Material characteristics

Topics:

Hysteresis
Creep

Extension under load

Power dissipation

Operation under reverse bias

Linearity CMA/SCMA/CMB

Linearity CSA

Thermal properties and temperature coefficients
Very high electrical field material data

Hysteresis

All piezoelectric materials exhibit a mechanical hysteresis as the strain does not follow the same track upon charging and discharging. The hysteresis is expressed as the maximum strain divided by the maximum difference between the two tracks. The mechanical hysteresis (in voltage) depends on the type of ceramics and can vary from 4% to 20%.

 
Another approach to strongly reduce the hysteresis effects consists of driving the piezoelectric actuator by controlling the transferred charge on the electrodes. 

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Creep

Piezoelectric materials exhibit a creep effect i.e. the material continues to expand for some time upon charging. Correspondingly the material does not immediately return to the initial strain level upon discharging. The creep effect for different actuator materials is compared in the following figure, where the time for reaching 100% strain is shown.
 
Creep always occurs in the same direction as the dimensional change produced by the voltage step. Typical values range from 1% to 20% with time constant between 10 and 100 seconds. 
 

 
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Extension under load

Piezoelectric actuators withstand very high axial pressure due to their solid-state nature. The properties of the actuator can vary to some extent depending on pre-stress or load conditions. This depends on the type of piezoelectric ceramic used. Some materials show stroke enhancement on mechanical loading, whereas other types are rather insensitive to load variations.
 

Mechanical pre-stressing of stacked actuators is recommended for many applications and is usually applied in a kind of spring mechanism. Pre-stressing stacked actuators result in advantages such as: 

• Optimisation of strain
• Compensation for tensile stress to prevent damage of the piezoelectric ceramic (sensitive to tensile stress).
• Increase of actuator stability against impact of bending or other non-axial forces.
• Only by applying mechanical pre-stress can piezoelectric actuators be operated with high dynamics (high frequency operations, pulsed operations), where high tensile stress potentially results from acceleration forces. Without pre-stress/pre-load the large alternating forces destroy immediately the actuator. 

Please note: The design of proper preload mechanisms is important. The main principle is to obtain high pre-stress forces together with as low as possible