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A nanocomposite approach to tailor electrocaloric effect in ferroelectric polymer
Polymer 54 (2013) 5299e5302
Contents lists available at ScienceDirect
Polymer journal homepage: www.elsevier.com/locate/polymer
Polymer communication
A nanocomposite approach to tailor electrocaloric effect in ferroelectric polymer Xiang-Zhong Chen a, b, Xinyu Li a, Xiao-Shi Qian a, Minren Lin a, Shan Wu a, Qun-Dong Shen b, Q.M. Zhang a, * a
Materials Research Institute, Department of Electrical Engineering, The Pennsylvania State University, University Park, PA 16802, USA Department of Polymer Science & Engineering, Key Laboratory of Mesoscopic Chemistry of MOE, School of Chemistry & Chemical Engineering, Nanjing University, Nanjing 210093, China b
a r t i c l e i n f o
a b s t r a c t
Article history: Received 28 May 2013 Received in revised form 17 July 2013 Accepted 21 July 2013 Available online 31 July 2013
The electrocaloric effect (ECE) is enhanced in ferroelectric relaxor terpolymer poly(vinylidene fluoride etrifluoroethyleneechlorofluoroethylene) (P(VDFeTrFEeCFE))/ZrO2 nanocomposites. It was observed that the interface effects between the polymer matrix and nano-fillers enhance the polarization response and provide additional electrocaloric entropy changes. As a consequence, the nanocomposites exhibit a larger ECE than that of the neat terpolymer, i.e., the adiabatic temperature change of the nanocomposite with 3 volume percent of nano-fillers is 120% of that of the neat terpolymer. The results, for the first time, demonstrate that ECE can be tailored and enhanced through nanocomposite approach in the ferroelectric polymers. Ó 2013 Elsevier Ltd. All rights reserved.
Keywords: Ferroelectric polymer Electrocaloric effect Nanocomposite
1. Introduction The electrocaloric effect (ECE) in ferroelectric materials has attracted a great deal of attention, recently, because the ECE is one of the most promising alternatives, due to its environmentalfriendliness and potentials of achieving high efficiency, to the cooling technology based on the vapor-compression approach [1e7]. Moreover, electrocaloric materials can be integrated into solid-state cooling devices for many applications such as on-chip cooling and temperature regulation for sensors and electronic devices. The electrocaloric effect refers to the entropy and/or temperature changes caused by changes of dipolar ordering state in a dielectric material when an electric field is applied or removed [8e 10]. It is widely acknowledged that ferroelectrics are the most suitable materials for this purpose because large reversible polarization can be induced by external electric fields. Ferroelectric polymers have shown their capability of generating very large ECE [2,11e14]. Composite approach, such as polymer blends and polymereinorganic nanoparticle composites, is one facile method of tuning polymer properties, and has demonstrated great success in
* Corresponding author. Tel.: þ1 814 863 8994. E-mail address: [email protected] (Q.M. Zhang). 0032-3861/$ e see front matter Ó 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.polymer.2013.07.052
enhancing the dielectric behavior of ferroelectric polymers [15e 25]. Especially, for the polymereinorganic nanoparticle composites, the interface effects may introduce additional polarization process that can result in an improved polarization response and elevated polarization level [17e21]. From the LandaueDevonshire (L-D) phenomenological theory, the isothermal entropy change DS from the ECE is directly proportional to the square of the electric displacement DD
DS ¼ 1=2bðDDÞ2
(1)
where b is a coefficient in the LeD theory [3,10]. Therefore, it may be expected that the ECE can also be enhanced through nanocomposite approach. This paper investigates the ECE in nanocomposites comprising surface-modified ZrO2 nanoparticles and a ferroelectric relaxor P(VDFeTrFEeCFE) terpolymer, which show a large ECE near room temperature [12]. Earlier studies revealed that the P(VDFeTrFEe CFE)/ZrO2 nanoparticle composites can exhibit higher induced polarization than the neat terpolymer due to interface effects [18]. Here we show that the interface effects induced polarization increase can be made use of to enhance the ECE too. The results, for the first time, demonstrate that the ECE can be tailored and enhanced through polymereinorganic nanoparticle nanocomposite approach in ferroelectric polymers.
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Fig. 1. An SEM image of P(VDFeTrFEeCFE)/ZrO2 nanocomposites.
2. Experimental P(VDFeTrFEeCFE) 62.5/29/8.5 was synthesized via a suspension polymerization method [26]. The ZrO2 nanoparticles (w25 nm, Alfa Aesar) was modified by 3-Phosphonopropionic acid (PPA, Sigma Aldrich), which improves the dispersion of nanoparticles in organic solvents and polymer matrix (Figs. S1 and S2 in Supplementary Materials) [17,19e21]. The ZrO2 nanoparticle dispersion was mixed with terpolymer solution (DMF as the solvent) in a proper ratio and then poured onto a clean glass slide and dried at 50 C. After the solvent evaporated, the composite film was peeled off and annealed at 105 C for 24 h to improve crystallinity and remove the residual solvent. The distribution of the nanoparticles in the polymer matrix was characterized by a Scanning Electronic Microscopy (SEM) FEI Nano 630. In this study, nanocomposites with up to 3 vol % ZrO2 nanoparticles were studied. As shown in earlier studies, the enhanced polarization in the nanocomposites reached saturation at
3 vol% ZrO2 nanoparticles. The melting and crystallization behaviors of the pure terpolymer and composites were measured by differential scanning calorimeter (DSC) (TA Q100) with a heating and cooling rate of 10 C/min. For dielectric measurements, gold electrodes were sputtered on both surfaces of the polymer films. The dielectric properties as a function of temperature were characterized using a precision LCR meter (HP 4284A) equipped with a temperature chamber (Delta 9023). Polarization-electric field (P-E) loops were measured using a modified Sawyer-Tower circuit at 10 Hz and room temperature. Some details of the ECE measurement setup have been described earlier [11,12]. In the ECE measurement, the heat generated by the ECE sample is compared with the heat generated by a standard reference resistor R, from which DS is determined. When a voltage, V, with a pulse time duration, t, applies to the resistor heater, it would produce a joule heat Qh ¼ (V2/R)t. The heat generated is detected by a heat flux sensor directly attached to the sample surface. Now, if the ECE film under an applied electric field (or removal of an applied field) also generates (or absorbs) the same amount of heat as detected by the same flux sensor, then the heat QECE from the ECE material is equal to Qh. From Qh ¼ QECE ¼ TDS, DS, the isothermal entropy change can be determined. If the ECE material has a heat capacity of cE, a density r, and a volume of U, the adiabatic temperature change can be obtained, DTECE ¼ QECE/(cErU). 3. Results and discussion Presented in Fig. 1 is an SEM image of the nanocomposite with 3 vol% of ZrO2 nanoparticles, which shows that ZrO2 nanoparticles are evenly distributed in the polymer matrix without forming large agglomeration. This is distinctively different from the nanocomposites prepared using un-surface modified ZrO2 nanoparticles. Fig. S2 indicates that ZrO2 nanoparticles without surface fictionalization tend to agglomerate in DMF. Fig. 2(a) and (b) presents the temperature dependence of the weak field dielectric properties of the terpolymer and its nanocomposite. The broad dielectric constant peaks shift progressively
Fig. 2. Temperature dependent dielectric constant and dielectric loss of pure terpolymer (a) and nanocomposite with 2% ZrO2 nanoparticles (b) at different frequencies (from 100 Hz to 1 MHz). (c) Unipolar P-E loops of terpolymer and nanocomposites measured at 10 Hz. (d) The induced polarization of terpolymer and nanocomposites as functions of applied electric field amplitude. The data was extracted from the P-E loops of .(c). The inset shows variation of induced polarization under 150 MV/m vs. ZrO2 content.
X.-Z. Chen et al. / Polymer 54 (2013) 5299e5302
Fig. 3. (a) The isothermal entropy change (DS) and adiabatic temperature change (DT) of terpolymer and nanocomposites as functions of the applied field amplitude at room temperature. (b) The relationship of entropy change (DS) and square of electrical displacement (DD)2. The solid lines are fitted results to determine b and drawn to guide eyes. (c) b as a function of the ZrO2 volume content.
toward higher temperature with frequency, showing typical relaxor characteristics. It is noteworthy that a new dielectric anomaly appears in the dielectric constants of nanocomposite at around 20 C, especially at low frequencies, which is believed to be caused by a low frequency relaxation process due to the heterogeneous nature of the composites. The anomaly was not observed in the terpolymereZrO2 nanocomposites studied earlier in which the nanoparticles were not modified [18], but observed in the other terpolymer nanocomposites with surface-modified nanoparticles [17,19,20]. The surface modification allows polymer to wet surfaces of the nano fillers and improves the interaction between the nanofillers and polymer matrix, which may enhance the interface effects. Fig. 2(c) presents P-E loops of the terpolymer and its nanocomposites under a unipolar electric field of 150 MV/m at 10 Hz. The nanocomposites exhibit higher polarization than that of the terpolymer in the field range measured, similar to the previous
5301
results [18]. The reasons for enhanced polarization behavior may lie in two aspects: first, nanoparticles may enhance the mobility of the polymer chains at the interfaces, which may reduce the energy barriers for the transformation from the nonpolar-molecular conformation to the polar conformation under electric field; second, stabilization of the polar phase in the polymer due to the nanoparticles may also enhance the polarization response because the polar phase has a higher polarization density than the nonpolar phase [18,27]. It should also be noted that the polarization of the nanocomposite at E ¼ 0 after one charge/discharge cycle is the same as terpolymer, indicating that the enhancement of polarization is intrinsic instead of caused by conduction. The nanocomposite with 1 vol% of ZrO2 nanoparticles shows the strongest polarization enhancement and enhancement gradually reaches saturation, as shown in Fig. 2(d). Earlier studies have shown that further increase of nanoparticle content will cause coalescence of interfaces and thus saturation of the interface effects [17,20]. The nanocomposite with 3 vol% nanoparticles exhibits an induced polarization of 6.9 mC/cm2 under 150 MV/m, which is 15% higher than that of neat terpolymer (6.0 mC/cm2). That value is also higher than the results reported previously (w6.4 mC/cm2), which was obtained in the nanocomposites with ZrO2 nanoparticles of no-surface modification [18]. The difference may lie in the fact that the composition of terpolymers from different batches varies. Fig. 3(a) compares the isothermal entropy change (DS) of the terpolymer and nanocomposites as functions of electric field at room temperature. As can be seen, nanocomposites show higher DS than that of the neat terpolymer at all electric fields measured, especially at higher electric field. For example, at 140 MV/m, a DS w46 J kg1 K1, corresponding to a DT w 9.2 C, is induced in the nanocomposite with 3 vol% of nanoparticles, about 20% higher than that of the pure terpolymer (DS w38 J kg1 K1, corresponding to a DT w 7.6 C). Eq. (1) is used to deduce the coefficient b from the experimental data in Figs. 2(d) and 3(a). As shown in Fig. 3(b), the terpolymer and nanocomposites display nearly linear relationships between the entropy change and square of electrical displacement. b obtained for the terpolymer and its nanocomposites with 1 vol%, 2 vol%, and 3 vol% ZrO2, are 3.57 107, 3.44 107, 3.27 107, and 3.37 107 J mK1 C2, respectively (Fig. 3(c)). The slightly decrease of b with the nanoparticle content indicates that the enhanced polarization due to the interface effects may not be as effective in generating ECE compared with that from the neat terpolymer. However, the large enhancement in polarization from the interface
Fig. 4. DSC traces of terpolymer and nanocomposites obtained during the cooling scan.
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effects suppresses the decrease in b and therefore the resulting ECE in the nanocomposites is still significantly enhanced. The superior dielectric and ferroelectric properties of the PVDF based electroactive polymers originate from their crystalline phases. To investigate the influence of the ZrO2 nanoparticles on the crystallization behavior of the polymer matrix, DSC measurements were carried out. Fig. 4 shows the DSC profiles of the polymer and nanocomposites during the cooling scan. The introduction of the nanoparticles into the polymer raised the crystallization temperature, from w99 C for terpolymer to w101 C for the nanocomposite containing 1 vol % ZrO2, indicating the nanoparticles possibly serve as nucleating agent in the crystallization of the polymer matrix. It is noted that the crystallization temperatures of the nanocomposites do not show changes as the volume content of ZrO2 nanoparticles increased further. The heat of melting DH increased from 23.2 J/g for the terpolymer to 24.1 J/g, 24.8 J/g and 24.6 J/g for the composites with 1 vol%, 2 vol% and 3 vol% ZrO2, respectively. The heat of melting, which is directly proportional to the crystallinity, varied slightly when the nanoparticle loading increased beyond 1 vol%, which suggests that the increased crystallinity is not the main reason for the enhanced EC effect observed. For example, assuming the isothermal entropy change DS is solely originated from the crystal phase, the neat terpolymer has a ratio of DS/DH of 1.64. For the composite with 3 vol% ZrO2, the increase in DS from the crystallinity is 40.3 J kg1 K1, which is much smaller than the observed DS ¼ 46 J kg1 K1, suggesting that the interface effects cause additional polarization responses which are the major reason for the enhanced ECE observed. 4. Conclusions In summary, for the first time, tailoring the electrocaloric effect in ferroelectric relaxor P(VDFeTrFEeCFE) terpolymer through inorganic nanoparticleepolymer nanocomposite approach was demonstrated. Compared with the terpolymer, the nanocomposite with 3 vol% of ZrO2 nanoparticles exhibits a DS and DT which are 120% of those of the neat terpolymer. Since the interface effects are sensitive to the nanoparticle interface properties which may be tailored over a broad range, the results here suggest the potential of employing nanocomposite approach to generate significant polarization response and ECE compared with the neat ferroelectric polymers. Acknowledgments This research was supported by Army Research Office under Grant No. W911NF-11-1-0534 (X. Z. Chen, M. Lin, and Q. Zhang) and by the U.S. DoE, Office of Basic Energy Sciences, Division of
Materials Science and Engineering under Award No. DE-FG0207ER46410 (X. Li and X. Qian). X. Z. Chen was also partially supported by Nanjing University, China. Appendix A. Supplementary material Supplementary material associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.polymer. 2013.07.052. References [1] Mischenko AS, Zhang Q, Scott JF, Whatmore RW, Mathur ND. Science 2006;311(5765):1270e1. [2] Neese B, Chu BJ, Lu SG, Wang Y, Furman E, Zhang QM. Science 2008;321(5890):821e3. [3] Lu SG, Zhang QM. Adv Mater 2009;21(19):1983e7. [4] Qian XS, Lu SG, Li XY, Gu HM, Chien LC, Zhang QM. Adv Funct Mater 2013. http://dx.doi.org/10.1002/adfm.201202686. [5] Mischenko AS, Zhang Q, Whatmore RW, Scott JF, Mathur ND. Appl Phys Lett 2006;89(24):242912. [6] Peng B, Fan H, Zhang Q. Adv Funct Mater 2013. http://dx.doi.org/10.1002/ adfm.201202525. [7] Moya X, Stern-Taulats E, Crossley S, González-Alonso D, Kar-Narayan S, Planes A, et al. Adv Mater 2013;25(9):1360e5. [8] Fatuzzo E, Merz WJ. Ferroelectricity. Amsterdam: North-Holland Publishing Company; 1967. [9] Mitsui T, Tatsuzaki I, Nakamura E. An introduction to the physics of ferroelectrics. London: Gordon and Breach; 1976. [10] Lines ME, Glass AM. Principles and applications of ferroelectrics and related materials. Oxford: Clarendon Press; 1977. [11] Lu SG, Rozic B, Zhang QM, Kutnjak Z, Li XY, Furman E, et al. Appl Phys Lett 2010;97(16):162904. [12] Li XY, Qian XS, Lu SG, Cheng JP, Fang Z, Zhang QM. Appl Phys Lett 2011;99(5): 052907. [13] Chen XZ, Qian XS, Li XY, Lu SG, Gu HM, Lin MR, et al. Appl Phys Lett 2012;100(22):222902. [14] Lu SG, Rozic B, Zhang QM, Kutnjak Z, Neese B. Appl Phys Lett 2011;98(12): 122906. [15] Chu BJ, Neese B, Lin MR, Lu SG, Zhang QM. Appl Phys Lett 2008;93(15): 152903. [16] Zhang QM, Li HF, Poh M, Xia F, Cheng ZY, Xu HS, et al. Nature 2002;419(6904): 284e7. [17] Li JJ, Seok SI, Chu BJ, Dogan F, Zhang QM, Wang Q. Adv Mater 2009;21(2):217e21. [18] Chu BJ, Lin MR, Neese B, Zhou X, Chen Q, Zhang QM. Appl Phys Lett 2007;91(12):122909. [19] Li JJ, Claude J, Norena-Franco LE, Il Seok S, Wang Q. Chem Mater 2008;20(20): 6304e6. [20] Li JJ, Khanchaitit P, Han K, Wang Q. Chem Mater 2010;22(18):5350e7. [21] Kim P, Doss NM, Tillotson JP, Hotchkiss PJ, Pan MJ, Marder SR, et al. ACS Nano 2009;3(9):2581e92. [22] Dang ZM, Lin YH, Nan CW. Adv Mater 2003;15(19):1625e9. [23] Arbatti M, Shan XB, Cheng ZY. Adv Mater 2007;19(10):1369e72. [24] He F, Lau S, Chan HL, Fan JT. Adv Mater 2009;21(6):710e5. [25] Yuan JK, Li WL, Yao SH, Lin YQ, Sylvestre A, Bai JB. Appl Phys Lett 2011;98(3): 032901. [26] Bauer F, Fousson E, Zhang QM. IEEE Trans Dielectr Electr Insul 2006;13(5): 1149e54. [27] Chu BJ, Lin MR, Neese B, Zhang QM. J Appl Phys 2009;105(1):014103.
Contents lists available at ScienceDirect
Polymer journal homepage: www.elsevier.com/locate/polymer
Polymer communication
A nanocomposite approach to tailor electrocaloric effect in ferroelectric polymer Xiang-Zhong Chen a, b, Xinyu Li a, Xiao-Shi Qian a, Minren Lin a, Shan Wu a, Qun-Dong Shen b, Q.M. Zhang a, * a
Materials Research Institute, Department of Electrical Engineering, The Pennsylvania State University, University Park, PA 16802, USA Department of Polymer Science & Engineering, Key Laboratory of Mesoscopic Chemistry of MOE, School of Chemistry & Chemical Engineering, Nanjing University, Nanjing 210093, China b
a r t i c l e i n f o
a b s t r a c t
Article history: Received 28 May 2013 Received in revised form 17 July 2013 Accepted 21 July 2013 Available online 31 July 2013
The electrocaloric effect (ECE) is enhanced in ferroelectric relaxor terpolymer poly(vinylidene fluoride etrifluoroethyleneechlorofluoroethylene) (P(VDFeTrFEeCFE))/ZrO2 nanocomposites. It was observed that the interface effects between the polymer matrix and nano-fillers enhance the polarization response and provide additional electrocaloric entropy changes. As a consequence, the nanocomposites exhibit a larger ECE than that of the neat terpolymer, i.e., the adiabatic temperature change of the nanocomposite with 3 volume percent of nano-fillers is 120% of that of the neat terpolymer. The results, for the first time, demonstrate that ECE can be tailored and enhanced through nanocomposite approach in the ferroelectric polymers. Ó 2013 Elsevier Ltd. All rights reserved.
Keywords: Ferroelectric polymer Electrocaloric effect Nanocomposite
1. Introduction The electrocaloric effect (ECE) in ferroelectric materials has attracted a great deal of attention, recently, because the ECE is one of the most promising alternatives, due to its environmentalfriendliness and potentials of achieving high efficiency, to the cooling technology based on the vapor-compression approach [1e7]. Moreover, electrocaloric materials can be integrated into solid-state cooling devices for many applications such as on-chip cooling and temperature regulation for sensors and electronic devices. The electrocaloric effect refers to the entropy and/or temperature changes caused by changes of dipolar ordering state in a dielectric material when an electric field is applied or removed [8e 10]. It is widely acknowledged that ferroelectrics are the most suitable materials for this purpose because large reversible polarization can be induced by external electric fields. Ferroelectric polymers have shown their capability of generating very large ECE [2,11e14]. Composite approach, such as polymer blends and polymereinorganic nanoparticle composites, is one facile method of tuning polymer properties, and has demonstrated great success in
* Corresponding author. Tel.: þ1 814 863 8994. E-mail address: [email protected] (Q.M. Zhang). 0032-3861/$ e see front matter Ó 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.polymer.2013.07.052
enhancing the dielectric behavior of ferroelectric polymers [15e 25]. Especially, for the polymereinorganic nanoparticle composites, the interface effects may introduce additional polarization process that can result in an improved polarization response and elevated polarization level [17e21]. From the LandaueDevonshire (L-D) phenomenological theory, the isothermal entropy change DS from the ECE is directly proportional to the square of the electric displacement DD
DS ¼ 1=2bðDDÞ2
(1)
where b is a coefficient in the LeD theory [3,10]. Therefore, it may be expected that the ECE can also be enhanced through nanocomposite approach. This paper investigates the ECE in nanocomposites comprising surface-modified ZrO2 nanoparticles and a ferroelectric relaxor P(VDFeTrFEeCFE) terpolymer, which show a large ECE near room temperature [12]. Earlier studies revealed that the P(VDFeTrFEe CFE)/ZrO2 nanoparticle composites can exhibit higher induced polarization than the neat terpolymer due to interface effects [18]. Here we show that the interface effects induced polarization increase can be made use of to enhance the ECE too. The results, for the first time, demonstrate that the ECE can be tailored and enhanced through polymereinorganic nanoparticle nanocomposite approach in ferroelectric polymers.
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Fig. 1. An SEM image of P(VDFeTrFEeCFE)/ZrO2 nanocomposites.
2. Experimental P(VDFeTrFEeCFE) 62.5/29/8.5 was synthesized via a suspension polymerization method [26]. The ZrO2 nanoparticles (w25 nm, Alfa Aesar) was modified by 3-Phosphonopropionic acid (PPA, Sigma Aldrich), which improves the dispersion of nanoparticles in organic solvents and polymer matrix (Figs. S1 and S2 in Supplementary Materials) [17,19e21]. The ZrO2 nanoparticle dispersion was mixed with terpolymer solution (DMF as the solvent) in a proper ratio and then poured onto a clean glass slide and dried at 50 C. After the solvent evaporated, the composite film was peeled off and annealed at 105 C for 24 h to improve crystallinity and remove the residual solvent. The distribution of the nanoparticles in the polymer matrix was characterized by a Scanning Electronic Microscopy (SEM) FEI Nano 630. In this study, nanocomposites with up to 3 vol % ZrO2 nanoparticles were studied. As shown in earlier studies, the enhanced polarization in the nanocomposites reached saturation at
3 vol% ZrO2 nanoparticles. The melting and crystallization behaviors of the pure terpolymer and composites were measured by differential scanning calorimeter (DSC) (TA Q100) with a heating and cooling rate of 10 C/min. For dielectric measurements, gold electrodes were sputtered on both surfaces of the polymer films. The dielectric properties as a function of temperature were characterized using a precision LCR meter (HP 4284A) equipped with a temperature chamber (Delta 9023). Polarization-electric field (P-E) loops were measured using a modified Sawyer-Tower circuit at 10 Hz and room temperature. Some details of the ECE measurement setup have been described earlier [11,12]. In the ECE measurement, the heat generated by the ECE sample is compared with the heat generated by a standard reference resistor R, from which DS is determined. When a voltage, V, with a pulse time duration, t, applies to the resistor heater, it would produce a joule heat Qh ¼ (V2/R)t. The heat generated is detected by a heat flux sensor directly attached to the sample surface. Now, if the ECE film under an applied electric field (or removal of an applied field) also generates (or absorbs) the same amount of heat as detected by the same flux sensor, then the heat QECE from the ECE material is equal to Qh. From Qh ¼ QECE ¼ TDS, DS, the isothermal entropy change can be determined. If the ECE material has a heat capacity of cE, a density r, and a volume of U, the adiabatic temperature change can be obtained, DTECE ¼ QECE/(cErU). 3. Results and discussion Presented in Fig. 1 is an SEM image of the nanocomposite with 3 vol% of ZrO2 nanoparticles, which shows that ZrO2 nanoparticles are evenly distributed in the polymer matrix without forming large agglomeration. This is distinctively different from the nanocomposites prepared using un-surface modified ZrO2 nanoparticles. Fig. S2 indicates that ZrO2 nanoparticles without surface fictionalization tend to agglomerate in DMF. Fig. 2(a) and (b) presents the temperature dependence of the weak field dielectric properties of the terpolymer and its nanocomposite. The broad dielectric constant peaks shift progressively
Fig. 2. Temperature dependent dielectric constant and dielectric loss of pure terpolymer (a) and nanocomposite with 2% ZrO2 nanoparticles (b) at different frequencies (from 100 Hz to 1 MHz). (c) Unipolar P-E loops of terpolymer and nanocomposites measured at 10 Hz. (d) The induced polarization of terpolymer and nanocomposites as functions of applied electric field amplitude. The data was extracted from the P-E loops of .(c). The inset shows variation of induced polarization under 150 MV/m vs. ZrO2 content.
X.-Z. Chen et al. / Polymer 54 (2013) 5299e5302
Fig. 3. (a) The isothermal entropy change (DS) and adiabatic temperature change (DT) of terpolymer and nanocomposites as functions of the applied field amplitude at room temperature. (b) The relationship of entropy change (DS) and square of electrical displacement (DD)2. The solid lines are fitted results to determine b and drawn to guide eyes. (c) b as a function of the ZrO2 volume content.
toward higher temperature with frequency, showing typical relaxor characteristics. It is noteworthy that a new dielectric anomaly appears in the dielectric constants of nanocomposite at around 20 C, especially at low frequencies, which is believed to be caused by a low frequency relaxation process due to the heterogeneous nature of the composites. The anomaly was not observed in the terpolymereZrO2 nanocomposites studied earlier in which the nanoparticles were not modified [18], but observed in the other terpolymer nanocomposites with surface-modified nanoparticles [17,19,20]. The surface modification allows polymer to wet surfaces of the nano fillers and improves the interaction between the nanofillers and polymer matrix, which may enhance the interface effects. Fig. 2(c) presents P-E loops of the terpolymer and its nanocomposites under a unipolar electric field of 150 MV/m at 10 Hz. The nanocomposites exhibit higher polarization than that of the terpolymer in the field range measured, similar to the previous
5301
results [18]. The reasons for enhanced polarization behavior may lie in two aspects: first, nanoparticles may enhance the mobility of the polymer chains at the interfaces, which may reduce the energy barriers for the transformation from the nonpolar-molecular conformation to the polar conformation under electric field; second, stabilization of the polar phase in the polymer due to the nanoparticles may also enhance the polarization response because the polar phase has a higher polarization density than the nonpolar phase [18,27]. It should also be noted that the polarization of the nanocomposite at E ¼ 0 after one charge/discharge cycle is the same as terpolymer, indicating that the enhancement of polarization is intrinsic instead of caused by conduction. The nanocomposite with 1 vol% of ZrO2 nanoparticles shows the strongest polarization enhancement and enhancement gradually reaches saturation, as shown in Fig. 2(d). Earlier studies have shown that further increase of nanoparticle content will cause coalescence of interfaces and thus saturation of the interface effects [17,20]. The nanocomposite with 3 vol% nanoparticles exhibits an induced polarization of 6.9 mC/cm2 under 150 MV/m, which is 15% higher than that of neat terpolymer (6.0 mC/cm2). That value is also higher than the results reported previously (w6.4 mC/cm2), which was obtained in the nanocomposites with ZrO2 nanoparticles of no-surface modification [18]. The difference may lie in the fact that the composition of terpolymers from different batches varies. Fig. 3(a) compares the isothermal entropy change (DS) of the terpolymer and nanocomposites as functions of electric field at room temperature. As can be seen, nanocomposites show higher DS than that of the neat terpolymer at all electric fields measured, especially at higher electric field. For example, at 140 MV/m, a DS w46 J kg1 K1, corresponding to a DT w 9.2 C, is induced in the nanocomposite with 3 vol% of nanoparticles, about 20% higher than that of the pure terpolymer (DS w38 J kg1 K1, corresponding to a DT w 7.6 C). Eq. (1) is used to deduce the coefficient b from the experimental data in Figs. 2(d) and 3(a). As shown in Fig. 3(b), the terpolymer and nanocomposites display nearly linear relationships between the entropy change and square of electrical displacement. b obtained for the terpolymer and its nanocomposites with 1 vol%, 2 vol%, and 3 vol% ZrO2, are 3.57 107, 3.44 107, 3.27 107, and 3.37 107 J mK1 C2, respectively (Fig. 3(c)). The slightly decrease of b with the nanoparticle content indicates that the enhanced polarization due to the interface effects may not be as effective in generating ECE compared with that from the neat terpolymer. However, the large enhancement in polarization from the interface
Fig. 4. DSC traces of terpolymer and nanocomposites obtained during the cooling scan.
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X.-Z. Chen et al. / Polymer 54 (2013) 5299e5302
effects suppresses the decrease in b and therefore the resulting ECE in the nanocomposites is still significantly enhanced. The superior dielectric and ferroelectric properties of the PVDF based electroactive polymers originate from their crystalline phases. To investigate the influence of the ZrO2 nanoparticles on the crystallization behavior of the polymer matrix, DSC measurements were carried out. Fig. 4 shows the DSC profiles of the polymer and nanocomposites during the cooling scan. The introduction of the nanoparticles into the polymer raised the crystallization temperature, from w99 C for terpolymer to w101 C for the nanocomposite containing 1 vol % ZrO2, indicating the nanoparticles possibly serve as nucleating agent in the crystallization of the polymer matrix. It is noted that the crystallization temperatures of the nanocomposites do not show changes as the volume content of ZrO2 nanoparticles increased further. The heat of melting DH increased from 23.2 J/g for the terpolymer to 24.1 J/g, 24.8 J/g and 24.6 J/g for the composites with 1 vol%, 2 vol% and 3 vol% ZrO2, respectively. The heat of melting, which is directly proportional to the crystallinity, varied slightly when the nanoparticle loading increased beyond 1 vol%, which suggests that the increased crystallinity is not the main reason for the enhanced EC effect observed. For example, assuming the isothermal entropy change DS is solely originated from the crystal phase, the neat terpolymer has a ratio of DS/DH of 1.64. For the composite with 3 vol% ZrO2, the increase in DS from the crystallinity is 40.3 J kg1 K1, which is much smaller than the observed DS ¼ 46 J kg1 K1, suggesting that the interface effects cause additional polarization responses which are the major reason for the enhanced ECE observed. 4. Conclusions In summary, for the first time, tailoring the electrocaloric effect in ferroelectric relaxor P(VDFeTrFEeCFE) terpolymer through inorganic nanoparticleepolymer nanocomposite approach was demonstrated. Compared with the terpolymer, the nanocomposite with 3 vol% of ZrO2 nanoparticles exhibits a DS and DT which are 120% of those of the neat terpolymer. Since the interface effects are sensitive to the nanoparticle interface properties which may be tailored over a broad range, the results here suggest the potential of employing nanocomposite approach to generate significant polarization response and ECE compared with the neat ferroelectric polymers. Acknowledgments This research was supported by Army Research Office under Grant No. W911NF-11-1-0534 (X. Z. Chen, M. Lin, and Q. Zhang) and by the U.S. DoE, Office of Basic Energy Sciences, Division of
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