shape memory piezoelectric actuator by control of the imprint … · 2008. 11. 6. · in this...

Ferroelectrics, 368:185–193, 2008Copyright © Taylor & Francis Group, LLCISSN: 0015-0193 print / 1563-5112 onlineDOI: 10.1080/00150190802368479

Shape Memory Piezoelectric Actuator by Controlof the Imprint Electrical Field

Y. KADOTA,1,∗ H. HOSAKA,1 AND T. MORITA1

1Graduate School of Frontier Sciences, University of Tokyo: 5-1-5 Kashiwanoha,Kashiwa 277-8563, Japan

The fundamental study of a shape memory piezoelectric actuator was conducted,using operated with a pulsed voltage, so that low energy consumption and low voltageoperation could be realized. To realize the shape memory operation of the piezoelectricactuator, an imprint electrical field was induced with a high electrical field of 3.5 kV/mmin a 150◦C environment. The memory effect of the displacement and the permittivity ofthe shape memory piezoelectric actuator resulted from the asymmetric butterfly curvecaused by the imprint electrical field. The actuator has two stable strain conditions andpermittivity values that were dependent on the direction of polarization, imparting theshape memory effect and permittivity memory effect. Furthermore, self-sensing opera-tion was realized by detection of the permittivity that corresponds to the position of theactuator.

Keywords Shape memory piezoelectric actuator; imprint; polarization reversal; shapememory

Introduction

Ferroelectric materials are utilized in various applications, due to their multifunctional prop-erties of piezoelectricity, large permittivity, nonvolatile charge, and electro-optical functions.As a nonvolatile memory, ferroelectric random access memory (FeRAM) has been inten-sively studied [1–3]. In this case, the remanent polarization in the ferroelectric materialis utilized as the direction of polarization for 1-bit information memory storage. Anotherapplication of ferroelectric materials is piezoelectric actuators, because the simple structureand high energy density are of great advantage for micro actuators and sensors [4–6]. Ingeneral, however, the piezoelectric strain is so small that a large voltage is indispensable.For the operation of conventional piezoelectric actuators, the driving voltage is consideredso that it does not exceed the coercive electrical field. This is due to polarization reversal,because control of the piezoelectric displacement becomes difficult, due to the change inthe sign of the piezoelectric coefficient. With perfectly reversed polarization, displacementof the piezoelectric results in the same position, because the piezoelectric strain versuselectrical field is symmetric (butterfly hysteresis curve), so that reversal of the piezoelectricpolarization is not useful for actuator application.

Received September 3, 2007; in final form March 13, 2008.∗Corresponding author. E-mail: [email protected]

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186/[424] Y. Kadota et al.

Figure 1. The principle of the memory effect with imprint electrical field.

Principle

In this paper, we propose a shape memory piezoelectric actuator that utilizes polarizationreversal; therefore, this approach is quite different from that of conventional piezoelectricactuators. A completely symmetric butterfly curve, due to the perfect reversal of polarization,indicates that the piezoelectric actuator does not have a memory effect. However, there aresome reports of asymmetric butterfly curves caused by the imprint electrical field [5–6]. Theimprint electrical field is an internal electrical field present in ferroelectric materials, and is awell-known phenomenon in the field of ferroelectric thin films. Although the precise originof the imprint electric field has not yet been clarified, it seems to be caused by several factors,such as trapped electrons or holes, lattice defects, and lattice mismatching, etc. Figure 1shows the principle of the memory effect induced by control of the imprint electrical field.With the imprint electrical field, the D-E hysteresis of the ferroelectric material shiftsto the direction of the axis of the electrical field and becomes asymmetric. For FeRAMapplication, this asymmetricity causes a serious failure; changing of the coercive voltageresults in different operation voltages for the reversal of the polarization. Therefore, manystudies have attempted to remove the imprint electrical field. However, this asymmetricitycauses memory effects not only for the polarization, but also for strain and permittivity.The controlled imprint electrical field also causes the strain butterfly curve and permittivitybutterfly curve shift in the direction of the horizontal axis. For this type of asymmetricbutterfly curve, the shape memory piezoelectric actuator has two different stable pointsof strain and permittivity at the point of 0 volts, depending on its polarization direction.In the other words, with the reversal of polarization, the piezoelectric strain (and also itspermittivity value) changes to another stable position, which is maintained even after theexternal electrical field becomes zero. In addition, the permittivity values of the shapememory piezoelectric actuator correspond to the piezoelectric strain conditions that depend

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Figure 2. The driving principle of the shape memory piezoelectric actuator.

on the polarization direction. This means that the shape memory piezoelectric actuator canrealize self-sensing by detection of its own permittivity.

There are some important advantages of shape memory piezoelectric actuators. Forexample, when a conventional piezoelectric actuator is utilized for a mechanical relayswitch, a continuous driving voltage is required to maintain the “on” or “off” conditions.In this case, the energy consumption is not zero, due to the leak current. Moreover, aconventional actuator requires a large dc voltage to maintain a certain position, so that largeelectric amplifiers are usually required.

In contrast, the shape memory piezoelectric actuator does not require application of avoltage to maintain the “on” or “off” states, because it has two stable positions depending onits polarization direction. When a change in the condition of the switch mode is required,it is realized using a pulsed voltage that reverses the polarization of the shape memorypiezoelectric actuator. After this operation, no electrical field is required, as shown in Fig.2. Therefore, the energy consumption to maintain the piezoelectric displacement is zero.

Another advantage of a shape memory piezoelectric actuator is low voltage operation.This claim may seem somewhat unusual, because the voltage usually required to reverse thepolarization is larger than that used to drive a conventional piezoelectric actuator. However,a large pulsed voltage can be easily generated by combining a small voltage source witha capacitor and transformer. After accumulation of the charge in the capacitor, the chargecan then be used through the transformer as a pulse-shaped voltage for the operation of theshape memory piezoelectric actuator.

The concept of the shape memory piezoelectric actuator is applicable for all piezo-electric actuators, and is not limited to certain piezoelectric material forms (thin films, bulkceramics, or single crystals), actuator types (bimorph, unimorph, or multilayered type), andmaterials [Pb(Zr,Ti)O3 (PZT), BaTiO3 or LiNbO3].


Figure 3. The measurement principle.

Experimental

Permittivity Detection Method

In order to confirm the memory effect of the permittivity of the shape memory piezoelectricactuator, a method to measure the bias field dependent permittivity hysteresis propertywas developed. A schematic diagram depicting the measurement principle is shown inFig. 3. Two different voltages are applied to the actuator; one is a low frequency and highamplitude triangular wave used to drive the actuator, and the other is a high frequency andlow amplitude sine wave for detection of the permittivity. The permittivity is defined asdifferential value of electrical displacement with respect to the electrical field, and it can becalculated from the relationship between the current amplitude and applied voltage usingthe following equation,

ε = d2πε0S

If V

where ε is the permittivity, I is the amplitude of the induced current, d is the thickness ofthe piezoelectric material, f is the frequency of the permittivity detection voltage, εo is theelectric constant, S is the area of the piezoelectric material, and V is the amplitude of thepermittivity detection voltage. By detecting the alternate current amplitude of the actuator,the bias field dependent permittivity can be detected. However, there are certain aspectsrequiring careful consideration. The amplitude of the permittivity detection voltage mustbe as small as possible in order not to affect the piezoelectric displacement. The frequencyof this voltage should be high, because the current signal is proportional to the permittivitydetection voltage frequency. At the same time, high frequency results in little affect on


Figure 4. The diagram of the measurement.

the piezoelectric displacement, because it is difficult for the actuator to respond to highfrequency signals.

Control of the Imprint Electrical Field

A PZT unimorph type actuator was used in the present investigation. This actuator wassupplied as a bimorph-type (Nihon Ceratec, LPD3713); however, a lead wire connectingthe two PZT layers was cut in order to make the actuator a unimorph type, so as to simplifyits operation. The actuator was comprised of a metal plate (37 × 13.4 mm, 0.2 mm thick)sandwiched between two PZT layers (28 × 13.4 mm, 0.2 mm thick), giving a total thicknessof 0.6 mm.

The operating voltage was applied from a function generator (NF, WF1946) with twooutput channels. The driving voltage (high amplitude and low frequency triangular wave)and the permittivity detecting voltage (low amplitude and high frequency sinusoidal wave)were provided from CH1 and CH2, respectively. The voltages were superimposed and ap-plied to the actuator through a voltage amplifier (NF, 4010) that had this additional function.One end of the actuator was clamped and its piezoelectric displacement was measured using


a laser displacement sensor (Keyence, LC2400). The ac current amplitude was simultane-ously detected using a current probe with a lock-in amplifier (Stanford Research Systems,SR530) and a reference signal of the permittivity detection voltage (CH2). The permittiv-ity was then calculated from the current amplitude using the actuator dimensions, and theamplitude and frequency of the permittivity detection voltage. A schematic diagram of theexperimental set-up is shown in Fig. 4.

To apply memory effect, the imprint electrical field was induced to the actuator with700 V dc voltage to the top electrode of the actuator for 2 h and 4 h at 150◦C in an electricaloven (Yamato, DKN302). The butterfly curves of strain and permittivity were then measuredusing a 150 Vop triangular voltage at 0.25 Hz to drive the actuator, in addition to a 1.5 Vopsinusoidal wave at 100 kHz for detection of the permittivity.

Results and Discussions

The displacement and change of permittivity resulting from the imprint electrical fieldtreatment are shown in Figs. 5 and 6, respectively. The change of the displacement butterfly

Figure 5. The displacement butterfly curves. (a) initial butterfly curve (b) the 2 hour imprint treated(c) the 4 hour imprint treated (d) the opposite direction 2 hour imprint treated.


Figure 6. The permittivity butterfly curves. (a) initial butterfly curve (b) the 4 hour imprint treated.

curve is shown in Fig. 5. The initial butterfly curve is symmetric and the actuator has onlyone stable condition at the point of zero electrical field. On the other hand, the butterflycurve after imprint treatment of the actuator is asymmetric, induced by the imprint electricalfield. Shape memory results, because of the asymmetricity; the actuator has changed to havetwo distinct stable positions according to the direction of the polarization. Comparison ofthe 2 h and 4 h treatments indicates that the additional time causes an additional shift of thebutterfly curve. Furthermore, voltage treatment in the opposite direction caused an oppositeshift. These results represent the controllability of the imprint electrical field of the actuator.

The change of the permittivity butterfly curve shown in Fig. 6 is similar to the displace-ment curve, in that the initial curve of the permittivity is symmetric, and after the imprinttreatment it becomes asymmetric, with two distinct values of permittivity depending on thepolarization state. However, in the case of permittivity, the lowering of the peak under acoercive field is the prime factor for the asymmetricity.

Pulse Operation of the Shape Memory Piezoelectric Actuator

Pulse operation of the shape memory piezoelectric actuator was attempted by alternateapplication of positive and negative pulse voltage at 150 V amplitude and 100 ms width.The permittivity detection voltage, a 1.5 Vop sine wave at 100 kHz, was simultaneouslyapplied, and the results are shown in Fig. 7. The shape memory piezoelectric actuatorexhibits two distinct stable values for both strain and permittivity. These values correspondto the values at the point of zero electrical field in the asymmetric butterfly curves of Figs. 5and 6. Moreover, in Fig. 7, each permittivity value with 0 V electrical field corresponds toeach strain condition. This indicates that by measuring the permittivity, the strain conditioncan be detected without the use of other sensors, that is, the actuator can perform self-sensingoperation.


Figure 7. Pulse operation of the shape memory piezoelectric actuator.

Figure 8. Various pulse operation. (a) various pulse time width under the constant amplitude (b)various pulse amplitude under the constant time width.


Various pulse shaped voltage operation was examined, as shown in Fig. 8. The actuatorused for this experiment was prepared using the 4 h imprint treatment. Figure 8(a) shows theshape memory displacement for various pulse width operations under constant amplitude.The result indicates that the actuator realizes shape memory with a pulse width of at least20 ms. Figure 8(b) shows the shape memory displacement for various pulse amplitudesunder a constant pulse width. The shape memory displacement increases in accordancewith the large amplitude. However, the shape memory decreases at amplitude of over 150V. The reason for this is considered to be that the trapped charge is removed by the highelectrical field and the imprint electrical field is decreased, thereby causing a decrease inshape memory displacement.

Conclusion

In summary, a shape memory piezoelectric actuator was proposed and its operation wasdemonstrated. The principle of the shape memory piezoelectric actuator is based on theimprint electrical field and the reversal of polarization. The imprint electrical field wascontrolled by high electrical field treatment in a high temperature environment. The butterflycurves for displacement and permittivity were shifted in the direction of the electrical field.With an imprint electrical field, the piezoelectric strain and permittivity had two stablepoints under a condition of zero electrical field, depending on the polarization direction.The direction of polarization was reversed by a pulse voltage, and this operation resulted ina piezoelectric displacement switch, and also in a change of permittivity. With pulse voltageoperation, the permittivity value indicated the piezoelectric displacement condition, therebyrealizing self-sensing operation.

Control of the imprint electrical field was performed under several conditions of highelectrical field and high temperature. Similar to a FeRAM imprint, this shape memory effectis thought to come from the induced charge at the boundary face between the ferroelectricmaterial and an electrode. Therefore, for the shape memory piezoelectric actuator, thematerial of the electrode should be taken into account. Some materials are thought to beineffective, due to an insufficient piezoelectric constant; however, these materials can bepromising candidates for shape memory piezoelectric actuators, because the characteristicsof the shape memory piezoelectric actuator do not only depend on the piezoelectric constant,but also depend on the butterfly hysteresis properties.

References

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3. K. Itoh, T. Watanabe, S. Kimura, and T. Sakata, Proc. of 2000 International Semiconductor Con-ference, 13–22 (2000).

4. T. Morita, Sens. and Actu. 291, 103–3 (2003).5. H. C. Lee, J. Y. Park, and J. U. Bu, IEEE Microwave and Wireless Components Letters 15, 202

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