Structure of Alumina

Aluminium oxide is a compound composed primarily of oxygen and aluminium molecules. It is a colourless crystalline substance that appears naturally in several forms, such as sapphire, which are usually composed of corundum. Alumina, or the most common crystal form of alumina, is corundum. Interestingly, under special conditions, corundum became the gem sapphire.

Alumina is also extracted from ores, such as bauxite found on the surface of some tropical and subtropical regions. Extraction and purification of alumina were performed using the Bayer process. Here, bauxite is crushed and dissolved in a sodium hydroxide solution. When stored at high temperatures, bauxite is converted to sodium aluminate, filtering out impurities such as red mud or slag, also known as Fe2O3 (ferric oxide). As the beer cooled, Al(OH) {alumina or also known as aluminium hydroxide} precipitated, and the silicate remained in the solution.

Preparation of Alumina and Physical appearances

Alumina particles are separated by heating or calcining Al(OH)3 at approximately 1100 °C. Pure bauxite has the appearance of aluminium oxide as white powder-like table salt or granulated sugar.

Aluminium oxide is used in certain types of illumination, such as sodium-vapour lamps, and the growing nanotechnology industry uses it as an electrical conductor in minuscule circuits. Aluminium oxide may be produced into filaments as fine as human hair, making it suitable for DNA filtration. Wire guides, machinery seals, metering devices, and high-temperature electrical insulators are just a few examples.

Aluminium oxide is an odourless white powdered material. Although it is non-toxic, airborne aluminium oxide dust can pose an industrial danger, therefore, extended exposure is best avoided by wearing a mask. Aluminium oxide is extremely heavy; a cube of aluminium oxide with a side length of 1 metre weighs approximately 7,200 lbs.

Aluminium oxide can be moulded or manufactured into robust, wear-resistant materials that can be used in a range of industrial applications.

Classification of the Structure of Alumina

The Al2O3 transition is obtained by thermal decomposition of Al-O-OH (Al is double-bonded to an oxygen atom and singly bonded to an OH atom). Boehmite has a complex structure and is still largely unknown. The two main types are described in detail in the structure, and we also get their shaping power. 

Comparisons with related Al2O3 transitions show how energy dissipation leads to structural disruptions and complex adhesions between some Al2O3 transitions. The results have an important understanding of the thermodynamically stable compounds.

Advanced Classification of the Structure of Alumina

The Al2O3 transitions are generally caused by the transition of lattice under high temperature and pressure and the placement of the crystal lattice. It also depends on the particle size and the thermodynamics of the alumina used. 

So, below are the types of the different structures of alumina discovered in the past decades with detailed analysis:-

• Boehmite:

Characteristics of transitional aluminates are particularly difficult due to their similar diffraction patterns, low crystallinity, and small particle size. The starting material for the alumina transition is boehmite (Al-O-OH), a hydroxide containing a sub-segment of cubic anions (ccp) O2-. Upon heating, some (but not all) hydrogen atoms migrate out of the structure in the form of water. This dehydration leaves holes in the crystal lattice occupied by aluminium cations. 

γ-Alumina is the least thermodynamically stable form of alumina, making it inconvenient for those wishing to use the material at elevated temperatures. The structure formed is likely a result of the “new” Tetrahedral conditions resulting from boehmite dehydration. There is currently no consensus on the composition of the other two forms of more commonly mentioned alumina.

• Spine-Like Structure:

Until the early 2000s, γ-alumina was consistently reported to have a lattice structure resembling a cubic spinel similar to Fe2O3 (iron oxide). The spinel is officially referred to as AB2X4, the X anion forms the ccp (cubic closed packed) network, and the cations A and B each occupy intermediate network locations. There is controversial evidence that Al can occupy non-spinel positions in the crystal lattice. 

Theoretical research supports alumina energy priority for tetrahedral sites. A series of DFT (Density functional theory) total energy calculations were performed to show that the low-energy spinel cubic unit cell (a = 7.887 Å) has two VSSs (Volatile Suspended Solids) separated at the maximum.

Although it has been used in theoretical studies, it can be oversimplified in explaining the internal shape of γ-alumina. A single spinel cell, such as the monoclinic system, has also been used as a bulk structure for some subsequent theoretical studies. VSS sites show significant thermal activation due to the large energy barrier to the movement of vacancies. 

• Hydrogenated Spinel Structure:

Numerous experimental and theoretical studies support the existence of interstitial hydrogen atoms in γ-alumina structures. Hydrogen atoms are mobile and can move between the metal oxide lattice positions. After hydrogen moves out of the structure, Al cations can take their place, resulting in an increasingly ordered and thermodynamically stable structure.

Conclusion

Some criticisms of the hydrogenated spinel structure of Al2O3 appear to be strictly semantic. It is known that different metal oxides exist at different hydrogen levels since the defects can act as hydrogen traps. This is generally a short-sighted claim, as the substance was called “alumina” a few years ago when the structure was thought to contain hydrogen. 

Alumina is found in many different stable phases, such as the cubic phase, the hexagonal phase, the rhombic phase, the monoclinic phase, and the eighth phase, the puzzle phase. It is crystalline and can form an almost hexagonal structure and the centre of an octagon. It can be a rhombus or a quadrangle and others. Each phase has a unique crystal structure and properties and is also known as the puzzle phase.