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Sand
Sand generally refers to the coarse-textured (less than 2-millimeter) mineral fraction of soil. Sand’s main job in a green storm water or "bio swale" soil mix is to provide high infiltration rates and resist compaction. Sand has a low surface area and hence a low “caption exchange capacity” (CEC), meaning it does very little to filter contaminants such as metal pollutants and plant nutrients. The large pore space found in sandy soil is also poor at holding water. This makes for good drainage, but can also make for sad-looking, nutrient- and water-starved plants in the garden. It’s best to select a coarse, gravelly sand for your soil mixture so that water can infiltrate at the desired rate (approximately one inch per hour). Washed, concrete sand is often recommended as it is coarse, clean, and a recycled product available locally in most areas. While sand may make up the majority of these mixes—frequently 60% or more by volume — compost is the real workhorse of any bio swale storm water treatment system.
Physical Properties of Soil
Soil Texture
The particles that make up soil are categorized into three groups by size – sand, silt, and clay. Sand particles are the largest and clay particles the smallest. Most soils are a combination of the three. The relative percentages of sand, silt, and clay are what give soil its texture. A clay loam texture soil, for example, has nearly equal parts of sand, slit, and clay. These textural separates result from the weathering process.
This is an image comparing the sizes of sand, silt, and clay together. Sand is the largest. Clay is the smallest.
There are 12 soil textural classes represented on the soil texture triangle. This triangle is used so that terms like “clay” or “loam” always have the same meaning. Each texture corresponds to specific percentages of sand, silt, or clay. Knowing the texture helps us manage the soil.
Soil Structure
Soil structure is the arrangement of soil particles into small clumps, called peds or aggregates. Soil particles (sand, silt, clay and even organic matter) bind together to form peds. Depending on the composition and on the conditions in which the peds formed (getting wet and drying out, or freezing and thawing, foot traffic, farming, etc.), the ped has a specific shape. They could be granular (like gardening soil), blocky, columnar, platy, massive (like modeling clay) or single-grained (like beach sand). Structure correlates to the pore space in the soil which influences root growth and air and water movement.
Soil Color
The color of soil is measured by its hue (actual color), value (how light and dark it is), and chroma (intensity).
Soil color is influenced primarily by soil mineralogy – telling us what is in a specific soil. Soils high in iron are deep orange-brown to yellowish-brown. Those soils that are high in organic matter are dark brown or black. Color can also tell us how a soil “behaves” – a soil that drains well is brightly colored and one that is often wet and soggy will have a mottled pattern of grays, reds, and yellows.
Soil texture is a classification instrument used both in the field and laboratory to determine soil classes based on their physical texture. Soil texture can be determined using qualitative methods such as texture by feel, and quantitative methods such as the hydrometer method. Soil texture has agricultural applications such as determining crop suitability and to predict the response of the soil to environmental and management conditions such as drought or calcium (lime) requirements. Soil texture focuses on the particles that are less than two millimeters in diameter which include sand, silt, and clay. The USDA soil taxonomy and WRB soil classification systems use 12 textural classes whereas the UK-ADAS system uses 11. These classifications are based on the percentages of sand, silt, and clay in the soil.
Soil texture classification
Soil texture triangle, showing the 12 major textural classes, and particle size scales as defined by the USDA.
In the United States, twelve major soil texture classifications are defined by the USDA. The twelve classifications are sand, loamy sand, sandy loam, loam, silt loam, silt, sandy clay loam, clay loam, silty clay loam, sandy clay, silty clay, and clay. Soil textures are classified by the fractions of each soil separate (sand, silt, and clay) present in a soil. Classifications are typically named for the primary constituent particle size or a combination of the most abundant particles sizes, e.g. "sandy clay" or "silty clay". A fourth term, loam, is used to describe equal properties of sand, silt, and clay in a soil sample, and lends to the naming of even more classifications, e.g. "clay loam" or "silt loam". Determining soil texture is often aided with the use of a soil texture triangle.[2] An example of a soil triangle is found on the right side of the page. One side of the triangle represents percent sand, the second side represents percent clay, and the third side represents percent silt. If you know the percentages of sand, clay, and silt in your soil sample, the triangle can be used to determine which of the twelve soil types you have. To do this, find your percentage of sand along the bottom of the triangle. Then follow the slanted line up to the left until you reach your percentage of clay. Where that point is will tell you what soil type you have. For example, if your soil is 70 percent sand and 10 percent clay then your soil is classified as a sandy loam. The same method can be used starting on any side of the soil triangle. If the texture by feel method was used to determine which type of soil you had, the triangle can also provide a rough estimate on the percentages of sand, silt, and clay in your soil. Chemical and physical properties of a soil are related to texture. Particle size and distribution will affect a soil's capacity for holding water and nutrients. Fine textured soils generally have a higher capacity for water retention, whereas sandy soils contain large pore spaces that allow leaching.
Soil separates
Particle size classifications used by different countries, diameters in μm
Soil separates are specific ranges of particle sizes. The smallest particles are clay particles and are classified as having diameters of less than 0.002 mm. Clay particles are plate-shaped instead of spherical, allowing for an increased specific surface area. The next smallest particles are silt particles and have diameters between 0.002 mm and 0.05 mm (in USDA soil taxonomy). The largest particles are sand particles and are larger than 0.05 mm in diameter. Furthermore, large sand particles can be described as coarse, intermediate as medium, and the smaller as fine. Other countries have their own particle size classifications.
Hand analysis is a simple and effective means to rapidly assess and classify a soil's physical condition. Correctly executed, the procedure allows for rapid and frequent assessment of soil characteristics with little or no equipment. It is thus an extremely useful tool for identifying spatial variation both within and between fields as well as identifying progressive changes and boundaries between soil map units (soil series). Texture by feel is a qualitative method, it does not provide exact values of sand, silt, and clay. Although qualitative, the texture by feel flowchart can be an accurate way for a scientist or interested individual to analyze the relative proportions of sand, silt, and clay. The texture by feel method involves taking a small sample of soil and making a ribbon. A ribbon can be made by taking a ball of soil and pushing the soil between your thumb and forefinger, squeezing it upward into a ribbon. Allow the ribbon to emerge and extend over the forefinger, breaking from its own weight. Measuring the length of the ribbon can help determine the amount of clay in the sample. After making a ribbon, excessively wet a small pinch of soil in the palm of your hand and rub in with your forefinger to determine the amount of sand in the sample. Soils that have a high percentage of sand, such as sandy loam, or sandy clay, have a gritty texture. Soils that have a high percentage of silt, such as silty loam or silty clay, feel smooth. Soils that have a high percentage of clay, such as clay loam, have a sticky feel. Although the texture by feel method takes practice, it is a useful way to determine soil texture, especially in the field.
Hydrometer Method
The hydrometer method of determining soil texture is a qualitative measurement providing estimates of the percent sand, clay, and silt in the soil. The hydrometer method was developed in 1927 and is still widely used today. This method requires a chemical compound, sodium hexa metaphsophate, which acts as a dispersing agent to separate soil aggregates. The soil is mixed with the sodium hexa meta phosphate solution on an orbital shaker overnight. The solution is transferred to one liter graduated cylinders and filled with water. The soil solution is mixed with a metal plunger to disperse the soil particles. The soil particles separate based on size and sink to the bottom. Sand particles are the largest (2.00 - 0.05 mm) and sink to the bottom of the cylinder first. Silt particles are the medium-sized (0.05 - 0.002 mm) and sink to the bottom of the cylinder after the sand. Clay particles are the smallest (<0.002 mm) and separate out above the silt layer. Measurements are taken using a soil hydrometer. A soil hydrometer measures that relative density of liquids (density of a liquid to the density of water). The hydrometer is lowered into the cylinder containing the soil mixture at different times, forty-five seconds to measure sand content, one and a half hours to measure silt content and between six and twenty-four hours (depending on the protocol used) to measure clay. The number on the hydrometer that is visible (above the soil solution) is recorded. A blank (containing only water and the dispersing agent) is used to calibrate the hydrometer. The values recorded from the readings are used to calculate the percent clay, silt and sand. The blank is subtracted from each of the three readings. The calculations are as follows: Percent silt = ( dried mass of soil - (sand hydrometer reading - blank reading) / (dried mass of soil) *100 Percent clay = (clay hydrometer reading - blank reading) / (dried mass of soil) *100 Percent sand = 100 - (percent clay + percent silt)
Additional Methods
There are several additional quantitative methods to determine soil texture. Some examples of these methods are the pipette method, the particulate organic matter (POM) method, and the rapid method
Flash Back of classification
The first classification, the International system, was first proposed by Albert Atterberg (1905), and was based on his studies in southern Sweden. Atterberg chose 20 μm for the upper limit of silt fraction because particles smaller than that size were not visible to the naked eye, the suspension could be coagulated by salts, capillary rise within 24 hours was most rapid in this fraction, and the pores between compacted particles were so small as to prevent the entry of root hairs .
XXX . XXX Soil moisture sensor
A simple soil moisture sensor for gardeners.
Soil moisture sensors measure the volumetric water content in soil. Since the direct gravimetric measurement of free soil moisture requires removing, drying, and weighting of a sample, soil moisture sensors measure the volumetric water content indirectly by using some other property of the soil, such as electrical resistance, dielectric constant, or interaction with neutrons, as a proxy for the moisture content. The relation between the measured property and soil moisture must be calibrated and may vary depending on environmental factors such as soil type, temperature, or electric conductivity. Reflected microwave radiation is affected by the soil moisture and is used for remote sensing in hydrology and agriculture. Portable probe instruments can be used by farmers or gardeners. Soil moisture sensors typically refer to sensors that estimate volumetric water content. Another class of sensors measure another property of moisture in soils called water potential; these sensors are usually referred to as soil water potential sensors and include tensiometers and gypsum blocks.
Technology
Technologies commonly used to indirectly measure volumetric water content (soil moisture) include)
Neutron moisture gauges: The moderator properties of water for neutrons are utilized to estimate soil moisture content between a source and detector probe.
Soil resistivity: Measuring how strongly the soil resists the flow of electricity between two electrodes can be used to determine the soil moisture content.
Galvanic cell: The amount of water present can be determined based on the voltage the soil produces because water acts as an electrolyte and produces electricity. The technology behind this concept is the galvanic cell.
Application
Agriculture
Measuring soil moisture is important for agricultural applications to help farmers manage their irrigation systems more efficiently. Knowing the exact soil moisture conditions on their fields, not only are farmers able to generally use less water to grow a crop, they are also able to increase yields and the quality of the crop by improved management of soil moisture during critical plant growth stages.
Landscape irrigation
In urban and suburban areas, landscapes and residential lawns are using soil moisture sensors to interface with an irrigation controller. Connecting a soil moisture sensor to a simple irrigation clock will convert it into a "smart" irrigation controller that prevents irrigation cycles when the soil is already wet, e.g. following a recent rainfall event. Golf courses are using soil moisture sensors to increase the efficiency of their irrigation systems to prevent over-watering and leaching of fertilizers and other chemicals into the ground.
Relatively cheap and simple devices that do not require a power source are available for checking whether plants have sufficient moisture to thrive. After inserting a probe into the soil for approximately 60 seconds, a meter indicates if the soil is too dry, moist or wet for plants.[
XXX . XXX 4% zero The World is Running Out of Sand
The little-known exploitation of this seemingly infinite resource could wreak political and environmental havoc
We hear a lot about the over-extraction of oil, but less about the consequences of the sand trade
hen people picture sand spread across idyllic beaches and endless deserts, they understandably think of it as an infinite resource. But as we discuss in a just-published perspective in the journal Science, over-exploitation of global supplies of sand is damaging the environment, endangering communities, causing shortages and promoting violent conflict. Skyrocketing demand, combined with unfettered mining to meet it, is creating the perfect recipe for shortages. Plentiful evidence strongly suggests that sand is becoming increasingly scarce in many regions. For example, in Vietnam domestic demand for sand exceeds the country’s total reserves. If this mismatch continues, the country may run out of construction sand by 2020, according to recent statements from the country’s Ministry of Construction. This problem is rarely mentioned in scientific discussions and has not been systemically studied. Media attention drew us to this issue. While scientists are making a great effort to quantify how infrastructure systems such as roads and buildings affect the habitats that surround them, the impacts of extracting construction minerals such as sand and gravel to build those structures have been overlooked. Two years ago we created a working group designed to provide an integrated perspective on global sand use. In our view, it is essential to understand what happens at the places where sand is mined, where it is used and many impacted points in between in order to craft workable policies. We are analyzing those questions through a systems integration approach that allows us to better understand socioeconomic and environmental interactions over distances and time. Based on what we have already learned, we believe it is time to develop international conventions to regulate sand mining, use and trade.
Moreover, we have found that these numbers grossly underestimate global sand extraction and use. According to government agencies, uneven record-keeping in many countries may hide real extraction rates. Official statistics widely underreport sand use and typically do not include non construction purposes such as hydraulic fracturing and beach nourishment.
Sand traditionally has been a local product. However, regional shortages and sand mining bans in some countries are turning it into a globalized commodity. Its international trade value has skyrocketed, increasing almost sixfold in the last 25 years.
Profits from sand mining frequently spur profiteerin .
Research shows that sand mining operations are affecting numerous animal species, including fish, dolphins, crustaceans and crocodiles. For example, the gharial (Gavialis gangeticus) – a critically endangered crocodile found in Asian river systems – is increasingly threatened by sand mining, which destroys or erodes sand banks where the animals bask.
Sand mining also has serious impacts on people’s livelihoods. Beaches and wetlands buffer coastal communities against surging seas. Increased erosion resulting from extensive mining makes these communities more vulnerable to floods and storm surges.
sand mining is reducing sediment supplies as drastically as dam construction, threatening the sustainability of the delta. It also is probably enhancing saltwater intrusion during the dry season, which threatens local communities’ water and food security.
Potential health impacts from sand mining are poorly characterized but deserve further study. Extraction activities create new standing pools of water that can become breeding sites for malaria-carrying mosquitoes.
Natural raw materials and materials are becoming more scarce
Raw materials are materials that are used to make or manufacture something. We differentiate between natural raw materials, semi manufactured products and synthetic materials. Natural raw materials are materials from nature such as fertile soil, oil, natural gas, ores, wood and other crops. Ores are rocks which contain usable minerals, such as lead, iron, copper, zinc, tin and precious metals. We get natural stone from quarries. Natural raw materials have become more scarce in the past few decades. Therefore, it is advised to become less dependent on these raw materials for your manufacturing and to focus more on renewable raw materials. A natural raw material which is less subject to scarcity is sand, which is often used in the construction sector but also as a raw material for making artistic and industrial glassware.
Metalworking: from semimanufactured products to finished products
Second only to oxygen, silicon is the most abundant element in Earth's crust. It is found in rocks, sand, clays and soils, combined with either oxygen as silicon dioxide, or with oxygen and other elements as silicates. Silicon's compounds are also found in water, in the atmosphere, in many plants, and even in certain animals. Silicon is the fourteenth element of the periodic table and is a Group IVA element, along with carbon germanium, tin and lead. Pure silicon is a dark gray solid with the same crystalline structure as diamond. Its chemical and physical properties are similar to this material. Silicon has a melting point of 2570° F (1410° C), a boiling point of 4271° F (2355° C), and a density of 2.33 g/cm3. When silicon is heated it reacts with the halogens (fluorine, chlorine, bromine, and iodine) to form halides. It reacts with certain metals to form silicides and when heated in an electric furnace with carbon, a wear resistant ceramic called silicon carbide is produced. Hydrofluoric acid is the only acid that affects silicon. At higher temperatures, silicon is attacked by water vapor or by oxygen to form a surface layer of silicon dioxide. When silicon is purified and doped with such elements as boron, phosphorus and arsenic, it is used as a semiconductor in various applications. For maximum purity, a chemical process is used that reduces silicon tetrachloride or trichlorosilane to silicon. Single crystals are grown by slowly drawing seed crystals from molten silicon. Silicon of lower purity is used in metallurgy as a reducing agent and as an alloying element in steel, brass, alumiinum, and bronze. When small amounts of silicon are added to aluminum, aluminum becomes easier to cast and also has improved strength, hardness, and other properties. In its oxide or silicate form, silicon is used to make concrete, bricks, glass, ceramics, and soap. Silicon metal is also the base material for making silicones used in such products as synthetic oils, caulks and sealers, and anti-foaming agents. In 1999, world production was around 640,000 metric tons (excluding China), with Brazil, France, Norway and the United States major producers. This is a continued decline compared to the last several years (653,000 tons in 1998 and 664,000 in 1997). Though data is not available, China is believed to be the largest producer, followed by the United States. One estimate puts China's production capacity as high as 400,000 metric tons per year, with over 400 producers. Exports from this country have increased in recent years. Consumption of silicon metal in the United States was roughly 262,000 metric tons, at a cost of 57 cents per pound. The annual growth rate during 1980-1995 was about 3.5% for silicon demand by the aluminum industry and about 8% by the chemical industry. Demand by the chemical industry (mainly silicones) was affected by the Asian economic crisis of the late 1990s.
Flash Back
Silicon was first isolated and described as an element in 1824 by a Swedish chemist, Jons Jacob Berzelius. An impure form was obtained in 1811. Crystalline silicon was first produced in 1854 using electrolysis.
The reaction between silica and carbon within an electric arc furnace produces silicon.
The type of furnace now used to make silicon, the electric arc furnace, was first invented in 1899 by French inventor Paul Louis Toussaint Heroult to make steel. The first electric arc furnace in the United States was installed in Syracuse, New York in 1905. In recent years, furnace technology, including the electrodes used for heating elements, has improved.
Raw Materials
Silicon metal is made from the reaction of silica (silicon dioxide, SiO2) and carbon materials like coke, coal and wood chips. Silica is typically received in the form of metallurgical grade gravel. This gravel is 99.5% silica, and is 3 x 1 or 6 x 1 in (8 x 3 cm or 15 x 3 cm) in size. The coal is usually of low ash content (1-3% to minimize calcium, aluminum, and iron impurities), contains around 60% carbon, and is sized to match that of the gravel. Wood chips are usually hardwood of 1/2 x 1/8 inch size (1 x. 3 cm size). All materials are received as specified by the manufacturer.
The Manufacturing Process
The basic process heats silica and coke in a submerged electric arc furnace to high temperatures. High temperatures are required to produce a reaction where the oxygen is removed, leaving behind silicon. This is known as a reduction process. In this process, metal carbides usually form first at the lower temperatures. As silicon is formed, it displaces the carbon. Refining processes are used to improve purity.
The Reduction Process
1 The raw materials are weighed and then placed into the furnace through the top using the fume hood, buckets, or cars. A typical batch contains 1000 lb (453 kg) each of gravel and chips, and 550 lb (250 kg) of coal. The lid of the furnace, which contains electrodes, is placed into position. Electric current is passed through the electrodes to form an arc. The heat generated by this arc (a temperature of 4000° F or 2350 ° C) melts the material and results in the reaction of sand with carbon to form silicon and carbon monoxide. This process takes about six to eight hours. The furnace is continuously charged with the batches of raw materials.
2 While the metal is in the molten state, it is treated with oxygen and air to reduce the amount of calcium and aluminum impurities. Depending on the grade, silicon metal contains 98.5-99.99% silicon with trace amounts of iron, calcium and aluminum.
Cooling/Crushing
3 Oxidized material, called slag, is poured off into pots and cooled. The silicon metal is cooled in large cast iron trays about 8 ft (2.4 m) across and 8 in (20 cm) deep. After cooling, the metal is dumped from the mold into a truck, weighed and then dumped in the storage pile. Dumping the metal from the mold to the truck breaks it up sufficiently for storage. Before shipping, the metal is sized according to customer specifications, which may require a crushing process using jaw or cone crushers.
Packaging
4 Silicon metal is usually packaged in large sacks or wooden boxes weighing up to 3,000 lb (1,361 kg). In powder form, silicon is packaged in 50-lb (23-kg) plastic pails or paper bags, 500-lb (227-kg) steel drums or 3,000-lb (1,361-kg) large sacks or boxes.
Quality Control
Statistical process control is used to ensure quality. Computer-controlled systems are used to manage the overall process and evaluate statistical data. The two major process parameters that must be controlled are amounts of raw materials used and furnace temperatures. Laboratory testing is used to monitor the chemical composition of the final product and to research methods to improve the composition by adjusting the manufacturing process. Quality audits and regular assessments of suppliers also ensure that quality is maintained from extraction of raw materials through shipping of the final product.
Byproducts/Waste
With statistical process control, waste is kept to a minimum. A byproduct of the process, silica fume, is sold to the refractory and cement industries to improve strength of their products. Silica fume also is used for heat insulation, filler for rubber, polymers, grouts and other applications. The cooled slag is broken down into smaller pieces and sold to other companies for further processing. Some companies crush it into sandblasting material. Because electric arc furnaces emit particulate emissions, manufacturers must also comply with the Environmental Protection Agency's (EPA) regulations.
The Future
Though industry analysts predicted demand for chemical-grade silicon by Western countries would increase at an annual average rate of about 7% until 2003, this growth may be slower due to recent economic declines in Asia and Japan. If supplies continue to outpace demand, prices may continue to drop. The outlook for the automotive market is positive, as more car makers switch to an aluminum-silicon alloy for various components. Other methods for making silicon are being investigated, including supercooling liquid to form bulk amorphous silicon and a hydrothermal method for making porous silicon powder for optical applications. Pure silicon has a negative temperature coefficient of resistance, since the number of free charge carriers increases with temperature. The electrical resistance of single crystal silicon significantly changes under the application of mechanical stress due to the piezoresistive effect.
Chemical
Silicon is a semiconductor, readily either donating or sharing its four outer electrons, allowing for many different forms of chemical bonding. Even though it is, similar to carbon, a relatively inert element, silicon still reacts with halogens and dilute alkalis, but most acids (except for some hyper-reactive combinations of nitric acid and hydrofluoric acid) have no known effect on it. However, having four bonding electrons gives it, like carbon, many opportunities to combine with other elements or compounds under the right circumstances.
Isotopes
isotopes of silicon
Naturally occurring silicon is composed of three stable isotopes, silicon-28, silicon-29, and silicon-30, with silicon-28 being the most abundant (92% natural abundance). Out of these, only silicon-29 is of use in NMR and EPR spectroscopy. Twenty radioisotopes have been characterized, with the most stable being silicon-32 with a half-life of 170 years, and silicon-31 with a half-life of 157.3 minutes. All of the remaining radioactive isotopes have half-lives that are less than seven seconds, and the majority of these have half lives that are less than one tenth of a second. Silicon does not have any known nuclear isomers. The isotopes of silicon range in mass number from 22 to 44. The most common decay mode of six isotopes with mass numbers lower than the most abundant stable isotope, silicon-28, is β+, primarily forming aluminium isotopes (13 protons) as decay products. The most common decay mode(s) for 16 isotopes with mass numbers higher than silicon-28 is β−, primarily forming phosphorus isotopes (15 protons) as decay products.
Occurrence
Measured by mass, silicon makes up 27.7% of the Earth's crust and is the second most abundant element in the crust, with only oxygen having a greater abundance. Silicon is usually found in the form of complex silicate minerals, and less often as silicon dioxide (silica, a major component of common sand). Pure silicon crystals are very rarely found in nature. The silicate minerals—various minerals containing silicon, oxygen and one or another metal—account for 90% of the mass of the Earth's crust. This is due to the fact that at the high temperatures characteristic of formation of the inner solar system, silicon and oxygen have a great affinity for each other, forming networks of silicon and oxygen, in chemical compounds of very low volatility. Since oxygen and silicon were the most common non-gaseus and non-metalic elements in the debris from supernova dust which formed the protoplanetary disk in the formation and evolution of the Solar System, they formed many complex silicates which accreted into larger rocky planetesimals that formed the terrestrial planets. Here, the reduced silicate mineral matrix entrapped the metals reactive enough to be oxidized (aluminum, calcium, sodium, potassium and magnesium). After loss of volatile gases, as well as carbon and sulfur via reaction with hydrogen, this silicate mixture of elements formed most of the Earth's crust. There silicates were of relatively low density with respect to iron, nickel, and other metals non-reactive to oxygen and thus a residuum of uncombined iron and nickel sank to the planet's core, leaving a thick mantle consisting mostly of magnesium and iron silicates above. Examples of silicate minerals in the crust include those in the pyroxene, amphibole, mica, and feldspar groups. These minerals occur in clay and various types of rock such as granite and sandstone. Silica occurs in minerals consisting of very pure silicon dioxide in different crystalline forms, quartz, agate amethyst, rock crystal, chalcedony, flint, jasper, and opal. The crystals have the empirical formula of silicon dioxide, but do not consist of separate silicon dioxide molecules in the manner of solid carbon dioxide. Rather, silica is structurally a network-solid consisting of silicon and oxygen in three-dimensional crystals, like diamond. Less pure silica forms the natural glass obsidian. Biogenic silica occurs in the structure of diatoms, radiolaria and siliceous sponges. Silicon is also a principal component of many meteorites, and also is a component of tektites, a silicate mineral of possibly lunar origin, or (if Earth-derived) which has been subjected to unusual temperatures and pressures, possibly from meteorite strike.
Production of free silicon
Alloys
Ferrosilicon, an iron-silicon alloy that contains varying ratios of elemental silicon and iron, accounts for about 80% of the world's production of elemental silicon, with China, the leading supplier of elemental silicon, providing 4.6 million tonnes (or 2/3 of the world output) of silicon, most of which is in the form of ferrosilicon. It is followed by Russia (610,000 t), Norway (330,000 t), Brazil (240,000 t) and the United States (170,000 t). Ferrosilicon is primarily used by the steel industry (see below). Aluminum-silicon alloys are heavily used in the aluminum alloy casting industry, where silicon is the single most important additive to aluminum to improve its casting properties. Since cast aluminum is widely used in the automobile industry, this use of silicon is thus the single largest industrial use of "metallurgical grade" pure silicon (as this purified silicon is added to pure aluminum, whereas ferrosilicon is never purified before being added to steel).
Metallurgical grade
Elemental silicon not alloyed with significant quantities of other elements, and usually > 95% is often referred to loosely as silicon metal. It makes up about 20% of the world total elemental silicon production, with less than 1 to 2% of total elemental silicon (5–10% of metallurgical grade silicon) ever purified to higher grades for use in electronics. Metallurgical grade silicon is commercially prepared by the reaction of high-purity silica with wood, charcoal, and coal in an electric arc furnace using carbon electrodes. At temperatures over 1,900 °C (3,450 °F), the carbon in the aforementioned materials and the silicon undergo the chemical reaction SiO2 + 2 C → Si + 2 CO. Liquid silicon collects in the bottom of the furnace, which is then drained and cooled. The silicon produced via this process is called metallurgical grade silicon and is at least 98% pure. Using this method, silicon carbide (SiC) may also form from an excess of carbon in one or both of the following ways: SiO2 + C → SiO + CO or SiO + 2 C → SiC + CO. However, provided the concentration of SiO2 is kept high, the silicon carbide can be eliminated by the chemical reaction 2 SiC + SiO2 → 3 Si + 2 CO. As noted above, metallurgical grade silicon "metal" has its primary use in the aluminum casting industry to make aluminum-silicon alloy parts. The remainder (about 45%) is used by the chemical industry, where it is primarily employed to make fumed silica. As of September 2008, metallurgical grade silicon costs about US$1.45 per pound ($3.20/kg), up from $0.77 per pound ($1.70/kg) in 2005.
Electronic grade
The use of silicon in semiconductor devices demands a much greater purity than afforded by metallurgical grade silicon. Very pure silicon (>99.9%) can be extracted directly from solid silica or other silicon compounds by molten salt electrolysis. This method, known as early as 1854 (see also FFC Cambridge process), has the potential to directly produce solar-grade silicon without any carbon dioxide emission at much lower energy consumption. Solar grade silicon cannot be used for semiconductors, where purity must be extreme in order to properly control the process. Bulk silicon wafers used at the beginning of the integrated circuit making process must first be refined to "nine nines" purity (99.9999999%), a process which requires repeated applications of refining technology. The majority of silicon crystals grown for device production are produced by the Czochralski process, (CZ-Si) since it is the cheapest method available and it is capable of producing large size crystals. However, single crystals grown by the Czochralski process contain impurities because the crucible containing the melt often dissolves. Historically, a number of methods have been used to produce ultra-high-purity silicon. Early silicon purification techniques were based on the fact that if silicon is melted and re-solidified, the last parts of the mass to solidify contain most of the impurities. The earliest method of silicon purification, first described in 1919 and used on a limited basis to make radar components during World War II, involved crushing metallurgical grade silicon and then partially dissolving the silicon powder in an acid. When crushed, the silicon cracked so that the weaker impurity-rich regions were on the outside of the resulting grains of silicon. As a result, the impurity-rich silicon was the first to be dissolved when treated with acid, leaving behind a more pure product. In zone melting, also called zone refining, the first silicon purification method to be widely used industrially, rods of metallurgical grade silicon are heated to melt at one end. Then, the heater is slowly moved down the length of the rod, keeping a small length of the rod molten as the silicon cools and re-solidifies behind it. Since most impurities tend to remain in the molten region rather than re-solidify, when the process is complete, most of the impurities in the rod will have been moved into the end that was the last to be melted. This end is then cut off and discarded, and the process repeated if a still higher purity is desired.
At one time, DuPont produced ultra-pure silicon by reacting silicon tetrachloride with high-purity zinc vapors at 950 °C, producing silicon by SiCl4 + 2 Zn → Si + 2 ZnCl2. However, this technique was plagued with practical problems (such as the zinc chloride byproduct solidifying and clogging lines) and was eventually abandoned in favor of the Siemens process. In the Siemens process, high-purity silicon rods are exposed to trichlorosilane at 1150 °C. The trichlorosilane gas decomposes and deposits additional silicon onto the rods, enlarging them because 2 HSiCl3 → Si + 2 HCl + SiCl4. Silicon produced from this and similar processes is called polycrystalline silicon. Polycrystalline silicon typically has impurity levels of less than one part per billion. In 2006 REC announced construction of a plant based on fluidized bed (FB) technology using silane: 3 SiCl4 + Si + 2 H2 → 4 HSiCl3, 4 HSiCl3 → 3 SiCl4 + SiH4, SiH4 → Si + 2 H2. The advantage of fluid bed technology is that processes can be run continuously, yielding higher yields than Siemens Process, which is a batch process. Today, silicon is purified by converting it to a silicon compound that can be more easily purified by distillation than in its original state, and then converting that silicon compound back into pure silicon. Trichlorosilane is the silicon compound most commonly used as the intermediate, although silicon tetrachloride and silane are also used. When these gases are blown over silicon at high temperature, they decompose to high-purity silicon. In addition, there exists the Schumacher process, which utilizes tribromosilane in place of trichlorosilane and fluid bed technology. It requires lower deposition temperatures, lower capital costs to build facilities and operate, no hazardous polymers nor explosive material, and no amorphous silicon dust waste, all of which are drawbacks of the Siemens Process. However, there are yet to be any major factories built on this process.
Compounds
Silicon forms binary compounds called silicides with many metallic elements whose properties range from reactive compounds, e.g. magnesium silicide, Mg2Si through high melting refractory compounds such as molybdenum disilicide, MoSi2.Unknown extension tag "ref"
Silicon carbide, SiC (carborundum) is a hard, high melting solid and a well known abrasive. It may also be sintered into a type of high-strength ceramic used in armor.
Silane, SiH4, is a pyrophoric gas with a similar tetrahedral structure to methane, CH4. When pure, it does not react with pure water or dilute acids; however, even small amounts of alkali impurities from the laboratory glass can result in a rapid hydrolysis.Unknown extension tag "ref" There is a range of catenated silicon hydrides that form a homologous series of compounds, SinH2n+2 where n = 2–8 (analogous to the alkanes). These are all readily hydrolyzed and are thermally unstable, particularly the heavier members.Unknown extension tag "ref"
Disilenes contain a silicon-silicon double bond (analogous to the alkenes) and are generally highly reactive requiring large substituent groups to stabilize them. A disilyne with a silicon-silicon triple bond was first isolated in 2004; although as the compound is non-linear, the bonding is dissimilar to that in alkynes.
Tetrahalides, SiX4, are formed with all of the halogens.Unknown extension tag "ref" Silicon tetrachloride, for example, reacts with water, unlike its carbon analogue, carbon tetrachloride.Unknown extension tag "ref" Silicon dihalides are formed by the high temperature reaction of tetrahalides and silicon; with a structure analogous to a carbene they are reactive compounds. Silicon difluoride condenses to form a polymeric compound, (SiF2)n.
Silicon dioxide is a high melting solid with a number of different crystal forms; the most familiar of which is the mineral quartz. In quartz each silicon atom is surrounded by four oxygen atoms that bridge to other silicon atoms to form a three dimensional lattice.Unknown extension tag "ref" Silica is soluble in water at high temperatures forming a range of compounds called monosilicic acid, Si(OH)4.Unknown extension tag "ref"
Under the right conditions monosilicic acid readily polymerizes to form more complex silicic acids, ranging from the simplest condensate, disilicic acid (H6Si2O7) to linear, ribbon, layer and lattice structures which form the basis of the many different silicate minerals and are called polysilicic acids {Six(OH)4–2x}n.Unknown extension tag "ref"
With oxides of other elements the high temperature reaction of silicon dioxide can give a wide range of glasses with various properties.Unknown extension tag "ref" Examples include soda lime glass, borosilicate glass and lead crystal glass.
Silicon sulfide, SiS2 is a polymeric solid (unlike its carbon analogue the liquid CS2).Unknown extension tag "ref"
Silicon forms a nitride, Si3N4 which is a ceramic.Unknown extension tag "ref" Silatranes, a group of tricyclic compounds containing five-coordinate silicon, may have physiological properties.
Many transition metal complexes containing a metal-silicon bond are now known, which include complexes containing SiHnX3−n ligands, SiX3 ligands, and Si(OR)3 ligands.
Silicones are large group of polymeric compounds with an (Si-O-Si) backbone. An example is the silicone oil PDMS (polydimethylsiloxane). These polymers can be crosslinked to produce resins and elastomers.Unknown extension tag "ref"
Many organosilicon compounds are known which contain a silicon-carbon single bond. Many of these are based on a central tetrahedral silicon atom, and some are optically active when central chirality exists. Long chain polymers containing a silicon backbone are known, such as polydimethysilylene (SiMe2)n. Polycarbosilane, n with a backbone containing a repeating -Si-Si-C unit, is a precursor in the production of silicon carbide fibers.
Applications
Compounds
Most silicon is used industrially without being separated into the element, and indeed often with comparatively little processing from natural occurrence. Over 90% of the Earth's crust is composed of silicate minerals. Many of these have direct commercial uses, such as clays, silica sand and most kinds of building stone. Thus, the vast majority of uses for silicon are as structural compounds, either as the silicate minerals or silica (crude silicon dioxide). For example, silica is an important part of ceramic brick. Silicates are used in making Portland cement which is used in building mortar and stucco, but more importantly combined with silica sand, and gravel (usually containing silicate minerals like granite), to make the concrete that is the basis of most of the very largest industrial building projects of the modern world. Unknown extension tag "ref" Silicate minerals are also in whiteware ceramics, an important class of products usually containing various types of fired clay (natural aluminum silicate). An example is porcelain which is based on silicate mineral kaolinite. Ceramics include art objects, and also domestic, industrial and building products. Traditional quartz-based soda-lime glass also functions in many of the same roles. More modern silicon compounds also function as high-technology abrasives and new high-strength ceramics based upon (silicon carbide), and also in superalloys. Alternating silicon-oxygen chains with hydrogen attached to the remaining silicon bonds form the ubiquitous silicon-based polymeric materials known as silicones. These compounds containing silicon-oxygen and occasionally silicon-carbon bonds have the capability to act as bonding intermediates between glass and organic compounds, and to form polymers with useful properties such as impermeability to water, flexibility and resistance to chemical attack. Silicones are often used in waterproofing treatments, molding compounds, mold-release agents, mechanical seals, high temperature greases and waxes, and caulking compounds. Silicone is also sometimes used in breast implants, contact lenses, explosives and pyrotechnics. Silly Putty was originally made by adding boric acid to silicone oil.
Alloys
Elemental silicon is added to molten cast iron as ferrosilicon or silicocalcium alloys in order to improve performance in casting thin sections, and to prevent the formation of cementite where exposed to outside air. The presence of elemental silicon in molten iron acts as a sink for oxygen, so that the steel carbon content, which must be kept within narrow limits for each type of steel, can be more closely controlled. Ferrosilicon production and use is a monitor of the steel industry, and although this form of elemental silicon is impure, it accounts for 80% of the world's use of free silicon. The properties of silicon itself can be used to modify alloys. Silicon's importance in aluminum casting is that a significantly high amount (12%) of silicon in aluminum forms a eutectic mixture which solidifies with very little thermal contraction. This greatly reduces tearing and cracks formed from stress as casting alloys cool to solidity. Silicon also significantly improves the hardness and thus wear-resistance of aluminum. Silicon is an important constituent of electrical steel, modifying its resistivity and ferromagnetic properties. Metallurgical grade silicon is silicon of 95–99% purity. About 55% of the world consumption of metallurgical purity silicon goes for production of aluminum-silicon alloys for aluminum part casts, mainly for use in the automotive industry. The reason for the high silicon use in these alloys is noted above. Much of the rest of metallurgical-grade silicon is used by the chemical industry for production of the important industrial product fumed silica. The remainder is used in production of other fine chemicals such as silanes and some types of silicones.
Electronics
semiconductor device fabrication
Since most elemental silicon produced remains as ferrosilicon alloy, only a relatively small amount (20%) of the elemental silicon produced is refined to metallurgical grade purity (a total of 1.3–1.5 million metric tons/year). The fraction of silicon metal which is further refined to semiconductor purity is estimated at only 15% of the world production of metallurgical grade silicon. However, the economic importance of this small very high-purity fraction (especially the ~ 5% which is processed to monocrystalline silicon for use in integrated circuits) is disproportionately large. Pure monocrystalline silicon is used to produce silicon wafers used in the semiconductor industry, in electronics and in some high-cost and high-efficiency photovoltaic applications. In terms of charge conduction, pure silicon is an intrinsic semiconductor which means that unlike metals it conducts electron holes and electrons which may be released from atoms within the crystal by heat, and thus increase silicon's electrical conductance with higher temperatures. Pure silicon has too low a conductance to be used as a circuit element in electronics without being doped with small concentrations of certain other elements. This process greatly increases its conductivity and adjusts its electrical response by controlling the number and charge (positive or negative) of activated carriers. Such control is necessary for transistors, solar cells, semiconductor detectors and other semiconductor devices, which are used in the computer industry and other technical applications. For example, in silicon photonics, silicon can be used as a continuous wave Raman laser medium to produce coherent light, though it is ineffective as an everyday light source. In common integrated circuits, a wafer of monocrystalline silicon serves as a mechanical support for the circuits, which are created by doping, and insulated from each other by thin layers of silicon oxide, an insulator which is easily produced by exposing the element to oxygen under the proper conditions. Silicon has become the most popular material to build both high power semiconductors and integrated circuits, because of all the elements, silicon is the semiconductor which can withstand the highest powers and temperatures without becoming dysfunctional due to avalanche breakdown, a process in which an electron avalanche is created by a chain reaction process where heat produces free electrons and holes, which in turn produce more current which produces more heat. In addition, the insulating oxide of silicon is not soluble in water, which gives it an advantage over germanium (an element with similar properties which can also be used in semiconductor devices) in certain type of fabrication techniques. Monocrystalline silicon is expensive to produce, and is usually only justified in production of integrated circuits, where tiny crystal imperfections can interfere with tiny circuit paths. For other uses, other types of pure silicon which do not exist as single crystals may be employed. These include hydrogenated amorphous silicon and upgraded metallurgical-grade silicon (UMG-Si) which are used in the production of low-cost, large-area electronics in applications such as Liquid crystal displays, and of large-area, low-cost, thin-film solar cells. Such semiconductor grades of silicon which are either slightly less pure than those used in integrated circuits, or which are produced in polycrystalline rather than monocrystalline form, make up roughly similar amount of silicon as are produced for the monocrystalline silicon semiconductor industry, or 75,000 to 150,000 metric tons per year. However, production of such materials is growing more quickly than silicon for the integrated circuit market. By 2013 polycrystalline silicon production, used mostly in solar cells, is projected to reach 200,000 metric tons per year, while monocrystalline semiconductor silicon production (used in computer microchips) remains below 50,000 tons/year.
Biological role
Although silicon is readily available in the form of silicates, very few organisms have a use for it. Diatoms, radiolaria and siliceous sponges use biogenic silica as a structural material to construct skeletons. In more advanced plants, the silica phytoliths (opal phytoliths) are rigid microscopic bodies occurring in the cell; some plants, for example rice, need silicon for their growth. Although silicon was proposed to be an ultra trace nutrition its exact function in the biology of animals is still under discussion. Higher organisms are only known to use it in very limited occasions in the form of silicic acid and soluble silicates. Silicon is currently under consideration for elevation to the status of a "plant beneficial substance by the Association of American Plant Food Control Officials (AAPFCO)." Silicon has been shown in university and field studies to improve plant cell wall strength and structural integrity, improve drought and frost resistance, decrease lodging potential and boost the plant's natural pest and disease fighting systems. Silicon has also been shown to improve plant vigor and physiology by improving root mass and density, and increasing above ground plant biomass and crop yields. Hypothetical silicon-based lifeforms are the subject of silicon biochemistry, in analogy with carbon-based lifeforms. Silicon, being below carbon in the periodic table, is thought to have similar enough properties that would make silicon-based life possible, but much different from life as we know it.
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