Silicon (Chemical symbol: Si)
Silicon was discovered by Jöns Jacob Berzelius, a Swedish chemist, in 1824 by heating potassium metal with silicon tetrafluoride and purifying it by repeated washing (Fig. 1). Silicon had previously been recognized as early as 1808, but attempts to isolate and purify it had been unsuccessful. At that time it was given the name silicium – from the Latin silex (hard stone or flint) and ending in “ium” to suggest a metal.
The current English name was first used in 1817 to conform with the physically similar elements, carbon and boron. Silicon is the eighth most common element in the universe by mass but usually occurs in combination with other elements on Earth. Over 90% of the Earth's crust is composed of silicate minerals, making silicon the second most abundant element in the Earth's crust (about 28% by mass) after oxygen. It is distributed throughout the universe in dusts, sands, planetoids and planets as various forms of silicon-dioxide (silica) or silicates.
Most silicon used commercially is combined with other elements. It takes the forms of clays, silica sand and stone; it is used in glassware; silicon carbide is used as abrasives; and silica is used in high-strength ceramics and furnace refractories. Elemental (uncombined) silicon is also important to the modern world economy. Most free silicon is used in the steel refining, aluminum-casting and fine chemical industries (often to make fused silica). A relatively small portion of very highly purified silicon is used in semiconductor electronics (<10%) and is essential to integrated circuits. Computers, cell phones and photovoltaic cells all depend on silicon. Silicon is also the basis of the important synthetic polymers called silicones. Silicon-based polymers are commonly used as alternatives to hydrocarbon-based products and are found in many everyday products, such as resins, lubricants, greases, and skin and hair products.
Perhaps most important to the field of metallurgy is silicon's use as an alloy of aluminum. It makes aluminum lighter and stronger. Castings and forgings are the primary use of silicon-aluminum alloys (Fig. 2). Silicon is the principle alloying element for aluminum and provides high fluidity and reduced shrinkage, resulting in good castability and weldability. It also reduces the thermal-expansion coefficient and increases wear resistance. The percentage of silicon in cast alloys can be up to 24%, but alloys made by powder metallurgy may contain as much as 50% silicon.
Here are a few important facts about silicon.
- Atomic number: 14 (Fig. 3)
- Melting point: 1687K (1414°C, 2577°F)
- Boiling point: 3538K (3265°C, 5909°F)
- Density: 2.3290 g/cm3
- Heat of fusion: 50.21 kJ/mole
- Heat of vaporization: 383 kJ/mole
- Molar heat capacity: 19.789 J/(mole·K)