1 Introduction

In order to promote the development of sensors, electrical transformers and high energy storage density devices, multiferroic materials have received much attention in recent decades [1, 2]. All multiferroic materials coexist of two or more ferroics (ferroelectric, ferromagnetic, and ferroelastic) [3, 4], providing an extra degree of freedom in practical applications. However, most multiferroic materials exhibit weak magneto-electric coupling effect and lose ferroelectric properties above room temperature [5, 6]. Therefore, it is a growing demand for developing a special material with high TC and excellent magneto-electric coupling effect.

Among various multiferroics, BiFeO3 (BFO) is the best choice for meeting those demands of device application, owing to its high ferroelectric Curie temperature (Tc ≈ 830 °C) and anti-ferromagnetic Néel temperature (TN ≈ 370 °C) [7, 8]. BiFeO3 has anti-ferromagnetic G-type of spin configuration character along [111]cubic or [001]hexagonal direction with pseudocubic or rhombohedral symmetry at room temperature [9]. With the 6s2 lone pair electron configuration of the Bi3+ ions and the partially filled d orbital of the Fe3+ ions, BiFeO3 has great ferroelectric and ferromagnetic properties, respectively [10, 11]. However, BiFeO3 has some inherent defects that limit its utilization in industry application. For instance, BiFeO3 exhibits serious leakage current characteristic, which stems from the volatilization of Bi2O3 [8], the reduction of Fe3+ [6] and the generation of secondary phase (Bi2Fe4O9 and Bi25FeO39) at sintering process [12, 13]. Besides, it also has a high dielectric loss and temperature instability. Thus, the greatest challenge for BiFeO3 system is how to increase the ferroelectric properties and reduce leakage current density to promote its practical application.

By now, several attempts had been performed in the last decade to overcome these shortcomings of BiFeO3 systems, such as substituting Bi-site by rare-earth ions [9], substituting Fe-site by transition-metal ions [2], fabricating its composites with another ABO3-type perovskite materials [6] and so on. Among them, fabricating composites and solid solutions with other oxides can suppress the impurity phases and show outstanding ferroelectric properties. Zhuang et al. [11] synthesized Dy-modified BiFeO3–PbTiO3 multiferroics ceramics and found that the magnetic morphotropic phase boundary (MPB) in the 0.66Bi1−xDyxFeO3 − 0.34PbTiO3 system, which significantly enhanced the dielectric and ferromagnetic performances of BiFeO3 system. Wei et al. [12] fabricated (1 − x)BiFeO3 − xBaTiO3 composite ceramics using the traditional solid-state sintering route and observed that the highest remnant polarization (Pr = 26.0 μC/cm2) at x = 0.3, however, the Pr decreased from 26.0 μC/cm2 for x = 0.3 to 8.2 μC/cm2 for x = 0.45, meaning that excessive BaTiO3 addition will decrease the ferroelectric properties of BiFeO3. Jawad et al. [14] prepared Bi1−xAl2xFe1−xO3 ceramics via a solution combustion method and demonstrated that the replacement of Al3+ at both sites created more oxygen vacancy, which resulted in an anomalous phenomenon in structural and resistivity, respectively. Azam et al. [15] synthesized the Bi1−xAl2xFe1−xO3 ceramics and found that the activation energies increased from 0.54 to 0.73 eV with the addition of Al. However, the BiFeO3-Al2O3 composite ceramics has received comparatively less attention.

In this study, the addition oxide Al2O3 was applied to form composite ceramics with BiFeO3, and the effects of Al2O3 addition on structural (crystal data and microstructure), leakage current characteristic, ferroelectric and dielectric properties of (1 − x)BiFeO3 − xAl2O3 composite ceramics were investigated.

2 Experimental details

(1 − x)BiFeO3 − xAl2O3 composite ceramics, where x = 0.00, 0.02, 0.04, 0.06, 0.08 and 0.10 (denoted as A0, A2, A4, A6, A8 and A10, respectively), were prepared via the solid-state sintering route. The high purity (≥ 99%) starting reagents of Fe2O3, Al2O3, and Bi2O3 (all from Sinopharm Group Co., Ltd.) were used in this experiment. The oxides of Fe2O3 and Bi2O3 were accurately weighed in the stoichiometric proportion of BiFeO3, the above ingredients were thoroughly mixed in the air for 1 h. The mixed powders were placed in alumina for calcining at 810° C for 2 h. The calcined bulks were again ground by ball milling for 12 h at a rate of 400 rpm with ethanol as a solvent. Then, the powder was sieved through 400 meshes and dried. The treated powder (BiFeO3) was thoroughly mixed with Al2O3 in the above ratio. Finally, the powders were uniaxially pressed under a pressure of 50 MPa to obtain the green disk samples with 10 mm diameter and 1–2 mm thickness and then sintered at 780 °C for 2 h. In order to reduce the volatilization of Bi2O3, all compacted pellets were placed in a sealed alumina crucible during the sintering process.

The phase purity and structure for (1 − x)BiFeO3 − xAl2O3 were detected by room temperature X-ray diffraction using a Rigaku diffractometer with Cu–Kα radiation (D/max-RB, Rigaku, Japan) The morphology of the sintered ceramics was evaluated via the scanning electron micrograph (SEM, TM-1000, Hitachi, Japan). Determination of relative density of all (1 − x)BiFeO3 − xAl2O3 samples by Archimedes method. The sintered pellets were polished, electroded with silver-conductive paste on both parallel faces and fired at 550 °C for 15 min for electrical measurements. The electrical parameters of all the samples were measured in temperatures (25–500 °C) f and frequency (5 × 102–108 Hz) by an impedance analyzer (Agilent 4294A, America). The leakage current density and polarization hysteresis loops were evaluated using a ferroelectric tester (Ferroelectric analyzer, TF2000, Germany).

3 Results and discussion

The XRD Patterns for all the (1 − x)BiFeO3 − xAl2O3 samples at room temperature are shown in Fig. 1. The diffraction peaks of all the samples were well-indexed with the JCPDS Card No: 86-1518 [16], indicating all the samples were rhombohedrally distorted perovskite structure with space group R3c. However, secondary phase diffraction peaks (Bi2Fe4O9 and Bi24A12O39) became more pronounced with the increase of the Al2O3 content, indicating that the addition of Al2O3 increased the formation of secondary phases during the sintering process. In addition, there were no diffraction peaks matched with Al2O3 (JCPDS Card No: 10-0173) in all samples, indicating that Al2O3 participated in the reaction to form Bi24A12O39, and the residual content of Al2O3 lower than the limit detection of XRD. An enlarged view of patterns in the 2θ range of 31.5°–32.5° and the lattice parameter of all the samples are displayed in Fig. 1b and Table 1, respectively. The results showed that with the increase of the Al2O3 content, there was no obvious major peak shifted, and the lattice parameter had little changed, indicating that the Al3+ did not enter the BiFeO3 lattice position [17]. As an unstable phase structure, BiFeO3 easily decomposed in the temperature range of 447–831 °C [18], so the Bi24A12O39 may be produced by the interaction between BiFeO3 and Al2O3 during the sintering process.

Fig. 1
figure 1

RT XRD patterns for the (1 − x)BiFeO3 − xAl2O3 samples

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Table 1 The lattice parameters and volume for the (1 − x)BiFeO3 − xAl2O3 samples
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Figure 2 shows the cross-sectional SEM image for all the (1 − x)BiFeO3 − xAl2O3 samples. It can be observed that the microstructure of the samples was changed with the increase of Al2O3 content. The pure BiFeO3 sample presented that the grain had an irregular shape with an average grain size of 1.49 μm, and a large number of pores existed in some large grains owing to that rapid grain growth in the sintering process. With the increase of the Al2O3 content, the grain morphology changed to a more uniform distribution with obviously decreased in average grain size (0.45 μm of A10 samples), and the microstructures become denser, indicating that the addition of Al2O3 inhibited the growth of BiFeO3 grain. It was possibly owing to the formation of secondary phases and the more refractory nature of Al2O3 [19]. In the A8 and A10 samples, there were many small grains attached around large grains, which caused the improvement of relative density from 89% for A0 to 93% for A10. High density and small grain size in bulk ceramics were believed to result in low leakage current density.

Fig. 2
figure 2

SEM images for the (1 − x)BiFeO3 − xAl2O3 samples: a A0, b A2, c A4, d A6, e A8, amd f A10

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Figure 3 showed the variation of dielectric constant (εr) and dielectric loss (tan δ) depended on frequency dependence (5 × 102–108 Hz) for all the (1 − x)BiFeO3 − xAl2O3 samples at room temperature. It can be observed that the magnitude of the permittivity of all the samples decreased with the increase of the frequency and became almost constant at high frequency. The high value of permittivity value at low frequency may be owing to the existed of different types of polarization factors, such as the and dipole moments, electronic and interfacial dislocations which were active in the low-frequency range. In addition, the dipole cannot follow high-frequency electric reversal. So, the value of high-frequency permittivity was less than that of the low-frequency one. [20]. At 106 Hz, the dielectric constant decreased monotonically with the increase of the Al2O3 content, maybe owing to the permittivity values of Al2O3 (εr = 9) [21] and Bi2Fe4O9 (εr = 23) [22] were less than pure BiFeO3 (εr = 96) [17]. The value of dielectric loss reduced drastically with the increase in frequency within the tested range, due to the loss of space charge polarization at high-frequency range [23, 24]. In Al2O3 addition ceramics, the dielectric loss increases with the increase of Al2O3 content. It is owing to the formation of more impurity phases in the sintering process. However, the dielectric loss of Al2O3 addition ceramics was less than pure BiFeO3 sample. This significant decreased in dielectric loss can be attributed to the denser structure observed in the SEM images which were beneficial for the dielectric properties of BiFeO3 based ceramics.

Fig. 3
figure 3

Frequency dependence of dielectric parameters for the (1 − x)BiFeO3 − xAl2O3 samples at RT: a εr and b tanδ

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The dielectric constant and dielectric loss as a function of temperature (25–500 °C) for the (1 − x)BiFeO3 − xAl2O3 samples at 1 MHz is illustrated in Fig. 4. All the Al2O3 addition samples presented a dielectric constant ε (T) peak. With the addition of the Al2O3, the TC decreased gradually from 475 °C for A2 to 390 °C for A10, the dielectric constant value decreased and the shape ε (T) peak became broad, it is owing to the generation of secondary phases and the increase of dielectric relaxor behavior in (1 − x)BiFeO3 − xAl2O3 samples. Despite these changes, the value of the dielectric loss was small at less than 350 °C, but then it suddenly increased in higher temperature range, revealing an improvement in dc conductivity at high temperature. The rapid increase of dielectric loss at high temperature may be related to the release of space charge [25].

Fig. 4
figure 4

Temperature dependence of dielectric parameters for the (1 − x)BiFeO3 − xAl2O3 samples at 1 MHz: a εr and b tan δ

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Figure 5 illustrates the room temperature leakage current density (J) as a function of applied voltages from  −1000 to 1000 V for all the (1 − x)BiFeO3 − xAl2O3 samples. Leakage current density was decreased significantly with the addition of Al2O3. The observed leakage current densities for the (1 − x)BiFeO3 − xAl2O3 samples were 8.01 × 10−6, 6.38 × 10−6, 1.30 × 10−6, 3.75 × 10−7, 1.97 × 10−7, 9.18 × 10−8 A/cm2 respectively at an applied voltage of 1000 V. The values demonstrated that leakage current density for A10 sample was decreased by about two orders of magnitude in comparison with pure BiFeO3 sample. The fine grains and the compact structure had a close relationship with the decrease of the leakage current density for (1 − x)BiFeO3 − xAl2O3 samples. The charges easily accumulated around the grain boundary to form small regions to increase resistance for ceramic. So, the decreased of the grain size will increase of the grain boundaries and resulted in a decrease in the leakage current density [26, 27] The leakage conduction mechanism can be used to explain the decrease of leakage current caused by the addition of Al2O3. Figure 6 shows the plot of log (J)–log (E) for (1 − x)BiFeO3 − xAl2O3 samples at the positive voltage. The plots of A0, A2, and A4 showed a linear behavior with a slope of approximately 1, revealing an Ohmic relationship conduction behavior [28]. However, with the increases of the Al2O3 content, the A6, A8 and, A10 samples showed a linear behavior with a slope of 2 in low voltage region, indicating a good agreement with SCLC theory [29]. Ramachandran et al. [30] and Ahn et al. [31] also found an SCLC conduction mechanism in BiFeO3 system ceramics. As for A6, A8 and A10 samples, the voltage ranges affected by the SCLC mechanism increased monotonously into 105, 164, and 241 V, respectively, meaning that the SCLC mechanism tended to expand in leakage current characteristic.

Fig. 5
figure 5

Leakage current densities for the (1 − x)BiFeO3 − xAl2O3 samples

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Fig. 6
figure 6

Leakage current mechanisms for the (1 − x)BiFeO3 − xAl2O3 samples

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The polarization hysteresis loops (P–E loops) measured for all the (1 − x)BiFeO3 − xAl2O3 samples at room temperature are shown in Fig. 7. The polarization hysteresis loops were measured at different electric fields (10, 15 and 20 kV/cm) with an operating frequency of 100 Hz. The pure BiFeO3 ceramic shown a leaky polarization hysteresis loop owing to its severe leakage current characteristics. This is very common in pure phase BiFeO3 ceramics. However, the ferroelectric hysteresis loops became more typical in shape with the increase of the Al2O3 content, indicating that the leakage current density was decreased as discussed above. However, with the further addition of Al2O3, the value of remnant polarization decreased from 0.61 for A6 to 0.35 μC/cm2 for A10, owing to the excessive addition of Al2O3 and the formation of secondary phase, which have no ferroelectric properties or lost ferroelectric properties above − 29 °C [18]. During the measurement process, it was hard to get an exact value of the saturated polarization hysteresis loops, owing to the electrical penetration of the samples occurred before full switching [32].

Fig. 7
figure 7

P–E loops for the (1 − x)BiFeO3 − xAl2O3 samples: a A 0, b A2, c A4, d A6, e A8, and f A10

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4 Conclusions

Lead-free (1 − x)BiFeO3 − xAl2O3 composites ceramics were synthesized using the standard solid-state reaction method. The effects of Al2O3 addition on structural (crystal data and microstructure), leakage current characteristics, ferroelectric and dielectric properties of the (1 − x)BiFeO3 − xAl2O3 composites were studied. Secondary phases (Bi2Fe4O9 and Bi24Al2O39) were increased with the addition of Al2O3. The average grain size significantly decreased from 1.49 μm for x = 0.00 to 0.45 μm for x = 0.10. The ceramics with the addition of Al2O3 showed smaller dielectric constant and dielectric loss compared with pure BiFeO3 sample. The TC decreased from 475 °C for A2 to 390 °C for A10. Leakage current density decreased significantly, and the leakage conduction mechanism was changed from Ohmic relationship to SCLC mechanism with the addition of Al2O3. In addition, the polarization hysteresis loops became more typical in shape with the increase of the Al2O3 content.