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Towards suppressing loss tangent: Effect of polydopamine coating layers on dielectric properties of core–shell barium titanate filled polyvinylidene fluoride composites
Composites Science and Technology 118 (2015) 198e206
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Composites Science and Technology journal homepage: http://www.elsevier.com/locate/compscitech
Towards suppressing loss tangent: Effect of polydopamine coating layers on dielectric properties of coreeshell barium titanate filled polyvinylidene fluoride composites Yuhan Li a, Jiajia Yuan a, Jian Xue b, Fanyi Cai b, Feng Chen a, **, Qiang Fu a, * a b
College of Polymer Science and Engineering, State Key Laboratory of Polymer Materials Engineering, Sichuan University, Chengdu 610065, China Nari Group Corporation State Grid Electric Power Research Institute, Nanjing 211000, China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 1 July 2015 Received in revised form 30 August 2015 Accepted 4 September 2015 Available online 8 September 2015
Barium titanate (BT) particles coated with different thickness of polydopamine (PDA) layers were incorporated into polyvinylidene fluoride (PVDF) to investigate influence of PDA coating layers on dielectric properties of composites. It is found that PDA coating layers not only effectively improved the interfacial interaction between BT particles and matrix but also significantly affected dielectric properties. PVDF composites loaded with modified BT exhibits superior dielectric properties in comparison with unmodified BT. Interfacial polarization is substantially suppressed because the catechol groups of PDA are able to constrain the mobility of nomadic charge carriers and ionizable hydroxyl groups on surface of BT particles. As a result, dependency of dielectric constant on frequency attenuates and tan d is lowered to below 0.050 (1 kHz). The effect of suppression on tan d tends to be more prominent as the thickness of PDA coating layer increases. The tan d derived from ionic relaxation polarization is also constrained due to the chelation of PDA catechol groups with migrated cations. The as-prepared composites possess high dielectric constant and ultralow tan d, making them promising for the industrial application as embedded capacitors. © 2015 Elsevier Ltd. All rights reserved.
Keywords: Polymer-matrix composites (PMCs) Functional composites Electrical properties Scanning electron microscopy (SEM)
1. Introduction Composites with high dielectric constant and low tan d have important application of the microelectronics industry, especially in the field of embedded capacitor [1e5]. Its appealing industrial prospects attract researchers to devote their efforts to developing dielectric composites which can meet requirements. Polymerbased composites have many advantages, such as good flexibility, easy processability, lightweight and low cost so that it is able to meet demands for miniaturization of microelectronics that has become a mainstream trend in high-dielectric composite materials in recent years. Polymer composites loaded with conductive fillers can achieve very high dielectric constant, but the materials are inevitably accompanied by a huge increase of tan d, which is a fatal disadvantage for its practical use [6e14]. Combining the superior characteristics of dielectric ceramic and polymers is expected to
* Corresponding author. ** Corresponding author. E-mail addresses: [email protected] (F. Chen), [email protected] (Q. Fu). http://dx.doi.org/10.1016/j.compscitech.2015.09.004 0266-3538/© 2015 Elsevier Ltd. All rights reserved.
prepare ideal dielectric composites that possess relatively high dielectric constant and ultralow tan d. On purpose of putting dielectric composites into practical use, researchers should put more attention on inhibiting tan d but not limiting to increase the dielectric constant. Dielectric ceramics have high dielectric constant and low tan d, but its compatibility with the polymer matrix is poor, so the composites often fail to achieve perfect combination of the two excellent properties. In recent years, researchers have opened up an idea of designing ceramic-polymer coreeshell structure, which is committed to improving compatibility between ceramics and polymer matrix in order to achieve an effective combination of the two excellent dielectric properties and mechanical properties. Ceramic particles can be coated by polymer layers through chemical grafting polymerization on surface. Xie et al. [15] grafted poly (methyl methacrylate) (PMMA) on BT through in situ atom transfer radical polymerization (ATRP) method, and obtained ceramicpolymer nanocomposites which have higher dielectric constant and lower tan d than pure PMMA. Composite with 76.88 wt% BT has a dielectric constant of 14.6 and tan d of 3.72 102. Yang et al. [16] coated polystyrene (PS) layer on BT surface via in situ reversible
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addition chain transfer polymerization (RAFT). The results showed that the dielectric constant of nanocomposites significantly increased and tan d was very low. The dielectric constant and tan d of nanocomposite with 47.69 vol% of BT were 24.51 and 0.013, respectively. Based on a similar idea, some researchers adopted RAFT polymerization in conjunction with thiol-ene click chemistry to graft PS or PMMA coating layer on BT. The obtained nanocomposites possessed high dielectric constant and low tan d. For instance, BT nanocomposites grafted with PMMA (42.9 vol%) have a dielectric constant and tan d value of 34.01 and 0.0316, respectively [17]. Also, Researchers have prepared BT nanocomposites with double-shell structure. Firstly, hyperbranched polyamide inner layer was grafted on BT and then PMMA outer layer was formed by ATRP polymerization. The results showed that the obtained nanocomposites have improves interfacial interaction, weak dependence of dielectric constant on frequency and relatively low tan d [18]. Judged from papers reported above, one can know that chemically grafting polymer coating layer on BT contributes to combine the excellent dielectric properties of ceramic particles and the advantages of polymer. Although the chemical grafting method can keep dielectric properties under control effectively, its process is somewhat tedious. Hence, some scientists prepared BT hybrid particles with coreeshell structure through self-polymerization of dopamine through a simple step. Lin et al. [19] reported PDAmodified BT hybrid fillers, which has effectively improved interfacial interaction, and the polymer composite exhibit more excellent dielectric properties compared to composite with unmodified fillers. Unfortunately, the authors did not study and analyze the influence of PDA on dielectric properties. Luo et al. [20] coated PDA with a thickness of ~20 nm on doped BT. The compatibility between fillers and PVDF matrix was obviously improved, and increase of dielectric constant was in accordance with RothereLichtenecker theoretical model. Although the dielectric constant and tan d have low frequency dependence in the range of 103 Hze105 Hz, these parameters display an attenuating trend as the frequency increases when the frequency is lower than 103 Hz. The result showed improved dielectric properties, but the impact of PDA on tan d is not clear. As can be seen from the above analysis, although PDA can significantly improve compatibility between ceramic fillers and polymer matrix, systematical study in the mechanism of its effect on dielectric properties of composites is needed. Herein, we systematically study the influence of PDA on dielectric properties of PVDF composites filled with BT particles coated with different thickness of PDA layers. Introduction of PDA coating layer effectively improves the compatibility between PVDF matrix and BT particles. The results of dielectric properties indicate that PDA could effectively inhibit interface polarization in frequency less than 1 kHz and suppress the tan d to an ultralow level. It is found that thicker coating layer of PDA is unfavorable to improve dielectric constant, so it is necessary to control coating thickness of PDA in order to obtain composites with excellent dielectric properties. 2. Experimental section 2.1. Materials BT powders (diameter ~1 mm, 99.9%, density 6.02 g cm3), Tris buffer (pH ¼ 8.5) and dopamine hydrochloride (DA) were purchased from Chengdu Best Reagent Co., Ltd. Ethanol was purchased from Chengdu Changzheng Chemical Reagent Company. Ammonia (25%) were purchased from Chengdu Jinshan Chemical Reagent Co., Ltd. N, N-dimethylformamide (DMF) was purchased from Tianjin Chemical Reagent Company. PVDF powders (Solef 6020, density 1.78 g cm3, melt mass-flow rate (MFR) < 2.0 g/10 min tested by ASTM D1238) were purchased from Shanghai Alliedneon Co., Ltd., China.
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2.2. Preparation of PDA coated BT particles Modification of BT was divided into three identical groups. In 400 mL of deionized water 6 g BT powder were added, and then 0.1 g, 0.25 g and 1 g of DA were added, respectively. The mixtures were stirred for 10 min by a disperser (IKA, T25 digital ULTRATURRAX®) with speed of 12,000 rpm. 100 mL of Tris buffer (50 mM, pH ¼ 8.5) was added, then the concentrations of DA were tuned to be 0.2 mg/mL, 0.5 mg/mL and 1 mg/mL, respectively. After magnetical stirring at room temperature for 24 h, the resulting mixtures were centrifuged and washed with deionized water for five times to remove free PDA and other impurities. The obtained samples were then air-dried in oven and named as [email protected], [email protected] and BT@PDA2, respectively. All of fillers were grinded by an agate mortar for 30 min to become a fine powder before use. 2.3. Preparation of PVDF composites The content of fillers in composites is expressed with volume fraction, hence, the required mass fraction needs to be calculated in the following equation:
u ¼ nrf
. rp þ n rf rp
where u, n, rp and rf is mass fraction, volume fraction, density of BT and density of PVDF, respectively. The volume fractions of BT and BT@PDA were set to 0 vol%, 10 vol%, 20 vol%, 30 vol%, 40 vol% and 50 vol%. The mass fraction of fillers can be figured out according to above equation. A desirable amount of PVDF and filler powders for each sample was carefully weighed by using scale; PVDF powders were dissolved in 60 mL of DMF in 80 C water bath for 30 min, then the preliminarily weighed filler powders were added and followed by vigorous stirring with disperser for 10 min (12,000 rpm); at the moment stirring halted the mixture was decanted into 400 mL of ethanol for co-precipitation; the flocculates were centrifuged and air-dried in oven to remove residual solvent; samples with a thickness of 0.2 mm for dielectric tests were prepared through hot compression at 210 C with 10 min plasticizing and 10 MPa of holding pressure. 2.4. Characterization Cyro-fractured morphology of composites and morphology of fillers were investigated on an inspect scanning electron microscope (SEM) instrument (FEI) with an acceleration voltage of 20 kV. Transmission electron microscope (TEM, FEI-Tecnai G2 F20 S-TWIN type) was utilized to obtain detailed morphology of BT-GO. Infrared spectroscopy (FTIR) using the Nicolet 6700 was carried out in transmission mode. The weight percentage of PDA coating layers on BT particles was studied by Thermal Gravimetric Analysis (TGA, TA instrument, Q500, USA) in air with a ramp rate of 10 C/min from room temperature to 700 C. Dielectric properties including dielectric constant and tan d were measured on Agilent HP4294A in a frequency range of 40 Hze10 MHz. 3. Results and discussion 3.1. Microstructure and morphology of BT@PDA Dopamine is prone to self-polymerization under alkaline condition. The product PDA has unique physical properties, especially its strong adhesion to surface of almost any types of materials [21]. With regard to this feature, BT particles were put into dopamine
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aqueous solution, and the pH was adjusted to ~8.5 with Tris buffer, then polymerization of dopamine facilitated the formation of PDA coating layers on the surface of BT particles (Fig. 1). The whole process was carried out in a simple way with no special hydrophilic treatment of BT particles and the reaction can be readily performed at ambient atmosphere and room temperature. By contrast, the process of using surface graft polymerization ATRP and RAFT to prepare BT hybrid fillers is much more complicated with harsh conditions [15,16,22e24]. Consequently, this method using selfpolymeric of dopamine to coat and modify BT has an unparalleled advantage. TGA curves can be used to quantitatively analyze the coating amount of PDA on BT surface (Fig. 2a). The as-obtained BT particles were not involved in hydrophilic treatment, and its weight loss curve is almost a horizontal straight line. The three kinds of BT particles coated with PDA start to occur weight loss near 200 C, the process of weight loss proceeds in a slow pace and there is no significant sudden acceleration, which is consistent with the weight loss feature of PDA as reported in our recent work [25]. This phenomenon indicates that the as-prepared BT hybrid particles indeed contain PDA. Samples of [email protected], [email protected] and BT@PDA2 have different residual content and temperature at weight loss equilibrium. The coating amount of PDA in the hybrid particles can be estimated by residual content. As shown in Fig. 2a, [email protected], [email protected] and BT@PDA2 which were obtained from coating reaction with 0.2, 0.5 and 2 mg/mL of DA have 0.7, 2 and 6 wt% of PDA, respectively. This result indicates that the amount and thickness of coating PDA can be easily controlled by simply adjusting the concentration of DA. Different thickness of PDA coating layers would result in different temperature at weight loss equilibrium. PDA coating layers perform carbonization through loss of groups during at elevated temperature, thicker PDA coating layer correspondingly leads to thicker carbonized layer, and thus it requires higher temperature to implement thorough pyrolysis of carbonized layer. FTIR spectra can be used to qualitatively analyze chemical composition of BT hybrid filler. As shown in Fig. 2b, the peak near 1450 cm1 for BT is assigned to stretching vibration of eCO2 3 from its residual component of BaCO3 [24]. Such peak for BT particles coated with PDA is significantly weakened. The possible reason is that BaCO3 carried out hydrolysis during preparation procedure. Peaks at 1510 cm1 and 1288 cm1 correspond to shear vibrational peak of NeH and stretching vibration of phenolic hydroxyl of PDA
[26]. More amount of coating PDA results in higher strength of the two peaks. Moreover, there emerges a peak in the vicinity of 1731 cm1 for samples of BT particles coated with PDA, which is stemmed from carbonyl peak (CaO) of quinone groups [27,28]. The FTIR results confirm the presence of PDA in modified BT particles and it can also deduced that the concentration of DA is able to regulate the amount of PDA coating layer. Fig. 3 shows the morphology of BT, [email protected], [email protected] and BT@PDA2. One can see that the diameter of BT particles have an average diameter of ~1 mm, the particle surface is smooth, and the shape is not perfectly spherical but with obvious angular structure. Several individual BT pellets are prone to closely clustering together in a face-on-face manner. Compared with the morphology of BT particles, particles of [email protected], [email protected] and BT@PDA2 have rounded surfaces, and the angular structure becomes obscure as the coating amount of PDA increases. This reflects changes in the thickness of PDA coating layer (Fig. 3bed). TEM images provide direct evidence that reflects the morphology of PDA coating layer on the surface of BT particles. Fig. 3eeg clearly display different thickness of PDA layers. The black opaque part is BT particle whereas the transparent substance on edge is the PDA cladding layer, which illustrates that BT hybrid particles with a coreeshell structure can be easily obtained from the self-polymerization of dopamine in alkaline condition. The spherical particles with diameter of less than 1 mm are found to be embedded in the PDA coating layer. The coating layer thickness on the surface of [email protected], [email protected] and BT@PDA2 are measured to be ~5.5 nm, ~8 nm and ~30 nm, respectively. It validates the results of TGA that coating amount of PDA increases as the initial concentration of DA increases. Fig. 4 shows morphology of fillers distributed in PVDF matrix. It can be judged from Fig. 4aed that the fillers are uniformly dispersed in composites even though the volume fraction of fillers is up to 40%. The reason for such good dispersion can be explained by the procedure of preparation. Clusters of fillers were firstly sonicated and then crushed into individual particles homogeneously dispersing in PVDF solution under the shearing force of high-speed stirring. In spite that BT or modified BT particles have the tendency of precipitation, immediate decantation of the mixture into ethanol enables BT or modified BT particles coprecipitated with PVDF instead of agglomeration. SEM images with high magnifications serve to reveal the interaction between fillers and matrix, it can be clearly observed that the interaction
Fig. 1. Schematic illustration of coating BT particles with PDA.
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Fig. 2. (a) TGA curves of BT, [email protected], [email protected] and BT@PDA2; (b) FTIR spectra of BT, [email protected], [email protected] and BT@PDA2.
Fig. 3. SEM morphology images of (a) BT, (b) [email protected], (c) [email protected] and (d) BT@PDA2 particles; TEM morphology images of (e) [email protected], (f) [email protected] and (g) BT@PDA2 particles.
Fig. 4. SEM morphology images of PVDF composites with 40 vol% of (a) and (e) BT, (b) and (f) [email protected], (c) and (g) [email protected], (d) and (h) BT@PDA2 with different magnifications.
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between BT particles without PDA coating layer and PVDF is poor (Fig. 4e). Smooth surface and angular structure of BT particles are clearly visible, and clear-cut gap between BT particles and PVDF matrix is observed. It can be concluded by careful comparison that BT particles coated with PDA have rounded rough surfaces and the boundary of PVDF matrix and filler is fuzzy (Fig. 4feh). Furthermore, thicker PDA coating layers on surface of BT particles, seemingly lead to more prominent adhesions between PVDF matrix and fillers. This indicates PDA coating layers have the ability to improve the interaction between the BT particles and PVDF matrix. The reason for this is attributed to the excellent adhesive property of PDA. It is reported that catechol groups of PDA is able to perform coordination bonds with the tetravalent Ti ions of BT which accounts for close adjunction [29]. The polar groups CeF of PVDF are prone to forming hydrogen bonding with PDA [25], so the PDA acts as an interface modifier between the PVDF matrix and BT particles. Good dispersion of modified BT particles in PVDF matrix and great interfacial interactions would play a positive role in dielectric properties. The influence of PDA coating layers on dielectric
properties will be discussed below. 3.2. Dielectric properties The variation of dielectric constant and tan d of four composites with frequency is shown in Fig. 5. For dielectric constant, PVDF composites filled with unmodified BT and PDA coated BT particles have similar trend. The dielectric constant of composites improves as the volume fraction of fillers increases. The dielectric constant exhibits weak dependence on frequency in the range of frequency 103 Hze105 Hz while it rapidly declines as the frequency excesses 105 Hz. The change of tan d can be divided into three regions: tan d decreases with the frequency increasing when the frequency is less than 103 Hz; when the frequency ranges from 103 Hz to 105 Hz, tan d keeps in a low level roughly less than 0.1 for four composites and it shows weak dependence on the frequency; when the frequency is above 105 Hz, tan d increases rapidly with frequency increasing. Such dielectric behaviors can be ascribed to distinguished dominant factors that influence the dielectric properties of
Fig. 5. Frequency dependence on dielectric constant of PVDF composites loaded with (a) BT, (c) [email protected], (e) [email protected] and (g) BT@PDA2; tan d of PVDF composites loaded with (b) BT, (d) [email protected], (f) [email protected] and (h) BT@PDA2.
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Fig. 6. Variation of dielectric constant of BT@PDA/PVDF composites at (a) 100 Hz, (b) 1 kHz, (c) 10 kHz and (d) 100 kHz.
composites in different frequency ranges. For frequency less than 103 Hz, the influence on dielectric properties is the combination of intrinsic dielectric properties of BT particles and the interfacial polarization between fillers and PVDF [30]. The establishment of interfacial polarization is able to keep up with the change of electric field applied in this frequency range, so the introduction of BT particles would result in the increase of dielectric constant and tan d. The time for establishing interfacial polarization would fall behind the change of the external electric field as the frequency increases, which results in decreasing dielectric constant and tan d. It is noteworthy that the interfacial polarization of composites filled with modified BT particles is suppressed because the frequency dependency of dielectric constant and tan d is weakened. Comparatively, dielectric constant and tan d for composites filled with unmodified BT particles (Fig. 5a and b) caused by interfacial polarization are very significant. What is more important is that higher coating amount of PDA contributes to higher suppression degree of interface polarization. Typically, as shown in Fig. 5g, the dielectric constant of BT@PDA2/PVDF composites has almost little dependence on frequency in the range of 103 Hze105 Hz. And the tan d is reduced to below 0.075 with frequency less than 103 Hz. Dielectric loss generally originates from relaxation and conductance [31,32], in current situation the influence of conductance is neglectable since both BT particle and PDA coating layer is of high resistance. Accordingly, it is reasonable to assert that the PDA coating layers play a pivotal role in suppressing tan d of composites by affecting relaxation. Suppression of tan d derived from relaxation of interfacial polarization lies in the fact that the ionizable hydroxyls BT particles and ions that contribute to free carriers can be effectively captured by catechol and anchored by coordinating reaction so that the concentration is substantially reduced and the
migration of free chargers is significantly restricted. Although the dielectric constant of composites filled with modified BT particles is slightly lower than composites filled with unmodified BT particles, the tan d in the range of frequency lower than 103 Hz has been significantly suppressed to a very level. From the perspective of practical use, slightly sacrificing the dielectric constant for acquiring ultralow tan d is acceptable. Frequency ranges from 103 Hz to 105 Hz, the intrinsic dielectric properties of BT particles overwhelmingly contribute to the dielectric properties of composites, so the frequency dependence of the dielectric constant and tan d is very weak. As the frequency surpasses 105 Hz, the dielectric constant rapidly diminishes and tan d drastically increases with frequency increasing [33]. At this point the dominant factor that affects dielectric properties of composites is dipolar relaxation polarization of PVDF matrix. Neat PVDF mainly contains non-polar a form of crystalline, but polar forms of crystalline like b and g can be deduced by ionedipole interaction [34,35]. The coating layer of PDA blocks ionedipole interaction between BT particles and PVDF matrix, leading to low amount of b and g crystalline deduced. This explains why BT/PVDF composites are tested to be higher than that of composites filled with PDA coated BT particles shown in Fig. 5. If the relaxation polarization time of PVDF dipoles mismatch with the change of the applied electric field, the dielectric constant would decrease as is shown in Fig. 5 roughly in the range of 105 Hze107 Hz. At the meantime, molecular friction caused by relaxation polarization of dipoles would give rise to increased tan d. However, samples filled with PDA coated BT particles tend to show lower tan d in the range of 105e107 Hz thanks to the blockage of ionedipole interaction. It can be concluded that one merit of our work is that the tan d could be tuned to be in an ultralow level in the range of 102 Hze105 Hz due
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Fig. 7. Variation of tan d of BT@PDA/PVDF composites at (a) 100 Hz, (b) 1 kHz, (c) 10 kHz and (d) 100 kHz.
to the duplex suppression effect of PDA coating layers on tan d. Fig. 6 shows the variation of dielectric constant of the four composites with different volume fractions at 100 Hz, 1 kHz, 10 kHz and 100 kHz, respectively. The black curve represents the BT/PVDF composite; its dielectric constant is superior to the other three composites with the modified BT particles at four different frequencies. Especially, regardless of changes of the volume fraction, the dielectric constant of the BT/PVDF composite was higher than the other three composites at 100 Hz (Fig. 6a). For composites filled with 10 and 20 vol% of fillers, the dielectric constant of four composites is basically similar at 1 kHz, 10 kHz and 100 kHz. For instance, the dielectric constant of the four composites filled with 20 vol% of BT, [email protected], [email protected] and BT@PDA2 (1 kHz) is measured to be 23.5, 22.0, 25.6 and 23.4, respectively. This is because the interfacial polarization effect of BT/PVDF composites is obvious at 100 Hz, which has a great contribution to dielectric constant. Meanwhile, the interface polarization of modified BT particles has been constrained by PDA coating layers. Under condition of frequency at 1 kHz, 10 kHz and 100 kHz (Fig. 6bed), the dielectric constant of the composites filled with 30, 40 and 50 vol% of [email protected], [email protected] and BT@PDA2 is lower than that of unmodified BT particles because of the introduction of PDA coating layers as discussed above. Higher coating amount of PDA layers gives rise to bigger difference of dielectric constant. For example, for composites filled with 50 vol% of BT, [email protected], [email protected] and BT@PDA2, the dielectric constant at 1 kHz is measured to be 72.0, 61.8, 62.0 and 55.6. As discussed above, the contribution of the intrinsic dielectric properties of BT particles is overwhelming in the range of 103 Hze105 Hz for the overall dielectric constant of the composites. But the PDA coating layers seem to limit the function of
BT particles, resulting in a slight decrease of dielectric constant. From the comparative analysis of the above data, it can be seen the coating amount of PDA in the BT@PDA2 is so high that leads to excessive negative impact on the dielectric constant. The coating amount of PDA in [email protected] and [email protected] is suitable to obtain relatively high dielectric constant. Fig. 7 shows the variation of tan d of the four composites with different volume fractions at 100 Hz, 1 kHz, 10 kHz and 100 kHz, respectively. As can be observed at the four frequencies, tan d of composites filled [email protected], [email protected] and BT@PDA2 is generally lower than composites filled with unmodified BT particles. This reflects that PDA coating layers have the capacity of suppressing tan d. In the range of 40 Hze105 Hz, the dominant factor affecting tan d is interfacial polarization and ionic relaxation polarization. When the frequency is less than 103 Hz, the interfacial polarization imposes prevailing impact on tan d. Because the nomadic charge carriers generated from ionizable hydroxyl groups accumulate at the interface, and then the mobility of charge carriers would cause leakage current which has a great contribution to the increase of tan d. Hence, at 100 Hz and 1 kHz (Fig. 7a and b), tan d of composites filled with unmodified BT particles is much higher than that of composites filled with [email protected], [email protected] and BT@PDA2. Moreover, with the increase of volume fraction of fillers, tan d of BT/ PVDF composites inclines to increase while the tan d of [email protected], [email protected] and BT@PDA2 composites fluctuates in a range of low level. The time for establishing the interfacial polarization is relatively long, so the tan d at lower frequency (100 Hz) is higher than values at 1 kHz and 10 kHz. Specifically, tan d of BT/PVDF composites at 100 Hz is higher than 0.100, and it increases to about 0.166 when the content BT particles increases to 50 vol%. By
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contrast, tan d of [email protected]/PVDF, [email protected]/PVDF and BT@PDA2/PVDF composites is constantly lower than ~0.080 at 100 Hz (Fig. 7a). Although the interface polarization is weakened at 1 kHz, the tan d of BT/PVDF composites is higher than 0.036, and it increases to about 0.055 when the content BT particles increases to 50 vol%. By contrast, tan d of [email protected]/PVDF, [email protected]/PVDF and BT@PDA2/PVDF composites is measured to be constantly lower than 0.033 (Fig. 7b). Under the condition of frequency at 10 kHz and 100 kHz, the ionic relaxation polarization of BT particles is the overwhelming contribution to tan d of composites. As shown in Fig. 2b, FTIR results demonstrate that BT particles contain hetero-ions which cause lattice defects, inducing certain ions to migrate away from its equilibrium position in external electric field. When removing the external electric field, these ions cannot instantly return to the original equilibrium position. Therefore, the establishment of ionic relaxation polarization actually involves a process of thermal relaxation which contributes to dielectric loss. The tan d of [email protected]/PVDF, [email protected]/PVDF and BT@PDA2/PVDF composites exhibits lower values than BT/PVDF composites at 10 kHz and 100 kHz, indicating that the ionic relaxation polarization has been also successfully suppressed by PDA coating layers. It is reported that the catechol groups in PDA have the ability to chelate with metal cations [29,36], and those ions migrating to the surface could be anchored through coordination with catechol groups of PDA so that dielectric loss originated from ionic relaxation polarization is suppressed. Specifically, the tan d of BT/PVDF composites excesses 0.031 (10 kHz) and 0.077 (100 kHz) while tan d of [email protected]/PVDF, [email protected]/PVDF and BT@PDA2/PVDF composites is generally lower than 0.032 and 0.072 at 10 kHz and 100 kHz, respectively.
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In summary, the dielectric properties of PVDF composites filled with PDA coated BT particles are superior to composites filled with unmodified BT particles. Dielectric loss induced by interfacial polarization and ionic relaxation polarization could be effectively constrained by PDA coating layers. It is revealed that thickness of PDA coating layers should be rationally tuned in order to exert optimal influence on dielectric constant and tan d. Effect of PDA coating layers on dielectric properties lies in two aspects: one one hand, the catechol groups of PDA are able to capture and restrict the mobility of the ionizable hydroxyl groups and the nomadic charge carriers on the surface of BT particles, which serves to suppress interfacial polarization in the frequency range of 40 Hze1 kHz; on the other hand, ionic relaxation polarization in the frequency range of 10 kHze100 kHz could also be substantially restricted due to the chelation of catechol groups with migrated cations. Composites prepared in this system have a high dielectric constant and ultralow tan d in the range of 102 Hze105 Hz. The composites are promising in the applications of energy storage device, particularly the embedded capacitors. Acknowledgment This work was supported by the National Natural Science Foundation of China (grant no.51173112 and grant no. 21274095) and State Grid Corporation of China. References [1] Q.M. Zhang, H. Li, M. Poh, F. Xia, Z.Y. Cheng, H. Xu, et al., An all-organic composite actuator material with a high dielectric constant, Nature 419 (6904) (2002) 284e287. [2] Y. Li, X. Huang, Z. Hu, P. Jiang, S. Li, T. Tanaka, Large dielectric constant and high thermal conductivity in poly(vinylidene fluoride)/barium titanate/silicon
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Contents lists available at ScienceDirect
Composites Science and Technology journal homepage: http://www.elsevier.com/locate/compscitech
Towards suppressing loss tangent: Effect of polydopamine coating layers on dielectric properties of coreeshell barium titanate filled polyvinylidene fluoride composites Yuhan Li a, Jiajia Yuan a, Jian Xue b, Fanyi Cai b, Feng Chen a, **, Qiang Fu a, * a b
College of Polymer Science and Engineering, State Key Laboratory of Polymer Materials Engineering, Sichuan University, Chengdu 610065, China Nari Group Corporation State Grid Electric Power Research Institute, Nanjing 211000, China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 1 July 2015 Received in revised form 30 August 2015 Accepted 4 September 2015 Available online 8 September 2015
Barium titanate (BT) particles coated with different thickness of polydopamine (PDA) layers were incorporated into polyvinylidene fluoride (PVDF) to investigate influence of PDA coating layers on dielectric properties of composites. It is found that PDA coating layers not only effectively improved the interfacial interaction between BT particles and matrix but also significantly affected dielectric properties. PVDF composites loaded with modified BT exhibits superior dielectric properties in comparison with unmodified BT. Interfacial polarization is substantially suppressed because the catechol groups of PDA are able to constrain the mobility of nomadic charge carriers and ionizable hydroxyl groups on surface of BT particles. As a result, dependency of dielectric constant on frequency attenuates and tan d is lowered to below 0.050 (1 kHz). The effect of suppression on tan d tends to be more prominent as the thickness of PDA coating layer increases. The tan d derived from ionic relaxation polarization is also constrained due to the chelation of PDA catechol groups with migrated cations. The as-prepared composites possess high dielectric constant and ultralow tan d, making them promising for the industrial application as embedded capacitors. © 2015 Elsevier Ltd. All rights reserved.
Keywords: Polymer-matrix composites (PMCs) Functional composites Electrical properties Scanning electron microscopy (SEM)
1. Introduction Composites with high dielectric constant and low tan d have important application of the microelectronics industry, especially in the field of embedded capacitor [1e5]. Its appealing industrial prospects attract researchers to devote their efforts to developing dielectric composites which can meet requirements. Polymerbased composites have many advantages, such as good flexibility, easy processability, lightweight and low cost so that it is able to meet demands for miniaturization of microelectronics that has become a mainstream trend in high-dielectric composite materials in recent years. Polymer composites loaded with conductive fillers can achieve very high dielectric constant, but the materials are inevitably accompanied by a huge increase of tan d, which is a fatal disadvantage for its practical use [6e14]. Combining the superior characteristics of dielectric ceramic and polymers is expected to
* Corresponding author. ** Corresponding author. E-mail addresses: [email protected] (F. Chen), [email protected] (Q. Fu). http://dx.doi.org/10.1016/j.compscitech.2015.09.004 0266-3538/© 2015 Elsevier Ltd. All rights reserved.
prepare ideal dielectric composites that possess relatively high dielectric constant and ultralow tan d. On purpose of putting dielectric composites into practical use, researchers should put more attention on inhibiting tan d but not limiting to increase the dielectric constant. Dielectric ceramics have high dielectric constant and low tan d, but its compatibility with the polymer matrix is poor, so the composites often fail to achieve perfect combination of the two excellent properties. In recent years, researchers have opened up an idea of designing ceramic-polymer coreeshell structure, which is committed to improving compatibility between ceramics and polymer matrix in order to achieve an effective combination of the two excellent dielectric properties and mechanical properties. Ceramic particles can be coated by polymer layers through chemical grafting polymerization on surface. Xie et al. [15] grafted poly (methyl methacrylate) (PMMA) on BT through in situ atom transfer radical polymerization (ATRP) method, and obtained ceramicpolymer nanocomposites which have higher dielectric constant and lower tan d than pure PMMA. Composite with 76.88 wt% BT has a dielectric constant of 14.6 and tan d of 3.72 102. Yang et al. [16] coated polystyrene (PS) layer on BT surface via in situ reversible
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addition chain transfer polymerization (RAFT). The results showed that the dielectric constant of nanocomposites significantly increased and tan d was very low. The dielectric constant and tan d of nanocomposite with 47.69 vol% of BT were 24.51 and 0.013, respectively. Based on a similar idea, some researchers adopted RAFT polymerization in conjunction with thiol-ene click chemistry to graft PS or PMMA coating layer on BT. The obtained nanocomposites possessed high dielectric constant and low tan d. For instance, BT nanocomposites grafted with PMMA (42.9 vol%) have a dielectric constant and tan d value of 34.01 and 0.0316, respectively [17]. Also, Researchers have prepared BT nanocomposites with double-shell structure. Firstly, hyperbranched polyamide inner layer was grafted on BT and then PMMA outer layer was formed by ATRP polymerization. The results showed that the obtained nanocomposites have improves interfacial interaction, weak dependence of dielectric constant on frequency and relatively low tan d [18]. Judged from papers reported above, one can know that chemically grafting polymer coating layer on BT contributes to combine the excellent dielectric properties of ceramic particles and the advantages of polymer. Although the chemical grafting method can keep dielectric properties under control effectively, its process is somewhat tedious. Hence, some scientists prepared BT hybrid particles with coreeshell structure through self-polymerization of dopamine through a simple step. Lin et al. [19] reported PDAmodified BT hybrid fillers, which has effectively improved interfacial interaction, and the polymer composite exhibit more excellent dielectric properties compared to composite with unmodified fillers. Unfortunately, the authors did not study and analyze the influence of PDA on dielectric properties. Luo et al. [20] coated PDA with a thickness of ~20 nm on doped BT. The compatibility between fillers and PVDF matrix was obviously improved, and increase of dielectric constant was in accordance with RothereLichtenecker theoretical model. Although the dielectric constant and tan d have low frequency dependence in the range of 103 Hze105 Hz, these parameters display an attenuating trend as the frequency increases when the frequency is lower than 103 Hz. The result showed improved dielectric properties, but the impact of PDA on tan d is not clear. As can be seen from the above analysis, although PDA can significantly improve compatibility between ceramic fillers and polymer matrix, systematical study in the mechanism of its effect on dielectric properties of composites is needed. Herein, we systematically study the influence of PDA on dielectric properties of PVDF composites filled with BT particles coated with different thickness of PDA layers. Introduction of PDA coating layer effectively improves the compatibility between PVDF matrix and BT particles. The results of dielectric properties indicate that PDA could effectively inhibit interface polarization in frequency less than 1 kHz and suppress the tan d to an ultralow level. It is found that thicker coating layer of PDA is unfavorable to improve dielectric constant, so it is necessary to control coating thickness of PDA in order to obtain composites with excellent dielectric properties. 2. Experimental section 2.1. Materials BT powders (diameter ~1 mm, 99.9%, density 6.02 g cm3), Tris buffer (pH ¼ 8.5) and dopamine hydrochloride (DA) were purchased from Chengdu Best Reagent Co., Ltd. Ethanol was purchased from Chengdu Changzheng Chemical Reagent Company. Ammonia (25%) were purchased from Chengdu Jinshan Chemical Reagent Co., Ltd. N, N-dimethylformamide (DMF) was purchased from Tianjin Chemical Reagent Company. PVDF powders (Solef 6020, density 1.78 g cm3, melt mass-flow rate (MFR) < 2.0 g/10 min tested by ASTM D1238) were purchased from Shanghai Alliedneon Co., Ltd., China.
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2.2. Preparation of PDA coated BT particles Modification of BT was divided into three identical groups. In 400 mL of deionized water 6 g BT powder were added, and then 0.1 g, 0.25 g and 1 g of DA were added, respectively. The mixtures were stirred for 10 min by a disperser (IKA, T25 digital ULTRATURRAX®) with speed of 12,000 rpm. 100 mL of Tris buffer (50 mM, pH ¼ 8.5) was added, then the concentrations of DA were tuned to be 0.2 mg/mL, 0.5 mg/mL and 1 mg/mL, respectively. After magnetical stirring at room temperature for 24 h, the resulting mixtures were centrifuged and washed with deionized water for five times to remove free PDA and other impurities. The obtained samples were then air-dried in oven and named as [email protected], [email protected] and BT@PDA2, respectively. All of fillers were grinded by an agate mortar for 30 min to become a fine powder before use. 2.3. Preparation of PVDF composites The content of fillers in composites is expressed with volume fraction, hence, the required mass fraction needs to be calculated in the following equation:
u ¼ nrf
. rp þ n rf rp
where u, n, rp and rf is mass fraction, volume fraction, density of BT and density of PVDF, respectively. The volume fractions of BT and BT@PDA were set to 0 vol%, 10 vol%, 20 vol%, 30 vol%, 40 vol% and 50 vol%. The mass fraction of fillers can be figured out according to above equation. A desirable amount of PVDF and filler powders for each sample was carefully weighed by using scale; PVDF powders were dissolved in 60 mL of DMF in 80 C water bath for 30 min, then the preliminarily weighed filler powders were added and followed by vigorous stirring with disperser for 10 min (12,000 rpm); at the moment stirring halted the mixture was decanted into 400 mL of ethanol for co-precipitation; the flocculates were centrifuged and air-dried in oven to remove residual solvent; samples with a thickness of 0.2 mm for dielectric tests were prepared through hot compression at 210 C with 10 min plasticizing and 10 MPa of holding pressure. 2.4. Characterization Cyro-fractured morphology of composites and morphology of fillers were investigated on an inspect scanning electron microscope (SEM) instrument (FEI) with an acceleration voltage of 20 kV. Transmission electron microscope (TEM, FEI-Tecnai G2 F20 S-TWIN type) was utilized to obtain detailed morphology of BT-GO. Infrared spectroscopy (FTIR) using the Nicolet 6700 was carried out in transmission mode. The weight percentage of PDA coating layers on BT particles was studied by Thermal Gravimetric Analysis (TGA, TA instrument, Q500, USA) in air with a ramp rate of 10 C/min from room temperature to 700 C. Dielectric properties including dielectric constant and tan d were measured on Agilent HP4294A in a frequency range of 40 Hze10 MHz. 3. Results and discussion 3.1. Microstructure and morphology of BT@PDA Dopamine is prone to self-polymerization under alkaline condition. The product PDA has unique physical properties, especially its strong adhesion to surface of almost any types of materials [21]. With regard to this feature, BT particles were put into dopamine
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aqueous solution, and the pH was adjusted to ~8.5 with Tris buffer, then polymerization of dopamine facilitated the formation of PDA coating layers on the surface of BT particles (Fig. 1). The whole process was carried out in a simple way with no special hydrophilic treatment of BT particles and the reaction can be readily performed at ambient atmosphere and room temperature. By contrast, the process of using surface graft polymerization ATRP and RAFT to prepare BT hybrid fillers is much more complicated with harsh conditions [15,16,22e24]. Consequently, this method using selfpolymeric of dopamine to coat and modify BT has an unparalleled advantage. TGA curves can be used to quantitatively analyze the coating amount of PDA on BT surface (Fig. 2a). The as-obtained BT particles were not involved in hydrophilic treatment, and its weight loss curve is almost a horizontal straight line. The three kinds of BT particles coated with PDA start to occur weight loss near 200 C, the process of weight loss proceeds in a slow pace and there is no significant sudden acceleration, which is consistent with the weight loss feature of PDA as reported in our recent work [25]. This phenomenon indicates that the as-prepared BT hybrid particles indeed contain PDA. Samples of [email protected], [email protected] and BT@PDA2 have different residual content and temperature at weight loss equilibrium. The coating amount of PDA in the hybrid particles can be estimated by residual content. As shown in Fig. 2a, [email protected], [email protected] and BT@PDA2 which were obtained from coating reaction with 0.2, 0.5 and 2 mg/mL of DA have 0.7, 2 and 6 wt% of PDA, respectively. This result indicates that the amount and thickness of coating PDA can be easily controlled by simply adjusting the concentration of DA. Different thickness of PDA coating layers would result in different temperature at weight loss equilibrium. PDA coating layers perform carbonization through loss of groups during at elevated temperature, thicker PDA coating layer correspondingly leads to thicker carbonized layer, and thus it requires higher temperature to implement thorough pyrolysis of carbonized layer. FTIR spectra can be used to qualitatively analyze chemical composition of BT hybrid filler. As shown in Fig. 2b, the peak near 1450 cm1 for BT is assigned to stretching vibration of eCO2 3 from its residual component of BaCO3 [24]. Such peak for BT particles coated with PDA is significantly weakened. The possible reason is that BaCO3 carried out hydrolysis during preparation procedure. Peaks at 1510 cm1 and 1288 cm1 correspond to shear vibrational peak of NeH and stretching vibration of phenolic hydroxyl of PDA
[26]. More amount of coating PDA results in higher strength of the two peaks. Moreover, there emerges a peak in the vicinity of 1731 cm1 for samples of BT particles coated with PDA, which is stemmed from carbonyl peak (CaO) of quinone groups [27,28]. The FTIR results confirm the presence of PDA in modified BT particles and it can also deduced that the concentration of DA is able to regulate the amount of PDA coating layer. Fig. 3 shows the morphology of BT, [email protected], [email protected] and BT@PDA2. One can see that the diameter of BT particles have an average diameter of ~1 mm, the particle surface is smooth, and the shape is not perfectly spherical but with obvious angular structure. Several individual BT pellets are prone to closely clustering together in a face-on-face manner. Compared with the morphology of BT particles, particles of [email protected], [email protected] and BT@PDA2 have rounded surfaces, and the angular structure becomes obscure as the coating amount of PDA increases. This reflects changes in the thickness of PDA coating layer (Fig. 3bed). TEM images provide direct evidence that reflects the morphology of PDA coating layer on the surface of BT particles. Fig. 3eeg clearly display different thickness of PDA layers. The black opaque part is BT particle whereas the transparent substance on edge is the PDA cladding layer, which illustrates that BT hybrid particles with a coreeshell structure can be easily obtained from the self-polymerization of dopamine in alkaline condition. The spherical particles with diameter of less than 1 mm are found to be embedded in the PDA coating layer. The coating layer thickness on the surface of [email protected], [email protected] and BT@PDA2 are measured to be ~5.5 nm, ~8 nm and ~30 nm, respectively. It validates the results of TGA that coating amount of PDA increases as the initial concentration of DA increases. Fig. 4 shows morphology of fillers distributed in PVDF matrix. It can be judged from Fig. 4aed that the fillers are uniformly dispersed in composites even though the volume fraction of fillers is up to 40%. The reason for such good dispersion can be explained by the procedure of preparation. Clusters of fillers were firstly sonicated and then crushed into individual particles homogeneously dispersing in PVDF solution under the shearing force of high-speed stirring. In spite that BT or modified BT particles have the tendency of precipitation, immediate decantation of the mixture into ethanol enables BT or modified BT particles coprecipitated with PVDF instead of agglomeration. SEM images with high magnifications serve to reveal the interaction between fillers and matrix, it can be clearly observed that the interaction
Fig. 1. Schematic illustration of coating BT particles with PDA.
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Fig. 2. (a) TGA curves of BT, [email protected], [email protected] and BT@PDA2; (b) FTIR spectra of BT, [email protected], [email protected] and BT@PDA2.
Fig. 3. SEM morphology images of (a) BT, (b) [email protected], (c) [email protected] and (d) BT@PDA2 particles; TEM morphology images of (e) [email protected], (f) [email protected] and (g) BT@PDA2 particles.
Fig. 4. SEM morphology images of PVDF composites with 40 vol% of (a) and (e) BT, (b) and (f) [email protected], (c) and (g) [email protected], (d) and (h) BT@PDA2 with different magnifications.
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between BT particles without PDA coating layer and PVDF is poor (Fig. 4e). Smooth surface and angular structure of BT particles are clearly visible, and clear-cut gap between BT particles and PVDF matrix is observed. It can be concluded by careful comparison that BT particles coated with PDA have rounded rough surfaces and the boundary of PVDF matrix and filler is fuzzy (Fig. 4feh). Furthermore, thicker PDA coating layers on surface of BT particles, seemingly lead to more prominent adhesions between PVDF matrix and fillers. This indicates PDA coating layers have the ability to improve the interaction between the BT particles and PVDF matrix. The reason for this is attributed to the excellent adhesive property of PDA. It is reported that catechol groups of PDA is able to perform coordination bonds with the tetravalent Ti ions of BT which accounts for close adjunction [29]. The polar groups CeF of PVDF are prone to forming hydrogen bonding with PDA [25], so the PDA acts as an interface modifier between the PVDF matrix and BT particles. Good dispersion of modified BT particles in PVDF matrix and great interfacial interactions would play a positive role in dielectric properties. The influence of PDA coating layers on dielectric
properties will be discussed below. 3.2. Dielectric properties The variation of dielectric constant and tan d of four composites with frequency is shown in Fig. 5. For dielectric constant, PVDF composites filled with unmodified BT and PDA coated BT particles have similar trend. The dielectric constant of composites improves as the volume fraction of fillers increases. The dielectric constant exhibits weak dependence on frequency in the range of frequency 103 Hze105 Hz while it rapidly declines as the frequency excesses 105 Hz. The change of tan d can be divided into three regions: tan d decreases with the frequency increasing when the frequency is less than 103 Hz; when the frequency ranges from 103 Hz to 105 Hz, tan d keeps in a low level roughly less than 0.1 for four composites and it shows weak dependence on the frequency; when the frequency is above 105 Hz, tan d increases rapidly with frequency increasing. Such dielectric behaviors can be ascribed to distinguished dominant factors that influence the dielectric properties of
Fig. 5. Frequency dependence on dielectric constant of PVDF composites loaded with (a) BT, (c) [email protected], (e) [email protected] and (g) BT@PDA2; tan d of PVDF composites loaded with (b) BT, (d) [email protected], (f) [email protected] and (h) BT@PDA2.
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Fig. 6. Variation of dielectric constant of BT@PDA/PVDF composites at (a) 100 Hz, (b) 1 kHz, (c) 10 kHz and (d) 100 kHz.
composites in different frequency ranges. For frequency less than 103 Hz, the influence on dielectric properties is the combination of intrinsic dielectric properties of BT particles and the interfacial polarization between fillers and PVDF [30]. The establishment of interfacial polarization is able to keep up with the change of electric field applied in this frequency range, so the introduction of BT particles would result in the increase of dielectric constant and tan d. The time for establishing interfacial polarization would fall behind the change of the external electric field as the frequency increases, which results in decreasing dielectric constant and tan d. It is noteworthy that the interfacial polarization of composites filled with modified BT particles is suppressed because the frequency dependency of dielectric constant and tan d is weakened. Comparatively, dielectric constant and tan d for composites filled with unmodified BT particles (Fig. 5a and b) caused by interfacial polarization are very significant. What is more important is that higher coating amount of PDA contributes to higher suppression degree of interface polarization. Typically, as shown in Fig. 5g, the dielectric constant of BT@PDA2/PVDF composites has almost little dependence on frequency in the range of 103 Hze105 Hz. And the tan d is reduced to below 0.075 with frequency less than 103 Hz. Dielectric loss generally originates from relaxation and conductance [31,32], in current situation the influence of conductance is neglectable since both BT particle and PDA coating layer is of high resistance. Accordingly, it is reasonable to assert that the PDA coating layers play a pivotal role in suppressing tan d of composites by affecting relaxation. Suppression of tan d derived from relaxation of interfacial polarization lies in the fact that the ionizable hydroxyls BT particles and ions that contribute to free carriers can be effectively captured by catechol and anchored by coordinating reaction so that the concentration is substantially reduced and the
migration of free chargers is significantly restricted. Although the dielectric constant of composites filled with modified BT particles is slightly lower than composites filled with unmodified BT particles, the tan d in the range of frequency lower than 103 Hz has been significantly suppressed to a very level. From the perspective of practical use, slightly sacrificing the dielectric constant for acquiring ultralow tan d is acceptable. Frequency ranges from 103 Hz to 105 Hz, the intrinsic dielectric properties of BT particles overwhelmingly contribute to the dielectric properties of composites, so the frequency dependence of the dielectric constant and tan d is very weak. As the frequency surpasses 105 Hz, the dielectric constant rapidly diminishes and tan d drastically increases with frequency increasing [33]. At this point the dominant factor that affects dielectric properties of composites is dipolar relaxation polarization of PVDF matrix. Neat PVDF mainly contains non-polar a form of crystalline, but polar forms of crystalline like b and g can be deduced by ionedipole interaction [34,35]. The coating layer of PDA blocks ionedipole interaction between BT particles and PVDF matrix, leading to low amount of b and g crystalline deduced. This explains why BT/PVDF composites are tested to be higher than that of composites filled with PDA coated BT particles shown in Fig. 5. If the relaxation polarization time of PVDF dipoles mismatch with the change of the applied electric field, the dielectric constant would decrease as is shown in Fig. 5 roughly in the range of 105 Hze107 Hz. At the meantime, molecular friction caused by relaxation polarization of dipoles would give rise to increased tan d. However, samples filled with PDA coated BT particles tend to show lower tan d in the range of 105e107 Hz thanks to the blockage of ionedipole interaction. It can be concluded that one merit of our work is that the tan d could be tuned to be in an ultralow level in the range of 102 Hze105 Hz due
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Fig. 7. Variation of tan d of BT@PDA/PVDF composites at (a) 100 Hz, (b) 1 kHz, (c) 10 kHz and (d) 100 kHz.
to the duplex suppression effect of PDA coating layers on tan d. Fig. 6 shows the variation of dielectric constant of the four composites with different volume fractions at 100 Hz, 1 kHz, 10 kHz and 100 kHz, respectively. The black curve represents the BT/PVDF composite; its dielectric constant is superior to the other three composites with the modified BT particles at four different frequencies. Especially, regardless of changes of the volume fraction, the dielectric constant of the BT/PVDF composite was higher than the other three composites at 100 Hz (Fig. 6a). For composites filled with 10 and 20 vol% of fillers, the dielectric constant of four composites is basically similar at 1 kHz, 10 kHz and 100 kHz. For instance, the dielectric constant of the four composites filled with 20 vol% of BT, [email protected], [email protected] and BT@PDA2 (1 kHz) is measured to be 23.5, 22.0, 25.6 and 23.4, respectively. This is because the interfacial polarization effect of BT/PVDF composites is obvious at 100 Hz, which has a great contribution to dielectric constant. Meanwhile, the interface polarization of modified BT particles has been constrained by PDA coating layers. Under condition of frequency at 1 kHz, 10 kHz and 100 kHz (Fig. 6bed), the dielectric constant of the composites filled with 30, 40 and 50 vol% of [email protected], [email protected] and BT@PDA2 is lower than that of unmodified BT particles because of the introduction of PDA coating layers as discussed above. Higher coating amount of PDA layers gives rise to bigger difference of dielectric constant. For example, for composites filled with 50 vol% of BT, [email protected], [email protected] and BT@PDA2, the dielectric constant at 1 kHz is measured to be 72.0, 61.8, 62.0 and 55.6. As discussed above, the contribution of the intrinsic dielectric properties of BT particles is overwhelming in the range of 103 Hze105 Hz for the overall dielectric constant of the composites. But the PDA coating layers seem to limit the function of
BT particles, resulting in a slight decrease of dielectric constant. From the comparative analysis of the above data, it can be seen the coating amount of PDA in the BT@PDA2 is so high that leads to excessive negative impact on the dielectric constant. The coating amount of PDA in [email protected] and [email protected] is suitable to obtain relatively high dielectric constant. Fig. 7 shows the variation of tan d of the four composites with different volume fractions at 100 Hz, 1 kHz, 10 kHz and 100 kHz, respectively. As can be observed at the four frequencies, tan d of composites filled [email protected], [email protected] and BT@PDA2 is generally lower than composites filled with unmodified BT particles. This reflects that PDA coating layers have the capacity of suppressing tan d. In the range of 40 Hze105 Hz, the dominant factor affecting tan d is interfacial polarization and ionic relaxation polarization. When the frequency is less than 103 Hz, the interfacial polarization imposes prevailing impact on tan d. Because the nomadic charge carriers generated from ionizable hydroxyl groups accumulate at the interface, and then the mobility of charge carriers would cause leakage current which has a great contribution to the increase of tan d. Hence, at 100 Hz and 1 kHz (Fig. 7a and b), tan d of composites filled with unmodified BT particles is much higher than that of composites filled with [email protected], [email protected] and BT@PDA2. Moreover, with the increase of volume fraction of fillers, tan d of BT/ PVDF composites inclines to increase while the tan d of [email protected], [email protected] and BT@PDA2 composites fluctuates in a range of low level. The time for establishing the interfacial polarization is relatively long, so the tan d at lower frequency (100 Hz) is higher than values at 1 kHz and 10 kHz. Specifically, tan d of BT/PVDF composites at 100 Hz is higher than 0.100, and it increases to about 0.166 when the content BT particles increases to 50 vol%. By
Y. Li et al. / Composites Science and Technology 118 (2015) 198e206
contrast, tan d of [email protected]/PVDF, [email protected]/PVDF and BT@PDA2/PVDF composites is constantly lower than ~0.080 at 100 Hz (Fig. 7a). Although the interface polarization is weakened at 1 kHz, the tan d of BT/PVDF composites is higher than 0.036, and it increases to about 0.055 when the content BT particles increases to 50 vol%. By contrast, tan d of [email protected]/PVDF, [email protected]/PVDF and BT@PDA2/PVDF composites is measured to be constantly lower than 0.033 (Fig. 7b). Under the condition of frequency at 10 kHz and 100 kHz, the ionic relaxation polarization of BT particles is the overwhelming contribution to tan d of composites. As shown in Fig. 2b, FTIR results demonstrate that BT particles contain hetero-ions which cause lattice defects, inducing certain ions to migrate away from its equilibrium position in external electric field. When removing the external electric field, these ions cannot instantly return to the original equilibrium position. Therefore, the establishment of ionic relaxation polarization actually involves a process of thermal relaxation which contributes to dielectric loss. The tan d of [email protected]/PVDF, [email protected]/PVDF and BT@PDA2/PVDF composites exhibits lower values than BT/PVDF composites at 10 kHz and 100 kHz, indicating that the ionic relaxation polarization has been also successfully suppressed by PDA coating layers. It is reported that the catechol groups in PDA have the ability to chelate with metal cations [29,36], and those ions migrating to the surface could be anchored through coordination with catechol groups of PDA so that dielectric loss originated from ionic relaxation polarization is suppressed. Specifically, the tan d of BT/PVDF composites excesses 0.031 (10 kHz) and 0.077 (100 kHz) while tan d of [email protected]/PVDF, [email protected]/PVDF and BT@PDA2/PVDF composites is generally lower than 0.032 and 0.072 at 10 kHz and 100 kHz, respectively.
[3]
[4]
[5]
[6] [7] [8]
[9] [10]
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4. Conclusions [16]
In summary, the dielectric properties of PVDF composites filled with PDA coated BT particles are superior to composites filled with unmodified BT particles. Dielectric loss induced by interfacial polarization and ionic relaxation polarization could be effectively constrained by PDA coating layers. It is revealed that thickness of PDA coating layers should be rationally tuned in order to exert optimal influence on dielectric constant and tan d. Effect of PDA coating layers on dielectric properties lies in two aspects: one one hand, the catechol groups of PDA are able to capture and restrict the mobility of the ionizable hydroxyl groups and the nomadic charge carriers on the surface of BT particles, which serves to suppress interfacial polarization in the frequency range of 40 Hze1 kHz; on the other hand, ionic relaxation polarization in the frequency range of 10 kHze100 kHz could also be substantially restricted due to the chelation of catechol groups with migrated cations. Composites prepared in this system have a high dielectric constant and ultralow tan d in the range of 102 Hze105 Hz. The composites are promising in the applications of energy storage device, particularly the embedded capacitors. Acknowledgment This work was supported by the National Natural Science Foundation of China (grant no.51173112 and grant no. 21274095) and State Grid Corporation of China. References [1] Q.M. Zhang, H. Li, M. Poh, F. Xia, Z.Y. Cheng, H. Xu, et al., An all-organic composite actuator material with a high dielectric constant, Nature 419 (6904) (2002) 284e287. [2] Y. Li, X. Huang, Z. Hu, P. Jiang, S. Li, T. Tanaka, Large dielectric constant and high thermal conductivity in poly(vinylidene fluoride)/barium titanate/silicon
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