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Synthesis and properties of barium ferrite nano-powders by chemical co-precipitation method
Accepted Manuscript Synthesis and properties of barium ferrite nano-powders by chemical co-precipitation method S.L. Hu, J. Liu, H.Y. Yu, Z.W. Liu PII: DOI: Reference:
S0304-8853(18)31694-9 https://doi.org/10.1016/j.jmmm.2018.10.044 MAGMA 64455
To appear in:
Journal of Magnetism and Magnetic Materials
Received Date: Revised Date: Accepted Date:
3 June 2018 22 August 2018 9 October 2018
Please cite this article as: S.L. Hu, J. Liu, H.Y. Yu, Z.W. Liu, Synthesis and properties of barium ferrite nanopowders by chemical co-precipitation method, Journal of Magnetism and Magnetic Materials (2018), doi: https:// doi.org/10.1016/j.jmmm.2018.10.044
This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Synthesis and properties of barium ferrite nano-powders by chemical co-precipitation method S.L. Hu1, J. Liu2, H.Y. Yu3, Z.W. Liu3. 1、School of Automotive Studies, Jiangxi College Of Applied Technology, Ganzhou 341000,China 2、School of Mechanical Engineering, Jiangxi College Of Applied Technology, Ganzhou 341000,China 3、School of Material Science and Engineering, South China University of Technology, Guangzhou 510640, China Abstract: Nano-powders with controllable particle size, excellent magnetic properties and thermal stability of barium hexaferrite (BaFe12O19) have been synthesized via a co-precipitation/calcination technique. The phase composition, morphology and magnetic/thermal properties of the products were systematically studied. XRD patterns reveal that a long co-precipitation reaction time (5 h) and high calcination temperature (1100C) are beneficial for the formation of BaFe12O19 phase and decreasing the tendency to agglomeration. SEM micrographs show that the products show a hexagonal flake-like particle shape and the size are well controlled and maintained at single-domain particle size range area(<460nm), above which the coercivity will decrease abruptly for the coupling exchange among particles. The products with jHc of 5934 Oe, temperature coefficient of remanence (αBr) of -0.176% K-1 and temperature coefficient of coercivity (βjHc) of 0.0427% K-1 were obtained when co-precipitated for 5 h and calcined at 900C for 2h. A high saturation magnetization of 66.9 emu/g was obtained when co-precipitated for 5 h and calcined at 1100C for 2 h, approaching the theoretical saturation magnetization (72 emu/g).
Key words: Nano-powders; Barium hexaferrite; Co-precipitation; Calcination; Single-domain particle
1. Introduction Barium hexaferrite, BaFe12O19, is a well-known permanent magnet with great technical importance and it has attracted an extensive attention for the last few decades. BaFe12O19 compound has a hexagonal structure with fairly large crystal anisotropy along c-axis. As a result, Ba ferrite has shown high intrinsic
Corresponding author, Tel: 020-22236906, E-mail: [email protected]
1
coercivity (6700 Oe), large saturation magnetization (72 emu/g) and high Curie temperature (450◦C)[1]. At present, it has been widely used in permanent magnetic materials, magnetic recording media materials and wave absorbing materials[2-5]. It has been reported that the magnetic properties of M-type BaFe12O19 ferrites are closely related to their particle size and morphology[6]. For this reason, the researchers have modified their properties by various approaches, for an example, preparing the nanocrystalline BaFe12O19 by different methods. In order to achieve highly homogeneous nano-particles of barium ferrite, various techniques such as chemical co-precipitation method[7], sol-gel method[8-10], glass crystallization method[11-12], hydrothermal method[13], micro-emulsion method[14] have been developed. Among them, the chemical co-precipitation method is a low cost technique suitable for the mass production compared to the other mentioned methods. However, many problems are still existing for this method. Usually, the particle size is not easy to control, the reaction and transformation are not complete, and the magnetic properties are not stable. In this work, nano-sized Ba ferrite powders with controllable particle size and good magnetic properties
have
been
synthesized
via
a
co-precipitation/calcination
technique.
Their
process-microstructure-properties relationships have been systematically investigated. 2. Experimental The ferrite precursors were obtained from aqueous mixtures of barium and ferric chlorides by co-precipitation technique. The detailed preparation process is as follows: guaranteed chemically grade ferric chloride (FeCl36H2O), barium chloride (BaCl22H2O) and sodium hydroxide (NaOH) were used as starting materials. Aqueous solutions of iron and barium chlorides with a Fe3+/Ba2+ molar ratio of 8 were co-precipitated by the addition of NaOH. The aqueous suspensions were stirred quickly (at constant 200 rpm) for 3 h, 4 h and 5 h to achieve good homogeneity and attain a stable pH (9) condition. The co-precipitated sample was washed to neutral by distilled water and dried at 50◦C for 12h subsequently. In order to form the hexaferrite phase, the dry precursors were heated at a rate of 20C /min up to different temperatures (700, 800, 900, 1000 and 1100C), where they were maintained for 2 h in a static air atmosphere. The phase structure was determined by X-ray diffraction (XRD, Philips X-pert) with Cu-Kα radiation. The microstructure and size of the products was characterized by scanning electronic microscope (SEM) (Nova NanoSEM 430). The microstructure was examined by transmission electron microscope (TEM, Philips F20) and high resolution TEM (HRTEM, Joel 3100). Magnetic/thermal properties were tested by 2
physical properties measurement system (PPMS-9, Quantum Design, USA) equipped with a 9 T vibrating sample magnetometer (VSM). The temperature dependence of magnetization was measured in the temperature range of 300-500 K. 3. Results and Discussions 3.1 Crystal structure The chemical co-precipitation method involves the following reaction: 12FeCl3 + BaCl2 + 38NaOH = 12Fe (OH)3 +38NaCl +Ba (OH)2, through which iron and barium hydroxide are obtained. When the precursors exposed to a high temperature (i.e. calcination), the following reactions will occur: 2Fe(OH)3 = Fe2O3 + 3H2O；Ba(OH)2 = BaO + H2O；BaO + Fe2O3 =BaO·Fe2O3；BaO·Fe2O3 + 5Fe2O3 = BaO·6Fe2O3 (BaFe12O19). Fig. 1 shows the XRD patterns for the samples co-precipitated for 3 h (a), 4 h (b) and 5 h (c) followed by calcining at different temperatures. For the samples co-precipitated for 3h (Fig. 1a) at a low calcination temperature of 700C, the relatively low peak intensity of BaFe12O19 phase indicates that only a small amount of crystallized BaFe12O19 phase formed. When calcined at a higher temperature (800-1100C), the peak intensity of BaFe12O19 phase enhanced significantly with increasing temperature. In addition, at a short co-precipitation reaction time of 3h, the α-Fe2O3 phase was found in all samples calcined at different temperatures (800-1100C). For the samples co-precipitated for 4 h (Fig. 1b), a higher calcination temperature promotes the formation of barium hexaferrite phase, but the impurity phase(α-Fe2O3) only disappeared at a temperature over 1000C. For the samples co-precipitated for 5 h (Fig. 1c), increasing the calcination temperature also enhances the formation of barium hexaferrite phase. In addition, Fig.1 also reveals that, to form BaFe12O19 phase, the calcination temperature above 800oC is required. This is consistent with the observations by Darja et al.[15] and Huang et al.[16]. The humps around 50 deg for all samples may be attributed to the Ba2Fe2O5 phase, which is a minor impurity.
3
Fig.1 XRD patterns for the samples co-precipitated for 3 h (a), 4 h (b) and 5 h (c) followed by calcining at different temperatures
3.2 Microstructure Fig.2 is the TEM images and diffraction patterns of the precursors obtained at different co-precipitation reaction time. From the images, the precursors present a flocculent structure and rings of light and dark are detected. Meanwhile, the rings of light and dark in the diffraction patterns become less obvious with increasing of the co-precipitation time, which indicated that the crystallization degree of the precursors decrease with the increasing of the reaction time. As mentioned above, the synthesis of precursors involves the following reaction: 12FeCl3 + BaCl2 + 38NaOH = 12Fe (OH)3↓+38NaCl +Ba (OH)2↓. On the one hand, due to the incomplete crystallization, some Ba ions will exist in a free state. On
4
the other hand, Ba (OH)2 precipitates belong to weak precipitates, the solubility of which is about 3.89g per 100g in water at 20◦C, Ba ions will be washed by distilled water before calcination. A conclusion can be made that most of the excess Ba did not participate in the chemical co-precipitation reaction combined with the XRD results shown above (no obvious Ba-rich phase was detected).
Fig.2 TEM images and diffraction patterns for precursors co-precipitated for 3 h(a), 4 h(b), 5 h(c)
The SEM micrographs of the samples co-precipitated for 3h and calcined at different temperatures are given in Fig. 3(a–d). It can be seen that the size of hexagonal flake-like nano-particles increases with the increase of calcination temperature. When the calcination temperature rises to 1100C, the particles no longer exist in a hexagonal flake-like structure but a rod-like structure (as shown in Fig. 3d). There is a huge difference between the two structures. Fig. 4 shows the SEM images of the samples co-precipitated for 5h and calcined at different temperatures. The size of hexagonal flake-like nano-particles are well controlled and maintained at single-domain particle size range area (4nm~460nm[17]), which is beneficial
5
for the improvement of coercivity. In addition, with the increase of calcination temperature, the aggregation of the particles tends to decrease. The formation of the hexagonal flake structure has been well studied[18] and the rod-like structure appeared only in the sample contained α-Fe2O3 phase at a high calcination temperature. It is, therefore, reasonable to consider that α-Fe2O3 phase has a significant effect on surface energy and results in the increasing agglomeration behavior of nanoparticles when co-precipitated for 3 h and calcined at 1100C for 2 h.
Fig.3 SEM micrographs for the samples co-precipitated for 3 h and calcined at different temperatures: (a) 800 C, (b) 900C, (c) 1000C, and (d) 1100C
6
Fig.4 SEM images for samples co-precipitated for 5 h and calcined at different temperatures: (a) 800C, (b) 900C, (c) 1000C, and (d) 1100C
Fig. 5a describes the dependences of average particle size d on the calcination temperature obtained form the precursors prepared by co-precipitation reaction for 3 h and 5 h, respectively. The average particle size of calcined nano-powders obtained by co-precipitation reaction for 3 h is larger than that of 5 h with same calcination temperature. When the calcination temperature is relatively low (800~900C), the average particle size of calcined powders increases slowly. When the calcination temperature is higher than 900C, the average particle size of the product increases sharply with the increase of temperature, but it is still smaller than the critical size of the single domain particles of BaFe12O19 (≈460 nm). Fig. 5b shows the detailed particle size distribution of the sample co-precipitated for 5 h and calcined at 900◦C for 2 h. Most particles have the size range from 64 to76 nm.
7
Fig.5 Dependence of the average particle size d on the calcination temperature for samples co-precipitated for 3 h and 5 h (a) and the detailed particle size distribution of the sample co-precipitated for 5 h and calcined at 900◦C (b)
3.3 Magnetic properties The hysteresis loops for the samples co-precipitated for 3 h (a) and 5 h (b) followed by calcining at different temperatures are given in Fig. 6. All the calcined nano-powders exhibit hard magnetic properties, but co-precipitation reaction time and calcination temperature have a great influence on magnetic properties. The hysteresis loop in second quadrant decreases sharply for powders co-precipitated for 3h and calcined at 800C for 2 h (Fig .6a), which directly leads to the decrease of coercivity. The hysteresis loop in second quadrant also decreases sharply for powders co-precipitated for 5h and calcined at 800C for 2 h (Fig .6b). It is considered that the low degree crystallization of BaFe12O19 phase at a relatively low calcination temperature is the main reason, which has been verified by the XRD results shown above. Meanwhile, the hysteresis loop of the powders becomes smoother with the increase of calcination temperature.
Fig.6 Hysteresis loops for samples co-precipitated for 3 h (a) and 5 h (b) followed by calcining at different temperatures
The results of saturation magnetization Ms, remnant magnetization Mr, coercivity jHc and average particle size d for the samples co-precipitated for 3 h and 5 h and calcined at different temperatures are summarized in Table 1. The values of Ms and Mr increase with the increase of calcination temperature. It is considered that the increasing volume fraction of BaFe12O19 phase and decreasing volume fraction of α-Fe2O3 contribute to the results. The nano-powders with coercivity (jHc) of 5934 Oe, saturation magnetization (Ms) of 61.8 emu/g, remnant magnetization (Mr) of 28.2 emu/g, and average particle size (d) of 70nm were obtained when co-precipitated for 5 h and calcined at 900C for 2 h. 8
The dependences of jHc and Ms on the calcination temperature are given in Fig.7. A high saturation magnetization of 66.9 emu/g is obtained when co-precipitated for 5 h and calcined at 1100C for 2 h, approaching the theoretical saturation magnetization (72 emu/g[19]). The maximum value of jHc is obtained when calcined at 900C for 2 h, no matter the coprecipitation reaction time is 3 h or 5 h and the maximum average particle size in the products is 196 nm, except for the rod-like shape, according to Table 1. It was reported that the value of the critical size of BaFe12O19 single-domain particle is 460nm and there is an inferior limit for the single-domain particle size of BaFe12O19(≈4 nm), below which a superparamagnetic state appears, characterized by zero coercivity[17]. So the products size are well controlled and maintained at single-domain particle size range area (4~460nm), above which the coercivity will decrease abruptly for the coupling exchange among particles. It is considered that the decrease of coercivity is closely related to the increase size of particle and coupling exchange among particles when calcined at high temperature over 900C.
Fig. 7 Dependences of jHc and Ms on the calcination temperature
Table 1 Saturation magnetization Ms, remnant magnetization Mr, coercivity jHc and particle size d for the samples co-precipitated for 3 h and 5 h and calcined at different temperatures Calcined temperature
co-precipitated for 3h
co-precipitated for 5h
Ms
jHc
Mr
d
Ms
jHc
Mr
d
( C)
(emu/g)
(Oe)
(emu/g)
(nm)
(emu/g)
(Oe)
(emu/g)
(nm)
800
45.6
3284
20.1
75
45.5
3927
19.3
63
900
52.9
5230
24.8
103
61.8
5934
28.2
70
9
1000
59.2
3939
27.0
196
65.8
5090
30.6
136
1100
58.7
3731
27.8
Rod-like shape
66.9
4311
31.9
153
3.4 Temperature stability The thermal stability of the calcined nano-powders can be evaluated by the temperature coefficients of remanence α and the temperature coefficients of coercivity β, where α=[ Br(500K) - Br(300K)] / [ Br(300K) ×(500-300)] ×100%, and β=[ jHc(500K ) - jHc(300K)]/[ jHc(300K)×(500-300)] ×100%, for the temperature of 300~500K, and low absolute values of the coefficients suggest high thermal stability[20]. BaFe12O19 has a hexagonal structure with fairly high Curie temperature (750K). In order to study the temperature stability of the nano-powders, the magnetic properties at high temperatures for the calcined nano-powders were characterized. Fig. 8 reveals magnetic hysteresis curves for nano-powders (precursors co-precipitated for 5h) calcined at 900C (a) and 1000C (b). Table 2 list the value of magnetic properties, the temperature coefficient of remanence (αBr), and the temperature coefficient of coercivity (βjHc) at 300K and 500K for samples co-precipitated for 5 h with calcined at 900C and 1000C. The value (βjHc) of both products are positive, due to the fact that the saturation magnetization of barium ferrite decreases more with temperature than that of anisotropy constant K1 in a low temperature range. Therefore, the coercivity of BaFe12O19 increases with the increase of temperature within a certain range, while most of other permanent magnetic materials decreases, which is an advantage of application in high temperature area[21]. For precursors co-precipitated for 5 h, subsequently calcined at 900C or 1000C, indicating an excellent high temperature property, for example, the αBr and βjHc values of the precursors calcined at 900C are -0.176 %K-1 and 0.0427 %K-1, respectively, and the the αBr and βjHc of the the precursors calcined at 1000C are -0.185%K-1 and 0.0734 %K-1, respectively. The results indicate that the coercivity of the two products is very stable at high temperature, which is suitable for magnetic recording medium.
10
Fig. 8 Magnetic hysteresis curves for nano-powders (precursors co-precipitated for 5h) calcined at 900C (a) and 1000C (b)
Table 2 Magnetic properties, temperature coefficient of remanence (αBr), temperature coefficient of coercivity (βjHc) at 300K and 500K for samples co-precipitated for 5h with calcined at 900C and 1000C Calcined
Br(emu/g)
jHc(Oe)
(BH)max(MGs·Oe)
temperature
αBr(%K-1)
βjHc(%K-1)
300K
500K
300K
500K
300K
500K
900
29.6
19.2
5202
5647
0.800
0.358
-0.176
0.0427
1000
29.0
18.3
3104
3560
0.723
0.323
-0.185
0.073
（C）
4. Conclusion Barium hexaferrite (BaFe12O19) nanoparticles with controllable particle size, excellent magnetic properties and thermal stability of have been synthesized by a co-precipitation/calcination technique： (1) The XRD results indicated that the formation of barium hexaferrite and α-Fe2O3 phase occurs simultaneously at relatively short co-precipitation reaction time(3 h or 4 h) followed by calcining at different temperatures. Meanwhile, increased calcining temperature (700-1100C) improved the crystallization of BaFe12O19 phase and resulted in the single phase. (2) The performance of BaFe12O19 particles is closely related to reaction time of chemical co-precipitation and calcination temperature. The particle size increased with increasing calcination temperature while co-precipitation reaction time is the same. Meanwhile, the particle size also increased with increasing co-precipitation reaction time while calcination temperature is the same. A high coercive force of 5934 Oe is obtained with a coprecipitation reaction time of 5 h and a calcination temperature of 900C. 11
(3) Using the precursors co-precipitated for 5 h and calcination temperature of 900C or 1000C, the ferrites with excellent high temperature properties can be obtained. The αBr and βjHc values of the precursors calcined at 900C are -0.176 %K-1 and 0.0427 %K-1, respectively, and the αBr and βjHc of the precursors calcined at 1000C are -0.185%K-1 and 0.0734 %K-1, respectively. These results indicate that they are suitable for using as magnetic recording medium materials.
Acknowledgement This work was supported by the Jiangxi Provincial Science and Technology Program (Grant No. GJJ171308) and the DongGuan Innovative Research Team Program (Grant No. 201536000200027)
References [1] M. Radwan, M.M. Rashad, M.M. Hessien. Synthesis and characterization of barium hexaferrite nanoparticles[J]. Journal of Materials Processing Technology, 2007, 181: 106~109 [2] Kubo O., Ido T., Yokoyama H. Properties of Ba ferrite particles for perpendicular magnetic recording media [J]. IEEE transactions on Magnetics, 1982, 18(6): 1122~1124 [3] Matsuoka M., Hoshi Y., Naoe M., et al. Formation of Ba-ferrite films with perpendicular magnetization by targets-faing type of sputtering [J]. IEEE transactions on Magnetics, 1982, 18(6): 1119~1121 [4] Fujiwara T., Isshiki M., Koike Y., Oguchi T. Recording performances of Ba-ferrite coated perpendicular magnetic tapes [J]. IEEE Transactions on Magnetics, 1982, 18(6):1200~1202 [5] Ido T., Kubo O., Yokoyama H. Coercivity for Ba-ferrite superfine particles [J]. IEEE Transactions on Magnetics, 1986, 22(5): 704~706 [6] Kuzmitcheva T. G., Olkhovik L. P., Shabatin V. P. Synthesis and properties of fine Ba-ferrite powders [J]. IEEE Transactions on Magnetics, 1995, 31(1): 800~803 [7] Yamamoto H., Isono M., Kobayashi T. Magnetic properties of Ba-Nd-Co system M-type ferrite fine particles prepared by controlling the chemical coprecipitation method [J]. Journal of Magnetism and Magnetic Materials, 2005, 295(1): 51~56 [8] Ishikawa A., Tanahashi K., Futamoto M. Magnetic and structural properties of Ba-ferrite films prepared by sol-gel processing [J]. Journal of Applied Physics, 1996,79(9): 7080~7083 12
[9] Liu W. T., Wu J. M. The effect of the vacuum extraction and the Fe/Ba ratio on the phase formation of barium ferrite thin film synthesized by sol-gel method. Mater [J]. Journal of Chemical Physics, 2001, 69(1-3): 148~153 [10] Mu G. H., Chen N., Pan X. F., et al. Microwave absorption properties of hollow microsphere/titania/M-type Ba ferrite nanocomposites [J]. Applied Physics Letters, 2007, 91(4): 43110~43113 [11] Taubert J. Hergt R., Müller R., et al. Phase separation in Ba-ferrite glass ceramics investigated by Faraday microscopy [J]. Journal of Magnetism and Magnetic Materials, 1997, 168(1-2): 187~195 [12] Muller R., Hergt R., Dutz S., et al. Nanocrystalline iron oxide and Ba ferrite particles in the superparamagnetism-ferromagnetism transition range with ferrofluid applications [J]. Journal of Physics: Condensed Matter, 2006, 18(38): S2527~S2542 [13] Nam S. C., Park S. D., Kim G. J. Preparation of Ba-ferrite particles using the SuperCritical water crystallization method [J]. Industrial & Engineering Chemistry Research, 2001, 7(1): 38~43 [14] Gubin S. P., Koksharov Y. A., Khomutov G. B., et al. Magnetic nanoparticles: Preparation methods, structure and properties [J]. Uspekhi Khimii, 2005, 37(1): 539-574 [15] Darja L., Miha D. The Low-temperatrue formation of barium hexaferrites [J]. Journal of the European Ceramic Society, 2006, 26(16): 3681~3686 [16] Huang J. G., Zhuang H. R., Li W. L. Optimization of the microstructure of low-temperature combustion-synthesized barium ferrite powder [J]. Journal of Magnetism and Magnetic Materials, 2003, 256(1-3): 390~395 [17] Rezlescu L., Rezlescu E., Popa P. D., et al. Fine barium hexaferrite powder prepared by the crystallization of glass [J]. Journal of Magnetism and Magnetic Materials, 1999, 193(1-3): 288-290 [18] Meng Y. Y., He M. H., Zeng Q, et al. Ultrafine Powders by a Sol-Gel Combustion Method Using Glycine Gels [J]. Journal of Alloys and Compounds, 2014, 583 :220-225 [19] Haneda K., Kojima H. Magnetization reversal process in chemically precipitated and ordinary prepared BaFe12O19 [J]. Journal of Applied Physics, 1973, 44(8): 3760-3762 [20] Liu Z. W., Davies H. A. Influence of Co substitution for Fe on the magnetic properties of nanocrystalline (Nd, Pr)-Fe-B based alloys [J]. Journal of Physics D: Applied Physics, 2006, 39(13): 2647~2653 13
[21] Yamamoto H., Isono M., Kobayashi T. Magnetic properties of Ba-Nd-Co System M-type ferrite fine particles prepared by controlling the chemical coprecipitation method [J]. Journal of Magnetism and Magnetic Materials, 2005, 295(1): 51~56
14
Highlights 1. Nano-particles with controllable particle size, excellent magnetic properties and thermal stability of barium hexaferrite (BaFe12O19) have been synthesized. 2. The average nano-particles size of the products are well controlled and maintained at single-domain particle size range area (4~460nm), above which the coercivity will decrease abruptly for the coupling exchange among particles. 3. Excess Ba content is necessary for the combination of single phase BaFe12O19 powder and where is the excess Ba going has been well explained. 4. It is well known that Debye-Scherrer formula and Williamson–Hall analysis are always used to calculate average grain size in many reports. In this work, in order to measure the average size of nano-particles but not grain, a more traditional but practical method was used, i.e. measuring some of the representative particle sizes based on a scale and then take an average.
15
S0304-8853(18)31694-9 https://doi.org/10.1016/j.jmmm.2018.10.044 MAGMA 64455
To appear in:
Journal of Magnetism and Magnetic Materials
Received Date: Revised Date: Accepted Date:
3 June 2018 22 August 2018 9 October 2018
Please cite this article as: S.L. Hu, J. Liu, H.Y. Yu, Z.W. Liu, Synthesis and properties of barium ferrite nanopowders by chemical co-precipitation method, Journal of Magnetism and Magnetic Materials (2018), doi: https:// doi.org/10.1016/j.jmmm.2018.10.044
This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Synthesis and properties of barium ferrite nano-powders by chemical co-precipitation method S.L. Hu1, J. Liu2, H.Y. Yu3, Z.W. Liu3. 1、School of Automotive Studies, Jiangxi College Of Applied Technology, Ganzhou 341000,China 2、School of Mechanical Engineering, Jiangxi College Of Applied Technology, Ganzhou 341000,China 3、School of Material Science and Engineering, South China University of Technology, Guangzhou 510640, China Abstract: Nano-powders with controllable particle size, excellent magnetic properties and thermal stability of barium hexaferrite (BaFe12O19) have been synthesized via a co-precipitation/calcination technique. The phase composition, morphology and magnetic/thermal properties of the products were systematically studied. XRD patterns reveal that a long co-precipitation reaction time (5 h) and high calcination temperature (1100C) are beneficial for the formation of BaFe12O19 phase and decreasing the tendency to agglomeration. SEM micrographs show that the products show a hexagonal flake-like particle shape and the size are well controlled and maintained at single-domain particle size range area(<460nm), above which the coercivity will decrease abruptly for the coupling exchange among particles. The products with jHc of 5934 Oe, temperature coefficient of remanence (αBr) of -0.176% K-1 and temperature coefficient of coercivity (βjHc) of 0.0427% K-1 were obtained when co-precipitated for 5 h and calcined at 900C for 2h. A high saturation magnetization of 66.9 emu/g was obtained when co-precipitated for 5 h and calcined at 1100C for 2 h, approaching the theoretical saturation magnetization (72 emu/g).
Key words: Nano-powders; Barium hexaferrite; Co-precipitation; Calcination; Single-domain particle
1. Introduction Barium hexaferrite, BaFe12O19, is a well-known permanent magnet with great technical importance and it has attracted an extensive attention for the last few decades. BaFe12O19 compound has a hexagonal structure with fairly large crystal anisotropy along c-axis. As a result, Ba ferrite has shown high intrinsic
Corresponding author, Tel: 020-22236906, E-mail: [email protected]
1
coercivity (6700 Oe), large saturation magnetization (72 emu/g) and high Curie temperature (450◦C)[1]. At present, it has been widely used in permanent magnetic materials, magnetic recording media materials and wave absorbing materials[2-5]. It has been reported that the magnetic properties of M-type BaFe12O19 ferrites are closely related to their particle size and morphology[6]. For this reason, the researchers have modified their properties by various approaches, for an example, preparing the nanocrystalline BaFe12O19 by different methods. In order to achieve highly homogeneous nano-particles of barium ferrite, various techniques such as chemical co-precipitation method[7], sol-gel method[8-10], glass crystallization method[11-12], hydrothermal method[13], micro-emulsion method[14] have been developed. Among them, the chemical co-precipitation method is a low cost technique suitable for the mass production compared to the other mentioned methods. However, many problems are still existing for this method. Usually, the particle size is not easy to control, the reaction and transformation are not complete, and the magnetic properties are not stable. In this work, nano-sized Ba ferrite powders with controllable particle size and good magnetic properties
have
been
synthesized
via
a
co-precipitation/calcination
technique.
Their
process-microstructure-properties relationships have been systematically investigated. 2. Experimental The ferrite precursors were obtained from aqueous mixtures of barium and ferric chlorides by co-precipitation technique. The detailed preparation process is as follows: guaranteed chemically grade ferric chloride (FeCl36H2O), barium chloride (BaCl22H2O) and sodium hydroxide (NaOH) were used as starting materials. Aqueous solutions of iron and barium chlorides with a Fe3+/Ba2+ molar ratio of 8 were co-precipitated by the addition of NaOH. The aqueous suspensions were stirred quickly (at constant 200 rpm) for 3 h, 4 h and 5 h to achieve good homogeneity and attain a stable pH (9) condition. The co-precipitated sample was washed to neutral by distilled water and dried at 50◦C for 12h subsequently. In order to form the hexaferrite phase, the dry precursors were heated at a rate of 20C /min up to different temperatures (700, 800, 900, 1000 and 1100C), where they were maintained for 2 h in a static air atmosphere. The phase structure was determined by X-ray diffraction (XRD, Philips X-pert) with Cu-Kα radiation. The microstructure and size of the products was characterized by scanning electronic microscope (SEM) (Nova NanoSEM 430). The microstructure was examined by transmission electron microscope (TEM, Philips F20) and high resolution TEM (HRTEM, Joel 3100). Magnetic/thermal properties were tested by 2
physical properties measurement system (PPMS-9, Quantum Design, USA) equipped with a 9 T vibrating sample magnetometer (VSM). The temperature dependence of magnetization was measured in the temperature range of 300-500 K. 3. Results and Discussions 3.1 Crystal structure The chemical co-precipitation method involves the following reaction: 12FeCl3 + BaCl2 + 38NaOH = 12Fe (OH)3 +38NaCl +Ba (OH)2, through which iron and barium hydroxide are obtained. When the precursors exposed to a high temperature (i.e. calcination), the following reactions will occur: 2Fe(OH)3 = Fe2O3 + 3H2O；Ba(OH)2 = BaO + H2O；BaO + Fe2O3 =BaO·Fe2O3；BaO·Fe2O3 + 5Fe2O3 = BaO·6Fe2O3 (BaFe12O19). Fig. 1 shows the XRD patterns for the samples co-precipitated for 3 h (a), 4 h (b) and 5 h (c) followed by calcining at different temperatures. For the samples co-precipitated for 3h (Fig. 1a) at a low calcination temperature of 700C, the relatively low peak intensity of BaFe12O19 phase indicates that only a small amount of crystallized BaFe12O19 phase formed. When calcined at a higher temperature (800-1100C), the peak intensity of BaFe12O19 phase enhanced significantly with increasing temperature. In addition, at a short co-precipitation reaction time of 3h, the α-Fe2O3 phase was found in all samples calcined at different temperatures (800-1100C). For the samples co-precipitated for 4 h (Fig. 1b), a higher calcination temperature promotes the formation of barium hexaferrite phase, but the impurity phase(α-Fe2O3) only disappeared at a temperature over 1000C. For the samples co-precipitated for 5 h (Fig. 1c), increasing the calcination temperature also enhances the formation of barium hexaferrite phase. In addition, Fig.1 also reveals that, to form BaFe12O19 phase, the calcination temperature above 800oC is required. This is consistent with the observations by Darja et al.[15] and Huang et al.[16]. The humps around 50 deg for all samples may be attributed to the Ba2Fe2O5 phase, which is a minor impurity.
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Fig.1 XRD patterns for the samples co-precipitated for 3 h (a), 4 h (b) and 5 h (c) followed by calcining at different temperatures
3.2 Microstructure Fig.2 is the TEM images and diffraction patterns of the precursors obtained at different co-precipitation reaction time. From the images, the precursors present a flocculent structure and rings of light and dark are detected. Meanwhile, the rings of light and dark in the diffraction patterns become less obvious with increasing of the co-precipitation time, which indicated that the crystallization degree of the precursors decrease with the increasing of the reaction time. As mentioned above, the synthesis of precursors involves the following reaction: 12FeCl3 + BaCl2 + 38NaOH = 12Fe (OH)3↓+38NaCl +Ba (OH)2↓. On the one hand, due to the incomplete crystallization, some Ba ions will exist in a free state. On
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the other hand, Ba (OH)2 precipitates belong to weak precipitates, the solubility of which is about 3.89g per 100g in water at 20◦C, Ba ions will be washed by distilled water before calcination. A conclusion can be made that most of the excess Ba did not participate in the chemical co-precipitation reaction combined with the XRD results shown above (no obvious Ba-rich phase was detected).
Fig.2 TEM images and diffraction patterns for precursors co-precipitated for 3 h(a), 4 h(b), 5 h(c)
The SEM micrographs of the samples co-precipitated for 3h and calcined at different temperatures are given in Fig. 3(a–d). It can be seen that the size of hexagonal flake-like nano-particles increases with the increase of calcination temperature. When the calcination temperature rises to 1100C, the particles no longer exist in a hexagonal flake-like structure but a rod-like structure (as shown in Fig. 3d). There is a huge difference between the two structures. Fig. 4 shows the SEM images of the samples co-precipitated for 5h and calcined at different temperatures. The size of hexagonal flake-like nano-particles are well controlled and maintained at single-domain particle size range area (4nm~460nm[17]), which is beneficial
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for the improvement of coercivity. In addition, with the increase of calcination temperature, the aggregation of the particles tends to decrease. The formation of the hexagonal flake structure has been well studied[18] and the rod-like structure appeared only in the sample contained α-Fe2O3 phase at a high calcination temperature. It is, therefore, reasonable to consider that α-Fe2O3 phase has a significant effect on surface energy and results in the increasing agglomeration behavior of nanoparticles when co-precipitated for 3 h and calcined at 1100C for 2 h.
Fig.3 SEM micrographs for the samples co-precipitated for 3 h and calcined at different temperatures: (a) 800 C, (b) 900C, (c) 1000C, and (d) 1100C
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Fig.4 SEM images for samples co-precipitated for 5 h and calcined at different temperatures: (a) 800C, (b) 900C, (c) 1000C, and (d) 1100C
Fig. 5a describes the dependences of average particle size d on the calcination temperature obtained form the precursors prepared by co-precipitation reaction for 3 h and 5 h, respectively. The average particle size of calcined nano-powders obtained by co-precipitation reaction for 3 h is larger than that of 5 h with same calcination temperature. When the calcination temperature is relatively low (800~900C), the average particle size of calcined powders increases slowly. When the calcination temperature is higher than 900C, the average particle size of the product increases sharply with the increase of temperature, but it is still smaller than the critical size of the single domain particles of BaFe12O19 (≈460 nm). Fig. 5b shows the detailed particle size distribution of the sample co-precipitated for 5 h and calcined at 900◦C for 2 h. Most particles have the size range from 64 to76 nm.
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Fig.5 Dependence of the average particle size d on the calcination temperature for samples co-precipitated for 3 h and 5 h (a) and the detailed particle size distribution of the sample co-precipitated for 5 h and calcined at 900◦C (b)
3.3 Magnetic properties The hysteresis loops for the samples co-precipitated for 3 h (a) and 5 h (b) followed by calcining at different temperatures are given in Fig. 6. All the calcined nano-powders exhibit hard magnetic properties, but co-precipitation reaction time and calcination temperature have a great influence on magnetic properties. The hysteresis loop in second quadrant decreases sharply for powders co-precipitated for 3h and calcined at 800C for 2 h (Fig .6a), which directly leads to the decrease of coercivity. The hysteresis loop in second quadrant also decreases sharply for powders co-precipitated for 5h and calcined at 800C for 2 h (Fig .6b). It is considered that the low degree crystallization of BaFe12O19 phase at a relatively low calcination temperature is the main reason, which has been verified by the XRD results shown above. Meanwhile, the hysteresis loop of the powders becomes smoother with the increase of calcination temperature.
Fig.6 Hysteresis loops for samples co-precipitated for 3 h (a) and 5 h (b) followed by calcining at different temperatures
The results of saturation magnetization Ms, remnant magnetization Mr, coercivity jHc and average particle size d for the samples co-precipitated for 3 h and 5 h and calcined at different temperatures are summarized in Table 1. The values of Ms and Mr increase with the increase of calcination temperature. It is considered that the increasing volume fraction of BaFe12O19 phase and decreasing volume fraction of α-Fe2O3 contribute to the results. The nano-powders with coercivity (jHc) of 5934 Oe, saturation magnetization (Ms) of 61.8 emu/g, remnant magnetization (Mr) of 28.2 emu/g, and average particle size (d) of 70nm were obtained when co-precipitated for 5 h and calcined at 900C for 2 h. 8
The dependences of jHc and Ms on the calcination temperature are given in Fig.7. A high saturation magnetization of 66.9 emu/g is obtained when co-precipitated for 5 h and calcined at 1100C for 2 h, approaching the theoretical saturation magnetization (72 emu/g[19]). The maximum value of jHc is obtained when calcined at 900C for 2 h, no matter the coprecipitation reaction time is 3 h or 5 h and the maximum average particle size in the products is 196 nm, except for the rod-like shape, according to Table 1. It was reported that the value of the critical size of BaFe12O19 single-domain particle is 460nm and there is an inferior limit for the single-domain particle size of BaFe12O19(≈4 nm), below which a superparamagnetic state appears, characterized by zero coercivity[17]. So the products size are well controlled and maintained at single-domain particle size range area (4~460nm), above which the coercivity will decrease abruptly for the coupling exchange among particles. It is considered that the decrease of coercivity is closely related to the increase size of particle and coupling exchange among particles when calcined at high temperature over 900C.
Fig. 7 Dependences of jHc and Ms on the calcination temperature
Table 1 Saturation magnetization Ms, remnant magnetization Mr, coercivity jHc and particle size d for the samples co-precipitated for 3 h and 5 h and calcined at different temperatures Calcined temperature
co-precipitated for 3h
co-precipitated for 5h
Ms
jHc
Mr
d
Ms
jHc
Mr
d
( C)
(emu/g)
(Oe)
(emu/g)
(nm)
(emu/g)
(Oe)
(emu/g)
(nm)
800
45.6
3284
20.1
75
45.5
3927
19.3
63
900
52.9
5230
24.8
103
61.8
5934
28.2
70
9
1000
59.2
3939
27.0
196
65.8
5090
30.6
136
1100
58.7
3731
27.8
Rod-like shape
66.9
4311
31.9
153
3.4 Temperature stability The thermal stability of the calcined nano-powders can be evaluated by the temperature coefficients of remanence α and the temperature coefficients of coercivity β, where α=[ Br(500K) - Br(300K)] / [ Br(300K) ×(500-300)] ×100%, and β=[ jHc(500K ) - jHc(300K)]/[ jHc(300K)×(500-300)] ×100%, for the temperature of 300~500K, and low absolute values of the coefficients suggest high thermal stability[20]. BaFe12O19 has a hexagonal structure with fairly high Curie temperature (750K). In order to study the temperature stability of the nano-powders, the magnetic properties at high temperatures for the calcined nano-powders were characterized. Fig. 8 reveals magnetic hysteresis curves for nano-powders (precursors co-precipitated for 5h) calcined at 900C (a) and 1000C (b). Table 2 list the value of magnetic properties, the temperature coefficient of remanence (αBr), and the temperature coefficient of coercivity (βjHc) at 300K and 500K for samples co-precipitated for 5 h with calcined at 900C and 1000C. The value (βjHc) of both products are positive, due to the fact that the saturation magnetization of barium ferrite decreases more with temperature than that of anisotropy constant K1 in a low temperature range. Therefore, the coercivity of BaFe12O19 increases with the increase of temperature within a certain range, while most of other permanent magnetic materials decreases, which is an advantage of application in high temperature area[21]. For precursors co-precipitated for 5 h, subsequently calcined at 900C or 1000C, indicating an excellent high temperature property, for example, the αBr and βjHc values of the precursors calcined at 900C are -0.176 %K-1 and 0.0427 %K-1, respectively, and the the αBr and βjHc of the the precursors calcined at 1000C are -0.185%K-1 and 0.0734 %K-1, respectively. The results indicate that the coercivity of the two products is very stable at high temperature, which is suitable for magnetic recording medium.
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Fig. 8 Magnetic hysteresis curves for nano-powders (precursors co-precipitated for 5h) calcined at 900C (a) and 1000C (b)
Table 2 Magnetic properties, temperature coefficient of remanence (αBr), temperature coefficient of coercivity (βjHc) at 300K and 500K for samples co-precipitated for 5h with calcined at 900C and 1000C Calcined
Br(emu/g)
jHc(Oe)
(BH)max(MGs·Oe)
temperature
αBr(%K-1)
βjHc(%K-1)
300K
500K
300K
500K
300K
500K
900
29.6
19.2
5202
5647
0.800
0.358
-0.176
0.0427
1000
29.0
18.3
3104
3560
0.723
0.323
-0.185
0.073
（C）
4. Conclusion Barium hexaferrite (BaFe12O19) nanoparticles with controllable particle size, excellent magnetic properties and thermal stability of have been synthesized by a co-precipitation/calcination technique： (1) The XRD results indicated that the formation of barium hexaferrite and α-Fe2O3 phase occurs simultaneously at relatively short co-precipitation reaction time(3 h or 4 h) followed by calcining at different temperatures. Meanwhile, increased calcining temperature (700-1100C) improved the crystallization of BaFe12O19 phase and resulted in the single phase. (2) The performance of BaFe12O19 particles is closely related to reaction time of chemical co-precipitation and calcination temperature. The particle size increased with increasing calcination temperature while co-precipitation reaction time is the same. Meanwhile, the particle size also increased with increasing co-precipitation reaction time while calcination temperature is the same. A high coercive force of 5934 Oe is obtained with a coprecipitation reaction time of 5 h and a calcination temperature of 900C. 11
(3) Using the precursors co-precipitated for 5 h and calcination temperature of 900C or 1000C, the ferrites with excellent high temperature properties can be obtained. The αBr and βjHc values of the precursors calcined at 900C are -0.176 %K-1 and 0.0427 %K-1, respectively, and the αBr and βjHc of the precursors calcined at 1000C are -0.185%K-1 and 0.0734 %K-1, respectively. These results indicate that they are suitable for using as magnetic recording medium materials.
Acknowledgement This work was supported by the Jiangxi Provincial Science and Technology Program (Grant No. GJJ171308) and the DongGuan Innovative Research Team Program (Grant No. 201536000200027)
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[21] Yamamoto H., Isono M., Kobayashi T. Magnetic properties of Ba-Nd-Co System M-type ferrite fine particles prepared by controlling the chemical coprecipitation method [J]. Journal of Magnetism and Magnetic Materials, 2005, 295(1): 51~56
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Highlights 1. Nano-particles with controllable particle size, excellent magnetic properties and thermal stability of barium hexaferrite (BaFe12O19) have been synthesized. 2. The average nano-particles size of the products are well controlled and maintained at single-domain particle size range area (4~460nm), above which the coercivity will decrease abruptly for the coupling exchange among particles. 3. Excess Ba content is necessary for the combination of single phase BaFe12O19 powder and where is the excess Ba going has been well explained. 4. It is well known that Debye-Scherrer formula and Williamson–Hall analysis are always used to calculate average grain size in many reports. In this work, in order to measure the average size of nano-particles but not grain, a more traditional but practical method was used, i.e. measuring some of the representative particle sizes based on a scale and then take an average.
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