When assessing the influence of the environment on piezo actuators two effects must be considered:
· Linear Thermal Expansion
· Temperature Dependency of the Piezo Effect
Linear Thermal Expansion
Thermal stability of piezoceramics is better than that of most other materials. Actuators and positioning systems consist of a combination of piezoceramics 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 lead zirconate titanate (PZT) ceramics is known to function down to almost zero kelvin, but the magnitude of the piezo coefficients is temperature dependent.
At liquid helium temperature piezo gain drops to approximately 10–20 % of its room temperature value.
Operating Temperatures for PI Piezoceramics
Piezoceramics must be poled to exhibit the piezo effect. A poled PZT ceramic may depole when heated above the maximum allowable operating temperature. The “rate” of depoling is related to the Curie temperature of the material. PI HVPZT actuators have a Curie temperature of 350°C and can be operated at up to 150°C. LVPZT actuators have a Curie temperature of 150°C and can be operated at up to 80°C. The new monolithic PICMA® actuators with their high Curie temperature of 320°C allow operating at temperatures of up to 150°C.
Closed-loop piezo positioning systems are less sensitive to temperature changes than open-loop systems. Optimum accuracy is achieved if the operating temperature is identical to the calibration temperature. If not otherwise specified, PI piezomechanics are calibrated at 22°C.
Piezo Operation in High Humidity
The polymer insulation materials used in piezoceramic actuators are sensitive to humidity. Water molecules diffuse through the polymer layer and can cause short circuiting of the piezoelectric layers. The insulation materials used in piezo actuators are sensitive to humidity. For higher humidity environments, PI offers special systems with waterproofed enclosed stacks, or integrated dry-air flushing mechanisms. A better solution are PICMA® actuators, which have ceramic-only insulation without any polymer covering and are thus less sensitive to water diffusion.
Piezo Operation in Inert Gas Atmospheres
Ceramic-insulated PICMA® actuators are also recommended for use in inert gases, such as helium. To reduce the danger of flashover with high-voltage piezos, the maximum operating voltage must be reduced. Semi-bipolar operation is recommended, because the average operating voltage can be kept very low.
Vacuum Operation of Piezo Actuators
All PI piezo actuators can be operated at pressures below 100 Pa (~1 torr). When piezo actuators are used in a vacuum, two factors must be considered:
· Dielectric stability
The dielectric breakdown voltage of a sample in a specific gas is a function of the pressure p times the electrode distance s. Air displays a high insulation capacity at atmospheric pressure and at very low pressures. The minimum breakdown voltage of ~300 V can be found at a ps-product of 1000 mm Pa (~10 mm torr).
That is why PICMA® actuators with a maximum operating voltage of 120 V can be used in any vacuum condition. However, the operation of HVPZT actuators with dielectric layer thicknesses of 0.2 – 1.0 mm and nominal voltages to 1000 V is not recommended in the pressure range of 100 – 50000 Pa (~1 – 500 torr).
Outgassing behavior varies from model to model depending on design. Ultra-high-vacuum options for minimum outgassing are available for many standard low-voltage and highvoltage piezo actuators. Best suited are PICMA® ceramics (see Fig. 39a), because they have no polymers and can withstand bakeout to 150°C.
All materials used in UHV-compatible piezo nanopositioners, including cables and connectors, are optimized for minimal outgassing rates. Materials lists are available on request.