What Is Alumina?
Alumina is a compound known as aluminum oxide, consisting of aluminum and oxygen. Its chemical formula is Al2O3, and it appears as a white solid with a high melting point. Alumina is renowned as the primary component in materials like corundum, ruby, and sapphire.
There are different forms of alumina, and two noteworthy ones are:
- α-Alumina: Recognized for its heat resistance, insulation, and chemical resistance. It finds applications in refractory materials, cutting tools, and abrasives.
- γ-Alumina: Possessing a large specific surface area, it is used as a catalyst support.
Applications of Alumina
Alumina is primarily derived from the calcination of aluminum hydroxide (Al(OH)3). It exhibits various crystal forms, with α-alumina being the most stable and versatile.
α-Alumina boasts an exceptionally high melting point, outstanding heat resistance, hardness, insulation properties, and chemical resistance. These attributes make it suitable for a diverse range of applications, including refractory materials, spark plugs, semiconductor substrates, grinding and polishing tools, and crucibles.
Properties of Alumina
Aluminum, the element with atomic number 13, forms aluminum oxide (chemical formula: Al2O3), commonly known as alumina. Abundant in the Earth’s crust, aluminum ranks as the third most plentiful element, following oxygen and silicon.
Naturally occurring in the form of corundum, it transforms into ruby and sapphire when infused with trace elements like iron, chromium, and titanium. Bauxite, a reddish-brown ore, serves as a rich source of aluminum. Dissolving bauxite in caustic soda generates sodium aluminate, which, upon hydrolysis, yields aluminum hydroxide. Sintering aluminum hydroxide culminates in the production of alumina.
Alumina exhibits diverse crystal structures, with α-alumina being notably stable. Subjecting γ-alumina to high temperatures triggers a transformation through intermediate alumina phases, ultimately leading to α-alumina. α-Alumina boasts a melting point exceeding 2000°C and exhibits remarkable heat resistance and hardness.
Toxicity of Alumina
Alumina itself is non-toxic and generally safe. However, when handling alumina in a dusty form, protective gear such as goggles and dust masks should be employed, as they may pose risks to the eyes, skin, and respiratory system. Studies have reported lung retention and inflammation in rats when alumina is inhaled.
With a Mohs hardness rating of 9, alumina offers exceptional wear resistance and durability, making it a valuable biomedical ceramic. It finds use in applications like artificial joints and bones, where long-term implantation in the human body necessitates harmlessness. Previous research has yielded negative results in biological safety tests, including acute toxicity and hemolysis tests. Alumina itself is also non-carcinogenic.
Distinguishing Ceramics and Alumina
Alumina falls within the category of ceramics, which encompasses inorganic materials. The term “ceramics” originally referred to products like pottery, glass, and cement. However, it has evolved to encompass all products formed by heating inorganic materials at high temperatures, resulting in their hardening. Materials used for ceramics include alumina, zirconia, silicon nitride, and silicon carbide.
Ceramics are categorized into two main types:
- Old Ceramics: Made from natural materials like clay and silica stone.
- Fine Ceramics: Crafted from materials that have been modified for increased purity or that have undergone artificial preparation, resulting in enhanced properties. Fine ceramics are highly functional and hold added value, finding essential use in various fields like electronics and biomaterials. Alumina, when highly purified, falls under the category of fine ceramics.