热塑型聚酰亚胺/聚偏氟乙烯全有机复合薄膜的制备及其介电储能

介电电容器是一种重要的电子器件[1] 聚合物介电电容器易加工、击穿场强高、储能性能高和损耗较低，得到了广泛的应用[2~4] 目前制造商用介电薄膜电容器的主要材料，是双轴拉伸聚丙烯(BOPP)材料 这种材料的击穿场强高达700 MV·m-1，但是其可释放能量密度只约为2 J·cm-3，难以满足使用要求[5,6] 因此，提高介电聚合物薄膜的储能性能是当前的研究重点

高分子聚合物具有优异的可加工性、良好的柔韧性、较高的击穿场强和较低的介电损耗，且能大面积成膜[7,8] 高分子聚合物，主要有聚丙烯(PP)，聚乙烯(PE)，聚甲基丙烯酸甲酯(PMMA)，聚碳酸酯(PC)，聚酰亚胺(PI)以及聚偏氟乙烯(PVDF)[9,10] 电介质薄膜的储能密度可表示为[11]

Ue=∫DrDmaxEDd

(1)

此式表明，电介质材料的击穿场强(Eb)、剩余电位移强度(Dr)和最大电位移(Dmax)是影响电介质薄膜储能密度的关键因素 因此，提高电介质材料储能密度的关键，是降低Dr和提高其击穿场强和Dmax PP、PC等线性介电聚合物虽然具有较大的击穿场强和较大的充放电速率，但是其非极性本质使其极化值较低、Dmax小和可释放储能密度较低 以PVDF为代表的铁电聚合物极化值较高，能提供较高的可释放能量密度[12,13] 但是，PVDF固有的高介电损耗使其充放电效率较低 这意味着，在能量转换过程中很大一部分转换为热能，使电容器升温和失效，不利于电容器的安全运行[14] 减少PVDF能量损失的方法，包括纳米复合、化学改性和聚合物共混等[15~17] 其中聚合物共混策略是一种既简单又经济有效的方法，能在不牺牲PVDF基聚合物可释放储能密度的情况下降低其能量损失[18] 线性介电聚合物/PVDF二元共混物受到了极大的关注 这种二元共混物，在理论上是一种低损耗线性聚合物 此外，线性介电聚合物能减弱相邻PVDF铁电体之间的耦合域，最大限度地减少铁电损失和能量损失 Yang等[18]将ABS与PVDF共混制备出均匀的复合薄膜，实现了性能的优化 本文选用具有优异的机械性、耐化学性、热稳定性的聚酰亚胺(PI)，将共沉淀法和热压法相结合制备PI/PVDF全有机复合薄膜，研究其储能性能

1 实验方法1.1 薄膜的制备

图1给出了全有机复合薄膜的制备流程 制备步骤：(1)将一定量的聚偏氟乙烯(PVDF)粉末和热塑型聚酰亚胺(PI)加入容积为4 mL的N,N-二甲基甲酰胺(DMF，分析纯)中，将其置于65℃的加热台上使其完全溶解；(2) 在500 mL烧杯中倒入200 mL纯水及200 mL无水乙醇，用磁力搅拌器搅拌，转速为550 r/min；(3) 将步骤(1)中的混合溶液缓慢滴加入步骤(2)的烧杯中，收集析出的絮状物；(4) 将絮状物抽滤(SHZ-D(III)循环水式多用真空泵)、烘干(烘箱，DZF-6020)后热压(热压机，YLJ-HP300)，烘干温度为60℃，时间为12 h，热压温度为155℃，热压时间为2 h 热压后得到全有机复合薄膜

图1



图1PI/PVDF复合薄膜的制备流程

Fig.1Preparation of PI/PVDF composite film

改变PI的加入量，可制备出不同配比的全有机复合薄膜 PI的加入量(质量分数)分别为PVDF的0%，5%，10%，15%，20%，100%，将制备出的样品分别标记为0/100，5/95，10/90，15/85，20/80，100/0

1.2 性能表征

用场发射扫描电子显微镜(SEM，Hitachi SU8010)分析复合薄膜的截面；用X-射线衍射(XRD)仪表征不同薄膜的晶体结构，测试条件为：Cu-Kα靶，波长0.154 nm，扫描角2θ的变化范围为5°~60°，扫描速率为0.1 (°)·s-1 用差示扫描量热法(DSC7020)记录复合材料的熔融与结晶行为，温度测试范围为90℃~190℃，加热速率为10℃·min-1 用阻抗分析仪(HP4294，Agilent)测试复合材料的室温介电性能 用介电耐压测试仪测试复合材料的击穿场强 用铁电测试系统(TF2000，Trek 10/10B-HS)测试位移-电场(D-E)回线

2 结果和讨论2.1 全有机复合薄膜的微观结构

图2给出了PI/PVDF全有机复合薄膜的截面SEM照片 从图2a~e可见，用该方法制备的全有机薄膜的厚度约为18 μm 与纯PVDF薄膜的截面(图2a)相比，PI的加入没有产生明显的空隙和孔洞(图2b~e)，复合薄膜的结构依旧比较致密，验证了共沉淀法与热压法相结合的优越性 增大PI的添加量则PI线性介电材料的特征更加明显，可释放储能密度急剧降低，因此只讨论PI在低添加量时的情况 图2f~i给出了20/80组分的SEM元素映射图，C元素和F元素属于PVDF，O元素和N元素属于PI 与预期的一样，O元素和N元素在20/80复合材料的断口处的分散相当均匀 综上所述，SEM测试结果表明，共沉淀法与热压法相结合制备的全有机复合薄膜结构均匀、致密

图2



图2PI/PVDF复合薄膜截面的SEM形貌和(f-i)20/80组分的SEM元素映射图

Fig.2Cross-sectional SEM of PI/PVDF composite film (a~e) and SEM element mapping of the 20/80 component (f~i)

PVDF薄膜的性能与其晶相结构紧密相关，PVDF 主要有α、β与γ相，其中α和γ相极性较小，铁电损耗较小，适用于储能领域[19~21] 全有机复合薄膜的晶相结构，如图3所示 可以看出，在纯PVDF衍射谱的18.4°和19.8°处出现了两个衍射强峰，分别对应(020)晶面和(021)晶面的α相，说明纯PVDF具有以α相为主的相结构 由PI/PVDF共混膜的XRD谱可见，PI的加入使18.4°处的衍射峰分裂成17.7°和18.5°这两个小衍射峰，分别归属于(100)晶面的α相衍射和(020)晶面的γ相衍射 PI的加入对PVDF薄膜的相结构没有较大影响，复合薄膜依旧是α相为主导，意味着复合薄膜应该较好的储能性能

图3



图3PI/PVDF复合薄膜的XRD谱

Fig.3XRD patterns of PI/PVDF composite film

为了进一步分析样品的结晶性能，DSC测试结果如图4a所示 从DSC曲线可观察到全有机复合薄膜的熔融峰(Tc)约为167℃ α-PVDF和β-PVDF的Tc均约为167℃，可见DSC测试不能完全区分PVDF薄膜的晶相，只能作为XRD测试的辅助[22,23] 结合上述XRD测试，可见全有机复合薄膜均是以α相为主导 随着PI加入量的增加共混物的Tc呈略微单调的上升趋势，表明PI在PVDF内部的相互作用促进了PVDF的成核，分子链的缠结作用使Tc的略微上升 分子链相互作用引起的阻碍效应，也反映在结晶度值上

图4



图4PI/PVDF复合薄膜的DSC曲线和结晶度

Fig.4DSC curve and crystallinity of PI/PVDF composite film (a) The melting DSC traces of samples, (b) Crystallinity of samples

根据DSC测试结果，可计算材料的结晶度[24]

Xc=?HC?H×100%

(2)

其中?HC为DSC测试中获得的材料的熔融热焓值，?H为100%结晶的PVDF的熔融热焓值(此处为纯α-PVDF的熔融热焓值93.07 J·g-1) 如图4b所示，随着PI含量的提高全有机复合薄膜的结晶度呈明显降低的趋势 例如，纯PVDF的结晶度为41.8%，20/80复合薄膜的结晶度仅为36.8%

2.2 全有机复合薄膜的电学性能

图5a给出了PI/PVDF复合薄膜的室温相对介电常数(εr)和介电损耗正切角(tanδ)随频率的变化曲线 用该方法制备的纯PVDF薄膜其室温介电常数约为13(@1k Hz)，随着PI添加量的增加复合薄膜的介电常数略降低 其原因是，PI的介电常数较低而PVDF的介电常数主要受晶相与结晶度影响，结晶度的降低使对应的介电常数降低 PI的加入对tanδ 的影响微弱，因为这种全有机薄膜具有较为致密的结构 图5b给出了用Weibull分布法计算的PI/PVDF全有机复合薄膜的击穿场强 Weibull分布反映薄膜发生介电击穿的概率，其计算方法为[25]

图5



图5PI/PVDF复合薄膜的介电和铁电性能

Fig.5Dielectric and ferroelectric performance of PI/PVDF composite film (a) room temperature dielectric constant εr and dielectric loss tanδ versus frequency, (b) weibull distribution, (c) D-E loops, (d) discharged energy density and charge-discharge efficiencies

PE=1-e-(EEb)β

(3)

其中E为测试时薄膜的击穿强度，P为在E下发生击穿的概率，Eb为击穿概率为63.2%时电场强度的大小，β为拟合直线斜率 由图5b可见，纯PVDF的击穿场强Eb为354 MV·m-1，PI的加入略微降低了薄膜的击穿场强，但是影响不大，因为低添加量时PI与PVDF良好的结合性，材料的致密度较高

图5c给出了PI/PVDF全有机复合薄膜在300 MV·m-1电场下的D-E曲线 可以看出，PI的加入使剩余电位移降低，最大电位移增大，且在PI/PVDF为5/95时达到饱和最大电位移 在300 MV·m-1电场下5/95全有机复合薄膜的Dr为1.3 μC·cm-2，Dmax为7.2 μC·cm-2，而在相同情况下纯PVDF薄膜的Dr为2.4 μC·cm-2，Dmax为6.5 μC·cm-2 Dr的减小反映了全有机复合薄膜内部较低的铁电损耗和电导损耗，因为PI和PVDF之间强的相互作用和PI较低的铁电损耗 XRD测试结果表明，复合薄膜中还有少量的γ相结构，有利于抑制薄膜的铁电损耗 因此，添加PI使Dr明显减小[22] 同时，PI的加入提高了Dmax 根据单极D-E曲线计算出PI/PVDF全有机薄膜的储能密度、可释放储能密度及充放电效率，结果在图5d中给出 纯PVDF在300 MV·m-1时可释放储能密度约为4.67 J·cm-3，5/95复合薄膜在300 MV·m-1时可释放储能密度可达6.52 J·cm-3，是纯PVDF的1.4倍 同时，PI/PVDF全有机复合薄膜的放电效率优于纯PVDF，在300 MV·m-1内PI/PVDF全有机复合薄膜的充放电效率可保持在50%以上，而纯PVDF的充放电效率在200 MV·m-1就急剧下降到50% 例如，在300 MV·m-1时5/95复合薄膜的充放电效率为50.4%，而纯PVDF的充放电效率仅为38.21% PI/PVDF全有机复合薄膜的高充放电效率，伴随着较高的放电能量密度

3 结论

(1) 将共沉淀法与热压法相结合制备的PI/PVDF薄膜，具有致密的结构

(2) 添加量较低的PI分散性良好且具有界面极化效应，加入PI使薄膜的εr略微降低、tanδ的变化较小

(3) PI的加入提高了PVDF薄膜的可释放储能密度，PI添加量为5%的复合薄膜在300 MV·m-1电场下可释放储能密度达到6.52 J·cm-3 在300 MV·m-1条件下5/95复合薄膜的充放电效率为50.4%

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