Materials by Michael F. Ashby & Hugh Shercliff & David Cebon

Author:Michael F. Ashby & Hugh Shercliff & David Cebon
Language: eng
Format: epub
ISBN: 9780080982816
Publisher: Elsevier Science
Published: 2014-02-01T16:00:00+00:00

Figure 14.20 A piezoelectric material has non-symmetrically distributed charge, giving it a natural dipole moment. The surface charge associated with this is neutralized by pick-up of ions, but if it is deformed, as at (b), the dipole moment changes, and the surfaces become charged. The inverse is also true: a field induces a change of shape, the basis of piezoelectric actuation, as at (c).

A strain, then, induces an electric field in a piezoelectric material. The inverse is also true: a field induces a strain. The field pulls the positive ions and pushes the negative ones, changing their spacing and so changing the shape of the crystal (Figure 14.20(c)). If a small strain produces a large field, then a large field will produce only a very small strain. But the strain is a linear function of field, allowing extremely precise, if small, displacements, used for positioning and actuation at the sub-micron scale.

Piezoelectric materials respond to a change in electric field faster than most materials respond to a stimulus of this or any other kind. Put them in a megahertz field and they respond with microsecond precision. That opens up many applications, some described later in this chapter. In particular, it opens up the world of ultrasonics—sound waves with frequencies starting at the upper limit of the human ear, 20 kHz, on up to 20,000 kHz and above.

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