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高介电复合材料通常选用有机物为基体,以便于小型化集成电路的加工过程,并应用于埋入式电子元件的制造。常见的有机基体有环氧树脂、聚甲基丙烯酸甲酯和聚偏氟乙烯等[1,2]。其中,聚偏氟乙烯(PVDF)具有介电性能十分突出、储能密度高、机械性能好以及耐化学腐蚀等优点,因而在高介电材料领域有广阔的应用前景[1]。在实际应用中,往往需要对PVDF进行改性。基于渗流理论和界面极化理论,向PVDF中加入石墨烯、碳纳米管和金属粒子等导电填料可有效改善材料的介电性能[2-3,6]。但是,目前高介电复合材料研究仍面临两个普遍的问题:(1)纳米粒子容易在基体中团聚。在外电场的作用下,纳米粒子的团聚会产生漏电流,致使材料介电损耗过大;(2)无机粒子与有机基体的相容性差[4-5]。较差的相容性会极大地影响基体与填料之间的粘接力。这两大问题会直接影响高介电复合材料的介电性能和力学性能,使材料难以满足器件性能的要求。
相较于上述单一结构的填料,构建核壳结构的纳米填料是解决分散性和相容性问题的有效途径之一,也是近几年该方向研究的热点[2-4]。Yang等[7]通过化学气相沉积制备了核壳结构的碳纳米管@无定形碳(CNTs @ AC)填料。与原始碳纳米管(P-CNTs)/PVDF纳米复合材料相比,CNTs @ AC-60/PVDF纳米复合材料的体积分数(fc)从7.16 %增加到11.81 %,介电损耗从83.66降低到1.35,表明包覆无定形碳后渗透行为被延迟,材料的介电性能明显改善。Zhang等[8]在PVDF中加入由介电碳涂层包覆的钛酸钡混合颗粒(BT @ C),材料的介电损耗没有明显的上升,电导率也低于10−5 S/m,50 %(质量分数)-BT @ C-2/PVDF-HFP的介电常数在1 kHz下上升到80,是纯PVDF-HFP(8.8)的10倍[8]。
在上述改性思路的基础上,本文通过引入具有核壳结构的Fe3O4@C的纳米填料的方案,来改性PVDF薄膜的介电性能,在提升材料介电常数的同时,仍然使材料保持较低的介电损耗和良好的机械性能[3,9]。与CNTs @ AC-60这种圆柱形填料相比,球形的纳米填料(Fe3O4@C)之间接触面积更小,团聚的倾向更小,故而膜材料的介电损耗更小[7,9,10];与填料BT @ C相比,Fe3O4@C中的Fe3O4核在介电、压电、储能和电磁等方面的潜力更佳[8,11];此外,与Fe3O4/PVDF以及其他常见的几种PVDF改性聚合物相比,Fe3O4@C/PVDF也具有更加优异的综合性能。本文选用Fe3O4作为核,可利用Fe3O4的本征特性改善PVDF的介电性能[4]。C壳层不仅可以促进Fe3O4磁性纳米粒子的均匀分散,还可以有效提高无机纳米粒子与有机基体之间的相容性,避免高填充量的无机填料损害材料的力学性能[8,10]。更重要的是核壳结构的纳米填料向基体中引入大量的异质界面,包括Fe3O4-C界面和C-PVDF界面。这些异质界面在外电场的作用下,聚集了大量的偶极子,强化了界面极化效应的影响,进一步改善材料的介电性能[6]。
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六水合三氯化铁(FeCl3·6H2O,分析纯)、二水合柠檬酸钠(Na3Cit·2H2O,分析纯)、乙酸钠(NaAc,分析纯);间苯二酚(分析纯)、甲磺酸(MSA,分析纯)、丙酮(分析纯)、氨水(NH3·H2O,28%)、聚偏氟乙烯(PVDF,分析纯)由上海泰坦科技有限公司提供。乙二醇(分析纯)、甲醛(分析纯)、过硫酸铵(APS,分析纯)、N,N-二甲基甲酰胺(DMF,分析纯)由上海凌峰化学试剂有限公司提供。高纯氮气(
99.8%)由上海思灵气体有限公司提供。
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首先,将5.0 g FeCl3·6H2O溶解在150 mL的乙二醇中。然后在剧烈搅拌下加入0.42 g Na3Cit·2H2O,继续搅拌1 h后,加入7.5 g NaAc,继续搅拌40 min,将所得悬浮液转移到四氟乙烯衬里的不锈钢水热釜中,200 ℃下保持10 h[12]。冷却至室温。得到黑色产物,并分别用乙醇和去离子水各洗涤3次。50 ℃下真空干燥12 h。即得纳米Fe3O4磁性纳米粒子。
将制得的0.5 g Fe3O4和280 mL H2O加入到500 mL烧瓶中,超声处理40 min。加入0.3 mL NH3·H2O和0.4 g间苯二酚。在室温下搅拌1 h,然后缓慢滴加0.6 mL甲醛溶液继续搅拌6 h,再升温至80 ℃搅拌5 h[13]。在磁铁的的作用下,收集得到的砖红色产物,该砖红色物质是Fe3O4@酚醛纳米球[12]。用65 ℃去离子水洗涤3次,所得产物在80 ℃空气中烘12 h。然后将产物在N2气氛下加热至600 ℃,加热速率为10 K/min,600 ℃保温3小时,冷却至室温得到的黑色固体即为Fe3O4@ C。
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取一定量的Fe3O4@C或者Fe3O4纳米粒子(见表1)和40 mL DMF于烧瓶中,超声分散1 h。然后加入4.00 g的PVDF粉末,70 ℃机械搅拌2 h,之后再超声分散1 h得到黑色均匀的液态混合物[3,14]。将液态混合物缓慢的倒入洁净干燥的表面皿内,延展开来,静置20 min后,80 ℃真空干燥24 h除去DMF,得到Fe3O4@C/PVDF或者Fe3O4/PVDF薄膜。15 MPa,180 ℃条件下热压15 min,保压冷却到室温。
200/% m(PVDF)/g m(Fe3O4@C)/g m(Fe3O4 )/g V(DMF)/mL 0 4.00 0 0 40 4 4.00 0.17 0.17 40 8 4.00 0.35 0.35 40 12 4.00 0.55 0.55 40 16 4.00 0.76 0.76 40 表 1 不同填料比的Fe3O4@C/PVDF和Fe3O4/PVDF复合薄膜的制备参数
Table 1. Preparation parameters of the preparation of Fe3O4@C/PVDF and Fe3O4/PVDF composite film
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Fe3O4和Fe3O4@C两种纳米粒子的X射线衍射图谱如图2所示。30.4°,35.7°,43.2°,53.8°,57.4°,63.1°处是Fe3O4的特征峰[12-13]。XRD特征峰有效的证明了Fe3O4核的成功合成。相较于Fe3O4粒子的XRD图谱,Fe3O4@C纳米粒子的曲线中多了72.0°处的一个弱峰,这个弱峰来自于C壳层上C原子组成的网状结构[13-14]。
在图3的拉曼光谱中,610 cm−1处的特征峰来自于Fe3O4。1 324 cm−1和1 582 cm−1处的峰来自C壳的五元和六元环之间的共轭结构[15-16]。结合图2 XRD的特征峰和图4TEM的粒子结构,说明Fe3O4核外的酚醛层经过高温碳化形成C层,并且成功的包裹在Fe3O4核外。
图4(a)示出了Fe3O4@C纳米粒子的核壳结构,纳米粒子球形基本保持均匀,表面光滑,直径在400 nm左右,其中Fe3O4核的半径在150 nm左右,C层的壳厚约60 nm。图4(b)和 图4(c)是Fe3O4@C纳米粒子界面附近的高倍透射电镜图。Fe3O4相中有明显的晶格条纹,晶面间距为0.25 nm。C相中没有发现明显的晶格条纹,这说明酚醛高温碳化后形成的C壳为无定形的碳,不具备晶格结构(如图4(c))。图4(d)为Fe3O4-C界面附近的电子衍射花样(SAED)。图案由一系列不同半径的同心圆光圈组成,光圈是由大量Fe3O4小晶粒产生的,光圈半径(R)与晶面间距(d)之间存在如下关系:R=Lλ/d=K/d,其中,K和L是仪器参数。0.25 nm的晶格条纹对应图4(d)中光圈半径为2 nm的(311)晶面。
这种核壳结构对于材料性能的影响是至关重要的。C壳层的作用主要体现在3个方面:(1)有助于Fe3O4纳米粒子的分散;(2)改善Fe3O4与有机基体之间的相容性,减小由于填料与PVDF基体之间的界面不相容引起的机械性能的降低[8,20];(3)增加材料中的异质界面,强化界面极化对材料介电常数的影响[18,23]。
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不同填料比的PVDF薄膜的FT-IR光谱见图5,1 400 cm−1至1180 cm−1间的峰是—CH2和C—C主链的弯曲振动峰[7]。1 060,980,877 cm−1三处峰与PVDF的C—C不对称伸缩振动有关[4,8]。792 cm−1和764 cm−1两处峰与α-PVDF相—CH2和—CF2的弯曲振动有关[4]。840 cm−1与β-PVDF相—CH2的弯曲振动有关。同时,可以发现随着填料的质量分数从0增加到16 %,PVDF中β相的峰逐渐变强,而α相的峰逐渐变弱。β相较α相具有更高的偶极矩、压电系数和介电常数[3]。在辐射、外应力或者成核剂等条件的作用下,α相可以转变为β相[4]。通常β相含量越多,偶极子的分子间运动受限越小,PVDF的介电常数越高[5]。
图 5 不同质量分数的Fe3O4@C/PVDF复合薄膜的红外光谱
Figure 5. FT-IR spectra of Fe3O4@C/PVDF composite films with different mass fractions of Fe3O4@C
图6示出了纯PVDF和12 %-Fe3O4@C/PVDF薄膜的X射线衍射图谱。2θ=17.7°、18.4°、20.8°、20.6°对应α-PVDF的(100)、(020)、(021)、(020)晶面[5,17]。此外,Fe3O4的5个特征峰(30.4°,35.7°,43.2°,57.4°,63.1°)也可以在Fe3O4@C/PVDF薄膜的XRD图中找到。但是由于PVDF的遮蔽,导致C壳层的特征峰2θ为72.0°处原本的弱峰未显示出来[11,22]。更为重要的是,在2θ为20.5°处出现一个新峰,它对应于β-PVDF的(200)晶面[17,21]。说明随着Fe3O4@C纳米粒子的加入,PVDF中β-PVDF的比例开始增多。本文中Fe3O4@C纳米粒子作为成核剂,促进了PVDF的α相和β相之间的晶相转变[5,12]。这一结果与图5的红外光谱的分析结果一致。
图7示出了不同填料比(质量分数)的Fe3O4@C纳米粒子在PVDF基体中的分散情况的扫描电镜图。可见,当填料的质量分数为4 %,8 %和12 %时,纳米粒子在PVDF基体中分散良好,没有团聚现象,但是当填料的质量分数增加到16 %时,纳米粒子开始出现部分的团聚。纳米粒子相互接触产生漏电流,导致材料介电损耗的增加。填料比接近12 %时,纳米粒子之间的距离虽然已经很接近,但是还没有相互接触。这样的分布状态可以在改善材料介电常数的同时,抑制电导损耗造成到的介电损耗的激增。总之,填料粒子在基体中的均匀分布是PVDF复合薄膜具有优良介电性能的结构基础[25-26]。图8示出了相同填料比下,Fe3O4/PVDF薄膜截面的SEM图,从图中可见许多Fe3O4团聚在一起。对比说明Fe3O4在基体中的分散性比Fe3O4@C差。这是由于碳壳层可有效地削弱Fe3O4纳米粒子之间的吸引力,促进填料的分散。
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本文中薄膜材料的介电性能的变化可由渗流理论和界面极化理论的解释。式(1)为解释渗流理论的经典公式,其中φ表示体积分数。为表达方便将体积分数换算成质量分数,得到式(2)。其中,临界阈值可以由实验数据结合线性拟合得出[15,17,29]。如图9(d)所示,材料的理论临界阈值为0.13,该实验结果与理论值一致。ε是PVDF基体的介电常数;wc是渗滤阈值处的质量分数;wfiller是Fe3O4 @ C的质量分数;s表示临界指数[6,30]。
图 9 不同质量分数Fe3O4@C/PVDF薄膜的介电常数,介电损耗,电导率随频率变化
Figure 9. Frequency dependence of dielectric constant, dielectric loss and electrical condatirity of the Fe3O4@C@/PVDF films with different mass fractions of Fe3O4@C
如图9(c)所示,常温下,随着导电填料的增加,复合薄膜的电导率持续增加[27]。填料的质量分数由12%增加到16%时,材料的电导率也由10−7 S/cm增加到10−5 S/cm,由绝缘体转变为导体。从复合材料的各个相区来看,纯的PVDF的电导率(1 kHz)在10−11~10−10 S/cm之间,属于是绝缘体,Fe3O4是一种过渡金属氧化物,其导电率优于PVDF,低于C壳[28]。基于界面极化理论,PVDF、C和Fe3O4是存在电导率梯度的异质相。在外电场的作用下,由于载流子在3种相中的运动速度不同,电荷容易在异质相界面附近聚集,进而在材料中形成大量的电偶极子。这些电偶极子会产生界面极化效应,提升材料的介电常数。通常材料中的异质相界面越多,界面极化效应越明显,介电性能提升越多[29]。
如图9(a)纯PVDF薄膜的介电常数为8(1 kHz)。随着Fe3O4@C粒子的加入,薄膜的介电常数持续增加。在填料低负载阶段,介电常数的增加主要得益于填料本身的高介电性和界面极化[13,15,17]。当填料的质量分数增加到12%左右时,材料的介电常数由52迅速增加到115。这是由于当基体中的填料质量分数接近临界阈值时,两个导电填料之间仅被极薄的绝缘基体隔开而形成微电容。根据渗流理论,基体中无数的微电容促使材料的介电常数急剧增加[21,23,31]。
当外界电场的频率在1 kHz附近时,薄膜的介电损耗基本保持稳定。由图9(b)的局部放大图可知,随着导电填料质量分数的增加,材料的介电损耗持续增加,但是基本还维持在较低的水平。外电场频率为1 kHz,当填料的质量分数增加到12%时,介电损耗仅为0.063。当填料的质量分数增加到16%时,介电损耗激增到0.111,是纯PVDF薄膜的近5倍(纯PVDF的介电损耗为0.026)。这是由于部分导电纳米粒子相互接触时,异质界面附近的阻塞电荷很容易通过隧道效应或直接欧姆接触而松弛,产生局部的漏电流造成的电导损耗[29,32]。此外,随着外界电场频率的增加,电偶极子逐渐无法跟上外电场的变化,产生弛豫,极化损耗开始随着频率的增加而增加[27,33-34]。表2中列举了多种PVDF基的高介电材料的性能参数,可见本文的Fe3O4@C/PVDF复合薄膜在介电常数和介电损耗等参数上有明显的优势。
Materials Test conditions wc/% φc/% Dielectric constant Dielectric loss Ref. Fe3O4@C/PVDF 25 ℃/1 kHz 12 115 0.06 This work Fe3O4/PVDF 25 ℃/1 kHz 4 32 0.11 This work Pure PVDF 25 ℃/1 kHz − 8 0.03 [5] Carbon fiber/PVDF 20 ℃/1 kHz 0.056 23 0.07 [33] GO/PVDF 25 ℃/1 kHz 2 28 0.2 [22] Ni(OH)2/NBT/PVDF 1 kHz 2.5 15 0.039 [14] BaTiO3@Al2O3/PVDF 25 ℃/1 kHz 2.5 10.56 0.64 [34] BT @ C-2/PVDF-HFP 1 kHz 50 80 0.29 [8] 表 2 各种PVDF复合材料的介电性能参数
Table 2. Dielectric properties of various PVDF composites
如图10所示,外电场频率为1kHz下,质量分数分别为0,4%,8%,12%和16%的Fe3O4/PVDF薄膜对应的介电常数分别是8,32,20,18和13。与Fe3O4@C/PVDF相比,Fe3O4/PVDF的介电常数的提高十分有限。在填料质量分数相同的条件下,Fe3O4@C/PVDF的介电常数要优于Fe3O4/PVDF。Fe3O4难以像Fe3O4@C一样在基体中形成大量的微电容体系,对介电常数的贡献有限。在介电损耗方面,Fe3O4/PVDF和Fe3O4@C/PVDF的差距更加明显。外电场频率为1kHz下,当质量分数为0,4%,8%,12%和16%时,对应的介电损耗分别是0.02,0.11,0.14,0.19和0.22。与纯PVDF相比,几种材料的介电损耗普遍增幅超过5倍。Fe3O4粒子间产生大量的漏电流,引发电导损耗的激增。上述对比说明了C壳层在改善材料整体介电性能方面的作用明显。
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Fe3O4@C/PVDF复合薄膜不仅具有优良的介电性能,还具有良好的力学性能。由图11可见,随着填料的加入,材料的杨氏模量和拉伸强度不断增加,并且在质量分数为12%时达到最大值。当质量分数超过12%之后,填料开始团聚,力学性能下降。这是由于:当填料负载处于渗流阈值之前时,纳米填料扎在基体分子链的间隙中,可以有效地阻碍分子链在外力作用下的滑移;同时C壳与PVDF相容性高,界面处缺陷少,不易产生裂纹。因此,材料在断裂前能承受的应力更大,机械性能更好[19-20,35]。与纯的PVDF薄膜(拉伸强度42.3 MPa;杨氏模量1.3 GPa)相比,12%-Fe3O4@C/PVDF薄膜的拉伸强度提高到76.6 MPa,杨氏模量增加到2.0 GPa。但是,当填料填料质量分数超过12%之后,团聚造成填料与基体界面处的缺陷增多,相容性变差,力学性能又开始降低[18,36-37]。除此之外,由图11可见,12%-Fe3O4@C/PVDF薄膜在保持较高拉伸强度的情况下,仍能具有良好的柔韧性,便于实际应用时的加工生产。
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核壳结构的Fe3O4@C纳米粒子可以有效改善材料的综合性能。通过Fe3O4/PVD和Fe3O4@C/PVDF的对比实验说明:C壳层有利于纳米填料的均匀分散,防止漏电流的产生,维持较低的介电损耗;核壳结构产生了更多的异质界面,有利于强化界面极化对介电常数的影响;同时C壳可以提高与有机基体之间的相容性,避免产生过多的结构缺陷,影响材料的机械性能。与Fe3O4/PVDF,碳纤维/PVDF和石墨/PVDF等材料相比,改性过后的Fe3O4@C/PVDF柔性薄膜介电常数增加到115(常温,1 kHz),介电损耗维持在0.063的较低水平,拉伸强度由42.3 MPa增加到76.6 MPa,杨氏模量由1.3 GPa增加到2.0 GPa,同时柔韧性良好,可作为一种柔性高介电薄膜应用于微型电容等电子元器件领域[23,28,38]。
核壳结构Fe3O4@C改性PVDF柔性薄膜的介电性能研究
Core/shell-Structured Fe3O4@C/PVDF Flexible Films with Improved Dielectric Properties
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摘要: 通过高温炭化将碳(C)壳包覆在纳米四氧化三铁(Fe3O4)表面,形成Fe3O4@C的核壳结构。与单一结构填料粒子相比,核壳结构的Fe3O4@C在有机基体中的分散更均匀,相容性更高;以此制得了具有高介电常数、低介电损耗的Fe3O4@C/聚偏氟乙烯(PVDF)柔性薄膜。与Fe3O4/PVDF、碳纤维/PVDF和石墨/PVDF等材料相比,本文制备的Fe3O4@C/PVDF柔性薄膜介电常数高达115(常温,1 kHz),而介电损耗维持在0.063的较低水平;同时,其拉升强度由纯PVDF的42.3 MPa增加到76.6 MPa,杨氏模量由1.3 GPa增加到2.0 GPa,可作为一种柔性高介电薄膜应用于微型电容等电子元器件领域。Abstract: The carbon shell was coated on the surface of ferroferric oxide via high temperature carbonization to form the core-shell structure of Fe3O4@C. Compared with the single structured filler particles, the core-shell structure of Fe3O4@C was more uniform and more compatible with the organic matrix. Thus, the Fe3O4@C/PVDF flexible film with high dielectric constant and low dielectric loss was obtained. Carbon shell plays a key role in improving the dielectric properties of materials. The carbon shell can not only be used as a transition layer between the Fe3O4 core and the PVDF matrix, but also strengthen the interfacial polarization of the material. The improvement of the dielectric properties of the materials is mainly based on percolation theory and interfacial polarization theory. According to the experiment, the percolation threshold of Fe3O4@C was 12%. During the experiment, it is necessary to accurately control the amount of added filler. At this packing ratio, the film was filled with countless microcapacitors and the dielectric constant of the material was increased rapidly. At the same time, the interfacial polarization of the electric dipole between different interfaces further enhanced the dielectric properties of the material. Compared with pure PVDF, Carbon Fiber/PVDF and GO/PVDF, the Fe3O4@C/PVDF flexible film prepared in this paper has a higher dielectric constant of 115 (normal temperature, 1 kHz) and lower dielectric loss of 0.063. Meanwhile, compared with pure PVDF, its tensile strength was increased from 42.3 MPa to 76.6 MPa, and the Young's modulus was increased from 1.3 GPa to 2.0 GPa. Thus, the prepared materials can be used as a flexible film with high dielectric in electronic components and other fields.
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表 1 不同填料比的Fe3O4@C/PVDF和Fe3O4/PVDF复合薄膜的制备参数
Table 1. Preparation parameters of the preparation of Fe3O4@C/PVDF and Fe3O4/PVDF composite film
200/% m(PVDF)/g m(Fe3O4@C)/g m(Fe3O4 )/g V(DMF)/mL 0 4.00 0 0 40 4 4.00 0.17 0.17 40 8 4.00 0.35 0.35 40 12 4.00 0.55 0.55 40 16 4.00 0.76 0.76 40 表 2 各种PVDF复合材料的介电性能参数
Table 2. Dielectric properties of various PVDF composites
Materials Test conditions wc/% φc/% Dielectric constant Dielectric loss Ref. Fe3O4@C/PVDF 25 ℃/1 kHz 12 115 0.06 This work Fe3O4/PVDF 25 ℃/1 kHz 4 32 0.11 This work Pure PVDF 25 ℃/1 kHz − 8 0.03 [5] Carbon fiber/PVDF 20 ℃/1 kHz 0.056 23 0.07 [33] GO/PVDF 25 ℃/1 kHz 2 28 0.2 [22] Ni(OH)2/NBT/PVDF 1 kHz 2.5 15 0.039 [14] BaTiO3@Al2O3/PVDF 25 ℃/1 kHz 2.5 10.56 0.64 [34] BT @ C-2/PVDF-HFP 1 kHz 50 80 0.29 [8] -
[1] HONGCHAO L, LI L. Crystalline structure, dielectric, and mechanical properties of simultaneously biaxially stretched polyvinylidene fluoride film[J]. Polymers for Advanced Technologies, 2018, 29(4): 4426-4432. [2] YANG C, HAO S J, DAI S L, et al. Nanocomposites of poly (vinylidene fluoride) - Controllable hydroxylated/carboxylated graphene with enhanced dielectric performance for large energy density capacitor[J]. Carbon, 2017, 117(Complete): 301-312. [3] LI W, SONG Z, QIAN J, et al. Enhancing conjugation degree and interfacial interactions to enhance dielectric properties of noncovalent functionalized graphene/poly (vinylidene fluoride) composites[J]. Carbon, 2018, 123(25): 372-378. [4] TIWARI V, SRIVASTAVA G. Effect of thermal processing conditions on the structure and dielectric properties of PVDF films[J]. Journal of Polymer Research, 2014, 21(11): 587-592. doi: 10.1007/s10965-014-0587-0 [5] GYANARANJAN S, NILADRI S, SWAIN S K. The effect of reduced graphene oxide intercalated hybrid organoclay on the dielectric properties of polyvinylidene fluoride nanocomposite films[J]. Applied Clay Science, 2018, 162: 69-82. doi: 10.1016/j.clay.2018.05.008 [6] 杨晓天, 魏永明, 许振良. PVDF成膜过程中溶剂分布的模型计算及其大孔形成机理[J]. 华东理工大学学报(自然科学版), 2007, 33(5): 610-614. doi: 10.3969/j.issn.1006-3080.2007.05.004
[7] YANG M, ZHAO H, HE D, et al. Constructing a continuous amorphous carbon interlayer to enhance dielectric performance of carbon nanotubes/polyvinylidene fluoride nanocomposites[J]. Carbon, 2017, 116(Complete): 94-102. [8] ZHANG X, CHENG T, MA Y, et al. BaTiO3@carbon/silicon carbide/poly (vinylidene fluoride-hexafluoropropylene) three-component nanocomposites with high dielectric constant and high thermal conductivity[J]. Composites Science & Technology, 2018, 14(21): 373-378. [9] WANG D, BAO Y, ZHA J W, et al. Improved dielectric properties of nanocomposites based on poly(vinylidene fluoride) and poly(vinyl alcohol)-functionalized graphene[J]. ACS Applied Materials & Interfaces, 2012, 4(11): 6273-6279. [10] CHO S B, NOH J S, PARK S J, et al. Morphological control of Fe3O4 particles via glycothermal process[J]. Journal of Materials Science, 2007, 42(13): 4877-4886. doi: 10.1007/s10853-006-0685-4 [11] XIE L, HUANG X, HUANG Y, et al. Core@double-shell structured BaTiO3-polymer nanocomposites with high dielectric constant and low dielectric loss for energy storage application[J]. Journal of Physical Chemistry C, 2013, 117(44): 22525-22537. doi: 10.1021/jp407340n [12] 刘久清, 许振良, 张耀. 聚偏氟乙烯(PVDF)中空纤维复合纳滤膜的研究I. 复合纳滤膜的制备[J]. 华东理工大学学报(自然科学版), 2006, 32(3): 241-244. doi: 10.3969/j.issn.1006-3080.2006.03.001
[13] ZHANG Z, KONG J. Novel magnetic Fe3O4@C nanoparticles as adsorbents for removal of organic dyes from aqueous solution[J]. Journal of Hazardous Materials, 2011, 193(none): 325-329. [14] YANG Y, LI Z, JI W, et al. Enhanced dielectric properties through using mixed fillers consisting of nano-barium titanate/nickel hydroxide for polyvinylidene fluoride based composites[J]. Composites Part A: Applied Science and Manufacturing, 2018, 104: 24-31. doi: 10.1016/j.compositesa.2017.10.024 [15] ZHANG C, CHI Q, DONG J, et al. Enhanced dielectric properties of poly(vinylidene fluoride) composites filled with nano iron oxide-deposited barium titanate hybrid particles[J]. Sci Rep, 2016, 6: 33508-33512. doi: 10.1038/srep33508 [16] LIU G, CHEN Y, GONG M, et al. Enhanced dielectric performance of PDMS-based three-phase percolative nanocomposite films incorporating a high dielectric constant ceramic and conductive multi-walled carbon nanotubes[J]. Journal of Materials Chemistry C, 2018, 6(40): 10829-10837. doi: 10.1039/C8TC03868F [17] 袁国林, 许振良, 魏永明. 热处理对PVDF-PFSA中空纤维共混超滤膜结构及性能的影响[J]. 华东理工大学学报(自然科学版), 2009, 35(4): 501-505. doi: 10.3969/j.issn.1006-3080.2009.04.001
[18] ZHANG Z, GU Y, WANG S, et al. Enhanced dielectric and mechanical properties in chlorine-doped continuous CNT sheet reinforced sandwich polyvinylidene fluoride film[J]. Carbon, 2016, 107: 405-414. doi: 10.1016/j.carbon.2016.05.068 [19] ROY S, THAKUR P, HOQUE N A, et al. Electroactive and high dielectric folic acid/pvdf composite film rooted simplistic organic photovoltaic self-charging energy storage cell with superior energy density and storage capability[J]. ACS Applied Materials & Interfaces, 2017, 9(28): 24198-24209. [20] FANG X L, LIU X Y, et al. Preparation and properties of thermostable well-functionalized graphene oxide/polyimide composite films with high dielectric constant, low dielectric loss and high strength via in situ polymerization[J]. Journal of Materials Chemistry A, 2015, 3(18): 10005-10012. doi: 10.1039/C5TA00943J [21] CHEN Y, QIAN J, ZHANG K, et al. In situ synthesis and characterization of fluorinated polybenzobisoxazole/silica-coated magnetic Fe3O4 nanocomposites exhibiting enhanced electromagnetic wave absorption property[J]. Polymer Composites, 2015, 36(5): 884-891. doi: 10.1002/pc.23007 [22] GUAN J, XING C, WANG Y, et al. Poly (vinylidene fluoride) dielectric composites with both ionic nanoclusters and well dispersed graphene oxide[J]. Composites Science and Technology, 2017, 138: 98-105. doi: 10.1016/j.compscitech.2016.11.012 [23] 俞丽芸. PVDF-SiO2中空纤维复合膜的制备和表征[J]. 华东理工大学学报(自然科学版), 2010, 36(1): 1-5. doi: 10.3969/j.issn.1008-7672.2010.01.001
[24] Modulation of electromagnetic wave absorption by carbon shell thickness in carbon encapsulated magnetite nanospindles-poly (vinylidene fluoride) composites[J]. Carbon, 2015, 95: 870-878. [25] GENG H, ZHOU Q, PAN Y, et al. Preparation of fluorine-doped, carbon-encapsulated hollow Fe3O4 spheres as an efficient anode material for Li-ion batteries[J]. Nanoscale, 2014, 6(7): 3889. doi: 10.1039/c3nr06409c [26] RAO S R, PRATAP K J, BABU V H. Dielectric and electrical properties of in se films[J]. 1986, 21(7): 913-918. [27] ZHANG Y, ZHANG T D, LIU L Z, et al. Sandwich-structured pvdf-based composite incorporated with hybrid Fe3O4@BN nanosheets for excellent dielectric properties and energy storage performance[J]. The Journal of Physical Chemistry C, 2018, 122(3): 1500-1512. [28] LI L, GAO P, GAI S L, et al. Ultra small and highly dispersed Fe3O4, nanoparticles anchored on reduced graphene for supercapacitor application[J]. Electrochimica Acta, 2016, 190: 566-573. doi: 10.1016/j.electacta.2015.12.137 [29] LI Y, CHEN D, LIU X, et al. Preparation of the PBOPy/PPy/Fe3O4 composites with high microwave absorption performance and thermal stability[J]. Composites Science & Technology, 2014, 100(21): 212-219. [30] FENG H, FANG X, LIU X, et al. Reduced polyaniline decorated reduced graphene oxide/polyimide nanocomposite films with enhanced dielectric properties and thermostability[J]. Composites Part A: Applied Science and Manufacturing, 2018, 109(4): 578-584. [31] KANG S J, PARK Y J, HWANG J, et al. Localized pressure-induced ferroelectric pattern arrays of semicrystalline poly(vinylidene fluoride) by microimprinting[J]. Advanced Materials, 2010, 19(4): 581-586. [32] HUANG X, JIANG P. Core-shell structured high-k polymer nanocomposites for energy storage and dielectric applications[J]. Advanced Materials, 2015, 27(3): 546-554. doi: 10.1002/adma.201401310 [33] DANG Z M, WU J P, XU H P, et al. Dielectric properties of upright carbon fiber filled poly(vinylidene fluoride) composite with low percolation threshold and weak temperature dependence[J]. Applied Physics Letters, 2007, 91(7): 2101-2106. [34] YAO M, YOU S, YONG P. Dielectric constant and energy density of poly (vinylidene fluoride) nanocomposites filled with core-shell structured BaTiO3@Al2O3 nanoparticles[J]. Ceramics International, 2016, 43(3): 1342-1347. [35] LIU J, QIAO S, LIU H, et al. Extension of the stober method to the preparation of monodisperse resorcinol-formaldehyde resin polymer and carbon spheres[M]. The Indianization of China: Harvard University Press, 2011. [36] FENG H, MA W, CUI Z K, et al. Core/shell-structured hyperbranched aromatic polyamide functionalized graphene nanosheets- poly (p-phenylene benzobisoxazole) nanocomposite films with improved dielectric properties and thermostability[J]. Journal of Materials Chemistry A, 2017, 5(18): 8705-8713. doi: 10.1039/C7TA00587C [37] ZHANG P T, HE J J, CUI Z K, et al. Preparation and characterization of STRG/PI composite films with optimized dielectric and mechanical properties[J]. Polymer, 2015, 65: 262-269. doi: 10.1016/j.polymer.2015.04.015 [38] WAN Y J, ZHU P L, YU S H, et al. Barium titanate coated and thermally reduced graphene oxide towards high dielectric constant and low loss of polymeric composites[J]. Composites Science and Technology, 2017, 141: 48-55. doi: 10.1016/j.compscitech.2017.01.010 -