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The effects of anti-ferroelectric nanofillers on the negative electrocaloric effects in Poly(vinylidene fluoride-trifluoroethylene) matrix composites
Solid State Sciences 90 (2019) 9–13
Contents lists available at ScienceDirect
Solid State Sciences journal homepage: www.elsevier.com/locate/ssscie
The effects of anti-ferroelectric nanofillers on the negative electrocaloric effects in Poly(vinylidene fluoride-trifluoroethylene) matrix composites
T
Sarir Uddina,∗, Guang-Ping Zhenga, Zhiyuan Jiangb,∗∗ a b
Department of Mechanical Engineering, The Hong Kong Polytechnic University, Hung Hom, Kowloon, Hong Kong National Institute of Metrology, Beijing, 100029, China
A R T I C LE I N FO
A B S T R A C T
Keywords: Negative electrocaloric effect Ferroelectric copolymers Anti-ferroelectric Bismuth sodium titanate
The electrocaloric effects (ECE) in the composites consisting of ferroelectric (FE) copolymer poly(vinylidene fluoride-co-trifluoroethylene) (P(VDF-TrFE)) and (Bi0.5Na0.5)TiO3-xBaTiO3 nanoparticles (BNT-xBT, x = 0, 0.06, 1.0) are investigated. The anti-ferroelectric (AFE) nanofillers are found to affect more significantly than the FE nanofillers on the order-to-disorder phase transition near 60 °C in the copolymer matrix, as revealed by differential scanning calorimetry. Nevertheless, the influences of AFE nanofillers on the negative ECE are much weaker than that of FE nanofillers, suggesting that the negative ECE observed might not be associated with the AFE states in the composites. Instead, based on the dynamic mechanical analyses on the effects of nanofillers on the order-to-disorder phase transition in the copolymer matrix, the structural glassy states induced by the interaction between the nanofillers and the copolymer matrix are proposed to account for the negative ECE in the composite. This work provides an effective approach in enhancing the negative ECE, which is of technological importance to the implementation of electrocaloric refrigeration.
1. Introduction Electrocaloric effect (ECE) is the reversible change in temperature (ΔT) of a polarizable dielectric material due to a changing applied electric field through that material under adiabatic conditions [1–3]. Small-scale and environmentally friendly cooling systems for microelectronic and digital systems can be manufactured based on those solid-state materials. Electrocaloric effect has been investigated in different ceramics and polymers since its discovery in 1930 by Kobeko and Kurchatov in order to find out an alternative to the conventional cooling technologies due to environmental concerns [4–7]. The ECE entropy change (ΔS) which is the dipolar disorder produced due to the ionic disorder inside the material by the applied electric field (ΔE) is directly proportional to the pyroelectric coefficient (ΔP/ΔT). Generally, the entropy of a dielectric material increases with the removal of an applied electric field, resulting in an effect of cooling defined by a positive ΔT. Large ECE in a material requires large entropy change of the material which may occur near structural or ferroelectric-to-paraelectric (FE-to-PE) phase transition temperature (Tc) [8]. For a workable EC cooling device the ECE should be maximum near room temperature. Ferroelectric copolymer P(VDF-TrFE) has a phase transition temperature near room temperature [9], above which an ECE of
∗
ΔT = 12 °C has been observed [8,9]. More importantly, those ECE behaviors were found to be significantly enhanced by the additions of ferroelectric ceramic nanoparticles. For examples, the ECE was found to be larger than 20 °C in the P(VDF-TrFE) matrix composites containing barium strontium titanate [10]. Nevertheless, there is very little research work on EC properties in the copolymer matrix composites containing anti-ferroelectric nanofillers [7]. Recently, a remarkable phenomenon in ECE, i.e., negative ECE, is of great interest. In some AFE materials such as (Bi0.5Na0.5)TiO3-xBaTiO3 the ECE has been found to be in contrast to that (conventional ECE) mentioned above, which exhibited a cooling effect in the materials under the application of an electric field and a heating effect with the removal of the electric field [11]. In other words, ΔT for those AFE materials is negative, which is opposite to that of the conventional ECE. In (Pb0.97La0.02)(Zr0.95Ti0.05)O3 thin films, the negative ECE was determined to be as large as −6 °C [12]. In the present study (1-x)(Bi0.5Na0.5)TiO3-xBaTiO3 (x = 0, 0.06, 1.0) ceramic powders were incorporated in co-polymer P(VDF-TrFE) to obtain ceramic-polymer composite thick films to enhance its dielectric and EC properties. In particular, the effects of AFE BNT-xBT nanofillers on negative ECEs are investigated. The mechanisms of negative ECE are thus explored by tuning the structural and polar states of P(VDF-TrFE)
Corresponding author. Corresponding author. E-mail addresses: [email protected] (S. Uddin), [email protected] (Z. Jiang).
∗∗
https://doi.org/10.1016/j.solidstatesciences.2019.01.008 Received 22 December 2018; Received in revised form 23 January 2019; Accepted 23 January 2019 Available online 23 January 2019 1293-2558/ © 2019 Elsevier Masson SAS. All rights reserved.
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with the additions of BNT-xBT. 2. Methods and materials Ceramic powders (1-x)Bi1/2Na1/2TiO3-xBaTiO3 (BNT-xBT) were synthesized by a chemical, sol-gel process. Solutions of Bi(CH3COO)3 (> 99.99% Aldrich), Na(CH3COO) (> 99% reagent grade SigmaAldrich), Ba(CH3COO)2 (> 99% reagent ACS Sigma-Aldrich) and strontium acetate (Sr(CH3COO)2) (> 99% reagent ACS Sigma-Aldrich) in stoichiometric ratios were separately prepared in acetic acid on hot plate stirred with a magnetic stirrer. The solution of titanium (IV) butoxide (C16H36O4Ti) (99% reagent grade Aldrich) was prepared in ethanol to which acetylacetone was added drop-wise in order to stabilize the solution. A yellowish colored stable solution was obtained by mixing the above-mentioned four solutions in a flask on a hot plate. A yellowish colored gel was obtained by keeping the stock solution on a hot plate and stirred at 100 °C for 4 h. The gel was dried on a hot plate at 100 °C and calcined at 800 °C for 2 h to obtain fine BNT-xBT powders. The BNT-xBT powders (10% in volume ratio to polymer) were uniformly dispersed in dimethylformamide (DMF) while keeping in ultrasonic bath for 30 min in order to prevent agglomeration. The copolymer P(VDF-TrFE)-52/48 mol% (PiezoTech, France) powders were added to the solution slowly under magnetic stirring on hot plate at 75 °C for 12 h in order to dissolve the co-polymer powders in the solution. A thick film of the composite material was obtained on a 0.1 mm thick copper foil by a casting technique. The film was dried in oven at 70 °C by evaporating the DMF and further heat-treated at 110 °C in a vacuum oven for 12 h. Finally the P(VDF-TrFE)-(1-x)BNT-xBT composite film with a thickness of ∼0.03 mm was obtained on the copper substrate. The volume fraction of BNT-xBT nanoparticles in the composite films is kept as 10% independent of the content x of BNT-xBT. The phase and microstructure analyses of the samples were carried out by using X-Ray diffractometer (XRD) (Rigaku Smartlab) with the Cu Ka (λ = 0.154 nm) radiation and a JSM-6490 (JEOL) field emission scanning electron microscope (SEM), respectively. The phase transitions were investigated by a dynamic mechanical analyzer (DMA), (Q800 TA-Instrument) and differential scanning calorimeter (DSC, TA Instruments Q200). The densities of the fabricated samples were determined by Archimedes principle using an electronic densitometer (MD 300s). Specific heat capacities of samples were measured using differential scanning calorimeter (DSC, TA instrument Q200). In order to measure ferroelectric properties silver paste was painted on to the top faces of samples. The temperature dependent polarization data were obtained by ferroelectric test system (TF Analyzer 2000E, aixACCT) at a frequency of 10 Hz after every 5 °C temperature interval in the range of (30–90 °C).
Fig. 1. (a–c) XRD patterns of P(VDF-TrFE)-(1-x)BNT-xBT thick films (x = 0, 0.06 and 1.0), showing a two-phases material. (b) The effect of BNT and BNT0.06BT nanoparticles on the (020) peaks for the β phase of P(VDF-TrFE) matrix.
3. Results and discussion 3.1. Phase and microstructure analyses The XRD patterns of the P(VDF-TrFE)-(1-x)BNT-xBT are shown in Fig. 1. The single-phase crystalline powders of (1-x)BNT-xBT are found to embed in the P(VDF-TrFE) copolymer matrix (Fig. 2(a)-(c)), resulting in two-phase composite materials. The relatively broader β(020) peak at 18.8° (Fig. 1(b)) and an obvious shoulder peak at 19.2° indicated the coexistence of non-polar α and polar β phases, respectively [13,14]. All the remaining peaks are corresponded to the (1-x)BNT-xBT nanoparticles [14,15]. The XRD patterns suggest that the crystal structures of P(VDF-TrFE) and nanoparticles are not significantly affected with each other and the nanoparticles are dispersed in the copolymer matrix as revealed by the scanning electron microscopic (SEM) images (Fig. 2(a)-(c)). The addition of BNT-xBT polar nanoparticles alternates the anti-ferroelectric or ferroelectric properties of P(VDF-TrFE) copolymer via no chemical reaction among BNT-xBT, VDF or TrEF but the BNT-xBT nanofillers may create a local electric field due to their
Fig. 2. SEM images of the cross-sections of P(VDF-TrFE)-(1-x)BNT-xBT nanocomposites with x = 1.0 (a), x = 0.06 (b), and x = 0 (c). The scale bars are 100 nm. (d) Heat-flow curves of P(VDF-TrFE)-(1-x)BNT-xBT nanocomposites.
spontaneous polarizations. The induced electric fields are believed to affect the conformation of polymer chains of P(VDF-TrFE) such as the trans-gauche-trans-gauche (TGTG) non-polar α phase and the all-trans (TTTT) polar β phases [16]. 3.2. Analyses on the phase transitions The phase transition characteristics of P(VDF-TrFE)-(1-x)BNT-xBT composite thick films are investigated via DSC and DMA. As shown in 10
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the exothermal peak in the heat-flow curve at about 52 °C just below the onset transition temperature of the phase transition. Such increase in the phase transition temperature of P(VDF-TrFE)-BNT is similar with that reported recently in the P(VDF-TrFE)-(Ba1-xSrx)TiO3 nano-composites [11], where the (Ba1-xSrx)TiO3 nanoparticles have the characteristics of diffusive polar phases. The effect of (1-x)BNT-xBT ceramic nanoparticles on the order-todisorder phase transition in the P(VDF-TrFE)-(1-x)BNT-xBT composites is further confirmed by a more sensitive DMA analysis (Fig. 3) at frequencies ranging from 0.1 to 4 Hz with a heating rate of 0.3 °C/min. The plots of storage modulus and internal friction (Q−1) versus temperature show anomalies at around 55 °C, indicating the phase transition in P(VDF-TrFE) which could result from the instabilities of polar domain walls under the applied mechanical stresses [17–19]. The internal friction versus temperature plots reveal obvious dispersion at the peak temperature T1, indicating the non-Arrhenius relaxation or relaxor behavior of the fabricated composite materials [18,19]. The non-Arrhenius relaxation phenomenon near the phase transition can be characterized by the Vogel–Fulcher–Tammann (VFT) relation as follows [20]:
Ea ⎤, f = fo exp ⎡− ⎢ ⎣ kB (T − T0 ) ⎥ ⎦
(1)
where fo is the attempted frequency, T is the absolute temperature of the Q−1 peak and kB is Boltzmann constant. T0 represents the Kauzmann temperature or static freezing temperature at which the entropy becomes zero, suggesting that the polar domains are in a glassy state between T0 and the phase transition temperature. The activation energies (Ea) can be determined from Eq. (1) as shown in Fig. 4, which are 0.04 eV, 0.06 eV and 0.13 for x = 0, 0.06 and 1.0, respectively, in comparison with that (Ea = 0.11 eV) for pristine P(VDF-TrFE). The activation energy of nanocomposite containing 10% volume fraction of BT nanoparticles is larger than that of nanocomposite containing the same volume fraction of BNT and BNT-0.06BT, in consistent with the DSC analyses that the BT nanoparticles could stabilize the polar ordered state of copolymer matrix and thus the phase transition temperature of P(VDF-TrFE)-BT is similar with that of pristine P(VDF-TrFE). The relatively lower values of Ea for P(VDF-TrFE)-BNT and P(VDF-TrFE)-BNT0.06BT indicate that the presence of AFE nanofiller significantly alters the polar domains in the copolymer matrix. As shown in the inset of Fig. 4(b), the Kauzmann temperatures for P (VDF-TrFE) and P(VDF-TrFE)-(1-x)BNT-xBT with x = 0, 0.06 and 1.0 are 25, 36, 35 and 20 °C, respectively. The significantly higher Kauzmann temperatures for P(VDF-TrFE)-BNT and P(VDF-TrFE)-BNT0.06BT as compared with those for P(VDF-TrFE) and P(VDF-TrFE)-BT suggest that the FE nanofillers could induce the glassy features of the polar domains in P(VDF-TrFE) copolymer, while they could stabilize the FE ordered phases as revealed by the above-mentioned DSC analyses. The effects of (1-x)BNT-xBT nanoparticles on the ferroelectric properties of P(VDF-TrFE)-(1-x)BNT-xBT nanocomposites could be closely related with the polar states of the nanoparticles. The localized electric fields near the ferroelectric BT nanoparticles might not be strong enough to fully alternate a P(VDF-TrFE) molecule chain from a non-polar conformation to the all-trans state. As a consequence, the ferroelectric properties of P(VDF-TrFE)-BT such as the remnant polarization and coercive field could be compatible with those of P(VDFTrFE). On the contrary, the AFE domains in anti-ferroelectric BNT or BNT-0.06BT nanoparticles, which consist of atomistic twinning structures, could be effective to modulate the dipolar moments or the orientation of -CF2 or -CH2 groups in the molecule chains, resulting in the enhanced AFE state of P(VDF-TrFE). Thereby the remnant polarization of P(VDF-TrFE)-BNT could be smaller than that of P(VDF-TrFE).
Fig. 3. (a–c) Frequency and temperature dependence of storage modulus (E) and mechanical loss (Q−1) of P(VDF-TrFE)-(1-x)BNT-xBT composites for x = 0, 0.06 and 1.0, respectively.
Fig. 3(b), the heat flows versus temperature plots show an obvious endothermal peak at about 60.2 °C indicating a phase transition from the polar ordered phase to paraelectric non-polar phase in the P(VDFTrFE) copolymers [7,9], which is suggested to be diffusive in nature. The phase transition temperature TP of P(VDF-TrFE)-BNT composite is observed to increase to 61.2 °C with the addition of AFE BNT ceramic powders to the P(VDF-TrFE) copolymer. As compared with that of P (VDF-TrFE)-BNT, the phase transition of P(VDF-TrFE)-BT composite is less affected by the addition of FE BT (x = 1.0) nanoparticles to the P (VDF-TrFE) copolymer. When the composition of (1-x)BNT-xBT ceramic nanoparticles is at the morphotropic phase boundary (MPB), i.e., x = 0.06, the phase transition temperature of P(VDF-TrFE)-(1-x) BNT-xBT can be found to decrease to 59.2 °C. Such decrease may be attributed to the inhomogeneity produced at the nano-scaled level in the morphologies of the BNT-0.06BT nanoparticles [17], where the AFE and FE crystal structures of the nanoparticles are not distinguishable, thereby reduce the ordering of the polar phase of P(VDF-TrFE). On the contrary, the increase in the phase transition temperature of P(VDFTrFE)-BNT could be attributed to the enhanced ordering state in P(VDFTrFE) because of the addition of AFE-phase BNT, manifesting itself by 11
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Fig. 5. (a) Plots of polarization versus temperature at different electric fields for P(VDF-TrFE)-BNT-0.06BT nanocomposites. (b–d) Temperature and electric field dependences of the ECE (ΔT) for the P(VDF-TrFE)-(1-x)BNT-xBT nanocomposites with x = 0, 0.06 and 1.0 respectively. T0 indicates the Kauzmann temperature.
transition temperature TP. The observed negative ECE below phase transition temperature TP is a consequence of the increase in polarization with temperature where (∂P/∂T)E has positive values and negative ΔT values can be obtained from the macroscopic thermodynamics relations (Eq. (2)). Negative ECE is analogous to the negative magnetocaloric effect reported in Ni-Mn-Sn alloys which cool down when entering a magnetic field region and warm up on leaving the external applied magnetic field [22]. With the same applied filed ΔE, the negative ECE (the maximum |ΔTmax| = 7.05 at ΔE = 1.0 MV/cm) in P(VDF-TrFE)-BT is much significant as compared with those in P(VDFTrFE)-BNT and P(VDF-TrFE)-BNT-0.06BT. It has been argued that the negative ECE could be resulted from the multi-phase transitions in the ferroelectric materials, especially in those with metastable AFE phases. However, our observation of the reduced negative ECE in P(VDF-TrFE)-BNT and P(VDF-TrFE)-BNT-0.06BT suggests that the AFE phases might not play a dominate role in the negative ECE in the P(VDF-TrFE)-(1-x)BNT-xBT nanocomposites, since the above-mentioned DSC and DMA analyses show that the AFE phase of P (VDF-TrFE) could be enhanced by the AFE nanofillers in the composites with x = 0 and 0.06. Instead, the well consistency between the critical temperature where the ECE changes from positive values to negative values and the Kauzmann temperature T0 suggests that the negative ECE in the nanocomposites might be related with their structural glassy states. In general, the P(VDF-TrFE) matrix contains the polar β phase, non-polar α phase and other non-polar phases such as the AFE phases. In P(VDF-TrFE)-BT, the FE BT nanofillers produce local electric fields at the nanoparticle-copolymer interfaces, which may contribute in the alignment of dipoles in (-CF2-CH2) monomer units [15]. However the induced polar states in the non-polar phases could be different with the intrinsic polar β phase. As a consequence, the structure of polar domains could become more disordered as compared with those in the pristine P(VDF-TrFE). In contrast, the conformation of non-polar AFE phase in the BNT nanofillers is compatible with those in the P(VDFTrFE) matrix, without much affect the structure of polar domains of β phase. Therefore, the structural glassy states induced by the interaction between the nanofillers and the copolymer matrix could be much significant in P(VDF-TrFE)-BT as compared with that in P(VDF-TrFE)-BNT and P(VDF-TrFE)-BNT-0.06BT, demonstrating by the low Kauzmann temperature T0 for P(VDF-TrFE)-BT. As shown in Fig. 6, the maximum temperature change of negative
Fig. 4. (a–b) The VFT plots of mechanical relaxation near the order-to-disorder phase transitions in P(VDF-TrFE)-(1-x)BNT-xBT nanocomposites. The inset in (b) show the Kauzmann temperatures T0 for P(VDF-TrFE)-(1-x)BNT-xBT and P (VDF-TrFE).
3.3. Electrocaloric properties The electrocaloric properties of the fabricated composite samples were measured via indirect method using Maxwell's thermodynamic relations (Eq. 2) [21].
Δ T= −
1 ρ
E2
∫ TC ( ∂∂TP )E dE , E1
(2)
where ρ is the density of composites, E1 and E2 are the initial and final applied electric fields, T is the ambient temperature and c is the specific heat capacity assumed to be remain constant in the given temperature range [9]. The values of (∂P/∂T)E were calculated from the forth-order polynomial fit of the Pmax versus T plots, where Pmax is the saturation polarization. The plots of Pmax versus T are shown in Fig. 5 (a), indicating a maximum change around 60 °C. The plots of temperature and electricfield dependent electrocaloric temperature changes (ΔT) for the P(VDFTrFE)-(1-x)BNT-xBT nanocomposites are shown in Fig. 5(b–d). In order to compare ΔT among the nanocomposites, we fix E1 = 100 kV/cm while E2 is changed from 200 kV/cm to as high as 1500 kV/cm, resulting in different ΔE = E2-E1 as indicated in Fig. 5(b–d). For the P (VDF-TrFE)-(1-x)BNT-xBT nanocomposites, negative ECE can be observed to occur between a temperature close to T0 and the phase12
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The structural glassy states induced by the interaction between the nanofillers and the copolymer matrix are proposed to account for the negative ECE in the composite thick films. The results demonstrate that the negative ECE can be enhanced by inducing the disordered structures in the polar domains of ferroelectrics. Acknowledgment This work was supported by the National Natural Science Foundation of China (No. 61805226). References [1] P.D. Thacher, J. Appl. Phys. 39 (1968) 1996. [2] M.D. Li, X.G. Tang, S.M. Zeng, Q.X. Liu, Y.P. Jiang, W.H. Li, J. Alloy. Comp. 747 (2018) 1053. [3] M.D. Li, X.G. Tang, S.M. Zeng, Q.X. Liu, Y.P. Jiang, T.F. Zhang, W.H. Li, ACS Sustain. Chem. Eng. 6 (2018) 8920. [4] P. Kobeko, I. Kurchatov, Physik 66 (1930) 192. [5] J.F. Scott, Annu. Rev. Mater. Res. 41 (2011) 229. [6] M. Valant, Prog. Mater. Sci. 57 (2012) 980. [7] X. Li, S.G. Lu, X.Z. Chen, H. Gu, X.S. Qian, Q.M. Zhang, J. Mater. Chem. C. 1 (2013) 23. [8] A.S. Mischenko, Q. Zhang, J.F. Scott, W.R. Whatmore, N.D. Mathur, Science 311 (2006) 1270. [9] B. Neese, B. Chu, S.G. Lu, Y. Wang, E. Furman, Q.M. Zhang, Science 321 (2008) 821. [10] G. Zhang, Q. Li, H. Gu, S. Jiang, K. Han, M.R. Gadinski, M.A. Haque, Q. Zhang, Q. Wang, Adv. Mater. 27 (2015) 1450. [11] X.C. Zheng, G.P. Zheng, Z. Lin, Z.Y. Jiang, J. Electroceram. 28 (2012) 20. [12] W. Geng, Y. Liu, X. Meng, L. Bellaiche, J.F. Scott, B. Dkhil, A. Jiang, Adv. Mater. 27 (2015) 3165. [13] X. Guana, Y. Zhangb, H. Li, J. Ou, Sensor. Actuator. 194 (2013) 228. [14] Z.Y. Jiang, X.C. Zheng, G.P. Zheng, RSC Adv. 5 (2015) 61946. [15] G.P. Zheng, S. Uddin, X. Zheng, J. Yang, J. Alloy. Comp. 663 (2016) 249. [16] V. Mittal, Synthesis Techniques for Polymer Nanocomposites, Wiley, 2014, pp. 132–134. [17] S. Uddin, G.P. Zheng, Y. Iqbal, R. Ubic, N.Y. Chan, H.L.W. Chan, Mater. Res. Express 1 (2014) 046102. [18] W. Jo, S. Schaab, E. Sapper, L.A. Schmitt, H.J. Kleebe, A.J. Bell, J. Appl. Phys. 110 (2011) 074106. [19] D. Viehland, S.J. Jang, L.E. Cross, M. Wuttig, J. Appl. Phys. 68 (1990) 2916. [20] K.J. Laidler, Chemical Kinetics, third ed., Harper & Row, 1987, pp. 42–44. [21] M.E. Lines, A.M. Glass, Principles and Applications of Ferroelectrics and Related Materials, Clarendon Press, Oxford, 1977, pp. 70–90. [22] B. Yu, M. Liu, P.W. Egolf, A. Kitanovski, Int. J. Refrig. 33 (2010) 1029.
Fig. 6. The log-log plots of the absolute values of the maximum ECE (|ΔTmax|) as a function of applied electric field (ΔE=E2-E1) with E1 = 100 kV/cm for the P (VDF-TrFE)-(1-x)BNT-xBT nanocomposites.
ECE versus ΔE plots are fitted as |ΔTmax|∼ Eb, where b is an exponent which characterizes the increase of electrocaloric strength with increasing applied field. Especially, P(VDF-TrFE)-BT exhibits a maximum negative ECE of more than 7 °C. The enhanced negative ECE in P(VDFTrFE)-BT as compared with those in P(VDF-TrFE)-BNT and P(VDFTrFE)-BNT-0.06BT is evident under a relatively high applied electric field (E2 > 700 kV/cm). 4. Conclusions Composite thick-film samples of P(VDF-TrFE)-(1-x)BNT-xBT (x = 0, 0.06, 1.0) on copper substrate were fabricated. DSC and DMA analyses were used to investigate the phase transition behavior of the nanocomposites. Under the same applied fields, the negative ECE in P(VDFTrFE)-BT is much significant in the nanocomposites with ferroelectric nanofillers as compared with those with anti-ferroelectric nanofillers.
13
Contents lists available at ScienceDirect
Solid State Sciences journal homepage: www.elsevier.com/locate/ssscie
The effects of anti-ferroelectric nanofillers on the negative electrocaloric effects in Poly(vinylidene fluoride-trifluoroethylene) matrix composites
T
Sarir Uddina,∗, Guang-Ping Zhenga, Zhiyuan Jiangb,∗∗ a b
Department of Mechanical Engineering, The Hong Kong Polytechnic University, Hung Hom, Kowloon, Hong Kong National Institute of Metrology, Beijing, 100029, China
A R T I C LE I N FO
A B S T R A C T
Keywords: Negative electrocaloric effect Ferroelectric copolymers Anti-ferroelectric Bismuth sodium titanate
The electrocaloric effects (ECE) in the composites consisting of ferroelectric (FE) copolymer poly(vinylidene fluoride-co-trifluoroethylene) (P(VDF-TrFE)) and (Bi0.5Na0.5)TiO3-xBaTiO3 nanoparticles (BNT-xBT, x = 0, 0.06, 1.0) are investigated. The anti-ferroelectric (AFE) nanofillers are found to affect more significantly than the FE nanofillers on the order-to-disorder phase transition near 60 °C in the copolymer matrix, as revealed by differential scanning calorimetry. Nevertheless, the influences of AFE nanofillers on the negative ECE are much weaker than that of FE nanofillers, suggesting that the negative ECE observed might not be associated with the AFE states in the composites. Instead, based on the dynamic mechanical analyses on the effects of nanofillers on the order-to-disorder phase transition in the copolymer matrix, the structural glassy states induced by the interaction between the nanofillers and the copolymer matrix are proposed to account for the negative ECE in the composite. This work provides an effective approach in enhancing the negative ECE, which is of technological importance to the implementation of electrocaloric refrigeration.
1. Introduction Electrocaloric effect (ECE) is the reversible change in temperature (ΔT) of a polarizable dielectric material due to a changing applied electric field through that material under adiabatic conditions [1–3]. Small-scale and environmentally friendly cooling systems for microelectronic and digital systems can be manufactured based on those solid-state materials. Electrocaloric effect has been investigated in different ceramics and polymers since its discovery in 1930 by Kobeko and Kurchatov in order to find out an alternative to the conventional cooling technologies due to environmental concerns [4–7]. The ECE entropy change (ΔS) which is the dipolar disorder produced due to the ionic disorder inside the material by the applied electric field (ΔE) is directly proportional to the pyroelectric coefficient (ΔP/ΔT). Generally, the entropy of a dielectric material increases with the removal of an applied electric field, resulting in an effect of cooling defined by a positive ΔT. Large ECE in a material requires large entropy change of the material which may occur near structural or ferroelectric-to-paraelectric (FE-to-PE) phase transition temperature (Tc) [8]. For a workable EC cooling device the ECE should be maximum near room temperature. Ferroelectric copolymer P(VDF-TrFE) has a phase transition temperature near room temperature [9], above which an ECE of
∗
ΔT = 12 °C has been observed [8,9]. More importantly, those ECE behaviors were found to be significantly enhanced by the additions of ferroelectric ceramic nanoparticles. For examples, the ECE was found to be larger than 20 °C in the P(VDF-TrFE) matrix composites containing barium strontium titanate [10]. Nevertheless, there is very little research work on EC properties in the copolymer matrix composites containing anti-ferroelectric nanofillers [7]. Recently, a remarkable phenomenon in ECE, i.e., negative ECE, is of great interest. In some AFE materials such as (Bi0.5Na0.5)TiO3-xBaTiO3 the ECE has been found to be in contrast to that (conventional ECE) mentioned above, which exhibited a cooling effect in the materials under the application of an electric field and a heating effect with the removal of the electric field [11]. In other words, ΔT for those AFE materials is negative, which is opposite to that of the conventional ECE. In (Pb0.97La0.02)(Zr0.95Ti0.05)O3 thin films, the negative ECE was determined to be as large as −6 °C [12]. In the present study (1-x)(Bi0.5Na0.5)TiO3-xBaTiO3 (x = 0, 0.06, 1.0) ceramic powders were incorporated in co-polymer P(VDF-TrFE) to obtain ceramic-polymer composite thick films to enhance its dielectric and EC properties. In particular, the effects of AFE BNT-xBT nanofillers on negative ECEs are investigated. The mechanisms of negative ECE are thus explored by tuning the structural and polar states of P(VDF-TrFE)
Corresponding author. Corresponding author. E-mail addresses: [email protected] (S. Uddin), [email protected] (Z. Jiang).
∗∗
https://doi.org/10.1016/j.solidstatesciences.2019.01.008 Received 22 December 2018; Received in revised form 23 January 2019; Accepted 23 January 2019 Available online 23 January 2019 1293-2558/ © 2019 Elsevier Masson SAS. All rights reserved.
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with the additions of BNT-xBT. 2. Methods and materials Ceramic powders (1-x)Bi1/2Na1/2TiO3-xBaTiO3 (BNT-xBT) were synthesized by a chemical, sol-gel process. Solutions of Bi(CH3COO)3 (> 99.99% Aldrich), Na(CH3COO) (> 99% reagent grade SigmaAldrich), Ba(CH3COO)2 (> 99% reagent ACS Sigma-Aldrich) and strontium acetate (Sr(CH3COO)2) (> 99% reagent ACS Sigma-Aldrich) in stoichiometric ratios were separately prepared in acetic acid on hot plate stirred with a magnetic stirrer. The solution of titanium (IV) butoxide (C16H36O4Ti) (99% reagent grade Aldrich) was prepared in ethanol to which acetylacetone was added drop-wise in order to stabilize the solution. A yellowish colored stable solution was obtained by mixing the above-mentioned four solutions in a flask on a hot plate. A yellowish colored gel was obtained by keeping the stock solution on a hot plate and stirred at 100 °C for 4 h. The gel was dried on a hot plate at 100 °C and calcined at 800 °C for 2 h to obtain fine BNT-xBT powders. The BNT-xBT powders (10% in volume ratio to polymer) were uniformly dispersed in dimethylformamide (DMF) while keeping in ultrasonic bath for 30 min in order to prevent agglomeration. The copolymer P(VDF-TrFE)-52/48 mol% (PiezoTech, France) powders were added to the solution slowly under magnetic stirring on hot plate at 75 °C for 12 h in order to dissolve the co-polymer powders in the solution. A thick film of the composite material was obtained on a 0.1 mm thick copper foil by a casting technique. The film was dried in oven at 70 °C by evaporating the DMF and further heat-treated at 110 °C in a vacuum oven for 12 h. Finally the P(VDF-TrFE)-(1-x)BNT-xBT composite film with a thickness of ∼0.03 mm was obtained on the copper substrate. The volume fraction of BNT-xBT nanoparticles in the composite films is kept as 10% independent of the content x of BNT-xBT. The phase and microstructure analyses of the samples were carried out by using X-Ray diffractometer (XRD) (Rigaku Smartlab) with the Cu Ka (λ = 0.154 nm) radiation and a JSM-6490 (JEOL) field emission scanning electron microscope (SEM), respectively. The phase transitions were investigated by a dynamic mechanical analyzer (DMA), (Q800 TA-Instrument) and differential scanning calorimeter (DSC, TA Instruments Q200). The densities of the fabricated samples were determined by Archimedes principle using an electronic densitometer (MD 300s). Specific heat capacities of samples were measured using differential scanning calorimeter (DSC, TA instrument Q200). In order to measure ferroelectric properties silver paste was painted on to the top faces of samples. The temperature dependent polarization data were obtained by ferroelectric test system (TF Analyzer 2000E, aixACCT) at a frequency of 10 Hz after every 5 °C temperature interval in the range of (30–90 °C).
Fig. 1. (a–c) XRD patterns of P(VDF-TrFE)-(1-x)BNT-xBT thick films (x = 0, 0.06 and 1.0), showing a two-phases material. (b) The effect of BNT and BNT0.06BT nanoparticles on the (020) peaks for the β phase of P(VDF-TrFE) matrix.
3. Results and discussion 3.1. Phase and microstructure analyses The XRD patterns of the P(VDF-TrFE)-(1-x)BNT-xBT are shown in Fig. 1. The single-phase crystalline powders of (1-x)BNT-xBT are found to embed in the P(VDF-TrFE) copolymer matrix (Fig. 2(a)-(c)), resulting in two-phase composite materials. The relatively broader β(020) peak at 18.8° (Fig. 1(b)) and an obvious shoulder peak at 19.2° indicated the coexistence of non-polar α and polar β phases, respectively [13,14]. All the remaining peaks are corresponded to the (1-x)BNT-xBT nanoparticles [14,15]. The XRD patterns suggest that the crystal structures of P(VDF-TrFE) and nanoparticles are not significantly affected with each other and the nanoparticles are dispersed in the copolymer matrix as revealed by the scanning electron microscopic (SEM) images (Fig. 2(a)-(c)). The addition of BNT-xBT polar nanoparticles alternates the anti-ferroelectric or ferroelectric properties of P(VDF-TrFE) copolymer via no chemical reaction among BNT-xBT, VDF or TrEF but the BNT-xBT nanofillers may create a local electric field due to their
Fig. 2. SEM images of the cross-sections of P(VDF-TrFE)-(1-x)BNT-xBT nanocomposites with x = 1.0 (a), x = 0.06 (b), and x = 0 (c). The scale bars are 100 nm. (d) Heat-flow curves of P(VDF-TrFE)-(1-x)BNT-xBT nanocomposites.
spontaneous polarizations. The induced electric fields are believed to affect the conformation of polymer chains of P(VDF-TrFE) such as the trans-gauche-trans-gauche (TGTG) non-polar α phase and the all-trans (TTTT) polar β phases [16]. 3.2. Analyses on the phase transitions The phase transition characteristics of P(VDF-TrFE)-(1-x)BNT-xBT composite thick films are investigated via DSC and DMA. As shown in 10
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the exothermal peak in the heat-flow curve at about 52 °C just below the onset transition temperature of the phase transition. Such increase in the phase transition temperature of P(VDF-TrFE)-BNT is similar with that reported recently in the P(VDF-TrFE)-(Ba1-xSrx)TiO3 nano-composites [11], where the (Ba1-xSrx)TiO3 nanoparticles have the characteristics of diffusive polar phases. The effect of (1-x)BNT-xBT ceramic nanoparticles on the order-todisorder phase transition in the P(VDF-TrFE)-(1-x)BNT-xBT composites is further confirmed by a more sensitive DMA analysis (Fig. 3) at frequencies ranging from 0.1 to 4 Hz with a heating rate of 0.3 °C/min. The plots of storage modulus and internal friction (Q−1) versus temperature show anomalies at around 55 °C, indicating the phase transition in P(VDF-TrFE) which could result from the instabilities of polar domain walls under the applied mechanical stresses [17–19]. The internal friction versus temperature plots reveal obvious dispersion at the peak temperature T1, indicating the non-Arrhenius relaxation or relaxor behavior of the fabricated composite materials [18,19]. The non-Arrhenius relaxation phenomenon near the phase transition can be characterized by the Vogel–Fulcher–Tammann (VFT) relation as follows [20]:
Ea ⎤, f = fo exp ⎡− ⎢ ⎣ kB (T − T0 ) ⎥ ⎦
(1)
where fo is the attempted frequency, T is the absolute temperature of the Q−1 peak and kB is Boltzmann constant. T0 represents the Kauzmann temperature or static freezing temperature at which the entropy becomes zero, suggesting that the polar domains are in a glassy state between T0 and the phase transition temperature. The activation energies (Ea) can be determined from Eq. (1) as shown in Fig. 4, which are 0.04 eV, 0.06 eV and 0.13 for x = 0, 0.06 and 1.0, respectively, in comparison with that (Ea = 0.11 eV) for pristine P(VDF-TrFE). The activation energy of nanocomposite containing 10% volume fraction of BT nanoparticles is larger than that of nanocomposite containing the same volume fraction of BNT and BNT-0.06BT, in consistent with the DSC analyses that the BT nanoparticles could stabilize the polar ordered state of copolymer matrix and thus the phase transition temperature of P(VDF-TrFE)-BT is similar with that of pristine P(VDF-TrFE). The relatively lower values of Ea for P(VDF-TrFE)-BNT and P(VDF-TrFE)-BNT0.06BT indicate that the presence of AFE nanofiller significantly alters the polar domains in the copolymer matrix. As shown in the inset of Fig. 4(b), the Kauzmann temperatures for P (VDF-TrFE) and P(VDF-TrFE)-(1-x)BNT-xBT with x = 0, 0.06 and 1.0 are 25, 36, 35 and 20 °C, respectively. The significantly higher Kauzmann temperatures for P(VDF-TrFE)-BNT and P(VDF-TrFE)-BNT0.06BT as compared with those for P(VDF-TrFE) and P(VDF-TrFE)-BT suggest that the FE nanofillers could induce the glassy features of the polar domains in P(VDF-TrFE) copolymer, while they could stabilize the FE ordered phases as revealed by the above-mentioned DSC analyses. The effects of (1-x)BNT-xBT nanoparticles on the ferroelectric properties of P(VDF-TrFE)-(1-x)BNT-xBT nanocomposites could be closely related with the polar states of the nanoparticles. The localized electric fields near the ferroelectric BT nanoparticles might not be strong enough to fully alternate a P(VDF-TrFE) molecule chain from a non-polar conformation to the all-trans state. As a consequence, the ferroelectric properties of P(VDF-TrFE)-BT such as the remnant polarization and coercive field could be compatible with those of P(VDFTrFE). On the contrary, the AFE domains in anti-ferroelectric BNT or BNT-0.06BT nanoparticles, which consist of atomistic twinning structures, could be effective to modulate the dipolar moments or the orientation of -CF2 or -CH2 groups in the molecule chains, resulting in the enhanced AFE state of P(VDF-TrFE). Thereby the remnant polarization of P(VDF-TrFE)-BNT could be smaller than that of P(VDF-TrFE).
Fig. 3. (a–c) Frequency and temperature dependence of storage modulus (E) and mechanical loss (Q−1) of P(VDF-TrFE)-(1-x)BNT-xBT composites for x = 0, 0.06 and 1.0, respectively.
Fig. 3(b), the heat flows versus temperature plots show an obvious endothermal peak at about 60.2 °C indicating a phase transition from the polar ordered phase to paraelectric non-polar phase in the P(VDFTrFE) copolymers [7,9], which is suggested to be diffusive in nature. The phase transition temperature TP of P(VDF-TrFE)-BNT composite is observed to increase to 61.2 °C with the addition of AFE BNT ceramic powders to the P(VDF-TrFE) copolymer. As compared with that of P (VDF-TrFE)-BNT, the phase transition of P(VDF-TrFE)-BT composite is less affected by the addition of FE BT (x = 1.0) nanoparticles to the P (VDF-TrFE) copolymer. When the composition of (1-x)BNT-xBT ceramic nanoparticles is at the morphotropic phase boundary (MPB), i.e., x = 0.06, the phase transition temperature of P(VDF-TrFE)-(1-x) BNT-xBT can be found to decrease to 59.2 °C. Such decrease may be attributed to the inhomogeneity produced at the nano-scaled level in the morphologies of the BNT-0.06BT nanoparticles [17], where the AFE and FE crystal structures of the nanoparticles are not distinguishable, thereby reduce the ordering of the polar phase of P(VDF-TrFE). On the contrary, the increase in the phase transition temperature of P(VDFTrFE)-BNT could be attributed to the enhanced ordering state in P(VDFTrFE) because of the addition of AFE-phase BNT, manifesting itself by 11
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Fig. 5. (a) Plots of polarization versus temperature at different electric fields for P(VDF-TrFE)-BNT-0.06BT nanocomposites. (b–d) Temperature and electric field dependences of the ECE (ΔT) for the P(VDF-TrFE)-(1-x)BNT-xBT nanocomposites with x = 0, 0.06 and 1.0 respectively. T0 indicates the Kauzmann temperature.
transition temperature TP. The observed negative ECE below phase transition temperature TP is a consequence of the increase in polarization with temperature where (∂P/∂T)E has positive values and negative ΔT values can be obtained from the macroscopic thermodynamics relations (Eq. (2)). Negative ECE is analogous to the negative magnetocaloric effect reported in Ni-Mn-Sn alloys which cool down when entering a magnetic field region and warm up on leaving the external applied magnetic field [22]. With the same applied filed ΔE, the negative ECE (the maximum |ΔTmax| = 7.05 at ΔE = 1.0 MV/cm) in P(VDF-TrFE)-BT is much significant as compared with those in P(VDFTrFE)-BNT and P(VDF-TrFE)-BNT-0.06BT. It has been argued that the negative ECE could be resulted from the multi-phase transitions in the ferroelectric materials, especially in those with metastable AFE phases. However, our observation of the reduced negative ECE in P(VDF-TrFE)-BNT and P(VDF-TrFE)-BNT-0.06BT suggests that the AFE phases might not play a dominate role in the negative ECE in the P(VDF-TrFE)-(1-x)BNT-xBT nanocomposites, since the above-mentioned DSC and DMA analyses show that the AFE phase of P (VDF-TrFE) could be enhanced by the AFE nanofillers in the composites with x = 0 and 0.06. Instead, the well consistency between the critical temperature where the ECE changes from positive values to negative values and the Kauzmann temperature T0 suggests that the negative ECE in the nanocomposites might be related with their structural glassy states. In general, the P(VDF-TrFE) matrix contains the polar β phase, non-polar α phase and other non-polar phases such as the AFE phases. In P(VDF-TrFE)-BT, the FE BT nanofillers produce local electric fields at the nanoparticle-copolymer interfaces, which may contribute in the alignment of dipoles in (-CF2-CH2) monomer units [15]. However the induced polar states in the non-polar phases could be different with the intrinsic polar β phase. As a consequence, the structure of polar domains could become more disordered as compared with those in the pristine P(VDF-TrFE). In contrast, the conformation of non-polar AFE phase in the BNT nanofillers is compatible with those in the P(VDFTrFE) matrix, without much affect the structure of polar domains of β phase. Therefore, the structural glassy states induced by the interaction between the nanofillers and the copolymer matrix could be much significant in P(VDF-TrFE)-BT as compared with that in P(VDF-TrFE)-BNT and P(VDF-TrFE)-BNT-0.06BT, demonstrating by the low Kauzmann temperature T0 for P(VDF-TrFE)-BT. As shown in Fig. 6, the maximum temperature change of negative
Fig. 4. (a–b) The VFT plots of mechanical relaxation near the order-to-disorder phase transitions in P(VDF-TrFE)-(1-x)BNT-xBT nanocomposites. The inset in (b) show the Kauzmann temperatures T0 for P(VDF-TrFE)-(1-x)BNT-xBT and P (VDF-TrFE).
3.3. Electrocaloric properties The electrocaloric properties of the fabricated composite samples were measured via indirect method using Maxwell's thermodynamic relations (Eq. 2) [21].
Δ T= −
1 ρ
E2
∫ TC ( ∂∂TP )E dE , E1
(2)
where ρ is the density of composites, E1 and E2 are the initial and final applied electric fields, T is the ambient temperature and c is the specific heat capacity assumed to be remain constant in the given temperature range [9]. The values of (∂P/∂T)E were calculated from the forth-order polynomial fit of the Pmax versus T plots, where Pmax is the saturation polarization. The plots of Pmax versus T are shown in Fig. 5 (a), indicating a maximum change around 60 °C. The plots of temperature and electricfield dependent electrocaloric temperature changes (ΔT) for the P(VDFTrFE)-(1-x)BNT-xBT nanocomposites are shown in Fig. 5(b–d). In order to compare ΔT among the nanocomposites, we fix E1 = 100 kV/cm while E2 is changed from 200 kV/cm to as high as 1500 kV/cm, resulting in different ΔE = E2-E1 as indicated in Fig. 5(b–d). For the P (VDF-TrFE)-(1-x)BNT-xBT nanocomposites, negative ECE can be observed to occur between a temperature close to T0 and the phase12
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The structural glassy states induced by the interaction between the nanofillers and the copolymer matrix are proposed to account for the negative ECE in the composite thick films. The results demonstrate that the negative ECE can be enhanced by inducing the disordered structures in the polar domains of ferroelectrics. Acknowledgment This work was supported by the National Natural Science Foundation of China (No. 61805226). References [1] P.D. Thacher, J. Appl. Phys. 39 (1968) 1996. [2] M.D. Li, X.G. Tang, S.M. Zeng, Q.X. Liu, Y.P. Jiang, W.H. Li, J. Alloy. Comp. 747 (2018) 1053. [3] M.D. Li, X.G. Tang, S.M. Zeng, Q.X. Liu, Y.P. Jiang, T.F. Zhang, W.H. Li, ACS Sustain. Chem. Eng. 6 (2018) 8920. [4] P. Kobeko, I. Kurchatov, Physik 66 (1930) 192. [5] J.F. Scott, Annu. Rev. Mater. Res. 41 (2011) 229. [6] M. Valant, Prog. Mater. Sci. 57 (2012) 980. [7] X. Li, S.G. Lu, X.Z. Chen, H. Gu, X.S. Qian, Q.M. Zhang, J. Mater. Chem. C. 1 (2013) 23. [8] A.S. Mischenko, Q. Zhang, J.F. Scott, W.R. Whatmore, N.D. Mathur, Science 311 (2006) 1270. [9] B. Neese, B. Chu, S.G. Lu, Y. Wang, E. Furman, Q.M. Zhang, Science 321 (2008) 821. [10] G. Zhang, Q. Li, H. Gu, S. Jiang, K. Han, M.R. Gadinski, M.A. Haque, Q. Zhang, Q. Wang, Adv. Mater. 27 (2015) 1450. [11] X.C. Zheng, G.P. Zheng, Z. Lin, Z.Y. Jiang, J. Electroceram. 28 (2012) 20. [12] W. Geng, Y. Liu, X. Meng, L. Bellaiche, J.F. Scott, B. Dkhil, A. Jiang, Adv. Mater. 27 (2015) 3165. [13] X. Guana, Y. Zhangb, H. Li, J. Ou, Sensor. Actuator. 194 (2013) 228. [14] Z.Y. Jiang, X.C. Zheng, G.P. Zheng, RSC Adv. 5 (2015) 61946. [15] G.P. Zheng, S. Uddin, X. Zheng, J. Yang, J. Alloy. Comp. 663 (2016) 249. [16] V. Mittal, Synthesis Techniques for Polymer Nanocomposites, Wiley, 2014, pp. 132–134. [17] S. Uddin, G.P. Zheng, Y. Iqbal, R. Ubic, N.Y. Chan, H.L.W. Chan, Mater. Res. Express 1 (2014) 046102. [18] W. Jo, S. Schaab, E. Sapper, L.A. Schmitt, H.J. Kleebe, A.J. Bell, J. Appl. Phys. 110 (2011) 074106. [19] D. Viehland, S.J. Jang, L.E. Cross, M. Wuttig, J. Appl. Phys. 68 (1990) 2916. [20] K.J. Laidler, Chemical Kinetics, third ed., Harper & Row, 1987, pp. 42–44. [21] M.E. Lines, A.M. Glass, Principles and Applications of Ferroelectrics and Related Materials, Clarendon Press, Oxford, 1977, pp. 70–90. [22] B. Yu, M. Liu, P.W. Egolf, A. Kitanovski, Int. J. Refrig. 33 (2010) 1029.
Fig. 6. The log-log plots of the absolute values of the maximum ECE (|ΔTmax|) as a function of applied electric field (ΔE=E2-E1) with E1 = 100 kV/cm for the P (VDF-TrFE)-(1-x)BNT-xBT nanocomposites.
ECE versus ΔE plots are fitted as |ΔTmax|∼ Eb, where b is an exponent which characterizes the increase of electrocaloric strength with increasing applied field. Especially, P(VDF-TrFE)-BT exhibits a maximum negative ECE of more than 7 °C. The enhanced negative ECE in P(VDFTrFE)-BT as compared with those in P(VDF-TrFE)-BNT and P(VDFTrFE)-BNT-0.06BT is evident under a relatively high applied electric field (E2 > 700 kV/cm). 4. Conclusions Composite thick-film samples of P(VDF-TrFE)-(1-x)BNT-xBT (x = 0, 0.06, 1.0) on copper substrate were fabricated. DSC and DMA analyses were used to investigate the phase transition behavior of the nanocomposites. Under the same applied fields, the negative ECE in P(VDFTrFE)-BT is much significant in the nanocomposites with ferroelectric nanofillers as compared with those with anti-ferroelectric nanofillers.
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