3)O3 piezoelectric thick films by aerosol deposition

Materials Science and Engineering B 170 (2010) 67–70

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Pb(Zr,Ti)O3 –Pb(Mn1/3 Nb2/3 )O3 piezoelectric thick films by aerosol deposition Jungho Ryu ∗ , Jong-Jin Choi, Byung-Dong Hahn, Woon-Ha Yoon, Byoung-Kuk Lee, Joon Hwan Choi, Dong-Soo Park Functional Ceramics Group, Korea Institute of Materials Science (KIMS), 66 Sangnam-Dong, Changwon, Gyeongnam 641-831, Republic of Korea

a r t i c l e

i n f o

Article history: Received 22 September 2009 Received in revised form 22 November 2009 Accepted 21 February 2010 Keywords: Piezoelectric Thick film PZT–PMnN Aerosol deposition (AD)

a b s t r a c t Piezoelectric thick films of Pb(Zr,Ti)O3 –Pb(Mn1/3 Nb2/3 )O3 (PZT–PMnN) with Zr:Ti ratios ranging from 0.45:0.55 to 0.60:0.40 were fabricated on a platinized silicon wafer by aerosol deposition (AD). All the films were deposited with a thickness of 10 ␮m with high density. By adding PMnN to 57:43 PZT, a dielectric constant as low as ∼660 was achieved while the effective piezoelectric constant was over 140 pC/N. PZT–PMnN with a Zr:Ti ratio of 57:43 thus showed a maximum piezoelectric voltage constant (g33 ) of 23.8 × 10−3 Vm/N and is a good candidate for high quality thick films for application to high-energy density or high sensitivity, piezoelectric energy harvesters and sensors. © 2010 Elsevier B.V. All rights reserved.

1. Introduction In the numerous applications for piezoelectric thin/thick films such as microactuators, transducers, and sensors, combining the surface of the micro-machined silicon wafer with piezoelectric thick films with a thickness range of 1–100 ␮m has resulted in novel devices such as microfluidic devices, micropumps, cantilever or bimorph accelerometers, sensors and energy harvester [1–4]. One of the most promising applications of piezoelectric films is their incorporation into energy harvesting from environmental sources to power wireless devices and components [4]. For mechanical energy harvesting devices, piezoelectric films require relatively thick films in order to obtain sufficient power generation. In addition to energy harvester applications, microelectromechanical system (MEMS) base sensor applications such as accelerometers, acoustic sensors and infrared detectors also require dense, crack-free piezoelectric thick films with thickness ranging from 1 to 100 ␮m [1–9]. In addition to the crack-free, dense and high quality properties for such devices, high piezoelectric characteristics, especially piezoelectric constant (dij ) and voltage constant (gij ), should be obtained from the films for the fabrication of high performance MEMS devices, since the generated energy density is proportional to those two parameters [10]. Since gij is inversely proportional to the dielectric constant, a lower dielectric constant with high d33 constant from the piezoelectric films is preferred. The best examples of such high-energy

density piezoelectric ceramics are compositions comprised of the Mn-doped, PbZrO3 –PbTiO3 –Pb(Zn1/3 Nb2/3 )O3 (PZT–PZN) and PbZrO3 –PbTiO3 –Pb(Mn1/3 Nb2/3 )O3 (PZT–PMnN) systems [10–16]. Islam et al reported the high-energy density characteristics of the Mn-doped PZT–PZN system [10,11]. This system showed very high-energy density compared to general PZT-based materials by Mn doping. Ryu et al. and Ise et al. reported the high power piezoelectric characteristics of PZT–PMnN piezoelectric system [13,14]. This system showed a moderate piezoelectric constant d33 and a lower dielectric constant which is important to obtain high-energy density from the piezoelectrics [13–16]. In both cases, the role of Mn in the PZT-relaxor (PZN or PMnN) system is to maintain d33 and lower the dielectric constant of the piezoelectrics. However, few reports have been published on high-energy density piezoelectric films. Recently, we fabricated hard piezoelectric (PZT–PZN–Mn) thick films (∼10 ␮m) using aerosol deposition (AD) technique for high power energy harvesting devices [11]. The Mn-doped piezoelectric thick films exhibited good feasibility for energy harvester and the composition of the piezoelectric films was controlled by AD. In this study, we fabricated 0.9Pb(Zr,Ti)O3 –0.1Pb(Mn1/3 Nb2/3 )O3 (PZT–PMnN) (t∼10 ␮m) on a platinized silicon substrate at room temperature (RT) and annealed at 700 ◦ C to determine the film’s feasibility for use in piezoelectric sensor and energy harvester applications. We obtained a superior piezoelectric voltage constant. In addition, the effect of the Zr:Ti ratio in this system on the piezoelectric characteristics was also investigated. 2. Experimental

∗ Corresponding author. E-mail address: [email protected] (J. Ryu). 0921-5107/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.mseb.2010.02.028

Reagent grade raw materials of PbO, ZrO2 , TiO2 , MnCO3 , and Nb2 O5 (99.9%, Sigma–Aldrich Co.) were used to prepare the start-

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ing PZT–PMnN powders for the AD films. The basic composition was 0.9Pb(Zrx Tiy )O3 –0.1Pb(Mn1/3 Nb2/3 )O3 , (x + y = 1), where the x:y ratio was varied from 0.45:0.55 to 0.60:0.40. The raw powder mixture was ball-milled using high purity 3Y-TZP (tetragonal zirconia polycrystalline) ball media with ethyl alcohol in a high-density polyethylene jar for 24 h. The mixed slurry was dried and calcined for 4 h in an alumina crucible for PZT–PMnN perovskite phase formation at 850 ◦ C. The calcined PZT–PMnN powders were crushed by ball milling for 10 h to obtain an appropriate particle size distribution for AD. The average particle size (d50 ) was 1.4 ␮m. The dried powders were mixed with the carrier gas (medical grade dried air) at a flow rate of 10 l/min to form an aerosol flow in the aerosol chamber. The aerosol flow was transported through a tube to a nozzle, accelerated and ejected from a rectangular-shape nozzle with orifices of 5 mm × 0.4 mm into a deposition chamber, which was evacuated by a rotary pump with a mechanical booster pump. The accelerated PZT–PMnN particles collided with the platinized silicon (Pt/Ti/SiO2 /Si) substrate, which was located 5 mm from the nozzle, and formed a dense piezoelectric film at RT. The area of the deposited film was ∼5 mm × 15 mm with an approximate thickness of 10 ␮m. Deposition rate of the films was ∼2 ␮m/scan. The asdeposited films were annealed at 700 ◦ C for 1 h in air atmosphere. The phases of the starting powders and the deposited films were identified by X-ray diffractometry (XRD: D-MAX 2200, Rigaku Co., Tokyo, Japan), while the microstructures of the films were observed using scanning electron microscopy (SEM: JSM-5800, JEOL Co., Tokyo, Japan). Pt-top electrodes with diameters of 0.5 and 3.0 mm were deposited on the film by DC sputtering method in order to measure the dielectric/ferroelectric properties and piezoelectric property (d33 ), respectively. The dielectric properties were measured 1 kHz to 1 MHz using an Agilent 4294 A impedance analyzer under 0.5 Vp−p oscillation, and polarization hysteresis loops were obtained with a standardized P-LC100-K ferroelectric test system at RT. Before measuring piezoelectric constant, d33 , the 700 ◦ Cannealed films were poled under 150 kV/cm electric field at 100 ◦ C for 30 min. The d33 of the 1-day aged films ware measured by using a Berlincourt-type d33 piezometer (PM300, Piezotest, UK) to record the direct effect of the piezoelectricity of the same samples [17]. 3. Results and discussion The crystal phases of the PZT–PMnN films prepared on the platinized silicon substrate were characterized by XRD and are depicted in Fig. 1(a) and (b) for the as-deposited and 700 ◦ Cannealed films, respectively. No other peaks except perovskite phase and Pt electrode (2 ∼ 40◦ )/Si wafer (2 ∼ 34◦ ) were observed in the as-deposited film, which showed perovskite patterns with weaker and broader patterns than those of the annealed films. Most AD-ceramic films show similar behaviors and such broad XRD peaks are known to indicate a film composition of nanosized grains, according to other previous reports on PZT-based thick films [11,18–20]. After annealing the film at 700 ◦ C for 1 h, the peaks became stronger and sharper than those of the asdeposited film, reflecting the grain growth and crystallization of the nano-grains and amorphous phase [19–22]. Since the AD process generally produces nano-grains and a amorphous phase due to the high-energy particle collisions during deposition, a post-annealing process is mandatory for ferroelectric thick films whose properties are related with grain/domain size.[19,20] A tetragonal peak splits near 2 = 44–46◦ were observed from the films with Zr:Ti = 53:47 or higher Ti content. This shows that the crystal structure transforms from rhombohedral to tetragonal structure with increasing Ti content. When the Zr/Ti ratio was decreased to 53/47, the tetragonal phase became dominant. Therefore, a morphotropic phase boundary (MPB), where the domain wall contributions are maximized, was assumed to exist within the Ti content range of 0.43–0.47.

Fig. 1. XRD patterns of (a) as-deposited and (b) 700 ◦ C-annealed PZT–PMnN films.

Fig. 2 shows the surface (a) and fractured cross-sectional (b) SEM micrographs of the 700 ◦ C-annealed PZT–PMnN (Zr:Ti = 0.57:0.43) thick film. All other compositions showed similar surface morphologies. The films were highly dense and no micro-cracking or pores were observed in the cross-sectional micrographs. From the cross-sectional micrographs of the films, the film thicknesses, which were controlled by nozzle scanning cycles, were measured to be in the vicinity of 10 ␮m. The film deposition rate, as calculated from the film thickness divided by the deposition time and area, was over 2 ␮m/min cm2 . The annealed PZT–PMnN films maintained good adhesion with the platinized silicon substrate, and showed no peeling. Fig. 3 depicts the variation of low field (0.5 Vp−p ) dielectric constant and loss factor as a function of frequency. With increasing frequency, the dielectric constants were slightly decreased but loss factors were maintained in the range of 0.02–0.05 for all the samples. In the case of Zr:Ti = 57:43 film, dielectric constant was decreased from ∼660 (at 1 kHz) to ∼500 (at 1 MHz) and the tan ı was less than 0.05 in the frequency range of 1 kHz–1 MHz. The dielectric properties were quite stable without any leakage up to 1 MHz. The dielectric loss factor was not appreciably affected by the Zr:Ti ratio, but dielectric constant was changed from ∼530 (Zr:Ti = 45:55 and 53:47) to ∼660 (Zr:Ti = 57:43), as presented in Fig. 4. Among the five compositions, the dielectric constant was maximized at Zr:Ti = 57:43, which was attributed to the increased domain contribution in the MPB composition [22,23]. However, all the samples showed a relatively lower dielectric constant com-

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Fig. 4. Dielectric constant and dielectric loss of the 700 ◦ C-annealed PZT–PMnN films at 1 kHz as a function of Ti content.

Fig. 2. SEM micrographs of 700 ◦ C-annealed PZT–PMnN (Zr:Ti = 0.57:0.43) films: (a) surface and (b) cross-sectional views.

pared to the PZT (εr 1170–1400) [20,22] or PZT–PZN (εr 1050–1270) [11,19] piezoelectric films. Fig. 5 shows the P–E curve for the PZT–PMnN films. As expected from the low field dielectric characteristics, the magnitude of the remnant (Pr ) and saturation (Ps ) polarization was highest for the Zr:Ti = 57:43 film. The trend of Pr and Ps as a function of the Zr:Ti ratio was exactly the same as that of the low field dielectric response. The effective piezoelectric constant, d33 , of the PZT–PMnN films was evaluated according to the Ti content and is plotted in Fig. 6.

Fig. 3. Dielectric constant and dielectric loss of the 700 ◦ C-annealed PZT–PMnN films as a function of frequency.

Fig. 5. Polarization versus applied electric field of the 700 ◦ C-annealed PZT–PMnN films at 100 Hz.

Since the energy harvesters or piezoelectric sensors use the direct piezoelectric effect, we measured the effective d33 of the films with a Berlincourt d33 meter [17]. The composition of Zr:Ti = 57:43 showed a maximum piezoelectric constant of over 140 pC/N, which was attributed to the co-existence of rhombohedral and tetrag-

Fig. 6. Effective d33 of the 700 ◦ C-annealed PZT–PMnN films as a function of Ti content.

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constant. Therefore, PZT–PMnN thick films are more attractive for sensors or energy harvesters using the direct piezoelectric effect. 4. Conclusions

Fig. 7. Effective g33 of the 700 ◦ C-annealed PZT–PMnN films as a function of Ti content. Table 1 Summary of the electric properties of PZT–PMnN films.

Acknowledgments This research was supported by a grant from the Fundamental R&D Program for Core Technology of Materials funded by the Ministry of Knowledge Economy, Republic of Korea.

Zr:Ti

εr (1 kHz) Tan ı (1 kHz) Pr (␮C/cm2 ) Pr (␮C/cm2 ) d33eff (pC/N) g33eff (mVm/N)

Highly dense piezoelectric ternary perovskite thick films of PZT–PMnN with Zr:Ti ratios ranging from 0.45:0.55 to 0.60:0.40 were fabricated by AD on a platinized silicon wafer. The thickness was controlled to approximately 10 ␮m. By adding PMnN to 57:43 PZT, a dielectric constant as low as ∼660 was achieved while the effective piezoelectric constant was over 140 pC/N. PZT–PMnN with a Zr:Ti ratio of 57:43 thus showed a maximum piezoelectric voltage constant (g33 ) of 23.8 × 10−3 Vm/N. It is believed that Mn addition is effective for maintain the effective piezoelectric constant (d33 ) but lower the dielectric constant. We therefore considered these PZT–PMnN thick films to be suitable for high-energy density, piezoelectric energy harvester and sensor applications.

60:40

57:43

53:47

49:51

45:55

596 0.045 5.32 38.31 80 15.16

664 0.030 13.08 57.42 140 23.81

591 0.029 8.96 45.09 85 16.24

529 0.029 4.35 35.74 65 13.88

533 0.022 3.35 29.81 55 11.65

onal phases at MPB region. It is known that randomly oriented PZT-based ceramics or thick films show maximum piezoelectric characteristics due to its phase co-existence [22,23]. It is believed that Mn addition is effective to decrease the dielectric constant, but is not strongly affecting the piezoelectric properties. Piezoelectric properties are sensitively changed by Zr:Ti ratio which changes crystal structure of the films. Fig. 7 shows a plot of effective piezoelectric voltage constant, g33 , versus the Zr:Ti ratio of the PZT–PMnN films. For sensor or energy harvester application, the piezoelectric voltage constant, gij , is important to estimate the generated voltage and output energy [10]. The effective g33 of the PZT–PMnN films was maximized at over 23 × 10−3 Vm/N for the Zr:Ti ratio of 57:43 as effective d33 . This high effective g33 value was almost the same as that for commercialized bulk piezoelectric ceramic and was attributed to the low dielectric constant of the film related to the hardening effect induced by Mn substitution. The electric properties of the PZT–PMnN films are summarized in Table 1. It is believed that Mn addition is effective for maintain the effective piezoelectric constant (d33 ) but lower the dielectric constant. Although undoped PZT thick films [20] or PZT–PZN thick films [11,19] have a good piezoelectric constant, their dielectric constant is generally over 1000, which lowers their piezoelectric voltage

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