Boosting the Recoverable Energy Density of Lead-Free Ferroelectric

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Functional Inorganic Materials and Devices

Boosting the Recoverable Energy Density of Lead-Free Ferroelectric Ceramic Thick Films through Artificially Induced Quasi-Relaxor Behavior Mahesh Peddigari, Haribabu Palneedi, Geon-Tae Hwang, Kyung Won Lim, Ga-Yeon Kim, Dae-Yong Jeong, and Jungho Ryu ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b05347 • Publication Date (Web): 01 Jun 2018 Downloaded from http://pubs.acs.org on June 1, 2018

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Boosting the Recoverable Energy Density of Lead-Free Ferroelectric Ceramic Thick Films through Artificially Induced Quasi-Relaxor Behavior Mahesh Peddigari†, Haribabu Palneedi†, Geon-Tae Hwang†, Kyung Won Lim†, Ga-Yeon Kim†, Dae-Yong Jeong‡* and Jungho Ryu§* †

Functional Ceramics Group, Korea Institute of Materials Science (KIMS), Changwon 51508, Republic

of Korea. ‡

Department of Materials Science & Engineering, Inha University, Incheon 22212, Republic of Korea.

§

School of Materials Science & Engineering, Yeungnam University, Gyeongan, Gyeongbuk 38541,

Republic of Korea.

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Abstract Dielectric ceramic film capacitors, which store energy in the form of an electric polarization, are promising for miniature pulsed power electronic device applications. For superior energy storage performance of the capacitors, large recoverable energy density along with high efficiency, high power density, fast charge/discharge rate, and good thermal/fatigue stability are desired. Herein, we present highly dense lead-free 0.942[Na0.535K0.480NbO3] - 0.058LiNbO3 (KNNLN) ferroelectric ceramic thick films (~5µm) demonstrating remarkable energy storage performance. The nanocrystalline KNNLN thick film fabricated by aerosol deposition (AD) process and annealed at 600oC displayed quasi-relaxor ferroelectric behavior, which is in contrast to the typical ferroelectric nature of the KNNLN ceramic in its bulk form. The AD film exhibited a large recoverable energy density of 23.4 J/cm3 with an efficiency of over 70% under the electric field of 1400 kV/cm. Besides, an ultrahigh power density of 38.8 MW/cm3 together with a fast discharge speed of 0.45 µs, good fatigue endurance (up to 106 cycles), and thermal stability in a wide temperature range of 20°C-160°C were also observed. Using AD process, we could make highly dense microstructure of the film containing nano-sized grains, which gave rise to the quasi-relaxor ferroelectric characteristics and the remarkable energy storage properties. Keywords: Lead-free; relaxor ferroelectric; thick film; energy storage density

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1. INTRODUCTION In the past few years, there has been increasing attention on the development of dielectric capacitors for pulsed power technology and advanced power electronics, which have applications in lasers, radar transmitters, hybrid electric vehicles, frequency inverters, and pace makers, and electric weapon systems.1–3 Dielectric materials with large recoverable energy density (Urec), high efficiency (η), fast charge/discharge rate, and good fatigue endurance and thermal stability are highly desired to realize capacitors with improved energy storage performance. Various dielectric materials based on polymers, glasses, and ceramics have been studied for developing high energy density capacitors.4–13 The energy storage density of a dielectric material is proportional to its permittivity (polarization) and dielectric breakdown strength (DBS). In this regard, polymer- and glass- based dielectric materials have been extensively studied due to their higher DBSs (>7000kV/cm).4–7 However, the requirement of such substantially high electric field intensities for the realization of high energy density raises several safety concerns over their usage as a dielectric medium in capacitors. Further, the relatively low permittivity and maximum operating temperature (4 J/cm3) was realized in KNN-based bulk ceramics due to the enhancement of DBS by reducing the grain size to submicron scale.2,42,43 In the present study, we report the energy storage properties of 0.942(K0.480Na0.535)NbO3-0.058LiNbO3 (KNNLN) thick films (~5 µm) grown by the AD process followed by their thermal annealing at 600oC. Due to the quasi-relaxor behavior of the KNNLN thick films, induced by their nanocrystalline microstructure, they exhibited highly enhanced recoverable energy density (23.4 J/cm3) and storage efficiency (70%) at an electric field of 1400 kV/cm along with high power density (38.8 MW/cm3 at 500 kV/cm) and fast discharge time of 0.45µs. 2.

EXPERIMENTAL SECTION

2.1. Materials and powder preparation method Initially, KNNLN powders with the composition of 0.942[Na0.535K0.480NbO3] - 0.058LiNbO3 were prepared by solid state reaction method using K2CO3, Na2CO3, Li2CO3, and Nb2O5 (>99% pure, Sigma Aldrich) as raw materials. Stoichiometric amounts of these chemicals were milled 6 ACS Paragon Plus Environment

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for 24h and the obtained powder was calcined at 850 oC for 4 h to induce the perovskite phase formation. Subsequently, the calcined powder was crushed and re-milled for 5 h to obtain adequate particle size for AD process. 2.2. Thick films deposition For the deposition of the KNNLN film, the re-milled powder was mixed with dry air (carrier gas) to form an aerosol. This particle suspension was fed (10 L/min flow rate) through a Laval type nozzle with a rectangular orifice (10×0.5 mm2) and sprayed onto the Pt(111)/Ti/SiO2/Si(100) substrate in a vacuum chamber (3.5 Torr). The deposition parameters such as scan speed and number of repetitions were optimized to obtain the desired film thickness of 5 µm. For comparison studies, KNNLN bulk ceramic was made with the same powder that was used for the film deposition by sintering at 1075oC for 4 h. 2.3. Characterization The phase of the KNNLN powder, bulk ceramic and thick films was examined using X-ray diffractometry (XRD, D-MAX 2200, Rigaku) and Raman spectrometry (LABRAM HR800, Horiba Jobin Yvon). The microstructure of the films was observed by means of scanning electron microscopy (SEM, JSM-5800, JEOL) and transmission electron microscopy (TEM, JEM-2100F, JEOL). For electrical measurements, circular Pt electrodes (0.5 mm in diameter) were sputtered on the KNNLN film. The frequency dependent (1kHz - 1MHz) dielectric properties of the films were measured using an impedance analyzer (4294A, Agilent Technologies). The temperature dependent ferroelectric hysteresis loops were recorded at 1 kHz with a ferroelectric tester (Precision LC-II, Radiant Technologies). An in-house designed high speed switching circuit was used to measure the time dependent charge-discharge profiles of the 7 ACS Paragon Plus Environment

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KNNLN thick film capacitors. A DC voltage was applied to the capacitor and the discharged energy across the load resistance was measured using an oscilloscope (Wavesurfer 44Xs-A, LeCroy). 3.

RESULTS AND DISCUSSION The results of the X-ray diffraction (XRD) and Raman spectroscopic analyses are shown in

Figure 2. The KNNLN bulk ceramic exhibited an orthorhombic perovskite phase, while both the as-deposited and 600oC annealed KNNLN thick films displayed a pure perovskite phase with a pseudocubic structure. The film annealed at 700oC showed an impurity phase that might have formed due to volatilization of alkaline elements at high temperature (Figure S1a, supporting information).

Figure 2. (a) XRD patterns and (b) Raman spectra of bulk ceramic, as-deposited and annealed thick films of KNNLN. The as-deposited film exhibited broad and low-intensity XRD peaks and Raman modes, due to the presence of amorphous phase and nanocrystallites. Annealing of the film leads to crystallinity improvement, resulting in the enhanced intensity of all the XRD peaks and Raman modes. For comparison, XRD pattern and Raman spectra of bulk KNNLN bulk ceramic prepared by conventional sintering are also shown.

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In the case of the as-deposited KNNLN film, all XRD peaks as well as Raman modes were broad due to the smaller crystallite sizes and highly disordered crystallinity resembling an amorphous phase formed due to particle fracturing and deformation by high kinetic energy collisions during film deposition by AD process. Also, the as-deposited film exhibited a slight {001} orientation, which may be attributed to the high compressive stress in the film generated by the particle collisions on the substrate.44–46 After annealing, the intensity of all the XRD peaks and Raman modes were increased indicating an improvement in crystallinity of the film. A relative improvement in the intensity of the {011} peak was observed as compared to the {001} peak, whereas a clear red shift (~3.2 cm-1) in the ν1 Raman mode was noticed with annealing (Figure S1b, supporting information). These results suggest that annealing relaxed the residual compressive stress in the film.

Figure 3. Bright-field TEM images of (a) as-deposited and (b) annealed KNNLN thick films showing a microstructure with nano-sized grains. Besides the improvement of crystallinity, an increase in grain size (from 30.8 ± 7 nm to 54 ± 9 nm) was also observed in the annealed KNNLN thick film. (c) Cross-sectional SEM image of the KNNLN thick film annealed at 600oC for 1h, showing dense structure without any pores. The microstructural images of the as-deposited and annealed KNNLN thick films are shown in Figure 3. The as-deposited film had an average grain size of 30.8 ± 7 nm (Figure 3a). The formation of smaller crystallites in the film is due to the high kinetic energy collisions of 9 ACS Paragon Plus Environment

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KNNLN particles causing fracture and plastic deformation during the AD process. Figure 3b displays a TEM image of the KNNLN thick film annealed at 600oC, revealing a clear increase of the grain size (54 ± 9 nm) by annealing. The thickness of the KNNLN thick films measured from the cross-sectional SEM micrograph (Figure 3c) was approximately 5 µm. The highly dense microstructure observed in the AD films was resulted from the effective arrangement of the fractured and deformed particles during the film deposition.

Figure 4. (a) Frequency dependent dielectric properties of KNNLN thick films and bulk ceramic measured in the frequency range of 1kHz-1MHz. There is a large enhancement of dielectric permittivity of the KNNLN film after annealing, which can be attributed to its improved crystallinity. (b) Temperature dependent dielectric properties of annealed KNNLN thick film measured at different frequencies showing the diffuse type transition around Curie temperature. The inset of (b) displays the modified Curie-Weiss law fit for the annealed thick film measured at a frequency of 100kHz. The higher γ value (1.90) is indicative of the relaxor ferroelectric behavior of the annealed film. In order to obtain high recoverable energy density, the material should possess high relative permittivity (εr), low dielectric loss (tanδ), and high DBS.47 Frequency dependence of εr and tanδ of the KNNLN bulk ceramic and films measured in the range of 1kHz - 1MHz are compared in Figure 4a. The as-deposited KNNLN thick film displayed lower values of εr (65 at 1 kHz) and tanδ (3.2% at 1 kHz) as compared to the annealed (εr = 679 and tanδ = 3.4% at 1 kHz)

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film and the bulk ceramic (εr = 752 and tanδ = 5.2% at 1 kHz). The lower dielectric constant of the as-deposited film can be correlated to the disordered crystallinity and smaller crystallite size, which can inhibit the domain wall motion. The motion of the domain walls reflects the electrical properties of the ferroelectrics and it can be enhanced with increasing grain size.48 In this study, the film annealed at 600oC exhibited a improved dielectric properties as compared to that of the films annealed at other temperatures (Figure 4a and Figure S2a, supporting information) due to the increased grain size and better crystallinity. The temperature dependent (30oC - 600oC) dielectric properties of the annealed KNNLN thick film as a function of frequency (10kHz - 1MHz) are shown in Figure 4b. In the bulk KNNLN ceramic, two clear phase transitions were observed at 66oC and 422oC, which correspond to the orthorhombic to tetragonal (TO-T) and tetragonal to cubic (TT-C, TC) phase transitions, respectively (Figure S2b, supporting information). Interestingly, the AD film showed good temperature stability (tanδ ≤ 0.08 at 10 kHz) up to 140oC without showing a phase transition (TO-T) at 66oC due to the deformed crystal structure, as understood from the XRD and TEM results. In addition, the annealed film showed frequency independent TC changes but broad peaks with a diffuse type transition at around the TT-C phase transition. This type of diffuse transition is a typical characteristic of relaxor behavior and is quantitatively analyzed using a modified Curie-Weiss law [1/εr – 1/ εrm = (T-Tm)γ/C], where C is the Curie-Weiss constant, εrm is maximum values of εr at a temperature of Tm, and γ is the diffuseness coefficient, which varies between 1 (normal ferroelectrics) and 2 (relaxor ferroelectrics).49 The diffuseness coefficient γ for the bulk KNNLN ceramic is estimated as 1.08, whereas it increased to 1.90 for the film (inset of Figure 4b). This indicates that quasi-relaxor ferroelectric behavior was induced artificially in the annealed AD thick film, which is due to the nano-sized grains as compared to the large grains

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in bulk. It is believed that if the crystallite/domain size is similar to the critical grain size for ferroelectricity, relaxor ferroelectric behaviors can be induced artificially, similar to the PNR effect. The Curie temperature (TC) of the film was determined to be 506oC from the permittivity maxima, which is >80oC higher than that of bulk KNNLN ceramics. This is attributed to the presence of in-plane compressive stress in the AD film.44–46 The recoverable energy density (Urec) of the KNNLN thick films was calculated from the P-E loops (dynamic method) using the following equation, Pmax

U rec =

∫ EdP 0 ≤ E ≤ E

max

Pr

(1)

where Emax, Pr and Pmax are the applied maximum electric field, remnant and maximum polarization values, respectively. The polarization loops of KNNLN thick films measured under a unipolar electric field with increasing peak values are shown in Figure 5a. The KNNLN thick films exhibited slim hysteresis loops with low remnant polarizations and high breakdown strengths as compared with the KNNLN ceramic (Figure S3a, supporting information). The asdeposited film has shown paraelectric-like hysteresis loop behavior with smaller values of maximum polarization (Pmax = 13.6 µC/cm2), and an Urec value of 3.8 J/cm3 (at 1200 kV/cm). The annealed film exhibited relaxor-like ferroelectric hysteresis loops with a Pmax of 62.1 µC/cm2 and an Urec of 23.4 J/cm3 at an electric field of 1400 kV/cm (Figure 5b). The Urec value of the annealed film is a 522% enhancement compared to that of the as-deposited film and a manifold enhancement relative to that of the bulk KNNLN ceramic (< 1 J/cm3) (Figure S3b, supporting information). Also, the obtianed Urec value in the present work is larger than the other reported relaxor thick films.20,21,23–25 The increase of Pmax and Urec after annealing is attributed to the improvement in both crystallinity and grain size of the KNNLN film. However, with increasing

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annealing temperature above 600oC, a reduction in the Urec value was observed. This might be due to the increase in the grain size and the formation of a secondary phase as a result of volatilization of alkali elements at higher annealing temperature, thus resulting in an increase of leakage current density and a decrease of breakdown strength and the energy density of the film (Figure S4, supporting information).

Figure 5. (a) Electric field dependence of unipolar P-E loops of the KNNLN thick films. The asdeposited film showed paraelectric-like behavior, whereas the annealed film shown the relaxorlike behavior with enhanced polarization values. These P-E curves were used calculate the (b) recoverable energy density and (c) energy storage efficiency at respective electric fields. Due to the induced relaxor-like ferroelectric behavior in the annealed film, larger values of Urec and η 13 ACS Paragon Plus Environment

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were obtained. (d) Weibull distribution plot of the KNNLN thick film annealed at 600oC for 1h, revealed the most probable breakdown field (α) of the film is 1240 kV/cm with the Weibull modulus (β) around 7.63. For practical applications, it is important to maintain larger efficiency (η = Urec/(Urec+Uloss)) under a high electric field along with high energy density, where Uloss is the integrated area of the P-E hysteresis loop.50,51 Under an identical electric field (1200 kV/cm), asdeposited films have shown a η of 48.6%, whereas the annealed film exhibited a greatly improved η of ~70% (Figure 5c). However, η decreased with a further increase of annealing temperature due to the rise in conduction and ferroelectric losses of the film (Figure S4, supporting information). In order to verify the reliability of the film under a high applied electric field, the DBS of the film annealed at 600oC is analyzed using a Weibull cumulative probability function,52   E β  P ( E ) = 1 − exp 1 −      α  

(2)

where P(E) is the cumulative probability of electric failure at the electric field E, α is the scale parameter obtained from the experimental breakdown strength with a cumulative failure probability of 63.2%, and β is the Weibull modulus, which represents the dispersion in the data. 52,53

A higher value of β represents less dispersion in the data. From Figure 5d, the experimental

DBS of the KNNLN film reaches a maximum value of 1500 kV/cm. The values of α and β extracted from the linear regressive fit of the data distribution and are 1240 kV/cm and 7.6, respectively. Strong electrical fatigue endurance and temperature stability are the other important parameters required for energy storage applications. Therefore, the fatigue behavior of the KNNLN thick film annealed at 600oC was measured up to 107 electric cycles with a frequency of 14 ACS Paragon Plus Environment

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1kHz using a triangle bipolar waveform at an electric field of 700 kV/cm. Figure 6a displays the unipolar P-E loops and the corresponding normalized Urec and η values of the KNNLN thick film measured after different electric cycles. The KNNLN thick film exhibited clear fatigue-free behavior up to 106 cycles and slight changes in Urec (∆Urec ~0.3%) and η (∆η ~1.6%) were observed after 107 cycles.

Figure 6. Unipolar P-E loops and corresponding normalized values of Urec and η measured at 700 kV/cm for (a) fatigue measurement (up to 107 electric cycles) examined at room temperature, and (b) thermal stability measurement (20oC – 160oC) of KNNLN thick film annealed at 600oC for 1h. The film exhibited good fatigue endurance (∆Urec ~0.3% and ∆η ~1.6%) to the applied

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electric cycles and thermal stability (∆Urec < 4% and ∆η ~ 8%) in the measured temperature range. Further, the thermal stability of the KNNLN thick film was investigated by measuring the unipolar P-E loops at different temperatures from 20oC to 160oC under an applied electric field of 700 kV/cm. From Figure 6b, it is noted that the P-E hysteresis loop of the film becomes slim up to a temperature of 60oC and this can be attributed to the enhanced dynamics of smaller crystallites (PNRs).54 Above this temperature, the area of the P-E hysteresis loop increases gradually with increasing temperature. The normalized values of Urec and η as a function of temperature are shown in Figure 6b. The film exhibited excellent temperature stability in the measured temperature range with only a slight change of ∆Urec < 4% and ∆η ~ 8%.

Figure 7. Charge-discharge profiles of KNNLN thick films measured at 500 kV/cm and at a load resistance of 1kΩ: (a) the estimated discharge energy density of the films from the time dependent charge-discharge curves and (b) corresponding power density as a function of time. The larger values of discharge energy density (7.7 J/cm3) and power density (38.8 MW/cm3) for the annealed film are attributed to its improved crystallinity, grain size and induced relaxor-like ferroelectric behavior.

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For the pulsed power and other high power applications, a short discharge time is required.1,55 The discharge energy density, discharge time (τ0.9), and power density of the KNNLN thick films were measured from a static method using a high speed capacitor discharge circuit (Figure S5a, supporting information). The time τ0.9 is defined as the time required for 90% discharge of the maximum value from the discharge curve. All the films were charged up to 500 kV/cm and the discharge energy was measured across the load resistance (1 kΩ). The time dependent discharge energy density (Urec) and power density (P =V2 (t) /R) of the KNNLN thick films were estimated from the charge-discharge profile, as shown in Figure S5b, supporting information. t

U rec = ∫ V (t ) I (t ) dt = 0

τV02 

1 − e 2vR 



2t

τ

  

(3)

where V(t) is the voltage at time t, I(t) is the current at time t, R is the load resistance, τ is the relaxation time, and v is the volume of the capacitor film. It is observed that the as-deposited film has a τ0.9 of 0.02 µs, and this is fast compared to the other films due to the paraelectric-like behavior arising from the nano-sized crystallites in amorphous phase. Therefore, the as-deposited film showed a small value of discharge energy density (0.4 J/cm3 at 500 kV/cm) and short discharge time (Figure 7a). With annealing, both the Urec and τ0.9 values are enhanced and a maximum value of Urec (7.7 J/cm3 at 500 kV/cm) with a τ0.9 of 0.45 µs was obtained for the film annealed at 600oC. The obtained τ0.9 values (0.02 µs- 0.45 µs) of the KNNLN thick films are faster than poly(vinylidene fluoride) (PVDF) based films (2.30 µs)55

and Pb0.97Y0.02

[(Zr0.6Sn0.4)0.925Ti0.075]O3 thin films (1.46 µs)50 and comparable to 0.5(Ba0.7Ca0.3)TiO30.5Ba(Zr0.2Ti0.8)O3 (BCT-BZT) based nanocomposites (~0.19 µs).56 The calculated power

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densities of KNNLN thick films are in a range of 37.6 MW/cm3– 38.8 MW/cm3 at 500 kV/cm (Figure 7b) and these values are superior to the reported values.55,56

4.

CONCLUSIONS

High density lead-free artificial quasi-relaxor KNNLN thick films with nano-sized grains were fabricated by an aerosol deposition process to prepare high energy storage density capacitors. The effect of annealing temperature on structural, microstructural, and electrical properties has been systematically studied. A large recoverable energy density of 23.4 J/cm3 with an efficiency of 70% (at 1400 kV/cm) and a high power density of 38.8 MW/cm3 (at 500 kV/cm) with a fast discharge speed of 0.45 µs were realized in the KNNLN thick film annealed at 600oC. In addition, the KNNLN thick film exhibited a high fatigue endurance (∆Urec ~ 0.3% and ∆η ~ 1% at 700 kV/cm) up to 107 electric cycles and high thermal stability (∆Urec < 4% and ∆η ~ 8% at 700 kV/cm) in a temperature range of 20oC-160oC. The achievements suggest that the ADKNNLN thick films are promising for high energy density capacitor applications.

ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS Publication website at DOI: XRD patterns of the KNNLN film annealed at 700 oC for 1h; variation in the position and corresponding FWHM of the ν1 mode of the KNNLN thick films as a function of annealing temperature; frequency dependent dielectric properties of as-deposited and annealed (500oC700oC) KNNLN thick films; temperature dependent dielectric properties and modified Curie-

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Weiss law fit (inset) of KNNLN bulk ceramic; bipolar and unipolar P-E loops of the KNNLN bulk ceramic; unipolar P-E loops and J-E characteristics of KNNLN as-deposited and annealed thick films; Schematic illustration of the high speed switching circuit, and charge-discharge profiles of KNNLN as-deposited and annealed thick films.

AUTHOR INFORMATION Corresponding Authors E-mail: [email protected] (D.-Y. Jeong) E-mail: [email protected] (J. Ryu)

ORCID Jungho Ryu: 0000-0002-4746-5791

Author Contributions All authors have given approval to the final version of the manuscript.

Notes The authors declare no competing financial interest.

Acknowledgements This study was supported by the Basic Science Research Program of the National Research Foundation of Korea (NRF) funded by the Ministry of Science, ICT & Future Planning (NRF2016R1A2B4011663) and internal R&D program of Yeungnam University (Grant No. 218A580001).

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Barber, P; Balasubramanian, S; Anguchamy, Y; Wibowo, A.; Gao, H.; Ploehn, H. J.; Loye, H. Polymer Composite and Nanocomposite Dielectric Materials for Pulse Power Energy Storage. Materials (Basel). 2009, 2, 1697-1733.

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