Ultra High Energy Density Nanocomposite Capacitors with Fast

Mar 6, 2013 - Ultra High Energy Density Nanocomposite Capacitors with Fast Discharge ... application such as radar, lasers, rail guns, defibrillators,...
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Letter pubs.acs.org/NanoLett

Ultra High Energy Density Nanocomposite Capacitors with Fast Discharge Using Ba0.2Sr0.8TiO3 Nanowires Haixiong Tang and Henry A. Sodano* Department of Materials Science and Engineering, University of Florida, Gainesville, Florida 32611, United States S Supporting Information *

ABSTRACT: Nanocomposites combining a high breakdown strength polymer and high dielectric permittivity ceramic filler have shown great potential for pulsed power applications. However, while current nanocomposites improve the dielectric permittivity of the capacitor, the gains come at the expense of the breakdown strength, which limits the ultimate performance of the capacitor. Here, we develop a new synthesis method for the growth of barium strontium titanate nanowires and demonstrate their use in ultra high energy density nanocomposites. This new synthesis process provides a facile approach to the growth of high aspect ratio nanowires with high yield and control over the stoichiometry of the solid solution. The nanowires are grown in the cubic phase with a Ba0.2Sr0.8TiO3 composition and have not been demonstrated prior to this report. The poly(vinylidene fluoride) nanocomposites resulting from this approach have high breakdown strength and high dielectric permittivity which results from the use of high aspect ratio fillers rather than equiaxial particles. The nanocomposites are shown to have an ultra high energy density of 14.86 J/cc at 450 MV/m and provide microsecond discharge time quicker than commercial biaxial oriented polypropylene capacitors. The energy density of our nanocomposites exceeds those reported in the literature for ceramic/ polymer composites and is 1138% greater than the reported commercial capacitor with energy density of 1.2 J/cc at 640 MV/m for the current state of the art biaxial oriented polypropylene. KEYWORDS: Nanocomposite, capacitor, nanowire, energy storage, breakdown strength, barium strontium titanate

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constituents alone. Additionally, the use of nanocomposites adds a degree of tunability to the capacitor allowing variation of parameters in addition to the film thickness.4−8 Currently, polymers have received the most attention for pulsed power due to their low loss, high breakdown strength, and the low manufacturing cost of the polymer films.2,9−12 For example, the current state of the art is biaxially oriented polypropylene (BOPP) which only offers an energy density of 1.2 J/cc in commercial applications due to its low dielectric permittivity (2.2),13 although the highest reported value at breakdown is 4.88 J/cm3 at 700 MV/m.14 New applications for high power density capacitors are appearing in the automobile and aerospace fields which require greater energy density with a lower operating voltage such that space and cost can be

igh power capacitors form a critical technology for numerous electronic applications; however, the current state of the art technologies suffer from low energy density making them bulky and costly.1−3 High power capacitors typically find use in DC applications such as power inverters and in pulsed power application such as radar, lasers, rail guns, defibrillators, and pace makers.1−3 With the pulsed power capacitors being a more technologically challenging application due to the requirement for low loss and fast discharge time, much of the attention in high power capacitors has been applied to this class of materials.2,3 One method to increase the energy density of these systems is through the use of polymer nanocomposites as an alternative to the polymeric and ceramic dielectrics commonly used for electrostatic energy storage.4−8 Nanocomposites derive their high energy density from the use of a high dielectric filler and a high breakdown strength polymer which when combined in the correct ratio and dispersion can achieve higher energy density than either of the © 2013 American Chemical Society

Received: October 7, 2012 Revised: January 20, 2013 Published: March 6, 2013 1373

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Figure 1. SEM images and XRD of nanowires and nanocomposites: (a) sodium titanate NWs, (b) Ba0.2Sr0.8TiO3 NWs, (c) TEM image of a single Ba0.2Sr0.8TiO3 nanowire, (d) representative HRTEM image showing clear crystal lattice fringes of the Ba0.2Sr0.8TiO3 nanowire, (e) typical EDS spectra of Ba0.2Sr0.8TiO3 nanowire, and (f) XRD patterns of BaxSr1−xTiO3 NWs with a different Ba molar ratio.

failure is driven by flaws. Poly(vinylidene fluoride) (PVDF) and its copolymers and terpolymers have attracted considerable interest in recent years due to the high dielectric permittivity and breakdown strength obtained from the neat polymer.2,9−12 However, these polymers exhibit saturation and low efficiency, which limit the ultimate energy density and commercial potential.10,19 Recently, it was found that the maximum energy stored in a PVDF film is highly influenced by the crystalline phase obtained through each material processing method.10 Li et al. demonstrated that the γ-PVDF phase can be obtained by quenching the film in ice water, and showed it exhibitted higher energy density than β-PVDF or α-PVDF due to improved breakdown strength and the absence of early polarization saturation.10 Traditionally, ferroelectric ceramics such as lead zirconate titanate (PZT), barium titanate (BaTiO3 or BTO), or relaxor ferroelectrics are the preferred choice as the dielectric material

minimized.5,15,16 Therefore, nanocomposites are very promising candidates for the next generation of high power capacitors that reach unprecedented energy density. The energy density of a dielectric material is equal to the integral Ue = ∫ EdD, where E is the electric field and D is the electric displacement or charge density.6 Therefore, in addition to obtaining a high breakdown strength, a high electric displacement or dielectric permittivity is a key factor in achieving a high energy density.12 Recently, it was demonstrated that energy density of a nanocomposite capacitor could be improved through the use of high aspect ratio nanowire fillers rather than nanoparticles.6,17,18 In addition, when considering a high dielectric material it is critical to avoid saturation of the polarization below the breakdown field since this can limit the total energy density.12 However the most critical parameter that defines the ultimate energy density of a material is the processing method and quality of the film, since 1374

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Figure 2. (a) FTIR spectra of BST NWs and functionalized BST NWs by ethylenediamine, (b) FTIR spectra PVDF before and after quenched technique, (c) top surface and (d) cross-section of 5% Ba0.2Sr0.8TiO3/PVDF nanocomposites.

in capacitors because of their high dielectric permittivity.5,6,8 However, being ferroelectric, they are defined by their high remnant polarization and saturation of the polarization, both of which lead to limited energy density.10,12,19 Other ceramics can be designed such that they have a nonferroelectric structure but high dielectric properties, such as BaxSr1−xTiO3 (BST), which can be tuned to obtain a desired hysteresis behavior by varying the ceramics stoichiometry.20,21 In the BaxSr1−xTiO3 solid solution, transfer occurs from the ferroelectric phase to the paraelectric phase when the Ba mole fraction decreases below 0.7 (x < 0.7) and shows little hysteresis behavior at room temperature and above.20,22 The use of high dielectric fillers such as BST can provide a high dielectric constant while eliminating the remnant polarization and ultimately the efficiency of the capacitor. Here, we report a novel approach to prepare high energy density nanocomposite capacitors with fast discharge speed. The nanocomposites are prepared with a high aspect ratio Ba0.2Sr0.8TiO3 nanowires (NWs) in PVDF and are quenched in ice water to achieve the highest breakdown strength and energy storage properties. To the best of our knowledge, these nanocomposites have the largest breakdown strength (>450 MV/m) and energy density than any nanocomposite incorporating a high dielectric filler reported in the current literature, while also having low hysteresis and submicrosecond discharge. The maximum energy density calculated from the D−E loop is 14.8 J/cc, exceeding the state of the art commercially available capacitors (1.2 J/cc at 640 MV/m) by more than an order of magnitude.13 This report will serve to disseminate a state-of-the-art method of preparing nanocomposites with exceptional energy density. Ba0.2Sr0.8TiO3 ceramics were chosen since they exhibit paraelectric properties, with a high dielectric permittivity and

little hysteresis behavior. 20,22 The precursor to the Ba0.2Sr0.8TiO3 phase is sodium titanate synthesized by hydrothermal reaction as shown in Figure 1a. The precursor nanowires are free-standing and have a high aspect ratio. The second hydrothermal process is designed to specifically maintain the morphology of the nanowires and as can be seen by Figure 1b that the morphology of Ba0.2Sr0.8TiO3 is preserved. Chemical composition of nanowires was studied by an energy-dispersive X-ray spectroscopy (EDX, GENESIS), as shown in Figure 1e. The successful transformation of BaxSr1−xTiO3 nanowires after diffusion of the Ba and Sr ions into the precursor NWs during the hydrothermal reaction is further confirmed due to the presence of only Ba, Sr, Ti, and O. It should be mentioned that it is hard to clearly observe the separate peaks of Ba and Ti, since the main peaks of Ba (Ledge) and Ti (K-edge) overlap in the energy range of 4.5−5 keV.23 The crystal structure verification of Ba0.2Sr0.8TiO3 is performed by X-ray diffraction (XRD). As shown in the XRD patterns of BaxSr1−xTiO3 NWs (Figure 1f), it clearly demonstrates the crystal structure of BaxSr1−xTiO3 NWs after diffusion of the Ba and Sr ions into the precursor nanowires. The (110) diffraction peak of BaxSr1−xTiO3 gradually shifts from 31.48 to 32.24° as the molar fraction of barium initially in solution decreases from 1 to 0.20, since the radius of a Ba2+ ion (1.61 Å) is larger compared to radius of a Sr2+ ion (1.44 Å). Using Bragg’s law, the lattice parameters of Ba0.2Sr0.8TiO3 NWs is calculated as 3.922 Å, which closely matches reported data (3.920 Å).24,25 Transmission electron microscopy operated at 200 kV (TEM, JEOL TEM-1011) was utilized to further analyze the microstructure and crystal structure of the Ba0.2Sr0.8TiO3 nanowires. Figure 1c shows the individual nanowire has a straight cylindrical shape and single crystalline structure with growth along the [100] axis, as evidenced by 1375

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Figure 3. Dielectric properties of the nanocomposites: (a) comparison of measured dielectric constant (at 1 kHz) of nanocomposites with different aspect ratios: BST NWs and BST NRs and (b) dielectric constant of different BST NWs volume fractions in PVDF from 1 kHz to 1 MHz.

Figure 4. Energy storage performance of the nanocomposite capacitor: (a) Weibull distribution and observed dielectric breakdown strength of nanocomposites with different volume fraction of BST, quenched PVDF, untreated PVDF, and commercial polypropylene films; (b) unipolar electric displacement−electric field (D−E) loops for nanocomposites with different BST NWs volume fractions; (c) energy density of the nanocomposite with different volume fraction as a function of the electric field calculated from D−E loops; (d) efficiencies of the nanocomposites with different volume fraction as a function of the electric field.

To increase the compatibility between the filler and matrix, the Ba0.2Sr0.8TiO3 NWs have been functionalized with amine groups, which are well-known to provide strong interaction with PVDF thus improving dispersion and the charge uniformity at the ceramic/polymer interface.5 Figure 2a shows the FITR spectrum of Ba0.2Sr0.8TiO3 NWs before and after treatment with ethylenediamine. The presence of the amine groups on the surface of the Ba0.2Sr0.8TiO3 NWs following functionalization is evidenced through the appearance of the reduced transmission at 1450 cm−1.26 After functionalizing the BST NWs by ehtehylenediamine, they can be homogenously dispersed in the PVDF polymer matrix, as shown in Figure 2c and d, which also shows the absence of

clear lattice fringes in the high-resolution TEM (HRTEM) image of Figure 1d. The interlayer distances between adjacent lattice fringes in the HRTEM image of Ba0.2Sr0.8TiO3 nanowire is measured to be 3.915 Å ± 0.005 Å, which is consistent with XRD data and corresponds closely to the reported distance between two adjacent [100] Ba0.2Sr0.8TiO3 crystal planes (3.920 Å).24,25 The Ba0.2Sr0.8TiO3 has a paraelectric phase and shows little hysteresis behavior at room temperature and above,20,22 which directly decreases the ferroelectric loss and increases the energy density of the nanocomposites. This result demonstrates a unique process for the growth of high aspect ratio nanowires of any BaxSr1−xTiO3 stoichiometry. 1376

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voids in the film following annealing and quenching. The quenching process is used to modify the crystallization of the PVDF in the nanocomposites. Therefore, FTIR has been collected for PVDF before and after the samples were quenched, and these data are provided in Figure 2b. From the FTIR spectrum, the quenched PVDF is primarily in the γ phase (840, 812, 775 cm−1),10 while the untreated PVDF is dominated by the β phase. This result is as expected and desirable,10 since the γ-PVDF has reduced ferroelectric loss and higher breakdown strength10 of the nanocomposites as shown in the following results and discussion. It is known that the breakdown strength of the nanocomposites decreases with increasing concentration of the filler, especially at high volume fraction.6,8 To improve the dielectric property of the nanocomposite while maintaining high breakdown strength, the nanocomposites are fabricated at a low volume fraction of Ba0.2Sr0.8TiO3 NWs ranging from 2.5% to 7.5%. In addition, the high aspect ratio Ba0.2Sr0.8TiO3 NWs are ground to form a low aspect ratio of Ba0.2Sr0.8TiO3 nanorods (NRs) to effectively demonstrate the effect of the filler aspect ratio on the dielectric constant of the composite. Figure 3a shows that the dielectric constant of the samples increases at low electric field with increasing volume fraction of the Ba0.2Sr0.8TiO3 NWs in the nanocomposites, since the Ba0.2Sr0.8TiO3 fillers have a higher dielectric constant than the PVDF matrix. Also, it is clearly demonstrated that the nanocomposites with BST NWs have higher dielectric constants than the samples with BST nanorods (NRs), which has also been demonstrated in prior research.6,17 Building off these prior findings, the high energy density capacitors developed here will use high aspect ratio Ba0.2Sr0.8TiO3 NWs. Figure 3b shows the relationship between the dielectric constant of the nanocomposite and the frequency. The dielectric constant decreases with increasing frequency due to the dipole mobility, which is not sufficiently mobile to displace as the frequency of the applied electric field exceeds the relaxation frequency.27 The dielectric breakdown strength is analyzed using a two parameter Weibull cumulative probability function: P(E) = 1 − exp[1 − (E/EBD)β], where P(E) is the cumulative probability of failure occurring at the electric field lower or equal to E.9,15 The EBD is the scale parameter for experimental breakdown strength with a 63.2% probability for failure, and β is the shape parameter associated with linear regressive fit of the data distribution. The dielectric breakdown strength is then extracted from a fit using Weibull failure statistics across at least 15 tests per sample. Figure 4a summarizes the characteristic breakdown strength of the nanocomposites with different volume fractions of BST, quenched PVDF, untreated PVDF, and commercial biaxial oriented polypropylene (BOPP). Following quenching of the PVDF film in ice water, the dielectric strength is improved to 536.1 MV/m, while the untreated PVDF is 443.8 MV/m. It should be noticed that the breakdown strength of the quenched PVDF is close to commercial BOPP with 646.8 MV/m from Milwek Company. As the volume fraction of the BST NWs increases from 2.5% to 7.5%, the dielectric breakdown strength decreases from 505.1 to 450.1 MV/m, since the introduction of the fillers into the polymer results in defects that initiate failure and decrease the breakdown strength. It should be noted that, while the breakdown decreases, all nanocomposites maintain a relatively high breakdown strength at low volume fraction of the filler,

which provides the opportunity for nanocomposites with very high energy density. Energy density is not only related to dielectric permittivity and the dielectric breakdown strength, but also related to the polarization and applied electric field. The energy density of the linear dielectric can be simply expressed as κEb2/2, where κ is the dielectric permittivity and Eb is the breakdown strength.4 Typically, linear dielectrics do not have high dielectric permittivity, which makes it difficult to obtain high energy density. For ferroelectric materials, the electric displacement is not linearly dependent on electric filed. Also, the polarization and dielectric permittivity of ferroelectric materials have strong dependence on a variety of external conditions, particularly the applied electric field for energy storage. Therefore, a Sawyer− Tower circuit is used to determine the polarization−electric field relationship and the true energy density of the nanocomposite. The energy density of the nanocomposites can be computed from the D−E loops based on the integration of the discharge curve of the D−E loop. The polarization loop was measured under a unipolar field with increasing peak electric field is shown in Figure 4b for the neat PVDF and each volume fraction of NWs. The addition of the Ba0.2Sr0.8TiO3 NWs into the PVDF polymers greatly increases the maximum polarization of the nanocomposites. Most notably, it increases sharply at high volume fraction of Ba0.2Sr0.8TiO3 NWs compared to pure PVDF. It should also be noted that the hysteresis increases slowly with increasing concentration of the filler; however, the hysteresis is considerably smaller than the results presented in prior research efforts utilizing nanocomposites with ferroelectric fillers.19,28 Hysteresis is very important since it leads to reduced efficiency and internal heating and ultimately limits the maximum energy density and operational frequency of the capacitor. Figure 4c shows the use of the BST nanowires leads to improved energy densities compared with neat PVDF polymer at a high electric field. For the composites with 7.5% Ba0.2Sr0.8TiO3 NWs, the energy density is 14.86 J/cc at 450 MV/m, which represents a 42.9% increase in comparison to the PVDF with an energy density of 10.4 J/cc at the same electric field. This energy density rivals or exceeds those reports for ceramic/polymer composites5,8,19,28,29 and is 1138% greater than the 1.2 J/cc energy density at 640MV/m for commercial BOPP.13 The measured breakdown strength over 450 MV/m is the highest breakdown strength reported for nanocomposites incorporating a high dielectric filler ceramic/polymer capacitor, all of which are well below 250 MV/m.5,8,28,29 For practical applications, it is desirable to not only have a high energy density, but to also maintain a high efficiency (η), since the energy losses in the capacitor leads to heating and, consequently, to detrimental effects on the performance and reliability of the capacitor. Figure 4d shows the efficiency (discharge energy/charge energy) of the nanocomposites with different NW concentrations at a high electric field, as calculated from the D−E loops in Figure 4b. It is clearly shown that the efficiency decreases with the applied electric filed, particularly above 100 MV/m, which is highly related to the conduction loss.30 As the concentration of the filler increases the efficiency of the capacitor decreases due to the larger hysteresis in the polarization. However at fields below 100 MV/m, the efficiency is greater than 90% and greater than 60% at an electric field of 450 MV/m, which are higher than the nanocomposite capacitors reported in the literature.19,28 The improved efficiency is attributed to the nonferroelectric 1377

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Figure 5. Typical (a) discharged energy density and (b) power density profiles for nanocomposites with 7.5% BST NWs and commercial BOPP, respectively. The load resistor RL is 20 kΩ, and the electrical field is 200 MV/m.

strength of the nanocomposites with absence of polarization saturation. The energy density with 7.5% Ba0.2Sr0.8TiO3 NWs reached 14.86 J/cc at 450 MV/m, which represents a 42.9% increase in comparison to the PVDF with an energy density of 10.4 J/cc at the same electric field. To the best of our knowledge, this energy density is the highest reported for any nanocomposite incorporating a high dielectric filler ceramic/ polymer capacitor in the literature. Furthermore, the discharge speed of the nanocomposites is only 2.3 μs across a 20 kΩ resistor, which is faster than commercial polypropylene at the same RC time constant. The capacitors have more than an order of magnitude higher energy density than commercial BOPP capacitors (1.2 J/cc at 640 MV/m) and exhibit faster time to peak power and vastly improved power density. Furthermore, the efficiency of the nanocomposite is high due to the use of a pyroelelctric phase of barium strontium titanate. The results presented here demonstrate that the use of high aspect ratio nanowires can be used to produce nanocomposite capacitors with greater performance than the neat polymers, thus providing a novel process for the development of future pulsed power capacitors.

structure of Ba0.2Sr0.8TiO3 NWs and the quenching process to obtain the γ-PVDF phase.10 The efficiency results demonstrate that the nanocomposites developed here can capitalize upon the combination of inorganic materials of large permittivity with polymers of high breakdown strength to achieve high energy density and high efficiency. A fast discharge time is required for the pulsed power applications that form a challenging application of high power capacitors.2,12,18 The discharge speed and discharged energy were measured using a specially designed, high-speed capacitor discharge circuit similar to that reported in the literature.2 In this circuit, the discharge energy is measured from a load resistor (RL) in series with the nanocomposite samples. To compare with the discharge speed of commercial capacitors, the nanocomposites and a commercial polypropylene film is designed to have the same capacitance 26 pF at 1 kHz. Both samples are charged to 200 MV/m followed by discharge across a 20 kΩ load, as shown in the Figure 5a. The experimental discharge time τ0.9 is defined as the time for the discharged energy in the load to reach 90% of the final value from the discharge profiles. It is demonstrated that the nanocomposites have a discharge speed of approximately 2.3 μs, which is faster than commercial polypropylene (2.8 μs) that is noted for its fast discharge. More notably, our nanocomposites can discharge more energy at the same electric field compared to polypropylene. It should be noted that the electric field was limited for safety and thus the delivered energy should not be the full energy density of the material. The rate of discharge is also primarily driven by the RC time constant, and thus the discharge rate is highly dependent on the load resistance, however the nanocomposites have faster discharge than commercial BOPP when the RC time constant is identical. The discharge speed demonstrates that nanocomposites can be applied to the design of pulsed power capacitors with ultrahigh energy density and fast discharge time. It can be seen that the energy density measured from discharge measurement (Figure 5a) and the D−E loop measurement (Figure 4c) has consistent results (around 3.5 J/cc at electrical field 200 MV/m). The power density calculated from the discharge curve is shown in Figure 5b and demonstrates the nanocomposites can reach 2.38 MW/cc at 0.6 μs, while the BOPP can only deliver 0.164 MW/ cc at 0.92 μs. In summary, we have developed a novel method to prepare high energy density nanocomposite capacitors with fast discharge speed. It was demonstrated that the quenched PVDF and Ba0.2Sr0.8TiO3 NWs can improve the breakdown

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ASSOCIATED CONTENT

S Supporting Information *

Details about Ba0.2Sr0.8TiO3 NWs synthesis, nanocomposites preparation, and materials characterization. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: hsodano@ufl.edu. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors thank Kynar Company for providing PVDF and Mirwec Film Inc. for providing BOPP films used for this work. REFERENCES

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