ABSTRACT
A material is known as polar if, in one way or another, it displays macroscopic electric polarization. All polar materials exhibit piezoelectricity: they develop non-zero electric polarization when strained and change shape when placed into an external electric field. Structures that have nonzero intrinsic polarization-also called spontaneous polarization-even when no strain is applied to them are known as pyroelectrics. The prefix pyro-from the Greek word for fire-which is used to indicate the permanent electric moment associated to these materials, arises from the fact that pyroelectricity was first discovered in certain crystals when they were warmed. However, it was soon established that these materials also had permanent dipole moments at lower temperatures as well, only they were “hidden” by the adsorption of polar ions on the faces of the samples. Once these ions were desorbed at high temperatures, the permanent moment would appear. Usually the existence of pyroelectricity is attributed to the reduced symmetry in the geometrical structure of certain crystalline materials. In practice, if we model a crystal as a superposition of microscopic dipoles, these dipoles will cancel each other in most cases due to the symmetry of the solid. However, if the crystal has reduced symmetry, the lower number of symmetry operations that are allowed could lead to an only partial cancellation of the internal dipolar fields, leaving the system electrically polar. Ferroelectrics are pyroelectrics in which the direction of spontaneous polarization can be changed (for example, rotated by 180°) by applying a mechanical deformation or an external electric field. All these phenomena find a great number of applications in modern technology, from sensors and actuators, to memory cells and spintronics devices. In general, pyro-and piezoelectric materials to be used in modern technological applications should display an excellent piezoelectric response, combined with high mechanical stability and low environmental impact. Existing materials, which can be broadly divided into the families of perovskite crystals, ceramics, and polymers, can only partially fulfill the aforementioned requirements. Lead zirconate titanate (PZT) ceramics materials, for example, are strong piezo-and pyroelectrics1,2 with spontaneous polarizations of 0.3 to 0.7 C/m2 (Coulomb/m2), but unfortunately they are also brittle, heavy, and toxic. On the other hand, polymers such as polyvinylidene fluoride (PVDF) are lightweight, flexible, and virtually inert, but display polar properties an order of magnitude weaker than those of PZT.3 In Table 21.1 we summarize the main characteristics of modern piezo-and pyroelectric materials, compared with the predicted polar properties of BN nanotubes, which we will discuss at length in this chapter.
