Research in the uses of silicon

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THE HISTORY OF SILICON

Baron Jons Jakob Berzelius was born near Linkoping in Sweden.  He studied medicine at the University of Uppsala, but became interested in chemistry .  He became a professor and lecturer at Stockholm in 1807.  In 1808, he joined the Stockholm Academy of Sciences, and in 1815, started a 17 year career as a chemistry professor at the Carolin Medico-Chirurgical Institute, also in Stockholm.  He became the permanent secretary of the Stockholm Academy of Sciences in 1818.  Berzelius was the first person to isolate silicon in 1823, and thus he is credited with its discovery.  Due to his contributions to science, he was named Baron by Charles XIV John (Swedish and Norwegian king) in 1835.  Berzelius, an accomplished scientist, discovered cerium, selenium, and thorium, and was the first person able to isolate silicon, zirconium, and titanium.  He created the chemical notation system which is still used, and also came up with the theory of radicals.  However, the achievement Berzelius is most recognized for is his measurement of atomic weights.

    Berzelius was able to isolate/identify silicon in 1924.  He began the reaction by heating (in a container with silica) potassium metal chips.  The reaction below resulted:

SiF4 + 4K --> 4KF + Si

(silicon tetrafluoride + potassium --> potassium fluoride + silicon)

The Si and KF mixture must be separated chemically.  Potassium fluoride will react with water at room temperature, though silicon will not.

KF + H2O <--->HF + KOH

(potassium fluoride + water <---> hydrofluoric acid + potassium hydroxide)

After this reaction, Berzelius was able to wash away all byproducts, and the pieces remaining were silicon.1

BACKGROUND

The Group 14 ( or IV) consists of five elements, Silicon, Si is a nonmetallic chemical element in the carbon family of elements. Silicon is the second most abundant element in the earth's crust, surpassed only by oxygen. Silicon does not occur uncombined in nature.  Its has four electrons in their valence shell with the configuration of ns2np2

Silicon was first isolated and described as an element in 1824 by Jöns Jacob Berzelius, a Swedish chemist. 

Sand and almost all rocks contain silicon combined with oxygen, forming silica. When silicon combines with other elements, such as iron, aluminum or potassium, a silicate is formed. Compounds of silicon also occur in the atmosphere, natural waters, many plants and in the bodies of some animals. 2

Silicon is the basic material used to make computer , , silicon diodes and other electronic circuits and switching devices because its atomic structure makes the element an ideal . Silicon is commonly , or mixed, with other elements, such as boron, phosphorous and arsenic, to alter its conductive properties1

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In its elemental  form, silicon has a gray color and a metallic luster which increases with the size of the crystal. It is similar to glass in that it is rather strong, very brittle, and prone to chipping. Even though it is a relatively inert element, silicon still reacts with  and dilute , but most  (except for a combination of  and ) do not affect it. Pure silicon has a negative  coefficient of , 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 .2

Silicon monoxide only exists in the gas phase at temperature above 2000 0C. When cooled, it disproportionates spontaneously to silicon dioxide and silicon.

                     2SiO (g)  SiO2 (s) + Si (s) 

PHYSICAL PROPERTIES:

Color is iron-black, dark silver-gray to bluish brown.

Luster is metallic.

Transparency: Crystals are opaque.

Crystal System is isometric; 4/m bar 3 2/m 

Crystal Habits are limited to microscopic crystals and inclusions.

Cleavage is absent.

Fracture is conchoidal.

Hardness is 7. 

Specific Gravity is approxiamtely 2.3. 

Streak is black

Other Characteristics: 

Associated Minerals are limited to  in which silicon has been found as inclusions.

Notable Natural Occurrences include Nuevo Potosi, Cuba; Tolbachik, Kamchatka and , Russia.

Best Field Indicator: Found with computer circuits etched on the surface! 3

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CHEMICAL PROPERTIES

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Silicon in the Environment (Occurrence)

Silicon is rarely found in nature in its uncombined form. In fact it is amazing how rare native silicon is with 25.7% of the Earth's crust being silicon. Silicon, binds strongly with oxygen and is nearly always found as silicon dioxide, SiO2 (), or as a  (SiO4) Silicon has been found as a native mineral only in volcanic exhalations and as tiny inclusions in .

Of growing interest in rock shops, however, are laboratory-grown silicon boules. Most such specimens are end fragments or flawed discards from the integrated circuit industry. Silicon boules are grown (pulled) from a molten state from a seed crystal, in such a way as to produce a single large crystal which must be completely without crystal defects, or the entire boule must be discarded. Modern techniques can create a single crystal several feet long and up to 10 inches in diameter. These large crystals are sliced into very thin wafers, upon which complex integrated circuits can be etched. The unused parts of the boule are often saved, and used as paperweights or sometimes cut into  or other decorative items.

The word silicon (which is taken from the latin word for flint) can be confused with other terms. One of these terms was already mentioned:  (SiO4-4). Silicates are minerals whose primary cation is the SiO4-4 ion group. Another confusing term is silica. Silica is a term used by geologists for SiO2 or silicon dioxide in any form whether it is in the form of , or any of the  members, or as a segment of the chemistry of a silicate, or even as silicon dioxide dissolved in water. A geologist might use the phrase, "The magma was rather poor in silica." Indicating an SiO2 content that was lower than expected. Yet another term is silicone. Silicone is a synthetic polymer that is made of silicon, carbon and oxygen and has many medical and some industrial purposes. 6

Production

Silicon is commercially prepared by the reaction of high-purity  with wood, charcoal, and coal, in an  using  . At temperatures over 1900 °C, the carbon reduces the silica to silicon according to the

                                          SiO2 + C → Si + CO2 

Liquid silicon collects in the bottom of the furnace, and 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, can form. However, provided the amount of SiO2 is kept high, silicon carbide may be eliminated, as explained by this equation:

                                       2 SiC + SiO2 → 3 Si + 2 CO

In 2005, metallurgical grade silicon cost about  0.77 per pound ($1.70/kg) 7

Common Chemical Compounds

12 compounds found containing Silicon

  •  
  • Formula: (CH3)3-Si-[O-Si(CH3)2]n-O-Si-(CH3)3
  •  
  • Formula: CaSiO3
  •  
  • Formula: (C2H5)4SiO4
  •  
  • Formula: (CH3O)4Si
  •  
  • Formula: SiO2
  •  
  • Formula: Si
  •  
  • Formula: C8H10O2S
  •  
  • Formula: SiC
  •  
  • Formula: SiH4
  •  
  • Formula: Na2SiO3.9H2O
  •  
  • Formula: 3MgO-4SiO2-H2O
  •  
  • Formula: Mg3Si4O10(OH)2

Silicon (IV) Oxide (Silica)

Silica or silicon (IV) oxide is found in both crystalline and armophous form. The crystalline form of silica is quartz. It is hard, brittle, clear and colourless solid with a giant molecular structure. Sand is powered quartz. When pure it is white, but usually appears yellowish due to contamination by iron (III) oxide. In the solid state, each silicon atom is bonded covalently to two or four oxygen atoms in a tetrahedral manner, while each oxygen is bonded to two silicon atoms.

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The repeating unit in silicon (IV) oxide is tetrahedral ‘SiO4’ unit. The ration of silicon to oxygen in the giant structure is 1:2. Hence, the empirical formula is ‘SiO2.  The giant structure is responsible for the high melting point and hardness of silicon (IV) oxide. When heated to about 870 0C, quartz passes into another crystalline form called tridynamite. At about 1740 0C a further change occurs, into cristobalite. The relationships between the three forms are as shown below:

Quartz  tridynamite  cristobalite

Silicon (IV) oxide is completely molten at about 1710 0C. However, the melting point is not sharp; liquefaction ...

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