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5 Silicon from Sand Next

5 Silicon from Sand Next time you step onto the beach, bend down, grab a handful of sand and admire the fact: By mass 47% of what you hold in your hand is the element silicon. The rest is simply oxygen. Remarkable! Silicon is the second most abundant element in the earth’s crust (27.7%) - only oxygen beats it - and can easily be extracted from white sand (SiO2) in a spectacular reaction in the school science laboratory. The Background Silicon Of all the elements, silicon is the closest chemically related to its neighbour carbon (the element of life) and can therefore form similar tetravalent covalent bonds. It is however much less reactive than carbon. Being a metalloid, its properties are intermediate between that of metals and non-metals. Silicon does not occur free in nature. It mainly occurs in minerals consisting of pure silicon dioxide (SiO2 , silica) in different crystalline forms (quartz, mica, opal) and as silicates. These minerals are found in clay, sand and various types of rock. Sand is basically tiny crystals of silica. Silicon is the principal component of most semiconductor devices (eg. solar panels), glass, cement and ceramics. It is often confused with the polymer substances known as silicones, which do contain silicon but are soft and rubber-like. Silicones have found uses as medical implants, bathroom sealants, silly putties, etc. When silicon is combined with carbon, it forms silicon carbide (SiC) or carborundum, the next hardest 63

substance after diamond. Furthermore, quartz crystals keep the time in our modern watches as its oscillations are extremely stable. Silicon is widely used in semiconductor processors as it forms n and p type transistors (MOSFETs) when bombarded with boron and phosphorous ions. The high-tech Silicon Valley region in California is aptly named after this element. Surely there would be no Microsoft ® , Apple ® or wealthy IT magnates without silicon. This once again confirms that Chemistry rules! Characteristics of Silicon In its crystalline form, silicon has a dark gray colour and a metallic lustre. It forms four covalent bonds with four other silicon atoms very similar to the hard diamond structure of carbon. It is not as hard due to the larger size of the Si atom which causes increased inter-atomic distances and thus weaker bonds. Even though it is a relatively inert element, silicon still reacts with halogens and dilute alkalis, but it is not affected by most acids (except conc. HCl). Production of silicon In order for us to get to pure silicon, the silicon has to be reduced from silica - the oxygen has to be removed. This is accomplished by heating a mixture of silica and carbon in the form of coke in an electric arc furnace using carbon electrodes. At temperatures over 2,000°C (3,600!), the carbon reduces the silica to silicon: SiO2 + C ! Si + CO2 Liquid silicon settles 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 up to 99% pure. This is not nearly pure enough for semiconductor manufacture so it is further refined with gaseous hydrochloric acid, fractionally distilled and reacted with hydrogen at 1,100". A complex crystal transformation process follows and the silicon rod known as a ‘boule’ is sliced up into wafers typically 0.775 mm thick. The electronic properties of the wafer is then modified through exposure to ion beams, UV light, hot gases and chemicals. A high-tech, complex “Silicon Valley” type of activity. The Reduction of Metals from their Oxides Thermite reactions In Thermite reactions metal oxides react with aluminium to produce the molten metal. It requires substantial activation energy to get going. 64