DoITPoMS

previous | next

Effect of temperature on the dielectric constant

For materials that possess permanent dipoles, there is a significant variation of the dielectric constant with temperature. This is due to the effect of heat on orientational polarisation.

Note: This animation requires Adobe Flash Player 8 and later, which can be downloaded here.

However, this does not mean that the dielectric constant will increase continually as temperature is lowered. There are several discontinuities in the dielectric constant as temperature changes. First of all, the dielectric constant will change suddenly at phase boundaries. This is because the structure changes in a phase change and, as we have seen above, the dielectric constant is strongly dependent on the structure. Whether κ will increase or decrease at a given phase change depends on the exact two phases involved.

There is also a sharp decrease in κ at a temperature some distance below the freezing point. Let us now examine the reason for this.

In a crystalline solid, there are only certain orientations permitted by the lattice. To switch between these different orientations, a molecule must overcome a certain energy barrier ΔE.

Energy barriers in a crystal lattice with no external electric field

 


When an electric field is applied, the potential energy of orientations aligned with the field is lowered while the energy of orientations aligned against the field is raised. This means that less energy is required to switch to orientations aligned with the field, and more energy required to switch to orientations aligned against the field.

Energy barriers in a crystal lattice with an external electric field

Therefore over time molecules will become aligned with the field. However, they must still overcome an energy barrier in order to do this. If a molecule possesses an energy less than the height of any energy barrier, it cannot cross the energy barrier therefore cannot change its orientation. Hence the orientational mode becomes “frozen out” and can no longer contribute to overall polarisation, leading to a drop in the dielectric constant.

These effects are summarised in the graph below.

Note: This animation requires Adobe Flash Player 8 and later, which can be downloaded here.



previous | next