ceramics heading

Chemistry only

Clay Ceramics

Sand or silica is mainly silicon dioxide, SiO2. It has a structure very similar to that of diamond, the structure of silica or sand is shown below. You should notice that the structure is very ordered with all the atoms arranged in a very regular way, we say it has a crystalline structure. Silica has a giant covalent structure with lots of strong bonds. It has a very high melting point due to its strong bonding and giant structure. It is an electrical insulator due to the fact that all the electrons are held tightly in covalent bonds and cannot move. It is very hard, again due to its strong bonding and giant structure.

silica or sand structure

potter using clay on a wheel A group of compounds called silicates are related to silica. Silicates like silica have structures based on the one shown above. However in silicates other atoms and ions maybe present as well as silicon and oxygen. Aluminosilicates are silicates where some of the silicon atoms are replaced by aluminium. Perhaps the best known aluminosilicate is clay. Clays can absorb lots of water and this allows parts of the structure to slide and move, this why it is possible to mould and shape clay when it is wet. However when the clay is heated or fired in a hot oven or kiln the water is forced out of the structure and a giant covalent structure is formed. Firing clays is used to make objects such as pottery, china and bricks. These materials made from firing clay at high temperatures are called clay ceramics.
Ceramic materials are all around us, bricks, glass and cement are ceramics that probably make up the building you are sat in, you may be having a drink from a ceramic cup or a glass tumbler while using your computer or laptop which has ceramic semi-conductors and insulators inside it. When most people think of ceramic they probably think of sinks, toilets, ceramic wall and floor tiles, roof tiles and pottery, while these are indeed ceramics, modern ceramics are very different from these rather old-fashioned idea of what a ceramic is. The image below shows some everyday ceramic materials, mostly these ceramics are made from clay and sand and are what we call clay ceramic materials.

ceramic montage

These traditional clay ceramics have giant structures, they maybe crystalline (made up of atoms or ions arranged in a very ordered regular way - Silica or sand is crystalline because its structure is very ordered) or non-crystalline (atoms or ions in the structure are arranged in a random or disordered fashion). Glass has a non-crystalline structure, sometimes these disordered structures are said to be amorphous. In a liquid the atoms move freely and are arranged in a disordere or amorphous manner. Imagine taking a liquid and instantly freezing it so that the atoms do not have time to arrange themselves into an ordered solid structure - you would end up with a solid but the atoms in it are arranged in a random way - glasses have structures like this. Glasses are often called super-cooled liquids!. It is this random arrangement of atoms in a glass which make it transparent.

Properties of ceramics

The properties of ceramics include: This list includes some very desirable properties however many ceramic have some less than desirable properties: Despite many of the very desirable properties that ceramics have the main problem is the fact that they are brittle. This limits many of their uses. Despite many advances in manufacturing techniques for ceramics, manufacturing ceramics which are free of defects in their structures, mainly micro-cracks and voids or holes which makes them brittle have been difficult and ultimately this limits their uses. If it was possible to manufacture ceramic materials free of these defects then we would have a new wonder material! The good news is that many of the problems with ceramics can be partly solved by using them in composite materials.

Common clay ceramic

Bricks are a ceramic. They are made by firing clay in an oven. Bricks are very strong in compression, imagine the weight pushing down on a single brick at the bottom of a tall wall or chimney. If the ceramic brick was replaced by a metal brick, then the metal brick would be crushed. Bricks also have high resistance to wear and tear. However bricks are brittle, drop one onto a hard surface and its likely to break. This is the main problem with ceramics. They are brittle.

space shuttle uses a ceramic material as a heat shield Similarly it is possible to place four tea cups under the wheels of a lorry or bus and lower the bus or lorry down onto the cups and they will not crush. However drop the cups and they will break, pottery like bricks are strong in compression but brittle. The heat shield on NASA's space shuttle is made of a ceramic material similar to a dinner plate. This "dinner plate" can withstand the heat produced when the space shuttle re-enters the Earth, but again they are very brittle and must be constantly checked to ensure they are no small cracks which could spread and lead to failure of the heat shield. When the space shuttle Columbia exploded on February 1, 2003 killing all astronauts on board, the accident was blamed on damaged ceramic tiles on its wing.

Glass

If silica or sand is heated up to around 16500C it melts to form a tacky, thick viscous liquid which can be moulded and shaped. If it is cooled quickly it will form glass. Silica is a crystalline solid with a giant structure containing many strong silicon-oxygen bonds, its structure is very similar to that of diamond. Once it begins to melt the ordered crystalline structure begins to break down and some of the silicon-oxygen bonds have been broken. If this tacky liquid is allowed to cool it will slowly reform its ordered crystalline structure, however if it is cooled quickly this ordered crystalline structure will not have time to form, instead solid glass will form. Its structure will be made up of a random arrangement of atoms, it's called a non-crystalline or amorphous structur. The structure resembles that of a liquid where the atoms are arranged randomly.

glass blowing

To lower the melting point of the silica, calcium carbonate and sodium carbonate are added to the silica. These two substances decompose as the temperature rises to give:

calcium carbonate(s) → calcium oxide (or lime)(s) + carbon dioxide(g)
CaCO3(s) → CaO(s) + CO2(g)
sodium carbonate (or soda)(s) → sodium oxide(s) + carbon dioxide(g)
Na2CO3(s) → Na2O(s) + CO2(g)
The glass produced here is called soda-lime glass and it is the most widely used form of glass, it is used to make bottles and windows. Unfortunately soda-lime glass expands a lot when heated and this often causes it to crack and break. However if boron trioxide is added to the silica this produces a glass with a higher melting point and a glass that expands very little when heated. This means it is unlikely to crack and break with changes in temperature. This glass is called borosilicate glass and it is used for oven ware, tea and coffee makers and for lab apparatus. You probably have some of this glass in your kitchen - it's sold under the name of Pyrex glass.

Key points

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