- Email: [email protected]
barium titanate nanocomposite films
Materials Chemistry and Physics 131 (2011) 387–392
Contents lists available at SciVerse ScienceDirect
Materials Chemistry and Physics journal homepage: www.elsevier.com/locate/matchemphys
Microstructure, thermal and dielectric properties of homogeneous bismaleimide-triazine/barium titanate nanocomposite films Xiaoliang Zeng a , Shuhui Yu a,∗ , Rong Sun a,∗ , Ruxu Du b a b
Shenzhen Institutes of Advanced Technology, Chinese Academy of Sciences, Shenzhen 518055, China Institute of Precision Engineering, The Chinese University of Hong Kong, Shatin, NT, Hong Kong, China
a r t i c l e
i n f o
Article history: Received 12 February 2011 Received in revised form 21 July 2011 Accepted 27 September 2011 Keywords: A. Composite materials A. Thin films D. Dielectric properties D. Thermal properties
a b s t r a c t Bismaleimide-triazine resin/barium titanate (BT/BaTiO3 ) nanocomposite films were prepared by mixing the nano-BaTiO3 particles into BT resin, followed by films casting and thermal cure. The surface modification of BaTiO3 nanoparticles with silane coupling agent results in excellent dispersion and enhances the interaction between BaTiO3 and the BT matrix. The derived nanocomposite films exhibit improved dielectric constant, while the dielectric loss remains at a low level (<0.05). For the nanocomposite film containing 70 wt% of BaTiO3 , the effective dielectric constant at room temperature reaches 23.63, which is about 7 times larger than that of pure BT resin, and the dielectric loss is only 0.0212 at 100 Hz. The dielectric properties of the nanocomposite films are nearly frequency-independent, which is attributed to the excellent dispersion of BaTiO3 nanoparticles in the BT matrix. The interaction between BaTiO3 and BT affects not only the phase transition of BaTiO3 , but also the thermal behavior of BT. Moreover, the nanocomposite films exhibit improved thermal resistance. The highest glass transition temperature (Tg ) is 261 ◦ C, indicating good reliability as dielectric materials in applications. © 2011 Elsevier B.V. All rights reserved.
1. Introduction Polymeric composites with nano functional particles as fillers exhibit tailorable mechanical, optical and/or electrical properties and find their applications in various fields. One of the promising applications is in the high-density packaging technology as embedded materials [1,2], in the form of dielectric film. A lot of research has been carried out to develop composite dielectrics with high dielectric constant [3–6]. With nanosized ferroelectric or conductive particles as fillers, the dielectric constant of the polymeric composites can be improved by tens or even hundreds of times, compared with the pure polymers [7]. Meanwhile, thermal stability of the composite should also be evaluated for industrial application, because a rise in temperature of the composite during operation will lead to a decomposition or distortion of the polymer matrix. The matrix polymers such as epoxy resin [8–10] and polyvinylidene fluoride (PVDF) [11–13], which are commonly used to fabricate the composites, have poor thermal stability, with their glass transition temperature lower than 150 ◦ C [14]. There are only a few articles reporting on the thermal behavior of the nanocomposites. Xie et al. [15] synthesized the polyimide/barium titanate composites through a colloidal process,
∗ Corresponding authors. Tel.: +86 755 86392104; fax: +86 755 86392194. E-mail addresses: [email protected], [email protected] (S. Yu), [email protected] (R. Sun). 0254-0584/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.matchemphys.2011.09.060
which exhibited desired dielectric properties. The temperature of 5% weight loss was 614 ◦ C, but the glass transition temperature Tg , which is important to evaluate the reliability of the nanocomposite, was not reported. Considering the bisphenol A dicyanate has better thermal stability compared with epoxy resin, Chao et al. [16] prepared BaTiO3 /bisphenol A dicyanate composite. Although the dielectric property of the composite has been investigated systematically, the thermal property has not been investigated. A high glass transition temperature (>225 ◦ C) was reported by Wang et al. [17] in the BaTiO3 /polyethersulfone composite with 50 vol.% BaTiO3 . However, the effect of introducing nano ceramic particles on the derived polymeric nanocomposites is lack in the literature. Both thermal and dielectric properties are dependent on the individual characteristics of the polymeric matrix and fillers, as well as the interface chemistry between them. In this study, bismaleimide-triazine resin (BT resin) was chosen as the matrix because it has excellent thermal stability and reliable electrical properties. For example, its Tg can be as high as 230 ◦ C, which is most desired for application purpose. Nanosized BaTiO3 was used as the filler. To enhance the distribution of BaTiO3 particles in BT resin, the surface of BaTiO3 particles was modified with silane coupling agent. The interaction mechanism between BT and BaTiO3 , thermal behavior and dielectric properties versus frequency and temperature were systematically investigated. Three models were employed and compared in order to explicate the interaction mechanism and to explain the dielectric behavior of the composites. The
388
X. Zeng et al. / Materials Chemistry and Physics 131 (2011) 387–392
coupling model theory was used to analyze the effects of BaTiO3 particles on the thermal stability of the composites. 2. Experimental 2.1. Materials 4,4 -Bismaleimidodiphenylmethane (BMI, Honghu Bismaleimide Resin Factory, China), 2,2 -bis (4-cyanatophenyl) propane (BCE, Heijang Kinlyuan Pharmaceutical Co., Ltd., China), and 2,2 -diallyl bisphenol A (DBA, Wuxi Resin Factory, China) were used as starting materials to prepare the modified BT resin. Barium titanate (BaTiO3 , Fenghua Advanced Technology Holding Co., Ltd., China) was used as ceramic particles. The average diameter of BaTiO3 is 100 nm. ␥-Glycidoxypropyl trimethoxysilane (KH-560, Sinopharm Chemical Reagent Co., Ltd., China) was used as modifier of BaTiO3 . The other materials and reagents were also purchased from Sinopharm Chemical Reagent Co., Ltd., China.
Fig. 1. FTIR spectra of (a) BaTiO3 and (b) BaTiO3 modified with KH-560.
2.2. Surface modification of BaTiO3 by KH-560 First, 5 g of BaTiO3 nanoparticles were suspended in 50 ml of 95% ethanol solution, to which 0.5 g of KH-560 were slowly added. The mixture was stirred for 12 h at 70 ◦ C. The modified BaTiO3 was isolated through centrifugation and washed three times with the ethanol. Finally, the modified BaTiO3 was dried at 50 ◦ C for 24 h to yield dried powder.
loss (tan ı) were measured using an impedance analyzer (Agilent 4294A) in the frequency range of 100 Hz to 1 MHz and in the temperature range from 25 ◦ C to 250 ◦ C. For dielectric measurements, the silver paste was painted on the surface of the nanocomposite films. The measured area was 12.0 mm in diameter and about 5 m in thickness. At least three points were measured to ensure repeatability.
2.3. Modification of BT resin by DBA 3. Results and discussion The modified BT was prepared by a melt method. Typically, 5 mol BMI, 3 mol BCE, and 2 mol DBA were mixed by stirring at 120–150 ◦ C for 30 min to form transparent, amber-colored liquid, and cooled to room temperature. 2.4. Preparation of BT/BaTiO3 nanocomposite films The BT/BaTiO3 nanocomposite films with different BaTiO3 loadings in the range of 10–70 wt% were fabricated with the following method. First, the modified BT resins were dissolved in an organic solvent (methyl ethyl ketone). Then, the surface-modified BaTiO3 nanoparticles were dispersed into the BT solution under stirring condition. The obtained BaTiO3 /BT suspension was then roll-coated on a Cu plate and dried at 80 ◦ C for 30 min to evaporate the solvent. Finally, the dried BT/BaTiO3 nanocomposite films were cured by the programmed heating process in the temperature range of 150–240 ◦ C/2 h. The thickness of the composite films was about 5 m measured with optical microscope. 2.5. Characterization The chemical status of the BaTiO3 powder after modification was investigated by Fourier transform infrared spectroscopy (FTIR) (Bruker Vertex 70). The samples were milled with potassium bromide (KBr) and then compressed into a thin pellet for measurement. The surface morphology of the composite samples was examined using scanning electron microscope (SEM) (4800S, Hitachi). The samples were coated with a thin Au layer to assist the viewing. The glass transition temperature (Tg ) of the BT/BaTiO3 nanocomposite films and pure BT resin were measured by the differential scanning calorimetry (DSC) at a heating rate of 10 ◦ C min−1 in nitrogen (TA Q20). The Tg is defined as the inflection of the heat capacity versus the temperature curve. The nanocomposite films, with the weight of 10–20 mg, were fabricated as thin and flat as possible to minimize the occurrence of thermal gradients. The dielectric properties including effective dielectric constant (εeff ) and dielectric
3.1. Surface modification of BaTiO3 nanoparticles In order to improve the compatibility between BT resin and BaTiO3 , KH-560 is selected as the coupling agent to modify the BaTiO3 . Fig. 1 presents the FTIR spectra of pristine BaTiO3 and BaTiO3 modified with KH-560. The weak absorption band of pristine BaTiO3 at 3500–3200 cm−1 is caused by the valence vibration of the O–H group on the BaTiO3 . The two broad absorption peaks at 582 cm−1 and 487 cm−1 are assigned to the characteristic peaks of BaTiO3 . Compared with the spectrum of pristine BaTiO3 (Fig. 1(a)), the spectrum of modified BaTiO3 (Fig. 1(b)) displays the new peaks at 2940 cm−1 , 2840 cm−1 , 1103 cm−1 , 1199 cm−1 , 904 cm−1 and 920 cm−1 . The absorption peaks at 2940 cm−1 and 2840 cm−1 are the valence stretching vibrations of aliphatic C–H, while the peaks at 1103 cm−1 and 1199 cm−1 are assigned to the Si–O–Si stretch and C–O–C stretch, respectively. The peaks appearing at 904 cm−1 and 920 cm−1 , are assigned to the epoxy unit of KH-560. Based on the above results, we can conclude that KH-560 successfully reacted on the BaTiO3 nanoparticles.
3.2. The morphology of the nanocomposite films Fig. 2 shows the SEM images of the surface morphology of BT/BaTiO3 nanocomposite films containing 70 wt% of BaTiO3 . As presented in the figure, the BaTiO3 with the average diameter of 100 nm can be seen clearly. Moreover, the BaTiO3 nanoparticles are homogeneously dispersed throughout the BT resin matrix, and no agglomeration or voids are observed. As illustrated in Scheme 1, after modification, the functional groups from KH-560 act as dispersants on the surface of BaTiO3 , of which the epoxy unit can induce an interaction with BT resin via a ring-opening reaction. As a result, the distribution of the BaTiO3 fillers in the BT resin matrix is enhanced, which is important to obtain desired dielectric and thermal properties.
X. Zeng et al. / Materials Chemistry and Physics 131 (2011) 387–392
389
Scheme 1. Interactions among BT resin, silicone KH-560, and BaTiO3 nanoparticles.
nanocomposite decreases to 257 ◦ C. According to the coupling model theory suggested by Roland and Ngai [18], the segmental motion of polymers below Tg is a cooperative process and has to overcome the resistance from the surrounding segments in order to accomplish the transformation between configurations. As more segments are restricted by the presence of BaTiO3 , the activation threshold for the motion of some segments becomes higher. Therefore, the nanocomposites have exhibited a higher Tg than the corresponding pure BT resin system. Similar results have been reported in the epoxy resin/kaolinite nanocomposite [19] and epoxy resin/TiO2 nanocomposite [20]. High glass transition temperature of our nanocomposites assures the application of them in high temperature environment. 3.4. The dielectric properties of the nanocomposite films
Fig. 2. SEM images of the surface morphology of BT/BaTiO3 nanocomposites: (a) pure BT resin; (b) 70 wt% BaTiO3 .
Fig. 4 presents the frequency-dependence of effective dielectric constant (εeff ) and dielectric loss (tan ı) of BT/BaTiO3 nanocomposite films at 25 ◦ C. As shown in Fig. 4(a), the εeff versus frequency curves are almost parallel to the frequency axis in the log scale. The nearly independence of the εeff on frequency can be attributed to the high degree of homogeneous dispersion of BaTiO3 . It has been commonly reported that the εeff usually decreases with increasing the frequency [12,21–23], which is ascribed to an interfacial relaxation. However, in this study, the BaTiO3 nanoparticles were modified with KH-560, which improved the compatibility between the particle surface and BT resin. As a result, the filler aggregation and formation of voids would be reduced. The increased compatibility and voids reduction can decrease the probability of interfacial relaxation, leading to the independence of the εeff on frequency. On the other hand, stable response of the dielectric properties of BT resin on the frequency is another factor that makes the εeff nearly independent on the frequency. As revealed in Fig. 4(b), the tan ı of nanocomposite films apparently increase with the increase of
3.3. Thermal properties of the nanocomposite films Fig. 3 shows DSC thermograms for all nanocomposites, and the corresponding Tg are summarized in Table 1, and the estimated error in Tg is about ±0.5◦ C. With respect to the Tg values of the BT resins, addition of BaTiO3 shifts the Tg to higher temperatures. The Tg reaches 261 ◦ C when the nanocomposite contains 50 wt% of BaTiO3 , which is improved by 28 ◦ C compared with the BT resin. At a weight fraction of 70 wt%, however, the Tg of Table 1 Tg results of BT resin and BT/BaTiO3 nanocomposites. Samples
Tg (◦ C)
BT resin BT/10%BaTiO3 BT/30%BaTiO3 BT/50%BaTiO3 BT/70%BaTiO3
233 233 241 261 257
± ± ± ± ±
0.5 0.4 0.6 0.3 0.5
Fig. 3. DSC thermograms of BT resin and BT/BaTiO3 nanocomposites.
390
X. Zeng et al. / Materials Chemistry and Physics 131 (2011) 387–392
Fig. 6. The experimental and theoretical dielectric constants of BT/BaTiO3 nanocomposites films at 100 Hz and room temperature.
Fig. 4. Effective dielectric constant (a) and loss tan ı (b) of BaTiO3 /BT resin dielectric composites versus frequency.
from 0.0096 to 0.00212 at 100 Hz as the BaTiO3 amounts increases from 0 wt% to 70 wt%. The increase of the tan ı for the composites may be due to the interfacial loss. Another exceptional feature of our results is that, for the BT/BaTiO3 nanocomposite films with the BaTiO3 amount above 30 wt%, the tan ı is about 0.02, which is nearly independent of BaTiO3 amount. This result suggests that, as the increase of the BaTiO3 amount, the small voids and pores do not apparently increase because of high degree of homogeneous dispensation of BaTiO3 , as presented in Fig. 2. The prediction of dielectric behavior of the composite is very important to its application. For illustration and comparison, three approximation models are employed in this study to further analyze the dielectric behavior of the nanocomposite at 100 Hz and room temperature. Maxwell–Garnett model [24]
εeff = ε1 measured frequency in the range of 1.0 kHz to 1.0 MHz. Although the tan ı increases with further increasing frequency, it is still less than 0.05, showing that the material has a potential application as dielectric in capacitor. Fig. 5 presents εeff and tan ı of BT/BaTiO3 nanocomposite films as a function of BaTiO3 weight fraction measured at 100 Hz. As shown in Fig. 5, when the BaTiO3 amount increases from 0 wt% to 70 wt%, the εeff rises from 3.4 to 23.63 at 100 Hz, which implies that the incorporation of BaTiO3 into BT resin is very effective way to increase its εeff . The tan ı of the nanocomposite films ranges
1+
3VBaTiO3 ˇ
1 − VBaTiO3 ˇ
where ˇ = (ε2 − ε1 )/(ε2 + 2ε1 ), εeff is the effective dielectric constant of the BT/BaTiO3 nanocomposites; ε1 = 3.4 and ε2 = 3000 are used as the dielectric constants of the BT resin and BaTiO3 nanoparticles, respectively; and, VBaTiO3 is the volume fraction of the BaTiO3 nanoparticles. VBaTiO3 = (WBaTiO3 /BaTiO3 )/((WBaTiO3 /BaTiO3 ) + (WBT resin /BT resin )), WBaTiO3 and WBT resin are the weight of BaTiO3 and BT resins, respectively. BaTiO3 = 5.7 g cm−3 and BT resin = 1.2 g cm−3 are used as the density of BaTiO3 and BT resins, respectively. Therefore, the weight fractions of BaTiO3 nanoparticles of 10 wt%, 30 wt%, 50 wt% and 70 wt% are converted to volume fractions of 2.3 vol.%, 8.4 vol.%, 17.6 vol.% and 33.3 vol.%, respectively. Lichtenecher logarithmic model [25] log ε = (1 − VBaTiO3 )log ε1 + VBaTiO3 log ε2
Fig. 5. Effective dielectric constant εeff and dielectric loss tan ı of BT/BaTiO3 nanocomposites as a function of BaTiO3 weight fraction measured at 100 Hz.
(1)
(2)
As shown in Fig. 6, the predicted values by Maxwell–Garnett model and Lichtenecher logarithmic model both deviate from the experimental data, revealing that these two models are not suitable to describe the εeff of BT resin/BaTiO3 nanocomposite films. The reason is that both Maxwell–Garnett and Lichtenecher logarithmic model are based on the hypothesis that spheroidal particles are ideally dispersed in the matrix. Obviously, this does not match the situation of cubic BaTiO3 dispersing in the BT resins. Meanwhile, these simple models do not take into account the influence of the particle–particle dipolar interactions or their effect on the surrounding medium, which become important at higher volume fractions.
X. Zeng et al. / Materials Chemistry and Physics 131 (2011) 387–392
391
To better model the εeff of a composite system over a wide range of volume fractions, Li et al. [26] developed a self-consistent effective medium theory (SC-EMT). With SC-EMT, the effects of interactions between particles and the inclusion of possible porosity in the composites can be modeled as a three-phase material, consisting of a polymer matrix, interfacial phase with a fixed thickness, and nanoparticles. The εeff can be expressed as εeff = ε1 + f2 (ε2 − ε1 )a2 + f3 (ε3 − ε1 )a3
(3)
where the εeff , ε1 , ε2 , ε3 is the dielectric constant of nanocomposites, BT resin, possible porosity and BaTiO3 , respectively; f2 and f3 are the volume fractions of interfacial phase and BaTiO3 , and a2 and a3 denote the electric-field concentration factor in each phase. As such, the volume fraction of the interfacial phase is given by: 3
f2 =
(r + l) − r 3 f3 r3
(4)
where r is the nanoparticle radius, l is the thickness of interfacial phase, and l/r = 0.1, corresponding to the particle size around 100 nm for typical exchange length of a few nanometers, with ε3 /ε1 ≈ 1000 corresponding to typical ratio of dielectric constant for ceramic and polymer, ε2 = (ε1 + ε3 )/2. The a2 and a3 can be expressed as ar = 1 − s[(εr − εeff )−1 εeff + s]
−1
r = 2, 3
(5)
where s is the component of the dielectric Eshelby tensor that is related to the depolarization factor. The predicted values by SC-EMT model is obtained by using Eqs. (3)–(5). As revealed in Fig. 6, the experimental data fit well with the SC-EMT model, with the depolarization factor (s) of 0.054, 0.260, 0.012, and 0.510, in response to the BaTiO3 fractions of 10 wt%, 30 wt%, 50 wt% and 70 wt%, respectively. It is interesting to note that the depolarization factor, s, increases with increasing the BaTiO3 fraction. This result significantly demonstrates that the particle–particle dipolar interactions or their effect on the surrounding medium get intense with the increase of nanoparticles volume fraction. Fig. 7 shows the temperature dependence of the εeff and tan ı for the BT/BaTiO3 nanocomposite films at 100 Hz. As shown in Fig. 7, the εeff and tan ı of the nanocomposite films with the BaTiO3 amount below or equal 10 wt% remain relatively unchanged over a broad temperature range of 25–250 ◦ C. The phenomenon indicates that the cured BT resin has extremely low and stable εeff and tan ı over a wide temperature range. When the BaTiO3 content is more than 10 wt%, the εeff and tan ı display much stronger temperature dependence over the same temperature range. Namely, the εeff increases with an increase of temperature. For example, for the nanocomposite with 70 wt% BaTiO3 , the εeff increases from 23.48 to 99.37, while the corresponding tan ı increases from 0.028 to 0.846, as the temperature changes from 25 to 250 ◦ C. The exhibited increased εeff and tan ı with increasing temperature can be attributed to a higher density of freely promoted charge carriers at higher temperatures [27]. Three obvious peak regions from 40 ◦ C to 70 ◦ C, from 140 ◦ C to 160 ◦ C, and from 220 ◦ C to 250 ◦ C are observed in both εeff and tan ı results, indicated as ˇ2 , ˇ1 , and ˛, respectively in Fig. 7. The three regions can be attributed to the contribution of phase transition of BT resin and BaTiO3 . In reference to the DSC result (Fig. 3), the ˛ relaxation is probably associated with the glass transition of BT resin. It has been suggested that this process is due to rotational motions of dipolar groups in the amorphous regions of the BT resin. But these rotational motions of dipolar groups are often frozen so that they cannot catch up with the frequency change of the electric field, leading to the formation of the peaks of the εeff and tan ı. The ˇ1 and ˇ2 relaxation has been associated with the phase transition behavior of BaTiO3 , because no such behavior is
Fig. 7. Temperature dependence of dielectric constant (a) and dielectric loss (b) at 100 Hz.
observed in pure BT resin, as showed in Fig. 7. It is well known that BaTiO3 undergoes phase transitions upon heating or cooling. Upon cooling, BaTiO3 transforms from cubic state to tetragonal, then to orthorhombic, and finally to rhombohedral one. In the case of pure BaTiO3 particles, these three phase transitions take place at about 120 (Curie temperature), 5, and −90 ◦ C, respectively, and are accompanied by sharp maxima in the dielectric constant and tan ı magnitude around the phase transition temperatures [28]. However, in the BaTiO3 /BT composite film, the phase transition temperatures of BaTiO3 at 120 and 8 ◦ C shift up to 150 and 50 ◦ C, respectively. The internal and interfacial stress should be responsible for the enhanced phase transition temperatures. Some factors such as grain size, microstructure, chemical stoichiometry, defects and internal and interfacial stress, etc., are found to have prominent influences on the phase transition in BaTiO3 [29–31]. Yang et al. [31,32] investigated the phase transition in BaTiO3 and PbTiO3 nanotube arrays. They found that optimized two-dimensional compressive stress as well as the size effect was responsible for the shift of phase temperature to higher temperatures. Li et al. [33] reported the Curie temperature of BaTiO3 nanotube above 150 ◦ C and also attributed this phenomenon to the stress introduced during film growth. In our study, due to the existence of BT resins, impurity and defect of BaTiO3 , internal and interfacial stress could also be produced in the BT resin/BaTiO3 nanocomposite films, which induce the shift of phase transition temperature. However, since the BaTiO3 itself possesses very complicated phase transition behaviors [34], much experimental and analysis are required to clarify the phase transition behavior of the BaTiO3 nanoparticles in the BT resin. The relevant investigation will be
392
X. Zeng et al. / Materials Chemistry and Physics 131 (2011) 387–392
carried out by using X-ray diffraction (XRD) and Raman analysis in our future work.
research funding from National S&T Major Project with the contact No. 2009ZX02038.
4. Conclusion
References
In this work, the BT resin/barium titanate (BT/BaTiO3 ) nanocomposite films with a high degree of dispersion of the BaTiO3 were successfully prepared, by modification of the BaTiO3 with KH560. The dielectric constant and loss increase with the increase of BaTiO3 . In the case of nanocomposite containing 70 wt% BaTiO3 , the dielectric constant and loss of the nanocomposites are 23.63 and 0.0212 at 100 Hz, respectively. Three models were used to analyze the effective dielectric constant of the films. The experimental results are in good agreement with the SC-EMT model. The increased polarization factor, s, indicates that the particle–particle dipolar interactions or their effect on the surrounding medium get intense with increasing BaTiO3 volume fraction. Three relaxation peak regions for the dielectric properties are observed from 40 ◦ C to 70 ◦ C, from 140 ◦ C to 160 ◦ C, and from 220 ◦ C to 250 ◦ C, which can be attributed to the contribution of phase transition of BaTiO3 and relaxation of BT resin. The internal and interfacial stress is responsible for the enhanced phase transition temperatures of BaTiO3 . The glass transition temperature of the nanocomposite films increase, with increasing the amount of BaTiO3 nanoparticles. When the nanocomposite film contains 50 wt% BaTiO3 , the Tg of the film is 261 ◦ C, showing good thermal stability. In view of its application, the superior thermal stability, high dielectric constant, and low dielectric loss make the BT/BaTiO3 nanocomposites potentially replace the currently dominate epoxy/BaTiO3 embedded capacitor.
[1] J.X. Lu, C.P. Wong, IEEE Trans. Dielectr. Electr. Insul. 15 (2008) 1322. [2] S. Ramesh, B.A. Shutzberg, C. Huang, G. Jie, E.P. Giannelis, IEEE Trans. Adv. Packag. Technol. 26 (2003) 17. [3] S. Georgea, M.T. Sebastian, Compos. Sci. Technol. 69 (2009) 1298. [4] C.C. Wu, Y.C. Chen, C.C. Su, C.F. Yang, Eur. Polym. J. 45 (2009) 1442. [5] S.H. Choi, J.M. Hong, K.H. Park, S.G. Oh, Mater. Lett. 61 (2007) 2478. [6] M. Konieczna, E. Markiewicz, J. Jurga, Polym. Eng. Sci. 50 (2010) 1613. [7] M.T. Sebastian, H. Jantunen, Int. J. Appl. Ceram. Technol. 7 (2010) 415. [8] M. Iijima, N. Sato, I.W. Lenggoro, H. Kamiya, Colloids Surf. A 352 (2009) 88–93. [9] Z.M. Dang, H.P. Xu, J. Bai, Compos. Sci. Technol. 68 (2008) 171. [10] Y. Kobayashi, D. Nagao, M. Konno, Polym. Compos. 31 (2010) 1179. [11] H.M. Jung, J.H. Kang, S.Y. Yang, J.C. Won, Y.S. Kim, Chem. Mater. 22 (2010) 450. [12] H.P. Xu, Z.M. Dang, N. Bing, Y. Wu, D. Yang, J. Appl. Phys. 107 (2010) 034105. [13] K. Pal, D.J. Kang, Z.X. Zhang, J.K. Kim, Langmuir 26 (2010) 3609. [14] N. Mukherjee, D. Wavhal, R.B. Timmons, Appl. Mater. Interf. 2 (2010) 397. [15] S.H. Xie, X.Z. Wei, Z.K. Xu, Y.Y. Xu, Composites Part A 36 (2005) 1152. [16] F. Chao, G. Liang, W. Kong, X. Zhang, Mater. Chem. Phys. 108 (2008) 306. [17] F.J. Wang, W. Li, M.S. Xue, J.P. Yao, J.S. Lu, Composites Part B 42 (2011) 87. [18] C.M. Roland, K.L. Ngai, Macromolecules 24 (1991) 5315. [19] X.N. Xia, X. Zeng, J. Liu, W. Xu, J. Appl. Polym. Sci. 118 (2010) 2461. [20] A. Chatterjee, M.S. Islam, Mater. Sci. Eng. A 487 (2008) 574. [21] Y. Lin, C.W. Nan, J. Wang, G. Liu, J. Wu, N. Cai, J. Am. Ceram. Soc. 87 (2004) 742. [22] S. Panteny, C.R. Bowen, R. Stevens, J. Mater. Sci. 41 (2006) 3845. [23] G.S. Wang, Appl. Mater. Interf. 2 (2010) 1290. [24] J.C. Maxwell-Garnett, Philos. Trans. R. Soc. Lond. A 203 (1904) 385. [25] K. Lichtenecker, Phys. Z. 25 (1924) 225. [26] J.Y. Li, L. Zhang, S. Ducharme, Appl. Phys. Lett. 90 (2007) 132901. [27] K.C. Kao, Dielectric Phenomena in Solids, Academic Press, London, 2004. [28] R. Popielarz, C.K. Chiang, R. Nozaki, J. Obrzut, Macromolecules 34 (2001) 5910. [29] S. Lee, C.A. Randall, Z.K. Liu, J. Am. Ceram. Soc. 91 (2008) 1748. [30] A. Jana, T.K. Kundu, Mater. Lett. 61 (2007) 1544. [31] Y. Yang, X. Wang, C. Sun, L. Li, J. Appl. Phys. 109 (2011) 014109. [32] Y. Yang, X. Wang, C. Sun, L. Li, J. Appl. Phys. 104 (2008) 124108. [33] C.L. Li, Z.H. Chen, D.F. Cu, Y.L. Zhou, H.B. Lu, C. Dong, F. Wu, H. Chen, J. Appl. Phys. 86 (1999) 4555. [34] T. Hoshina, H. Kakemoto, T. Tsurumi, S. Wada, J. Appl. Phys. 99 (2006) 054311.
Acknowledgements The authors acknowledge the financial support from National Natural Science Foundation (No. 20971086 and 50807038) and the
Contents lists available at SciVerse ScienceDirect
Materials Chemistry and Physics journal homepage: www.elsevier.com/locate/matchemphys
Microstructure, thermal and dielectric properties of homogeneous bismaleimide-triazine/barium titanate nanocomposite films Xiaoliang Zeng a , Shuhui Yu a,∗ , Rong Sun a,∗ , Ruxu Du b a b
Shenzhen Institutes of Advanced Technology, Chinese Academy of Sciences, Shenzhen 518055, China Institute of Precision Engineering, The Chinese University of Hong Kong, Shatin, NT, Hong Kong, China
a r t i c l e
i n f o
Article history: Received 12 February 2011 Received in revised form 21 July 2011 Accepted 27 September 2011 Keywords: A. Composite materials A. Thin films D. Dielectric properties D. Thermal properties
a b s t r a c t Bismaleimide-triazine resin/barium titanate (BT/BaTiO3 ) nanocomposite films were prepared by mixing the nano-BaTiO3 particles into BT resin, followed by films casting and thermal cure. The surface modification of BaTiO3 nanoparticles with silane coupling agent results in excellent dispersion and enhances the interaction between BaTiO3 and the BT matrix. The derived nanocomposite films exhibit improved dielectric constant, while the dielectric loss remains at a low level (<0.05). For the nanocomposite film containing 70 wt% of BaTiO3 , the effective dielectric constant at room temperature reaches 23.63, which is about 7 times larger than that of pure BT resin, and the dielectric loss is only 0.0212 at 100 Hz. The dielectric properties of the nanocomposite films are nearly frequency-independent, which is attributed to the excellent dispersion of BaTiO3 nanoparticles in the BT matrix. The interaction between BaTiO3 and BT affects not only the phase transition of BaTiO3 , but also the thermal behavior of BT. Moreover, the nanocomposite films exhibit improved thermal resistance. The highest glass transition temperature (Tg ) is 261 ◦ C, indicating good reliability as dielectric materials in applications. © 2011 Elsevier B.V. All rights reserved.
1. Introduction Polymeric composites with nano functional particles as fillers exhibit tailorable mechanical, optical and/or electrical properties and find their applications in various fields. One of the promising applications is in the high-density packaging technology as embedded materials [1,2], in the form of dielectric film. A lot of research has been carried out to develop composite dielectrics with high dielectric constant [3–6]. With nanosized ferroelectric or conductive particles as fillers, the dielectric constant of the polymeric composites can be improved by tens or even hundreds of times, compared with the pure polymers [7]. Meanwhile, thermal stability of the composite should also be evaluated for industrial application, because a rise in temperature of the composite during operation will lead to a decomposition or distortion of the polymer matrix. The matrix polymers such as epoxy resin [8–10] and polyvinylidene fluoride (PVDF) [11–13], which are commonly used to fabricate the composites, have poor thermal stability, with their glass transition temperature lower than 150 ◦ C [14]. There are only a few articles reporting on the thermal behavior of the nanocomposites. Xie et al. [15] synthesized the polyimide/barium titanate composites through a colloidal process,
∗ Corresponding authors. Tel.: +86 755 86392104; fax: +86 755 86392194. E-mail addresses: [email protected], [email protected] (S. Yu), [email protected] (R. Sun). 0254-0584/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.matchemphys.2011.09.060
which exhibited desired dielectric properties. The temperature of 5% weight loss was 614 ◦ C, but the glass transition temperature Tg , which is important to evaluate the reliability of the nanocomposite, was not reported. Considering the bisphenol A dicyanate has better thermal stability compared with epoxy resin, Chao et al. [16] prepared BaTiO3 /bisphenol A dicyanate composite. Although the dielectric property of the composite has been investigated systematically, the thermal property has not been investigated. A high glass transition temperature (>225 ◦ C) was reported by Wang et al. [17] in the BaTiO3 /polyethersulfone composite with 50 vol.% BaTiO3 . However, the effect of introducing nano ceramic particles on the derived polymeric nanocomposites is lack in the literature. Both thermal and dielectric properties are dependent on the individual characteristics of the polymeric matrix and fillers, as well as the interface chemistry between them. In this study, bismaleimide-triazine resin (BT resin) was chosen as the matrix because it has excellent thermal stability and reliable electrical properties. For example, its Tg can be as high as 230 ◦ C, which is most desired for application purpose. Nanosized BaTiO3 was used as the filler. To enhance the distribution of BaTiO3 particles in BT resin, the surface of BaTiO3 particles was modified with silane coupling agent. The interaction mechanism between BT and BaTiO3 , thermal behavior and dielectric properties versus frequency and temperature were systematically investigated. Three models were employed and compared in order to explicate the interaction mechanism and to explain the dielectric behavior of the composites. The
388
X. Zeng et al. / Materials Chemistry and Physics 131 (2011) 387–392
coupling model theory was used to analyze the effects of BaTiO3 particles on the thermal stability of the composites. 2. Experimental 2.1. Materials 4,4 -Bismaleimidodiphenylmethane (BMI, Honghu Bismaleimide Resin Factory, China), 2,2 -bis (4-cyanatophenyl) propane (BCE, Heijang Kinlyuan Pharmaceutical Co., Ltd., China), and 2,2 -diallyl bisphenol A (DBA, Wuxi Resin Factory, China) were used as starting materials to prepare the modified BT resin. Barium titanate (BaTiO3 , Fenghua Advanced Technology Holding Co., Ltd., China) was used as ceramic particles. The average diameter of BaTiO3 is 100 nm. ␥-Glycidoxypropyl trimethoxysilane (KH-560, Sinopharm Chemical Reagent Co., Ltd., China) was used as modifier of BaTiO3 . The other materials and reagents were also purchased from Sinopharm Chemical Reagent Co., Ltd., China.
Fig. 1. FTIR spectra of (a) BaTiO3 and (b) BaTiO3 modified with KH-560.
2.2. Surface modification of BaTiO3 by KH-560 First, 5 g of BaTiO3 nanoparticles were suspended in 50 ml of 95% ethanol solution, to which 0.5 g of KH-560 were slowly added. The mixture was stirred for 12 h at 70 ◦ C. The modified BaTiO3 was isolated through centrifugation and washed three times with the ethanol. Finally, the modified BaTiO3 was dried at 50 ◦ C for 24 h to yield dried powder.
loss (tan ı) were measured using an impedance analyzer (Agilent 4294A) in the frequency range of 100 Hz to 1 MHz and in the temperature range from 25 ◦ C to 250 ◦ C. For dielectric measurements, the silver paste was painted on the surface of the nanocomposite films. The measured area was 12.0 mm in diameter and about 5 m in thickness. At least three points were measured to ensure repeatability.
2.3. Modification of BT resin by DBA 3. Results and discussion The modified BT was prepared by a melt method. Typically, 5 mol BMI, 3 mol BCE, and 2 mol DBA were mixed by stirring at 120–150 ◦ C for 30 min to form transparent, amber-colored liquid, and cooled to room temperature. 2.4. Preparation of BT/BaTiO3 nanocomposite films The BT/BaTiO3 nanocomposite films with different BaTiO3 loadings in the range of 10–70 wt% were fabricated with the following method. First, the modified BT resins were dissolved in an organic solvent (methyl ethyl ketone). Then, the surface-modified BaTiO3 nanoparticles were dispersed into the BT solution under stirring condition. The obtained BaTiO3 /BT suspension was then roll-coated on a Cu plate and dried at 80 ◦ C for 30 min to evaporate the solvent. Finally, the dried BT/BaTiO3 nanocomposite films were cured by the programmed heating process in the temperature range of 150–240 ◦ C/2 h. The thickness of the composite films was about 5 m measured with optical microscope. 2.5. Characterization The chemical status of the BaTiO3 powder after modification was investigated by Fourier transform infrared spectroscopy (FTIR) (Bruker Vertex 70). The samples were milled with potassium bromide (KBr) and then compressed into a thin pellet for measurement. The surface morphology of the composite samples was examined using scanning electron microscope (SEM) (4800S, Hitachi). The samples were coated with a thin Au layer to assist the viewing. The glass transition temperature (Tg ) of the BT/BaTiO3 nanocomposite films and pure BT resin were measured by the differential scanning calorimetry (DSC) at a heating rate of 10 ◦ C min−1 in nitrogen (TA Q20). The Tg is defined as the inflection of the heat capacity versus the temperature curve. The nanocomposite films, with the weight of 10–20 mg, were fabricated as thin and flat as possible to minimize the occurrence of thermal gradients. The dielectric properties including effective dielectric constant (εeff ) and dielectric
3.1. Surface modification of BaTiO3 nanoparticles In order to improve the compatibility between BT resin and BaTiO3 , KH-560 is selected as the coupling agent to modify the BaTiO3 . Fig. 1 presents the FTIR spectra of pristine BaTiO3 and BaTiO3 modified with KH-560. The weak absorption band of pristine BaTiO3 at 3500–3200 cm−1 is caused by the valence vibration of the O–H group on the BaTiO3 . The two broad absorption peaks at 582 cm−1 and 487 cm−1 are assigned to the characteristic peaks of BaTiO3 . Compared with the spectrum of pristine BaTiO3 (Fig. 1(a)), the spectrum of modified BaTiO3 (Fig. 1(b)) displays the new peaks at 2940 cm−1 , 2840 cm−1 , 1103 cm−1 , 1199 cm−1 , 904 cm−1 and 920 cm−1 . The absorption peaks at 2940 cm−1 and 2840 cm−1 are the valence stretching vibrations of aliphatic C–H, while the peaks at 1103 cm−1 and 1199 cm−1 are assigned to the Si–O–Si stretch and C–O–C stretch, respectively. The peaks appearing at 904 cm−1 and 920 cm−1 , are assigned to the epoxy unit of KH-560. Based on the above results, we can conclude that KH-560 successfully reacted on the BaTiO3 nanoparticles.
3.2. The morphology of the nanocomposite films Fig. 2 shows the SEM images of the surface morphology of BT/BaTiO3 nanocomposite films containing 70 wt% of BaTiO3 . As presented in the figure, the BaTiO3 with the average diameter of 100 nm can be seen clearly. Moreover, the BaTiO3 nanoparticles are homogeneously dispersed throughout the BT resin matrix, and no agglomeration or voids are observed. As illustrated in Scheme 1, after modification, the functional groups from KH-560 act as dispersants on the surface of BaTiO3 , of which the epoxy unit can induce an interaction with BT resin via a ring-opening reaction. As a result, the distribution of the BaTiO3 fillers in the BT resin matrix is enhanced, which is important to obtain desired dielectric and thermal properties.
X. Zeng et al. / Materials Chemistry and Physics 131 (2011) 387–392
389
Scheme 1. Interactions among BT resin, silicone KH-560, and BaTiO3 nanoparticles.
nanocomposite decreases to 257 ◦ C. According to the coupling model theory suggested by Roland and Ngai [18], the segmental motion of polymers below Tg is a cooperative process and has to overcome the resistance from the surrounding segments in order to accomplish the transformation between configurations. As more segments are restricted by the presence of BaTiO3 , the activation threshold for the motion of some segments becomes higher. Therefore, the nanocomposites have exhibited a higher Tg than the corresponding pure BT resin system. Similar results have been reported in the epoxy resin/kaolinite nanocomposite [19] and epoxy resin/TiO2 nanocomposite [20]. High glass transition temperature of our nanocomposites assures the application of them in high temperature environment. 3.4. The dielectric properties of the nanocomposite films
Fig. 2. SEM images of the surface morphology of BT/BaTiO3 nanocomposites: (a) pure BT resin; (b) 70 wt% BaTiO3 .
Fig. 4 presents the frequency-dependence of effective dielectric constant (εeff ) and dielectric loss (tan ı) of BT/BaTiO3 nanocomposite films at 25 ◦ C. As shown in Fig. 4(a), the εeff versus frequency curves are almost parallel to the frequency axis in the log scale. The nearly independence of the εeff on frequency can be attributed to the high degree of homogeneous dispersion of BaTiO3 . It has been commonly reported that the εeff usually decreases with increasing the frequency [12,21–23], which is ascribed to an interfacial relaxation. However, in this study, the BaTiO3 nanoparticles were modified with KH-560, which improved the compatibility between the particle surface and BT resin. As a result, the filler aggregation and formation of voids would be reduced. The increased compatibility and voids reduction can decrease the probability of interfacial relaxation, leading to the independence of the εeff on frequency. On the other hand, stable response of the dielectric properties of BT resin on the frequency is another factor that makes the εeff nearly independent on the frequency. As revealed in Fig. 4(b), the tan ı of nanocomposite films apparently increase with the increase of
3.3. Thermal properties of the nanocomposite films Fig. 3 shows DSC thermograms for all nanocomposites, and the corresponding Tg are summarized in Table 1, and the estimated error in Tg is about ±0.5◦ C. With respect to the Tg values of the BT resins, addition of BaTiO3 shifts the Tg to higher temperatures. The Tg reaches 261 ◦ C when the nanocomposite contains 50 wt% of BaTiO3 , which is improved by 28 ◦ C compared with the BT resin. At a weight fraction of 70 wt%, however, the Tg of Table 1 Tg results of BT resin and BT/BaTiO3 nanocomposites. Samples
Tg (◦ C)
BT resin BT/10%BaTiO3 BT/30%BaTiO3 BT/50%BaTiO3 BT/70%BaTiO3
233 233 241 261 257
± ± ± ± ±
0.5 0.4 0.6 0.3 0.5
Fig. 3. DSC thermograms of BT resin and BT/BaTiO3 nanocomposites.
390
X. Zeng et al. / Materials Chemistry and Physics 131 (2011) 387–392
Fig. 6. The experimental and theoretical dielectric constants of BT/BaTiO3 nanocomposites films at 100 Hz and room temperature.
Fig. 4. Effective dielectric constant (a) and loss tan ı (b) of BaTiO3 /BT resin dielectric composites versus frequency.
from 0.0096 to 0.00212 at 100 Hz as the BaTiO3 amounts increases from 0 wt% to 70 wt%. The increase of the tan ı for the composites may be due to the interfacial loss. Another exceptional feature of our results is that, for the BT/BaTiO3 nanocomposite films with the BaTiO3 amount above 30 wt%, the tan ı is about 0.02, which is nearly independent of BaTiO3 amount. This result suggests that, as the increase of the BaTiO3 amount, the small voids and pores do not apparently increase because of high degree of homogeneous dispensation of BaTiO3 , as presented in Fig. 2. The prediction of dielectric behavior of the composite is very important to its application. For illustration and comparison, three approximation models are employed in this study to further analyze the dielectric behavior of the nanocomposite at 100 Hz and room temperature. Maxwell–Garnett model [24]
εeff = ε1 measured frequency in the range of 1.0 kHz to 1.0 MHz. Although the tan ı increases with further increasing frequency, it is still less than 0.05, showing that the material has a potential application as dielectric in capacitor. Fig. 5 presents εeff and tan ı of BT/BaTiO3 nanocomposite films as a function of BaTiO3 weight fraction measured at 100 Hz. As shown in Fig. 5, when the BaTiO3 amount increases from 0 wt% to 70 wt%, the εeff rises from 3.4 to 23.63 at 100 Hz, which implies that the incorporation of BaTiO3 into BT resin is very effective way to increase its εeff . The tan ı of the nanocomposite films ranges
1+
3VBaTiO3 ˇ
1 − VBaTiO3 ˇ
where ˇ = (ε2 − ε1 )/(ε2 + 2ε1 ), εeff is the effective dielectric constant of the BT/BaTiO3 nanocomposites; ε1 = 3.4 and ε2 = 3000 are used as the dielectric constants of the BT resin and BaTiO3 nanoparticles, respectively; and, VBaTiO3 is the volume fraction of the BaTiO3 nanoparticles. VBaTiO3 = (WBaTiO3 /BaTiO3 )/((WBaTiO3 /BaTiO3 ) + (WBT resin /BT resin )), WBaTiO3 and WBT resin are the weight of BaTiO3 and BT resins, respectively. BaTiO3 = 5.7 g cm−3 and BT resin = 1.2 g cm−3 are used as the density of BaTiO3 and BT resins, respectively. Therefore, the weight fractions of BaTiO3 nanoparticles of 10 wt%, 30 wt%, 50 wt% and 70 wt% are converted to volume fractions of 2.3 vol.%, 8.4 vol.%, 17.6 vol.% and 33.3 vol.%, respectively. Lichtenecher logarithmic model [25] log ε = (1 − VBaTiO3 )log ε1 + VBaTiO3 log ε2
Fig. 5. Effective dielectric constant εeff and dielectric loss tan ı of BT/BaTiO3 nanocomposites as a function of BaTiO3 weight fraction measured at 100 Hz.
(1)
(2)
As shown in Fig. 6, the predicted values by Maxwell–Garnett model and Lichtenecher logarithmic model both deviate from the experimental data, revealing that these two models are not suitable to describe the εeff of BT resin/BaTiO3 nanocomposite films. The reason is that both Maxwell–Garnett and Lichtenecher logarithmic model are based on the hypothesis that spheroidal particles are ideally dispersed in the matrix. Obviously, this does not match the situation of cubic BaTiO3 dispersing in the BT resins. Meanwhile, these simple models do not take into account the influence of the particle–particle dipolar interactions or their effect on the surrounding medium, which become important at higher volume fractions.
X. Zeng et al. / Materials Chemistry and Physics 131 (2011) 387–392
391
To better model the εeff of a composite system over a wide range of volume fractions, Li et al. [26] developed a self-consistent effective medium theory (SC-EMT). With SC-EMT, the effects of interactions between particles and the inclusion of possible porosity in the composites can be modeled as a three-phase material, consisting of a polymer matrix, interfacial phase with a fixed thickness, and nanoparticles. The εeff can be expressed as εeff = ε1 + f2 (ε2 − ε1 )a2 + f3 (ε3 − ε1 )a3
(3)
where the εeff , ε1 , ε2 , ε3 is the dielectric constant of nanocomposites, BT resin, possible porosity and BaTiO3 , respectively; f2 and f3 are the volume fractions of interfacial phase and BaTiO3 , and a2 and a3 denote the electric-field concentration factor in each phase. As such, the volume fraction of the interfacial phase is given by: 3
f2 =
(r + l) − r 3 f3 r3
(4)
where r is the nanoparticle radius, l is the thickness of interfacial phase, and l/r = 0.1, corresponding to the particle size around 100 nm for typical exchange length of a few nanometers, with ε3 /ε1 ≈ 1000 corresponding to typical ratio of dielectric constant for ceramic and polymer, ε2 = (ε1 + ε3 )/2. The a2 and a3 can be expressed as ar = 1 − s[(εr − εeff )−1 εeff + s]
−1
r = 2, 3
(5)
where s is the component of the dielectric Eshelby tensor that is related to the depolarization factor. The predicted values by SC-EMT model is obtained by using Eqs. (3)–(5). As revealed in Fig. 6, the experimental data fit well with the SC-EMT model, with the depolarization factor (s) of 0.054, 0.260, 0.012, and 0.510, in response to the BaTiO3 fractions of 10 wt%, 30 wt%, 50 wt% and 70 wt%, respectively. It is interesting to note that the depolarization factor, s, increases with increasing the BaTiO3 fraction. This result significantly demonstrates that the particle–particle dipolar interactions or their effect on the surrounding medium get intense with the increase of nanoparticles volume fraction. Fig. 7 shows the temperature dependence of the εeff and tan ı for the BT/BaTiO3 nanocomposite films at 100 Hz. As shown in Fig. 7, the εeff and tan ı of the nanocomposite films with the BaTiO3 amount below or equal 10 wt% remain relatively unchanged over a broad temperature range of 25–250 ◦ C. The phenomenon indicates that the cured BT resin has extremely low and stable εeff and tan ı over a wide temperature range. When the BaTiO3 content is more than 10 wt%, the εeff and tan ı display much stronger temperature dependence over the same temperature range. Namely, the εeff increases with an increase of temperature. For example, for the nanocomposite with 70 wt% BaTiO3 , the εeff increases from 23.48 to 99.37, while the corresponding tan ı increases from 0.028 to 0.846, as the temperature changes from 25 to 250 ◦ C. The exhibited increased εeff and tan ı with increasing temperature can be attributed to a higher density of freely promoted charge carriers at higher temperatures [27]. Three obvious peak regions from 40 ◦ C to 70 ◦ C, from 140 ◦ C to 160 ◦ C, and from 220 ◦ C to 250 ◦ C are observed in both εeff and tan ı results, indicated as ˇ2 , ˇ1 , and ˛, respectively in Fig. 7. The three regions can be attributed to the contribution of phase transition of BT resin and BaTiO3 . In reference to the DSC result (Fig. 3), the ˛ relaxation is probably associated with the glass transition of BT resin. It has been suggested that this process is due to rotational motions of dipolar groups in the amorphous regions of the BT resin. But these rotational motions of dipolar groups are often frozen so that they cannot catch up with the frequency change of the electric field, leading to the formation of the peaks of the εeff and tan ı. The ˇ1 and ˇ2 relaxation has been associated with the phase transition behavior of BaTiO3 , because no such behavior is
Fig. 7. Temperature dependence of dielectric constant (a) and dielectric loss (b) at 100 Hz.
observed in pure BT resin, as showed in Fig. 7. It is well known that BaTiO3 undergoes phase transitions upon heating or cooling. Upon cooling, BaTiO3 transforms from cubic state to tetragonal, then to orthorhombic, and finally to rhombohedral one. In the case of pure BaTiO3 particles, these three phase transitions take place at about 120 (Curie temperature), 5, and −90 ◦ C, respectively, and are accompanied by sharp maxima in the dielectric constant and tan ı magnitude around the phase transition temperatures [28]. However, in the BaTiO3 /BT composite film, the phase transition temperatures of BaTiO3 at 120 and 8 ◦ C shift up to 150 and 50 ◦ C, respectively. The internal and interfacial stress should be responsible for the enhanced phase transition temperatures. Some factors such as grain size, microstructure, chemical stoichiometry, defects and internal and interfacial stress, etc., are found to have prominent influences on the phase transition in BaTiO3 [29–31]. Yang et al. [31,32] investigated the phase transition in BaTiO3 and PbTiO3 nanotube arrays. They found that optimized two-dimensional compressive stress as well as the size effect was responsible for the shift of phase temperature to higher temperatures. Li et al. [33] reported the Curie temperature of BaTiO3 nanotube above 150 ◦ C and also attributed this phenomenon to the stress introduced during film growth. In our study, due to the existence of BT resins, impurity and defect of BaTiO3 , internal and interfacial stress could also be produced in the BT resin/BaTiO3 nanocomposite films, which induce the shift of phase transition temperature. However, since the BaTiO3 itself possesses very complicated phase transition behaviors [34], much experimental and analysis are required to clarify the phase transition behavior of the BaTiO3 nanoparticles in the BT resin. The relevant investigation will be
392
X. Zeng et al. / Materials Chemistry and Physics 131 (2011) 387–392
carried out by using X-ray diffraction (XRD) and Raman analysis in our future work.
research funding from National S&T Major Project with the contact No. 2009ZX02038.
4. Conclusion
References
In this work, the BT resin/barium titanate (BT/BaTiO3 ) nanocomposite films with a high degree of dispersion of the BaTiO3 were successfully prepared, by modification of the BaTiO3 with KH560. The dielectric constant and loss increase with the increase of BaTiO3 . In the case of nanocomposite containing 70 wt% BaTiO3 , the dielectric constant and loss of the nanocomposites are 23.63 and 0.0212 at 100 Hz, respectively. Three models were used to analyze the effective dielectric constant of the films. The experimental results are in good agreement with the SC-EMT model. The increased polarization factor, s, indicates that the particle–particle dipolar interactions or their effect on the surrounding medium get intense with increasing BaTiO3 volume fraction. Three relaxation peak regions for the dielectric properties are observed from 40 ◦ C to 70 ◦ C, from 140 ◦ C to 160 ◦ C, and from 220 ◦ C to 250 ◦ C, which can be attributed to the contribution of phase transition of BaTiO3 and relaxation of BT resin. The internal and interfacial stress is responsible for the enhanced phase transition temperatures of BaTiO3 . The glass transition temperature of the nanocomposite films increase, with increasing the amount of BaTiO3 nanoparticles. When the nanocomposite film contains 50 wt% BaTiO3 , the Tg of the film is 261 ◦ C, showing good thermal stability. In view of its application, the superior thermal stability, high dielectric constant, and low dielectric loss make the BT/BaTiO3 nanocomposites potentially replace the currently dominate epoxy/BaTiO3 embedded capacitor.
[1] J.X. Lu, C.P. Wong, IEEE Trans. Dielectr. Electr. Insul. 15 (2008) 1322. [2] S. Ramesh, B.A. Shutzberg, C. Huang, G. Jie, E.P. Giannelis, IEEE Trans. Adv. Packag. Technol. 26 (2003) 17. [3] S. Georgea, M.T. Sebastian, Compos. Sci. Technol. 69 (2009) 1298. [4] C.C. Wu, Y.C. Chen, C.C. Su, C.F. Yang, Eur. Polym. J. 45 (2009) 1442. [5] S.H. Choi, J.M. Hong, K.H. Park, S.G. Oh, Mater. Lett. 61 (2007) 2478. [6] M. Konieczna, E. Markiewicz, J. Jurga, Polym. Eng. Sci. 50 (2010) 1613. [7] M.T. Sebastian, H. Jantunen, Int. J. Appl. Ceram. Technol. 7 (2010) 415. [8] M. Iijima, N. Sato, I.W. Lenggoro, H. Kamiya, Colloids Surf. A 352 (2009) 88–93. [9] Z.M. Dang, H.P. Xu, J. Bai, Compos. Sci. Technol. 68 (2008) 171. [10] Y. Kobayashi, D. Nagao, M. Konno, Polym. Compos. 31 (2010) 1179. [11] H.M. Jung, J.H. Kang, S.Y. Yang, J.C. Won, Y.S. Kim, Chem. Mater. 22 (2010) 450. [12] H.P. Xu, Z.M. Dang, N. Bing, Y. Wu, D. Yang, J. Appl. Phys. 107 (2010) 034105. [13] K. Pal, D.J. Kang, Z.X. Zhang, J.K. Kim, Langmuir 26 (2010) 3609. [14] N. Mukherjee, D. Wavhal, R.B. Timmons, Appl. Mater. Interf. 2 (2010) 397. [15] S.H. Xie, X.Z. Wei, Z.K. Xu, Y.Y. Xu, Composites Part A 36 (2005) 1152. [16] F. Chao, G. Liang, W. Kong, X. Zhang, Mater. Chem. Phys. 108 (2008) 306. [17] F.J. Wang, W. Li, M.S. Xue, J.P. Yao, J.S. Lu, Composites Part B 42 (2011) 87. [18] C.M. Roland, K.L. Ngai, Macromolecules 24 (1991) 5315. [19] X.N. Xia, X. Zeng, J. Liu, W. Xu, J. Appl. Polym. Sci. 118 (2010) 2461. [20] A. Chatterjee, M.S. Islam, Mater. Sci. Eng. A 487 (2008) 574. [21] Y. Lin, C.W. Nan, J. Wang, G. Liu, J. Wu, N. Cai, J. Am. Ceram. Soc. 87 (2004) 742. [22] S. Panteny, C.R. Bowen, R. Stevens, J. Mater. Sci. 41 (2006) 3845. [23] G.S. Wang, Appl. Mater. Interf. 2 (2010) 1290. [24] J.C. Maxwell-Garnett, Philos. Trans. R. Soc. Lond. A 203 (1904) 385. [25] K. Lichtenecker, Phys. Z. 25 (1924) 225. [26] J.Y. Li, L. Zhang, S. Ducharme, Appl. Phys. Lett. 90 (2007) 132901. [27] K.C. Kao, Dielectric Phenomena in Solids, Academic Press, London, 2004. [28] R. Popielarz, C.K. Chiang, R. Nozaki, J. Obrzut, Macromolecules 34 (2001) 5910. [29] S. Lee, C.A. Randall, Z.K. Liu, J. Am. Ceram. Soc. 91 (2008) 1748. [30] A. Jana, T.K. Kundu, Mater. Lett. 61 (2007) 1544. [31] Y. Yang, X. Wang, C. Sun, L. Li, J. Appl. Phys. 109 (2011) 014109. [32] Y. Yang, X. Wang, C. Sun, L. Li, J. Appl. Phys. 104 (2008) 124108. [33] C.L. Li, Z.H. Chen, D.F. Cu, Y.L. Zhou, H.B. Lu, C. Dong, F. Wu, H. Chen, J. Appl. Phys. 86 (1999) 4555. [34] T. Hoshina, H. Kakemoto, T. Tsurumi, S. Wada, J. Appl. Phys. 99 (2006) 054311.
Acknowledgements The authors acknowledge the financial support from National Natural Science Foundation (No. 20971086 and 50807038) and the