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Silicon

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Silicon
Silicon
General Info
Atomic Symbol Atomic symbol::Si
Atomic Number Atomic number::14
Atomic Weight Atomic weight::28.0855 g/mol
Chemical series Metalloids
Appearance
Sample of Silicon.jpg
Group, Period, Block 14,3,p
Electron configuration [Ne] 3s2, 3p2
Electrons per shell
Electron shell Silicon.png
CAS number CAS number::7440-21-3
Physical properties
Phase solid
Density [[Density::2.3290 g·cm−3 g/ml]]
Melting point Melting point::1687 K
Boiling point Boiling point::3538 K
Isotopes of Silicon
iso NA half-life DT DE (MeV) DP
28Si 92.23% 28Si is stable with 14 neutrons.
29Si 4.67% 29Si is stable with 15 neutrons.
30si 3.1% 30Si is stable with 16 neutrons.
32Si syn 170 y β- 13.020 32P
All properties are for STP unless otherwise stated.


Silicon is a chemical element with the atomic number 14, and the symbol Si. It is the second most abundant element on Earth comprising approximately 25.7% of the crust.[1] Due to its low reverse leakage current silicon is used as the main component when making semiconductor devices like glass, ceramics, and cement.

Properties

Silicon is a metalloid, meaning that it has a silver shine to it, and that it is a moderate conductor, and shares some other properties of non-metals. It is found in the third row, meaning that it has 3 shells, and it is in the fourth column. When silicon is in its crystalline form, it has a charcoal hue and a metallic luster. It is actually quite strong, but brittle and easy to chip, making it like glass. When it does chip it creates very sharp edges, also making it like glass.

Although it is a rather nonreactive element, it does react with dilute alkalies and halogens. For the most part, however, acids do not affect it even in the slightest. Pure silicon melts at 1410 °C and boils at 2355 °C. [2]

In nature, Silicon does not occur on its own. It is only found in combination with other elements. The most common compound that it is found in is silicon dioxide, which is a combination of oxygen and silicon. This compound is called silica, and is the main substance in sand, and is also a major ingredient in feldspar, pyroxene, and quartz. Lava is also mostly made of silica.

Occurrences

Silicon makes up around 27% of the earth, making it the second most abundant element, coming in second to oxygen. It is also predicted that silicon is the seventh most abundant element in the universe.[3]In nature, Silicon does not normally occur on its own. In fact, it almost never is found by itself, it is always found in at least a slight compound with other elements. 100% silicon is not found anywhere in nature. It is only found in combination with other elements.

The most common compound that it is found in is silicon dioxide, which is a combination of oxygen and silicon. This compound is called silica, and is the main substance in sand, and is also a major ingredient in feldspar, pyroxene, and quartz. Silica has 17 different crystalline forms. It is made when oxygen and silicon are exposed to each other. A extremely thin layer of 'native oxide' is made on the surface when air hits silicon under ambient conditions. At higher temperatures and alternate environments are both used in growing controlled layers of silicon dioxide on silicon. These minerals are often found in sand, clay, and other various types of rock. Lava is also mostly made of silica. Silica is also important in many cells that require it to retain rigidity in cells walls, such as horsetails or sponges.

Silicon can also be commercially produced. By combining coal, charcoal, and wood with high purity silica, and placing them in an electric arc furnace and using carbon electrodes. [4]

Another place silicon is commonly found is in meteorites, which is why scientists believe that it is one of the most abundant elements in the universe.

History

In the very beginning of the 1800's, silicon was not considered and element at all, it was instead considered a compound. In 1811 Thenard and Gay Lussac tried to obtain the amorphous silicon by placing silicon tetrafluoride in with warm potassium, but were not successful in obtaining pure silicon, so they thought that it was still a compound. [5]

Then in 1824, a man by the name of Jons Berzeius of Sweden was able to produce amorphous silicon using the same general idea and method used by Thenard and Gay Lussac 13 years earlier. Berzeius is generally credited with being the first person to retrieve pure silicon. [6]

An interesting side note is that because of the fact that silicon plays such an important role in semiconductors and high tech devices, there is a high-tech region in California called Silicon Valley, where a large amount of production and creation of technological devices goes on.

Uses

Because of Silicon unique properties, it is used in many different things. Because of its high melting points it is used in alloys and parts that are around high temperatures. It is also a great semiconductor which makes it used in a lot of electronic parts. Also because of its great abundance, it is very cheap and affordable, making it used for many different things.

The largest user of silicon is the automobile industry in producing different pieces for cars called "casts." This alone uses around 55% of the world's pure silicon consumption per year. Silicon is excellent for this because it is not completely rigid, allowing it to bend some with the car, and it doesn't overheat very easily. The second largest consumption of silicon is the production of silicone (polysiloxanes), which uses another 40% of the worlds silicon consumption. Silicone is used in many different things. They can be used in pastes, since silicon is very moldable. They can also be used in parts that are constantly in high heat, because of silicon's very high tolerance to heat. The remaining 5% of silicon consumption is used in a variety of different fields. One of the fields is semiconductors, in which silicon is mixed with a variety of different elements because of its low cost and semiconductability. Silicon can also be used to make extremely precise lasers. It is also used widely in LCDs and in solar panels. Silicon is a very important part of both concrete and brick, and is also used to create Portland cement. Silicon is also used in small amounts to resist rusting from seawater.

One of silicon's most important uses is in many electronic devices, such as computer ships, integrated circuits, solar cells, and transistors. It is used in all of these devices because of the fact that silicon is a good semiconductor. They are also used in these things because of the fact that silicon has a very high melting point, making it so that has the same properties even at very high temperatures. Silicon is also combined with other elements such as arsenic, gallium, and phosphorus and turned into compounds. These compounds are used in solar cells, transistors, and other electronic devices. Silicon is also commonly used in alloys such as brass, bronze, and copper. The reason for this is that these metals are commonly used as semiconductors. Since silicon is an excellent semi-conductor it is commonly used as a base element in alloys, and then depending on what metal is added, the alloy can become either more or less conductive as well as taking on many other characteristics. Silicon is one of the most valuable elements in the electronics industry.

Isotopes

Silicon has plenty of well know of isotopes, with their mass number being anywhere from 44 to 22. The most abundant isotope, 28Si, which consists of 92.23%, 29Si, which makes up 4.67%, and 30Si, making up 3.1%, are all stable. Silicon-32 is a isotope that is highly radioactive, and it is produced by argon decay. It has a half-life that has been estimated at approximately 132 years, making its entire life 264 years, and it decays by means of beta emission, which turns it into 32P, and then into 32S.

Isolation

Because of the fact that the use of silicon in semiconductors is so great, there are times where silicon is needed, but in a more purified state. Normal silicon found in nature is not pure enough to suit the needs of certain semiconductors, which is why isolation, or purification, is needed. There is not normally a need to make silicon in laboratories, due to the fact that it is in such abundance. It can be found almost anywhere, is in incredibly cheap to purchase. To make silicon chemists go through the treatment of silica, SiO2, with pure graphite in an electric furnace.

SiO2 + 2C → Si + 2CO

Using these conditions, silicon carbide, SiC, will be made. However, it is possible to make even purer forms of silicon. If you keep the amount of SiO2 high, the silicon carbide can be eliminated.

2SiC + SiO2 → 3Si + 2CO

An even purer form of silicon can be made still. Using the reaction of SiCl4 with hydrogen, using zone refining of the resultant silicon.

SiCl4 + 2H2 → Si + 4HCl

When not using chemical means to purify silicon, engineers during World War II figured out a way to physically purify silicon. They discovered that when silicon was crushed into a powder and mixed with acids, the silicon would begin to dissolve. When the silicon is dissolved, the purest forms of silicon, and diluted silicon begin to separate. At that point it is easy to take the pure silicon and throw out the diluted silicon. Another physical way to purify silicon is by first creating a bar of raw silicon. Then one end is super heated, until it is molten. The bar is then heated from one end to the other. Because pure silicon and diluted silicon separate under heat, when the process is finished the low-grade silicon is at one end, and the pure silicon is at the other. If and even higher-grade silicon is wanted, the process is done again.

Hazards of Silicon

The only real hazard that silicon poses to humans is the ability to cause a disease called silicosis. This disease is commonly found in miners and stone cutters. When silicon is being broken or ground, it releases siliceous dust that, when inhaled in large quantities, can give someone silicosis. Small particles of silica (glass) get inhaled and enter the lungs. Symptoms include shortness of breath, fatigue, severe coughing, loss of appetite, and chest pains. Each year there are over 250 cases of silicosis in the U.S. and unfortunately, there is no known cure for it. Luckily silicosis is easily preventable by following simple safety rules and wearing face masks.

Crystallization

Silicon single crystal made by Czochralski method

Nearly all silicon crystals grown for device production are made using the Czochralski method, since it is the most readily available method, and the cheapest. Silicon crystals grown by this method, however, contain certain impurities since the crucible dissolves. For some electronic devices the Czochralkski method of growing silicon crystals does not produce pure enough crystals, this is being especially true for items using high power applications. For these things, float-zone silicon, also known as FZ-Si, can be used in its place.

References