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Effects of microwave sintering power on microstructure, dielectric, ferroelectric and magnetic properties of bismuth ferrite ceramics
Journal of Alloys and Compounds 554 (2013) 64–71
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Journal of Alloys and Compounds journal homepage: www.elsevier.com/locate/jalcom
Effects of microwave sintering power on microstructure, dielectric, ferroelectric and magnetic properties of bismuth ferrite ceramics Wei Cai a,⇑, Chunlin Fu a,b, Wenguang Hu a, Gang Chen a, Xiaoling Deng a a b
School of Metallurgical and Materials Engineering, Chongqing University of Science and Technology, Chongqing 401331, China State Key Laboratory of Electronic Thin Films and Integrated Devices, University of Electronic Science and Technology of China, Chengdu 610054, China
a r t i c l e
i n f o
Article history: Received 28 August 2012 Received in revised form 20 November 2012 Accepted 24 November 2012 Available online 2 December 2012 Keywords: Bismuth ferrite Microwave sintering Dielectric Ferroelectric Magnetic
a b s t r a c t Multiferroic bismuth ferrite ceramics were fabricated via microwave sintering. The microstructure, dielectric, ferroelectric and magnetic properties of bismuth ferrite ceramics sintered at different microwave powers are characterized by X-ray diffraction, scanning electron microscope, impedance analyzers, ferroelectric test system and vibrating sample magnetometer. Bismuth ferrite ceramics sintered at 3.4 kW is single phase and has dense structure and uniform grains. The lower microwave sintering power for bismuth ferrite ceramics is benefit for the decrease of its dielectric loss. The remnant polarization and coercive electric field of bismuth ferrite ceramics decrease with the increasing of microwave sintering power. The remnant polarization and the coercive electric field of bismuth ferrite ceramics decrease simultaneously as frequency increases. The leakage current of bismuth ferrite ceramics increases with the increase of microwave sintering power. Bismuth ferrite ceramics prepared by microwave sintering exhibit typical antiferromagnetic behaviors and the remnant magnetization and coercive magnetic field increase as the microwave sintering power increases. It is inferred that the optimum microwave sintering power for bismuth ferrite ceramics is 3.4 kW. Ó 2012 Elsevier B.V. All rights reserved.
1. Introduction In recent years, multiferroic materials have received great attention because of the coexistence of ferroelectricity, (anti)ferromagnetism and ferroelasticity in a certain range of temperature [1,2]. Among them, bismuth ferrite (BiFeO3, short for BFO) is the only material exhibiting magnetic and ferroelectric properties at room temperature. BFO multiferroic materials demonstrates some promising and potential applications for ferroelectric random access memory, spintronics, solar energy devices and magnetoelectric devices (such as electrically controlled microwave phase shifters, broadband magnetic field sensors and magnetoelectric memory cells) due to its large remnant polarization (95 lC/ cm2) [3], the high ferroelectric Curie temperature (830 °C), the relative high antiferromagnetic Neel transition temperature (370 °C) [4], multiferroic property and the narrow band gap (2.3 eV) [5]. But these practical applications of BFO materials have been hindered because of its large leakage current as a result of the small
⇑ Corresponding author. Address: School of Metallurgical and Materials Engineering, Chongqing University of Science and Technology, Huxi Town, Shapingba, Chongqing 401331, China. Tel.: +86 23 65023479; fax: +86 23 65023706. E-mail address: [email protected] (W. Cai). 0925-8388/$ - see front matter Ó 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.jallcom.2012.11.154
amounts of Fe2+ and oxygen vacancies and difficulty in synthesizing single-phase BFO. BFO ceramics can be fabricated by conventional solid state reaction [6]. But in the process of solid state reaction, the kinetics of phase formation often leads to impurity phases, such as Bi2Fe4O9, Bi25FeO39, Bi46Fe2O72 and Bi2O3 due to the volatilization of some reactants and phase decompositions at high temperature [7]. In order to stabilize the perovskite phase and enhance the electrical, magnetic properties of BFO, a wide variety of dopants (such La, Nd, Sm, Gd, Nb [8,9], Ru [10], Al [11], Co [12], Ba [13], Ce [14], F [15] and so on) and other perovskites (such as BaTiO3 [16], CaTiO3 [17] and so on) have been added. Moreover, many alternative methods have been attempted to prepare singlephase BFO, including rapid liquid-phase sintering [18,19], sparkplasma sintering [20,21], hydrothermal synthesis [22,23], sol–gel method [24], and others [25]. The high heating rate for synthesis of single-phase BFO materials is necessary according to the kinetics of phase formation. Microwave sintering for ceramics is superior to conventional sintering owing to its unique characteristics, such as rapid heating, enhanced densification rate and improved microstructure [26,27]. Microwave heating differs significantly from conventional heating. In the microwave sintering process, the heat is generated internally within the material instead of originating from external sources, and hence there is an inverse heating profile [28]. The heating is very rapid as the material is heated by energy conversion rather than by energy transfer, which occurs in conven-
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tional techniques. Murty et al. [25] prepared the nanocrystalline BFO ceramics with weak ferromagnetism by high energy ball milling and microwave sintering. But there are still small amounts of secondary phase (Bi2Fe4O9) in the BFO ceramics. In this study, BFO ceramics were prepared by microwave sintering. Effects of microwave sintering power on microstructure, dielectric, ferroelectric and magnetic properties of BFO ceramics were studied in detail. Based on our findings, the optimum microwave sintering power for BFO ceramics was obtained.
2. Experimental BFO ceramics were prepared by microwave sintering process. The starting raw chemicals were high purity Bi2O3 (P99.9%, Sinopharm Group Co. Ltd.) and Fe2O3 (P99.9%, Sinopharm Group Co. Ltd.) powders. Bi2O3 and Fe2O3 were carefully weighed in stoichiometric proportion (1:1) and were added into ball milling jar, then milled for 2 h in distilled water and zirconia media. After the slurry was dried, the mixture was calcined in an alumina crucible at 600 °C for 2 h in muffle furnace (TM-0617P, Michem, China). The calcined powders were remilled for 2 h and then dried. The powders added with 7 wt.% binder were compacted into disk-shaped pellets with a diameter of 10.0 mm and thickness of 1.0 mm at 20 MPa pressure. The green pellets were sintered using a microwave sintering furnace (WLD3S-09, Nanjing Sanle, China, 4 kW, 2.45 GHz, single mode). Fig. 1 is the schematic illustration of the microwave sintering system. These green pellets were placed into the Al2O3 crucible filled with Al2O3 hollow spheres. And then the Al2O3 crucible was placed into the mullite sagger filled with Al2O3 fiber cotton. The several SiC rods as auxiliary heating materials were introduced into the Al2O3 fiber cotton. The temperature was measured with infrared thermometer. The microwave sintering temperature was controlled by adjusting microwave power. We initially prepared the samples sintered at 3 and 3.3 kW microwave powers. But the result shows that the sample sintered at the low microwave power (<3.4 kW) can not form the ceramics. Therefore, the BFO ceramics were sintered at different microwave powers (3.4–4 kW) for 35 min (The time is the interval from open-microwave to close-microwave) in air. The BFO ceramics sintered at 3.4, 3.7 and 4 kW are denoted as BFO1, BFO2 and BFO3, respectively. The crystal structure of ceramic sample was confirmed by X-ray diffractometer (XRD, DX-2700, Dandong Fangyuan, China) with Cu Ka (k = 0.15418 nm) radiation in a wide range of 2h (20° 6 2h 6 80°). The density of the BFO ceramics was measured by Archimede’s method in distilled water at room temperature. Surface morphology of the sintered samples was examined by scanning electron microscope (SEM, S-3700N, Hitachi, Japan). The elemental content for the sample was determined by energy-dispersive spectroscopy (EDS, ApolloII, EDAX, USA). In order to measure the dielectric and ferroelectric properties, silver paste was painted on the polished sintered samples as the electrodes and fired at 500 °C for 15 min. The capacitances of the ceramics were determined by impedance analyzer (LCR, HP 4980A, Agilent, USA) at 1 V/mm from 60 to 160 °C. The dielectric constant was calculated from the capacitance using the following equation:
e ¼ Cd=e0 A
3. Results and discussion 3.1. Microstructure Fig. 2 shows the XRD patterns of BFO ceramics sintered at different microwave powers. It is seen that the main diffraction peaks in BFO1, BFO2 and BFO3 ceramics can be successfully indexed as a rhombohedral distortion perovskite structure (BiFeO3, JCPDS Card No. 71-2494). No secondary phase is observed in the BFO1 ceramics. But there are small amount of secondary phase identified as Bi2Fe4O9 (JCPDS Card No. 20-0836 marked with ⁄in Fig. 2) in the BFO2 and BFO3 ceramics, which is commonly observed in this system [22,29]. The result indicates that the higher microwave power (>3.4 kW) corresponding to the higher sintering temperature could lead to the formation of impurity phase. Table 1 shows the density of BFO ceramics sintered at different microwave powers. It is found that the relative density of BFO1 ceramics is the maximum (95.2%) and the density decreases with the increase of microwave sintering power. It indicates that microwave sintering for BFO ceramics with higher density is feasible and the relative low microwave sintering power is better for forming dense structure. The surface morphologies of BFO ceramics sintered at different microwave powers are shown in Fig. 3(a)–(c). Firstly, BFO1 ceramics is dense, but there are some pores in BFO2 and BFO3 ceramics. The result indicates that the density of BFO ceramics decreases with the increase of microwave sintering power. Secondly, the grains of BFO ceramics prepared by microwave sintering are more uniform than that of the BFO ceramics sintered by conventional muffle furnace [30,31]. The average grain sizes of BFO1, BFO2 and BFO3 ceramics by lineal intercept procedure are 0.6, 1 and 1.6 lm, respectively, and the grain size of BFO ceramics pre-
ð1Þ
where C is the capacitance (F), e0 the free space dielectric constant value (8.85 1012 F/m), A represents the capacitor area (m2) and d represents the thickness (m) of the ceramics. The ferroelectric and leakage measurements were performed out using ferroelectric test system (TF2000e, aixACCT, Germany). The magnetic hysteresis loop was conducted using vibrating sample magnetometer (VSM, EV-11, Microsense, USA).
Fig. 2. XRD patterns of the BFO ceramics sintered at different microwave powers.
Table 1 Density of BFO ceramics prepared by microwave sintering.
Fig. 1. Schematic diagram of microwave sintering.
Sample
Density (g/cm3)
Relative density (%)
BFO1 BFO2 BFO3
7.91 7.77 7.51
95.2 93.5 90.4
Theoretical density of BFO at room temperature is 8.31 g/cm3.
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Fig. 3. Surface morphologies (a) BFO1, (b) BFO2, (c) BFO3 and elemental content, (d) A region at BFO1, (e) B region at BFO2, (f) C region at BFO3 and (g) D region at BFO3 sintered at different microwave powers.
pared by microwave sintering is much smaller than that of the ceramics sintered by conventional muffle furnace [30,31]. The result indicates that the single-phase BFO ceramics with smaller and more uniform grain can be prepared by microwave sintering.
The elemental contents for BFO ceramics sintered at different microwave powers are shown in Fig. 3(d)–(g). Firstly, it is found that there is the greater deviation of oxygen content for BFO1, BFO2 and BFO3 ceramics. It may be because that EDS is only a
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Fig. 4. The temperature dependences of dielectric constant and dielectric loss for BFO ceramics sintered at different microwave powers and measured at different frequencies: (a) 10 kHz, (b) 100 kHz, (c) 500 kHz, (d) 1000 kHz, (e) BFO1, (f) BFO2 and (g) BFO3.
semi-quantitative method. The elemental content of light element such as oxygen is too difficult to determine accurately. Secondly, it is seen that the atomic ratio of Bi: Fe is 1:1 according to the EDS result of A region for BFO1 ceramics (shown in Fig. 3(d)). It indicates that the phase of BFO1 ceramics is BiFeO3. But the atomic ra-
tio of Bi: Fe at B region for BFO2 and C region for BFO3 is 1.02:1 and 1.01:1, respectively (shown in Fig. 3(e) and (f)). It is worthwhile to note that the atomic ratio of Bi: Fe at D region for BFO3 ceramics is 0.87:1. These results indicate that impurity phase may be present in BFO ceramics sintered at higher microwave power. Moreover,
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the oxygen content decreases with the increasing of microwave sintering power (shown in Fig. 3(d)–(f)). It indicates that the higher microwave sintering power, that is, the higher sintering temperature may lead to the increase of oxygen vacancy. 3.2. Dielectric properties Fig. 4 shows the temperature dependence of dielectric properties for BFO ceramics sintered at different microwave powers and measured at different frequencies. Firstly, the dielectric constant of BFO ceramics sintered at 3.4–4 kW increases simultaneously
with the increase of temperature (60160 °C). But the Neel transition temperature and the ferroelectric Curie temperature of BFO are out of range of our test system (>160 °C) [32] so that these temperatures can not been determined. The dielectric loss of BFO1 and BFO2 ceramics measured at 101000 kHz increases with the increase of temperature (shown in Fig. 4(e) and (f)). But there is the peak of dielectric loss for BFO3 ceramics and the corresponding temperature at the peak of dielectric loss shifts to the higher temperature region as frequency increases (shown in Fig. 4(g)). It indicates that there is obvious frequency dispersion for BFO3 ceramics. The result is similar to the results presented by Dai [33]. Secondly,
Fig. 5. Room temperature hysteresis loops of BFO ceramics sintered at different microwave powers and measured at two frequncies: (a) 100 Hz and (b) 500 Hz.
Fig. 6. Room temperature hysteresis loops of BFO ceramics sintered at different microwave powers and measured at various frequencies: (a) BFO1, (b) BFO2 and (c) BFO3.
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the dielectric constant of BFO1, BFO2 and BFO3 ceramics decreases as frequency increases. It is due to the different polarization mechanisms at different frequencies. At low frequency, electron displacement polarization, ion displacement polarization, turningdirection polarization and space charge polarization contribute to dielectric constant. At high frequency, dielectric constant just results from the electron displacement polarization [34]. Moreover, it is found that the dielectric constant of BFO3 ceramics at 101000 kHz is larger than that of the BFO1 and BFO2 ceramics. It is attributed to the effect of grain size on dielectric constant. It is well known that the dielectric constant of the grain is larger than that of the grain boundary [35]. As mentioned above, the grain size of BFO3 ceramics is the maximum, and the ratio of the grain boundary in BFO3 ceramics is the minimum, hence the dielectric constant is the maximum. The dielectric loss of BFO1 ceramics measured at 500 and 1000 kHz is the minimum among the BFO ceramics (shown in Fig. 4(c) and (d)), and the dielectric loss of BFO1 ceramics is lower than that of BFO2 ceramics when frequency is 10 and 100 kHz (shown in Fig. 4(a) and (d)). It results from the density, oxygen vacancy concentration and secondary phase of BFO ceramics sintered at different microwave powers. Firstly, according to results of Archimede’s method and surface morphologies, the density of BFO1 ceramics is more than that of BFO2 and BFO3 ceramics. Secondly, there are oxygen vacancies caused by the volatilization of bismuth in the preparation process of BFO ceramics. The lower microwave sintering power corresponding to the lower sintering temperature leads to the lower oxygen vacancy concentration. Thirdly, the existence of secondary phase (Bi2Fe4O9) for BFO2 and BFO3 leads to the increase of dielectric loss. Therefore, the denser structure, lower oxygen vacancy concentration and no secondary phase for BFO1 ceramics makes its dielectric loss lower. 3.3. Ferroelectric properties Fig. 5 shows room temperature hysteresis loops of BFO ceramics sintered at different microwave powers and measured at two frequencies. Firstly, the lossy type hysteresis loops of BFO ceramics indicate that there is low electrical resistivity for BFO ceramics prepared by microwave sintering. The large loss is related to the high leakage in BFO ceramics. It is well known that the valence fluctuation of iron ions (i.e., from 3+ to 2+) and the oxygen vacancy formed by the volatilization of bismuth lead to large leakage in the processing of BFO [8,36]. Secondly, the remnant polarization (2Pr) of BFO1, BFO2 and BFO3 ceramics at 100 Hz is 4.64, 3.76 and 3.15 lC/cm2, respectively. It indicates that the remnant polarization of BFO ceramics decreases with the increasing of microwave sintering power. It is due to density and impurity effects of BFO ceramics. On the one hand, as mentioned above, BFO1 ceramics is denser than BFO2 and BFO3 ceramics. There are more ferroelectric domains in BFO1 ceramics with denser structure, which enhance the effective contribution to total polarization. On the other hand, Bi2Fe4O9 is paraelectric phase at room temperature because the Curie temperature of Bi2Fe4O9 is 23 °C [37]. Therefore, the remnant polarization of BFO2 and BFO3 ceramics with paraelectric Bi2Fe4O9 secondary phase is lower than that of BFO1 ceramics. It is noteworthy that the decreased extent of remnant polarization for BFO ceramics measured at 500 Hz with the increase of microwave sintering power is much lower than that measured at 100 Hz (shown in Fig. 5(b)). Thirdly, the coercive electric field (2Ec) of BFO1, BFO2 and BFO3 ceramics is 19.42, 15.07 and 14.94 kV/cm, respectively. The result indicates the coercive electric field of BFO ceramics decreases with the increase of microwave sintering power. It could be attributed to grain size. Energy barrier for switching ferroelectric domain must be broken through and it increases with the decrease of grain size. So the reversal polariza-
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tion process of a ferroelectric domain is more difficult inside a small grain than in a large grain [38]. According to fig. 3, the grain size of BFO ceramics increases as microwave sintering power increases. Fig. 6 shows the room temperature hysteresis loops of BFO ceramics sintered at different microwave powers and measured at various frequencies. It is found that the hysteresis loops of BFO ceramics become slimmer with the increasing of frequency, which indicates that the remnant polarization and the coercive electric field decrease as frequency increases. The strong frequency dependence for BFO ceramics indicates that the polarization of space charge caused by oxygen vacancy is the dominant. At low frequency, there are electron displacement, ion displacement, turning-direction polarization and space charge polarization in BFO ceramics. But as frequency increases, the space charge polarization can not keep up with change of electric field so that the remnant polarization and the coercive field decrease [39]. Fig. 7 shows the leakage current density of BFO ceramics sintered at different microwave powers. It is found that the leakage current density (J) of BFO ceramics increases with the increase of microwave sintering power. The leakage current density of BFO3 ceramics is much higher than that of BFO1 and BFO2 ceramics. It could be attributed to three reasons. Firstly, the denser structure of BFO ceramics sintered at lower microwave power leads to the lower leakage current for BFO1 and BFO2 ceramics. Secondly, the current leakage of BFO ceramics was indeed decreased with the decreasing of grain size [40]. As mentioned above, the grain size of BFO ceramics increases with the increase of microwave power so that leakage current increases as microwave sintering power increases. Thirdly, the lower oxygen vacancy concentration of BFO ceramics sintered at lower microwave sintering power leads to the decrease of the leakage. According to the above analysis result, it is evident that the BFO1 ceramics have excellent electric properties. Hence, the optimum microwave sintering power for BFO ceramics is 3.4 kW.
3.4. Magnetic properties Fig. 8 shows the room temperature magnetic hysteresis loops of BFO ceramics sintered at different microwave powers. According to Fig. 8, it is clear that the BFO ceramics exhibits a typical antiferromagnetic behavior, i.e. M depending linearly on H and no hysteresis of M on H. The result is consistent with other reports [41–43]. Although the crystal structure of BFO makes the appearance of
Fig. 7. Leakage current of BFO ceramics sintered at different microwave powers.
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Fig. 8. Room temperature magnetic hysteresis loop of BFO ceramics sintered at different microwave powers: (a) BFO1, (b) BFO2 and (c) BFO3.
weak ferromagnetism arising from the canting of the antiferromagnetic sublattices, the spiral spin structure leads to a cancellation of the macroscopic magnetization [36]. But Murty et al. prepared the BFO ceramics with 20 nm average grain size by microwave sintering and found that the sample has ferromagnetic order similar to BiFeO3 thin films [25]. It is due to that the grain sizes of BFO ceramics sintered at 3.4–4 kW are much larger than the wavelength (62 nm) of the incommensurate cycloid spin structure [44]. Moreover, it is found that the remnant magnetization (2Mr) and coercive magnetic field (2Hc) of BFO1, BFO2 and BFO3 ceramics are 5 105 emu/g and 6.3 Oe, 1 104 emu/g and 10 Oe, 1.4 104 emu/g and 18 Oe, respectively. The result indicates that the remnant magnetization and coercive magnetic field of BFO ceramics increase as the microwave sintering power increases. It may be due to the existence of secondary phase (Bi2Fe4O9) for BFO2 and BFO3 ceramics. 4. Conclusions BFO multiferroic ceramics were prepared by microwave sintering. The crystal structure, surface morphologies, dielectric, ferroelectric and magnetic properties of BFO ceramics sintered at different microwave powers were investigated. The BFO ceramics sintered at 3.4 kW is single phase, and the denser structure and smaller and more uniform grains are obtained. The dielectric loss of BFO ceramics sintered at 3.4 kW is lower than that of the samples sintered at higher microwave power. The remnant polarization and coercive electric field of BFO ceramics decrease with the increasing of microwave sintering power. Moreover, the remnant polarization and the coercive electric field of BFO ceramics decrease as frequency increases, which may be attributed to different
polarization mechanisms. The leakage current density of BFO ceramics increases with the increase of microwave sintering power. It is the common-effect of grain size, density and oxygen vacancy. BFO ceramics prepared by microwave sintering exhibit typical antiferromagnetic behaviors and the remnant magnetization and coercive magnetic field increase as the microwave sintering power increases. Acknowledgements This work was supported by the National Natural Science Foundation of China (51102288), Natural Science Foundation of Chongqing, China (CSTC2010BB4286 and CSTC2011BA4027), Open Research Fund of State Key Laboratory of Electronic Thin Films and Integrated Devices (UESTC) (KFJJ201104), the Science and Technology Research Project of Chongqing Education Committee of Chongqing (KJ121408) and Research Foundation of Chongqing University of Science and Technology (CK2010Z05). References [1] X.W. Tang, J.M. Dai, X.B. Zhu, J.C. Lin, Q. Chang, D.J. Wu, W.H. Song, Y.P. Sun, J. Am. Ceram. Soc. 95 (2012) 538. [2] J.G. Wu, J. Wang, D.Q. Xiao, J.G. Zhu, ACS Appl. Mater. Inter. 4 (2012) 1182. [3] J. Wang, J.B. Neaton, H. Zheng, V. Nagarajan, S.B. Ogale, B. Liu, D. Viehland, D.G. Schlom, U.V. Waghmare, N.A. Spaldin, K.M. Rabe, M. Wuttig, R. Ramesh, Science 299 (2003) 1719. [4] A.Z. Simões, M.A. Ramirez, C.R. Foschini, F. Moura, J.A. Varela, E. Longo, Ceram. Int. 38 (2012) 3841. [5] P. Chen, X.S. Xu, C. Koenigsmann, A.C. Santulli, S.S. Wong, J.L. Musfeldt Nano, Letters 11 (2010) 4526. [6] A.K. Pradhan, K. Zhang, D. Hunter, J.B. Dadson, G.B. Louttis, P. Bhattacharya, R. Katiyar, J. Zhang, D.J. Sellmyer, U.N. Roy, Y. Cui, A. Burger, J. Appl. Phys. 97 (2005) 093903.
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Journal of Alloys and Compounds journal homepage: www.elsevier.com/locate/jalcom
Effects of microwave sintering power on microstructure, dielectric, ferroelectric and magnetic properties of bismuth ferrite ceramics Wei Cai a,⇑, Chunlin Fu a,b, Wenguang Hu a, Gang Chen a, Xiaoling Deng a a b
School of Metallurgical and Materials Engineering, Chongqing University of Science and Technology, Chongqing 401331, China State Key Laboratory of Electronic Thin Films and Integrated Devices, University of Electronic Science and Technology of China, Chengdu 610054, China
a r t i c l e
i n f o
Article history: Received 28 August 2012 Received in revised form 20 November 2012 Accepted 24 November 2012 Available online 2 December 2012 Keywords: Bismuth ferrite Microwave sintering Dielectric Ferroelectric Magnetic
a b s t r a c t Multiferroic bismuth ferrite ceramics were fabricated via microwave sintering. The microstructure, dielectric, ferroelectric and magnetic properties of bismuth ferrite ceramics sintered at different microwave powers are characterized by X-ray diffraction, scanning electron microscope, impedance analyzers, ferroelectric test system and vibrating sample magnetometer. Bismuth ferrite ceramics sintered at 3.4 kW is single phase and has dense structure and uniform grains. The lower microwave sintering power for bismuth ferrite ceramics is benefit for the decrease of its dielectric loss. The remnant polarization and coercive electric field of bismuth ferrite ceramics decrease with the increasing of microwave sintering power. The remnant polarization and the coercive electric field of bismuth ferrite ceramics decrease simultaneously as frequency increases. The leakage current of bismuth ferrite ceramics increases with the increase of microwave sintering power. Bismuth ferrite ceramics prepared by microwave sintering exhibit typical antiferromagnetic behaviors and the remnant magnetization and coercive magnetic field increase as the microwave sintering power increases. It is inferred that the optimum microwave sintering power for bismuth ferrite ceramics is 3.4 kW. Ó 2012 Elsevier B.V. All rights reserved.
1. Introduction In recent years, multiferroic materials have received great attention because of the coexistence of ferroelectricity, (anti)ferromagnetism and ferroelasticity in a certain range of temperature [1,2]. Among them, bismuth ferrite (BiFeO3, short for BFO) is the only material exhibiting magnetic and ferroelectric properties at room temperature. BFO multiferroic materials demonstrates some promising and potential applications for ferroelectric random access memory, spintronics, solar energy devices and magnetoelectric devices (such as electrically controlled microwave phase shifters, broadband magnetic field sensors and magnetoelectric memory cells) due to its large remnant polarization (95 lC/ cm2) [3], the high ferroelectric Curie temperature (830 °C), the relative high antiferromagnetic Neel transition temperature (370 °C) [4], multiferroic property and the narrow band gap (2.3 eV) [5]. But these practical applications of BFO materials have been hindered because of its large leakage current as a result of the small
⇑ Corresponding author. Address: School of Metallurgical and Materials Engineering, Chongqing University of Science and Technology, Huxi Town, Shapingba, Chongqing 401331, China. Tel.: +86 23 65023479; fax: +86 23 65023706. E-mail address: [email protected] (W. Cai). 0925-8388/$ - see front matter Ó 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.jallcom.2012.11.154
amounts of Fe2+ and oxygen vacancies and difficulty in synthesizing single-phase BFO. BFO ceramics can be fabricated by conventional solid state reaction [6]. But in the process of solid state reaction, the kinetics of phase formation often leads to impurity phases, such as Bi2Fe4O9, Bi25FeO39, Bi46Fe2O72 and Bi2O3 due to the volatilization of some reactants and phase decompositions at high temperature [7]. In order to stabilize the perovskite phase and enhance the electrical, magnetic properties of BFO, a wide variety of dopants (such La, Nd, Sm, Gd, Nb [8,9], Ru [10], Al [11], Co [12], Ba [13], Ce [14], F [15] and so on) and other perovskites (such as BaTiO3 [16], CaTiO3 [17] and so on) have been added. Moreover, many alternative methods have been attempted to prepare singlephase BFO, including rapid liquid-phase sintering [18,19], sparkplasma sintering [20,21], hydrothermal synthesis [22,23], sol–gel method [24], and others [25]. The high heating rate for synthesis of single-phase BFO materials is necessary according to the kinetics of phase formation. Microwave sintering for ceramics is superior to conventional sintering owing to its unique characteristics, such as rapid heating, enhanced densification rate and improved microstructure [26,27]. Microwave heating differs significantly from conventional heating. In the microwave sintering process, the heat is generated internally within the material instead of originating from external sources, and hence there is an inverse heating profile [28]. The heating is very rapid as the material is heated by energy conversion rather than by energy transfer, which occurs in conven-
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tional techniques. Murty et al. [25] prepared the nanocrystalline BFO ceramics with weak ferromagnetism by high energy ball milling and microwave sintering. But there are still small amounts of secondary phase (Bi2Fe4O9) in the BFO ceramics. In this study, BFO ceramics were prepared by microwave sintering. Effects of microwave sintering power on microstructure, dielectric, ferroelectric and magnetic properties of BFO ceramics were studied in detail. Based on our findings, the optimum microwave sintering power for BFO ceramics was obtained.
2. Experimental BFO ceramics were prepared by microwave sintering process. The starting raw chemicals were high purity Bi2O3 (P99.9%, Sinopharm Group Co. Ltd.) and Fe2O3 (P99.9%, Sinopharm Group Co. Ltd.) powders. Bi2O3 and Fe2O3 were carefully weighed in stoichiometric proportion (1:1) and were added into ball milling jar, then milled for 2 h in distilled water and zirconia media. After the slurry was dried, the mixture was calcined in an alumina crucible at 600 °C for 2 h in muffle furnace (TM-0617P, Michem, China). The calcined powders were remilled for 2 h and then dried. The powders added with 7 wt.% binder were compacted into disk-shaped pellets with a diameter of 10.0 mm and thickness of 1.0 mm at 20 MPa pressure. The green pellets were sintered using a microwave sintering furnace (WLD3S-09, Nanjing Sanle, China, 4 kW, 2.45 GHz, single mode). Fig. 1 is the schematic illustration of the microwave sintering system. These green pellets were placed into the Al2O3 crucible filled with Al2O3 hollow spheres. And then the Al2O3 crucible was placed into the mullite sagger filled with Al2O3 fiber cotton. The several SiC rods as auxiliary heating materials were introduced into the Al2O3 fiber cotton. The temperature was measured with infrared thermometer. The microwave sintering temperature was controlled by adjusting microwave power. We initially prepared the samples sintered at 3 and 3.3 kW microwave powers. But the result shows that the sample sintered at the low microwave power (<3.4 kW) can not form the ceramics. Therefore, the BFO ceramics were sintered at different microwave powers (3.4–4 kW) for 35 min (The time is the interval from open-microwave to close-microwave) in air. The BFO ceramics sintered at 3.4, 3.7 and 4 kW are denoted as BFO1, BFO2 and BFO3, respectively. The crystal structure of ceramic sample was confirmed by X-ray diffractometer (XRD, DX-2700, Dandong Fangyuan, China) with Cu Ka (k = 0.15418 nm) radiation in a wide range of 2h (20° 6 2h 6 80°). The density of the BFO ceramics was measured by Archimede’s method in distilled water at room temperature. Surface morphology of the sintered samples was examined by scanning electron microscope (SEM, S-3700N, Hitachi, Japan). The elemental content for the sample was determined by energy-dispersive spectroscopy (EDS, ApolloII, EDAX, USA). In order to measure the dielectric and ferroelectric properties, silver paste was painted on the polished sintered samples as the electrodes and fired at 500 °C for 15 min. The capacitances of the ceramics were determined by impedance analyzer (LCR, HP 4980A, Agilent, USA) at 1 V/mm from 60 to 160 °C. The dielectric constant was calculated from the capacitance using the following equation:
e ¼ Cd=e0 A
3. Results and discussion 3.1. Microstructure Fig. 2 shows the XRD patterns of BFO ceramics sintered at different microwave powers. It is seen that the main diffraction peaks in BFO1, BFO2 and BFO3 ceramics can be successfully indexed as a rhombohedral distortion perovskite structure (BiFeO3, JCPDS Card No. 71-2494). No secondary phase is observed in the BFO1 ceramics. But there are small amount of secondary phase identified as Bi2Fe4O9 (JCPDS Card No. 20-0836 marked with ⁄in Fig. 2) in the BFO2 and BFO3 ceramics, which is commonly observed in this system [22,29]. The result indicates that the higher microwave power (>3.4 kW) corresponding to the higher sintering temperature could lead to the formation of impurity phase. Table 1 shows the density of BFO ceramics sintered at different microwave powers. It is found that the relative density of BFO1 ceramics is the maximum (95.2%) and the density decreases with the increase of microwave sintering power. It indicates that microwave sintering for BFO ceramics with higher density is feasible and the relative low microwave sintering power is better for forming dense structure. The surface morphologies of BFO ceramics sintered at different microwave powers are shown in Fig. 3(a)–(c). Firstly, BFO1 ceramics is dense, but there are some pores in BFO2 and BFO3 ceramics. The result indicates that the density of BFO ceramics decreases with the increase of microwave sintering power. Secondly, the grains of BFO ceramics prepared by microwave sintering are more uniform than that of the BFO ceramics sintered by conventional muffle furnace [30,31]. The average grain sizes of BFO1, BFO2 and BFO3 ceramics by lineal intercept procedure are 0.6, 1 and 1.6 lm, respectively, and the grain size of BFO ceramics pre-
ð1Þ
where C is the capacitance (F), e0 the free space dielectric constant value (8.85 1012 F/m), A represents the capacitor area (m2) and d represents the thickness (m) of the ceramics. The ferroelectric and leakage measurements were performed out using ferroelectric test system (TF2000e, aixACCT, Germany). The magnetic hysteresis loop was conducted using vibrating sample magnetometer (VSM, EV-11, Microsense, USA).
Fig. 2. XRD patterns of the BFO ceramics sintered at different microwave powers.
Table 1 Density of BFO ceramics prepared by microwave sintering.
Fig. 1. Schematic diagram of microwave sintering.
Sample
Density (g/cm3)
Relative density (%)
BFO1 BFO2 BFO3
7.91 7.77 7.51
95.2 93.5 90.4
Theoretical density of BFO at room temperature is 8.31 g/cm3.
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Fig. 3. Surface morphologies (a) BFO1, (b) BFO2, (c) BFO3 and elemental content, (d) A region at BFO1, (e) B region at BFO2, (f) C region at BFO3 and (g) D region at BFO3 sintered at different microwave powers.
pared by microwave sintering is much smaller than that of the ceramics sintered by conventional muffle furnace [30,31]. The result indicates that the single-phase BFO ceramics with smaller and more uniform grain can be prepared by microwave sintering.
The elemental contents for BFO ceramics sintered at different microwave powers are shown in Fig. 3(d)–(g). Firstly, it is found that there is the greater deviation of oxygen content for BFO1, BFO2 and BFO3 ceramics. It may be because that EDS is only a
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Fig. 4. The temperature dependences of dielectric constant and dielectric loss for BFO ceramics sintered at different microwave powers and measured at different frequencies: (a) 10 kHz, (b) 100 kHz, (c) 500 kHz, (d) 1000 kHz, (e) BFO1, (f) BFO2 and (g) BFO3.
semi-quantitative method. The elemental content of light element such as oxygen is too difficult to determine accurately. Secondly, it is seen that the atomic ratio of Bi: Fe is 1:1 according to the EDS result of A region for BFO1 ceramics (shown in Fig. 3(d)). It indicates that the phase of BFO1 ceramics is BiFeO3. But the atomic ra-
tio of Bi: Fe at B region for BFO2 and C region for BFO3 is 1.02:1 and 1.01:1, respectively (shown in Fig. 3(e) and (f)). It is worthwhile to note that the atomic ratio of Bi: Fe at D region for BFO3 ceramics is 0.87:1. These results indicate that impurity phase may be present in BFO ceramics sintered at higher microwave power. Moreover,
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the oxygen content decreases with the increasing of microwave sintering power (shown in Fig. 3(d)–(f)). It indicates that the higher microwave sintering power, that is, the higher sintering temperature may lead to the increase of oxygen vacancy. 3.2. Dielectric properties Fig. 4 shows the temperature dependence of dielectric properties for BFO ceramics sintered at different microwave powers and measured at different frequencies. Firstly, the dielectric constant of BFO ceramics sintered at 3.4–4 kW increases simultaneously
with the increase of temperature (60160 °C). But the Neel transition temperature and the ferroelectric Curie temperature of BFO are out of range of our test system (>160 °C) [32] so that these temperatures can not been determined. The dielectric loss of BFO1 and BFO2 ceramics measured at 101000 kHz increases with the increase of temperature (shown in Fig. 4(e) and (f)). But there is the peak of dielectric loss for BFO3 ceramics and the corresponding temperature at the peak of dielectric loss shifts to the higher temperature region as frequency increases (shown in Fig. 4(g)). It indicates that there is obvious frequency dispersion for BFO3 ceramics. The result is similar to the results presented by Dai [33]. Secondly,
Fig. 5. Room temperature hysteresis loops of BFO ceramics sintered at different microwave powers and measured at two frequncies: (a) 100 Hz and (b) 500 Hz.
Fig. 6. Room temperature hysteresis loops of BFO ceramics sintered at different microwave powers and measured at various frequencies: (a) BFO1, (b) BFO2 and (c) BFO3.
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the dielectric constant of BFO1, BFO2 and BFO3 ceramics decreases as frequency increases. It is due to the different polarization mechanisms at different frequencies. At low frequency, electron displacement polarization, ion displacement polarization, turningdirection polarization and space charge polarization contribute to dielectric constant. At high frequency, dielectric constant just results from the electron displacement polarization [34]. Moreover, it is found that the dielectric constant of BFO3 ceramics at 101000 kHz is larger than that of the BFO1 and BFO2 ceramics. It is attributed to the effect of grain size on dielectric constant. It is well known that the dielectric constant of the grain is larger than that of the grain boundary [35]. As mentioned above, the grain size of BFO3 ceramics is the maximum, and the ratio of the grain boundary in BFO3 ceramics is the minimum, hence the dielectric constant is the maximum. The dielectric loss of BFO1 ceramics measured at 500 and 1000 kHz is the minimum among the BFO ceramics (shown in Fig. 4(c) and (d)), and the dielectric loss of BFO1 ceramics is lower than that of BFO2 ceramics when frequency is 10 and 100 kHz (shown in Fig. 4(a) and (d)). It results from the density, oxygen vacancy concentration and secondary phase of BFO ceramics sintered at different microwave powers. Firstly, according to results of Archimede’s method and surface morphologies, the density of BFO1 ceramics is more than that of BFO2 and BFO3 ceramics. Secondly, there are oxygen vacancies caused by the volatilization of bismuth in the preparation process of BFO ceramics. The lower microwave sintering power corresponding to the lower sintering temperature leads to the lower oxygen vacancy concentration. Thirdly, the existence of secondary phase (Bi2Fe4O9) for BFO2 and BFO3 leads to the increase of dielectric loss. Therefore, the denser structure, lower oxygen vacancy concentration and no secondary phase for BFO1 ceramics makes its dielectric loss lower. 3.3. Ferroelectric properties Fig. 5 shows room temperature hysteresis loops of BFO ceramics sintered at different microwave powers and measured at two frequencies. Firstly, the lossy type hysteresis loops of BFO ceramics indicate that there is low electrical resistivity for BFO ceramics prepared by microwave sintering. The large loss is related to the high leakage in BFO ceramics. It is well known that the valence fluctuation of iron ions (i.e., from 3+ to 2+) and the oxygen vacancy formed by the volatilization of bismuth lead to large leakage in the processing of BFO [8,36]. Secondly, the remnant polarization (2Pr) of BFO1, BFO2 and BFO3 ceramics at 100 Hz is 4.64, 3.76 and 3.15 lC/cm2, respectively. It indicates that the remnant polarization of BFO ceramics decreases with the increasing of microwave sintering power. It is due to density and impurity effects of BFO ceramics. On the one hand, as mentioned above, BFO1 ceramics is denser than BFO2 and BFO3 ceramics. There are more ferroelectric domains in BFO1 ceramics with denser structure, which enhance the effective contribution to total polarization. On the other hand, Bi2Fe4O9 is paraelectric phase at room temperature because the Curie temperature of Bi2Fe4O9 is 23 °C [37]. Therefore, the remnant polarization of BFO2 and BFO3 ceramics with paraelectric Bi2Fe4O9 secondary phase is lower than that of BFO1 ceramics. It is noteworthy that the decreased extent of remnant polarization for BFO ceramics measured at 500 Hz with the increase of microwave sintering power is much lower than that measured at 100 Hz (shown in Fig. 5(b)). Thirdly, the coercive electric field (2Ec) of BFO1, BFO2 and BFO3 ceramics is 19.42, 15.07 and 14.94 kV/cm, respectively. The result indicates the coercive electric field of BFO ceramics decreases with the increase of microwave sintering power. It could be attributed to grain size. Energy barrier for switching ferroelectric domain must be broken through and it increases with the decrease of grain size. So the reversal polariza-
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tion process of a ferroelectric domain is more difficult inside a small grain than in a large grain [38]. According to fig. 3, the grain size of BFO ceramics increases as microwave sintering power increases. Fig. 6 shows the room temperature hysteresis loops of BFO ceramics sintered at different microwave powers and measured at various frequencies. It is found that the hysteresis loops of BFO ceramics become slimmer with the increasing of frequency, which indicates that the remnant polarization and the coercive electric field decrease as frequency increases. The strong frequency dependence for BFO ceramics indicates that the polarization of space charge caused by oxygen vacancy is the dominant. At low frequency, there are electron displacement, ion displacement, turning-direction polarization and space charge polarization in BFO ceramics. But as frequency increases, the space charge polarization can not keep up with change of electric field so that the remnant polarization and the coercive field decrease [39]. Fig. 7 shows the leakage current density of BFO ceramics sintered at different microwave powers. It is found that the leakage current density (J) of BFO ceramics increases with the increase of microwave sintering power. The leakage current density of BFO3 ceramics is much higher than that of BFO1 and BFO2 ceramics. It could be attributed to three reasons. Firstly, the denser structure of BFO ceramics sintered at lower microwave power leads to the lower leakage current for BFO1 and BFO2 ceramics. Secondly, the current leakage of BFO ceramics was indeed decreased with the decreasing of grain size [40]. As mentioned above, the grain size of BFO ceramics increases with the increase of microwave power so that leakage current increases as microwave sintering power increases. Thirdly, the lower oxygen vacancy concentration of BFO ceramics sintered at lower microwave sintering power leads to the decrease of the leakage. According to the above analysis result, it is evident that the BFO1 ceramics have excellent electric properties. Hence, the optimum microwave sintering power for BFO ceramics is 3.4 kW.
3.4. Magnetic properties Fig. 8 shows the room temperature magnetic hysteresis loops of BFO ceramics sintered at different microwave powers. According to Fig. 8, it is clear that the BFO ceramics exhibits a typical antiferromagnetic behavior, i.e. M depending linearly on H and no hysteresis of M on H. The result is consistent with other reports [41–43]. Although the crystal structure of BFO makes the appearance of
Fig. 7. Leakage current of BFO ceramics sintered at different microwave powers.
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Fig. 8. Room temperature magnetic hysteresis loop of BFO ceramics sintered at different microwave powers: (a) BFO1, (b) BFO2 and (c) BFO3.
weak ferromagnetism arising from the canting of the antiferromagnetic sublattices, the spiral spin structure leads to a cancellation of the macroscopic magnetization [36]. But Murty et al. prepared the BFO ceramics with 20 nm average grain size by microwave sintering and found that the sample has ferromagnetic order similar to BiFeO3 thin films [25]. It is due to that the grain sizes of BFO ceramics sintered at 3.4–4 kW are much larger than the wavelength (62 nm) of the incommensurate cycloid spin structure [44]. Moreover, it is found that the remnant magnetization (2Mr) and coercive magnetic field (2Hc) of BFO1, BFO2 and BFO3 ceramics are 5 105 emu/g and 6.3 Oe, 1 104 emu/g and 10 Oe, 1.4 104 emu/g and 18 Oe, respectively. The result indicates that the remnant magnetization and coercive magnetic field of BFO ceramics increase as the microwave sintering power increases. It may be due to the existence of secondary phase (Bi2Fe4O9) for BFO2 and BFO3 ceramics. 4. Conclusions BFO multiferroic ceramics were prepared by microwave sintering. The crystal structure, surface morphologies, dielectric, ferroelectric and magnetic properties of BFO ceramics sintered at different microwave powers were investigated. The BFO ceramics sintered at 3.4 kW is single phase, and the denser structure and smaller and more uniform grains are obtained. The dielectric loss of BFO ceramics sintered at 3.4 kW is lower than that of the samples sintered at higher microwave power. The remnant polarization and coercive electric field of BFO ceramics decrease with the increasing of microwave sintering power. Moreover, the remnant polarization and the coercive electric field of BFO ceramics decrease as frequency increases, which may be attributed to different
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