Temperature influence on dielectric energy storage of nanocomposites

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CERAMICS INTERNATIONAL

Ceramics International 41 (2015) 1807–1813 www.elsevier.com/locate/ceramint

Temperature influence on dielectric energy storage of nanocomposites Md Rajib, Mohammad Arif Ishtiaque Shuvo, Hasanul Karim, Diego Delfin, Samia Afrin, Yirong Linn Department of Mechanical Engineering, The University of Texas at El Paso, El Paso 79968, TX, United States Received 15 August 2014; accepted 25 September 2014 Available online 11 October 2014

Abstract The demand for high energy density dielectric capacitor devices is increasing due to their significant role in stationary power systems, mobile devices and pulse power applications. The polymer film based dielectric capacitor is still one of the most widely used energy storage devices owing primarily to its high energy density and low cost. To further enhance the energy density, high dielectric constant ceramic inclusions have been embedded into the polymer matrix; however, the relationship between temperature and energy density has not yet been fully investigated. Therefore, in this paper, a commonly used composites with barium titanate (BaTiO3) nanoparticles embedded in polyvinylidene fluoride (PVDF) matrix were fabricated using a solution casting method in order to explore their energy densities within a temperature range from 20 1C to 120 1C. Scanning Electron Microscopy (SEM) and X-ray Diffraction (XRD) were used for material morphology and crystal structure characterization. Capacitance and breakdown strength were measured throughout the temperature range in order to determine the energy density of the samples with 10%, 20%, 30% and 40% volume fractions. It is found that nanocomposites with 30% volume fraction displayed the highest energy density of 5.79 J/cm3 at 50 1C. & 2014 Elsevier Ltd and Techna Group S.r.l. All rights reserved.

Keywords: Nanocomposites; Dielectric; Energy storage

1. Introduction The excellent energy storing capabilities of capacitors have lent themselves to increasing use as energy storage devices [1]. Due to their excellent dielectric properties, easy accessibility, and low cost, dielectric capacitors are the most common type of capacitors in use [1]. Dielectric capacitors come in two types, polymer capacitors and ceramic capacitors, for different application needs. Compared to polymer-based capacitors, ceramicbased capacitors have higher relative dielectric permittivity; however, they have lower breakdown strength that limits the energy density and performance of the capacitor [1]. To mitigate this issue, the approach of fabricating nanocomposites has been implemented by many researchers aiming at combining the high relative dielectric permittivity of ceramic and the high breakdown strength of polymer. With this approach, dielectric capacitors can n

Corresponding author. Tel.: þ1 915 747 6015. E-mail address: [email protected] (Y. Lin).

http://dx.doi.org/10.1016/j.ceramint.2014.09.127 0272-8842/& 2014 Elsevier Ltd and Techna Group S.r.l. All rights reserved.

be manufactured by inclusion of ceramic filler materials with higher relative dielectric permittivity into polymer matrix that has higher breakdown strength in order to achieve a higher energy density capacitor [2–4]. Several different ceramic filler materials have been employed to fabricate a dielectric capacitor: barium titanate (BaTiO3), lithium niobate (LiNbO3), silicone nitrate (Si3N4), silicon oxide (SiO2), aluminum oxide (Al2O3), zinc oxide (ZnO), lead titanate (PbTiO3) and lead zirconate titanate (Pb(ZrxTi1
x)O3) [5]. Amongst many ceramic materials, barium titanate (BaTiO3), a ferroelectric ceramic, is the most commonly used due to its stable properties, high relative dielectric permittivity, piezoelectric characteristics, polycrystalline form and compliance with the environmental safety policies [2,6,7]. PVDF is the most commonly used polymer dielectric materials due to its spontaneous polarization, which results in higher relative dielectric permittivity and higher breakdown strength [8,9]. By integrating the excellent dielectric properties of BaTiO3 and PVDF, researchers have improved the dielectric properties of the dielectric capacitor. Dang et al. displayed improvement of

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dielectric properties by chemical surface modification of BaTiO3 nanoparticles [10]. Using dopamine, Song et al. strengthened the interfacial adhesion of polymer matrix and thus improved the dielectric properties of BaTiO3/PVDF nanocomposites [11]. Dou et al. investigated a technique for improving the dielectric strength for BaTiO3/PVDF nanocomposites by using titanate coated barium titanate nanoparticles [12]. However, dielectric capacitors with higher volume fractions of ceramic filler material still require more understanding to improve their dielectric properties; thus it is imperative to analyze the influence of elevated temperature on energy storage properties to better understand its performance under elevated temperatures. Therefore, this paper will focus on the characterization of the energy storage properties of PVDF nanocomposites with different volume fraction of BaTiO3 nanoparticles and the influence of temperature on dielectric properties. All the samples were fabricated with 10%, 20%, 30% and 40% volume fractions of ceramic filler materials. Sample morphology and crystal structure at different temperatures were analyzed by using the SEM and XRD. Energy density was calculated for every sample with the measured relative dielectric permittivity and breakdown strengths at different temperatures ranging from 20 1C to 120 1C; the highest result was recorded at 50 1C for the sample with 30% volume fraction of BaTiO3 nanoparticles. 2. Experimental procedures 2.1. Materials Commercial BaTiO3 nanoparticles (Advanced Materials, 5622ON-01), Polyvinylidene fluoride (PVDF) (Aldrich, Mw  53,4000) and Dimethylformamide (DMF) (Acros, 99.8%) were used as the starting materials to fabricate nanocomposites. Average size of the BaTiO3 nanoparticles was 100 nm, as indicated by the vendor. 2.2. Fabrication of BaTiO3/PVDF nanocomposites Nanocomposites were prepared using a solution casting method with volume fractions ranging from 10% to 40% to investigate the influence of ceramic filler materials on energy density [14]. PVDF

and DMF were mixed at a 1: 10 weight ratio and heated up to 80 1C for 30 min to fully dissolve the PVDF. Nanocomposites were prepared by dispersing BaTiO3 nanoparticles into PVDF/DMF solution by manual stirring and horn sonication (Branson, S-450A) until a homogeneous mixture was obtained. Subsequently, the solution was casted onto a Polytetrafluoroethylene (PTFE) film and dried at 80 1C for 1 h. In order to achieve a consistent thickness over the entire film, nanocomposites were hot pressed at 150 1C for 15 min under a constant pressure of 1 t (Carver, 3850). Finally, top and bottom surfaces of nanocomposites were coated with silver paint as electrodes for dielectric testing. The fabrication process of the nanocomposites is schematically shown in Fig. 1. 2.3. Characterization and testing of BaTiO3/PVDF nanocomposites The crystal structure of the BaTiO3 nanoparticles was determined using a Bruker D8 Discover XRD with Cu Kα radiation. The morphology of the samples was analyzed using S-4800 SEM. Dielectric properties were analyzed at 20 1C, 50 1C, 75 1C, 100 1C and 120 1C, respectively. Dielectric capacitances of the nanocomposites were tested at different frequency levels ranging from 20 Hz to 1 MHz using HP 4284A LCR meter. Later, relative dielectric permittivity was also calculated for different frequencies and temperatures. Breakdown strength was calculated at the mentioned temperature ranges with a high DC voltage supply (up to 30 kV) according to ASTM standard (ASTM D149-09) [13]. Energy density was calculated for all the dielectric capacitor samples with different volume fractions of ceramic filler materials. 3. Results and discussion The morphologies of fabricated nanocomposites of BaTiO3 nanoparticles are shown in Fig. 2(a)–(d) with volume fraction of 10%, 20%, 30%, and 40% respectively. With the increase of volume fraction, more BaTiO3 particles are shown in the same area. All samples have shown a homogenous dispersion of BaTiO3 nanoparticles into the PVDF polymer matrix. Note

Fig. 1. Schematic diagram for fabrication process of BaTiO3/PVDF nanocomposites.

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Fig. 2. SEM images of BaTiO3/PVDF nanocomposites: (a) 10% BaTiO3/PVDF, (b) 20%BaTiO3/PVDF, (c) 30% BaTiO3/PVDF, and (d) 40% BaTiO3/PVDF nanocomposites, (e) BaTiO3 particles.

the nanoparticles have average diameter of 100 nm which is consistent with the manufacturer’s information. Agglomeration of the ceramic fillers will significantly lower the energy density [14]. Previous study has shown that the dispersion of inorganic fillers in these fluorinated polymers is always problematic due to the low surface energy of the polymers [14]. The combination of a suitable concentration of PVDF in DMF and the sonication method were found to be critical for uniform dispersion BaTiO3 nanoparticles in PVDF matrix [14]. The crystal structures of the BaTiO3/PVDF nanocomposites were determined utilizing a Bruker D8 Discover XRD using Cu Kα radiation after testing samples at different temperatures ranging from 20 1C to 120 1C. XRD patterns for the nanocomposites are shown in Fig. 3. BaTiO3/PVDF nanocomposites displayed a tetragonal phase and excellent crystallinity for all samples tested within the temperature ranging from 20 1C to 120 1C. The calculated parameters for these BaTiO3/PVDF nanocomposites, a¼ 3.99454 Å and c¼ 4.02411 Å, are similar to values previously reported in literature [15,16]. For all samples the entire diffraction peaks match those indicated by the standard card JCPDS (No. 05-0626) pertaining to this tetragonal structure. No other peaks were determined, indicating all BaTiO3 utilized for the nanocomposites before and after testing had excellent crystallinity; the fabrication and testing procedures did not influence the crystal structures of the BaTiO3 used in this study. The relative dielectric permittivity of the nanocomposites at different frequency ranges and temperatures were indirectly determined by the relationship of capacitance and relative dielectric permittivity, defined by the following equation [24]: εr ¼

Cd ε0 A

ð1Þ

Fig. 3. XRD patterns of BaTiO3/PVDF nanocomposites at different temperatures.

where C is the capacitance (F), measured by inductance capacitance-resistance (LCR) meter, d is the thickness (m) of the nanocomposites, ε0 is the permittivity of space (ε0 E 8.854  10
12 F/m) and A is the surface area (m2) of the electrode. Once the capacitance is measured, the relative dielectric permittivity can be determined. In order to measure the capacitance, all samples were tested using an LCR meter (HP 4284A) at different frequency levels ranging from 20 Hz to 1 MHz. The capacitance of a dielectric capacitor is influenced by the volume fraction of the ceramic filler material and frequency. As expected, higher volume fractions of ceramic filler material increased the capacitance due to the higher relative dielectric permittivity of ceramic fillers. At 20 Hz testing frequency, all the samples displayed higher capacitance; however,

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an abrupt drop in capacitance occurs within the lower frequency level range from 20 Hz to 100 Hz due to the nature of ferroelectric materials, i.e., material phase and lattice structure [17]. The degree of crystallinity, existence of permanent dipoles, mobility of free charge and defects, all, contribute to this response. At high frequency, the electric response of a sample lags behind the applied field and creates loss [25]. After 100 Hz frequency, capacitance linearly decreased with the increase of frequency from 100 Hz to 1 MHz. All the samples with different volume fractions of BaTiO3 nanoparticles displayed the same trend of change in capacitance. The relative dielectric permittivity for all dielectric capacitor samples is influenced by the sample’s loading concentration of ceramic filler materials under different frequency levels and elevated operating temperatures. The influence of BaTiO3 nanoparticles on relative dielectric permittivity of the nanocomposites was analyzed from 20 Hz to 1 MHz frequency at room temperature, which is shown in Fig. 4(a). With higher volume fractions of BaTiO3 nanoparticles, the relative dielectric permittivity for all samples increased due to the high relative dielectric permittivity of the ceramic filler material, but relative dielectric permittivity decreases with increasing frequency from 20 Hz to 1 MHz. A sharp change in relative dielectric permittivity is observed within the frequency ranges between 20 Hz and 100 Hz; however, at higher level of frequency, small variation occurs in the relative dielectric permittivity. With increase of frequency, the change of relative dielectric permittivity varies from two to eight times lower than its value at 20 Hz depending on the volume fraction of BaTiO3. For 30% BaTiO3/PVDF composites, the relative dielectric permittivity at 1 MHz is eight times lower than its calculated relative dielectric permittivity at 20 Hz. The calculated difference of relative dielectric permittivity is 902.59 at

20 Hz. This phenomenon can be described with Clausius– Mosotti relation, which states that the dielectric constant is related to polarizability of the material. Polarizability of a dielectric material changes with the frequency of the applied electric field which contributes to the variation of the capacitance as well as the relative dielectric permittivity. At low frequency, the dipole orientation of the nano-composites is not as affected as it does at high frequency, therefore, contributing to high capacitance value. At higher frequency, the dipoles cannot maintain their orientation with the alternating field, resulting in a lower relative dielectric permittivity. In order to analyze the temperature influence on dielectric properties of a sample, the relative dielectric permittivity was also tested at different temperature levels under the same frequency for all the samples with different volume fractions of ceramic filler material, as shown in Fig. 4(b). As standard 1 kHz frequency has been chosen and from the obtained plot it is shown that temperature also has a strong influence on the relative dielectric permittivity of the dielectric capacitor samples [26]. For all the samples, a higher relative dielectric permittivity was obtained at elevated temperature due to higher dipole movement potential energy. Moreover, it has been found in previous work that alternating current conductivity is influenced by the inhomogeneity of the nano-composites by changing the volume fraction of BaTiO3. This leads to the inhomogeneous conduction from the interfacial polarization between PVDF and BaTiO3. The rise in relative dielectric permittivity with increasing temperature at 1 kHz for different volume fraction seems to be mainly due to the inhomogeneous conduction for this interfacial polarization in addition to the domain contribution of the composites [20]. Among different samples, nanocomposites with 40% volume fraction displayed a higher relative dielectric permittivity at all the tested

Fig. 4. Relative dielectric permittivity of BaTiO3/PVDF nanocomposites with 0% to 40% volume fractions: (a) between a frequency ranges of 20 Hz–1 MHz at room temperature and, (b) at 1 kHz for temperatures 20 1C, 50 1C, 75 1C, 100 1C and 120 1C.

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two volume fraction values, the decrease in breakdown strength is about a half of the 30% volume fraction value. These drops in breakdown strength may have occurred due to percolation effect at high volume faction nanocomposites [23]. Due to the ultra-fine particle size of BaTiO3, there will be increasingly important surface effect, which might have an influence on the motion of charge carriers [21]. The internal stresses in the composites also play a role in the motion of charge carriers. With more ultra-fine ceramic filler material, more surface effect and internal stresses would increase; therefore, higher volume fraction samples would provide lower dielectric breakdown strength. Moreover, ceramic filler materials have orders of magnitude lower breakdown strength compared with polymer which also contributes to the lower breakdown strength value. As seen in Fig. 5(b), breakdown strengths decrease with increasing temperature for all the samples’ various volume fractions. For 10%, 20%, 30%, and 40% BaTiO3/PVDF composites, decrease in resulted breakdown strength tested from 20 1C to 120 1C temperature was 74%, 62%, 67%, and 37%, respectively. This relationship can be explained with the resistive behavior of the nano-composites. Previous study on the resistivity behavior of BaTiO3/PVDF nanocomposites found that the nanocomposites appear to offer some restriction to the electrons’ motion through the field and as a result the current is not properly channeled through the amorphous region [22]. This restriction of motion increases with the increase of temperature, resulting higher amount of current flow as the electrons can move with greater ease. Therefore, lower breakdown value is observed at higher temperature for the same sample composition. With both relative dielectric permittivity and breakdown strength determined, the following equation was used to calculate

temperatures, and there was significant increase in the relative dielectric permittivity from 20 1C to 120 1C. Following the dielectric capacitance and relative dielectric permittivity testing, the breakdown strengths of the samples were also tested to determine the energy density under higher voltage ranges and elevated temperatures for all samples with different volume fractions of filler ceramic material. The breakdown strength were measured according to the ASTM D149-09 standard [13] and determined using the following equation: E bd ¼

V bd d

ð2Þ

where E bd is the breakdown strength of the material measured in MV/m, V bd is the breakdown voltage and d is the thickness of the capacitor. For breakdown strength testing of dielectric capacitor, test setup included a 30 kV high voltage power supply (Acopian PO3HP2), digital oscilloscope (Rigol DS1102E), 20 MHz function/arbitrary waveform generator (Agilent 33220A) and a digital multimeter (FLUKE 16 Multimeter). During breakdown tests, all samples were kept inside a 300 mL beaker filled with silicon oil to limit moisture influence on testing results and sparking while applying high voltage. The breakdown strength testing was conducted at different temperatures from 20 1C to 120 1C for all the samples with different volume fraction of ceramic filler material and results are shown in Fig. 5. The inclusion of ceramic filler material increased the relative dielectric permittivity of the sample but decreased the breakdown strength of the sample due to its lower dielectric strength as shown in Fig. 5(a). The breakdown strength at a given temperature decreases significantly more than a half of the 10% volume fraction value with respect to the 40% volume fraction value. The rapid decrease of breakdown strength occurred from 30% to 40% volume fraction. Between these

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Fig. 5. Breakdown strength of BaTiO3/PVDF nanocomposites: (a) for 0% to 40% volume fraction of filler ceramic material, (b) at 20 1C, 50 1C, 75 1C, 100 1C, and 120 1C.

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7 20°C 50°C 75°C 100°C 120°C

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Fig. 6. Energy density of BaTiO3/PVDF nanocomposites: (a) for 0% to 40% volume fraction of ceramic filler material, (b) at 20 1C, 50 1C, 75 1C, 100 1C and 120 1C.

the energy density of samples with the calculated relative dielectric permittivity and breakdown strengths [14]: W¼

1 εr ε0 E 2db 2

ð3Þ

where W is the volume averaged energy density measured in J/cm3, E db is the breakdown strength of the dielectric capacitor, εr is the relative dielectric permittivity of the dielectric materials and ε0 the dielectric permittivity of vacuum. Energy density is proportional to the relative dielectric permittivity and breakdown strength of the dielectric capacitor. Improved relative dielectric permittivity and breakdown strength greatly influenced the energy density of the capacitor. Though, a higher volume fraction of ceramic material increases the relative dielectric permittivity, it also decreases the dielectric breakdown strength of the capacitor, thus lead to the existence of an optimal volume fraction for maximum energy density for all tested samples, as shown in Fig. 6. Among different dielectric capacitors, higher energy density was calculated for the sample with 30% volume fraction of BaTiO3 and the energy density is 5.17 J/cm3, which represents about 297% increase in comparison to the PVDF with an energy density of 1.3 J/cm3 at the same electric field. The highest energy density was also calculated for the same sample at 50 1C. Note that this energy density exceeds those reports for polymer based capacitors and is 330% greater than the 1.2 J/cm3 energy density for commercial biaxial oriented polypropylene [3,18,19]. 4. Conclusion In this work, dielectric nanocomposites samples were fabricated by combining the high relative dielectric permittivity of BaTiO3 nanoparticles, and higher breakdown strength of PVDF dielectric polymer, in order to improve the energy density of capacitor at elevated temperatures. Different volume fractions of

ceramic filler materials were used to analyze its effect on dielectric properties and all the samples were tested at elevated temperature to investigate temperature influence on the dielectric properties of the samples. Nanocomposite with 30% volume fraction of BaTiO3 demonstrated the highest energy density at all temperatures from 20 1C to 120 1C under the frequency levels 20 Hz to 1 MHz. The highest energy density of 5.79 J/cm3 was calculated at 50 1C for this nanocomposites. Thus, these results indicate that the improvement of energy density of samples could be achieved with certain volume fraction of ceramic filler materials at elevated temperatures. Acknowledgement This work is supported by the National Science Foundation (NSF) under NSF-PREM Grant No. DMR-1205302. References [1] P. Barber, S. Balasubramanian, Y. Anguchamy, S. Gong, A. Wibowo, H. Gao, H. Ploehn, H.C. Zur Loye, Polymer composite and nanocomposite dielectric materials for pulse power energy storage, Materials 2 (4) (2009) 1697–1733. [2] M. Mendoza, M.A. Rahaman Khan, M.A. Ishtiaque Shuvo, A. Guerrero, Y. Lin, Development of lead-free nanowire composites for energy storage applications, ISRN Nanomater. 2012 (2012). [3] P. Kim, N.M. Doss, J.P. Tillotson, P.J. Hotchkiss, M.J. Pan, S.R. Marder, J. Li, J.P. Calame, J.W. Perry, High energy density nanocomposites based on surface-modified BaTiO3 and a ferroelectric polymer, ACS Nano 3 (9) (2009) 2581–2592. [4] Y.P. Mao, S.Y. Mao, Z.G. Ye, Z.X. Xie, L.S. Zheng, Size-dependences of the dielectric and ferroelectric properties of BaTiO3/polyvinylidene fluoride nanocomposites, J. Appl. Phys. 108 (1) (2010) 014102. [5] Z. Tian, X. Wang, L. Shu, T. Wang, T.H. Song, Z. Gui, L. Li, Preparation of nano BaTiO3‐based ceramics for multilayer ceramic capacitor application by chemical coating method, J. Am. Ceram. Soc. 92 (4) (2009) 830–833.

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