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, C^4-. 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.Fact|date=October 2007

: 4Al + 3C → Al₄C₃

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.Fact|date=October 2007

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. [cite journal | author = Urena, S. Gomez De, Gil, Escalera and Baldonedo | title = Scanning and transmission electron microscopy study of the microstructural changes occurring in aluminium matrix composites reinforced with SiC particles during casting and welding: interface reactions | year = 1999 | journal = Journal of Microscopy | volume = 196 | issue = 2 | pages = 124–136 | doi = 10.1046/j.1365-2818.1999.00610.x] 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. [cite web | title = A359/SiC/xxp: A359 Al alloy reinforced with irregularly shaped SiC particles | url = http://mmc-assess.tuwien.ac.at/data/prm/duralcan/a359_sic.htm | author = Guillermo Requena | publisher = MMC-ASSESS Metal Matrix Composites | accessdate = 2007-10-07]

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.Fact|date=October 2007

: Al₄C₃ + 12H₂O → 4Al(OH)₃ + 3CH₄

Applications

Aluminium carbide particles finely dispersed in aluminium matrix lower the tendency of the material to creep, especially in combination with silicon carbide particles. [cite journal | journal = Acta Technica CSAV | author = S.J. Zhu, L.M. Peng, Q. Zhou, Z.Y. Ma, K. Kucharova, J. Cadek | title = Creep behaviour of aluminium strengthened by fine aluminium carbide particles and reinforced by silicon carbide particulates DS Al-SiC/Al4C3composites | issue = 5 | year = 1998 | pages = 435–455 | url = http://www.iee.cas.cz/acta/98_5a.htm | format = abstract]

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. [US patent | 6033789] Approximately, it has the hardness of topaz. [E. Pietsch, ed.: "Gmelins Hanbuch der anorganischen Chemie: Aluminium, Teil A", Verlag Chemie, Berlin, 1934–1935.]

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

References