介電電容器是一種重要的電子器件[1] 聚合物介電電容器易加工、擊穿場強高、儲能性能高和損耗較低，得到了廣泛的應用[2~4] 目前制造商用介電薄膜電容器的主要材料，是雙軸拉伸聚丙烯(BOPP)材料 這種材料的擊穿場強高達700 MV·m-1，但是其可釋放能量密度只約為2 J·cm-3，難以滿足使用要求[5,6] 因此，提高介電聚合物薄膜的儲能性能是當前的研究重點

高分子聚合物具有優(yōu)異的可加工性、良好的柔韌性、較高的擊穿場強和較低的介電損耗，且能大面積成膜[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)，聚酰亞胺(PI)以及聚偏氟乙烯(PVDF)[9,10] 電介質薄膜的儲能密度可表示為[11]

Ue=∫DrDmaxEDd

(1)

此式表明，電介質材料的擊穿場強(Eb)、剩余電位移強度(Dr)和最大電位移(Dmax)是影響電介質薄膜儲能密度的關鍵因素 因此，提高電介質材料儲能密度的關鍵，是降低Dr和提高其擊穿場強和Dmax PP、PC等線性介電聚合物雖然具有較大的擊穿場強和較大的充放電速率，但是其非極性本質使其極化值較低、Dmax小和可釋放儲能密度較低 以PVDF為代表的鐵電聚合物極化值較高，能提供較高的可釋放能量密度[12,13] 但是，PVDF固有的高介電損耗使其充放電效率較低 這意味著，在能量轉換過程中很大一部分轉換為熱能，使電容器升溫和失效，不利于電容器的安全運行[14] 減少PVDF能量損失的方法，包括納米復合、化學改性和聚合物共混等[15~17] 其中聚合物共混策略是一種既簡單又經濟有效的方法，能在不犧牲PVDF基聚合物可釋放儲能密度的情況下降低其能量損失[18] 線性介電聚合物/PVDF二元共混物受到了極大的關注 這種二元共混物，在理論上是一種低損耗線性聚合物 此外，線性介電聚合物能減弱相鄰PVDF鐵電體之間的耦合域，最大限度地減少鐵電損失和能量損失 Yang等[18]將ABS與PVDF共混制備出均勻的復合薄膜，實現了性能的優(yōu)化 本文選用具有優(yōu)異的機械性、耐化學性、熱穩(wěn)定性的聚酰亞胺(PI)，將共沉淀法和熱壓法相結合制備PI/PVDF全有機復合薄膜，研究其儲能性能

1 實驗方法1.1 薄膜的制備

圖1給出了全有機復合薄膜的制備流程 制備步驟：(1)將一定量的聚偏氟乙烯(PVDF)粉末和熱塑型聚酰亞胺(PI)加入容積為4 mL的N,N-二甲基甲酰胺(DMF，分析純)中，將其置于65℃的加熱臺上使其完全溶解；(2) 在500 mL燒杯中倒入200 mL純水及200 mL無水乙醇，用磁力攪拌器攪拌，轉速為550 r/min；(3) 將步驟(1)中的混合溶液緩慢滴加入步驟(2)的燒杯中，收集析出的絮狀物；(4) 將絮狀物抽濾(SHZ-D(III)循環(huán)水式多用真空泵)、烘干(烘箱，DZF-6020)后熱壓(熱壓機，YLJ-HP300)，烘干溫度為60℃，時間為12 h，熱壓溫度為155℃，熱壓時間為2 h 熱壓后得到全有機復合薄膜

圖1



圖1PI/PVDF復合薄膜的制備流程

Fig.1Preparation of PI/PVDF composite film

改變PI的加入量，可制備出不同配比的全有機復合薄膜 PI的加入量(質量分數)分別為PVDF的0%，5%，10%，15%，20%，100%，將制備出的樣品分別標記為0/100，5/95，10/90，15/85，20/80，100/0

1.2 性能表征

用場發(fā)射掃描電子顯微鏡(SEM，Hitachi SU8010)分析復合薄膜的截面；用X-射線衍射(XRD)儀表征不同薄膜的晶體結構，測試條件為：Cu-Kα靶，波長0.154 nm，掃描角2θ的變化范圍為5°~60°，掃描速率為0.1 (°)·s-1 用差示掃描量熱法(DSC7020)記錄復合材料的熔融與結晶行為，溫度測試范圍為90℃~190℃，加熱速率為10℃·min-1 用阻抗分析儀(HP4294，Agilent)測試復合材料的室溫介電性能 用介電耐壓測試儀測試復合材料的擊穿場強 用鐵電測試系統(tǒng)(TF2000，Trek 10/10B-HS)測試位移-電場(D-E)回線

2 結果和討論2.1 全有機復合薄膜的微觀結構

圖2給出了PI/PVDF全有機復合薄膜的截面SEM照片 從圖2a~e可見，用該方法制備的全有機薄膜的厚度約為18 μm 與純PVDF薄膜的截面(圖2a)相比，PI的加入沒有產生明顯的空隙和孔洞(圖2b~e)，復合薄膜的結構依舊比較致密，驗證了共沉淀法與熱壓法相結合的優(yōu)越性 增大PI的添加量則PI線性介電材料的特征更加明顯，可釋放儲能密度急劇降低，因此只討論PI在低添加量時的情況 圖2f~i給出了20/80組分的SEM元素映射圖，C元素和F元素屬于PVDF，O元素和N元素屬于PI 與預期的一樣，O元素和N元素在20/80復合材料的斷口處的分散相當均勻 綜上所述，SEM測試結果表明，共沉淀法與熱壓法相結合制備的全有機復合薄膜結構均勻、致密

圖2



圖2PI/PVDF復合薄膜截面的SEM形貌和(f-i)20/80組分的SEM元素映射圖

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

PVDF薄膜的性能與其晶相結構緊密相關，PVDF 主要有α、β與γ相，其中α和γ相極性較小，鐵電損耗較小，適用于儲能領域[19~21] 全有機復合薄膜的晶相結構，如圖3所示 可以看出，在純PVDF衍射譜的18.4°和19.8°處出現了兩個衍射強峰，分別對應(020)晶面和(021)晶面的α相，說明純PVDF具有以α相為主的相結構 由PI/PVDF共混膜的XRD譜可見，PI的加入使18.4°處的衍射峰分裂成17.7°和18.5°這兩個小衍射峰，分別歸屬于(100)晶面的α相衍射和(020)晶面的γ相衍射 PI的加入對PVDF薄膜的相結構沒有較大影響，復合薄膜依舊是α相為主導，意味著復合薄膜應該較好的儲能性能

圖3



圖3PI/PVDF復合薄膜的XRD譜

Fig.3XRD patterns of PI/PVDF composite film

為了進一步分析樣品的結晶性能，DSC測試結果如圖4a所示 從DSC曲線可觀察到全有機復合薄膜的熔融峰(Tc)約為167℃ α-PVDF和β-PVDF的Tc均約為167℃，可見DSC測試不能完全區(qū)分PVDF薄膜的晶相，只能作為XRD測試的輔助[22,23] 結合上述XRD測試，可見全有機復合薄膜均是以α相為主導 隨著PI加入量的增加共混物的Tc呈略微單調的上升趨勢，表明PI在PVDF內部的相互作用促進了PVDF的成核，分子鏈的纏結作用使Tc的略微上升 分子鏈相互作用引起的阻礙效應，也反映在結晶度值上

圖4



圖4PI/PVDF復合薄膜的DSC曲線和結晶度

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

根據DSC測試結果，可計算材料的結晶度[24]

Xc=?HC?H×100%

(2)

其中?HC為DSC測試中獲得的材料的熔融熱焓值，?H為100%結晶的PVDF的熔融熱焓值(此處為純α-PVDF的熔融熱焓值93.07 J·g-1) 如圖4b所示，隨著PI含量的提高全有機復合薄膜的結晶度呈明顯降低的趨勢 例如，純PVDF的結晶度為41.8%，20/80復合薄膜的結晶度僅為36.8%

2.2 全有機復合薄膜的電學性能

圖5a給出了PI/PVDF復合薄膜的室溫相對介電常數(εr)和介電損耗正切角(tanδ)隨頻率的變化曲線 用該方法制備的純PVDF薄膜其室溫介電常數約為13(@1k Hz)，隨著PI添加量的增加復合薄膜的介電常數略降低 其原因是，PI的介電常數較低而PVDF的介電常數主要受晶相與結晶度影響，結晶度的降低使對應的介電常數降低 PI的加入對tanδ 的影響微弱，因為這種全有機薄膜具有較為致密的結構 圖5b給出了用Weibull分布法計算的PI/PVDF全有機復合薄膜的擊穿場強 Weibull分布反映薄膜發(fā)生介電擊穿的概率，其計算方法為[25]

圖5



圖5PI/PVDF復合薄膜的介電和鐵電性能

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為測試時薄膜的擊穿強度，P為在E下發(fā)生擊穿的概率，Eb為擊穿概率為63.2%時電場強度的大小，β為擬合直線斜率 由圖5b可見，純PVDF的擊穿場強Eb為354 MV·m-1，PI的加入略微降低了薄膜的擊穿場強，但是影響不大，因為低添加量時PI與PVDF良好的結合性，材料的致密度較高

圖5c給出了PI/PVDF全有機復合薄膜在300 MV·m-1電場下的D-E曲線 可以看出，PI的加入使剩余電位移降低，最大電位移增大，且在PI/PVDF為5/95時達到飽和最大電位移 在300 MV·m-1電場下5/95全有機復合薄膜的Dr為1.3 μC·cm-2，Dmax為7.2 μC·cm-2，而在相同情況下純PVDF薄膜的Dr為2.4 μC·cm-2，Dmax為6.5 μC·cm-2 Dr的減小反映了全有機復合薄膜內部較低的鐵電損耗和電導損耗，因為PI和PVDF之間強的相互作用和PI較低的鐵電損耗 XRD測試結果表明，復合薄膜中還有少量的γ相結構，有利于抑制薄膜的鐵電損耗 因此，添加PI使Dr明顯減小[22] 同時，PI的加入提高了Dmax 根據單極D-E曲線計算出PI/PVDF全有機薄膜的儲能密度、可釋放儲能密度及充放電效率，結果在圖5d中給出 純PVDF在300 MV·m-1時可釋放儲能密度約為4.67 J·cm-3，5/95復合薄膜在300 MV·m-1時可釋放儲能密度可達6.52 J·cm-3，是純PVDF的1.4倍 同時，PI/PVDF全有機復合薄膜的放電效率優(yōu)于純PVDF，在300 MV·m-1內PI/PVDF全有機復合薄膜的充放電效率可保持在50%以上，而純PVDF的充放電效率在200 MV·m-1就急劇下降到50% 例如，在300 MV·m-1時5/95復合薄膜的充放電效率為50.4%，而純PVDF的充放電效率僅為38.21% PI/PVDF全有機復合薄膜的高充放電效率，伴隨著較高的放電能量密度

3 結論

(1) 將共沉淀法與熱壓法相結合制備的PI/PVDF薄膜，具有致密的結構

(2) 添加量較低的PI分散性良好且具有界面極化效應，加入PI使薄膜的εr略微降低、tanδ的變化較小

(3) PI的加入提高了PVDF薄膜的可釋放儲能密度，PI添加量為5%的復合薄膜在300 MV·m-1電場下可釋放儲能密度達到6.52 J·cm-3 在300 MV·m-1條件下5/95復合薄膜的充放電效率為50.4%

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Polymer dielectrics having high dielectric constant, high temperature capability, and low loss are attractive for a broad range of applications such as film capacitors, gate dielectrics, artificial muscles, and electrocaloric cooling. Unfortunately, it is generally observed that higher polarization or dielectric constant tends to cause significantly enhanced dielectric loss. It is therefore highly desired that the fundamental physics of all types of polarization and loss mechanisms be thoroughly understood for dielectric polymers. In this Perspective, we intend to explore advantages and disadvantages for different types of polarization. Among a number of approaches, dipolar polarization is promising for high dielectric constant and low loss polymer dielectrics, if the dipolar relaxation peak can be pushed to above the gigahertz range. In particular, dipolar glass, paraelectric, and relaxor ferroelectric polymers are discussed for the dipolar polarization approach.

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Nanjing:

Nanjing University of Aeronautics and Astronautics, 2017

楊 路.

聚偏氟乙烯基鐵電復合材料的制備及其電性能研究

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南京航空航天大學, 2017

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核殼結構納米纖維摻雜PMMA/PVDF基復合介質的儲能特性研究

1

2021