Structural and functional properties of screen-printed PZT PVDF-TrFE composites

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1 Available online at Sensors and Actuators A 143 (2008) Structural and functional properties of screen-printed PZT composites Matthias Dietze, Mohammed Es-Souni Institute of Materials & Surface Technology, University of Applied Science Kiel, Grenzstr. 3, Kiel, Germany Received 9 August 2007; received in revised form 6 November 2007; accepted 18 November 2007 Available online 22 November 2007 Abstract Thick films of 0 3 composites of lead zirconate titanate (PZT) particulate and polyvinylidene-trifluoroethylene () copolymer have been produced by screen-printing on indium tin-oxide (ITO)-coated glass substrates. The microstructure was investigated using scanning electron microscopy and X-ray diffraction. The dependences of the dielectric properties of the composites on PZT volume fraction are reported and analyzed in terms of an analytical model. The pyroelectric properties were determined using dynamical experimental setups, and are given in terms of pyroelectric coefficient and figures of merit. The piezoelectric response was investigated as a function of the PZT volume fraction using a laser vibrometer. The piezoelectric charge coefficient, d 33,eff, is shown to exhibit a minimum at a PZT volume fraction of 40% which arises from the piezoelectric charge coefficients of the pure constituents being of opposite sign. It is thought that screen-printing of these composites constitutes a cost-effective way of producing structured functional thin films for pyroelectric and piezoelectric applications Elsevier B.V. All rights reserved. Keywords: Polymer matrix composites (PMCs); Pyroelectric properties; Smart materials 1. Introduction In recent years composites of polymers and ceramic or metal gained increasing interest among the materials research community. A particular class of composites consists of a polymer matrix in which randomly distributed particles of metal or ceramic are embedded without being connected to each others. These composites are usually referred to as 0 3 composites. This name arises from the concept of connectivity, developed by Newnham et al. [1] in the late 1970s. The main advantages of this type of composites are the possibility to tailor their properties by adjusting the volume content of the ceramic or metal phase, and the fairly low processing temperatures which open many possibilities for integration with thermally sensitive substrates or devices. 0 3 polymer ceramic composites, especially of the ferroelectric polymer PVDF and its copolymers and the ferroelectric ceramics such as PZT and PMN-PT have been the subject of Corresponding author. Tel.: ; fax: address: me@fh-kiel.de (M. Es-Souni). some research work [2 4], and detailed studies were reported on their dielectric, piezoelectric and pyroelectric properties [5 8]. It was shown that the dielectric constant increases with increasing ceramic volume fraction and that these composites have promising properties for applications in thermal imaging sensors and transducers [9,10]. A main feature of such composites is the possibility of suppressing the piezoelectric response concomitantly promoting the pyroelectric properties and vice versa [6,9]. This behavior arises from the coupling of the piezoelectric properties of the ferroelectric polymeric matrix and ceramic inclusions which possess piezoelectric charge coefficients of opposite signs. This allows to process composites with a vanishing effective piezoelectric coefficient. In terms of pyroelectric applications (infrared sensing) this is very useful, since the design of pyroelectric sensors becomes simple, as expensive design measures for compensating piezoelectric noise become superfluous. Depending on the structure required, these composites are usually prepared by spin-coating a mixed polymer-particles solution [5], solvent casting such a solution for the production of tapes or hot pressing a mixture of polymer and ceramic particles [6,8]. For the batch production of sensor or actuator /$ see front matter 2007 Elsevier B.V. All rights reserved. doi: /j.sna

2 330 M. Dietze, M. Es-Souni / Sensors and Actuators A 143 (2008) devices each of these methods has specific disadvantages. The scale-up of spin-coating process can be difficult, while hot pressing is a non-continuous and time-consuming process. Another drawback is the need of an additional structuring step for sensor production. A more suitable method for batch production is screen-printing which is widely used for the processing of structured thick ceramic films for sensor and actuator purposes [11]. In this paper we demonstrate that screen-printing can be applied to PZT composites for the processing of structured thick films at low processing temperatures. The microstructures obtained as well as the dielectric, pyroelectric and piezoelectric properties and their dependencies on the PZT volume fraction are reported. To our knowledge no previous work had dealt with the screen-printing of 0 3 composites, so that the results obtained in this work will only be compared to spin-coated thick films [5] and hot-pressed free-standing foils [6,8]. 2. Experimental The materials used in this study were a commercial PZT powder (PZ21) from Ferroperm-piezo (Denmark) and polyvinylidenefluoride-trifluoroethylene () copolymer powder (56:44, Solvay Solexis S.A.S, France). First a 7 vol.% polymer solution was prepared by dissolving the copolymer powder in a solvent. A suitable solvent for screen-printing should have a low vapor pressure at room temperature to avoid evaporation from the paste. We choose N,N-dimethylformamide (DMF, Fluka) as solvent. DMF has a vapor pressure at room temperature of 3.7 hpa which is quite low compared to that of other solvents used for PVDF copolymers such as methylethylketone (105 hpa). Subsequently an appropriate amount of ceramic powder was dispersed in this solution by stirring and ultrasonication. After dispersion the solvent was simply evaporated under increased temperature and stirring until the paste had reached a suitable viscosity for printing; no organic binders were added to the slurry to achieve a printable paste. A paste with 20 vol.% PZT and a dynamic viscosity of 39 Pa s and 9 Pa s at a shear rate of 20 s 1 and 100 s 1, respectively (measured with Rheomat RM180) lead to 5 m thick films. Thickness of the screen-printed structures can be varied by adjusting the viscosity of the paste. Different structures with PZT volume contents from 20 to 50% were screen-printed on ITO-coated glass substrates. The samples were kept on a hotplate at 130 C for 15 min to evaporate the solvent, and subsequently cured at 140 C for 12 h to ensure that all the solvent is removed and to increase crystallinity of the copolymer. Poling of the samples was conducted by corona discharge method. This method is a more suitable poling method for composites than the usually applied contact poling, because the high electric field necessary to pole the composites can lead to electrical breakdown on weak spots when using contact poling [12,13]. The samples were poled by corona poling at a dc voltage of 17.5 kv for 2 h at 120 C, followed by poling at room temperature for 1 h. After poling Au front electrodes of 3 mm diameter for dielectric and piezoelectric measurements and 0.5 mm diameter electrodes for ferroelectric hysteresis and pyroelectric characterization were sputtered. The microstructures of the samples were investigated by means of a scanning electron microscope (SEM) and X- ray diffraction. The roughness profiles were obtained using roughness tester (Hommel Tester T1000). The dielectric properties were measured in the frequency range from 500 Hz to 13 MHz using a computer-controlled impedance analyzer (Agilent 4192). A Sawyer-Tower setup was used to measure ferroelectric hysteresis at a frequency of 100 Hz and room temperature. Dynamical pyroelectric properties were measured using a self-made experimental setup including a computer-controlled lock-in amplifier, frequency generator, current source and voltmeter. Sinusoidal thermal excitation of the samples was performed using a thermoelectric heater/cooler element (Peltier element) at a frequency of 0.02 Hz and a T of 1.5 K. A detailed description of the measurement procedure of the pyroelectric coefficient can be found in our previous work [5]. The volumetric specific heat of the films was determined from differential scanning calorimetry (DSC) measurements (Setaram DSC131) of free-standing films. Piezoelectric measurements were performed on poled samples by means of a self-made experimental setup, including a computer-controlled laser vibrometer (Polytec OFV 353 sensor head and OFV 3001 vibrometer controller), lock-in amplifier, frequency generator and a voltage source. The vibrometer was operated in the most sensitive velocity range (1 mm s 1 V 1 ). The effective thickness mode piezoelectric coefficients, d 33,eff, of the composites were determined at 5 khz, outside mechanical resonances. 3. Results and discussion 3.1. Microstructure Square thick film structures down to 200 m and film thickness of 5 10 m could be printed using the composite pastes. The thickness of the films was controlled by adjusting the viscosity of the paste via controlling of solvent evaporation. Fig. 1 shows a cross-sectional SEM image of a screenprinted composite with 20 vol.% PZT. The PZT particles are well distributed in the polymer matrix as can be inferred from the back-scattered electron image where the PZT particles appear bright. The surface of the composite is slightly rough (Ra = 0.2 m), typical for screen-printed structures. Samples with higher volume content of PZT show increased roughness, too. The X-ray diffraction patterns from unpoled and poled samples with 20 vol.% PZT are shown in Fig. 2. For comparison, diffraction patterns from the substrate with the ITO layer are also shown. The peaks at 2Θ values of 31,36 and 52 belong to the ITO layer. The peaks at 22,32,39,45 and 56 can be attributed to PZT (PDF ). The (200/110)

3 M. Dietze, M. Es-Souni / Sensors and Actuators A 143 (2008) Fig. 1. Back-scattered (BS)-scanning electron microscopy (SEM) micrograph of the cross-section of a composite with 20 vol.% PZ21. peak appears at around 19 [14]. After poling, the peak of the polymer shifts to higher angles, which has also been reported for the copolymer composition 75:25 VDF-TrFE [15]. Additionally, a splitting of the peak is observed. This splitting is usually observed [16] for copolymers in the ferroelectric state with compositions near 50:50. We suggest that upon poling the coordination of the chains tends to be more perfect, and because of this the (2 0 0) and (1 1 0) peaks become separated. The polymer is thus well poled by the procedure described above. In case of PZT it is known that poling changes the ratio of intensity for the (2 0 0/0 0 2) peaks of PZT at around 45 [17]. The fairly small shift observed in Fig. 2 suggests that the PZT particles are only partially poled and would either require a stronger poling field or a somewhat conducting matrix for complete poling Dielectric properties The dependence of the dielectric constant, ε, and dielectric loss, tan δ, of the composites on the volume fraction of PZT is shown in Fig. 3. As expected ε increases from 31 to 71 with Fig. 3. Dielectric constant (filled symbols) and dielectric loss tan δ (open symbols) of composites at 1 khz as a function of ceramic volume content. The dotted line indicates the calculated dielectric constant with the Bruggeman model. increasing PZT volume content up to 50%, while tan δ decreases. This result is obviously due to the increasing contribution of PZT to the dielectric properties of the composites, since PZT has a substantially higher dielectric constant and lower loss than. Up to 40 vol.% PZT the screen-printed composites show slightly higher dielectric constants compared to similar composites from literature prepared by hot pressing [7]. The experimental data are compared to the well-known Bruggeman model [18] for 0 3 composites. For low volume contents of up to 30 vol.% the measured data agrees well with the model, but at higher ceramic loading there is an increasing discrepancy between predicted and measured data. SEM images of crosssections of composites with ceramic volume content of 50 vol.% show some porosity in the printed composite, especially at the interface with the substrate (Fig. 4). We attribute the discrepancy of measured and calculated data to this porosity, because of the lower ε of air. Furthermore the porosity is expected to reduce the contact area between composite and electrode, therefore leading to lower effective dielectric constants, without however affecting the magnitude of tan δ. Fig. 2. X-ray diffraction from substrate and sample with 20 vol.% of PZ21. The peaks are indexed, P denotes polymer, C ceramic. Fig. 4. Back-scattered (BS)-scanning electron microscopy (SEM) micrograph of the cross-section of a composite with 50 vol.% PZ21. The arrows highlight porosity.

4 332 M. Dietze, M. Es-Souni / Sensors and Actuators A 143 (2008) Fig. 5. Polarization-electric field loops of poled and unpoled composite with 30 vol.% PZT. A hysteresis loop of a pure, spin-coated copolymer sample is shown for comparison Ferroelectric properties Ferroelectric polarization-electric field (P-E) loops of unpoled and poled samples with 30 vol.% PZT are shown in Fig. 5. For comparison a hysteresis loop of a spin-coated pure copolymer sample is shown, too. The pure copolymer sample shows a P-E loop with a high coercive field (E c )of about 0.4 MV/cm and a saturation polarization (P sat ) of about 7 C/cm 2. These values are in good agreement with those reported in Ref. [19] for the same composition, where E c of MV/cm and P sat of 7.1 C/cm 2 were found. The composite samples have a slightly smaller coercive field, probably due to the smaller volume fraction of the polymer phase. Both composite samples show higher P sat than the pure copolymer, which can be attributed to the contribution of PZT. Poling leads to increasing P sat to a value of about 10.5 C/cm 2. The change in the shape of the hysteresis, especially the open shape at saturation, may be attributed to space charge effects (e.g. due to charge injection during poling) at the interface and inner porosity Pyroelectric properties Pyroelectricity is a property of crystals possessing a spontaneous polarization P s. The dielectric displacement D of a polar material subjected to an electric field E can be expressed as [20]: D = εe + P s (1) where ε is the permittivity. Small temperature changes T affect D as follows: D T = P s T + E ε (2) T P s / T is defined as the true pyroelectric coefficient, p, under the constraints of constant electric field E and constant elastic stress. The change of polarization with temperature is usually given by P i = p i T (3) Fig. 6. Pyroelectric coefficient, p, as a function of PZT volume content. The pyroelectric coefficient is a vector, but practically it is treated as a scalar when the electrodes are arranged normal to the poling direction. The effective pyroelectric coefficient of the composites is shown in Fig. 6. Samples with a PZT volume content of 30 vol.% show the highest pyroelectric coefficient of about 80 Cm 2 K 1. This is an increase of about 60% compared to the values of pure copolymer reported in literatures [5,15]. The decrease of the pyroelectric coefficient for higher PZT volume fractions can be explained by increasing porosity as discussed above. Although the pyroelectric coefficient of the screen-printed samples is slightly lower than that of spin-coated thick films of the same material reported in a previous work [5], it is still higher than that of similar composites reported in literatures [6,8]. In the present work a copolymer with optimized pyroelectric properties served as the matrix, and this should explain the higher values of the pyroelectric coefficient obtained in comparison to previous work [6,8], where a PVDF/TrFE copolymer of ratio 70:30 with a smaller pyroelectric coefficient [14] was used. Furthermore corona poling might be a better poling method for this kind of composites than contact poling used in Refs. [6,8]. To describe the materials performance for pyroelectric sensors several figures of merit (FOM) are considered. The FOM commonly used is the voltage FOM given by F V = p c (4) ε 0 ε r where p is the pyroelectric coefficient, c is the volumetric specific heat, ε 0 and ε r are the permittivity of free space and relative permittivity of the material, respectively. The so-called detectivity FOM is the most important FOM for pyroelectric detectors if the noise is dominated by the ac dielectric loss, tan δ, of the pyroelectric element. p F D = c (5) ε 0 ε r tan δ To increase the FOM of a pyroelectric material it is necessary to increase p while decreasing c, ε r and tan δ. Composites of

5 M. Dietze, M. Es-Souni / Sensors and Actuators A 143 (2008) Table 1 Volumetric specific heat, dielectric and pyroelectric properties of the screen-printed composites investigated Screen-print Spin-coat 20 vol.% PZ21 30 vol.% PZ21 40 vol.% PZ21 50 vol.% PZ21 20 vol.% PZ21 (Ref. [5]) PZT thick film (Ref. [21]) c (MJ m 3 K 1 ) ε r tan δ p ( Cm 2 K 1 ) F V (m 2 /C) F D ( 10 6 Pa 1/2 ) The results from a spin-coated thick film of a previous work which has shown the best performance and a PZT thick film from literature are also shown for comparison. ceramics and polymers are possibilities to fulfill these requirements. Table 1 summarizes the measured values of ε r, tan δ and c as well as the calculated FOMs, and compares them to those of a spin-coated composite [5] and a PZT thick film from literature [21]. The measured volumetric specific heat for the screen-printed composites is much lower than those reported for spin-coated samples. This might be explained by lower density of screenprinted structures. Because of the lower measured volumetric specific heat the current and the detectivity FOM for samples with 30 vol.% PZT are higher than those of spin-coated samples, although the spin-coated samples have higher pyroelectric coefficients. Compared to PZT thick film from literature [21] the screen-printed films with 30 vol.% PZT show much better voltage figures of merit and only little lower detectivity FOM, emphasizing their applicability for uncooled pyroelectric sensors Piezoelectric properties While pyroelectricity describes the relation between electrical displacement and temperature changes, piezoelectricity relates mechanical stress X or deformation S to the electrical displacement D and electrical field E. D = d ij X + ε X ij E (7) S = s E ij T + d ije (8) The quantities d ij, the piezoelectric charge coefficient, ε ij the permittivity, and s ij, the stiffness are all tensors. The piezoelectric charge coefficient, which relates the dielectric displacement to stress and the strain to the electric field, is a common parameter to describe the piezoelectric performance of a material. In the present work we consider only d 33,eff, the effective piezoelectric coefficient measured in thickness mode, i.e. perpendicular to the electrode surface. As the polymer matrix and the PZT particles show opposite sign of the piezoelectric coefficient, there should be a composition where the effective piezoelectric coefficient vanishes if both phases are poled in the same direction. Fig. 7 shows the measured modulus of the effective piezoelectric coefficient of the composites. A minimum can be seen at 40 vol.% PZT, similar to the results of Lam et al. [8]. The overall piezoelectric response is quiet small (less than 10 pm/v), suggesting only small sensi- Fig. 7. Modulus of effective charge coefficient, d 33, of the composites as a function of PZT volume content. tivity of the material for pressure changes or vibrations if used as a pyroelectric sensor. 4. Conclusion PZT thick film composites with PZT volume fractions between 20 and 50 vol.% PZT were produced by screen-printing on ITO-coated glass substrates. The thickness of the printed films was controlled by adjusting the viscosity of the paste. SEM images suggested a random distribution of the PZT particles in the printed structures. Samples with high volume content of PZT show some porosity on the boundary between substrate and film. X-ray diffraction indicates successful poling of the polymer matrix, while complete poling of the PZT particles was not achieved. The dielectric properties could be tuned by varying the volume fraction of the PZT particles. A higher dielectric constant of up to 71 and a dielectric loss down to could be obtained at 1 khz for a PZT volume fraction of 50%. The highest pyroelectric coefficients was obtained for screen-printed samples with 30 vol.% PZT. The value of about 80 Cm 2 K 1 is 60% higher than that of the pure polymer, leading to a detectivity figure of merit that is only slightly lower than that of PZT thick films. It should be emphasized that the processing temperature of the composites

6 334 M. Dietze, M. Es-Souni / Sensors and Actuators A 143 (2008) is much lower compared to pure ceramic materials [21,22]. This is a great advantage, because of the compatibility with silicon technology and low melting temperature substrates. The piezoelectric measurements show only a small piezoelectric response of the samples which should make mechanical compensation for piezoelectric noise superfluous. This result, together with the demonstrated possibility of screen-printing, low processing temperatures and corona poling make these composites suitable for large-scale production of low-cost pyroelectric detectors. Acknowledgements The Financial supports of this work by the German Federal Ministry of Education and Research (BMBF), Grant No X 04, and the Regional EU Funds of the Land of Schleswig Holstein are acknowledged. References [1] R.E. Newnham, D.P. Skinner, L.E. Cross, Connectivity and piezoelectric pyroelectric composites, Mater. Res. Bull. 13 (1978) [2] V. Bobnar, B. Vodopivec, Z. Kutnjak, M. Kosec, A. Levstik, B. Hilczer, Dielectric and calorimetric studies of PLZT-P(VDF/TrFE) ceramics copolymer composite, Ferroelectrics 304 (2004) [3] C.K. Wong, F.G. Shin, Electrical conductivity enhanced dielectric and piezoelectric properties of ferroelectric 0 3 composites, J. Appl. Phys. 97 (2005) [4] K.W. Kwok, C.K. Wong, R. Zheng, F.G. Shin, ac poling study of lead zirconate titanate/vinylidene fluorode trifluoroethylene composites, Appl. Phys. A 81 (2005) [5] M. Dietze, J. Krause, C.-H. Solterbeck, M. Es-Souni, Thick film polymer ceramic composites for pyroelectric applications, J. Appl. Phys. 101 (2007) [6] B. Ploss, W.Y. Ng, H.L.W. Chan, B. Ploss, C.L. Choy, Poling study of PZT/P(VDF-TrFE) composites, Compos. Sci. Technol. 61 (2001) [7] K.H. Lam, H.K.W. Chan, H.S. Luo, Q.R. Yin, Z.W. Yin, C.L. Choy, Dielectric properties of 65PMN-35PT/P(VDF-TrFE) 0-3 composites, Micro. Eng. 66 (2003) [8] K.H. Lam, H.L.W. Chan, Piezoelectric and pyroelectric properties of 65PMN-35PT/P(VDF-TrFE) 0 3 composites, Compos. Sci. Technol. 65 (2005) [9] B. Ploss, B. Ploss, F.G. Shin, H.L.W. Chan, C.L. Choy, Pyroelectric or piezoelectric compensated ferroelectric composites, Appl. Phys. Lett. 76 (2000) [10] S.T. Lau, K. Li, H.L.W. Chan, PT/(P(VDF-TrFE) nanocomposites for ultrasonic transducer applications, Ferroelectrics 304 (2004) [11] F. Ménil, H. Debéda, C. Lucat, Screen-printed thick-films: from materials to functional devices, J. Eur. Ceram. Soc. 25 (2005) [12] D. Waller, T. Iqbal, A. Safari, Poling of lead zirconate titanate ceramics and flexible piezoelectric composites by the corona discharge technique, J. Am. Ceram. Soc. 72 (1989) [13] S.N. Fedosov, A.E. Sergeeva, Corona poling of ferroelectric and nonlinear optical polymers, Moldavian J. Phys. Sci. 1 (2002) [14] Y. Kubouchi, Y. Kumetani, T. Yagi, T. Masudda, A. Nakajima, Structure and dielectric properties of vinylidene fluoride copolymers, Pure Appl. Chem. 61 (1989) [15] K.J. Kim, G.B. Kim, Curie transition, ferroelectric crystal structure and ferroelectricity of a VDF/TrFE (75/25) copolymer. 2. The effect of poling on Curie transition and ferroelectric crystal structure, Polymer 38 (1997) [16] T. Yamada, T. Ueda, T. Kitayama, Ferroelectric-to-paraelectric phase transition of vinylidene fluoride-trifluoroethylene copolymer, J. Appl. Phys. 52 (1981) [17] M.H. Lee, A. Halliyal, R.E. Newnham, Poling of coprecipitated lead titanate-epoxy 0 3 piezoelectric composites, J. Am. Ceram. Soc. 72 (1989) [18] D.A.G. Bruggeman, Berechnung verschiedener physikalischer Konstanten von heterogenen Substanzen, Ann. Phys. Lpz. 24 (1935) [19] R. Köhler, N. Neumann, G. Hofmann, Pyroelectric single-element and linear-array sensors based on P(VDF/TrFE) thin films, Sens. Actuator A 45 (1994) [20] A.J. Moulson, H.M. Herbert, Electroceramics, Chapman and Hall, London, 1990, pp [21] M. Es-Souni, M. Kuhnke, A. Piorra, C.-H. Solterbeck, Pyroelectric and piezoelectric properties of thick PZT films produced by a new sol gel route, J. Eur. Ceram. Soc. 25 (2005) [22] K. Yao, X. He, Y. Xu, M. Chen, Screen-printed piezoelectric ceramic thick films with sintering additives introduced through a liquid-phase approach, Sens. Actuator A 118 (2005) Biographies Matthias Dietze graduated in materials science, speciality ceramic materials on the Technical University Freiberg, Germany. He is currently pursuing his PhD in the Institute of Materials and Surface Technology of the University of Applied Science Kiel on functional composite materials. Prof. Es-Souni obtained his PhD (Doctorat es Sciences) in solid state physics, speciality surface sciences from the University of Strasbourg, Institut Le Bel, France, in He is currently the chair of the Institute of Materials and Surface Technology of the University of Applied Science Kiel.