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Microstructure, enhanced electric and magnetic properties of Bi0.9La0.1FeO3 ceramics prepared by microwave sintering
Journal of Alloys and Compounds 774 (2019) 61e68
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Microstructure, enhanced electric and magnetic properties of Bi0.9La0.1FeO3 ceramics prepared by microwave sintering Wei Cai a, b, *, Rongli Gao a, b, Chunlin Fu a, b, Liangwen Yao a, Gang Chen a, b, Xiaoling Deng a, b, Zhenhua Wang a, b, Xianlong Cao a, b, Fengqi Wang a a
School of Metallurgy and Materials Engineering, Chongqing University of Science and Technology, University Town, Shapingba District, Chongqing, 401331, China Chongqing Key Laboratory of Nano-Micro Composite Material and Device, University Town, Shapingba District, Chongqing, 401331, China
b
a r t i c l e i n f o
a b s t r a c t
Article history: Received 24 July 2018 Received in revised form 22 September 2018 Accepted 25 September 2018 Available online 28 September 2018
Bi0.9La0.1FeO3 (short for BLFO) ceramics was fabricated by conventional solid-state reaction and microwave sintering. Effects of microwave sintering temperature on microstructure, electric and magnetic properties have been systematically investigated. The XRD results indicate that BLFO ceramics sintered at 800 C and 820 C are single rhombohedral distortion perovskite phase, and there is small amount of secondary phase (Bi2Fe4O9) in the ceramic sample sintered at 780 C. As microwave sintering temperature rises, the densification of BLFO ceramics enhances and the grain size increases, but the homogeneity of grain becomes worse. Microwave sintering temperature has little effect on room-temperature dielectric constant. The leakage current density decreases with the rise of sintering temperature, which is closely related to enhanced densification. The leakage mechanism of BLFO ceramics changes from bulklimited Poole-Frenkel emission to space-charge-limited bulk conduction as sintering temperature rises. The remnant polarization of BLFO ceramics sintered at 820 C is much higher than that of the samples sintered at 800 C and 780 C, and the coercive electric field changes little with sintering temperature. The results indicate the higher sintering temperature is benefit for the ferroelectricity of BLFO ceramics. The magnetic hysteresis loops of BLFO ceramics indicate that the ceramic samples sintered at 800 C and 820 C show a typical ferromagnetic behavior at room temperature. Compared with BLFO ceramics sintered at 800 C, the spontaneous magnetization and saturation magnetization of BLFO ceramics sintered at 820 C are higher, and its remnant magnetization and coercive magnetic field are lower. © 2018 Elsevier B.V. All rights reserved.
Keywords: Microwave sintering Electrical properties Magnetic properties Bismuth ferrite Lanthanum
1. Introduction Multiferroic materials have drawn huge interest due to the coexistence of ferroelectricity, ferromagnetism and/or ferroelasticity. BiFeO3 (short for BFO) is the most important roomtemperature single phase multiferroic materials with a high Neel (TN - 370 C) and Curie (TC - 830 C) temperature [1]. But BFO has high leakage current due to the reduction of Fe3þ to Fe2þ and the formation of oxygen vacancies during the sintering process, and it is difficult to fabricate single phase BFO owing to narrow temperature range of phase stabilization [2,3]. To solve the above
* Corresponding author. Permanent address: School of Metallurgy and Materials Engineering, Chongqing University of Science and Technology, University Town, Shapingba District, Chongqing, 401331, China. E-mail address: [email protected] (W. Cai). https://doi.org/10.1016/j.jallcom.2018.09.316 0925-8388/© 2018 Elsevier B.V. All rights reserved.
problems, a wide variety of dopants (such as La, Nd, Sm, Gd, Ru, Ba, Ce and so on [2,4]) have been added to improve the phase stability and reduce oxygen defects of BFO. Among dopants, the partial substitution of La3þ for Bi3þ on A sites of BiFeO3 has been found to be an effective method to suppress impurity phase and improve electric properties [5e7]. Pu et al. found that the introduction of La3þ can reduce the dielectric loss of BFO and enhance the ferroelectric and piezoelectric properties [5]. Yotburt et al. found that La3þ significantly enhances the breakdown field and decreases the leakage current density [6]. Moreover, many alternative preparation methods such as rapid liquid-phase sintering [8], spark-plasma sintering [9], sol-gel method [10] and so on have been used to obtain single phase BFO. According to reaction kinetics of BiFeO3, it is obvious that the lower sintering temperature and shorter dwelling time are helpful for preparation of single phase BFO with low leakage current. Compared to conventional sintering method, microwave sintering method has many advantages such as rapid-
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heating rate, obviously shortening of the processing time and decreasing of sintering temperature, which results from selfheating by absorption of microwave power. Therefore, microwave sintering is suitable for preparing BFO ceramics with lower leakage current and is good to gain fine and uniform grain and high densification. The effectiveness of microwave sintering in the decreasing of sintering temperature and dwelling time which results in higher density and fine and uniform grain of BFO ceramics has already been demonstrated in our previous work, but its ferroelectricity is not good [11]. There are the other reports on microstructure, electric and magnetic properties of Pure, Gddoping and La and Mn co-doping BFO ceramics fabricated by microwave sintering [12e15]. Prasad et al. prepared BiFeO3 nanoceramics by high energy ball milling and microwave sintering and found that nanocrystalline BiFeO3 has higher electrical resistivity, remnant polarization and shows ferromagnetic behavior compared with a conventionally sintered microcrystalline BiFeO3 [12]. Kolte et al. prepared nanocrystalline BiFeO3 ceramics by sol-gel method following microwave sintering and found that microwave sintering can significantly make the sintering temperature and time reduce and fine grained BiFeO3 ceramics are highly dense and insulating (R- 1.8 GU cm) [15]. As mentioned above, La is an important dopant to improve the phase stability, electric and magnetic properties of BFO. But there are few reports on La-doped BFO ceramics prepared by microwave sintering. In the paper, we fabricated Bi0.9La0.1FeO3 (short for BLFO) ceramics by microwave sintering and studied systematically the effects of sintering temperature on its microstructure, electric and magnetic properties.
heat insulator. And then the alumina crucible was placed into the mullite sagger filled with alumina fiber cotton. The SiC rods as auxiliary heating materials were introduced into the alumina fiber cotton. The temperature was measured by infrared thermometer. 2.2. Characterization XRD patterns were obtained by X-ray diffractometer (Rigaku, Smartlab, Japan) with Cu Ka (l ¼ 0.15418 nm) radiation in range of 2q (20e60 ). The surface morphology of ceramic sample was observed by scanning electron microscope (Hitachi, S-3700N, Japan). The density of ceramics was measured by Archimedes method. For electric measurements, the ceramic pellets were coated with silver paste and fired at 500 C for 30 min. The temperature dependences of dielectric constant and loss were measured by impedance analyzer (Agilent, HP4980A, USA) in a temperature range from 25 C to 480 C. The ferroelectric hysteresis loops and leakage current curves were performed out using ferroelectric test system (aixACCT, TF2000E, Germany). To separate leakage and dielectric contributions, the positive-up-negativedown (PUND) method was employed [16]. The magnetic hysteresis loops were conducted using vibrating sample magnetometer (Changchun Yingpu, VSM-300, China). 3. Results and discussion 3.1. Microstructure The XRD patterns of the ceramic samples sintered at different
2. Material and methods 2.1. Preparation Bi0.9La0.1FeO3 ceramics were fabricated by conventional solidstate method and microwave sintering. The raw materials Bi2O3, La2O3 and Fe2O3 powder were weighed in stoichiometric proportion and then milled for 6 h in distilled water and zirconia media with planetary ball mill to form slurry. After the slurry was dried, the mixture consisting of Bi2O3, La2O3 and Fe2O3 was added with 7 wt% binder (15% polyvinyl alcohol solution) and then pressed into disk-shaped green pellets with a diameter of 10.0 mm and thickness of 1.0 mm at 15 MPa pressure. The green pellets were then sintered at 780e820 C for 30 min (heating rate is 30 C/min) respectively by a microwave sintering furnace (HAMiLab-M1500, SYNO THERM, 3 kW, 2.45 GHz). Fig. 1 shows the schematic illustration of microwave sintering system. The green pellet was placed into the alumina crucible filled with alumina hollow spheres as
Fig. 2. XRD patterns of BLFO ceramics sintered at different temperature.
Fig. 1. Schematic diagram of microwave sintering.
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temperature are shown in Fig. 2. Firstly, it is found that the main diffraction peaks of the ceramic samples sintered at 780e820 C are indexed as rhombohedral distortion perovskite structure (BiFeO3, R3c, JCPDS Card No. 71e2494). But there is small amount of secondary phase (indexed as Bi2Fe4O9) in the sample sintered at 780 C. The result indicates that the higher microwave sintering temperature is beneficial to form single phase. Secondly, the lattice constants of ceramic samples sintered at 780e820 C are obtained and shown in Table 1 according to Bragg equation. The result shows that the lattice constants of BLFO ceramics decreases slightly with the rise of microwave sintering temperature, which may be due to that La3þ with smaller ionic radius (0.106 nm) substitutes for more Bi3þ with larger ionic radius (0.108 nm) and the volatilization of Bi3þ is more serious when microwave sintering temperature is higher. To find out the influence of microwave sintering temperature on densification of BLFO ceramics, the relative density of BLFO ceramics are measured by Archimedes method. The relative density of BLFO ceramics sintered at 780 C, 800 C and 820 C is 91.2%, 93.6% and 95.6% respectively, which indicates that higher sintering temperature is helpful to obtain higher densification. In order to learn more about the microstructure of BLFO ceramics sintered at different temperature, SEM was used to get the surface morphology images (shown in Fig. 3). Firstly, it is seen that there are some pores in BLFO ceramics sintered at 780 C and 800 C, and the ceramic sample sintered at 820 C shows obviously denser surface, which further indicates that the higher sintering temperature can enhance the densification. Secondly, the average grain size of BLFO ceramics sintered at 780 C, 800 C and 820 C obtained by lineal intercept method is 2.1 mm, 2.8 mm and 3.1 mm respectively. Moreover, it is noteworthy that there is abnormal grain growth in BLFO ceramics sintered at 800 C and 820 C compared with the sample sintered at 780 C, which suggests that the homogeneity of grain becomes worse with the rise of sintering temperature. 3.2. Dielectric properties The frequency dependences of dielectric constant and loss for BLFO ceramics are shown in Fig. 4. Firstly, the dielectric constant initially decreases and then remains nearly constant as the frequency increases. The variation of dielectric constant reveals the dispersion due to Maxwell-Wagner type of interfacial polarization [17]. Generally, interfacial, ionic and dipole polarizations respond to a low frequency region. At higher frequency, the interfacial and dipole polarization could not follow the change of electric field and relax down [6] so that the dielectric constant decreases. Secondly, there is little difference in the room-temperature dielectric constant of BLFO ceramics sintered at 780e820 C. Moreover, the dielectric loss of BLFO ceramics increases with the rise of sintering temperature when frequency is below 800 Hz, and when frequency is above 9000 Hz, the dielectric loss of BLFO ceramics sintered at 800 C is the highest and that of the sample sintered at 820 C is the lowest. The temperature dependences of dielectric constant and loss of BLFO ceramics are shown in Fig. 5. The dielectric constant and loss
Table 1 The lattice constants of BLFO ceramics sintered at different temperature.
Sintering temperature ( C)
780 800 820
Lattice constants (nm) a
c
0.5572 0.5567 0.5561
1.3978 1.3948 1.3935
Fig. 3. SEM micrographs of BLFO ceramics sintered at different temperature (a) 780 C, (b) 800 C and (c) 820 C.
of BLFO ceramics sintered at 780e820 C increase simultaneously with the rise of temperature and a sharp increase of dielectric constant and loss starts when temperature is above 300 C. But in test temperature range (25e480 C), we could not find any dielectric anomaly or phase transition for all the samples. The result is consistent with Sen's and Das's results [17,18].
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Fig. 4. Frequency dependences of dielectric constant and loss of BLFO ceramics sintered at different temperature.
based ceramics originates mainly from charge defects such as oxygen vacancies and oxygen vacancy results from volatility of Bi and the transition from Fe3þ to Fe2þ [22,23]. It suggests that the low sintering temperature and short dwelling time are beneficial to the decrease of oxygen vacancy. On the one hand, microwave sintering temperature (780e820 C) for our BLFO ceramics is lower than that of conventional sintering and rapid liquid-phase sintering, and the dwelling time of microwave sintering (30 min) is obviously shorter than that of conventional sintering (1e2 h), which results in much lower leakage current density of BLFO ceramics. On the other hand, the densification of our BLFO ceramics fabricated by microwave sintering (especially for 820 C for 30 min) is much better than that of BLFO ceramics prepared by conventional sintering [19,20]. It is noteworthy the higher microwave sintering temperature for our BLFO ceramics results in the decrease of leakage current density. Generally speaking, the high sintering temperature can make oxygen vacancy increase so that the leakage current increases. But the densification of ceramic sample is the other important factor that
Fig. 5. Temperature dependences of dielectric constant and loss of BLFO ceramics sintered at different temperature (a) 780 C, (b) 800 C and (c) 820 C.
The J-E curves of BLFO ceramics sintered at different temperature are shown in Fig. 6. Firstly, it is seen that the leakage current density of BLFO ceramics decreases with the rise of sintering temperature and the leakage current density of the ceramic sample sintered at 820 C is obviously lower than that of BLFO ceramics fabricated by conventional sintering [19,20] and rapid liquid-phase sintering [21]. It is well known that the leakage current of BFO-
affects leakage current. As mentioned above, the higher microwave sintering temperature for BLFO ceramics is good to gain denser structure, which results in lower leakage current. Although the higher sintering temperature could make oxygen vacancy increase, the effect of densification on leakage current of BLFO ceramics is more than that of oxygen vacancy so that the leakage current decreases with the rise of microwave sintering temperature. Secondly,
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2 pffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi 3 ft e eE=pε0 K 5 J ¼ AE exp4 kB T
K¼
Fig. 6. Leakage current density versus electric field curves of BLFO ceramics sintered at different temperature.
it is found that there are obviously asymmetric J-E curves of BLFO ceramics sintered at 800 C and 820 C under positive and negative electric field and the J-E curves significantly shift to the positive direction of electric field axis as sintering temperature rises. The result suggests that there is obvious internal bias field, which may result from the increase of defect such as oxygen vacancy. Therefore, this also proves that oxygen vacancy of BLFO ceramics increases with the rise of sintering temperature. In general, there are some leakage mechanisms such as Ohmic conduction, interface-limited Schottky emission, space-chargelimited bulk conduction (SCLC), bulk-limited Poole-Frenkel emission (PF) [24,25]. They could be described by the following. Ohmic conduction can be expressed by:
J ¼ enmE
(1)
where e is the electronic charge, n is the number of charge carriers and m is the carrier mobility. Interface-limited Schottky emission is expressed by:
2 pffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi 3 fB e eE=4pε0 K 5 J ¼ A*T 2 exp4 kB T
K¼
e3 k2B T 2
4pε0 a2
e3 k2B T 2
pε0 a2
9mεr ε0 2 E 8d
(2)
(3)
(4)
where εr is the relative dielectric permittivity, ε0 is the permittivity of free space, m is the carrier mobility and d is the film thickness. Bulk-limited Poole-Frenkel emission (PF) is expressed by:
(5)
(6)
where A is a constant, ft is the trap ionization energy, ε0 is the permittivity of free space, K is the optical dielectric constant, kB is the Boltzmann constant and T is the temperature and a is the linear fitting slope. To understand the leakage mechanism in BLFO ceramics sintered at different temperature, logJ versus logE curves at negative electric field are plotted in Fig. 7. Firstly, it is clear that there are two and three different regions manifested by different slopes in the plots. For BLFO ceramics sintered at 780 C, the slope (a) at lower electric field (<9 kV/cm) is 1.307, which is characteristic of Ohmic conduction, and the slope increases to 3.023 when electric field is between 9 kV/cm and 19 kV/cm, which indicates that the leakage mechanism is neither Ohmic conduction nor SCLC. To confirm the leakage mechanism, the plotted ln (J/E) versus (E1/2) curve at higher electric field (9e19 kV/cm) (shown in Fig. 8(a)) shows obvious linear characteristics, which indicates that the Poole-Frenkel mechanism dominates leakage behavior of BLFO ceramics sintered at 780 C according to Eq. (5) when electric field is between 9 kV/cm and 19 kV/cm. The logJ-logE curves of BLFO ceramics sintered at 800 C and 820 C are similar, each curve can be divided into three regions. The slope of logJ-logE curve of BLFO ceramics sintered at 800 C and 820 C in region I is 0.621 and 0.377, respectively. To confirm the leakage mechanism in region I, the lnJE1/2 curves are plotted (shown in Fig. 8(b) and (c)), it is seen that the curves show obvious linear characteristics, which indicates that the Schottky emission mechanism dominates according to Eq. (2). For BLFO ceramics sintered at 800 C and 820 C, the region II corresponds to Ohmic conduction mechanism and the region III corresponds to the SCLC mechanism characterized by a slope of 2.0, which may be attributed to the increase in grain boundary density. Oxygen vacancies in BLFO ceramics is the origin of space charge. Oxygen vacancies create deep-trap energy levels in the band gap for activated electrons to be mobile. Thus, more oxygen vacancies can generate more free carriers, which results in the SCLC mechanism in BLFO ceramics sintered at higher temperature (800 C and 820 C).
where A* is Richardson constant, fB is Schottky barrier height and K is optical dielectric constant, kB is the Boltzmann constant and T is the temperature and a is the linear fitting slope. Space-chargelimited bulk conduction (SCLC) is expressed by:
JSCLC ¼
65
Fig. 7. logJ versus logE curves of BLFO ceramics sintered at 780e820 C.
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ferroelectric polarization in BLFO ceramics, PUND pulse measurement was taken which distinguishes ferroelectric polarization from other effects such as leakage current and accumulated charge at the grain boundaries [26,27]. Room-temperature P-E hysteresis loops of BLFO ceramics obtained by PUND method are shown in Fig. 9. The P-E loops of all the samples show typical and saturated ferroelectric loop characteristic. The remnant polarization (2Pr) and coercive electric field (2Ec) of BLFO ceramics sintered at 780 C, 800 C and 820 C are 0.416 mC/cm2 and 53.6 kV/cm, 0.624 mC/cm2 and 55.1 kV/ cm, 1.942 mC/cm2 and 55.1 kV/cm, respectively. The result suggests that the higher sintering temperature can obviously enhance ferroelectric property of BLFO ceramics. It is worthwhile to note that the remnant polarization of BLFO ceramics sintered at 820 C is far higher than that of BLFO ceramics sintered at 800 C and 780 C, which is attributed to the higher densification and purity of phase in BLFO ceramics sintered at 820 C. 3.4. Magnetic properties According to XRD results, the ceramic samples sintered at 800 C and 820 C are single BLFO phase. Therefore, the roomtemperature magnetization hysteresis loops (M-H) for BLFO ceramics sintered at 800 C and 820 C are shown in Fig. 10. It is well known that BiFeO3 possesses antiferromagnetic G-type magnetic structure ordering subjected to a cycloidal modulation of 62 nm, which leads to a cancellation of the macroscopic magnetization [28], i.e. M depending linearly on H and no hysteresis of M on H [29]. Compared with pure BFO, it is clear that the M-H curves for BLFO ceramics sintered at 800 C and 820 C show obvious nonlinear characteristics, which indicates that there is weak ferromagnetism in BLFO ceramics and the spontaneous magnetization contributes to the total magnetization. It is because that the substitution of La3þ on A sites collapses the space-modulated spin structure of BiFeO3 [30]. The magnetization linearly increases with the further increase of magnetic field (H), reflecting the basic antiferromagnetic nature of our BLFO specimens. The spontaneous magnetization (Ms) were obtained by extrapolating the linear portion of M-H curves to H ¼ 0. The remnant magnetization (Mr) and coercivity (Hc) of BLFO ceramics sintered at 800 C and 820 C are 0.0026 emu/g and 326.1 Oe, 0.0023 emu/g and 239.25 Oe, and the spontaneous magnetization (Ms) of the sample sintered at 820 C and 800 C is 0.012 emu/g and 0.0052 emu/g respectively. The result indicates that although the remnant magnetization of BLFO ceramics
Fig. 8. Various fits of the leakage current data (a) ln(J/E)-E1/2 curve of BLFO ceramics sintered at 780 C, (b) lnJ-E1/2 curve of BLFO ceramics sintered at 800 C, (c) lnJ-E1/2 curve of BLFO ceramics sintered at 820 C.
3.3. Ferroelectric properties Aa result of relatively high conductivity of BFO-based ceramics, conventional polarization-electric field (P-E) hysteresis loop maybe not give correct information on polarization. To confirm the
Fig. 9. Room-temperature hysteresis loops of BLFO ceramics sintered at different temperature.
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References
Fig. 10. Magnetic hysteresis loops of BLFO ceramics sintered at different temperature. The inset is the enlarged M-H curves.
sintered at 820 C is slightly lower than that of the sample sintered at 800 C, the spontaneous magnetization of the sample sintered at 820 C is much higher. Therefore, although the average grain size (2.8e3.1 mm) of our BLFO ceramics fabricated by microwave sintering (800 C and 820 C for 30 min) is still much larger than the spin cycloid (62 nm) [31], the decrease of grain size of BLFO ceramics is benefit to enhancement of its magnetic properties. 4. Conclusions BLFO ceramics was fabricated by microwave sintering. The effects of sintering temperature on microstructure, electric and magnetic properties of BLFO ceramics have been investigated. Relative higher sintering temperature contributes to form single phase BLFO ceramics. The rise of sintering temperature enhances the densification and increases the grain size. The enhancement of densification in BLFO ceramics caused by the rise of sintering temperature makes the leakage current density obviously decrease. As sintering temperature rises, the leakage mechanism of BLFO ceramics changes from bulk-limited Poole-Frenkel emission to space-charge-limited bulk conduction. The remnant polarization of BLFO ceramics sintered at 820 C is much higher than that of the samples sintered at lower temperature. BLFO ceramics sintered at 800 C and 820 C show weak ferromagnetism. Compared with the sample sintered at 800 C, the spontaneous magnetization and saturation magnetization of the ceramic sample sintered at 820 C are higher, and the remnant magnetization and coercive magnetic field is slightly lower. These results indicate the higher microwave sintering temperature is benefit to enhancing the ferroelectric and magnetic properties and decreasing the leakage current of BLFO ceramics. Acknowledgements This work was supported by Excellent Talent Project in University of Chongqing (Grant No. 2017e35), the Science and Technology Innovation Project of Social Undertakings and People's Livelihood Guarantee of Chongqing (Grant No. cstc2017shmsA0192), the Program for Innovation Teams in University of Chongqing (Grant No. CXTDX201601032) and the Chongqing Research Program of Basic Research and Frontier Technology (Grant No. CSTC2018jcyjAX0416, CSTC2016jcyjA0175, CSTC2016jcyjA0349).
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[28] L.L. Pan, Q. Yuan, Z.Z. Liao, L.L. Qin, J. Bi, D.J. Gao, J.T. Wu, H. Wu, Z.G. Ye, Superior room-temperature magnetic field-dependent magnetoelectric effect in BiFeO3-based multiferroic, J. Alloys Compd. 762 (2018) 184e189. [29] G.L. Yuan, S.W. Or, H.L.W. Chan, Structural transformation and ferroelectriceparaelectric phase transition in Bi1-xLaxFeO3 (x¼0-0.25) multiferroic ceramics, J. Phys. D Appl. Phys. 40 (2007) 1196e1200. [30] D.V. Karpinsky, I.O. Troyanchuk, M. Tovar, V. Sikolenko, V. Efimov,
A.L. Kholkin, Evolution of crystal structure and ferroic properties of La-doped BiFeO3 ceramics near the rhombohedral-orthorhombic phase boundary, J. Alloys Compd. 555 (2013) 101e107. [31] N.T. Liu, R.H. Liang, X.B. Zhao, Y.Y. Zhang, Z.Y. Zhou, X.D. Tang, X.L. Dong, Tailoring domain structure through manganese to modify the ferroelectricity, strain and magnetic properties of lead-free BiFeO3-based multiferroic ceramics, J. Alloys Compd. 740 (2018) 470e476.
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Journal of Alloys and Compounds journal homepage: http://www.elsevier.com/locate/jalcom
Microstructure, enhanced electric and magnetic properties of Bi0.9La0.1FeO3 ceramics prepared by microwave sintering Wei Cai a, b, *, Rongli Gao a, b, Chunlin Fu a, b, Liangwen Yao a, Gang Chen a, b, Xiaoling Deng a, b, Zhenhua Wang a, b, Xianlong Cao a, b, Fengqi Wang a a
School of Metallurgy and Materials Engineering, Chongqing University of Science and Technology, University Town, Shapingba District, Chongqing, 401331, China Chongqing Key Laboratory of Nano-Micro Composite Material and Device, University Town, Shapingba District, Chongqing, 401331, China
b
a r t i c l e i n f o
a b s t r a c t
Article history: Received 24 July 2018 Received in revised form 22 September 2018 Accepted 25 September 2018 Available online 28 September 2018
Bi0.9La0.1FeO3 (short for BLFO) ceramics was fabricated by conventional solid-state reaction and microwave sintering. Effects of microwave sintering temperature on microstructure, electric and magnetic properties have been systematically investigated. The XRD results indicate that BLFO ceramics sintered at 800 C and 820 C are single rhombohedral distortion perovskite phase, and there is small amount of secondary phase (Bi2Fe4O9) in the ceramic sample sintered at 780 C. As microwave sintering temperature rises, the densification of BLFO ceramics enhances and the grain size increases, but the homogeneity of grain becomes worse. Microwave sintering temperature has little effect on room-temperature dielectric constant. The leakage current density decreases with the rise of sintering temperature, which is closely related to enhanced densification. The leakage mechanism of BLFO ceramics changes from bulklimited Poole-Frenkel emission to space-charge-limited bulk conduction as sintering temperature rises. The remnant polarization of BLFO ceramics sintered at 820 C is much higher than that of the samples sintered at 800 C and 780 C, and the coercive electric field changes little with sintering temperature. The results indicate the higher sintering temperature is benefit for the ferroelectricity of BLFO ceramics. The magnetic hysteresis loops of BLFO ceramics indicate that the ceramic samples sintered at 800 C and 820 C show a typical ferromagnetic behavior at room temperature. Compared with BLFO ceramics sintered at 800 C, the spontaneous magnetization and saturation magnetization of BLFO ceramics sintered at 820 C are higher, and its remnant magnetization and coercive magnetic field are lower. © 2018 Elsevier B.V. All rights reserved.
Keywords: Microwave sintering Electrical properties Magnetic properties Bismuth ferrite Lanthanum
1. Introduction Multiferroic materials have drawn huge interest due to the coexistence of ferroelectricity, ferromagnetism and/or ferroelasticity. BiFeO3 (short for BFO) is the most important roomtemperature single phase multiferroic materials with a high Neel (TN - 370 C) and Curie (TC - 830 C) temperature [1]. But BFO has high leakage current due to the reduction of Fe3þ to Fe2þ and the formation of oxygen vacancies during the sintering process, and it is difficult to fabricate single phase BFO owing to narrow temperature range of phase stabilization [2,3]. To solve the above
* Corresponding author. Permanent address: School of Metallurgy and Materials Engineering, Chongqing University of Science and Technology, University Town, Shapingba District, Chongqing, 401331, China. E-mail address: [email protected] (W. Cai). https://doi.org/10.1016/j.jallcom.2018.09.316 0925-8388/© 2018 Elsevier B.V. All rights reserved.
problems, a wide variety of dopants (such as La, Nd, Sm, Gd, Ru, Ba, Ce and so on [2,4]) have been added to improve the phase stability and reduce oxygen defects of BFO. Among dopants, the partial substitution of La3þ for Bi3þ on A sites of BiFeO3 has been found to be an effective method to suppress impurity phase and improve electric properties [5e7]. Pu et al. found that the introduction of La3þ can reduce the dielectric loss of BFO and enhance the ferroelectric and piezoelectric properties [5]. Yotburt et al. found that La3þ significantly enhances the breakdown field and decreases the leakage current density [6]. Moreover, many alternative preparation methods such as rapid liquid-phase sintering [8], spark-plasma sintering [9], sol-gel method [10] and so on have been used to obtain single phase BFO. According to reaction kinetics of BiFeO3, it is obvious that the lower sintering temperature and shorter dwelling time are helpful for preparation of single phase BFO with low leakage current. Compared to conventional sintering method, microwave sintering method has many advantages such as rapid-
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heating rate, obviously shortening of the processing time and decreasing of sintering temperature, which results from selfheating by absorption of microwave power. Therefore, microwave sintering is suitable for preparing BFO ceramics with lower leakage current and is good to gain fine and uniform grain and high densification. The effectiveness of microwave sintering in the decreasing of sintering temperature and dwelling time which results in higher density and fine and uniform grain of BFO ceramics has already been demonstrated in our previous work, but its ferroelectricity is not good [11]. There are the other reports on microstructure, electric and magnetic properties of Pure, Gddoping and La and Mn co-doping BFO ceramics fabricated by microwave sintering [12e15]. Prasad et al. prepared BiFeO3 nanoceramics by high energy ball milling and microwave sintering and found that nanocrystalline BiFeO3 has higher electrical resistivity, remnant polarization and shows ferromagnetic behavior compared with a conventionally sintered microcrystalline BiFeO3 [12]. Kolte et al. prepared nanocrystalline BiFeO3 ceramics by sol-gel method following microwave sintering and found that microwave sintering can significantly make the sintering temperature and time reduce and fine grained BiFeO3 ceramics are highly dense and insulating (R- 1.8 GU cm) [15]. As mentioned above, La is an important dopant to improve the phase stability, electric and magnetic properties of BFO. But there are few reports on La-doped BFO ceramics prepared by microwave sintering. In the paper, we fabricated Bi0.9La0.1FeO3 (short for BLFO) ceramics by microwave sintering and studied systematically the effects of sintering temperature on its microstructure, electric and magnetic properties.
heat insulator. And then the alumina crucible was placed into the mullite sagger filled with alumina fiber cotton. The SiC rods as auxiliary heating materials were introduced into the alumina fiber cotton. The temperature was measured by infrared thermometer. 2.2. Characterization XRD patterns were obtained by X-ray diffractometer (Rigaku, Smartlab, Japan) with Cu Ka (l ¼ 0.15418 nm) radiation in range of 2q (20e60 ). The surface morphology of ceramic sample was observed by scanning electron microscope (Hitachi, S-3700N, Japan). The density of ceramics was measured by Archimedes method. For electric measurements, the ceramic pellets were coated with silver paste and fired at 500 C for 30 min. The temperature dependences of dielectric constant and loss were measured by impedance analyzer (Agilent, HP4980A, USA) in a temperature range from 25 C to 480 C. The ferroelectric hysteresis loops and leakage current curves were performed out using ferroelectric test system (aixACCT, TF2000E, Germany). To separate leakage and dielectric contributions, the positive-up-negativedown (PUND) method was employed [16]. The magnetic hysteresis loops were conducted using vibrating sample magnetometer (Changchun Yingpu, VSM-300, China). 3. Results and discussion 3.1. Microstructure The XRD patterns of the ceramic samples sintered at different
2. Material and methods 2.1. Preparation Bi0.9La0.1FeO3 ceramics were fabricated by conventional solidstate method and microwave sintering. The raw materials Bi2O3, La2O3 and Fe2O3 powder were weighed in stoichiometric proportion and then milled for 6 h in distilled water and zirconia media with planetary ball mill to form slurry. After the slurry was dried, the mixture consisting of Bi2O3, La2O3 and Fe2O3 was added with 7 wt% binder (15% polyvinyl alcohol solution) and then pressed into disk-shaped green pellets with a diameter of 10.0 mm and thickness of 1.0 mm at 15 MPa pressure. The green pellets were then sintered at 780e820 C for 30 min (heating rate is 30 C/min) respectively by a microwave sintering furnace (HAMiLab-M1500, SYNO THERM, 3 kW, 2.45 GHz). Fig. 1 shows the schematic illustration of microwave sintering system. The green pellet was placed into the alumina crucible filled with alumina hollow spheres as
Fig. 2. XRD patterns of BLFO ceramics sintered at different temperature.
Fig. 1. Schematic diagram of microwave sintering.
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temperature are shown in Fig. 2. Firstly, it is found that the main diffraction peaks of the ceramic samples sintered at 780e820 C are indexed as rhombohedral distortion perovskite structure (BiFeO3, R3c, JCPDS Card No. 71e2494). But there is small amount of secondary phase (indexed as Bi2Fe4O9) in the sample sintered at 780 C. The result indicates that the higher microwave sintering temperature is beneficial to form single phase. Secondly, the lattice constants of ceramic samples sintered at 780e820 C are obtained and shown in Table 1 according to Bragg equation. The result shows that the lattice constants of BLFO ceramics decreases slightly with the rise of microwave sintering temperature, which may be due to that La3þ with smaller ionic radius (0.106 nm) substitutes for more Bi3þ with larger ionic radius (0.108 nm) and the volatilization of Bi3þ is more serious when microwave sintering temperature is higher. To find out the influence of microwave sintering temperature on densification of BLFO ceramics, the relative density of BLFO ceramics are measured by Archimedes method. The relative density of BLFO ceramics sintered at 780 C, 800 C and 820 C is 91.2%, 93.6% and 95.6% respectively, which indicates that higher sintering temperature is helpful to obtain higher densification. In order to learn more about the microstructure of BLFO ceramics sintered at different temperature, SEM was used to get the surface morphology images (shown in Fig. 3). Firstly, it is seen that there are some pores in BLFO ceramics sintered at 780 C and 800 C, and the ceramic sample sintered at 820 C shows obviously denser surface, which further indicates that the higher sintering temperature can enhance the densification. Secondly, the average grain size of BLFO ceramics sintered at 780 C, 800 C and 820 C obtained by lineal intercept method is 2.1 mm, 2.8 mm and 3.1 mm respectively. Moreover, it is noteworthy that there is abnormal grain growth in BLFO ceramics sintered at 800 C and 820 C compared with the sample sintered at 780 C, which suggests that the homogeneity of grain becomes worse with the rise of sintering temperature. 3.2. Dielectric properties The frequency dependences of dielectric constant and loss for BLFO ceramics are shown in Fig. 4. Firstly, the dielectric constant initially decreases and then remains nearly constant as the frequency increases. The variation of dielectric constant reveals the dispersion due to Maxwell-Wagner type of interfacial polarization [17]. Generally, interfacial, ionic and dipole polarizations respond to a low frequency region. At higher frequency, the interfacial and dipole polarization could not follow the change of electric field and relax down [6] so that the dielectric constant decreases. Secondly, there is little difference in the room-temperature dielectric constant of BLFO ceramics sintered at 780e820 C. Moreover, the dielectric loss of BLFO ceramics increases with the rise of sintering temperature when frequency is below 800 Hz, and when frequency is above 9000 Hz, the dielectric loss of BLFO ceramics sintered at 800 C is the highest and that of the sample sintered at 820 C is the lowest. The temperature dependences of dielectric constant and loss of BLFO ceramics are shown in Fig. 5. The dielectric constant and loss
Table 1 The lattice constants of BLFO ceramics sintered at different temperature.
Sintering temperature ( C)
780 800 820
Lattice constants (nm) a
c
0.5572 0.5567 0.5561
1.3978 1.3948 1.3935
Fig. 3. SEM micrographs of BLFO ceramics sintered at different temperature (a) 780 C, (b) 800 C and (c) 820 C.
of BLFO ceramics sintered at 780e820 C increase simultaneously with the rise of temperature and a sharp increase of dielectric constant and loss starts when temperature is above 300 C. But in test temperature range (25e480 C), we could not find any dielectric anomaly or phase transition for all the samples. The result is consistent with Sen's and Das's results [17,18].
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Fig. 4. Frequency dependences of dielectric constant and loss of BLFO ceramics sintered at different temperature.
based ceramics originates mainly from charge defects such as oxygen vacancies and oxygen vacancy results from volatility of Bi and the transition from Fe3þ to Fe2þ [22,23]. It suggests that the low sintering temperature and short dwelling time are beneficial to the decrease of oxygen vacancy. On the one hand, microwave sintering temperature (780e820 C) for our BLFO ceramics is lower than that of conventional sintering and rapid liquid-phase sintering, and the dwelling time of microwave sintering (30 min) is obviously shorter than that of conventional sintering (1e2 h), which results in much lower leakage current density of BLFO ceramics. On the other hand, the densification of our BLFO ceramics fabricated by microwave sintering (especially for 820 C for 30 min) is much better than that of BLFO ceramics prepared by conventional sintering [19,20]. It is noteworthy the higher microwave sintering temperature for our BLFO ceramics results in the decrease of leakage current density. Generally speaking, the high sintering temperature can make oxygen vacancy increase so that the leakage current increases. But the densification of ceramic sample is the other important factor that
Fig. 5. Temperature dependences of dielectric constant and loss of BLFO ceramics sintered at different temperature (a) 780 C, (b) 800 C and (c) 820 C.
The J-E curves of BLFO ceramics sintered at different temperature are shown in Fig. 6. Firstly, it is seen that the leakage current density of BLFO ceramics decreases with the rise of sintering temperature and the leakage current density of the ceramic sample sintered at 820 C is obviously lower than that of BLFO ceramics fabricated by conventional sintering [19,20] and rapid liquid-phase sintering [21]. It is well known that the leakage current of BFO-
affects leakage current. As mentioned above, the higher microwave sintering temperature for BLFO ceramics is good to gain denser structure, which results in lower leakage current. Although the higher sintering temperature could make oxygen vacancy increase, the effect of densification on leakage current of BLFO ceramics is more than that of oxygen vacancy so that the leakage current decreases with the rise of microwave sintering temperature. Secondly,
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2 pffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi 3 ft e eE=pε0 K 5 J ¼ AE exp4 kB T
K¼
Fig. 6. Leakage current density versus electric field curves of BLFO ceramics sintered at different temperature.
it is found that there are obviously asymmetric J-E curves of BLFO ceramics sintered at 800 C and 820 C under positive and negative electric field and the J-E curves significantly shift to the positive direction of electric field axis as sintering temperature rises. The result suggests that there is obvious internal bias field, which may result from the increase of defect such as oxygen vacancy. Therefore, this also proves that oxygen vacancy of BLFO ceramics increases with the rise of sintering temperature. In general, there are some leakage mechanisms such as Ohmic conduction, interface-limited Schottky emission, space-chargelimited bulk conduction (SCLC), bulk-limited Poole-Frenkel emission (PF) [24,25]. They could be described by the following. Ohmic conduction can be expressed by:
J ¼ enmE
(1)
where e is the electronic charge, n is the number of charge carriers and m is the carrier mobility. Interface-limited Schottky emission is expressed by:
2 pffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi 3 fB e eE=4pε0 K 5 J ¼ A*T 2 exp4 kB T
K¼
e3 k2B T 2
4pε0 a2
e3 k2B T 2
pε0 a2
9mεr ε0 2 E 8d
(2)
(3)
(4)
where εr is the relative dielectric permittivity, ε0 is the permittivity of free space, m is the carrier mobility and d is the film thickness. Bulk-limited Poole-Frenkel emission (PF) is expressed by:
(5)
(6)
where A is a constant, ft is the trap ionization energy, ε0 is the permittivity of free space, K is the optical dielectric constant, kB is the Boltzmann constant and T is the temperature and a is the linear fitting slope. To understand the leakage mechanism in BLFO ceramics sintered at different temperature, logJ versus logE curves at negative electric field are plotted in Fig. 7. Firstly, it is clear that there are two and three different regions manifested by different slopes in the plots. For BLFO ceramics sintered at 780 C, the slope (a) at lower electric field (<9 kV/cm) is 1.307, which is characteristic of Ohmic conduction, and the slope increases to 3.023 when electric field is between 9 kV/cm and 19 kV/cm, which indicates that the leakage mechanism is neither Ohmic conduction nor SCLC. To confirm the leakage mechanism, the plotted ln (J/E) versus (E1/2) curve at higher electric field (9e19 kV/cm) (shown in Fig. 8(a)) shows obvious linear characteristics, which indicates that the Poole-Frenkel mechanism dominates leakage behavior of BLFO ceramics sintered at 780 C according to Eq. (5) when electric field is between 9 kV/cm and 19 kV/cm. The logJ-logE curves of BLFO ceramics sintered at 800 C and 820 C are similar, each curve can be divided into three regions. The slope of logJ-logE curve of BLFO ceramics sintered at 800 C and 820 C in region I is 0.621 and 0.377, respectively. To confirm the leakage mechanism in region I, the lnJE1/2 curves are plotted (shown in Fig. 8(b) and (c)), it is seen that the curves show obvious linear characteristics, which indicates that the Schottky emission mechanism dominates according to Eq. (2). For BLFO ceramics sintered at 800 C and 820 C, the region II corresponds to Ohmic conduction mechanism and the region III corresponds to the SCLC mechanism characterized by a slope of 2.0, which may be attributed to the increase in grain boundary density. Oxygen vacancies in BLFO ceramics is the origin of space charge. Oxygen vacancies create deep-trap energy levels in the band gap for activated electrons to be mobile. Thus, more oxygen vacancies can generate more free carriers, which results in the SCLC mechanism in BLFO ceramics sintered at higher temperature (800 C and 820 C).
where A* is Richardson constant, fB is Schottky barrier height and K is optical dielectric constant, kB is the Boltzmann constant and T is the temperature and a is the linear fitting slope. Space-chargelimited bulk conduction (SCLC) is expressed by:
JSCLC ¼
65
Fig. 7. logJ versus logE curves of BLFO ceramics sintered at 780e820 C.
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ferroelectric polarization in BLFO ceramics, PUND pulse measurement was taken which distinguishes ferroelectric polarization from other effects such as leakage current and accumulated charge at the grain boundaries [26,27]. Room-temperature P-E hysteresis loops of BLFO ceramics obtained by PUND method are shown in Fig. 9. The P-E loops of all the samples show typical and saturated ferroelectric loop characteristic. The remnant polarization (2Pr) and coercive electric field (2Ec) of BLFO ceramics sintered at 780 C, 800 C and 820 C are 0.416 mC/cm2 and 53.6 kV/cm, 0.624 mC/cm2 and 55.1 kV/ cm, 1.942 mC/cm2 and 55.1 kV/cm, respectively. The result suggests that the higher sintering temperature can obviously enhance ferroelectric property of BLFO ceramics. It is worthwhile to note that the remnant polarization of BLFO ceramics sintered at 820 C is far higher than that of BLFO ceramics sintered at 800 C and 780 C, which is attributed to the higher densification and purity of phase in BLFO ceramics sintered at 820 C. 3.4. Magnetic properties According to XRD results, the ceramic samples sintered at 800 C and 820 C are single BLFO phase. Therefore, the roomtemperature magnetization hysteresis loops (M-H) for BLFO ceramics sintered at 800 C and 820 C are shown in Fig. 10. It is well known that BiFeO3 possesses antiferromagnetic G-type magnetic structure ordering subjected to a cycloidal modulation of 62 nm, which leads to a cancellation of the macroscopic magnetization [28], i.e. M depending linearly on H and no hysteresis of M on H [29]. Compared with pure BFO, it is clear that the M-H curves for BLFO ceramics sintered at 800 C and 820 C show obvious nonlinear characteristics, which indicates that there is weak ferromagnetism in BLFO ceramics and the spontaneous magnetization contributes to the total magnetization. It is because that the substitution of La3þ on A sites collapses the space-modulated spin structure of BiFeO3 [30]. The magnetization linearly increases with the further increase of magnetic field (H), reflecting the basic antiferromagnetic nature of our BLFO specimens. The spontaneous magnetization (Ms) were obtained by extrapolating the linear portion of M-H curves to H ¼ 0. The remnant magnetization (Mr) and coercivity (Hc) of BLFO ceramics sintered at 800 C and 820 C are 0.0026 emu/g and 326.1 Oe, 0.0023 emu/g and 239.25 Oe, and the spontaneous magnetization (Ms) of the sample sintered at 820 C and 800 C is 0.012 emu/g and 0.0052 emu/g respectively. The result indicates that although the remnant magnetization of BLFO ceramics
Fig. 8. Various fits of the leakage current data (a) ln(J/E)-E1/2 curve of BLFO ceramics sintered at 780 C, (b) lnJ-E1/2 curve of BLFO ceramics sintered at 800 C, (c) lnJ-E1/2 curve of BLFO ceramics sintered at 820 C.
3.3. Ferroelectric properties Aa result of relatively high conductivity of BFO-based ceramics, conventional polarization-electric field (P-E) hysteresis loop maybe not give correct information on polarization. To confirm the
Fig. 9. Room-temperature hysteresis loops of BLFO ceramics sintered at different temperature.
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References
Fig. 10. Magnetic hysteresis loops of BLFO ceramics sintered at different temperature. The inset is the enlarged M-H curves.
sintered at 820 C is slightly lower than that of the sample sintered at 800 C, the spontaneous magnetization of the sample sintered at 820 C is much higher. Therefore, although the average grain size (2.8e3.1 mm) of our BLFO ceramics fabricated by microwave sintering (800 C and 820 C for 30 min) is still much larger than the spin cycloid (62 nm) [31], the decrease of grain size of BLFO ceramics is benefit to enhancement of its magnetic properties. 4. Conclusions BLFO ceramics was fabricated by microwave sintering. The effects of sintering temperature on microstructure, electric and magnetic properties of BLFO ceramics have been investigated. Relative higher sintering temperature contributes to form single phase BLFO ceramics. The rise of sintering temperature enhances the densification and increases the grain size. The enhancement of densification in BLFO ceramics caused by the rise of sintering temperature makes the leakage current density obviously decrease. As sintering temperature rises, the leakage mechanism of BLFO ceramics changes from bulk-limited Poole-Frenkel emission to space-charge-limited bulk conduction. The remnant polarization of BLFO ceramics sintered at 820 C is much higher than that of the samples sintered at lower temperature. BLFO ceramics sintered at 800 C and 820 C show weak ferromagnetism. Compared with the sample sintered at 800 C, the spontaneous magnetization and saturation magnetization of the ceramic sample sintered at 820 C are higher, and the remnant magnetization and coercive magnetic field is slightly lower. These results indicate the higher microwave sintering temperature is benefit to enhancing the ferroelectric and magnetic properties and decreasing the leakage current of BLFO ceramics. Acknowledgements This work was supported by Excellent Talent Project in University of Chongqing (Grant No. 2017e35), the Science and Technology Innovation Project of Social Undertakings and People's Livelihood Guarantee of Chongqing (Grant No. cstc2017shmsA0192), the Program for Innovation Teams in University of Chongqing (Grant No. CXTDX201601032) and the Chongqing Research Program of Basic Research and Frontier Technology (Grant No. CSTC2018jcyjAX0416, CSTC2016jcyjA0175, CSTC2016jcyjA0349).
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