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Basics Of Piezo Materials |
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16 Mar, 2015 |
Piezo Ceramic |
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The classification of piezo materials
Piezo materials are generally divided into soft and hard piezo materials as follows:
Soft piezo materials are used on condition that high coupling or high charge sensitivity, such as level sensor or other low-power transducer. These materials have characters of high relative dieletric constant, large Electro-mechanical coupling coefficient, large piezoelectric constant and low mechanical quality constant. But they are unsuitable for high power applications because of the working temperature beyond their bearable range.
Hard piezo materials are used when high power characteristics are required. The stability of property makes them ideal for high-power applications such as the high-voltage energy in ultrasonic cleaners, sonar devices, etc. The higher mechanical quality factor reduces mechanical loss and enables a lower operating temperature.
The constitution of piezo materials
Piezoelectricity is the property of nearly all materials that have a non-centrosymmetric crystal structure.
Some naturally occuring crystalline materials that possess these properties are quartz and tourmaline. Some
aritficially produced piezoelectric materials are Rochelle salt, ammonium
dihydrogen phosphate and lithium sulphate. Another class of materials possessing these
properties is polarized piezo ceramic materials. In contrast to the naturally occurring piezo-electric
crystals, piezo ceramic materials have a polycrystalline structure.
Piezoelectric effect of piezo materials
Piezoelectricity is based on the ability of
certain ceramics to generate an piezo charge by mechanically loaded with
pressure, the phenomenon is known as "direct piezoelectric effect”.
This unique characteristics is reversible, these piezo materials become deformed due to
electric field, which is generally called "
inverse piezoelectric effect".
The polarity of the charge depends on
the orientation of the piezoelectric material relative to
the direction of the pressure.
The polarizing process of piezo materials
After sintering process, the areas of a ceramic body which
consisting
of elementary cells of uniform
dipole direction, and shows an arbitrary
(statistical distribution) orientation, in other words,
the body of ceramic is isotropic and
has no piezoelectric properties.
So the piezoelectric properties can
be originated by polarizing. In this
process, the piezo material body is placed in a strong electric DC field which forces the
electric dipoles to become aligned in the
direction of field. They will maintain this
orientation even when the DC field is no
longer applied, which is called “remanent polarization”. After the polarizing process is finished, piezo materials are produced.
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