From a chemical point of view, advanced ceramics – like traditional ceramics – consist of compounds between metallic and non-metallic elements: oxides such as aluminium oxide and zirconium oxide, nitrides such as silicon nitride and titanium nitride, and carbides such as silicon carbide. The advanced ceramics family can be described as stable and functional materials. Mechanically stable in the sense of stiff, strong and hard materials that can withstand high temperatures. And functional in that one material combines several properties, or ‘can do a trick’. For example, the combination of thermal conductivity and electrical insulation is not often found in metals, but it is in the advanced ceramic material aluminium nitride. And lead zirconate titanate can change shape when an electrical voltage is applied over it.
For ceramic oxides, it is common to abbreviate their names by putting the letter a at the end. Alumina is short for aluminium oxide, zirconia for zirconium oxide, yttria for yttrium oxide and magnesia for magnesium oxide.
Advanced ceramics are strong and hard because of the strong chemical bonds between the elements. These bonds can be ionic in nature, such as between positively and negatively charged ions that strongly attract each other, with magnesia as an example. On the other hand, these bonds can also be of a covalent nature, such as in silicon carbide, where the non-metals silicon and carbon share a common pair of electrons to form a strong bond. Most types of advanced ceramics have a mixture of ionic and covalent bonds.
These strong bonds are also the cause of the stiff (non-elastic) behaviour of advanced ceramics. For crystalline materials such as ceramics, but also for a material such as metal or glass, the stiffness is strongly linked to the binding energy between the atoms of the material. In the steady state, the atoms are firmly in position, and they vibrate somewhat around this equilibrium state. If you pull the material, the atoms move further apart, and if you release the material, the atoms return to their original position.
The brittle fracture behaviour of ceramics can also be explained by these strong bonds. Due to the strong, directed covalent or ionic atomic bonds, no plastic deformation can occur in the material as is the case with metals. This means that ceramics immediately break when the applied (tensile) force exceeds a critical limit. This limit value depends on the already existing defect size: cracks and other irregularities, which are mainly on the surface, cause a high stress concentration locally. These can be (micro) cracks that are already present in the material from the start, or that arise during use.
Remember: only when you apply tensile and bending loads, you pull a surface crack further apart, allowing it to grow into a larger crack. This does not happen if you subject the ceramic to a compressive load – in a sense, you could say that you are actually closing such a surface crack. Ceramics are therefore much more resistant to compressive forces than to tensile and bending forces.
The influence of temperature
It takes a lot of energy to break strong ionic or covalent bonds, so ceramics have a high melting point. For example, alumina melts at a temperature as high as 2054 °C, and titanium carbide even at 3067 °C. Materials with a high melting point generally have a low thermal expansion coefficient, which for advanced ceramics is on the order of 10-6 per degree Celsius.
In an oxygen-containing environment, silicon nitride and silicon carbide can be used up to 1600 °C. Above this temperature, these materials begin to oxidise. For ceramic oxides, the maximum operating temperature is a little higher. These materials are not sensitive to oxidation as they are already in oxide form. The phenomenon of creep – deformation due to low but continuous loading at high temperature – does begin to occur at these extremely high temperatures.
The combination of poor electrical conductivity and good thermal conductivity makes aluminium nitride (AlN) a curious but useful ceramic material. Diamond has the same properties, but aluminium nitride is more practical, for example in electronics.
The ability to transport heat from a hot to a cold environment is important directly underneath a processor in a computer that heats up during computation and needs to dissipate this heat. Isn’t it convenient that aluminium nitride is not only a good heat conductor, but also a poor electrical conductor! Substrates for electronics and heat sinks are typical applications of aluminium nitride in ICT.
Heat conduction in aluminium nitride occurs via lattice vibrations (phonons). This type of thermal conduction is favoured in materials that consist of light atoms, have strong covalent atomic bonds and a simple crystal structure. In short, conditions that favour the propagation of phonons. Of course, products made of this material must not contain any pores, other imperfections or impurities that hinder the transport of phonons, but careful ceramic processing steps can provide a solution here.
Aluminium nitride is a good heat conductor and silicon carbide is a little less effective. Zirconia has a low thermal conductivity and is therefore ideally suited in heat-insulating thermal barrier coatings for turbines.
By far the majority of ceramics are not electrically conductive. And that comes in handy when using, for example, porcelain or alumina as insulators in spark plugs or in the suspension of high-voltage cables. Electrical conduction requires free, flowing electrons, which are ‘stuck’ in the ionic and covalent bonds of the ceramics. (Other) electroceramics such as barium titanate and lead zirconate titanate (PZT) have special properties: these materials are not electrically conductive, but become polarised when an electric field is applied over them. Silicon carbide belongs to the semiconductor class; these are materials that are only electrically conductive when a certain critical amount of energy is added to them, for example in the form of heat. Only exotic ceramic materials such as the superconductor YBa2Cu3O7-x have an extremely high electrical conductivity under extreme conditions.