Silicon

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2 samples of elemental silicon, image provided by woelen.

Silicon is a metalloid element with the atomic number 14 and the chemical symbol Si. It is an extremely common element, but it is difficult to get ahold of it. However, due to its numerous semiconductor applications, it is relatively easy to get silicon at extremely high purities, such as 99.9999%.

Properties

Physical properties

Silicon is a grayish metalloid with a slight blue tint. It is exceptionally light, to the point that it is sometimes described as feeling hollow. It is a semiconductor, so electricity will pass through it, albeit with great resistance. It crystallizes in the cubic crystal system, though amorphous forms exist. It is brittle and prone to chipping.

Chemical properties

Silicon is highly resistant to all acids, but dissolves readily in strong bases. Sodium hydroxide produces sodium silicate and hydrogen gas when reacted with silicon. A large part of silicon's chemical resistance comes from the formation of a passivation layer of silicon dioxide.

Availability

Silicon can be easily obtained from the dies found on many electronic devices, such as microprocessors and transistors (the late models, earlier and first generation usually had either germanium or germanium-silicon). Sometimes it can also be found on other devices, such as hard drive readers, and alone on electronic boards. Silicon dies are extremely fragile, and will shatter if you try to cut the, also being prone to chipping. A better way to separate the silicon die is to heat the device it's glued to, either with a flame or an air gun, which will cause it to fall off. CPU dies are generally covered in a multicolored protection layer, which requires removal, to increase the purity of the silicon. Another source are discarded silicon solar panels.

Larger quantities of very pure silicon can be bought from websites such as GalliumSource, 40$/30-35 g.

Preparation

An easy means of obtaining relatively pure elemental silicon in a home setting is by composing a thermite mixture of finely powdered silicon dioxide and aluminum, which can be initiated with a high temperature ignition source, often in the form of magnesium ribbon. Because this reaction is not particularly vigorous and therefore not self-sustaining, those that use this method of silicon extraction often add an additional component to the mixture to raise the temperature and prevent the reaction from stopping. For example, rather than a stoichiometric ratio of aluminum powder and silicon dioxide, a mixture of 9 parts silicon dioxide, 10 parts aluminum, and 12 parts sulfur by mass[1], can be used. Another method to sustain the reaction is through the addition of an oxidiser (e.g. sodium nitrate[2]), though this will likely lower the yield by removing available aluminum. The resultant slag mixture from the thermite reaction can be broken up to reveal pieces of elemental silicon, which can be cleaned by a short immersion in hydrochloric acid. Caution must be taken in this step, as any aluminum sulfide produced earlier will react to form hydrogen sulfide, a dangerous and very potent-smelling gas which has effects similar to that of cyanide.

Projects

  • Silicides
  • Tribromosilane
  • Silane synthesis (dangerous!)
  • Silicon alkoxides

Handling

Safety

Silicon is nontoxic, as are most of its compounds. Inhaled silicon and silicon dioxide may cause silicosis, however, if inhaled in large quantities.

Storage

No special storage is required. Silicon will slowly form an extremely thin protective oxide layer in open air, however this does not affect its purity significantly.

Disposal

Silicon is non-toxic to the environment. It will slowly oxidize to silicon dioxide in air.

References

  1. As used by mrhomescientist in this video: https://www.youtube.com/watch?v=73YmP_JSrlU
  2. As tested by No Tears Only Dreams Now.

Relevant Sciencemadness threads