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Vertically-aligned lead-free BCTZY nanofibers with enhanced electrical properties for flexible piezoelectric nanogenerators
Applied Surface Science 469 (2019) 283–291
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Full Length Article
Vertically-aligned lead-free BCTZY nanofibers with enhanced electrical properties for flexible piezoelectric nanogenerators ⁎
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Yinghong Wua,b, Fei Maa,b, Jingkui Qua, , Yang Luoa,b, Caixia Lva, Qiang Guoa, , Tao Qia a National Engineering Laboratory for Hydrometallurgical Cleaner Production Technology, Institute of Process Engineering, Chinese Academy of Sciences, Beijing 100190, PR China b University of Chinese Academy of Sciences, Beijing 100190, PR China
A R T I C LE I N FO
A B S T R A C T
Keywords: Lead-free BCTZY nanofibers Electrospinning Vertical alignment Piezoelectric nanogenerators
Flexible lead-free (Ba0.85Ca0.15)(Ti0.9Zr0.1)O3-0.2 mol%Y nanofibers (BCTZY NFs) were synthesized by electrospinning and their corresponding nanogenerators (NGs) with vertical alignments were fabricated. The lowtemperature sintering properties of BCTZY NFs were investigated, to optimize their synthesis path and minimize the thermal energy consumption during the sintering process. The continuity, flexibility, and stability of the BCTZ-based NFs were improved by adding Y3+. Moreover, the temperature-evolved Raman spectra displayed a high Curie temperature of 280 °C for BCTZY NFs, which was far higher than that of about 90 °C for BCTZ-based ceramic bulks, owing to the discontinuous physical property of NFs. The dielectric, ferroelectric, and piezoelectric properties of the vertically aligned BCTZY NFs/PDMS were estimated and compared with those of BCTZ NFs/PDMS composites, to verify the advantages of vertical alignments and the donor doping effect of Y3+. Vertically aligned BCTZY NF-based NGs showed an average VOC of 3.0 V and ISC of 85 nA by finger tapping, suggesting their potential applications in tiny energy harvesting.
1. Introduction Owing to their characteristic of transforming mechanical energy into electrical energy (and vice versa), piezoelectric materials have been regarded as one of the most promising energy suppliers in actuators, sensors, and energy harvesting devices [1–3]. Piezoelectric nanomaterials, especially for flexible nanofibers (NFs) and nanowires (NWs), are currently attracting intensive attention because of their unique features (like lightweight nature, flexibility, and high mechanical robustness) that are beneficial for developing miniaturized devices, such as flexible piezoelectric nanogenerators (NGs) that can easily harvest energy from negligible vibrations, small air flow, as well as tiny body movements [4–6]. Therefore, it is of great importance to find a suitable method to synthesize flexible NFs and NF-based NGs. Among various NFs synthesis techniques, electrospinning is considered as one of the most efficient methods due to the following advantages: Apart from versatility, economy, and the capability of operating at room temperature, it can spin ultra-fine and nanoscale organic/ inorganic fibers and simplify the measurement procedure by directly fabricating piezoelectric NFs on substrates [7–9]. More significantly, the uniaxially aligned NFs can be obtained by increasing the rotation speed of the roller during the electrospinning process (Fig. S1) [10–12].
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This is very useful for the fabrication of vertically aligned NGs with superior piezoelectric properties, as reported by some literatures [11–13]. For instance, a pressure of 0.1 Pa was sufficient to generate an open-circuit voltage of 0.1 mV for the well-aligned PVDF-TrFE NFs according to Persato et al.’s study [11]. Furthermore, the well-aligned Pb(Zr,Ti)O3 (PZT) NWs prepared by Xu et al. showed an output voltage of 0.7 V and a current density of 4 mA/cm−2 with an average power density of 2.8 mW/cm−3 [13]. In fact, PZT has even played the dominating role in the applications of piezoelectric NFs and NWs, because of its outstanding piezoelectric properties and high output voltages [14,15]. However, it is necessary to develop lead-free NFs substitutes, owing to the huge amount of toxic PbO violating the principles of green and sustainable development [16]. It was found that the vertically-aligned lead-free BaTiO3 (BT) NF-based NGs exhibited much better piezoelectric properties than NGs with randomly or horizontally aligned BT NFs, since the achieved piezoelectric potentials of the former being transmitted to electrodes more efficiently than those of the latter [12]. Moreover, (Ba,Ca)(Ti,Zr)O3 (BCTZ) has attracted much attention in recent years and is considered one of the most potential alternatives, as its piezoelectric coefficient d33 has been reported to be as high as that of PZT and much higher than that of BT [17]. Hence, it is interesting to
Corresponding authors. E-mail address: [email protected] (J. Qu).
https://doi.org/10.1016/j.apsusc.2018.10.229 Received 13 July 2018; Received in revised form 24 September 2018; Accepted 28 October 2018 Available online 29 October 2018 0169-4332/ © 2018 Elsevier B.V. All rights reserved.
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find out how vertically-aligned BCTZ NF-based NGs will affect the electrical characteristics. Besides, many studies have reported that the introduction of Cu2+ [18], Li+ [19], and Y3+ [20] into BCTZ ceramics can perfect their comprehensive properties like the sintering properties, the Curie temperature (TC), and the electrical properties. Up to now, however, few literatures have investigated the effect of the doping elements on the enhanced performance of the BCTZ-based NFs and NGs. Therefore, in this study, uniaxially aligned BCTZY NFs were prepared by electrospinning and their corresponding vertically-aligned NGs were further manufactured. The low-temperature sintering properties of BCTZY NFs were investigated by TG/DSC, XRD and FTIR, to optimize their sintering process and minimize the thermal energy consumption. The morphology, flexibility, and crystallinity of these sintered NFs were examined by SEM and FETEM measurements. The phase transition and TC of the BCTZY NFs were confirmed by the temperature-evolved Raman spectra. The dielectric, ferroelectric, and piezoelectric properties of the vertically aligned BCTZY NFs/PDMS and the electrical output of its NGs by finger tapping were evaluated to verify their potential application in tiny energy harvesting.
composite was obtained by cutting these NFs along the red region in Fig. 1(d) and rotating it 90° clockwise. (3) Two ITO-PET films used as electrodes were attached to both ends of the NFs/PDMS to form BCTZY NF-based NGs. (4) Copper wires were attached to the electrodes, with conductive silver paste, to test the electrical output of these NGs. Before manufacturing and testing, the obtained NF-based NGs were subjected to corona poling under the corona charges of 12.0 kV at 25 °C for 30 min. (As reference substances, vertically aligned BCTZ NFs and their NGs were also synthesized by the above method) 2.3. Characterization techniques Thermo-gravimetric/differential scanning calorimetry (TG/DSC, SDTQ600, TA) was conducted to analyze the crystallization process of the BCTZY NFs and design their heat-treatment procedure. X-ray diffraction (XRD, X’pert Pro MPD, PANalytical) and Fourier transform infrared spectroscopy (FTIR, TENSOR Ⅱ, BRUKER) were performed to confirm the crystalline phase and molecular structure of the BCTZY NFs. Scanning electron microscopy (SEM, JSM 6700M, JEOF) and Fieldemission transmission electron microscopy (FETEM, JEM-2100F, JEOL) were used to determine the physical morphologies and micro-structures of the BCTZY NFs. Raman spectroscopy (LabRam HR Evolution, HORIBA) was performed to study the molecular structure and phase changes of the BCTZY NFs at different temperatures. An impedance analyzer (HP4294A, Agilent, America) and a ferroelectric tester (WS-2000, Radiant, America) were used to examine the dielectric and ferroelectric properties, respectively, of the BCTZY NFs/ PDMS. A quasi-static piezoelectric coefficient d33 testing meter (ZJ–3A, Institute of Acoustics, Chinese Academy of Sciences, China) was used to measure the d33 of the polarized BCTZY NFs/PDMS. An electrochemical workstation (CHI760E, America) was utilized to estimate the electrical output of the BCTZY NF–based NGs. FlexiFore Multi-Handle (MELF) System (Teksan) was utilized for monitoring the force (14 ± 1 N) during the output test.
2. Experimental 2.1. Preparation of BCTZY NFs Uniaxially aligned (Ba0.85Ca0.15)(Ti0.9Zr0.1)O3-0.2 mol%Y (BCTZY) NFs were fabricated by combining the sol-gel method and the electrospinning technique, as shown in Fig. 1(a–c). All the raw materials were analytically pure and used without any further purification. In accordance with the typical procedure, requisite quantities of ethanol (EtOH), acetic acid (HAc), and acetylacetone (Hacac) were first mixed. Tetrabutyl titanate, barium acetate, calcium acetate monohydrate, zirconium acetylacetonate, and yttrium acetate tetrahydrate were then sequentially added in stoichiometric ratios and continuously stirred at 40 °C for 12 h. Later, PVP was dissolved into the above solution. After constant stirring at 60 °C for 12 h, a yellow transparent solution was obtained and used as the electrospinning precursor. This precursor was loaded into a plastic syringe and spun by an electrospinning setup, where the voltage, pinhead-to-collector distance, spinning rate, and rotation speed were 15 kV, 15 cm, 0.9 mL/h, and 1000 rpm, respectively. Finally, the as-spun NFs were calcined at 450 °C for 1 h and sintered at 700 °C for 2 h to obtain the sintered BCTZY NFs.
3. Results and discussion 3.1. Low-temperature sintering properties In contrast to the temperature of 1500 °C used for BCTZ-based ceramic bulks by the conventional solid-state reaction method [20], the sintering temperature (∼700 °C) for the BCTZ-based NFs by electrospinning is much lower, which means a significant reduction in thermal energy consumption for the synthesis process. Thus, it is important to clarify the low-temperature sintering properties and mechanisms of the BCTZY NFs to optimize their synthesis path and minimize the thermal energy consumption during the sintering process. The TG/DSC measurement of the as-spun NFs was performed at a rate of 5 °C/min in air
2.2. Fabrication of BCTZY NF-based NGs The vertically aligned BCTZY NF-based NGs were fabricated following the steps in Fig. 1(d and e): (1) The sintered BCTZY NFs were penetrated by the PDMS mixtures (mPDMS: m(curing agent) = 10:1) and cured at 80 °C for 2 h. (2) The vertically aligned BCTZY NFs/PDMS
Fig. 1. Fabrication procedure schematic for the BCTZY-based NGs. 284
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products such as BaO, CaO, ZrOx, and TiOx. Although the XRD results at 430 °C displayed as little information as those at 230 °C, the changes in the molecular structures were quite apparent, as shown in Fig. 3(b), such as the vanishing of –CH2–/–CH3 and C]O. This suggested the conversion of various carbon chains into amorphous carbon and could well explain the black and smooth surfaces of the BCTZY NFs obtained at 430 °C. When the temperature reached 600 °C, a second peak and a continuous weight loss were observed in Fig. 2, which was mainly ascribed to the decarbonization of organics and the crystallization of inorganic NFs. The decarbonization process could be verified by the color change of the NFs, from black to white, as seen in Fig. 4. On the other hand, the formation of crystalline BaO, ZrO2, TiO2, BTO, and BCT at 600 °C could be proved by the XRD results in Fig. 3(a). Almost all peaks related to the carbon-chain structures in the FTIR spectra disappeared as most of the carbon was burnt off at this temperature. Nevertheless, 600 °C was not high enough for BCTZ-based NFs to form a pure perovskite structure. As the temperature increased further to 700 °C, the growth of inorganic phases and the formation of BCTZY NFs were completed, because no obvious weight loss was observed in the TG curve. In addition, a pure perovskite structure without any other phase could be easily observed at this temperature, in the XRD patterns. Fig. 3(b) exhibits a typical FTIR spectrum of the BCZT-based NFs, and their surfaces are quite smooth and flexible, as shown in Fig. 4. The low sintering temperature of 700 °C for pure-phase BCTZY NFs is primarily due to the low reaction potential barrier of the intermediate metal oxides and the low energy barrier for ion diffusion at the molecular level [22]. Based on the above discussion, it is clear that the low-temperature sintering process of BCTZY NFs generally contains five periods, including the evaporation of solvents, the decomposition of organic compounds, the formation of intermediate products, the decarbonisation of organics and crystallization of inorganics, and the formation and growth of pure perovskite structures. This mechanism can be suitable for all BCTZ-based NFs fabricated by electrospinning, and thus will minimize the thermal energy consumption and promote the sustainable development of the BCTZ-based NFs systems. However, the microstructures, phase transformations, and electrical properties may differ with the addition of various doping elements. Therefore, it will be interesting to find out how the addition of yttrium (Y) element, in this study, will impact the morphologies, phase transitions, and electrical performances of the BCTZ-based NFs.
Fig. 2. TG/DSC curves of the as-spun BCTZY NFs.
and their curves shown in Fig. 2 were divided into several temperature periods. The XRD and FTIR patterns were plotted in Fig. 3 to study the changes in the crystalline phases and molecular structures at these temperature periods. The photographs of NFs sintered at various temperatures and their corresponding schematic diagrams of the reaction products are presented in Fig. 4. As shown in Fig. 2, when the temperature was below 230 °C, the evaporation of solvents such as EtOH, HAc, and Hacac caused the weight loss (∼10%) and a small amount of heat adsorption. Hence, no obvious XRD characteristic peak was detected in Fig. 3(a) for the asspun NFs and NFs sintered at 230 °C. However, the FTIR spectra provided much structural information such as the details of –OH, –CH2–, C]O, and CeO bonds, since very few reactions occurred during this period and almost all chemicals maintained their original structures. Consequently, the morphology of BCTZY NFs sintered at 230 °C in Fig. 4 shows little change, expect for the slightly deeper color, in comparison with the as-spun NFs. It should be mentioned that some CaCO3 may be produced during this period, as the decomposition temperature of Ca (COOCH3)2 is 160 °C. A sharp exothermic peak and a huge weight loss of ∼ 30% were observed in the TG/DSC curves, in the temperature range of 230–430 °C, suggesting the decomposition of organics [21]. In this period, the PVP was decomposed into amorphous carbon, while the raw chemicals reacted with O2 and formed amorphous intermediate
Fig. 3. (a) XRD patterns and (b) FTIR spectra of BCTZY NFs sintered at various temperatures. 285
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Fig. 4. Photographs of BCTZY NFs sintered at various temperatures and schematic diagrams of the formation process of BCTZY NFs.
smoothness (less NFs with beads) of sintered BCTZY NFs in Fig. 5(d) are much better than those of the BCTZ NFs sintered at the same conditions in Fig. 5(b), demonstrating the potentially superior piezoelectric and ferroelectric properties of the former [11–13]. This happens probably because of the addition of the Y element, which is often regarded as a useful method to increase the spinnability, flexibility and stability of Zrbased NFs prepared by electrospinning [23,24]. Hence, adding a small amount of Y3+ into the BCTZ-based system can be conducive to the morphology and diameter distribution of the NFs, and might be advantageous for the potential electrical properties. Since SEM shows only the surface morphology and relative macrostructure of the BCTZY NFs, it is necessary to further confirm its micro and crystalline structures to deeply study the physical properties of the BCTZY NFs. Thus, FETEM and the related SAED characterizations were performed on the sintered BCTZY NFs, and the results are presented in Fig. 6(a and b). Apparently, the inter-planar distances of 0.283 nm and 0.405 nm marked in the inset of Fig. 6(a) match the [1 1 0] and [1 0 0] crystallographic planes, respectively, of the pure perovskite BCTZY,
3.2. Microstructures and phase transitions To identify the morphology and diameter distributions of the BCTZY NFs, representative SEM images before and after sintering were displayed in Fig. 5. All these patterns were compared with those of pure BCTZ NFs to investigate the potential effects of Y3+ on their morphologies. It can be seen that the as-spun BCTZ and BCTZY NFs were continuous and ultra-long without any pore or bead, in Fig. 5(a and c). An average diameter of ∼300 nm (BCTZ) or ∼350 nm (BCTZY) with smooth and uniform surfaces could be observed for both NFs, under high magnification, owing to the cylindrical geometries and amorphous characteristic of these fibers [21]. After being sintered at 700 °C, the surfaces of these NFs became relatively rough and partly contractive, and majority of them were reduced to 200–300 nm in diameter, as shown in Fig. 5(e and f). This phenomenon was primarily due to the volatilization of organics and the generation of crystalline grains during the sintering process. Nevertheless, it can be noticed that the alignment (more aligned NFs), continuity (less fractured NFs), and
Fig. 5. (a-d) SEM images and (e-f) size distributions of BCTZ and BCTZY NFs before and after sintering at 700 °C. 286
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Fig. 6. (a) FETEM, (c) SAED, (c) EDS, (d-j) STEM mapping images of the sintered BCTZY NFs.
which is consistent with the above XRD results. The good crystallinity of these NFs can be proved by the regular atomic arrangement. The SAED pattern in Fig. 6(b) shows lattice spacings of 0.400 nm, 0.401 nm, and 0.284 nm corresponding to the [0 0 1], [1 0 0], and [1 0 1] orientations, respectively, where the angle of ∼ 90° implies the formation of a quasi-tetragonal structure in this unit cell. Moreover, to identify the element ratios and distributions of the BCTZY NFs, the EDS spectrum and STEM mapping patterns of these NFs are displayed in Fig. 6(c–i). It is evident that all elements related to Ca, Ti, Zr, Ba, and Y are collected by EDS, and the homogeneous distribution of these elements can be observed in the mapping images. Although the characteristic peak of Y element is small and its corresponding color signal in Fig. 6(i) is very weak, both of them indicate the successful synthesis of the sintered BCTZY NFs. It is generally accepted that the asymmetry of body-centred ions is responsible for the piezoelectric properties of perovskites. The formation of electric dipoles (caused by the disorder of the body-centred ions) can result in the asymmetrical tetragonal phases of the BCZT-based structures being Raman-active [25]. Accordingly, the temperatureevolved Raman spectroscopy was carried out in the range from 20 °C to 300 °C, to investigate the phase transition and TC of the BCTZY NFs. As seen in Fig. 7, the intensities of various modes changed on increasing the temperature. The peaks at 184 and 264 cm−1 were ascribed to the [A1(TO)], while others such as the ones at 307, 521, and 726 cm−1 corresponded to [E(LO + TO),B1], [E(TO),A(TO)], and [E(LO),A(LO)],
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respectively [26]. Additionally, all signals belonged to the tetragonal (T) phase, and partly to the orthorhombic (O) phase (expect for the modes at 473 cm−1) [27]. When the measured temperature was 20 °C, the mode at 307 cm−1 was much sharper than the others, but no signal was detected at 287
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473 cm−1, manifesting that only the O phase existed in BCTZY NFs. As the temperature increased to 60 °C, the peak intensity at 307 cm−1 gradually decreased while a mode appeared at 473 cm−1, which indicated the appearance of the T phase and the coexistence of the O and T phases. With the temperature further increasing to 180 °C, the mode at 307 cm−1 disappeared, but the peak at 473 cm−1 became much sharper. This is a symbol of a single T phase forming at this period. Most of these above peaks merged with each other or even disappeared at 280 °C, and there was no obvious change on increasing the temperature further, to 300 °C. Those intensity and crystal structure changes versus temperature implied a diffused first-order ferroelectric to para-ferroelectric phase transition occurring, and thus the TC of BCTZY NFs was ∼280 °C which was slightly lower than that of the pure BCTZ NFs (∼300 °C) both in Fig. S2 and in Fu et al.’s study [26]. This was mainly on account of the donor doping effect of Y3+, by which the piezoelectric, ferroelectric, and electromechanical properties could be improved while TC was decreased [28]. However, what is more significant and worth noting is that, the TC of the BCTZY NFs in this study was much higher than that of BCZT-based piezoceramic bulks (∼90 °C). The reason for this huge difference could be explained by the continuous or discontinuous physical properties of various piezoelectric materials [29]. For ceramic bulks, the compact structure and micro-crystals demonstrated a less-diffusion phase transition, continuous physical properties, and further, a fast transformation of the thermal and electrical properties with change in temperatures. While NFs with nanoceramic crystals and 1D geometric boundaries illustrated a discontinuous physical property and a reduction of free energy, which further led to an enhancement of TC. From the above analysis, it can be concluded that introducing Y3+ into the BCTZ NFs system could not only improve their morphology and stability, but also maintain the favourable crystallinity and high TC for the BCTZ-based NFs. As a result, it is interesting to further study what influence will Y element have in the electrical properties of the obtained BCTZ-based NFs/PDMS and NGs.
impact on the dielectric properties of the BCTZ-based NFs. On the other hand, the εr values at ∼30 (BCTZ NFs/PDMS) and ∼50 (BCTZY NFs/ PDMS, ∼6.4 vol% of BCTZY) in Fig. 8(a) were not only higher than that at 2 of BTNF-R (7.6 vol% of randomly aligned BaTiO3 Nanofibers) and comparable with that at 40 of BTNF-V (7.6 vol% of vertically aligned BaTiO3 Nanofibers) reported by Yan et al [12], but also better than that at 22 for randomly aligned BT/PVDF NFs (20 vol% of BT) in Zhang et al’s study [30] and even that at ∼12 for the perpendicular epoxy/BT fiber composites (2 vol% of BT) in Ávila et al’s study [31]. To explain this phenomenon, we fabricated a randomly aligned BCTZY NFs/PDMS composite (BCTZY-R) by dispersing the ground NFs powder into PDMS and compared its cross section and vertical section with that of the vertically aligned BCTZY NFs/PDMS composite (BCTZY-V). As shown in Fig. 9(a–c), NFs of BCTZY-R in both the cross and vertical sections were disordered and discontinuous. However, many “white points” could be found in the cross section in Fig. 9(e), and well-aligned and continuous NFs were seen in Fig. 9(f), because of the artificial synthesis of BCTZY-V following the steps in Fig. 1. The oriented alignment and continuity of NFs are of great importance to the advancement of electrical properties, as the electric field along with the NFs alignment orientations will be more efficient [31]. In contrast to εr, the tanδ values of the BCTZ NFs/ PDMS and BCTZY NFs/PDMS were quite low, although both of them were higher than that of the pure PDMS in Fig. 8(b). It is known that a smaller tanδ value usually indicates a smaller energy loss, and subsequently, better dielectric and piezoelectric properties [32]. In addition, the promoting effect of the Y element in the BCTZ-based NFs system was obvious for its higher εr value and lower tanδ value compared to those of BCTZ NFs/PDMS. To clarify the relationship between the doping level of Y3+ and dielectric properties, the effect of Y3+ contents on εr and tanδ were systematically studied. As shown in Fig. 8(c), the εr value increased first with increasing the Y3+ contents from 0 to 0.20 mol%. It is known that too many doping contents always lower electrical properties. Thus, it is reasonable that the εr value slightly decreased when the content of Y3+ further increased to 0.25 mol%. Unlike εr, the change tendency of tanδ was opposite. Therefore, 0.2 mol % of Y3+ could be regarded as the optimal doping content in this study. Knowing that the addition of Y3+ can effectively enhance the dielectric properties, it was assumed that the ferroelectricity of BCTZY NFs/PDMS might also be improved, compared to that of BCTZ NFs/ PDMS composites. The P–E ferroelectric hysteresis loop curves at room temperature and 5 Hz in Fig. 10(a) fully verify this assumption. It can be seen that the curves of these two composites present relatively typical shapes that are similar to other literatures [33,34], justifying the reason to consider them as ferroelectric materials. Moreover, the
3.3. Enhanced electrical properties As typical parameters in dielectricity, the dielectric constant (εr) and dielectric loss (tanδ) were often used to assess the ability of various materials to be polarized. Fig. 8 presents the εr and tanδ of the neat PDMS, BCTZ NFs/PDMS (vertically aligned), and BCTZY NFs/PDMS (vertically aligned), as functions of frequency. It can be seen that theεr and tanδ values of pure PDMS were negligible in the frequency range of 40–106 Hz, which means that the introduction of PDMS may have little
Fig. 8. (a) Dielectric constant and (b) dielectric loss of PDMS, BCTZ NFs/PDMS, and BCTZY NFs/PDMS, as functions of frequency; (c) effect of Y3+ contents on dielectric properties at 10 kHz. 288
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Fig. 9. SEM images of cross section and vertical section of (a-c) randomly and vertically (d-f) aligned BCTZY NFs/PDMS.
remnant polarization (Pr) value of 0.151 μC/cm2 for BCTZY NFs/PDMS was apparently higher than that of 0.077 μC/cm2 for BCTZ NFs/PDMS, while the coercive field (EC) value of 3.20 kV/cm for the former was inversely lower than that of 4.05 kV/cm for the latter. These two comparative results adequately manifested the superior ferroelectric properties of the BCTZY system arising from the donor doping effect of Y3+. However, a common phenomenon that happens in many multiferroic materials was also observed in this work, wherein a low-quality hysteresis was presented in the P–E loops owing to the high conduction causing large leakage [35]. These above characteristics made BCTZbased NFs/PDMS typical lossy dielectrics, similar to many other multiferroics. It also should be mentioned that the much lower Pr of BCTZbased NFs/PDMS, compared to that of their corresponding ceramics, mainly stem from NFs having lower density but more defects. Piezoelectric properties, as one of the most significant characteristics, must be investigated for all piezoelectric materials. In this study, the d33 piezoelectric coefficients of various NFs/PDMS composites, before and after corona poling, were examined and summarized in Fig. 10(b). Similar to the dielectric results, pure PDMS showed almost
no piezoelectricity, regardless of whether being polarized or not. Nevertheless, with adding BCTZ-based NFs into PDMS, the d33 value increased significantly to 22.5 pC/N for BCTZ NFs/PDMS and 30.6 pC/ N for BCTZY NFs/PDMS, after corona poling, as shown in Fig. 10(b). That is, the sufficiently polarized BCTZ-based NFs, instead of PDMS, played the key role in the obtained piezoelectric properties that can be comparable with 25 pC/N of the PVDF/NKN (50 vol% of NKN) reported by Kato and Kakimoto [36]. Besides, the higher d33 of BCTZY NFs/ PDMS, compared to that of BCTZ NFs/PDMS, was ascribed to the positive effect of the Y element on the electrical properties for the BCTZbased system. In this case, flexible piezoelectric NGs based on vertically aligned BCTZ NFs/PDMS and BCTZY NFs/PDMS were fabricated, and their output voltage (VOC) and current (ISC) changes by finger tapping were tested to further illustrate their potential prospects in tiny energy harvesting. As plotted in Fig. 11, both positive and negative signals of the VOC and ISC were observed, where the positive and negative signals were generated from the tapping and releasing motion, respectively. An average VOC of 1.9 V and ISC of 48 nA for BCTZ NF-based NGs were
Fig. 10. (a) P–E ferroelectric hysteresis loops at 5 Hz and (b) Piezoelectric coefficient d33 before and after corona poling, of BCTZ NFs/PDMS and BCTZY NFs/PDMS. 289
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efficiently improved, mainly because of the vertical alignment of these NFs and the donor doping effect of Y3+. The average VOC of 3.0 V and ISC of 85 nA obtained for the vertically aligned BCTZY NF-based NGs by finger tapping indicated their potential applications in tiny energy harvesting.
achieved, which were higher than those of randomly aligned BCTZ NFbased NGs (0.8 V and 7 nA, respectively), obtained by repeatedly tapping and releasing fingers in Kou et al.’s study [37]. It is primarily because of the vertically aligned NFs accumulating more electric charges and being more compliant to mechanical stress [3 2 3],[38]. More intriguingly, the average VOC and ISC further increased to 3.0 V and 85 nA in Fig. 11(c and d), when adding a small amount of Y3+ into BCTZ. There were two possible factors contributing to this improvement. One was the donor doping effect of Y3+, which could effectively perfect the piezoelectricity and other related electrical properties, similar to the donor effect of Ba2+ in Ghasemian et al.’s study [39]. The other was that the Y element was often considered as an effective additive to promote the flexibility and stability of Zr-based NFs [23,24]. Thus, BCTZY NF-based NGs with high piezoelectricity could enhance the power generation and further be a power-storage microsystem for tiny energy harvesting.
Acknowledgments This work was supported by the Key Research Program of Frontier Sciences of Chinese Academy of Sciences (Grant No. QYZDJ-SSWJSC021), the Science and Technology Service Network Plan of Chinese Academy of Sciences (Grant No. KFJ-STS-ZDTP-040), and the Key Research Program of Chinese Academy of Sciences (Grant No. ZDRWZS-2018-1). Declarations of interest Author declares that there is no conflict of interest.
4. Conclusions
Appendix A. Supplementary material
In summary, lead-free BCTZY NFs and its vertically-aligned NGs were synthesized. The sintering path of BCTZY NFs was optimized and its thermal energy consumption was minimized by clarifying the lowtemperature sintering properties of these NFs. Owing to their discontinuous physical properties, a high TC of 280 °C was obtained for BCTZY NFs, which was ∼190 °C higher than that of BCTZ-based ceramic bulks. Furthermore, the electrical properties such as dielectricity, ferroelectricity, and piezoelectricity of BCTZY NFs/PDMS were
Supplementary data to this article can be found online at https:// doi.org/10.1016/j.apsusc.2018.10.229. References [1] Y. Qin, X. Wang, Z. Wang, Microfibre-nanowire hybrid structure for energy
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Full Length Article
Vertically-aligned lead-free BCTZY nanofibers with enhanced electrical properties for flexible piezoelectric nanogenerators ⁎
T
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Yinghong Wua,b, Fei Maa,b, Jingkui Qua, , Yang Luoa,b, Caixia Lva, Qiang Guoa, , Tao Qia a National Engineering Laboratory for Hydrometallurgical Cleaner Production Technology, Institute of Process Engineering, Chinese Academy of Sciences, Beijing 100190, PR China b University of Chinese Academy of Sciences, Beijing 100190, PR China
A R T I C LE I N FO
A B S T R A C T
Keywords: Lead-free BCTZY nanofibers Electrospinning Vertical alignment Piezoelectric nanogenerators
Flexible lead-free (Ba0.85Ca0.15)(Ti0.9Zr0.1)O3-0.2 mol%Y nanofibers (BCTZY NFs) were synthesized by electrospinning and their corresponding nanogenerators (NGs) with vertical alignments were fabricated. The lowtemperature sintering properties of BCTZY NFs were investigated, to optimize their synthesis path and minimize the thermal energy consumption during the sintering process. The continuity, flexibility, and stability of the BCTZ-based NFs were improved by adding Y3+. Moreover, the temperature-evolved Raman spectra displayed a high Curie temperature of 280 °C for BCTZY NFs, which was far higher than that of about 90 °C for BCTZ-based ceramic bulks, owing to the discontinuous physical property of NFs. The dielectric, ferroelectric, and piezoelectric properties of the vertically aligned BCTZY NFs/PDMS were estimated and compared with those of BCTZ NFs/PDMS composites, to verify the advantages of vertical alignments and the donor doping effect of Y3+. Vertically aligned BCTZY NF-based NGs showed an average VOC of 3.0 V and ISC of 85 nA by finger tapping, suggesting their potential applications in tiny energy harvesting.
1. Introduction Owing to their characteristic of transforming mechanical energy into electrical energy (and vice versa), piezoelectric materials have been regarded as one of the most promising energy suppliers in actuators, sensors, and energy harvesting devices [1–3]. Piezoelectric nanomaterials, especially for flexible nanofibers (NFs) and nanowires (NWs), are currently attracting intensive attention because of their unique features (like lightweight nature, flexibility, and high mechanical robustness) that are beneficial for developing miniaturized devices, such as flexible piezoelectric nanogenerators (NGs) that can easily harvest energy from negligible vibrations, small air flow, as well as tiny body movements [4–6]. Therefore, it is of great importance to find a suitable method to synthesize flexible NFs and NF-based NGs. Among various NFs synthesis techniques, electrospinning is considered as one of the most efficient methods due to the following advantages: Apart from versatility, economy, and the capability of operating at room temperature, it can spin ultra-fine and nanoscale organic/ inorganic fibers and simplify the measurement procedure by directly fabricating piezoelectric NFs on substrates [7–9]. More significantly, the uniaxially aligned NFs can be obtained by increasing the rotation speed of the roller during the electrospinning process (Fig. S1) [10–12].
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This is very useful for the fabrication of vertically aligned NGs with superior piezoelectric properties, as reported by some literatures [11–13]. For instance, a pressure of 0.1 Pa was sufficient to generate an open-circuit voltage of 0.1 mV for the well-aligned PVDF-TrFE NFs according to Persato et al.’s study [11]. Furthermore, the well-aligned Pb(Zr,Ti)O3 (PZT) NWs prepared by Xu et al. showed an output voltage of 0.7 V and a current density of 4 mA/cm−2 with an average power density of 2.8 mW/cm−3 [13]. In fact, PZT has even played the dominating role in the applications of piezoelectric NFs and NWs, because of its outstanding piezoelectric properties and high output voltages [14,15]. However, it is necessary to develop lead-free NFs substitutes, owing to the huge amount of toxic PbO violating the principles of green and sustainable development [16]. It was found that the vertically-aligned lead-free BaTiO3 (BT) NF-based NGs exhibited much better piezoelectric properties than NGs with randomly or horizontally aligned BT NFs, since the achieved piezoelectric potentials of the former being transmitted to electrodes more efficiently than those of the latter [12]. Moreover, (Ba,Ca)(Ti,Zr)O3 (BCTZ) has attracted much attention in recent years and is considered one of the most potential alternatives, as its piezoelectric coefficient d33 has been reported to be as high as that of PZT and much higher than that of BT [17]. Hence, it is interesting to
Corresponding authors. E-mail address: [email protected] (J. Qu).
https://doi.org/10.1016/j.apsusc.2018.10.229 Received 13 July 2018; Received in revised form 24 September 2018; Accepted 28 October 2018 Available online 29 October 2018 0169-4332/ © 2018 Elsevier B.V. All rights reserved.
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find out how vertically-aligned BCTZ NF-based NGs will affect the electrical characteristics. Besides, many studies have reported that the introduction of Cu2+ [18], Li+ [19], and Y3+ [20] into BCTZ ceramics can perfect their comprehensive properties like the sintering properties, the Curie temperature (TC), and the electrical properties. Up to now, however, few literatures have investigated the effect of the doping elements on the enhanced performance of the BCTZ-based NFs and NGs. Therefore, in this study, uniaxially aligned BCTZY NFs were prepared by electrospinning and their corresponding vertically-aligned NGs were further manufactured. The low-temperature sintering properties of BCTZY NFs were investigated by TG/DSC, XRD and FTIR, to optimize their sintering process and minimize the thermal energy consumption. The morphology, flexibility, and crystallinity of these sintered NFs were examined by SEM and FETEM measurements. The phase transition and TC of the BCTZY NFs were confirmed by the temperature-evolved Raman spectra. The dielectric, ferroelectric, and piezoelectric properties of the vertically aligned BCTZY NFs/PDMS and the electrical output of its NGs by finger tapping were evaluated to verify their potential application in tiny energy harvesting.
composite was obtained by cutting these NFs along the red region in Fig. 1(d) and rotating it 90° clockwise. (3) Two ITO-PET films used as electrodes were attached to both ends of the NFs/PDMS to form BCTZY NF-based NGs. (4) Copper wires were attached to the electrodes, with conductive silver paste, to test the electrical output of these NGs. Before manufacturing and testing, the obtained NF-based NGs were subjected to corona poling under the corona charges of 12.0 kV at 25 °C for 30 min. (As reference substances, vertically aligned BCTZ NFs and their NGs were also synthesized by the above method) 2.3. Characterization techniques Thermo-gravimetric/differential scanning calorimetry (TG/DSC, SDTQ600, TA) was conducted to analyze the crystallization process of the BCTZY NFs and design their heat-treatment procedure. X-ray diffraction (XRD, X’pert Pro MPD, PANalytical) and Fourier transform infrared spectroscopy (FTIR, TENSOR Ⅱ, BRUKER) were performed to confirm the crystalline phase and molecular structure of the BCTZY NFs. Scanning electron microscopy (SEM, JSM 6700M, JEOF) and Fieldemission transmission electron microscopy (FETEM, JEM-2100F, JEOL) were used to determine the physical morphologies and micro-structures of the BCTZY NFs. Raman spectroscopy (LabRam HR Evolution, HORIBA) was performed to study the molecular structure and phase changes of the BCTZY NFs at different temperatures. An impedance analyzer (HP4294A, Agilent, America) and a ferroelectric tester (WS-2000, Radiant, America) were used to examine the dielectric and ferroelectric properties, respectively, of the BCTZY NFs/ PDMS. A quasi-static piezoelectric coefficient d33 testing meter (ZJ–3A, Institute of Acoustics, Chinese Academy of Sciences, China) was used to measure the d33 of the polarized BCTZY NFs/PDMS. An electrochemical workstation (CHI760E, America) was utilized to estimate the electrical output of the BCTZY NF–based NGs. FlexiFore Multi-Handle (MELF) System (Teksan) was utilized for monitoring the force (14 ± 1 N) during the output test.
2. Experimental 2.1. Preparation of BCTZY NFs Uniaxially aligned (Ba0.85Ca0.15)(Ti0.9Zr0.1)O3-0.2 mol%Y (BCTZY) NFs were fabricated by combining the sol-gel method and the electrospinning technique, as shown in Fig. 1(a–c). All the raw materials were analytically pure and used without any further purification. In accordance with the typical procedure, requisite quantities of ethanol (EtOH), acetic acid (HAc), and acetylacetone (Hacac) were first mixed. Tetrabutyl titanate, barium acetate, calcium acetate monohydrate, zirconium acetylacetonate, and yttrium acetate tetrahydrate were then sequentially added in stoichiometric ratios and continuously stirred at 40 °C for 12 h. Later, PVP was dissolved into the above solution. After constant stirring at 60 °C for 12 h, a yellow transparent solution was obtained and used as the electrospinning precursor. This precursor was loaded into a plastic syringe and spun by an electrospinning setup, where the voltage, pinhead-to-collector distance, spinning rate, and rotation speed were 15 kV, 15 cm, 0.9 mL/h, and 1000 rpm, respectively. Finally, the as-spun NFs were calcined at 450 °C for 1 h and sintered at 700 °C for 2 h to obtain the sintered BCTZY NFs.
3. Results and discussion 3.1. Low-temperature sintering properties In contrast to the temperature of 1500 °C used for BCTZ-based ceramic bulks by the conventional solid-state reaction method [20], the sintering temperature (∼700 °C) for the BCTZ-based NFs by electrospinning is much lower, which means a significant reduction in thermal energy consumption for the synthesis process. Thus, it is important to clarify the low-temperature sintering properties and mechanisms of the BCTZY NFs to optimize their synthesis path and minimize the thermal energy consumption during the sintering process. The TG/DSC measurement of the as-spun NFs was performed at a rate of 5 °C/min in air
2.2. Fabrication of BCTZY NF-based NGs The vertically aligned BCTZY NF-based NGs were fabricated following the steps in Fig. 1(d and e): (1) The sintered BCTZY NFs were penetrated by the PDMS mixtures (mPDMS: m(curing agent) = 10:1) and cured at 80 °C for 2 h. (2) The vertically aligned BCTZY NFs/PDMS
Fig. 1. Fabrication procedure schematic for the BCTZY-based NGs. 284
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products such as BaO, CaO, ZrOx, and TiOx. Although the XRD results at 430 °C displayed as little information as those at 230 °C, the changes in the molecular structures were quite apparent, as shown in Fig. 3(b), such as the vanishing of –CH2–/–CH3 and C]O. This suggested the conversion of various carbon chains into amorphous carbon and could well explain the black and smooth surfaces of the BCTZY NFs obtained at 430 °C. When the temperature reached 600 °C, a second peak and a continuous weight loss were observed in Fig. 2, which was mainly ascribed to the decarbonization of organics and the crystallization of inorganic NFs. The decarbonization process could be verified by the color change of the NFs, from black to white, as seen in Fig. 4. On the other hand, the formation of crystalline BaO, ZrO2, TiO2, BTO, and BCT at 600 °C could be proved by the XRD results in Fig. 3(a). Almost all peaks related to the carbon-chain structures in the FTIR spectra disappeared as most of the carbon was burnt off at this temperature. Nevertheless, 600 °C was not high enough for BCTZ-based NFs to form a pure perovskite structure. As the temperature increased further to 700 °C, the growth of inorganic phases and the formation of BCTZY NFs were completed, because no obvious weight loss was observed in the TG curve. In addition, a pure perovskite structure without any other phase could be easily observed at this temperature, in the XRD patterns. Fig. 3(b) exhibits a typical FTIR spectrum of the BCZT-based NFs, and their surfaces are quite smooth and flexible, as shown in Fig. 4. The low sintering temperature of 700 °C for pure-phase BCTZY NFs is primarily due to the low reaction potential barrier of the intermediate metal oxides and the low energy barrier for ion diffusion at the molecular level [22]. Based on the above discussion, it is clear that the low-temperature sintering process of BCTZY NFs generally contains five periods, including the evaporation of solvents, the decomposition of organic compounds, the formation of intermediate products, the decarbonisation of organics and crystallization of inorganics, and the formation and growth of pure perovskite structures. This mechanism can be suitable for all BCTZ-based NFs fabricated by electrospinning, and thus will minimize the thermal energy consumption and promote the sustainable development of the BCTZ-based NFs systems. However, the microstructures, phase transformations, and electrical properties may differ with the addition of various doping elements. Therefore, it will be interesting to find out how the addition of yttrium (Y) element, in this study, will impact the morphologies, phase transitions, and electrical performances of the BCTZ-based NFs.
Fig. 2. TG/DSC curves of the as-spun BCTZY NFs.
and their curves shown in Fig. 2 were divided into several temperature periods. The XRD and FTIR patterns were plotted in Fig. 3 to study the changes in the crystalline phases and molecular structures at these temperature periods. The photographs of NFs sintered at various temperatures and their corresponding schematic diagrams of the reaction products are presented in Fig. 4. As shown in Fig. 2, when the temperature was below 230 °C, the evaporation of solvents such as EtOH, HAc, and Hacac caused the weight loss (∼10%) and a small amount of heat adsorption. Hence, no obvious XRD characteristic peak was detected in Fig. 3(a) for the asspun NFs and NFs sintered at 230 °C. However, the FTIR spectra provided much structural information such as the details of –OH, –CH2–, C]O, and CeO bonds, since very few reactions occurred during this period and almost all chemicals maintained their original structures. Consequently, the morphology of BCTZY NFs sintered at 230 °C in Fig. 4 shows little change, expect for the slightly deeper color, in comparison with the as-spun NFs. It should be mentioned that some CaCO3 may be produced during this period, as the decomposition temperature of Ca (COOCH3)2 is 160 °C. A sharp exothermic peak and a huge weight loss of ∼ 30% were observed in the TG/DSC curves, in the temperature range of 230–430 °C, suggesting the decomposition of organics [21]. In this period, the PVP was decomposed into amorphous carbon, while the raw chemicals reacted with O2 and formed amorphous intermediate
Fig. 3. (a) XRD patterns and (b) FTIR spectra of BCTZY NFs sintered at various temperatures. 285
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Fig. 4. Photographs of BCTZY NFs sintered at various temperatures and schematic diagrams of the formation process of BCTZY NFs.
smoothness (less NFs with beads) of sintered BCTZY NFs in Fig. 5(d) are much better than those of the BCTZ NFs sintered at the same conditions in Fig. 5(b), demonstrating the potentially superior piezoelectric and ferroelectric properties of the former [11–13]. This happens probably because of the addition of the Y element, which is often regarded as a useful method to increase the spinnability, flexibility and stability of Zrbased NFs prepared by electrospinning [23,24]. Hence, adding a small amount of Y3+ into the BCTZ-based system can be conducive to the morphology and diameter distribution of the NFs, and might be advantageous for the potential electrical properties. Since SEM shows only the surface morphology and relative macrostructure of the BCTZY NFs, it is necessary to further confirm its micro and crystalline structures to deeply study the physical properties of the BCTZY NFs. Thus, FETEM and the related SAED characterizations were performed on the sintered BCTZY NFs, and the results are presented in Fig. 6(a and b). Apparently, the inter-planar distances of 0.283 nm and 0.405 nm marked in the inset of Fig. 6(a) match the [1 1 0] and [1 0 0] crystallographic planes, respectively, of the pure perovskite BCTZY,
3.2. Microstructures and phase transitions To identify the morphology and diameter distributions of the BCTZY NFs, representative SEM images before and after sintering were displayed in Fig. 5. All these patterns were compared with those of pure BCTZ NFs to investigate the potential effects of Y3+ on their morphologies. It can be seen that the as-spun BCTZ and BCTZY NFs were continuous and ultra-long without any pore or bead, in Fig. 5(a and c). An average diameter of ∼300 nm (BCTZ) or ∼350 nm (BCTZY) with smooth and uniform surfaces could be observed for both NFs, under high magnification, owing to the cylindrical geometries and amorphous characteristic of these fibers [21]. After being sintered at 700 °C, the surfaces of these NFs became relatively rough and partly contractive, and majority of them were reduced to 200–300 nm in diameter, as shown in Fig. 5(e and f). This phenomenon was primarily due to the volatilization of organics and the generation of crystalline grains during the sintering process. Nevertheless, it can be noticed that the alignment (more aligned NFs), continuity (less fractured NFs), and
Fig. 5. (a-d) SEM images and (e-f) size distributions of BCTZ and BCTZY NFs before and after sintering at 700 °C. 286
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Fig. 6. (a) FETEM, (c) SAED, (c) EDS, (d-j) STEM mapping images of the sintered BCTZY NFs.
which is consistent with the above XRD results. The good crystallinity of these NFs can be proved by the regular atomic arrangement. The SAED pattern in Fig. 6(b) shows lattice spacings of 0.400 nm, 0.401 nm, and 0.284 nm corresponding to the [0 0 1], [1 0 0], and [1 0 1] orientations, respectively, where the angle of ∼ 90° implies the formation of a quasi-tetragonal structure in this unit cell. Moreover, to identify the element ratios and distributions of the BCTZY NFs, the EDS spectrum and STEM mapping patterns of these NFs are displayed in Fig. 6(c–i). It is evident that all elements related to Ca, Ti, Zr, Ba, and Y are collected by EDS, and the homogeneous distribution of these elements can be observed in the mapping images. Although the characteristic peak of Y element is small and its corresponding color signal in Fig. 6(i) is very weak, both of them indicate the successful synthesis of the sintered BCTZY NFs. It is generally accepted that the asymmetry of body-centred ions is responsible for the piezoelectric properties of perovskites. The formation of electric dipoles (caused by the disorder of the body-centred ions) can result in the asymmetrical tetragonal phases of the BCZT-based structures being Raman-active [25]. Accordingly, the temperatureevolved Raman spectroscopy was carried out in the range from 20 °C to 300 °C, to investigate the phase transition and TC of the BCTZY NFs. As seen in Fig. 7, the intensities of various modes changed on increasing the temperature. The peaks at 184 and 264 cm−1 were ascribed to the [A1(TO)], while others such as the ones at 307, 521, and 726 cm−1 corresponded to [E(LO + TO),B1], [E(TO),A(TO)], and [E(LO),A(LO)],
20°C 60°C 100°C 140°C 180°C 220°C 260°C 280°C 300°C
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5000 4000 3000 2000
184 264 307 521 473
1000 0
200
400
726 600
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Fig. 7. Temperature evolved Raman spectra of the BCTZY NFs.
respectively [26]. Additionally, all signals belonged to the tetragonal (T) phase, and partly to the orthorhombic (O) phase (expect for the modes at 473 cm−1) [27]. When the measured temperature was 20 °C, the mode at 307 cm−1 was much sharper than the others, but no signal was detected at 287
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473 cm−1, manifesting that only the O phase existed in BCTZY NFs. As the temperature increased to 60 °C, the peak intensity at 307 cm−1 gradually decreased while a mode appeared at 473 cm−1, which indicated the appearance of the T phase and the coexistence of the O and T phases. With the temperature further increasing to 180 °C, the mode at 307 cm−1 disappeared, but the peak at 473 cm−1 became much sharper. This is a symbol of a single T phase forming at this period. Most of these above peaks merged with each other or even disappeared at 280 °C, and there was no obvious change on increasing the temperature further, to 300 °C. Those intensity and crystal structure changes versus temperature implied a diffused first-order ferroelectric to para-ferroelectric phase transition occurring, and thus the TC of BCTZY NFs was ∼280 °C which was slightly lower than that of the pure BCTZ NFs (∼300 °C) both in Fig. S2 and in Fu et al.’s study [26]. This was mainly on account of the donor doping effect of Y3+, by which the piezoelectric, ferroelectric, and electromechanical properties could be improved while TC was decreased [28]. However, what is more significant and worth noting is that, the TC of the BCTZY NFs in this study was much higher than that of BCZT-based piezoceramic bulks (∼90 °C). The reason for this huge difference could be explained by the continuous or discontinuous physical properties of various piezoelectric materials [29]. For ceramic bulks, the compact structure and micro-crystals demonstrated a less-diffusion phase transition, continuous physical properties, and further, a fast transformation of the thermal and electrical properties with change in temperatures. While NFs with nanoceramic crystals and 1D geometric boundaries illustrated a discontinuous physical property and a reduction of free energy, which further led to an enhancement of TC. From the above analysis, it can be concluded that introducing Y3+ into the BCTZ NFs system could not only improve their morphology and stability, but also maintain the favourable crystallinity and high TC for the BCTZ-based NFs. As a result, it is interesting to further study what influence will Y element have in the electrical properties of the obtained BCTZ-based NFs/PDMS and NGs.
impact on the dielectric properties of the BCTZ-based NFs. On the other hand, the εr values at ∼30 (BCTZ NFs/PDMS) and ∼50 (BCTZY NFs/ PDMS, ∼6.4 vol% of BCTZY) in Fig. 8(a) were not only higher than that at 2 of BTNF-R (7.6 vol% of randomly aligned BaTiO3 Nanofibers) and comparable with that at 40 of BTNF-V (7.6 vol% of vertically aligned BaTiO3 Nanofibers) reported by Yan et al [12], but also better than that at 22 for randomly aligned BT/PVDF NFs (20 vol% of BT) in Zhang et al’s study [30] and even that at ∼12 for the perpendicular epoxy/BT fiber composites (2 vol% of BT) in Ávila et al’s study [31]. To explain this phenomenon, we fabricated a randomly aligned BCTZY NFs/PDMS composite (BCTZY-R) by dispersing the ground NFs powder into PDMS and compared its cross section and vertical section with that of the vertically aligned BCTZY NFs/PDMS composite (BCTZY-V). As shown in Fig. 9(a–c), NFs of BCTZY-R in both the cross and vertical sections were disordered and discontinuous. However, many “white points” could be found in the cross section in Fig. 9(e), and well-aligned and continuous NFs were seen in Fig. 9(f), because of the artificial synthesis of BCTZY-V following the steps in Fig. 1. The oriented alignment and continuity of NFs are of great importance to the advancement of electrical properties, as the electric field along with the NFs alignment orientations will be more efficient [31]. In contrast to εr, the tanδ values of the BCTZ NFs/ PDMS and BCTZY NFs/PDMS were quite low, although both of them were higher than that of the pure PDMS in Fig. 8(b). It is known that a smaller tanδ value usually indicates a smaller energy loss, and subsequently, better dielectric and piezoelectric properties [32]. In addition, the promoting effect of the Y element in the BCTZ-based NFs system was obvious for its higher εr value and lower tanδ value compared to those of BCTZ NFs/PDMS. To clarify the relationship between the doping level of Y3+ and dielectric properties, the effect of Y3+ contents on εr and tanδ were systematically studied. As shown in Fig. 8(c), the εr value increased first with increasing the Y3+ contents from 0 to 0.20 mol%. It is known that too many doping contents always lower electrical properties. Thus, it is reasonable that the εr value slightly decreased when the content of Y3+ further increased to 0.25 mol%. Unlike εr, the change tendency of tanδ was opposite. Therefore, 0.2 mol % of Y3+ could be regarded as the optimal doping content in this study. Knowing that the addition of Y3+ can effectively enhance the dielectric properties, it was assumed that the ferroelectricity of BCTZY NFs/PDMS might also be improved, compared to that of BCTZ NFs/ PDMS composites. The P–E ferroelectric hysteresis loop curves at room temperature and 5 Hz in Fig. 10(a) fully verify this assumption. It can be seen that the curves of these two composites present relatively typical shapes that are similar to other literatures [33,34], justifying the reason to consider them as ferroelectric materials. Moreover, the
3.3. Enhanced electrical properties As typical parameters in dielectricity, the dielectric constant (εr) and dielectric loss (tanδ) were often used to assess the ability of various materials to be polarized. Fig. 8 presents the εr and tanδ of the neat PDMS, BCTZ NFs/PDMS (vertically aligned), and BCTZY NFs/PDMS (vertically aligned), as functions of frequency. It can be seen that theεr and tanδ values of pure PDMS were negligible in the frequency range of 40–106 Hz, which means that the introduction of PDMS may have little
Fig. 8. (a) Dielectric constant and (b) dielectric loss of PDMS, BCTZ NFs/PDMS, and BCTZY NFs/PDMS, as functions of frequency; (c) effect of Y3+ contents on dielectric properties at 10 kHz. 288
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Fig. 9. SEM images of cross section and vertical section of (a-c) randomly and vertically (d-f) aligned BCTZY NFs/PDMS.
remnant polarization (Pr) value of 0.151 μC/cm2 for BCTZY NFs/PDMS was apparently higher than that of 0.077 μC/cm2 for BCTZ NFs/PDMS, while the coercive field (EC) value of 3.20 kV/cm for the former was inversely lower than that of 4.05 kV/cm for the latter. These two comparative results adequately manifested the superior ferroelectric properties of the BCTZY system arising from the donor doping effect of Y3+. However, a common phenomenon that happens in many multiferroic materials was also observed in this work, wherein a low-quality hysteresis was presented in the P–E loops owing to the high conduction causing large leakage [35]. These above characteristics made BCTZbased NFs/PDMS typical lossy dielectrics, similar to many other multiferroics. It also should be mentioned that the much lower Pr of BCTZbased NFs/PDMS, compared to that of their corresponding ceramics, mainly stem from NFs having lower density but more defects. Piezoelectric properties, as one of the most significant characteristics, must be investigated for all piezoelectric materials. In this study, the d33 piezoelectric coefficients of various NFs/PDMS composites, before and after corona poling, were examined and summarized in Fig. 10(b). Similar to the dielectric results, pure PDMS showed almost
no piezoelectricity, regardless of whether being polarized or not. Nevertheless, with adding BCTZ-based NFs into PDMS, the d33 value increased significantly to 22.5 pC/N for BCTZ NFs/PDMS and 30.6 pC/ N for BCTZY NFs/PDMS, after corona poling, as shown in Fig. 10(b). That is, the sufficiently polarized BCTZ-based NFs, instead of PDMS, played the key role in the obtained piezoelectric properties that can be comparable with 25 pC/N of the PVDF/NKN (50 vol% of NKN) reported by Kato and Kakimoto [36]. Besides, the higher d33 of BCTZY NFs/ PDMS, compared to that of BCTZ NFs/PDMS, was ascribed to the positive effect of the Y element on the electrical properties for the BCTZbased system. In this case, flexible piezoelectric NGs based on vertically aligned BCTZ NFs/PDMS and BCTZY NFs/PDMS were fabricated, and their output voltage (VOC) and current (ISC) changes by finger tapping were tested to further illustrate their potential prospects in tiny energy harvesting. As plotted in Fig. 11, both positive and negative signals of the VOC and ISC were observed, where the positive and negative signals were generated from the tapping and releasing motion, respectively. An average VOC of 1.9 V and ISC of 48 nA for BCTZ NF-based NGs were
Fig. 10. (a) P–E ferroelectric hysteresis loops at 5 Hz and (b) Piezoelectric coefficient d33 before and after corona poling, of BCTZ NFs/PDMS and BCTZY NFs/PDMS. 289
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efficiently improved, mainly because of the vertical alignment of these NFs and the donor doping effect of Y3+. The average VOC of 3.0 V and ISC of 85 nA obtained for the vertically aligned BCTZY NF-based NGs by finger tapping indicated their potential applications in tiny energy harvesting.
achieved, which were higher than those of randomly aligned BCTZ NFbased NGs (0.8 V and 7 nA, respectively), obtained by repeatedly tapping and releasing fingers in Kou et al.’s study [37]. It is primarily because of the vertically aligned NFs accumulating more electric charges and being more compliant to mechanical stress [3 2 3],[38]. More intriguingly, the average VOC and ISC further increased to 3.0 V and 85 nA in Fig. 11(c and d), when adding a small amount of Y3+ into BCTZ. There were two possible factors contributing to this improvement. One was the donor doping effect of Y3+, which could effectively perfect the piezoelectricity and other related electrical properties, similar to the donor effect of Ba2+ in Ghasemian et al.’s study [39]. The other was that the Y element was often considered as an effective additive to promote the flexibility and stability of Zr-based NFs [23,24]. Thus, BCTZY NF-based NGs with high piezoelectricity could enhance the power generation and further be a power-storage microsystem for tiny energy harvesting.
Acknowledgments This work was supported by the Key Research Program of Frontier Sciences of Chinese Academy of Sciences (Grant No. QYZDJ-SSWJSC021), the Science and Technology Service Network Plan of Chinese Academy of Sciences (Grant No. KFJ-STS-ZDTP-040), and the Key Research Program of Chinese Academy of Sciences (Grant No. ZDRWZS-2018-1). Declarations of interest Author declares that there is no conflict of interest.
4. Conclusions
Appendix A. Supplementary material
In summary, lead-free BCTZY NFs and its vertically-aligned NGs were synthesized. The sintering path of BCTZY NFs was optimized and its thermal energy consumption was minimized by clarifying the lowtemperature sintering properties of these NFs. Owing to their discontinuous physical properties, a high TC of 280 °C was obtained for BCTZY NFs, which was ∼190 °C higher than that of BCTZ-based ceramic bulks. Furthermore, the electrical properties such as dielectricity, ferroelectricity, and piezoelectricity of BCTZY NFs/PDMS were
Supplementary data to this article can be found online at https:// doi.org/10.1016/j.apsusc.2018.10.229. References [1] Y. Qin, X. Wang, Z. Wang, Microfibre-nanowire hybrid structure for energy
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