case of carbon fibre, it reacts with the aluminium matrix at temperatures above 500 °C; better wetting of the fibre and inhibition of chemical reaction can be achieved by coating it with e.g. titanium boride.

Applications
Aluminium carbide particles finely dispersed in aluminium matrix lower the tendency of the material to creep, especially in combination with silicon carbide particles.

Aluminium carbide can be used as an abrasive in high-speed cutting tools. It has approximately the same hardness as topaz.