Lead-free piezoceramic cymbal actuator

Sensors and Actuators A 125 (2006) 393–397

Lead-free piezoceramic cymbal actuator K.H. Lam ∗ , X.X. Wang, H.L.W. Chan Department of Applied Physics and Materials Research Centre, The Hong Kong Polytechnic University, Hunghom, Kowloon, Hong Kong, China Received 17 January 2005; received in revised form 22 August 2005; accepted 22 August 2005 Available online 29 September 2005

Abstract Lead-free piezoelectric ceramic 0.90(Bi1/2 Na1/2 )TiO3 -0.05(Bi1/2 K1/2 )TiO3 -0.05BaTiO3 (abbreviated as BNT-BKT-BT5) is used as the driving element in a cymbal actuator with titanium endcaps. Both the electrical and mechanical properties of the lead-free piezoceramic cymbal are compared with that of the lead zirconate titanate (PZT) cymbal. It is found that the performance of the lead-free ceramic cymbal actuator is comparable to those fabricated using hard PZT ceramic driving element because BNT-BKT-BT5 has reasonable piezoelectric coefficients and low density. © 2005 Elsevier B.V. All rights reserved. Keywords: Lead-free ceramics; Cymbal actuator

1. Introduction The most widely used piezoelectric ceramics are leadbased ceramics, especially Pb(Zr,Ti)O3 (PZT), because of their superior piezoelectric properties. With concern in the environmental pollution of PbO evaporation, lead-free piezoelectric ceramics have recently attracted considerable interest to replace the lead-based material systems [1]. A (Bi1/2 Na1/2 )TiO3 (Bi1/2 K1/2 )TiO3 -BaTiO3 ternary system (BNT-BKT-BT) has been found to be a promising lead-free piezoelectric composition. This BNT-BT and BNT-BKT5 system has a rhombohedral (FR )—tetragonal (FT ) morphotropic phase boundary (MPB) [2,3]. For compositions near the MPB, high piezoelectric and dielectric properties have been obtained. The cymbal actuator is an actuator in which a piezoelectric disc is sandwiched between two truncated conical metal endcaps made of brass or titanium (Fig. 1). It has a high displacement compared to other types of piezoelectric actuators [4,5], and has been extensively studied for various applications such as servo displacement transducer and pulse drive motor [5]. The piezoelectric disc in a cymbal actuator acts as a driving element and the metal endcaps have the function of enlarging the displacement by converting the transverse shrinkage of the piezoelectric disc into a longitudinal displacement. When the ∗

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0924-4247/$ – see front matter © 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.sna.2005.08.028

cymbal actuator is driven by an electric field parallel to the poling direction of the piezoelectric disc, the radial motion of the disc is converted into flexural and rotational motions in the metal endcaps as shown in Fig. 1 [6]. Supported by the stiffness of metal, a large resultant displacement can be obtained. Thus, the displacement is created by a combination of flexural and rotational motions [7]. 2. Materials used BNT-BKT-BT5 ceramic discs of 10 mm diameter and 0.9 mm thickness were prepared by the conventional mixed oxide technique [3]. To compare the performance of the lead-free ceramics, lead-based ceramic discs with identical dimensions fabricated using PKI PZT powder (Piezo Kinetics, USA) were also prepared. Two types of PZT ceramic powders PZT802 and PZT552 were used. PZT802 is a hard piezoelectric material while PZT552 is a soft PZT. A summary of poling conditions of various materials used in this work is shown in Table 1. After poling, the samples were short-circuit annealed at room temperature for 8 h to remove the injected charges. The density, ρ, of the samples was measured using a method based on the Archimedes principle. The piezoelectric coefficient, d33 of both the ceramic discs and the cymbal actuators were measured by a d33 meter (ZJ-3B) which was supplied by the Beijing Institute of Acoustics, Academia Sinica. The hydrostatic coefficient, dh , of the samples was measured using a hydrostatic

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Fig. 1. Schematic diagram of a cymbal actuator. The solid arrows show the directions of displacements when the cymbal actuator is driven by an electric field parallel to the poling direction of the piezoelectric disc.

Table 1 Poling conditions of various driving elements of the cymbal actuator

Poling field (kV/mm) Poling temperature (◦ C) Poling time (min)

the lead-free ceramic is quite high, which is comparable to the soft PZT.

BNT-BKT-BT5

PZT802

PZT552

4.5 80 5

6.0 140 30

3.0 120 30

chamber made of PMMA [8]. To obtain a stable measuring environment, silicone oil was used instead of air. After d33 and dh have been determined, d31 can be calculated by dh − d33 (1) 2 The impedance and phase spectra of the samples were measured using an impedance/gain phase analyzer (Agilent 4294A). The electromechanical properties, kt , of the samples were determined at room temperature following the IEEE standard on piezoelectricity [9] 
π fr π fa − f r 2 kt = (2) tan 2 fa 2 fa d31 =

3. Cymbal actuators After poling and characterization, ceramic discs of 10 mm diameter and ∼0.9 mm thick were used as the driving element in the cymbal actuators. The truncated conical metal endcaps were fabricated by a die punching method. The endcap material is a ∼0.3 mm thick titanium sheet. The driving element and endcaps were bonded with an epoxy (Araldite LY5138/HY5138). A specially designed pressing mould was used to provide a uniform distribution of pressure on the bonding area and maintain the shape of the actuator. The photograph of the cymbal actuator is shown in Fig. 2. The performance of the actuator was evaluated by electrical and mechanical measurements. The electrical characteristics were related to the resonance mode of the actuator and were measured by an HP4294A impedance analyzer.

where fr and fa are the resonance and anti-resonance frequencies of the thickness mode of the sample. The properties of the ceramics are compared in Table 2. The density of the lead-free ceramic is the lowest compared with PZT ceramics. For the piezoelectric coefficients, the soft PZT has the highest values while the d31 coefficient of the lead-free ceramic is comparable to that of the hard PZT. The kt value of Table 2 Properties of various driving elements of the cymbal actuator

(kg/m3 )

ρ d33 (pC/N) d31 (pC/N) kt

BNT-BKT-BT5

PZT802

PZT552

5880 150 −65 0.49

7573 215 −82 0.39

7552 580 −265 0.51 Fig. 2. Photograph of the cymbal actuator.

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Table 3 Summary of the fundamental resonance frequency of cymbal actuators fabricated using different driving elements in which three determine methods are compared Fundamental resonance frequency, f1 (kHz)

Calculation (thin plate theory) Simulation (ANSYS) Experimental

BNT-BKT-BT5

PZT802

PZT552

91.9 95.2 94.2

82.6 88.7 85.1

82.8 89.1 85.9

After bonding the endcaps on the ceramic, the first resonance frequency of the actuator is associated with the flextensional mode of the endcaps. The first resonance frequency of the cymbal actuator can be estimated using the thin plate vibration theory [10].  t Y f1 = 0.47 2 (3) a ρ(1 − σ 2 ) Using Eq. (3), the first resonance frequency can be estimated and listed in Table 3. It depends on the geometry (the thickness t and the radius a) and the density ρ of the sample, Young’s modulus Y (=116 GPa) and Poisson’s ratio σ (=0.36) of the titanium endcap. Besides using analytic calculation, the first vibration mode of the cymbal actuator can also be simulated by the finite element method (FEM) using the commercial FEM program ANSYS. The simulation (Fig. 3) shows how the cymbal actuator vibrates under its first vibration mode with frequency fr (Table 3). This mode is the (0, 1) or “umbrella” flexural mode of the caps [11]. To verify the estimation by Eq. (2) and the ANSYS simulation, the electrical properties of the cymbal actuator are determined using the resonance technique. Fig. 4 shows the fundamental mode (fr ∼ 95 kHz) of the BNT-BKT-BT5 cymbal actuator which is the first resonance mode in the impedance spectrum. The measured fundamental resonance frequency of the cymbal actuator is quite close to that estimated using Eq. (3) and by FEM simulation as shown in Table 3. When comparing fr in cymbals with different driving elements, it is found that the fundamental resonance frequency in the BNT-BKT-BT5 cymbal actuator is the highest compared with that of the PZT cymbal actuators. It can be explained using the thin plate theory (Eq. (3)) as the resonance frequency is inversely proportional to its density. To evaluate the electrical performance of the cymbal actuator, the piezoelectric d33 coefficient, the effective coupling coefficient and the fastest response time were calculated. The effective coupling coefficient, keff , describes the conversion of energy

Fig. 4. The fundamental mode of a BNT-BKT-BT5 cymbal actuator. Table 4 Comparison between the cymbal actuators fabricated using different driving elements

d33 (pC/N) keff t(FRT) (␮s)

BNT-BKT-BT5

PZT802

PZT552

1550 0.17 10.3

1600 0.16 11.8

3550 0.17 11.6

from electrical to mechanical form or vice versa. The fastest response time (FRT) is defined as the time for the device to achieve the precise response without over-shooting and ringing. Those parameters were determined by the series and parallel resonant frequencies of the fundamental mode of the cymbal actuator using the following equations [9]: 
2 fr keff = 1 − (4) fa t(FRT) =

1 fr

(5)

The electrical properties of the cymbal actuators fabricated using different driving elements are summarized in Table 4. As expected, the PZT552 cymbal actuator has the highest d33 coefficient because of the soft PZT552 ceramic has the highest piezoelectric coefficients. The values of keff are comparable for the cymbal actuators fabricated using the lead-free and lead-based driving elements. For t(FRT) , the BNT-BKT-BT5 cymbal actuator has the shortest response

Fig. 3. The endcap deflection during the first vibration mode of the cymbal actuator (dash line: undeformed shape; solid line: deformed shape).

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Both results show that the mechanical performance of the lead-free cymbal actuator is comparable to that of the hard PZT cymbal. The axial displacement d of an ideal cymbal actuator without considering the thermal displacement dependence can be estimated by the following equation [6]:    2 1/2   (dc + d31 Edc − dt ) d=2 l2 − − h + d33 Etp   2 (6)

Fig. 5. ac displacement of cymbal actuators with different piezoelectric driving elements as a function of position on the surface of endcap.

time (10.3 ␮s) because of its highest fundamental resonance frequency. The mechanical characteristic was related to the displacement of the actuator. The displacement can be divided into two types: ac and dc. The ac displacement was measured by a Polytec “outplane vibrometer” (Model no.: OFV-3300-2). During the measurement, the laser scans the surface of the endcap while the cymbal actuator was driven by a constant ac electric field of 0.1 kV/mm at 10 kHz. The dc axial displacement was measured by a Fotonic sensor (MTI-2000) consists of a non-contact type fibre-optic probe and the actuator was driven by a dc voltage. The measurement location was the centre of the endcap. Figs. 5 and 6 show the ac and dc displacements induced in the cymbal actuators fabricated using different piezoelectric driving elements. In Fig. 5, the resultant profile is similar in shape to that of the endcap. As expected, the ac displacement of the PZT552 cymbal actuator which has the highest piezoelectric coefficients is the highest. The displacement of the BNT-BKT-BT5 cymbal actuator is similar to that of the PZT802 cymbal actuator. Similar trend is observed in the dc displacements measured for the three cymbal actuators as shown in Fig. 6.

where dc is the cavity diameter, E the applied electric field, dt the diameter of the top part of the endcap, h the cavity depth and tp the thickness of the driving element (see Fig. 1). As the second term in Eq. (6) is much smaller than the first term, the axial displacement d is significantly affected by the magnitudes of the d31 coefficient for given actuator geometry. Since the d31 coefficient of BNT-BKT-BT5 ceramic is similar to that of PZT802 ceramic, the resultant displacement in the corresponding cymbal actuators is comparable. 4. Conclusion Cymbal actuator made of the lead-free BNT-BKT-BT5 has been studied. The effective electromechanical properties of the BNT-BKT-BT5 cymbal actuator are comparable to that of the hard PZT cymbal actuator. Because of its low density, the lead-free cymbal has higher fundamental resonance frequency compared with the lead-based cymbals and hence has a faster response time. Although the d33 coefficient of BNTBKT-BT5 ceramic is low compared with both PZT ceramics, the resultant mechanical performance of the lead-free cymbal is comparable to that of the hard PZT cymbal actuator because of the good d31 coefficient. It is envisaged that BNTBKT-BT5 ceramics with reasonable good piezoelectric properties has the potential to become the next generation actuator material. Acknowledgements This work was supported by the Hong Kong Research Grants Council (PolyU 5317/04E) and by the Centre for Smart Materials of the Hong Kong Polytechnic University. References

Fig. 6. dc displacement of cymbal actuators with different piezoelectric driving elements as a function of dc electric field.

[1] T. Takenaka, Ultrason. Technol. 8 (2001) 2 (in Japanese). [2] H. Nagata, M. Yoshida, Y. Makiuchi, T. Takenaka, Large piezoelectric constant and high curie temperature of lead-free piezoelectric ceramic ternary system based on bismuth sodium titanate-bismuth titanate near the morphotropic phase boundary, Jpn. J. Appl. Phys. 42 (2003) 7401– 7403. [3] X.X. Wang, X.G. Tang, H.L.W. Chan, Electromechanical and ferroelectric properties of (Bi1/2 Na1/2 )TiO3 -(Bi1/2 K1/2 )TiO3 -BaTiO3 lead-free piezoelectric ceramics, Appl. Phys. Lett. 85 (1) (2004) 91–93. [4] A. Dogan, J.F. Fernandez, K. Uchino, R.E. Newnham, The “cymbal” electromechanical actuator, Proc. IEEE Symp. 2 (1996) 213– 216. [5] K. Uchino, Piezoelectric Actuators and Ultrasonic Motors, Kluwer Academic Publishers, USA, 1997.

K.H. Lam et al. / Sensors and Actuators A 125 (2006) 393–397 [6] J.F. Fernandez, A. Dogan, J.T. Fielding, K. Uchino, R.E. Newnham, Tailoring the performance of ceramic–metal piezocomposite actuators, ‘cymbals’, Sens. Actuators A 65 (1998) 228–237. [7] A. Dogan, K. Uchino, R.E. Newnham, Composite piezoelectric transducer with truncated conical endcaps “cymbal”, IEEE Trans. Ultrason. Ferroelect. Freq. Contr. 44 (1997) 597–605. [8] H. Taunaumang, I.L. Guy, H.L.W. Chan, Electromechanical properties of 1-3 piezoelectric ceramic/piezoelectric polymer composites, J. Appl. Phys. 76 (1994) 484–489. [9] IEEE Standard on Piezoelectricity, ANSI/IEEE Std. 176 (1987). [10] L.E. Kinsler, A.R. Frey, A.B. Coopens, J.V. Sanders, Fundamentals of Acoustics, 3rd ed., Wiley, New York, 1982, p. 92. [11] J.F. Tressler, W. Cao, K. Uchino, R.E. Newnham, Ceramic–metal composite transducer for underwater acoustic applications, Proc. IEEE Symp. 2 (1996) 561–564.

Biographies K.H. Lam is a PhD candidate in the Department of Applied Physics, The Hong Kong Polytechnic University, Hong Kong, China. He received the BSc and MPhil degrees in applied physics from the Hong Kong Polytechnic Uni-

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versity in 2000 and 2002, respectively. He has worked on the PMN-PT ceramics and their composites in his MPhil thesis. He is currently working on the applications of piezoelectric materials. X.X. Wang received his PhD degrees in electronic materials and engineering from Shzuoka University of Japan in 2001 on Japanese Monbusho Scholarship. He is currently a research fellow at Hong Kong Polytechnic University. His most recent research interest is in research and development of lead-free piezoelectric ceramics and devices. H.L.W. Chan was born in Hong Kong in 1948. She received the BSc and MPhil degrees in physics from The Chinese University of Hong Kong in 1970 and 1974, respectively, and the PhD degree in material science from Macquarie University, Australia in 1987. Dr. Chan was a research scientist at CSIRO Division of Applied Physics in Sydney, N.S.W., Australia, during 1987–1991. She then worked at GEC-Marconi Australia for 1 year as a senior acoustic designer before she returned to Hong Kong in 1992. Dr. Chan is presently a chair professor and head of the Department of Applied Physics at The Hong Kong Polytechnic University. Her research interests are processing and characterization of ferroelectric ceramics, polymers and composites for developing the applications of ferroelectric materials in ultrasonic transducers, integrated pyroelectric sensors and arrays.