The characteristic of
converting mechanical energy to electrical energy, and from electric
energy to mechanical energy is called piezoelectric. In other words,
piezoelectric materials will expand or contract when subjected to a
voltage, and generate a voltage when subjected to a pressure.
Generally, ceramics are comprised of fine crystals. Each crystal is
comprised of atoms with a positive or negative electrical charge. Most
ceramics have well balanced positive and negative electric charges.
However, some dielectric ceramics called "ferroelectrics" have
unbalanced positive and negative electric charges in the crystals, even
under natural conditions, resulting in biased electric charges
(spontaneous polarization). |
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| Immediately after being
subjected to firing, the ferroelectric ceramics will develop
spontaneous polarization with random polar axes. As a whole, the
ceramics seem to have well balanced positive and negative electric
charges. However, with the application of a high DC voltage, the polar
axes generated by spontaneous polarization are aligned in a uniform
direction, which cannot be cancelled even if the voltage is removed.
The process to align the polar axes of spontaneous polarization is
called the polarizing process. |
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If the polarizing process is
applied to ferroelectric ceramics, piezoelectric ceramics are produced.
When an external voltage is applied to piezoelectric ceramics, the
centers of the positive and negative electrical charges in the ceramics
are individually attracted or repelled by the external electric
charges, causing the ceramics to expand or contract.
On the other hand, applying a pressure to piezoelectric ceramics
generates positive and negative electric charges on oppsing face of the
piezoelectric ceramic. Conversely, if a tensile force to the same
material, the polarity of the electric charges will be reversed.
As described above, piezoelectric ceramics enable mutual conversion
between electric energy and mechanical energy by utilizing polarization
of the crystals. |
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