Silica and alumina
Silica (SiO₂) is made of SiO₄ tetrahedra: a small silicon surrounded by 4 oxygen at the corners of a pyramid. Each oxygen is shared between two tetrahedra, so the ratio is 1 Si to 2 O. Silica has several crystal forms (polymorphs): quartz, tridymite and cristobalite. At about 573 °C quartz changes slightly from α to β form and its size jumps a little. This is why fired ceramics with quartz can crack if they are heated or cooled too fast near this temperature.
Alumina (Al₂O₃) in its stable form, corundum, has big oxygen atoms packed in layers and small Al³⁺ ions in two-thirds of the gaps between them. Its bonds are strong, so alumina is very hard (9 on Mohs scale), melts near 2050 °C, resists chemicals and does not conduct electricity. Rubies and sapphires are alumina with a trace of other elements. Uses: spark-plug insulators, cutting tools, bone implants, furnace parts.
Aluminium silicates and clay minerals
Aluminium silicates are compounds of Al, Si and O. Most clays are clay minerals, which are aluminium silicates in thin layers. The common one is kaolinite, Al₂Si₂O₅(OH)₄. Each layer has a silica sheet (tetrahedra) stuck to an alumina sheet (Al with O and OH). Layers are stacked like pages of a book.
Between the layers, water can enter. Water acts like a thin lubricant, so the layers slide over each other. That is why wet clay is plastic (can be shaped and keeps its shape). When it dries the water leaves, the layers lock, and the clay is hard.
On firing, kaolinite loses its water (about 450 to 600 °C), and at about 1000 to 1200 °C it changes into mullite (3Al₂O₃·2SiO₂) and silica. Mullite is made of long needle crystals. It makes porcelain and firebrick strong and heat-proof. Feldspars are other aluminium silicates, used as flux because they melt at lower temperature and glue the grains together.
Oxide materials
Oxide ceramics are made of a metal and oxygen. They are already "burnt", so they do not burn more in hot air.
- Magnesia (MgO): rock-salt structure, CN 6, melts at about 2850 °C. Used for furnace bricks and linings.
- Zirconia (ZrO₂): changes shape (monoclinic to tetragonal) near 1170 °C and cracks when it does. Adding a little yttria (Y₂O₃) keeps it stable ("stabilised zirconia"). Used in oxygen sensors, tough knives and dental crowns.
- Barium titanate (BaTiO₃): its charges can be pushed by an electric field. Used in capacitors in electronics.
- Titania (TiO₂): bright white pigment and in sunscreens.
Their strong ionic bonds give high melting points and hardness; their lack of free electrons makes them insulators (although some, like stabilised zirconia, let oxygen ions move at high temperature).
Non-oxide materials
Non-oxide ceramics contain carbon, nitrogen or boron and no oxygen. Their bonds are mostly covalent, which are very strong and point in fixed directions.
- Silicon carbide (SiC): diamond-like network. Very hard, conducts heat well, resists thermal shock. Used in abrasives, brake discs and furnace heaters.
- Silicon nitride (Si₃N₄): strong and tough at high temperature. Used in ball bearings and turbine parts.
- Aluminium nitride (AlN): conducts heat but not electricity. Used in electronics.
- Boron nitride (BN): soft hexagonal form is used as a high-temperature lubricant; hard cubic form is for cutting.
Non-oxides are hard to make: they must be fired in nitrogen or argon, otherwise oxygen in air would burn them. SiC forms a thin SiO₂ skin in air, which protects it.
Key formulas and definitions
- Silica: SiO₂ (SiO₄ tetrahedra, corners shared)
- Alumina: Al₂O₃ (corundum)
- Kaolinite: Al₂Si₂O₅(OH)₄
- Mullite: 3Al₂O₃·2SiO₂
- Non-oxides: SiC, Si₃N₄, AlN, BN
- Quartz α → β change at about 573 °C
Worked examples
1. Why is the formula of silica SiO₂ even though each Si has 4 oxygen neighbours?
Each oxygen is shared by two silicons. So each Si owns 4 × ½ = 2 oxygen. The formula is SiO₂.
2. Why can wet clay be moulded but dry clay cannot?
Water enters between the silica–alumina sheets and acts like a lubricant, so sheets slide and the clay keeps its shape when pressed. When dry, the sheets lock.
3. Why must silicon nitride be fired in nitrogen?
In air, oxygen would react with it at high temperature and spoil it. Firing in nitrogen prevents that.
4. Kaolinite changes to mullite on firing. Name two other things given off or formed.
Water vapour is given off (loss of OH as H₂O) at about 450 to 600 °C, and silica is formed along with mullite at about 1000 to 1200 °C.
Common mistakes
- Writing silica as SiO₄ instead of SiO₂. The SiO₄ is the building block, and the corners are shared.
- Thinking wet clay is soft because the clay particles dissolve. They stay solid; water only lets the sheets slide.
- Saying all ceramics are oxides. Silicon carbide, silicon nitride and boron nitride are non-oxide ceramics.
- Thinking alumina and corundum are different. Corundum is the stable crystal form of alumina.