Question

One aspect of material science and engineering is the investigation of relationships that exist between the structures and properties of materials. By structure, we mean how some internal component(s) of the material is arranged.

Ceramics possess certain unique properties that are not present in other types of materials due to its microstructure. Critically evaluate and express your opinion on what you think are the desirable characteristics of ceramics compared to other types of materials. Justify your arguments by giving examples of the applications of ceramics.

Answer 1

In metals, for example, atoms are relatively weakly bonded (which is why most metals are fairly soft); their electrons are shared between them in a kind of sea that can "wash" right through them, which is (simplistically speaking) why they conduct electricity and heat. A material like rubber, on the other hand, is made of long-chain molecules (polymers) that are very weakly attached to one another; that's why raw, white, latex rubber is so stretchy and why black, vulcanized rubber (like that used in car tires) is harder and stronger, because heat-and-sulfur treatment makes strong cross-links form between the polymer chains, holding them tightly together. All the electrons are locked up in bonds of various kinds (none are free to carry an electric current), and that's why rubber is generally a good insulator.

Ceramics are different again. Their atoms are ionically bonded (like sodium and chlorine in sodium chloride, common salt), which holds them firmly in place (making ceramics hard and strong) and locks up all their electrons (so, unlike in metals, there are no free electrons to carry heat or electricity). Metals can bend, stretch, and be drawn into wires because their rows of regularly packed atoms will slide past one another. But in a ceramic, there are no rows of atoms; the atoms are either locked in a regularly repeating three-dimensional crystal or randomly arranged to make what's called an amorphous solid (a solid without a neat and tidy, internal crystalline structure). If you whack a lump of metal with a hammer, the mechanical energy you supply is dissipated as layers of atoms jump past one another; in other words, the metal bends out of shape. If you whack a ceramic such as glass, there's nowhere for that energy to go—no way for the glass to deform and soak up the blow—so it shatters instead. This explains why ceramics are both hard and brittle.

THERMAL PROPERTIES:
On heating, ceramics expand this is generally known as thermal expansion. Thermal expansion is due to conduction of heat. From this, we know that ceramics conduct heat but withstand high temperature. Major thermal properties of ceramics are

• insulating properties
• semi-conducting properties
• thermal conductivity
• thermal expansion
• thermal shock resistance

Thermal conduction is varied due to the certain factors. These factors are internal porosity, grain boundaries and impurities. By controlling these factors we can increase or decrease the thermal conductivity. Conduction is due to the movement of electrons. If the grain boundaries and porosity is less then the conduction will be high. Aluminium nitride and silicon carbide have a high level of thermal conductivity and metals to have high thermal conductivity. Zircon blocks have low thermal conductivity hence it is used in kiln walls. Metals conduct.