Enhanced Pyroelectric and Piezoelectric Figure of Merit of Porous Bi0 ...

J. Am. Ceram. Soc., 93 [7] 1957–1964 (2010) DOI: 10.1111/j.1551-2916.2010.03651.x r 2010 The American Ceramic Society

Journal

Enhanced Pyroelectric and Piezoelectric Figure of Merit of Porous Bi0.5(Na0.82K0.18)0.5TiO3 Lead-Free Ferroelectric Thick Films Haibo Zhang,w,z,y Shenglin Jiang,z and Koji Kajiyoshi*,y z

Department of Electronic Science and Technology, Huazhong University of Science and Technology, Wuhan 430074, China

y

Research Laboratory of Hydrothermal Chemistry, Faculty of Science, Kochi University, Kochi 780-8520, Japan

and structural properties, several NBT-based solid solutions were formulated. It has been reported that NBT ceramics modified with BaTiO3 (BT)8 or Bi0.5K0.5TiO3 (KBT)3 showed improved dielectric and piezoelectric properties because there existed the corresponding rhombohedral–tetragonal morphotropic phase boundary (MPB). Sasaki et al.2 also reported their work on NBT–KBT system and showed that maximum electromechanical coupling factor and dielectric constant were obtained around the MPB region. Recently, Takenaka and Sakata9 found the SrTiO31PbTiO31CaTiO3-doped NBT ceramics with MPB composition have a moderate dielectric constant of 240 and a high pyroelectric coefficient of 120 mC/m21C, which make the figure of merit of NBT ceramics comparable with that of lead zirconate titanate (PZT) ceramics. In our previous studies,10 we have synthesized NKBT lead-free piezoelectric thick films with the composition near the MPB by screen printing, and the thick films exhibit the high remanent polarization and longitudinal effective piezoelectric coefficient d33 up to 28.3 mC/cm2 and 109 pm/V. Most recently,11 we prepared MnO-doped NKBT thick films with thickness of 40 mm using screen printing on Pt-electroded alumina substrates. The strong pyroelectric coefficient of 3.8 104 C/m21C was observed in 1.0 mol% MnO-doped thick films and the calculated detectivity figure of merit as high as 1.1 105 Pa0.5, which can be comparable to that of the commonly used lead-based materials. As a continuation of our earlier promising work, in the present work, we try to further improve the pyroelectric and piezoelectric figures of merit of NKBT thick films by introducing pores in the films. Pyroelectric materials have attracted significant attention because of their application in uncooled infrared detectors, which can be operated at ambient temperature without heavy cooling systems.12 To evaluate the quality of a pyroelectric material, different figures of merit exist, depending upon the device requirement. The figure of merit FD for specific detectivity of pyroelectric IR sensors is an important parameter to evaluate the quality of a pyroelectric material, which can be expressed by FD 5 p/(Cv(ertg d)1/2) where p is the pyroelectric coefficient, Cv is the volume specific heat, er and tg d are the relative dielectric constant and dielectric loss,13 respectively. Accordingly, to obtain high figure of merit FD for specific detectivity, it is necessary to maximize pyroelectric coefficient p and to lower the dielectric constant, volume specific heat, and dielectric loss. A possible method to lower dielectric constant and increase specific detectivity is the introduction of pores into the films. Recently, Seifert14 have synthesized PbxCa1xTiO3 (PCT) porous ferroelectric thin films by controlling the nucleation and growth of the grains during rapid thermal annealing of the materials. Their result demonstrated that the PCT thin films possess reduced dielectric constant as low as 55 for the porous structure caused by an intermediate Pb–Ca–Ti–fluorite phase before perovskite crystallization. Suyal et al.15 also have deposited PZT and PCT porous thin films which show the figure merit of FD as high as 250 mC/m21C.

Bi0.5(Na0.82K0.18)0.5TiO3(NKBT) lead-free ferroelectric thick films with various porosities have been produced by screen printing. Their microstructures, pyroelectric and piezoelectric properties were investigated with variation of porosity. The results show that the relative dielectric constant of the resulting 90 lm NKBT thick films with 19% and 32% porosity was down to 161 and 56, respectively. The pyroelectric voltage figure of merit (Fv) and detectivity figure of merit (FD) of NKBT thick films were increased from 10.2 1013 to 19.7 1013 Cm/J and 1.1 105 to 3.8 105 Pa0.5. Moreover, the hydrostatic voltage constant and hydrostatic figure of merit of the NKBT thick films with 32% porosity reached 81 103 V/mPa and 8200 1015 Pa1 respectively. The decreasing of relative dielectric constant, volume specific heat, and piezoelectric coefficient with increasing porosity was responsible for the improved pyroelectric and piezoelectric figures of merit of the NKBT thick films. Furthermore, screen printing with the addition of controlled fraction of organic vehicle in pastes has been proved to be an alternative method for fabrication of porous pyroelectric and piezoelectric thick films. Our work demonstrates that introduction of pores in ferroelectric thick films creates a matrix void composite resulting in high figures of merit for pyroelectric and piezoelectric applications. I. Introduction

R

on lead-free ferroelectric and piezoelectric ceramics or crystals has been more and more intense in the last few years due to restriction of the use of certain hazardous substances in electrical and electronic equipments (RoHS).1 Bismuth sodium titanate, (Bi0.5Na0.5)TiO3 (NBT), is considered to be an excellent candidate as a key material of lead-free piezoelectric material because NBT is strongly ferroelectric with relatively large remanent polarization. In recent years, considerable efforts have been put into the fabrication and characterization of NBT-based ceramics. In particular, NBT-based ceramics have been investigated intensively on their various properties, including piezoelectric,2 ferroelectric,3 and pyroelectric4 properties. In addition to these electronic properties, the thermal behavior,5 compressibility,6 and optical behavior7 of NBT-based ceramics have also been reported. However, this material has a drawback of high conductivity and high coercive field which cause problems in the poling process. To improve its electrical ESEARCH

D. Damjanovic—contributing editor

Manuscript No. 26902. Received October 1, 2009; approved January 8, 2010. This research was partially supported by China Postdoctoral Science Foundation (No. 20090460933), National High Technology Research and Development Program of China (No. 2007AA03Z120), and the National Natural Science Foundation of China (No. 60777043). *Member, The American Ceramic Society. w Author to whom correspondence should be addressed. e-mail: changhb@gmail. com

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Moreover, for ultrasonic transducer application of the piezoelectric materials, the acoustical impedance decreases with the increasing porosity, and as a result, the transfer of acoustical energy to water or biological tissues can be improved with the decreasing acoustical impedance.16 The hydrostatic figure of merit, dhgh, is defined to evaluate the effect of the piezoelectric materials used as underwater acoustics,17 and the hydrostatic piezoelectric voltage coefficient, gh, can be calculated by dh/er. Therefore, the hydrostatic figure of merit can increase with the decreasing dielectric constant er. Furthermore, an increase in the amount of pores leads to a decrease in the transverse piezoelectric coefficient (d31) relative to the longitudinal piezoelectric coefficient (d33),18 which produces an increase in the hydrostatic strain coefficient, dh ( 5 d3312d31), thus higher electrical charges are generated per unit hydrostatic stress. Recently, Zeng et al.19 reported that the hydrostatic figure of merit, dhgh for 41% porous PZT ceramics were 10 times more than that of 95% dense PZT. Moreover, Lee et al.20 fabricated porous lead zirconate titanate–lead zinc niobate (PZT–PZN) piezoelectric ceramics with a high degree of pore alignment using directional freeze casting of ceramic/camphene slurry. Their results show that the PZT–PZN porous ceramics containing pores aligned parallel to the poling direction exhibited an extremely high hydrostatic figure of merit of 161 019 1015 Pa1, which was about 1300 times higher than that (124 1015 Pa1) of the dense sample. However, up to now, most of these studies were performed on the bulk form of pyroelectric or piezoelectric porous ceramics. There is little work reported to date on the fabrication of porous piezoelectric thick or thin films. Recently, only several attempts at the fabrication of porous lead-based thin film have been made for pyroelectric application.15 There is a dearth of literatures on the fabrication of porous lead-free ferroelectric thick or thin films, as well as its pyroelectric and piezoelectric properties. Compared with commonly used bulk ceramics, the thick films exhibit much lower relative dielectric constant, and maintain the high remanent polarization and low dielectric loss simultaneously. Furthermore, the porous thick films offer low volume specific heat because of the introduction of pores resulting from the burning of the polymers in screen-printing process. A variety of manufacturing methods are currently available for the production of porous ferroelectric and piezoelectric ceramics, such as replamine process,21 burned-out polymer process,22 polymer replication method,18 and solid-free form fabrication techniques.23 As a most commonly used method to produce porous ferroelectric and piezoelectric with different pore size and porosities, the burned-out polymer method can be made by burning pore formers during the sintering process with different sizes and contents, such as starch, PVA, PVB, and yeast. In addition, screen printing is the most widely used thick film deposition technique, and the print pastes contain 20–40 vol% polymers of the binder, solvent, plasticizer, and the dispersing agent. Moreover, the polymers such as PVA, PVB, and ethyl cellulose used as pore formers in burned-out polymer method are just the binder polymers in screen-printing process of thick films. Thus, we expect that the controlled content of polymers added in print paste can introduce the controlled porosity in ferroelectric thick films. Therefore, we try to reduce the dielectric constant, volume specific heat, and transverse piezoelectric coefficient by introducing pores to improve the pyroelectric and piezoelectric figures of merit of ferroelectric thick films. In present study, we have fabricated NKBT lead-free porous thick films deposited by screen printing. Specifically, the effects of porous microstructure on the dielectric and ferroelectric properties were studied. And then the pyroelectric properties, such as pyroelectric coefficient, voltage, and detectivity figures of merit were investigated. Moreover, piezoelectric properties, such as hydrostatic piezoelectric strain coefficient (dh), hydrostatic piezoelectric voltage coefficient (gh), and hydrostatic figure of merit (dhgh), were also explored.

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II. Experimental Procedure Conventional solid-state reaction method was used to synthesize the NKBT ceramic powders. Commercially available reagent grade metal oxides and carbonate powders TiO2, Bi2O3, Na2CO3, and K2CO3 were mixed according to chemical formula Bi0.5(Na0.82K0.18)0.5TiO3. These oxide and carbonate powders were mixed in ethanol and ball milled for 24 h. After calcining at 8501C for 4 h, the powders were again ball milled for 48 h with the addition of ethanol. After the second ball milling procedure, the ground ceramic powders were dried at 601C for 24 h. The ground NKBT powders were ball milled for 4 h with the addition of 20–40 wt% of organic vehicle. The organic vehicle usually consists of a binder (ethyl cellulose), a solvent (a-terpineol), a plasticizer (polyethylene glycol), and a dispersing agent (butoxyethoxyethyl acetate). The organic vehicle also serves as a pore former during the sintering of the NKBT thick films. After the third ball milling, the viscosity of the prepared screen-printable pastes was measured by viscosimeter and adjusted in the range of 20–80 Pa s for shear rate 181 s. NKBT layers were screen printed with a 320 mesh screen mask on Ptelectroded alumina substrates (20 mm 15 mm 0.5 mm). The printed wet layers were dried at 1201C for 5 min after leveling in air at room temperature for 5 min. In order to remove the organic vehicle the layers were fired at 5501C for 5 min with a heating rate of 1801C/min in rapid thermal processor. The final sintering of the piezoelectric thick films was performed at 10001– 11001C for 30 min in air. These processes from printing to sintering were repeated 12 times to obtain about 90-mm-thick leadfree ferroelectric films. The organic vehicle burns out during sintering process forming cavities which increase the porosity of sintered NKBT thick films. Pore fraction, size, and distribution of pore size as well as morphology are controlled by the amount of added organic vehicle. The microstructures of the thick films were examined by fieldemission scanning electron microscopy (FE-SEM, Sirion 200, FEI Ltd., Eindhoven, the Netherlands). The crystal structures of the unpoled samples were analyzed using X-ray diffraction (X’Pert PRO, PANalytical B. V., Almelo, the Netherlands) with y–2y configuration and CuKa radiation (l 5 1.5406 A˚, 40 kV, 30 mA). In order to determine the phase structure of the ceramics, fine scanning was recorded with a step interval of 0.011 in the ranges of 461–491 and 551–611. The density of NKBT thick film was calculated from its weight and volume. The film weight was obtained from the sample weight by subtracting the weight of the substrate, and the film volume was obtained from the surface shape of the film. The film thickness was achieved using Form Talysurf Series 2 profilometer (Rank Taylor Hobson Ltd., Leicester, UK). The relative density of the film was calculated based on the pure NKBT. For dielectric, ferroelectric, pyroelectric and piezoelectric measurements, after the surface of the thick film was polished, platinum top electrodes with diameters of 0.8 mm and thickness of 0.3 mm were sputtered onto the sintered NKBT layers. The dielectric, ferroelectric, and pyroelectric properties were measured before and after poling of NKBT thick films. The NKBT thick films were poled by using 100–600 kV/cm applied field at 1001C for 10 min in silicone oil. The films are short-circuited for 24 h after poling in order to allow the release of excess charges injected during the poling process. Moreover, in order to avoid confusing loosely bound space charge with true polarization, the temperature cycled several times before the record of pyroelectric current of the thick films. Electrical measurements were performed using a LF impedance analyzer (HP 4192A, Agilent Technologies Inc., Santa Clara, CA) for dielectric constant, a modified Sawyer-Tower circuit for ferroelectric hysteresis measurements. The pyroelectric current, ip, was measured by a pA meter (KEITHLEY 6485, Keithley Instruments Inc., OH) while raising the temperature under vacuum at a constant rate of 1.01C/min over the 0–1001C range using a programmed temperature control system.24 The parameters dh, gh, and dhgh were calculated from the measured d33, d31, and er values and the equations discussed in the introduction.

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Enhanced Pyroelectric and Piezoelectric Figure of Merit

The transverse piezoelectric coefficient d31 of NKBT thick films was measured directly from the transverse expansion of the cantilever beams, and a dual-beam laser interferometer for the longitudinal piezoelectric charge coefficient, d33 measurements an oscilloscope is used as the final data output instead of the lock-in amplifier. The detailed process of the piezoelectric measurement can refer to the previous literatures.25,26

III. Results and Discussions The SEM morphologies of the unpolished surface and cross section of NKBT thick films with various contents of organic vehicle fired at 11001C for 0.5 h are shown in Fig. 1. It can be

(a)

seen that the thick films with about 90 mm in thickness contained porous structures. In addition, the porosity increased with an increase of the content of organic vehicle at a fixed sintering temperature. The cross-sectional view of the porous film showed that there are no pores going from the top to the bottom of the films. Moreover, the grain size of NKBT thick film is rather regular and without obvious presence of secondary phases. The grain size was approximately 1.1 mm, which was much larger than the grain in NKBT thin film.27 The volume fraction of pores P is calculated from the measured density of the film with respect to the bulk, and it is defined as P 5 (r0r)/r0, where r0, r are the density of the bulk and thick film samples, respectively. In fact, owing to the added organic vehicle in green films, a lot of pores appeared in the film when it is sintered. Generally, the

(b)

NKBT Pt

Al2O3 20µm

(c)

50µm

(d)

NKBT

Pt

Al2O3 20µm (e)

50µm (f)

NKBT

Pt

Al2O3 20µm

50µm

Fig. 1. SEM micrographs of the NKBT thick films: (a) surface and (b) cross section with porosity of 32%, (c) surface and (d) cross section with porosity of 19%, (e) surface and (f) cross section of dense film.

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Fig. 2. (a) XRD patterns in the 2y range of 201–701 and (b) the fine scanning XRD patterns in the selected 2y ranges of 461–491 and 551–611 of NKBT thick films with various porosities.

porosity, pore shape, and pore sizes can be controlled by adding organic vehicle and the sintering processing. In this work, the named dense films still content 5% pores, but it is much denser than the thick films with porosity of 19% and 32%. It is well known from the early works in porous ceramics that the spatial arrangement between the pores and the ceramic phase can be defined according to the volume fraction of porosity in the ceramics, and two possible connectivities (i.e. 3–3 and 3–0) can be formed. As the porosity volume fraction increased there is increased connectivity between pores and the mean pore size increases, and there can also be different fractions of closed and open pores, changing the connectivity of pores from 3–3 to a 3–0 type.16 The microstructure observations demonstrate that the thick films are considered 3–0 composites with mixed 3–3 connectivity. All NKBT thick films with various porosities present the pure perovskite structures and the coexistence of rhombohedral and tetragonal phases as shown in Fig. 2. The sintering does not change the crystalline structures of NKBT thick films and no second phase was observed. Figure 2(b) gives the fine scanning XRD patterns in the selected 2y ranges of 461–491 and 551–611 of NKBT thick films with various porosities. The rhombohedral symmetry and tetragonal symmetry of the NKBT thick films at room temperature are characterized by a (002)/(200) peak splitting near 461 and a (211)/(112) peak splitting near 581, respectively.28 The existence of the MPB between rhombohedral NBT and tetragonal KBT phases of the NKBT thick films is beneficial to the high piezoelectric and ferroelectric properties. It is well known that the volatilization of Na and K in NKBT ceramics sintered at high temperature is a complicated process, the compositional change caused by the volatilization could deteriorate the electrical properties of the ceramics.28 However, the XRD measurement of the NKBT thick films demonstrates that the samples exhibit pure perovskite structures and the coexistence of rhombohedral and tetragonal phases. Compared with the conventional ceramic processing, the relative shorter time of the sintering of the NKBT thick films is believed to be responsible for the reduction of the deviation of the compositional change. The existence of the MPB between rhombohedral NBT and tetragonal KBT phases of the NKBT thick films is beneficial to the high piezoelectric and ferroelectric properties. Figure 3 shows the poling field dependence of relative dielectric constant and dielectric loss of NKBT thick films with various porosities at 10 kHz and room temperature. With respect to the condition before the poling process, poling of the thick films had produced about a 23% reduction in relative dielectric constant which was down to the values given in Fig. 3. Apart from the increasing poling field, the increasing porosity was also responsible for the decreases of relative dielectric constant of NKBT thick films. After poling at

Fig. 3. Poling field dependence of relative dielectric constant and dielectric loss of NKBT thick films with various porosities at 10 kHz in room temperature.

applied field of 600 kV/cm for 15 min, the dielectric constant of NKBT thick films with 19% and 32% porosities were down to 161 and 56, respectively, whereas it is as much as 603 for dense NKBT thick films. The dielectric constants of the porous NKBT thick films can be calculated according to the following function29 ( er ¼ er;P¼0

1

P2=3 2=3

Ks

" 1

1

2=3 P1=3 er;P¼0 1 Ks þ 1

#) (1)

where er is the relative dielectric constant of the thick films with pores, er, P 5 0 the dielectric constant of thick films without pores, P the porosity, and Ks a parameter that is dependent on the shape of a pore. For a spherical pore, Ks 5 1, for an oval pore, Ks 5 0.5. Because the dielectric constant of dense NKBT thick films is very high, the above equation can be simplified as ! P2=3 er ¼ er;P¼0 1 2=3 (2) Ks In present study, using the hypothetical value (Ks 5 0.5), the calculated relative dielectric constant of the NKBT thick films with 5%, 19%, and 32% porosities were 78% er, P 5 0, 47% er, P 5 0, and 26% er, P 5 0. That is to say, according to Eq. (2) as the porosity increase from 5% to 19% and 32%, the relative

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dielectric constant should reduce 40% and 67%, respectively. However, the measured dielectric constant reduced 73% and 90% when the porosity of NKBT thick films increased to 19% and 32%. Compared with porous bulk ceramics, the much larger reduction of dielectric constants with increasing porosity for porous NKBT thick films was ascribed to different dielectric mechanism of thick films and bulk ceramics. Compared with the ceramics, the dielectric constant of NKBT thick films can be influenced by several effects, including clamping with the substrate, residual stress, and interfacial layer between the film and bottom electrode. As has been demonstrated by Xu et al.,30 there are both intrinsic and extrinsic contributions to the dielectric and ferroelectric responses of ferroelectric materials at room temperature. It is believed that about 25%–50% of the dielectric response of ferroelectric films at room temperature was from extrinsic sources.30 The extrinsic contribution to the dielectric constant of NKBT thick films was mainly attributed to 1801 domain wall motion, which was significant influenced by the clamping of the substrate, residual stress, and interfacial layer. Therefore, the measured dielectric constant of the NKBT thick films dramatically decreased with the increasing porosity. The dielectric losses at 10 kHz for both porous films and dense films were 1.9%–4.2%. In addition, the dielectric loss of porous NKBT thick films is slightly higher than that of dense NKBT thick films at room temperature, which is consistent with the lead magnesium niobate–lead zinc niobate (PMN–PZT) porous ceramics reported by Zeng et al.31 The increase in amplitude of dielectric loss is much lower than the decrease in amplitude of relative dielectric constant for NKBT thick films through introduction of pores. The dielectric strength of the films must have played a role as well, since the more porous film could not withstand a higher field than 600 kV/cm, the saturation of polarization has not been achieved. The existence of pores in NKBT thick films caused the reduction of the dielectric strength due to the enhanced field concentration and discharge in pores. Moreover, the pores could also influence the dielectric strength under high ac or dc field because the field concentrations and space–charge accumulation in the pores of the thick films. Therefore, in order to withstand high poling field, only the low molecular weight polymer (20–40 wt%) was used for further studies. Figure 4 illustrates P–E hysteresis loops of the NKBT thick films with various porosities at room temperature. The coercive field remarkably increases from 56 to 93 kV/cm as the porosity increasing from 5% to 32%, whereas the remanent polarization decreases from 25.1 to 21.6 mC/cm2. These results suggest that the NKBT thick films with high porosity are very hard to pole as

Fig. 4. P–E hysteresis loops of the NKBT thick films with various porosities at room temperature.

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Fig. 5. Poling field dependence of pyroelectric coefficient and relative dielectric constant of NKBT thick films with various porosities.

compared to dense thick films, which is in line with the results by Suyal et al.15 But Li et al.32 found the remanent polarization decreased with the increase of the porosity of PZT ceramics, whereas the coercive field remained unchanged. It may be due to the small grain size of about 50 nm for porous PZT. Domain wall motion and thus polarizability is known to be significantly reduced in small grained materials.33 However, the SEM figures show that there was no change in grain size of NKBT thick films with various porosities. Thus, the increase of coercive field was not caused by the reduction of grain size but by the increase of stress resulting from the pore of the thick films.31 Because the microscopic stress and strain can inhibit the movement of domain walls, resulting in the increase of the coercive field of the NKBT thick films. The increase in microscopic stress and strain resulting from the enhanced stress near the pores was responsible for the increase of coercive field with increasing porosity. Figure 5 demonstrates that the increase in poling field increases the pyroelectric coefficient of the NKBT thick films. The pyroelectric coefficient, p, can be written as p 5 dPr/dT, where the Pr is remanent polarization, and the reduction in p is caused by the reduction in dPr/dT. It means that the remanent polarization changed less per unit temperature with the increasing porosity, and the pyroelectric coefficient can decrease with the increasing porosity. This supports the ferroelectric measurements that show a small reduction in the remanent polarization due to the porosity of the films. For the samples having 32% porosity, the increase in the poling field from 100 to 600 kV/cm increases the pyroelectric coefficient from 1.5 to 4.2 104 C/ m2K. Whereas, for the dense thick films, the increase of poling field from 100 to 600 kV/cm increases the pyroelectric coefficient from 3.2 to 4.6 104 C/m2K. These results suggest that the introduction of pores slightly decreases the pyroelectric coefficient after the NKBT thick film poled by using 600 kV/cm applied field. This behavior can be explained by the fact that the films with higher porosity will have a proportionally reduced charge density and hence a reduced pyroelectric coefficient.34 Whereas, the introduction of pores remarkably decreases the dielectric constant of the NKBT thick films as shown in Fig. 2. The reduction proportion of pyroelectric coefficient has been shown to be smaller than the reduction of dielectric constant, so the p/er value shows an increase for the porosity range investigated, which is also essential in order to get high pyroelectric detectivity figures of merit according to the previous formula. To improve pyroelectric voltage response it is necessary to maximize pyroelectric coefficient p and lower the relative dielectric constant, to increase the pyroelectric voltage figure of merit

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Fig. 6. Poling field dependence of voltage and detectivity figures of merit of NKBT thick films with various porosities.

FV which can be calculated by FV ¼

p CV er e0

(3)

The figures of merit Fv and FD for NKBT thick films are shown in Fig. 6. The Fv and FD values of NKBT thick films with 32% porosity can be increased to 19.7 1013 Cm/J and 3.8 105 Pa0.5 with increase in the poling field from 100 to 600 kV/cm, whereas, these values are 10.2 1013 Cm/J and 1.1 105 Pa0.5, respectively, for dense NKBT thick films. The presence of the pores decreases the pyroelectric coefficient, however, the reduction of pyroelectric coefficient is much less than the reduction of dielectric constant of the NKBT thick films,15 thus the p/er value shows an increase with the increasing porosity. Moreover, the volume specific heat of NKBT thick films also decreases with the increase of porosity. The volume specific heat, CV, can be defined as4 CV ¼ rCS

(4)

where r denotes bulk density and CS denotes specific heat. According to the above equation, the room-temperature volume specific heat of the NKBT thick films with 19% and 32% porosity are 2.27 J/K cm3, 1.90, respectively, whereas the roomtemperature volume specific heat of the dense NKBT thick film

Fig. 7. The effective longitudinal coefficient d33 piezoelectric coefficient as a function of dc electric field of the NKBT thick films with various porosities.

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is 2.63 J/K cm3 according to the results demonstrated by Abe et al.4 The longitudinal piezoelectric coefficient was measured using a dual-beam laser interferometer by applying the 5 kHz ac electrical field with the amplitude of 0.10 kV/cm. The effective longitudinal d33 piezoelectric coefficient as a function of dc electric field of the NKBT thick films with various porosities is shown in Fig. 7. The piezoelectric coefficient d33 demonstrates a clear switching behavior. The effective longitudinal piezoelectric coefficient was found to slightly decrease under the high dc bias Edc. This typical behavior of the NKBT thick films could come from the decrease of the number of domain walls, which reduces the extrinsic contribution to piezoelectric coefficient. There are two effects can be responsible for the decreases of d33, one is the suppression of the intrinsic d33 at high dc bias Edc, the other is the displacement of the residual domain wall by the driving ac electrical field.35 Moreover, the piezoelectric thick films are rigidly clamped on one face to Pt/Al2O3 substrates, and this constraint limits the movement piezoelectric layer as it attempts to expand or contract when or voltage is applied. Therefore, the clamping effect caused by substrates reduces the measured piezoelectric coefficient, d33, of the NKBT thick film. This reduction is ascribed to the influence of the d31 component in the thick film when a deformation of the structure occurs, by either the direct or indirect piezoelectric effect. The measured effective parameter d33 and the effect of clamping to the substrate are as following function36 d33 ¼

S3 2d S E ¼ d33 E 31 13E E3 ðS11 þ S12 Þ

(5)

, where S13E, S11E, and S12E are the elastic compliance, and d33 d31 are the free longitudinal, transversal piezoelectric coefficient, respectively. Since d31 < 0, S13Eo0, S11E1S12E40, the measured effective d33 is always smaller than that in free materials. As follows from Torah et al.36 there is approximately 74% reduction in the measured d33 value. The maximum effective longitudinal piezoelectric coefficient of the NKBT thick films decreases with the increase of porosity as expected. The decreases of piezoelectric coefficient was attributed to the decreases of grain size resulting from the enhanced stress near the pores by Zeng et al.31 However, there are no obvious changes in the grain size of the NKBT thick films as shown in Fig. 1. It is believed that 1801 wall motion contribute only to dielectric behavior, while non-1801 wall motion contribute to the elastic compliance and piezoelectric coefficients.30,37 The stress can inhibit the movement of non-1801 domain walls which results in suppression of piezoelectric response. So we proposed that the decreases of piezoelectric coefficient with the increase porosity were ascribed to the restriction of non-1801 domain wall motion by the stress enhancements in the vicinity of pores. The poling field

Fig. 8. Poling field dependence of longitude and transverse piezoelectric coefficient of NKBT thick films with various porosities.

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Fig. 9. (a) Poling field dependence of hydrostatic voltage constant (b) and hydrostatic figures of merit of NKBT thick films with various porosities.

dependence of longitude and transverse piezoelectric coefficient of NKBT thick films with various porosities are shown in Fig. 8 which demonstrate that there is a more rapid decrease of d31 with increasing porosity compared to d33. Figure 9 depicts poling field dependence of hydrostatic voltage constant and hydrostatic figures of merit of NKBT thick films with various porosities. The reduced piezoelectric coefficient d31 and relative dielectric constant, er of the porous NKBT thick films led to an increase in hydrostatic voltage constant and large improvements are observed for NKBT thick films with 32% porosity compared to the dense NKBT thick films, as seen in Fig. 9(a). The hydrostatic figure of merit can be calculated by following formula: gh dh ¼ ðd33 þ 2d31 Þ2 =er

(6)

The improved d3312d31 and reduced er of porous NKBT thick films lead to significant improvements in the figure of merit when compared with dense thick films. As shown in Fig. 9(b), the NKBT thick film with 32% porosity shows an extremely high hydrostatic figure of merit of 8200 1015 Pa1, which is about 40 times higher than that of the dense thick films. The hydrostatic figure of merit is also higher than that of porous PMN–PZT ceramic (4000 1015 Pa1)31 but much less than that of (PZT–PZN) ceramic with highly aligned pores (161 019 1015 Pa1).20 This is associated with the clamping effect of the substrates as well as pore geometry and pore distributions which were not well controlled in NKBT thick films.

IV. Conclusions It has been shown that the NKBT thick films with different porosity have been fabricated by screen printing. The screen printing with controlled fraction of organic vehicle in paste has been proved to be an excellent method for fabrication of porous pyroelectric and piezoelectric thick films. The dielectric, ferroelectric, pyroelectric and piezoelectric properties of NKBT thick films with various porosities were investigated as a function of poling field. The introduction of pores in NKBT thick films remarkably reduced the relative dielectric constant, volume specific heat and piezoelectric coefficient d31. The pyroelectric coefficient, p and piezoelectric coefficient, d33 also slightly decreased with the increasing porosity. The pyroelectric voltage figure of merit, detectivity figure of merit, hydrostatic voltage constant and hydrostatic figure of merit were obviously improved by introducing a porous microstructure in NKBT thick films. The pyroelectric voltage figure of merit, Fv and detectivity figures of merit, FD of NKBT thick films were increased from 10.2 1013 to 19.7 1013 Cm/J and 1.1 105 to 3.8 105 Pa0.5, respectively. This makes the NKBT thick films excellent

candidates for pyroelectric uncooled infrared detectors. The hydrostatic voltage constant and hydrostatic figure of merit of NKBT thick films with 32% porosity reached 81 103 V/mPa and 8200 1015 Pa1, respectively, which prove that porous NKBT thick films are an ideal for lead-free piezoelectric transducer applications. Moreover, the hydrostatic voltage constant and the hydrostatic figure of merit of screen-printed NKBT thick films can be further improved by tailoring the pore geometry and pore distribution.

References 1

E. Hollenstein, M. Davis, D. Damjanovic, and N. Setter, ‘‘Piezoelectric Properties of Li- and Ta-Modified (K0.5Na0.5)NbO3 Ceramics,’’ Appl. Phys. Lett., 87 [18] 182905, 3pp (2005). 2 A. Sasaki, T. Chiba, Y. Mamiya, and E. Otsuki, ‘‘Dielectric and Piezoelectric Properties of (Bi0.5Na0.5)TiO3–(Bi0.5K0.5)TiO3 Systems,’’ Jpn. J. Appl. Phys., 38 [9B] 5564–7 (1999). 3 V. A. Isupov, ‘‘Ferroelectric Na0.5Bi0.5TiO3 and K0.5Bi0.5TiO3 Perovskites and Their Solid Solutions,’’ Ferroelectrics, 315, 123–47 (2005). 4 J. Abe, M. Kobune, T. Nishimura, T. Yazaw, and Y. Nakai, ‘‘Effects of Manganese Addition on Pyroelectric Properties of (Bi0.5Na0.5TiO3)0.94(BaTiO3)0.06 Ceramics,’’ Integr. Ferroelectr., 80 [1] 87–95 (2006). 5 Z. H. Zhou, J. M. Xue, W. Z. Li, J. Wang, H. Zhu, and J. M. Miao, ‘‘Ferroelectric and Electrical Behavior of (Na0.5Bi0.5)TiO3 Thin Films,’’ Appl. Phys. Lett., 85 [5] 804–6 (2004). 6 J. Kreisel and A. M. Glazer, ‘‘Estimation of the Compressibility of Na0.5Bi0.5 TiO3 and Related Perovskite-Type Titanates,’’ J. Phys.: Condens. Mat., 12 [46] 9689–98 (2000). 7 T. V. Kruzina, V. M. Duda, and J. Suchanicz, ‘‘Peculiarities of Optical Behaviour of Na0.5Bi0.5TiO3 Single Crystals,’’ Mater. Sci. Eng. B, 87 [1] 48–52 (2001). 8 H. Nagata, M. Yoshida, Y. Makiuchi, and T. Takenaka, ‘‘Large Piezoelectric Constant and High Curie Temperature of Lead-Free Piezoelectric Ceramic Ternary System Based on Bismuth Sodium Titanate–Bismuth Potassium Titanate–Barium Titanate near the Morphotropic Phase Boundary,’’ Jpn. J. Appl. Phys., 42 [12] 7401–3 (2003). 9 T. Takenaka and K. Sakata, ‘‘Dielectric, Piezoelectric and Pyroelectric Properties of (BiNa)1/2TiO3-Based Ceramics,’’ Ferroelectrics, 95 [1] 153–6 (1989). 10 H. B. Zhang, S. L. Jiang, and Y. K. Zeng, ‘‘Piezoelectric Property in Morphotropic Phase Boundary Bi0.5(Na0.82K0.18)0.5TiO3 Lead Free Thick Film Deposited by Screen Printing,’’ Appl. Phys. Lett., 92 [15] 152901, 3pp (2008). 11 H. B. Zhang, S. L. Jiang, and K. Kajiyoshi, ‘‘Pyroelectric and Dielectric Properties of Mn Modified 0.82Bi0.5Na0.5TiO3–0.18Bi0.5K0.5TiO3 Lead-Free Thick Films,’’ J. Am. Ceram. Soc., 92 [9] 2147–50 (2009). 12 S. B. Lang, ‘‘Pyroelectricity: From Ancient Curiosity to Modern Imaging Tool,’’ Phys. Today, 58 [8] 31–6 (2005). 13 R. W. Whatmore, ‘‘Pyroelectric Devices and Materials,’’ Rep. Prog. Phys., 49 [12] 1335–86 (1986). 14 A. Seifert, ‘‘The Influence of Porosity on the Electromechanical and Pyroelectric Properties of Ca-Modified PbTiO3 Thin Films,’’ J. Sol–Gel Sci. Technol., 16 [1–2] 13–20 (1999). 15 G. Suyal, A. Seifert, and N. Setter, ‘‘Pyroelectric Nanoporous Films: Synthesis and Properties,’’ Appl. Phys. Lett., 81 [6] 1059–61 (2002). 16 F. Levassort, J. Holc, E. Ringgaard, T. Bove, M. Kosec, and M. Lethiecq, ‘‘Fabrication, Modelling and Use of Porous Ceramics for Ultrasonic Transducer Applications,’’ J. Electroceram., 19 [1] 125–37 (2007). 17 C. R. Bowen, A. Perry, A. C. F. Lewis, and H. Kara, ‘‘Processing and Properties of Porous Piezoelectric Materials with High Hydrostatic Figures of Merit,’’ J. Eur. Ceram. Soc., 24 [2] 541–5 (2004). 18 M. J. Creedon and W. A. Schulze, ‘‘Axially Distorted 3–3 Piezoelectric Composites for Hydrophone Applications,’’ Ferroelectrics, 153 [1–4, Part 3] 333–9 (1994).

1964 19


T. Zeng, X. Dong, C. Mao, Z. Zhou, and H. Yang, ‘‘Effects of Pore Shape and Porosity on the Properties of Porous PZT 95/5 Ceramics,’’ J. Eur. Ceram. Soc., 27 [4] 2025–9 (2007). 20 S. H. Lee, S. H. Jun, H. E. Kim, and Y. H. Koh, ‘‘Piezoelectric Properties of PZT-Based Ceramic with Highly Aligned Pores,’’ J. Am. Ceram. Soc., 91 [6] 1912– 5 (2008). 21 D. P. Skinner, R. E. Newnham, and L. E. Cross, ‘‘Flexible Composite Transducers,’’ Mater. Res. Bull., 13 [6] 599–607 (1978). 22 K. Rittenmyer, T. Shrout, W. A. Schulze, and R. E. Newnham, ‘‘Piezoelectric 3–3 Composites,’’ Ferroelectrics, 41 [1–4] 189–95 (1982). 23 M. Allahverdi, S. C. Danforth, M. Jafari, and A. Safari, ‘‘Processing of Advanced Electroceramic Components by Fused Deposition Technique,’’ J. Eur. Ceram. Soc., 21 [10–11] 1485–90 (2001). 24 S. B. Liu, M. D. Liu, S. L. Jiang, C. R. Li, Y. K. Zeng, Y. Q. Huang, and D. X. Zhou, ‘‘Fabrication of SiO2-Doped Ba0. 85Sr0.15TiO3 Glass–Ceramic Films and the Measurement of Their Pyroelectric Coefficient,’’ Mater. Sci. Eng. B, 99 [1–3] 511–5 (2003). 25 I. Kanno, S. Fujii, T. Kamada, and R. Takayama, ‘‘Piezoelectric Properties of C-Axis Oriented Pb(Zr,Ti)O3 Thin Films,’’ Appl. Phys. Lett., 70 [11] 1378–80 (1997). 26 A. L. Kholkin, C. Wutchrich, D. V. Taylor, and N. Setter, ‘‘Interferometric Measurements of Electric Field-Induced Displacements in Piezoelectric Thin Films,’’ Rev. Sci. Instrum., 67 [5] 1935–41 (1996). 27 T. Yu, K. W. Kwok, and H. L. W. Chan, ‘‘The Synthesis of Lead-Free Ferroelectric Bi0.5Na0.5TiO3–Bi0.5K0.5TiO3 Thin Films by Sol–Gel Method,’’ Mater. Lett., 61 [10] 2117–20 (2007). 28 Y. R. Zhang, J. F. Li, and B. P. Zhang, ‘‘Enhancing Electrical Properties in NBT–KBT Lead-Free Piezoelectric Ceramics by Optimizing Sintering Temperature,’’ J. Am. Ceram. Soc., 91 [8] 2716–9 (2008).

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H. Banno, ‘‘Effects of Shape and Volume Fraction of Closed Pores on Dielectric, Elastic, and Electromechanical Properties of Dielectric and Piezoelectric Ceramics—A Theoretical Approach,’’ Am. Ceram. Soc. Bull., 66 [9] 1332–7 (1987). 30 F. Xu, S. Trolier-McKinstry, W. Ren, B. Xu, Z. L. Xie, and K. J. Hemker, ‘‘Domain Wall Motion and Its Contribution to the Dielectric and Piezoelectric Properties of Lead Zirconate Titanate Films,’’ J. Appl. Phys., 89 [2] 1336–48 (2001). 31 T. Zeng, X. Dong, C. Mao, S. Chen, and H. Chen, ‘‘Preparation and Properties of Porous PMN–PZT Ceramics Doped with Strontium,’’ Mater. Sci. Eng. B, 135 [1] 50–4 (2006). 32 J. F. Li, K. Takagi, M. Ono, W. Pan, R. Watanabe, A. Almajid, and M. Taya, ‘‘Fabrication and Evaluation of Porous Piezoelectric Ceramics and PorosityGraded Piezoelectric Actuators,’’ J. Am. Ceram. Soc., 86 [7] 1094–8 (2003). 33 G. Arlt and N. A. Pertsev, ‘‘Force Constant and Effective Mass of 901 Domain Walls in Ferroelectric Ceramics,’’ J. Appl. Phys., 70 [4] 2283–9 (1991). 34 G. Suyal and N. Setter, ‘‘Enhanced Performance of Pyroelectric Microsensors through the Introduction of Nanoporosity,’’ J. Eur. Ceram. Soc., 24 [2] 247–51 (2004). 35 N. Bassiri-Gharb, I. Fujii, E. Hong, S. Trolier-McKinstry, D. Taylor, and D. Damjanovic, ‘‘Domain Wall Contributions to the Properties of Piezoelectric Thin Films,’’ J. Electroceram., 19 [1] 47–65 (2007). 36 R. N. Torah, S. P. Beeby, and N. M. White, ‘‘Experimental Investigation into the Effect of Substrate Clamping on the Piezoelectric Behaviour of Thick-Film PZT Elements,’’ J. Phys. D. Appl. Phys., 37 [7] 1074–8 (2004). 37 M. Budimir, D. Damjanovic, and N. Setter, ‘‘Qualitative Distinction in Enhancement of the Piezoelectric Response in PbTiO3 in Proximity of Coercive Fields: 901 Versus 1801 Switching,’’ J. Appl. Phys., 101 [10] 104119, 4pp (2007). &