English

• Electrocaloric effect (ECE) describes the isothermal entropy change (ΔS) or adiabatic temperature change (ΔT) in polar materials induced by the polarization change under an external electric field^[1-3]. Discovered by Kobeco and Kurtschatov in Rochelle salt in the 1930s, ECE was undervalued for a long time because a small temperature change was observed in many ferroelectrics. However, giant ECE was found in PbZr_0.95Ti_0.05O₃ ceramic film at ca. 222 ℃ in 2006 and in polyvinylidene difluoride (PVDF)-based ferroelectric polymers near room temperature in 2008^[4-6]. These studies arise great interest and lay the groundwork for the development of solid-state refrigeration devices based on ECE. ECE is considered an emerging solid-state refrigeration technology in terms of its high efficiency and environmental friendliness compared with traditional vapor compression cooling technology^[7-8].

  Over the past decade, the ECE of polar polymers receives growing attention due to their flexibility, lightweight, processability, and high breakdown strength^[9-11]. However, the heat transfer efficiency of polymers is too poor to satisfy the demand of device applications^[12-13]. Therefore, to achieve larger thermal conductivity and higher ECE, composite materials have been explored^[14-16]. Fillers with high thermal conductivity can improve the heat transfer of polymers. At the same time, the interfaces between the polymer matrix and fillers may generate an interfacial coupling effect and offer an additional benefit to improve ECE^[17].

  The ferroelectric-paraelectric (FE-PE) transition in ferroelectric copolymers is a first-order transition at the Curie transition. Around the Curie temperature (T_C), the ECE of ferroelectric polymers is the largest. Poly(vinylidene fluoride‑trifluoroethylene) (55/45) (P(VDF-TrFE) (55/45)) has the lowest T_C (73 ℃) among the P(VDF-TrFE) copolymers, making it suitable for low-temperature ECE. P(VDF-TrFE) (55/45) used in this work is a typical ferroelectric polymer that is frequently investigated in the field of ECE. To enhance the thermal conductivity and ECE of P(VDF-TrFE) (55/45) ferroelectric copolymer, a small amount of 0.75BiFeO₃-0.25BaTiO₃ (BFO-BTO) nanofibers with a diameter of about 100 nm were added. The results showed that ECE was slightly enhanced by adding only 2% BFO-BTO nanofibers to the copolymer. What is more important, the thermal conductivity of the copolymer was enhanced by nearly two times by adding the nanofibers.

  1.   Experimental

  1.1   Reagent

  Bismuth nitrate pentahydrate (Bi(NO₃)₂·5H₂O), ferric nitrate nonahydrate (Fe(NO₃)₃·9H₂O), barium acetate (Ba(Ac)₂), tetrabutyl titanate (TTBT), citric acid (CA), acetylacetone (Hacac), glacial acetic acid (HAc), and N, N-dimethylformamide (DMF) were purchased from Sinopharm Chemical Reagent Co., Ltd. Pyrrolidone (PVP) was purchased from Tianjin Bodi Chemical Limited Industry. P(VDF-TrFE) (55/45) copolymer was purchased from Piezotech Arkema.

  1.2   Preparation of composite films

  BFO-BTO nanofibers^[18] were prepared using the electrospinning process. 2.985 9 g Bi(NO₃)₂·5H₂O, 2.487 0 g Fe(NO₃)₃·9H₂O, 0.698 3 g TTBT, 0.524 1 g Ba(Ac)₂, 2.480 9 g CA, 0.235 3 g Hacac, 3.589 2 g HAc, 1.973 3 g PVP, and 25 g DMF were added into a conical flask and stirred for 30 min. Subsequently, 1.973 3 g PVP was added to the mixture and stirred overnight to obtain a homogeneous precursor solution. The electrospinning process was carried out using a custom-made electrospinning machine. The applied voltage of the electrospinning process was about 20 kV. Raw nanofibers were collected and dried at 200 ℃ for 1 h and 350 ℃ for 2 h before being calcined at 600 ℃ for 2 h to obtain BFO-BTO nanofibers.

  1.6 g P(VDF-TrFE) (55/45) powders and 5 mL DMF were mixed and stirred for 2 h. After that, BFO-BTO nanofibers were mixed into the copolymer solution. The fibers were broken down into smaller sizes and dispersed in the solution with the help of ultrasound and violent stirring. The homogeneous suspension was cast on a glass plate and dried in an oven at 70 ℃ for 2 h. The dried film was peeled off from the glass plate and treated in a vacuum oven at 135 ℃ for 5 h to eliminate the residual solvent and promote crystallinity. The volume fractions^[19] of BFO-BTO nanofibers in the composite films were 0%, 1%, 1.5%, 2%, and 2.5%, respectively. The thickness of the films was about 20 μm.

  1.3   Characterization

  The morphologies of BFO-BTO nanofibers, copolymer, and composite films were analyzed by a scanning electron microscope (SEM, SU8200, HITACHI, Japan, the testing voltage was 1.5 kV) and a field emission transmission electron microscopy (FETEM, JEM-2100F, JEOL, Japan, the accelerating speed was 200 kV). The crystal structure of the specimens was examined by X-ray diffraction (XRD) using an X-ray diffractometer (Ultima Ⅳ, Rigaku, Japan, the incident source was Cu Kα, λ=0.154 187 nm, the operating voltage and current were 40 kV and 40 mA, respectively, the scanning range was 10°-60°, and the scanning speed was 10 (°)·min^-1). The chain conformation of the copolymer and composite films was investigated by an attenuated total reflectance-Fourier transform infrared spectrophotometer (ATR-FTIR, Nicolet 8700, Thermo Nicolet, USA). The specific heat capacity (c_p) of the films was measured by differential scanning calorimetry (DSC, 204F1 Phoenix, NETZSCH, Germany, N₂ atmosphere, and the heating and cooling speed was 10 ℃·min^-1). The thermal conductivity of the films was measured by a laser flash diffusivity apparatus (LAF467, NETZSCH, Germany). For electrical tests, gold electrodes with diameters of 6 mm (for dielectric tests) or 3 mm (for ferroelectric tests) were coated on the upper and bottom surfaces of the films in a sputter coater (EMS150T, Electron Microscopy Sciences, USA). The frequency and temperature dependence of the dielectric properties of the materials were measured using an LCR meter (E4980, Agilent Technology, USA) equipped with a furnace (The testing temperature range was 30-140 ℃, and the heating and cooling speed was 2 ℃·min^-1). The polarization vs electric filed hysteresis curves (P‑E loops) were studied using a modified Sawyer-Tower circuit (Polyktech, State College, USA, the testing frequency was 1 000 Hz).

  2.   Results and discussion

  Fig. 1a shows the SEM image of the calcined BFO-BTO nanofibers, and the diameter of the nanofibers was quite uniform and was around 100 nm. The TEM image in Fig. 1b shows that nanofibers consist of small crystal grains, causing the rough surface of the fibers observed in SEM images. The XRD pattern (Fig. 1c) indicates the perovskite crystal structure of nanofibers^[19].

  图 1

  

  图 1.  (a) SEM image, (b) TEM image, and (c) XRD pattern of calcined BFO-BTO nanofibers

  下载: 全尺寸图片 幻灯片

  Fig. 2a and 2b show the surface and cross-sectional SEM images of the copolymer and composite films, a braid-like crystalline microstructure was observed on the surface of all films. For the composite films, we can see the claviform fibers were uniformly distributed in the composite films without obvious aggregation, as shown in Fig. 2a. In the cross-sectional image shown in Fig. 2b, only the ends of the nanofibers can be observed and the fibers were not aggregated according to the image. The XRD patterns of the copolymer and composite films are shown in Fig. 2c. The peaks at 18.7° and 19.1° are assigned to the (110) and (200) planes of β-phase of the copolymer (PDF No.42-1649), while the peaks at 22.2°, 32.6°, 39.1°, 45.4°, 51.2°, and 56.4° were associated with BFO-BTO nanofibers (PDF No.05-0626)^[20]. On the FTIR spectra shown in Fig. 2d, the peaks at 840 and 1 286 cm^-1, which were from T_m > 4 (All-trans conformation was larger than 4) and T₃G chain conformations of the copolymer, had strong intensity. The FTIR results are consistent with the XRD results that the copolymer is mainly in β-phase^[21].

  图 2

  

  图 2.  (a) Surface and (b) cross-sectional SEM images, (c) XRD patterns, and (d) FTIR spectra of the copolymer and composite films

  下载: 全尺寸图片 幻灯片

  Fig. 3 presents the temperature dependence and frequency dependence of the dielectric properties of the copolymer and composite films. The T_C (ca. 73 ℃) did not vary with the addition of BFO-BTO nanofibers, as shown in Fig. 3a. As shown in Fig. 3b, as the amount of BFO-BTO nanofibers in composite films increased, the dielectric constant of the composite films was slightly enhanced. This can be ascribed to the mechanisms that the dielectric constant of BFO-BTO nanofibers was much higher than the polymer matrix and the addition of BFO-BTO nanofibers induced interfacial effects in composite films, which enhanced the dielectric properties^[22].

  图 3

  

  图 3.  (a) Temperature dependence at 1 kHz and (b) frequency dependence of the dielectric properties at room temperature of the copolymer and composite films

  下载: 全尺寸图片 幻灯片

  Fig. 4 shows the P-E loops of the copolymer and composite films at room temperature and around T_C measured under an electric field of 100 MV·m^-1. At room temperature, the films were in a ferroelectric state and the P-E loops with large hysteresis can be observed (Fig. 4a). Because only a small amount of fibers was added to the copolymer, the variation of the polarization response between these films was minor. While at a higher temperature that near and above the FE-PE transition, the P-E loops became slim, and the remanent polarization diminished, as shown in Fig. 4b-4f. From the P-E loops measured at different temperatures, the variation of the polarization with temperature can be obtained, as presented in Fig. 5a. According to the Maxwell relations, the reversible ΔS and ΔT can be deduced as^[23-24]:

  图 4

  

  图 4.  (a) P-E loops measured at 1 kHz of the copolymer and composite films at room temperature; (b-f) P-E loops measured at 1 kHz of the copolymer and composite films at a temperature below, near, and above the FE-PE transition

  下载: 全尺寸图片 幻灯片

  图 5

  

  图 5.  (a) Polarization responses (Inset: c_p vs T curves of the composite films with 0% and 2.5% BFO-BTO), (b) ΔS vs T curves and (c) ΔT vs T curves of the copolymer and composite films at an electric field of 100 MV•m^-1

  下载: 全尺寸图片 幻灯片

  $ \Delta S=\int_{E_1}^{E_2}\left(\frac{\partial D}{\partial T}\right) \mathrm{d} E $

  (1)

  $ \Delta T=-\frac{T}{\rho} \int_{E_1}^{E_2} \frac{1}{c_p}\left(\frac{\partial D}{\partial T}\right) \mathrm{d} E $

  (2)

  Herein, D is the electric displacement, which is approximately equal to P (polarization) for ferroelectric polymers; T is the temperature; E is the electric field strength; ρ is the density of the material. The c_p of the composite films with 0% and 2.5% BFO-BTO was characterized and presented in the inset in Fig. 5a. The ΔS and ΔT as a function of T at 100 MV·m^-1 can be calculated, as shown in Fig. 5b and 5c. From 75 to 100 ℃, ΔS and ΔT gradually decreased. The difference between ΔS or ΔT of the copolymer and composites was minor. For example, the copolymer film had a ΔS of 61.4 kJ·m^-3·K^-1 and a ΔT of 8.1 ℃ under an electric field of 100 MV·m^-1 at 75 ℃, whereas the composite film with 2% BFO-BTO had a ΔS of 67.8 kJ·m^-3·K^-1 and a ΔT of 8.5 ℃. Consequently, the addition of BFO-BTO nanofibers did not change or even slightly enhanced the ECE of the copolymer. This can be ascribed to the increase in dielectric constant and polarization with the addition of nanofibers, which allows for a greater change in polarization near the T_C and thus an enhancement in the ECE.

  The thermal conductivities of the copolymer and composite films at 75 ℃ were measured, as illustrated in Fig. 6. The thermal conductivity of the composite film with 0% BFO-BTO was 0.11 W·m^-1·K^-1. The composite films had a thermal conductivity from 0.2 to 0.22 W·m^-1·K^-1, which was nearly twice that of the composite film with 0% BFO-BTO. This may be due to the higher thermal conductivity of BFO-BTO ceramic nanofibers than copolymers, upon the addition of nanofibers to the copolymer, the nanofibers can act as a thermal path and the dispersed fibers formed a thermal conductivity network to promote heat transfer^[25]. For fiber-filled composites, their thermal conductivity of the composite was given^[26]:

  图 6

  

  图 6.  Thermal conductivity of the copolymer and composite films at 25 and 75 ℃

  下载: 全尺寸图片 幻灯片

  $ \frac{1}{k_{\mathrm{c}}}=\frac{1}{\sqrt{C\left(k_{\mathrm{p}}-k_{\mathrm{f}}\right)\left[k_{\mathrm{p}}+B\left(k_{\mathrm{f}}-k_{\mathrm{p}}\right)\right]}} \ln \frac{\sqrt{k_{\mathrm{p}}+B\left(k_{\mathrm{f}}-k_{\mathrm{p}}\right)}+0.5 B \sqrt{C\left(k_{\mathrm{p}}-k_{\mathrm{f}}\right)}}{\sqrt{k_{\mathrm{p}}+B\left(k_{\mathrm{f}}-k_{\mathrm{p}}\right)}-0.5 B \sqrt{C\left(k_{\mathrm{p}}-k_{\mathrm{f}}\right)}}+\frac{1-B}{k_{\mathrm{p}}} $

  (3)

  Where, $B=\sqrt{\frac{3 \varphi}{2}}, C=-4 \sqrt{\frac{2}{3 \varphi}} $; k_c, k_p, k_f are the thermal conductivity of the composite film, polymer, and nanofiber, respectively. φ is the volume fraction of nanofibers.

  If the thermal conductivity of the polymer matrix is much smaller than that of filler materials, in the low φ range (φ < 0.667), the effective thermal conductivity of composites can be approximated by:

  $ k_{\mathrm{c}} \approx \frac{k_{\mathrm{p}}}{1-B} $

  (4)

  After substituting the thermal conductivity of the copolymer (0.17 W·m^-1·K^-1) and the nanofiber (8 W·m^-1·K^-1) into the above formula^[27], the thermal conductivity of the composite film was calculated as 0.19 W·m^-1·K^-1 when the φ was 1% and 0.20 W·m^-1·K^-1 when the φ was 2%, which was consistent with estimated values.

  The thermal conductivity of copolymer and composite films at 25 ℃ was also measured, as shown in Fig. 6. The thermal conductivity of the copolymer film was 0.17 W·m^-1·K^-1, which is consistent with prior studies, and that of the composite films was in the range of 0.22 to 0.24 W·m^-1·K^{-1 [14, 28]}. The results suggested that by adding only a small amount of BFO-BTO nanofibers, the thermal conductivity of the copolymer can be greatly improved. The enhancement of thermal conductivity by adding nanofibers is beneficial for electrocaloric application because good heat dissipation is necessary.

  3.   Conclusions

  In summary, we prepared BFO-BTO nanofibers by the electrospinning process. The nanofibers consisted of small crystal grains and had a perovskite crystal structure. The diameter of nanofibers was quite uniform, which was around 100 nm. The composites of P(VDF-TrFE) (55/45) and the nanofibers were prepared by the solution casting method. Microstructure analysis indicated that the fibers were uniformly distributed in the composite film without obvious aggregation. Subsequently, the ECE values of the copolymer and composite films with BFO-BTO nanofibers were investigated. We demonstrate that the composite film with 2% BFO-BTO had the optimal ECE properties with ΔS of 67.8 kJ·m^-3·K^-1 and ΔT of 8.5 ℃ at an electric field of 100 MV·m^-1. By adding a small amount of BFO-BTO nanofibers to the copolymer, the composite films had a notable enhancement (about two times) in thermal conductivity, which can be ascribed to the formation of the thermal conductivity network in composite films. Besides, the high thermal conductivity of composite films remained relatively steady as the temperature rises.

  
  
• 1. [1]

     Grünebohm A, Ma Y B, Marathe M, Xu B X, Albe K, Kalcher C, Meyer K C, Shvartsman V V, Lupascu D C, Ederer C. Origins of the inverse electrocaloric effect[J]. Energy Technol., 2018, 6(8):  1491-1511. doi: 10.1002/ente.201800166

  2. [2]

     Li B, Kawakita Y, Ohira-Kawamura S, Sugahara T, Wang H, Wang J F, Chen Y N, Kawaguchi S I, Kawaguchi S, Ohara K. Colossal barocaloric effects in plastic crystals[J]. Nature, 2019, 567(7749):  506-510. doi: 10.1038/s41586-019-1042-5

  3. [3]

     Shi J Y, Han D L, Li Z C, Yang L, Lu S G, Zhong Z F, Chen J P, Zhang Q M, Qian X S. lectrocaloric cooling materials and devices for zero-global-warming-potential, high-efficiency refrigeration[J]. Joule, 2019, 3(5):  1200-1225. doi: 10.1016/j.joule.2019.03.021

  4. [4]

     Kobeko P, Kurtschatov J. Pielectric properties of Rochelle salt crystals[J]. Zeitschrift Fur Physik, 1930, 66(3/4):  192-205.

  5. [5]

     Mischenko A S, Zhang Q M, Scott J F, Whatmore R W, Mathur N D. Giant electrocaloric effect in thin-film PbZr_0.95Ti_0.05O₃[J]. Science, 2006, 311(5765):  1270-1271. doi: 10.1126/science.1123811

  6. [6]

     Neese B, Chu B J, Lu S G, Wang Y, Furman E, Zhang Q M. Large electrocaloric effect in ferroelectric polymers near room temperature[J]. Science, 2008, 321(5890):  821-823. doi: 10.1126/science.1159655

  7. [7]

     Han F, Bai Y, Qiao L J, Guo D. A systematic modification of the large electrocaloric effect within a broad temperature range in rare earth doped BaTiO₃ ceramics[J]. J. Mater. Chem. C, 2016, 4(9):  1842-1849. doi: 10.1039/C5TC04209G

  8. [8]

     Jia Y B, Sungtaek Y. A solid-state refrigerator based on the electrocaloric effect[J]. Appl. Phys. Lett., 2012, 100(24):  242901. doi: 10.1063/1.4729038

  9. [9]

     Scott J. Electrocaloric materials[J]. Ann. Rev. Mater. Res., 2011, 41:  229240.

  10. [10]

      Chen X, Xu W H, Lu B, Zhang T, Wang Q, Zhang Q M. Towards electrocaloric heat pump-A relaxor ferroelectric polymer exhibiting large electrocaloric response at low electric field[J]. Appl. Phys. Lett., 2018, 113(11):  113902. doi: 10.1063/1.5048599

  11. [11]

      Jian X D, Lu B, Li D D, Yao Y B, Tao T, Liang B, Guo J H, Zeng Y J, Chen J L, Lu S G. Direct measurement of large electrocaloric effect in Ba (Zr_xTi_1-x) O₃ ceramics[J]. ACS Appl. Mater. Interfaces, 2018, 10(5):  4801-4807. doi: 10.1021/acsami.7b15933

  12. [12]

      Choy C. Thermal conductivity of polymers[J]. Polymer, 1977, 18(10):  984-1004. doi: 10.1016/0032-3861(77)90002-7

  13. [13]

      Dos Santos W, Iguchi C Y, Gregorio Jr R. Thermal properties of poly (vinilidene fluoride) in the temperature range from 25 to 210℃[J]. Polym. Test., 2008, 27(2):  204-208. doi: 10.1016/j.polymertesting.2007.10.005

  14. [14]

      Chen X Z, Li X Y, Qian X S, Lin M, Wu S, Shen Q D, Zhang Q M. A nanocomposite approach to tailor electrocaloric effect in ferroelectric polymer[J]. Polymer, 2013, 54(20):  5299-5302. doi: 10.1016/j.polymer.2013.07.052

  15. [15]

      Zhang G Z, Li Q, Gu H M, Jiang S L, Han K, Gadinski M R, Haque M A, Zhang Q M, Wang Q. Ferroelectric polymer nanocomposites for room-temperature electrocaloric refrigeration[J]. Adv. Mater., 2015, 27(8):  1450-1454. doi: 10.1002/adma.201404591

  16. [16]

      Zhang G Z, Fan B Y, Zhao P, Liu Y, Liu F H, Jiang S L, Zhang S L, Li H L, Wang Q. Ferroelectric polymer nanocomposites with complementary nanostructured fillers for electrocaloric cooling with high power density and great efficiency[J]. ACS Appl. Energy Mater., 2018, 1(3):  1344-1354. doi: 10.1021/acsaem.8b00052

  17. [17]

      Aziguli H, Chen X, Liu Y, Yang G, Yu P, Wang Q. Enhanced electrocaloric effect in lead-free organic and inorganic relaxor ferroelectric composites near room temperature[J]. Appl. Phys. Lett., 2018, 112(19):  193902. doi: 10.1063/1.5028459

  18. [18]

      Zhang X Y, Yang X, Wang Z P, Pan Q, Chu B J, Zuo R Z. Enhanced magnetoelectric response of Mn doped BiFeO₃ based multiferroic ceramics[J]. J. Mater. Sci.-Mater. Electron, 2022, 33:  15520-15532. doi: 10.1007/s10854-022-08458-5

  19. [19]

      Hu X P, Che Y P, Zhang Z, Shen Q D, Chu B J. BiFeO₃-BaTiO₃/P (VDF TrFE) multifunctional polymer nanocomposites[J]. ACS Appl. Electron. Mater., 2021, 3(2):  743-751. doi: 10.1021/acsaelm.0c00926

  20. [20]

      Liu J, Zhou Y, Hu X P, Chu B J. Flexoelectric effect in PVDF-based copolymers and terpolymers[J]. Appl. Phys. Lett., 2018, 112(23):  232901. doi: 10.1063/1.5028344

  21. [21]

      Zhou Y, Liu J, Hu X P, Chu B J, Chen S T, Salem D. Flexoelectric effect in PVDF-based polymers[J]. IEEE Trans. Dielectr. Electr. Insul., 2017, 24(2):  727-731. doi: 10.1109/TDEI.2017.006273

  22. [22]

      Chu B J, Lin M R, Neese B, Zhou X, Chen Q, Zhang Q M. Large enhancement in polarization response and energy density of poly (vinylidene fluoride-trifluoroethylene-chlorofluoroethylene) by interface effect in nanocomposites[J]. Appl. Phys. Lett., 2007, 91(12):  122909. doi: 10.1063/1.2786839

  23. [23]

      Rožič B, Malič B, Uršič H, Holc H, Kosec M, Neese B, Zhang Q M, Kutnjak Z. Direct measurements of the giant electrocaloric effect in soft and solid ferroelectric materials[J]. Ferroelectrics, 2010, 405(1):  26-31. doi: 10.1080/00150193.2010.482884

  24. [24]

      Kumar A, Thakre A, Jeong D Y, Ryu J. Prospects and challenges of the electrocaloric phenomenon in ferroelectric ceramics[J]. J. Mater. Chem. C, 2019, 7(23):  6836-6859. doi: 10.1039/C9TC01525F

  25. [25]

      Yusuf A H, Chen L, Geng X W, Jiang Z Y, Zhang F, Luo A B, Hu H L. Electrocaloric effect of structural configurated ferroelectric polymer nanocomposites for solid state refrigeration[J]. ACS Appl. Mater. Interfaces, 2021, 13:  46681-46693. doi: 10.1021/acsami.1c13614

  26. [26]

      Tavman I H, Akmcl H. Transverse thermal conductivity of fiber reinforced polymer composites[J]. Int. Commun. Heat Mass Transf., 2000, 27(2):  253-261. doi: 10.1016/S0735-1933(00)00106-8

  27. [27]

      Di C, Pan J H, Dong S T, Lv Y Y, Yan X J, Zhou J, Yao A H, Lu H, Gusev V E, Chen Y F, Lu M H. Ultralow cross-plane lattice thermal conductivity caused by Bi-O/Bi-O interfaces in natural superlatticelike single crystals[J]. CrystEngComm, 2019, 21:  6261. doi: 10.1039/C9CE01139K

  28. [28]

      Wong Y W, Hui N M, Ong E L, Chan H L W, Choy C L. Specific heat and thermal diffusivity of vinylidene fluoride/trifluoroethylene copolymers[J]. J. Appl. Polym. Sci., 2003, 89(12):  3160-3166. doi: 10.1002/app.12551

• 

  图 1  (a) SEM image, (b) TEM image, and (c) XRD pattern of calcined BFO-BTO nanofibers

  下载: 全尺寸图片 幻灯片
  

  图 2  (a) Surface and (b) cross-sectional SEM images, (c) XRD patterns, and (d) FTIR spectra of the copolymer and composite films

  下载: 全尺寸图片 幻灯片
  

  图 3  (a) Temperature dependence at 1 kHz and (b) frequency dependence of the dielectric properties at room temperature of the copolymer and composite films

  下载: 全尺寸图片 幻灯片
  

  图 4  (a) P-E loops measured at 1 kHz of the copolymer and composite films at room temperature; (b-f) P-E loops measured at 1 kHz of the copolymer and composite films at a temperature below, near, and above the FE-PE transition

  下载: 全尺寸图片 幻灯片
  

  图 5  (a) Polarization responses (Inset: c_p vs T curves of the composite films with 0% and 2.5% BFO-BTO), (b) ΔS vs T curves and (c) ΔT vs T curves of the copolymer and composite films at an electric field of 100 MV•m^-1

  下载: 全尺寸图片 幻灯片
  

  图 6  Thermal conductivity of the copolymer and composite films at 25 and 75 ℃

  下载: 全尺寸图片 幻灯片
•