Significantly enhanced dielectric constant and energy storage properties in polyimide/reduced BaTiO3 composite films with excellent thermal stability

In this work, reduced BaTiO3 (rBT) particles with a large number of defects sintered in a reducing atmosphere (95N2/5H2) were introduced into polyimide (PI) matrix without using any modifier or surfactant components. The rBT/PI composite films fabricated by an in situ polymerization method showed significantly enhanced dielectric constant and energy storage density. The dielectric constant of the rBT/PI composite with 30 wt% rBT reached up to 31.6, while maintaining lower loss (tg δ = 0.031@1000 kHz) compared to pure PI (εr = 4.1). Its energy storage density (9.7 J cm−3 at 2628 kV cm−1) was enhanced by more than 400% over that of pure PI (1.9 J cm−3 at 3251 kV cm−1), and was greater than the energy density of the best commercial biaxially-oriented-polypropylenes (BOPP) (1.2 J cm−3 at 6400 kV cm−1). The energy storage efficiency was around 90% due to the linear dielectric performance of rBT/PI composite films. The improved dielectric constant and energy storage density could be attributed to the combined effect of the interface interaction between two phases and the surface defects of rBT induced by the reducing atmosphere. Therefore, rBT/PI composite films with high dielectric constant, energy storage density and storage efficiency may have potential applications in the preparation of embedded capacitors.


Introduction
Polymer materials with high dielectric constant play critical roles in the modern information and electronic industry as embedded capacitors and charge-storage devices because they are light weight, multifunction-integrated and miniaturizationfriendly. 1-5 Pure polymer materials have many merits including excellent exibility, low-temperature processing, high electric breakdown strength, and so on. 6 However, their low dielectric constant (typically less than 10) impedes their practical applications in capacitors. 7 A promising strategy to enhance the dielectric constant of polymer materials is to incorporate ferroelectric materials with high dielectric constant (such as titanium dioxide (TiO 2 ), 8,9 BaTiO 3 , 10,11 Pb(Zr,Ti)O 3 , 12,13 BaSrTiO 3 , 14,15 K 0.5 Na 0.5 NbO 3 (ref. 16 and 17) and CaCu 3 Ti 4 O 12 (ref. 18 and 19)) into a polymer matrix to form composites. [20][21][22][23] This approach has been extensively studied for various polymer matrices including poly(vinylidene uoride) (PVDF), polymethylmethacrylate (PMMA), 24 polyimides 25 and epoxy resins. 26 The dielectric constant of ferroelectric/polymer composite system can be improved by a few times compared to the pure polymer when a high content of ferroelectric particles is incorporated (>50 vol%). Unfortunately, the breakdown strength and energy storage density of these composites are low. [27][28][29] Therefore, it is still a challenge to nd an effective way to achieve both high dielectric constant and breakdown strength.
Among the many polymers, polyimides (PI) are widely used as packaging materials, insulating layers, circuit boards and interlayer dielectrics due to their high tensile strength, superior mechanical properties, high glass transition temperature, good resistance to solvents and excellent thermal stability. [30][31][32][33] PI is now considered to be a promising candidate for polymer composite dielectrics with good temperature stability. For example, Li et al. reported that titanium oxide/PI composites exhibited dielectric constant (3 r ) of 10.6 and low dielectric loss of <0.03 with 10 wt% high-aspect-ratio titanium oxide nanowires. 34 The dielectric properties for some recently reported BT/ PI composites are shown in Fig. 1. From these reported results, it can be seen that it is difficult to simultaneously achieve high dielectric constant and low loss (low dielectric breakdown strength) in PI-based composites. In addition to the dielectric properties at room temperature, it is necessary to systematically investigate the thermal stability of PI composites for dielectric capacitors in practical application.
Compared to BaTiO 3 ceramics fabricated in air, BaTiO 3 materials sintered in reducing atmosphere exhibit excellent dielectric properties due to their semiconducting properties. 35 In this work, reduced BaTiO 3 (rBT) particles were introduced into PI matrix to form rBT/PI composite lms by in situ polymerization. Notably, the composites were obtained through simply mixing the precursor solution (PAA) and rBT suspension without using any modier or surfactant components. The dielectric and energy storage properties of rBT/PI composite lms were systematically studied. Signicantly increased dielectric constant and energy storage density were realized in the as-prepared composite lms.

Preparation of reduced BaTiO 3 (rBT)
rBaTiO 3 powder was prepared by a solid-state reaction described by the following equation: High purity metal oxides or carbonates (BaCO 3 , TiO 2 ) were used as the raw materials in this work. These powders were weighed and ball-milled in a polyethylene bottle with ethyl alcohol and zirconia balls for 24 h, and then dried in an oven at 60 C for 6 hours. The resulting powder was calcined at 1000 C for 4 h in air. The calcined powder was ball-milled once in ethyl alcohol for 24 h, and then dried in an oven at 60 C. Finally, the reduced BT powder was prepared by heat treatment at 1250 C for 2 h in a reducing atmosphere of N 2 and H 2 (95/5).

Preparation of rBT/PI composite lms
The rBT/PI composites were prepared by in situ polymerization. First, ODA was dispersed into DMF solvent, and stirred until ODA was completely dissolved in DMF. Then, PMDA was added slowly to ensure complete dissolution, and stirred for 12 h. The as-prepared rBT powder was added into DMF solvent with the contents of 0 wt%, 5 wt%, 10 wt%, 20 wt%, 30 wt%, 40 wt% and 50 wt%, to form a series of suspensions. The suspensions were ultrasonicated for 1 h and then stirred for 12 h. Subsequently, the two solutions were mixed together, ultrasonicated for 1 h and then stirred it for 12 h. Finally, the rBT/PI composite lms were prepared via solution casting method on the ITO substrate. The lms were subsequently vacuum-dried at 80 C for 2 h to volatilize the solvent, and then dried at 100 C/2 h, 120 C/2 h, 150 C/1 h, 200 C/1 h, 250 C/1 h and 300 C/1 h to convert completely into rBT/PI composite lms. The detailed experimental procedure is described in Fig. 2.

Characterization of structure and dielectric properties
The surface and cross-section morphology of the samples were examined by eld emission scanning electron microscope (FESEM; JEOL, JSM-6701F) equipped with energy dispersive spectroscopy (EDS, OXFORD). Phase structure of samples was analyzed by an X-ray diffractometer using Cu-Ka radiation (D8 Advanced, Bruker, Germany). Thermal gravimetric analyzer (TGA; TA, STA 449C) was used to perform the thermal analysis of composite lms from room temperature to 800 C with a heating rate of 10 C min À1 under Ar ow. Bonding energy of elements in BT was measured using X-ray photoelectron spectroscopy (XPS; Kratos, Axis Supra) with Al Ka radiation (hn ¼ 1486.6 eV). Both sides of composite lms were sputtered with gold (1 mm diameter and 60 nm thick) as electrodes for measurement of electrical properties. A TH2828S LCR meter was used to measure dielectric properties of composites from frequency 0.1 kHz to 1000 kHz at room temperature. The electric breakdown strength (E b ) was measured using a high-voltage tester (ET2671A, China) at room temperature. During breakdown strength measurements, at least 8 specimens were selected for calculating the average E b . The polarization-electric (P-E) eld loops for rBT/PI composites were measured using a Premier II ferroelectric test system (Radiant Technologies, Inc. Albuquerque, NM).

Results and discussion
3.1. Structure characterization X-ray diffraction (XRD) patterns of BT and rBT powders are shown in Fig. 3(a). The rened scanning (2q ¼ 40-50 ) patterns are also shown in Fig. 3(b). The reduced BaTiO 3 had similar XRD pattern as pure BaTiO 3 phase. All samples belonged to Fig. 1 The dielectric constant and loss of BT/PI composite materials reported in literature. [36][37][38][39][40][41][42][43] typical perovskite structure, and no secondary phases were detected. The rBT exhibited a tendency to transform from tetragonal phase to cubic phase, which can be seen from the   was white, as shown in the inset of Fig. 3(a). According to many reported results, the blue color of rBaTiO 3 powder was attributed to the presence of Ti 3+ formed from Ti 4+ , which may be related to the following two mechanisms: (1) a direct donordoping process: 2Ti 4+ + H 2 / 2H + + 2Ti 3+ ; (2) the loss of oxygen during heat treatment in an atmosphere with low oxygen partial pressure: The transition of Ti 4+ to Ti 3+ and the existence of oxygen vacancies can be further conrmed by XPS analysis, as shown in Fig. 4(b)-(e). The observed XPS spectra of BaTiO 3 and rBaTiO 3 were consistent with the reported data in literature. 48,49 The Ti 2p peak of BaTiO 3 was split into 4 peaks, which were located at 465.0 eV and 459.8 eV corresponding to Ti 2p3/2 and Ti 2p3/2 peaks of Ti 4+ , and 463.9 eV and 458.26 eV corresponding to Ti 2p1/2 and Ti 2p1/2 peaks of Ti 3+ , respectively. 50 The Ti 2p peak of rBaTiO 3 can also be deconvoluted into 4 peaks, two of which were located at 464.4 eV and 459.3 eV consistent with the Ti 2p3/2 peaks of Ti 4+ , and the other two peaks were located at 463.5 eV and 458.1 eV, corresponding to the Ti 2p1/2 peaks of Ti 3+ . The ratio of Ti 3+ /Ti 4+ increased from 1.  Table 1. The area of Ti 3+ and absorbed O of rBaTiO 3 increased signicantly compared to that of BaTiO 3 (as shown in the Fig. 4(b)-(e) and Table 1), suggesting that most Ti 4+ ions were reduced to Ti 3+ ions and the oxygen vacancies increased under the reducing atmosphere (N 2 /H 2 ). 51 According to the XPS survey spectra of BT and rBT powders, the elemental concentration were determined by a Thermo Avantage soware. The Ba, Ti, O, and C atomic percentages for BT and rBT powders (BT/rBT) are about 8.85%/7.65% (Ba 3d5 ), 8.72%/8.33% (Ti 2p ), 45.54%/40.29% (O 1s ), and 36.89%/34.94% (C 1s ), respectively. It is found that Ba/Ti ratios is close to 1, which is in agreement with BaTiO 3 molecular formula, whereas rBT show a difference of Ti/O ratios, suggesting that there exist many O vacancies.  (211) peaks as compared to rBaTiO 3 particles. 52 Moreover, the relative intensities of rBT diffraction peaks gradually increased with rBaTiO 3 concentrations. The amorphous structure of pure PI was clearly observed by the broad peak at around 2q ¼ 18 , which was due to the accumulation of PI polymer chains. However, the XRD pattern of polyimide in rBT/PI composite was different from that of pure  polyimide. The peak intensities of PI slowly decreased, which was attributed to the high content of rBaTiO 3 llers in the composites. This result suggested that the crystallinity of the pure PI was affected by rBT particles. Fig. 6(a) shows the SEM images of the rBaTiO 3 particles. The rBaTiO 3 particles exhibit plate-like shape, whose particle sizes about 200-500 nm (length or width). The fractured crosssections of pure PI and rBT/PI composites with different ller fractions are presented in Fig. 6(b)-(h). The fabricated rBT/PI composite lms were approximately 7 mm to 14 mm in thickness. Under SEM observation, rBT particles showed no obvious aggregation and were homogeneously dispersed in the PI matrix even when the ller content was up to about 50 wt%. The homogeneous dispersion of the rBT particles contributed to the good dielectric properties of rBT/PI composites. 53 The thermal stability of pure PI and rBT/PI composites was investigated by TG and DSC, and the curves are shown in Fig. 7. In Fig. 7(a), the TG curves of the rBT/PI composites showed   a similar trend as pure PI in the temperature range from 20 to 800 C, indicating that the rBT powders had no inuence on the degradation mechanism of PI matrix. 54 Pure PI and rBT/PI composites started to degrade at 510 C, which may be mainly due to the decomposition of the PI network. 55 The values of weight loss rate of composites decreased with the increase in rBT loading, ranging from 31.6% (x ¼ 0 wt%), 30.0% (x ¼ 5 wt%), 28.2% (x ¼ 10 wt%), 26.3% (x ¼ 20 wt%), 22.9% (x ¼ 30 wt%), 21.8% (x ¼ 40 wt%), to 22.5% (x ¼ 50 wt%). Overall, the rBT/PI composites had better thermal stability compared to pure PI, which was ascribed to the homogeneous distribution of rBT particles in the PI matrix. 56 The rBT particles acted as barriers in the composites. Consequently, the volatile byproducts formed during the pyrolysis could not escape. 57 As shown in Fig. 7(b), the crystallization temperature of pure PI was about 621.5 C. With increasing rBT contents, the crystallization temperature shied to higher temperature, for example, 653.2 C (x ¼ 5 wt%), 683.9 C (x ¼ 10 wt%), 640.7 C (x ¼ 20 wt%), 673.4 C (x ¼ 30 wt%), 659.8 C (x ¼ 40 wt%), 632.9 C (x ¼ 50 wt%), respectively. The crystallization temperature of rBT/PI composite lms rose with increasing rBT content, indicating that the crystallization process was promoted in the polymer matrix through the introduction of rBT powders. 58

Dielectric properties of the rBT/PI composite lms
The frequency dependence of dielectric constant and dielectric loss for rBT/PI composites with various mass fractions is illustrated in Fig. 8. The dielectric constant and dielectric loss of rBT/PI composites exhibited a decreasing trend with increase in frequency. This could be due to the response characteristics of different molecule groups and different chemical structures in different frequency ranges. 59 Especially, the dielectric loss was relatively high in the low frequency range. The production of dielectric loss can be ascribed to the accumulation of many free charges at the internal interfaces between rBT and PI matrix. The interface polarization increases with the accumulation of free charges under the applied electric eld. 60 In the low frequency range, the charges have enough time to accumulate on the interfaces between rBT and PI matrix, which leads to high dielectric loss. In the high frequency range, the interfacial polarization cannot respond to the change in frequency, and thus, the dielectric constant and loss would decrease. 61 In the insets of Fig. 8(a) and (b), the dielectric constant and dielectric loss of composite samples changed signicantly with different rBT llers. When the content of rBT increased up to 40 wt%, the dielectric constant reached its maximum of about 33.4 at 1000 kHz, which is considerably higher than that of pure PI (3 r ¼ 4.1). Subsequently, the dielectric constant sharply decreased to 23.9 with 50 wt% of rBT content. Correspondingly, the dielectric loss of samples was lower (tg d < 0.031) when the rBT content was below 30 wt%. It was found that the rBT/PI composite with the loading of 30 wt% was the best candidate (3 r ¼ 31.6, tg d ¼ 0.031@1000 kHz) among all the tested composites.
The rBT/PI composite sample with 30 wt% was selected for comparison with the corresponding non-reduced composite lled with BaTiO 3 particles. Dependence of dielectric constant and dielectric loss on frequencies for rBT/PI and BT/PI composites is shown in Fig. 8(c) and (d). The dielectric constant of rBT/PI composite lm (3 r ¼ 31.6) was evidently higher than the BT/PI composite (3 r ¼ 18.8) at 1000 kHz, while maintaining a relatively lower loss (tg d ¼ 0.031) compared to that of BT/PI composite (tg d ¼ 0.027). Fig. 9 shows the effect of temperature on the dielectric properties of rBT/PI composites. The dependence of dielectric constant and dielectric loss on temperature ranging from 25 to 370 C at 1000 kHz was investigated. The dielectric constant of the rBT/PI composites showed no signicant uctuations over the entire temperature range. With increase in temperature, the dielectric constant increased slightly. This may be due to the high temperature resistance of PI. The dielectric loss of rBT/PI composites increased sharply with increase in temperature from 25 to 370 C when rBT concentration was above 40 wt%. At lower content of rBT, the dielectric loss maintained a lower value over a wide temperature range (tg d < 0.08). Generally, the statistical thermal motion of dipole and the charge distribution decide the dielectric properties of the composites. 62 In Fig. 9(a)  and (b), the dielectric constant of composites, measured at 350 C, gradually increased with rBT content, and reached its maximum of about 40.2. Dielectric loss showed a lower value (<0.147) when rBT concentration was below 40 wt% than at high concentrations of rBT.
Dielectric breakdown strength (E b ) of composite materials is a crucial parameter to realize capacitor applications. Fig. 10(a) shows the average breakdown strength of rBT/PI composites with different rBT contents. The detailed E b values at room temperature are listed in Table 2. Clearly, the E b values gradually decreased with increasing rBT content, as shown in Fig. 10(b), ranging from 3251 kV cm À1 (x ¼ 0 wt%), 3115 (x ¼ 5 wt%), 3064 kV cm À1 (x ¼ 10 wt%), 2810 kV cm À1 (x ¼ 20 wt%),  to 2628 kV cm À1 (x ¼ 30 wt%), respectively. With further increase in rBT content, the E b values reduced signicantly from 2179 kV cm À1 (x ¼ 40 wt%) to 1406 kV cm À1 (x ¼ 50 wt%).
The polarization-electric eld (P-E) curve for a representative composite sample (x ¼ 30 wt%) is presented in Fig. 11(a). It can be seen from the curve that at the electric eld of 500 kV cm À1 , the maximum polarization reached 1.616 mC cm À2 , and the remanent polarization (P r ) remained at a very low value, almost close to zero. A large energy-storage efficiency of about 90% (energy loss ¼ 10%) was achieved in the composite lm. P-E curves at higher electric elds (>500 kV cm À1 ) are not provided due to limitation of instruments. Similar trend was also observed in other composite samples (not shown here). According to the P-E loop, it was evident that the rBT/PI composite lm possessed linear dielectric performance. Therefore, its energy storage density (U e ) was calculated using the following formula: where 3 r and 3 0 are the relative dielectric constant and vacuum permittivity, respectively. E b is the dielectric breakdown strength. The calculated results of energy storage density for different samples are shown in Fig. 12(b) and Table 2. Based on the overall consideration of E b and 3 r , a high energy density of 9.7 J cm À3 was achieved at 2628 kV cm À1 in rBT/PI composite lled with 30 wt% rBT particles. The energy density of the rBT/ PI composites was enhanced by more than 400% over that of pure PI (1.9 J cm À3 at 3251 kV cm À1 ). Moreover, the energy density of rBT/PI composites was greater than the energy density of BOPP (1.2 J cm À3 at 6400 kV cm À1 ). 63,64 Therefore, such marked enhancement of energy storage density (U e ) was a result of introducing rBT particles in the polymer matrix to form the corresponding composite lms. Moreover, another favorable property of rBT/PI composites was their ultrahigh energy storage efficiency. These results indicate that rBaTiO 3 can effectively improve the dielectric constant and energy storage density of PI-based composites.
The above results can be explained according to the proposed polarization mechanism occurring in rBT/PI composites, as shown in Fig. 12. As discussed earlier, there are numerous defects inside the reduced BaTiO 3 , which are caused by lattice defects including a large number of oxygen vacancies and reduced Ti 3+ ions. When rBT particles are introduced into PI to form composites, many space-charge polarization dipoles occur on the interface between rBT particles and PI due to a concentration gradient, which then contribute to the dielectric constant of composites. 65,66 As shown in Fig. 8(a), a signicant enhancement in dielectric constant was clearly observed at about 30 wt% rBT content. Moreover, the presence of these defects in rBT would facilitate the interface interaction between ceramic particles and polymer, which play an important role in dielectric loss and dielectric breakdown strength properties of composites. 67,68 In this work, -COOH chains in PI This journal is © The Royal Society of Chemistry 2019 matrix likely reacted with surface -OH groups of BT to form ester group. The existence of defects in rBT enabled the two phases to bond tightly to each other. These two factors were benecial for the higher dielectric breakdown strength of rBT/PI composites at a certain content of rBT. However, the increase in rBT content not only enlarged the distance between rBT particles, but also extended the internal stress between the two phases. This may lead to the appearance of microcracking, and consequently lower the breakdown strength. Therefore, when the rBT content was increased up to 40 wt%, the dielectric breakdown strength of rBT/PI composite lms decreased signicantly.

Conclusions
In summary, reduced BaTiO 3 (rBT) particles obtained by heating under reducing atmosphere were introduced into PI polymer matrix to form composite lms by in situ polymerization method. It was found that the rBT with surface defects played an important role in enhancing dielectric and energy storage properties of PI-based composites. A signicant increase in dielectric constant (3 r ¼ 31.6@1000 kHz) was realized for the composite sample with 30 wt% rBT, while maintaining a relatively low dielectric loss (tg d ¼ 0.031) and high breakdown strength (2628 kV cm À1 ). The measured energy density for the sample with 30 wt% rBT was as high as 9.7 J cm À3 , which was much higher than that of pure PI (1.9 J cm À3 ). These results suggest that the rBT/PI composites could have potential applications in embedded capacitors.

Conflicts of interest
There are no conicts of interest to declare.