ABSTRACT
This chapter offers the features of the application of tensors to physical properties. It considers the heating of crystals, this process can give rise to expansions and sometimes contractions of different magnitudes in different directions. The expansions in three dimensions in a crystal constitute strain and the thermal expansion tensor is a second-rank tensor which relates the strain to the change in temperature. The thermal expansion tensor is essentially a strain tensor which arises from the internal forces consequent on a change of temperature. The chapter considers all pyroelectric materials which are also piezoelectric effect. The piezoelectric effect can be measured under differing conditions, notably under constant stress or under constant strain. Propagation of the acoustic wave produces strain in the crystal and it is the periodic nature of the strain field which sets up the diffraction effects.
