Effect of BaTiO3 nanowires on dielectric properties and energy storage density of polyimide composite films

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Effect of BaTiO3 nanowires on dielectric properties and energy storage density of polyimide composite films M. Wang, W.L. Li, Y. Feng, Y.F. Hou, T.D. Zhang, W.D. Fei, J.H. Yin

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Cite this article as: M. Wang, W.L. Li, Y. Feng, Y.F. Hou, T.D. Zhang, W.D. Fei, J.H. Yin, Effect of BaTiO3 nanowires on dielectric properties and energy storage density of polyimide composite films, Ceramics International, http://dx.doi.org/10.1016/j.ceramint.2015.07.153 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting galley proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

Effect of BaTiO3 nanowires on dielectric properties and energy storage density of polyimide composite films M. Wang,a W. L. Li,a Y. Feng,a Y. F. Hou,a T. D. Zhang,a W. D. Fei,a J. H. Yin,*b a b

School of Materials Science and Engineering, Harbin Institute of Technology, Harbin 150001, P. R. China

Key Laboratory of Engineering Dielectrics and Its Application, Ministry of Education, Harbin University of Science and Technology, Harbin 150080, P. R. China

Abstract BaTiO3 nanowires/polyimide (BT-NWs/PI) and BaTiO3 nanoparticles/polyimide (BT-NPs/PI) composites with low volume (İ10 vol%) fraction of ceramic fillers were successfully prepared by in-situ dispersive polymerization. The BT-NWs were synthesized by a simple one-step hydrothermal method. The microstructure, dielectric properties, and energy storage density of the composites were studied. It was found that the two types of fillers were dispersed homogeneously in the PI matrix. Compared with BT-NPs/PI composites, the dielectric constant of composites filled by BT-NWs was larger at the same concentration. Stronger interfacial polarization mainly determined the dielectric properties that we observed on BT-NWs/PI composites. Furthermore, the energy storage density of the composites was significantly enhanced by BT-NWs, and energy storage density of 1.06 J/cm3 was obtained under an electric field of 2200 kV/cm with the BT-NWs content of 2 vol%, which was 37% lager than that of the pure PI. The results indicate that the introduced wire-like ceramic fillers contribute to the enhancement of the dielectric properties and energy storage density of the composites.

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Keywords: BaTiO3 nanowires; polyimide; dielectric properties; energy storage density *Authors to whom correspondence should be addressed. E-mail: [email protected] Fax:/Tel: +86-451-86390777

1. Introduction With high dielectric constant, easy processability and low cost, composite materials consisting of a polymer matrix and high dielectric constant filler have significant potential applications in modern electric power systems, such as capacitors, communications devices and actuators [1-5]. The most classical approach to enhance the dielectric constant of the polymer composites is to introduce high dielectric constant ceramic filler (e.g., BaTiO3, PbTiO3 and CaCu3Ti4O12) into the polymer [6-8]. To obtain ceramic/polymer composites with high dielectric properties, however, a large volume fraction (˚50 vol%) of ceramic fillers are usually required and this may cause low flexibility, high mass density and poor mechanical strength of the composites [9,10]. Therefore, ceramic/polymer composites with high dielectric performance and low volume fraction could become a big challenge. To overcome these limitations of the composites, recently, some promising studies have been performed based on high aspect ratio dielectric ceramic fillers [11-15]. Many models have shown that high aspect ratio fillers can improve the dielectric constant of the composites [16,17]. And some studies have also demonstrated that fillers with high 2

aspect ratio can improve the dielectric constant of the composites more efficiently as compared to the spherical nanoparticles [14,18]. These meaningful investigations have led many researchers to follow the route to prepare high dielectric constant composites. Due to the challenges in synthesizing nanowires, however, high aspect ratio ceramic fillers in the polymer composites still have not been studied as extensively as compared to that of spherical nanoparticles. Although many researches have been investigated on the influence of nanowires on the dielectric constant, energy storage density and ferroelectric properties, studies about the composites filled by low volume fraction (≤10 vol%) nanowires has been barely investigated. Composites with a low ceramic filler volume (≤10 vol%), a relatively high dielectric constant and a low loss are our purpose. In this study, polyimide (PI) was chosen as the matrix because it has extensive applications due to its superior mechanical, electrical and thermal stable properties [19,20]. A one-step hydrothermal method was used to prepare the BaTiO3 nanowires (BT-NWs), which is simple, time-saving and low cost. Dielectric constant of the composites filled by BT-NWs was improved at low volume fraction (≤10 vol%) and the influence of BT-NWs on the dielectric properties under band (102-107 Hz) has been studied. The energy storage density of composites with different filler contents has also been researched.

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2. Experimental procedure 2.1. Materials Pyromellitic

dianhydride

(PMDA),

4,4′-oxydianiline

(ODA)

and

N,N-dimethylacetamide (DMAc) were obtained from Shanghai Chemical Reagent Company (China). BaTiO3 nanoparticles (BT-NPs) with average diameter in 100 nm were purchased from Aladdin Industrial Corporation. BaTiO3 nanowires (BT-NWs) were synthesized via a simple one-step hydrothermal method. Polyethylene glycol (PEG 6000), potassium hydroxide (KOH), tetrabutyl titanate (TBOT), barium hydroxide octahydrate (Ba(OH)2·8H2O), acetic acid, formic acid, and ethanol were received from Shanghai Chemical Corp. All chemicals were used without further purification. 2.2. Preparation of BT-NWs and PI composites The BT-NWs were prepared by a simple one-step hydrothermal method as following operations: PEG 6000 (1.5 g) was dissolved into ethanol (21 mL). A solution of ethanol (15 mL) containing TBOT (1.5 mmol) was then added with vigorously stirring for 0.5 h. And then, KOH alkaline solution (20 mL, 2.25 g) of ethanol and deionized water was added. Subsequently, the mixture solution was stirred vigorously for 0.5 h and then transferred into a Teflon-lined stainless-steel autoclave with a capacity of 100 mL. All the above processes were carried out at room temperature. Then, Ba(OH)2·8H2O (0.473 g) was added. The autoclave was heated at 200 ℃ for 12 h and subsequently cooled to room temperature. The product was centrifuged and washed by formic acid, ethanol, and deionized water several times, 4

and dried in a vacuum oven at 80 ć for 12 h [21]. BaTiO3/PI composites were prepared via in-situ dispersive polymerization as following operations: DMAc (20 mL) and appropriate amount BT-NWs (or BT-NPs) were placed into a three-neck round-bottom under vigorously stirred under ultrasonication for 2 h at room temperature. Then, ODA (1.5 g) was added to the solution and stirred for 0.5 h. Subsequently, PMDA (1.65 g) was added by several batches into the solution and stirred for 24 h to prepare poly (amic acid) (PAA) solution. The resulting mixture was cast onto a clean glass plate and then thermally imidized at 80 ć for 5 h, 120 ć, 200 ć and 300 ć for 2 h, respectively. As a result, BT-NWs/PI or BT-NPs/PI composites with a thickness of 40 μm were obtained (as shown in scheme 1). 2.3. Characterization The dispersion morphology of the BT-NWs (or BT-NPs) in PI matrix were investigated by a HELIOS NanoLab 600i scanning electron microscope (SEM). The transmission electron microscope (TEM) image of the synthesized BT-NWs was performed on a JEOL JEM-2100. The sample structure was acquired in 2θ scan mode by a Philips X'Pert X-ray diffractometer (XRD), using Cu Kα radiation at 40 kV and 40 mA. The dielectric measurements were performed on a frequency range from 102 to 107 Hz using a broadband dielectric spectral instrument (Agilent 4294A). The polarization-electric field (P-E) hysteresis loops were measured at 100 Hz using a Radiant Technologies Precision work station. Before dielectric and (P-E) hysteresis loop measurements, silver electrodes were coated on both sides of the sample. 5

3. Results and discussion 3.1. Characterization of BT-NWs and BT-NPs The XRD patterns of BT-NWs and BT-NPs are shown in Fig. 1a. It can be seen that all the characteristic peaks can be assigned to the tetragonal phase of BaTiO3 without any impurity phase, which is compared with JCPDS (No.: 81-2203) reference database. Fig. 1b shows the SEM micrograph of the BT-NPs, which are clearly visible with rounded shapes with the size of about 100 nm. The SEM micrograph of the BT-NWs is shown in Fig. 1c, which shows that the products consist of BT-NWs. The TEM image of a typical nanowire is also shown in Fig. 1d. The diameter of this nanowire is about 35 nm and length is about 1300 nm. The aspect ratio of this nanowire is about 37. 3.2 Characterization of composites Fig. 2 shows the XRD patterns of pure PI, BT-NPs/PI and BT-NWs/PI composites. The XRD pattern of pure PI shows a broad peak centered at about 18°, which is a typical amorphous structure. The XRD patterns of the composites exhibit both the characteristic peaks of BaTiO3 and pure PI, which demonstrates that BaTiO3 is completely filled in the polyimide matrix. Fig. 3 shows the SEM images of the fresh fractured cross-section of the pure PI and composites. Fig. 3a is the microstructure of pure PI, which shows that a continuous phase is formed by the PI molecules. The images of Fig. 3b-c present both the BT-NPs and BT-NWs are uniformly distributed throughout the PI matrix without aggregation when the filler content is 10 vol%. However, there is a difference 6

between the above two type composites. The interface between BT-NWs and PI matrix is more vague as compared to that of BT-NPs, which indicating the more excellent compatibility because of the huge interfacial effect between the PI matrix and BT-NWs than that of BT-NPs. 3.3. Dielectric properties of the composites The dependences of the dielectric constant of composites with different contents on the frequency are shown in Fig. 4. It can be clearly seen that the dielectric constant of the composites increase with the filler (BT-NPs, BT-NWs) content increasing. The dielectric constant is 6.6 at 100 Hz when the content of BT-NWs filler is 5 vol%, which is 94% higher than that of pure PI (3.4) and 22% higher than that of 10 vol% BT-NPs/PI (5.4). The dielectric constant of 10 vol% BT-NWs/PI composite is 7.3 at 100 Hz, which is 115% higher than that of pure PI. Interestingly, compared with the BT-NPs/PI composites, the dielectric constant of BT-NWs/PI films shows obviously improvement at relatively lower frequency (İ104 Hz). However, with the increasing frequency (ı104 Hz), the dielectric constant ratio of BT-NWs/PI to BT-NPs/PI composites gradually diminishes. This possibly results from the stronger interfacial polarization existed in the BT-NWs/PI composites, and the interfacial polarization on the dielectric constant decreasing with frequency increasing [22]. Fig. 5 displays the dependences of dielectric loss of composites with different filler contents on the frequency. The dielectric loss of the BT-NWs/PI composites decreases sharply with increasing frequency at lower frequencies and then increases lightly at higher frequencies, whereas it is not very obviously for that of the 7

BT-NPs/PI composites. It is generally believed that the dielectric response at high frequency is mainly associated with dipolar relaxation, while interfacial polarization and conductivity are contributed significantly to the dielectric response at low frequency. Thus, the large variation trends of the dependence of dielectric constant (Fig. 4b) and dielectric loss (Fig. 5b) of composites with different filler contents on frequency indicate that the interfacial polarization in the BT-NWs/PI composites should be much stronger than those in BT-NPs/PI composites. Fig. 6 displays the frequency dependences of alternating current (AC) electrical conductivity of composites with different filler contents at room temperature. For both types of composites, the conductivity exhibits strong dependence on frequency because of its insulating nature. The conductivity of pure PI and 10 vol% BT-NPs/PI composites is 5.4×10-11 and 5.5×10-10 S m-1 at 100 Hz, respectively. The conductivity of 10 vol% BT-NWs/PI composite is 4.8×10-9 S m-1 at 100 Hz, which is much greater than that of BT-NPs/PI composites. This may result from that the BT-NWs are easier to reach to the percolation threshold value than that of BT-NPs in the polymer matrix at the same content because of its shape and high aspect ratio. Although the AC conductivity has an obviously increasing trend with the increasing BT-NWs content (0-10 vol%), the AC conductivity of BT-NWs/PI composite still remains nearly frequency dependent, which indicates that no conductive network was formed. In the view of the previous data on dielectric loss and conductivity, we find that the dielectric properties of BT-NWs/PI composites are similar to that of polymer/conductive fillers: higher dielectric loss and conductivity. Possible reasons 8

for this are as follows. In the paper, BT-NWs are prepared by a hydrothermal method. During the course of hydrothermal reactions, some oxygen vacancies should be introduced to the lattice of the BT-NWs. Thus, a few shallow donors would be formed around the oxygen vacancies. The shallow donors are easy to generate charge carriers under electric field. The character of the carriers generated by shallow donors is similar to that in the conductive fillers [12]. The only difference is that the concentration of carriers generated by shallow donors is lower compared with that in the conductive fillers. Thus, higher dielectric loss and conductivity are observed for BT-NWs/PI composites. 4. Energy storage density of composites Although PI is non-ferroelectric material, the relationship between the polarization and electric field of the BaTiO3/PI composites exhibits a nonlinear characteristic because BaTiO3 is a ferroelectric. Thus, the energy storage density of BaTiO3/PI composites could be calculated from the P-E loops. The P-E loops for composites with different filler contents are measured at frequency of 100 Hz. The P-E loops of the composites are shown in Fig. 7a. It could be found that the maximum and remnant polarization of composites filled by different contents are greater than that of pure PI. Particularly, the polarization of the BT-NWs/PI composites is greatly enhanced. For example, the maximum polarization of the composites with 2 vol% BT-NWs is 1.27 μC/cm2 under the electric field of 2200 kV/cm, which is 74% higher than that of pure PI (0.73 μC/cm2) and 12% higher than that of 5 vol% BT-NPs/PI composite (1.13 μC/cm2). As a typical ferroelectric, BaTiO3 in the polymer matrix 9

exhibit large polarization under the applied electric field. Besides, the distribution of electric field in the composites is distorted because the dielectric constant of BaTiO3 is much larger than that of PI, which leads to higher electric field and larger polarization in the matrix. The energy storage density is calculated according to the formula U=ĥEdD [13]. The energy storage density of composites filled by various contents as function of electric field is shown in Fig. 7b. With the electric filed increasing, the energy storage density of all samples is enhanced obviously and the uptrend of the energy storage density for composites is more significant than that for pure PI. Under the electric field of 2200 kV/cm, as is shown in the inset, the maximal energy storage density of 1.06 J/cm3 was obtained in the composite with 2 vol% BT-NWs, which is 37% higher than that of the pure PI and 6% higher than that of composite with 5 vol% BT-NPs. The leakage phenomenon and unsaturated loops are observed in the composites with 5 vol% and 10 vol% BT-NWs, as shown in the inset of Fig.7a, which result in the lower energy storage density in these composites. Thus, the results indicate that more does not necessarily mean better, and there is a certain volume fraction for the best performance. 5. Conclusions It is found that the dielectric constant of the BT-NWs/PI composite is obviously larger than that of the BT-NPs/PI composite and pure PI. The dielectric constant of the 5 vol% BT-NWs/PI composite is 6.6 at 100 Hz, which is 94% higher than that of pure PI (3.4) and 22% higher than that of 10 vol% BT-NPs/PI (5.4). In addition, the 10

conductivity of the BT-NWs/PI composites increases slowly with an increase in filling concentration, indicating that a good insulating nature. The maximal energy storage density of 1.06 J/cm3 is obtained at 2200 kV/cm in the composite with 2 vol% BT-NWs, which is 37% higher than that of the pure PI. 6. Acknowledgements This work was supported by Open Project of Key Laboratory of Engineering Dielectrics and Its Application (Grant No. DJZ201401) 7. References [1] P. Barber, S. Balasubramanian, Y. Anguchamy, S. Gong, A. Wibowo, H. Gao, H.J. Ploehn, H.C. zur Loye, Polymer composite and nanocomposite dielectric materials for pulse power energy storage, Mater. 2 (2009) 1697-1733. [2] Z.M. Dang, J.K. Yuan, J.W. Zha, T. Zhou, S. T. Li, G.H. Hu, Fundamentals, processes and applications of high-permittivity polymer–matrix composites, Prog. Mater. Sci. 57 (2012) 660-723. [3] Z.M. Dang, T. Zhou, S.H. Yao, J.K. Yuan, J.W. Zha, H.T. Song, J.Y. Li, Q. Chen, W.T. Yang, J. Bai, Advanced calcium copper titanate/polyimide functional hybrid films with high dielectric permittivity, Adv. Mater. 21 (2009) 2077-2082. [4] J. Li, S.I. Seok, B. Chu, F. Dogan, Q. Zhang, Q. Wang, Nanocomposites of ferroelectric polymers with TiO2 nanoparticles exhibiting significantly enhanced electrical energy density, Adv. Mater. 21 (2009) 217-221. [5] Q.G. Chi, C.H. Zhang, X. Wang, J. Sun, L. Gao, X. Wang, Q.Q. Lei, Dielectric properties of PI hybrid film doped by CaCu3Ti3.95Zr0.05O12 ceramics with different particle sizes, Ceram. Int. 40 (2014) 15045-15049. [6] W. Yang, S. Yu, R. Sun, R. Du, Nano and microsize effect of CCTO fillers on the dielectric behavior of CCTO/PVDF composites, Acta. Mater. 59 (2011) 5593-5602. [7] S.H. Xie, B.K. Zhu, X.Z. Wei, Z.K. Xu, Y.Y. Xu, Polyimide/BaTiO3 composites with controllable dielectric properties, Compos. Part A-Appl. S, 36 (2005) 1152-1157. [8] S.F. Wang, Y.R. Wang, K.C. Cheng, Y.P. Hsaio, Characteristics of polyimide/barium titanate composite films, Ceram. Int. 35 (2009) 265-268. [9] M. Arbatti, X. Shan, Z.Y. Cheng, Ceramic-Polymer Composites with High Dielectric Constant, Adv. Mater. 19 (2007) 1369-1372. [10] Q. Chi, J. Sun, C. Zhang, G. Liu, J. Lin, Y. Wang, X. Wang, Q. Lei, Enhanced dielectric performance of amorphous calcium copper titanate/polyimide hybrid film, J. Mater. Chem. C, 2 (2014) 172-177. [11] A.B. Da Silva, M. Arjmand, U. Sundararaj, R.E.S. Bretas, Novel composites of copper nanowire/PVDF with superior dielectric properties, Polym. 55 (2014) 226-234. 11

[12] Y. Feng, W.L. Li, Y.F. Hou, Y. Yu, W.P. Cao, T.D. Zhang, W.D. Fei, Enhanced dielectric properties of PVDF-HFP/BaTiO3-nanowire composites induced by interfacial polarization and wire-shape, J. Mater. Chem. C. 3 (2015) 1250-1260. [13] S.H. Liu, J.W. Zhai, J.W. Wang, S.X. Xue, W.Q. Zhang, Enhanced energy storage density in poly(vinylidene fluoride) nanocomposites by a small loading of suface-hydroxylated Ba0.6Sr0.4TiO3 nanofibers, Acs. Appl. Mater. Inter. 6 (2014) 1533-1540. [14] H. Tang, Z. Zhou, H.A. Sodano, Relationship between BaTiO3 nanowire aspect ratio and the dielectric permittivity of nanocomposites, Acs. Appl. Mater. Inter. 6 (2014) 5450-5455. [15] H. Tang, H.A. Sodano, Ultra high energy density nanocomposite capacitors with fast discharge using Ba0.2Sr0.8TiO3 nanowires, Nano. Lett. 13 (2013) 1373-1379. [16] M.C. Araújo, C.M. Costa, S. Lanceros-Méndez, Evaluation of dielectric models for ceramic/polymer composites: Effect of filler size and concentration, J. Non-Cryst. Solids. 387 (2014) 6-15. [17] S.K. Patil, M.Y. Koledintseva, R.W. Schwartz, W. Huebner, Prediction of effective permittivity of diphasic dielectrics using an equivalent capacitance model, J. Appl. Phys. 104 (2008) 074108. [18] H. Tang, Y. Lin, H.A. Sodano, Synthesis of high aspect ratio BaTiO3 nanowires for high energy density nanocomposite capacitors, Adv. Energy. Mater. 3 (2013) 451-456. [19] U.A. Yano K, Okada A, Synthesis and properties of polyimide-clay hybrid films, J. Polym. Sci. Pol. Chem. 35 (1997) 2289-2294. [20] C. Wang, Q.H. Wang, T.M. Wang, Simple method for preparation of porous polyimide film with an ordered surface based on in situ self-assembly of polyamic acid and silica microspheres, Langmuir, 26 (2010) 18357-18361. [21] J. Yang, J. Zhang, C. Liang, M. Wang, P. Zhao, M. Liu, J. Liu, R. Che, Ultrathin BaTiO 3 nanowires with high aspect ratio: a simple one-step hydrothermal synthesis and their strong microwave absorption, Acs. Appl. Mater. Inter. 5 (2013) 7146-7151. [22] Y. Feng, J.H. Yin, M.H. Chen, M.X. Song, B. Su, Q.Q. Lei, Effect of nano-TiO2 on the polarization process of polyimide/TiO2 composites, Mater. Lett. 96(2013) 113-116.

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Figure captions Scheme 1 Schematic diagram of the preparation of BT-NWs/PI composites Fig. 1 (a) XRD pattern of BT-NWs and BT-NWs, SEM image of (b) BT-NPs, (c) BT-NWs, (d) TEM image of the BT-NW Fig. 2 XRD patterns of PI, BT-NPs/PI and BT-NWs /PI composites Fig. 3 Cross section SEM images, (a) pure PI, (b) 10 vol% BT-NPs/PI, (c) 10 vol% BT-NWs/PI Fig. 4 Dependences of the dielectric constant of (a) BT-NPs/PI composites and (b) BT-NWs/PI composites with different filler contents on the frequency at room temperature Fig. 5 Dependences of the tan δ of (a) BT-NPs /PI composites and (b) BT-NWs /PI composites with different filler contents on the frequency at room temperature Fig. 6 Frequency dependences of the electrical conductivity of BT-NPs /PI composites (a) and BT-NWs/PI composites (b) with different filler contents at room temperature Fig. 7 (a) The P-E loops of the composites with different filler contents, and the inset figure is the P-E loops of the composites with 5 vol% and 10 vol% BT-NWs. (b) The energy storage density of the composites with different filler contents, and the inset figure is the comparison of energy storage density of 2 vol% BT-NWs/PI, 2 vol% and 5 vol% BT-NPs/PI at 2200 kV/cm

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FIG7