Aluminium carbide
Chemical compound
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Through Chemical formula, Aluminium and Carbon
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
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Reactions
Aluminium carbide hydrolyses with evolution of methane and considerable amounts of heat.
- Al4C3 + 12 H2O → 4 Al(OH)3 + 3 CH4
The addition of 10g of Al4C3 to 10g of water was found to reach a maximum temperature of 387 °F (197 °C) at a time of 11 minutes in one study, while in another the addition of 20g of Al4C3 to 10g of water reached a maximum temperature of 280 °F (138 °C). Reversing the order and adding 10g of water to 20g of Al4C3 reached a maximum temperature of 349 °F (176 °C).
Similar reactions occur with other protic reagents:
- Al4C3 + 12 HCl → 4 AlCl3 + 3 CH4
Composites
Reactive hot isostatic pressing (hipping) at ≈40 megapascals (390 atm) of the appropriate mixtures of Ti, Al4C3-graphite, for 15 hours at 1,300 °C (2,370 °F) yields predominantly single-phase samples of Ti2AlC0.5N0.5, 30 hours at 1,300 °C (2,370 °F) yields predominantly single-phase samples of Ti2AlC (Titanium aluminium carbide).
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.
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.
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 (932 °F); 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.