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 bonds and giant structure. It is an electrical insulator due to the fact that all the electrons are held tightly in covalent bonds and so cannot move. It ;like diamond is very hard, again due to its strong bonding and giant structure.
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.
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)
(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 disordered 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:
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 it is likely to break. This is the main problem with ceramics. They are brittle.
Similarly it is possible to place four finew china 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 or shatter. 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 the astronauts on board, the accident was blamed on damaged ceramic tiles on its wing.
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 as mentioned above is a crystalline solid with a giant covalent structure containing many strong silicon-oxygen bonds. Once it begins to melt the ordered crystalline structure begins to break down and some of the silicon-oxygen bonds break. 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 the atoms are arranged in a very disordered random fashion, as found in a liquid, but the atoms are unable to move freely as you might expect in a liquid. This arrangement of atoms is called a solid glass.
To lower the melting point of the silica used in making glass, calcium carbonate and sodium carbonate are added to the silica. These two substances decompose as the temperature inside the oven or kiln begins to rise. They decompose to give: