Influence of Crystallization Time on Energy-Storage Density and Efficiency of Lead-Free Bi 0 . 5 ( Na 0 . 8 K 0 . 2 ) 0 . 5 TiO 3 Thin Films

1 School of Engineering Physics, Ha Noi University of Science and Technology, 1 Dai Co Viet Road, Hanoi 100000, Vietnam 2International Training Institute for Materials Science, Hanoi University of Science and Technology, 1 Dai Co Viet Road, Hanoi 100000, Vietnam 3Graduate University of Science and Technology, Vietnam Academy of Science and Technology, 18 Hoang Quoc Viet Road, Hanoi 100000, Vietnam 4Institute of Natural Products Chemistry, Vietnam Academy of Science and Technology, 18 Hoang Quoc Viet Road, Hanoi 100000, Vietnam


Introduction
Recently, people have faced big global challenges such as the air pollution, climate change, and the energy scarcity.For the sustainable development, the traditional fossil fuel energy resources should be replaced by new energy generation technologies, such as wind and solar and thermal energy with cleaner renewable sources.Hence, the development of the high energy-storage materials and devices has been paid increasing attention.The dielectric capacitors owning numerous good properties such as ultrafast charge-discharge rate, high energy-storage density ( reco ), good temperature stability, and low cost have been widely studied in the past decades.They have been used in a lot of modern electrical equipment, such as high power microwaves and electromagnetic armor.[1][2][3][4][5].Because of the integrating and miniaturizing tendency of electronic devices, an ideal capacitor needs to possess high energy-storage density, rapid discharge, and high energy-storage efficiency () [1,6,7].Theoretically, the dielectric materials having the small remnant polarization ( r ), high saturated polarization ( m ), and high breakdown strength (BDS) are the most potential candidates [1].By far, majority of studies on film capacitors have been just focused on lead-based materials (PZT) [5,[8][9][10][11][12].However, owing to containing high Pb content, PZT-based materials are not environmentally friendly and likely harm the human health.Hence, the developments of lead-free materials with high  reco and  are an urgent demand for the applications of pulsed power capacitors.Currently, lead-free ceramics, such as (K 0.5 Na 0.5 )NbO 3 (KNN)-based [13], BaTiO 3 (BT)based [14], SrTiO3 (ST)-based [3], and (Bi 0.5 Na 0.5 )TiO 3 (BNT)-based [15], have attracted much attention because of their good energy-storage properties.Among them, leadfree (Na 0.5 Bi 0.5 )TiO 3 (NBT) ferroelectric materials have been extensively investigated for mainly the past 10 years [16].Smolenski and his coworkers [17] revealed NBT as typical perovskite-type ferroelectrics in 1961.When modified by other ABO 3 phases, the critical temperature at which the AF-AFE phase transition occurs could be reduced with an increase of ferroelectric properties.For example, NBT-xBT lead-free thick films [18] were studied on the temperaturedependence of the  reco and leakage current performance.Authors attained the enhancement of the BDS and the  m - r value for NBT-0.06BT,resulting in the improvements of energy-storage properties (the maximum  reco and  of 2.3 J/cm 3 and 45%, respectively at under 550 kV/cm).Xu and his coworkers [19] obtained the solid solubility of BNTBT with NBN and optimized energy-storage properties with  reco = 1.36 J/cm 3 and  = 73.9% at NBN content of 0.02.For the same system, Lu et al. [20], thanks to the La and Zr codoping, enhanced the energy-storage capacity of the BNTBT (the maximum  reco was 1.21 J/cm 3 at 100 kV/cm).In the report [20], the energy-storage density of BNBT-xKN ceramics was explored with the rise of KN addition.Authors indicated that the energy-storage density is increased radically and reached the maximum value of 0.89 J/cm 3 at 100 kV/cm for BNBT-0.06KN.(1 − x)BNTBT-xNN ceramics [15], prepared via the conventional solid state reaction method, exhibit more slim P-E loops with increasing the NN amount.Therefore, the  reco was enhanced significantly and reached the highest value of 0.71 J/cm 3 for x = 0.10 at 7 kV/mm.Being similar AFE ceramic, (1−x)(Na 0.5 Bi 0.5 )TiO 3 −xSrTiO 3 (NBT-ST) system was reported by Cao et al. [21].The 0.7NBT-0.3STexhibited excellent temperature stability in its energy-storage density from the room temperature to 120 ∘ C with the maximum  reco value of 0.65 J/cm 3 obtained at 65 kV/cm.Via a solid-solid state route, 0.475BNT-0.525BCTZ-xwt% MgO ceramics [22] were prepared and we investigated their phase structure, microstructure and dielectric and energy-storage properties with the focus on optimizing properties for capacitor applications.Thanks to significant enhancement of the dielectric breakdown strength in these ceramics with the addition of MgO, their discharge energy density was strongly improved.With a high dielectric breakdown strength of 15.67 kV/mm, the sample with x = 5 showed the highest maximum  reco of 1.04 J/cm 3 .Comparing to bulk ceramics, the ferroelectric films having the higher BDS could release the drawbacks of bulk ceramics and be suitable for the high energy-storage applications.NBT films [23] synthesized on LNO/Si (100) substrates by using a sol-gel technique show the excellent energy-storage properties with the  reco of 12.4 J/cm 3 and  of 43% at 1200 kV/cm.Zhang et al. [24] markedly enhanced the energy-storage properties of 0.7NBT-0.3STrelaxor films by the Mn 2+ substitution for Ti 4+ .With appropriate Mn doping (1mol.%), the highest  reco value of 27 J/cm 3 was gained thanks to the high BDS value of 1894 kV/cm.
The above studies indicated that the energy-storage density was significantly improved by enlarging the difference value between  m and  r and/or increasing the value of breakdown strength (BDS).Enhancement of  reco value in lead-free thin films has gained the progress, but their performances were still far inferior to those of lead-based materials.The processing conditions of NBT-based films (such as the annealing temperature and crystallization time), which contribute closely to the quality and cost of products, were rarely studied.In the present study, we fabricated leadfree Bi 0.5 (Na 0.8 K 0.2 ) 0.5 TiO 3 (abbreviated as BNKT) films via a sol-gel method on Pt/Ti/SiO 2 /Si substrates and investigated the energy-storage properties of BNKT films annealed at 700 ∘ C as a function of annealing time (10 min, 20 min, 40 min, and 60 min).We found that ferroelectric and energystorage properties were enhanced when the annealing time was increased.
Here, the BNKT precursor solutions are derived from sodium nitrate (NaNO 3 , ≥99%, Sigma-Aldrich), potassium nitrate (KNO 3 , ≥99%, Sigma-Aldrich), bismuth nitrate (Bi(NO 3 ) 3 ⋅5H 2 O, ≥98%, Sigma-Aldrich), and titanium isopropoxide (Ti[i-OPr] 4 , 99%, Sigma-Aldrich).Acetic acid and 2-methoxyethanol were chosen as cosolvent, and acetylacetones were chosen as ligands.During the process, titanium isopropoxide was at first dissolved in acetylacetone to prevent its hydrolysis.In order to compensate the possible Na and K loss during high temperature annealing, we added their excess amounts of 30% and 20%, respectively.The mixture was stirred at 70 ∘ C for 6 hours constantly to form the final solution until a 0.4 M transparent and stable yellow precursor solution was obtained.Each layer of the BNKT films was formed by spin coating 0.4 M precursor on substrates at 4000 rpm for 30 s, followed by pyrolysis at 400 ∘ C for 10 min.The process was repeated until the BNKT thin films with the required coating layers were obtained.Finally, BNKT thin films were annealed at 700 ∘ C in different periods (10 min, 20 min, 40 min, and 60 min) to obtain the ferroelectric phase.For the convenience, these films were named as T10, T20, T40, and T60, respectively.
Characteristics of the films as the cross-sectional and surface morphologies were detected by a field emission scanning electron microscope (FE-SEM, Hitachi S4800) and atomic force microscopy (AFM, Bruker Dimension ICON).The crystal structures of BNKT thin films were characterized by a Bruker D5005 diffractometer using Cu-K radiation ( = 1.5406Å).P−E hysteresis loops were measured under the applied voltages ranging from −25 V to 25 V, frequency of 1000 Hz by using a TF Analyzer 2000 ferroelectric tester (aixACCT Systems GmbH, Germany).

Results and Discussion
The BNKT films, after a synthesis, were investigated to check the surface morphology and roughness by using the SEM and AFM techniques.The two-dimensional (2D) and threedimensional (3D) AFM images for the BNKT films scanned over a surface area of 40 m × 40 m were described in detail in Figures 1(a)-1(d).The AFM images revealed the densely packed, crack-free, uniform surfaces with the grains possessing columnar shape.The roots mean square surface roughness ( rms ) was calculated and shown in the Table 1. rms possesses the values of 5.5, 4.8, 4.6, and 3.8 nm corresponding to the BNKT films: T10, T20, T40, and T60.The low  rms values of the film surfaces evidence a smooth morphology.The decrease of  rms with annealing time is mainly due to the growth of BNKT crystallite with smaller grain boundaries merged into larger ones.The surface morphology of the BNKT thin films, therefore, becomes denser with an increase of the annealing time.This AFM result was also consistent with the analysis exposed by the XRD characterization and FE-SEM micrographs.In Figure 1(e), we demonstrated the FE-SEM micrograph of sample T60 annealed in 60 min.This thin film was shown to exhibit a dense and smooth surface consisting of tetragonal grain size particles without any traces of cracks detected.Figure 1(f) displayed the cross-sectional FE-SEM image of the T60 film.
Based on this analysis, we can determine thicknesses of films, around 300 nm.
The BNKT thin film structure was measured with a -2 mode in steps of 0.02 ∘ .Figure 2(a) displays the XRD patterns of the BNKT films associated with the conditions of annealed at 700 ∘ C for different durations.The angular peak positions of the XRD signals are observed at around 2: 31.2 ∘ , 39.8 ∘ , 47.3 ∘ , and 58.0 ∘ corresponding to the BNKT bulk [24].With the surpassed intensity, the (111) orientation located at 2 ∼ 39.8 ∘ is the mixture of orientations, deriving from the Pt coated substrate.All other peaks belong to a single-phase perovskite structure with no other impurity phases detected, which agrees with our recent report [25] on influence of crystallization temperature on multiferroic behaviour of BNKT films.This observation also matches our previous works [26,27] where they indicated that BNKT thin films possess simultaneously the rhombohedral and tetragonal symmetries.Among the typical orientations of the perovskite structure, the (110)/(200) peaks possess high intensity and therefore are the preferred orientations of the films.These intensity peaks were demonstrated in Figures 2(b) and 2(c) with the 2 ranges: (b) 30.0 ∘ -34.0 ∘ and (c) 46.0 ∘ -48.5 ∘ .Analysed result shows that the (200) peak intensity of the BNKT films obviously tends to rise when annealing time increases from 10 min to 60 min.The full width at half maximum (FWHM) evaluated from the XRD patterns declines with increasing annealing time.This proved that the grain size enlarged as the annealing time was increased.To specify, we evaluated the grain size of films by using Scherrer equation [28]: where D, K, , , and  are grain size, constant of crystallite shape (normally taken as 0.9), wave length, FWHM, and  Bragg angle, respectively.The grain size was calculated and illustrated as a function of the annealing time in the Table 1.D value increased significantly from 44.9 nm for the film T10 to 49.7 nm for the film T60.Furthermore, the XRD analysis about the film structure was confirmed by Raman technique with higher sensitivity.Figures 3(a)-3(d) show the Raman spectra of BNKT films as a function of crystallization time in the wavenumber ranges of 100-1000 cm −1 at room temperature.Lorentzian fitting technique was employed to expose the typical peaks of a perovskite structure.The Raman result is consistent with previous work on BNKTS ceramics [29], where it reported that A 1 (TO1) mode at ∼138 cm −1 is assigned to the Asite vibrations.For the film, this mode corresponds to the wavenumber of 118 cm −1 .We also observed the B 1 (TO1) mode at 281 cm −1 , assigned to O-Ti-O bending motion [30].Especially, Chen and his coworkers [29,31] revealed the modes at higher wavenumbers, such as the A 1 (TO3) at 525 cm −1 and E(LO) at 615 cm −1 characterizing for vibrations of TiO 6 oxygen octahedral.For our study, these modes are located at 514 cm −1 and 590 cm −1 , respectively.The wavenumber differences may be caused by the residual stress formed by film/substrate interface.Similar behaviour is also observed in our recent works [25,32].
Ferroelectric properties of the BNKT films were examined by using a TF Analyzer 2000 ferroelectric tester at 1 kHz at room temperature.Figures 4(a)-4(d) illustrate the P-E loops for the BNKT films with different crystallization times.In Figure 4, the  m ,  r , and  m - r values of the films were evaluated and analysed.An obvious enhancement of the ferroelectricity can be observed as the annealing time increases.When being annealed in 10 min, BNKT films show a relatively poor ferroelectricity with small  m ,  r , and  m - r values of around 18.9 C/cm 2 , 4.1 C/cm 2 , and 14.8 C/cm 2 , respectively.These values continuously increase as the annealing time is raised.Especially, at annealing time of 60 min, the  m ,  r , and  m - r reach their highest values of around 28.9 C/cm 2 , 7.9 C/cm 2 , and 21.0 C/cm 2 , respectively.The increase in the ferroelectric properties matches up the grain size change and phase transition observed from the XRD analysis.With the annealing time ≥ 60 min, the intermediate pyrochlore phase completely transformed into the perovskite phase, and therefore the polarization in the film is improved.Furthermore, BNKT films revealed a great difference between the maximum polarization ( m ) and remnant polarization ( r ).Ahn and his coworkers [33] also reported similar behaviour, which resulted from the strong transform in the domain configuration between the states: (i) applying the electric field and (ii) removing the electric field.Because of the tensile stress at the film/substrate interface, domains may easily switch their original states as the electric field decreases to zero, leading to the small net polarization.By contrast, at a high electric field, BNKT films reach the spontaneous polarization of the bulk materials thanks to the domain reorientation induced by the applied electric field.A high ( m −  r ) value is a useful parameter for the energystorage devices.
The energy-storage properties of the BNKT films were calculated based on the schematic diagram in Figure 5(a). reco is the recoverable electrical-energy density stored in the dielectric films and calculated from P-E hysteresis loops by the following expression [34]: where E refers to the applied electric field and  m and  r are the maximum and remnant polarization values, respectively. loss is the energy corresponding to the inherent hysteresis in the material.It is defined as numerical integration of closed area of the hysteresis loops (Figure 5(a)).The total energy ( total ) needs to charge a ferroelectric capacitor which is the sum of dielectric losses ( loss ) and recoverable ( reco ) energy.Thus,  loss and  total can be represented as Based on (2) and (3),  value of the material can be calculated as follows: Figure 5(b) demonstrates energy-storage density ( reco ), energy loss density ( loss ), and energy-storage efficiency () of the BNKT films as a function of crystallization time at applied electric field  appl of 400 kV/cm.While  reco and  loss experience an increasing trend with a rise of crystallization time, the opposite is true of .The BNKT films, annealed in 10 min, possess the small  reco and  loss values of 2.2 J/cm 3 and 1.2 J/cm 3 , respectively. reco continuously increases from 2.2 J/cm 3 to 2.9 J/cm 3 as crystallization time rises from 10 min to 60 min.Similarly,  loss reaches the highest value of about 2.0 J/cm 3 . reco is obviously enhanced with an increase of annealing time.This matches up the improved polarization as analysed in Figure 4.But  follows a downward trend from 65.0% to 59.3%.This behaviour is related to an increase of the coercive electric field, caused by oxygen vacancies.
When the BNKT films are annealed at high temperature, Asite metals (such as Bi 3+ , Na + , K + ) are evaporated, forming oxygen vacancies.With such  reco and  values, the BNKT films are comparable with other lead-free films in terms of energy-storage ability and have potential application for electrostatic energy-storage devices.

Conclusions
Lead-free Bi 0.5 (Na 0.80 K 0.20 ) 0.5 TiO 3 ferroelectric films have been successfully fabricated on Pt/Ti/SiO 2 /Si substrates via a spin coating assisted sol-gel method.The experimental results showed that all the films have well crystallized corresponding to the single-phase perovskite structure and no other impurity phases are detected.BNKT thin films obtained a significant enhancement in ferroelectric properties and energy-storage density by increasing the crystallization time. m and  r of films reached their highest values of around 28.9 C/cm 2 and 7.9 C/cm 2 , respectively, thanks to rising the crystallization time from 10 min to 60 min.Because of large ( m −  r ) polarization difference,  reco got the maximum value of 2.9 J/cm 3 .The obtained advantages are due to the intermediate pyrochlore phase which is completely transformed into the perovskite phase.

Figure 3 :
Figure 3: Raman spectra of BNKT films with different annealing times in the wavenumber ranges of 100-1000 cm −1 .

𝑃 𝑟 0 𝐸𝑑𝑃Figure 4 :
Figure 4: P-E ferroelectric hysteresis loops of BNKT films with different crystallization times at the same applied electric field of 400 kV/cm.

Figure 5 :
Figure 5: (a) Schematic diagram for the calculation of energy-storage properties of ferroelectric films, (b) energy-storage density ( reco ), energy loss density ( loss ), and energy-storage efficiency () of BNKT films at applied electric field E appl of 400 kV/cm.

Table 1 :
The grain size and roots mean square roughness as a function of the annealing time.