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Jul 27, 2018 - High breakdown strength and outstanding piezoelectric performance in flexible. PVDF based percolative nanocomposites through the ...
Accepted Manuscript High breakdown strength and outstanding piezoelectric performance in flexible PVDF based percolative nanocomposites through the synergistic effect of topological-structure and composition modulations Lu Yang, Qiuying Zhao, Ying Hou, Rujie Sun, Meng Cheng, Mingxia Shen, Shaohua Zeng, Hongli Ji, Jinhao Qiu PII: DOI: Reference:

S1359-835X(18)30309-9 https://doi.org/10.1016/j.compositesa.2018.07.039 JCOMA 5134

To appear in:

Composites: Part A

Received Date: Revised Date: Accepted Date:

27 May 2018 27 July 2018 31 July 2018

Please cite this article as: Yang, L., Zhao, Q., Hou, Y., Sun, R., Cheng, M., Shen, M., Zeng, S., Ji, H., Qiu, J., High breakdown strength and outstanding piezoelectric performance in flexible PVDF based percolative nanocomposites through the synergistic effect of topological-structure and composition modulations, Composites: Part A (2018), doi: https://doi.org/10.1016/j.compositesa.2018.07.039

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High breakdown strength and outstanding piezoelectric performance in flexible PVDF based percolative nanocomposites through the synergistic effect of topological-structure and composition modulations Lu Yanga,b,Qiuying Zhaob,Ying Hou c, Rujie Sun d,Meng Chengb,Mingxia Shena*, Shaohua Zenga,Hongli Jib,e,Jinhao Qiub,e* a

College of Mechanics and Materials, Hohai University, Nanjing 210098, China State Key Laboratory of Mechanics and Control of Mechanical Structures, Nanjing University of Aeronautics and Astronautics, Nanjing 210016, China c Department of Physics, College of Science, East China University of Science and Technology, Shanghai 200237, China d Bristol Composites Insitute (ACCIS), University of Bristol, BS8 1TR, UK e College of Aerospace Engineering, Nanjing University of Aeronautics and Astronautics, Nanjing 210016, China b

Abstract Over decades, the fabricaion of flexible poly(vinylidene fluoride) (PVDF) based percolative nanocomposites with high piezoelectric performance is of great concern from both academia and industry. However, the issue of sharply declined breakdown strength in percolative nanocomposites poses an obstacle to realizing the full potential of conductive nanofillers in enhancing piezoelectricity. Herein, we demonstrated that through proper topological structure and composition modulations, notably improved breakdown strength and piezoelectric performance can be achieved in PVDF based percolative nanocomposites. By constructing a sandwiched structure where a layer of high breakdown strength is intercalated between layers containing high

*

Corresponding author. Tel.: Fax: +862584891123. E-mail address: [email protected] (J.H. Qiu), [email protected] (M.X. Shen).

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content (near percolation threshold) of conductive nanofillers, the breakdown strength of overall nanocomposites is significantly strengthened and thus allows for sufficient poling of outer layers, fulfilling the great potential of conductive nanofillers in yielding piezoelectricity enhancement. A super high piezoelectric coefficient d33 of 48 pC/N is obtained in the optimized sandwich nanocomposites.

1. Introduction

Over decades, piezoelectric materials have trigged enormous interest due to their potential applications in sensors, transducers and energy harvesters [1]. Among the various piezoelectric materials, piezoelectric poly(vinylindene fluoride) (PVDF) is of great concern due to the advantages of easy processing, cost effectiveness and light weight, which enable the fabrication of flexible, mechanically durable and portable electronic devices based on piezoelectric materials [2]. However, the piezoelectric response of PVDF is generally low (i.e.,piezoelectric coefficient d33=18pC/N), which largely hinders its applications in modern electronics requiring superior performance [3]. To enhance the piezoelectricity of PVDF, one important strategy is to introduce conductive nanoparticles represented by carbon nanotubes (CNTs) into PVDF [4-8]. The existence of conductive nanoparticles in PVDF matrix can induce interface effect and facilitate the piezoelectric phase formation, leading to enhanced piezoelectric response [8-10]. For example, the CNTs/PVDF nanocomposites were reported to have a piezoelectric coefficient d33 exceeding 25pC/N [8,9].

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Apart from the strong interfacial coupling effect, the introduced conductive nanofillers can also lead to concentrated local electric field, allowing for a more efficient polarization under relatively low electric field [9]. Thus, compared with pure PVDF, a lower poling electric field is required to induce a high piezoelectricity for the PVDF based percolative nanocomposites. However, on the other hand, the existence of conductive nanofillers may also cause a quick or easy breakdown failure in nanocomposites due to the easily formed conductive paths and serve local electric field concentration [10]. In fact, for the PVDF based percolative nanocomposites designed for piezoelectric application, the nanofillers loading is always strictly limited to far below the percolation threshold (around 1wt.%) to avoid the easy breakdown failure. For instance, it can be concluded from the published literatures that the content of conductive nanofillers represented by CNTs is generally below 0.2 wt.% in the piezoelectric PVDF based percolative nanocomposites [8-10]. Such tiny loading level makes it hard to realize the full potential of conductive nanofillers in enhancing the piezoelectricity of PVDF. To fulfill the potential of conductive nanofillers in inducing piezoelectric increment, the key point lies in improving the breakdown strength of nanocomposites. In addressing the quickly declined breakdown strength issue, various approaches including core-shell nanostructure and nanofiller alignment have been proposed, among which the strategy of core-shell nanostructure is especially popular [11-13]. By introducing a buffer layer (TiO2, Ag, etc.) outside conductive fillers to construct a core-shell nanostructure, it is possible to confine the mobility of free electrons, mitigate local electric field concentration, and reduce the probability of forming conductive network, resulting in enhanced breakdown strength [10,12]. However, it is worthy noting that the

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introduced buffer layers may also lower the efficiency of conductive nanofillers in inducing piezoelectric performance enhancement, particularly under low poling electric fields. This means that,as compared with the pristine conductive nanofillers, a much higher poling electric fields may be required for the core-shell nanofillers to produce a similar level of piezoelectricity, which in turn pose an obstacle to obtain superior piezoelectricity. Thus, exploring an effective strategy to simultaneously enhance the breakdown strength of percolative nanocomposites while retain the efficiency of conductive nanofillers in strengthening piezoelectricity still remains to be a challenge. Much more recently, a topological structure strategy has been developed to fabricate dielectric nanocomposites for energy storage application where high dielectric constant and breakdown strength are required [14,15]. By constructing a particular topological structure (sandwich or multilayer) comprising of a layer of high breakdown strength intercalated between layers of high dielectric constant, one can achieve both higher dielectric constant and higher breakdown strength than those of a single layer [15,16]. Such topological-structure modulated nanocomposites exhibit great promise in combining synergistically the complementary properties of each layer. Inspired by above mentioned works concerning energy storage nanocomposites, we hypothesize that the topological structure strategy will be efficient and convenient to simultaneously enhance breakdown strength and polarization. Herein, the topological structure strategy is expanded to fabricate piezoelectric nanocomposites. We show that, through properly composition tailoring and topological structure design, both high breakdown strength and high efficiency of nanofillers in yielding improved piezoelectricity can be achieved in a sandwiched nanocomposites. In this

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sandwich structure, a layer of high breakdown strength acts as middle layer and two layer containing high content (near the percolation threshold) of conductive nanofillers serve as outer layers. The middle layer acts as a strong barrier to increase the path tortuosity in the growth process of electrical trees during breakdown, diminish the probability of forming conductive paths and protect the nanocomposite from global breakdown, contributing to higher breakdown strength. Consequently, the outer layers loaded with high content of conductive nanofillers can then be polarized under an enough high electric field, realizing the full potential in inducing piezoelectric enhancement. As will be shown in this paper, the topological-structure modulated nanocomposites with optimally tailored composition present outstanding piezoelectric coefficient d33 of around 48 pC/N at room temperature, which is among the best data reported so far. In this work, a novel three-dimensional CNTs based nanohybrid (3D-CNTs) reported recently was chosen as the conductive nanofillers [17]. The novel 3D-CNTs hybrids are comprised of CNTs, graphene and manganese oxide (MnO2) and exhibit a nanostructure analogous to a branched tree where CNTs acts as the trunk while graphene and MnO2 serve as the branches. Compared with typical one-dimensional or two-dimensional nanofillers such as CNTs and graphene, these 2

3D-CNTs exhibit larger specific surface area (80-105 m /g) and better compatibility with PVDF, and thus present superior ability in strengthening the interfacial coupling effect and consequently piezoelectric performance enhancement [18-23]. Moreover, as verified in our recent work, by simply tuning the density/content of MnO2 shielding layer in 3D-CNTs, we can easily tailor the breakdown strength as well as the efficiency of 3D-CNTs in inducing piezoelectricity enhancement. It is found that for the 3D-CNTs nanohybrids, those with a lower MnO2 content

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(23wt.%, CM23) present higher efficiency in inducing enhanced piezoelectricity, while those containing high content of MnO2 (66wt.%, CM66) exhibit good capability in remaining the high breakdown strength of nanocomposites [17]. Thus, we adopted CM23 and CM66 as the nanofillers for the outer layers and middle layer of sandwiched structure, respectively. Through modulating the composition content in each layer of sandwich structure, one can tune the breakdown strength and efficiency in inducing enhanced piezoelectricity. It is also worth noting that a three-step method based on solution-casting, hot-pressing and subsequently rolling is adopted here to fabricate the sandwich-structured nanocomposites, which is different from the previous reported literatures that generally used a two-step fabrication method (solution-casting and hot-pressing) [15,16]. The subsequent rolling process not only is advantageous to align the nanofillers within the matrix and thus further enhance the breakdown strength of nanocomposites, but also is critical to promote the piezoelectric phase formation. The results revealed that the obtained rolled sandwiched nanocomposites present dramatically enhanced breakdown strength of over 130 MV/m. Additionally, the breakdown strength of sandwich-structured nanocomposites were even comparable to that of the middle layer, which might be mainly ascribed to the trapping of free charge induced by the additional interfaces between the adjacent layers. The notably improved breakdown strength enabled the poling process of outer layers under electric field between 40 MV/m and 80 MV/m, realizing the great potential of CM23 in inducing superior piezoelectricity. After poling under 70 MV/m, a maximum piezoelectric constant of 48 pC/N is achieved in the optimized sandwiched nanocomposites. Additionally, the resultant nanocompsoites retain good piezoelectric performance over a wide

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o

o

temperature range from 10 C to 60 C, implying the great promise toward practical applications in sensors and energy harvesters. For the first time, we demonstrated that through the synergistic effect of topological-structure and composition modulations, high breakdown strength and outstanding piezoelectricity can be achieved in PVDF based nanocomposites, opening up a novel avenue to further enhance the piezoelectric performance of PVDF based percolative nanocomposites.

2.Experimental

2.1Synthesis of 3D-CNTs hybrids and nanocomposites

Two kinds of 3D-CNTs hybrids including CM23 and CM66 (see supporting information Fig.S1) were synthesized using a modified one-pot synthesis as described in our previous reported work [17]. The sandwich nanocomposites were fabricated via a three-step process based on solution-casting, hot-pressing and subsequently rolling. First, 3D-CNTs/PVDF nanocomposites with different loadings were prepared into films with a thickness of 20-25um using a solution-casting method as illustrated in previous reported work. Afterwards, three pieces of films o

were stacked together as designed and compression molded at 200 C for 1h. Next, a rolling process was applied to the obtained nanocomposites through a rolling machine with a temperature o

of 65 C and speed of 23 r/min. The structure of CM23/PVDF-CM66/PVDF-CM23/PVDF was denoted as L-H-L for short. The resultant sandwiched nanocomposites with a thickness of 22 ± 2 um were marked as Lx-Hy-Lx, in which x (2, 4) and y (0,1,2) stands for ten times the weight

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fraction of CM23 (0.2wt.%, 0.4wt.%) and CM66 (0wt.%, 0.1wt.%, 0.2wt.%) with respect to the PVDF matrix, respectively .

2.2 Sample characterization

Field emission scanning electron microscope (FESEM, Hitachi SU8010) was performed to characterize the morphology of nanocomposites. Transmission electron microscope (TEM, JEM-1400) was adopted to observe the distribution of nanofillers within the matrix. Fourier transform-infrared measurements were carried out using a Nicolet 6700 FT-IR spectrometer. The X-ray diffraction (XRD) experiments of nanocomposites were performed via an Oxford Xcalibur diffracto meter equipped with an ONYX CCD area detector, wherein the X-ray wavelength was Cu Kα (0.1542 nm). The melting and crystallization behaviors of nanocomposites were recorded by a differential scanning calorimetry (DSC; DSC7020, Japan) in the temperature range between 25 °C to 200 °C with a heating rate of 10 °C/min. The room temperature dielectric properties of nanocomposites were measured employing an Impedance Analyzer (HP4294, Agilent, USA). The electric breakdown strength test was carried out via a dielectric withstand voltage test (Beijing Electromechanical Research Institute Supesvoltage Technique) at a ramping rate of 200 V/s and a limiting current of 5 mA. Polarization-electric field (P-E) hysteresis loops were obtained with a ferroelectric testing system (RT66A, Radiant Technologies, USA) equipped with a high voltage interface (Trek 609B, Trek, USA). The piezoelectric strain coefficients (d33) were characterized using a piezo d33 meter (ZJ-3A, Institute of Acoustics, Chinese Academy of Sciences, China).

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3.Results and discussions

3.1 Morphology and structure of the nanocomposites

The topological structure modulated nanocomposites films were fabricated by an optimized three-step process, through which the good flexibility of PVDF can be well retained (see Fig.S2, supporting information). The three-step process is illustrated schematically in Fig.1a. First, a solution-casting process was adopted to prepare the 3D-CNTs/PVDF nanocomposites films with a homogenous 3D-CNTs distribution (see SEM images in Fig.1 b and TEM images presented supporting information Fig.S3), which has also been evidenced in our previous work. As the second step, three layers of the solution-casting nanocomposites were stacked to undergo a compressing process, after which a sandwiched-structure with distinct boundaries between the adjacent layers was obtained (see Fig.1 c). It should be pointed out that for the multilayer structure, a critical issue is the delamination occurring in the interface region between neighboring layers [24]. In this case, no obvious delamination can be observed, suggesting the good integration of layers in nanocomoposites. Next, a rolling process was applied to the three-layered nanocomposites, as displayed in Fig.1a. As expected, a highly oriented fibrillar structure of nanocomposites is achieved, which is in accordance with our previous work (see Fig1.c) [25, 26]. The shear force provided by rolling not only can induce a parallel orientation of nanofillers along the rolling direction, giving rise to enhanced breakdown strength, but also is essential to obtain a high fraction of piezoelectric phase, which will be discussed in the following part.

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Fig. 1 (a) Schematic illustration of the fabrication process of sandwich nanocomposites. Cross-section SEM morphologies of the (b) solution-casted L2 nanocomposite film, (c) hot-pressed L2-H2-L2 nanocomposites and (d) rolled L2-H2-L2 nanocomposites. The white dots circled in (a) represent the nanofillers.

Based on the processing conditions, the semi-crystalline PVDF may crystallize into five phases including , , among which phase is of critical importance to the piezoactivity of PVDF [27]. In general, two characterization methods including XRD and FTIR are often combined to examine the crystalline phase of PVDF. Fig.2 a and b give the XRD and FTIR patterns of sandwiched nanocomposites obtained from hot pressing and rolling, respectively. From both XRD and FTIR patterns, it is clear that a mixture of  and characteristic peaks can be observed in the hot pressed nanocomposites while in the case of rolled nanocomposites, mainly the characteristic peaks corresponding to -phase are found. This indicates that rolling is capable of promoting -phase transition and increasing the relative fraction of -phase. A further FTIR analysis (see supporting information Fig. S4) revealed that the -phase content is improved by almost 30% after the rolling process and all the rolled samples present a high relative proportion of phase (F) of 90% (Fig.2 c). It should be signified that under the rolling conditions in this work, the three-layered sandwich structure appeared to have negative but marginal effect on the -phase transition. Compared with the individual layers, a slight increment of -phase content (3%) is found in the three-layered nanocomposites (see Fig.2c), which may be explained by the stress distribution mechanism while a further and deeper investigation is demanded to fully explain this phenomenon.

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Apart from the fraction of phase (F), crystallinity (Xc) is another critical factor that directly determines the piezoelectric properties of PVDF [28]. Fig.S5 displays the DSC melting traces of rolled nanocomposites, from which one can obtain the melting heat to calculate the crystallinity of samples (see supporting information). Based on the FTIR and DSC results, the crystallinity of phase (X,X=Xc * F) of nanocomposites can be deduced and the results are shown in Fig.2d. Clearly, all the rolled samples exhibit a similar high level of X(40%)which will ensure their good piezoactivity as discussed below.

Fig. 2 (a) FTIR and (b) XRD spectra of L2-H0-L2 and L2-H2-H2 nanocomposites obtained from hot pressing and rolling, respectively. (c) The relative fraction of -phase in sandwich nanocomposites obtained from hot pressing and rolling, respectively. The inset gives the relative fraction of -phase in constitute layers obtained from hot pressing and rolling, respectively. (d) The crystallinity of -phase in sandwich nanocomposites obtained from hot pressing and rolling, respectively.

3.2 Dielectric properties and breakdown strength of nanocomposites

As has been well established in previous work, the addition of conductive nanofillers with high specific area can significantly enhance the dielectric constant of PVDF due to the interface effect and micro-capacitor mechanism [29,30]. However, on the other hand, the enhanced dielectric constant is also accompanied by a large dielectric conduction loss and low breakdown strength, which may seriously weaken the insulating nature of PVDF and render quick breakdown failure under external electric field [31,32].

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Similar to the traditional conductive nanofillers such as CNTs and graphene, the addition of 3D-CNTs into PVDF can also result in enhanced dielectric constant while significantly increasing conduction loss and sharply decreasing breakdown strength, particularly for the CM23 containing low MnO2 content. For instance, with a small loading of 0.2 wt.% CM23, the room temperature dielectric constant of nanocomposites is significantly raised to 70 (100 Hz), nearly 7 times that of pure PVDF. Meanwhile, significantly enlarged dielectric loss and reduced breakdown strength in the nanocomposites are also found accordingly. Recent phase-modeling and experimental results demonstrate that the orientation of nanofillers parallel to the nanocomposite surface is favorable for enhancing the breakdown strength [33-35]. However, even if the CM23 nanoparticles were aligned parallel to the nanocomposite surface (perpendicular to the electric field) through a rolling process, the breakdown strength of obtained CM23/PVDF nanocomposites still quickly declined to be below 18MV/m with a small loading of only 0.2 wt.%, and these CM23/PVDF nanocomposites with further loadings behave as conductors rather than dielectrics. To suppress the dielectric conduction loss and improve the breakdown strength, in this work, a sandwiched structure composed of high breakdown strength layer interacted between two CM23/PVDF layers is designed. As revealed in Fig.3b, due to the stronger shielding effect of high MnO2 content in CM66, the accordingly hot-pressed and rolled nanocomposites present higher level of breakdown strength of over 80 and 200MV/m, respectively (loadings≤0.2 wt.%). With such a relatively high breakdown strength, the introduced CM66/PVDF central layer is assumed to be capable of inhibiting the long-distance conduction of charge carrier and thus lowering the conduction loss as well leakage current [15,16]. As expected, rather low dielectric loss under low

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frequency (mainly caused by conduction loss) are observed in all the rolled sandwiched nanocomposites (tan70), it is assumed that higher electric field is concentrated in the middle Hx layer with a relatively lower dielectric constant (