Fabrication of Al2O3-coated BiFeO3 particles and fine-grained ceramics with improved electric properties

(1-x)BiFeO3@xAl2O3 ceramics with x = 0, 2.5, 5, and 7.5 mol% were prepared via the Stöber coating method. The effects of Al2O3 coating on microstructure, dielectric, and ferroelectric properties had been investigated. At x = 5 and 7.5, the samples had a great Al2O3-coating effect. XRD results indicated that excessive Al2O3 coating increased the formation of secondary phases (Bi2Fe4O9 and Bi24A12O39). At x = 7.5, the sample had the highest relative density (95.8%) and lowest loss tangent (0.02 at 1 kHz ). Compared with the pure BiFeO3 sample, the Al2O3-coated samples had improved Bi-O strength and less oxygen vacancy, and the reduction of Fe3+ was decreased. The leakage current density decreased gradually. At x = 5, the sample had the highest Pr value (1.53 µC/cm2). These electric properties changes were ascribed to the generation of secondary phases, the fine grains, and the fewer vacancies.


Introduction
The multiferroic materials, which show the coexistence of multiferroic orders (ferroelectric, ferromagnetic, or ferroelasticity), can achieve the mutual control of the electric and magnetic signals. In recent years, multiferroic materials become one of the hottest research topics, and the multiferroic materials have a great application in electric devices such as sensors, spintronics, multi-state memories, and optoelectronic devices [1][2][3]. Those materials may either simplify the operation of present device structures or offer a new architecture [4][5][6].
Among various multiferroics, BiFeO 3 (BFO) is the most concerning multiferroic materials, owing to its highly G-type anti-ferromagnetic Néel temperature (T N * 640 K), highest ferroelectric Curie temperature (T C * 1103 K) [7], and large saturated polarization value (P S * 100 lC/cm 2 ) [8]. The high saturated polarization value comes from the stereochemical activity of Bi 6s 2 lone pair electrons hybridizing with the empty 6p 0 orbitals of Bi 3? and 2p 6 elections of O 2ion [9]. BiFeO 3 contains a G-type of anti-ferromagnetic ordering with a cycloidal modulation of 64 nm along the [110] direction and results in the concentration of macroscopic magnetization [2,4]. In 2003, BiFeO 3 has been proved enormously stimulating and had inspired both new fundamental physics and exciting device applications [10]. Catalan et al. [11] summarized both the basic physics and unresolved aspects of BiFeO 3 . Recently, Wang et al. [12] prepared BiFeO 3 -based lead-free ceramics with a high recoverable energy density (W rec * 2.1 J/cm 3 ). Gumiel et al. [13] found the switchable photovoltaic current and both diode effect in BiFeO 3 single crystals, indicating that the BiFeO 3 -based materials have widespread potential applications. However, BiFeO 3 has a series of shortcomings required for device applications, such as large leakage current density, high loss tangent, and weak magneto-electric coupling effects [3,9]. These defects stem from the volatilization of Bi [14], the valence variation of Fe 3? /Fe 2? , and the generation of secondary phases [15]. Those defects make it difficult to obtain saturated hysteresis polarization loops in the BiFeO 3 sample. It is also hard to synthetic phase-pure BiFeO 3 ceramics with high density, because BiFeO 3 is vulnerable to decompose into Bi 25 FeO 39 and Bi 2 Fe 4 O 9 in the range of 677-827°C [16].
In order to solve those defects, some attempts have been performed. Song et al. [17] prepared BiFeO 3 ceramics via a two-step solid-state sintering method at 830°C and found that the ceramics contained secondary phases, and the microstructure was inhomogeneous with a low density, resulting in a decrease of ferroelectric properties. Wang et al. [18] prepared BiFeO 3 ceramic with larger remnant polarization (0.892 lC/cm 2 ) and decreased leakage current density (6.58 9 10 -8 A/cm 2 ) via the spark plasma sintering method under an oxidizing atmosphere. Perez-Maqueda et al. [19] synthesized the high-density nanostructured BiFeO 3 ceramic with a nanometric grain size of * 20 nm via flash sintering technology. However, these sintering methods were more expensive and more complicated. It is well known that the Stöber coating is a quite effective way to modify the surface of ceramic particles and to form the core-shell structure. Wang et al. [20] prepared La 2 O 3 -coated BaTiO 3 ceramics and found that the BaTiO 3 @La 2 O 3 sample had outstanding temperaturestable dielectric properties. Liu et al. [21] prepared Al 2 O 3 -coated BaTiO 3 ceramics and found that the BaTiO 3 @Al 2 O 3 sample had fine grains and improve energy storage properties. Aluminum has been confirmed helpful in refining the grain size of BaTiO 3 ceramics, the fine grains make it possible to decrease leakage current density and improve the electric properties of ceramics. However, there are few studies focus on the fabrication of coating BiFeO 3 ceramics and its electric properties research [22].
In this work, Al 2 O 3 -coated BiFeO 3 particles were prepared via the Stö ber coating method. We fabrication fine-grained (1-x)BiFeO 3 @xAl 2 O 3 ceramics with different coating contents, and the microstructure, dielectric and ferroelectric properties, and leakage current density of all (1-x)BiFeO 3 @xAl 2 O 3 samples were investigated.

Experimental details
(1-x)BiFeO 3 @xAl 2 O 3 samples, where x = 0, 2.5, 5, and 7.5 mol% (denoted as A0, A2.5, A5, A7.5, respectively), were prepared via the Stöber coating method. Reagent pure Bi 2 O 3 (C 99%, Sinopharm Group Co., Ltd.), Fe 2 O 3 (C 99%, Sinopharm Group Co., Ltd.), and Al(NO 3 ) 3 Á9H 2 O (C 99%, Sinopharm Group Co., Ltd.) were used as starting materials. The BiFeO 3 powder was synthesized via the solid-state reaction method as described in Ref [23]. Firstly, the BiFeO 3 powder was added in 50 ml alcohol, then ultrasonicated, and stirred for 1 h to break up the BiFeO 3 agglomeration. The Al(NO 3 ) 3 Á9H 2 O was dissolved in 30 ml alcohol, and then the Al(NO 3 ) 3 Á9H 2 O solution was added into BiFeO 3 suspension slurries drop by drop. After titration, the suspension was aged for 2 h under stirring. Subsequently, the suspension was dried at 80°C for 12 h. The dried powder was calcined at 550°C for 30 min, and the complex oxidecoated BiFeO 3 was fabricated. The coated powders were pressed into pellets with 10 mm in diameter and 1-2 mm in thickness using a 4 wt% polyvinyl alcohol binder. In order to reduce the evaporation of Bi 2 O 3 , the pellets were placed in a sealed corundum crucible and sintered at 780°C for 2 hours. The schematic illustration of the preparation of (1x)BiFeO 3 @xAl 2 O 3 ceramic is shown in Fig. 1.
The phase-purity of powders and ceramics was analyzed using the Rigaku Smart-Lab X-ray diffraction (RT-XRD, DMX-2200, Rigaku, Tokyo, JP) with Cu-Ka radiation (k = 1.5405 Å , 40 kV, and 75 mA). Micrographs of all the samples were measured using Field emission scanning electron micrograph (FESEM, TM-1000, Hitachi, JP). The Archimedes method was used to measure the relative densities of all samples. The X-ray photoelectron spectroscopy (ESCALAB 250Xi, Thermo, USA) was used to investigate the valence state of ions of all samples. For electric properties measurements, Ag-conductive paste was painted on both sides and fired at 550°C for 15 min. The dielectric constant (e r ) and dielectric loss (tan d) were measured using an impedance analyzer (Agilent 4294A, Agilent, USA) in a wide frequency range (10 2 -10 7 Hz). The polarization hysteresis loops (P-E loops) and leakage current density were measured via the ferroelectric analyzer (Radiant Technologies, premier-II, USA). All the sintered pellets were polished to a thickness of 0.2 mm for the measurements of P-E loops. The grain surface morphology of (1-x)BiFeO 3 @xAl 2 O 3 particles was changed with the increase of the Al 2 O 3coating content. The pure BiFeO 3 particles (Fig. 2a) had a smooth surface. With the increase of the Al 2 O 3coating content, A5 and A7.5 particles were tightly coated by small grain and had a rough surface. Through EDS analysis, the A0 particle only contained Bi, Fe, and O elements, indicating pure BiFeO 3 . The small grain coated on the A7.5 particle (position 4) had only the Al and O elements, indicating that the Al 2 O 3 -coated samples were successfully prepared. However, positions 2 and 3 contained Bi, Fe, Al, and O elements, owing to the penetrability of the X-ray, the internal Bi, and Fe elements were also detected. Figure 2f shows the XRD patterns of all particles after calcined at 550°C, and the patterns were well indexed with the peaks in JCPDS Card No: 74-2016 (space group R3c), revealing that the main phase of all the samples was BiFeO 3 . However, there were no diffraction peaks match with Al 2 O 3 , because Al 2 O 3 was uniformly coated on the sample and the highest content was only 2.57 wt%. In short, the Al 2 O 3 -coated BiFeO 3 particles were prepared.

Results and discussion
The XRD patterns of all the (1-x)BiFeO 3 @xAl 2 O 3 sintered ceramics are shown in Fig. 3. It can be observed that the pure BiFeO 3 sample contained a little secondary phase, owing to BiFeO 3 is vulnerable to decompose into Bi 25 FeO 39 and Bi 2 Fe 4 O 9 in the range of 677-827°C. The BiFeO 3 decomposition process can be expressed by formula (1) [16]. With the increase of the Al 2 O 3 -coating content, the secondary phases (Bi 2 Fe 4 O 9 and Bi 24 A1 2 O 39 ) became more pronounced. Bi 24 A1 2 O 39 may be produced by the reaction of Al 2 O 3 and BiFeO 3 during the sintering process as formula (2). During the sintering process, Al 2 O 3 coating on the surface reacted with a part of BiFeO 3 to generate secondary phases, which will further improve the coating effect. However, it will also bring some serious changes to the electric properties of the (1-x)BiFeO 3 @xAl 2 O 3 ceramics, owing to the generation of secondary phases.
The cross-sectional SEM images of all the (1x)BiFeO 3 @xAl 2 O 3 sintered ceramics are shown in Fig. 4. The microstructure was changed after the coating of the Al 2 O 3 . The pure BiFeO 3 sample (A0) presented the large grains which had an irregular shape and a large number of porosities between the grains. With the increase of the Al 2 O 3 -coating content, the A2.5 sample had a mixture of larger and small grains, indicating that the coating effect is poor when the content of Al 2 O 3 was 2.5 mol%. The A5 and A7.5 samples exhibited more fine grains, which may be the Al 2 O 3 coated on the surface of the BiFeO 3 and  Indicating that the coating of Al 2 O 3 could improve the sintering property of BiFeO 3 ceramics. Figure 5 shows the XPS spectra of all the (1x)BiFeO 3 @xAl 2 O 3 samples. The spectra were simulated and fitted via the XPS SPEAK 41 analysis software. The C, O, Al, Bi, and Fe elements are observed in Fig. 5a. The C1s peaks located 282.6 eV was used to rectify the binding energy of the XPS spectra [24]. Figure 5b shows two peaks near 156.73 eV and 162.04 eV, respectively, which related to the Bi-O bonds. With the increase of the Al 2 O 3 -coating content, the binding energy of Bi-O bonds in (1x)BiFeO 3 @xAl 2 O 3 samples gradually increased, indicating that the Bi-O bond strength in the oxygen octahedron was improved, which leads to a more stable perovskite structure for (1-x)BiFeO 3 @xAl 2 O 3 samples [25]. After coating with Al 2 O 3 , the internal BiFeO 3 grains were protected, and the volatilization of Bi was suppressed during the sintering process, which caused the improvement of the Bi-O bond strength. Besides, Fig. 5c shows the fitted Fe 2p 3/2 narrow-scan spectra of all samples. The Fe 2p XPS spectra exhibited two peaks near 707.84 eV for Fe 2p 3/2 and 722.35 eV for Fe 2p 1/2 which corresponded to the Fe-O bonds [26]. A satellite peak was present in the middle of these two peaks (near * 717 eV), indicating that the existence of Fe 3? oxidation state [27]. On the basis of the ratio of the fitted peak areas, the concentration ratios of Fe 3? to Fe 2? in A0, A2.5,   Figure 5d shows the fitted O1s narrow-scan XPS spectra of all samples, and there were two subpeaks near 527.4 eV and 529.4 eV, indicating two types of oxygen atoms. The former peak (O F ) can be attributed to the main peak of oxygen atoms in BiFeO 3 lattice, and the latter peak (O L ) was related to the presence of oxygen vacancy [28]. Combined with the fitted results, the concentration ratios of O F to O L in A0, A2.5, A5, and A7.  (3) and (4) [29].
Therefore, it was easy to conclude that the Al 2 O 3 coating was helpful to improve the Bi-O strength and decrease the reduction of Fe 3? , which resulted in a decrease of the vacancy concentration of (1x)BiFeO 3 @xAl 2 O 3 sample. Figure 6 depicts the frequency dependence of dielectric constant (e r ) and loss tangent (tan r) of all the (1-x)BiFeO 3 @xAl 2 O 3 samples. It was observed in the plots that the value of dielectric constant decreased with the increase in the frequency and then becomes almost constant. The higher dielectric constant at low frequency is a characteristic for all the dielectric materials, the Maxwell-Wegner model can be used to describe this behavior [30,31]. With the increase of the Al 2 O 3 -coating content, the dielectric constant value of A5 and A7.5 samples changed little with the increase of the frequency, because these samples had fewer vacancies and higher relative density. At 1 kHz, the dielectric constants of A0, A2.5, A5, and A7. 5 [32] were less than that of pure BiFeO 3 [33] and the generation of secondary phases during the sintering process. The value of loss tangent decreased with the increase in frequency. Comparison with the pure BiFeO3 sample, the A7.5 samples had the lowest loss tangent (0.02 at 1 kHz) because the Al 2 O 3 -coated sample had higher relative density, smaller grains size, and fewer oxygen vacancies. These were beneficial for the dielectric properties of (1-x)BiFeO 3 @xAl 2 O 3 samples.
The characteristics of leakage current density versus applied electric field (J-E) of all the (1x)BiFeO 3 @xAl 2 O 3 samples are shown in Fig. 7. All the samples were measured from -10 to 10 kV/cm. With the increase of the Al 2 O 3 -coating content, the leakage current densities of (1-x)BiFeO 3 @xAl 2 O 3 samples decreased gradually. At the applied electric field of 10 kV/cm, the leakage current densities of A0, A2.5, A5, and A7.5 samples were 5.70 -6 9 10 -6 , 4.61 9 10 -6 , 1.66 9 10 -7 , and 4.44 9 10 -8 A/cm 2 , respectively. In comparison with the pure BiFeO 3 sample, the leakage current density of A5and A7.5 samples was decreased about two orders of magnitude. Because A5and A7.5 samples had a great Al 2 O 3coating effect resulting in denser microstructure, fine grains, and fewer vacancies. The fine-grains ceramic had a longer grain boundary to delay charge transmission, and the charges were easily accumulated around the small grains to increase the resistance of ceramics [34].
The polarization hysteresis loops (P-E loops) of all the (1-x)BiFeO 3 @xAl 2 O 3 samples are shown in Fig. 8. The P-E loops were recorded at 50 Hz. All samples presented a saturated polarization hysteresis loop. With the increase of the Al 2 O 3 -coating content, the remnant polarization value (P r ) of (1-x)BiFeO 3 @xAl 2-O 3 samples increase at first and then decreased. The remnant polarization values of A0, A2.5, A5, and A7.5 samples were 0.93, 1.37, 1.53, and 0.92 lC/cm 2 , respectively, under a maximum alternating-current electric field (E m ) of 100 kV/cm. A5 sample had the highest remnant polarization value, owing to the great Al 2 O 3 -coating effect, the sample had denser microstructure, improved Bi-O bond strength, and less vacancy. However, the A7.5 sample had lower P r value than that of A2.5 and A5 samples, owing to the excessive Al 2 O 3 coating will generate too many secondary phases (Bi 2 Fe 4 O 9 and Bi 24 A1 2 O 39 ). These secondary phases will loss of ferroelectric properties above -29°C (Bi 2 Fe 4 O 9 ) [35]. All the (1-x)BiFeO 3 @-xAl 2 O 3 samples were hard to measurer the exact value of the P-E loops, because the electric coercive  (E C ) of BiFeO 3 is more than 200 kV/cm [36], and the electrical penetration occurred before full switching. Besides, all the P-E loops were not closed, which were also related to the low-electric field breakdown, those behaviors were common in the BiFeO 3 system [37,38].

Conclusions
In summary, (1-x)BiFeO 3 @xAl 2 O 3 particles with x = 0, 2.5, 5, and 7.5 mol% were prepared via the Stö ber coating method using Al(NO 3 ) 3 Á9H 2 O. SEM images revealed that the A5 and A7.5 samples had a great Al 2 O 3 -coating effect. After sintering into ceramics, the excessive Al 2 O 3 -coating sample generated many secondary phases (Bi 2 Fe 4 O 9 and Bi 24 A1 2-O 39 ). With the increase of the Al 2 O 3 -coating content, the relative density increased from 87% for pure BiFeO 3 ceramic to 96% for A7.5 ceramic, the dielectric constant and loss tangent were decreased. Compared with the pure BiFeO 3 sample, the A5 and A7.5 had increasing Bi-O strength, the less reduction of Fe 3? , and less oxygen vacancy. A5 samples had the highest P r value (1.53 lC/cm 2 ). Besides, the leakage current density decreased gradually with the increase of the Al 2 O 3 -coating content.