Energy storage properties of 0.87BaTiO3-0.13Bi(Zn2/3(Nb0.85Ta0.15)1/3)O3 multilayer ceramic capacitors with thin dielectric layers

Multilayer ceramic capacitors (MLCCs) for energy storage applications require a large discharge energy density and high discharge/charge efficiency under high electric fields. Here, 0.87BaTiO3-0.13Bi(Zn2/3(Nb0.85Ta0.15)1/3)O3 (BTBZNT) MLCCs with double active dielectric layers were fabricated, and the effects of inner electrode and sintering method on the energy storage properties of BTBZNT MLCCs were investigated. By using the pure Pt as inner electrode instead of Ag0.6Pd0.4 alloys, an alternating current (AC) breakdown strength (BDS) enhancement from 1047 to 1500 kV/cm was achieved. By investigating the leakage current behavior of BTBZNT MLCCs, the Pt inner electrode and two-step sintering method (TSS) were confirmed to enhance the Schottky barrier and minimize the leakage current density. With relatively high permittivity, dielectric sublinearity, and ultra-high BDS, the Pt TSS BTBZNT MLCCs exhibited a surprisingly discharge energy density (Udis) of 14.08 J/cm3. Moreover, under an operating electric field of 400 kV/cm, the MLCCs also exhibited thermal stability with Udis variation < ±8% over a wide temperature (t) range from -50 to 175 °C and cycling reliability with Udis reduction < 0.3% after 3000 charge-discharge cycles. These remarkable performances make Pt TSS BTBZNT MLCCs promising for energy storage applications.

Such MLCCs with thin dielectric layers are typically subjected to high AC operating electric fields to store and release a large amount of energy, and the possible thermal breakdown may occur when heat generated from the leakage current exceeds the heat lost [29]. The leakage current density (J) of the MLCCs is reported strongly dependent on the thermodynamic temperature (T) and electric field (E), and the Schottky emission model [30] is used to describe these leakage current behaviors, as shown in Eq. (2): where e is the effective electron mass, m  is the effective Richardson constant, k is the Boltzmann constant, h is the Plank constant, φ B is the Schottky barrier height, ε 0 is the vacuum permittivity, and ε r is the relative permittivity at optical frequency. In an ideal Schottky barrier [31] (no electrode-dielectric interface states considered), the difference between the metal work function (φ m , φ Pt = 5.6 eV, φ Ni = 5.1 eV [32], φ Pd = 5.5 eV [33], m = Pt, Ni, Pd, etc.) and the semiconductor affinity (χ, χ BaTiO3 = 3.9 eV) is the Schottky barrier height of the electrode-dielectric interface [34], as shown in Eq. (3): However, the electrode-dielectric counter-diffusion is inevitable during the sintering process, and asformed interfacial alloys [35][36][37] of minor work functions (φ Ba = 2.7 eV, φ Ti = 4.3 eV [31]) lower the Schottky barrier height. An effective approach to depress the formation of alloy is using a smart sintering method [38]. By increasing the heating rate from 200 to 3000 ℃/h, the interfacial alloy became thinner, and the Schottky barrier height was improved from 1.14 to 1.27 eV [39] in BaTiO 3 MLCCs. Therefore, this indicates that by using inner electrodes with large work function and a suitable sintering method, the Schottky barrier of MLCCs can be heightened, and thus, the leakage current density can be minimized.
In this study, BTBZNT MLCCs with thin dielectric layers using different inner electrodes and sintering methods were fabricated. The effects of inner electrode and sintering method on the energy storage property of BTBZNT MLCCs were investigated.

2 MLCCs processing
The calcined powders were mixed in a solution with 27 wt% ethyl acetate (AR), 27 wt% ethyl alcohol (AR), and 1 wt% triglycerides as dispersant. The suspension solution was then ball-milled using zirconia balls for 24 h. The organics, including 10 wt% polyvinyl butyral (aerospace grade) as binder, 2 wt% polyethylene glycol (AR) and 2 wt% butyl benzyl phthalate (AR, Aladdin Biochemical Technology Co., Ltd., China) as plasticizer, were added, and the slurry was then ball-milled for an additional 24 h. All the organics except butyl benzyl phthalate were purchased from Sinopharm Chemical Reagent Co., Ltd., China. The well-milled slurry was separated and left to stand for 24 h to remove air. After roll-to-roll tape casting with a test coater (CMD-S1.7/D/4.OH/3-778, Yasui Seiki Corp, Japan), continuous tapes with thickness variation < ±0.2 μm 294 J Adv Ceram 2020, 9(3): 292-302 www.springer.com/journal/40145 were formed. The Pt paste (MC-Pt100, GRIKIN Advanced Material Co., Ltd., China) was then screen printed on the green tapes with a size of 2.7 mm × 3.8 mm to serve as the inner electrode with an automated printing machine (NPM-1Y01, Yodogawa NCC Co., Ltd., Japan). Electrode patterned tapes were stacked with a 0.3 mm off-set from each other so that the effective electrode area was 2.7 mm × 3.2 mm. The active dielectric layer quantity and dielectric thickness were controlled using an automated laminating machine (NSM-1Y01, Yodogawa NCC Co., Ltd., Japan). After these processes, the BTBZNT MLCCs with double active dielectric layers (D ≈ 5 μm) were fabricated via the two-step sintering method (TSS, held at t = 1185 ℃ for 1 min and then at t = 1015 ℃ for 3 h) or the one-step sintering method (OSS, held at t = 1115 ℃ for 2 h) in air.

3 Characterization and testing method
The micro-structure was characterized via scanning electron microscopy (SEM; Supra 40/40vp, Carl Zeiss Corp, Germany) at an operating voltage of 15 kV and high-resolution transmission electron microscopy (HR-TEM; JEM-2010F, JEOL Ltd., Japan) at an operating voltage of 200 kV. The phase structures were detected via X-ray diffraction (XRD; D8 advance-A25, Bruker Co., Ltd., Germany). The temperature dependence of ε r and dielectric loss (tanδ) were measured via an impedance analyzer (HP4278A; Hewlett-Packard, USA) with a temperature controller (Delta Design 9023, Cohu Semiconductor Equipment Group, USA). Hysteresis loops were measured using a ferroelectric measuring system (TF ANALYZER 2000E, aixACCT SystemsGmbH, Germany) at a frequency ( f ) of 10 Hz. The ε r versus E curves were measured using a power device analyzer/curve tracer (Agilent B1505A; Agilent Technologies, USA) with an applied maximum direct current (DC) voltage (E = 400 kV/cm) and AC bias voltage (E = 10 kV/cm) at f = 10 Hz. The AC BDS at f = 10 Hz was measured using a high-voltage tester (YD2670B, Yangzi Electronic Co., Ltd., China). Unless specifically mentioned, all characterization and testing methods were carried out at room temperature (RT, t = 25 ℃). Figure 1 shows the micro-structure of the Pt BTBZNT MLCCs. The two-step sintering method [40], a universal method for preparing nanocrystalline ceramics, helped to decrease G of the Pt BTBZNT MLCCs from 346 to 271 nm.   (002) and (200) peaks were detected, implying that all the dopants diffused into BaTiO 3 lattices forming a homogenous perovskite solid solution with symmetrical crystalline structure. The broader (002) peak of the Pt TSS MLCCs (full width at half maximum, 2θ FWHM = 0.142°) than the Pt OSS MLCCs (2θ FWHM = 0.104°) implied finer grains according to Debye-Scherrer equation, which is consistent with SEM results. Figure 3 shows the temperature dependence of the ε r , dielectric loss, and temperature capacitance coefficients of the BTBZNT MLCCs. Because of the existence of PNRs in the grains [11], the BTBZNT MLCCs have a relatively high permittivity (ε r ≈ 1200) and a low dielectric loss (tanδ < 0.5% at f = 100 kHz) at RT. Broad Curie peaks can be observed in all MLCCs, and they arise from the random fields created by cation disorder because of the size and charge differences that disrupt the development of long-range polar ordering [41,42]. The Curie temperature (T C ) shifts to a higher value with increasing frequency, exhibiting the relaxor phenomenon with a strong diffuse phase transition [43]. The modified Curie-Weiss law [44] is used to describe the diffuse phase transition and is given by Eq. (4):

Results and discussion
where ε m is the maximum ε r and t m is t corresponding to ε m . C' is the modified Curie-Weiss constant and γ is an indicator of the diffuseness degree. By changing the temperature dependence of ε r at f = 1 kHz into the forms ln(1/ε -1/ε m ) and ln(t -t m ), linear regressions with excellent correlations (goodness of fit, R 2 > 0.999) could be realized, as shown in Fig. 3 (insets). The large γ > 1.7 indicates the strong diffuse phase transition in the BTBZNT MLCCs. Because of the broad Curie peaks around RT, the temperature capacitance coefficients (TCCs) between 100 and 1 kHz are maintained at < ±15 % in the range of t = (-55) -125 ℃, which meets the Electronics Industries Association (EIA) X7R specification. The dielectric nonlinearity, which is the nonlinear reduction of the permittivity with increasing electric field, frequently degrades the energy storage properties of ceramics. The ε r versus electric field and electric field capacitance coefficient (ECC) curves of the BTBZNT MLCCs are shown in Fig. 4. The gradual variation in ε r with increasing E in the range of t = (-50) -150 ℃ indicates the weak dielectric nonlinearity. The Johnson's phenomenological expression based on the Devonshire's phenomenological theory [45] is adopted to evaluate the dielectric nonlinearity, as shown in Eq. (5): where ε r (E) is the ε r at E, ε r (0) is the ε r at E = 0 kV/cm, and αε r (0) 3 is an indicator of the degree of dielectric nonlinearity. By changing ε r versus E curves into the forms (ε r (0)/ε r (E)) 3 and E 2 , linear regressions (R 2 > 0.996) were achieved, as shown in Fig. 4. The αε r (0) 3 is one order lower than the SrTiO 3 -surface-modified BaTiO 3 relaxor ferroelectrics (αε r (0) 3 ≈ 9×10 -4 ) [46] with an approximate similar G, demonstrating the dielectric sublinearity of the BTBZNT MLCCs. Moreover, a further reduction in αε r (0) 3 with increasing temperature significantly benefits the MLCC's hightemperature energy storage properties, as shown in Fig. 4 (insets). Because all the MLCCs exhibited similar dielectric property, the effects of inner electrode and sintering method were negligible, considering of the uncertain effective electrode area and dielectric thickness after sintering. Figure 5 shows the complex impedance spectra and three series resistor-capacitance (3RC) equivalent circuit fitting of the BTBZNT MLCCs. By fitting the experimental data with 3RC equivalent circuit [47,48] as shown in Fig. 5, which were assigned to grain, grain-boundary, and electrode-dielectric interface. The Arrhenius relationship is adopted to evaluate the activation energy (E a ) of the insulation resistance (R ins ), where R 0 is the characteristic resistance. By changing the Arrhenius relationship into the forms ln(R ins ) and 1000/T as shown in Fig. 5. The E a corresponding to the interface insulation resistance of the Pt TSS BTBZNT MLCC (E a = 2.85 eV) was higher than that of the Pt OSS BTBZNT MLCC (E a = 2.26 eV) and the AgPd TSS BTBZNT MLCC (E a = 2.70 eV). The insulation resistance of grain (R g ), grain-boundary (R gb ), and interface (R int ) of the BTBZNT MLCCs at various temperatures are listed in Table 1.
To further investigate the conduction mechanisms, current-voltage measurements were performed. Figure  6 shows the leakage current curves and the Schottky thermionic emission model fitting of the BTBZNT MLCCs. Despite the approximate maximum leakage current density (J max = 2.15 μA/cm 2 ) at T = 348 K, the higher J was detected for the AgPd TSS MLCC (J max = 44.5 μA/cm 2 ) than the Pt OSS MLCC (J max = 27.8 μA/cm 2 ) and the Pt TSS MLCC (J max = 24.2 μA/cm 2 ) at T = 423 K, implying different φ B according to Eq. (1). Leakage current data under E > 250 kV/cm were adopted and linear correlations of ln(J/T 2 ) and E 1/2 could be realized, guaranteeing the reliability of the Schottky thermionic emission model fitting results. The φ B of the AgPd TSS, Pt OSS, and Pt TSS MLCC, evaluated from the plots of extrapolated values of ln(J/T 2 ) for E→0 versus 1000/T in Fig. 6 (insets), were 0.407, 0.431, and 0.489 eV, respectively.
The higher φ B was obtained for the Pt TSS than AgPd TSS MLCC. It can be explained by the difference of the work function between Pt (φ m = 5.6 eV) and Ag 0.6 Pd 0.4 alloy (φ m = 4.6 eV), which was evaluated by  the Freeouf empirical model [49] as given by Eq. (7):

Table 1 Comparison between the insulation resistances of the BTBZNT MLCCs
where φ M and φ m are the work functions of metal M and m, respectively, and x is the concentration of the metal. In addition, a higher φ B was also obtained for the Pt TSS than Pt OSS MLCC, indicating that the two-step sintering method suppressed the formation of the interface (the thickness decreased from 35 to 30 nm as shown in Fig. 7), and thus heightened the Schottky barrier. Figure 8 shows the hysteresis loops and the energy storage property of the BTBZNT MLCCs. The Pt TSS BTBZNT MLCC had an exceptional E max = 1500 kV/cm, which was higher than the Pt OSS (E max = 1301 kV/cm) and AgPd TSS (E max = 1047 kV/cm) BTBZNT MLCC. Thus it was confirmed that using Pt and TSS were effective approaches minimizing the leakage current density, and thus enhancing the breakdown strengthen. The Weibull distribution was used to evaluate the AC where P is the cumulative probability of failure, α is a scale parameter characterizing the breakdown strength, β is a shape parameter indicating the dispersion of the data, and E is the critical electric field above which breakdown occurs. The BDS data are ranked in ascending order and the values of P are estimated by Eq. (9): where i is the rank and N is the total number of samples. α and β are then determined via linear regression of ln[-ln(1-P)] and lnE, as shown in Fig. 8 Fig. 8(d). Because of the enhanced E max , the U max of the TSS BTBZNT MLCC was improved from 10.1 to 14.1 J/cm 3 using Pt instead of AgPd inner electrode. The Pt TSS BTBZNT MLCC also had an excellent thermal stability and the cycling reliability of over 3000 charge-discharge cycles, as shown in Fig. 9, which were generally superior to the reported energy storage MLCCs [6,9,[50][51][52][53][54], as listed in Table 2.

Conclusions
BTBZNT double-layer ceramic capacitors with D ≈ 5 μm using the Pt or AgPd inner electrode were successfully fabricated via a one-step or two-step sintering method, and the Schottky thermionic emission was confirmed to dominate the conduction mechanism of the BTBZNT MLCCs. By investigating the leakage current behaviors of the BTBZNT MLCCs, using the Pt and TSS were proved to be effective approaches heightening φ B , minimizing J, and thus enhancing BDS. The Pt TSS BTBZNT MLCC had the remarkable energy property with an E max = 1500 kV/cm and a U max = 14.1 J/cm 3 , and an excellent thermal stability with variation < ±8% at t = (-50) -150 ℃ and E = 400 kV/cm. The Pt TSS BTBZNT MLCC was generally superior than recently reported energy storage MLCCs, and the improvements in their electric properties further indicate that they have high potential for energy storage applications.