Aluminum oxide, commonly known as alumina, is a chemical compound formed from aluminum and oxygen with the formula Al₂O₃. It is refined primarily from bauxite ore. Aluminum oxide is not a gas; it exists as a white, odorless solid under standard temperature and pressure conditions. This solid form is responsible for its widespread use in industrial and commercial applications.
The Physical State of Aluminum Oxide
Aluminum oxide’s stable existence as a solid is a consequence of its atomic structure and strong chemical bonds. The compound is held together by strong ionic bonds formed between aluminum cations (Al³⁺) and oxygen anions (O²⁻). This strong electrostatic attraction locks the ions into a rigid three-dimensional structure known as a crystal lattice.
The most stable and common crystalline form is alpha-alumina, which is the mineral corundum. In this structure, the oxygen ions form a densely packed arrangement, with the aluminum ions occupying specific spaces. Disrupting this tightly bound structure requires a substantial input of energy, which explains why the substance remains a stable solid.
Extreme Conditions and Phase Change
To overcome the inter-atomic forces holding the crystal lattice together, aluminum oxide must be subjected to extremely high temperatures. The material has a very high melting point of approximately 2,072°C (3,762°F), a temperature far exceeding that of most metals and ceramics. Only at this temperature does the solid transition into a liquid state.
The liquid form of alumina is also stable, requiring temperatures approaching 3,000°C (around 2,977°C or 5,391°F) to vaporize into a gas. This transition is highly abnormal and only occurs in specialized industrial processes or laboratory settings. While it is theoretically possible for aluminum oxide to exist as a gas, it is never found in that state naturally on Earth.
Common Applications of Aluminum Oxide
The hardness and high thermal stability of aluminum oxide make it an invaluable material across many industries. Its natural crystalline form, corundum, is known for its hardness, second only to diamond, making it an excellent abrasive. It is used in materials like sandpaper, grinding wheels, and polishing compounds.
The high melting point makes it suitable for refractory applications, such as lining furnaces and kilns to withstand intense heat. Its electrical insulating properties, combined with its ability to conduct heat, lead to its use in electronics as an insulator and in spark plug bodies. In its purest, clear form, it is the base material for precious gemstones like rubies and sapphires, where trace impurities provide the color.