Aluminum carbide , chemical formula Al4Si3 , is the carbide of aluminum . It has the appearance of light yellow to brown crystals. It is stable up to 1400 °C. It decomposes in water with the production of methane.
Structure
Aluminum carbide has an unusual crystal structure consisting of alternating layers of Al 2 Si and Al 2 Si 2 . Each aluminum atom is coordinated with 4 carbon atoms to give a tetrahedral arrangement. Carbon atoms are present in 2 different binding environments; one is a distorted octahedron of 6 Al atoms at a distance of 217 pm . The second is a distorted triangular dipole structure of 4 Al atoms at 190–194 pm and a fifth Al atom at 221 pm. [3] [4] Other carbides ( IUPAC nomenclature: methides) also exhibit complex structures.
reactions
Aluminum carbide hydrolyzes with the evolution of methane. The reaction proceeds at room temperature but is rapidly accelerated by heating.
Al 4 Si 3 + 12 H 2 O → 4 Al (OH) 3 + 3 CH 4
Similar reactions occur with other protic reagents:
Al 4 Si 3 + 12 HCl → 4 AlCl 3 + 3 CH 4
Reactive hot isostatic pressing (hipping) at 40 MPa of a suitable mixture of Ti, Al 4 Si 3 graphite, predominately Ti 2 Alc 0.5 N 0.5 for 15 h at 1300 °C, 30 h at 1300 °C yields single-phase samples Does C give single-phase samples mainly of Ti 2 AlC (titanium Al 4 Si 3).
Preparation
Al 4 Si 3 is prepared by the direct reaction of aluminum and carbon in an electric arc furnace. [3]
4Al + 3C → Al 4C3
An alternative reaction starts with alumina, but this is less favorable due to the production of carbon monoxide.
2 Al 2 O 3 + 9 C → Al 4 C 3 + 6 CO
Silicon carbide also reacts with aluminum to obtain Al 4 C 3 . This conversion limits the mechanical applications of SiC, as Al 4 C is more brittle than
4 Al + 3 SiC → Al 4 Si 3 + 3 Si
In aluminum-matrix composites reinforced with silicon carbide, chemical reactions between silicon carbide and molten aluminum generate a layer of Al4Si3 on the silicon carbide particles, which reduces the material’s strength, although it does reduce the instability of the SiC particles. Increases. [8] This tendency can be reduced by coating the silicon carbide particles with a suitable oxide or nitride, preoxidation of the particles to form a silica coating, or by using a layer of sacrificial metal. [9]
An aluminum-Al 4 Si 3 composite material can be made by mechanical alloying, mixing aluminum powder with graphite particles.
incident
Small amounts of Al 4 Si 3 are a common impurity of technical calcium carbide. In the electrolytic manufacturing of aluminum, aluminum carbide is formed as a corrosion product of graphite electrodes.
In metal matrix composites based on aluminum matrix reinforced with non-metal carbides (silicon carbide, boron carbide, etc.) or carbon fiber, aluminum carbide often forms as an unwanted product. In the case of carbon fiber, it reacts with the aluminum matrix at temperatures above 500 °C; Improved wetting of the fiber and inhibition of chemical reaction can be achieved by coating it with titanium boride.
Application
Aluminum carbide particles are finely dispersed in the aluminum matrix, reducing the material’s tendency to creep, especially in combination with silicon carbide particles. [11 1]
Al 4 Si 3 can be used as an abrasive in high speed cutting tools. [12] It has almost the same hardness as topaz.
