Aluminium carbide

Aluminium carbide, chemical formula Al4C3, is a carbide of aluminium. It has the appearance of pale yellow to brown crystals with complex lattice structure. It is stable up to 1400 °C. Its density is 2.36 g/cm³. It is a methide: the carbon atoms are present in the lattice as discrete carbon anions, C4-. Its CAS number is 12656-43-8 and 1299-86-1.

Preparation
Aluminium carbide is prepared by heating a mixture of aluminium and carbon at 1000 °C.

Occurrence
Small amount of aluminium carbide is 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 metal carbides (silicon carbide, boron carbide, etc.) or carbon fibers, 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 eg. titanium boride.

In silicon carbide reinforced aluminium-matrix composites, eg. Duralcan, the chemical reactions between silicon carbide and molten aluminium create a layer of aluminium carbide on the silicon carbide particles, which decreases the strength of the material, though if 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.

Reactions
As a methide, aluminium carbide generates methane when subjected to water or dilute acids. It is thus used as a chemical reagent for measuring tritium content in water. The measured water reacts with the carbide; the resulting tritium-laced methane then fills the measuring chamber.

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

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

Aluminium carbide can be used as an abrasive in high-speed cutting tools. Approximately, it has the hardness of topaz.

Aluminium carbide is also used in pyrotechnics, e.g. to achieve the firefly effect.