alpha alumina (Al2O3) with the trading name of Alumond is the final step in the thermal decomposition of hydroxides. Aluminium oxide exists in many forms, α, χ, η, δ, κ, θ, γ, ρ; these arise during the heat treatment of aluminium hydroxide or aluminium oxy hydroxide. The most thermodynamically stable form is α-aluminium oxide.
After calcination, the alumina particles appear as crystallite agglomerates. the size of alpha alumina crystallites typically vary from 0.5 to 10 µm. The greater the degree of calcination, the bigger the crystallites will be.
The diversity of applications for Alumond could be explained by the surprising range of properties that can be obtained depending on their quality. These include their purity, refractoriness, sinterability, chemical inertness in both oxidizing and reducing environments and in both acidic and basic environments, hardness, wear and abrasion resistance, mechanical resistance in heat, dimensional stability, high thermal conductivity, transparency, electric resistivity, low dielectric loss and high permittivity, ionic conductivity of beta alumina.
The two main characteristics of calcined aluminas are the size of their crystallites and their impurity content. The market divides calcined aluminas into 4 families based on their soda content :
- Na2O between 3 000 and 7 000 ppm : Standard aluminas.
- Between 1 000 and 3 000 ppm: Intermediate aluminas. Here the soda content has been lowered by modifying the conditions of either precipitation of gibbsite or calcination.
- Between 300 and 1 000 ppm: Low soda aluminas, These are obtained by washing the precursor or by soda extraction during calcination.
- Below 100 ppm: High purity aluminas. These are generally obtained by a process other than Bayer.
After that the reactivity of alumina is determined in relation to its specific surface area, conversely proportional to the size of the crystallites. That is, the smaller the crystallites (the greater the specific surface area), the more reactive the alumina will be. The applications of a given alumina are mainly defined on the basis of these two characteristics.
The main applications for Alumond include: refractories, glass and enamel, tiles and porcelains, mechanical, electric and electronic ceramics etc.