Question

Explain why polymers have a higher coefficient of thermal expansion than ceramics in terms of atomic bonding.

How does particle gradation (i.e., size distribution) affect the properties of composites?

Answer 1

Answer-

1st part:

The coefficient of thermal expansion is a measure of the change in the dimensions of a matter due to change in temperature. The higher the coefficient of thermal expansion, more easily the matter melts with rising temperature and the lesser the coefficient of thermal expansion, more is the thermal resistance of the matter.

Now, in polymers, the atomic bonding is due to covalent bonds while in ceramics, both the covalent and the ionic bonds are present. This makes the bonding in ceramics stronger than that in polymers. Therefore, polymers have a higher coefficient of thermal expansion than ceramics (as a consequence of which we see polymers melting faster with increase in temperature).

2nd part:

Particle gradation/size distribution has a direct effect on the properties of composites. The main properties being effected are the conductivity and the flexural strength (bend strength) and the effects are as below:

1) The conductivity of composites containing one kind of the same size conductive fillers improves but the flexural strength of composites decreases as the filler size increases.

2) The composites containing two kinds of different size conductive fillers have higher conductivity and flexural strength than that of the composites containing only one kind of the large size conductive filler(>90micrometre).

3) The flexural strength of conductive composites containing two kinds of different size conductive fillers is lower than that of composites containing only one kind of small size conductive fillers(<45micrometre).