Antiferroelectric Thin-Film Capacitors with High Energy-Storage

Nov 25, 2015 - exhibiting a low energy-storage efficiency (η) of 45.4%.11. Figure 1 reveals the typical polarization behavior versus the electric fie...
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Antiferroelectric Thin-Film Capacitors with High Energy-Storage Densities, Low Energy Losses, and Fast Discharge Times Chang Won Ahn, Gantsooj Amarsanaa, Sung Sik Won, Song A Chae, Dae Su Lee,† and Ill Won Kim* Department of Physics and EHSRC, University of Ulsan, Ulsan 680-749, Republic of Korea S Supporting Information *

ABSTRACT: We demonstrate a capacitor with high energy densities, low energy losses, fast discharge times, and high temperature stabilities, based on Pb0.97Y0.02[(Zr0.6Sn0.4)0.925Ti0.075]O3 (PYZST) antiferroelectric thin-films. PYZST thin-films exhibited a high recoverable energy density of Ureco = 21.0 J/cm3 with a high energy-storage efficiency of η = 91.9% under an electric field of 1300 kV/cm, providing faster microsecond discharge times than those of commercial polypropylene capacitors. Moreover, PYZST thinfilms exhibited high temperature stabilities with regard to their energy-storage properties over temperatures ranging from room temperature to 100 °C and also exhibited strong charge− discharge fatigue endurance up to 1 × 107 cycles. KEYWORDS: solid-state dielectric capacitors, energy storage, antiferroelectrics, thin-films, fatigue

O

ver the past several decades, dielectric materials with high energy-storage densities have received increased attention because of their potential application within capacitors for modern electronics and electrical power systems. High power capacitors typically find use in electronic applications such as power inverters, and in pulsed power applications such as radar, lasers, medical defibrillators, and pacemakers.1−3 With pulsed power capacitors being more technologically challenging in application because of their requirement for low energy losses (the same meaning as high-efficiency) and fast discharge times, much of the attention with regard to high power capacitors has been applied toward these types of materials.2−6 Dielectrics for high power capacitors have been primarily focused on linear dielectric materials (DEs), ferroelectric materials (FEs), and antiferroelectric materials (AFEs). Among these, the energy-storage densities of AFEs are typically higher than those of FEs and DEs, because of their high saturated polarization and near-zero remnant polarization.7−9 Moreover, AFEs also possess fast charge−discharge speeds and good fatigue endurance because of their unique field-induced switching capabilities between the AFE and FE phase.10 Therefore, it could be concluded that AFEs are more suitable for application in pulsed capacitors with high energy-storage densities. However, typical AFEs exhibit quite low efficiencies, potentially below 50%. For example, Pb0.97La0.02(Zr0.98Ti0.02)O3 (PLZT) films exhibit relatively high energy-storage densities of 13.3 J/cm3 under an electric field of E = 1000 kV/cm while exhibiting a low energy-storage efficiency (η) of 45.4%.11 Figure 1 reveals the typical polarization behavior versus the electric field for AFEs. The stored electric energy density of a capacitor, U, is given by © XXXX American Chemical Society

Figure 1. Schematic illustration of the energy-storage characteristics for antiferroelectric materials in a unipolar P−E loop.

U=

∫ E dP

(1)

where E is the electric field, and P is the polarization. When the electric field increases from zero to the maximum value Emax, the polarization increases to the maximum value Pmax, and electric energy is stored in the capacitor as Ustore. The recoverable electric energy density Ureco is then released during discharging from Emax to zero, represented by the green-colored Received: September 17, 2015 Accepted: November 25, 2015

A

DOI: 10.1021/acsami.5b08786 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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Figure 2. (a) X-ray diffraction patterns of PYZST thin-films deposited via pulsed laser deposition onto Pt(111)/TiO2/SiO2/Si(100) substrates. (b) Scanning electron microscopy image of the cross-sectional view of the PYZST thin-film.

Figure 3. Polarization switching and energy-storage performance of the PYZST thin-film capacitors: (a) bipolar P−E hysteresis loops of the PYZST thin-film at an electric field of 200 kV/cm, demonstrating room temperature antiferroelectric behavior; (b) unipolar P−E hysteresis loops of the PYZST thin-films under various electric fields; (c) recoverable electric energy density (Ureco) of the PYZST thin-films as a function of the electric field calculated from unipolar P−E loops in b; (d) energy-storage efficiency (η) of PYZST as a function of electric field.

The low η of AFEs is due to the large hysteresis of coercive fields (ΔEc) during the phase transition between the AFE and FE phases. Capacitor charging and discharging proceeds by supplying and removing unipolar bias, respectively. The large Uloss within AFE capacitors occurred from ΔEc as a result of the phase transition between the AFE and FE phases when supplying and removing unipolar bias. Therefore, to enhance η within AFE capacitors, the ΔEc of AFEs should be reduced. Furthermore, to increase the energy-storage performance of AFE materials at lower fields, the threshold field from the AFE to FE state should be reduced, and P max should be

area in Figure 1. The electric energy loss density Uloss is the difference between Ustore and Ureco in eq 2, represented by the red-colored area in Figure 1. Ureco = Ustore − Uloss

(2)

The energy-storage efficiency η can be defined as η=

Ureco 100(%) Ustore

(3) B

DOI: 10.1021/acsami.5b08786 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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Figure 4. Temperature stability and fatigue endurance of the energy-storage performance of the PYZST thin-film capacitors: (a) recoverable electric energy density (Ureco) and the electric energy loss density (Uloss) as a function of temperature; (b) energy-storage efficiency (η) as a function of temperature at an electric field of 1300 kV/cm. (c) Ureco and (d) η as a function of charging/discharging cycles up to 1 × 107 under various ambient temperatures. The electric field was 1300 kV/cm and the charge/discharge frequency was 1 kHz.

following oxides: PbO, Y2O3, ZrO2, SnO, and TiO2. PYZST AFE thin-films were deposited onto Pt(111)/Ti/SiO2/Si(100) substrates by pulsed laser deposition using an ArF excimer laser with an oxygen partial pressure of 50 mTorr. The as-grown samples were annealed by direct insertion within a tube furnace at 700 °C for 60 min under an oxygen atmosphere for crystallization of PYZST. The thickness of final annealed film was about 500 nm. The chemical composition of PYZST thin film determined by electron probe microanalysis was Pb0.979±0.020Y0.023±0.004Zr0.560±0.019Sn0.372±0.008Ti0.077±0.010O3. The detailed experimental procedure is described in the Supporting Information. Figure 2a reveals the indexed X-ray diffraction patterns of the PYZST thin-films. The PYZST thin-films feature a single perovskite phase with a pseudocubic structure on the Pt(111)/ TiO2/SiO2/Si(100) substrates. The XRD patterns indicated that the PYZST thin-films were polycrystalline and randomly oriented. Figure 2b presents a cross-sectional scanning electron microscopy image of a PYZST thin-film deposited onto Pt(111)/TiO2/SiO2/Si(100). The PYZST thin-films featured dense and columnar grains due to heterogeneous nucleation growth with the film thickness being approximately 500 nm, as seen from the cross-sectional SEM image. High phase purity, uniformity, and density of the PYZST thin-films are important with regard to the dielectric breakdown strength and energy

simultaneously enhanced. Among AFE materials, Sn-modified Pb(Zr,Ti)O3 AFE materials exhibit relatively lower threshold fields and higher Pmax values compared to general PbZrO3 (PZ)-based AFE materials.12,13 Moreover, substitution of donor ions within Pb(Zr,Sn,Ti)O3 ceramics, such as Y3+ or La3+ substitution for Pb2+ sites, provided a drastic decrease in hysteresis and stabilized the ferroelectric phase.12−14 To design a proper AFE material for commercial high-power capacitor applications, high energy-storage densities, efficiencies, and temperature-dependent stabilities of the energystorage performance must be satisfied simultaneously; polarization-fatigue endurance is important as well. It is well-known that Pb(Zr,Ti)O3-based ferroelectric thin-films exhibit a gradual degradation of Pmax against domain switching driven by an externally applied electric field.15,16 The degradation of Pmax inevitably leads to a decrease in both Ureco and η. Hence, the improvement of polarization-fatigue endurance is very important for commercial high power capacitor applications. In this work, we demonstrated a high energy density, low loss, fast discharging, and highly stable capacitor based on Pb0.97Y0.02[(Zr0.6Sn0.4)0.925Ti0.075]O3 (PYZST) AFE thin films, which demonstrated a low ΔEc, low threshold fields, and a large Pmax. Y- and Sn-modified lead zirconate titanate ceramic target was prepared by conventional solid state reaction method using the C

DOI: 10.1021/acsami.5b08786 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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Figure 5. Discharging time of the PYZST thin-film capacitor: (a) power density and (b) discharge energy density profiles as a function of time measured via direct discharging of the PYZST thin-film to a 1 kΩ load resistor (RL). The applied electric field was 1300 kV/cm. The experimental discharge time τ0.9 was defined as the time required for the discharge energy within the load to reach 90% of its final value from the discharge profile.

Uloss both increased slightly but did not drastically change as shown in Figure 4a. Changes to Ureco were less than 8% when the temperature increased from 30 to 100 °C. Furthermore, η also exhibited a relatively high value of 81.8% at 100 °C, as shown in Figure 4b. These results demonstrated the excellent thermal stability of the PYZST thin films. A high fatigue endurance is necessary for long-term stability during the capacitor charge−discharge cycling process. Hence, the polarization fatigue behavior of the PYZST thin films in unipolar P−E loops was investigated as a function of the charge−discharge cycles up to 1 × 107 cycles at various temperatures using a 1 kHz pulse signal under a 1300 kV/cm electric field. The P-E loops of the virgin PYZST thin-films and after 1 × 107 charge−discharge cycles are displayed in Figure S4 of the Supporting Information. Figure 4c, d shows the variation of Ureco and η during fatigue testing. Ureco and η were calculated from the unipolar P−E loop results of the polarization fatigue endurance test. As shown in Figure 4c, the reduction of Ureco after 1 × 107 switching cycles was less than 2.5% at three experimental ambient temperatures, specifically at room temperature, 50 °C, and 100 °C. The reduction of η was also less than 5.5% under the same conditions. This indicated that the PYZST thin-film capacitors exhibited high fatigue endurance up to 1 × 107 switching cycles under a working temperature range from room temperature to 100 °C. It is known that unipolar pulses produce no or less fatigue compared to bipolar pulses for FE materials, while AFEs show better fatigue endurance than FEs under bipolar electrical cycling.19,20 The fatigue mechanism for ABO3-type FE materials was proposed via the following two aspects.21,22 One aspect was the long-range diffusion of defects via an applied external electric field, such as oxygen vacancies in ABO3-type perovskite structures or switching-induced charge injection from the nearest electrode, which was the main cause for local phase decomposition and polarization fatigue for both AFEs and FEs. The other aspect was domain wall pinning under repeated domain switching. The improved unipolar fatigue endurance compared to bipolar systems was attributed to less severe switching-induced charge injection from the nearest electrode. A higher applied electric field would improve fatigue performance, because larger electric fields enable easier domain switching and depinning. Fatigue endurance could also be improved through doping with donor ions at the A- or B-sites.23,24 For example, yttriummodified Pb(Zr,Ti)O3 (PZT) thin films exhibited more than 10

loss properties of the thin-film capacitors for high energystorage applications. Figure 3a presents the P−E loops of the PYZST films, which were measured at room temperature under an applied field of 200 kV/cm at 1 kHz. The Pmax at 200 kV/cm for the PYZST thin-film was 32.3 μC/cm2, and the threshold field (EA‑F) for phase switching between the AFE and FE states was 105 kV/ cm, which was much lower than that of the PZ (463 kV/cm) or PLZT (310 kV/cm) films.17,18 Moreover, PYZST thin-films exhibited quite small ΔEc values of 15 kV/cm compared to PZ (210 kV/cm) or PLZT (100 kV/cm) films.17,18 These low EA‑F and ΔEc values should facilitate high recoverable electric energy densities (Ureco) and efficiencies (η) at relatively low electric fields for the PYZST thin-film capacitors. Figure 3b presents the unipolar P−E hysteresis loops of the PYZST thin-film under various electric fields up to 1800 kV/ cm. The Ureco and η were obtained by using numerical integration of the area between the polarization axis and the switching curve of the unipolar P−E hysteresis loops, as explained in Figure 1. The calculated Ureco values of the PYZST thin-films are presented in Figure 3c. As expected, Ureco clearly increased with an increase in the electric field. A maximum value of 32.7 J/cm3 was obtained at 1800 kV/cm. Beyond this value, a higher field of 1850 kV/cm led to the breakdown of the PYZST thin-films. The Ureco value of the PYZST thin-films at 1000 kV/cm was 14.6 J/cm3, which was similar to the values measured for PZ and PLZT AFE films as listed in Table S2. However, PYZST thin-films exhibited relatively high efficiencies (η) as shown in Figure 3d. The average value of η under various electric fields ranging from 200 kV/cm to 1800 kV/cm was about 90%. The η value for PYZST thin-films at 1000 kV/cm was 91.3%, which was an exceedingly high value among AFE films as listed in Table S2. The bipolar P−E hysteresis loops and energy-storage performances of the PYZST thin-film capacitors were also measured as shown in Figure S1. The energy-storage performances (Ureco, Uloss, and η) under bipolar polarization switching were similar to those of unipolar polarization switching due to the nature of AFE materials. The recoverable electric energy density (Ureco), electric energy loss density (Uloss), and the charge−discharge efficiency (η) were calculated from the unipolar P-E hysteresis loops measured under an electric field of 1300 kV/cm at different temperatures (Figure S3). The results are displayed in Figure 4a, b. With increasing temperatures up to 100 °C, Ureco and D

DOI: 10.1021/acsami.5b08786 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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project. C.W.A. and I.W.K. cowrote the manuscript. All authors viewed and commented on the manuscripts.

times higher polarization fatigue endurance compared to the fatigue properties of pure PZT films.23 Fast discharge times are required for the pulsed power applications of energy-storage capacitors.2,25−28 The discharging speed of the PYZST thin-films was measured with a highspeed capacitor discharge circuit (Figure S6). The discharged energy was measured through a 1 kΩ load resistor (RL) in series with the PYZST capacitor; the experimental charge− discharge currents and applied voltage profiles are displayed in Figure S5. From the discharge current and applied voltage profiles, the time dependence of the power density and Ureco during discharging was calculated as shown in Figure 5. The discharge time τ0.9 was defined as the time for the discharged energy in the load to reach 90% of its final value from the discharge profiles. The τ0.9 of the PYZST thin-film capacitor was 1.46 μs for a 1 kΩ load resistor. This value was comparable to poly(vinylidene fluoride) (PVDF)-based films and commercial biaxial-oriented polypropylene films for high power pulse capacitors.2,29 In summary, a large recoverable energy-storage density, high energy-storage efficiency, fast discharge time, high temperature stability, and high fatigue endurance were simultaneously achieved for polycrystalline Pb0.97Y0.02[(Zr0.6Sn0.4)0.925Ti0.075]O3 (PYZST) antiferroelectric thin-film capacitors. These capacitors exhibited relatively low threshold fields for phase switching from antiferroelectric to ferroelectric states, and also exhibited a small hysteresis of coercive fields during the phase transition between the antiferroelectric and ferroelectric phases compared to other antiferroelectric materials. The excellent energystorage performance of PYZST thin-films may render them to be promising materials for applications in high reliability energy-storage devices.

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Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This research was supported by the Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Education (2015R1D1A3A01019470 and 2014R1A1A4A01004404). This work was supported by the Priority Research Centers Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Education (2009-0093818).

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.5b08786. Additional experimental data; chemical compositional analysis; bipolar P−E hysteresis loops and energy-storage performance; DC leakage current density; unipolar P−E hysteresis loops under various ambient temperatures; variation in unipolar P−E hysteresis loops prior to and after fatigue testing; charging/discharging current and applied voltage profiles to measure the discharging time; and schematic of the discharge experiment (PDF)

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ABBREVIATIONS PYZST:Pb0.97Y0.02[(Zr0.6Sn0.4)0.925Ti0.075]O3 DEs:linear dielectric materials FEs:ferroelectric materials AFEs:antiferroelectric materials

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Tel: +82-52-259-2323. Fax: +82-52-259-1693. Present Address †

D.S.L. is currently at Global Technology Center, Measurement Technology Group, Samsung Electro-Mechanics, Suwon, Gyeonggi-do 16674, Republic of Korea Author Contributions

C.W.A. designed this research and carried out electrical energystorage experiments for PYZST thin-film capacitors. D.S.L. fabricated PYSZT thin-films using PLD and carried out characterization such as SEM and XRD experiments. G.A., S.S.W., and S.A.C. helped electrical energy-storage experiments. All authors participated in data analysis. I.W.K. supervised the E

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DOI: 10.1021/acsami.5b08786 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX