B O L E T Í N DE LA S O C I E D A D ESPAÑOLA DE

Cerámica y Vidrio A R T I C U L O

• • •

Piezoelectric ceramics and their applications for actuators E. HENNIG

PI Ceramic GmbH, Germany

The paper gives a review of the complex problems for the application of piezoelectric actuators. The ferroelectric and piezoelectric properties of PZT ceramic are discussed. The specific behavior of piezoelectric actuators under large signal conditions and the advantages and shortcomings of the different actuator designs are discussed. Finally, trends in several new actuator applications are presented.

Key words: piezoelectric ceramic, ferroelectric ceramic, PZT, piezoelectric actuator

Cerám^icas piezoeléctricas y su aplicación como actuadores

Este trabajo revisa los problemas complejos en la aplicación de los actuadores piezoeléctricos. Se discuten las propiedades ferroe-léctricas y piezoeléctricas de las cerámicas de PZT. Se aborda la respuesta específica de los actuadores piezoeléctricos bajo condi­ciones de señal grande, así como las ventajas e inconvenientes de los diferentes diseños de actuadores. Finalmente, se presentan las tendencias en algunas aplicaciones novedosas en actuadores.

Palabras clave: cerámicas piezoeléctricas, cerámicas ferroeléctricas, PZT, actuadores piezoeléctricas

1. INTRODUCTION

Now 50 years have passed since the piezoelectric effect of barium-titanate ceramics was firstly reported (1). This was the starting point for a very expansive development of new pie­zoelectric materials and compositions worldwide as well as the industrial application of the piezoelectric effect. Intensive studies of the reaction mechanism and the ferroelectric and piezoelectric behavior were carried out during this time. Based on this history, a large number of piezoelectric ceramic materials with different properties are commercially available now. The annual production quantity of piezoelectric ceramic materials in Europe has reached more than 400 tons until now. A further growing can be expected in future comprising new applications of piezoelectric actuators.

Piezoelectric actuators have entered a wide variety of appli­cations in the micropositioning technology in the last years because of their ability to convert directly electrical energy into mechanical energy. Depending on the type of actuator such as stacked actuators, contractors, bending elements, tubes or piezoelectrically driven motors, movements from nanometers up to some millimeters can be realized with theo­retical unlimited resolution or forces up to several kN can be generated. New developments in piezoelectric materials and actuator designs as well as new electronic driving concepts will open new fields of applications. However, tomorrow's applications of piezoelectric actuator systems also require a better understanding of the complex working behavior of the piezoelectric actuators and a new quality in the cooperation of material scientists, manufacturers and mechanical and elec­tronic engineers.

2. PIEZOELECTRIC AND FERROELECTRIC PROPERTIES OF PZT

Piezoelectric ceramics based on modifications of the solid solution of lead-zirconate-titanate (PZT) are the most often used materials for actuator devices. Actuators made of such ceramic are often referred to as piezoelectric actuators. From the physical point of view, however, this denotation is not fully correct and may cause confusion. For a better understan­ding, the origin of the piezoelectric effect is discussed below.

It is well known that piezo ceramics have been poled to get a net polarization and exhibit a macroscopic piezoelectric effect, respectively. For this purpose a strong electric field is apphed to the piezo ceramic at certain temperature and for a certain time. Under the influence of the electric field, the domains (regions with parallel orientation of the dipoles), align close to the field direction and roughly stay in alignment after tunning off the electric field below the Curie temperatu­re. The total strain (deformation) of a poled ceramic in parallel to the electric field can be expressed as:

Sst - Q33(Pr + P3) ' - QssPr' + 2 Q33 P^P3 + Q33P32 [1]

Where Q33 is the electrostrictive coefficient, P̂ . is the rema­nent polarization and P3 is the induced polarization. Q33Pi.̂ is the remanent deformation after poling of the ceramic. Q33P3^ is the true electrostrictive effect of the ceramic that can be neglected because of its insignificance. The piezoelectric effect is given by:

^ 3 - 2 Q33 ^r ^3 ~ S33P3 ' ^33-^3 [2]

Bol. Soc. Esp. Cerám. Vidrio, 37 [2-3] 167-171 (1998) 167

Where d33 is given by d^^ = 833^ g33. The strain S3 is the elec­tric field induced deformation for constant remanent polariza­tion. Therefore, a linear behavior of piezoelectric ceramic occurs only for small signal driving conditions because they do not affect the state of remanent polarization.

In contrast, piezoelectric actuators are usually operated with strong electric fields and high mechanical loads to achieve large displacements or large forces. This large signal excitation causes complex domain reorientations resulting in a strong nonlinear behavior of the piezoelectric, elastic and dielectric properties of the material (2-4). Furthermore, the temperature dependence of the domain mobility additionally contributes to the nonlinearity of the material as well as aging effects which are also connected to changes in domain configuration.

Therefore, the large signal operation benefit of large actua­tor deflection is won at the nonlinearity and hysteresis. That means, the expansion of an actuator at a given voltage depends on whether it was previously operated at a higher or a lower voltage. Furthermore, the hysteresis behavior is also the cause of internal losses, because the dielectric permittivity and the dielectric loss tangent depend also strongly on the applied electric field, too. The domain reorientations influence the elastic and stiffness behavior of a piezoelectric actuator, because they can also be switched by mechanical stresses. In other words, a piezoelectric actuator does not behave like a simple spring.

-120 0 200 400 600 800 1000

Applied Voltage [V]

3. ACTUATOR DESIGN

3.1 General considerations

Fig.l. Field-deflection-hysteresis curves of a stacked actuator for increasing static mechanical stress

As stated above, piezoelectric ceramics are well suited for creating actuator devices. Piezoelectric actuators provide several advantages such as displacements ranging from the sub-nanometer range up to some millimeters, high resolution, zero stiction-friction, generation of forces up to several thou­sand Newtons and fast response (5). Different types of piezo­electric actuators were developed in the last years. Many of them have found industrial applications in the meantime. New developments in electric power amplifiers and sensor technology have supported this process.

Some typical actuator constructions and their advantages and shortcomings will be discussed in the following. Examples of their typical working behavior will be given.

3.2. Stacked actuators

The well-known stacked design of actuators uses the longi­tudinal deformation of piezoelectric bodies. That means, the electric field and the deformation are in parallel with the direction of polarization. A certain number of ceramic discs or layers are connected mechanically in series and electrically in parallel. Therefore, a summation of the deformation of each single layer takes place. Depending on the production met­hod, traditionally stacked actuators and multilayer actuators can be distinguished. In traditionally stacked actuators, the individual ceramic disks are usually glued together with thin metallic electrodes between two disks which feed in the ope­ration voltage. The disk thickness usually ranges from 0.3 to 1 mm. This design allows a wide variety of geometrical dimen­sions and has no restrictions for the used materials. Multilayer

actuators are produced in cofiring processes known from mul­tilayer capacitors (6). The layer thickness ranges usually bet­ween 20 and 100 pm.

Stacked actuators are normally operated with the same polarity as the original poling field. Therefore, an elongation occurs. If they are operated with opposite polarity, a shrinka­ge occurs, but there is a risk of depolarization. The maximum electric field for operating piezoelectric actuators is in the order of 2 kV/mm. With this field strength a strain of up to 0.13 % can be achieved. For higher electric fields dielectric breakdown can occur. Furthermore, the expected lifetime of an actuator will decrease with increasing of the operating elec­trical field. Depending on the layer thickness the maximum operating voltage ranges between 40 and 200 V for multilayer actuators and between 600 and 2000 V for traditionally stac­ked actuators, if there are no restrictions from the insulation strength. Therefore the actuators are also often referred as low voltage and high voltage actuators, respectively.

As mentioned above, the expansion and stiffness behavior of piezoelectric actuators strongly depend on the electrical and mechanical boundary conditions. This complex response is caused by domain reorientations due to the poling effect cau­sed by the electric field and the depoling effect caused by mechanical stresses parallel to the polarization (4). Fig.l shows some typical hysteresis curves measured with increa­sing static mechanical loads. The shape of the field-deflection-hysteresis loop is strongly dependent on the loading force. The deflection at the maximum operating voltage also depends on the applied stress as shown in Fig.2 for three dif­ferent materials. Fig.3 shows the stiffness of an actuator dependent on the mechanical load which was numerical cal-

168 Boletín de la Sociedad Española de Cerámica y Vidr io . Vo l . 37 Núms. 2-3 Marzo-Junio 1998

PIEZOELECTRIC CERAMICS AND THEIR APPLICATIONS FOR ACTUATORS

30

25 +

20

15 O '43 O O»

â 10 -A—Sampfei

-Sample 4

'Samples

30 60 90

Applied Stress [MPa] 120

Fig.2. Maximum deflection in dependence on the applied static stress of stacked actuators with same dimensions but made of different materials

A special stacked actuator for movements in x, y, and z directions is described in (7). This actuator consists of two parts using the shear effect and one part using the longitudi­nal effect.

3.3. Laminated actuators

Laminated actuators consist of a certain number of thin cera­mic strips bonded together. Unlike stacked actuators, the strips are acting mechanically in parallel. Both traditionally lamina­ted actuators and multilayer actuators are available. Because of the used transverse piezo effect, the shrinkage of these devices is perpendicular to both the direction of polarization and the electric field. The maximum contraction is a function of the operating field and the length of the strips, while the number of strips arranged in parallel determines the stiffness and the stability of the actuator. Due to the low tensile strength of pie­zoelectric materials, only small forces can be generated.

Contractors are also used in the «moonie» actuator (8). The piezoelectric contracting elements are connected to special shaped metal caps having a cavity on its inner surface. The caps transform and an amplify the lateral deformation of the ceramic plate into an axial motion. Both the transverse effect and the longitudinal effect contribute to the axial motion of the «moonie».

200

160

E 3.

in

c

120

CO

—st i f fness (OV)

^ Stiffness (1000 V}

-A—DIspi. (OV)

40

20 »=•

0 r E m o j5 a. Q

-60 ö> 3

-20

+ -40

O -80 if>

< -100

-120 0 50 100

Applied Stress [MPa]

Fig.3. Displacement and static stiffness of a stacked actuator in dependence on applied static stress for different control voltages

culated from the absolute displacement in Fig.L With increa­sing stresses domain reorientations will be hindered more and more resulting in an increasing stiffness. The depoling effect caused by mechanical stress in parallel to the polarization direction can be seen from Fig.4. A remarkable remanent strain was observed for the shorted sample.

3.4. Tube actuators

Thin walled piezoelectric ceramic tubes were among the first piezoelectric actuators to be commonly used. They employ the transverse effect, similar to the laminated actua­tors. With structured electrodes, and one end of the tube clam­ped, movements with high resolutions in three dimensions can be realized at the free end. Therefore, they are now used in special appHcations such as scanning probe microscopes.

3.5. Bending actuators

Considering the production numbers, bending actuators are the most industrial applied piezoelectric actuators at presence. The used principle of piezoelectric bending elements has been well- known for a long time (9) and many different designs are available today allowing for an easy adaption to the applica­tion requirements. Bending elements exist as strips as well as circular and square plates. With bending element's deflections up to several millimeters can be realized while forces are hmi-ted to several Newtons. The large deflection comes at the expense of low stiffness and low resonant frequency.

The mostly used bending element for actuator apphcations is the parallel bender. It consists of two piezoelectric layers bonded to both sides of a passive middle layer. This construc­tion provides deflections to both sides with the operating vol­tage of the piezoelectric layers always in parallel to the polari­zation direction. Fig.5 shows the typical field-deflection-hyste­resis curves of such a bending element when only one piezoe­lectric layer is operated . The achievable deflection depends on the electrical boundary condition of the second layer, too.

With the improvement of the multilayer technology, multi­layer bending actuators become more and more important because of their low operating voltage.

Boletín de la Sociedad Española de Cerámica y Vidrio. Vol. 37 Núms. 2-3 Marzo-Junio 1998 169

Applied Stress [MPa] Fig.4. Deflection-stress-hysteresis curves of a stacked actuator for different electrical boundary conditions

300

200

c

O

ë S -100

-200

-300

Voltage [V]

-•—y-(open) -A—y+ (open)

•*—y- (short) -K—y+(short)

Fig.5. Deflection-voltage-hysteresis curves of a parallel bender element with three layers for different electrical boundary conditions

In contrast to laminated actuators, bending actuators can only be operated with electrical fields up to 1 kV/mm, due to the large internal stress. Fig.6 show the simulated stress distri­bution of a three layer parallel bender (clamped at one side) at the cross section (positive sign means tensile stress). Two cri­tical regions can be seen. One is near the bonding layer bet­

ween the active piezoelectric layer and the middle layer and the second is at the outer side of the passive piezoelectric layer.

Recently a new bending device was realized which combi­nes the advantages of stacked actuators and bending actuators (10). The device employs the longitudinal effect instead of the transverse effect. A thin layer of multilayer stacked actuator is bonded to a passive supporting layer. Compared to a classical bending element, forces up to 100 N can be generated with low operating voltage.

4. TRENDS IN ACTUATOR APPLICATIONS

4.1. General considerations

Piezoelectric materials are ideal to satisfy the requirements of today's trend to smaller, faster and more accurate products. Especially in the semiconductor technology, the structures are getting smaller and smaller by the year. This requires new handling tools as well as measuring systems providing reso­lution in the sub-nanometer range as well as high stability. Modern optical information systems use micromachined com­ponents and optical fibers with diameters in the range of micrometers on one hand and long distance information transfer via satellites at the other. For both systems precise and stable positioning and the suppression of disturbances are necessary.

A new insight of pollution control has taken place worldwi­de. Therefore, the reduction of energy consumption is one of the main problems in future. Manifold efforts are undertaken by the car and aircraft industry to reduce the fuel consump­tion. To improve the Hfe quality, noise cancellation and vibra­tion supression are main challenges.

Many investigations are conducted in this fields at presence. The following discussion will emphasize several new applica­tions of piezoelectric actuators that will gain importance in the near future.

4.2. Adaptive mechanics

Piezoelectric driven flexure positioners are superior to tra­ditional motion systems in terms of straightness, runout, stiff­ness and lack of stiction/friction. Mechanical tolerances, stresses in the base material, and non parallel coupling bet­ween the actuator axis and the motion axis cause an unwan­ted runout in axes other than the command axis in the range of several nanometers. An integration of a multi axes com­pensation system can reduce this runout to less than 0.5 nanometers (5). Similiar arrangements of piezoelectric actua­tors will be used for vibration supression of platforms for IC manufacturing (11).

Lightweight truss structures based on fiber reinforced poly­mers are now developed for satellite technologies and for tur­boprop engine supports of aircraft. Tests have shown, that unwanted vibrations can be sufficiently suppressed if one or more piezoelectric controlled struts will be integrated (5,12,13).

Other developments deal with the improvement of the aerodynamic efficiency and the reduction of vibration and acoustic emissions (12,14). Developments of adaptive helico-ter rotor blades as well as the decoupling of the engine from the cockpit are in progress.

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PIEZOELECTRIC CERAMICS AND THEIR APPLICATIONS FOR ACTUATORS

30

20 fiMMf

CS a. S 10 • — J

0) (^ 0) U ^. * r f

CO

75 -10 X <

-20

-30

Position C)jm]

Fig.6. Calculated stress distribution over a cross section of a parallel bender with three layers each 150 }im in thickness

Another main goal is the active noise suppression of planar structures (12) with the help of piezoelectric plates bonded to or integrated in the structure. Both the radiated and the trans­mitted sound will be canceled if the piezoelectric elements are operated in a closed loop real-time regime.

4.3. Piezoelectrically driven valves

For several years the development of piezoelectric driven valves is in progress and several types are now on the market (15-19).

While bender actuators are the favorite ones for pneumatic valves, stacked actuators are mostly investigated for hydrau­lic valves. Several prototypes have shown that piezos can sig­nificantly improve the performances of valves. Since the envi­ronmental conditions in hydraulic and pneumatic valves are very harsh and the requirements of lifetime and reliability are high further investigations of adapted actuator constructions, improved reliability and new concepts of protection coatings are necessary.

The automotive industry is investing large sums in the development of piezoelectrically controlled fuel injection val­ves. The goal is a significant reduction of the fuel consumption in the near future.

4.4. Adaptive Optics

Piezoelectric actuators can also be used to improve the qua­lity of optical systems. State-of-the-art adaptive optics systems are deformable mirrors, fast tip/tilt correction systems and controllable focal length systems. Further improvements of the systems will extend their applications. Fiber optic techno­logy can open new fields of applications for piezoelectric actuators. First industrial applications of piezoelectric benders in fiber optic switches for communication networks are now on the market (20).

5. SUMMARY

Many developments for new applications of piezoelectric actuators are now in progress. Continuing improvement in the performance and rehability of piezoceramic materials and actuator design have already opend new markets. The next steps that have to be taken are: the international stan­dardization of test methods for actuator materials and compo­nents and the estabhshment of a very close cooperation bet­ween material scientists, manufactures, mechanical and elec­tronical designers.

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