- Email: [email protected]
Direct ink writing of 3–3 piezoelectric composite
Journal of Alloys and Compounds 620 (2015) 125–128
Contents lists available at ScienceDirect
Journal of Alloys and Compounds journal homepage: www.elsevier.com/locate/jalcom
Direct ink writing of 3–3 piezoelectric composite Ya-Yun Li a,⇑, Long-Tu Li a, Bo Li b a b
State Key Laboratory of New Ceramics and Fine Processing, School of Materials Science and Engineering, Tsinghua University, Beijing 100084, PR China Research Institute for Advanced Materials, Graduate School at Shenzhen, Tsinghua University, Shenzhen 518055, PR China
a r t i c l e
i n f o
Article history: Received 14 July 2014 Received in revised form 15 September 2014 Accepted 15 September 2014 Available online 28 September 2014 Keywords: 3–3 Piezoelectric composite Direct ink writing PLZT Figure of merit
a b s t r a c t Three dimensional (3D) ceramic structures form a construction of piezoelectric composite would be prepared with direct ink writing method (DIW). The preparation of aqueous based lead zirconate titanate lanthanum (PLZT) inks and the principle of DIW were systematically investigated. The ink with solids volume fraction about 70 vol% by aging 48 h reveals shear-thinning behavior and proper viscoelastic properties. As shown by scanning electron microscopy (SEM), PLZT samples sintered at 1200 °C for 4 h in a lead-rich atmosphere yielded best microstructures which were densified with relative density exceed 98%. The test of X-ray diffraction (XRD) reveals that the main phase of sintered samples is rhombohedral Pb0.93La0.07(Zr0.65Ti0.35)0.9825O3. 3–3 Piezoelectric composite formed by 3D PLZT ceramics filled with epoxy resin at 40 °C have higher hydrostatic figure of merit dhgh (4112 1015 m2/N) than that of the monolithic PLZT piezoelectric ceramics (365 1015 m2/N). Piezoelectric ceramic–polymer composites can be widely used for the applications like underwater acoustic transducers in having combined hardness and electric properties of piezoelectric ceramics and flexibility, low density and low acoustic impedance of polymers. Ó 2014 Elsevier B.V. All rights reserved.
1. Introduction Nowadays, more and more attention has been paid to the applications of piezoelectric composite for its enhanced magnetoelectric effect and piezoelectric property [1–5]. The polymer is a promising candidate to fabricate piezoelectric composite for its flexibility, low density and the low acoustic impedance, which are of interest for the applications such as medical imaging, acoustic transducers, and non-destructive evaluation [6–8]. Since Newnham [9] first put forward the phase connectivity concept, composites with various connectivity patterns such as 0–3, 1–3 and 3–3 have been widely used for sensor, transducer and actuator applications [8,10–13]. Among the above mentioned connectivity patterns, the 3–3 composites have both the piezoelectric ceramics and the polymer, which are connected in three dimensions. For acoustic transducer applications, the important parameters of the 3–3 composite are the hydrostatic voltage constant (gh), the hydrostatic strain constant (dh) and the hydrostatic figure of merit (dhgh) [14]. The value of dhgh indicates the signal to noise ratio and describes the acoustic matching degree, which has been found to be better in the 3–3 piezoelectric composites than those of the monolithic piezoelectric ceramics due to the polymer phase. There
⇑ Corresponding author. E-mail address: [email protected] (Y.-Y. Li). http://dx.doi.org/10.1016/j.jallcom.2014.09.124 0925-8388/Ó 2014 Elsevier B.V. All rights reserved.
are several fabrication methods of piezoelectric composites such as the dice and fill method [15,16], UV laser micromachining method [17] and the BURPS method [18]. Direct ink writing (DIW) method is a novel ceramic forming technique, which is one of the solid freeform fabrication techniques used to produce the piezoelectric composite. Solid freeform fabrication allows one to design and rapidly fabricate materials in complex 3D shapes without the need for any dies, lithographic masks, or expensive tooling, which will simplify the process of material preparation and realize the product integrated manufacture [19]. Compared with other solid freeform fabrication techniques such as selective laser sintering [20] and stereolithography [21], DIW can produce a 3D architecture at room temperature without any laser or ultraviolet light. DIW method is a fabrication technique that uses a high solid volume ink and a computer controlled moving stage, which moves a three dimensional (3D) ceramic device via the computer-aided design instruction. Complex 3D architectures such as structures with high aspect ratio walls can be constructed in a layer-by-layer build sequence [22]. Both the shape and the composition of micro zone can be controlled in each individual layer with various materials. Therefore, DIW has a potential application value in the fabrication of catalytic materials [23], tissue engineering scaffolds [24–26] and especially the piezoelectric composites [27,28].
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Y.-Y. Li et al. / Journal of Alloys and Compounds 620 (2015) 125–128
In this paper, we developed a design of aqueous based suspensions of lead zirconate titanate lanthanum (PLZT) inks and fabricated the 3D piezoelectric woodpile structures with various structural parameters by using DIW method. The microstructure and the phase composition of the sintered samples were observed to confirm the samples are fully sintered. 3–3 PLZT–Epoxy composites were fabricated after the ceramic infiltrated with Epoxy. Measuring and calculating the electrical parameters (d33, d31, dh, gh, e33/ e0, dhgh) to show that 3–3 PLZT–Epoxy formed by DIW performs better than the PLZT solid disks, which will make an important application in transducers and hydrophones.
2. Experimental 2.1. Ink’s preparation PLZT (Pb0.93La0.07(Zr0.65Ti0.35)0.9825O3) powder (Bao Ding HongSheng Acoustics Electron Apparatus Co., Ltd.) with average particle size of 10 lm were mixed with a 1 wt% deflocculant sodium citrate dihydrate (AR, Beijing Yili Fine Chemicals). After attrition milling for 24 h, the mixture was dried at 70 °C for 24 h. Deionized water was served as solvent, with a 1 wt% deflocculant sodium citrate dehydrate that was evaluated dispersing the PLZT powder in the solvent. A certain amount of PLZT powders were added into the mixture with ultrasound treatment and the final suspension had a solid volume fraction about 70 vol%. After 48 h aging, the PLZT ceramic ink turned to be better with less bubbles and the powders dispersed uniformly.
2.2. Direct write equipment and processing As Fig. 1 shows, the direct ink writing equipment consists of two parts, one is the computer-controlled system which designs the structure and translates the mobile path into code language and the other is the output device which receives the motion instructions to complete the fabrication process. The computer system working as the input device can design structures with various shapes and sizes are the foundation of the whole forming process. The output device is a robotic deposition apparatus (Aerotech Inc., A3200, Pittshurgh, PA) with a mechanical movement precision of 10 nm. In this experiment, we designed several woodpile structures with various sizes and change those into the code language. The robotic deposition apparatus started to build the PLZT woodpile ceramic structures layer-by-layer after receiving the instructions. Considering the high volume fraction, the green samples were dried in the air for 48 h at room temperature with little drying shrinkage. These dried samples were sintered in a Pb-rich atmosphere at 800 °C for 1 h with a heating rate of 150 °C/h, then at 1200 °C for 4 h with a heating rate of 150 °C/h and cooling along with the furnace to the room temperature. The sintered PLZT woodpile structures were filled with Bisphenol A Epoxy resin and bath heating at 40 °C for 4 h. 3–3 PLZT–Epoxy composites were produced after the resin cured completely.
2.3. Polarization processing The composites were printed with silver paste on both sides and poled in a silicone oil bath at 80 °C under a dc field of 3.5 kV/mm for 20 min. As a benchmark, solid PLZT disks were formed by dry pressing and sintered with the same processing, then poled at 5 kV/mm for 20 min at 120 °C.
2.4. Characterization The plots of viscosity as a function of the shear stress and storage modulus as a function of shear stress of the ink were measured by a controlled stress rheometer (Physica MRC300 Modular Compact Rheometer, Germany) from 1 Pa to 1000 Pa with a fixed frequency of 1 kHz at the room temperature. XRD pattern of the sintered sample was characterized using a diffractometer (Rigaku, D/max 2500, Japan). The optical morphology of sintered sample was showed using a micro-computer tomography (Axio Imager. Z1m, Carl Zeiss Shanghai Co. Ltd., China) and the microstructure tomography was observed by using the scanning electronic microscope (SSX-550, Shimadzu, Japan). The sintered shrinkage was calculated by measuring the sizes of the sintered samples. The actual density of the sintered samples was tested by the Archimedes drainage method. The dielectric constant of the samples were measured with an impedance analyzer (HP 5278A). Finally, d33 measurements were performed by using a ZJ-3A quasi-static d33 tester, while the d31 measurements were performed by a PiezoMeter System PM300.
3. Results and discussion 3.1. Rheological performance results The inks used in direct wring method should satisfy several criteria to realize the fabrication of the 3D structures. Firstly, they should solidify immediately once they are deposited on the substrate to maintain their shapes. Secondly, the inks need to have a shear thinning behavior [29] when the shear stress above a certain critical shear stress, which can be described by Herschel–Bulkley [30] equation:
s ¼ sy þ KDn
ð1Þ
s is the shear stress, K is the viscosity parameter, D is the shear rate, and n is the characterization of the Newtonian fluid behavior deviation index (for shear thinning fluid, n < 1). Fig. 2(a) is the plot of ink’s stress on shear rate, close to that of the classic shear thinning fluid. Through fitting out the value, we find that the value of n is 0.3 (Fig. 2(b)) confirming that the ink have a shear thinning behavior. Fig. 2(c and d) shows that when the shear stress less than 20 Pa, the ink deforms elastically, its viscosity varies linearly with the increasing of the shear stress, while the storage modulus (G0 ) remains the same. When the shear stress exceeds 20 Pa, the viscosity decreases rapidly and reaches a steady value at 110 Pa. The plot of storage modulus indicates the similar changing trend, which insures the shear-thinning behavior of the ink. 3.2. Morphology and composition characterization Fig. 3 describes the top view of sintered samples with various designed sizes including 8 mm, 10 mm and 15 mm, which reflect the structure keeps the linear shape. Fig. 3(b) shows the sectional view that the woodpile structure has small collapses, and hardly any cracking or bending. Fig. 3(c and d) shows both the 8 mm sample and the 10 mm sample has a filament diameter of 280 lm, and Fig. 3(e) shows the 15 mm sample has a filament diameter of 480 lm. It is obviously that the direct writing method has an advantage in the structure design. Fig. 4(a) shows the surface SEM image of the sintered sample. It is obviously that the sample is fully sintered in this sintering condition with few pores and the grains which are growing completely with a uniform grain size. The actual density of the sintered sample is 7.74 g/cm3 and the relative density is 99.3%. As a result, the average sintered shrinkage is 14%. The test of X-ray diffraction (XRD) reveals that the main phase of the sintered sample is rhombohedral Pb0.93La0.07(Zr0.65Ti0.35)0.9825O3 (Fig. 4(b)). 3.3. Electrical performance characterization
Fig. 1. Experimental set-up for the direct ink writing process.
Measuring and calculating the electrical parameters (d33, d31, dh, gh, e33/e0, dhgh) of the solid PLZT disks and the PLZT–Epoxy composites after polarization, shown in Table 1:
Y.-Y. Li et al. / Journal of Alloys and Compounds 620 (2015) 125–128
127
Fig. 2. (a) Plot of ink’s shear stress on shear rate, (b) fitting curve of shear stress as a function of shear rate, (c) plot of storage modulus as a function of shear stress and (d) plot of viscosity as a function of shear stress.
Fig. 3. (a) The top view of sintered woodpile samples with different designed size (8 mm, 10 mm, 15 mm), (b) the sectional view of 8 mm woodpile structure, (c) the 8 mm sample with filament diameter of 280 lm, (d) the 10 mm sample with filament of 280 lm and (e) the 15 mm sample with filament of 480 lm.
dh ¼ d33 þ 2d31
ð2Þ
g h ¼ dh =e33
ð3Þ
Compared with traditional 3–3 fabrication method, for example, the replamineform process [31], the BURPS [18] method and the fiber/polymer processing method [32], DIW can make an abudant skeleton and their interconnection can be controlled in a more convenient way because it can produce piezoelectric scaffold rapidly without any tools. The value of dhgh up to 4020 1015 m2/N in the 3–3 PZT/polymer fabricated with injection method [33], while 3–3 PLZT/polymer achieved by direct ink
writing technique has a figure of dhgh about 4112 1015 m2/N, which can compare favourably with the PZT/polymer. From Table 1, we can see the PLZT disk have a higher d33, while a lower dhgh. All the PLZT–Epoxy composites have a higher value of dhgh since the polymer phase can reduce both the force perpendicular to the poling direction on the ceramics and the coupling of d33 and d31. And the improvements in all figures of merit can be achieved by using a high compliance polymer phase due to increased stress into the active piezoceramic phase [8]. Moreover, the polymer can further reduce the dielectric constant and the density of materials.
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Fig. 4. (a) Surface SEM image of the sintered sample and (b) XRD pattern of the sintered sample.
Table 1 The electrical properties of monolithic PLZT ceramics and PLZT–Epoxy composites.
PLZT disk PLZT–Epoxy composite
PLZT (vol%)
d33 (pC/N)
d31 (pC/N)
dh (pC/N)
gh (103 V m/N)
dhgh (1015 m2/N)
e33/e0
q (g/cm3)
100 39 48 55
481 335 343 347
200 121 120 123
81 93 103 101
4.5 41 40 35
365 3813 4112 3525
1986 256 291 325
7.74 3.40 3.65 3.77
Variation of dielectric and piezoelectric properties of the PLZT– Epoxy composites with volume fraction PLZT is shown in Table 1. The values of q, d33 and e33/e0 are increasing with the increase of the PLZT volume fraction. And the composite with PLZT volume fraction of 48% has a maximum value of dhgh (4112 1015 m2/ N) because of the sintered porous PLZT scaffold with about 50% open porosity and minimum closed porosity give maximum degree of interconnectivity between ceramic and polymer phases and optimum piezoelectric properties. DIW has an advantage on controlling the various degree of polymerization with the help of structure design in CAD. 4. Conclusions PLZT woodpile structures with various sizes are produced by the direct ink writing method. 3–3 PLZT–Epoxy composites filled with Epoxy will have a better electrical performance compared with PLZT solid disks and can be widely used in the manufacture of acoustic transducers. Compared with the existing method to fabricate the 3–3 composite such as the BURPS technique, the direct ink writing method can produce the piezoelectric scaffold conveniently and rapidly without any dies or lithographic masks. It also has merits of smart design ability which opens a potential route for the design and fabrication of piezoelectric devices with various connectivity (2–2, 1–3, etc.). References [1] Y. Zhang, G.X. Liu, H.D. Shi, W.L. Xiao, Y.D. Zhu, M.Z. Li, J. Liu, J. Alloys Comp. 613 (2014) 93–95. [2] N. Jaitanong, R. Yimnirun, H.R. Zeng, G.R. Li, Q.R. Yin, A. Chaipanich, Mater. Lett. 130 (2014) 146–149. [3] H.Q. Shen, Y.G. Wang, D. Xie, J.H. Cheng, J. Alloys Comp. 610 (2014) 11–14. [4] E. Tufekcioglu, A. Dogan, Sens. Actuators, A 216 (2014) 355–363. [5] L. Chen, P. Li, Y.M. Wen, Y. Zhu, J. Alloys Comp. 606 (2014) 15–20.
[6] D.Y. Xu, S.F. Huang, X. Cheng, Constr. Build. Mater. 65 (2014) 543–550. [7] Z. Ahmad, A. Prasad, K. Prasad, Physica B 404 (2009) 3637–3644. [8] C.R. Bowen, A. Perry, H. Kara, S.W. Mahon, J. Eur. Ceram. Soc. 21 (2001) 1463– 1467. [9] R.E. Newnham, D.P. Skinner, L.E. Cross, Mater. Res. Bull. 13 (1978) 325–326. [10] J.Y. Zhao, W. Ren, H.F. Ji, L.Y. Wang, X.Q. Wu, P. Shi, Ferroelectrics 465 (2014) 7–12. [11] R. Xie, Y. Zhao, K.C. Zhou, D. Zhang, Y. Wang, H.L.W. Chan, J. Am. Ceram. Soc. 97 (2014) 2590–2595. [12] Y. Xu, J.F. Li, J. Ma, C.W. Nan, J. Appl. Phys. 110 (2011) 044104. [13] D. Zhou, K.H. Lam, Y. Chen, Q. Zhang, Y.C. Chiu, H. Luo, J. Dai, H.L.W. Chan, Sens. Actuators, A 182 (2012) 95–100. [14] H. Kara, A. Perry, R. Stevens, Ferroelectrics 265 (2002) 317–332. [15] C. Richard, H.S. Lee, D. Guyomar, Ultrasonics 42 (2004) 417–424. [16] L.L. Li, S.J. Zhang, Z. Xu, X.C. Geng, T.R. Shrout, Phys. Status Solidi A 210 (2013) 1888–1891. [17] D.W. Zeng, K. Li, K.C. Yung, H.L.W. Chan, C.L. Choy, C.S. Xie, Appl. Phys. A 78 (2004) 415–421. [18] H. Kara, R. Ramesh, S. Steven, C.R. Bowen, IEEE Trans. Ferroelectr. Freq. Control 50 (2003) 289–295. [19] E. Sachs, M. Cima, P. Williams, D. Brancazio, J. Cornie, J. Eng. Ind. 114 (1992) 481–488. [20] J.M. Williams, A. Adewunmi, R.M. Schek, C.L. Flanagan, P.H. Krebsbach, S.E. Feinberg, S.J. Hollister, S. Das, Biomaterials 26 (2005) 4817–4827. [21] F.P.W. Melchels, J. Feijen, D.W. Grijpma, Biomaterials 31 (2010) 6121–6130. [22] J.A. Lewis, Adv. Funct. Mater. 16 (2006) 2193–2204. [23] Y.Y. Li, B. Li, L.T. Li, Mod. Phys. Lett. B 28 (2014) 1450051. [24] J.G. Dellinger, J.A. Eurell, R.D. Jamison, J. Biomed. Mater. Res. Part A 76A (2006) 366–376. [25] S. Michna, W. Wu, J.A. Lewis, Biomaterials 26 (2005) 5632–5639. [26] Y. Zhang, L.J. Chen, J. Zeng, K.C. Zhou, D. Zhang, Mater. Sci. Eng., C 39 (2014) 143–149. [27] M.L. Young, R. Rao, J.D. Almer, D.R. Haeffner, J.A. Lewis, D.C. Dunand, Acta Mater. 57 (2009) 2362–2375. [28] C.S. Marchi, M. Kouzeli, R. Rao, J.A. Lewis, D.C. Dunand, Scripta Mater. 49 (2003) 861–866. [29] J.A. Lewis, Curr. Opin. Solid State Mater. Sci. 6 (2002) 240–245. [30] W.H. Herschel, R. Bulkley, Kolloid Zeitschrify 39 (1926) 291–300. [31] R.A. White, J.N. Weber, E.W. White, Science 176 (1972) 922. [32] B. Jadidian, M. Allahverdi, F. Mohammadi, A. Safari, J. Electroceram. 8 (2002) 209–214. [33] Y.C. Chen, S. Wu, Ceram. Int. 30 (2004) 69–74.
Contents lists available at ScienceDirect
Journal of Alloys and Compounds journal homepage: www.elsevier.com/locate/jalcom
Direct ink writing of 3–3 piezoelectric composite Ya-Yun Li a,⇑, Long-Tu Li a, Bo Li b a b
State Key Laboratory of New Ceramics and Fine Processing, School of Materials Science and Engineering, Tsinghua University, Beijing 100084, PR China Research Institute for Advanced Materials, Graduate School at Shenzhen, Tsinghua University, Shenzhen 518055, PR China
a r t i c l e
i n f o
Article history: Received 14 July 2014 Received in revised form 15 September 2014 Accepted 15 September 2014 Available online 28 September 2014 Keywords: 3–3 Piezoelectric composite Direct ink writing PLZT Figure of merit
a b s t r a c t Three dimensional (3D) ceramic structures form a construction of piezoelectric composite would be prepared with direct ink writing method (DIW). The preparation of aqueous based lead zirconate titanate lanthanum (PLZT) inks and the principle of DIW were systematically investigated. The ink with solids volume fraction about 70 vol% by aging 48 h reveals shear-thinning behavior and proper viscoelastic properties. As shown by scanning electron microscopy (SEM), PLZT samples sintered at 1200 °C for 4 h in a lead-rich atmosphere yielded best microstructures which were densified with relative density exceed 98%. The test of X-ray diffraction (XRD) reveals that the main phase of sintered samples is rhombohedral Pb0.93La0.07(Zr0.65Ti0.35)0.9825O3. 3–3 Piezoelectric composite formed by 3D PLZT ceramics filled with epoxy resin at 40 °C have higher hydrostatic figure of merit dhgh (4112 1015 m2/N) than that of the monolithic PLZT piezoelectric ceramics (365 1015 m2/N). Piezoelectric ceramic–polymer composites can be widely used for the applications like underwater acoustic transducers in having combined hardness and electric properties of piezoelectric ceramics and flexibility, low density and low acoustic impedance of polymers. Ó 2014 Elsevier B.V. All rights reserved.
1. Introduction Nowadays, more and more attention has been paid to the applications of piezoelectric composite for its enhanced magnetoelectric effect and piezoelectric property [1–5]. The polymer is a promising candidate to fabricate piezoelectric composite for its flexibility, low density and the low acoustic impedance, which are of interest for the applications such as medical imaging, acoustic transducers, and non-destructive evaluation [6–8]. Since Newnham [9] first put forward the phase connectivity concept, composites with various connectivity patterns such as 0–3, 1–3 and 3–3 have been widely used for sensor, transducer and actuator applications [8,10–13]. Among the above mentioned connectivity patterns, the 3–3 composites have both the piezoelectric ceramics and the polymer, which are connected in three dimensions. For acoustic transducer applications, the important parameters of the 3–3 composite are the hydrostatic voltage constant (gh), the hydrostatic strain constant (dh) and the hydrostatic figure of merit (dhgh) [14]. The value of dhgh indicates the signal to noise ratio and describes the acoustic matching degree, which has been found to be better in the 3–3 piezoelectric composites than those of the monolithic piezoelectric ceramics due to the polymer phase. There
⇑ Corresponding author. E-mail address: [email protected] (Y.-Y. Li). http://dx.doi.org/10.1016/j.jallcom.2014.09.124 0925-8388/Ó 2014 Elsevier B.V. All rights reserved.
are several fabrication methods of piezoelectric composites such as the dice and fill method [15,16], UV laser micromachining method [17] and the BURPS method [18]. Direct ink writing (DIW) method is a novel ceramic forming technique, which is one of the solid freeform fabrication techniques used to produce the piezoelectric composite. Solid freeform fabrication allows one to design and rapidly fabricate materials in complex 3D shapes without the need for any dies, lithographic masks, or expensive tooling, which will simplify the process of material preparation and realize the product integrated manufacture [19]. Compared with other solid freeform fabrication techniques such as selective laser sintering [20] and stereolithography [21], DIW can produce a 3D architecture at room temperature without any laser or ultraviolet light. DIW method is a fabrication technique that uses a high solid volume ink and a computer controlled moving stage, which moves a three dimensional (3D) ceramic device via the computer-aided design instruction. Complex 3D architectures such as structures with high aspect ratio walls can be constructed in a layer-by-layer build sequence [22]. Both the shape and the composition of micro zone can be controlled in each individual layer with various materials. Therefore, DIW has a potential application value in the fabrication of catalytic materials [23], tissue engineering scaffolds [24–26] and especially the piezoelectric composites [27,28].
126
Y.-Y. Li et al. / Journal of Alloys and Compounds 620 (2015) 125–128
In this paper, we developed a design of aqueous based suspensions of lead zirconate titanate lanthanum (PLZT) inks and fabricated the 3D piezoelectric woodpile structures with various structural parameters by using DIW method. The microstructure and the phase composition of the sintered samples were observed to confirm the samples are fully sintered. 3–3 PLZT–Epoxy composites were fabricated after the ceramic infiltrated with Epoxy. Measuring and calculating the electrical parameters (d33, d31, dh, gh, e33/ e0, dhgh) to show that 3–3 PLZT–Epoxy formed by DIW performs better than the PLZT solid disks, which will make an important application in transducers and hydrophones.
2. Experimental 2.1. Ink’s preparation PLZT (Pb0.93La0.07(Zr0.65Ti0.35)0.9825O3) powder (Bao Ding HongSheng Acoustics Electron Apparatus Co., Ltd.) with average particle size of 10 lm were mixed with a 1 wt% deflocculant sodium citrate dihydrate (AR, Beijing Yili Fine Chemicals). After attrition milling for 24 h, the mixture was dried at 70 °C for 24 h. Deionized water was served as solvent, with a 1 wt% deflocculant sodium citrate dehydrate that was evaluated dispersing the PLZT powder in the solvent. A certain amount of PLZT powders were added into the mixture with ultrasound treatment and the final suspension had a solid volume fraction about 70 vol%. After 48 h aging, the PLZT ceramic ink turned to be better with less bubbles and the powders dispersed uniformly.
2.2. Direct write equipment and processing As Fig. 1 shows, the direct ink writing equipment consists of two parts, one is the computer-controlled system which designs the structure and translates the mobile path into code language and the other is the output device which receives the motion instructions to complete the fabrication process. The computer system working as the input device can design structures with various shapes and sizes are the foundation of the whole forming process. The output device is a robotic deposition apparatus (Aerotech Inc., A3200, Pittshurgh, PA) with a mechanical movement precision of 10 nm. In this experiment, we designed several woodpile structures with various sizes and change those into the code language. The robotic deposition apparatus started to build the PLZT woodpile ceramic structures layer-by-layer after receiving the instructions. Considering the high volume fraction, the green samples were dried in the air for 48 h at room temperature with little drying shrinkage. These dried samples were sintered in a Pb-rich atmosphere at 800 °C for 1 h with a heating rate of 150 °C/h, then at 1200 °C for 4 h with a heating rate of 150 °C/h and cooling along with the furnace to the room temperature. The sintered PLZT woodpile structures were filled with Bisphenol A Epoxy resin and bath heating at 40 °C for 4 h. 3–3 PLZT–Epoxy composites were produced after the resin cured completely.
2.3. Polarization processing The composites were printed with silver paste on both sides and poled in a silicone oil bath at 80 °C under a dc field of 3.5 kV/mm for 20 min. As a benchmark, solid PLZT disks were formed by dry pressing and sintered with the same processing, then poled at 5 kV/mm for 20 min at 120 °C.
2.4. Characterization The plots of viscosity as a function of the shear stress and storage modulus as a function of shear stress of the ink were measured by a controlled stress rheometer (Physica MRC300 Modular Compact Rheometer, Germany) from 1 Pa to 1000 Pa with a fixed frequency of 1 kHz at the room temperature. XRD pattern of the sintered sample was characterized using a diffractometer (Rigaku, D/max 2500, Japan). The optical morphology of sintered sample was showed using a micro-computer tomography (Axio Imager. Z1m, Carl Zeiss Shanghai Co. Ltd., China) and the microstructure tomography was observed by using the scanning electronic microscope (SSX-550, Shimadzu, Japan). The sintered shrinkage was calculated by measuring the sizes of the sintered samples. The actual density of the sintered samples was tested by the Archimedes drainage method. The dielectric constant of the samples were measured with an impedance analyzer (HP 5278A). Finally, d33 measurements were performed by using a ZJ-3A quasi-static d33 tester, while the d31 measurements were performed by a PiezoMeter System PM300.
3. Results and discussion 3.1. Rheological performance results The inks used in direct wring method should satisfy several criteria to realize the fabrication of the 3D structures. Firstly, they should solidify immediately once they are deposited on the substrate to maintain their shapes. Secondly, the inks need to have a shear thinning behavior [29] when the shear stress above a certain critical shear stress, which can be described by Herschel–Bulkley [30] equation:
s ¼ sy þ KDn
ð1Þ
s is the shear stress, K is the viscosity parameter, D is the shear rate, and n is the characterization of the Newtonian fluid behavior deviation index (for shear thinning fluid, n < 1). Fig. 2(a) is the plot of ink’s stress on shear rate, close to that of the classic shear thinning fluid. Through fitting out the value, we find that the value of n is 0.3 (Fig. 2(b)) confirming that the ink have a shear thinning behavior. Fig. 2(c and d) shows that when the shear stress less than 20 Pa, the ink deforms elastically, its viscosity varies linearly with the increasing of the shear stress, while the storage modulus (G0 ) remains the same. When the shear stress exceeds 20 Pa, the viscosity decreases rapidly and reaches a steady value at 110 Pa. The plot of storage modulus indicates the similar changing trend, which insures the shear-thinning behavior of the ink. 3.2. Morphology and composition characterization Fig. 3 describes the top view of sintered samples with various designed sizes including 8 mm, 10 mm and 15 mm, which reflect the structure keeps the linear shape. Fig. 3(b) shows the sectional view that the woodpile structure has small collapses, and hardly any cracking or bending. Fig. 3(c and d) shows both the 8 mm sample and the 10 mm sample has a filament diameter of 280 lm, and Fig. 3(e) shows the 15 mm sample has a filament diameter of 480 lm. It is obviously that the direct writing method has an advantage in the structure design. Fig. 4(a) shows the surface SEM image of the sintered sample. It is obviously that the sample is fully sintered in this sintering condition with few pores and the grains which are growing completely with a uniform grain size. The actual density of the sintered sample is 7.74 g/cm3 and the relative density is 99.3%. As a result, the average sintered shrinkage is 14%. The test of X-ray diffraction (XRD) reveals that the main phase of the sintered sample is rhombohedral Pb0.93La0.07(Zr0.65Ti0.35)0.9825O3 (Fig. 4(b)). 3.3. Electrical performance characterization
Fig. 1. Experimental set-up for the direct ink writing process.
Measuring and calculating the electrical parameters (d33, d31, dh, gh, e33/e0, dhgh) of the solid PLZT disks and the PLZT–Epoxy composites after polarization, shown in Table 1:
Y.-Y. Li et al. / Journal of Alloys and Compounds 620 (2015) 125–128
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Fig. 2. (a) Plot of ink’s shear stress on shear rate, (b) fitting curve of shear stress as a function of shear rate, (c) plot of storage modulus as a function of shear stress and (d) plot of viscosity as a function of shear stress.
Fig. 3. (a) The top view of sintered woodpile samples with different designed size (8 mm, 10 mm, 15 mm), (b) the sectional view of 8 mm woodpile structure, (c) the 8 mm sample with filament diameter of 280 lm, (d) the 10 mm sample with filament of 280 lm and (e) the 15 mm sample with filament of 480 lm.
dh ¼ d33 þ 2d31
ð2Þ
g h ¼ dh =e33
ð3Þ
Compared with traditional 3–3 fabrication method, for example, the replamineform process [31], the BURPS [18] method and the fiber/polymer processing method [32], DIW can make an abudant skeleton and their interconnection can be controlled in a more convenient way because it can produce piezoelectric scaffold rapidly without any tools. The value of dhgh up to 4020 1015 m2/N in the 3–3 PZT/polymer fabricated with injection method [33], while 3–3 PLZT/polymer achieved by direct ink
writing technique has a figure of dhgh about 4112 1015 m2/N, which can compare favourably with the PZT/polymer. From Table 1, we can see the PLZT disk have a higher d33, while a lower dhgh. All the PLZT–Epoxy composites have a higher value of dhgh since the polymer phase can reduce both the force perpendicular to the poling direction on the ceramics and the coupling of d33 and d31. And the improvements in all figures of merit can be achieved by using a high compliance polymer phase due to increased stress into the active piezoceramic phase [8]. Moreover, the polymer can further reduce the dielectric constant and the density of materials.
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Fig. 4. (a) Surface SEM image of the sintered sample and (b) XRD pattern of the sintered sample.
Table 1 The electrical properties of monolithic PLZT ceramics and PLZT–Epoxy composites.
PLZT disk PLZT–Epoxy composite
PLZT (vol%)
d33 (pC/N)
d31 (pC/N)
dh (pC/N)
gh (103 V m/N)
dhgh (1015 m2/N)
e33/e0
q (g/cm3)
100 39 48 55
481 335 343 347
200 121 120 123
81 93 103 101
4.5 41 40 35
365 3813 4112 3525
1986 256 291 325
7.74 3.40 3.65 3.77
Variation of dielectric and piezoelectric properties of the PLZT– Epoxy composites with volume fraction PLZT is shown in Table 1. The values of q, d33 and e33/e0 are increasing with the increase of the PLZT volume fraction. And the composite with PLZT volume fraction of 48% has a maximum value of dhgh (4112 1015 m2/ N) because of the sintered porous PLZT scaffold with about 50% open porosity and minimum closed porosity give maximum degree of interconnectivity between ceramic and polymer phases and optimum piezoelectric properties. DIW has an advantage on controlling the various degree of polymerization with the help of structure design in CAD. 4. Conclusions PLZT woodpile structures with various sizes are produced by the direct ink writing method. 3–3 PLZT–Epoxy composites filled with Epoxy will have a better electrical performance compared with PLZT solid disks and can be widely used in the manufacture of acoustic transducers. Compared with the existing method to fabricate the 3–3 composite such as the BURPS technique, the direct ink writing method can produce the piezoelectric scaffold conveniently and rapidly without any dies or lithographic masks. It also has merits of smart design ability which opens a potential route for the design and fabrication of piezoelectric devices with various connectivity (2–2, 1–3, etc.). References [1] Y. Zhang, G.X. Liu, H.D. Shi, W.L. Xiao, Y.D. Zhu, M.Z. Li, J. Liu, J. Alloys Comp. 613 (2014) 93–95. [2] N. Jaitanong, R. Yimnirun, H.R. Zeng, G.R. Li, Q.R. Yin, A. Chaipanich, Mater. Lett. 130 (2014) 146–149. [3] H.Q. Shen, Y.G. Wang, D. Xie, J.H. Cheng, J. Alloys Comp. 610 (2014) 11–14. [4] E. Tufekcioglu, A. Dogan, Sens. Actuators, A 216 (2014) 355–363. [5] L. Chen, P. Li, Y.M. Wen, Y. Zhu, J. Alloys Comp. 606 (2014) 15–20.
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