Smart Technologies
Providing a general introduction to intelligent or smart materials, systems and machines, this non-mathematical guide highlights the broad range of applications that can benefit from the concept.
TABLE OF CONTENTS Smart Technologies
Chapter 6: Piezoelectric Materials
Tim King,
School of Mechanical Engineering, University of Leeds, Leeds LS2 9JT,
Sirindhorn International Institute of Technology, Thammasat University, Pathum Thani 12121,
School of Mechanical Engineering, University of Leeds, Leeds LS2 9JT,
UK.
Michele Pozzi, Sirindhorn International Institute of Technology, Thammasat University, Pathum Thani 12121,
Thailand.
6.1 Introduction to Piezoelectricity
The phenomenon of piezoelectricity has been known for more than a century. In fact, in 1880 Jacques and Pierre Curie reported to the Acad mie de Sciences about a new discovery: suitable compression of a hemihedral [*] crystal with oblique faces leads to the development of electrical charges on the surfaces. Such crystals were already known for the phenomenon of pyroelectricity [ ]: electrical charges develop on the surfaces normal to the hemihedral axis following heating or cooling. The Curie brothers realized that the underlying explanation was identical: the variation in temperature causes a dimensional change, which is ultimately responsible for the electrical charges.
6.1.1 Crystallography of Piezoelectricity
Piezoelectricity can be observed in some crystals which lack a centre of symmetry. If the elementary unit (or cell ) of the crystal lattice is such that the centre of gravity of its positive charges does not coincide with that of its negative charges it creates a permanent dipole [ ] the cell is electrically asymmetric as illustrated in Fig. 6.1. A macroscopic electrical polarization is observed if the dipoles are aligned throughout the crystal. Applying external mechanical stress will strain the dipoles which alters the polarization so that electrical charges appear on the surface of the crystal. This is termed the direct piezoelectric effect (Fig. 6.2(a)). Conversely, applying an external electric field...
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