As the temperature of piezoelectrics is raised, the tetragonal crystalline structure will change to a cubic crystalline structure at a certain temperature (transformation point). This temperature is referred to as "Curie point" or "Curie temperature." When the tetragonal crystalline structure is changed to the cubic crystalline structure, the material will lose piezoelectricity. Therefore, once a piezoelectric material is exposed to a temperature higher than the Curie point, it does not recover piezoelectricity even if it is restored to room temperature (This state is called "dipole"). To recover the piezoelectricity, the material must be polarized with application of a high DC voltage.