Q1: Which of the following is not a natural abrasive ?

ANS:A - Silicon carbide

Silicon carbide (SiC) is a compound composed of silicon and carbon. It is represented by the chemical formula SiC. Silicon carbide is a hard, brittle crystalline compound with a structure similar to that of diamond. It occurs naturally in small quantities in meteorites, but the vast majority of silicon carbide used today is produced synthetically. Here are some key points about silicon carbide:

1. Structure: Silicon carbide has a tetrahedral crystal structure, where each silicon atom is bonded to four carbon atoms and each carbon atom is bonded to four silicon atoms. This structure gives silicon carbide its exceptional hardness and thermal conductivity.
2. Hardness: Silicon carbide is one of the hardest known materials, with a Mohs hardness of around 9-9.5. This makes it highly resistant to abrasion and wear, making it suitable for use in abrasive applications such as grinding wheels, sandpaper, and cutting tools.
3. Thermal conductivity: Silicon carbide has excellent thermal conductivity, even at high temperatures. This property, combined with its hardness, makes it useful in applications requiring high thermal conductivity and heat resistance, such as heat exchangers, thermal management systems, and refractory materials.
4. Wide range of applications: Due to its unique combination of properties, silicon carbide finds applications in various industries, including electronics, automotive, aerospace, ceramics, and metallurgy. It is used as a semiconductor material in electronic devices such as diodes and transistors, as an abrasive in grinding and cutting tools, as a refractory material in kiln furniture and crucibles, and as a component in high-temperature applications such as heating elements and furnace linings.
5. Production: Silicon carbide can be produced through several methods, including reaction of silicon dioxide and carbon in an electric furnace (carbothermal reduction), reaction of silica sand and petroleum coke in an electric furnace (Acheson process), and by chemical vapor deposition (CVD). The synthetic production of silicon carbide allows for the control of its properties and the production of customized materials for specific applications.