Alumina vs. Corundum

Attribute	Alumina	Corundum
Chemical Formula	Al2O3	Al2O3
Crystal Structure	Hexagonal	Hexagonal
Color	White, gray, or brown	Colorless, white, gray, or brown
Hardness	9 on Mohs scale	9 on Mohs scale
Luster	Vitreous	Vitreous
Transparency	Transparent to translucent	Transparent to translucent
Density	3.97 g/cm3	3.97 g/cm3
Melting Point	2,072°C (3,762°F)	2,072°C (3,762°F)
Uses	Refractory material, ceramics, abrasives	Refractory material, ceramics, abrasives

Introduction

Alumina and corundum are two closely related materials that share many similarities but also have distinct differences. Both alumina and corundum are forms of aluminum oxide (Al₂O₃), but they differ in terms of crystal structure, physical properties, and applications. In this article, we will explore the attributes of alumina and corundum, highlighting their similarities and differences.

Crystal Structure

Alumina and corundum have different crystal structures, which significantly impact their properties. Alumina typically exists in a hexagonal crystal structure, known as alpha-alumina, which gives it excellent mechanical strength and thermal stability. On the other hand, corundum has a trigonal crystal structure, also known as alpha-corundum, which contributes to its high hardness and resistance to wear.

Physical Properties

While alumina and corundum share some physical properties, they also exhibit distinct characteristics. Both materials have high melting points, with alumina melting at around 2,072 degrees Celsius and corundum at approximately 2,050 degrees Celsius. Additionally, both alumina and corundum are electrically insulating and chemically inert, making them suitable for various applications in harsh environments.

However, one of the key differences between alumina and corundum lies in their hardness. Corundum is renowned for its exceptional hardness, ranking 9 on the Mohs scale, just below diamond. This hardness makes corundum ideal for applications that require abrasion resistance, such as grinding wheels and cutting tools. Alumina, while still relatively hard, has a lower hardness rating of around 8 on the Mohs scale.

Another notable difference is the color of alumina and corundum. Alumina is typically white or off-white, while corundum can occur in a wide range of colors, including red (ruby), blue (sapphire), yellow, green, and even colorless. These vibrant colors are due to the presence of impurities within the corundum crystal lattice.

Applications

Both alumina and corundum find extensive applications across various industries due to their unique properties. Alumina, with its excellent mechanical strength and thermal stability, is commonly used in the production of high-performance ceramics, electrical insulators, and refractory materials. It is also utilized as a catalyst support in the chemical industry and as a filler in composite materials.

Corundum, on the other hand, is highly valued for its hardness and abrasion resistance. It is widely employed in the manufacturing of grinding and polishing materials, such as sandpaper and abrasive discs. Additionally, corundum's gemstone varieties, such as ruby and sapphire, are highly sought after for their beauty and durability in jewelry.

Furthermore, corundum has applications in the field of optics. Its transparency and high refractive index make it suitable for lenses, windows, and laser components. Corundum is also used as a substrate for the growth of gallium nitride (GaN) crystals, which are crucial for the production of blue and white LEDs.

Production and Availability

Alumina and corundum can be produced through different methods, each tailored to their specific applications. Alumina is commonly obtained from bauxite ore through the Bayer process, which involves crushing, grinding, and refining the ore to extract pure alumina. This process makes alumina widely available and relatively cost-effective.

Corundum, on the other hand, is typically produced through the fusion of high-purity alumina in electric arc furnaces at extremely high temperatures. The resulting corundum is then crushed and sorted into different sizes for various applications. While corundum production requires more specialized techniques, it remains readily available due to its high demand in industries such as abrasives and optics.

Conclusion

In conclusion, alumina and corundum are closely related materials with distinct attributes. While both share the same chemical composition of aluminum oxide, their crystal structures, physical properties, and applications set them apart. Alumina's hexagonal crystal structure provides it with excellent mechanical strength and thermal stability, making it suitable for ceramics and refractory materials. Corundum, with its trigonal crystal structure and exceptional hardness, finds applications in abrasives, optics, and gemstones. Understanding the similarities and differences between alumina and corundum is crucial for selecting the appropriate material for specific industrial and commercial purposes.