Silicon For Solar
Posted July 8, 2008on:
Silicon is the second most common element in the Earth’s crust, comprising 25.7% of the Earth’s crust by weight. It was discovered in 1824 by the Swedish chemist Jons Jakob Berzelius. It is shiny, dark gray with a tint of blue. Silicon, atomic number of 14, is a semi-metallic or metalloid, because it has several of the metallic characteristics. Silicon is never found in its natural state, but rather in combination with oxygen as a silicate ion (SiO4) in silica-rich rocks such as obsidian, granite, diorite, and sandstone. Feldspar and quartz are the most significant silicate minerals. Silicon alloys with a variety of metals, including iron, aluminum, copper, nickel, manganese and ferrochromium.
The name silicon comes from the Latin word silicis which means flint.
Silica is processed into two intermediate products- silicon and ferrosilicon. Silicon is known in the ferroalloy and chemical industries as “silicon metal.” The ultra pure form of silicon (>99.99% Si) is distinguished from silicon metal by the term “semiconductor-grade silicon.” The terms “silicon metal” and “silicon” are used interchangeably.
Silicon is used in ceramics and in making glass. Ferrosilicon is crushed into a variety of forms and sold as bulk metal. Depending on its intended use, it can be mixed with aluminum and calcium. It is a very heavy alloy. When it comes into contact with moist air or water, an explosive chemical reaction occurs in which hydrogen is released. Consequently there are very strict laws about the shipping of ferrosilicon it must be kept perfectly clean and dry.
Silicon conducts electricity, but not as well as a metal such as copper or silver. This physical property makes silicon an important commodity in the computer manufacturing business.
Ferrosilicon accounts for 53% of the annual silicon consumption in the United States; pure silicon accounts for the remaining 47%.
Silica is in human connective tissues, bones, teeth, skin, eyes, glands and organs. It is a major constituent of collagen which helps keep our skin elastic, and it helps calcium in maintaining bone strength. Silica dust in mines has caused silicosis or a lung disease in miners. Wetting the area being mined and application of good ventilation has reduced the danger of lung disease. Some organisms like sponges and some plants use silicon to create structural support.
Silicon compounds are the most significant component of the Earth’s crust. Silicon is recovered from an abundant resource: sand. Most pure sand is quartz, silicon dioxide (SiO2). Since sand is plentiful, easy to mine and relatively easy to process, it is the primary ore source of silicon. Some silicon is also retrieved from two other silicate minerals, talc and mica. The metamorphic rock, quartzite, is another source (quartzite is metamorphosed sandstone). All combined, world resources of silicon are plentiful and will supply demand for many decades to come.
The United States has plentiful sand, quartzite, talc and mica resources. The majority of the silica produced in the U.S. is produced East of the Mississippi River and in the Northwest. The U.S. also imports silicon from Norway, Russia, Brazil, Canada, and from a number of other countries.
Most silicon chips are made of single crystal silicon, which has a very low resistance to electron flow. However, silicon can exist in different forms (just as carbon can exist in the form of diamond, graphite, soot and buckminsterfullerine). As well as single crystal silicon, other forms are Poly-silicon and Amorphous silicon.
Polysilicon (p-si) is short for Polycrystalline Silicon, which is a form of silicon composed of many crystals, as opposed to Amorphous Silicon (a-si), which is an unordered form with a random internal structure. Electrons have a hard time moving quickly in amorphous silicon so the transistors in active matrix displays have to be relatively large, blocking quite a lot of light, and are relatively slow to switch. Polysilicon would be more efficient so why, then, do most active matrix LCD panels use a-si?
Making an LCD requires the silicon to be deposited on a transparent material (the substrate) and depositing p-si proved to require too high a temperature (typically 650 deg plus) to make it a practical proposition, so attention switched to a-si which could be deposited at a much lower temperature (380 deg ) allowing glass to be used as the substrate.
The speed of p-si was a strong lure, however, and eventually P-si panels were produced using quartz glass, which, though very expensive, could stand the heat of the deposition process. Small panels using quartz are used in LCD projectors and some other small applications such as camcorders and digital cameras. A major advantage of getting poly-silicon onto the glass is that the driver chips can be produced in the same process, saving cost and space and improving reliability.
Various companies have developed methods of using lasers to create p-si transistors on glass. The process is that amorphous silicon is deposited on the glass at low temperature and the silicon is then heated with a very short pulse from a laser that avoids heating the glass excessively.
As new, cheaper methods, using still lower temperatures, are developed to produce p-si displays, a-si displays will be replaced by p-si units which require less power, are brighter, more responsive, have a higher resolution and require less external circuitry to make them operate.
Ferrosilicon alloys are used to improve the strength and quality of iron and steel products. Tools, for instance, are made of steel and ferrosilicon.
In addition to tool steels, an example of “alloy steels,” ferrosilicon is used in the manufacture of stainless steels, carbon steels, and other alloy steels (e.g., high-strength, low-alloy steels, electrical steels, and full-alloy steels).
An alloy steel refers to all finished steels other than stainless and carbon steels. Stainless steels are used when superior corrosion resistance, hygiene, aesthetic, and wear-resistance qualities are needed.
Carbon steels are used extensively in suspension bridges and other structural support material, and in automotive bodies, to name a few. Silicon is also added to aluminum to create a stronger alloy. The largest consumers of silicon metal are the aluminum and chemical industries.
Silicon is used in the aluminum industry to improve castability and weldability, not to add strength as noted in the text. Silicon-aluminum alloys tend to have relatively low strength and ductility, so other metals, especially magnesium and copper, are often added to improve strength.
In the chemicals industry, silicon metal is the starting point for the production of silianes, silicones, fumed silica, and semiconductor-grade silicon. Silanes are the used to make silicone resins, lubricants, anti-foaming agents, and water-repellent compounds. Silicones are used as lubricants, hydraulic fluids, electrical insulators, and moisture-proof treatments.
Semiconductor-grade silicon is used in the manufacture of silicon chips and solar cells. Fumed silica is used as a filler in the cement and refractory materials industries, as well as in heat insulation and filling material for synthetic rubbers, polymers and grouts.
Other silicon materials are used in the production of advanced ceramic materials, including silicon carbide, silicon nitride, and sialons. Silicon carbide is also used as an abrasive material, a refractory agent, and in steel manufacturing.
Substitutes and Alternative Sources
There are relatively few options to replace silicon in its applications. Germanium and gallium arsenide can be used as semiconductors in place of silicon. In some applications, a small number of metal alloys, such as silicomanganese and aluminum, can substitute for ferrosilicon.