Transverse 1-3 piezoelectric ceramic/polymer composite with multi-layered PZT ceramic blocks

Transcription

1 Sensors and Actuators A 134 (2007) Transverse 1-3 piezoelectric ceramic/polymer composite with multi-layered PZT ceramic blocks Chang-Bun Yoon a, Sung-Mi Lee a, Seung-Ho Lee a, Hyoun-Ee Kim a,, Kyung-Woo Lee b a School of Materials Science and Engineering, Seoul National University, Seoul , Republic of Korea b Kyungwon Ferrite Ind. Co., Ltd., Chungwang-dong, Shiheung-si, Kyonggi-do , Republic of Korea Received 1 August 2005; received in revised form 13 April 2006; accepted 1 May 2006 Available online 27 June 2006 Abstract An ultrasonic transducer with transverse 1-3 connectivity was fabricated by using multi-layer piezoelectric blocks. The multi-layer ceramic bodies were composed of piezoelectrically active PZN-PZT layers and electrically conducting PZN-PZT/Ag layers. The multi-layer piezoelectric blocks were fabricated by co-extruding and then CNC-machining the thermoplastic body, which consisted of five piezoelectric layers interleaved with four conducting layers. After binder burnout and sintering, the thicknesses of the piezoelectric layer and the electrode were 280 and 70 m, respectively. The blocks were arranged vertically, and the space between them was filled with an epoxy, in order to fabricate a PZT/epoxy composite with 1-3 connectivity. The apparent piezoelectric coefficient ( d 33,App ) of the PZT/epoxy composites utilizing the transverse (d 31 ) mode was 6000 pc/n, and the displacement was 4.5 m at 800 V (length of specimen: 16 mm). These values increased linearly with decreasing thickness and increasing length of the layers. The principal resonance frequency was 60 khz, at which the displacement was 270 nm with the applied voltage of 10 V ( d 33,App 27, 000 pc/n) Elsevier B.V. All rights reserved. Keywords: Piezoelectric composite; PZN-PZT; Ultrasonic transducer 1. Introduction Piezoelectric ceramics have been extensively used for actuators and transducers [1]. Among the various piezoelectric ceramics, lead zirconate titanate (PZT) has been widely used as the active ceramic phase, since this ceramic offers excellent electromechanical properties, such as a high piezoelectric charge coefficient and a high electromechanical coupling coefficient [2,3]. In the field of ultrasonic transducer applications, PZT/ polymer composites have been particularly prevalent, because their properties are superior to those of monolithic PZT ceramics. These composites offer many unique and attractive features which enable the piezoelectric response to be enhanced, by designing the connectivity of the component phases [4 6]. One of the most promising designs for ultrasonic sensing applications is the PZT/polymer composite with 1-3 connectiv- Corresponding author. address: kimhe@snu.ac.kr (H.-E. Kim). ity, which exhibits low density, a high hydrostatic piezoelectric response, a high electromechanical coupling factor, and improved impedance matching [7 9]. To achieve high electromechanical properties and a moderate acoustic impedance in ultrasonic sensing applications, the volume fraction of the ceramic in the composite should be maintained in the range of vol.% [10 13]. For many emerging applications, such as large area projectors and adaptive materials for fluid borne noise control, more advanced transducers are needed, which provides high power radiation with a large surface displacement, and which has a wide frequency range (off-resonance frequency) while being operated at a low voltage [14]. However, conventional longitudinal piezoceramic polymer composites with one piezoelectric component, such as those which provide 1-3 or 2-2 connectivity, cannot meet these requirements, because their piezoelectric d h coefficient is limited by the simple longitudinal piezoelectric configuration of the polycrystalline ceramics [15 17]. Therefore, in this paper, we introduce a new piezoelectric PZT/polymer composite with transverse 1-3 connectivity, using the piezoelectric transverse mode. In order to increase /$ see front matter 2006 Elsevier B.V. All rights reserved. doi: /j.sna

2 the apparent piezoelectric properties of the composite, multilayer piezoelectric blocks with a high length-to-thickness ratio were used. A multilayer thermoplastic body, consisting of five piezoelectric layers and four conducting layers fabricated by the co-extrusion process [18], was machined into blocks with dimensions of 2 mm 2mm 19 mm. After thermal treatment for binder burnout and densification, the multi-layer PZN-PZT blocks were arrayed vertically and the space between them was filled with an epoxy. The transverse 1-3 piezo-composite exhibited a linear increase in its effective piezoelectric coefficient with decreasing thickness and increasing length of the blocks. This piezo-composite was able to be used in an under-water ultrasonic transmitter application which needs high power and a wide frequency range. 2. Experimental procedure C.-B. Yoon et al. / Sensors and Actuators A 134 (2007) The transverse 1-3 piezo-composite was fabricated using multi-layer piezoelectric blocks, composed of five piezoelectric layers and four electrically conducting layers. The piezoelectric part was composed of low-temperature sinterable PZN-PZT, while the conducting part was composed of a mixture of PZN- PZT and Ag. The composition of the PZN-PZT was fixed at Pb((Zn 1/3 Nb 2/3 ) 0.2 (Zr 0.5 Ti 0.5 ) 0.8 )O 3 and that of the PZN- PZT/Ag was fixed at 45 wt.% PZN-PZT and 55 wt.% Ag, based on our previous research [19 21]. Highly pure PbO, ZnO, TiO 2 and Nb 2 O 5 (all purity 99.9%, Aldrich Chemical Co. Inc., Milwaukee, WI), and ZrO 2 (99%, Aldrich Chemical Co. Inc.) were used as the starting materials. These powders were mixed by ball milling for 24 h using zirconia balls and ethanol as the media. After ball milling, they were dried on a hot plate and subsequently calcined at 850 C for 4 h. The calcined powder was then ball-milled again for 72 h. About 60 vol.% of the dried powders were blended with thermoplastic binders, consisting of EEA (EEA 6182; Union Carbide, Danbury, CT) and IBMA (Paraloid B67; Rohm and Haas, Philadelphia, PA) using a heated high shear-mixer (Jeongsung Inc., Seoul, Korea) at 110 C. Processing aids were also added to ensure a consistent apparent viscosity during blending. After compounding, the PZN-PZT and PZN-PZT/Ag compounds were warm-pressed at 110 C and extruded through a reduction die with a cross section of 24 mm 4 mm or 24 mm 1 mm. An assembly composed of five PZN-PZT layers (each layer having a thickness of 4 mm) interspersed with four PZN-PZT/Ag layers (1 mm thickness) was extruded through a reduction die with a cross section of 24 mm 2 mm to generate multilayer sheets. The produced thermoplastic multi-layer PZN- PZT sheet was machined using a mini-cnc machine (Modela; Roland DGA Corp., Japan) to produce 2 mm 2mm 19 mm (width width length) samples [17,18]. The binders were completely removed by heating the samples in a furnace at a slow heating rate up to 500 C in air to prevent the formation of defects. Following this, the samples were sintered at 900 C for 4 h in air. To minimize the PbO loss from the specimens during sintering, a PbO-rich atmosphere was maintained by placing an equimolar mixture of PbO and ZrO 2 in the crucible. The outer electrodes were prepared by apply- Fig. 1. Multi-layer piezoelectric block: (A) schematic illustration showing the application of the electric field to convert the transverse mode into the longitudinal one and (B) optical micrograph of the fabricated multi-layer block. ing a thin silver paste on the face of the PZN-PZT layers, as shown in Fig. 1(A). The PZT blocks were arranged vertically, the space between them was filled with an epoxy (Spurrs epoxy, Polysciences, Inc., Warrington, PA) and subsequently they were cured at 70 C for 8 h. Thereafter, the composite was poled in a silicone oil bath at room temperature by applying an electric field of 3 kv/mm for 20 min, and then aged for 24 h prior to testing. A conducting layer of TiC Ni, sintered at 1500 C for 1 h in a high vacuum, was bonded on the face of the piezo-composite with an electrically conducting epoxy (D-723S, Fujikurakasei Co., Tokyo, Japan). The piezoelectric properties of the piezocomposite were measured using a quasistatic piezoelectric d 33 meter (model ZJ-3D, Institute of Acoustics, Beijing, China). The field-induced displacement was monitored at the center of the poled specimens by using a displacement probe (DT/2/S, Solartron, UK) and digital laser vibrometer (OFV552, Polytec, Waldbronn, Germany). 3. Results and discussion Piezo-composites with 1-3 connectivity were fabricated using multilayer blocks, which were operated in transverse piezoelectric mode (d 31 mode). The blocks were composed of five piezoelectric PZN-PZT layers and four conducting PZN-

3 482 C.-B. Yoon et al. / Sensors and Actuators A 134 (2007) Table 1 Piezoelectric properties of the transverse 1-3 piezo-composite Parameter Volume (%) Dimensions (mm mm mm) Density (g/cm 3 ) d 33 (pc/n) Displacement at 800 V ( m) PZN-PZT ceramic Transverse multi-layer block Transverse 1-3 piezo-composite w/o plate ± Transverse 1-3 piezo-composite with plate ± PZT/Ag layers. The PZN-PZT showed good electromechanical properties, even when it was sintered at low temperatures (<900 C). Considering the electrical conductivity, densification mismatch, and viscosities of thermoplastic compounds, the electrically conducting layer was fabricated by mixing 45% PZN-PZT and 55% Ag. The electromechanical properties of the PZN-PZT ceramics were as follows: piezoelectric coefficient, d 33 = 510 pc/n, d 31 = 230 pc/n, electromechanical coupling coefficient, k p = 0.65, and relative dielectric coefficient, K T = The electrical conductivity of the 45% PZN- PZT/55% Ag mixture was comparable to that of pure silver [19 21]. The multi-layer piezoelectric block, composed of five 280 m-thick piezoelectric PZN-PZT layers and four 70 mthick conducting layers, had dimensions of 1.7 mm 1.7 mm 16 mm (width width length), as shown in Fig. 1. Fig. 1(A) schematically shows the configuration of the electrodes which converts the transverse mode into the longitudinal one. The electrodes were inserted alternately in order to apply a high electric field to each layer, and in such a way that half of the electrodes extended to the bottom, while the other half extended to the top. Fig. 1(B) shows the optical microscopy image of the sintered multilayer piezoelectric block, which was composed of five piezoelectric layers and four conducting layers. The green bodies with dimensions of 2 mm 2mm 19 mm shrank to dense bodies with dimensions of 1.7 mm 1.7 mm 16 mm, indicating a 40 vol.% shrinkage during sintering. When the piezoelectric properties were measured vertically, the apparent piezoelectric coefficient ( d 33,App ) of a single multi-layer piezoelectric block was about 7000 pc/n. The transverse 1-3 piezo-composite was fabricated with these piezoelectric blocks, by filling in the space between the blocks with epoxy, as shown in Fig. 2. Fig. 2(A) shows a schematic illustration of the 1-3 transverse piezo-composite with a conducting facial TiC Ni plate. The blocks were arranged vertically, and then the empty space was filled with epoxy, to form a PZT/epoxy composite with 1-3 connectivity, as shown in Fig. 2(B). This piezo-composite consisted of 18% piezoelectric multi-layer block (active part) and 82% epoxy (passive part). The actual aspect ratio of the piezo-composite (length/thickness ratio of each layer) was more than 60. To improve its piezoelectric response, TiC Ni facial plates with a high Young s modulus were attached to both sides of the piezo-composite. The measured elastic modulus and density of the sintered TiC Ni plate were 480 GPa and 5.2 g/cm 3, respectively. The apparent piezoelectric properties of the composite are shown in Table 1. The piezoelectric block showed an apparent piezoelectric coefficient ( d 33,App ) of 7000 pc/n and a displacement of 6 m at 800 V under a DC electric field, while the pure PZN-PZT showed an apparent d 33 value and displacement of 510 pc/n and 0.4 m, respectively, at the same voltage. The apparent d 33 of the piezo-composite without the conducting plates (TiC Ni) was 4700 ± 240 pc/n. The piezoelectric coefficient appeared to decrease due to the clamping effect of the passive epoxy matrix. When the conducting plates were attached to both faces, the piezoelectric coefficient increased to 6010 ± 80 pc/n. This improvement was attributed to the improvement in the stress transfer between the piezoelectric block and the polymer [13,14]. The displacements of the composite with respect to the applied voltage are shown in Fig. 3. The PZT block showed a displacement of 6 m at 800 V (Fig. 3(A)) which increased Fig. 2. Transverse 1-3 piezo-composite fabricated using multi-layer PZN-PZT blocks: (A) schematic illustration and (B) 1-3 piezo-composite with 18% PZT/82% polymer.

4 C.-B. Yoon et al. / Sensors and Actuators A 134 (2007) Fig. 3. The uni-polar displacements of the (A) multi-layer PZN-PZT block, (B) 1-3 piezo-composite without the facial plates and (C) 1-3 piezo-composite with the facial TiC Ni plates. linearly in proportion to the voltage, while the 1-3 composite without the facial plates showed a displacement of 3.5 m at 800 V (Fig. 3(B)). The displacement of the 1-3 composite with the facial plates was 4.5 m at the same voltage (Fig. 3(C)), which was higher than that of the 1-3 composite without the facial plates. These results coincided with the behaviors of the piezoelectric coefficient. The apparent piezoelectric coefficient and displacement of a multi-layer PZT block using the transverse mode can be estimated by means of the following equations, taking the force balance between the piezoelectric materials and the electrodes into consideration [14,22]: E 1 A 1 L apparent d 33,block = d 31 E 1 A 1 + E 2 A 2 t (1) L 33,block = d 33,block V (2) where subscripts 1 and 2 refer to the PZN-PZT and PZN- PZT/Ag, respectively, d 31 the transverse piezoelectric coefficient, L the length of the piezoelectric block, t the thickness of each layer, V the applied voltage, A the area of each material, and L 33 is the longitudinal displacement of the specimen. Based on these equations, it can be concluded the displacement of an actuator using transverse mode is amplified by a factor of L/t. Therefore, even though the transverse piezoelectric coefficient (d 31 ) is less than half of the longitudinal coefficient (d 33 ) for the same material, a very high displacement can be achieved by utilizing the d 31 mode. The calculated apparent d 33 value ( d 33,App ) and displacement of the piezoelectric block were 8100 pc/n and 6.5 m at 800 V, respectively. The measured apparent d 33 value (7000 pc/n) and displacement (6.0 m) were reasonably close to the calculated ones. Similarly, the piezoelectric properties of the composite were obtained from the following equation [22]: d 33,composite = E 3 A 3 E 3 A 3 + E 4 A 4 d 33,block (3) Fig. 4. (A) Impedance characteristic of the multi-layer block (B) longitudinal displacement of the multi-layer block as a function of the electrical frequency at 10 V. where subscripts 3 and 4 refer to the PZT block and epoxy, respectively. The calculated d 33 value ( d 33, App ) of the 1-3 composite was 7000 pc/n, while the measured d 33 value of the composite without the facial plates was 4700 pc/n. With the facial plates, the ( d 33, App ) increased to 6000 pc/n, which is similar to the calculated values, indicating that the performance of the piezoelectric composite almost reached its full potential. The resonance characteristics of the multi-layer PZT block are shown in Fig. 4. The impedence variation of the PZT block as a function of the applied frequency under 0.5 V p p low voltage conditions is shown in Fig. 4(A). Because the piezo-composite is driven at or around the resonance frequency, the performances of the piezo-composite are dependent on the resonance characteristics. The resonance and anti-resonance frequencies of the block were 92 and 98 khz, respectively. The displacement of the multi-layer block was measured as a function of frequency with the applied voltage of 10 V p p, as shown in Fig. 4(B). The displacement increased remarkably at the resonance frequency. The displacement of the multi-layer block at the resonance frequency was measured to be 500 nm at 10 V under an AC configuration. The resonance characteristics of the 1-3 composite are shown in Fig. 5. The principle resonance frequency of the 1-3 piezocomposite was between 59 and 62 khz as shown in Fig. 5(A). This resonance frequency appeared to be the d 33 mode of the composite, but was actually the d 31 mode of the multi-layer blocks. Fig. 5(B) shows the longitudinal displacement of the 1-3 composite as a function of the applied frequency at 10 V. Many resonance frequencies were observed for the 1-3 composite in different frequency ranges, and these were attributed to the various components (PZT, electrode, and epoxy) and the geometric effect (1-3 structure). The principal resonance frequency of this piezo-composite was 60 khz which almost coincided with the value by the impedance method. At the resonance frequency, the displacement was 270 nm at 10 V ( d 33, App 27, 000 pc/n). The displacement of the 1-3 composite was about half of that of the multi-layer block. The lower displacement of the piezocomposite compared with the multi-layer block was considered to be a result of the vibration damping caused by the passive

5 484 C.-B. Yoon et al. / Sensors and Actuators A 134 (2007) Conclusions A piezoelectric ceramic/polymer composite with transverse 1-3 connectivity was fabricated using multi-layer piezoelectric blocks, each composed of five 280 m-thick piezoelectric layers and four 70 m-thick conducting layers. The multi-layer PZT blocks were arrayed vertically and the space between them was filled with epoxy. The transverse 1-3 piezo-composite exhibited a linear increase in its effective piezoelectric coefficient with decreasing thickness and/or increasing length of the layers. The measured piezoelectric coefficients (d 33 and d h )ofthe piezo-composite with a length of 16 mm were about 6000 and 4100 pc/n, respectively. Fig. 5. (A) Impedance characteristic of the 1-3 piezo-composite (B) longitudinal displacement of the 1-3 composite as a function of the electrical frequency at 10 V. epoxy matrix. One thing to be noted is that the base displacement of the composite is about 30 nm at all frequencies. The displacements of the 1-3 composite (width length thickness 16 mm 16 mm 16 mm) in the x (d 31 )-, y (d 32 )-, and z (d 33 )-directions at ±100 V and 200 Hz are shown in Fig. 6. The piezo-composite generated a sine motion at its surface, when a sine voltage was applied. The maximum displacement in the z- direction was 0.6 m at 100 V. The apparent d 33 value ( d 33,App ), calculated directly from the slope of the plot of the strain versus the electric field, was 6000 pc/n, which is very close to the value obtained by the static method (Table 1). The displacements in the x- and y-directions were 0.35 and 0.16 m, respectively, which correspond to an apparent d 31 value of 3500 pc/n and an apparent d 32 value of 1600 pc/n. The displacements in the x- and y-directions were in opposite directions, due to the arrangement of the multi-layer block (Fig. 2). Therefore, these two movements cancel each other out, thereby yielding a high hydrostatic piezoelectric coefficient. The calculated hydrostatic piezoelectric coefficient of the 1-3 composite was 4100 pc/n. Fig. 6. Displacements of the transverse 1-3 piezo-composite in the x, y, and z-directions as a function of time at 200 Hz and ±100 V (200 V p p ). References [1] B. Jaffe, W.R. Cook, H. Jaffe, Piezoelectric Ceramics, Academic Press, New York, 1971, pp [2] K. Uchino, Ferroelectric Devices, Marcel Dekker, Inc., 2000, pp [3] Y. Xu, Ferroelectric Materials and Their Application, North-Holland, Amsterdam, Netherlands, 1991, pp [4] J.F. Tressler, S. Alkpu, A. Dogan, R.E. Newnham, Functional composites for sensors, actuators and transducers, Composites, Part A 30 (4) (1999) [5] R.E. Newnham, D.P. Skinner, L.E. Cross, Connectivity and piezoelectricpyroelectric composites, Mater. Res. Bull. 13 (5) (1978) [6] S. Sripada, J. Unsworth, M. Krishnamurty, Y.S. Ng, PZT/polymer composite for medical ultrasound, Mater. Res. Bull. 31 (6) (1996) [7] A.J. Moulson, J.M. Herbert, Electroceramics, Chapman and Hall, London, UK, 1990, pp [8] V.F. Janas, A. Safari, Overview of fine-scale piezoelectric ceramic/polymer composite processing, J. Am. Ceram. Soc. 78 (11) (1995) [9] W.A. Smith, A.A. Shaulov, Composite piezoelectrics: basic research to a practical device, Ferroelectrics 87 (1998) [10] H. Kara, R. Ramesh, R. Stevens, C.R. Bowen, Porous PZT ceramics for receiving transducers, IEEE Trans. Ultrason., Ferroelectron. Freq. Control 50 (3) (2003) [11] T.R. Gururaja, A. Safari, R.E. Newnham, L.E. Cross, Piezoelectric ceramic polymer composites for transducer applications, in: L.M. Levinson (Ed.), Electronic Ceramics, Marcel Dekker, New York, USA, 1987, pp [12] W.A. Smith, B.A. Auld, Modeling 1-3 composite piezoelectrics: thicknessmode oscillations, IEEE Trans. Ultrason., Ferroelectron. Freq. Control 38 (1) (1991) [13] J. Bennett, G. Hayward, Design of 1-3 piezoelectric hydrophones using finite element analysis, IEEE Trans. Ultrason., Ferroelectron. Freq. Control 44 (3) (1997) [14] Q.M. Zhang, J. Chen, H. Wang, J. Zhao, L.E. Cross, M.C. Trottier, A new transverse piezoelectric mode 2-2 piezocomposite for underwater transducer applications, IEEE Trans. Ultrason., Ferroelectron. Freq. Control 42 (4) (1995) [15] G.M. Lous, I.A. Cornejo, T.F. McNulty, A. Safari, S.C. Danforth, Fabrication of piezoelectric ceramic/polymer composite transducers using fused deposition of ceramics, J. Am. Ceram. Soc. 83 (1) (2000) [16] T.R. Gururaja, Piezoelectrics for medical ultrasonic imaging, Am. Ceram. Soc. Bull. 73 (5) (1994) [17] Y.-H. Koh, C.-B. Yoon, S.-M. Lee, H.-E. Kim, Thermoplastic green machining for the fabrication of a piezoelectric ceramic/polymer composite with 2-2 connectivity, J. Am. Ceram. Soc. 88 (4) (2005) [18] C.-B. Yoon, Y.-H. Koh, G.-T. Park, H.-E. Kim, Multilayer actuator composed of PZN-PZT and PZN-PZT/Ag fabricated by co-extrusion process, J. Am. Ceram. Soc. 88 (6) (2005) [19] S.-M. Lee, C.-B. Yoon, S.-H. Lee, H.-E. Kim, Effect of lead zinc niobate addition on sintering behavior and piezoelectric properties of lead zirconate titanate ceramic, J. Mater. Res. 19 (9) (2004)

6 C.-B. Yoon et al. / Sensors and Actuators A 134 (2007) [20] S.-B. Seo, S.-H. Lee, C.-B. Yoon, G.-T. Park, H.-E. Kim, Low-temperature sintering and piezoelectric properties of 0.6Pb(Zr 0.47 Ti 0.53 )O 3 0.4Pb(Zn 1/3 Nb 2/3 )O 3 ceramics, J. Am. Ceram. Soc. 87 (7) (2004) [21] C.-B. Yoon, S.-H. Lee, S.-M. Lee, H.-E. Kim, Co-firing of PZN- PZT flextensional actuators, J. Am. Ceram. Soc 87 (9) (2004) [22] A.C. Ugural, Mechanics of Materials, McGraw-Hill, New York, USA, 1993, pp Biographies Chang-Bun Yoon received his BS and MS degrees in 2000 and in 2002, respectively, from the School of Materials Science and Engineering at the Seoul National University, where he is currently a PhD candidate. His research interests include the areas of piezoelectric ceramics and composites for ultrasonic motors and actuators. Sung-Mi Lee received her BS degree in the Department of Materials Science and Engineering at Hong-Ik University in Korea in Since 2003, she is in the School of Materials Science and Engineering at Seoul National University for PhD degree. She is investigating on processing and properties of piezoelectric PZN-PZT materials. Seung-Ho Lee is a graduate student in the Materials Science and Engineering at the Seoul National University in Korea. He earned his BS degree in Materials Science and Engineering from Seoul National University in His current research interests are relaxor-pzt solid solution ceramics and lead-free piezoelectric ceramics. Hyoun-Ee Kim is a Professor of the School of Materials Science and Engineering at the Seoul National University since Before joining to the SNU, he worked as a Research Scientist in Metals and Ceramic Division at the Oak Ridge National Lab in USA. He received his BS degree from Seoul National University in Korea in 1981 and PhD degree in Ceramic Engineering from the Ohio State University in His research interests include processing and characterization of piezoelectric ceramics and films. Utilization of the piezoelectric materials for ultrasonic motors and transducers are also included in his research field. Kyung-Woo Lee is a Chief Executor of R&D Center at the Kyunngwon Ferrite Industry Co., Ltd. since Before joining to the Kyungwon, he worked as a Research Scientist in Thin Films Technology Division at the KIST (Korea Institute of Science and Technology). He received his MS degree in Electrical Engineering from Yonsei University (Korea) in His research interests include processing and characterization of ceramics, sensors and actuators using piezoelectric effect.