Monocrystalline alumina, often referred to as “single crystal alumina” or simply “sapphire” in some contexts, is a type of alumina (Al₂O₃) that has a single, continuous crystal structure. This is in contrast to polycrystalline materials, which are made up of numerous small crystalline regions or grains with different orientations.
Monocrystalline alumina exhibits unique properties due to its singular crystal structure, including:
- Optical clarity: Monocrystalline alumina is transparent in its pure form and can transmit light over a wide range of wavelengths, from ultraviolet to infrared. This makes it useful for applications like sapphire windows and watch crystals.
- High hardness: Sapphire ranks 9 on the Mohs scale of mineral hardness, making it one of the hardest known materials, surpassed only by diamond.
- Thermal stability: Monocrystalline alumina has a high melting point and maintains its strength at high temperatures.
- Chemical resistance: Sapphire is chemically inert and doesn’t react with most acids, bases, and other chemical agents.
- Electrical insulating properties: Monocrystalline alumina is an excellent electrical insulator.
Due to these properties, monocrystalline alumina finds application in various fields, including:
- Electronics: Substrates for some semiconductor devices.
- Optics: Windows, lenses, and optical components, especially in harsh environments.
- Watch industry: Scratch-resistant watch faces.
- Medical: Surgical tools and endoscope optics.
The method commonly used to produce large single crystals of sapphire is known as the Kyropoulos method or other bulk crystal growth techniques.