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Synthesis of barium hexaferrite nano-particles via mechano-combustion route
Journal of Alloys and Compounds 431 (2007) 331–336
Synthesis of barium hexaferrite nano-particles via mechano-combustion route A. Ataie ∗ , S.E. Zojaji School of Metallurgy and Materials Engineering, P.O. Box: 14395-553, University of Tehran, Tehran, Iran Received 25 April 2006; received in revised form 23 May 2006; accepted 23 May 2006 Available online 30 June 2006
Abstract Nano-size particles of barium hexaferrite have been synthesized by what is termed mechano-combustion after milling of intermediate products obtained in the sol–gel combustion process using nitrate–citrate gels prepared from metal nitrates and citric acid. The effects of precursor milling conditions on the phase evolution, crystallite size and annealing behavior of the products were investigated using XRD technique. The XRD results indicate the presence of ␥-Fe2 O3 as a major phase and BaCO3 as a minor phase in as-burnt powder. Analysis of the XRD patterns also confirms the transformation of ␥- to ␣-Fe2 O3 via mechanical milling; the rate of this transformation is strongly affected by milling conditions. Barium hexaferrite starts to form at 700 ◦ C and fully formed at 1000 ◦ C for milled sample under the optimum milling conditions. XRD and DTA/TGA results showed that an intermediate precursor milling accelerates the formation of magnetic phase. SEM micrograph of the sample milled for 120 h and then annealed at 1000 ◦ C exhibited nano-size particles of BaFe12 O19 with mean particle size of around 100 nm. © 2006 Elsevier B.V. All rights reserved. Keywords: Sol–gel combustion; Mechanical milling; X-ray diffraction; Nano-size particles
1. Introduction Barium hexaferrite, BaFe12 O19 , as an oxide ceramic magnet with interesting magnetic properties is a proper material for fabrication of permanent magnet as well as for perpendicular high density recording media [1]. Barium hexaferrite is produced mainly by a conventional mixed oxide ceramic method that involves calcination of mixture of starting materials at around 1200 ◦ C [2]. However, in order to achieve highly homogeneous nano-particles of barium hexaferrite, non-conventional routes such as chemical co-precipitation method [3], hydrothermal method [4], sol–gel method [5], glass crystallization method [6], micro-emulsion method [7] have been also developed. Nowadays, the combination of chemical routes and high-energy ball milling method is considered as a new technique for producing of nano-crystalline magnetic ceramics [8]. Synthesis of nano-size particles of barium hexaferrite by mechano-chemical method which includes sol–gel combustion technique and intermediate mechanical milling is focused in this
∗
Corresponding author. Tel.: +98 21 88012999; fax: +98 21 88006076. E-mail address: [email protected] (A. Ataie).
0925-8388/$ – see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.jallcom.2006.05.085
study. The effects of the milling conditions on the characteristics of the products have been investigated. 2. Experimental procedure The starting materials were iron nitrate, barium nitrate, citric acid and ammonia, all of analytic purity. Appropriate amount of Fe (NO3 )3 ·9H2 O and Ba(NO3 )2 , in a Fe/Ba molar ratio of 11 were dissolved in a minimum amount of deionized water. The molar ratio of citric acid to total moles of nitrate ions was adjusted at 1:1. Details of the sol–gel combustion synthesis procedure to obtain as-burnt powder have been described previously [9,10]. The as-burnt powder has been milled under different conditions using planetary ball mill (FP-2). Effect of milling parameters, i.e. ball diameter, ball to powder mass ratio and milling time on the phase composition and morphology of the products has been studied. The processed samples have been subjected to annealing in air at 700, 800, 900 and 1000 ◦ C for 1 h. Tables 1 and 2 summarize the milling and annealing conditions, respectively. Phase composition and morphology of the samples were characterized by X-ray diffraction (XRD Philips PW-1730) using Cu k␣ radiation and scanning electron microscopy (SEM Cambridge S360) techniques, respectively. The mean crystallite size of the samples was measured by X-ray line-broadening technique employing the Williamson-Hall formula. The transformation temperatures find out with differential thermal analysis/thermo gravimetery analysis (DTA/TGA Dupont) technique with a heating rate of 10 ◦ C/min. Magnetic properties were measured by a vibrating sample magnetometer (VSM) with a maximum applied field of 1100 kA/m.
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Table 1 Milling conditions of samples, i.e. milling time t, balls diameter D, ball to powder mass ratio r Sample
t (h)
D (mm)
r
MD(x)
40
20
MR(x)
40
x = 5, 10, 20, 5, and 10 5 and 10
Mt(x)
x = 10, 20, 40, 80, 120
5 and 10
x = 10, 15, 20, 25 20
Table 2 Annealing conditions of the processed samples Sample
Milling parameters
Annealing temperature (◦ C)
H(x) HM(x)
– t = 120 h, r = 20, D = 5 and 10 mm
x = 700, 800, 900, 1000 x = 700, 800, 900, 1000
3. Results and discussion Fig. 1 shows the XRD pattern of as-burnt powder, which indicates the presence of ␥-Fe2 O3 as a major phase and some small reflections of minor phases such as BaCO3 and BaFe12 O19 . The interaction between very reactive Ba2+ ions and CO or CO2 , generated from the decomposition of citric acid, leads to the formation of BaCO3 . Fig. 2 shows the XRD patterns of MD(5), MD(10), MD(20) and MD(5 and 10) samples which are milled with a different size of balls under the given experimental conditions. Analysis of the XRD patterns confirms the complete transformation of ␥- to ␣Fe2 O3 via mechanical milling for samples MD(5) and MD(10). In samples MD(20) and MD(5 and 10), ␥ → ␣ transformation occurs partially and ␥-Fe2 O3 was not fully eliminated. The same results were also reported by Rams et al. [11]. Milling process causes to contacting of two or more maghemite (␥-Fe2 O3 ) particles. The applied energy to ␥-Fe2 O3 particles cause to take apart them and the clean broken surfaces are desired positions for nucleation of ␣-Fe2 O3 . Generally, this transformation occurs during annealing of as-burnt powder at around 700 ◦ C [11]. It believes that, mechanical milling causes to this transformation take places at room temperature probably due to decrease of
Fig. 1. X-ray diffraction pattern of as-burnt powder.
Fig. 2. X-ray diffraction patterns of (a) MD(5), (b) MD(10), (c) MD(20) and (d) MD(5 and 10) samples.
mean crystallite size of ␥-Fe2 O3 . The mean crystallite size of the above samples varied from 13 to 90 nm; the smallest size was obtained for sample MD(5 and 10). Fig. 3 shows the XRD patterns of MR(10), MR(15), MR(20) and MR(25) samples which are milled with a different ball to powder mass ratio under the given experimental conditions. Analysis of the XRD patterns reveals the partial transformation of ␥- to ␣-Fe2 O3 via mechanical milling for all samples. The first reflection of barium hexaferrite (1 1 4) was appeared clearly in MR(20) sample. Normally, this reflection appears by annealing of as-burnt powder at 800 ◦ C [9,10]. The mean crystallite size of ␣-Fe2 O3 was not influenced significantly by the ball to powder mass ratio; the minimum size of 13 nm was measured for sample MR(20). Fig. 4 shows the XRD patterns of Mt(10), Mt(20), Mt(40), Mt(80) and Mt(120) samples which are milled for various milling times under the given experimental conditions. XRD results show the ␥- to ␣-Fe2 O3 transformation via mechanical milling. The major peaks of ␥-Fe2 O3 were reduced continuously with increasing milling time, but finally not disappeared, even after a milling time of 120 h. The first peak of ␣-Fe2 O3 (1 0 4) was quite broadening on increasing milling time due to the reduction of crystallite size. The first reflection of barium hexaferrite (1 1 4) was appeared clearly for sample Mt(20) and its intensity increases by increasing the milling time. Mean crys-
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increasing the annealing temperature from 800 to 900 ◦ C, only the first peak of ␣-Fe2 O3 (1 0 4) reflected weakly. The completion of barium hexaferrite occurs at 1000 ◦ C. Fig. 6 shows the XRD patterns of sample milled for 120 h under the given experimental conditions, Mt(120) after annealing at various temperatures. Analysis of the XRD patterns confirms that for milled sample, barium hexaferrite starts to form at a relatively low temperature of 700 ◦ C and fully formed at 900 ◦ C. Although, the milled sample behaves in a similar manner to non-milled sample during annealing, however, the formation and completion temperatures of barium hexaferrite for milled sample are almost 100 ◦ C lower than those of non-milled sample. The DTA/TGA traces for Mt(120) sample are shown in Fig. 7. The DTA curve indicates four peaks. The endothermic peak at around 200 ◦ C could be corresponded to the dehydration of the absorbed water. The first exothermic peak at about 325 ◦ C with a weight loss of ∼2% could be due to the decomposition of
Fig. 3. X-ray diffraction patterns of (a) MR(10), (b) MR(15), (c) MR(20) and (d) MR(25) samples.
tallite size of samples was affected by the milling time and the sample milled for 80 h exhibits a minimum size of 12 nm. This behavior could be explained by two mechanisms that occur in milling process: cold welding and rupture [12]. It seems before 80 h, rupture is the dominant mechanism and from 80 to 120 h, the rupture mechanism replaced with cold welding mechanism. These results are in good agreement with results reported by other investigators [13]. From the results obtained, it seems the single phase of barium hexaferrite could not be formed during the mechanical milling under the given experimental conditions, but the intermediate milling process could accelerates the formation of barium hexaferrite as a single phase during subsequent annealing [11]. Fig. 5 shows the XRD patterns of as-burnt powder (nonmilled sample) after it was annealed at 700, 800, 900 and 1000 ◦ C. In sample H(700), the major phases are ␣- and ␥Fe2 O3 . By increasing the annealing temperature to 800 ◦ C, BaFe12 O19 appears as a major phase and ␣-Fe2 O3 and BaFe2 O4 as minor phases are detected. It is believed that by increasing the annealing temperature, BaCO3 decomposes and Ba2+ liberated reacts with ␥- or ␣-Fe2 O3 to form a small amount of barium monoferrite, BaFe2 O4 . The reaction between barium monoferrite and iron oxide (␥- or ␣-Fe2 O3 ) leads to formation of barium hexaferrite. The amount of barium hexaferrite increases monotonically with increasing the annealing temperature. By
Fig. 4. X-ray diffraction patterns of (a) Mt(10), (b) Mt(20), (c) Mt(40), (d) Mt(80) and (e) Mt(120) samples.
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Fig. 5. X-ray diffraction patterns of (a) H(700), (b) H(800), (c) H(900) and (d) H(1000) samples.
unreacted citric acid remained after combustion. The second exothermic peak at 605 ◦ C might be attributed to the decarboxylation of BaCO3 formed during combustion, which has been reported to take place at 1050 ◦ C for pure carbonate and at around 800 ◦ C for a mixture of carbonate and iron oxide [14]. This peak also could be assigned to simultaneous formation of barium monoferrite (BaFe2 O4 ). The last broadened exothermic peak at 815 ◦ C with a very small weight loss (<0.1%) could be considered as a solid-state reaction attributed to the gradual formation of barium hexaferrite. Previous work showed that barium hexaferrite forms at 880 ◦ C for non-milled sample [10]. The DTA/TGA results revealed that intermediate precursor milling reduces the formation temperature of barium hexaferrite from 880 to 815 ◦ C. The DTA/TGA results are in well agreement with the XRD results. SEM micrographs of the samples H(1000) and HM(1000) exhibit nano-size particles of barium hexaferrite (Fig. 8), the mean particle size for milled sample is smaller than that of nonmilled sample. From the SEM micrographs, the mean particle size of samples H(1000) and HM(1000) were measured almost 160 and 100 nm, respectively. Fig. 9 shows the magnetization curves for samples H(1000) and HM(1000) which are non-milled and milled for 120 h and then calcined at 1000 ◦ C. The squareness ratio, Mr /Ms for both samples is almost 0.5 that is equal to the value expected for
Fig. 6. X-ray diffraction patterns of (a) HM(700), (b) HM(800), (c) HM(900) and (d) HM(1000) samples.
randomly oriented assembly of uniaxial single-domain particles undergoing magnetization reversal by coherent rotation [15]. It is believed that the chemical phases presented in the specimens as well as the degree of the crystallinity of the magnetic phase are the main reasons for the difference in saturation magnetization values. The higher saturation magnetization value (67.1 emu/g) of sample HM(1000) in comparison with that of
Fig. 7. DTA/TGA traces for the milled sample for 120 h; Mt(120).
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335
Fig. 9. Magnetization curves for samples H(1000) and HM(1000).
4. Conclusion Nano-size particles of barium hexaferrite have been synthesized at a relatively low temperature by mechano-combustion method. Considering the (1) crystallite size of the as milled products and (2) appearance of the first reflection of barium hexaferrite (1 1 4) in the XRD patterns and its intensity, the ball diameter of 5 and 10, ball to powder mass ratio of 20 and milling time of 120 h may be determined as an optimum milling conditions. Single phase of barium hexaferrite could not be formed during the mechanical milling under the given experimental conditions. For the formation of pure BaFe12 O19 , a subsequent heat treatment at 900 ◦ C or above is required. SEM micrographs revealed that for sample annealed at 1000 ◦ C, the mean particle size of barium hexaferrite decreased from 160 nm in non-milled sample to 100 nm in milled sample. An intermediate milling accelerates the formation of single phase barium hexaferrite and enhances the magnetic properties. Acknowledgment
Fig. 8. SEM micrographs of (a) non-milled and (b) milled samples annealed at 1000 ◦ C.
The financial support of this work by Nano Technology Development Project of Ministry of Science, Research and Technology of I.R. Iran is gratefully acknowledged. References
H(1000) (61.2 emu/g) could be a consequence of the relatively higher amount of barium hexaferrite phase and its higher degree of crystallinity. A very high coercivity value of 493.5 kA/m of sample HM(1000) could be due to the effect of intermediate milling in grain refinement and single-domain nature of the barium hexaferrite particles. Existence of a few multi-domain particles could be responsible to a relatively low coercivity value (405.9 kA/m) of H(1000) sample. The VSM results are correlated well with the results obtained from DTA/TGA and XRD. It could be concluded that intermediate milling accelerates the formation of single phase barium hexaferrite and enhances the magnetic properties.
[1] H. Kojima, in: E.P. Wohlfarth (Ed.), Ferromagnetic Materials, vol. 3, NorthHolland, Amsterdam, 1982, pp. 305–391 (Chapter 5). [2] H. Stablin, in: E.P. Wohlfarth (Ed.), Ferromagnetic Materials, vol. 3, NorthHolland, Amsterdam, 1982, pp. 441–602 (Chapter 7). [3] H. Sakai, K. Hanawa, K. Aoyagi, IEEE Trans. Magn. 28 (6) (1992) 3355–3362. [4] J.F. Wang, C.B. Ponton, R. Grossinger, I.R. Harris, J. Alloys Compd. 369 (2004) 170–174. [5] W. Zhong, W. Ding, N. Zhang, J. Hong, Q. Yan, Y. Du, J. Magn. Magn. Mater. 168 (1997) 196–202. [6] L. Rezlescu, E. Rezlescu, P.D. Popar, N. Rezlescu, J. Magn. Magn. Mater. 193 (1993) 288–290. [7] X. Liu, J. Wang, L.M. Gan, S.C. Ng, J. Ding, J. Magn. Magn. Mater. 184 (1998) 344–354.
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J. Ding, X.Y. Liu, J. Wang, Y. Shi, Mater. Lett. 44 (2000) 19–22. A. Mali, A. Ataie, Ceram. Int. 30 (2004) 1979–1983. A. Mali, A. Ataie, J. Alloys Compd. 399 (2005) 245–250. E.E. Rams, R.M. Garcia, E. Reguera, H.M. Sanchez, H.Y. Madeira, J. Phys. D: Appl. Phys. 33 (2000) 2708–2815. [12] M.S.E. Eskandarany, Mechanical Alloying for Advance Engineering Materials, William Andrew Publications, 2001.
[13] F. Zhang, W.A. Kaczmarek, L. Lu, M.O. Lai, J. Alloys Compd. 307 (2000) 249–253. [14] G. Benito, M.P. Morales, J. Requena, V. Raposo, M. Vazquez, J.S. Moya, J. Magn. Magn. Mater. 234 (2001) 65–72. [15] E.C. Stoner, E.P. Wohlfarth, Phil. Trans. Roy. Soc. London, Ser. A 240 (1948) 599–604.
Synthesis of barium hexaferrite nano-particles via mechano-combustion route A. Ataie ∗ , S.E. Zojaji School of Metallurgy and Materials Engineering, P.O. Box: 14395-553, University of Tehran, Tehran, Iran Received 25 April 2006; received in revised form 23 May 2006; accepted 23 May 2006 Available online 30 June 2006
Abstract Nano-size particles of barium hexaferrite have been synthesized by what is termed mechano-combustion after milling of intermediate products obtained in the sol–gel combustion process using nitrate–citrate gels prepared from metal nitrates and citric acid. The effects of precursor milling conditions on the phase evolution, crystallite size and annealing behavior of the products were investigated using XRD technique. The XRD results indicate the presence of ␥-Fe2 O3 as a major phase and BaCO3 as a minor phase in as-burnt powder. Analysis of the XRD patterns also confirms the transformation of ␥- to ␣-Fe2 O3 via mechanical milling; the rate of this transformation is strongly affected by milling conditions. Barium hexaferrite starts to form at 700 ◦ C and fully formed at 1000 ◦ C for milled sample under the optimum milling conditions. XRD and DTA/TGA results showed that an intermediate precursor milling accelerates the formation of magnetic phase. SEM micrograph of the sample milled for 120 h and then annealed at 1000 ◦ C exhibited nano-size particles of BaFe12 O19 with mean particle size of around 100 nm. © 2006 Elsevier B.V. All rights reserved. Keywords: Sol–gel combustion; Mechanical milling; X-ray diffraction; Nano-size particles
1. Introduction Barium hexaferrite, BaFe12 O19 , as an oxide ceramic magnet with interesting magnetic properties is a proper material for fabrication of permanent magnet as well as for perpendicular high density recording media [1]. Barium hexaferrite is produced mainly by a conventional mixed oxide ceramic method that involves calcination of mixture of starting materials at around 1200 ◦ C [2]. However, in order to achieve highly homogeneous nano-particles of barium hexaferrite, non-conventional routes such as chemical co-precipitation method [3], hydrothermal method [4], sol–gel method [5], glass crystallization method [6], micro-emulsion method [7] have been also developed. Nowadays, the combination of chemical routes and high-energy ball milling method is considered as a new technique for producing of nano-crystalline magnetic ceramics [8]. Synthesis of nano-size particles of barium hexaferrite by mechano-chemical method which includes sol–gel combustion technique and intermediate mechanical milling is focused in this
∗
Corresponding author. Tel.: +98 21 88012999; fax: +98 21 88006076. E-mail address: [email protected] (A. Ataie).
0925-8388/$ – see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.jallcom.2006.05.085
study. The effects of the milling conditions on the characteristics of the products have been investigated. 2. Experimental procedure The starting materials were iron nitrate, barium nitrate, citric acid and ammonia, all of analytic purity. Appropriate amount of Fe (NO3 )3 ·9H2 O and Ba(NO3 )2 , in a Fe/Ba molar ratio of 11 were dissolved in a minimum amount of deionized water. The molar ratio of citric acid to total moles of nitrate ions was adjusted at 1:1. Details of the sol–gel combustion synthesis procedure to obtain as-burnt powder have been described previously [9,10]. The as-burnt powder has been milled under different conditions using planetary ball mill (FP-2). Effect of milling parameters, i.e. ball diameter, ball to powder mass ratio and milling time on the phase composition and morphology of the products has been studied. The processed samples have been subjected to annealing in air at 700, 800, 900 and 1000 ◦ C for 1 h. Tables 1 and 2 summarize the milling and annealing conditions, respectively. Phase composition and morphology of the samples were characterized by X-ray diffraction (XRD Philips PW-1730) using Cu k␣ radiation and scanning electron microscopy (SEM Cambridge S360) techniques, respectively. The mean crystallite size of the samples was measured by X-ray line-broadening technique employing the Williamson-Hall formula. The transformation temperatures find out with differential thermal analysis/thermo gravimetery analysis (DTA/TGA Dupont) technique with a heating rate of 10 ◦ C/min. Magnetic properties were measured by a vibrating sample magnetometer (VSM) with a maximum applied field of 1100 kA/m.
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Table 1 Milling conditions of samples, i.e. milling time t, balls diameter D, ball to powder mass ratio r Sample
t (h)
D (mm)
r
MD(x)
40
20
MR(x)
40
x = 5, 10, 20, 5, and 10 5 and 10
Mt(x)
x = 10, 20, 40, 80, 120
5 and 10
x = 10, 15, 20, 25 20
Table 2 Annealing conditions of the processed samples Sample
Milling parameters
Annealing temperature (◦ C)
H(x) HM(x)
– t = 120 h, r = 20, D = 5 and 10 mm
x = 700, 800, 900, 1000 x = 700, 800, 900, 1000
3. Results and discussion Fig. 1 shows the XRD pattern of as-burnt powder, which indicates the presence of ␥-Fe2 O3 as a major phase and some small reflections of minor phases such as BaCO3 and BaFe12 O19 . The interaction between very reactive Ba2+ ions and CO or CO2 , generated from the decomposition of citric acid, leads to the formation of BaCO3 . Fig. 2 shows the XRD patterns of MD(5), MD(10), MD(20) and MD(5 and 10) samples which are milled with a different size of balls under the given experimental conditions. Analysis of the XRD patterns confirms the complete transformation of ␥- to ␣Fe2 O3 via mechanical milling for samples MD(5) and MD(10). In samples MD(20) and MD(5 and 10), ␥ → ␣ transformation occurs partially and ␥-Fe2 O3 was not fully eliminated. The same results were also reported by Rams et al. [11]. Milling process causes to contacting of two or more maghemite (␥-Fe2 O3 ) particles. The applied energy to ␥-Fe2 O3 particles cause to take apart them and the clean broken surfaces are desired positions for nucleation of ␣-Fe2 O3 . Generally, this transformation occurs during annealing of as-burnt powder at around 700 ◦ C [11]. It believes that, mechanical milling causes to this transformation take places at room temperature probably due to decrease of
Fig. 1. X-ray diffraction pattern of as-burnt powder.
Fig. 2. X-ray diffraction patterns of (a) MD(5), (b) MD(10), (c) MD(20) and (d) MD(5 and 10) samples.
mean crystallite size of ␥-Fe2 O3 . The mean crystallite size of the above samples varied from 13 to 90 nm; the smallest size was obtained for sample MD(5 and 10). Fig. 3 shows the XRD patterns of MR(10), MR(15), MR(20) and MR(25) samples which are milled with a different ball to powder mass ratio under the given experimental conditions. Analysis of the XRD patterns reveals the partial transformation of ␥- to ␣-Fe2 O3 via mechanical milling for all samples. The first reflection of barium hexaferrite (1 1 4) was appeared clearly in MR(20) sample. Normally, this reflection appears by annealing of as-burnt powder at 800 ◦ C [9,10]. The mean crystallite size of ␣-Fe2 O3 was not influenced significantly by the ball to powder mass ratio; the minimum size of 13 nm was measured for sample MR(20). Fig. 4 shows the XRD patterns of Mt(10), Mt(20), Mt(40), Mt(80) and Mt(120) samples which are milled for various milling times under the given experimental conditions. XRD results show the ␥- to ␣-Fe2 O3 transformation via mechanical milling. The major peaks of ␥-Fe2 O3 were reduced continuously with increasing milling time, but finally not disappeared, even after a milling time of 120 h. The first peak of ␣-Fe2 O3 (1 0 4) was quite broadening on increasing milling time due to the reduction of crystallite size. The first reflection of barium hexaferrite (1 1 4) was appeared clearly for sample Mt(20) and its intensity increases by increasing the milling time. Mean crys-
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increasing the annealing temperature from 800 to 900 ◦ C, only the first peak of ␣-Fe2 O3 (1 0 4) reflected weakly. The completion of barium hexaferrite occurs at 1000 ◦ C. Fig. 6 shows the XRD patterns of sample milled for 120 h under the given experimental conditions, Mt(120) after annealing at various temperatures. Analysis of the XRD patterns confirms that for milled sample, barium hexaferrite starts to form at a relatively low temperature of 700 ◦ C and fully formed at 900 ◦ C. Although, the milled sample behaves in a similar manner to non-milled sample during annealing, however, the formation and completion temperatures of barium hexaferrite for milled sample are almost 100 ◦ C lower than those of non-milled sample. The DTA/TGA traces for Mt(120) sample are shown in Fig. 7. The DTA curve indicates four peaks. The endothermic peak at around 200 ◦ C could be corresponded to the dehydration of the absorbed water. The first exothermic peak at about 325 ◦ C with a weight loss of ∼2% could be due to the decomposition of
Fig. 3. X-ray diffraction patterns of (a) MR(10), (b) MR(15), (c) MR(20) and (d) MR(25) samples.
tallite size of samples was affected by the milling time and the sample milled for 80 h exhibits a minimum size of 12 nm. This behavior could be explained by two mechanisms that occur in milling process: cold welding and rupture [12]. It seems before 80 h, rupture is the dominant mechanism and from 80 to 120 h, the rupture mechanism replaced with cold welding mechanism. These results are in good agreement with results reported by other investigators [13]. From the results obtained, it seems the single phase of barium hexaferrite could not be formed during the mechanical milling under the given experimental conditions, but the intermediate milling process could accelerates the formation of barium hexaferrite as a single phase during subsequent annealing [11]. Fig. 5 shows the XRD patterns of as-burnt powder (nonmilled sample) after it was annealed at 700, 800, 900 and 1000 ◦ C. In sample H(700), the major phases are ␣- and ␥Fe2 O3 . By increasing the annealing temperature to 800 ◦ C, BaFe12 O19 appears as a major phase and ␣-Fe2 O3 and BaFe2 O4 as minor phases are detected. It is believed that by increasing the annealing temperature, BaCO3 decomposes and Ba2+ liberated reacts with ␥- or ␣-Fe2 O3 to form a small amount of barium monoferrite, BaFe2 O4 . The reaction between barium monoferrite and iron oxide (␥- or ␣-Fe2 O3 ) leads to formation of barium hexaferrite. The amount of barium hexaferrite increases monotonically with increasing the annealing temperature. By
Fig. 4. X-ray diffraction patterns of (a) Mt(10), (b) Mt(20), (c) Mt(40), (d) Mt(80) and (e) Mt(120) samples.
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Fig. 5. X-ray diffraction patterns of (a) H(700), (b) H(800), (c) H(900) and (d) H(1000) samples.
unreacted citric acid remained after combustion. The second exothermic peak at 605 ◦ C might be attributed to the decarboxylation of BaCO3 formed during combustion, which has been reported to take place at 1050 ◦ C for pure carbonate and at around 800 ◦ C for a mixture of carbonate and iron oxide [14]. This peak also could be assigned to simultaneous formation of barium monoferrite (BaFe2 O4 ). The last broadened exothermic peak at 815 ◦ C with a very small weight loss (<0.1%) could be considered as a solid-state reaction attributed to the gradual formation of barium hexaferrite. Previous work showed that barium hexaferrite forms at 880 ◦ C for non-milled sample [10]. The DTA/TGA results revealed that intermediate precursor milling reduces the formation temperature of barium hexaferrite from 880 to 815 ◦ C. The DTA/TGA results are in well agreement with the XRD results. SEM micrographs of the samples H(1000) and HM(1000) exhibit nano-size particles of barium hexaferrite (Fig. 8), the mean particle size for milled sample is smaller than that of nonmilled sample. From the SEM micrographs, the mean particle size of samples H(1000) and HM(1000) were measured almost 160 and 100 nm, respectively. Fig. 9 shows the magnetization curves for samples H(1000) and HM(1000) which are non-milled and milled for 120 h and then calcined at 1000 ◦ C. The squareness ratio, Mr /Ms for both samples is almost 0.5 that is equal to the value expected for
Fig. 6. X-ray diffraction patterns of (a) HM(700), (b) HM(800), (c) HM(900) and (d) HM(1000) samples.
randomly oriented assembly of uniaxial single-domain particles undergoing magnetization reversal by coherent rotation [15]. It is believed that the chemical phases presented in the specimens as well as the degree of the crystallinity of the magnetic phase are the main reasons for the difference in saturation magnetization values. The higher saturation magnetization value (67.1 emu/g) of sample HM(1000) in comparison with that of
Fig. 7. DTA/TGA traces for the milled sample for 120 h; Mt(120).
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335
Fig. 9. Magnetization curves for samples H(1000) and HM(1000).
4. Conclusion Nano-size particles of barium hexaferrite have been synthesized at a relatively low temperature by mechano-combustion method. Considering the (1) crystallite size of the as milled products and (2) appearance of the first reflection of barium hexaferrite (1 1 4) in the XRD patterns and its intensity, the ball diameter of 5 and 10, ball to powder mass ratio of 20 and milling time of 120 h may be determined as an optimum milling conditions. Single phase of barium hexaferrite could not be formed during the mechanical milling under the given experimental conditions. For the formation of pure BaFe12 O19 , a subsequent heat treatment at 900 ◦ C or above is required. SEM micrographs revealed that for sample annealed at 1000 ◦ C, the mean particle size of barium hexaferrite decreased from 160 nm in non-milled sample to 100 nm in milled sample. An intermediate milling accelerates the formation of single phase barium hexaferrite and enhances the magnetic properties. Acknowledgment
Fig. 8. SEM micrographs of (a) non-milled and (b) milled samples annealed at 1000 ◦ C.
The financial support of this work by Nano Technology Development Project of Ministry of Science, Research and Technology of I.R. Iran is gratefully acknowledged. References
H(1000) (61.2 emu/g) could be a consequence of the relatively higher amount of barium hexaferrite phase and its higher degree of crystallinity. A very high coercivity value of 493.5 kA/m of sample HM(1000) could be due to the effect of intermediate milling in grain refinement and single-domain nature of the barium hexaferrite particles. Existence of a few multi-domain particles could be responsible to a relatively low coercivity value (405.9 kA/m) of H(1000) sample. The VSM results are correlated well with the results obtained from DTA/TGA and XRD. It could be concluded that intermediate milling accelerates the formation of single phase barium hexaferrite and enhances the magnetic properties.
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