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The direct piezoelectric effect was
discovered by the Curie brothers in 1880.
They showed that when a weight was placed on a quartz crystal, charges
appeared on the crystal surface; the magnitude of the charge was proportional
to the weight. In 1881, the converse
piezoelectric effect was illustrated; when a voltage was applied to the
crystal, the crystal deformed due to the lattice strains caused by the
effect. The strain reversed when the
voltage was reversed. The
piezoelectric effect can, thereby, provide a coupling between an electrical
circuit and the mechanical properties of the crystal. Under the proper conditions, a “good”
piezoelectric resonator can stabilize the frequency of an oscillator circuit.
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Of the 32 crystal classes, 20 exhibit the
piezoelectric effect (but only a few of these are useful). Piezoelectric crystals lack a center of
symmetry. When a force deforms the lattice,
the centers of gravity of the positive and negative charges in the crystal
can be separated so as to produce surface charges. The figure shows one example (from Kelvin’s
qualitative model) of the effect in quartz.
Each silicon atom is represented by a plus, and each oxygen atom by a
minus. When a strain is applied so as
to elongate the crystal along the Y-axis, there are net movements of negative
charges to the left and positive charges to the right (along the X-axis).
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When a crystal has a center of symmetry,
i.e., when the properties of the crystal are the same in both directions
along any line in the crystal, no piezoelectric effect can occur.
Electrostriction, however, exists in all dielectric solids. It is a deformation
quadratic in the applied electric field (whereas, piezoelectricity is a
linear effect; reversal of the electric field reverses the mechanical
deformation.) Biased electrostriction,
where small electric field variations are superimposed on a constant
component, is phenomenologically equivalent to linear piezoelectricity; this
artifice may be used with nonpiezoelectric crystals such as silicon, but the
coupling depends upon the bias, and is often small.
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R. A. Heising,
Quartz Crystals for Electrical Cicuits - Their Design and Manufacture, D. Van
Nostrand Co., New York, pp. 16-20, 1946.
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