Corundum

2.1.1 Chemistry, Types and Characteristics of Corundum

Corundum is crystalline aluminum oxide, Al2O3, and also known as alumina. Al2O3 is also the active element in the natural abrasive material emery [LEWI76, p. 9] and in the gemstones sapphire and ruby. Aluminum oxide occurs in at least five modifications, a-Al2O3, P-Al2O3, y-Al2O3, 5-Al2O3, and e-Al2O3. The tech­nologically most important modification is the rhombohedral crystalline a-Al2O3, being the basis for corundum (Fig. 2.2). The proportion of covalent atom bonds to

ionic atom bonds is 40-60 % [MULL01, p. 25]. The cubic form y-Al2O3 (clay, German “Tonerde”) is the basis for vitrified bonds and other ceramic products.

Industry applies several corundum types with different chemical composition and manufacturing routes:

• Molten or fused corundum, such as

— Brown corundum (also known as brown fused alumina, BFA, semi-friable corundum, regular corundum),

— White corundum (also known as pure white alumina),

— Pink corundum,

— Ruby corundum (also known as red alumina),

— Zirconium corundum (also known as alumina zirconia),

— Mono crystalline corundum (also known as single crystal alumina),

— Micro crystalline corundum,

— Hollow sphere corundum (also known as bubble alumina)

• Sintered corundum

• Sol-Gel corundum

Table 2.1 gives the chemical composition of several types of molten corundum.