- Email: [email protected]
Poly(vinylidene fluoride) composites with an ultra high dielectric constant: A comparative study
Journal Pre-proof Flexible La1.5Sr0.5NiO4/Poly(vinylidene fluoride) composites with an ultra high dielectric constant: A comparative study Keerati Meeporn, Prasit Thongbai PII:
S1359-8368(19)32092-X
DOI:
https://doi.org/10.1016/j.compositesb.2019.107738
Reference:
JCOMB 107738
To appear in:
Composites Part B
Received Date: 10 May 2019 Revised Date:
7 September 2019
Accepted Date: 30 December 2019
Please cite this article as: Meeporn K, Thongbai P, Flexible La1.5Sr0.5NiO4/Poly(vinylidene fluoride) composites with an ultra high dielectric constant: A comparative study, Composites Part B (2020), doi: https://doi.org/10.1016/j.compositesb.2019.107738. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2019 Published by Elsevier Ltd.
1
Flexible La1.5Sr0.5NiO4/Poly(vinylidene fluoride) Composites with an Ultra High Dielectric Constant: A Comparative Study
Keerati Meeporn a, Prasit Thongbai b,c,*
a
Materials Science and Nanotechnology Program, Faculty of Science, Khon Kaen University, Khon Kaen 40002, Thailand b
Department of Physics, Faculty of Science, Khon Kaen University, Khon Kaen 40002, Thailand
c
Institute of Nanomaterials Research and Innovation for Energy (IN–RIE),
NANOTEC–KKU RNN on Nanomaterials Research and Innovation for Energy, Khon Kaen University, Khon Kaen,40002 , Thailand
* Corresponding author’s email: [email protected] (P. Thongbai)
2 ABSTRACT Poly(vinylidene fluoride) (PVDF)–based polymer composites filled with high– permittivity La1.5Sr0.5NiO4 (LSNO) particles have been fabricated to produce a composite material with excellent dielectric performance and good mechanical flexibility. Micro–sized (µLSNO) and nano–sized LSNO (nLSNO) particles are used separately as fillers. The dielectric properties of the composites are determined by their particle sizes, resultant interfacial area, and the interparticle distances between the LSNO particles. The percolation threshold of the nLSNO/PVDF composite is lower than that of the µLSNO/PVDF composite. A greatly enhanced dielectric response with a high ε′ ≈ 565.9 at 1 kHz and room temperature and an effectively suppressed low loss tangent (tanδ ≈ 0.28) are obtained in the nLSNO/PVDF composite with a filler volume fraction of 30 vol %. The greatly enhanced ε′ value of nLSNO/PVDF can be explained by a combination of the interfacial polarization at the semiconducting nLSNO–insulating PVDF interfaces, an intrinsically giant ε′ response in nLSNO particles, and shorter interparticle distances between nLSNO particles in the PVDF matrix, while the retained low tanδ value is primarily attributed to the formation of a micro–capacitor microstructure. Keywords: Dielectric polymer composites; Giant dielectric properties; Colossal permittivity; Interparticle distance; Micro−capacitor.
3 1.
Introduction Poly(vinylidene fluoride) (PVDF)–based polymer composites with a high
dielectric constant (ε′) are of great interest because of their great potential in modern electronic technology, including microelectromechanical devices, charge storage capacitors,
nano–energy
harvesters,
flexible
ferroelectric
memories,
and
portable/wearable sensors [1-4]. Polymer–based composites offer superior chemical resistance, ease of processing, and low cost. Moreover, they also provide adjustable electrical, mechanical, and thermal properties. To date, many innovative approaches have been focused on the enhancement of the dielectric properties of polymer–based composites through the fabrication of hierarchical structured [5, 6], cross–linked polymer treatments [7, 8], and surface–modification of inorganic/organic fillers [6, 9, 10]. However, the dielectric permittivity (ε′) values of some resultant composite materials are still low, which is one of the important parameters that limits their practical applications, especially in charge-storage capacitor applications. In addition, polymer–based composites can possess a high ε′ value of ≥100 when the volume fraction of a filler (f) is ≥ 50 vol % [10, 11]. Nevertheless, the relatively high–f results in poor mechanical performance, low flexibility, and high weight. These conditions are undesirable for modern electronic applications. Importantly, the excellent dielectric properties and the good mechanical performance of polymer–based composites are determined by both fillers and polymer matrices. Consequently, the identification of suitable fillers and polymer matrices is critical for these purposes. A great number of polymers have been used as matrices, including polystyrene (PS), polyimide (PI), poly(methyl methacrylate) (PMMA), polydimethylsiloxane (PDMS), polypropylene (PP), and epoxy [1, 6]. However, their low ε′ values (2–5) limit the enhancement of the dielectric properties of the resultant
4 composites. To resolve this issue, poly(vinylidene fluoride) (PVDF), as well as its copolymers with the highest ε′ (≥10) and high breakdown strength, have been considered as the most promising polymer matrix materials for dielectric applications [1, 6, 12]. Greatly increased dielectric response of polymers can also be accomplished by using conductive nanomaterials to develop high–k polymer nanocomposites. When f value of a conductive filler is lower than that of the percolation threshold (f < fc), both ε′ and loss tangent (tanδ) continuously increase with increasing f. For example, the ε′ values at 103 Hz of Au/PVDF and Ag/PVDF nanocomposites increased from ∼10 to ∼21 and to ∼42.7, respectively, when fAu = 0 → 0.0125 and fAg = 0 → 0.12 [13, 14]. However, the rapid increase in ε′ is usually observed when f is approached the percolation threshold (f → fc) [15]. Unfortunately, the dramatically increased ε′ value is usually accompanied by a large increase in tanδ. For example, the ε′ value of the Ni/PVDF composites strongly increased to ∼400 when fNi→ fc, while tanδ greatly increased (> 0.15) [15]. Several materials with giant dielectric ε′ values have been reported [16-21]. Among them, La2-xSxNiO4 (LSNO) ceramics offer superior dielectric performance with an extremely large ε′ (105–106), and an ε′ of approximately 104 can be observed even in a high frequency region (109 Hz) [22, 23]. It was demonstrated that the giant dielectric response in LSNO ceramics is produced by an intrinsic property, which is attributed to polaron hopping between Ni2+ and Ni3+. The giant ε′ of a filler material that resulted from an intrinsic effect is more beneficial to a composite approach than the giant ε′ that resulted from the extrinsic effect at grain boundaries. Therefore, the LSNO ceramic is a promising effective filler for polymer–based composite systems.
5 In this article, we report a promising composite system with a large ε′, low tanδ, and good mechanical flexibility by a simple combination of a PVDF matrix and an LSNO filler. The dielectric properties of the LSNO/PVDF composites were investigated. The effect of LSNO particle sizes, interfacial area and interparticle distances on the dielectric properties of PVDF–based composites filled with two kinds of LSNO of average sizes as a function of the volume fraction of filler (f) are also discussed. 2.
Experimental details 2.1 Preparation of two kinds of La1.5Sr0.5NiO4 (LSNO) ceramic powders The preparation of micro–sized LSNO (µLSNO) powder was conducted using
a solid–state reaction method. Analar-grade La2O3 (Sigma–Aldrich), SrCO3 (Sigma– Aldrich), and NiO (Sigma–Aldrich) were used as starting materials in stoichiometric amounts and ball–milled in ethanol for 24 h. The resulting ball–milled mixture was dried and calcined at 1200 °C for 12 h to obtain a pure phase of µLSNO powder. Nano–sized LSNO (nLSNO) powder was prepared by a chemical combustion method following a process previously reported [24, 25]. 2.2 Fabrication of µLSNO/PVDF and nLSNO/PVDF composites The PVDF–based composites using LSNO powders with different sizes of
µLSNO and nLSNO fillers were fabricated using a solution processing method [24]. First, a PVDF matrix solution was produced by dissolving the PVDF in N,N dimethylformamide (DMF) (RCI Labscan). Second, two kinds of LSNO fillers were added separately to the PVDF solution with stirring for 4 h at 60 oC to form
µLSNO/PVDF and nLSNO/PVDF composite systems and to prevent agglomerates until the highly viscous solution of polymer composites was obtained. After that, the
6 composite solutions were further sonicated in order to enhance the dispersion of ceramic fillers. Usually, the highest dipolar moment and largest dielectric response were obtained in the β–PVDF phase [26], which can be achieved by casting the solution of polar β–PVDF solvents. It was clearly shown that the processing conditions have an effect on the volume fraction of β–PVDF phase [27, 28]. To obtain the large fraction of β–PVDF phase, the temperature should not be higher than 70 oC [29]. On the other hand, highly porous PVDF films would be formed when the temperature was lower than 70 oC [3]. Thus, in this work, the composite solutions were dried at ≈70 °C for 6 h to obtain the β–PVDF phase and to minimize the pores as much as possible. The β–PVDF phase can also be induced by uniaxial stretching of the α–PVDF phase [30]. Thus, the uniaxial compressive stress might also induce the β–PVDF phase. Finally, to obtain dense films with designed β–PVDF structure, the dried composite were uniaxial pressed at ≈10 MPa at 200 °C for 0.5 h. 2.3 Characterization techniques The microstructures of µLSNO and nLSNO powders were examined by using transmission electron microscopy (TEM) (ZEISS, EM902). The morphologies and phase compositions of the two kinds of LSNO powders and PVDF–based composites were characterized by using scanning electron microscopy (SEM) (SEC, SNE4500M) and X–ray diffraction (XRD) (PANalytical, EMPYREAN). Focused ion beam (FIB) technique and field–emission scanning electron microscopy (FESEM) (FEI, Helios NanoLab G3 CX) were used to reveal the dispersion of fillers in the PVDF matrix and to map the distribution of all elements in the inner opened–surface. The outer surfaces of the composite samples were removed by using focused ion beams (FIBs). The dispersion of fillers was also characterized using an optical microscope (OLYMPUS,
7 BX51). Phase formations of PVDF polymer were identified using a Fourier transformed infrared spectroscopy (FTIR) (Bruker, TENSOR27). The thermal properties of the PVDF–based composites were evaluated by using thermo– gravimetric analysis (TGA) (Pyris Diamond, Perkin Elmer). 2.4 Dielectric and mechanical measurements The electrodes on the composite samples were painted with silver paste to perform the dielectric measurements. A KEYSIGHT E4990A Impedance Analyzer was used to investigate the dielectric properties in the frequency and temperature ranges of 102–106 Hz and -60–100 oC. Tensile test was carried out on a universal testing machine (UTM) (INTSTRON, 5567A) with stretching rate of 5 mm/min. The samples were cut into rectangle shape with size of 40×5 mm2.
3.
Results and discussion Previously, in studies on variations in microstructures with supplementing
conducting and semiconducting ceramic fillers that were reported, the dielectric properties of the polymer-based composites could be affected by the dispersion state, the geometric shape, size and interfacial area. Fig. 1(a)–(d) reveal the morphologies of the semiconducting LSNO ceramic fillers with different particle sizes for the µLSNO and nLSNO ceramics using TEM and SEM techniques. As shown in Fig. 1(a) and 1(b), the average sizes of the µLSNO and the nLSNO particles are ∼1.89 µm and ∼180 nm, respectively. Hard agglomeration of nLSNO particles bonding together with a solid bridge is observed in the TEM image. Similarly, the SEM images of these two LSNO powders are shown in Fig. 1(c−d), respectively. A small agglomeration of nLSNO particles can be seen. For the effects of particles sizes, nanosized particles have a relatively larger surface area (interfacial area) when compared to that of the
8 larger particles with the same volume fraction [31]. Therefore, it is expected that the semiconducting nLSNO with a smaller particle size that has been used as a filler can enhance interfacial polarization in nLSNO/PVDF composites. Figs. 2(a) and 2(b) show SEM images of the fractured cross–sections of the PVDF polymer and nLSNO/PVDF composites with f = 0.1, respectively. The pure PVDF film prepared via casting the PVDF solution in DMF and hot–pressing at 200 °C for 0.5 h formed a tight surface with a continuous phase. The dispersion states of the nLSNO/PVDF composite with f = 0.10 are shown in Fig. 2(b). The fractured surfaces of the composites show a random distribution of the nLSNO filler in the PVDF matrix. Interestingly, as shown in the inset of (b), the nLSNO/PVDF composite with f = 0.30 still exhibits good flexibility. This can be attributed to the beneficial effect of a good dispersion of nLSNO particles in the PVDF matrix. To reveal the dispersion states of nLSNO and µLSNO particles in the overall large area, the nLSNO/PVDF and µLSNO/PVDF composite films with f = 0.3 were characterized using an optical microscope. Obviously, as shown in Figs. 2(c) and 2(d), the dispersion of the nLSNO particles in the PVDF polymer is better than that of the
µLSNO particles. Furthermore, the dispersion of the fillers in the deep inner part of the composite films was also analyzed. The outer–surface layer of the composite films was removed by FIBs. Figs. 2(e) and 2(f) display FESEM micrograph of the nLSNO/PVDF and µLSNO/PVDF composite films with f = 0.3, showing good distribution of the nLSNO particles in the PVDF matrix. Fig. 3 shows the FESEM image and elemental mapping for all elements (i.e., C, F, La, Sr, Ni, and O) of the nLSNO/PVDF composite with f = 0.3. All elements are homogeneously dispersed. The crystalline structures of the nLSNO filler, the pure PVDF film, and the PVDF–based composites with different volume fractions of nLSNO are shown in Fig.
9 4(a). The XRD pattern of the nLSNO powder shows that the LSNO phase with a tetragonal structure can be observed without any impurity phases. The crystal planes are well–indexed to the (101), (004), (103), (110), (112), (006), (114), (200), (211), (116), (107), (204), (213), (215), (218) and (220) planes, which can be compared to the standard La1.5Sr0.5NiO4 pattern (JCPDS No. 32–1241). The XRD pattern of a pure PVDF film demonstrates the mixing of α, β and γ phases of PVDF [3]. The diffraction peaks at 2θ = 18.50° and 20.25° are assigned to the (020) plane of the γ phase and to the (110) and (200) planes of the β phase, respectively [3]. The diffraction peaks at 2θ = 35.97° and 39.02° are assigned to the (200) and (002) planes, respectively, of the α phase [32]. For the composites, both LSNO and PVDF phases appear in the XRD patterns of the nLSNO/PVDF composites, and no impurity phase is observed. However, due to the strong intensity of the XRD diffraction peaks of the LSNO phase, the XRD peaks for the PVDF phase are not clearly seen in the composites with a high–volume fraction of nLSNO filler. The FTIR spectra of the pure PVDF film and PVDF−based composites demonstrate the combination of α–, β– and γ–phases of PVDF, as shown in Fig. 4(b). The absorbance bands at 766, 976 and 1179 cm-1 are assigned to the α phase [3]. The β–phase characteristics are observed at 840 and 878 cm-1 [3]. The absorbance bands at 1402 and 1970 cm-1 are related to the in–plane bending or scissoring of CH2 and the bending of C−C−C, respectively [33]. To quantify the fraction of the β phase, it is assumed that only the α– and β–phases exist and FTIR follows the Lambert−Beer law. The relative fraction of the β phase (
=
/
) can be calculated using Eq. (1) [34]:
,
(1)
10 where
and
are the measured absorbance at 766 and 840 cm-1, corresponding to
the α and β phases.
and
are the absorption coefficients at the respective
wavenumber, the values of which are 6.1 x 104 and 7.7 x 104 cm2mol-1, respectively. The
of the nLSNO/PVDF composite films with f = 0 (pure PVDF), 0.05, 0.1,
0.2, 0.3, and 0.4 are 34, 37, 45, 50, 64, and 59 %, while
for the µLSNO/PVDF
composite films are 34, 35, 41, 41, 40, and 36 %, respectively. Obviously, values of all composite films are larger than that of a pure PVDF film. Furthermore, for both composite systems,
increases with increasing f from 0 to 0.2, and then
slightly decreases when f = 0.3. Notably,
of the nLSNO/PVDF composites is
larger than that of the µLSNO/PVDF composites, indicating the advantages of nanoparticles. Fig. 4(c) shows the thermal stabilities of the pure PVDF film and the nLSNO/PVDF composites with various volume fractions of nLSNO. The TGA curves for the PVDF film and composites show a single–stage decomposition of PVDF. The temperature at 5 % mass–loss increases with f, which are 432, 464, 467, 470, 471 and 473 °C for the composites with f = 0, 0.05, 0.10, 0.20, 0.30 and 0.40, respectively. Consequently, the nLSNO particles enhance the thermal stability of the PVDF polymer. The origin of the dielectric response in PVDF–based composites using µLSNO and nLSNO particles as fillers is explained not only based on the interfacial area in the LSNO/PVDF composites but also by the interparticle distances between the LSNO particles. Fig. 5 shows the average interparticle distances (d) between the filler particles used for the µLSNO/PVDF and nLSNO/PVDF composites. The d value is calculated by using d = r[(4π/f)1/3-2)] [35], where f and r are the volume fraction and the average radius of LSNO particles, respectively. The d value of the µLSNO/PVDF
11 and nLSNO/PVDF composites decreases from ∼2.24 → 0.18 µm, and from ∼213 → 16.9 nm with increasing f from 0.05 → 0.4, respectively. Generally, at a given particular LSNO loading, a decrease in the particle size of a LSNO filler causes an increase in the number of particles per unit volume in the PVDF–based composites. The interparticle distance between the adjacent nLSNO particles, which is separated by a thin layer of PVDF polymer, is therefore shorter than that of the µLSNO particles. This is expected to have a great influence on the dielectric properties of the composites [25, 31]. The dielectric constants (ε′) and loss tangents (tanδ) at 1 kHz of the
µLSNO/PVDF and nLSNO/PVDF composites as a function of the filler volume fraction are shown in Fig. 6(a) and 6(b). The ε′ values of the nLSNO/PVDF composites with f = 0.05, 0.1, 0.2, 0.3, and 0.4 are 27.6, 40.3, 98.4, 565.9, and 2353.1, while the ε′ values for the µLSNO/PVDF composites are 15.4, 25.1, 28.7, 56.0, and 85.7, respectively. Obviously, for the same filler loading, the nLSNO/LSNO composite exhibits a higher ε′ value than that of the µLSNO/PVDF composite. As depicted in the inset of Fig. 2(b) and Fig. 6, good flexibility and high ε′ ≈ 565.9 and low tanδ ≈ 0.28 (at 1 kHz and RT) are achieved in the nLSNO/PVDF composite with f = 0.3. This enhanced ε′ value is higher than that of the pure PVDF matrix by ∼57−fold. Moreover, it should be noted that the ε′ for the nLSNO/PVDF composites is greater than those of other PVDF–based two and three−phase composites that have even lower filler loadings, such as BaTiO3/PVDF [36], CaCu3Ti4O12/PVDF [37], Ba(Fe0.5Nb0.5)O3/PVDF [38], Na0.5Bi0.5Cu3Ti4O12/PVDF [39], Ag/BaTiO3/PVDF [40] and SiO2/CaCu3Ti4O12/PVDF [41]. Therefore, the flexible nLSNO/PVDF composites with such dielectric performance are potential composite materials for dielectric charge storage capacitor and electromagnetic wave absorption applications.
12 Distinct dielectric behavior is observed in these two composite systems. Rapid changes in the ε′ and tanδ at approximately f = 0.3 are observed in the nLSNO/PVDF composites, which may suggest the formation of a percolation network in this percolative composite system [15, 42]. Alternatively, this behavior is not observed in the µLSNO/PVDF composites. The ε′ and tanδ values for the µLSNO/PVDF composites continuously increase over the entire filler loading range studied (f = 0.05 – 0.4). This indicates that the formation of a percolation network in the
µLSNO/PVDF composites may occur at f ≥ 0.4 [42]. Thus, the percolation threshold (fc) of the µLSNO/PVDF composites is higher than that of the nLSNO/PVDF composites.
This
result
confirms
that
the
percolation
threshold
of
the
high−permittivity semiconductive oxide/PVDF composite system is dependent on the particle size of filler used, as reported in metal particle/PVDF composite systems [14, 15, 42]. Generally, the highest ε′ value and largest dipolar moment of a PVDF polymer were achieved in the β–PVDF phase [3, 26]. Thus, the greatly enhanced ε′ value of the nLSNO/PVDF composite films may be correlated to the variation in F(β) of the PVDF matrix. The correlation between ε′ and F(β) is illustrated in the inset of Fig. 6(b). Obviously, the ε′ value of the nLSNO/PVDF composites with f = 0 – 0.3 increases with increasing F(β). This result clearly indicates that the relative fraction of β–PVDF phase is one of the most important factors that can cause an increase in the dielectric response of PVDF–based composites. When f = 0.4, ε′ largely increases, while F(β) decreased slightly. This is likely due to the dominant effect of Ohmic conduction associated with the percolation and Maxwell–Wagner–Sillars (MWS) effects [43-45]. This effect is usually occurred in several kinds of heterogeneous
13 materials, in which free charges trapped at the interfaces between large–ε′ oxide particles/clusters and an insulating matrix. The dielectric response behaviors in the nLSNO/PVDF composites are further described by considering the frequency−dependent behavior of the dielectric properties. The frequency dependence of the ε′ and tanδ at RT for the nLSNO/PVDF composites with f = 0.05 − 0.4 is shown in Fig. 7. The overall result shows that the ε′ and tanδ values increase with increased loading of nLSNO over the measured frequency range. The ε′ of the nLSNO/PVDF composites with f ≤ 0.2 is independent of the frequency from 102 to 105 Hz, while the ε′ values of nLSNO particles and PVDF polymer are also independent of the frequency in this range. This clearly indicates that the enhanced dielectric response of the composites with f ≤ 0.2 is primarily contributed from the intrinsic ε′ values of nLSNO particles and PVDF polymer. Although the interfacial effect can partially contribute to the dielectric response, it is not dominant in the composites with f ≤ 0.2, especially for the
µLSNO/PVDF composites. This causes a continuous increase in the ε′ for composites with f = 0.05 → 0.2, as displayed in the inset of Fig. 6(a). By increasing the f to 0.3, ε′ of the composite changes with increasing frequency and the step–like decrease in ε′ appears at approximately 104 Hz, indicating the dominant effect of the extrinsic effect, i.e., the interfacial polarization [25]. This assumption is supported by the observed rapid increase in ε′, as shown in Fig. 6(a). It is worth noting that the tanδ value for the nLSNO/PVDF composite with f = 0.3 is suppressed to lower than 0.5 over the measured frequency range. It is important to note that although the suppressed tanδ values are lower than 0.5 over a wide frequency range, the obtained tanδ values are still high for most of the applications of
14 a dielectric material. Further research work is needed to reduce tanδ into the level of applications. AC conductivity (σ) as a function of frequency and volume fraction of nLSNO for the nLSNO/PVDF composites is shown in Fig. 8(a–b), respectively. When f ≤ 0.3, the frequency dependence of σ shows a similar trend with an increasing level of f from 0 to 0.3. The low–frequency σ values of these composites (f = 0.05 – 0.3) are suppressed. Thus, the significantly enhanced ε′ and the suppressed low tanδ value of the nLSNO/PVDF composite with f = 0.3 should be due to the formation of micro– capacitors in the composite [25, 46]. Inset of Fig. 8(b), the conductivity of the polymer composites that causes an increase in the dielectric loss (ε″) or the imaginary part of complex dielectric permittivity (ε*), i.e., ε″ = tanδ×ε′, can also be caused by the MWS process, i.e., accumulated free charges at internal interface boundaries or at the interface of sample–electrode [44]. Therefore, incorporation of nLSNO in the PVDF polymer can cause increases in both of the ac conductivity and the intensity of the MWS polarization, which would be increasingly predominant as the frequency decreased [43]. This is associated with the contribution of dc conduction to the low– frequency dielectric response. The existence of nLSNO particles can also promote the formation of β–PVDF phase, resulting in an enhanced ε′ value. By further increasing the f to 0.4, ε′ becomes independent of the frequency in the measured frequency range as observed in the composites with f ≤ 0.2, but this may be caused by different mechanisms. The frequency–independent ε′ of the composite with f = 0.4 is likely due to the step–like decrease in ε′ shifts to a higher frequency range of > 106 Hz due to stronger interfacial polarization, which is supported by the large increase in tanδ and ε′′, as shown in Fig. 7(b) and the inset of Fig. 8(b). Such a strong interfacial polarization usually occurs in the microstructure of a percolative–
15 composite system when the loading of semiconductive filler reaches the percolation threshold (f → fc). As shown in Fig. 8(a–b), a sudden transition in the σ occurs when f > 0.30, which suggests the formation of a percolation network, resulting in an insulator–conductor transition [47, 48]. This leads to Ohmic and non–Ohmic conduction. The former occurs due to the contact of semiconducting nLSNO particles. The latter may result from the barrier–tunneling effect between two adjacent semiconducting nLSNO particles, which is separated by a PVDF layer with a critical thickness. This confirms the occurrence of a percolation network within the nLSNO/PVDF composite. The temperature dependence of the dielectric properties (i.e., ε′, tanδ, and ε′′) for the nLSNO/PVDF and µLSNO/PVDF composite films with f = 0.3 is illustrated in Fig. 9. The ε′ values in the temperature range of 0–100 oC continuously increase with increasing temperature, corresponding to the linear increase in tanδ. In a relative–low temperature range, the rapid decrease in ε′ is observed, corresponding the existence of a tanδ peak, which is similar to that observed in the pure PVDF polymer [2, 44]. This type of dielectric relaxation is assigned as the β–relaxation [44]. As shown in the inset of Fig. 9(a), the temperature dependence of ε″ for the nLSNO/PVDF composite shows the thermally activated β–relaxation behavior. The relaxation ε″–peak shifts to high frequencies with increasing temperature from -60 to 20 oC. This is originated at the amorphous regions related to the glass transition, resulting from the cooperative segmental motions. When the temperature increased, the greatly enhanced ε″ is observed, especially in the low–frequency range. This behavior indicates the contribution of dc conduction to ε′. This is also contributed by the MWS effect. According to the effects of particle sizes and related interparticle distances, the nLSNO/PVDF composites filled with smaller particles have higher interfacial areas
16 and shorter interparticle distances (Fig. 5). As shown in Fig. 6(a), the ε′ value of the nLSNO/PVDF composites with f = 0.05 – 0.1 is slightly higher than that of the
µLSNO/PVDF composites. This result is partially caused by a light interfacial effect that is due to the larger interfacial area of the nLSNO particles even though the interfacial polarization is still not dominated in the composites with such low volume fractions. Another reason is that the intrinsic dielectric permittivity of the nLSNO particles might be higher than that of the µLSNO particles. Thus, by incorporating with the same f value, the dielectric permittivity of the nLSNO particles should be higher. Unfortunately, the intrinsic dielectric permittivity of LSNO powders is very difficult to measure. By increasing the f to 0.2, the dielectric permittivity of the nLSNO/PVDF composite diverges from the µLSNO/PVDF composite. This is because the interparticle distance between nLSNO fillers has been reduced to less than 100 nm. The micro–capacitor effect then becomes dominant. When f = 0.3, the interparticle distances of filler particles in the nLSNO/PVDF and µLSNO/PVDF composites are approximately 36 and 384 nm, respectively. The dielectric permittivity of the nLSNO/PVDF composite is therefore much larger than that of the
µLSNO/PVDF composite due to a large valued micro–capacitor that is formed and to a thinner PVDF layer between adjacent nLSNO particles, giving rise to the capacitance value of micro–capacitors. Furthermore, the strong interfacial polarization is too strong, and gives rise to the greatly enhanced dielectric permittivity. At f = 0.4, as we have mentioned, the extreme increase in the dielectric permittivity is attributed to the formation of a percolation network [15], which results in sudden increases in the dielectric permittivity, loss tangent and conductivity. It is important to note that the µLSNO/PVDF composites filled with larger particles require a greater f value to reach the percolation threshold and short interparticle distance, which adversely
17 affects the mechanical properties of the composites. Thus, using nLSNO as filler can possibly benefit both the dielectric properties and the mechanical properties. For application in modern smart microelectronics, besides the improved dielectric properties, the flexibility and mechanical strength of dielectric composite materials are also important. These mechanical properties would be largely degraded by incorporating of high–permittivity ceramic particles with high–volume fractions (f ≥ 0.5) into polymer. Fig. 10 illustrates the typical stress–strain curves of the pure PVDF polymer and nLSNO/PVDF composite films with f = 0.1 – 0.3. The insets show the tensile strength and modulus. The tensile strength tends to decrease with increasing f, while the modulus of the PVDF polymer can be enhanced by incorporating with nLSNO particles. It is ∼1.35 GPa for the composite with f = 0.3, which is larger than that of the pure PVDF film (∼0.54 GPa). By considering the area under the stress– strain curves, it can be estimated that the toughness of the nLSNO/PVDF composite films tends to decrease with increasing f. However, as shown in the inset of Fig. 2(b) and Fig. 10, the nLSNO/PVDF composite film with f = 0.3 is still flexible.
4.
Conclusion Promising LSNO/PVDF composites have been successfully fabricated by
solution processing and hot–pressing methods. The µLSNO and nLSNO particles are used as fillers to investigate the effects of particle size, interfacial area and interparticle distances on the dielectric behaviors of PVDF–based composites. The particle size has an influence on the percolation threshold of the LSNO/PVDF composites. Abrupt changes in the dielectric properties and electrical conductivity are observed only in the nLSNO/PVDF composites, while the dielectric properties of the
µLSNO/PVDF composites changed continuously. Nevertheless, by optimization of
18 the filler volume fraction, the nLSNO/PVDF composite with f = 0.3 shows good flexibility and exhibits a high ε′ ≈ 565.9 and a low tanδ ≈ 0.28 (at 1 kHz and RT), while the µLSNO/PVDF composite exhibits a very low ε′ ≈ 56.0. The results show that the intrinsically high ε′ value of the nLSNO and µLSNO particles primarily contributes to the dielectric response in the composites when f ≤ 0.2 and f ≤ 0.4, respectively. The large increase in the dielectric response in the nLSNO/PVDF composites with f ≥ 0.3 is attributed to the combined effects of the interfacial polarization at the interface of nLSNO–PVDF with much larger interfacial areas and to the shorter interparticle distances between nLSNO particles compared to those for
µLSNO particles in the composites. The lowered tanδ value can be attributed by the formation of a micro–capacitor microstructure. These flexible nLSNO/PVDF composites with such great dielectric performance are potential composite materials for dielectric charge storage capacitors and for electromagnetic wave absorption applications.
Acknowledgments This work was financially supported by the Synchrotron Light Research Institute, Khon Kaen University, and the Thailand Research Fund (TRF) [Grant No. BRG6180003]. This work has been partially supported by the Research Network NANOTEC (RNN) program of the National Nanotechnology Center (NANOTEC), NSTDA, Ministry of Science and Technology and Khon Kaen University, Thailand. K.M. would like to thank the Thailand Research Fund under the Royal Golden Jubilee Ph.D. Program [Grant No. PHD/0191/2556] for his Ph.D. scholarship.
19
References [1] Prateek, V.K. Thakur, R.K. Gupta, Recent Progress on Ferroelectric PolymerBased Nanocomposites for High Energy Density Capacitors: Synthesis, Dielectric Properties, and Future Aspects, Chem Rev 116(7) (2016) 4260-317. [2] Z.-M. Dang, J.-K. Yuan, J.-W. Zha, T. Zhou, S.-T. Li, G.-H. Hu, Fundamentals, processes and applications of high-permittivity polymer–matrix composites, Prog. Mater. Sci. 57(4) (2012) 660-723. [3] P. Martins, A.C. Lopes, S. Lanceros-Mendez, Electroactive phases of poly(vinylidene fluoride): Determination, processing and applications, Prog. Polym. Sci. 39(4) (2014) 683-706. [4] X. Lin, L. Fan, D. Ren, Z. Jiao, P. Coates, W. Yang, Enhanced dielectric properties of immiscible poly (vinylidene fluoride)/low density polyethylene blends by inducing multilayered and orientated structures, Composites Part B: Engineering 114 (2017) 58-68. [5] M. Arbatti, X. Shan, Z.Y. Cheng, Ceramic–Polymer Composites with High Dielectric Constant, Advanced Materials 19(10) (2007) 1369-1372.
20 [6] H. Li, F. Liu, B. Fan, D. Ai, Z. Peng, Q. Wang, Nanostructured FerroelectricPolymer Composites for Capacitive Energy Storage, Small Methods 2(6) (2018) 1700399. [7] Z. Pu, X. Zheng, Y. Tian, L. Hu, J. Zhong, Flexible Ultrahigh-Temperature Polymer-Based Dielectrics with High Permittivity for Film Capacitor Applications, Polymers 9(11) (2017) 596. [8] C. Wang, W.-Y. Lee, R. Nakajima, J. Mei, D.H. Kim, Z. Bao, Thiol–ene CrossLinked Polymer Gate Dielectrics for Low-Voltage Organic Thin-Film Transistors, Chemistry of Materials 25(23) (2013) 4806-4812. [9] T. Zhou, J.W. Zha, R.Y. Cui, B.H. Fan, J.K. Yuan, Z.M. Dang, Improving dielectric properties of BaTiO3/ferroelectric polymer composites by employing surface hydroxylated BaTiO3 nanoparticles, ACS Appl Mater Interfaces 3(7) (2011) 2184-8. [10] S. Luo, S. Yu, R. Sun, C.-P. Wong, Nano Ag-Deposited BaTiO3 Hybrid Particles as Fillers for Polymeric Dielectric Composites: Toward High Dielectric Constant and Suppressed Loss, ACS Applied Materials & Interfaces 6(1) (2014) 176-182. [11] S. Dash, R.N.P. Choudhary, M.N. Goswami, Enhanced dielectric and ferroelectric properties of PVDF-BiFeO3 composites in 0–3 connectivity, J. Alloys Compd. 715 (2017) 29-36. [12] X. Chen, X. Han, Q.-D. Shen, PVDF-Based Ferroelectric Polymers in Modern Flexible Electronics, Advanced Electronic Materials 3(5) (2017) 1600460. [13] A. Toor, H. So, A.P. Pisano, Improved Dielectric Properties of Polyvinylidene Fluoride Nanocomposite Embedded with Poly(vinylpyrrolidone)-Coated Gold Nanoparticles, ACS Applied Materials & Interfaces 9(7) (2017) 6369-6375.
21 [14] N. Phromviyo, N. Chanlek, P. Thongbai, S. Maensiri, Enhanced dielectric permittivity with retaining low loss in poly(vinylidene fluoride) by incorporating with Ag nanoparticles synthesized via hydrothermal method, Appl. Surf. Sci. 446 (2018) 59-65. [15] C.W. Nan, Y. Shen, J. Ma, Physical Properties of Composites Near Percolation, Annu. Rev. Mater. Res. 40(1) (2010) 131-151. [16] K. Meeporn, N. Chanlek, P. Thongbai, Effects of DC bias on non-ohmic sampleelectrode
contact
and
grain
boundary
responses
in
giant-permittivity
La1.7Sr0.3Ni1−xMgxO4 ceramics, RSC Advances 6(94) (2016) 91377-91385. [17] K. Meeporn, T. Yamwong, S. Pinitsoontorn, V. Amornkitbamrung, P. Thongbai, Grain size independence of giant dielectric permittivity of CaCu3Ti4−xScxO12 ceramics, Ceram. Int. 40(10, Part A) (2014) 15897-15906. [18] K. Meeporn, T. Yamwong, P. Thongbai, La1.7Sr0.3NiO4 nanocrystalline powders prepared by a combustion method using urea as fuel: Preparation, characterization, and their bulk colossal dielectric constants, Japanese Journal of Applied Physics 53(6S) (2014) 06JF01. [19] S. Wang, Y.-D. Hou, M.-P. Zheng, H.-Y. Ge, M.-K. Zhu, H. Yan, Roomtemperature dielectric relaxation and magnetic properties in rutile-type FeTiNbO6, Materials Research Bulletin 48(9) (2013) 3098-3102. [20] Y.Y. Liu, X.M. Chen, X.Q. Liu, L. Li, Giant dielectric response and relaxor behaviors induced by charge and defect ordering in Sr(Fe1⁄2Nb1⁄2)O3 ceramics, Applied Physics Letters 90(19) (2007) 192905. [21] P. Zheng, J.L. Zhang, Y.Q. Tan, C.L. Wang, Grain-size effects on dielectric and piezoelectric properties of poled BaTiO3 ceramics, Acta Materialia 60(13-14) (2012) 5022-5030.
22 [22] S. Krohns, P. Lunkenheimer, C. Kant, A.V. Pronin, H.B. Brom, A.A. Nugroho, M. Diantoro, A. Loidl, Colossal dielectric constant up to gigahertz at room temperature, Appl. Phys. Lett. 94(12) (2009) 122903. [23] S. Krohns, P. Lunkenheimer, S. Meissner, A. Reller, B. Gleich, A. Rathgeber, T. Gaugler, H.U. Buhl, D.C. Sinclair, A. Loidl, The route to resource-efficient novel materials, Nat Mater 10(12) (2011) 899-901. [24] K. Meeporn, S. Maensiri, P. Thongbai, Abnormally enhanced dielectric permittivity in poly(vinylidene fluoride)/nanosized-La2NiO4−δ films, Applied Surface Science 380 (2016) 67-72. [25] K. Meeporn, P. Thongbai, T. Yamwong, S. Maensiri, Greatly enhanced dielectric permittivity in La1.7Sr0.3NiO4/poly(vinylidene fluoride) nanocomposites that retained a low loss tangent, RSC Advances 7(28) (2017) 17128-17136. [26] P. Martins, J.S. Nunes, G. Hungerford, D. Miranda, A. Ferreira, V. Sencadas, S. Lanceros-Méndez, Local variation of the dielectric properties of poly(vinylidene fluoride) during the α- to β-phase transformation, Phys. Lett. A 373(2) (2009) 177180. [27] R.E. Sousa, J. Nunes-Pereira, J.C.C. Ferreira, C.M. Costa, A.V. Machado, M.M. Silva, S. Lanceros-Mendez, Microstructural variations of poly(vinylidene fluoride cohexafluoropropylene) and their influence on the thermal, dielectric and piezoelectric properties, Polym. Test. 40 (2014) 245-255. [28] P. Costa, J. Silva, V. Sencadas, C.M. Costa, F.W.J. van Hattum, J.G. Rocha, S. Lanceros-Mendez, The effect of fibre concentration on the α to β-phase transformation, degree of crystallinity and electrical properties of vapour grown carbon nanofibre/poly(vinylidene fluoride) composites, Carbon 47(11) (2009) 25902599.
23 [29] C. Ribeiro, C.M. Costa, D.M. Correia, J. Nunes-Pereira, J. Oliveira, P. Martins, R. Gonçalves, V.F. Cardoso, S. Lanceros-Méndez, Electroactive poly(vinylidene fluoride)-based structures for advanced applications, Nat. Protoc. 13 (2018) 681. [30] C. Ben Osman, S. Nowak, A. Garcia-Sanchez, Y. Charles, S. Ammar, S. Mercone, F. Mammeri, In situ monitored stretching induced α to β allotropic transformation of flexible poly(vinylidene fluoride)-CoFe2O4 hybrid films: The role of nanoparticles inclusion, Eur. Polym. J. 84 (2016) 602-611. [31] Z. Wang, M. Fang, H. Li, Y. Wen, C. Wang, Y. Pu, Enhanced dielectric properties in poly(vinylidene fluoride) composites by nanosized Ba(Fe0.5Nb0.5)O3 powders, Composites Science and Technology 117 (2015) 410-416. [32] W. Li, Q. Meng, Y. Zheng, Z. Zhang, W. Xia, Z. Xu, Electric energy storage properties of poly(vinylidene fluoride), Appl. Phys. Lett. 96(19) (2010) 192905. [33] B. Luo, X. Wang, Y. Wang, L. Li, Fabrication, characterization, properties and theoretical analysis of ceramic/PVDF composite flexible films with high dielectric constant and low dielectric loss, Journal of Materials Chemistry A 2(2) (2014) 510519. [34] R. Jr, M. Cestari, Effect of crystallization temperature on the crystalline phase content and morphology of poly(vinylidene fluoride), Journal of Polymer Science Part B: Polymer Physics 32 (1994) 859-870. [35] K.S. Deepa, M.T. Sebastian, J. James, Effect of interparticle distance and interfacial area on the properties of insulator-conductor composites, Applied Physics Letters 91(20) (2007) 202904. [36] M.-F. Lin, V.K. Thakur, E.J. Tan, P.S. Lee, Surface functionalization of BaTiO3 nanoparticles and improved electrical properties of BaTiO3/polyvinylidene fluoride composite, RSC Advances 1(4) (2011) 576.
24 [37] A. Srivastava, K.K. Jana, P. Maiti, D. Kumar, O. Parkash, Poly(vinylidene fluoride)/ CaCu3Ti4O12 and La doped CaCu3Ti4O12 composites with improved dielectric and mechanical properties, Materials Research Bulletin 70 (2015) 735-742. [38] Z. Wang, M. Fang, H. Li, Y. Wen, C. Wang, Y. Pu, Enhanced dielectric properties in poly(vinylidene fluoride) composites by nanosized Ba(Fe0.5Nb0.5)O3 powders, Composites Science and Technology 117 (2015) 410-416. [39] Y.-l. Su, C. Sun, W.-q. Zhang, H. Huang, Fabrication and dielectric properties of Na0.5Bi0.5Cu3Ti4O12/poly(vinylidene fluoride) composites, J. Mater. Sci. 48(23) (2013) 8147-8152. [40] L. Ren, X. Meng, J.-W. Zha, Z.-M. Dang, Coulomb block effect inducing distinctive
dielectric
properties
in
electroless
plated
barium
titanate@silver/poly(vinylidene fluoride) nanocomposites, RSC Advances 5(80) (2015) 65167-65174. [41] H. Tang, Z. Zhou, C.C. Bowland, H.A. Sodano, Synthesis of calcium copper titanate (CaCu 3 Ti 4 O 12 ) nanowires with insulating SiO2 barrier for low loss high dielectric constant nanocomposites, Nano Energy 17 (2015) 302-307. [42] C.-W. Nan, Physics of inhomogeneous inorganic materials, Prog. Mater. Sci. 37(1) (1993) 1-116. [43] A. Maceiras, C.M. Costa, A.C. Lopes, M. San Sebastián, J.M. Laza, J.L. Vilas, J.L.G. Ribelles, R. Sabater i Serra, A. Andrio Balado, S. Lanceros-Méndez, L.M. León, Dielectric relaxation dynamics of high-temperature piezoelectric polyimide copolymers, Applied Physics A 120(2) (2015) 731-743. [44] A.C. Lopes, C.M. Costa, R.S.i. Serra, I.C. Neves, J.L.G. Ribelles, S. LancerosMéndez,
Dielectric
relaxation,
ac
conductivity
and
electric
modulus
in
25 poly(vinylidene fluoride)/NaY zeolite composites, Solid State Ionics 235 (2013) 4250. [45] A.C. Lopes, M.P. Silva, R. Gonçalves, M.F.R. Pereira, G. Botelho, A.M. Fonseca, S. Lanceros-Mendez, I.C. Neves, Enhancement of the Dielectric Constant and Thermal Properties of α-Poly(vinylidene fluoride)/Zeolite Nanocomposites, The Journal of Physical Chemistry C 114(34) (2010) 14446-14452. [46] J. Fu, Y. Hou, Q. Wei, M. Zheng, M. Zhu, H. Yan, Advanced FeTiNbO6/poly(vinylidene fluoride) composites with a high dielectric permittivity near the percolation threshold, J. Appl. Phys. 118(23) (2015) 235502. [47] F. He, S. Lau, H.L. Chan, J. Fan, High Dielectric Permittivity and Low Percolation Threshold in Nanocomposites Based on Poly(vinylidene fluoride) and Exfoliated Graphite Nanoplates, Adv. Mater. 21(6) (2009) 710-715. [48] P.W. Jaschin, R. Bhimireddi, K.B.R. Varma, Enhanced Dielectric Properties of LaNiO3/BaTiO3/PVDF: A Three-Phase Percolative Polymer Nanocrystal Composite, ACS Appl Mater Interfaces 10(32) (2018) 27278-27286.
26 LIST OF FIGURE CAPTIONS Fig. 1.
(a–b) TEM images of LSNO ceramic powders for µLSNO and nLSNO. (c–
d) SEM images of LSNO ceramic powders for µLSNO and nLSNO. Fig. 2.
(a−b) SEM images of fractured surfaces of pure PVDF films and
nLSNO/PVDF composites with f = 0.10; inset of (b) shows the flexibility of nLSNO/PVDF composite f = 0.3. (c−d) Optical microscope images showing the dispersion sates of LSNO fillers in µLSNO/PVDF and nLSNO/PVDF composites with f = 0.3, respectively. (e−f) SEM images of the inner opened−surfaces for µLSNO/PVDF and nLSNO/PVDF composites with f = 0.3, respectively. Fig. 3.
(a−b) FESEM image and elemental mapping analysis for all elements of
nLSNO/PVDF composite with f = 0.3. (c−h) EDS elemental mapping images of C, F, La, Sr, Ni, and O, respectively. Fig. 4.
(a) XRD patterns of nLSNO powder, pure PVDF, and nLSNO/PVDF
composites with various f. (b) FTIR spectra of (1) pure PVDF film and (2−6) nLSNO/PVDF composites with f = 0.05, 0.10, 0.20, 0.30, and 0.40, respectively. (c) TGA curves of pure PVDF and nLSNO/PVDF composites with various volume fractions of nLSNO. Fig. 5.
Variation of interparticle distance as a function of volume fractions of filler
(f) for µLSNO/PVDF and nLSNO/PVDF composites. Fig. 6.
Variation of (a) dielectric constant (ε′) and (b) loss tangent (tanδ) as a
function of filler volume fraction (f) for µLSNO/PVDF and nLSNO/PVDF composites; insets of (a) and (b) show the enlarged scale from f = 0.05 to 0.2 and the correlation between the dielectric constant (ε′) and F(β) of the nLSNO/PVDF composites, respectively.
27 Fig. 7.
Frequency dependence of (a) dielectric constant (ε′) and (b) loss tangent
(tanδ) for nLSNO/PVDF composites with various volume fractions of nLSNO. Fig. 8.
(a) Frequency dependence of ac conductivity (σ) of nLSNO/PVDF
composites with various volume fractions of nLSNO. (b) The ac conductivity (σ) as a function of the volume fraction of nLSNO; inset shows the frequency dependence of the imaginary part (ε′′) of the complex dielectric permittivity (ε*). Fig. 9.
Temperature dependence of the (a) dielectric constant (ε′) and (b) loss
tangent (tanδ) at 1 kHz for nLSNO/PVDF and µLSNO/PVDF composites with f = 0.3; inset shows the frequency dependence of ε′′ at different temperatures. Fig. 10. Tensile stress−strain properties of pure PVDF film and nLSNO/PVDF composite with f = 0.10 − 0.30; insets shows the results of tensile strength and Young's modulus for the pure PVDF film and nLSNO/PVDF composite.
28 Fig. 1
29 Fig. 2
30 Fig. 3
31
(215)
(118) (220)
(200)
f = 0.40 f = 0.30 f = 0.20 f = 0.10 f = 0.05
*
β (110)
γ(020) β(200)
10
α(200) α(002)
20
(b)
■
(6) (5) (4)
30
▼
40
2θ (Deg) (c)
♦♦ ■ ▼
▼
pure PVDF film
50
60
70
100
90 80
(3)
mass (%)
Transmittance (%)
(211) (116) (107) (204) (213)
nLSNO
(112)
(004)
(101)
Intensity (arb. unit)
* PVDF
(006) (114)
(103)
(a)
(110)
Fig. 4
(2) (1)
▼ α phase
1400
840 766
40
■ γ phase
30
1070 1179
♦ β phase
1200
60 50
976 1401
70
878
1000
800 -1
Wave number (cm )
f = 0.40 f = 0.30 f = 0.20 f = 0.10 f = 0.05 pure PVDF
200 300 400 500 600 700 800 o
Temperature ( C)
32
Interparticle distance, d (µm)
Fig. 5
2.4 µLSNO/PVDF nLSNO/PVDF
2.0 1.6 1.2 0.8 0.4 0.0 0.0
0.1
0.2
0.3
Volume fraction (f)
0.4
33 Fig. 6
34 Fig. 7
Dielectric Constant (ε')
(a) 105 f=0 f = 0.30
4
10
f = 0.05 f = 0.40
f = 0.10
f = 0.20
3
10
2
10
1
10
2
10
3
4
10
5
10
6
10
10
Frequency (Hz)
(b) 10 Loss Tangent (tanδ)
4
f=0 f = 0.30
3
10
f = 0.05 f = 0.40
f = 0.10
f = 0.20
2
10
1
10
0
10
-1
10
-2
10
2
10
3
10
4
10
Frequency (Hz)
5
10
6
10
35 Fig. 8 (b) 1.0x10-4
-2
10
f = 0.05 f = 0.40
f = 0.10
f = 0.20
-4
10
-6
10
-8
10
-10
10
10
7
10
5
-5
8.0x10
-5
6.0x10
ε''
f=0 f = 0.30
Conductivity, σ (S.cm-1)
Conductivity, σ (S.cm-1)
(a) 100
-5
4.0x10
10
3
10
1
10 -5
2.0x10
f=0 f = 0.10 f = 0.30
f = 0.05 f = 0.20 f = 0.40
-1
10
2
10
3
10
4
10
5
10
6
Frequency (Hz)
0.0 2
10
3
10
4
10
Frequency (Hz)
5
10
6
10
0.0
0.1
0.2
0.3
Volume fraction (f)
0.4
36
Fig. 9
700
f = 0.3 @ 1 kHz
500
o
-20 C o 20 C o 60 C o 70 C o 90 C
400 300
ε''
Dielectric Constant (ε')
(a)
200 100 0
100
2
3
4
5
6
Log(f)
µLSNO/PVDF nLSNO/PVDF
30 -60
-40
-20
0
20
40
60
80
100
Temperature (°C)
Loss Tangent (tanδ)
(b)
0.5
µLSNO/PVDF nLSNO/PVDF
0.4 0.3 0.2 0.1 0.0
f = 0.3 @ 1 kHz
-60 -40 -20 0 20 40 60 80 100 Temperature (°C)
37
Fig. 10
Pure PVDF f = 0.10 f = 0.20 f = 0.30
24 20 16
8 4
30 25 20 15 10 5 0
0
10
20
30
Volume fraction (f)
0
0
5
Modulus (MPa)
12 Stress (MPa)
Tensile Stress (MPa)
28
1500 1200 900 600 300 0
0
10
20
30
Volume fraction (f)
10
Tensile Strain (%)
15
20
S1359-8368(19)32092-X
DOI:
https://doi.org/10.1016/j.compositesb.2019.107738
Reference:
JCOMB 107738
To appear in:
Composites Part B
Received Date: 10 May 2019 Revised Date:
7 September 2019
Accepted Date: 30 December 2019
Please cite this article as: Meeporn K, Thongbai P, Flexible La1.5Sr0.5NiO4/Poly(vinylidene fluoride) composites with an ultra high dielectric constant: A comparative study, Composites Part B (2020), doi: https://doi.org/10.1016/j.compositesb.2019.107738. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2019 Published by Elsevier Ltd.
1
Flexible La1.5Sr0.5NiO4/Poly(vinylidene fluoride) Composites with an Ultra High Dielectric Constant: A Comparative Study
Keerati Meeporn a, Prasit Thongbai b,c,*
a
Materials Science and Nanotechnology Program, Faculty of Science, Khon Kaen University, Khon Kaen 40002, Thailand b
Department of Physics, Faculty of Science, Khon Kaen University, Khon Kaen 40002, Thailand
c
Institute of Nanomaterials Research and Innovation for Energy (IN–RIE),
NANOTEC–KKU RNN on Nanomaterials Research and Innovation for Energy, Khon Kaen University, Khon Kaen,40002 , Thailand
* Corresponding author’s email: [email protected] (P. Thongbai)
2 ABSTRACT Poly(vinylidene fluoride) (PVDF)–based polymer composites filled with high– permittivity La1.5Sr0.5NiO4 (LSNO) particles have been fabricated to produce a composite material with excellent dielectric performance and good mechanical flexibility. Micro–sized (µLSNO) and nano–sized LSNO (nLSNO) particles are used separately as fillers. The dielectric properties of the composites are determined by their particle sizes, resultant interfacial area, and the interparticle distances between the LSNO particles. The percolation threshold of the nLSNO/PVDF composite is lower than that of the µLSNO/PVDF composite. A greatly enhanced dielectric response with a high ε′ ≈ 565.9 at 1 kHz and room temperature and an effectively suppressed low loss tangent (tanδ ≈ 0.28) are obtained in the nLSNO/PVDF composite with a filler volume fraction of 30 vol %. The greatly enhanced ε′ value of nLSNO/PVDF can be explained by a combination of the interfacial polarization at the semiconducting nLSNO–insulating PVDF interfaces, an intrinsically giant ε′ response in nLSNO particles, and shorter interparticle distances between nLSNO particles in the PVDF matrix, while the retained low tanδ value is primarily attributed to the formation of a micro–capacitor microstructure. Keywords: Dielectric polymer composites; Giant dielectric properties; Colossal permittivity; Interparticle distance; Micro−capacitor.
3 1.
Introduction Poly(vinylidene fluoride) (PVDF)–based polymer composites with a high
dielectric constant (ε′) are of great interest because of their great potential in modern electronic technology, including microelectromechanical devices, charge storage capacitors,
nano–energy
harvesters,
flexible
ferroelectric
memories,
and
portable/wearable sensors [1-4]. Polymer–based composites offer superior chemical resistance, ease of processing, and low cost. Moreover, they also provide adjustable electrical, mechanical, and thermal properties. To date, many innovative approaches have been focused on the enhancement of the dielectric properties of polymer–based composites through the fabrication of hierarchical structured [5, 6], cross–linked polymer treatments [7, 8], and surface–modification of inorganic/organic fillers [6, 9, 10]. However, the dielectric permittivity (ε′) values of some resultant composite materials are still low, which is one of the important parameters that limits their practical applications, especially in charge-storage capacitor applications. In addition, polymer–based composites can possess a high ε′ value of ≥100 when the volume fraction of a filler (f) is ≥ 50 vol % [10, 11]. Nevertheless, the relatively high–f results in poor mechanical performance, low flexibility, and high weight. These conditions are undesirable for modern electronic applications. Importantly, the excellent dielectric properties and the good mechanical performance of polymer–based composites are determined by both fillers and polymer matrices. Consequently, the identification of suitable fillers and polymer matrices is critical for these purposes. A great number of polymers have been used as matrices, including polystyrene (PS), polyimide (PI), poly(methyl methacrylate) (PMMA), polydimethylsiloxane (PDMS), polypropylene (PP), and epoxy [1, 6]. However, their low ε′ values (2–5) limit the enhancement of the dielectric properties of the resultant
4 composites. To resolve this issue, poly(vinylidene fluoride) (PVDF), as well as its copolymers with the highest ε′ (≥10) and high breakdown strength, have been considered as the most promising polymer matrix materials for dielectric applications [1, 6, 12]. Greatly increased dielectric response of polymers can also be accomplished by using conductive nanomaterials to develop high–k polymer nanocomposites. When f value of a conductive filler is lower than that of the percolation threshold (f < fc), both ε′ and loss tangent (tanδ) continuously increase with increasing f. For example, the ε′ values at 103 Hz of Au/PVDF and Ag/PVDF nanocomposites increased from ∼10 to ∼21 and to ∼42.7, respectively, when fAu = 0 → 0.0125 and fAg = 0 → 0.12 [13, 14]. However, the rapid increase in ε′ is usually observed when f is approached the percolation threshold (f → fc) [15]. Unfortunately, the dramatically increased ε′ value is usually accompanied by a large increase in tanδ. For example, the ε′ value of the Ni/PVDF composites strongly increased to ∼400 when fNi→ fc, while tanδ greatly increased (> 0.15) [15]. Several materials with giant dielectric ε′ values have been reported [16-21]. Among them, La2-xSxNiO4 (LSNO) ceramics offer superior dielectric performance with an extremely large ε′ (105–106), and an ε′ of approximately 104 can be observed even in a high frequency region (109 Hz) [22, 23]. It was demonstrated that the giant dielectric response in LSNO ceramics is produced by an intrinsic property, which is attributed to polaron hopping between Ni2+ and Ni3+. The giant ε′ of a filler material that resulted from an intrinsic effect is more beneficial to a composite approach than the giant ε′ that resulted from the extrinsic effect at grain boundaries. Therefore, the LSNO ceramic is a promising effective filler for polymer–based composite systems.
5 In this article, we report a promising composite system with a large ε′, low tanδ, and good mechanical flexibility by a simple combination of a PVDF matrix and an LSNO filler. The dielectric properties of the LSNO/PVDF composites were investigated. The effect of LSNO particle sizes, interfacial area and interparticle distances on the dielectric properties of PVDF–based composites filled with two kinds of LSNO of average sizes as a function of the volume fraction of filler (f) are also discussed. 2.
Experimental details 2.1 Preparation of two kinds of La1.5Sr0.5NiO4 (LSNO) ceramic powders The preparation of micro–sized LSNO (µLSNO) powder was conducted using
a solid–state reaction method. Analar-grade La2O3 (Sigma–Aldrich), SrCO3 (Sigma– Aldrich), and NiO (Sigma–Aldrich) were used as starting materials in stoichiometric amounts and ball–milled in ethanol for 24 h. The resulting ball–milled mixture was dried and calcined at 1200 °C for 12 h to obtain a pure phase of µLSNO powder. Nano–sized LSNO (nLSNO) powder was prepared by a chemical combustion method following a process previously reported [24, 25]. 2.2 Fabrication of µLSNO/PVDF and nLSNO/PVDF composites The PVDF–based composites using LSNO powders with different sizes of
µLSNO and nLSNO fillers were fabricated using a solution processing method [24]. First, a PVDF matrix solution was produced by dissolving the PVDF in N,N dimethylformamide (DMF) (RCI Labscan). Second, two kinds of LSNO fillers were added separately to the PVDF solution with stirring for 4 h at 60 oC to form
µLSNO/PVDF and nLSNO/PVDF composite systems and to prevent agglomerates until the highly viscous solution of polymer composites was obtained. After that, the
6 composite solutions were further sonicated in order to enhance the dispersion of ceramic fillers. Usually, the highest dipolar moment and largest dielectric response were obtained in the β–PVDF phase [26], which can be achieved by casting the solution of polar β–PVDF solvents. It was clearly shown that the processing conditions have an effect on the volume fraction of β–PVDF phase [27, 28]. To obtain the large fraction of β–PVDF phase, the temperature should not be higher than 70 oC [29]. On the other hand, highly porous PVDF films would be formed when the temperature was lower than 70 oC [3]. Thus, in this work, the composite solutions were dried at ≈70 °C for 6 h to obtain the β–PVDF phase and to minimize the pores as much as possible. The β–PVDF phase can also be induced by uniaxial stretching of the α–PVDF phase [30]. Thus, the uniaxial compressive stress might also induce the β–PVDF phase. Finally, to obtain dense films with designed β–PVDF structure, the dried composite were uniaxial pressed at ≈10 MPa at 200 °C for 0.5 h. 2.3 Characterization techniques The microstructures of µLSNO and nLSNO powders were examined by using transmission electron microscopy (TEM) (ZEISS, EM902). The morphologies and phase compositions of the two kinds of LSNO powders and PVDF–based composites were characterized by using scanning electron microscopy (SEM) (SEC, SNE4500M) and X–ray diffraction (XRD) (PANalytical, EMPYREAN). Focused ion beam (FIB) technique and field–emission scanning electron microscopy (FESEM) (FEI, Helios NanoLab G3 CX) were used to reveal the dispersion of fillers in the PVDF matrix and to map the distribution of all elements in the inner opened–surface. The outer surfaces of the composite samples were removed by using focused ion beams (FIBs). The dispersion of fillers was also characterized using an optical microscope (OLYMPUS,
7 BX51). Phase formations of PVDF polymer were identified using a Fourier transformed infrared spectroscopy (FTIR) (Bruker, TENSOR27). The thermal properties of the PVDF–based composites were evaluated by using thermo– gravimetric analysis (TGA) (Pyris Diamond, Perkin Elmer). 2.4 Dielectric and mechanical measurements The electrodes on the composite samples were painted with silver paste to perform the dielectric measurements. A KEYSIGHT E4990A Impedance Analyzer was used to investigate the dielectric properties in the frequency and temperature ranges of 102–106 Hz and -60–100 oC. Tensile test was carried out on a universal testing machine (UTM) (INTSTRON, 5567A) with stretching rate of 5 mm/min. The samples were cut into rectangle shape with size of 40×5 mm2.
3.
Results and discussion Previously, in studies on variations in microstructures with supplementing
conducting and semiconducting ceramic fillers that were reported, the dielectric properties of the polymer-based composites could be affected by the dispersion state, the geometric shape, size and interfacial area. Fig. 1(a)–(d) reveal the morphologies of the semiconducting LSNO ceramic fillers with different particle sizes for the µLSNO and nLSNO ceramics using TEM and SEM techniques. As shown in Fig. 1(a) and 1(b), the average sizes of the µLSNO and the nLSNO particles are ∼1.89 µm and ∼180 nm, respectively. Hard agglomeration of nLSNO particles bonding together with a solid bridge is observed in the TEM image. Similarly, the SEM images of these two LSNO powders are shown in Fig. 1(c−d), respectively. A small agglomeration of nLSNO particles can be seen. For the effects of particles sizes, nanosized particles have a relatively larger surface area (interfacial area) when compared to that of the
8 larger particles with the same volume fraction [31]. Therefore, it is expected that the semiconducting nLSNO with a smaller particle size that has been used as a filler can enhance interfacial polarization in nLSNO/PVDF composites. Figs. 2(a) and 2(b) show SEM images of the fractured cross–sections of the PVDF polymer and nLSNO/PVDF composites with f = 0.1, respectively. The pure PVDF film prepared via casting the PVDF solution in DMF and hot–pressing at 200 °C for 0.5 h formed a tight surface with a continuous phase. The dispersion states of the nLSNO/PVDF composite with f = 0.10 are shown in Fig. 2(b). The fractured surfaces of the composites show a random distribution of the nLSNO filler in the PVDF matrix. Interestingly, as shown in the inset of (b), the nLSNO/PVDF composite with f = 0.30 still exhibits good flexibility. This can be attributed to the beneficial effect of a good dispersion of nLSNO particles in the PVDF matrix. To reveal the dispersion states of nLSNO and µLSNO particles in the overall large area, the nLSNO/PVDF and µLSNO/PVDF composite films with f = 0.3 were characterized using an optical microscope. Obviously, as shown in Figs. 2(c) and 2(d), the dispersion of the nLSNO particles in the PVDF polymer is better than that of the
µLSNO particles. Furthermore, the dispersion of the fillers in the deep inner part of the composite films was also analyzed. The outer–surface layer of the composite films was removed by FIBs. Figs. 2(e) and 2(f) display FESEM micrograph of the nLSNO/PVDF and µLSNO/PVDF composite films with f = 0.3, showing good distribution of the nLSNO particles in the PVDF matrix. Fig. 3 shows the FESEM image and elemental mapping for all elements (i.e., C, F, La, Sr, Ni, and O) of the nLSNO/PVDF composite with f = 0.3. All elements are homogeneously dispersed. The crystalline structures of the nLSNO filler, the pure PVDF film, and the PVDF–based composites with different volume fractions of nLSNO are shown in Fig.
9 4(a). The XRD pattern of the nLSNO powder shows that the LSNO phase with a tetragonal structure can be observed without any impurity phases. The crystal planes are well–indexed to the (101), (004), (103), (110), (112), (006), (114), (200), (211), (116), (107), (204), (213), (215), (218) and (220) planes, which can be compared to the standard La1.5Sr0.5NiO4 pattern (JCPDS No. 32–1241). The XRD pattern of a pure PVDF film demonstrates the mixing of α, β and γ phases of PVDF [3]. The diffraction peaks at 2θ = 18.50° and 20.25° are assigned to the (020) plane of the γ phase and to the (110) and (200) planes of the β phase, respectively [3]. The diffraction peaks at 2θ = 35.97° and 39.02° are assigned to the (200) and (002) planes, respectively, of the α phase [32]. For the composites, both LSNO and PVDF phases appear in the XRD patterns of the nLSNO/PVDF composites, and no impurity phase is observed. However, due to the strong intensity of the XRD diffraction peaks of the LSNO phase, the XRD peaks for the PVDF phase are not clearly seen in the composites with a high–volume fraction of nLSNO filler. The FTIR spectra of the pure PVDF film and PVDF−based composites demonstrate the combination of α–, β– and γ–phases of PVDF, as shown in Fig. 4(b). The absorbance bands at 766, 976 and 1179 cm-1 are assigned to the α phase [3]. The β–phase characteristics are observed at 840 and 878 cm-1 [3]. The absorbance bands at 1402 and 1970 cm-1 are related to the in–plane bending or scissoring of CH2 and the bending of C−C−C, respectively [33]. To quantify the fraction of the β phase, it is assumed that only the α– and β–phases exist and FTIR follows the Lambert−Beer law. The relative fraction of the β phase (
=
/
) can be calculated using Eq. (1) [34]:
,
(1)
10 where
and
are the measured absorbance at 766 and 840 cm-1, corresponding to
the α and β phases.
and
are the absorption coefficients at the respective
wavenumber, the values of which are 6.1 x 104 and 7.7 x 104 cm2mol-1, respectively. The
of the nLSNO/PVDF composite films with f = 0 (pure PVDF), 0.05, 0.1,
0.2, 0.3, and 0.4 are 34, 37, 45, 50, 64, and 59 %, while
for the µLSNO/PVDF
composite films are 34, 35, 41, 41, 40, and 36 %, respectively. Obviously, values of all composite films are larger than that of a pure PVDF film. Furthermore, for both composite systems,
increases with increasing f from 0 to 0.2, and then
slightly decreases when f = 0.3. Notably,
of the nLSNO/PVDF composites is
larger than that of the µLSNO/PVDF composites, indicating the advantages of nanoparticles. Fig. 4(c) shows the thermal stabilities of the pure PVDF film and the nLSNO/PVDF composites with various volume fractions of nLSNO. The TGA curves for the PVDF film and composites show a single–stage decomposition of PVDF. The temperature at 5 % mass–loss increases with f, which are 432, 464, 467, 470, 471 and 473 °C for the composites with f = 0, 0.05, 0.10, 0.20, 0.30 and 0.40, respectively. Consequently, the nLSNO particles enhance the thermal stability of the PVDF polymer. The origin of the dielectric response in PVDF–based composites using µLSNO and nLSNO particles as fillers is explained not only based on the interfacial area in the LSNO/PVDF composites but also by the interparticle distances between the LSNO particles. Fig. 5 shows the average interparticle distances (d) between the filler particles used for the µLSNO/PVDF and nLSNO/PVDF composites. The d value is calculated by using d = r[(4π/f)1/3-2)] [35], where f and r are the volume fraction and the average radius of LSNO particles, respectively. The d value of the µLSNO/PVDF
11 and nLSNO/PVDF composites decreases from ∼2.24 → 0.18 µm, and from ∼213 → 16.9 nm with increasing f from 0.05 → 0.4, respectively. Generally, at a given particular LSNO loading, a decrease in the particle size of a LSNO filler causes an increase in the number of particles per unit volume in the PVDF–based composites. The interparticle distance between the adjacent nLSNO particles, which is separated by a thin layer of PVDF polymer, is therefore shorter than that of the µLSNO particles. This is expected to have a great influence on the dielectric properties of the composites [25, 31]. The dielectric constants (ε′) and loss tangents (tanδ) at 1 kHz of the
µLSNO/PVDF and nLSNO/PVDF composites as a function of the filler volume fraction are shown in Fig. 6(a) and 6(b). The ε′ values of the nLSNO/PVDF composites with f = 0.05, 0.1, 0.2, 0.3, and 0.4 are 27.6, 40.3, 98.4, 565.9, and 2353.1, while the ε′ values for the µLSNO/PVDF composites are 15.4, 25.1, 28.7, 56.0, and 85.7, respectively. Obviously, for the same filler loading, the nLSNO/LSNO composite exhibits a higher ε′ value than that of the µLSNO/PVDF composite. As depicted in the inset of Fig. 2(b) and Fig. 6, good flexibility and high ε′ ≈ 565.9 and low tanδ ≈ 0.28 (at 1 kHz and RT) are achieved in the nLSNO/PVDF composite with f = 0.3. This enhanced ε′ value is higher than that of the pure PVDF matrix by ∼57−fold. Moreover, it should be noted that the ε′ for the nLSNO/PVDF composites is greater than those of other PVDF–based two and three−phase composites that have even lower filler loadings, such as BaTiO3/PVDF [36], CaCu3Ti4O12/PVDF [37], Ba(Fe0.5Nb0.5)O3/PVDF [38], Na0.5Bi0.5Cu3Ti4O12/PVDF [39], Ag/BaTiO3/PVDF [40] and SiO2/CaCu3Ti4O12/PVDF [41]. Therefore, the flexible nLSNO/PVDF composites with such dielectric performance are potential composite materials for dielectric charge storage capacitor and electromagnetic wave absorption applications.
12 Distinct dielectric behavior is observed in these two composite systems. Rapid changes in the ε′ and tanδ at approximately f = 0.3 are observed in the nLSNO/PVDF composites, which may suggest the formation of a percolation network in this percolative composite system [15, 42]. Alternatively, this behavior is not observed in the µLSNO/PVDF composites. The ε′ and tanδ values for the µLSNO/PVDF composites continuously increase over the entire filler loading range studied (f = 0.05 – 0.4). This indicates that the formation of a percolation network in the
µLSNO/PVDF composites may occur at f ≥ 0.4 [42]. Thus, the percolation threshold (fc) of the µLSNO/PVDF composites is higher than that of the nLSNO/PVDF composites.
This
result
confirms
that
the
percolation
threshold
of
the
high−permittivity semiconductive oxide/PVDF composite system is dependent on the particle size of filler used, as reported in metal particle/PVDF composite systems [14, 15, 42]. Generally, the highest ε′ value and largest dipolar moment of a PVDF polymer were achieved in the β–PVDF phase [3, 26]. Thus, the greatly enhanced ε′ value of the nLSNO/PVDF composite films may be correlated to the variation in F(β) of the PVDF matrix. The correlation between ε′ and F(β) is illustrated in the inset of Fig. 6(b). Obviously, the ε′ value of the nLSNO/PVDF composites with f = 0 – 0.3 increases with increasing F(β). This result clearly indicates that the relative fraction of β–PVDF phase is one of the most important factors that can cause an increase in the dielectric response of PVDF–based composites. When f = 0.4, ε′ largely increases, while F(β) decreased slightly. This is likely due to the dominant effect of Ohmic conduction associated with the percolation and Maxwell–Wagner–Sillars (MWS) effects [43-45]. This effect is usually occurred in several kinds of heterogeneous
13 materials, in which free charges trapped at the interfaces between large–ε′ oxide particles/clusters and an insulating matrix. The dielectric response behaviors in the nLSNO/PVDF composites are further described by considering the frequency−dependent behavior of the dielectric properties. The frequency dependence of the ε′ and tanδ at RT for the nLSNO/PVDF composites with f = 0.05 − 0.4 is shown in Fig. 7. The overall result shows that the ε′ and tanδ values increase with increased loading of nLSNO over the measured frequency range. The ε′ of the nLSNO/PVDF composites with f ≤ 0.2 is independent of the frequency from 102 to 105 Hz, while the ε′ values of nLSNO particles and PVDF polymer are also independent of the frequency in this range. This clearly indicates that the enhanced dielectric response of the composites with f ≤ 0.2 is primarily contributed from the intrinsic ε′ values of nLSNO particles and PVDF polymer. Although the interfacial effect can partially contribute to the dielectric response, it is not dominant in the composites with f ≤ 0.2, especially for the
µLSNO/PVDF composites. This causes a continuous increase in the ε′ for composites with f = 0.05 → 0.2, as displayed in the inset of Fig. 6(a). By increasing the f to 0.3, ε′ of the composite changes with increasing frequency and the step–like decrease in ε′ appears at approximately 104 Hz, indicating the dominant effect of the extrinsic effect, i.e., the interfacial polarization [25]. This assumption is supported by the observed rapid increase in ε′, as shown in Fig. 6(a). It is worth noting that the tanδ value for the nLSNO/PVDF composite with f = 0.3 is suppressed to lower than 0.5 over the measured frequency range. It is important to note that although the suppressed tanδ values are lower than 0.5 over a wide frequency range, the obtained tanδ values are still high for most of the applications of
14 a dielectric material. Further research work is needed to reduce tanδ into the level of applications. AC conductivity (σ) as a function of frequency and volume fraction of nLSNO for the nLSNO/PVDF composites is shown in Fig. 8(a–b), respectively. When f ≤ 0.3, the frequency dependence of σ shows a similar trend with an increasing level of f from 0 to 0.3. The low–frequency σ values of these composites (f = 0.05 – 0.3) are suppressed. Thus, the significantly enhanced ε′ and the suppressed low tanδ value of the nLSNO/PVDF composite with f = 0.3 should be due to the formation of micro– capacitors in the composite [25, 46]. Inset of Fig. 8(b), the conductivity of the polymer composites that causes an increase in the dielectric loss (ε″) or the imaginary part of complex dielectric permittivity (ε*), i.e., ε″ = tanδ×ε′, can also be caused by the MWS process, i.e., accumulated free charges at internal interface boundaries or at the interface of sample–electrode [44]. Therefore, incorporation of nLSNO in the PVDF polymer can cause increases in both of the ac conductivity and the intensity of the MWS polarization, which would be increasingly predominant as the frequency decreased [43]. This is associated with the contribution of dc conduction to the low– frequency dielectric response. The existence of nLSNO particles can also promote the formation of β–PVDF phase, resulting in an enhanced ε′ value. By further increasing the f to 0.4, ε′ becomes independent of the frequency in the measured frequency range as observed in the composites with f ≤ 0.2, but this may be caused by different mechanisms. The frequency–independent ε′ of the composite with f = 0.4 is likely due to the step–like decrease in ε′ shifts to a higher frequency range of > 106 Hz due to stronger interfacial polarization, which is supported by the large increase in tanδ and ε′′, as shown in Fig. 7(b) and the inset of Fig. 8(b). Such a strong interfacial polarization usually occurs in the microstructure of a percolative–
15 composite system when the loading of semiconductive filler reaches the percolation threshold (f → fc). As shown in Fig. 8(a–b), a sudden transition in the σ occurs when f > 0.30, which suggests the formation of a percolation network, resulting in an insulator–conductor transition [47, 48]. This leads to Ohmic and non–Ohmic conduction. The former occurs due to the contact of semiconducting nLSNO particles. The latter may result from the barrier–tunneling effect between two adjacent semiconducting nLSNO particles, which is separated by a PVDF layer with a critical thickness. This confirms the occurrence of a percolation network within the nLSNO/PVDF composite. The temperature dependence of the dielectric properties (i.e., ε′, tanδ, and ε′′) for the nLSNO/PVDF and µLSNO/PVDF composite films with f = 0.3 is illustrated in Fig. 9. The ε′ values in the temperature range of 0–100 oC continuously increase with increasing temperature, corresponding to the linear increase in tanδ. In a relative–low temperature range, the rapid decrease in ε′ is observed, corresponding the existence of a tanδ peak, which is similar to that observed in the pure PVDF polymer [2, 44]. This type of dielectric relaxation is assigned as the β–relaxation [44]. As shown in the inset of Fig. 9(a), the temperature dependence of ε″ for the nLSNO/PVDF composite shows the thermally activated β–relaxation behavior. The relaxation ε″–peak shifts to high frequencies with increasing temperature from -60 to 20 oC. This is originated at the amorphous regions related to the glass transition, resulting from the cooperative segmental motions. When the temperature increased, the greatly enhanced ε″ is observed, especially in the low–frequency range. This behavior indicates the contribution of dc conduction to ε′. This is also contributed by the MWS effect. According to the effects of particle sizes and related interparticle distances, the nLSNO/PVDF composites filled with smaller particles have higher interfacial areas
16 and shorter interparticle distances (Fig. 5). As shown in Fig. 6(a), the ε′ value of the nLSNO/PVDF composites with f = 0.05 – 0.1 is slightly higher than that of the
µLSNO/PVDF composites. This result is partially caused by a light interfacial effect that is due to the larger interfacial area of the nLSNO particles even though the interfacial polarization is still not dominated in the composites with such low volume fractions. Another reason is that the intrinsic dielectric permittivity of the nLSNO particles might be higher than that of the µLSNO particles. Thus, by incorporating with the same f value, the dielectric permittivity of the nLSNO particles should be higher. Unfortunately, the intrinsic dielectric permittivity of LSNO powders is very difficult to measure. By increasing the f to 0.2, the dielectric permittivity of the nLSNO/PVDF composite diverges from the µLSNO/PVDF composite. This is because the interparticle distance between nLSNO fillers has been reduced to less than 100 nm. The micro–capacitor effect then becomes dominant. When f = 0.3, the interparticle distances of filler particles in the nLSNO/PVDF and µLSNO/PVDF composites are approximately 36 and 384 nm, respectively. The dielectric permittivity of the nLSNO/PVDF composite is therefore much larger than that of the
µLSNO/PVDF composite due to a large valued micro–capacitor that is formed and to a thinner PVDF layer between adjacent nLSNO particles, giving rise to the capacitance value of micro–capacitors. Furthermore, the strong interfacial polarization is too strong, and gives rise to the greatly enhanced dielectric permittivity. At f = 0.4, as we have mentioned, the extreme increase in the dielectric permittivity is attributed to the formation of a percolation network [15], which results in sudden increases in the dielectric permittivity, loss tangent and conductivity. It is important to note that the µLSNO/PVDF composites filled with larger particles require a greater f value to reach the percolation threshold and short interparticle distance, which adversely
17 affects the mechanical properties of the composites. Thus, using nLSNO as filler can possibly benefit both the dielectric properties and the mechanical properties. For application in modern smart microelectronics, besides the improved dielectric properties, the flexibility and mechanical strength of dielectric composite materials are also important. These mechanical properties would be largely degraded by incorporating of high–permittivity ceramic particles with high–volume fractions (f ≥ 0.5) into polymer. Fig. 10 illustrates the typical stress–strain curves of the pure PVDF polymer and nLSNO/PVDF composite films with f = 0.1 – 0.3. The insets show the tensile strength and modulus. The tensile strength tends to decrease with increasing f, while the modulus of the PVDF polymer can be enhanced by incorporating with nLSNO particles. It is ∼1.35 GPa for the composite with f = 0.3, which is larger than that of the pure PVDF film (∼0.54 GPa). By considering the area under the stress– strain curves, it can be estimated that the toughness of the nLSNO/PVDF composite films tends to decrease with increasing f. However, as shown in the inset of Fig. 2(b) and Fig. 10, the nLSNO/PVDF composite film with f = 0.3 is still flexible.
4.
Conclusion Promising LSNO/PVDF composites have been successfully fabricated by
solution processing and hot–pressing methods. The µLSNO and nLSNO particles are used as fillers to investigate the effects of particle size, interfacial area and interparticle distances on the dielectric behaviors of PVDF–based composites. The particle size has an influence on the percolation threshold of the LSNO/PVDF composites. Abrupt changes in the dielectric properties and electrical conductivity are observed only in the nLSNO/PVDF composites, while the dielectric properties of the
µLSNO/PVDF composites changed continuously. Nevertheless, by optimization of
18 the filler volume fraction, the nLSNO/PVDF composite with f = 0.3 shows good flexibility and exhibits a high ε′ ≈ 565.9 and a low tanδ ≈ 0.28 (at 1 kHz and RT), while the µLSNO/PVDF composite exhibits a very low ε′ ≈ 56.0. The results show that the intrinsically high ε′ value of the nLSNO and µLSNO particles primarily contributes to the dielectric response in the composites when f ≤ 0.2 and f ≤ 0.4, respectively. The large increase in the dielectric response in the nLSNO/PVDF composites with f ≥ 0.3 is attributed to the combined effects of the interfacial polarization at the interface of nLSNO–PVDF with much larger interfacial areas and to the shorter interparticle distances between nLSNO particles compared to those for
µLSNO particles in the composites. The lowered tanδ value can be attributed by the formation of a micro–capacitor microstructure. These flexible nLSNO/PVDF composites with such great dielectric performance are potential composite materials for dielectric charge storage capacitors and for electromagnetic wave absorption applications.
Acknowledgments This work was financially supported by the Synchrotron Light Research Institute, Khon Kaen University, and the Thailand Research Fund (TRF) [Grant No. BRG6180003]. This work has been partially supported by the Research Network NANOTEC (RNN) program of the National Nanotechnology Center (NANOTEC), NSTDA, Ministry of Science and Technology and Khon Kaen University, Thailand. K.M. would like to thank the Thailand Research Fund under the Royal Golden Jubilee Ph.D. Program [Grant No. PHD/0191/2556] for his Ph.D. scholarship.
19
References [1] Prateek, V.K. Thakur, R.K. Gupta, Recent Progress on Ferroelectric PolymerBased Nanocomposites for High Energy Density Capacitors: Synthesis, Dielectric Properties, and Future Aspects, Chem Rev 116(7) (2016) 4260-317. [2] Z.-M. Dang, J.-K. Yuan, J.-W. Zha, T. Zhou, S.-T. Li, G.-H. Hu, Fundamentals, processes and applications of high-permittivity polymer–matrix composites, Prog. Mater. Sci. 57(4) (2012) 660-723. [3] P. Martins, A.C. Lopes, S. Lanceros-Mendez, Electroactive phases of poly(vinylidene fluoride): Determination, processing and applications, Prog. Polym. Sci. 39(4) (2014) 683-706. [4] X. Lin, L. Fan, D. Ren, Z. Jiao, P. Coates, W. Yang, Enhanced dielectric properties of immiscible poly (vinylidene fluoride)/low density polyethylene blends by inducing multilayered and orientated structures, Composites Part B: Engineering 114 (2017) 58-68. [5] M. Arbatti, X. Shan, Z.Y. Cheng, Ceramic–Polymer Composites with High Dielectric Constant, Advanced Materials 19(10) (2007) 1369-1372.
20 [6] H. Li, F. Liu, B. Fan, D. Ai, Z. Peng, Q. Wang, Nanostructured FerroelectricPolymer Composites for Capacitive Energy Storage, Small Methods 2(6) (2018) 1700399. [7] Z. Pu, X. Zheng, Y. Tian, L. Hu, J. Zhong, Flexible Ultrahigh-Temperature Polymer-Based Dielectrics with High Permittivity for Film Capacitor Applications, Polymers 9(11) (2017) 596. [8] C. Wang, W.-Y. Lee, R. Nakajima, J. Mei, D.H. Kim, Z. Bao, Thiol–ene CrossLinked Polymer Gate Dielectrics for Low-Voltage Organic Thin-Film Transistors, Chemistry of Materials 25(23) (2013) 4806-4812. [9] T. Zhou, J.W. Zha, R.Y. Cui, B.H. Fan, J.K. Yuan, Z.M. Dang, Improving dielectric properties of BaTiO3/ferroelectric polymer composites by employing surface hydroxylated BaTiO3 nanoparticles, ACS Appl Mater Interfaces 3(7) (2011) 2184-8. [10] S. Luo, S. Yu, R. Sun, C.-P. Wong, Nano Ag-Deposited BaTiO3 Hybrid Particles as Fillers for Polymeric Dielectric Composites: Toward High Dielectric Constant and Suppressed Loss, ACS Applied Materials & Interfaces 6(1) (2014) 176-182. [11] S. Dash, R.N.P. Choudhary, M.N. Goswami, Enhanced dielectric and ferroelectric properties of PVDF-BiFeO3 composites in 0–3 connectivity, J. Alloys Compd. 715 (2017) 29-36. [12] X. Chen, X. Han, Q.-D. Shen, PVDF-Based Ferroelectric Polymers in Modern Flexible Electronics, Advanced Electronic Materials 3(5) (2017) 1600460. [13] A. Toor, H. So, A.P. Pisano, Improved Dielectric Properties of Polyvinylidene Fluoride Nanocomposite Embedded with Poly(vinylpyrrolidone)-Coated Gold Nanoparticles, ACS Applied Materials & Interfaces 9(7) (2017) 6369-6375.
21 [14] N. Phromviyo, N. Chanlek, P. Thongbai, S. Maensiri, Enhanced dielectric permittivity with retaining low loss in poly(vinylidene fluoride) by incorporating with Ag nanoparticles synthesized via hydrothermal method, Appl. Surf. Sci. 446 (2018) 59-65. [15] C.W. Nan, Y. Shen, J. Ma, Physical Properties of Composites Near Percolation, Annu. Rev. Mater. Res. 40(1) (2010) 131-151. [16] K. Meeporn, N. Chanlek, P. Thongbai, Effects of DC bias on non-ohmic sampleelectrode
contact
and
grain
boundary
responses
in
giant-permittivity
La1.7Sr0.3Ni1−xMgxO4 ceramics, RSC Advances 6(94) (2016) 91377-91385. [17] K. Meeporn, T. Yamwong, S. Pinitsoontorn, V. Amornkitbamrung, P. Thongbai, Grain size independence of giant dielectric permittivity of CaCu3Ti4−xScxO12 ceramics, Ceram. Int. 40(10, Part A) (2014) 15897-15906. [18] K. Meeporn, T. Yamwong, P. Thongbai, La1.7Sr0.3NiO4 nanocrystalline powders prepared by a combustion method using urea as fuel: Preparation, characterization, and their bulk colossal dielectric constants, Japanese Journal of Applied Physics 53(6S) (2014) 06JF01. [19] S. Wang, Y.-D. Hou, M.-P. Zheng, H.-Y. Ge, M.-K. Zhu, H. Yan, Roomtemperature dielectric relaxation and magnetic properties in rutile-type FeTiNbO6, Materials Research Bulletin 48(9) (2013) 3098-3102. [20] Y.Y. Liu, X.M. Chen, X.Q. Liu, L. Li, Giant dielectric response and relaxor behaviors induced by charge and defect ordering in Sr(Fe1⁄2Nb1⁄2)O3 ceramics, Applied Physics Letters 90(19) (2007) 192905. [21] P. Zheng, J.L. Zhang, Y.Q. Tan, C.L. Wang, Grain-size effects on dielectric and piezoelectric properties of poled BaTiO3 ceramics, Acta Materialia 60(13-14) (2012) 5022-5030.
22 [22] S. Krohns, P. Lunkenheimer, C. Kant, A.V. Pronin, H.B. Brom, A.A. Nugroho, M. Diantoro, A. Loidl, Colossal dielectric constant up to gigahertz at room temperature, Appl. Phys. Lett. 94(12) (2009) 122903. [23] S. Krohns, P. Lunkenheimer, S. Meissner, A. Reller, B. Gleich, A. Rathgeber, T. Gaugler, H.U. Buhl, D.C. Sinclair, A. Loidl, The route to resource-efficient novel materials, Nat Mater 10(12) (2011) 899-901. [24] K. Meeporn, S. Maensiri, P. Thongbai, Abnormally enhanced dielectric permittivity in poly(vinylidene fluoride)/nanosized-La2NiO4−δ films, Applied Surface Science 380 (2016) 67-72. [25] K. Meeporn, P. Thongbai, T. Yamwong, S. Maensiri, Greatly enhanced dielectric permittivity in La1.7Sr0.3NiO4/poly(vinylidene fluoride) nanocomposites that retained a low loss tangent, RSC Advances 7(28) (2017) 17128-17136. [26] P. Martins, J.S. Nunes, G. Hungerford, D. Miranda, A. Ferreira, V. Sencadas, S. Lanceros-Méndez, Local variation of the dielectric properties of poly(vinylidene fluoride) during the α- to β-phase transformation, Phys. Lett. A 373(2) (2009) 177180. [27] R.E. Sousa, J. Nunes-Pereira, J.C.C. Ferreira, C.M. Costa, A.V. Machado, M.M. Silva, S. Lanceros-Mendez, Microstructural variations of poly(vinylidene fluoride cohexafluoropropylene) and their influence on the thermal, dielectric and piezoelectric properties, Polym. Test. 40 (2014) 245-255. [28] P. Costa, J. Silva, V. Sencadas, C.M. Costa, F.W.J. van Hattum, J.G. Rocha, S. Lanceros-Mendez, The effect of fibre concentration on the α to β-phase transformation, degree of crystallinity and electrical properties of vapour grown carbon nanofibre/poly(vinylidene fluoride) composites, Carbon 47(11) (2009) 25902599.
23 [29] C. Ribeiro, C.M. Costa, D.M. Correia, J. Nunes-Pereira, J. Oliveira, P. Martins, R. Gonçalves, V.F. Cardoso, S. Lanceros-Méndez, Electroactive poly(vinylidene fluoride)-based structures for advanced applications, Nat. Protoc. 13 (2018) 681. [30] C. Ben Osman, S. Nowak, A. Garcia-Sanchez, Y. Charles, S. Ammar, S. Mercone, F. Mammeri, In situ monitored stretching induced α to β allotropic transformation of flexible poly(vinylidene fluoride)-CoFe2O4 hybrid films: The role of nanoparticles inclusion, Eur. Polym. J. 84 (2016) 602-611. [31] Z. Wang, M. Fang, H. Li, Y. Wen, C. Wang, Y. Pu, Enhanced dielectric properties in poly(vinylidene fluoride) composites by nanosized Ba(Fe0.5Nb0.5)O3 powders, Composites Science and Technology 117 (2015) 410-416. [32] W. Li, Q. Meng, Y. Zheng, Z. Zhang, W. Xia, Z. Xu, Electric energy storage properties of poly(vinylidene fluoride), Appl. Phys. Lett. 96(19) (2010) 192905. [33] B. Luo, X. Wang, Y. Wang, L. Li, Fabrication, characterization, properties and theoretical analysis of ceramic/PVDF composite flexible films with high dielectric constant and low dielectric loss, Journal of Materials Chemistry A 2(2) (2014) 510519. [34] R. Jr, M. Cestari, Effect of crystallization temperature on the crystalline phase content and morphology of poly(vinylidene fluoride), Journal of Polymer Science Part B: Polymer Physics 32 (1994) 859-870. [35] K.S. Deepa, M.T. Sebastian, J. James, Effect of interparticle distance and interfacial area on the properties of insulator-conductor composites, Applied Physics Letters 91(20) (2007) 202904. [36] M.-F. Lin, V.K. Thakur, E.J. Tan, P.S. Lee, Surface functionalization of BaTiO3 nanoparticles and improved electrical properties of BaTiO3/polyvinylidene fluoride composite, RSC Advances 1(4) (2011) 576.
24 [37] A. Srivastava, K.K. Jana, P. Maiti, D. Kumar, O. Parkash, Poly(vinylidene fluoride)/ CaCu3Ti4O12 and La doped CaCu3Ti4O12 composites with improved dielectric and mechanical properties, Materials Research Bulletin 70 (2015) 735-742. [38] Z. Wang, M. Fang, H. Li, Y. Wen, C. Wang, Y. Pu, Enhanced dielectric properties in poly(vinylidene fluoride) composites by nanosized Ba(Fe0.5Nb0.5)O3 powders, Composites Science and Technology 117 (2015) 410-416. [39] Y.-l. Su, C. Sun, W.-q. Zhang, H. Huang, Fabrication and dielectric properties of Na0.5Bi0.5Cu3Ti4O12/poly(vinylidene fluoride) composites, J. Mater. Sci. 48(23) (2013) 8147-8152. [40] L. Ren, X. Meng, J.-W. Zha, Z.-M. Dang, Coulomb block effect inducing distinctive
dielectric
properties
in
electroless
plated
barium
titanate@silver/poly(vinylidene fluoride) nanocomposites, RSC Advances 5(80) (2015) 65167-65174. [41] H. Tang, Z. Zhou, C.C. Bowland, H.A. Sodano, Synthesis of calcium copper titanate (CaCu 3 Ti 4 O 12 ) nanowires with insulating SiO2 barrier for low loss high dielectric constant nanocomposites, Nano Energy 17 (2015) 302-307. [42] C.-W. Nan, Physics of inhomogeneous inorganic materials, Prog. Mater. Sci. 37(1) (1993) 1-116. [43] A. Maceiras, C.M. Costa, A.C. Lopes, M. San Sebastián, J.M. Laza, J.L. Vilas, J.L.G. Ribelles, R. Sabater i Serra, A. Andrio Balado, S. Lanceros-Méndez, L.M. León, Dielectric relaxation dynamics of high-temperature piezoelectric polyimide copolymers, Applied Physics A 120(2) (2015) 731-743. [44] A.C. Lopes, C.M. Costa, R.S.i. Serra, I.C. Neves, J.L.G. Ribelles, S. LancerosMéndez,
Dielectric
relaxation,
ac
conductivity
and
electric
modulus
in
25 poly(vinylidene fluoride)/NaY zeolite composites, Solid State Ionics 235 (2013) 4250. [45] A.C. Lopes, M.P. Silva, R. Gonçalves, M.F.R. Pereira, G. Botelho, A.M. Fonseca, S. Lanceros-Mendez, I.C. Neves, Enhancement of the Dielectric Constant and Thermal Properties of α-Poly(vinylidene fluoride)/Zeolite Nanocomposites, The Journal of Physical Chemistry C 114(34) (2010) 14446-14452. [46] J. Fu, Y. Hou, Q. Wei, M. Zheng, M. Zhu, H. Yan, Advanced FeTiNbO6/poly(vinylidene fluoride) composites with a high dielectric permittivity near the percolation threshold, J. Appl. Phys. 118(23) (2015) 235502. [47] F. He, S. Lau, H.L. Chan, J. Fan, High Dielectric Permittivity and Low Percolation Threshold in Nanocomposites Based on Poly(vinylidene fluoride) and Exfoliated Graphite Nanoplates, Adv. Mater. 21(6) (2009) 710-715. [48] P.W. Jaschin, R. Bhimireddi, K.B.R. Varma, Enhanced Dielectric Properties of LaNiO3/BaTiO3/PVDF: A Three-Phase Percolative Polymer Nanocrystal Composite, ACS Appl Mater Interfaces 10(32) (2018) 27278-27286.
26 LIST OF FIGURE CAPTIONS Fig. 1.
(a–b) TEM images of LSNO ceramic powders for µLSNO and nLSNO. (c–
d) SEM images of LSNO ceramic powders for µLSNO and nLSNO. Fig. 2.
(a−b) SEM images of fractured surfaces of pure PVDF films and
nLSNO/PVDF composites with f = 0.10; inset of (b) shows the flexibility of nLSNO/PVDF composite f = 0.3. (c−d) Optical microscope images showing the dispersion sates of LSNO fillers in µLSNO/PVDF and nLSNO/PVDF composites with f = 0.3, respectively. (e−f) SEM images of the inner opened−surfaces for µLSNO/PVDF and nLSNO/PVDF composites with f = 0.3, respectively. Fig. 3.
(a−b) FESEM image and elemental mapping analysis for all elements of
nLSNO/PVDF composite with f = 0.3. (c−h) EDS elemental mapping images of C, F, La, Sr, Ni, and O, respectively. Fig. 4.
(a) XRD patterns of nLSNO powder, pure PVDF, and nLSNO/PVDF
composites with various f. (b) FTIR spectra of (1) pure PVDF film and (2−6) nLSNO/PVDF composites with f = 0.05, 0.10, 0.20, 0.30, and 0.40, respectively. (c) TGA curves of pure PVDF and nLSNO/PVDF composites with various volume fractions of nLSNO. Fig. 5.
Variation of interparticle distance as a function of volume fractions of filler
(f) for µLSNO/PVDF and nLSNO/PVDF composites. Fig. 6.
Variation of (a) dielectric constant (ε′) and (b) loss tangent (tanδ) as a
function of filler volume fraction (f) for µLSNO/PVDF and nLSNO/PVDF composites; insets of (a) and (b) show the enlarged scale from f = 0.05 to 0.2 and the correlation between the dielectric constant (ε′) and F(β) of the nLSNO/PVDF composites, respectively.
27 Fig. 7.
Frequency dependence of (a) dielectric constant (ε′) and (b) loss tangent
(tanδ) for nLSNO/PVDF composites with various volume fractions of nLSNO. Fig. 8.
(a) Frequency dependence of ac conductivity (σ) of nLSNO/PVDF
composites with various volume fractions of nLSNO. (b) The ac conductivity (σ) as a function of the volume fraction of nLSNO; inset shows the frequency dependence of the imaginary part (ε′′) of the complex dielectric permittivity (ε*). Fig. 9.
Temperature dependence of the (a) dielectric constant (ε′) and (b) loss
tangent (tanδ) at 1 kHz for nLSNO/PVDF and µLSNO/PVDF composites with f = 0.3; inset shows the frequency dependence of ε′′ at different temperatures. Fig. 10. Tensile stress−strain properties of pure PVDF film and nLSNO/PVDF composite with f = 0.10 − 0.30; insets shows the results of tensile strength and Young's modulus for the pure PVDF film and nLSNO/PVDF composite.
28 Fig. 1
29 Fig. 2
30 Fig. 3
31
(215)
(118) (220)
(200)
f = 0.40 f = 0.30 f = 0.20 f = 0.10 f = 0.05
*
β (110)
γ(020) β(200)
10
α(200) α(002)
20
(b)
■
(6) (5) (4)
30
▼
40
2θ (Deg) (c)
♦♦ ■ ▼
▼
pure PVDF film
50
60
70
100
90 80
(3)
mass (%)
Transmittance (%)
(211) (116) (107) (204) (213)
nLSNO
(112)
(004)
(101)
Intensity (arb. unit)
* PVDF
(006) (114)
(103)
(a)
(110)
Fig. 4
(2) (1)
▼ α phase
1400
840 766
40
■ γ phase
30
1070 1179
♦ β phase
1200
60 50
976 1401
70
878
1000
800 -1
Wave number (cm )
f = 0.40 f = 0.30 f = 0.20 f = 0.10 f = 0.05 pure PVDF
200 300 400 500 600 700 800 o
Temperature ( C)
32
Interparticle distance, d (µm)
Fig. 5
2.4 µLSNO/PVDF nLSNO/PVDF
2.0 1.6 1.2 0.8 0.4 0.0 0.0
0.1
0.2
0.3
Volume fraction (f)
0.4
33 Fig. 6
34 Fig. 7
Dielectric Constant (ε')
(a) 105 f=0 f = 0.30
4
10
f = 0.05 f = 0.40
f = 0.10
f = 0.20
3
10
2
10
1
10
2
10
3
4
10
5
10
6
10
10
Frequency (Hz)
(b) 10 Loss Tangent (tanδ)
4
f=0 f = 0.30
3
10
f = 0.05 f = 0.40
f = 0.10
f = 0.20
2
10
1
10
0
10
-1
10
-2
10
2
10
3
10
4
10
Frequency (Hz)
5
10
6
10
35 Fig. 8 (b) 1.0x10-4
-2
10
f = 0.05 f = 0.40
f = 0.10
f = 0.20
-4
10
-6
10
-8
10
-10
10
10
7
10
5
-5
8.0x10
-5
6.0x10
ε''
f=0 f = 0.30
Conductivity, σ (S.cm-1)
Conductivity, σ (S.cm-1)
(a) 100
-5
4.0x10
10
3
10
1
10 -5
2.0x10
f=0 f = 0.10 f = 0.30
f = 0.05 f = 0.20 f = 0.40
-1
10
2
10
3
10
4
10
5
10
6
Frequency (Hz)
0.0 2
10
3
10
4
10
Frequency (Hz)
5
10
6
10
0.0
0.1
0.2
0.3
Volume fraction (f)
0.4
36
Fig. 9
700
f = 0.3 @ 1 kHz
500
o
-20 C o 20 C o 60 C o 70 C o 90 C
400 300
ε''
Dielectric Constant (ε')
(a)
200 100 0
100
2
3
4
5
6
Log(f)
µLSNO/PVDF nLSNO/PVDF
30 -60
-40
-20
0
20
40
60
80
100
Temperature (°C)
Loss Tangent (tanδ)
(b)
0.5
µLSNO/PVDF nLSNO/PVDF
0.4 0.3 0.2 0.1 0.0
f = 0.3 @ 1 kHz
-60 -40 -20 0 20 40 60 80 100 Temperature (°C)
37
Fig. 10
Pure PVDF f = 0.10 f = 0.20 f = 0.30
24 20 16
8 4
30 25 20 15 10 5 0
0
10
20
30
Volume fraction (f)
0
0
5
Modulus (MPa)
12 Stress (MPa)
Tensile Stress (MPa)
28
1500 1200 900 600 300 0
0
10
20
30
Volume fraction (f)
10
Tensile Strain (%)
15
20