Properties of Step-down Multilayer Piezo Stack Transformers Using PNN-PMN-PZT Ceramics with CeO 2 Addition

Transcription

1 Journal of the Korean Physical Society, Vol. 58, No. 3, March 2011, pp Properties of Step-down Multilayer Piezo Stack Transformers Using PNN-PMN-PZT Ceramics with CeO 2 Addition Insung Kim, Minsoo Kim, Soonjong Jeong, Jaesung Song and Hyeonkyu Joo Battery and Piezoelectric Research Center, KERI, Changwon , Korea Vo Viet Thang Department of Energy Conversion, University of Science and Technology, Daejeon , Korea Alexandru Muller National Institute for Research and Development in Microtechnologies, Romania (Received 8 March 2010, in final form 1 July 2010) In order to fabricate multilayer piezo stack transformers, we investigated the influences of the sintering temperature and the dielectric and piezoelectric properties of Pb(Ni 1/3 Nb 2/3 )- Pb(Mn 1/3 Nb 2/3 )-Pb(Zr 0.505Ti 0.495)O 3 (PNN-PMN-PZT) ceramics with CeO 2 additives due to their effects of grain growth suppression. Meanwhile, the effects of CeO 2 additives on the microstructure and the electrical properties of PNN-PMN-PZT were investigated in detail. In addition, multilayer piezo stack transformers with the composition of PNN-PMN-PZT were fabricated, and the characteristics were measured for various load resistances at resonance frequency. The voltage stepup ratio was continuously changed with increasing input voltage and load resistance. The output voltages and powers were increased with increasing input voltage at matching impedance. The temperature rise of the multilayer piezo stack transformer increased with increasing input voltage and load resistance. These results indicate that the optimized properties of ceramics, d 33 = 274 pc/n, K p = 0.55, Q m = 2581, ɛ r = 1474, density = g/cm 3, and tanδ = , were obtained at a sintering temperature of 1250 C. Meanwhile, multilayer piezo stack transformers a sintered at 1250 C showed favorable characteristics with a power of 5 W at 150 Ω. PACS numbers: j, s, d Keywords: PNN-PMN-PZT, Step-down, Multilayer, Piezo stack transformer, CeO 2 DOI: /jkps I. INTRODUCTION Piezoelectric transformers have been studied for many years, and piezoelectric ceramics used for high-power device, such as transformers, actuators and ultrasonic motors, have been extensively studied [1, 2]. Among them, piezoelectric transformers have been actively researched mainly to increase the step-up ratio and the output power [3]. Multilayer piezoelectric transformers have the advantages of high density, large planar electro-mechanical coupling factor (K p ) and high mechanical quality factor (Q m ), but they are subject to de-lamination, and the need for an increased sintering temperature make the electrodes costly. This is of particular concern during the co-firing process when a mismatch in the sintering of the two materials (the ceramic and the internal elec- kimis@keri.re.kr; Fax: trode) can cause interfacial delamination, cracking and other defects [4]. Therefore, an efficient and stable process is necessary to manufacture multilayer piezoelectric transformers. Since piezoelectric transformers should not overheat during use piezoelectric ceramics should have a high factor Q m and a low dielectric loss tanδ. Furthermore, the piezoelectric transformers must to have good transform efficiency, which means their electromechanical coupling factor k p must be high [5]. PNN-PMN-PZT ceramics have been reported to be suitable for high-power piezoelectric transformer due to their excellent electromechanical properties [6]. It is well known that rareearth doping can improve the mechanical properties of ceramics due to their grain growth suppression effect, their purifying effect on the interface between grains, effective solution, and their strengthening and toughening effect [7]. Variations in the Zr/Ti ratios and several dopants of either a donor or a acceptor type added separately on together to PZT systems, such as CeO 2 [8],

2 Properties of Step-down Multilayer Piezo Stack Transformers Using Insung Kim et al MnO 2 [9] and so on, have been intensively investigated in order to improve electrical properties. However, for applications to high-power piezoelectric transformers, high electrical properties are not enough, the better manufacturing processes are also required. In this study, in order to easily fabricate multilayer piezoelectric transformers, we studied the influences of the piezoelectric and the dielectric properties of PNN- PMN-PZT ceramics with CeO 2 addition; then, we manufactured step-down multilayer piezo stack transformers by using the composite ceramics. The mechanisms for the changes in the electrical properties with CeO 2 doping are also discussed. We expect these work to lead to a promising composition for multilayer piezo stack transformer applications. The characteristics of multilayer piezo stack transformers were also studied for various load resistances at the resonance frequency. The multilayer piezo stack transformer can be successfully fabricated in the 5 W class by using composited ceramics. II. EXPERIMENTS AND DISCUSSION 1. Electric and Piezoelectric Properties of PNN-PMN-PZT with CeO 2 Addition Industrial-grade raw powders of PbO (99%), ZrO 2 (99%), TiO 2 (98%), NiO (99%), Nb 2 O 5 (99.5%), MnO 2 (99%), and CeO 2 (99.5%) were used as the raw materials to prepare the powders and ceramics with a composition of 0.01Pb(Ni 1/3 Nb 2/3 )O Pb(Mn 1/3 Nb 2/3 )O Pb(Zr Ti )O 3 (abbreviated as PNN-PMN- PZT), and to these specimens, χ wt% CeO 2 (χ = 0.0, 0.1, 0.2, 0.3, 0.4, and 0.5) was added by using a conventional method. The mixtures were milled in ethanol using zirconia balls in a polyethylene jar for 24 h. The mixed slurry was dried at 80 C for 24 h and calcined at 850 C for 2 h. After drying, powders were added to a 10-wt% polyvinyl alcohol (PVA) solution and were then pressed into plates with diameters of 18 mm under a lead pressure of 100 MPa. The Pressed plates were sintered at 1250 C for 4 h in a sealed alumina crucible under a lead atmosphere. Subsequently, the sintered disks were polished, and silver-paste electrodes were fired at 700 C. The ceramics were poled at 120 C for 30 min under a DC electric field of 3 kv/mm in silicone oil. The bulk densities of the sintered specimens were measured by using the Archimedes method. The dielectric properties were obtained by measuring the capacitance and the dielectric loss at 1 khz at room temperature by using an impedance analyzer (HP4294A). The piezoelectric constant (d 33 ) was measured using a quasi-static piezoelectric d 33 meter (Model ZJ-3d, Institute of Acoustics Academic Sinica, China). The electromechanical coupling factor (k p ) and the mechanical quality factor Fig. 1. XRD pattern of the PNN-PMN-PZT ceramics sintered at 1250 C with different CeO 2 amounts. (Q m ) were determined by using the resonance and antiresonance technique, on the basis of IEEE standards, with an impedance analyzer (HP4294A). Figure 1 shows the XRD pattern of the PNN-PMN- amounts. All diffraction peaks can be related to a pure perovskite phase when the concentration of CeO 2 addition is below 0.4 wt%. However, with further increases in CeO 2 addition, the characteristic diffraction of CeO 2 is detected, which means that a small amounting of CeO 2 can be effectively incorporated into the lattice of the perovskite structure, but extra CeO 2 tends to accumulate in the ceramics. Thus, one can conclude that the solution limit of CeO 2 in the ceramics is about 0.4 wt%. Generally, the diffraction peaks at 2θ = 45 can be associated with the (002) and (200) reflections of the tetragonal phase and with the (200) reflection of the rhombohedral phase [10]. Figure 2 shows the SEM photographs of PNN-PMN- amounts. The PNN-PMN-PZT ceramics without CeO 2 have a homogeneous microstructure and well-grown grains. However, the specimen doped with 0.2 wt% CeO 2 exhibited more uniform structure. Furthermore, increasing amount of CeO 2 above 0.3 wt% reduced the grain size gradually. From the above results, one can conclude that CeO 2 has a solubility of about 0.2 wt% in the PNN-PMN-PZT system. Ce ions may be homogenously dissolved in the PNN-PMN-PZT system when the CeO 2 addition is below 0.2 wt%, but further addition of CeO 2 will inhibit the grain growth due to accumulation of Ce ions at the grain boundary [11]. Figure 3 shows the k p and the Q m of PNN-PMN- amounts. When the content of CeO 2 is wt%, k p increases, and Q m decreases. The variations may be due to CeO 2 being a dopant with both soft and hard charac-

3 -582- Journal of the Korean Physical Society, Vol. 58, No. 3, March 2011 Fig. 4. tanδ and density of PNN-PMN-PZT ceramics sintered at 1250 C with different CeO2 amounts. Fig. 2. SEM photographs of PNN-PMN-PZT ceramics sintered at 1250 C with different CeO2 amount: (a) 0.0 wt%, (b) 0.1 wt%, (c) 0.2 wt%, (d) 0.3 wt%, (e) 0.4 wt%, and (f) 0.5 wt%. Fig. 5. r and d33 of PNN-PMN-PZT ceramics sintered at 1250 C with different CeO2 amount. Fig. 3. kp and Qm of PNN-PMN-PZT ceramics sintered at 1250 C with different CeO2 amounts. teristics and to Ce4+ being reduced from the Ce4+ to the Ce3+ state during calcination or sintering [12]. Ce3+ occupying the A site of the perovskite structure it leads to lead vacancies that make the mobility of the domain wall easier. Subsequently, kp increases, but Qm decreases. As the content of CeO2 is increased from 0.3 to 0.5 wt%, kp gradually decreases. The decrease in kp may be caused by a decrease in grain size while excess CeO2 will stay in the boundary and form a grain boundary layer, which cause a decrease in Qm [13]. Figure 4 shows the tanδ and the density of PNN-PMN- PZT ceramics sintered at 1250 C with different CeO2 amounts. When the content of CeO2 is in the range of wt%, the density increases. Their result indicates that the proper addition of CeO2 can enhance the density of the ceramics. According to the report of Hou et al. [14], the variation in SEM data is related to the result for the density. When the content of CeO2 is at 0.2 wt%, well-grown grains with relatively homogenous sizes can be seen in Fig. 2. When the content of CeO2 is in the range of wt%, tanδ increases gradually with increasing content of CeO2. This is caused by the creation of oxygen vacancies, which pins the movement of the ferroelectric domain walls and results in an increased tanδ. Figure 5 shows the r and the d33 of PNN-PMNPZT ceramics sintered at 1250 C with different CeO2 amounts. When the content of CeO2 is increased from 0 to 0.2 wt%, d33 increases. Then, when the content of CeO2 is in the range of wt%, r increase gradually with increasing content of CeO2. As a result, the PNN-PMN-PZT ceramics with 0.2 wt% CeO2 sintered at 1250 have maximum values of d33, kp, Qm, density, r,

4 Properties of Step-down Multilayer Piezo Stack Transformers Using Insung Kim et al and tanδ, which are 274 pc/n, 0.55, 2581, g/cm3, 1474, and , respectively. This composition can be effectively practical used for high quality multilayer piezo stack transformer applications. We used ceramics with optional compositions to fabricate multilayer piezo stack transformers, and we studied their characteristics. 2. Fabrication of a Step-down Multilayer Piezo Stack Transformer In order to fabricate the multilayer piezo stack transformer, we added powders to a 10-wt% polyvinyl alcohol (PVA) solution then, we pressed the powders into plates with dimensions of mm under a pressure of 100 MPa. The pressed plates were sintered at 1250 C for 4 h in a sealed alumina crucible under a lead atmosphere. Subsequently, the sintered plates were polished to a thickness of 1.0 mm, and silver-paste electrodes were fired at 700 C. The ceramics were poled at 120 C for 30 min under a DC electric field of 3 kv/mm in silicone oil. Then, the manufactural piezoelectric ceramic plates were piled up. Electrical contact of each PZT layer was achieved by introducing an insulator between the layers and interconnecting them electrically on the stacks surface. A schematic diagram, the poling direction and a multilayer piezo stack transformer are presented in Figs. 6(a), (b), and (c), respectively. When the input voltage is applied to the dot part, a radial extensional vibration is generated. Then, an output voltage appears at the ring part. The inputs between the layers are connected in series to increase the impedance while the outputs are connected in parallel to increase the output capacitance [15]. The dielectric and the piezoelectric properties of the multilayer piezo stack transformer were measured 24 h after poling by using the IEEE resonance technique [16]. The electrical properties of the multilayer piezo stack transformer were measured using a setup that consisted of an oscilloscope (Tektronix TDS 2014), a function generator (Agilent 33120A), and a power amplifier (NF 4055), and for various load resistances and driving frequencies, the electrical characteristics of the multilayer piezo stack transformer were investigated. Figure 7 shows the voltage step-up ratio of a multilayer piezo stack transformer as a function of frequency for different load resistances under a constant output power of 2 W. The step-up ratio increased to 0.70 with increasing load resistance from 150 Ω to 300 Ω. Also, the driving frequency showing the maximum step-up ratio increased with increasing load resistance. This phenomenon can be illustrated by the increased elastic vibration of a multilayer piezo stack transformer due to the increased load resistance. The maximum voltage step-up ratios of the multilayer piezo stack transformer at operating frequencies from 69 to 71 khz were 0.36, 0.52, and 0.70, respectively, at load resistances of 150, 200, and 300 Ω. Fig. 6. (Color online) Schematic diagram of the multilayer piezo stack transformer: (a) schematic diagram (b) poling direction, and (c) multilayer piezo stack transformer. Fig. 7. Voltage step-up ratio of a multilayer piezo stack transformer as a function of frequency for different load resistances under a constant output power of 2 W. Figure 8 shows the output power of a multilayer piezo stack transformer as a function of the input voltage and the load resistance at resonance frequency. The maximum output powers of the multilayer piezo stack trans-

5 -584- Journal of the Korean Physical Society, Vol. 58, No. 3, March 2011 reflection for the rhombohedral phase. The characteristics of multilayer piezo stack transformers were studied at load resistances ranging from 150 Ω to 300 Ω. The maximum voltage step-up ratios of the multilayer piezo stack transformers at operating frequencies from 69 to 71 khz were 0.36, 0.52, and 0.70, respectively, at the load resistances of 150, 200, and 300 Ω. The multilayer piezo stack transformers exhibited excellent properties for the 5W class in terms of output power. REFERENCES Fig. 8. Output power of a multilayer piezo stack transformer as a function of input voltage for various load resistances at resonance frequency. formers were 5.36, 4.3, and 3.6 W at load resistances of 150, 200, and 300 Ω, respectively. The manufactured multilayer piezo stack transformers could be used up to 5 W at 150 Ω because the temperature rise from room temperature should be less than about 20 C for stable operation. III. CONCLUSION In this research, multilayer piezo stack transformers with high quality have been successfully manufactured using 0.01Pb(Ni 1/3 Nb 2/3 )O Pb(Mn 1/3 Nb 2/3 )O Pb(Zr Ti )O 3 ceramics with CeO 2. As a result, the PNN-PMN-PZT ceramics with 0.2 wt% CeO 2 sintered at 1250 C had the maximum values of d 33, k p, Q m, density, ɛ r, and tanδ, which were 274 pc/n, 0.55, 2581, g/cm 3, 1474, and , respectively. Meanwhile, all diffractions could be identified as the pure perovskite phase when the concentration of CeO 2, addition was below 0.4 wt% and the diffractions at 2θ = 45 could be referred as to the (002) and (200) reflections of for the tetragonal phase and the (200) [1] B. H. Kim and J. H. An, J. Korean Phys. Soc. 44, 346 (2004). [2] H. K. Joo, I. S. Kim, J. S. Song, S. J. Jeong, M. S. Kim and S. H. Jeon, J. Korean Phys. Soc. 54, 877 (2009). [3] J. Hu, Y. Fuda, M. Katsuno and T. Yoshida, Jpn. J. Appl. Phys. 38, 3208 (1999). [4] J. H. Jean and C. R. Chany, J. Am. Ceram. Soc. 80, 2401 (1997). [5] L. Sun, C. Feng, Q. C. Sun and H. Zhou, Mater. Sci. Eng., B 122, 61 (2005). [6] H. K. Joo, I. S. Kim, S. J. Jeong, M. S. Kim and J. S. Song, J. Korean Phys. Soc. 56, 374 (2010). [7] Y. D. Hou, M. K. Zhu, F. Gao, H. Wu, B. Wang, H. Yang and C. S. Tian, J. Am. Ceram. Soc. 87, 847 (2004). [8] D. Sun, Z. Zhang, C. Liu, Q. Zhang, D. He and X. Lu, J. Rare Earths 23, 137 (2005). [9] C. Xu, X. Ai, C. Huang and J. Deng, J. Rare Earths 18, 73 (2000). [10] P. Lu, W. Zou, Y. Hou. Q. Ma and C. Wang, J. Rare Earths 25, 176 (2007). [11] R. Zhang, Z. Yang, X. Chao and C. Kang, Ceram. Int. 35, 199 (2009). [12] A. Garg and D. C. Agrawal, J. Mater. Sci. - Mater. Electron. 10, 649 (1999). [13] Y. D. Hou, P. X. Lu, M. K. Zhu, X. M. Song, J. L. Tang, B. Wang and H. Yan, Mater. Sci. Eng. 116, 104 (2005). [14] Y. D. Hou, M. K. Zhu, H. Wang, B. Wang, H. Yang and C. S. Tian, Mater. Lett. 58, 1508 (2004). [15] J. Y. Yoo, K. J. Kim, C. G. Lee, L. H. Hwang, D. S. Paik, H. S. Yoon and H. W. Choi, Sens. Actuators, A 137, 81 (2007). [16] IEEE Standard on Piezoelectricity (American National Standards Institute, Washington DC, 1976).