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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
Ultra high vacuum compatibility
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%.
Material
Hysteresis (%)
NCE57
19
NCE59
13
NCE46
20
Please note:
If it is important to know the exact displacement of the actuator at a given
voltage, it is then recommended to use a sensor system, e.g. strain gauge
mounted on the actuator (static applications) or an integrated piezo-sensor
(dynamic applications) as a feedback system.
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
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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.
Creep for NCE51 (PCM51), NCE57 (S1), NCE59 (S2) and NCE46 (H1)
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Extension under load
Piezoelectric actuators withstand very high axial pressure due to their solidstate 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 stiffness of
the pre-load mechanism. A too-high stiffness of the pre-load will reduce the
stroke of the actuator.
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Power dissipation
Piezoelectric elements are essentially capacitors. At temperatures well below
the Curie temperature their internal resistance is in the order of 1010 Ohms.
Consequently, under static operation virtually no current is drawn nor power
consumed to maintain a state of activation. Power is only required to change
the voltage on the PZT element.
Whereas a perfect capacitor would dissipate no energy in charging and
discharging, piezoelectric ceramics dissipate energy in the form of heat
proportional to the dissipation factor (tan δ), the tangent of the loss angle for
the material. The mechanism is similar to that by which any elastic material
such as a rubber band becomes hot when stretched repeatedly. For comparison
between the materials, the dissipation factor is usually specified for low
electrical fields and at 1000Hz. Soft PZT materials have large dissipation factors
in the order of 2% to 4% and hard PZT materials have dissipation factors in the
order of 0.5%.
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The power dissipated by a piezoelectric element with a capacitance C, driven at
a voltage V and frequency f can be calculated from the equation:
P = 6,28 x f x C x tan(δ) x V 2
The resultant temperature rise will depend on factors such as the heat capacity
of the device and what means exist for transferring that heat to the
surroundings by convection, conduction and radiation. Parts with a large
volume/surface ratio are more subject to self-heating in dynamic conditions
than parts with a large surface/volume ratio.
With soft PZT materials, the capacitance may increase rapidly with temperature
due to increase in the dielectric constant approaching the Curie temperature.
Consequently caution is necessary when running at high frequency to avoid
thermal runaway by self-heating that might damage the actuator.
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Operation under reverse bias
In addition to the normal hysteresis curve AB when the applied voltage is
positive, the butterfly diagram CDEFG defines the behaviour of the material
through a complete cycle of positive and negative operating electric fields.
Negative electric fields produce negative strain along curve C until at the
depoling field (coercive field) the extension suddently turns positive following
the curve D. The process is repeated along curves EFG when the electric field is
made positive again. The “butterfly” diagram provides a complete
characterization of the depoling and repoling process.
Most hard piezoelectric materials can only be poled or depoled at elevated
temperatures so once poled, they can be used with either forward or reverse
bias without difficulty.
Soft piezoelectric materials are easily depoled when subjected to an electrical
field opposite to the poling direction. The effect of cycling between positive and
negative voltages for various piezoelectric materials is shown in the following
figures:
NCE57 (S1)
NCE59 (S2) and NCE46 (H1)
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NCE51
Butterfly loop, NCE51
Please note:
The coercive field / depoling field depend on the temperature.
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Linearity CMA/SCMA/CMB
The stroke versus applied voltage relationship for piezo electric actuators is not
perfectly linear as predicted by the piezoelectric equations. Typical
performances are shown in the following figures. As it can be seen, the
extension vs voltage curve is actually slightly S-shaped. At low voltage, the
curve for increasing voltage is concave upward and the shape is close to
quadratic. At the high voltage end (>>3KV/mm) the electric field strength is
approaching the limit where no further alignment of the electric dipoles inside
the material can occur.
NCE57 (S1)
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The “Corrective Coefficient vs Electrical Field Strength“ figure can be used to
accurately estimate the free stroke for electrical field strength lower than
3KV/mm. This graph plots the ratio between the stroke obtained by a linear
approximation to the experiment value for different materials and different
electrical field strength.
NCE57 (S1), NCE46 (H1) and CE59 (S2)
The following example explains how to use the "corrective coefficient vs
electrical field strength" curve:
Example:
Let's consider the following stack:
Material: NCE46
Dimensions: 5 x 5 x 60 (mm)
Active layers thickness: 67µm
Max Operating Voltage: 200V
Stroke @ 200V: 58µm
What would the performances of this actuator be at 40V?
Step 1:
Calculate the operating electrical field.
At 40V, the electrical field strength is 40V/66,7µm=0,6KV/mm.
Step 2:
Calculate stroke assuming performances are linear with the electrical field
strength:
Free stroke @ 40 V = Free stroke @ 200V x (40/200) = 58 x 40/200 =
11,6µm
Step 3:
On the graph, draw a vertical line starting from the calculated electrical field
strength [0.6kV/mm]. From the intersection between this line and the H1
curve, draw a horizontal line. The intersection between this line and the Y-axis
gives a value of ~ 0,7.This value is the corrective coefficient.
Step 4:
Multiply the free stroke and blocking force values obtained in step 2 by the
corrective coefficient to get the estimated performances at 40V.
Free stroke @ 40V: 11,6 x 0,7 = 8,12µm
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Linearity CSA
The stroke versus applied voltage relationship for shear plates is not linear.
Typical measurements are shown in the following figure. As it can be seen, the
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displacement increases when the actuator is used close to the maximum
recommended voltage.
The polynomial trend follows the experimental relationship:
with δ being the displacement, t the height of the actuator and E the applied
electrical field (Voltage / height).
More displacement is even achievable at higher voltage, however above the
maximum recommended voltage the actuator starts to degrade slowly.
Experimental data and trend for two CSAP02 samples.
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Thermal properties and temperature coefficients
Generalities
The performance characteristics of the electric and piezoelectric properties are
affected by temperature variations. Each piezoelectric material is affected
differently by temperature changes, according to the method of manufacture
and chemical composition of the material. Piezoelectric materials should be used
well below their Curie temperature for the poling to be stable. Any conditions
that raise the temperature close to the Curie temperature will cause the
piezoelectric material to become partially or completely depolarised and
severely degrade its piezoelectric properties. For applications that require
operation at elevated temperature a material with a sufficiently high Curie
temperature should be chosen.
The mechanical and electrical properties of piezoelectric ceramic are greatly
reduced at cryogenic temperatures. When piezoelectric actuators are cooled
down to cryogenic temperatures, the piezoelectric ceramic behaves like a very
hard piezoelectric material featuring:
 Strong reduction of electrical capacitance
 Reduction of loss factor/reduced hysteresis
 Reduced strain coefficients d33 and d31
 Strong increase of the coercive field strength
The last point means, that at low temperatures a piezoelectric actuator becomes
extremely stable against electrical depoling and other destabilizing effect. So a
much wider bipolar operation compared to room temperature is possible now.
Thereby, the loss in stroke for low temperature can be partially compensated
for.
A less known parameter is the thermal expansion coefficient for ceramics,
important to consider when designing devices where piezoelectric actuators will
be part of a composite structure and where the other elements of constructions
are e.g metals. The thermal expansion coefficient for ceramics is similar to
many ceramics and glasses and is typically in the range of 10 -5 to 10-6/°C. A
major difference with common materials is that the thermal expansion
coefficient is anisotropic with respect to the poling direction.
Temperature coefficients
The changes in various material properties with temperature are shown in the
following tables.
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Stroke measurements at cryogenic temperatures with NCE59
Low voltage actuators
Cryogenic measurements have been performed on a stack made of NCE59
material. The piezoelectric actuator, which dimensions is 5mm x 5mm x 20mm,
was tested at room temperature and at liquid nitrogen temperature (77K). In
both cases, the actuator was energized to 150 V (this corresponds to field
strength of 3 kV/mm) and then looped between +150V and -150V. These data
were used to produce the displacement vs. applied voltage curve at 77K and at
room temperature as shown in the following figures.
As it can be seen the strain at 77 K is approximately reduced to half size
at room temperature. Besides, a strong increase of the coercive field, it can also
be observed that the actuator exhibits a fairly linear voltage-displacement
characteristic at negative voltage. The piezoelectric actuator becomes extremely
stable against electrical depoling and the loss in stroke at low temperature could
be partially compensated by using a wide bipolar operation.
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Very high electrical field material data
In some applications the customer wants to archive maximum strain from the
piezo electric element only by applying a very high electrical field. In some
cases the maximum recommended field strength of 3kV/mm may be exceeded
e.g. for short-term use applications or static applications. Operating field of
4kV/mm is normally acceptable, however testing is recommended.
In the figure below is shown how the strain evolves with electric field for our
different materials up to a maximum electrical field strength of 9kV/mm. The
fall back in applying a very high electric field is that the actuator lifetime is
reduced drastically.
The data in the figure are only of informative character and we recommend to
contact our R&D before designing actuators based on very high electrical field.
NCE57 (S1), NCE59 (S2), NCE46 (H1), NCE5 (PCM51)
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Ultra high vacuum compatibility
It is widely acknowledged that PZT materials are UHV compatible. When
stacking such PZT elements the use of glue, solder, flux and wires could
compromise the otherwise inherent compatibility for device level products. It is
the purpose of this paper to give an estimate on outgassed species and rate
from a Noliac Stacked Ceramic Multilayer Actuator (SCMA).
For this purpose a SCMA consisting of 4 CMA’s and 2 endplates has been
prepared. The overall dimensions of the SCMA are 5mm x 5mm x 10mm,
having 5 glue joints each of 20mm length. The SCMA was equipped with
standard buswire and 2 Kapton coated multi-core wires of 200mm length each.
Cleaning:
The excess glue is mechanically removed during the stacking process and
during the curing. After curing the SCMA was brushed with a glass brush to
remove any glue residues and wiped clean with acetone.
Test conditions:
The outgassing test was performed at Outgassing Services International, in
accordance with the ASTM E 1559 measurement method.
Chamber pressure: 10-10 torr = 1,33*10 -10 mbar.
Measurement temperature: 23 oC.
Test results:
• Total outgassed mass after 24 hours was 54.6=g/cm 2
• Outgassing rate after 24 hours was 2,0*10 -10 g/g/s equivalent to 4,6*10-11
g/cm2 /s. The outgassing rates are illustrated in the graphs below.
• Outgassed species was found to be predominantly water (98.7% by mass),
but small traces of acetone and possibly fluorocarbons were also found.
Analysis:
The presence of acetone most likely arises from a cleaning procedure performed
after sample assembly and is therefore not considered as severe. The traces of
what is believed to be fluorocarbons are presumed to originate from the glue
used for the stacking, but the amounts detected are very low.
Conclusion:
By additional cleaning procedures and by using UHV compatible wires, the
tested specimen exhibited only very low outgassing levels, proving the UHV
compatibility of stacked actuators.
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