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nano-aluminum composites
SCT-22071; No of Pages 6 Surface & Coatings Technology xxx (2017) xxx–xxx
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Performance and microstructure characteristics in polyimide/nano-aluminum composites Xiaoxu Liu a,d,⁎, Yanpeng Li a, Yuanyuan Liu b, Duo Sun b, Wenmao Guo a, Xiaonan Sun a, Yu Feng d, Hongyan Chi a,b, Xiuhong Li c, Feng Tian c, Bo Su b, Jinghua Yin b,⁎⁎ a
Heilongjiang University of Science and Technology, Harbin 150040, China School of Applied Science, Harbin University of Science and Technology, Harbin 150040, China Shanghai Institute of Applied Physics, Chinese Academy of Sciences, Shanghai 201800, China d Harbin Institute of Technology, Harbin 150001, China b c
a r t i c l e
i n f o
Article history: Received 31 August 2016 Revised 16 January 2017 Accepted in revised form 24 January 2017 Available online xxxx Keywords: Polyimide Aluminum particles Composite Dielectric Mechanical properties
a b s t r a c t Polyimide (PI) composites with high dielectric permittivity have received a great deal of attention in the embedded capacitors and energy-storage devices due to its excellent thermal stability and good mechanical properties. In this study, nano-aluminum (Al) particles were introduced into PI to prepare promising PI/nano-Al composite. The results indicated that the dielectric constant of the composite films increased with the increase of nano-Al contents and the highest dielectric constant was 15.7 for a composite film incorporating 15 wt% nano-Al. The microstructures of PI/nano-Al composite have been investigated by scanning electron microscopy (SEM), translation electron microscopy(TEM), synchrotron radiation small angle X-ray scattering (SAXS) and wide-angle Xray diffraction (XRD). The effects of mixture doping concentration on volume resistivity and loss tangent are analyzed. The correlation effects of the Al nanoparticles on the different factors which influence the dielectric performance in PI matrix such as microstructure, resistivity, and interface of the composites were discussed in detail. This composite film would be possessing potential application in flexible energy-storage devices. © 2017 Published by Elsevier B.V.
1. Introduction Recently, polymeric based composites associated with high dielectric permittivity and low dielectric loss, and high volume resistivity have been in increasing demands owing to continuous development towards the miniaturization and multifunctionalities of apparatus used in high charge-storage capacitors [1,2] and microelectronic components, such as capacitors embedded in flexible printing circuit [3,4]. Adding high permittivity ceramic fillers or conductive fillers are the two common strategies to fabricate high permittivity polymer based composites [5,6]. The drawback of ceramic filler polymer based composites is the high concentration of fillers will lead to a dramatic decrease of mechanical properties [7–9]. Compared to ceramic fillers of polymer based composites, the disadvantage of conductive filler is consequently increased dielectric loss [10]. Polyimide (PI) as excellent insulating polymer at a high temperature have attracted wide interest as a basis enhancing PI properties and extending their applications, [11–15] which innovative materials have been employed for various contemporary applications ⁎ Correspondence to: X. Liu, Heilongjiang University of Science and Technology, Harbin 150040, China. ⁎⁎ Corresponding author. E-mail address: [email protected] (X. Liu).
such as insulation materials, frequency conversion motors, and proton conductive membranes. The dielectric, insulating and mechanical properties of pure PI films do not quite meet the requirements to be used as an insulating material in high charge-storage capacitors. In the same way, PI can be improved by introducing the high permittivity ceramic fillers or conductive fillers into its matrix. Especially, the addition of inert inorganic oxides such as BaTiO3 [8], Calcium Copper Titanate (CCTO) [6], CNT [16] and others [17] in PI matrix has attracted considerable attention as hybrid materials due to enhanced their dielectric property enhancements and mechanical stability improvements. Metal and polyimide composites were researched for polyimide dielectric performance enhancement. For example, Dang et al., have obtained PI and Ag composite films with high dielectric constant and excellent thermal stability as good candidate for energy storage devices [18]. Wang et al., researched SiO2 coated Ag and PI based composites, and obtained remarkably improved high thermal conductivity and dielectric performance [19]. Yang et al., discovered that by coating the surface of CCTO nanoparticles with Ag, the dielectric permittivity of PI/CCTO@Ag composites is significantly increased to 103 (100 Hz) at 3 vol% filler loading. The surface coating of nano-fillers is useful for enhancing dielectric performance [20]. To our best knowledge, a low cost metal Al nanoparticles (nano-Al) with nature oxide Al2O3 coating for nanofillers of polyimide matrix has not yet been reported.
http://dx.doi.org/10.1016/j.surfcoat.2017.01.095 0257-8972/© 2017 Published by Elsevier B.V.
Please cite this article as: X. Liu, et al., Surf. Coat. Technol. (2017), http://dx.doi.org/10.1016/j.surfcoat.2017.01.095
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X. Liu et al. / Surface & Coatings Technology xxx (2017) xxx–xxx
In this paper, a low cost metal Al nanoparticles (nano-Al) with inartificial Al2O3 coating for nanofiller of polyimide matrix was synthesized by using in-situ polymerization, which the nano-Al was modified by employing surface chemical reaction. The distribution of nano-Al particles was observed by the scanning electron microscope (SEM) and transmission electron microscope (TEM). The dielectric, thermal and mechanical properties of the PI/nano-Al composites were investigated by dielectric tests, thermal gravimetric analysis (TGA), and tensile tests, respectively. The as-synthesized PI/nano-Al composites show high electric, superior electrical and mechanical properties. Meanwhile, PI composite with 15 wt% nano-Al loading has been exhibited higher dielectric constant (15.7) among the all of tested PI composites. The insitu polymerization process is critical in dispersing the nano-Al particles into the PI matrix homogeneously to ensure the good electrical and mechanical properties of the PI/nano-Al films. 2. Experimental details 2.1. Fabrication of the PI/nano-Al composite samples PI/nano-Al composites were prepared by using in-situ polymerization. The nano-Al surface modification was carried out by using the dry toluene reflux method and described in detail in reference [21]. First, modified nano-Al and N,N-dimethylacetamide (DMAC) were added into a three-opening round-bottomed flask and the flask was placed in an ultrasonic bath with continuous stir. The mechanical stirrer and ultrasonic wave were simultaneously utilized until a stable suspension was obtained. Then 4, 4′-oxy dianiline (ODA) was added into the flask and dissolved in the suspension. (The mixture of nano-Al particles and pyromellitic dianhydride in the flask exposed in ultrasonic bath for 2 h in DMAC solvent). Finally, the PMDA was divided into five portions and one portion was added into the suspension at one time to ensure the complete dissolution of the portion before adding another one, until all five portions were added. Then polyamic acid (PAA) suspension was stirred for 4 h at this viscosity until the suspension turns to yellow. The yellow PAA was cast onto a glass dish using a doctor blade. Composite films were obtained after forming, heat treatments and imidization. The resulted films are light yellow, transparent with thicknesses about 40 μm. The nano-Al doping concentrations in all composite films are 1, 3, 5, 10 and 15 wt%, respectively. 2.2. Measurements The cross-section SEM images were obtained by a JEOL field-emission scanning electron microscope under operating voltage of 15 kV, model JSM-6700F. The small angle X-ray scattering (SAXS) tests were carried out at Shanghai Synchrotron Radiation Facility, by using a wavelength of 0.124 nm, a sample to detector distance of 5 m, and an exposure time of 10 s. The 2D scattering patterns were collected on a CCD camera, and the intensity vs. scattering angle is obtained by integrating the data from the 2D scattering patterns. TEM-HRTEM was carried out on TEM (JEOL JEM-2010). X-ray diffraction (XRD) measurements were performed at a Rigaku D/max-rB X-ray diffractometer with Cu Kα (λ = 0.15418 nm) incident radiation. The diffraction patterns were collected at room temperature in the 2θ ranges of 10 to 60°. The dielectric constant of hybrid PI films was tested using an impedance analyzer (Aglient 4294A) with 16451B Dielectric Test Fixture in the frequency range of 1–107 Hz. The DC volume resistivity measurements are performed using a Keithley electrometer with 8009 resistivity measurement kit at a voltage of 500 V. 3. Results and discussion The morphology of nano-Al filler was tested TEM. As seen in Fig. 2a, the spherical nano-Al size is about 10 nm. The nano-Al has the passivated oxide layer (a kind of insulating layer) around its core surface shown
in Fig. 1b. The dispersion of the nano-Al within the polyimide matrix was further investigated and the SEM micrographs of the fractured section of the composites prepared by in situ polymerization were shown in Fig. 1c–f.it can be seen that a homogeneous dispersion of the nanoparticles in the PI matrix are separated in low doping (3% doping) sample, and the nano-Al particle sizes of the composite are smaller than 100 nm. Meanwhile a few agglomerations with polyimide containing nano-Al particles are observed as indicated in the SEM image (see Fig. 1b) at higher doping (10% doping) sample. With loading increase of Al nanoparticles, the microstructure change is observed to occur up to 10 wt%, which a number of micro-size layer-like clusters with polyimide molecules coating nano-Al particles begin to appear from place to place and their magnitudes are found to further increase. (see Fig. 1e, f) The formation of large size particle clusters in polymer matrix is determined by the kinetics of composite preparation. The attractive van der Waals forces between nanoparticles are relatively weak because of the long particle-to-particle distance at low nanoparticle loading, which may be the main reason why the nanoscale dispersion is obtained for the composites with nanoparticle loading 3 wt%. With further increasing the Al nanoparticle loading, the particle-to-particle distance decreases obviously and the more attractive van der Waals forces begin to create agglomeration of the particles from place to place. More interesting, the Al nanoparticles still uniformly dispersed in micro-size clusters, which these cluster formed many layers with parallel orientation at leading of 10% nano-Al. This is beneficial for increasing the overall performances of PI/nano-Al composites. For further confirmation of dispersion of nano-Al, the microstructures of PI/nano-Al composites with content 3 wt% were characterized using TEM technique in Fig. 2 a, b. It can be observed that the nano-Al particles (black spots) are homogeneously dispersed in the polyimide matrix with the particle size in nano-scale, and the particles size is about 10 nm. The FFT pattern of HRTEM revealed the crystalline nature of nano-Al and the (111) plane of aluminium were observed (inset of Fig. 2b). Meanwhile, the EDX pattern obtained from TEM (Fig. 2a) indicated the existence of abundant aluminium and slight oxygen. The homogeneous dispersion of Al particles is attributed to the coaction of mechanical stirring and ultrasonic wave which is an effective way to produce the stable suspension. XRD patterns for the PI/nano-Al with 3 and 10 wt% doping, and pure PI film were given in Fig. 2c. In the XRD patterns of PI/nano-Al composite with the presence of Al particles, the strong diffraction peak of Al is at 2θ = 38.5 and 44.7° (PDF:040787). A wide peak in the range of 18.2° in the PI/nano-Al spectrum clearly demonstrates the existence of the amorphous structure of polyimide, which in another way indicates the effectiveness of our synthesizing polyimide method. In the pure film XRD spectrum, on the other hand, the position of the wide peak is shifted to the left slightly in Fig. 2c, this indicates that the crystalline of PI matrix was enhanced. The thermogravimetric analysis (TGA) was examined to evaluate the thermal stability of the PI/nano-Al composites with different Al content. Results are shown in Fig. 2d. It can be found that the thermal stability of the PI/nano-Al with 3% Al content have the highest value in decomposition temperature, about 551.1 °C, among three of the composite films tested when 5 wt% mass lose is reached. This superior in thermal stability of PI/nano-Al with 3% Al content also indicates a rather good compatibility between the inorganic particles and the organic matrix with addition of Al. As a useful method for testing microstructure of polymer based composite materials, small angle X-ray scattering (SAXS) technology has been used to characterize composite polymers in our previously research [22]. A two-dimensional (2D) SAXS pattern of the PI/nano-Al composite is shown in the Fig. 3a. The PI/nano-Al composites with various inorganic doping concentration have quite similar SAXS patterns, which are all isotropous 2D patterns of scattering intensity with scattering angle. This indicates that ordered microstructure is not presences in PI/nano-Al composites. SAXS data can be analyzed to get the particle size distribution (PSD), unordered fractal structure and interface information for PI composites. The method [23] used to estimate PSD with
Please cite this article as: X. Liu, et al., Surf. Coat. Technol. (2017), http://dx.doi.org/10.1016/j.surfcoat.2017.01.095
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Fig. 1. (a, b) A TEM images of the nano-Al. (c, d) SEM images of a low and high magnification cross-section SEM image of the PI/nano-Al composites with content 3 wt%. (e, f) A low and high magnification cross-section SEM image of the PI/nano-Al composite with content 10 wt%.
SAXS data is Tangent-by-Tangent (TBT) method, and we use it here to estimate the PSD of PI/nano-Al composites (as shown in Fig. 4b). There is quite similar in PSD of PI/nano-Al composites with 1, 3, 5, 10 and 15 wt% inorganic doping concentrations. A distinctive peak appears in all composite films about 7–8 nm in particle size and we attribute the peak to the real nano-Al particles. When doping concentration is N5 wt%, the normalized volume fraction of distinctive peak was obviously decreased. This can be explained as aggregation nano-Al particles. The PSD results in particle sizes are consistent with the particles sizes estimated from SEM and TEM images. As in our previous report [24], the fractal dimension parameter D is used to quantify mass or surface changes of scatters. The ln(I(q)) vs. ln(q) plots for all composites film samples, as shown in Fig. 3c, clearly show that mass fractal(Dm) and surface fractal (Ds) coexist in the two specimens, from the slope changes. The mass and surface fractal data of five films from curve fitting were
collected and are shown in the inset of Fig. 3c. With increasing of doping concentrations, the surface fractals of the composite films increase, indicating that the surfaces of composite films become rougher, and the mass fractals of composite films decrease, indicating that the composite structures become looser. The SAXS intensity plots of the all PI/nano-Al composite films are shown negative deviations in Fig. 3d, according to the Porod's theorem of SAXS theory. It is believed that the interaction between organic polymer molecular chains and inorganic nanoparticles in the interfaces is responsible for the negative deviation in Porod plots. This interaction can attribute that abundant unsaturated bonds, hydrogen and organic groups (silane couplings) existing on the surface of nano Al can easily connect with the PI matrix by ionic and hydrogen bonds [25]. The permittivity, loss tangent and volume resistivity were also tested. The dielectric properties of the PI/nano-Al composites ranging from 100 Hz to 10,000 kHz at room temperature were investigated for the
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Fig. 2. (a, b) A low and high magnification cross-section TEM image of the PI/nano-Al composites with content 3 wt%. (c) XRD patterns of the pure PI and the PI/nano-Al composites with 5 and 10 wt% inorganic doping concentration. (d) TGA cures of PI/nano-Al composites with 1, 3, 5, 10 and 15 wt% inorganic doping concentration.
Fig. 3. (a) Typical two dimensional image of SAXS of a PI/nano-Al composite; (b) The PSD of the PI/nano-Al composites with different dopings; (c) Typical ln(I(q)) versus ln(q) plots of the PI/nano-Al composites; (d) Porod's curves of the PI/nano-Al composites with various inorganic doping concentrations.
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Fig. 4. Dielectric properties of the PI/Nano-Al composites with various inorganic doping concentrations (a) dependence of permittivity on frequency, (b) dependence of loss tangent on frequency, (c) Variations of dielectric permittivity and loss tangent of the composites as a function of doping concentrations at 103 Hz and room temperature, (d) Dependence of volume resistivity on doping concentrations.
frequency dependence (Fig. 4a and b). The dielectric permittivity of the all composites only has a weak dependence on frequency (Fig. 4a). The dielectric permittivity of the composites with the nano-Al content lower than 5% shows very small frequency dependence. Only when the nanoAl content exceeds 5%, the dielectric permittivity of the composite film shows a slight decrease with the increasing frequency. At the same time, the dielectric loss of the composites shows a frequency dependence at a wider range of the filler contents at room temperature (Fig. 4b). The dielectric loss increases slowly with the increasing nanoAl contents. At the frequency of 1000 kHz, the dielectric loss of the 15 wt% composite is only 0.005. Fig. 4c illustrates the dielectric permittivity and dielectric loss of the PI/nano-Al composites with various inorganic doping concentration measured at 1 kHz and room temperature. It shows that the dielectric permittivity gradually increases with the increasing amount of nano-Al in the polyimide matrix. The dielectric permittivity reached to 15.7 when the content of nano-Al is 15 wt%. The
observed increase of the dielectric permittivity is due to the higher polarization of nano-Al compared to that of polyimide matrix. Meanwhile, the dielectric loss of the composites increases slightly with the increasing content of nano-Al. However, the dielectric loss remains very low (b 0.005) even at a high nano-Al content of 15 wt%. The high dielectric permittivity and low dielectric loss make the PI/nano-Al composites attractive for practical applications. High volume resistivity of PI/nano-Al composites with various inorganic doping concentration were retained, even insulation performance of PI/nano-Al composites with 1and 3 wt% content is better than pure PI (as shown in Fig. 4d). Incorporating nano-Al nanoparticles into PI polymer matrix can improve composites' mechanical properties, as well. Typical stress–strain curves of PI/Nano-Al films which exhibit superior mechanical properties are shown in Fig. 5. The tensile modulus of PI/Nano-Al film containing 10 wt% of Nano-Al nanoparticles is 38% better than that of pure PI film. The tensile strength of a PI/Nano-Al film containing 3 wt% Nano-
Fig. 5. (a) The stress-strain curve of the PI/nano-Al composites; (b) the tensile strengths and tensile modulus of the PI/nano-Al composites with various inorganic doping concentrations.
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Al nanoparticles is 192 MPa (about 58% greater than that of a pure PI films). Increasing Nano-Al nanoparticle doping concentration from 1 to15 wt% decreases PI/nano-Al films' tensile strength down to 105.3 MPa. The PI/Nano-Al composite films' tensile modulus shows the inverse trend as Nano-Al nanoparticle doping concentration increases. These effects from PI/nano-Al composite can be mainly attributed to the strong interactions between PI and modified nano-Al via forming ionic and hydrogen bonds etc., as well as the fine dispersion and a certain extent of orientation of PI molecular chains, leading to less stress convergence during the elongation process. Significant increase was noted with 1% nano-Al, beyond which the modulus and strength decreased, implying that the amount of nano-Al was in excessive. Similarly, it has been mentioned that the optimum loading is estimated to be around 1% with reactive nanofillers in polymer matrix [26]. Besides, nano-Al particles in PI matrix act like “anchors”, much like sands in cement, which restrict PI polymer movements and deformation under stress. As Nano-Al nanoparticle doping concentration increases, however, another effect would gradually sets in as the distance between Nano-Al nanoparticles gets smaller. When the Nano-Al nanoparticle doping concentration reach certain level (it is about 15 wt% in this study), the pure PI films between Nano-Al nanoparticles become thin and “anchors” change to “grinders” under stress—damaging thin PI films between doped nanoparticles. Thus, at Nano-Al nanoparticle doping larger than 15 wt%, the monolayer PI/nano-Al composite tensile strength is less than that of pure PI. 4. Conclusions The in-situ polymerization technique is applied for homogeneously dispersing nano-Al fillers into polyimide matrix. The PI/nano-Al nanocomposites exhibit high dielectric permittivity with relatively high volume resistivity and a low dielectric loss, good mechanical properties, and excellent thermal stability. A high dielectric permittivity (15.7) is obtained at 103 Hz when the concentration of the nano-Al filler reaches 15 wt%, which is about 4 times higher than that of pure PI matrix. The dielectric permittivity could be tuned by controlling the amount of nano-Al particles. The composites also show a weak dependence of dielectric properties on the testing frequency. It is believed that the functional PI/nano-Al composites with these advantages could be applied in future high-technology fields. Acknowledgement The authors would like to acknowledge support from the National Natural Science Foundation of China (Grant No. 51307046), Natural Science Foundation of Heilongjiang Province of China (Grant No. E2016062), the China Postdoctoral Science Foundation (General Financial Grant No. 2014M561345), the Heilongjiang Postdoctoral Science Foundation (LBH-Z14105), the Scientific Research Foundation for the
Returned Overseas Chinese Scholars of the State Education Ministry (No. 20151098), the University Nursing Program for Young Scholars with Creative Talents in Heilongjiang Province (No. 2015082), postdoctoral scientific research developmental fund of Heilongjiang Province (LBH-Q14144),and the Open Project Program of the Key Laboratory for Photonic and Electric Band Gap Materials of the Ministry of Education of Harbin Normal University (No. PEBM201405). References [1] Y. Yang, B.-P. Zhu, Z.-H. Lu, Z.-Y. Wang, C.-L. Fei, D. Yin, R. Xiong, J. Shi, Q.-G. Chi, Q.-Q. Lei, Appl. Phys. Lett. 102 (2013) 042904. [2] S.A. Harich, D. Dai, C.C. Wang, X. Yang, S.D. Chao, R.T. Skodje, Nature 419 (2002) 281–284. [3] W. Xu, Y. Ding, S. Jiang, J. Zhu, W. Ye, Y. Shen, H. Hou, Eur. Polym. J. 59 (2014) 129–135. [4] L.A. Fredin, Z. Li, M.T. Lanagan, M.A. Ratner, T.J. Marks, Adv. Funct. Mater. 23 (2013) 3560–3569. [5] X. Huang, C. Kim, P. Jiang, Y. Yin, Z. Li, J. Appl. Phys. 105 (2009) 014105. [6] 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, Adv. Mater. 21 (2009) 2077–2082. [7] Z.-M. Dang, Y.-Q. Lin, H.-P. Xu, C.-Y. Shi, S.-T. Li, J. Bai, Adv. Funct. Mater. 18 (2008) 1509–1517. [8] W. Xu, Y. Ding, S. Jiang, W. Ye, X. Liao, H. Hou, Polym. Compos. 37 (2016) 794–801. [9] X. Liu, J. Yin, Y. Kong, M. Chen, Y. Feng, Z. Wu, B. Su, Q. Lei, Thin Solid Films 544 (2013) 54–58. [10] L. Wang, Z.-M. Dang, Appl. Phys. Lett. 87 (2005) 042903. [11] 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. [12] L. Weng, L.W. Yan, H.X. Li, L.Z. Liu, J. Nanosci. Nanotechnol. 16 (2016) 1638–1644. [13] F. Ali, S. Saeed, S.S. Shah, F. Rahim, L. Duclaux, J.M. Leveque, L. Reinert, Recent Pat. Nanotechnol. (2016). [14] Z. Xu, X. Zhuang, C. Yang, J. Cao, Z. Yao, Y. Tang, J. Jiang, D. Wu, X. Feng, Adv. Mater. 28 (2016) 1981–1987. [15] X. Liu, J. Yin, Y. Kong, M. Chen, Y. Feng, K. Yan, X. Li, B. Su, Q. Lei, Thin Solid Films 544 (2013) 352–356. [16] Y. Senoo, N. Nishizawa, Y. Sakakibara, K. Sumimura, E. Itoga, H. Kataura, K. Itoh, Opt. Express 17 (2009) 20233–20241. [17] Y. Zhou, H. Wang, Appl. Phys. Lett. 102 (2013) 132901. [18] Z.-M. Dang, B. Peng, D. Xie, S.-H. Yao, M.-J. Jiang, J. Bai, Appl. Phys. Lett. 92 (2008) 112910. [19] Y. Zhou, L. Wang, H. Zhang, Y. Bai, Y. Niu, H. Wang, Appl. Phys. Lett. 101 (2012) 012903. [20] Y. Yang, H.L. Sun, D. Yin, Z.H. Lu, J.H. Wei, R. Xiong, J. Shi, Z.Y. Wang, Z.Y. Liu, Q.Q. Lei, J. Mater. Chem. A 3 (2015) 4916–4921. [21] X. Huang, C. Kim, Z. Ma, P. Jiang, Y. Yin, Z. Li, J. Polym. Sci. B Polym. Phys. 46 (2008) 2143–2154. [22] X. Liu, J. Yin, M. Chen, W. Bu, W. Cheng, Z. Wu, Nanosci. Nanotechnol. Lett. 3 (2011) 226–229. [23] W. Wang, X. Chen, Q. Cai, G. Mo, L.S. Jiang, K. Zhang, Z.J. Chen, Z.H. Wu, W. Pan, Eur. Phys. J. B 65 (2008) 57–64. [24] L. Xiao-Xu, Y. Jing-Hua, S. Dao-Bin, B. Wen-Bin, C. Wei-Dong, W. Zhong-Hua, Chin. Phys. Lett. 27 (2010) 096103. [25] G.Y. Shengtao Li, G. Chen, Jianying Li, Suna Bai, Lisheng Zhong, Yunxia Zhang, Qingquan Lei, IEEE Trans. Dielectr. Electr. Insul. 17 (2010). [26] L.-B. Zhang, J.-Q. Wang, H.-G. Wang, Y. Xu, Z.-F. Wang, Z.-P. Li, Y.-J. Mi, S.-R. Yang, Compos. A: Appl. Sci. Manuf. 43 (2012) 1537–1545.
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Contents lists available at ScienceDirect
Surface & Coatings Technology journal homepage: www.elsevier.com/locate/surfcoat
Performance and microstructure characteristics in polyimide/nano-aluminum composites Xiaoxu Liu a,d,⁎, Yanpeng Li a, Yuanyuan Liu b, Duo Sun b, Wenmao Guo a, Xiaonan Sun a, Yu Feng d, Hongyan Chi a,b, Xiuhong Li c, Feng Tian c, Bo Su b, Jinghua Yin b,⁎⁎ a
Heilongjiang University of Science and Technology, Harbin 150040, China School of Applied Science, Harbin University of Science and Technology, Harbin 150040, China Shanghai Institute of Applied Physics, Chinese Academy of Sciences, Shanghai 201800, China d Harbin Institute of Technology, Harbin 150001, China b c
a r t i c l e
i n f o
Article history: Received 31 August 2016 Revised 16 January 2017 Accepted in revised form 24 January 2017 Available online xxxx Keywords: Polyimide Aluminum particles Composite Dielectric Mechanical properties
a b s t r a c t Polyimide (PI) composites with high dielectric permittivity have received a great deal of attention in the embedded capacitors and energy-storage devices due to its excellent thermal stability and good mechanical properties. In this study, nano-aluminum (Al) particles were introduced into PI to prepare promising PI/nano-Al composite. The results indicated that the dielectric constant of the composite films increased with the increase of nano-Al contents and the highest dielectric constant was 15.7 for a composite film incorporating 15 wt% nano-Al. The microstructures of PI/nano-Al composite have been investigated by scanning electron microscopy (SEM), translation electron microscopy(TEM), synchrotron radiation small angle X-ray scattering (SAXS) and wide-angle Xray diffraction (XRD). The effects of mixture doping concentration on volume resistivity and loss tangent are analyzed. The correlation effects of the Al nanoparticles on the different factors which influence the dielectric performance in PI matrix such as microstructure, resistivity, and interface of the composites were discussed in detail. This composite film would be possessing potential application in flexible energy-storage devices. © 2017 Published by Elsevier B.V.
1. Introduction Recently, polymeric based composites associated with high dielectric permittivity and low dielectric loss, and high volume resistivity have been in increasing demands owing to continuous development towards the miniaturization and multifunctionalities of apparatus used in high charge-storage capacitors [1,2] and microelectronic components, such as capacitors embedded in flexible printing circuit [3,4]. Adding high permittivity ceramic fillers or conductive fillers are the two common strategies to fabricate high permittivity polymer based composites [5,6]. The drawback of ceramic filler polymer based composites is the high concentration of fillers will lead to a dramatic decrease of mechanical properties [7–9]. Compared to ceramic fillers of polymer based composites, the disadvantage of conductive filler is consequently increased dielectric loss [10]. Polyimide (PI) as excellent insulating polymer at a high temperature have attracted wide interest as a basis enhancing PI properties and extending their applications, [11–15] which innovative materials have been employed for various contemporary applications ⁎ Correspondence to: X. Liu, Heilongjiang University of Science and Technology, Harbin 150040, China. ⁎⁎ Corresponding author. E-mail address: [email protected] (X. Liu).
such as insulation materials, frequency conversion motors, and proton conductive membranes. The dielectric, insulating and mechanical properties of pure PI films do not quite meet the requirements to be used as an insulating material in high charge-storage capacitors. In the same way, PI can be improved by introducing the high permittivity ceramic fillers or conductive fillers into its matrix. Especially, the addition of inert inorganic oxides such as BaTiO3 [8], Calcium Copper Titanate (CCTO) [6], CNT [16] and others [17] in PI matrix has attracted considerable attention as hybrid materials due to enhanced their dielectric property enhancements and mechanical stability improvements. Metal and polyimide composites were researched for polyimide dielectric performance enhancement. For example, Dang et al., have obtained PI and Ag composite films with high dielectric constant and excellent thermal stability as good candidate for energy storage devices [18]. Wang et al., researched SiO2 coated Ag and PI based composites, and obtained remarkably improved high thermal conductivity and dielectric performance [19]. Yang et al., discovered that by coating the surface of CCTO nanoparticles with Ag, the dielectric permittivity of PI/CCTO@Ag composites is significantly increased to 103 (100 Hz) at 3 vol% filler loading. The surface coating of nano-fillers is useful for enhancing dielectric performance [20]. To our best knowledge, a low cost metal Al nanoparticles (nano-Al) with nature oxide Al2O3 coating for nanofillers of polyimide matrix has not yet been reported.
http://dx.doi.org/10.1016/j.surfcoat.2017.01.095 0257-8972/© 2017 Published by Elsevier B.V.
Please cite this article as: X. Liu, et al., Surf. Coat. Technol. (2017), http://dx.doi.org/10.1016/j.surfcoat.2017.01.095
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In this paper, a low cost metal Al nanoparticles (nano-Al) with inartificial Al2O3 coating for nanofiller of polyimide matrix was synthesized by using in-situ polymerization, which the nano-Al was modified by employing surface chemical reaction. The distribution of nano-Al particles was observed by the scanning electron microscope (SEM) and transmission electron microscope (TEM). The dielectric, thermal and mechanical properties of the PI/nano-Al composites were investigated by dielectric tests, thermal gravimetric analysis (TGA), and tensile tests, respectively. The as-synthesized PI/nano-Al composites show high electric, superior electrical and mechanical properties. Meanwhile, PI composite with 15 wt% nano-Al loading has been exhibited higher dielectric constant (15.7) among the all of tested PI composites. The insitu polymerization process is critical in dispersing the nano-Al particles into the PI matrix homogeneously to ensure the good electrical and mechanical properties of the PI/nano-Al films. 2. Experimental details 2.1. Fabrication of the PI/nano-Al composite samples PI/nano-Al composites were prepared by using in-situ polymerization. The nano-Al surface modification was carried out by using the dry toluene reflux method and described in detail in reference [21]. First, modified nano-Al and N,N-dimethylacetamide (DMAC) were added into a three-opening round-bottomed flask and the flask was placed in an ultrasonic bath with continuous stir. The mechanical stirrer and ultrasonic wave were simultaneously utilized until a stable suspension was obtained. Then 4, 4′-oxy dianiline (ODA) was added into the flask and dissolved in the suspension. (The mixture of nano-Al particles and pyromellitic dianhydride in the flask exposed in ultrasonic bath for 2 h in DMAC solvent). Finally, the PMDA was divided into five portions and one portion was added into the suspension at one time to ensure the complete dissolution of the portion before adding another one, until all five portions were added. Then polyamic acid (PAA) suspension was stirred for 4 h at this viscosity until the suspension turns to yellow. The yellow PAA was cast onto a glass dish using a doctor blade. Composite films were obtained after forming, heat treatments and imidization. The resulted films are light yellow, transparent with thicknesses about 40 μm. The nano-Al doping concentrations in all composite films are 1, 3, 5, 10 and 15 wt%, respectively. 2.2. Measurements The cross-section SEM images were obtained by a JEOL field-emission scanning electron microscope under operating voltage of 15 kV, model JSM-6700F. The small angle X-ray scattering (SAXS) tests were carried out at Shanghai Synchrotron Radiation Facility, by using a wavelength of 0.124 nm, a sample to detector distance of 5 m, and an exposure time of 10 s. The 2D scattering patterns were collected on a CCD camera, and the intensity vs. scattering angle is obtained by integrating the data from the 2D scattering patterns. TEM-HRTEM was carried out on TEM (JEOL JEM-2010). X-ray diffraction (XRD) measurements were performed at a Rigaku D/max-rB X-ray diffractometer with Cu Kα (λ = 0.15418 nm) incident radiation. The diffraction patterns were collected at room temperature in the 2θ ranges of 10 to 60°. The dielectric constant of hybrid PI films was tested using an impedance analyzer (Aglient 4294A) with 16451B Dielectric Test Fixture in the frequency range of 1–107 Hz. The DC volume resistivity measurements are performed using a Keithley electrometer with 8009 resistivity measurement kit at a voltage of 500 V. 3. Results and discussion The morphology of nano-Al filler was tested TEM. As seen in Fig. 2a, the spherical nano-Al size is about 10 nm. The nano-Al has the passivated oxide layer (a kind of insulating layer) around its core surface shown
in Fig. 1b. The dispersion of the nano-Al within the polyimide matrix was further investigated and the SEM micrographs of the fractured section of the composites prepared by in situ polymerization were shown in Fig. 1c–f.it can be seen that a homogeneous dispersion of the nanoparticles in the PI matrix are separated in low doping (3% doping) sample, and the nano-Al particle sizes of the composite are smaller than 100 nm. Meanwhile a few agglomerations with polyimide containing nano-Al particles are observed as indicated in the SEM image (see Fig. 1b) at higher doping (10% doping) sample. With loading increase of Al nanoparticles, the microstructure change is observed to occur up to 10 wt%, which a number of micro-size layer-like clusters with polyimide molecules coating nano-Al particles begin to appear from place to place and their magnitudes are found to further increase. (see Fig. 1e, f) The formation of large size particle clusters in polymer matrix is determined by the kinetics of composite preparation. The attractive van der Waals forces between nanoparticles are relatively weak because of the long particle-to-particle distance at low nanoparticle loading, which may be the main reason why the nanoscale dispersion is obtained for the composites with nanoparticle loading 3 wt%. With further increasing the Al nanoparticle loading, the particle-to-particle distance decreases obviously and the more attractive van der Waals forces begin to create agglomeration of the particles from place to place. More interesting, the Al nanoparticles still uniformly dispersed in micro-size clusters, which these cluster formed many layers with parallel orientation at leading of 10% nano-Al. This is beneficial for increasing the overall performances of PI/nano-Al composites. For further confirmation of dispersion of nano-Al, the microstructures of PI/nano-Al composites with content 3 wt% were characterized using TEM technique in Fig. 2 a, b. It can be observed that the nano-Al particles (black spots) are homogeneously dispersed in the polyimide matrix with the particle size in nano-scale, and the particles size is about 10 nm. The FFT pattern of HRTEM revealed the crystalline nature of nano-Al and the (111) plane of aluminium were observed (inset of Fig. 2b). Meanwhile, the EDX pattern obtained from TEM (Fig. 2a) indicated the existence of abundant aluminium and slight oxygen. The homogeneous dispersion of Al particles is attributed to the coaction of mechanical stirring and ultrasonic wave which is an effective way to produce the stable suspension. XRD patterns for the PI/nano-Al with 3 and 10 wt% doping, and pure PI film were given in Fig. 2c. In the XRD patterns of PI/nano-Al composite with the presence of Al particles, the strong diffraction peak of Al is at 2θ = 38.5 and 44.7° (PDF:040787). A wide peak in the range of 18.2° in the PI/nano-Al spectrum clearly demonstrates the existence of the amorphous structure of polyimide, which in another way indicates the effectiveness of our synthesizing polyimide method. In the pure film XRD spectrum, on the other hand, the position of the wide peak is shifted to the left slightly in Fig. 2c, this indicates that the crystalline of PI matrix was enhanced. The thermogravimetric analysis (TGA) was examined to evaluate the thermal stability of the PI/nano-Al composites with different Al content. Results are shown in Fig. 2d. It can be found that the thermal stability of the PI/nano-Al with 3% Al content have the highest value in decomposition temperature, about 551.1 °C, among three of the composite films tested when 5 wt% mass lose is reached. This superior in thermal stability of PI/nano-Al with 3% Al content also indicates a rather good compatibility between the inorganic particles and the organic matrix with addition of Al. As a useful method for testing microstructure of polymer based composite materials, small angle X-ray scattering (SAXS) technology has been used to characterize composite polymers in our previously research [22]. A two-dimensional (2D) SAXS pattern of the PI/nano-Al composite is shown in the Fig. 3a. The PI/nano-Al composites with various inorganic doping concentration have quite similar SAXS patterns, which are all isotropous 2D patterns of scattering intensity with scattering angle. This indicates that ordered microstructure is not presences in PI/nano-Al composites. SAXS data can be analyzed to get the particle size distribution (PSD), unordered fractal structure and interface information for PI composites. The method [23] used to estimate PSD with
Please cite this article as: X. Liu, et al., Surf. Coat. Technol. (2017), http://dx.doi.org/10.1016/j.surfcoat.2017.01.095
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Fig. 1. (a, b) A TEM images of the nano-Al. (c, d) SEM images of a low and high magnification cross-section SEM image of the PI/nano-Al composites with content 3 wt%. (e, f) A low and high magnification cross-section SEM image of the PI/nano-Al composite with content 10 wt%.
SAXS data is Tangent-by-Tangent (TBT) method, and we use it here to estimate the PSD of PI/nano-Al composites (as shown in Fig. 4b). There is quite similar in PSD of PI/nano-Al composites with 1, 3, 5, 10 and 15 wt% inorganic doping concentrations. A distinctive peak appears in all composite films about 7–8 nm in particle size and we attribute the peak to the real nano-Al particles. When doping concentration is N5 wt%, the normalized volume fraction of distinctive peak was obviously decreased. This can be explained as aggregation nano-Al particles. The PSD results in particle sizes are consistent with the particles sizes estimated from SEM and TEM images. As in our previous report [24], the fractal dimension parameter D is used to quantify mass or surface changes of scatters. The ln(I(q)) vs. ln(q) plots for all composites film samples, as shown in Fig. 3c, clearly show that mass fractal(Dm) and surface fractal (Ds) coexist in the two specimens, from the slope changes. The mass and surface fractal data of five films from curve fitting were
collected and are shown in the inset of Fig. 3c. With increasing of doping concentrations, the surface fractals of the composite films increase, indicating that the surfaces of composite films become rougher, and the mass fractals of composite films decrease, indicating that the composite structures become looser. The SAXS intensity plots of the all PI/nano-Al composite films are shown negative deviations in Fig. 3d, according to the Porod's theorem of SAXS theory. It is believed that the interaction between organic polymer molecular chains and inorganic nanoparticles in the interfaces is responsible for the negative deviation in Porod plots. This interaction can attribute that abundant unsaturated bonds, hydrogen and organic groups (silane couplings) existing on the surface of nano Al can easily connect with the PI matrix by ionic and hydrogen bonds [25]. The permittivity, loss tangent and volume resistivity were also tested. The dielectric properties of the PI/nano-Al composites ranging from 100 Hz to 10,000 kHz at room temperature were investigated for the
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Fig. 2. (a, b) A low and high magnification cross-section TEM image of the PI/nano-Al composites with content 3 wt%. (c) XRD patterns of the pure PI and the PI/nano-Al composites with 5 and 10 wt% inorganic doping concentration. (d) TGA cures of PI/nano-Al composites with 1, 3, 5, 10 and 15 wt% inorganic doping concentration.
Fig. 3. (a) Typical two dimensional image of SAXS of a PI/nano-Al composite; (b) The PSD of the PI/nano-Al composites with different dopings; (c) Typical ln(I(q)) versus ln(q) plots of the PI/nano-Al composites; (d) Porod's curves of the PI/nano-Al composites with various inorganic doping concentrations.
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Fig. 4. Dielectric properties of the PI/Nano-Al composites with various inorganic doping concentrations (a) dependence of permittivity on frequency, (b) dependence of loss tangent on frequency, (c) Variations of dielectric permittivity and loss tangent of the composites as a function of doping concentrations at 103 Hz and room temperature, (d) Dependence of volume resistivity on doping concentrations.
frequency dependence (Fig. 4a and b). The dielectric permittivity of the all composites only has a weak dependence on frequency (Fig. 4a). The dielectric permittivity of the composites with the nano-Al content lower than 5% shows very small frequency dependence. Only when the nanoAl content exceeds 5%, the dielectric permittivity of the composite film shows a slight decrease with the increasing frequency. At the same time, the dielectric loss of the composites shows a frequency dependence at a wider range of the filler contents at room temperature (Fig. 4b). The dielectric loss increases slowly with the increasing nanoAl contents. At the frequency of 1000 kHz, the dielectric loss of the 15 wt% composite is only 0.005. Fig. 4c illustrates the dielectric permittivity and dielectric loss of the PI/nano-Al composites with various inorganic doping concentration measured at 1 kHz and room temperature. It shows that the dielectric permittivity gradually increases with the increasing amount of nano-Al in the polyimide matrix. The dielectric permittivity reached to 15.7 when the content of nano-Al is 15 wt%. The
observed increase of the dielectric permittivity is due to the higher polarization of nano-Al compared to that of polyimide matrix. Meanwhile, the dielectric loss of the composites increases slightly with the increasing content of nano-Al. However, the dielectric loss remains very low (b 0.005) even at a high nano-Al content of 15 wt%. The high dielectric permittivity and low dielectric loss make the PI/nano-Al composites attractive for practical applications. High volume resistivity of PI/nano-Al composites with various inorganic doping concentration were retained, even insulation performance of PI/nano-Al composites with 1and 3 wt% content is better than pure PI (as shown in Fig. 4d). Incorporating nano-Al nanoparticles into PI polymer matrix can improve composites' mechanical properties, as well. Typical stress–strain curves of PI/Nano-Al films which exhibit superior mechanical properties are shown in Fig. 5. The tensile modulus of PI/Nano-Al film containing 10 wt% of Nano-Al nanoparticles is 38% better than that of pure PI film. The tensile strength of a PI/Nano-Al film containing 3 wt% Nano-
Fig. 5. (a) The stress-strain curve of the PI/nano-Al composites; (b) the tensile strengths and tensile modulus of the PI/nano-Al composites with various inorganic doping concentrations.
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Al nanoparticles is 192 MPa (about 58% greater than that of a pure PI films). Increasing Nano-Al nanoparticle doping concentration from 1 to15 wt% decreases PI/nano-Al films' tensile strength down to 105.3 MPa. The PI/Nano-Al composite films' tensile modulus shows the inverse trend as Nano-Al nanoparticle doping concentration increases. These effects from PI/nano-Al composite can be mainly attributed to the strong interactions between PI and modified nano-Al via forming ionic and hydrogen bonds etc., as well as the fine dispersion and a certain extent of orientation of PI molecular chains, leading to less stress convergence during the elongation process. Significant increase was noted with 1% nano-Al, beyond which the modulus and strength decreased, implying that the amount of nano-Al was in excessive. Similarly, it has been mentioned that the optimum loading is estimated to be around 1% with reactive nanofillers in polymer matrix [26]. Besides, nano-Al particles in PI matrix act like “anchors”, much like sands in cement, which restrict PI polymer movements and deformation under stress. As Nano-Al nanoparticle doping concentration increases, however, another effect would gradually sets in as the distance between Nano-Al nanoparticles gets smaller. When the Nano-Al nanoparticle doping concentration reach certain level (it is about 15 wt% in this study), the pure PI films between Nano-Al nanoparticles become thin and “anchors” change to “grinders” under stress—damaging thin PI films between doped nanoparticles. Thus, at Nano-Al nanoparticle doping larger than 15 wt%, the monolayer PI/nano-Al composite tensile strength is less than that of pure PI. 4. Conclusions The in-situ polymerization technique is applied for homogeneously dispersing nano-Al fillers into polyimide matrix. The PI/nano-Al nanocomposites exhibit high dielectric permittivity with relatively high volume resistivity and a low dielectric loss, good mechanical properties, and excellent thermal stability. A high dielectric permittivity (15.7) is obtained at 103 Hz when the concentration of the nano-Al filler reaches 15 wt%, which is about 4 times higher than that of pure PI matrix. The dielectric permittivity could be tuned by controlling the amount of nano-Al particles. The composites also show a weak dependence of dielectric properties on the testing frequency. It is believed that the functional PI/nano-Al composites with these advantages could be applied in future high-technology fields. Acknowledgement The authors would like to acknowledge support from the National Natural Science Foundation of China (Grant No. 51307046), Natural Science Foundation of Heilongjiang Province of China (Grant No. E2016062), the China Postdoctoral Science Foundation (General Financial Grant No. 2014M561345), the Heilongjiang Postdoctoral Science Foundation (LBH-Z14105), the Scientific Research Foundation for the
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Please cite this article as: X. Liu, et al., Surf. Coat. Technol. (2017), http://dx.doi.org/10.1016/j.surfcoat.2017.01.095