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PVDF composites regulated by coupling agent
Materials Today Communications 21 (2019) 100705
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Dielectrical properties of graphite nanosheets/PVDF composites regulated by coupling agent
T
Xue Lia, Xiaoming Wanga,b, Ling Wenga,b,*, Yang Yub, Xiaorui Zhanga,b,*, Lizhu Liua,b, Cheng Wanga a b
School of Materials Science and Engineering, Harbin University of Science and Technology, Harbin, 150080, PR China Key Laboratory of Engineering Dielectrics and Its Application, Ministry of Education, Harbin University of Science and Technology, Harbin, 150080, PR China
A R T I C LE I N FO
A B S T R A C T
Keywords: Coupling agent PVDF Composites
nanosheets (GNs) with different coupling agents to enhance the dielectric properties of GNs/PVDF composite membranes, and the modified results were characterized by FT-IR. The dielectrical properties and the breakdown field strength of composites film was characterized. The results indicated that the dielectric constant of the composite increases, with the increasing of GNs volume fraction. Moreover, the coupling agent modifies the GNs to confine the carriers of the GNs, which reduces the dielectric loss and increases the breakdown field strength. Since PFOES contains CeF, higher affinity for PVDF leads to higher dielectric constant, and modification of GNs with PFOES and KH570 improves the compatibility of GNs and PVDF, which leads to higher breakdown field strength. When the graphite nanosheets (5 wt%) was modified with coupling agent KH570, the dielectric constant was increased by an order of magnitude to 167 compared with PVDF, the breakdown field strength was also increased to 85.39 kV/mm, and the storage density reached 5.37655. J/cm3; when used PFOES modified graphite nanosheets, the dielectric constant was as high as 295 when the amount was 5 wt%, and the breakdown field strength was increased to 68.64 kV/mm, which is 3.3 times that of pure PVDF. The storage density is 6.13,691 J/cm3. It is worth noting that the use of 3 wt% KH570 modified graphite nanosheets in PVDF has significantly improved the dielectric constant while maintaining dielectric loss comparable to 1 wt% (The dielectric constant reached 80 at 100 Hz.). Different coupling agents have different effects on the properties of composites due to the functionalization of GNs, which provides some ideas for further research on capacitors and other electrons.
1. Introductions Polymer composites with high dielectric properties are appreciated due to their unique combination of mechanical flexibility and tunable dielectric properties [1,2]. The common fillers are mainly high dielectric inorganic ceramic particles and conductive fillers [3,4]. When conductive filler is used to enhance the dielectric film, high dielectric constant can be obtained because the carriers provided by conductive filler can be polarized in an electric field. However, these carriers must be subjected to a confinement treatment, otherwise the dielectric film may leak and the dielectric loss will be very high [5–12]. Metal particles and carbon materials are usually used as conductive fillers [13,14]. Carbon materials represented by graphite nanosheets, carbon nanotubes and graphene are in hot topics [15,16]. The dielectric films prepared by using these materials as fillers have many advantages such as high dielectric constant and good mechanical properties. But
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these materials should be coated or modified. Organic materials such as coupling agents which could coat the carbon material can significantly improve the breakdown field strength of the film, reduce its dielectric loss, and greatly improve its insulation capacity [17–20]. An el al. added graphene oxide (GO) to PVDF, and the dielectric constant of PVDF-based composites increased with increasing GO content [21]. Xu et al used two different coupling agents (KH550/MDI) to functionalize the graphene oxide (GO) and added it to the phenyl silicone rubber. The results showed that GO was well dispersed in phenyl silicone rubber after modification. The authors studied the effects of functional graphene oxide on the dielectric behaviour, thermal transmittance, optical transmittance and mechanical properties of the composites [22]. Liu et al. used different coupling agents (ATPS/GTPS) to modify the graphene core-shell structure, the dielectric constant was significantly enhanced, and the dispersion of the filler in the matrix was improved [23]. Huang et al. used PFOES to modify graphene and added
Corresponding authors at: School of Materials Science and Engineering, Harbin University of Science and Technology, Harbin, 150080, PR China. E-mail addresses: [email protected] (X. Li), [email protected] (L. Weng), [email protected] (X. Zhang).
https://doi.org/10.1016/j.mtcomm.2019.100705 Received 20 August 2019; Received in revised form 13 October 2019; Accepted 14 October 2019 Available online 15 October 2019 2352-4928/ © 2019 Published by Elsevier Ltd.
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3 h. A series of composite films with different mass fraction were obtained.To the preparation of good composite membrane, into the homemade mold, using 30 min under 180 °C hot pressing plate vulcanizing machine, pressure to 10 Mpa. Put in the mold. Keep at constant temperature and constant pressure for 30 min. Close the machine, let cool to room temperature and remove the prepared composite film.
it to PVDF. Compared with unmodified graphene, the addition of PFOES increased the dispersion of graphene in PVDF. Moreover, the dielectric constant, piezoelectric constant, and mechanical properties of PVDF-based composites are improved [24]. Wang et al. used silica to cover the surface of graphene oxide and then modified epoxy resin (EP). As a result, the surface not only improved the flame retardancy and thermal stability of EP, but also made EP have the advantages of high dielectric constant and low dielectric loss [25]. Fang et al. proposed a novel and simple method was to synthesize high dielectric constant NH2 functionalized and carboxyl functionalized graphene oxide (PPDCFGO)/polyimide (PI) composite films. The dielectric constant increases up to 36.9 with an increasing amount of PPD-CFGO, higher than that of the pure PI polymer by a factor of 12.5, while the dielectric loss is only 0.0075 and the breakdown strength still remains at a high level [26]. From other studies, it can be found that the functional group of the silane coupling agent can interact with a specific chemical part in the polymer matrix, can improve the dispersibility of graphene, effectively reduce the agglomeration of graphene [27–34]. Moreover, the coupling agent can also limit the carriers of graphene, thereby increase the dielectric constant of the polymer, reduce the dielectric loss, and increase the breakdown field strength. However, due to the need for functionalize the graphene using a coupling agent, graphene needs to be converted into graphene oxide for reduction. The surface of the reduced graphene oxide contains a large amount of oxidized groups, so that it provides a much smaller carrier than graphene. In order to retain the number of carriers and improve the dispersibility of graphene, the graphite nanosheets was graft modified with coupling agent. Although the surface is graft-modified by coupling agent, the carrier of the graphite nanosheets is sufficiently large due to its multilayer structure. This solves the dispersion problem to increase the breakdown field strength of the composite while having sufficient carriers to improve the dielectric properties of the composite. Based on previous studies, it has been found that different coupling agents have different effects on the properties of the composite. Considering the characteristics of PVDF, a functionalized modifier containing a fluorine functional group can improve the compatibility and miscibility between the modified layer and PVDF, because the fluorine atom group and the corresponding molecular chain contained in the PVDF may be intertwined with other atomic groups [35–40]. So we used PFOES (Heptadecafluorodecyl trimethoxysilane.) with CeF groups and KH570 (γ-methacryloxypropyltrimethoxysilane) with CeF groups to study the effects of different coupling agents on the properties of PVDF matrix composites.
2.3. Performance characterization and testing methods The cross-sectional microstructure of the composite film doped with different coupling agent modified graphite nanosheets was observed and characterized by FEI SIRION 200 scanning electron microscope.The composite film scanning sample was brittle cracked in liquid nitrogen to make a section, cut to a certain size and stuck to an aluminum block with conductive adhesive, after spraying gold, a test sample was made. The dielectric properties of the composite film were measured by BDS 4000 broadband dielectric/impedance spectrometer. The test frequency range is 10−1 Hz to107 Hz and the test temperature is room temperature. Before testing, an Al electrode with a diameter of 20 mm should be plated on both sides of the film sample. The breakdown field strength of the composite film was measured by HT-5/20 breakdown voltage tester and boosted at a rate of 1 kV/s. Each sample was tested at 5 points, and the average of the 5 points was taken as the electric breakdown field strength of the film. 3. Results and discussion 3.1. Structural characterization In order to determine the grafting of GNs, the GNs and the modified GNs were characterized by FT-IR. The infrared spectrum of the graphite nanosheets and the modified graphite nanosheets was shown in Fig. 1. It could be found that in the unmodified GNs, 3430 cm−1 was the stretching vibration peak of −OH and 2925 cm−1 was −CH2- of stretching vibration peak. In the infrared spectrum of the modified GNs the −OH stretching vibration peak at a wavelength of 3430 cm−1 appeared. At 2925 cm-1 and 2850 cm−1, there are overlapping peaks of −CH2- and −CH-. The wavelength of 1380 cm−1 was the stretching vibration peak of CeF peculiar to the PFOES, and 1620 cm−1 was a stretching vibration peak of C]C peculiar to KH570, and 1020 cm−1 and 1125 cm−1 were stretching vibration peaks of Si-O bonds. The characteristic absorption peak (Si-O) of silane coupling agent appears in the modified GNs, and the stretching vibration peak of CeF or C]C unique to the two coupling agents also appeared, which indicated that PFOES or KH570 was grafted successfully onto the GNs. Composite materials with different doping amounts were prepared
2. Experimental 2.1. Preparation of GNs and modified GNs The required quality of graphite was put in the 1000 °C heating with 15 s, vermicular graphite was produced. The vermicular graphite was stirred by ultrasound for 2 h in solution with a volume ratio of 1: 1 of deionized water and ethanol. 0.02 g of coupling agent was added, and removed the overall solution to preheat to 80 °C, stirred for 8 h. The solution was filtered., again in 80 °C oven for 2 days, to the solvent volatilization completely,dry preservation. The coupling agent modified GNs can be obtained. 2.2. preparation of composites A certain amount of graphite powder was weighed and added into a three-mouth bottle containing 50 ml N, N-dimethylformamide (DMF) solution. After 1 h ultrasonic dispersion, 10.25 g PVDF (PVDF) powder was added to the bottle, and the mixture was continuously stirred for 3 h under ultrasonic state. A colloidal solution of the composite is obtained. After the prepared colloidal solution was evacuated to eliminate air bubbles, the film was cast by a casting method and dried at 80 °C for
Fig. 1. FT-IR of modified GNs. 2
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Fig. 4. Dependence of dielectric constant (a) and dielectric loss (b) of GNs/ PVDF on frequency.
doping is sufficient.
4. Electric properties Fig. 2. SEM of PVDF-based composites materials with filler content of 5 wt% (Filler of a is GNs,the scanning multiple of 5000. Filler of b is KH570-GNs, the scanning multiple of 5000. Filler of c is PFOES-GNs,the scanning multiple of 5000.).
4.1. Dielectric properties The relation between the permittivity and frequency of PVDF-based composites doped with unmodified GNs was shown in Fig. 4(a). It can be seen that the permittivity increases with the increasing of GNs. Moreover, the dielectric constant of the composite material does not change much compared with pure PVDF when the doped amount of graphite nanosheets is 1 wt%. When the doped amount of graphite nanosheets is greater than or equal to 3 wt%, the dielectric constant increases by an order of magnitude compared with pure PVDF. When the frequency is 100 Hz, the dielectric constant exceeds 102. However, as shown in Fig. 4(b), the correspond dielectric loss changes with the frequency, and the dielectric loss increases with the GNs incorporation increased. Moreover, the dielectric loss increased greatly when the doped amount of the graphite nanosheets was greater than or equal to 3 wt%, which was corresponding to the change of the dielectric constant of the composite material, indicating that the dielectric constant was positively correlated with the dielectric loss. As the carrier provided by the graphite nanosheets increases with the addition of the graphite nanosheets, the distance between the graphite particles is close enough to form a conductive channel, resulted in the leakage phenomenon that the loss was so large. Fig.5(a) shows that the variation trend of the dielectric constant with the addition amount of the KH570 modified graphite nanosheets is basically consistent with that of the GNs/PVDF composite. Therefore, the functionalization of the KH570 functional group has little effect on the dielectric constant of the composite. However in Fig. 5(b) can be found that the dielectric loss although also increases with the increase of graphite content, but the minimum dielectric loss is around 0.1. It can be seen that the dielectric constant of the composite is as high as 80 or more (100 Hz) when the content of KH570-GNs is 3 wt%, and the dielectric loss of the composite is at least 0.1. This trait of high dielectric constant and low dielectric loss indicates that this ratio of composite materials has certain practical value. We observed the change in dielectric constant of the composite after PFOES modified GNs in Fig. 6(a), which is consistent with Figs. 4(a) and 5(a). As the dielectric constant of GNs increases, the dielectric constant
by mixing GNs and modified GNs into polyvinylidene fluoride (PVDF) with mass ratios of 1 wt%,3 wt%,5 wt% and 7 wt%. Then, scanning electron microscopy (SEM) was used to determine the morphology of the composites, and SEM was observed in the cross-section of the composites. From Fig. 2(a), it can be seen that the dispersion effect of pure GNs in PVDF is general, while the dispersion of GNs in PVDF with different coupling agents has been significantly improved, as shown in Figs. 2(b) and 2(c). Due to the effect of coupling agent, GNs with better dispersion effect are uniformly dispersed in various positions of PVDF, and the phenomenon of leakage caused by mutual superposition between GNS is avoided. It can be seen from the XRD (Fig. 3) diffraction pattern that the 2θ at 18.3° and 19.9° shown α phase of PVDF, and the 20.6° and the 26.6° at diffraction peaks were the β phase and γ phase of PVDF respectively This indicates that the pure phase phase was the α phase, the β phase and the γ phase multiphase coexisted. However we can found that 2θ in these composite film diffraction peak of 18.3° and 19.9° gradually weakened, 20.6° and 26.6° gradually stronger, and the PVDF α phase gradually transformed to the β phase with the addition of GNs and modified GNs. And β-PVDF has good dielectric properties, which is beneficial to obtain high dielectric constant29. Experiments show that the addition of filler has a great influence on the crystallization properties of the matrix, which is one of the main reasons for the formation of β-PVDF and the increase of dielectric constant when the amount of
Fig. 5. Dependence of dielectric constant (a) and dielectric loss (b) of KH570GNs/PVDF on frequency.
Fig. 3. XRD of modified PVDF-based composites. 3
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Fig. 6. Dependence of dielectric constant (a) and dielectric loss (b) of PFOESGNs/PVDF on frequency.
increases significantly when the amount of GNs added was 3 wt%. This is because the dielectric constant of the composites is mainly determined by the dipole polarization of the PVDF matrix when the doping amount of the graphite nanosheets is low, and it is not affected by the frequency in the frequency range tested in the experiment. Therefore, the dielectric constant of composite material does not change much with frequency. As the content of graphite nanosheets filler increased to 3 wt%, near the percolation threshold, the internal electron polarization increased. As for the relationship between the dielectric loss and frequency of composite materials, we can see from the figure that the dielectric loss decreases first with frequency and then increases. At low frequency, dipole polarization varies with electric field without delay and polarization loss. Leakage loss is the main cause of loss, and leakage loss decreases with the increase of frequency. As the frequency continues to rise, the polarization relaxes and generates polarization loss, which increases with the increase of frequency and replaces leakage loss as the leading factor of dielectric loss. Fig. 7(a) and Fig. 7(b) are graphs showing the dielectric constant and dielectric loss as a function of frequency for a PVDF composite having a GNs content of 5 wt%, respectively. It can be seen from Fig. 7(a) that the dielectric constant increases greatly after the addition of GNs to PVDF. This is because the carrier polarization is stronger, the dielectric constant of the PVDF composite film is stronger, and the number of carriers is the doping amount of GNs is proportional. The change in dielectric constant after using the coupling agent is not significant because the compatibility between GNs and PVDF is improved by the coupling agent under the action of electric field. At the same time, the number of carriers provided by GNs is not affected by the functionalization of the coupling agent. We can see in Fig. 7(b) that the dielectric loss increases with the increase of GNs. However, after the addition of the coupling agent, the dielectric loss is reduced. This is because PFOES and KH570 functionalize the GNs, limiting the carriers and avoiding leakage. Therefore, the dielectric loss is reduced in the case where the dielectric constant does not change much.
Fig. 8. The conductivity of PVDF-based composites depends on the frequency.
Fig. 8(d) is a plot of conductivity versus frequency for a PVDF-based composite with a GNs content of 5 wt%. All conductivity is in accordance with the rules: σac (ω) = ω8 (s < 1 (0–103 Hz),s > 1 (f > 103 Hz)). It can be seen from Fig. 8(a) that as the GNs increase, the electrical conductivity of the composite material gradually increases, as high as 10−2 S/cm, and is close to a straight line. The figure shows that as the GNs increase, the PVDF-based composite material gradually changes from an insulator to a conductor. This is because the addition of GNs causes the composite to experience severe leakage, which is similar to conductor performance. Although the material has a high dielectric constant, it loses its insulation and has no application value.While Fig. 8(b) and (c) respectively modify the graphite nanosheets with two coupling agents, the conductivity also increases with the increase of GNs. This is because the composite material is weakened by the insulation properties, but the electrical conductivity does not become a straight line, indicating that the composite material still has a certain insulation property. As can be seen in Fig. 8(d), the modification of the coupling agent results in a decrease in electrical conductivity. This is because the use of coupling agents can improve the compatibility between GNs and PVDF, while coupling agent functionalized GNs also limits the free migration of carriers in GNs. It plays an important role in improving the insulation of composite materials.
4.3. Breakdown performance In order to better compare the influence of two coupling agents on the electrical properties of composite materials, a breakdown test was conducted on the composite materials, and the results were shown in Fig. 9. Look at Fig. 9(a)–(e), it can be seen that the addition of graphite nanosheets causes a sudden drop in the breakdown field strength of the composite. This was because the addition of the graphite nanosheets makes the carriers easier to move, and more carriers were introduced with the movement of the carriers under the action of the high electric field, resulted in a decrease in the breakdown field strength. As can be seen from Fig. 9(c)–(e), the breakdown field strength increases after the coupling agent is modified. And not only improves the compatibility between GNs and PVDF, but also provides carriers for GNs to limit free carrier migration, slower carrier movement and increase kinetic energy, resulted in increased breakdown field strength. Beacause the specific surface area of graphite nanosheets was too large in combination with Fig. 9(b)–(f). As the graphite nanosheets increased, when the doped amount of the graphite nanosheets reaches 7 wt%, the breakdown field strength is decreased due to the difficulty in dispersion. Compared the influence of the interface formed by different coupling agents (KH570 and PFOES) on the breakdown field strength of composites. It can be
4.2. Electrical conductivity In order to understand the conductivity of the composite material, the conductivity of the composite material was tested to obtain Fig. 8. Fig. 8(a)–(c) are plots of conductivity versus frequency for GNs/PVDF, KH570-GNs/PVDF, and PFOES-GNs/PVDF composites, respectively.
Fig. 7. Dependence of dielectric constant (a) and dielectric loss (b) of PVDFbased composites on frequency. 4
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Fig. 11. Energy storage density of composite materials.
4.5. Energy storage density According to the performance analysis above, the dielectric constant and the breakdown field strength at a frequency of 100 Hz (normal frequency) were selected. The calculation of the storage density was performed using Eq. 1. It is concluded that the storage density is preferably a composite material with a GNs content of 5 wt%, which is plotted as shown in Fig. 11.
Fig. 9. Figures (a)–(e) show the relationship between the breakdown field strength and the composite mass fraction. Figures (b)(d) and (f) are cross-sectional SEM of 20,000 times the corresponding composite.
u= ε 0 ε, E2 /2
found that KH570, which has weak affinity with PVDF, has better carrier-restricting ability, so when KH570 is used, the breakdown field strength of the composite is higher than that of the coupling agent (PFOES) with higher affinity for PVDF. The KH570-GNs/PVDF composite with 5 wt% doping has the highest breakdown field strength of 85.39 kV/mm, which is 4 times that of pure film. It is 10 times the breakdown field strength of the same amount of doped graphite nanosheets/PVDF composites. This indicates that for the breakdown field strength of PVDF-based composites, it is preferred to PFOES.
(1)
Where U is the energy storage density, εr is the relative dielectric constant, and εo is 8.85 × 10−12 F/m. According to the results in the figure, it can be seen that the energy storage density of the composite increases slightly after adding GNs, but it was not obvious. The used of coupling agents founded that the storage density of the composites increased significantly because the coupling agent not only improveed the compatibility between GNs and PVDF, but also provided carriers for the confinement. This results in a decrease in the dielectric loss of the composite and an increase in the breakdown field strength. For the dielectric constant, the effect of used a coupling agent PFOES with higher affinity for PVDF is preferred over the use of the coupling agent KH570. This is because the dielectric constant of the composite was the dipole polarization of PVDF itself, except for the influence of carriers. PFOES contains CFe groups, which makes PFOES and PVDF more compatible, and can improve the compatibility between GNs and PVDF. It enhances the polarity of the composite itself and enhances the polarization, so the effect of increasing the dielectric constant was better than that of KH570. For the breakdown field strength, the KH570 modification is better than the PFOES. This indicates that the interface adjustment of KH570 is more effective in limiting the free migration of carriers. The maximum storage density of PFOES-GNs/PVDF is 6.613691 times higher than pure PVDF (0.01965). In general, if PVDF-based composites pursue higher dielectric constants and higher energy storage densities, then we can use the more affinity coupling agent PFOES to modify the composite, if the PVDF-based composites are more High breakdown field strength, then we can consider the use of the coupling agent KH570 for function.
4.4. Impedance analysis It can be seen from the Fig. 10 that the structure of the analog plate capacitor can be formed between two opposing GNs, the GNs are the two plates of the capacitor, and the PVDF substrate between the two microchips is a dielectric. The two plates can store charge, control the electron transfer rate, increase the dielectric constant, enhance the storage charge capacity, and reduce the electron transfer rate, thereby increasing the breakdown field strength of the composite. Moreover, it can be seen from the figure that the fitting in (a) of the figure is better, so the composite material modified with the coupling agent KH570 has a higher breakdown fieldstrength Fig. 10.
5. Conclusion In this work, we have prepared PVDF-based composites doped with GNs, but due to severe leakage, the dielectric loss is too high, and the breakdown field strength is too low, thus losing the insulation performance and original application value. Therefore, different coupling agents PFOES and KH570 were used to functionalize GNS to improve the compatibility between GNS and PVDF and to improve the dispersion of GNS in PVDF. After the addition of the coupling agent, the
Fig. 10. Figures (a) and (b) show the Nyquist diagrams of 5 wt% KH570-GNs/ PVDF and PFOES-GNs/PVDF. 5
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electrical properties of the composite are improved, resulting in a high dielectric, low loss composite. After the addition of the coupling agent, the electrical properties of the composite are improved, resulting in a high dielectric, low loss composite. As can be seen from the above, a higher dielectric constant can be obtained by using a coupling agent PFOES having a higher affinity with PVDF than KH570. The coupling agent KH570 allows the GNs/PVDF composite to achieve higher energy storage density, lower dielectric loss and higher breakdown field strength. It is worth noting that the PVDF composites containing 3 wt% KH570-GNs have excellent dielectric properties and have certain reference value when selecting materials such as flexible capacitors. This study provides a reference for adjusting the interface between GNS and PVDF from a coupling agent.
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[20]
Declaration of Competing Interest [21]
We declare that we have no financial and personal relationships with other people or organizations that can inappropriately influence our work, there is no professional or other personal interest of any nature or kind in any product, service and/or company that could be construed as influencing the position presented in, or the review of, the manuscript entitled, “Dielectrical properties of Graphite Nanosheets/ PVDF Composites Regulated by Coupling agent”.
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Acknowledgment [26]
This work was financially supported by the National Nature Science Foundation of China (No. 51677045, 51177030, 51603057), the Harbin science and Techonology Innovation Talents Project (No. 2016RAQXJ059).
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Dielectrical properties of graphite nanosheets/PVDF composites regulated by coupling agent
T
Xue Lia, Xiaoming Wanga,b, Ling Wenga,b,*, Yang Yub, Xiaorui Zhanga,b,*, Lizhu Liua,b, Cheng Wanga a b
School of Materials Science and Engineering, Harbin University of Science and Technology, Harbin, 150080, PR China Key Laboratory of Engineering Dielectrics and Its Application, Ministry of Education, Harbin University of Science and Technology, Harbin, 150080, PR China
A R T I C LE I N FO
A B S T R A C T
Keywords: Coupling agent PVDF Composites
nanosheets (GNs) with different coupling agents to enhance the dielectric properties of GNs/PVDF composite membranes, and the modified results were characterized by FT-IR. The dielectrical properties and the breakdown field strength of composites film was characterized. The results indicated that the dielectric constant of the composite increases, with the increasing of GNs volume fraction. Moreover, the coupling agent modifies the GNs to confine the carriers of the GNs, which reduces the dielectric loss and increases the breakdown field strength. Since PFOES contains CeF, higher affinity for PVDF leads to higher dielectric constant, and modification of GNs with PFOES and KH570 improves the compatibility of GNs and PVDF, which leads to higher breakdown field strength. When the graphite nanosheets (5 wt%) was modified with coupling agent KH570, the dielectric constant was increased by an order of magnitude to 167 compared with PVDF, the breakdown field strength was also increased to 85.39 kV/mm, and the storage density reached 5.37655. J/cm3; when used PFOES modified graphite nanosheets, the dielectric constant was as high as 295 when the amount was 5 wt%, and the breakdown field strength was increased to 68.64 kV/mm, which is 3.3 times that of pure PVDF. The storage density is 6.13,691 J/cm3. It is worth noting that the use of 3 wt% KH570 modified graphite nanosheets in PVDF has significantly improved the dielectric constant while maintaining dielectric loss comparable to 1 wt% (The dielectric constant reached 80 at 100 Hz.). Different coupling agents have different effects on the properties of composites due to the functionalization of GNs, which provides some ideas for further research on capacitors and other electrons.
1. Introductions Polymer composites with high dielectric properties are appreciated due to their unique combination of mechanical flexibility and tunable dielectric properties [1,2]. The common fillers are mainly high dielectric inorganic ceramic particles and conductive fillers [3,4]. When conductive filler is used to enhance the dielectric film, high dielectric constant can be obtained because the carriers provided by conductive filler can be polarized in an electric field. However, these carriers must be subjected to a confinement treatment, otherwise the dielectric film may leak and the dielectric loss will be very high [5–12]. Metal particles and carbon materials are usually used as conductive fillers [13,14]. Carbon materials represented by graphite nanosheets, carbon nanotubes and graphene are in hot topics [15,16]. The dielectric films prepared by using these materials as fillers have many advantages such as high dielectric constant and good mechanical properties. But
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these materials should be coated or modified. Organic materials such as coupling agents which could coat the carbon material can significantly improve the breakdown field strength of the film, reduce its dielectric loss, and greatly improve its insulation capacity [17–20]. An el al. added graphene oxide (GO) to PVDF, and the dielectric constant of PVDF-based composites increased with increasing GO content [21]. Xu et al used two different coupling agents (KH550/MDI) to functionalize the graphene oxide (GO) and added it to the phenyl silicone rubber. The results showed that GO was well dispersed in phenyl silicone rubber after modification. The authors studied the effects of functional graphene oxide on the dielectric behaviour, thermal transmittance, optical transmittance and mechanical properties of the composites [22]. Liu et al. used different coupling agents (ATPS/GTPS) to modify the graphene core-shell structure, the dielectric constant was significantly enhanced, and the dispersion of the filler in the matrix was improved [23]. Huang et al. used PFOES to modify graphene and added
Corresponding authors at: School of Materials Science and Engineering, Harbin University of Science and Technology, Harbin, 150080, PR China. E-mail addresses: [email protected] (X. Li), [email protected] (L. Weng), [email protected] (X. Zhang).
https://doi.org/10.1016/j.mtcomm.2019.100705 Received 20 August 2019; Received in revised form 13 October 2019; Accepted 14 October 2019 Available online 15 October 2019 2352-4928/ © 2019 Published by Elsevier Ltd.
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3 h. A series of composite films with different mass fraction were obtained.To the preparation of good composite membrane, into the homemade mold, using 30 min under 180 °C hot pressing plate vulcanizing machine, pressure to 10 Mpa. Put in the mold. Keep at constant temperature and constant pressure for 30 min. Close the machine, let cool to room temperature and remove the prepared composite film.
it to PVDF. Compared with unmodified graphene, the addition of PFOES increased the dispersion of graphene in PVDF. Moreover, the dielectric constant, piezoelectric constant, and mechanical properties of PVDF-based composites are improved [24]. Wang et al. used silica to cover the surface of graphene oxide and then modified epoxy resin (EP). As a result, the surface not only improved the flame retardancy and thermal stability of EP, but also made EP have the advantages of high dielectric constant and low dielectric loss [25]. Fang et al. proposed a novel and simple method was to synthesize high dielectric constant NH2 functionalized and carboxyl functionalized graphene oxide (PPDCFGO)/polyimide (PI) composite films. The dielectric constant increases up to 36.9 with an increasing amount of PPD-CFGO, higher than that of the pure PI polymer by a factor of 12.5, while the dielectric loss is only 0.0075 and the breakdown strength still remains at a high level [26]. From other studies, it can be found that the functional group of the silane coupling agent can interact with a specific chemical part in the polymer matrix, can improve the dispersibility of graphene, effectively reduce the agglomeration of graphene [27–34]. Moreover, the coupling agent can also limit the carriers of graphene, thereby increase the dielectric constant of the polymer, reduce the dielectric loss, and increase the breakdown field strength. However, due to the need for functionalize the graphene using a coupling agent, graphene needs to be converted into graphene oxide for reduction. The surface of the reduced graphene oxide contains a large amount of oxidized groups, so that it provides a much smaller carrier than graphene. In order to retain the number of carriers and improve the dispersibility of graphene, the graphite nanosheets was graft modified with coupling agent. Although the surface is graft-modified by coupling agent, the carrier of the graphite nanosheets is sufficiently large due to its multilayer structure. This solves the dispersion problem to increase the breakdown field strength of the composite while having sufficient carriers to improve the dielectric properties of the composite. Based on previous studies, it has been found that different coupling agents have different effects on the properties of the composite. Considering the characteristics of PVDF, a functionalized modifier containing a fluorine functional group can improve the compatibility and miscibility between the modified layer and PVDF, because the fluorine atom group and the corresponding molecular chain contained in the PVDF may be intertwined with other atomic groups [35–40]. So we used PFOES (Heptadecafluorodecyl trimethoxysilane.) with CeF groups and KH570 (γ-methacryloxypropyltrimethoxysilane) with CeF groups to study the effects of different coupling agents on the properties of PVDF matrix composites.
2.3. Performance characterization and testing methods The cross-sectional microstructure of the composite film doped with different coupling agent modified graphite nanosheets was observed and characterized by FEI SIRION 200 scanning electron microscope.The composite film scanning sample was brittle cracked in liquid nitrogen to make a section, cut to a certain size and stuck to an aluminum block with conductive adhesive, after spraying gold, a test sample was made. The dielectric properties of the composite film were measured by BDS 4000 broadband dielectric/impedance spectrometer. The test frequency range is 10−1 Hz to107 Hz and the test temperature is room temperature. Before testing, an Al electrode with a diameter of 20 mm should be plated on both sides of the film sample. The breakdown field strength of the composite film was measured by HT-5/20 breakdown voltage tester and boosted at a rate of 1 kV/s. Each sample was tested at 5 points, and the average of the 5 points was taken as the electric breakdown field strength of the film. 3. Results and discussion 3.1. Structural characterization In order to determine the grafting of GNs, the GNs and the modified GNs were characterized by FT-IR. The infrared spectrum of the graphite nanosheets and the modified graphite nanosheets was shown in Fig. 1. It could be found that in the unmodified GNs, 3430 cm−1 was the stretching vibration peak of −OH and 2925 cm−1 was −CH2- of stretching vibration peak. In the infrared spectrum of the modified GNs the −OH stretching vibration peak at a wavelength of 3430 cm−1 appeared. At 2925 cm-1 and 2850 cm−1, there are overlapping peaks of −CH2- and −CH-. The wavelength of 1380 cm−1 was the stretching vibration peak of CeF peculiar to the PFOES, and 1620 cm−1 was a stretching vibration peak of C]C peculiar to KH570, and 1020 cm−1 and 1125 cm−1 were stretching vibration peaks of Si-O bonds. The characteristic absorption peak (Si-O) of silane coupling agent appears in the modified GNs, and the stretching vibration peak of CeF or C]C unique to the two coupling agents also appeared, which indicated that PFOES or KH570 was grafted successfully onto the GNs. Composite materials with different doping amounts were prepared
2. Experimental 2.1. Preparation of GNs and modified GNs The required quality of graphite was put in the 1000 °C heating with 15 s, vermicular graphite was produced. The vermicular graphite was stirred by ultrasound for 2 h in solution with a volume ratio of 1: 1 of deionized water and ethanol. 0.02 g of coupling agent was added, and removed the overall solution to preheat to 80 °C, stirred for 8 h. The solution was filtered., again in 80 °C oven for 2 days, to the solvent volatilization completely,dry preservation. The coupling agent modified GNs can be obtained. 2.2. preparation of composites A certain amount of graphite powder was weighed and added into a three-mouth bottle containing 50 ml N, N-dimethylformamide (DMF) solution. After 1 h ultrasonic dispersion, 10.25 g PVDF (PVDF) powder was added to the bottle, and the mixture was continuously stirred for 3 h under ultrasonic state. A colloidal solution of the composite is obtained. After the prepared colloidal solution was evacuated to eliminate air bubbles, the film was cast by a casting method and dried at 80 °C for
Fig. 1. FT-IR of modified GNs. 2
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Fig. 4. Dependence of dielectric constant (a) and dielectric loss (b) of GNs/ PVDF on frequency.
doping is sufficient.
4. Electric properties Fig. 2. SEM of PVDF-based composites materials with filler content of 5 wt% (Filler of a is GNs,the scanning multiple of 5000. Filler of b is KH570-GNs, the scanning multiple of 5000. Filler of c is PFOES-GNs,the scanning multiple of 5000.).
4.1. Dielectric properties The relation between the permittivity and frequency of PVDF-based composites doped with unmodified GNs was shown in Fig. 4(a). It can be seen that the permittivity increases with the increasing of GNs. Moreover, the dielectric constant of the composite material does not change much compared with pure PVDF when the doped amount of graphite nanosheets is 1 wt%. When the doped amount of graphite nanosheets is greater than or equal to 3 wt%, the dielectric constant increases by an order of magnitude compared with pure PVDF. When the frequency is 100 Hz, the dielectric constant exceeds 102. However, as shown in Fig. 4(b), the correspond dielectric loss changes with the frequency, and the dielectric loss increases with the GNs incorporation increased. Moreover, the dielectric loss increased greatly when the doped amount of the graphite nanosheets was greater than or equal to 3 wt%, which was corresponding to the change of the dielectric constant of the composite material, indicating that the dielectric constant was positively correlated with the dielectric loss. As the carrier provided by the graphite nanosheets increases with the addition of the graphite nanosheets, the distance between the graphite particles is close enough to form a conductive channel, resulted in the leakage phenomenon that the loss was so large. Fig.5(a) shows that the variation trend of the dielectric constant with the addition amount of the KH570 modified graphite nanosheets is basically consistent with that of the GNs/PVDF composite. Therefore, the functionalization of the KH570 functional group has little effect on the dielectric constant of the composite. However in Fig. 5(b) can be found that the dielectric loss although also increases with the increase of graphite content, but the minimum dielectric loss is around 0.1. It can be seen that the dielectric constant of the composite is as high as 80 or more (100 Hz) when the content of KH570-GNs is 3 wt%, and the dielectric loss of the composite is at least 0.1. This trait of high dielectric constant and low dielectric loss indicates that this ratio of composite materials has certain practical value. We observed the change in dielectric constant of the composite after PFOES modified GNs in Fig. 6(a), which is consistent with Figs. 4(a) and 5(a). As the dielectric constant of GNs increases, the dielectric constant
by mixing GNs and modified GNs into polyvinylidene fluoride (PVDF) with mass ratios of 1 wt%,3 wt%,5 wt% and 7 wt%. Then, scanning electron microscopy (SEM) was used to determine the morphology of the composites, and SEM was observed in the cross-section of the composites. From Fig. 2(a), it can be seen that the dispersion effect of pure GNs in PVDF is general, while the dispersion of GNs in PVDF with different coupling agents has been significantly improved, as shown in Figs. 2(b) and 2(c). Due to the effect of coupling agent, GNs with better dispersion effect are uniformly dispersed in various positions of PVDF, and the phenomenon of leakage caused by mutual superposition between GNS is avoided. It can be seen from the XRD (Fig. 3) diffraction pattern that the 2θ at 18.3° and 19.9° shown α phase of PVDF, and the 20.6° and the 26.6° at diffraction peaks were the β phase and γ phase of PVDF respectively This indicates that the pure phase phase was the α phase, the β phase and the γ phase multiphase coexisted. However we can found that 2θ in these composite film diffraction peak of 18.3° and 19.9° gradually weakened, 20.6° and 26.6° gradually stronger, and the PVDF α phase gradually transformed to the β phase with the addition of GNs and modified GNs. And β-PVDF has good dielectric properties, which is beneficial to obtain high dielectric constant29. Experiments show that the addition of filler has a great influence on the crystallization properties of the matrix, which is one of the main reasons for the formation of β-PVDF and the increase of dielectric constant when the amount of
Fig. 5. Dependence of dielectric constant (a) and dielectric loss (b) of KH570GNs/PVDF on frequency.
Fig. 3. XRD of modified PVDF-based composites. 3
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Fig. 6. Dependence of dielectric constant (a) and dielectric loss (b) of PFOESGNs/PVDF on frequency.
increases significantly when the amount of GNs added was 3 wt%. This is because the dielectric constant of the composites is mainly determined by the dipole polarization of the PVDF matrix when the doping amount of the graphite nanosheets is low, and it is not affected by the frequency in the frequency range tested in the experiment. Therefore, the dielectric constant of composite material does not change much with frequency. As the content of graphite nanosheets filler increased to 3 wt%, near the percolation threshold, the internal electron polarization increased. As for the relationship between the dielectric loss and frequency of composite materials, we can see from the figure that the dielectric loss decreases first with frequency and then increases. At low frequency, dipole polarization varies with electric field without delay and polarization loss. Leakage loss is the main cause of loss, and leakage loss decreases with the increase of frequency. As the frequency continues to rise, the polarization relaxes and generates polarization loss, which increases with the increase of frequency and replaces leakage loss as the leading factor of dielectric loss. Fig. 7(a) and Fig. 7(b) are graphs showing the dielectric constant and dielectric loss as a function of frequency for a PVDF composite having a GNs content of 5 wt%, respectively. It can be seen from Fig. 7(a) that the dielectric constant increases greatly after the addition of GNs to PVDF. This is because the carrier polarization is stronger, the dielectric constant of the PVDF composite film is stronger, and the number of carriers is the doping amount of GNs is proportional. The change in dielectric constant after using the coupling agent is not significant because the compatibility between GNs and PVDF is improved by the coupling agent under the action of electric field. At the same time, the number of carriers provided by GNs is not affected by the functionalization of the coupling agent. We can see in Fig. 7(b) that the dielectric loss increases with the increase of GNs. However, after the addition of the coupling agent, the dielectric loss is reduced. This is because PFOES and KH570 functionalize the GNs, limiting the carriers and avoiding leakage. Therefore, the dielectric loss is reduced in the case where the dielectric constant does not change much.
Fig. 8. The conductivity of PVDF-based composites depends on the frequency.
Fig. 8(d) is a plot of conductivity versus frequency for a PVDF-based composite with a GNs content of 5 wt%. All conductivity is in accordance with the rules: σac (ω) = ω8 (s < 1 (0–103 Hz),s > 1 (f > 103 Hz)). It can be seen from Fig. 8(a) that as the GNs increase, the electrical conductivity of the composite material gradually increases, as high as 10−2 S/cm, and is close to a straight line. The figure shows that as the GNs increase, the PVDF-based composite material gradually changes from an insulator to a conductor. This is because the addition of GNs causes the composite to experience severe leakage, which is similar to conductor performance. Although the material has a high dielectric constant, it loses its insulation and has no application value.While Fig. 8(b) and (c) respectively modify the graphite nanosheets with two coupling agents, the conductivity also increases with the increase of GNs. This is because the composite material is weakened by the insulation properties, but the electrical conductivity does not become a straight line, indicating that the composite material still has a certain insulation property. As can be seen in Fig. 8(d), the modification of the coupling agent results in a decrease in electrical conductivity. This is because the use of coupling agents can improve the compatibility between GNs and PVDF, while coupling agent functionalized GNs also limits the free migration of carriers in GNs. It plays an important role in improving the insulation of composite materials.
4.3. Breakdown performance In order to better compare the influence of two coupling agents on the electrical properties of composite materials, a breakdown test was conducted on the composite materials, and the results were shown in Fig. 9. Look at Fig. 9(a)–(e), it can be seen that the addition of graphite nanosheets causes a sudden drop in the breakdown field strength of the composite. This was because the addition of the graphite nanosheets makes the carriers easier to move, and more carriers were introduced with the movement of the carriers under the action of the high electric field, resulted in a decrease in the breakdown field strength. As can be seen from Fig. 9(c)–(e), the breakdown field strength increases after the coupling agent is modified. And not only improves the compatibility between GNs and PVDF, but also provides carriers for GNs to limit free carrier migration, slower carrier movement and increase kinetic energy, resulted in increased breakdown field strength. Beacause the specific surface area of graphite nanosheets was too large in combination with Fig. 9(b)–(f). As the graphite nanosheets increased, when the doped amount of the graphite nanosheets reaches 7 wt%, the breakdown field strength is decreased due to the difficulty in dispersion. Compared the influence of the interface formed by different coupling agents (KH570 and PFOES) on the breakdown field strength of composites. It can be
4.2. Electrical conductivity In order to understand the conductivity of the composite material, the conductivity of the composite material was tested to obtain Fig. 8. Fig. 8(a)–(c) are plots of conductivity versus frequency for GNs/PVDF, KH570-GNs/PVDF, and PFOES-GNs/PVDF composites, respectively.
Fig. 7. Dependence of dielectric constant (a) and dielectric loss (b) of PVDFbased composites on frequency. 4
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Fig. 11. Energy storage density of composite materials.
4.5. Energy storage density According to the performance analysis above, the dielectric constant and the breakdown field strength at a frequency of 100 Hz (normal frequency) were selected. The calculation of the storage density was performed using Eq. 1. It is concluded that the storage density is preferably a composite material with a GNs content of 5 wt%, which is plotted as shown in Fig. 11.
Fig. 9. Figures (a)–(e) show the relationship between the breakdown field strength and the composite mass fraction. Figures (b)(d) and (f) are cross-sectional SEM of 20,000 times the corresponding composite.
u= ε 0 ε, E2 /2
found that KH570, which has weak affinity with PVDF, has better carrier-restricting ability, so when KH570 is used, the breakdown field strength of the composite is higher than that of the coupling agent (PFOES) with higher affinity for PVDF. The KH570-GNs/PVDF composite with 5 wt% doping has the highest breakdown field strength of 85.39 kV/mm, which is 4 times that of pure film. It is 10 times the breakdown field strength of the same amount of doped graphite nanosheets/PVDF composites. This indicates that for the breakdown field strength of PVDF-based composites, it is preferred to PFOES.
(1)
Where U is the energy storage density, εr is the relative dielectric constant, and εo is 8.85 × 10−12 F/m. According to the results in the figure, it can be seen that the energy storage density of the composite increases slightly after adding GNs, but it was not obvious. The used of coupling agents founded that the storage density of the composites increased significantly because the coupling agent not only improveed the compatibility between GNs and PVDF, but also provided carriers for the confinement. This results in a decrease in the dielectric loss of the composite and an increase in the breakdown field strength. For the dielectric constant, the effect of used a coupling agent PFOES with higher affinity for PVDF is preferred over the use of the coupling agent KH570. This is because the dielectric constant of the composite was the dipole polarization of PVDF itself, except for the influence of carriers. PFOES contains CFe groups, which makes PFOES and PVDF more compatible, and can improve the compatibility between GNs and PVDF. It enhances the polarity of the composite itself and enhances the polarization, so the effect of increasing the dielectric constant was better than that of KH570. For the breakdown field strength, the KH570 modification is better than the PFOES. This indicates that the interface adjustment of KH570 is more effective in limiting the free migration of carriers. The maximum storage density of PFOES-GNs/PVDF is 6.613691 times higher than pure PVDF (0.01965). In general, if PVDF-based composites pursue higher dielectric constants and higher energy storage densities, then we can use the more affinity coupling agent PFOES to modify the composite, if the PVDF-based composites are more High breakdown field strength, then we can consider the use of the coupling agent KH570 for function.
4.4. Impedance analysis It can be seen from the Fig. 10 that the structure of the analog plate capacitor can be formed between two opposing GNs, the GNs are the two plates of the capacitor, and the PVDF substrate between the two microchips is a dielectric. The two plates can store charge, control the electron transfer rate, increase the dielectric constant, enhance the storage charge capacity, and reduce the electron transfer rate, thereby increasing the breakdown field strength of the composite. Moreover, it can be seen from the figure that the fitting in (a) of the figure is better, so the composite material modified with the coupling agent KH570 has a higher breakdown fieldstrength Fig. 10.
5. Conclusion In this work, we have prepared PVDF-based composites doped with GNs, but due to severe leakage, the dielectric loss is too high, and the breakdown field strength is too low, thus losing the insulation performance and original application value. Therefore, different coupling agents PFOES and KH570 were used to functionalize GNS to improve the compatibility between GNS and PVDF and to improve the dispersion of GNS in PVDF. After the addition of the coupling agent, the
Fig. 10. Figures (a) and (b) show the Nyquist diagrams of 5 wt% KH570-GNs/ PVDF and PFOES-GNs/PVDF. 5
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electrical properties of the composite are improved, resulting in a high dielectric, low loss composite. After the addition of the coupling agent, the electrical properties of the composite are improved, resulting in a high dielectric, low loss composite. As can be seen from the above, a higher dielectric constant can be obtained by using a coupling agent PFOES having a higher affinity with PVDF than KH570. The coupling agent KH570 allows the GNs/PVDF composite to achieve higher energy storage density, lower dielectric loss and higher breakdown field strength. It is worth noting that the PVDF composites containing 3 wt% KH570-GNs have excellent dielectric properties and have certain reference value when selecting materials such as flexible capacitors. This study provides a reference for adjusting the interface between GNS and PVDF from a coupling agent.
[14] [15]
[16]
[17] [18] [19]
[20]
Declaration of Competing Interest [21]
We declare that we have no financial and personal relationships with other people or organizations that can inappropriately influence our work, there is no professional or other personal interest of any nature or kind in any product, service and/or company that could be construed as influencing the position presented in, or the review of, the manuscript entitled, “Dielectrical properties of Graphite Nanosheets/ PVDF Composites Regulated by Coupling agent”.
[22] [23]
[24]
[25]
Acknowledgment [26]
This work was financially supported by the National Nature Science Foundation of China (No. 51677045, 51177030, 51603057), the Harbin science and Techonology Innovation Talents Project (No. 2016RAQXJ059).
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