Core-Shell Structure and Dielectric Properties of Ba 0.6 Sr 0.4 TiO 3 @ Fe 2 O 3 Ceramics Prepared by Co-Precipitation Method

: Ba 0.6 Sr 0.4 TiO 3 (BST) ceramic materials have been widely used in the ﬁeld of multilayer ceramic capacitors. Surface modiﬁcation through the surface coating to form a heterogeneous layer could effectively improve the dielectric properties. In this work, BST powders were prepared by a co-precipitation method. The effects of reaction conditions on the microstructure of the BST powder were investigated. The reaction temperatures signiﬁcantly affected the morphology of BST powder, and the rhombic-type particles were obtained with the reaction temperature around 80 ◦ C. Meanwhile, the BST@Fe 2 O 3 was prepared by the chemical precipitation method using BST powders with rhombic-type microstructure as “core”, and the so-called “core-shell” microstructure was conﬁrmed in the BST@Fe 2 O 3 powder. Then, BST@x wt%Fe 2 O 3 (x = 2.5, 5, 7.5, and 10, denoting the different content of Fe 2 O 3 ) ceramics were further prepared, and the inﬂuence of “core-shell” structure on the phase structure, microstructure, and dielectric properties was investigated. With the increasing of Fe 2 O 3 content, the ferroelectric–paraelectric phase transition temperature shifts toward lower temperatures, and dielectric peaks gradually become broad and frequency-dependent, which may be due to inconsistent chemical composition from core to shell.


Introduction
Ba 0.6 Sr 0.4 TiO 3 (BST) ceramic materials have been widely used in multilayer ceramic capacitors, infrared detectors, DRAM memories, and phased-array antenna, etc., due to their advantages of having a high dielectric constant, small dielectric loss, low leakage current, and large dielectric breakdown strength [1][2][3][4][5][6][7]. Many researchers have been focused on improving the dielectric properties of BST ceramics, such as doping, multiphase recombination, and surface modification. Among them, surface modification through the surface coating to form a heterogeneous coating layer has attracted attention extensively [8]. Tian [9] et al. found that Ba 0.75 Sr 0.25 TiO 3 powders coated with Mg 0.9 Zn 0.1 O to form a "core-shell structure" could prevent the growth of BST particles in the processing of sintering and improve the dielectric tuning performance significantly. Wang et al. [10] found that Ba 0.6 Sr 0.4 TiO 3 -ZnNb 2 O 6 composites coated with Al 2 O 3 as shell structure on the surface of BST powder can inhibit the formation of the second phase BaNb 3.6 O 10 effectively, which also improves the dielectric temperature performance dramatically. Zhang et al. [11] prepared BaTiO 3 @SiO 2 /PVDF composite and found that the breakdown strength was dramatically enhanced. Hence, constructing a "core-shell structure" with a different composition from the surface to the interior of the grain could improve the dielectric properties effectively. Besides that, it has already been confirmed that a "core-shell structure" can also improve magnetoelectric properties and photocatalytic activity. Reaz [12] et al. have successfully synthesized superior-quality BaTiO 3 /iron oxide CSNP by the physiochemical synthesis process, and the observed superparamagnetic response due to the thin nanolayer of iron oxide has a wide variety of device applications. They also investigated the tunable magnetic properties of magneto-luminescent ZnO/iron oxide core-shell nanostructures using low-cost sonochemical synthesis [13]. Taufique et al. synthesized ZnO-CuO nanocomposites with improved photocatalytic activity for environmental and energy applications [14].
According to the previous studies, the "core-shell structure" can be influenced by the factors of the characteristics of coating ions and the preparation process. Kishi et al. [15] found that different coating ions have different diffusion rates in the solid solution, which could lead to the ununiform composition in the micro-area. Simultaneously, Kim et al. [16] found that the increase in milling time facilitated the shell formation leading to the increased shell portion in the core-shell grain.
Therefore, in this work, BST powder was prepared by the co-precipitation method, and then Fe 2 O 3 was coated on the surface to form the Ba 0.6 Sr 0.4 TiO 3 @Fe 2 O 3 structure. The effects of reaction conditions on the microstructure and reaction mechanism were investigated. Besides, the influence of microstructure and composition on the dielectric properties for Ba 0.6 Sr 0.4 TiO 3 @Fe 2 O 3 ceramics was also studied.

Preparation of BST Powders
BST powders were prepared by the co-precipitation method according to the formula of Ba 0.6 Sr 0.4 TiO 3 . Barium nitrate (Ba(NO 3 ) 2 ), strontium nitrate (Sr(NO 3 ) 2 ), and tetrabutyl titanate (Ti(C 4 H 9 O) 4 ) were used as raw materials, oxalic acid (H 2 C 2 O 4 ) as a precipitant and deionized water and ethanol as solvents. Firstly, an appropriate amount of H 2 C 2 O 4 was dissolved in the mixture of deionized water, and ethanol Ti(C 4 H 9 O) 4 was dissolved in ethanol. Secondly, these solutions were mixed, and ammonium hydroxide (NH 3 ·H 2 O) was added in the mixed solution drop by drop with continual stirring to keep the pH at 3 and finally formed the titanium oxalate (H 2 TiO(C 2 O 4 ) 2 ) solution. After that, Ba(NO 3 ) 2 and (Sr(NO 3 ) 2 were dissolved in deionized water at 80 • C, and then the mixture of Ba(NO 3 ) 2 and (Sr(NO 3 ) 2 was slowly added to the H 2 TiO(C 2 O 4 ) 2 solution with stirring continually at 80 • C for 2 h to obtain the BST precipitate. After aging for 24 h at room temperature, the precipitate was filtered, and the filtrate was collected and dried in an oven at 100 • C. Finally, the resulting precursor was calcined at 800 • C for 2 h to obtain the BST powders.

Preparation of BST@Fe 2 O 3 Powders and Ceramics
The BST@x wt% Fe 2 O 3 powders with x = 2.5, 5, 7.5, and 10 were obtained using the coprecipitation method. Firstly, the BST powders were dispersed in the mixture of deionized water and C 2 H 5 OH with ultrasonic vibration. The Fe(NO 3 ) 3 ·9H 2 O with corresponding stoichiometric ratio were dissolved in the deionized water, stirred continuously, and then poured in the suspension of BST slowly. Secondly, NH 3 ·H 2 O was added to the mixture drop by drop with continual stirring to control the pH between 9 and 10 [17] to obtain the precipitate BST@Fe(OH) 3 . Finally, after centrifugally washed and dried in an oven at 100 • C, the powders were decomposed at 600 • C for 2 h to obtain BST@Fe 2 O 3 powders. After that, the BST@Fe 2 O 3 powders were mixed with 6% PVA and pressed into pellets of 12 mm diameter and 1 mm thickness by uniaxial pressing. These pellets were sintered in air at 1200 • C for 3 h.

Characterization
The phase structure of powders and sintered pellets were investigated by X-ray diffraction (XRD; D8 Advance, Bruker AXS, Billerica, MA, USA) operated at 40 kV with CuKa1 radiation (k = 0.15406 nm) and 2θ range from 20 to 90 degree. The microstructures of the powder and sintered pellets were observed by field-emission scanning electron microscopy (FE-SEM; S-4800, Hitachi, Tokyo, Japan). The core-shell structure and elemental compositions were detected by transmission electron microscopy (TEM; JEM-200CX, Hitachi) equipped with energy-dispersive X-ray spectroscopy (EDS; FeatureMax, Oxford Instruments, Abingdon, UK) operated at 200 kV. Fourier-transform infrared spectra were obtained using a Bruker Tensor-II FT-IR spectrometer (FTIR; Tensor27, Bruker Germany, Berlin, Germany). Differential scanning calorimetry (DSC) and thermogravimetric analysis (TGA) were carried out using a synchronous thermal analyzer (SDT-Q600, TA Instruments Inc., New Castle, DE, USA) with nitrogen flow (50 mL/min) over a temperature range of 30-1000 • C and at a heating rate of 10 • C/min.
For measuring the electrical properties, silver electrode paste was applied on both surfaces of the sintered pellets followed by firing at 550 • C for 30 min. The dielectric characteristics were measured by an impedance analyzer (Agilent 4294 A) at a frequency range of 1 kHz to 1 MHz at −150 • C-200 • C. Figure 1 shows the TGA and DSC curves of the BST precursor prepared by the coprecipitation method. From the TGA curve, it can be observed that the actual weight loss of the BST powder was 48%, which is well-matched with the theoretical weight loss of 49%. From the DSC curve, it was obvious that the physically absorbed water of Ba 1−x Sr x TiO(C 2 O 4 ) 2 ·4H 2 O removed from 25 • C to 240 • C, corresponding to an exothermic peak at 101 • C and Ba 1−x Sr x TiO(C 2 O 4 ) 2 , decomposed to form Ba 1−x Sr x CO 3 and TiO 2 at 240 • C to 470 • C, accompanied by an exothermic peak at 337 • C. Meanwhile, Ba 1−x Sr x CO 3 and TiO 2 diffused with each other to form Ba 0.6 Sr 0.4 TiO 3 at 470 • C to 700 • C, along with an exothermic peak at 584 • C. Up to 730 • C and above, the pure phase BST powder was produced. Therefore, the preparation of Ba 0.6 Sr 0.4 TiO 3 required the following reactions:

Effect of Calcining Temperature on IR Transmittance Spectra and Microstructure of BST
In order to further confirm the reaction mechanism of BST powder, the infrared spectra of BST precursor dried at 100 • C, calcined at 500 • C and 800 • C were measured respectively, as shown in Figure 2. It can be seen that the absorption peaks of 2332 cm −1 , 1397 cm −1, and 875 cm −1 corresponded to the stretching vibration of CO 2 and carbonate, indicating that the BST precursor powder decomposes into carbonate, which is in agreement with Equation (5). The absorption peak of 507 cm −1 corresponds to the stretching vibration of Ti-O, suggesting that the pure phase BST powder is formed after calcination at 800 • C.

Effect of Calcining Temperature on IR Transmittance Spectra and Microstructure of BST
In order to further confirm the reaction mechanism of BST powder, the infrared spectra of BST precursor dried at 100 °C, calcined at 500 °C and 800 °C were measured respectively, as shown in Figure 2. It can be seen that the absorption peaks of 2332 cm −1 , 1397 cm −1, and 875 cm −1 corresponded to the stretching vibration of CO2 and carbonate, indicating that the BST precursor powder decomposes into carbonate, which is in agreement with Equation (5). The absorption peak of 507 cm −1 corresponds to the stretching vibration of Ti-O, suggesting that the pure phase BST powder is formed after calcination at 800 °C.  Figure 3 shows the XRD pattern and SEM images of BST powders calcined at 800 °C for 2 h. All the major diffraction peaks in the XRD pattern could be indexed according to the BST perovskite phase, and no secondary phase could be detected, indicating that a solid solution had been formed (Figure 3a). SEM images show a rhombic hexahedron growth with fusiform-type grains composed of a number of tiny sub-round particles, as shown in Figure 3b,c. Generally, the reaction conditions in the co-precipitation process

Effect of Calcining Temperature on IR Transmittance Spectra and Microstructure of BST
In order to further confirm the reaction mechanism of BST powder, the infrared spectra of BST precursor dried at 100 °C, calcined at 500 °C and 800 °C were measured respectively, as shown in Figure 2. It can be seen that the absorption peaks of 2332 cm −1 , 1397 cm −1, and 875 cm −1 corresponded to the stretching vibration of CO2 and carbonate, indicating that the BST precursor powder decomposes into carbonate, which is in agreement with Equation (5). The absorption peak of 507 cm −1 corresponds to the stretching vibration of Ti-O, suggesting that the pure phase BST powder is formed after calcination at 800 °C.  Figure 3 shows the XRD pattern and SEM images of BST powders calcined at 800 °C for 2 h. All the major diffraction peaks in the XRD pattern could be indexed according to the BST perovskite phase, and no secondary phase could be detected, indicating that a solid solution had been formed (Figure 3a). SEM images show a rhombic hexahedron growth with fusiform-type grains composed of a number of tiny sub-round particles, as shown in Figure 3b,c. Generally, the reaction conditions in the co-precipitation process  Figure 3 shows the XRD pattern and SEM images of BST powders calcined at 800 • C for 2 h. All the major diffraction peaks in the XRD pattern could be indexed according to the BST perovskite phase, and no secondary phase could be detected, indicating that a solid solution had been formed (Figure 3a). SEM images show a rhombic hexahedron growth with fusiform-type grains composed of a number of tiny sub-round particles, as shown in Figure 3b,c. Generally, the reaction conditions in the co-precipitation process significantly influence the morphology of the final powder, such as the addition of different polymeric dispersants [18], the emission of gases during the pyrolysis process, and using C 2 O 4 2− as the capping agent could make anisotropic crystal growth [19]. According to the growth mechanism of the crystal, the high reaction temperature contributes to the more uniform surface energy of each lattice plane, leading to the rhombic hexahedron crystal growth followed by secondary nucleation on the crystal surface, eventually forming poly-crystalline particles with sizes ranging from 0.3 to 1.6 µm [20].
significantly influence the morphology of the final powder, such as the addition of different polymeric dispersants [18], the emission of gases during the pyrolysis process, and using C2O4 2− as the capping agent could make anisotropic crystal growth [19]. According to the growth mechanism of the crystal, the high reaction temperature contributes to the more uniform surface energy of each lattice plane, leading to the rhombic hexahedron crystal growth followed by secondary nucleation on the crystal surface, eventually forming poly-crystalline particles with sizes ranging from 0.3 to 1.6 μm [20].  Figure 4 shows the XRD spectra with peak intensity in log scale of BST@Fe2O3 composite powders with different Fe2O3 coating amounts calcined at 600 °C for 2 h [21]. It was clear that the main phase structure was still perovskite Ba0.6Sr0.4TiO3 in the samples (PDF#34-0411). When the coating content of Fe2O3 was lower than 5 wt%, it was difficult to detect the diffraction peaks of Fe2O3. When the coating content of Fe2O3 was more than 5 wt%, the second phase Fe2O3 corresponding to diffraction peaks of 2θ = 24.2°, 33.2°, 35.7° was apparent (PDF#85-0599), and the intensity of the diffraction peaks increased with the amount of Fe2O3 [22].  Figure 4 shows the XRD spectra with peak intensity in log scale of BST@Fe 2 O 3 composite powders with different Fe 2 O 3 coating amounts calcined at 600 • C for 2 h [21]. It was clear that the main phase structure was still perovskite Ba 0.6 Sr 0.4 TiO 3 in the samples (PDF#34-0411). When the coating content of Fe 2 O 3 was lower than 5 wt%, it was difficult to detect the diffraction peaks of Fe 2 O 3 . When the coating content of Fe 2 O 3 was more than 5 wt%, the second phase Fe 2 O 3 corresponding to diffraction peaks of 2θ = 24.2 • , 33.2 • , 35.7 • was apparent (PDF#85-0599), and the intensity of the diffraction peaks increased with the amount of Fe 2 O 3 [22].

Effect of Fe 2 O 3 Coating on the Microstructure of BST@Fe 2 O 3 Composite Powders
The effect of coating on the microstructure of BST was studied, which is shown in the SEM images of BST and BST@ 10% Fe 2 O 3 powders ( Figure 5). Compared to the pure BST powder with regular morphology, clear edges, and smooth surface (Figure 5c), the encapsulated BST@Fe 2 O 3 powder had obvious flocculent particles deposited on its surface, and the blurred edges, as shown in Figure 5d.
The TEM associated with the EDS was further employed to further investigate the microstructure of BST@ 10 wt% Fe 2 O 3 , as shown in Figures 6 and 7, respectively. The BST particles were surrounded with a large number of flocculent crystalline particles, displaying an overall uniform cohesive coating layer with a size ranging from about dozens to 100 nm ( Figure 6). The relevant EDS spectra associated with elemental mapping show that the Fe element is enriched in the edge (Figure 7), indicating that the "core-shell" structure was well-formed.  The effect of coating on the microstructure of BST was studied, which is shown in the SEM images of BST and BST@ 10% Fe2O3 powders ( Figure 5). Compared to the pure BST powder with regular morphology, clear edges, and smooth surface (Figure 5c), the encapsulated BST@Fe2O3 powder had obvious flocculent particles deposited on its surface, and the blurred edges, as shown in Figure 5d. The TEM associated with the EDS was further employed to further investigate the microstructure of BST@ 10 wt% Fe2O3, as shown in Figures 6 and 7, respectively. The BST  The effect of coating on the microstructure of BST was studied, which is shown in the SEM images of BST and BST@ 10% Fe2O3 powders ( Figure 5). Compared to the pure BST powder with regular morphology, clear edges, and smooth surface (Figure 5c), the encapsulated BST@Fe2O3 powder had obvious flocculent particles deposited on its surface, and the blurred edges, as shown in Figure 5d. The TEM associated with the EDS was further employed to further investigate the microstructure of BST@ 10 wt% Fe2O3, as shown in Figures 6 and 7, respectively. The BST  Figure 8 shows the XRD spectra of BST@Fe 2 O 3 ceramics sintered at 1200 • C for 2 h. It was evident that with an increase of Fe 2 O 3 , the second phase emerged in the form of Ba 2 (Fe 2 Ti 4 O 13 ) (PDF#87-1480), which indicated that the high sintered temperature encourages the reaction between Fe 2 O 3 and BST perovskite phase to form the Ba 2 (Fe 2 Ti 4 O 13 ) phase. Figure 9 shows the surface morphology of BST@Fe 2 O 3 ceramics with different Fe 2 O 3 content. The microstructure of BST@Fe 2 O 3 ceramics displayed a relatively uniform grain size, but poor density was clear, as shown in Figure 9. In general, a big gap would generate between rhombic-type grains [23][24][25][26][27], which would lead to the migration of grain boundary being more difficult, followed by the appetence of pores in the ceramics. Besides that, Fe 2 O 3 coated on the surface of BST would inhibit the migration of grain boundary in the processing of sintering, thus leading to reduced grain size and density [28]. particles were surrounded with a large number of flocculent crystalline particles, displaying an overall uniform cohesive coating layer with a size ranging from about dozens to 100 nm ( Figure 6). The relevant EDS spectra associated with elemental mapping show that the Fe element is enriched in the edge (Figure 7), indicating that the "core-shell" structure was well-formed.  Figure 8 shows the XRD spectra of BST@Fe2O3 ceramics sintered at 1200 °C for 2 h. It was evident that with an increase of Fe2O3, the second phase emerged in the form of Ba2(Fe2Ti4O13) (PDF#87-1480), which indicated that the high sintered temperature encourages the reaction between Fe2O3 and BST perovskite phase to form the Ba2(Fe2Ti4O13) phase. Figure 9 shows the surface morphology of BST@Fe2O3 ceramics with different Fe2O3 content. The microstructure of BST@Fe2O3 ceramics displayed a relatively uniform grain size, but poor density was clear, as shown in Figure 9. In general, a big gap would generate particles were surrounded with a large number of flocculent crystalline particles, displaying an overall uniform cohesive coating layer with a size ranging from about dozens to 100 nm ( Figure 6). The relevant EDS spectra associated with elemental mapping show that the Fe element is enriched in the edge (Figure 7), indicating that the "core-shell" structure was well-formed.  Figure 8 shows the XRD spectra of BST@Fe2O3 ceramics sintered at 1200 °C for 2 h. It was evident that with an increase of Fe2O3, the second phase emerged in the form of Ba2(Fe2Ti4O13) (PDF#87-1480), which indicated that the high sintered temperature encourages the reaction between Fe2O3 and BST perovskite phase to form the Ba2(Fe2Ti4O13) phase. Figure 9 shows the surface morphology of BST@Fe2O3 ceramics with different Fe2O3 content. The microstructure of BST@Fe2O3 ceramics displayed a relatively uniform grain size, but poor density was clear, as shown in Figure 9. In general, a big gap would generate

BST@Fe 2 O 3 Ceramics: Dielectric Properties
To investigate the effect of coating on the dielectric properties of BST ceramics, the temperature dependence from −150 • C to 200 • C of the dielectric constant ε' (T) and the loss tangent tan δ(T) for different BST@Fe 2 O 3 ceramics is plotted in Figure 10 under various measurement frequencies from 1 kHz to 1 MHz. Obviously, with the increase of Fe 2 O 3 content, the dielectric peaks of the ferroelectric-paraelectric phase transition shifted toward low temperatures from 11 • C for pure BST to −49 • C for BST@7.5%wt Fe 2 O 3 ceramics. Ferroelectric-paraelectric phase transition peaks also gradually became broad and frequency-dependent, which was probably due to the particular "core-shell" Crystals 2021, 11, 623 8 of 11 structure of BST@Fe 2 O 3 . The so-called "core-shell" structure makes it difficult for Fe 3+ to diffuse into the core of BST, thus resulting in inconsistent chemical composition from core to shell, which leads to the broaden and dispersive ferroelectric-paraelectric phase transition. In addition, dielectric constant ε' obtained a maximum at 7.5 wt% Fe 2 O 3 , which could be explained by the grain size effects, clearly seen in Figure 9a-d. Previous reports have proved that the reduced grain size in slightly Fe 2 O 3 doped BST ceramics resulted in the reduction of dielectric properties and broadening of the dielectric peak [29]. While for the ceramics with 10 wt% Fe 2 O 3 , the dielectric properties drop at all, which may be due to the combination of impurity and lower density (Figure 9e). Except that, as a whole, the tan δ increased with increasing Fe 2 O 3 content. The tan δ also decreased with the increase in frequency. between rhombic-type grains [23][24][25][26][27], which would lead to the migration of grain boundary being more difficult, followed by the appetence of pores in the ceramics. Besides that, Fe2O3 coated on the surface of BST would inhibit the migration of grain boundary in the processing of sintering, thus leading to reduced grain size and density [28].  between rhombic-type grains [23][24][25][26][27], which would lead to the migration of grain boundary being more difficult, followed by the appetence of pores in the ceramics. Besides that, Fe2O3 coated on the surface of BST would inhibit the migration of grain boundary in the processing of sintering, thus leading to reduced grain size and density [28].  plained by the grain size effects, clearly seen in Figure 9a-d. Previous reports have proved that the reduced grain size in slightly Fe2O3 doped BST ceramics resulted in the reduction of dielectric properties and broadening of the dielectric peak [29]. While for the ceramics with 10 wt% Fe2O3, the dielectric properties drop at all, which may be due to the combination of impurity and lower density (Figure 9e). Except that, as a whole, the tan δ increased with increasing Fe2O3 content. The tan δ also decreased with the increase in frequency. Figure 10. Temperature dependence of dielectric permittivity ε' and dielectric loss tan δ for the BST@Fe 2 O 3 ceramics with different coating amounts (a) 0 wt%, (b) 2.5 wt%, (c) 5 wt%, (d) 7.5 wt%, and (e) 10 wt% from 1 kHz to 1 MHz.

Conclusions
In this work, the BST and BST@Fe 2 O 3 powders were successfully prepared by the co-precipitation method. The reaction temperatures had a significant effect on the morphology of the BST powder. As the reaction temperature around 80 • C, the rhombic-type particles were obtained, which were suitable to serve as the "core" and the so-called "core-shell" microstructure, as confirmed in the BST@Fe 2 O 3 powders. In addition, BST@x wt%Fe 2 O 3 ceramics were further prepared, and the influence of "core-shell" structure on the phase structure, microstructure, and dielectric properties was investigated. The results showed that with the increasing of Fe 2 O 3 content, the ferroelectric-paraelectric phase transition temperature shifts toward lower temperatures, and due to the inconsistent chemical composition from core to shell, dielectric peaks gradually become broad and frequency-dependent.