Use of cofired monolithic
piezo-ceramic stacks and chips

Overview

Contents

1.

2.

Selecting a proper piezo actuator system
Guideline
Power supplies

Overview: main operating parameters
Piezoactuators PSt 150, HPSt 150, PCh 150, HPCh 150

3.

12

LV-Piezostacking technologies (isi)In stack insulation

6.

6

LV-Piezostacking technologies (osi)
On stack insulation

5.

5

Ceramic piezo actuators
Handling and mounting

4.

3
4

13

LV quasi monolithic isi piezostacks
LV: Low Voltage

2 monolithic piezo-ceramic stacks and chips

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1. Selecting a proper piezo actuator system

Guideline:
The main pre-requisite for selecting suitable piezo components is the precise definition of
the needed operation profile by the user.
Any supplier of piezo-mechanical components will highly appreciate precise specificationsof the requested components.
Putting definite numbers on the needed piezo-parameters is helpful to avoid over-sizing
and mismatch. Poorly selected system components are ineffective and therefore
expensive.
Please try to analyze the needs for operating your mechanics successfully according to thefollowing:
A, what shift/stroke shall be achieved?
B, what force variation shall be generated by the piezo action ?
C, what static preload is acting on the actuator from the beginning ?
D, what is the desired maximum operation frequency ?
E, what is the desired stroke at maximum frequency (D) ?
F, what is the desired max. frequency at maximum stroke (see A) ?
G, shortest achievable rise-/fall-time?
H, what external masses shall be attached to the actuator ?
A, to C, allow an actuator selection for low dynamic operation
D, to H, aims for the best match for the designated dynamic operation.

Selecting the amplifier
The above selection process results in a piezo-actuator of distinct voltage range and electrical capacitance. Only amplifiers with a matched voltage
range should be considered for use.
Do not use amplifiers providing higher voltage!

The dynamic operation profile D, to H, defines theneeded current levels (I peak and I average).
When the power consumption of the actuator exceeds the Watt-range, self-heating of the piezo-ceramics can occur.
See brochure: First Steps towards Piezoaction

For details see brochure:“Piezoaction in practice”
Piezoaction in practice

Thermograph of a dynamically operated piezo stack

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monolithic piezo-ceramic stacks and chips

3Selecting power supplies

1. Rule of thumb: Use only power supplies with
voltage ranges matched to the actuator ratings.
Power consumption, current levels:
Piezo-stacks behave like capacitors in the subresonant frequency range.
Therefore, a current flows only during a voltage
variation for altering the position and/or force statusof an actuator.
2. Rule of thumb: Define precisely the electricalcurrent or power level to get the required dynamics
of your actuating system (rise-/falltimes, see brochure “electronics”).
Do not oversize significantly the max. current andpower ratings of the used amplifier to avoid annecessary stress in the piezosystem.

f~I
1/욼 T ~ I

Do not misinterpret catalogue data: Not all
operating specifications can be realized at thesame time due to simple physical facts.
● Maximum displacement/shift/stroke andmaximum force generation /max. blocking force
cannot be generated at the same time, only
either-or.
● The maximum actuator shifts (strokes) shownin data sheet are only valid under constant loadconditions (no force variation!).
● Two values for stroke are stated in the data
sheetA, for unipolar activation 0V/+Umax
B, for semibipolar operation -U/+Umax
The semi-bipolar operation increases the openloop stroke of a stack by 20 - 30%.

resonance frequency:
For precise positioning and short settling time:
Stay well below “resonance frequency”.

4 monolithic piezo-ceramic stacks and chips

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2. Overview: main operating parameters

Piezoactuators
PSt 150, HPSt 150, PCh 150, HPCh 150
Max. Voltage ranges: -30V thru +150V
Typical operating schemes:
Unipolar range: 0V/+150V
for high power applications
Semi-bipolar range: -30V thru +150V
for quasistatic low power applications withenhanced stroke/blocking force

Strokes A/B
Max. ratings shown in data sheet
A, for max. semi bipolarB, for max. unipolar activation
Example:Max. stroke: PSt 150/7/20 VS12
A/B = 27µm/20µm
Stiffness
Inverse compliance,– measured for open loop voltage control operation– measured with a static preload of at least of– 10% of actuator’s maximum load.
Blocking force/max force generation
Means maximum force generation of an actuator.
Depends on the applied voltage variation.
Stated values refer to maximum semi-bipolar
operation.
Maximum uni-polar operation gives about 30%
lower values

Actuator’s poling

Piezo-actuators are poled components from factory
Usually, the positive pole is indicated e.g. by
red insulation wires , dots , longer wire

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Resonant frequency– Refers to axial mode
– Defined for one side fixed piezo-element– (other modes like planar diameter not taken into)
Capacitances– Measured at low field excitation at room
– temperature– Manufacturing tolerances up to +/- 20%– Capacitance can depend on high field excitation– and temperaturemonolithic piezo-ceramic stacks and chips

5

3. Ceramic piezo actuators

The handling of piezo-actuators during mounting and operation is ruled by the specific mechanical properties of ceramics like brittleness, low tensile stress damage threshold on one
side and its sensitive electrical structure on the other side.
Stack type actuators are mechanically coupled
only via the end faces e.g. by clamping, glueing
or other methods. The end-faces are electricallyinsulated.
No mechanical contact/impact shall be applied
to the side faces.
The side faces bear the supply-electrodes tocontact electrically the internal layer electrodesof the capacitor structure. The supply electrodes
are wired by pig-tails near the actuator socket.
Standard actuators PSt 150 are finally coatedwith a high quality polymer and are therefore
more resistant to mechanical and otherinfluences than bare ceramic components.

Piezo-stacks can bear high compressive load
forces, but they are very sensitive to bending-,
torsion-, shear-forces and especially to tensile
forces. Attention has to be paid to this fact not only
during the final operation of such stacks, but during
all handling and mounting steps in advance tooperation.
The design of any attached mechanics shall aim forpure axial loading of the actuators without any
compromise. Imperfect solutions can lead tostrongly reduced performance and reliability.
Pay attention not only to the static force balances,
but also to dynamic influences (acceleration forces
resulting in bending motion or torque).

motionmounting side (ceramic)

polymercoatedsensitive side
no mechanical contact!

Mounting scheme of piezo-actuators

6 monolithic piezo-ceramic stacks and chips

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Piezo actuators: Handling and mounting

Mechanical preloading:
Preloading is achieved in most cases by using anelastic spring for resetting the moving part of an
actuator.
Mechanical preloading of piezo ceramics is necessary to allow a push-pull-operation of an actuator.
This is to handle externally applied tensile forces
caused by static or dynamic driving conditions.
A properly designed mechanical preloading ofactuators reduces further microscopic tilting of theactuator end-faces. Coherent optical arrangements
like tuneable etalons/resonator structures do notneed then additional guiding mechanism for shifts inthe micro-meter range.

moving top

preload spring

hold-downof springpiezo-stack

Operating stack actuators with reduced strain
A beginner in piezo actuation usually seeks for an
actuator type, what’s maximum rating stroke covers
rather exactly the needed range. This implies
consequently, that the actuator is operated then up
to maximum voltage, maximum electrical fieldstrength, maximum strain and thereby for dynamic
applications with maximum power consumption andpower losses.
The use of an “oversized” longer actuator can havea lot of advantages compared to the above
mentioned “maximum ratings” operation:● The long-term reliability is improved by the use of
reduced electrical field strength (driving voltage<< max. voltage rating).
● The reduced mechanical strain results in a lower
power consumption and self-heating.
Vice versa higher oscillation frequencies can beachieved by applying the same power as with the
maximum rating type actuators.
● Power supplies with reduced output voltages canbe used, providing higher currents (=> higher
actuator oscillation frequency) and lower costs.
This reduced strain strategy finds its limits, whenhigher resonance frequencies, stiffness are a mustor the stack length is restricted by space limitation.

Schematic of a preload spring mechanism

Preload forces can be high as 50% of the load
capability of actuators or even more, to handle
extreme driving conditions (e.g. pulsed operationor symmetrical push-pull arrangements).
The “golden rule” of preloading:Use preload springs of lowest possible stiffness(large compliance).
A good attempt is to select for a spring’s stiffness in the % range of actuator’s stiffness.
Then, no loss of stroke will occur even whenhigh preload forces are applied (details: See“force generation” in main cata logue).

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monolithic piezo-ceramic stacks and chips

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Piezo actuators: Handling and mounting

The achievable performance and reliability of an actuator must be seen always in context
with the interaction with the operated mechanical system and the electrical driving characteristics.
Poorly designed mechanics like low stiffness
coupling to the actuator, friction, wrong preloading,wrong force coupling, misalignment of coupling
faces from actuator to mechanics reducesignificantly usable stroke, accuracy, force
generation and make the use of piezo actuator
more or less worthless. Poor designing impact
further actuator’s long-term reliability.
Fig. A shows the consequences of inhomogeneous
high force loading: A stack with excessive edge
squeezing/pressure of the ceramic-stack by an
improper force coupling (Fig. C and D). Cracks are
generated within the active ceramic section
resulting in electrical break down and arcing.

Coupling of actuator and mechanics
Optimum actuation performance is achieved by following a few simple rules● The coupling face of the mechanics shall covercompletely actuator’s end faces to achieve
maximum force transfer (Fig. B). The contact
force shall be homogeneously distributed overthe contact area.
● When a high load pressure is applied, thecoupling faces of actuator and mechanics faces
shall be absolutely plain (eg. by grinding) to avoid
local overload of the ceramic front face.
● The resulting load force vector shall coincide withactuator’s axis. Within a virtual cylinder of+/- 10% of actuator’s cross-section (Fig. A) to
avoid excessive bending and shear stress. Forcemisalignment tolerance becomes more criticalfor increased ratios actuator length / diameter.
For high dynamic operation, actuator’s axis shallfurther hit the centre of mass of the attachedmechanics to avoid dynamic torque.

mechanics

piezo-stack움Fig. A: Failed actuator stack, caused by a wrong coupling to
the actuated mechanics. Local edge pressure exceeded
ceramic’s stability with subsequent electrical break down
after approx. 800 hours.

8 monolithic piezo-ceramic stacks and chips

Fig. B: Perfect plain-plain coupling of piezo-stack and attached mechanics by floating axis orientation of the mechanics.
Acceptable tolerance 움 : See above

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Piezo actuators: Handling and mounting● When the mechanical partner can read just itself

by a free suspension (floating axis) according
actuator’s plane face, no problems will occur.
● Coupling of piezo actuators to guided

mechanisms (axis orientation not floating).
One of most widespread design mistake iscoupling a plain-faced actuator directly to aplainfaced guided mechanism (Fig. C) Even theslightest misalignment between the orientationsof both plains leads immediately to edge
squeezing with very high local spot pressuresand subsequent ceramic damaging.

moving partIn a similar way, the plain-plain coupling of anaxially acting stack with a rotating lever
arrangement will lead to a fundamental edge
squeezing situation in any case (Fig. D).
Do not try this!
In the above cases, it is a must to decouple
the axis orientations by using spherical/plain
coupling or flex hinges or other means!
● The above requirements are valid at any timeand any state of the system during set up andoperation.

moving partwrong

moving part

critical edge
squeezingcorrect

correct

piezo-stack

piezo-stack

piezo-stack

움

움

움

Fig. C: Perfect plain-plain coupling of piezo-stack and attached mechanics by floating axis orientation of the mechanics.
Acceptable tolerance 움 : See above

pivot

wrong
critical edge
squeezing

pivot

correct

Fig. D: Perfect plain-plain coupling of piezo-stack and attached mechanics by floating axis orientation of the mechanics.
Acceptable tolerance 움 : See above

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monolithic piezo-ceramic stacks and chips

9Piezo actuators: Handling and mounting

Bonding of piezo stacks and chips to mechanicalparts:
Piezoceramic elements are often bonded to mechanical counterparts by glueing.
The quality of the glue-line is important both for the
motion transfer quality and reliability of the piezomechanical system.
“Quality” means in this context, that the glue-line
must not be “too hard” or “too soft”.
Especially too hard glue line may result in rather highin-plane tensile stress within the brittle piezo-ceramic components
A, due to the in-plane d31 piezo-contraction of a
PZT-layer coupled with the axial (out of plane)d33-mode mostly used for piezo-chip and stackapplications.

inactive PZT-layer

activated PZT-layer

d33

d31

Fig. A: Action of a free PZT-layer

B, by varying temperatures as a consequence ofpotentially different thermal expansion coefficients of the couples materials.
The planar CTE of poled piezo-ceramics is about
5 ppm/°C.
For applications within a range of -20 °C/+60 °C,
any good quality standard epoxy can be used. The
cure temperature shall be held rather low to avoid
thermally induced stress, when the CTEs of thejoined parts is remarkably different. The thickness ofthe glue lines shall be held rather thin (< 50 µm) to
avoid a “softening” of the coupling quality.
For “exotic” driving conditions like very low or very
high temperature, vacuum application etc. special
adhesive formulations and glueing techniques arerecommended.
Ask PIEZOMECHANIK for support.

inactive PZT-layer
activated PZT-layer

hard glueline

Fig. B: Hindered expansion of a PZT-layer by clamping bya hard glue line together with a rigid substratePiezo-chips
Too hard and too thin glue-lines increase the “d31clamping” effect, what can hinder or block the axial
motion especially of piezo-chips (Fig. B,C).
For coherent optical applications, unwanted tiltingof mounted optical element will then occur. Further,thin optical elements can be distorted by a hardcoupling to a piezo-chip (bending structure).
Only when the d31-planar contraction is nothindered, a thin piezo-chip will show the maximum
axial d33-expansion and planar displacement.

10 monolithic piezo-ceramic stacks and chips

activated PZT-layer

substrate

Fig. C: Bending structure by gluing a PZT-layer onto a thin
substrate.

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Piezo actuators: Handling and mounting

Contamination: Piezo stacks are electrically highly active elements with high electricalfields at stack’s surface.

For mounting stacks into metal constructions,ensure sufficiently large insulation gaps between
stack’s side-faces and the surrounding mechanics.
A PZT-stack-structure is rather sensitive to any
contamination by conductive species like electrolytes. Leakage currents will set on, leading to electro corrosion and subsequent short circuiting of
the actuator.
Contact to aqueous liquids shall be avoided.

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On the other hand stacks with polymer coating
can be operated within non-aqueous fluids (e.g.
Diesel fuel, transformer oil, silicon oil), what can beused for high efficient cooling of dynamically operated stacks.
Do not touch bare stacks with bare fingers.
Cleaning of coated and uncoated stacks shall bedone only with 100% iso-propanole.
Never use acetone!

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4. LV-Piezostacking technologies (osi)

Piezo stacks are electrically highly active elements with high electrical fields at stack’s
surface. Among the spectrum of low voltage multilayer actuators, the osi-based elements
have found the widest applications. The patent for this technology has expired since a few
years. The main step is to use a fine glass filament fused onto the PZT-ceramic stack,where the internal layer electrode comes to surface and shall be separated electrically fromthe counter-polar supply electrode. The glass filaments are well separated from each otherand do not form a dense (inflexible) glass layer (Fig. A)● Due to the absence of structure borne tensile
100 %internal electrodestress within the ceramics, the osi-stacks can be
operated with high strain rates without
mechanical preload (as long as no externaltensile forces are present).
Surface protection
The sidefaces of standard osi-stacks are coated bypolymers.

PZT-layers

glass insulation
supply electrodeThe coating protects the brittle ceramics against“less skillful handling”, mechanical attack or
chemical contamination.

Fig. A: Schematic of the osi-technique using fused-on glass
fibers to terminate internal electrodes on stack’s surface

The main highlights of the osi-technique are● 100% active PZT-ceramic cross-section andstack volume for maximum stroke and force
generation (blocking force).
● Homogeneous strain within the PZT-volume,no intrinsic mechanical tensile stress by active/
inactive PZT-boundaries resulting in ceramic
cracking and electrical break down.
● The absence if intrinsic inhomogeneous stressresults in low tilting of the actuator endfaces(useful for coherent-optical arrangements of ringactuators).
● High electrical reliability:

Osi-actuators show a dense PZT-ceramic surface
without side cuts or slots as the isi-techniquedoes. No risk of rupturing the supply electrodes
at slots/cuts within the side-faces of the stack.

12 monolithic piezo-ceramic stacks and chips

Fig. B: Different kinds of stack coatingsSpecial coatings are available to handle exotic
driving conditions like cryogenic temperatures,vacuum / UHV, immersion in (non-aqueous) liquids.
Polymers are much more flexible than PZT-ceramics
and do therefore not affect adversely the piezoaction.

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5. LV-Piezostacking technologies (isi)On the contrary: A thicker coating acts as a kind of
reinforcement and “ruggedizing”, making the stacks
less sensitive to bending or tilting forces than an
uncoated ceramic stack (Fig.).

F
Piezo-stackD

Fig.: Schematic of a bending experimentwww.piezomechanik.com

A piezo-stack PSt 150/2x3/20 with a ceramiccross-section of 2 x 3 mm has been bended bya force F (distance D of the supporting points:
15 mm, bended stack’s dimension: 2 mm).
Ruggedizing
The coated ceramic stack withstands a force F of
80 Newtons before breaking, whereas the uncoatedstack can bear only 38 Newtons!
Thick coated stacks show a better resistance toa robust handling or sub-optimum mechanicaldesigns. Bending oscillations of longer stacks witha critical ratio of length/cross-section are damped
away.
Osi-Technology is used by PIEZMECHANIK for
PSt 150/HPSt 150 actuators.

monolithic piezo-ceramic stacks and chips

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Cofired multilayer piezo-ceramic technologies: In-Stack-Insulation (isi)The isi-piezo-stack design is a more simple insulating strategy, well-known since the earlydays of low voltage actuation in the 70ies of the past century.
The necessary electrical insulation step is done inside the stack: The PZT-layers are notcompletely metallized. A small insulation gap ensures the electrical separation between theinternal electrode and the external supply electrode (Fig. A).

internal
electrodeinsulation gap(no internal electroding

supplyelectrodeThis leads potentially to mechanical cracking of theceramic and electrical break down of the stackduring long-term action, when potential crack
propagation is not compensated for by special
techniques.

Fig. A: In stack insulation scheme.

onset of crack
propagadionat side sots

piezo-stack

It is easily seen, that the PZT-ceramic of the stack isonly partially active and a steep transition from
active to inactive PZT-ceramic is created:
A pronounced mechanical tensile stress at theactive/inactive boundary region occurs, when the
PZT-stack is activated and expanding.

Further:
The isi-structure with inside slots is more sensitive
to bending forces, because the notches act as
starting point of ceramic cracking (Fig. B).
The designs of any coupled mechanics has to be
optimized for strict axial loading of isi-stacks.
Attention should be paid to any mechanical influences resulting in bending modes especially of stacks
with large ratio of length/diameter.

D

Fig. B: Bending-induced ceramic cracking of isi-stacks withside slots

Mechanical preloading
In addition to the above-mentioned techniques,
the internal stress-problem of isi-actuators is
usually handled by the application of a large axial
preload or prestress even for static or low dynamic
applications.
Supply electrodes
The supply electrodes of isi-structures bridge theside slots/cuts in the stack structure. For simple arrangements, the side electrodes tend to rupture in
the vicinity of the side slots, when the stacks areoperated with high strain and dynamics. The actuator will be deactivated then.
The optimum of highly reliable side electrodes isachieved by using metal mesh.
(The osi-techniques does not need such strategies
simply due to the absence of the side cuts/slots).

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6. LV quasi monolithic isi piezostacks

No cracking of the ceramic occurs for rather thinelements. This means: When the axial thickness ofthe multi-layer element is held within 2-3 mm(“Piezo-chips”).
Longer stacks are manufactured by glueing suchmonolithic piezo-chips. The gluelines between thechips provide stress relaxation.
The mechanical properties of such longer stacks
depend to some extent on the quality of the gluejoints.

Chip-based quasi-monolithic-stack

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monolithic piezo-ceramic stacks and chips

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PIEZOMECHANIK is a globally recognized supplier of first-class piezo systems.
Our actuator specialists are excellent connoisseurs of the current actuator scene.
This allows you to point out certain intricacies of the topic, which you will not find in the usual company scripts.
PIEZOMECHANIK successfully provides advice and development contributions even for unorthodox
piezoaktorian applications, some of which go far beyond the classical approaches.
We get for you from actuators what is really in it.

Stress reduction by expansion slots
For producing monolithic multilayer-stacks withlengths > 3 mm, slots are prepared into the ceramicparallel to the PZT-layers in the critical active/inactive ceramic section (Fig. ?).

internal
electrodeactivePZT
volumeside cuts/slots

supply electrode

Piezomechanik · Dr. Lutz Pickelmann GmbH
Berg-am-Laim-Str. 64 · D-81673 Munich · Phone ++ 49 /89 /4 3155 83 · Fax ++ 49 /89 /4 316412
e-mail: info@piezomechanik.com · http://www.piezomechanik.com

Stand: März 2021

Fig. ??: Bending-induced ceramic cracking of isi-stacks withside slots