The Properties and Uses of Silicon Metal

Silicon metal is a grey and lustrous semi-conductive metal that is used to manufacture steel, solar cells, and microchips. Silicon is the second most abundant element in the earth's crust (behind only oxygen) and the eighth-most common element in the universe. Nearly 30 percent of the weight of the earth's crust can be attributed to silicon.

The element with atomic number 14 naturally occurs in silicate minerals, including silica, feldspar, and mica, which are major components of common rocks such as quartz and sandstone. A semi-metal (or metalloid), silicon possesses some properties of both metals and non-metals.

Like water - but unlike most metals - silicon contracts in its liquid state and expands as it solidifies. It has relatively high melting and boiling points, and when crystallized forms a diamond cubic crystal structure. Critical to silicon's role as a semiconductor and its use in electronics is the element's atomic structure, which includes four valence electrons that allow silicon to bond with other elements readily.

Properties

• Atomic Symbol: Si
• Atomic Number: 14
• Element Category: Metalloid
• Density: 2.329g/cm3
• Melting Point: 2577°F (1414°C)
• Boiling Point: 5909°F (3265°C)
• Moh’s Hardness: 7

History

The Swedish chemist Jons Jacob Berzerlius is credited with first isolating silicon in 1823. Berzerlius accomplished this by heating metallic potassium (which had only been isolated a decade earlier) in a crucible along with potassium fluorosilicate. The result was amorphous silicon.

Making crystalline silicon, however, required more time. An electrolytic sample of crystalline silicon would not be made for another three decades. The first commercialized use of silicon was in the form of ferrosilicon.

Following Henry Bessemer's modernization of the steelmaking industry in the mid 19th century, there was great interest in steel metallurgy and research in steelmaking techniques. By the time of the first industrial production of ferrosilicon in the 1880s, the importance of silicon in improving ductility in pig iron and deoxidizing steel was fairly well understood.

Early production of ferrosilicon was done in blast furnaces by reducing silicon-containing ores with charcoal, which resulted in silvery pig iron, a ferrosilicon with up to 20 percent silicon content.

The development of electric arc furnaces at the beginning of the 20th century allowed not only greater steel production, but also more ferrosilicon production. In 1903, a group specializing in making the ferroalloy (Compagnie Generate d'Electrochimie) began operations in Germany, France and Austria and, in 1907, the first commercial silicon plant in the US was founded.

Steelmaking was not the only application for silicon compounds commercialized before the end of the 19th century. To produce artificial diamonds in 1890, Edward Goodrich Acheson heated aluminum silicate with powdered coke and incidentally produced silicon carbide (SiC).

Three years later Acheson had patented his production method and founded Carborundum Company (carborundum being the common name for silicon carbide at the time) for the purpose of making and selling abrasive products.

By the early 20th century, silicon carbide's conductive properties had also been realized, and the compound was used as a detector in early ship radios. A patent for silicon crystal detectors was granted to GW Pickard in 1906.

In 1907, the first light emitting diode (LED) was created by applying voltage to a silicon carbide crystal. Through the 1930s silicon use grew with the development of new chemical products, including silanes and silicones. The growth of electronics over the past century has also been inextricably linked to silicon and its unique properties.

While the creation of the first transistors - the precursors to modern microchips - in the 1940s relied on germanium, it was not long before silicon supplanted its metalloid cousin as a more durable substrate semiconductor material. Bell Labs and Texas Instruments began commercially producing silicon-based transistors in 1954. 

The first silicon integrated circuits were made in the 1960s and, by the 1970s, silicon-containing processors were had been developed. Given that silicon-based semiconductor technology forms the backbone of modern electronics and computing, it should be no surprise that we refer to the hub of activity for this industry as 'Silicon Valley.'

(For a detailed look at the history and development of Silicon Valley and microchip technology, I highly recommend the American Experience documentary entitled Silicon Valley). Not long after unveiling the first transistors, Bell Labs' work with silicon led to a second major breakthrough in 1954: The first silicon photovoltaic (solar) cell.

Prior to this, the thought of harnessing energy from the sun to create power on earth was believed impossible by most. But just four years later, in 1958, the first satellite powered by silicon solar cells was orbiting the earth. 

By the 1970s, commercial applications for solar technologies had grown to terrestrial applications such as powering lighting on offshore oil-rigs and railroad crossings. Over the past two decades, the use of solar energy has grown exponentially. Today, silicon-based photovoltaic technologies account for about 90 percent of the global solar energy market.

Production

The majority of silicon refined each year - about 80 percent - is produced as ferrosilicon for use in iron and steelmaking. Ferrosilicon can contain anywhere between 15 and 90 percent silicon depending on the requirements of the smelter.

The alloy of iron and silicon is produced using a submerged electric arc furnace via reduction smelting. Silica-rich ore and a carbon source such as coking coal (metallurgical coal) are crushed and loaded into the furnace along with scrap iron.

At temperatures over 1900°C (3450°F), carbon reacts with the oxygen present in the ore, forming carbon monoxide gas. The remaining iron and silicon, meanwhile, then combine to make molten ferrosilicon, which can be collected by tapping the base of the furnace. Once cooled and hardened, the ferrosilicon can then be shipped and used directly in iron and steel manufacturing.

The same method, without the inclusion of iron, is used to produce metallurgical grade silicon that is greater than 99 percent pure. Metallurgical silicon is also used in steel smelting, as well as the manufacture of aluminum cast alloys and silane chemicals.

Metallurgical silicon is classified by the impurity levels of iron, aluminum, and calcium present in the alloy. For example, 553 silicon metal contains less than 0.5 percent of each iron and aluminum, and less than 0.3 percent calcium.

About 8 million metric tonnes of ferrosilicon is produced each year globally, with China accounting for about 70 percent of this total. Large producers include Erdos Metallurgy Group, Ningxia Rongsheng Ferroalloy, Group OM Materials, and Elkem.

An additional 2.6 million metric tonnes of metallurgical silicon - or about 20 percent of total refined silicon metal - is produced annually. China, again, accounts for about 80 percent of this output. A surprise to many is that solar and electronic grades of silicon account for just a small amount (less than two percent) of all refined silicon production. To upgrade to solar-grade silicon metal (polysilicon), the purity must increase to upwards of 99.9999% (6N) pure silicon. It is done via one of three methods, the most common being the Siemens process.

The Siemens Process involves chemical vapor deposition of a volatile gas known as trichlorosilane. At 1150°C (2102°F) trichlorosilane is blown over a high purity silicon seed mounted at the end of a rod. As it passes over, high purity silicon from the gas is deposited onto the seed.

Fluid bed reactor (FBR) and upgraded metallurgical grade (UMG) silicon technology are also used to enhance the metal to polysilicon suitable for the photovoltaic industry. Two hundred thirty thousand metric tonnes of polysilicon were produced in 2013. Leading producers include GCL Poly, Wacker-Chemie, and OCI.

Finally, to make electronics grade silicon suitable for the semiconductor industry and certain photovoltaic technologies, polysilicon must be converted to ultra-pure monocrystal silicon via the Czochralski process. To do this, the polysilicon is melted in a crucible at 1425°C (2597°F) in an inert atmosphere. A rod mounted seed crystal is then dipped in the molten metal and slowly rotated and removed, giving time for the silicon to grow on the seed material.

The resulting product is a rod (or boule) of single crystal silicon metal that can be as high as 99.999999999 (11N) percent pure. This rod can be doped with boron or phosphorous as required to tweak the quantum mechanical properties as required. The monocrystal rod can be shipped to clients as is, or sliced into wafers and polished or textured for specific users.

Applications

While roughly ten million metric tonnes of ferrosilicon and silicon metal are refined each year, the majority of silicon used commercially is actually in the form of silicon minerals, which are used in the manufacture of everything from cement, mortars, and ceramics, to glass and polymers.

Ferrosilicon, as noted, is the most commonly used form of metallic silicon. Since its first use around 150 years ago, ferrosilicon has remained an important deoxidizing agent in the production of carbon and stainless steel. Today, steel smelting remains the largest consumer of ferrosilicon.

Ferrosilicon has a number of uses beyond steelmaking, though. It is a pre-alloy in the production of magnesium ferrosilicon, a nodulizer used to produce ductile iron, as well as during the Pidgeon process for refining high purity magnesium. Ferrosilicon can also be used to make heat and corrosion resistant ferrous silicon alloys as well as silicon steel, which is used in the manufacture of electro-motors and transformer cores.

Metallurgical silicon can be used in steelmaking as well as an alloying agent in aluminum casting. Aluminum-silicon (Al-Si) car parts are lightweight and stronger than components cast from pure aluminum. Automotive parts such as engine blocks and tire rims are some of the most commonly cast aluminum silicon parts.

Nearly half of all metallurgical silicon is used by the chemical industry to make fumed silica (a thickening agent and desiccant), silanes (a coupling agent) and silicone (sealants, adhesives, and lubricants). Photovoltaic grade polysilicon is primarily used in the making of polysilicon solar cells. About five tons of polysilicon is needed to make one megawatt of solar modules.

Currently, polysilicon solar technology accounts for more than half of the solar energy produced globally, while monosilicon technology contributes approximately 35 percent. In total, 90 percent of the solar energy used by humans is collected by silicon-based technology.

Monocrystal silicon is also a critical semiconductor material found in modern electronics. As a substrate material used in the production of field-effect transistors (FETs), LEDs and integrated circuits, silicon can be found in virtually all computers, mobile phones, tablets, televisions, radios, and other modern communication devices. It is estimated that more than one-third of all electronic devices contain silicon-based semiconductor technology.

Finally, the hard alloy silicon carbide is used in a variety of electronic and non-electronic applications, including synthetic jewelry, high-temperature semiconductors, hard ceramics, cutting tools, brake discs, abrasives, bulletproof vests, and heating elements.

Sources:

A Brief History of Steel Alloying and Ferroalloy Production. 
URL: http://www.urm-company.com/images/docs/steel-alloying-history.pdf
Holappa, Lauri, and Seppo Louhenkilpi. 

On the Role of Ferroalloys in Steelmaking. June 9-13, 2013. The thirteenth International Ferroalloys Congress. URL: http://www.pyrometallurgy.co.za/InfaconXIII/1083-Holappa.pdf