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polyvinyl alcohol composite films
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Enhanced dielectric properties of homogeneous Ti3C2Tx MXene@SiO2/ polyvinyl alcohol composite films Wei Wana,b,∗, Meizhen Taoa, Hailin Caoa, Yuqing Zhaoa, Junrong Luoc, Jian Yangb,∗∗, Tai Qiub a Hunan Engineering Laboratory for Preparation Technology of Polyvinyl Alcohol Fiber Materials, Huaihua Key Laboratory for Preparation of Ceramic Materials and Devices, College of Chemistry and Materials Engineering, Huaihua University, Huaihua, 418000, PR China b College of Materials Science and Engineering, Nanjing Tech University, Nanjing, 210009, PR China c College of Biological and Food Engineering, Huaihua University, Huaihua, 418000, PR China
A R T I C LE I N FO
A B S T R A C T
Keywords: Polyvinyl alcohol Ti3C2Tx MXene Composite films Dielectric properties SiO2
Exploring flexible dielectrics with high dielectric constant and low loss is pertinent for many applications in electronic devices. In the present work, a non-ferroelectric polymer, polyvinyl alcohol (PVA), was used to fabricate homogeneous dielectric composite films with a relatively high dielectric constant and a low dielectric loss by using 2D nano sheets (Ti3C2Tx MXene) as the filler. To limit dielectric loss, SiO2 was coated onto the surface of MXene to provide interfacial barrier effect and suppress dielectric loss. MXene@SiO2/PVA composite films showed lower dielectric losses at low frequencies (from 20 Hz to ~ 10 kHz) compared with MXene/PVA composite films. MXene@SiO2/PVA composite films with 2.5 wt% MXene loading and 5 wt% (with respect to MXene content) SiO2 coating had a dielectric constant of 27.2 (a 292.5% rise compared to neat PVA film) and a dielectric loss of only 0.057 (a 259.6% reduction compared to MXene/PVA composite film) at 100 Hz and room temperature (RT). In addition, this SiO2-coated composite film had stable dielectric properties (dielectric constant and loss change from 27.2 to 29.3 and 0.057 to 0.104, respectively) in the temperature range of RT to 60 °C. This work provides a promising way to fabricate PVA-based dielectric composites with excellent dielectric properties for practical applications in electronics.
1. Introduction
ceramics fillers, especially relaxor ferroelectrics (such as BaTiO3, Ba0.5Sr0.5TiO3, and Ba0.95Ca0.05Zr0.15Ti0.85O3) into polymer matrices can generate relatively high dielectric constant materials [10–12]. However, to obtain high dielectric constant, a large amount (typically ~40 vol%) of ceramics are typically necessary, which deteriorates the flexibility and always results in relatively high dielectric loss and reduced breakdown strength [6,13]. Incorporating conductive fillers into polymer matrices can obtain a high dielectric constant with lower loadings of fillers based on Maxwell-Wagner-Sillars (MWS) polarization [14,15]. Much effort has been devoted to develop conductive filler/ polymer composite dielectric materials [16–18]. In recent years, two-dimensional (2D) materials like graphene [19,20], MoS2 [21], BN [22], and Bi2Te3 [23] have drawn tremendous attention in the field of nano dielectric composites due to their unique chemical and physical properties. Recently, a new family of 2D materials named MXenes have gained wide attention owing to their unique structure and excellent properties such as good hydrophilicity, high
Flexible dielectrics with a high dielectric constant and low loss have drawn significant attention for potential applications in many electronic devices such as energy storage capacitors, flexible electronics, energy harvesters, and power systems [1–6]. Many ceramics (e.g., BaTiO3 and SrTiO3) possess excellent dielectric properties but are usually fabricated in a complex forming process combined with hightemperature sintering and suffer from delicate properties [7,8]. Even though most polymers have good flexibility, their ultra-low dielectric constant values (usually lower than 10) restricts the practical use of them in many electronics applications. For instance, the most successful, commercially used dielectric polymer, polypropylene, shows a dielectric constant of ~3, leading to a low energy density of ~2 Jcm−3 [9]. To prepare materials with good flexibility and excellent dielectric properties, incorporating ceramic or conductive fillers into polymers to construct binary composites are better alternatives. Introducing
∗
Corresponding author. Hunan Engineering Laboratory for Preparation Technology of Polyvinyl Alcohol Fiber Materials, Huaihua Key Laboratory for Preparation of Ceramic Materials and Devices, College of Chemistry and Materials Engineering, Huaihua University, Huaihua, 418000, PR China. ∗∗ Corresponding author. E-mail addresses: [email protected] (W. Wan), [email protected] (J. Yang). https://doi.org/10.1016/j.ceramint.2020.02.179 Received 28 January 2020; Received in revised form 17 February 2020; Accepted 17 February 2020 0272-8842/ © 2020 Published by Elsevier Ltd.
Please cite this article as: Wei Wan, et al., Ceramics International, https://doi.org/10.1016/j.ceramint.2020.02.179
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Fig. 1. Scheme of the preparation process of the MXene@SiO2/PVA composite film.
magnetic stirring for 30 min. Then 1 g Ti3AlC2 powder was slowly added to the mixture solution and magnetically stirred at 35 °C for 24 h, after which the mixture was washed through four cycles of ethyl alcohol addition, centrifugation (3500 rpm, 5 min), and decanting. The as-obtained pasted product was dissolved into deionized water (300 times the ratios of product to DI water) and exfoliated by ultrasonication for 1 h. Finally, the dilute suspension was centrifuged (3500 rpm, 1h), and the dark green supernatant was collected.
electrical conductivity, good chemical stability and environmental friendliness [24,25]. MXenes are 2D transition metal carbide/nitride materials derived from their precursor Mn+1AXn (or MAX, where M represents an early transition metal, A is a group A element, X is C or N element, and n = 1, 2 or 3) by selectively etching A atoms using hydrofluoric acid (or hydrochloric acid combined with fluorides), followed by ultrasonic delamination [26,27]. To date, Mxenes have been reported as promising materials for applications in batteries [28,29], hydrogen storage [30], chemical adsorption [31], energy conversion [32], and dielectrics. Tu et al. [33] first used MXene as the filler to construct Ti3C2Tx MXene/polyvinylidene fluoride (PVDF)-based polymer composites. The composites exhibited a dielectric constant of up to 105 and a loss of ~4 (at 1 kHz test frequency) near the percolation limit (~15.0 wt% MXene content). However, the dielectric loss caused by interface leakage conduction from conductive fillers can be relatively high. To reduce dielectric losses, multilayer structural MXene/ PVDF films [14], sandwich structural PVDF/MXene/2D BN nanocomposites [15], and gradient sandwich structural MXene/PVDF nanocomposites [9] were constructed and showed relatively good dielectric properties. To ensure consistency of the as-prepared composite films, the preparation process for constructing inhomogeneous structural films should be very precisely controlled. Moreover, it is well known that PVDF and most PVDF-based polymers are relaxor ferroelectrics, which will show strong piezoelectric effects when used in electronic devices [34]. In the present work, a non-ferroelectric and biodegradable polymer, polyvinyl alcohol (PVA), was used as the matrix, and homogeneous MXene@SiO2/polyvinyl alcohol (PVA) composite films were prepared by using 2D Ti3C2Tx MXene nano sheets as the filler and SiO2 as the dielectric loss inhibitor. The composite films exhibited desirable dielectric properties, namely, a dielectric constant of 27.2 and a low dielectric loss of only 0.057 at 100 Hz. The results of this work provide a facile route to create promising high performance flexible dielectric materials for practical applications in advanced electronic components.
2.2. Ti3C2Tx MXene@SiO2/PVA composite film fabrication To decrease the dielectric loss of Ti3C2Tx MXene/PVA composite films, the surface of Ti3C2Tx MXene was coated with SiO2. 150 mL of Ti3C2Tx MXene suspension was dispersed into 150 mL ethyl alcohol and magnetically stirred for 30 min. Under continuous magnetic stirring, 6 mL of 28% ammonium hydroxide solution and 0.026 g (for 5 wt% SiO2 coating, with respect to MXene content) or 0.052 g (for 10 wt% SiO2 coating) of TEOS (Sinopharm chemical reagent co. LTD) were consecutively added into the above mixed suspension and further stirred for 2 h. Finally, the resultant suspension was filtered and washed with deionized water until the pH of the filtrate approached seven with subsequent dilution of the filtered product into 150 mL deionized water. Ti3C2Tx MXene@SiO2/PVA composite films were fabricated by a solution coating method. First, 10 mL of MXene@SiO2 suspension and a certain amount of PVA (PVA-1799, Aladdin Industrial Corporation, 99.9%) solution (10 wt% concentration) were added into a nylon jar and ball-milled for 30 min at 180 rpm. The mixed suspension was then deaerated by ultrasonication. Finally, the mixed suspension was coated onto a polyethylene plate with subsequent oven-drying at 40 °C. 2.3. Characterization and measurements X-ray diffraction (XRD) results of the samples were identified by an X-ray diffractometer (Rigaku Industrial Corp., Japan, CuKα, 40 kV, 100 mA). Microstructures were observed under a field emission scanning electron microscope (FE-SEM, Zeiss Sigma HD, Germany) coupled with an energy-dispersive X-ray spectrometer (EDS, X-MaxN, Oxford Instruments, UK). For dielectric measurements, MXene@SiO2/PVA composite films were coated with a conductive platinum thin film electrode by employing ion sputtering coating equipment (GVC-1200, Beijing Gevee-Tech Co. Ltd., China, sputtering parameters: 10 mA, 100 s and two times). Fig. 1 shows the scheme of the preparation
2. Experimental 2.1. Ti3C2Tx MXene suspension preparation Ti3AlC2 (~38 μm) powders were used as the precursor for MXene synthesis. The Ti3C2Tx suspension was prepared according to the modified Michael's method [27]. First, 0.666 g LiF (Aladdin Industrial Corporation, 99.9%) was dissolved into 10 mL 6 M HCl solution with 2
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Fig. 2. (a) FE-SEM micrograph and (b) XRD pattern of exfoliated Ti3C2Tx MXene after coating with SiO2; the inset in (b) show the enlarged view of Ti3C2Tx MXene after coating with SiO2; elemental mapping of (c) Ti , (d) C and (e) Si for image (a).
Fig. 3. (a) FE-SEM micrograph of the surface of the MXene@SiO2/PVA composite film with 2.5 wt% MXene loading; (b) photograph of the MXene@SiO2/PVA composite film and (c) plates after coating with Pt electrode.
process of the MXene@SiO2/PVA composite film. Dielectric properties were measured using a Keysight E4990A precision impedance analyzer over a frequency range from 20 Hz to 1 MHz with a parallel equivalent circuit.
coating with SiO2. It is evident that Ti3C2Tx has an open stacked sheet structure (typical structure for MXene). The diffraction peak at 2θ ≈ 9° (Fig. 2b) corresponds to the structure of Ti3C2Tx MXene and no diffraction peaks for SiO2 were observed. The very weak diffuse scattering peak at 2θ ≈ 20° (shown by the red dashed line) indicates the SiO2 coating is amorphous as SiO2 obtained by the precipitation method usually possesses a non-crystalline structure [35,36]. From the inset in Fig. 2b, it is known that SiO2 coating prepared by the precipitation
3. Results and discussion Fig. 2a shows the microstructure of the exfoliated Ti3C2Tx after 3
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black glossy surface as Ti3C2Tx MXene is black. After coating with the Pt electrode via surface sputtering of the composite film, the Pt thin film shows gray and white colors with metallic luster, indicating sufficient coating of the Pt electrode onto the composite film surface. Fig. 4a presents the dielectric constant under 100 Hz test frequency of MXene/PVA composite films without and with SiO2 coating as a function of MXene loading. The dielectric constant increases with MXene content for MXene/PVA composite films and MXene@SiO2/PVA composite films, owing to the enhanced interfacial polarization (MWS polarization) between the conductive MXene nano sheets and insulating PVA matrix or between MXene nano sheets and insulating SiO2 coating. Increases of dielectric constant from 28.5 to 151.3, 27.2 to 111.4, and 17.7 to 84.1 for the MXene/PVA composite film, MXene@5 wt% SiO2/ PVA composite film, and MXene@10 wt% SiO2/PVA composite film, respectively, were observed as MXene loading increased from 2.5 wt% to 10 wt%. Compared with MXene/PVA composite films, MXene@ SiO2/PVA composite films exhibit slightly lower dielectric constants, which decrease with increased SiO2 loading as SiO2 has intrinsically low polarity and a low dielectric constant of ~3.5 [38]. Fig. 4b shows the dielectric loss of the as-obtained samples under 100 Hz test frequency. For MXene/PVA composite films, increases MXene content can enhance MWS polarization, but at the same time, the increased conductivity and interfacial relaxation results in increased dielectric loss [39]. However, it can be seen that MXene@SiO2/ PVA composite films have lower dielectric loss as SiO2 insulating coating isolates the high loss of conductive MXene nano sheets due to the interfacial barrier effect. The insulation of SiO2 coating layer can restrict the mobility of interfacial charge carriers and prevent the current percolation between MXene nano sheets and PVA matrix [40]. It is clearly that the suppression effect on dielectric loss is very remarkable at 2.5 wt% MXene loading. When the MXene loading exceeds 2.5 wt%, the dielectric loss of MXene@SiO2/PVA composite films increases gradually with MXene loading, which could be caused by increased interfacial relaxation, space charges or imperfections (such as structural defects, voids and cracks) at higher MXene content [37,41]. Compared with the MXene/PVA composite film with 2.5 wt% MXene loading, the MXene@SiO2/PVA composite film with 2.5 wt% MXene loading and 5 wt% nano SiO2 coating shows a dielectric loss of only 0.057 at 100 Hz, which is a 259.6% reduction, while the dielectric constant slightly decreases from 28.5 to 27.2. Also, it should be noted that the MXene@SiO2/PVA composite film with 2.5 wt% MXene loading and 10 wt% nano SiO2 coating also has desirable dielectric properties, namely an ultra-low dielectric loss of only 0.017 and a relatively high dielectric constant of 17.7 at 100 Hz. Table 1 provides a comparison of dielectric properties of MXene/polymer composites with different polymer matrices and structures. It is known that this homogeneous MXene@SiO2/PVA composite film has comparable and even superior dielectric properties to other inhomogeneous MXene/polymer dielectric composites previously reported [9,15]. Fig. 5 shows the dielectric constant, loss, and AC conductivity of the as-prepared films as a function of frequency. All films exhibit gradual decreases in dielectric constant with an increase in frequency. The declining trend is different at lower frequency (from 20 Hz to 1 kHz) for all films. Pure PVA film has the slowest change of dielectric constant among all films. Once MXene nanosheets were filled in the PVA matrix,
Fig. 4. (a) Dielectric constant and (b) loss vs MXene loading for MXene/PVA composite films without and with SiO2 coating under 100 Hz frequency.
method has layer structure with nano scale dimension, which is consistent with the reported literatures [36,37]. Elemental mapping of C, Ti, and Si (Fig. 2c-e) suggests that SiO2 was coated onto the surfaces of Ti3C2Tx MXene with a relatively homogeneous distribution. Fig. 3a is the FE-SEM micrograph of the surface of the Ti3C2Tx MXene@SiO2/PVA composite film with 2.5 wt% Ti3C2Tx MXene loading. The composite film shows a relatively dense structure without stomatal defects, and MXene@SiO2 nanosheets were dispersed into the PVA matrix without obvious MXene@SiO2 nanosheets agglomeration, revealing good compatibility between the PVA polymer matrix and MXene@SiO2 nanosheets. Fig. 3b and c shows the photographs of the as-prepared Ti3C2Tx MXene@SiO2/PVA composite film and testing samples after coating with the Pt electrode. The composite film has a smooth and
Table 1 Comparison of dielectric properties of MXene/polymer composites with different polymer matrix and structure. Types of MXene/polymer composites MXene/P[VDF-TrFE-CFE] Sandwich structural PVDF/MXene/2D BN nanocomposites Gradient sandwich structural MXene/PVDF composites Homogeneous MXene/PVA composites Homogeneous MXene@ 5 wt% SiO2/PVA composites Homogeneous MXene@ 10 wt% SiO2/PVA composites
Dielectric constant 5
~10 ~18 26 28.5 27.2 17.7
4
Dielectric loss
Test frequency
~4 ~0.029 0.04 0.148 0.057 0.017
1 kHz 1 kHZ 100 Hz 100 Hz 100 Hz 100 HZ
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Fig. 5. (a) Dielectric constant, (b) loss and (c) AC conductivity vs frequency for the as-prepared films.
Fig. 6. (a) Dielectric constant , (b) loss and (c) AC conductivity vs temperature for the as-prepared films under 100 Hz frequency.
the dielectric constant decrease more rapidly at low frequency, suggesting the presence of strong MWS polarization between the MXene nano sheets and PVA matrix at low frequency. When SiO2 was coated onto MXene nano sheets, the declining rate of dielectric constant decelerates, which may be attributed to the weakening MWS polarization caused by the insulating SiO2 coating. MXene@SiO2/PVA composite
films with 2.5 wt% MXene loading and 5 wt% SiO2 coating even showed a higher dielectric constant than that of the MXene/PVA composite film over a frequency range from 1 kHz to 100 kHz. At the frequency range from 20 Hz to about 100 kHz, MXene@SiO2/ PVA composite films show lower dielectric loss than MXene/PVA composite films, indicating effective suppression of loss owing to the 5
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Science Foundation of China (No.: 2018JJ3411), Scientific Research Fund of Hunan Provincial Education Department (No.: 18B493) and Open Fund of Hunan Engineering Laboratory for Preparation Technology of Polyvinyl Alcohol Fiber Materials (No.: HGI201802).
interfacial barrier effect caused by the insulating SiO2 coating. MXene@ SiO2/PVA composite films even show lower dielectric loss than neat PVA film at frequencies below 500 Hz, suggesting effective suppression of conductive paths in MXene@SiO2/PVA composite films. Moreover, there is no observed dielectric loss at the frequency below 70 Hz for the MXene@SiO2/PVA composite films with 2.5 wt% MXene loading and 10 wt% SiO2 coating. This is generally because the dielectric loss value is too small to be within the measuring accuracy range of the impedance analyzer [42]. This provides further indication of effective dielectric loss suppression at low frequencies by coating SiO2 onto the MXene surface. The increase in AC conductivity with frequency is a nearly linear trend for all four films, indicating the high insulating ability of these films [43]. The AC conductivity increases after Ti3C2Tx MXene nano sheets were incorporated into the polymer matrix, attributable to the highly conductive nature of MXene [27]. When SiO2 was coated onto the surface of MXene, the AC conductivity decreases due to the highly insulative nature of SiO2. The low AC conductivity of the MXene@SiO2/ PVA composite film indicates the effective interfacial barrier effect of the SiO2 coating. The trends in AC conductivity fact correspond to the dielectric loss results. Fig. 6 shows the temperature dependence of the dielectric constant, loss, and AC conductivity of the MXene/PVA film and the MXene@ 5 wt % SiO2/PVA composite film. When the temperature is below 60 °C, both films show a stable dielectric constant and conductivity and a slow increase in dielectric loss, which suggests that the polarization of the interface and the PVA matrix is stable in this temperature range and the movement of the space charges is difficult. In addition, it is known that in the temperature range from RT to 60 °C, the MXene@ 5 wt% SiO2/ PVA composite film shows a similar dielectric constant to the MXene/ PVA film, but the former's dielectric loss is much lower. When the temperature exceeds 60 °C, the polarization of the interfaces and the PVA matrix increase rapidly, leading to a significant increase in dielectric constant. Moreover, the movement of the space charges becomes easier, resulting in a rapid increase in conductivity and loss. In summary, the MXene@ 5 wt% SiO2/PVA composite film has better dielectric properties below 60 °C compared with the MXene/PVA film, which is advantageous for practical applications.
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4. Conclusions In summary, this work presented a novel strategy for preparing PVA-based dielectric composite films with a relatively high dielectric constant and low loss. 2D nano sheets (Ti3C2Tx MXene) were used as the filler and SiO2 was coated onto the surface of MXene so as to generate interfacial barrier effects and suppress dielectric loss. MXene@ SiO2/PVA composite films show lower dielectric loss at low frequency compared to MXene/PVA composite film and neat PVA film. The MXene@SiO2/PVA composite film with 2.5 wt% MXene loading and 5 wt% SiO2 coating showed a dielectric constant of 27.2 and a low dielectric loss of only 0.057 at 100 Hz and room temperature. Moreover, this SiO2-coated composite film had stable dielectric properties over the temperature range from room temperature to 60 °C. Therefore, this work provides a promising way to fabricate polymerbased dielectric composites with excellent dielectric properties for practical applications in electronics. Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Acknowledgements This work was financially supported by Hunan Provincial Natural 6
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Enhanced dielectric properties of homogeneous Ti3C2Tx MXene@SiO2/ polyvinyl alcohol composite films Wei Wana,b,∗, Meizhen Taoa, Hailin Caoa, Yuqing Zhaoa, Junrong Luoc, Jian Yangb,∗∗, Tai Qiub a Hunan Engineering Laboratory for Preparation Technology of Polyvinyl Alcohol Fiber Materials, Huaihua Key Laboratory for Preparation of Ceramic Materials and Devices, College of Chemistry and Materials Engineering, Huaihua University, Huaihua, 418000, PR China b College of Materials Science and Engineering, Nanjing Tech University, Nanjing, 210009, PR China c College of Biological and Food Engineering, Huaihua University, Huaihua, 418000, PR China
A R T I C LE I N FO
A B S T R A C T
Keywords: Polyvinyl alcohol Ti3C2Tx MXene Composite films Dielectric properties SiO2
Exploring flexible dielectrics with high dielectric constant and low loss is pertinent for many applications in electronic devices. In the present work, a non-ferroelectric polymer, polyvinyl alcohol (PVA), was used to fabricate homogeneous dielectric composite films with a relatively high dielectric constant and a low dielectric loss by using 2D nano sheets (Ti3C2Tx MXene) as the filler. To limit dielectric loss, SiO2 was coated onto the surface of MXene to provide interfacial barrier effect and suppress dielectric loss. MXene@SiO2/PVA composite films showed lower dielectric losses at low frequencies (from 20 Hz to ~ 10 kHz) compared with MXene/PVA composite films. MXene@SiO2/PVA composite films with 2.5 wt% MXene loading and 5 wt% (with respect to MXene content) SiO2 coating had a dielectric constant of 27.2 (a 292.5% rise compared to neat PVA film) and a dielectric loss of only 0.057 (a 259.6% reduction compared to MXene/PVA composite film) at 100 Hz and room temperature (RT). In addition, this SiO2-coated composite film had stable dielectric properties (dielectric constant and loss change from 27.2 to 29.3 and 0.057 to 0.104, respectively) in the temperature range of RT to 60 °C. This work provides a promising way to fabricate PVA-based dielectric composites with excellent dielectric properties for practical applications in electronics.
1. Introduction
ceramics fillers, especially relaxor ferroelectrics (such as BaTiO3, Ba0.5Sr0.5TiO3, and Ba0.95Ca0.05Zr0.15Ti0.85O3) into polymer matrices can generate relatively high dielectric constant materials [10–12]. However, to obtain high dielectric constant, a large amount (typically ~40 vol%) of ceramics are typically necessary, which deteriorates the flexibility and always results in relatively high dielectric loss and reduced breakdown strength [6,13]. Incorporating conductive fillers into polymer matrices can obtain a high dielectric constant with lower loadings of fillers based on Maxwell-Wagner-Sillars (MWS) polarization [14,15]. Much effort has been devoted to develop conductive filler/ polymer composite dielectric materials [16–18]. In recent years, two-dimensional (2D) materials like graphene [19,20], MoS2 [21], BN [22], and Bi2Te3 [23] have drawn tremendous attention in the field of nano dielectric composites due to their unique chemical and physical properties. Recently, a new family of 2D materials named MXenes have gained wide attention owing to their unique structure and excellent properties such as good hydrophilicity, high
Flexible dielectrics with a high dielectric constant and low loss have drawn significant attention for potential applications in many electronic devices such as energy storage capacitors, flexible electronics, energy harvesters, and power systems [1–6]. Many ceramics (e.g., BaTiO3 and SrTiO3) possess excellent dielectric properties but are usually fabricated in a complex forming process combined with hightemperature sintering and suffer from delicate properties [7,8]. Even though most polymers have good flexibility, their ultra-low dielectric constant values (usually lower than 10) restricts the practical use of them in many electronics applications. For instance, the most successful, commercially used dielectric polymer, polypropylene, shows a dielectric constant of ~3, leading to a low energy density of ~2 Jcm−3 [9]. To prepare materials with good flexibility and excellent dielectric properties, incorporating ceramic or conductive fillers into polymers to construct binary composites are better alternatives. Introducing
∗
Corresponding author. Hunan Engineering Laboratory for Preparation Technology of Polyvinyl Alcohol Fiber Materials, Huaihua Key Laboratory for Preparation of Ceramic Materials and Devices, College of Chemistry and Materials Engineering, Huaihua University, Huaihua, 418000, PR China. ∗∗ Corresponding author. E-mail addresses: [email protected] (W. Wan), [email protected] (J. Yang). https://doi.org/10.1016/j.ceramint.2020.02.179 Received 28 January 2020; Received in revised form 17 February 2020; Accepted 17 February 2020 0272-8842/ © 2020 Published by Elsevier Ltd.
Please cite this article as: Wei Wan, et al., Ceramics International, https://doi.org/10.1016/j.ceramint.2020.02.179
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Fig. 1. Scheme of the preparation process of the MXene@SiO2/PVA composite film.
magnetic stirring for 30 min. Then 1 g Ti3AlC2 powder was slowly added to the mixture solution and magnetically stirred at 35 °C for 24 h, after which the mixture was washed through four cycles of ethyl alcohol addition, centrifugation (3500 rpm, 5 min), and decanting. The as-obtained pasted product was dissolved into deionized water (300 times the ratios of product to DI water) and exfoliated by ultrasonication for 1 h. Finally, the dilute suspension was centrifuged (3500 rpm, 1h), and the dark green supernatant was collected.
electrical conductivity, good chemical stability and environmental friendliness [24,25]. MXenes are 2D transition metal carbide/nitride materials derived from their precursor Mn+1AXn (or MAX, where M represents an early transition metal, A is a group A element, X is C or N element, and n = 1, 2 or 3) by selectively etching A atoms using hydrofluoric acid (or hydrochloric acid combined with fluorides), followed by ultrasonic delamination [26,27]. To date, Mxenes have been reported as promising materials for applications in batteries [28,29], hydrogen storage [30], chemical adsorption [31], energy conversion [32], and dielectrics. Tu et al. [33] first used MXene as the filler to construct Ti3C2Tx MXene/polyvinylidene fluoride (PVDF)-based polymer composites. The composites exhibited a dielectric constant of up to 105 and a loss of ~4 (at 1 kHz test frequency) near the percolation limit (~15.0 wt% MXene content). However, the dielectric loss caused by interface leakage conduction from conductive fillers can be relatively high. To reduce dielectric losses, multilayer structural MXene/ PVDF films [14], sandwich structural PVDF/MXene/2D BN nanocomposites [15], and gradient sandwich structural MXene/PVDF nanocomposites [9] were constructed and showed relatively good dielectric properties. To ensure consistency of the as-prepared composite films, the preparation process for constructing inhomogeneous structural films should be very precisely controlled. Moreover, it is well known that PVDF and most PVDF-based polymers are relaxor ferroelectrics, which will show strong piezoelectric effects when used in electronic devices [34]. In the present work, a non-ferroelectric and biodegradable polymer, polyvinyl alcohol (PVA), was used as the matrix, and homogeneous MXene@SiO2/polyvinyl alcohol (PVA) composite films were prepared by using 2D Ti3C2Tx MXene nano sheets as the filler and SiO2 as the dielectric loss inhibitor. The composite films exhibited desirable dielectric properties, namely, a dielectric constant of 27.2 and a low dielectric loss of only 0.057 at 100 Hz. The results of this work provide a facile route to create promising high performance flexible dielectric materials for practical applications in advanced electronic components.
2.2. Ti3C2Tx MXene@SiO2/PVA composite film fabrication To decrease the dielectric loss of Ti3C2Tx MXene/PVA composite films, the surface of Ti3C2Tx MXene was coated with SiO2. 150 mL of Ti3C2Tx MXene suspension was dispersed into 150 mL ethyl alcohol and magnetically stirred for 30 min. Under continuous magnetic stirring, 6 mL of 28% ammonium hydroxide solution and 0.026 g (for 5 wt% SiO2 coating, with respect to MXene content) or 0.052 g (for 10 wt% SiO2 coating) of TEOS (Sinopharm chemical reagent co. LTD) were consecutively added into the above mixed suspension and further stirred for 2 h. Finally, the resultant suspension was filtered and washed with deionized water until the pH of the filtrate approached seven with subsequent dilution of the filtered product into 150 mL deionized water. Ti3C2Tx MXene@SiO2/PVA composite films were fabricated by a solution coating method. First, 10 mL of MXene@SiO2 suspension and a certain amount of PVA (PVA-1799, Aladdin Industrial Corporation, 99.9%) solution (10 wt% concentration) were added into a nylon jar and ball-milled for 30 min at 180 rpm. The mixed suspension was then deaerated by ultrasonication. Finally, the mixed suspension was coated onto a polyethylene plate with subsequent oven-drying at 40 °C. 2.3. Characterization and measurements X-ray diffraction (XRD) results of the samples were identified by an X-ray diffractometer (Rigaku Industrial Corp., Japan, CuKα, 40 kV, 100 mA). Microstructures were observed under a field emission scanning electron microscope (FE-SEM, Zeiss Sigma HD, Germany) coupled with an energy-dispersive X-ray spectrometer (EDS, X-MaxN, Oxford Instruments, UK). For dielectric measurements, MXene@SiO2/PVA composite films were coated with a conductive platinum thin film electrode by employing ion sputtering coating equipment (GVC-1200, Beijing Gevee-Tech Co. Ltd., China, sputtering parameters: 10 mA, 100 s and two times). Fig. 1 shows the scheme of the preparation
2. Experimental 2.1. Ti3C2Tx MXene suspension preparation Ti3AlC2 (~38 μm) powders were used as the precursor for MXene synthesis. The Ti3C2Tx suspension was prepared according to the modified Michael's method [27]. First, 0.666 g LiF (Aladdin Industrial Corporation, 99.9%) was dissolved into 10 mL 6 M HCl solution with 2
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Fig. 2. (a) FE-SEM micrograph and (b) XRD pattern of exfoliated Ti3C2Tx MXene after coating with SiO2; the inset in (b) show the enlarged view of Ti3C2Tx MXene after coating with SiO2; elemental mapping of (c) Ti , (d) C and (e) Si for image (a).
Fig. 3. (a) FE-SEM micrograph of the surface of the MXene@SiO2/PVA composite film with 2.5 wt% MXene loading; (b) photograph of the MXene@SiO2/PVA composite film and (c) plates after coating with Pt electrode.
process of the MXene@SiO2/PVA composite film. Dielectric properties were measured using a Keysight E4990A precision impedance analyzer over a frequency range from 20 Hz to 1 MHz with a parallel equivalent circuit.
coating with SiO2. It is evident that Ti3C2Tx has an open stacked sheet structure (typical structure for MXene). The diffraction peak at 2θ ≈ 9° (Fig. 2b) corresponds to the structure of Ti3C2Tx MXene and no diffraction peaks for SiO2 were observed. The very weak diffuse scattering peak at 2θ ≈ 20° (shown by the red dashed line) indicates the SiO2 coating is amorphous as SiO2 obtained by the precipitation method usually possesses a non-crystalline structure [35,36]. From the inset in Fig. 2b, it is known that SiO2 coating prepared by the precipitation
3. Results and discussion Fig. 2a shows the microstructure of the exfoliated Ti3C2Tx after 3
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black glossy surface as Ti3C2Tx MXene is black. After coating with the Pt electrode via surface sputtering of the composite film, the Pt thin film shows gray and white colors with metallic luster, indicating sufficient coating of the Pt electrode onto the composite film surface. Fig. 4a presents the dielectric constant under 100 Hz test frequency of MXene/PVA composite films without and with SiO2 coating as a function of MXene loading. The dielectric constant increases with MXene content for MXene/PVA composite films and MXene@SiO2/PVA composite films, owing to the enhanced interfacial polarization (MWS polarization) between the conductive MXene nano sheets and insulating PVA matrix or between MXene nano sheets and insulating SiO2 coating. Increases of dielectric constant from 28.5 to 151.3, 27.2 to 111.4, and 17.7 to 84.1 for the MXene/PVA composite film, MXene@5 wt% SiO2/ PVA composite film, and MXene@10 wt% SiO2/PVA composite film, respectively, were observed as MXene loading increased from 2.5 wt% to 10 wt%. Compared with MXene/PVA composite films, MXene@ SiO2/PVA composite films exhibit slightly lower dielectric constants, which decrease with increased SiO2 loading as SiO2 has intrinsically low polarity and a low dielectric constant of ~3.5 [38]. Fig. 4b shows the dielectric loss of the as-obtained samples under 100 Hz test frequency. For MXene/PVA composite films, increases MXene content can enhance MWS polarization, but at the same time, the increased conductivity and interfacial relaxation results in increased dielectric loss [39]. However, it can be seen that MXene@SiO2/ PVA composite films have lower dielectric loss as SiO2 insulating coating isolates the high loss of conductive MXene nano sheets due to the interfacial barrier effect. The insulation of SiO2 coating layer can restrict the mobility of interfacial charge carriers and prevent the current percolation between MXene nano sheets and PVA matrix [40]. It is clearly that the suppression effect on dielectric loss is very remarkable at 2.5 wt% MXene loading. When the MXene loading exceeds 2.5 wt%, the dielectric loss of MXene@SiO2/PVA composite films increases gradually with MXene loading, which could be caused by increased interfacial relaxation, space charges or imperfections (such as structural defects, voids and cracks) at higher MXene content [37,41]. Compared with the MXene/PVA composite film with 2.5 wt% MXene loading, the MXene@SiO2/PVA composite film with 2.5 wt% MXene loading and 5 wt% nano SiO2 coating shows a dielectric loss of only 0.057 at 100 Hz, which is a 259.6% reduction, while the dielectric constant slightly decreases from 28.5 to 27.2. Also, it should be noted that the MXene@SiO2/PVA composite film with 2.5 wt% MXene loading and 10 wt% nano SiO2 coating also has desirable dielectric properties, namely an ultra-low dielectric loss of only 0.017 and a relatively high dielectric constant of 17.7 at 100 Hz. Table 1 provides a comparison of dielectric properties of MXene/polymer composites with different polymer matrices and structures. It is known that this homogeneous MXene@SiO2/PVA composite film has comparable and even superior dielectric properties to other inhomogeneous MXene/polymer dielectric composites previously reported [9,15]. Fig. 5 shows the dielectric constant, loss, and AC conductivity of the as-prepared films as a function of frequency. All films exhibit gradual decreases in dielectric constant with an increase in frequency. The declining trend is different at lower frequency (from 20 Hz to 1 kHz) for all films. Pure PVA film has the slowest change of dielectric constant among all films. Once MXene nanosheets were filled in the PVA matrix,
Fig. 4. (a) Dielectric constant and (b) loss vs MXene loading for MXene/PVA composite films without and with SiO2 coating under 100 Hz frequency.
method has layer structure with nano scale dimension, which is consistent with the reported literatures [36,37]. Elemental mapping of C, Ti, and Si (Fig. 2c-e) suggests that SiO2 was coated onto the surfaces of Ti3C2Tx MXene with a relatively homogeneous distribution. Fig. 3a is the FE-SEM micrograph of the surface of the Ti3C2Tx MXene@SiO2/PVA composite film with 2.5 wt% Ti3C2Tx MXene loading. The composite film shows a relatively dense structure without stomatal defects, and MXene@SiO2 nanosheets were dispersed into the PVA matrix without obvious MXene@SiO2 nanosheets agglomeration, revealing good compatibility between the PVA polymer matrix and MXene@SiO2 nanosheets. Fig. 3b and c shows the photographs of the as-prepared Ti3C2Tx MXene@SiO2/PVA composite film and testing samples after coating with the Pt electrode. The composite film has a smooth and
Table 1 Comparison of dielectric properties of MXene/polymer composites with different polymer matrix and structure. Types of MXene/polymer composites MXene/P[VDF-TrFE-CFE] Sandwich structural PVDF/MXene/2D BN nanocomposites Gradient sandwich structural MXene/PVDF composites Homogeneous MXene/PVA composites Homogeneous MXene@ 5 wt% SiO2/PVA composites Homogeneous MXene@ 10 wt% SiO2/PVA composites
Dielectric constant 5
~10 ~18 26 28.5 27.2 17.7
4
Dielectric loss
Test frequency
~4 ~0.029 0.04 0.148 0.057 0.017
1 kHz 1 kHZ 100 Hz 100 Hz 100 Hz 100 HZ
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Fig. 5. (a) Dielectric constant, (b) loss and (c) AC conductivity vs frequency for the as-prepared films.
Fig. 6. (a) Dielectric constant , (b) loss and (c) AC conductivity vs temperature for the as-prepared films under 100 Hz frequency.
the dielectric constant decrease more rapidly at low frequency, suggesting the presence of strong MWS polarization between the MXene nano sheets and PVA matrix at low frequency. When SiO2 was coated onto MXene nano sheets, the declining rate of dielectric constant decelerates, which may be attributed to the weakening MWS polarization caused by the insulating SiO2 coating. MXene@SiO2/PVA composite
films with 2.5 wt% MXene loading and 5 wt% SiO2 coating even showed a higher dielectric constant than that of the MXene/PVA composite film over a frequency range from 1 kHz to 100 kHz. At the frequency range from 20 Hz to about 100 kHz, MXene@SiO2/ PVA composite films show lower dielectric loss than MXene/PVA composite films, indicating effective suppression of loss owing to the 5
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Science Foundation of China (No.: 2018JJ3411), Scientific Research Fund of Hunan Provincial Education Department (No.: 18B493) and Open Fund of Hunan Engineering Laboratory for Preparation Technology of Polyvinyl Alcohol Fiber Materials (No.: HGI201802).
interfacial barrier effect caused by the insulating SiO2 coating. MXene@ SiO2/PVA composite films even show lower dielectric loss than neat PVA film at frequencies below 500 Hz, suggesting effective suppression of conductive paths in MXene@SiO2/PVA composite films. Moreover, there is no observed dielectric loss at the frequency below 70 Hz for the MXene@SiO2/PVA composite films with 2.5 wt% MXene loading and 10 wt% SiO2 coating. This is generally because the dielectric loss value is too small to be within the measuring accuracy range of the impedance analyzer [42]. This provides further indication of effective dielectric loss suppression at low frequencies by coating SiO2 onto the MXene surface. The increase in AC conductivity with frequency is a nearly linear trend for all four films, indicating the high insulating ability of these films [43]. The AC conductivity increases after Ti3C2Tx MXene nano sheets were incorporated into the polymer matrix, attributable to the highly conductive nature of MXene [27]. When SiO2 was coated onto the surface of MXene, the AC conductivity decreases due to the highly insulative nature of SiO2. The low AC conductivity of the MXene@SiO2/ PVA composite film indicates the effective interfacial barrier effect of the SiO2 coating. The trends in AC conductivity fact correspond to the dielectric loss results. Fig. 6 shows the temperature dependence of the dielectric constant, loss, and AC conductivity of the MXene/PVA film and the MXene@ 5 wt % SiO2/PVA composite film. When the temperature is below 60 °C, both films show a stable dielectric constant and conductivity and a slow increase in dielectric loss, which suggests that the polarization of the interface and the PVA matrix is stable in this temperature range and the movement of the space charges is difficult. In addition, it is known that in the temperature range from RT to 60 °C, the MXene@ 5 wt% SiO2/ PVA composite film shows a similar dielectric constant to the MXene/ PVA film, but the former's dielectric loss is much lower. When the temperature exceeds 60 °C, the polarization of the interfaces and the PVA matrix increase rapidly, leading to a significant increase in dielectric constant. Moreover, the movement of the space charges becomes easier, resulting in a rapid increase in conductivity and loss. In summary, the MXene@ 5 wt% SiO2/PVA composite film has better dielectric properties below 60 °C compared with the MXene/PVA film, which is advantageous for practical applications.
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4. Conclusions In summary, this work presented a novel strategy for preparing PVA-based dielectric composite films with a relatively high dielectric constant and low loss. 2D nano sheets (Ti3C2Tx MXene) were used as the filler and SiO2 was coated onto the surface of MXene so as to generate interfacial barrier effects and suppress dielectric loss. MXene@ SiO2/PVA composite films show lower dielectric loss at low frequency compared to MXene/PVA composite film and neat PVA film. The MXene@SiO2/PVA composite film with 2.5 wt% MXene loading and 5 wt% SiO2 coating showed a dielectric constant of 27.2 and a low dielectric loss of only 0.057 at 100 Hz and room temperature. Moreover, this SiO2-coated composite film had stable dielectric properties over the temperature range from room temperature to 60 °C. Therefore, this work provides a promising way to fabricate polymerbased dielectric composites with excellent dielectric properties for practical applications in electronics. Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Acknowledgements This work was financially supported by Hunan Provincial Natural 6
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