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polydimethylsiloxane nanocomposite films and revealing their dielectric and impedance properties
Ceramics International 45 (2019) 8713–8720
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Ceramics International journal homepage: www.elsevier.com/locate/ceramint
Synthesis of three phase graphene/titania/polydimethylsiloxane nanocomposite films and revealing their dielectric and impedance properties
T
Saira Ishaqa,b,c, Farah Kanwala,∗∗, Shahid Atiqd, Mahmoud Moussab,c, Dusan Losicb,c,∗ a
Institute of Chemistry, University of the Punjab, Lahore, 54590, Pakistan School of Chemical Engineering, The University of Adelaide, Adelaide, 5005, SA, Australia c The ARC Research Hub for Graphene Enabled Industry Transformation, The University of Adelaide, Adelaide, 5005, SA, Australia d Centre of Excellence in Solid State Physics, University of the Punjab, Lahore, 54590, Pakistan ∗ Corresponding author. School of Chemical Engineering, The University of Adelaide, Adelaide, 5005, SA, Australia. b
A R T I C LE I N FO
A B S T R A C T
Keywords: Graphene Titania Polydimethylsiloxane Dielectric permittivity AC conductivity
Compounds and composites with high dielectric performance are in high demand in electronic industry due to their ability to store more charge and lower energy dissipation. In this paper we present synthesis of flexible three phase nanocomposite films of graphene/titania/polydimethylsiloxane with reasonably high real part of permittivity and very low imaginary part of permittivity. Dielectric permittivity of polydimethylsiloxane was raised by addition of graphene and rutile titania used as conducting and ceramic additives, respectively added in various weight ratios. Dielectric studies of synthesized three phase graphene/titania/polydimethylsiloxane nanocomposite films with weight/weight of 2:15:20 exhibits dielectric permittivity of 34.8 even at very high frequency of 2 MHz. While loss tangent of the same three phase graphene/titania/polydimethylsiloxane nanocomposite film is as low as 0.01 and its AC conductivity is 4.3 × 10−5 Sm−1. Complex impedance and complex electric modulus of three phase graphene/titania/polydimethylsiloxane nanocomposite films also confirm its capacitive behaviour. Based on these properties and results we propose that these type of three phase graphene/ titania/polydimethylsiloxane nanocomposite films can be used as useful dielectric materials in energy storage devices.
1. Introduction Polymer based composites bearing high dielectric constant (ɛ') as well as low dielectric loss (ɛ") have gain considerable attention in past few years due to their flexibility, mechanical strength and less amount required for miniaturization of energy storage devices including transistors, gate dielectrics and other portable flexible electronics [1–3]. However, challenge is to keep balance between high ɛ' and low ɛ". Recently two approaches are being used to obtain flexible dielectric composites. First is to increase ɛ' of polymer by addition of conducting fillers which can increase ɛ' by following percolative phenomenon. Second is addition of ceramic fillers having high dielectric permittivity. Both approaches have advantages as well as disadvantages. Former has advantage of elevation of ɛ' even with very small concentration but conducting fillers also cause increase of ɛ" due to development of well aligned pathways which increase leakage current density. As far as ceramic fillers are concerned, although they increase ɛ' of polymer but
they need to be added in high concentration which causes brittleness of polymer thus reducing its mechanical strength [4,5]. Another strategy may be to create barriers between conducting layers of conducting fillers to avoid leakage current [6,7]. Wang et al., reported synthesis of PVA coated graphene to use as filler in PVDF matrix to avoid direct contact of graphene sheets with each other to reduce leakage current. As a result of this coating, at 100 Hz, ɛ' was increased from 100 to 230 and loss tangent (tanδ) was decreased to 0.5 [8]. Similarly Li et al., synthesized insulating PANI coated graphene fillers into poly(methyl methacrylate) (PMMA). Insulating coating increased ɛ' up to 40.6 from 20 and decreased tanδ from 1250 to 1.12 at 1000 Hz [9]. Another approach may be to incorporate both conducting and ceramic fillers in polymers to achieve composite with desired properties. Some researchers have recently reported such ternary composites. Guan et al., synthesized ternary carboxyl functionalized multiwalled carbon nanotubes/barium titanate/polydimethylsiloxane (cMWCNT/BT/PDMS) composites having ɛ' of 423 at 100 Hz and tanδ of 8.4 at 1 kHz [10]. Qi
∗∗
Corresponding author. E-mail addresses: [email protected] (F. Kanwal), [email protected] (D. Losic).
https://doi.org/10.1016/j.ceramint.2019.01.194 Received 14 November 2018; Received in revised form 23 January 2019; Accepted 24 January 2019 Available online 25 January 2019 0272-8842/ © 2019 Elsevier Ltd and Techna Group S.r.l. All rights reserved.
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Fig. 1. Scheme of the synthesis of G/TiO2/PDMS nanocomposite films.
Australia, Australia) were supplied from an Australian mining site. Polydimethylsiloxane (PDMS) prepolymer (Sylgard 184) and curing agent (Sylgard 184) were purchased from Dow Corning Corportaion (USA). Potassium permanganate (97% KMnO4, Sigma-Aldrich), phosphoric acid (85% H3PO4, Chem-Supply), sulfuric acid (98% H2SO4, Chem-Supply), hydrogen peroxide (30% H2O2, Chem-Supply), hydrochloric acid (35% HCl, Chem-Supply) and titanium(IV) oxide (TiO2, rutile, 99.98% trace metal basis, Aldrich) were used without further purification.
et al., reported synthesis of polyamide11/barium titanate/carbon nanotube (PA11/BaTiO3/CNT) nanocomposites having ɛ' of 16.2 at 1000 Hz and tanδ of 0.15 at 100 Hz [11]. In this research work, we report synthesis of three phase graphene/ titania/polydimethysiloxane (G/TiO2/PDMS) nanocomposite films in which graphene sheets were segregated from each other due to presence of ceramic filler and explore their dielectric and impedance properties. To make three phase composite we initially explore and optimize each component considering their different properties. Firstly, we optimized addition of graphene fillers where weight concentration of graphene was varied until it reached its percolation fraction. Then keeping it fixed, concentration of ceramic filler (rutile titania, TiO2) was added till maximum ɛ' was achieved. Titania has high ɛ' of 114 at room temperature (RT) [12], but it has disadvantage of mechanical instability [13]. However, when both conducting and ceramic additives are used together it can further rise ɛ' of the polymer. So to address this limitation we used graphene as conducting filler due to its high aspect ratio, large surface area, high electrical conductivity and high modulus [14]. PDMS, a silicon based non conducting elastomer, was selected as base polymer matrix, due to its excellent flexibility [15]. Graphene sheets were found to form electrodes of many microcapacitors formed and ceramic filler and polymer matrix acted like dielectric medium between electrodes. Tanδ was suppressed due to random distribution of graphene sheets, which are not continuously aligned to circumvent tunneling conductance that contributes to tanδ. Mostly dielectric materials have low ɛ' at elevated frequencies, but currently synthesized G/ TiO2/PDMS nanocomposite films is proposed to have sustained high ɛ' even at high frequency, while still maintaining low tanδ. Morphological and chemical composition of prepared composite was characterized by FESEM following by testing their comparative dielectric properties and precision impedance at frequency range of 20 Hz - 2 MHz by using copper electrodes.
2.2. Synthesis of G/TiO2/PDMS nanocomposite films Oxidation of graphite powder was carried out by a previously reported method to form graphene oxide (GO) [16]. GO was converted to graphene by reduction of GO in the presence of hydrazine hydrate which was used as reducing agent [17]. To synthesize three phase G/ TiO2/PDMS nanocomposite film, graphene and rutile TiO2 in w/w of 1:2 were mixed together during magnetic stirring in Polydimethylsiloxane (PDMS) prepolymer (Sylgard 184) and curing agent (Sylgard 184) taken in 10:1. The mixture was then poured in petri dish followed by degassing by using vacuum pump. Mixture was oven dried at 50 °C for 24 h to obtain G/TiO2/PDMS nanocomposite film. A series of nanocomposite films of G/TiO2/PDMS was synthesized by above described method in which graphene concentration was increased in order to get optimum value of its concentrations while w/w of TiO2, PDMS prepolymer and curing agent were kept constant. After making dielectric studies of all synthesized G/TiO2/PDMS nanocomposite films, graphene weight in G/TiO2/PDMS nanocomposite film giving maximum ɛ' and minimum tanδ was chosen as optimum and a new series of G/TiO2/PDMS nanocomposite films was fabricated by changing weight of rutile TiO2 while concentrations of other components were kept fixed. Fig. 1 depicts scheme of formation of G/TiO2/ PDMS nanocomposite film. Sample codes and w/w of graphene, rutile titania and polymer in all synthesized G/TiO2/PDMS nanocomposite films are summarized in Table 1.
2. Materials and methods 2.1. Materials Commercial natural graphite rocks (Uley, Eyre Peninsula, South 8714
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with previous reports [20]. Thermal stability of graphene, rutile TiO2, neat PDMS, GTPDMS-II and GTPDMS-VI nanocomposite films was characterized by TGA carried out in temperature range of 30–900 °C and is presented in Fig. 4. TGA graphs show that graphene starts degrading at about 380 °C and completely degrades at 590 °C. Rutile TiO2 is thermally stable, even at 900 °C it does not degrade. Neat PDMS film degrades up to 45% at 520 °C and after which its weight becomes constant. For GTPDMS-II and GTPDMS-VI, constant weight loss was observed above 400 °C that is due to degradation of PDMS which occurs as a result of breakage of SieSi bonds which produces silyl radicals which strike with CeH and SieO to evolve gases like H2 and CH4. This evolution of gases causes weight loss between 300 and 550 °C [21]. However GTPDMS-II is thermally more stable than neat PDMS due to presence of graphene and TiO2. With further increase in concentration of TiO2 thermal stability is further increased in GTPDMS-VI. Maximum weight loss for neat PDMS, GTPDMS-II and GTPDMS-VI is 46%, 43% and 29%, respectively.
Table 1 Sample codes with w/w fraction of components of ternary G/TiO2/ PDMS nanocomposite films. Sample code
w/w fraction of G/TiO2/PDMS
Neat PDMS GTPDMS-I GTPDMS-II GTPDMS-III GTPDMS-IV GTPDMS-V GTPDMS-VI GTPMS-VII
0:0:100 1:2:20 2:2:20 3:2:20 2:5:20 2:10:20 2:15:20 2:20:20
2.3. Characterization Field emission scanning electron microscopy (FESEM, Quanta 450, FEI, USA) of neat PDMS and three phase G/TiO2/PDMS nanocomposite films was carried out to explicit their morphology. Structural analyses of neat PDMS and three phase G/TiO2/PDMS nanocomposite films were made by using X-ray diffraction (XRD, 600 Miniflex, Rigaco, Japan). Fourier transform infrared (FTIR) spectroscopy (Nicolet 6700 Thermo Fisher) was carried out to locate the functional groups in the neat PDMS and synthesized G/TiO2/PDMS nanocomposite films. Thermal gravimetric analyzer (TGA, Q500, TA Instruments, USA) was used to make thermal analyses under air by heating all films from RT to 900 °C while heating them at rate of 10 °Cmin−1 Dielectric and impedance analyses of neat PDMS and all G/TiO2/PDMS nanocomposite films at RT were done by using precision impedance analyzer (Wayer Kerr, 6500B) at frequency range of 20 Hz - 2 MHz by using copper electrodes.
3.2. Dielectric and impedance studies of G/TiO2/PDMS nanocomposite films The ɛ' values of neat PDMS and all synthesized G/TiO2/PDMS nanocomposite films are plotted against log f in Fig. 5a. It is clear from the plots that values of ɛ' are high at low frequencies due to enhanced Maxwell Wagner Sillars (MWS) effect or interfacial polarization (IP). IP or MWS effect is increased in low frequency region. When electric field is applied induced and permanent dipoles have enough time to align themselves. With increase in frequency, ɛ' decreases rapidly and becomes low and steady at high frequency region as at high frequency region dipoles do not have enough time for alignment in proper orientation, this phenomenon is called polarization relaxation phenomenon [22]. These results indicate that at elevated frequency, ɛ' becomes frequency independent due to microcapacitor structure model [5]. ɛ' values of synthesized nanocomposite elevates with rising graphene w/w till GBPDMS-II. GTPDMS-II has ɛ΄ of 19.5 at 2 MHz that is about 1.63 times higher than ɛ΄ of neat PDMS having ɛ' value of 12 at 2 MHz. Rise in conductivity and IP causes elevation of value of ɛ'. Rise of IP results due to accumulation of free charges at interfaces of components, which results in formation of dipoles. Rise of IP is caused by both factors; enhanced interfacial area and homogenous distribution of additives [22,23]. Microcapacitors were created and explored within three phase nanocomposite films where conducting graphene sheets act as electrodes and rutile TiO2 and PDMS provide insulating paths. As w/w of graphene increases more microcapacitors are formed to rise ɛ' values of the three phase nanocomposite films. A stage reaches where further rise of graphene fraction causes decrease of ɛ' due to development of well connected and efficient conducting pathways which results in rise of leakage current causing depression of ɛ' [24]. ɛ' value of three phase G/ TiO2/PDMS nanocomposite films further rises with raising weight of rutile TiO2 and after reaching a maximum value at percolation weight fraction, it starts depressing with further increasing TiO2 fraction. GTPDMS-VI bears highest ɛ' value of 34.8 among neat PDMS and all G/ TiO2/PDMS nanocomposite films. Exhibited ɛ' value of GTPDMS-VI is about 3 times greater than ɛ' value of neat PDMS film at 2 MHz. Improvement in ɛ' is caused due to addition of rutile TiO2 nanoparticles having good ɛ' instead of ɛ' of PDMS or graphene from GBPDMS-IV to GBPDMS-VI. This improvement of ɛ' is caused by formation of efficient network to transport charge thus resulting in improved conductivity. Network meant for transport of charge becomes more efficient with homogenous dispersion of rutile TiO2 in PDMS and it is in accordance with previous findings [25,26]. However when w/w of rutile TiO2 is further increased ɛ' value in GBPDMS-VII decreases due to formation of big lumps as is visible in Fig. 2g. Large rutile TiO2 aggregations may destruct charge transportation network resulting in decrease in value of
3. Results and discussions 3.1. Characterization of G/TiO2/PDMS nanocomposite films The FESEM images depicting typical morphology of synthesized G/ TiO2/PDMS nanocomposite films are presented in Fig. 2. Images (Fig. 2a–g) clearly indicate the presence of all three components of nanocomposite. It is evident that conducting graphene layers in PDMS matrix are not continuously connected, due to presence of ceramic nanoparticles. Graphene sheets and rutile TiO2 nanoparticles are homogenously distributed in the PDMS. Nanoparticles of rutile TiO2 are distributed on graphene layers and they are also embedded in PDMS. Fig. 2g clearly shows that as weight of rutile TiO2 increases, its nanoparticles start to coagulate and form aggregates. A comparison of XRD patterns of neat PDMS and all synthesized G/ TiO2/PDMS nanocomposite films is shown in Fig. 3a. A very broad peak at 22° in XRD pattern of neat PDMS indicates that PDMS is in amorphous form [18]. Peaks for rutile TiO2 present at 27.9° (110), 36.9° (101), 39.9° (200), 41.8° (111), 44.8° (210), 54.7° (211), and 56.5° (220), 62.8° (002), 64.8° (310) and 69.6° (112) are according to JCPDS card no. 77-0441 [19]. In all synthesized G/TiO2/PDMS nanocomposites films, characteristic peak for graphene does not appears because of very broad peak for PDMS. With increasing amount of both graphene and rutile TiO2 fillers, characteristic peak of PDMS at 22° becomes less intense suggesting that graphene and rutile TiO2 fillers are interacting with each other [16]. FTIR spectra of neat PDMS and all synthesized G/TiO2/PDMS nanocomposite films are compared in Fig. 3b. Characteristic peak at 785 cm−1 corresponds to SieC, while peaks at 1007 cm−1 and 1080 cm−1 correspond to SieOeSi. Peak at 1257 cm−1 is attributed to deformation vibration of two eCH3 linked with Si while Peak at 2960 cm−1 represents stretching vibration of CeH. No new peak is observed in FTIR spectra of G/TiO2/PDMS compared with the FTIR spectrum of neat PDMS membrane which shows that the graphene and TiO2 are only physically blended with PDMS. This result is in agreement 8715
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Fig. 2. FESEM images of synthesized G/TiO2/PDMS nanocomposite films with different composition (a) GTPDMS-I (b) GTPDMS-II (c) GTPDMS-III (d) GTPDMS-IV (e) GTPDMS-V (f) GTPDMS-VI (g) GTPDMS-VII.
ɛ'. Values of ɛ' of neat PDMS, GTPDMS-II and GTPDMS-VI taken at a frequency range are compared in Table 2. Fig. 5b shows that with increase in frequency, tanδ of PDMS and three phase G/TiO2/PDMS nanocomposite films drops. Tanδ rises with rise in w/w of conducting graphene, due to reason that graphene sheets
become well aligned and connected to each other with increasing concentration and increase leakage current density to increase energy dissipation i.e., tanδ. As soon as electric field is passed conducting graphene layers may cause passage of electric current through them and some transition from electrical energy to thermal energy takes place 8716
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Fig. 3. (a) XRD patterns of neat PDMS and G/TiO2/PDMS nanocomposites films (b) FTIR spectra of neat PDMS and G/TiO2/PDMS nanocomposite films.
explain the process of conduction in PDMS and G/TiO2/PDMS nanocomposite films. Fig. 7a–b show real part of complex impedance (Z′) and imaginary part of complex impedance (Z″) of neat PDMS and three phase G/TiO2/PDMS nanocomposite films plotted as function of log f. Plots show that both parts of complex impedance decrease when frequency is raised and becomes constant at higher frequencies. This behaviour is in agreement with previous research reports [27]. Conduction is increased at high frequencies as more free space charges exist there [28]. The dielectric modulus explains the electrical transport within the dielectric medium. Fig. 8a–b shows real part (M′) and imaginary part (M″) of complex electric modulus plotted as function of log f for PDMS and synthesized three phase G/TiO2/PDMS nanocomposite films. When frequency is raised M′ also rises till reaches a maximum value called asymptotic value M∞ convincing about capacitive nature of G/TiO2/ PDMS nanocomposite films. This result indicates that process of relaxation lies over a long frequency range. Fig. 8b describes that M″ rises and after reaching a highest value M″max starts to drop with further elevation in frequency [29]. It may be explained by considering that when frequency is low charge carriers may move over long distances with alteration of electric field until M″max attains, then when frequency becomes high and electric field alters rapidly charge carriers can move to short distances only as they do not get enough time for alignment [30]. For GTPDMS-VII, M″max moves to relatively smaller frequency region depicting that conduction mechanism is increased due to better conducting network development due to large aggregations of rutile TiO2 nanoparticles. M″max shows that charge movement transits from long to short range from low to high frequencies respectively due to reason that free charges may move from large distance to short distance with increasing in frequency [27]. The ɛ' and tanδ values of synthesized three phase G/TiO2/PDMS nanocomposite film with previously reported ternary composites are compared in Table 3. Although in table previously reported ternary composites seem to have more ɛ' their values are reported at low frequencies. It is clear from the comparison that our synthesized ternary G/TiO2/PDMS nanocomposite has high dielectric constant even at very high frequency of 2 MHz which is usually difficult to attain. Tanδ of recently reported ternary G/TiO2/PDMS nanocomposite is still very low as compared to previously reported ternary composites. So we propose
Fig. 4. TG curves of graphene, TiO2, neat PDMS and G/TiO2/PDMS nanocomposites films (GTPDMS-II, GTPDMS-VI).
[24]. GTPDMS-II has as low tanδ as 0.02 at 2 MHz. However, tanδ further drops when weight of rutile TiO2 increases and reaches to 0.01 for GTPDMS-VI as shown in Fig. 5b and Table 2. But when TiO2 fraction is further increased from GTPDMS-VI to GTPDMS-VII tanδ rises many times. This rise in tanδ with rising weight of rutile TiO2 is due to increase in conductivity due to improved network formation. These results are in agreement with results available in previous literature [25,26]. Fig. 6 shows σac of PDMS and three phase G/TiO2/PDMS nanocomposite films. It shows that with rise of w/w of conducting graphene σac increases due to development of more conducting pathways. Further increase of σac is caused due to increased w/w of rutile TiO2 fraction. This increased conductivity is also responsible for enhanced tanδ. Improved σac of three phase G/TiO2/PDMS nanocomposites with greater weight of TiO2 in GTPDMS-VII can be explained due to formation of a network for transport of charge. These results are in agreement with published research reports in literature [26]. Complex impedance spectroscopy (CIS) study was performed to 8717
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Fig. 5. (a) Real part of permittivity (ɛ΄) of neat PDMS and G/TiO2/PDMS nanocomposites films (b) Loss tangent (tanδ) of neat PDMS and G/TiO2/PDMS nanocomposites films. Table 2 Dielectric permittivity (ɛ'), loss tangent (tanδ) and AC conductivity (σac) of neat PDMS, GTPDMS-II and GTPDMS-VI. Frequency (Hz)
Dielectric permittivity (ɛ') Neat PDMS
20 40 100 103 104 105 106 2 × 106
6.5 4.0 3.8 3.2 3.1 3.1 3.0 2.99
GTPDMS-II 96.7 89.6 36.6 25 21.6 20.1 19.5 19.5
AC conductivity (σac) (Sm−1)
Loss tangent (tanδ) GTPDMS-VI 108.3 84.1 37 36.5 35.8 35.1 34.8 34.8
Neat PDMS 0.5 0.23 0.05 0.01 0.01 0.01 0.01 0.01
GTPDMS-II 1.7 1.3 0.28 0.18 0.08 0.03 0.02 0.02
GTPDMS-VI 0.56 0.4 0.17 0.04 0.02 0.01 0.01 0.01
Neat PDMS −10
7.3 × 10 5.7 × 10−9 2.9 × 10−9 2.2 × 10−9 1.8 × 10−8 2.6 × 10−7 2.5 × 10−6 4.6 × 10−6
GTPDMS-II
GTPDMS-VI
−7
6.8 × 10−8 7.9 × 10−8 3.6 × 10−8 8.7 × 10−8 4.6 × 10−7 2.6 × 10−6 2.2 × 10−5 4.3 × 10−5
1.8 × 10 2.7 × 10−7 6.2 × 10−8 2.4 × 10−7 1 × 10−6 4.3 × 10−6 2.5 × 10−5 4.6 × 10−5
where it was 7.2 at such high frequency. Similarly, tanδ of synthesized G/TiO2/PDMS nanocomposite film is 0.01 i.e., much lower as compared to 0.10 i.e., previously reported G/TiO2/PVA nanocomposite films. Thus, synthesized G/TiO2/PDMS nanocomposite films have better dielectric properties than previously reported research results and indicating their potential use as an efficient dielectric material for embedded capacitors. Comparisons also suggest that although fillers are same in both composites i.e., G/TiO2/PVA and G/TiO2/PDMS, difference of ɛ' is purely based on choice of polymer so PDMS is better than PVA to be used as polymer matrix in dielectric materials. Results from this work also suggest that although polymers have low ɛ' and fillers play a key role in determining the dielectric properties of composites, yet selection of polymer is also of utmost importance and changing the polymer can cause visible variations in dielectric properties and flexibility of dielectric composite material. 4. Conclusions Flexible dielectric ternary graphene/titania/Polydimethylsiloxane nanocomposite films with high ɛ' and low tanδ were fabricated by varying w/w of conducting and ceramic additives. As first step, three phase G/TiO2/PDMS nanocomposite films were fabricated by changing weights of conducting graphene while keeping fixed weight of rutile TiO2 and PDMS. After making dielectric studies, w/w of graphene better dielectric characteristics was chosen and some more three phase G/TiO2/PDMS nanocomposite films were prepared while keeping weights of graphene and PDMS fixed and changing weights of rutile TiO2. Results explain that three phase G/TiO2/PDMS nanocomposite film with w/w of 2:15:20 has high ɛ' of 34.8, tanδ of 0.01 and σac of 4.3 × 10−5 at 2 MHz. M∗ and Z∗ also elucidate that three phase G/ TiO2/PDMS nanocomposite film with w/w of 2:15:20 exhibits
Fig. 6. AC conductivity (σac) of neat PDMS and G/TiO2/PDMS nanocomposites films.
ternary G/TiO2/PDMS nanocomposite as efficient dielectric material to be used in electronics like dielectric gates in transistors and embedded capacitors. We made a comparison of our results with reported results in literature on ternary composites [36]. Although previous findings showed ɛ' 330 i.e., that is much higher compared with synthesized G/TiO2/ PDMS nanocomposite films at low frequency of 20 Hz. However, at higher frequency of 2 MHz synthesized G/TiO2/PDMS nanocomposite film has more ɛ' of 34.8 i.e., higher than our previously reported results 8718
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Fig. 7. (a) Real part of impedance (Z′) of neat PDMS and G/TiO2/PDMS nanocomposites films (b) Imaginary part of impedance (Z″) of neat PDMS and G/TiO2/PDMS nanocomposites films.
Fig. 8. (a) Real part of electric modulus (M′) of neat PDMS and G/TiO2/PDMS nanocomposites films (b) Imaginary part of electric modulus (M″) of neat PDMS and G/ TiO2/PDMS nanocomposites films. Table 3 Comparison of dielectric permittivity (ɛ') and loss tangent (tanδ) of G/TiO2/PDMS nanocomposite film with previously reported ternary composites. Ternary dielectric composite cMWCNT/BT/PDMS PBCNCs-3D Ag/BT/PVDF ReZnO/(BT + PVDF) mCNT/mBT/PVDF P(VDF-TrFE-CFE)/BNNS/BST BT@GO/PVDF BT@RGO/PVDF G/TiO2/PVA G/TiO2/PDMS
Frequency (Hz) 100/1000 1000/100 1000 100 100 1000 1000 1000 2 × 106 2 × 106
Dielectric permittivity (ɛ') 2
423 (10 Hz) 16.2 (1000 Hz) 160 (1 kHz) 175 (102 Hz) 109 (102 Hz) 52.7 27 110 7.2 34.8
Loss tangent (tanδ)
References
0.8 0.15 0.11 0.4 0.06 < 0.052 0.09 0.24 0.10 0.01
[10] [11] [31] [32] [33] [34] [35] [35] [36] This work
funding the research project under International Research Support Initiative Program (IRSIP). The authors acknowledge the support of the Australian Research Council (IH 150100003) ARC Research Hub for Graphene Enabled Industry Transformation. We acknowledge the Institute of Chemistry, University of the Punjab, Lahore, Pakistan for providing research facilities. We are thankful to Centre of Excellence in Solid State Physics, University of the Punjab, Lahore, Pakistan for providing facilities for dielectric spectroscopy.
improved dielectric and impedance characteristics. The proposed three phase graphene/titania/Polydimethylsiloxane nanocomposites are good dielectric candidates be used in electronic devices. Competing interest statement Authors have no competing interests to declare. Acknowledgments We are thankful to Higher Education Commission of Pakistan for 8719
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References
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Synthesis of three phase graphene/titania/polydimethylsiloxane nanocomposite films and revealing their dielectric and impedance properties
T
Saira Ishaqa,b,c, Farah Kanwala,∗∗, Shahid Atiqd, Mahmoud Moussab,c, Dusan Losicb,c,∗ a
Institute of Chemistry, University of the Punjab, Lahore, 54590, Pakistan School of Chemical Engineering, The University of Adelaide, Adelaide, 5005, SA, Australia c The ARC Research Hub for Graphene Enabled Industry Transformation, The University of Adelaide, Adelaide, 5005, SA, Australia d Centre of Excellence in Solid State Physics, University of the Punjab, Lahore, 54590, Pakistan ∗ Corresponding author. School of Chemical Engineering, The University of Adelaide, Adelaide, 5005, SA, Australia. b
A R T I C LE I N FO
A B S T R A C T
Keywords: Graphene Titania Polydimethylsiloxane Dielectric permittivity AC conductivity
Compounds and composites with high dielectric performance are in high demand in electronic industry due to their ability to store more charge and lower energy dissipation. In this paper we present synthesis of flexible three phase nanocomposite films of graphene/titania/polydimethylsiloxane with reasonably high real part of permittivity and very low imaginary part of permittivity. Dielectric permittivity of polydimethylsiloxane was raised by addition of graphene and rutile titania used as conducting and ceramic additives, respectively added in various weight ratios. Dielectric studies of synthesized three phase graphene/titania/polydimethylsiloxane nanocomposite films with weight/weight of 2:15:20 exhibits dielectric permittivity of 34.8 even at very high frequency of 2 MHz. While loss tangent of the same three phase graphene/titania/polydimethylsiloxane nanocomposite film is as low as 0.01 and its AC conductivity is 4.3 × 10−5 Sm−1. Complex impedance and complex electric modulus of three phase graphene/titania/polydimethylsiloxane nanocomposite films also confirm its capacitive behaviour. Based on these properties and results we propose that these type of three phase graphene/ titania/polydimethylsiloxane nanocomposite films can be used as useful dielectric materials in energy storage devices.
1. Introduction Polymer based composites bearing high dielectric constant (ɛ') as well as low dielectric loss (ɛ") have gain considerable attention in past few years due to their flexibility, mechanical strength and less amount required for miniaturization of energy storage devices including transistors, gate dielectrics and other portable flexible electronics [1–3]. However, challenge is to keep balance between high ɛ' and low ɛ". Recently two approaches are being used to obtain flexible dielectric composites. First is to increase ɛ' of polymer by addition of conducting fillers which can increase ɛ' by following percolative phenomenon. Second is addition of ceramic fillers having high dielectric permittivity. Both approaches have advantages as well as disadvantages. Former has advantage of elevation of ɛ' even with very small concentration but conducting fillers also cause increase of ɛ" due to development of well aligned pathways which increase leakage current density. As far as ceramic fillers are concerned, although they increase ɛ' of polymer but
they need to be added in high concentration which causes brittleness of polymer thus reducing its mechanical strength [4,5]. Another strategy may be to create barriers between conducting layers of conducting fillers to avoid leakage current [6,7]. Wang et al., reported synthesis of PVA coated graphene to use as filler in PVDF matrix to avoid direct contact of graphene sheets with each other to reduce leakage current. As a result of this coating, at 100 Hz, ɛ' was increased from 100 to 230 and loss tangent (tanδ) was decreased to 0.5 [8]. Similarly Li et al., synthesized insulating PANI coated graphene fillers into poly(methyl methacrylate) (PMMA). Insulating coating increased ɛ' up to 40.6 from 20 and decreased tanδ from 1250 to 1.12 at 1000 Hz [9]. Another approach may be to incorporate both conducting and ceramic fillers in polymers to achieve composite with desired properties. Some researchers have recently reported such ternary composites. Guan et al., synthesized ternary carboxyl functionalized multiwalled carbon nanotubes/barium titanate/polydimethylsiloxane (cMWCNT/BT/PDMS) composites having ɛ' of 423 at 100 Hz and tanδ of 8.4 at 1 kHz [10]. Qi
∗∗
Corresponding author. E-mail addresses: [email protected] (F. Kanwal), [email protected] (D. Losic).
https://doi.org/10.1016/j.ceramint.2019.01.194 Received 14 November 2018; Received in revised form 23 January 2019; Accepted 24 January 2019 Available online 25 January 2019 0272-8842/ © 2019 Elsevier Ltd and Techna Group S.r.l. All rights reserved.
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Fig. 1. Scheme of the synthesis of G/TiO2/PDMS nanocomposite films.
Australia, Australia) were supplied from an Australian mining site. Polydimethylsiloxane (PDMS) prepolymer (Sylgard 184) and curing agent (Sylgard 184) were purchased from Dow Corning Corportaion (USA). Potassium permanganate (97% KMnO4, Sigma-Aldrich), phosphoric acid (85% H3PO4, Chem-Supply), sulfuric acid (98% H2SO4, Chem-Supply), hydrogen peroxide (30% H2O2, Chem-Supply), hydrochloric acid (35% HCl, Chem-Supply) and titanium(IV) oxide (TiO2, rutile, 99.98% trace metal basis, Aldrich) were used without further purification.
et al., reported synthesis of polyamide11/barium titanate/carbon nanotube (PA11/BaTiO3/CNT) nanocomposites having ɛ' of 16.2 at 1000 Hz and tanδ of 0.15 at 100 Hz [11]. In this research work, we report synthesis of three phase graphene/ titania/polydimethysiloxane (G/TiO2/PDMS) nanocomposite films in which graphene sheets were segregated from each other due to presence of ceramic filler and explore their dielectric and impedance properties. To make three phase composite we initially explore and optimize each component considering their different properties. Firstly, we optimized addition of graphene fillers where weight concentration of graphene was varied until it reached its percolation fraction. Then keeping it fixed, concentration of ceramic filler (rutile titania, TiO2) was added till maximum ɛ' was achieved. Titania has high ɛ' of 114 at room temperature (RT) [12], but it has disadvantage of mechanical instability [13]. However, when both conducting and ceramic additives are used together it can further rise ɛ' of the polymer. So to address this limitation we used graphene as conducting filler due to its high aspect ratio, large surface area, high electrical conductivity and high modulus [14]. PDMS, a silicon based non conducting elastomer, was selected as base polymer matrix, due to its excellent flexibility [15]. Graphene sheets were found to form electrodes of many microcapacitors formed and ceramic filler and polymer matrix acted like dielectric medium between electrodes. Tanδ was suppressed due to random distribution of graphene sheets, which are not continuously aligned to circumvent tunneling conductance that contributes to tanδ. Mostly dielectric materials have low ɛ' at elevated frequencies, but currently synthesized G/ TiO2/PDMS nanocomposite films is proposed to have sustained high ɛ' even at high frequency, while still maintaining low tanδ. Morphological and chemical composition of prepared composite was characterized by FESEM following by testing their comparative dielectric properties and precision impedance at frequency range of 20 Hz - 2 MHz by using copper electrodes.
2.2. Synthesis of G/TiO2/PDMS nanocomposite films Oxidation of graphite powder was carried out by a previously reported method to form graphene oxide (GO) [16]. GO was converted to graphene by reduction of GO in the presence of hydrazine hydrate which was used as reducing agent [17]. To synthesize three phase G/ TiO2/PDMS nanocomposite film, graphene and rutile TiO2 in w/w of 1:2 were mixed together during magnetic stirring in Polydimethylsiloxane (PDMS) prepolymer (Sylgard 184) and curing agent (Sylgard 184) taken in 10:1. The mixture was then poured in petri dish followed by degassing by using vacuum pump. Mixture was oven dried at 50 °C for 24 h to obtain G/TiO2/PDMS nanocomposite film. A series of nanocomposite films of G/TiO2/PDMS was synthesized by above described method in which graphene concentration was increased in order to get optimum value of its concentrations while w/w of TiO2, PDMS prepolymer and curing agent were kept constant. After making dielectric studies of all synthesized G/TiO2/PDMS nanocomposite films, graphene weight in G/TiO2/PDMS nanocomposite film giving maximum ɛ' and minimum tanδ was chosen as optimum and a new series of G/TiO2/PDMS nanocomposite films was fabricated by changing weight of rutile TiO2 while concentrations of other components were kept fixed. Fig. 1 depicts scheme of formation of G/TiO2/ PDMS nanocomposite film. Sample codes and w/w of graphene, rutile titania and polymer in all synthesized G/TiO2/PDMS nanocomposite films are summarized in Table 1.
2. Materials and methods 2.1. Materials Commercial natural graphite rocks (Uley, Eyre Peninsula, South 8714
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with previous reports [20]. Thermal stability of graphene, rutile TiO2, neat PDMS, GTPDMS-II and GTPDMS-VI nanocomposite films was characterized by TGA carried out in temperature range of 30–900 °C and is presented in Fig. 4. TGA graphs show that graphene starts degrading at about 380 °C and completely degrades at 590 °C. Rutile TiO2 is thermally stable, even at 900 °C it does not degrade. Neat PDMS film degrades up to 45% at 520 °C and after which its weight becomes constant. For GTPDMS-II and GTPDMS-VI, constant weight loss was observed above 400 °C that is due to degradation of PDMS which occurs as a result of breakage of SieSi bonds which produces silyl radicals which strike with CeH and SieO to evolve gases like H2 and CH4. This evolution of gases causes weight loss between 300 and 550 °C [21]. However GTPDMS-II is thermally more stable than neat PDMS due to presence of graphene and TiO2. With further increase in concentration of TiO2 thermal stability is further increased in GTPDMS-VI. Maximum weight loss for neat PDMS, GTPDMS-II and GTPDMS-VI is 46%, 43% and 29%, respectively.
Table 1 Sample codes with w/w fraction of components of ternary G/TiO2/ PDMS nanocomposite films. Sample code
w/w fraction of G/TiO2/PDMS
Neat PDMS GTPDMS-I GTPDMS-II GTPDMS-III GTPDMS-IV GTPDMS-V GTPDMS-VI GTPMS-VII
0:0:100 1:2:20 2:2:20 3:2:20 2:5:20 2:10:20 2:15:20 2:20:20
2.3. Characterization Field emission scanning electron microscopy (FESEM, Quanta 450, FEI, USA) of neat PDMS and three phase G/TiO2/PDMS nanocomposite films was carried out to explicit their morphology. Structural analyses of neat PDMS and three phase G/TiO2/PDMS nanocomposite films were made by using X-ray diffraction (XRD, 600 Miniflex, Rigaco, Japan). Fourier transform infrared (FTIR) spectroscopy (Nicolet 6700 Thermo Fisher) was carried out to locate the functional groups in the neat PDMS and synthesized G/TiO2/PDMS nanocomposite films. Thermal gravimetric analyzer (TGA, Q500, TA Instruments, USA) was used to make thermal analyses under air by heating all films from RT to 900 °C while heating them at rate of 10 °Cmin−1 Dielectric and impedance analyses of neat PDMS and all G/TiO2/PDMS nanocomposite films at RT were done by using precision impedance analyzer (Wayer Kerr, 6500B) at frequency range of 20 Hz - 2 MHz by using copper electrodes.
3.2. Dielectric and impedance studies of G/TiO2/PDMS nanocomposite films The ɛ' values of neat PDMS and all synthesized G/TiO2/PDMS nanocomposite films are plotted against log f in Fig. 5a. It is clear from the plots that values of ɛ' are high at low frequencies due to enhanced Maxwell Wagner Sillars (MWS) effect or interfacial polarization (IP). IP or MWS effect is increased in low frequency region. When electric field is applied induced and permanent dipoles have enough time to align themselves. With increase in frequency, ɛ' decreases rapidly and becomes low and steady at high frequency region as at high frequency region dipoles do not have enough time for alignment in proper orientation, this phenomenon is called polarization relaxation phenomenon [22]. These results indicate that at elevated frequency, ɛ' becomes frequency independent due to microcapacitor structure model [5]. ɛ' values of synthesized nanocomposite elevates with rising graphene w/w till GBPDMS-II. GTPDMS-II has ɛ΄ of 19.5 at 2 MHz that is about 1.63 times higher than ɛ΄ of neat PDMS having ɛ' value of 12 at 2 MHz. Rise in conductivity and IP causes elevation of value of ɛ'. Rise of IP results due to accumulation of free charges at interfaces of components, which results in formation of dipoles. Rise of IP is caused by both factors; enhanced interfacial area and homogenous distribution of additives [22,23]. Microcapacitors were created and explored within three phase nanocomposite films where conducting graphene sheets act as electrodes and rutile TiO2 and PDMS provide insulating paths. As w/w of graphene increases more microcapacitors are formed to rise ɛ' values of the three phase nanocomposite films. A stage reaches where further rise of graphene fraction causes decrease of ɛ' due to development of well connected and efficient conducting pathways which results in rise of leakage current causing depression of ɛ' [24]. ɛ' value of three phase G/ TiO2/PDMS nanocomposite films further rises with raising weight of rutile TiO2 and after reaching a maximum value at percolation weight fraction, it starts depressing with further increasing TiO2 fraction. GTPDMS-VI bears highest ɛ' value of 34.8 among neat PDMS and all G/ TiO2/PDMS nanocomposite films. Exhibited ɛ' value of GTPDMS-VI is about 3 times greater than ɛ' value of neat PDMS film at 2 MHz. Improvement in ɛ' is caused due to addition of rutile TiO2 nanoparticles having good ɛ' instead of ɛ' of PDMS or graphene from GBPDMS-IV to GBPDMS-VI. This improvement of ɛ' is caused by formation of efficient network to transport charge thus resulting in improved conductivity. Network meant for transport of charge becomes more efficient with homogenous dispersion of rutile TiO2 in PDMS and it is in accordance with previous findings [25,26]. However when w/w of rutile TiO2 is further increased ɛ' value in GBPDMS-VII decreases due to formation of big lumps as is visible in Fig. 2g. Large rutile TiO2 aggregations may destruct charge transportation network resulting in decrease in value of
3. Results and discussions 3.1. Characterization of G/TiO2/PDMS nanocomposite films The FESEM images depicting typical morphology of synthesized G/ TiO2/PDMS nanocomposite films are presented in Fig. 2. Images (Fig. 2a–g) clearly indicate the presence of all three components of nanocomposite. It is evident that conducting graphene layers in PDMS matrix are not continuously connected, due to presence of ceramic nanoparticles. Graphene sheets and rutile TiO2 nanoparticles are homogenously distributed in the PDMS. Nanoparticles of rutile TiO2 are distributed on graphene layers and they are also embedded in PDMS. Fig. 2g clearly shows that as weight of rutile TiO2 increases, its nanoparticles start to coagulate and form aggregates. A comparison of XRD patterns of neat PDMS and all synthesized G/ TiO2/PDMS nanocomposite films is shown in Fig. 3a. A very broad peak at 22° in XRD pattern of neat PDMS indicates that PDMS is in amorphous form [18]. Peaks for rutile TiO2 present at 27.9° (110), 36.9° (101), 39.9° (200), 41.8° (111), 44.8° (210), 54.7° (211), and 56.5° (220), 62.8° (002), 64.8° (310) and 69.6° (112) are according to JCPDS card no. 77-0441 [19]. In all synthesized G/TiO2/PDMS nanocomposites films, characteristic peak for graphene does not appears because of very broad peak for PDMS. With increasing amount of both graphene and rutile TiO2 fillers, characteristic peak of PDMS at 22° becomes less intense suggesting that graphene and rutile TiO2 fillers are interacting with each other [16]. FTIR spectra of neat PDMS and all synthesized G/TiO2/PDMS nanocomposite films are compared in Fig. 3b. Characteristic peak at 785 cm−1 corresponds to SieC, while peaks at 1007 cm−1 and 1080 cm−1 correspond to SieOeSi. Peak at 1257 cm−1 is attributed to deformation vibration of two eCH3 linked with Si while Peak at 2960 cm−1 represents stretching vibration of CeH. No new peak is observed in FTIR spectra of G/TiO2/PDMS compared with the FTIR spectrum of neat PDMS membrane which shows that the graphene and TiO2 are only physically blended with PDMS. This result is in agreement 8715
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Fig. 2. FESEM images of synthesized G/TiO2/PDMS nanocomposite films with different composition (a) GTPDMS-I (b) GTPDMS-II (c) GTPDMS-III (d) GTPDMS-IV (e) GTPDMS-V (f) GTPDMS-VI (g) GTPDMS-VII.
ɛ'. Values of ɛ' of neat PDMS, GTPDMS-II and GTPDMS-VI taken at a frequency range are compared in Table 2. Fig. 5b shows that with increase in frequency, tanδ of PDMS and three phase G/TiO2/PDMS nanocomposite films drops. Tanδ rises with rise in w/w of conducting graphene, due to reason that graphene sheets
become well aligned and connected to each other with increasing concentration and increase leakage current density to increase energy dissipation i.e., tanδ. As soon as electric field is passed conducting graphene layers may cause passage of electric current through them and some transition from electrical energy to thermal energy takes place 8716
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Fig. 3. (a) XRD patterns of neat PDMS and G/TiO2/PDMS nanocomposites films (b) FTIR spectra of neat PDMS and G/TiO2/PDMS nanocomposite films.
explain the process of conduction in PDMS and G/TiO2/PDMS nanocomposite films. Fig. 7a–b show real part of complex impedance (Z′) and imaginary part of complex impedance (Z″) of neat PDMS and three phase G/TiO2/PDMS nanocomposite films plotted as function of log f. Plots show that both parts of complex impedance decrease when frequency is raised and becomes constant at higher frequencies. This behaviour is in agreement with previous research reports [27]. Conduction is increased at high frequencies as more free space charges exist there [28]. The dielectric modulus explains the electrical transport within the dielectric medium. Fig. 8a–b shows real part (M′) and imaginary part (M″) of complex electric modulus plotted as function of log f for PDMS and synthesized three phase G/TiO2/PDMS nanocomposite films. When frequency is raised M′ also rises till reaches a maximum value called asymptotic value M∞ convincing about capacitive nature of G/TiO2/ PDMS nanocomposite films. This result indicates that process of relaxation lies over a long frequency range. Fig. 8b describes that M″ rises and after reaching a highest value M″max starts to drop with further elevation in frequency [29]. It may be explained by considering that when frequency is low charge carriers may move over long distances with alteration of electric field until M″max attains, then when frequency becomes high and electric field alters rapidly charge carriers can move to short distances only as they do not get enough time for alignment [30]. For GTPDMS-VII, M″max moves to relatively smaller frequency region depicting that conduction mechanism is increased due to better conducting network development due to large aggregations of rutile TiO2 nanoparticles. M″max shows that charge movement transits from long to short range from low to high frequencies respectively due to reason that free charges may move from large distance to short distance with increasing in frequency [27]. The ɛ' and tanδ values of synthesized three phase G/TiO2/PDMS nanocomposite film with previously reported ternary composites are compared in Table 3. Although in table previously reported ternary composites seem to have more ɛ' their values are reported at low frequencies. It is clear from the comparison that our synthesized ternary G/TiO2/PDMS nanocomposite has high dielectric constant even at very high frequency of 2 MHz which is usually difficult to attain. Tanδ of recently reported ternary G/TiO2/PDMS nanocomposite is still very low as compared to previously reported ternary composites. So we propose
Fig. 4. TG curves of graphene, TiO2, neat PDMS and G/TiO2/PDMS nanocomposites films (GTPDMS-II, GTPDMS-VI).
[24]. GTPDMS-II has as low tanδ as 0.02 at 2 MHz. However, tanδ further drops when weight of rutile TiO2 increases and reaches to 0.01 for GTPDMS-VI as shown in Fig. 5b and Table 2. But when TiO2 fraction is further increased from GTPDMS-VI to GTPDMS-VII tanδ rises many times. This rise in tanδ with rising weight of rutile TiO2 is due to increase in conductivity due to improved network formation. These results are in agreement with results available in previous literature [25,26]. Fig. 6 shows σac of PDMS and three phase G/TiO2/PDMS nanocomposite films. It shows that with rise of w/w of conducting graphene σac increases due to development of more conducting pathways. Further increase of σac is caused due to increased w/w of rutile TiO2 fraction. This increased conductivity is also responsible for enhanced tanδ. Improved σac of three phase G/TiO2/PDMS nanocomposites with greater weight of TiO2 in GTPDMS-VII can be explained due to formation of a network for transport of charge. These results are in agreement with published research reports in literature [26]. Complex impedance spectroscopy (CIS) study was performed to 8717
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Fig. 5. (a) Real part of permittivity (ɛ΄) of neat PDMS and G/TiO2/PDMS nanocomposites films (b) Loss tangent (tanδ) of neat PDMS and G/TiO2/PDMS nanocomposites films. Table 2 Dielectric permittivity (ɛ'), loss tangent (tanδ) and AC conductivity (σac) of neat PDMS, GTPDMS-II and GTPDMS-VI. Frequency (Hz)
Dielectric permittivity (ɛ') Neat PDMS
20 40 100 103 104 105 106 2 × 106
6.5 4.0 3.8 3.2 3.1 3.1 3.0 2.99
GTPDMS-II 96.7 89.6 36.6 25 21.6 20.1 19.5 19.5
AC conductivity (σac) (Sm−1)
Loss tangent (tanδ) GTPDMS-VI 108.3 84.1 37 36.5 35.8 35.1 34.8 34.8
Neat PDMS 0.5 0.23 0.05 0.01 0.01 0.01 0.01 0.01
GTPDMS-II 1.7 1.3 0.28 0.18 0.08 0.03 0.02 0.02
GTPDMS-VI 0.56 0.4 0.17 0.04 0.02 0.01 0.01 0.01
Neat PDMS −10
7.3 × 10 5.7 × 10−9 2.9 × 10−9 2.2 × 10−9 1.8 × 10−8 2.6 × 10−7 2.5 × 10−6 4.6 × 10−6
GTPDMS-II
GTPDMS-VI
−7
6.8 × 10−8 7.9 × 10−8 3.6 × 10−8 8.7 × 10−8 4.6 × 10−7 2.6 × 10−6 2.2 × 10−5 4.3 × 10−5
1.8 × 10 2.7 × 10−7 6.2 × 10−8 2.4 × 10−7 1 × 10−6 4.3 × 10−6 2.5 × 10−5 4.6 × 10−5
where it was 7.2 at such high frequency. Similarly, tanδ of synthesized G/TiO2/PDMS nanocomposite film is 0.01 i.e., much lower as compared to 0.10 i.e., previously reported G/TiO2/PVA nanocomposite films. Thus, synthesized G/TiO2/PDMS nanocomposite films have better dielectric properties than previously reported research results and indicating their potential use as an efficient dielectric material for embedded capacitors. Comparisons also suggest that although fillers are same in both composites i.e., G/TiO2/PVA and G/TiO2/PDMS, difference of ɛ' is purely based on choice of polymer so PDMS is better than PVA to be used as polymer matrix in dielectric materials. Results from this work also suggest that although polymers have low ɛ' and fillers play a key role in determining the dielectric properties of composites, yet selection of polymer is also of utmost importance and changing the polymer can cause visible variations in dielectric properties and flexibility of dielectric composite material. 4. Conclusions Flexible dielectric ternary graphene/titania/Polydimethylsiloxane nanocomposite films with high ɛ' and low tanδ were fabricated by varying w/w of conducting and ceramic additives. As first step, three phase G/TiO2/PDMS nanocomposite films were fabricated by changing weights of conducting graphene while keeping fixed weight of rutile TiO2 and PDMS. After making dielectric studies, w/w of graphene better dielectric characteristics was chosen and some more three phase G/TiO2/PDMS nanocomposite films were prepared while keeping weights of graphene and PDMS fixed and changing weights of rutile TiO2. Results explain that three phase G/TiO2/PDMS nanocomposite film with w/w of 2:15:20 has high ɛ' of 34.8, tanδ of 0.01 and σac of 4.3 × 10−5 at 2 MHz. M∗ and Z∗ also elucidate that three phase G/ TiO2/PDMS nanocomposite film with w/w of 2:15:20 exhibits
Fig. 6. AC conductivity (σac) of neat PDMS and G/TiO2/PDMS nanocomposites films.
ternary G/TiO2/PDMS nanocomposite as efficient dielectric material to be used in electronics like dielectric gates in transistors and embedded capacitors. We made a comparison of our results with reported results in literature on ternary composites [36]. Although previous findings showed ɛ' 330 i.e., that is much higher compared with synthesized G/TiO2/ PDMS nanocomposite films at low frequency of 20 Hz. However, at higher frequency of 2 MHz synthesized G/TiO2/PDMS nanocomposite film has more ɛ' of 34.8 i.e., higher than our previously reported results 8718
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Fig. 7. (a) Real part of impedance (Z′) of neat PDMS and G/TiO2/PDMS nanocomposites films (b) Imaginary part of impedance (Z″) of neat PDMS and G/TiO2/PDMS nanocomposites films.
Fig. 8. (a) Real part of electric modulus (M′) of neat PDMS and G/TiO2/PDMS nanocomposites films (b) Imaginary part of electric modulus (M″) of neat PDMS and G/ TiO2/PDMS nanocomposites films. Table 3 Comparison of dielectric permittivity (ɛ') and loss tangent (tanδ) of G/TiO2/PDMS nanocomposite film with previously reported ternary composites. Ternary dielectric composite cMWCNT/BT/PDMS PBCNCs-3D Ag/BT/PVDF ReZnO/(BT + PVDF) mCNT/mBT/PVDF P(VDF-TrFE-CFE)/BNNS/BST BT@GO/PVDF BT@RGO/PVDF G/TiO2/PVA G/TiO2/PDMS
Frequency (Hz) 100/1000 1000/100 1000 100 100 1000 1000 1000 2 × 106 2 × 106
Dielectric permittivity (ɛ') 2
423 (10 Hz) 16.2 (1000 Hz) 160 (1 kHz) 175 (102 Hz) 109 (102 Hz) 52.7 27 110 7.2 34.8
Loss tangent (tanδ)
References
0.8 0.15 0.11 0.4 0.06 < 0.052 0.09 0.24 0.10 0.01
[10] [11] [31] [32] [33] [34] [35] [35] [36] This work
funding the research project under International Research Support Initiative Program (IRSIP). The authors acknowledge the support of the Australian Research Council (IH 150100003) ARC Research Hub for Graphene Enabled Industry Transformation. We acknowledge the Institute of Chemistry, University of the Punjab, Lahore, Pakistan for providing research facilities. We are thankful to Centre of Excellence in Solid State Physics, University of the Punjab, Lahore, Pakistan for providing facilities for dielectric spectroscopy.
improved dielectric and impedance characteristics. The proposed three phase graphene/titania/Polydimethylsiloxane nanocomposites are good dielectric candidates be used in electronic devices. Competing interest statement Authors have no competing interests to declare. Acknowledgments We are thankful to Higher Education Commission of Pakistan for 8719
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References
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