Description of Piezoelectric Ceramics

Basic Knowledge of Ceramic Resonators CERALOCK®
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).  
Polycrystall with random polar 

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.
polarization gif cartoon film

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.

causing the ceramics to expand or contract gif cartoon film

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.
mutual conversion between electric energy and mechanical energy gif cartoon film

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