Fundamentals ... (cont.)
The product You were Looking for was moved
Click Here for new Address
Two effects must be considered:
Linear Thermal Expansion
Thermal stability of PZT ceramics is better than that of most other materials (steel, aluminum, etc.). Fig 39 shows the behavior of several types of PZT ceramics used by PI. The curves only describe the behavior of the PZT ceramics. Actuators and positioning systems consist of a combination of PZT ceramics and other materials and their overall behavior differs accordingly.
Temperature Dependency of the Piezo Effect
Piezo translators work in a wide temperature range. The piezo effect in PZT ceramics is known to function down to almost zero kelvin. For several reasons the magnitude of the piezoelectric effect (piezo gain) is dependent on the temperature.
At liquid helium temperature piezo gain drops to approximately 20% of its room-temperature value. See Fig. 38, for temperature dependency.
PZT ceramics must be poled to exhibit the piezo effect. A poled PZT may depole when heated above the maximum allowable operating temperature. The "rate" of depoling is related to the Curie temperature of the material. PI HVPZTs have a Curie temperature of 250 °C and can be operated up to 150 °C (with the high-temperature option). LVPZTs have a Curie temperature of 150 °C and can be operated up to 80 °C. See "Options," see link in the "PZT Actuators" section, for temperature range modifications.
PZT Operation in Normal Atmospheres
The insulation materials used in standard piezo actuators are sensitive to humidity. These PZTs are not recommended in environments with high relative humidity (more than 60%). For higher humidity environments, PI offers special systems with enclosed stacks, or integrated dry-air flushing mechanisms.
PZT Operation in Inert Gas Atmospheres
Piezo actuators can be damaged if operated at maximum drive voltage in a helium or argon atmosphere. Low-voltage actuators are recommended for these conditions. To reduce the risk of dielectric breakdown, the PZTs should be operated at minimum possible voltage (HVPZTs: < 300 V, LVPZTs: < 80 V). Semi-bipolar operation helps to further reduce the electric field strength, while producing reasonable displacement.
Vacuum Operation of PZTs
All PI piezo actuators can be operated at pressures below 0.1 hPa (~0.1 torr). When piezo actuators are used in a vacuum, two factors must be considered:
I. Dielectric stability
I. The dielectric breakdown strength of a gas is a function of pressure. Air displays a high insulation capacity at atmospheric pressure and below 0.1 hPa (~0.1 torr). However, in the corona area range from 100 to 0.1 hPa (~100 to 0.1 torr), its insulation properties are greatly degraded. PZTs should not be operated in this range because an electric breakdown may occur.
II. Outgassing (of the insulation materials) may limit the use of PZTs in applications where contamination or virtual leaks are an issue. Outgassing behavior varies from model to model depending on design. High-vacuum options for minimum outgassing are available for several standard LVPZTs and HVPZTs (see "Options," see link ff., in the "PZT Actuators" section for details). UHV (ultra-high-vacuum) compatible PZT flexure positioners are available on request.
Fig. 39. Linear thermal expansion of several PZT materials.
Fig. 38. Temperature dependency of the piezo effect.