Material of the month – silicon

Materials World magazine
,
1 Mar 2015

This month, Anna Ploszajski explores the chemical element silicon and its applications, from the cosmetic industry to soda lime glass. 

Silicon is a tetravalent metalloid, meaning that it has four electrons in its outer shell, and exhibits properties of metals and non-metals, which led to some confusion during its discovery. In 1787, Antoine Lavoisier pondered whether the well-known material silica was in fact the oxide of an unknown fundamental element. Sir Humphry Davy attempted to isolate the element in 1808 and named the illusive substance silicium, with the -um suffix to indicate that he believed it was metallic. It wasn’t until chemist and mineralogist, Thomas Thompson, from Scotland, came along that its non-metallic properties were realised and he renamed it silicon in 1817 following the recognised pattern of solid non-metals boron and carbon.

In 1823, chemist Jöns Jacob Berzelius, from Sweden, created the first known preparation of pure amorphous silicon. Although Frenchmen Joseph-Louis Gay-Lussac and Louis-Jacques Thenard succeeded in the preparation some 12-years previous, they did not realise what they had made and their substance was impure. Berzelius repeated their work and added numerous washing steps to purify the final material. Berzelius is generally credited with silicon’s isolation and discovery. Then, in 1854, Henri Deville prepared the crystalline form of silicon by electrolysis of impure sodium aluminium chloride, which contained 10% silicon. This process produced aluminum silicide, and the aluminium was removed with water to leave pure silicon crystals. Since then, silicon has lent its name to technological centres worldwide, such as Silicon Valley in California and the somewhat more modestly dubbed Silicon Roundabout in central London. These names pay tribute to silicon’s huge impact on the local economy and business there.

Characteristics

Pure silica materials formed of silicon dioxide (SiO2) can exist as different crystal forms, such as quartz, agate, amethyst, flint, jasper and opal. These naturally occur in silicon-based minerals, called silicates, which are found in granite and sandstone. More than 90% of the Earth’s crust is made up of silicate minerals, making silicon one of the most abundant elements, second only to oxygen. These minerals dominate the terrestrial planets because silicon and oxygen were the most common non-metallic elements in the supernova dust debris that originally formed our solar system some 4.6 billion years ago. Under extreme high-temperature conditions, they form network solids and incorporate reactive aluminium, calcium, sodium, potassium and magnesium. These solids eventually accreted into the inner planets we know today. 

Industrially, silicon is graded by its purity and application, as metallurgical grade (95% pure), solar grade (>99.9999% pure, called 6N) and electrical grade (99.9999999% pure, also known as 9N). Metallurgical grade silicon is produced when high-purity silica feedstock reacts with carbon in an electric arc furnace. The high temperatures (>1,900oC) facilitate the reduction of silica to pure silicon, which is drained and cooled to release carbon monoxide as a waste product. 

Purification methods of metallurgical grade silicon exploit the tendency for impurities to concentrate in molten silicon, so that most of the impurities end up in the part of the mass that is last to solidify. The zone refining method involves a thin strip heater that is slowly moved along a rod of metallurgical grade silicon, melting a small section as it advances down the length. The impurities prefer to stay in the molten section than re-solidify behind the heater. These impurities concentrate at the end of the rod, which is the last to be melted. This end is cut off and discarded, and the process can be repeated for further purification. 

A major application of metallurgical grade silicon is in aluminium alloys. In moderate quantities (<13% silicon), silicon and aluminium form a eutectic mixture that solidifies with very little thermal contraction. This convenient property makes silicon-aluminium alloys favoured in casting applications, common in the automobile industry, since tears and cracks that arise from thermal stresses during solidification are eliminated. The silicon also provides an improved hardness and wear-resistance to the product alloy. 

Metallurgical grade silicon is a major player in the steel industry for similar reasons.

Ferrosilicon, an iron-silicon alloy, is added to molten cast iron to improve casting, particularly for thin section components, and it also helps to prevent the formation of the embrittling cementite phase. The silicon acts as an oxygen sink, such that the carbon content of the alloy can be much more closely controlled. 

Applications

Aside from metallurgy, a large portion of metallurgical grade silicon is employed by the chemical industry to make fumed silica. Synthesised by flame pyrolysis, fumed silica comprises microscopic droplets of amorphous silica, which aggregate and fuse into branched 3D chains, that further agglomerate into tertiary particles. The resulting powder has very low density and high surface area, and exhibits useful thixotropic (time-dependent thinning) and viscosity increasing properties. These characteristics give fumed silica application as a thickener, anti-caking agent or viscosity adjuster in paints, coatings, printing inks and adhesives.  It is used in the cosmetics industry for its light-diffusing properties and as a mild abrasive in toothpaste. 

Solar grade silicon is used in photovoltaic cells, which harness energy from the sun to create electrical power and promise a future of carbon-neutral energy. The majority is made by the Siemens process, where chemical vapour deposition is used to grow high purity (9N–11N) polycrystalline silicon on the surface of silicon seed rods. The higher the purity of the silicon in a solar cell, the more efficient the device, but the more expensive the Siemens process becomes. Alternative processes, such as the fluidised bed reactor or modified FFC Cambridge process (see Material of the Month, Titanium, October 2014) promise to save 90% of the energy required to make solar grade silicon and drive down the price of commercial photovoltaic cell technologies. 

Electrical grade silicon must have purity above 9N for complete control over the quantum properties of the single crystal wafers used in integrated circuitry. This is achieved in the Czochralski process, whereby a single crystal is grown from a seed crystal, which is slowly rotated and withdrawn from a bath of molten silicon at 1,425oC. Dopants such as boron or phosphorus can be added to tailor the electronic properties of the resultant crystal. 

In integrated circuit boards, the silicon acts as a mechanical support, and doping the silicon in particular patterns creates semiconductor devices such as transistors and logic gates. Doping silicon with small concentrations of other elements, such as boron or phosphorus, adjusts its electrical response by controlling the number and charge of active carriers. The silicon can also be oxidised in thin layers to insulate the individual components and avoid interference. 

Aside from elemental silicon, the material also features in all sorts of materials that we rely on day to day. Most of these are well deserving of their own Material of the Month tribute, but here are just a few... 

Surrounding silicates

If you are reading this sitting inside a building, you will be surrounded by silicates and silica sand. Calcium silicates are used for making Portland cement, which, when combined with silica sand and gravel, makes concrete. While you may be gazing out of a window, you’re still looking at silica, as it forms a large component of common soda-lime glass. Since glass is both reflective and refractive, different geometries of cut glass can be used to make optical lenses and prisms, and can be pulled into long, thin fibres used in fibre optic telecommunications. Other uses of glass fibres include glass wool as a thermal insulator and fibreglass, adding strength to a thermosetting plastic in a structural composite. 

Many of the silicon-based ceramics have amassed a number of applications. For example, silicon carbide (SiC) in powder form is an abrasive, and SiC grains can be sintered into a high-strength, hard ceramic used in bullet-proof vests. Its properties also make this material ideal for reinforced carbon fibres that form a composite used in car brakes. Single crystal SiC is used in high-temperature, high-voltage semiconductor devices such as diodes, logic gates, switches and LEDs. Rarely found naturally, the mineral form of SiC, called moissanite, is similar to diamond in its transparency, refractive index and hardness, making it a cheaper synthetic alternative for jewellery. Finally, SiC could enable the mass-production of graphene, since epitaxial graphene results from heating SiC to high temperatures (1,100oC) under low pressures (10–9 bar). Silicon nitride (Si3N4) has applications as widespread as ball bearings in machinery that are 79% lighter than tungsten carbide equivalents, high-temperature components in hydrogen/oxygen rocket engines, abrasive cutting tools, an electrical insulator in electronics and as the sensitive cantilever component in atomic wforce microscopy. 

Silicones are polymers with alternating silicon and oxygen atoms along the backbone, with organic side groups attached to the silicon atoms. When cross-linked, these chains form polymeric resins and elastomers, and a large number of favourable and diverse properties sees silicone materials applied in a range of settings. These silicones exhibit low chemical reactivity, have high flexibility and are hydrophobic, making them ideal for watertight seals, particularly since they don’t support microbiological growth. The electrical conductivity of silicone can be tailored, and is used to insulate spark plug wires and prevent misfires in automobiles as well as electronics in hostile conditions, such as satellites. Silicone grease is a common lubricant for vehicle brakes and bicycle chains due to its high-temperature stability and insolubility in water. Low toxicity, flexibility, low thermal conductivity and thermal stability from -100oC up to 250oC make silicone an excellent material for cookware, kitchen utensils, and moulds for both baking and ice cube trays. 

Finally, silicone is highly biocompatible, lending its use in all sorts of medical applications such as contact lenses, dressings and implants. In 2012, a health scare concerning silicone breast implants rocked the cosmetic surgery industry across Europe and South America. French company Poly Implant Prothèse used industrial grade silicone in its implants, rather than the medical grade material regarded as the safe industry standard. The bogus implants had twice the rupture rate of others, causing silicone gel to leak into the bodies of the patients, forming additional scar tissue and causing irritation of the area, pain and inflammation. Jean-Claude Mas, founder of the company, is currently serving four years in jail for fraud. 

The ubiquitous use of elemental silicon in electronics unanimously renders it one of the most influential elements of the 21st Century. But on top of this, silicon combines with other elements in three major materials categories – metals, ceramics and polymers – that touch our daily lives in some unexpected ways. In these guises it features in brittle, flexible, abrasive, hydrophobic, insulating, semiconducting, strengthening, optical and photovoltaic materials. Silicon is the Swiss army knife of the periodic table.