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Electrical and mechanical properties of polyimide composite films reinforced by ultralong titanate nanotubes
Surface & Coatings Technology 360 (2019) 13–19
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Electrical and mechanical properties of polyimide composite films reinforced by ultralong titanate nanotubes
T
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He Zhaoa, Chen Yangc, Na Lic, Jinghua Yina,b, , Yu Fengb, Yuanyuan Liua, Jialong Lia, ⁎⁎ Yanpeng Lia, Dong Yuec, Congcong Zhua, Xiaoxu Liud, a
School of Materials Science and Engineering, Harbin University of Science and Technology, Harbin 150080, China Key Laboratory of Engineering Dielectric and Its Application, Ministry of Education, Harbin University of Science and Technology, Harbin 150040, China c Heilongjiang University of Science and Technology, Harbin 150027, China d School of Material Science and Engineering, Shaanxi University of Science and Technology, Xi'an 710021, China b
A R T I C LE I N FO
A B S T R A C T
Keywords: Polyimide Composite Dielectric Titanate nanotubes Mechanical properties
Ultralong titanate nanotubes (TNs) were successfully synthesized by using hydrothermal method. The TNs were introduced into polyimide (PI) to prepare a kind of promising PI/TNs composite films. The effects of the doping contents on electrical and mechanical properties of the PI/TNs composites were investigated. Results show that the electrical and mechanical properties of the PI/TNs composites are improved remarkably compared to that of the pure PI. With only 0.25 wt% TNs loading, the elongation at break can reach 60.3%, about 235.1% greater than that of the pure PI. In addition, the corona-resistant lifetime of the PI/TNs composite doped with TNs content of 3 wt% improved about ten times higher than the pure PI. The compatibility and interaction between TNs and PI matrix were investigated by the scanning electron microscopy (SEM) and synchrotron radiation small angle X-ray scattering (SAXS). How the microstructure and interface of the PI/TNs composites influence the electrical and mechanical performances was discussed in detail. This study on the PI/TNs composites provides a possibility for realizing a potential application in the high strength insulation devices.
1. Introduction Nowadays, polymer-based composites using as advanced functional materials have attracted more and more attention [1–10] due to their relatively excellent combining properties of organic component, such as flexibility, low dielectric properties, high breakdown strength, durability and super thermal stability, etc. [11–14]. In addition, it was found that the incorporation between inorganic nanoparticles and polymer matrix can affect the mechanical properties of polymer significantly [15–17]. However, the incorporation is closely relative to the characteristics of nanoparticles as well as their interaction with the polymer matrix to improve polymer properties, mainly including nanoparticles type, size, shape and doping concentration. Especially, the addition of inorganic fillers such as 0-D nanoparticles, 1-D nanowires, nanofibers and nanotubes, 2-D nanosheets into polymer matrix can obviously enhance some composite properties [18–22]. It is well known that PI as typical engineering polymer materials due to its excellent electrical insulation, thermal properties and chemical resistances, has been widely used as insulation materials, microelectronics, even
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aerospace and aircraft parts in the past years [23–25]. However, the relatively low electrical and mechanical properties for the pure PI limit its demands in some special applications [26]. To overcome the above limits, the inorganic nanoparticles such as Al2O3 [27], SiO2 [28], AlN [29], TiO2 [30], BaTiO3 [15] and the others [31] have been introduced into the PI matrix to enhance their mechanical and electrical properties. At present, an urgently solved problem is the poor dispersibility resulted in the restacking and aggregation of nanoparticles in PI matrix. Besides, the interfaces between polymer and nanoparticles has aroused researcher's concerns, such as Lewis pointed out that the interaction region was a conductive layer [32], Nelson also built a dielectric bilayer structure model [33], and Tanaka further constructed a famous multicore model theory of the interface [34]. All those provide great inspiration for researchers to study the interfaces between organic and inorganic phases. More important, the interfaces are considered as a major factor relative to the mechanical and electrical properties of the composites. Titanate nanotubes (TNs), as the typical 1-D materials, have been widely used in photocatalysts, drug delivery and solar cells due to its large surface areas, chemical inertness, excellent thermal
Correspondence to: J. Yin, School of Materials Science and Engineering, Harbin University of Science and Technology, Harbin 150080, China. Corresponding author. E-mail addresses: [email protected] (J. Yin), [email protected] (X. Liu).
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https://doi.org/10.1016/j.surfcoat.2019.01.013 Received 31 August 2018; Received in revised form 7 December 2018; Accepted 6 January 2019 Available online 07 January 2019 0257-8972/ © 2019 Elsevier B.V. All rights reserved.
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Fig. 1. Schematic flow of in-situ polymerization preparing processes for PI/TNs composite film.
causes Van der Waals forces to be increased, resulting in TNs agglomerating into entangled aggregates in the synthesis process. Therefore, the decomposition of these initial aggregates and the release of independent and separated nanotubes without significant destroying can be achieved by long-term stirring in water suspensions. In a typical preparation, 0.2 g of titanium dioxide nanoparticles were added to a 30 mL of 10 M NaOH solution, stirring for 1 h to form a stable suspension. To ensure adequate internal pressure, put the stable suspension in a 45 mL PTFE beaker, and then was placed in a Teflon lined autoclave at 130 °C for 24 h. Rotational speed plays an important role in the formation of titanate nanotube length. It was synthesized at a rotating speed of 500 rpm under a static condition. The obtained white flocculation deposits were collected after being cooled to room temperature. Then, the white deposits were treated for 24 h in a 0.1 M hydrochloric acid (HCL) to complete ion exchanges. Samples were centrifuged repeatedly with deionized water and alcohol until PH reached 7. Finally drying the samples at 60 °C for 24 h. After repeating the above experiments for 3 times, 0.6 g TNs can be obtained.
stability and low cost of preparation since its discovery in 1997 [35,36]. For example, Mikesova et al., obtained the excellent mechanical properties in polypropylene/titanate nanotubes composite with 5 wt% titanate nanotubes, with an improvement of the stiffness and microhardness about 27% and 33%, respectively [37], indicating the mechanical properties of nanocomposites is closely relative to the changes of polymer matrix supramolecular structure. Porras et al., reported that the PEO/CS (6:1) based composites with 25 wt% titanate nanotubes exhibited 2.6 times as hard as the neat polymer blend and 3.4 times stiffer [38]. Olariu et al., reported that the PI based composites doped with the modified TiO2 nanotubes of 10 wt% and 20 wt% exhibited an improved glass transition temperature from 205 to 249 °C and an enhanced dielectric constant about 5.34 at 20 wt% TNs loading [39]. These results show that the combination of titanate nanotubes (TNs) with polymer matrix plays a critical role for improving the dielectric, thermal, and mechanical properties of resulting composites. But, lots of research efforts concentrate on the thermal and mechanical behaviors of polymer composites at a high TNs loading and with TNs length only several hundred nanometers. Compared with ordinary length TNs, the ultralong TNs can introduce long migration paths of charge carriers to improve breakdown strength of the composites [40,41]. In addition, the ultralong TNs have larger aspect ratio, which can form a large number of interfaces between the TNs and inorganic phase to promote the exchange coupling effect of the dipolar interface layer, resulting in high polarization level and dielectric response [42]. To our best knowledge, the interfacial behavior and the electrical, mechanical properties of PI based composites with superior length TNs at ultralow contents have not been reported. In this study, the ultralong titanate nanotubes with a long length (4–6 μm) were successfully synthesized using a hydrothermal method with stirring. The effects of the incorporation between TNs and PI matrix on the mechanical and electrical properties of PI composites were investigated. The PI/TNs composites exhibit good electrical properties, especially showing a significantly enhancement in mechanical properties at a very low TNs doping. The synthesis of PI/TNs composite films with superior mechanical properties provides a new way for developing high strength polyimide based composites.
2.2. In-situ synthesis of PI/TNs composite film The TNs (0.6 g) was dispersed in N,N‑dimethylacetamide (DMAC) (100 mL) by ultrasonic mixing for 3 h. After that no deposits was found, indicating that the TNs have been dispersed in the DMAC very well. Equivalent molar ratios of 3 g 4,4′‑oxydianiline (ODA) and 3.28 g pyromellitic dianhydride (PMDA) were dissolved in the TNs/DMAC dispersion solution to make the solutions containing 0.25, 0.5, 1 and 3 wt % of TNs, respectively. When the molar ratio of PMDA and ODA is 1.01:1, the viscosity of solution reaches the maximum. Sufficient molecular weight polyamic acid (PAA)/TNs were obtained. The PAA/TNs was cast onto a glass dish using a doctor blade, thereafter dried at 80 °C overnight. Then the PAA/TNs films were thermally imidized at 120 °C, 180 °C, 240 °C, 300 °C, 350 °C for 1 h, respectively. The light yellow, soft and transparent PI/TNs films with thicknesses about 40 μm were obtained (as shown in Fig. 1). 2.3. Measurements
2. Experimental details
The morphology of TNs and the cross-section images were observed by FEI quanta200 scanning electron microscope (SEM). The micromorphology of TNs was tested by model Hitachi H-7650 TEM. The molecular bonds and subsurface composition of the samples are revealed by Bruker EQUINOX-55 FTIR spectrometer (FTIR) and Philips X'Pert diffractometer (XRD). The interfaces of the PI/TNs were studied by small angle X-ray scattering (SAXS) at beam line 1W2A at Beijing
2.1. Synthesis of titanate-nanotubes The ultralong titanate nanotubes (TNs) were synthesized by using a hydrothermal method [43]. Dispersion is a key factor to affect the lengths of resulted titanate nanotubes. The larger aspect ratio of TNs 14
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bands). To characterize the microstructure of the composites after doping TNs, small angle X-ray scattering (SAXS) was carried out. A typical twodimensional (2D) SAXS pattern of the PI/TNs composites is shown in Fig. 4a. We know that scattering with clear boundaries follows the Porod's law in ideal two-phase systems. According to Porod's law, the thickness of interface between inorganic particles and organic polymer can be calculated by Porod's equation [47].
Synchrotron Radiation Facility (with the storage ring operated at 2.5 GeV and current about 80 mA). The dielectric measurements were carried out in a frequency range from 1 Hz to 106 Hz using NOVO control Dielectric Spectrometer (CONCEPT 40). Prior to dielectric measurements, aluminum electrodes were deposited onto both surfaces of the PI/TNs films by sputtering. The AC breakdown properties were performed according to IEC 243. The films were placed between two standard electrodes in silicone oil, and the voltage increased at a rate of 1 kV/s until breakdown to occur. The Corona resistant of PI/TNs films was operated at an AC voltage of 1.7 kV (under an electric field of 60 kV/mm) with a rod-plate electrode system (IEC60343). 0.1 mm air gap was maintained between one side of the specimen and the rod electrode. The tensile strength and elongation at break were measured by the XLD series liquid sieve electronic stretching device according to GB/T1354-92 specimens at a tensile rate of 50 mm/min. The length and the width of samples is 50 mm and 10 mm, respectively. Dynamic mechanical analyses (DMA) were performed using a Q800 DMA analyzer with heating steps from 30 °C to 380 °C at 3 °C/min. Differential scanning calorimetry (DSC) was carried out on METTLER TOLEDO model DSC-1 TA Q800.
ln[q3I (q)] = ln k ′ − δ 2q2
(1)
E=
(2)
2π σ
where E is the average thickness of the interfaces, σ is the absolute value of the slope and intercept of the slope curve segment. The plots of ln[q3I(q)] versus q2 from the PI/TNs composite films with different TNs doping concentrations are given in Fig. 4b. All of the curves show negative deviation, meaning that obvious interface layers exhibit between TNs and PI matrix. It can be seen that the thickness of the interface layers decreases with the increase of TNs doping contents. It indicated that heavy doping induced drastic change of electronic energy states. The ln(I(q)) vs. ln(q) plots of the PI/TNs thin films are shown in Fig. 4c, where there are mass fractals (Dm) to exist between the two phases. By gathering the data from curve fitting of four composite films, both Dm and E changes with the addition of that doping contents are shown together in Fig. 4d, where E is reduced from 4.62 to 2.19. With the increase of TNs doped contents, the mass fractal gradually increases, indicating that the composite structures become denser and the TNs has good dispersibility in PI matrix. SAXS results show that there is a strong interaction between TNs and PI matrix, which plays an important role in the properties of materials. The dielectric constant, loss tangent and AC conductivity were also tested here. With the increasing of frequency, the dielectric constant increase obviously in the tested frequency range of 1–106 Hz as shown in Fig. 5a, with an increased dielectric constant of 4.6 for the only 1% TNs loading. In contrast, when doping TNs up to 3%, the dielectric constant decreases. It is because that the relative dielectric constants of the composites are mainly affected by interfacial polarization and space charge caused by doping the TNs in the PI matrix. However, when doping TNs up to the critical value (3%), the heterogeneous cluster formed will blur the interface and make it difficult to conduct interface polarization, ultimately resulting in a loss of dielectric constant, which is consistent with the SAXS detection. In addition, a large number of heterostructures can be formed when embedding the TNs into PI matrix. The dielectric confinement and the strong self-polarization induced by the heterostructures located between the insulation and the semiconductor result in a significant reduction in the dielectric constant. The dielectric loss increases with the addition of TNs doping contents compared with the pure PI in the low frequency region (< 300 Hz), and then it will rapidly increase when increasing the frequency more than 104 Hz due to the orientation polarization of dielectric materials. The dielectric loss of the PI/TNs composites at 1000 Hz is very low to remain at 0.006. The variation of AC conductivity with frequency is also given as shown in Fig. 5c. With respect to the pure PI, the conductivity of composite films is slightly increased, maintaining around 10−12 S/ cm at 100 Hz. However, all of the conductivity for the composite films don't exhibit a conductive platform, indicating that the introducing TNs into the PI matrix don't change the insulation properties of the composites. The short-time breakdown strength and breakdown time (long time corona) for PI/TNs films were tested as two key parameters of insulating materials electrical properties. As shown in Fig. 5d, increasing TNs doping content from 0.25 to 3 wt%, the breakdown strength of PI/TNs first increases to 149 kV/mm, then decreases down to 100 kV/mm. Only the 0.5 wt% TNs loading can realize a breakdown strength of 25% higher than that of pure PI. In response, the corona aging time goes up with the increase of TNs doping concentration. Due to the formation of corona aging barrier layer in the composites with
3. Results and discussions Fig. 2a shows the SEM image of TNs, where the TNs are uniform and have good dispersibility. The lengths of the TNs are about 4–6 μm, longer than that in previous reports [20,40,41]. The EDS of TNs is given in Fig. 2b, it shows the presence of Ti and O elements. Fig. 2c–d shows the TEM image of the TNs, clearly displaying the open structure of the nanotubes. The inner diameter and the wall thickness of TNs is 15.1 nm and 9.0 nm, respectively. The cross-section SEM images of cold fractured PI/TNs composite films containing 0.5 and 3 wt% TNs are shown in Fig. 2e–f, where the TNs are uniformly distributed in the PI matrix without obvious aggregation (see in the red dotted line area). The rough cross-section indicates a well compatibility to exhibit in the composite attributing to the strong interfacial effect between the PI matrix and TNs. When increasing the TNs content to 3%, very few agglomerations in PI matrix containing TNs are observed and some defects appear due to the influence of the TNs larger size. To analyze the subsurface composition effects of TNs on the microstructures of the composite films, the phase properties and molecular bond structures of the composite films are tested, as shown in Fig. 3. The XRD pattern for the PI film and PI/TNs composite films are given in Fig. 3a, all with a broad peak observed at 2θ = 18°, indicating the existence of amorphous regions in the composites. The broad peak of PI/TNs with 0.25 wt% becomes narrower and has a little left shift compared with that of pure PI, which means that the crystalline of PI matrix was enhanced. When adding the content of TNs, the peak widened gradually, i.e. the more adding, the more disordered for the distribution of polyimide chains, leading to the formation of matrix defects, which is consistent with the SEM image. As seen from the XRD pattern of the PI/TNs, it can be found that no exist of the crystalline region mainly attributes to the formation of bulky pendant groups in the polyimide matrix disrupted the arrangements of polymer chains and obligated them to be amorphous [44–46], in accordance with the formation of amorphous TNs. However, the melting endothermic peak cannot be observed in all of the samples by using DSC (see Fig. S1). Fig. 3b shows the FTIR spectroscopy of the PI/TNs film, where the CeN stretching and the C]O bonding characteristic peaks occur at 1376 cm−1 and 726 cm−1, respectively. The asymmetric and symmetric C]O stretching characteristic peaks of imide groups exhibit at 1776 cm−1 and 1725 cm−1. The complete disappearance of CeNH characteristic absorption peak at 1650 cm−1 indicates that the imidization was fully, dedicating that doping TNs doesn't change the polyimide properties. However, due to the low TNs doping in the PI matrix, no a clear characteristic peak occurs around 400–750 cm−1 (TieO 15
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Fig. 2. (a) The SEM image of TNs (b) the EDS of TNs (c, d) the TEM images of TNs; the cross-section SEM images of the PI/TNs films with (e) 0.25 wt% and (f) 3% wt % doping concentration. (For interpretation of the references to color in this figure, the reader is referred to the web version of this article.)
particular structure, the corona aging time of PI/TNs film doped with 3% is up to 30 h, 10 times longer than that of pure PI, conforming with the other PI/TiO2 films corona resistant properties [48,49]. The mechanical properties of polymer can be improved by doping
TNs into polymer matrix have been reported in previous studies [40,41]. The mechanical properties of PI/TNs composite films are investigated, and the effects of TNs doping on the properties of the composite films are studied as well. Typical stress–strain curves of PI/
Fig. 3. (a) XRD patterns of TNs, PI, and the PI/TNs composites with 0.25, 0.5, 1, and 3 wt% inorganic doping concentration. (b) FTIR spectra of the PI/TNs composites with 0.25, 0.5, 1, and 3 wt% inorganic doping concentration. 16
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Fig. 4. (a) Typical 2-D image of SAXS, (b) The plots of ln[q3I(q)] versus q2, (c) Typical ln(I(q)) versus ln(q) plots, (d) the mass fractal and interface from PI/TNs composite films with different TNs doping concentrations.
TNs composite films with superior mechanical properties are shown in Fig. 6. Increasing TNs doping content from 0.25 to 3 wt%, the tensile strength of PI/TNs composite films first increases to 135 MPa and then decreases down to 110 MPa. The tensile strength of the 0.25 wt% PI/
TNs is 35% higher than that of pure PI. The variation trend of the fracture elongation is basically consistent with tensile strength. Meanwhile, the elongation at break of PI/TNs film containing 0.25 wt% TNs is 60.3%, higher 235.1% than that of pure PI. It indicates that the minor
Fig. 5. Dielectric properties of the PI/TNs composites with various inorganic doping concentrations (a) dependence of permittivity on frequency, (b) dependence of loss tangent on frequency, (c) the AC conductivity, (d) breakdown strength and corona resistance time as a function of the concentration. 17
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Fig. 6. (a) The stress-strain curve of the PI/TNs composites; (b) the tensile strengths and elongation at break of the PI/TNs composites with various inorganic doping concentrations.
aging life. SAXS results show that PI/TNs composite films have mass fractal, indirectly confirming that TNs has good dispersibility in PI matrix. The negative deviation of the Porod's curve indicates that interfacial layers with strong interaction between TNs and PI matrix were formed in the composite films, leading to the above excellent performance of PI/TNs composite films. The corona aging life of a 3% doped PI/TNs film is 30 h, 10 times longer than that of pure PI film. A PI/TNs composite film with 0.25 wt% TNs loading, exhibits the tensile strength of 135 MPa and the elongation at break of 60.3%, higher 35% and 235.1% than that of pure PI film, respectively. The super length of TNs introduced can cause a large number of interfaces between the TNs and inorganic phase, which is directly relative to the improvement of the electrical and mechanical properties of composite films. Due to the low cost, simple production, outstanding electrical and mechanical properties for the PI/TNs composite films, it is possible to apply to electrical insulation, microelectronic packaging device and high strength flexible device where such materials have shown promise.
doping of TNs (0.25 wt%) into PI matrix can greatly improve mechanical performance and reduce the cost in practical applications, mainly attributing to the strong interactions between TNs and PI matrix. Furthermore, the fine dispersion of TNs and a certain degree of PI molecular chains orientation can result in less stress concentration during the tensile process. Thus, the TNs limit the movements and deformation of polymers under stress. We believe that the TNs not only help to strengthen the composites, but also play a net role, to keep PI polymers together and avoid sliding under pressure. However, as the TNs doping content reach a certain level (about 0.25 wt% in this study), the distance between TNs become smaller and the TNs orientation alignment become more random, resulting in relative separately thinner layers of PI matrix to be formed and reducing the tensile strength and the elongation at break of the PI/TNs composite films. The SEM images of the tensile fracture surfaces of the PI/TNs composites are supplemented in Fig. S2, which confirms that the supporting stress transferred from the polyimide matrix to the TNs is significantly enhanced. DMA reflecting the amount of elastic energy stored in composites and energy dissipated during strain, was used to illustrate the reinforcement effect of TNs on the polyimide composites [50–52]. Fig. S3a shows the effects of TNs fraction on the dynamic storage modulus. Due to the softening of polymer chains with increasing temperature, the storage modulus of PI and PI/TNs composites decreases. It is evident that the storage modulus of PI/TNs film containing 0.25 wt% of TNs loading exhibits the maximum and then decreases with increasing filler concentration. The storage modulus of pure PI at 30 °C is 1420 MPa, whereas all of the PI/TNs composites are higher than PI, especially that of 0.25 wt% TNs loading can reach 2120 MPa. The curves of loss modulus versus temperature of PI and PI/TNs composites are shown in Fig. S3b. The results show that the loss modulus of PI increase significantly after adding a little of TNs (0.25–0.5 wt%). This remarkable enhancement indicates the homogeneous dispersion of TNs in the polyimide matrix. Therefore, it is possible for the TNs to capture and inhibit the movement of PI molecular chains and promote the transfer of external stress. Fig. S3c show the temperature dependence of the stiffness of the pure PI and the PI/TNs composites. The composites with TNs content lower than 1% show smaller stiffness compared with the pure PI. It can be concluded that the addition of small amount of TNs can greatly reduce the stiffness and improve flexibility, thus enhancing the overall mechanical properties of the composites.
Acknowledgements The authors would like to acknowledge support from the National Natural Science Foundation of China (Grant No. 51777047 and 51337002), and the Beijing Synchrotron Radiation Facility in China, Key Laboratory of Engineering Dielectrics and Its Application (Harbin University of Science and Technology) and Ministry of Education of China (Grant No. KF20171111). Appendix A. Supplementary data Supplementary data to this article can be found online at https:// doi.org/10.1016/j.surfcoat.2019.01.013. References [1] J. Hao, N. Li, X. Ma, X. Liu, X. Liu, Y. Li, H. Xu, J. Zhao, Mater. Lett. 144 (2015) 50–53. [2] J.-l. Li, J.-h. Yin, T. Ji, Y. Feng, Y.-y. Liu, H. Zhao, Y.-p. Li, C.-c. Zhu, D. Yue, B. Su, X.-x. Liu, Mater. Lett. 234 (2019) 74–78. [3] X.X. Liu, J.H. Yin, Y.N. Kong, M.H. Chen, Y. Feng, K. Yan, X.H. Li, B. Su, Q.Q. Lei, Thin Solid Films 544 (2013) 352–356. [4] X. Liu, D. Chao, D. Su, S. Liu, L. Chen, C. Chi, J. Lin, Z.X. Shen, J. Zhao, L. Mai, Y. Li, Nano Energy 37 (2017) 108–117. [5] M. Chen, J. Yin, X. Liu, Y. Feng, B. Su, Q. Lei, Thin Solid Films 544 (2013) 116–119. [6] Y. Feng, J.H. Yin, M.H. Chen, X.X. Liu, B. Su, W.D. Fei, Q.Q. Lei, IEEE Trans. Dielectr. Electr. Insul. 21 (2014) 1501–1508. [7] P.M. Lee, Z. Wang, X. Liu, Z. Chen, E. Liu, Thin Solid Films 584 (2015) 85–89. [8] X. Liu, J. Yin, M. Chen, W. Bu, W. Cheng, Z. Wu, Nanosci. Nanotechnol. Lett. 3 (2011) 226–229. [9] P. Zhang, J. Zhao, K. Zhang, R. Bai, Y. Wang, C. Hua, Y. Wu, X. Liu, H. Xu, Y. Li, Compos. A: Appl. Sci. Manuf. 84 (2016) 428–434. [10] X. Liu, J. Yin, Y. Kou, M. Chen, L. Yuanyuan, J. Li, N. Zhang, Y. Lei, Z. Wu, B. Su, Nanosci. Nanotechnol. Lett. 7 (2015) 262–267. [11] Z. Xu, X. Zhuang, C. Yang, J. Cao, Z. Yao, Y. Tang, J. Jiang, D. Wu, X. Feng, Adv.
4. Conclusion The superior length TNs with the lengths about 4–6 μm and the wall thickness of 9 nm were synthesized by hydrothermal method. Advanced PI/TNs composites films were prepared by using in-situ polymerization method under low TNs doping, achieving excellent mechanical properties, low dielectric permittivity, high breakdown strength and corona 18
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Electrical and mechanical properties of polyimide composite films reinforced by ultralong titanate nanotubes
T
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He Zhaoa, Chen Yangc, Na Lic, Jinghua Yina,b, , Yu Fengb, Yuanyuan Liua, Jialong Lia, ⁎⁎ Yanpeng Lia, Dong Yuec, Congcong Zhua, Xiaoxu Liud, a
School of Materials Science and Engineering, Harbin University of Science and Technology, Harbin 150080, China Key Laboratory of Engineering Dielectric and Its Application, Ministry of Education, Harbin University of Science and Technology, Harbin 150040, China c Heilongjiang University of Science and Technology, Harbin 150027, China d School of Material Science and Engineering, Shaanxi University of Science and Technology, Xi'an 710021, China b
A R T I C LE I N FO
A B S T R A C T
Keywords: Polyimide Composite Dielectric Titanate nanotubes Mechanical properties
Ultralong titanate nanotubes (TNs) were successfully synthesized by using hydrothermal method. The TNs were introduced into polyimide (PI) to prepare a kind of promising PI/TNs composite films. The effects of the doping contents on electrical and mechanical properties of the PI/TNs composites were investigated. Results show that the electrical and mechanical properties of the PI/TNs composites are improved remarkably compared to that of the pure PI. With only 0.25 wt% TNs loading, the elongation at break can reach 60.3%, about 235.1% greater than that of the pure PI. In addition, the corona-resistant lifetime of the PI/TNs composite doped with TNs content of 3 wt% improved about ten times higher than the pure PI. The compatibility and interaction between TNs and PI matrix were investigated by the scanning electron microscopy (SEM) and synchrotron radiation small angle X-ray scattering (SAXS). How the microstructure and interface of the PI/TNs composites influence the electrical and mechanical performances was discussed in detail. This study on the PI/TNs composites provides a possibility for realizing a potential application in the high strength insulation devices.
1. Introduction Nowadays, polymer-based composites using as advanced functional materials have attracted more and more attention [1–10] due to their relatively excellent combining properties of organic component, such as flexibility, low dielectric properties, high breakdown strength, durability and super thermal stability, etc. [11–14]. In addition, it was found that the incorporation between inorganic nanoparticles and polymer matrix can affect the mechanical properties of polymer significantly [15–17]. However, the incorporation is closely relative to the characteristics of nanoparticles as well as their interaction with the polymer matrix to improve polymer properties, mainly including nanoparticles type, size, shape and doping concentration. Especially, the addition of inorganic fillers such as 0-D nanoparticles, 1-D nanowires, nanofibers and nanotubes, 2-D nanosheets into polymer matrix can obviously enhance some composite properties [18–22]. It is well known that PI as typical engineering polymer materials due to its excellent electrical insulation, thermal properties and chemical resistances, has been widely used as insulation materials, microelectronics, even
⁎
aerospace and aircraft parts in the past years [23–25]. However, the relatively low electrical and mechanical properties for the pure PI limit its demands in some special applications [26]. To overcome the above limits, the inorganic nanoparticles such as Al2O3 [27], SiO2 [28], AlN [29], TiO2 [30], BaTiO3 [15] and the others [31] have been introduced into the PI matrix to enhance their mechanical and electrical properties. At present, an urgently solved problem is the poor dispersibility resulted in the restacking and aggregation of nanoparticles in PI matrix. Besides, the interfaces between polymer and nanoparticles has aroused researcher's concerns, such as Lewis pointed out that the interaction region was a conductive layer [32], Nelson also built a dielectric bilayer structure model [33], and Tanaka further constructed a famous multicore model theory of the interface [34]. All those provide great inspiration for researchers to study the interfaces between organic and inorganic phases. More important, the interfaces are considered as a major factor relative to the mechanical and electrical properties of the composites. Titanate nanotubes (TNs), as the typical 1-D materials, have been widely used in photocatalysts, drug delivery and solar cells due to its large surface areas, chemical inertness, excellent thermal
Correspondence to: J. Yin, School of Materials Science and Engineering, Harbin University of Science and Technology, Harbin 150080, China. Corresponding author. E-mail addresses: [email protected] (J. Yin), [email protected] (X. Liu).
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https://doi.org/10.1016/j.surfcoat.2019.01.013 Received 31 August 2018; Received in revised form 7 December 2018; Accepted 6 January 2019 Available online 07 January 2019 0257-8972/ © 2019 Elsevier B.V. All rights reserved.
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Fig. 1. Schematic flow of in-situ polymerization preparing processes for PI/TNs composite film.
causes Van der Waals forces to be increased, resulting in TNs agglomerating into entangled aggregates in the synthesis process. Therefore, the decomposition of these initial aggregates and the release of independent and separated nanotubes without significant destroying can be achieved by long-term stirring in water suspensions. In a typical preparation, 0.2 g of titanium dioxide nanoparticles were added to a 30 mL of 10 M NaOH solution, stirring for 1 h to form a stable suspension. To ensure adequate internal pressure, put the stable suspension in a 45 mL PTFE beaker, and then was placed in a Teflon lined autoclave at 130 °C for 24 h. Rotational speed plays an important role in the formation of titanate nanotube length. It was synthesized at a rotating speed of 500 rpm under a static condition. The obtained white flocculation deposits were collected after being cooled to room temperature. Then, the white deposits were treated for 24 h in a 0.1 M hydrochloric acid (HCL) to complete ion exchanges. Samples were centrifuged repeatedly with deionized water and alcohol until PH reached 7. Finally drying the samples at 60 °C for 24 h. After repeating the above experiments for 3 times, 0.6 g TNs can be obtained.
stability and low cost of preparation since its discovery in 1997 [35,36]. For example, Mikesova et al., obtained the excellent mechanical properties in polypropylene/titanate nanotubes composite with 5 wt% titanate nanotubes, with an improvement of the stiffness and microhardness about 27% and 33%, respectively [37], indicating the mechanical properties of nanocomposites is closely relative to the changes of polymer matrix supramolecular structure. Porras et al., reported that the PEO/CS (6:1) based composites with 25 wt% titanate nanotubes exhibited 2.6 times as hard as the neat polymer blend and 3.4 times stiffer [38]. Olariu et al., reported that the PI based composites doped with the modified TiO2 nanotubes of 10 wt% and 20 wt% exhibited an improved glass transition temperature from 205 to 249 °C and an enhanced dielectric constant about 5.34 at 20 wt% TNs loading [39]. These results show that the combination of titanate nanotubes (TNs) with polymer matrix plays a critical role for improving the dielectric, thermal, and mechanical properties of resulting composites. But, lots of research efforts concentrate on the thermal and mechanical behaviors of polymer composites at a high TNs loading and with TNs length only several hundred nanometers. Compared with ordinary length TNs, the ultralong TNs can introduce long migration paths of charge carriers to improve breakdown strength of the composites [40,41]. In addition, the ultralong TNs have larger aspect ratio, which can form a large number of interfaces between the TNs and inorganic phase to promote the exchange coupling effect of the dipolar interface layer, resulting in high polarization level and dielectric response [42]. To our best knowledge, the interfacial behavior and the electrical, mechanical properties of PI based composites with superior length TNs at ultralow contents have not been reported. In this study, the ultralong titanate nanotubes with a long length (4–6 μm) were successfully synthesized using a hydrothermal method with stirring. The effects of the incorporation between TNs and PI matrix on the mechanical and electrical properties of PI composites were investigated. The PI/TNs composites exhibit good electrical properties, especially showing a significantly enhancement in mechanical properties at a very low TNs doping. The synthesis of PI/TNs composite films with superior mechanical properties provides a new way for developing high strength polyimide based composites.
2.2. In-situ synthesis of PI/TNs composite film The TNs (0.6 g) was dispersed in N,N‑dimethylacetamide (DMAC) (100 mL) by ultrasonic mixing for 3 h. After that no deposits was found, indicating that the TNs have been dispersed in the DMAC very well. Equivalent molar ratios of 3 g 4,4′‑oxydianiline (ODA) and 3.28 g pyromellitic dianhydride (PMDA) were dissolved in the TNs/DMAC dispersion solution to make the solutions containing 0.25, 0.5, 1 and 3 wt % of TNs, respectively. When the molar ratio of PMDA and ODA is 1.01:1, the viscosity of solution reaches the maximum. Sufficient molecular weight polyamic acid (PAA)/TNs were obtained. The PAA/TNs was cast onto a glass dish using a doctor blade, thereafter dried at 80 °C overnight. Then the PAA/TNs films were thermally imidized at 120 °C, 180 °C, 240 °C, 300 °C, 350 °C for 1 h, respectively. The light yellow, soft and transparent PI/TNs films with thicknesses about 40 μm were obtained (as shown in Fig. 1). 2.3. Measurements
2. Experimental details
The morphology of TNs and the cross-section images were observed by FEI quanta200 scanning electron microscope (SEM). The micromorphology of TNs was tested by model Hitachi H-7650 TEM. The molecular bonds and subsurface composition of the samples are revealed by Bruker EQUINOX-55 FTIR spectrometer (FTIR) and Philips X'Pert diffractometer (XRD). The interfaces of the PI/TNs were studied by small angle X-ray scattering (SAXS) at beam line 1W2A at Beijing
2.1. Synthesis of titanate-nanotubes The ultralong titanate nanotubes (TNs) were synthesized by using a hydrothermal method [43]. Dispersion is a key factor to affect the lengths of resulted titanate nanotubes. The larger aspect ratio of TNs 14
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bands). To characterize the microstructure of the composites after doping TNs, small angle X-ray scattering (SAXS) was carried out. A typical twodimensional (2D) SAXS pattern of the PI/TNs composites is shown in Fig. 4a. We know that scattering with clear boundaries follows the Porod's law in ideal two-phase systems. According to Porod's law, the thickness of interface between inorganic particles and organic polymer can be calculated by Porod's equation [47].
Synchrotron Radiation Facility (with the storage ring operated at 2.5 GeV and current about 80 mA). The dielectric measurements were carried out in a frequency range from 1 Hz to 106 Hz using NOVO control Dielectric Spectrometer (CONCEPT 40). Prior to dielectric measurements, aluminum electrodes were deposited onto both surfaces of the PI/TNs films by sputtering. The AC breakdown properties were performed according to IEC 243. The films were placed between two standard electrodes in silicone oil, and the voltage increased at a rate of 1 kV/s until breakdown to occur. The Corona resistant of PI/TNs films was operated at an AC voltage of 1.7 kV (under an electric field of 60 kV/mm) with a rod-plate electrode system (IEC60343). 0.1 mm air gap was maintained between one side of the specimen and the rod electrode. The tensile strength and elongation at break were measured by the XLD series liquid sieve electronic stretching device according to GB/T1354-92 specimens at a tensile rate of 50 mm/min. The length and the width of samples is 50 mm and 10 mm, respectively. Dynamic mechanical analyses (DMA) were performed using a Q800 DMA analyzer with heating steps from 30 °C to 380 °C at 3 °C/min. Differential scanning calorimetry (DSC) was carried out on METTLER TOLEDO model DSC-1 TA Q800.
ln[q3I (q)] = ln k ′ − δ 2q2
(1)
E=
(2)
2π σ
where E is the average thickness of the interfaces, σ is the absolute value of the slope and intercept of the slope curve segment. The plots of ln[q3I(q)] versus q2 from the PI/TNs composite films with different TNs doping concentrations are given in Fig. 4b. All of the curves show negative deviation, meaning that obvious interface layers exhibit between TNs and PI matrix. It can be seen that the thickness of the interface layers decreases with the increase of TNs doping contents. It indicated that heavy doping induced drastic change of electronic energy states. The ln(I(q)) vs. ln(q) plots of the PI/TNs thin films are shown in Fig. 4c, where there are mass fractals (Dm) to exist between the two phases. By gathering the data from curve fitting of four composite films, both Dm and E changes with the addition of that doping contents are shown together in Fig. 4d, where E is reduced from 4.62 to 2.19. With the increase of TNs doped contents, the mass fractal gradually increases, indicating that the composite structures become denser and the TNs has good dispersibility in PI matrix. SAXS results show that there is a strong interaction between TNs and PI matrix, which plays an important role in the properties of materials. The dielectric constant, loss tangent and AC conductivity were also tested here. With the increasing of frequency, the dielectric constant increase obviously in the tested frequency range of 1–106 Hz as shown in Fig. 5a, with an increased dielectric constant of 4.6 for the only 1% TNs loading. In contrast, when doping TNs up to 3%, the dielectric constant decreases. It is because that the relative dielectric constants of the composites are mainly affected by interfacial polarization and space charge caused by doping the TNs in the PI matrix. However, when doping TNs up to the critical value (3%), the heterogeneous cluster formed will blur the interface and make it difficult to conduct interface polarization, ultimately resulting in a loss of dielectric constant, which is consistent with the SAXS detection. In addition, a large number of heterostructures can be formed when embedding the TNs into PI matrix. The dielectric confinement and the strong self-polarization induced by the heterostructures located between the insulation and the semiconductor result in a significant reduction in the dielectric constant. The dielectric loss increases with the addition of TNs doping contents compared with the pure PI in the low frequency region (< 300 Hz), and then it will rapidly increase when increasing the frequency more than 104 Hz due to the orientation polarization of dielectric materials. The dielectric loss of the PI/TNs composites at 1000 Hz is very low to remain at 0.006. The variation of AC conductivity with frequency is also given as shown in Fig. 5c. With respect to the pure PI, the conductivity of composite films is slightly increased, maintaining around 10−12 S/ cm at 100 Hz. However, all of the conductivity for the composite films don't exhibit a conductive platform, indicating that the introducing TNs into the PI matrix don't change the insulation properties of the composites. The short-time breakdown strength and breakdown time (long time corona) for PI/TNs films were tested as two key parameters of insulating materials electrical properties. As shown in Fig. 5d, increasing TNs doping content from 0.25 to 3 wt%, the breakdown strength of PI/TNs first increases to 149 kV/mm, then decreases down to 100 kV/mm. Only the 0.5 wt% TNs loading can realize a breakdown strength of 25% higher than that of pure PI. In response, the corona aging time goes up with the increase of TNs doping concentration. Due to the formation of corona aging barrier layer in the composites with
3. Results and discussions Fig. 2a shows the SEM image of TNs, where the TNs are uniform and have good dispersibility. The lengths of the TNs are about 4–6 μm, longer than that in previous reports [20,40,41]. The EDS of TNs is given in Fig. 2b, it shows the presence of Ti and O elements. Fig. 2c–d shows the TEM image of the TNs, clearly displaying the open structure of the nanotubes. The inner diameter and the wall thickness of TNs is 15.1 nm and 9.0 nm, respectively. The cross-section SEM images of cold fractured PI/TNs composite films containing 0.5 and 3 wt% TNs are shown in Fig. 2e–f, where the TNs are uniformly distributed in the PI matrix without obvious aggregation (see in the red dotted line area). The rough cross-section indicates a well compatibility to exhibit in the composite attributing to the strong interfacial effect between the PI matrix and TNs. When increasing the TNs content to 3%, very few agglomerations in PI matrix containing TNs are observed and some defects appear due to the influence of the TNs larger size. To analyze the subsurface composition effects of TNs on the microstructures of the composite films, the phase properties and molecular bond structures of the composite films are tested, as shown in Fig. 3. The XRD pattern for the PI film and PI/TNs composite films are given in Fig. 3a, all with a broad peak observed at 2θ = 18°, indicating the existence of amorphous regions in the composites. The broad peak of PI/TNs with 0.25 wt% becomes narrower and has a little left shift compared with that of pure PI, which means that the crystalline of PI matrix was enhanced. When adding the content of TNs, the peak widened gradually, i.e. the more adding, the more disordered for the distribution of polyimide chains, leading to the formation of matrix defects, which is consistent with the SEM image. As seen from the XRD pattern of the PI/TNs, it can be found that no exist of the crystalline region mainly attributes to the formation of bulky pendant groups in the polyimide matrix disrupted the arrangements of polymer chains and obligated them to be amorphous [44–46], in accordance with the formation of amorphous TNs. However, the melting endothermic peak cannot be observed in all of the samples by using DSC (see Fig. S1). Fig. 3b shows the FTIR spectroscopy of the PI/TNs film, where the CeN stretching and the C]O bonding characteristic peaks occur at 1376 cm−1 and 726 cm−1, respectively. The asymmetric and symmetric C]O stretching characteristic peaks of imide groups exhibit at 1776 cm−1 and 1725 cm−1. The complete disappearance of CeNH characteristic absorption peak at 1650 cm−1 indicates that the imidization was fully, dedicating that doping TNs doesn't change the polyimide properties. However, due to the low TNs doping in the PI matrix, no a clear characteristic peak occurs around 400–750 cm−1 (TieO 15
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Fig. 2. (a) The SEM image of TNs (b) the EDS of TNs (c, d) the TEM images of TNs; the cross-section SEM images of the PI/TNs films with (e) 0.25 wt% and (f) 3% wt % doping concentration. (For interpretation of the references to color in this figure, the reader is referred to the web version of this article.)
particular structure, the corona aging time of PI/TNs film doped with 3% is up to 30 h, 10 times longer than that of pure PI, conforming with the other PI/TiO2 films corona resistant properties [48,49]. The mechanical properties of polymer can be improved by doping
TNs into polymer matrix have been reported in previous studies [40,41]. The mechanical properties of PI/TNs composite films are investigated, and the effects of TNs doping on the properties of the composite films are studied as well. Typical stress–strain curves of PI/
Fig. 3. (a) XRD patterns of TNs, PI, and the PI/TNs composites with 0.25, 0.5, 1, and 3 wt% inorganic doping concentration. (b) FTIR spectra of the PI/TNs composites with 0.25, 0.5, 1, and 3 wt% inorganic doping concentration. 16
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Fig. 4. (a) Typical 2-D image of SAXS, (b) The plots of ln[q3I(q)] versus q2, (c) Typical ln(I(q)) versus ln(q) plots, (d) the mass fractal and interface from PI/TNs composite films with different TNs doping concentrations.
TNs composite films with superior mechanical properties are shown in Fig. 6. Increasing TNs doping content from 0.25 to 3 wt%, the tensile strength of PI/TNs composite films first increases to 135 MPa and then decreases down to 110 MPa. The tensile strength of the 0.25 wt% PI/
TNs is 35% higher than that of pure PI. The variation trend of the fracture elongation is basically consistent with tensile strength. Meanwhile, the elongation at break of PI/TNs film containing 0.25 wt% TNs is 60.3%, higher 235.1% than that of pure PI. It indicates that the minor
Fig. 5. Dielectric properties of the PI/TNs composites with various inorganic doping concentrations (a) dependence of permittivity on frequency, (b) dependence of loss tangent on frequency, (c) the AC conductivity, (d) breakdown strength and corona resistance time as a function of the concentration. 17
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Fig. 6. (a) The stress-strain curve of the PI/TNs composites; (b) the tensile strengths and elongation at break of the PI/TNs composites with various inorganic doping concentrations.
aging life. SAXS results show that PI/TNs composite films have mass fractal, indirectly confirming that TNs has good dispersibility in PI matrix. The negative deviation of the Porod's curve indicates that interfacial layers with strong interaction between TNs and PI matrix were formed in the composite films, leading to the above excellent performance of PI/TNs composite films. The corona aging life of a 3% doped PI/TNs film is 30 h, 10 times longer than that of pure PI film. A PI/TNs composite film with 0.25 wt% TNs loading, exhibits the tensile strength of 135 MPa and the elongation at break of 60.3%, higher 35% and 235.1% than that of pure PI film, respectively. The super length of TNs introduced can cause a large number of interfaces between the TNs and inorganic phase, which is directly relative to the improvement of the electrical and mechanical properties of composite films. Due to the low cost, simple production, outstanding electrical and mechanical properties for the PI/TNs composite films, it is possible to apply to electrical insulation, microelectronic packaging device and high strength flexible device where such materials have shown promise.
doping of TNs (0.25 wt%) into PI matrix can greatly improve mechanical performance and reduce the cost in practical applications, mainly attributing to the strong interactions between TNs and PI matrix. Furthermore, the fine dispersion of TNs and a certain degree of PI molecular chains orientation can result in less stress concentration during the tensile process. Thus, the TNs limit the movements and deformation of polymers under stress. We believe that the TNs not only help to strengthen the composites, but also play a net role, to keep PI polymers together and avoid sliding under pressure. However, as the TNs doping content reach a certain level (about 0.25 wt% in this study), the distance between TNs become smaller and the TNs orientation alignment become more random, resulting in relative separately thinner layers of PI matrix to be formed and reducing the tensile strength and the elongation at break of the PI/TNs composite films. The SEM images of the tensile fracture surfaces of the PI/TNs composites are supplemented in Fig. S2, which confirms that the supporting stress transferred from the polyimide matrix to the TNs is significantly enhanced. DMA reflecting the amount of elastic energy stored in composites and energy dissipated during strain, was used to illustrate the reinforcement effect of TNs on the polyimide composites [50–52]. Fig. S3a shows the effects of TNs fraction on the dynamic storage modulus. Due to the softening of polymer chains with increasing temperature, the storage modulus of PI and PI/TNs composites decreases. It is evident that the storage modulus of PI/TNs film containing 0.25 wt% of TNs loading exhibits the maximum and then decreases with increasing filler concentration. The storage modulus of pure PI at 30 °C is 1420 MPa, whereas all of the PI/TNs composites are higher than PI, especially that of 0.25 wt% TNs loading can reach 2120 MPa. The curves of loss modulus versus temperature of PI and PI/TNs composites are shown in Fig. S3b. The results show that the loss modulus of PI increase significantly after adding a little of TNs (0.25–0.5 wt%). This remarkable enhancement indicates the homogeneous dispersion of TNs in the polyimide matrix. Therefore, it is possible for the TNs to capture and inhibit the movement of PI molecular chains and promote the transfer of external stress. Fig. S3c show the temperature dependence of the stiffness of the pure PI and the PI/TNs composites. The composites with TNs content lower than 1% show smaller stiffness compared with the pure PI. It can be concluded that the addition of small amount of TNs can greatly reduce the stiffness and improve flexibility, thus enhancing the overall mechanical properties of the composites.
Acknowledgements The authors would like to acknowledge support from the National Natural Science Foundation of China (Grant No. 51777047 and 51337002), and the Beijing Synchrotron Radiation Facility in China, Key Laboratory of Engineering Dielectrics and Its Application (Harbin University of Science and Technology) and Ministry of Education of China (Grant No. KF20171111). Appendix A. Supplementary data Supplementary data to this article can be found online at https:// doi.org/10.1016/j.surfcoat.2019.01.013. References [1] J. Hao, N. Li, X. Ma, X. Liu, X. Liu, Y. Li, H. Xu, J. Zhao, Mater. Lett. 144 (2015) 50–53. [2] J.-l. Li, J.-h. Yin, T. Ji, Y. Feng, Y.-y. Liu, H. Zhao, Y.-p. Li, C.-c. Zhu, D. Yue, B. Su, X.-x. Liu, Mater. Lett. 234 (2019) 74–78. [3] X.X. Liu, J.H. Yin, Y.N. Kong, M.H. Chen, Y. Feng, K. Yan, X.H. Li, B. Su, Q.Q. Lei, Thin Solid Films 544 (2013) 352–356. [4] X. Liu, D. Chao, D. Su, S. Liu, L. Chen, C. Chi, J. Lin, Z.X. Shen, J. Zhao, L. Mai, Y. Li, Nano Energy 37 (2017) 108–117. [5] M. Chen, J. Yin, X. Liu, Y. Feng, B. Su, Q. Lei, Thin Solid Films 544 (2013) 116–119. [6] Y. Feng, J.H. Yin, M.H. Chen, X.X. Liu, B. Su, W.D. Fei, Q.Q. Lei, IEEE Trans. Dielectr. Electr. Insul. 21 (2014) 1501–1508. [7] P.M. Lee, Z. Wang, X. Liu, Z. Chen, E. Liu, Thin Solid Films 584 (2015) 85–89. [8] X. Liu, J. Yin, M. Chen, W. Bu, W. Cheng, Z. Wu, Nanosci. Nanotechnol. Lett. 3 (2011) 226–229. [9] P. Zhang, J. Zhao, K. Zhang, R. Bai, Y. Wang, C. Hua, Y. Wu, X. Liu, H. Xu, Y. Li, Compos. A: Appl. Sci. Manuf. 84 (2016) 428–434. [10] X. Liu, J. Yin, Y. Kou, M. Chen, L. Yuanyuan, J. Li, N. Zhang, Y. Lei, Z. Wu, B. Su, Nanosci. Nanotechnol. Lett. 7 (2015) 262–267. [11] Z. Xu, X. Zhuang, C. Yang, J. Cao, Z. Yao, Y. Tang, J. Jiang, D. Wu, X. Feng, Adv.
4. Conclusion The superior length TNs with the lengths about 4–6 μm and the wall thickness of 9 nm were synthesized by hydrothermal method. Advanced PI/TNs composites films were prepared by using in-situ polymerization method under low TNs doping, achieving excellent mechanical properties, low dielectric permittivity, high breakdown strength and corona 18
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