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Hexagonal ferrite powder synthesis using chemical coprecipitation
Materials Chemistry and Physics 104 (2007) 1–4
Hexagonal ferrite powder synthesis using chemical coprecipitation Hsing-I Hsiang ∗ , Ren-Qian Yao Particulate Materials Research Center, Department of Resources Engineering, National Cheng Kung University, Tainan, Taiwan, ROC Received 12 April 2006; received in revised form 13 October 2006; accepted 19 February 2007
Abstract The formation mechanism of 3BaO·2CoO·12Fe2 O3 (Co2 Z), 2BaO·2CoO·6Fe2 O3 (Co2 Y) and BaO·6Fe2 O3 (BaM) powders were prepared using chemical coprecipitation methods in this study using X-ray diffraction (XRD), thermo-gravimetry (TG), differential thermal analysis (DTA) and Fourier transform infrared spectroscopy (FTIR). It was found that the BaM phase was formed directly through the reaction of the preceding -Fe2 O3 and amorphous BaCO3 for BaM precursor. For the Co2 Y precursor, the intermediate phase, BaM, was obtained through the reaction of the earlier formed BaFe2 O4 and ␣-Fe2 O3 . The Co2 Y phase was obtained through a BaM and BaFe2 O4 reaction. However, for the Co2 Z precursors, the BaM phase was obtained directly from the BaCO3 and amorphous iron hydroxide reaction, with no ␣-Fe2 O3 and BaFe2 O4 formed as intermediates. Co2 Z phase was obtained through the reaction of the two previous formed BaM and Co2 Y phases. © 2007 Elsevier B.V. All rights reserved. Keywords: Hexagonal ferrites; Chemical coprecipitation; Co2 Z; Co2 Y
1. Introduction Hexagonal ferrites, such as BaM, Co2 Y and Co2 Z are very attractive materials for magnetic recording media and high frequency circuits. BaM is an important hard magnetic material because of its large coercive force, which results from the large magnetic anisotropy [1]. However, Co2 Y and Co2 Z ferrites exhibit a higher dispersion frequency than that of nickel ferrites because of the magneto-plumbite magnetic anisotropy can be used in high frequency circuits [2]. Hsiang [3] and Bai et al. [4] observed that multilayer chip inductors made of Co2 Z and Co2 Y ferrites exhibited excellent magnetic properties. A single Co2 Z phase is difficult to produce using solid state reaction, requiring a temperature of at least 1200 ◦ C, due to its complicated structures [5]. The Co2 Z phase usually coexists with some of the M, Y, W and spinel phases [6]. The Co2 Y phase has been regarded as an intermediate phase during Co2 Z ferrite preparation [7]. Therefore, the formation temperature of a pure Co2 Y phase was lower than that of the Co2 Z phase. Bai et al. [4] prepared Co2 Y ferrite using a solid state reaction. They observed that Co2 Y became the major phase, accompanied with intermediate phases, BaFe2 O4 and BaM at 1000 ◦ C and that a single phase of Co2 Y was obtained at 1050 ◦ C. ∗
Corresponding author. Tel.: +886 6 2757575x62821; fax: +886 6 2380421. E-mail address: [email protected] (H.-I. Hsiang).
0254-0584/$ – see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.matchemphys.2007.02.030
Many wet chemical methods have been proposed for preparing hexagonal ferrite particles, such as chemically derived citrate precursor [8], stearic acid sol–gel [9], inorganic sol–gel method [10] and the self-propagation method [11] to decrease the synthesis temperature. The coprecipitation method is attractive for large-scale powder production because of good stochiometric control and homogeneous mixing on the atomic scale. However, reports on the formation mechanisms of Co2 Z, Co2 Y and BaM ferrites prepared using the coprecipitation method have not been made to the best of our knowledge. In this study, the formation mechanism of Co2 Z, Co2 Y and BaM powders prepared using chemical coprecipitation methods were investigated using XRD, TG, DTA and FTIR. 2. Experimental procedure 2.1. Preparation of powders 3BaO·2CoO·12Fe2 O3 , 2BaO·2CoO·6Fe2 O3 , BaO·6Fe2 O3 powders were prepared using coprecipitation methods. The starting materials were reagentgrade purity oxide powders (Ba(NO3 )2 , Co(NO3 )2 ·6H2 O, Fe(NO3 )2 ·9H2 O, NaOH and Na2 CO3 ). Desired amounts of Ba(NO3 )2 , Co(NO3 )2 ·6H2 O, Fe(NO3 )3 ·9H2 O were mixed to yield a clear aqueous solution. The mixed solution was added to an aqueous solution of NaOH/Na2 CO3 using vigorous stirring. The pH value reached around 13 after titration completion. After precipitation, the co-precipitated powders were filtered and washed with deionized water two times, followed by washing with 99.5 ethyl alcohol and filtering. The washed powders were dried at 80 ◦ C and then calcined at 500–1250 ◦ C for 2 h.
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Fig. 3. TG profiles for the as-precipitated precursors: (a) Co2 Z, (b) Co2 Y and (c) BaM. Fig. 1. XRD patterns of the as-precipitated: (a) BaM, (b) Co2 Z and (c) Co2 Y precursors.
2.2. Powder characterization The thermal properties of the samples were measured using DTA/TG (Netzsch STA 409C). Crystalline phase and chemical composition of the calcined specimens were characterized by an X-ray diffractometer with a Cu K␣ (Siemens D5000) and FTIR spectrometer (Bruker EQUINOX 55), respectively.
3. Results and discussion 3.1. Crystalline phase of the precursors Fig. 1 shows the XRD patterns of the as-precipitated BaM, Co2 Z and Co2 Y precursors. The as-precipitate BaM precursor was amorphous. The precursors of Co2 Z and Co2 Y were both a mixture of crystalline BaCO3 and amorphous phase. However, the crystallite size of the BaCO3 estimated using the Scherrer equation for the Co2 Z precursor (29 nm) was smaller than that for the Co2 Y precursor (60 nm). This result suggests that the crystallite size of BaCO3 may be related to the molar ratio of Co2+ /Ba2+ in the precursors, and a higher Co2+ /Ba2+ ratio lead to bigger crystallite sizes of BaCO3 . Fig. 2 shows the FTIR spectra of the as-precipitated BaM, Co2 Z and Co2 Y precursors. The main absorption bands at 693, 857, 1059 and 1454 cm−1 are attributed to the carbonate groups. The absorption bands at 1384, 1454 and 1749 cm−1 resulted
Fig. 2. FTIR spectra of the as-precipitated: (a) Co2 Y, (b) Co2 Z and (c) BaM precursors.
from the COO group. The absorption band at around 3400 cm−1 is associated with the OH group. These results reveal that the asprecipitate BaM, Co2 Z and Co2 Y precursors were all mixtures of BaCO3 and iron and cobalt hydroxides, which give good correlation with the crystalline phase observed in the XRD. 3.2. Thermal behavior of the precursors The TG profiles for the as-precipitated precursors are shown in Fig. 3. For the Co2 Y precursor, two distinct weight loss steps around 100–500 and 780–950 ◦ C were observed, which were attributed to the decomposition of hydroxides and barium carbonate, respectively. However, the weight loss for the Co2 Z and BaM precursors occurred gradually and finished around 710 and 650 ◦ C, respectively. The barium carbonate decomposition temperature for the Co2 Y precursor was higher than that for the Co2 Z and BaM precursors and may result from the larger BaCO3 crystallite size as suggested by the XRD results. Fig. 4 is the DTA diagram of the as-precipitated precursors. Roos investigated the reaction process of chemically precipitated barium ferrites using DTA and observed that a large exothermic peak occurred at 760 ◦ C due to the formation of a BaM phase [12]. The exothermic peak due to the crystallization of BaM phase for the Co2 Z and BaM precursors occurred around 710 ◦ C in the DTA curve. However, no exothermic peak was observed for the Co2 Y precursor.
Fig. 4. DTA diagram of the as-precipitated precursors: (a) Co2 Z, (b) BaM and (c) Co2 Y.
H.-I. Hsiang, R.-Q. Yao / Materials Chemistry and Physics 104 (2007) 1–4
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Fig. 5. XRD patterns for the Co2 Y precursors calcined at various temperatures: (a) 600 ◦ C, (b) 700 ◦ C and (c) 800 ◦ C.
3.3. Reaction processes of the precursors 3.3.1. Co2 Y precursor The XRD patterns for the Co2 Y precursors calcined at various temperatures are shown in Fig. 5. After calcining at 600 ◦ C, only BaCO3 phase was observed (Fig. 5a). Beyond BaCO3 , the new phase, ␣-Fe2 O3 due to the decomposition of iron hydroxide, the BaFe2 O4 phase peaks accompanied by a significant BaM phase were observed after calcinations at 700 ◦ C, as shown in Fig. 5b. The formation mechanism of the BaM solid state reaction has been shown to take place in two main steps: (1) the decomposition of BaCO3 accompanied with the formation of monoferrite and (2) the diffusion of Ba2+ into iron oxide [13]: BaCO3 + Fe2 O3 → BaFe2 O4 + CO2 BaFe2 O4 + 5Fe2 O3 → BaFe12 O19 Crystalline BaFe2 O4 was reported as an essential intermediate phase for the formation of BaM using a solid state reaction between BaCO3 and Fe2 O3 . In the case of the Co2 Y precursor, the ␣-Fe2 O3 phase was observed as an intermediate phase, which resulted from the conversion of amorphous iron hydroxide during calcination. It is well known that the phase transformation that occurs during calcination gives rise to transformed ␣-Fe2 O3 powder, which has undergone considerable aggregation and crystal growth [14]. The above characteristics are detrimental to the formation of nano-sized ␣-Fe2 O3 powder. Therefore, the transformed ␣-Fe2 O3 powder, which possesses larger crystallite size, then, reacted with BaCO3 to form BaFe2 O4 and BaFe12 O19 phases following the solid state reaction mechanism. At 1000 ◦ C, Co2 Y phase became the main phase, but the intermediate phase, BaFe2 O4 , still remained (Fig. 5c), which is supported by the study of Pullar et al. [15]. 3.3.2. Co2 Z precursor For the Co2 Z precursor calcined at 500 ◦ C, the iron hydroxide decomposed into amorphous Fe2 O3 (Fig. 6a). The crystallization of Fe2 O3 was inhibited due to the intimate mixture of amorphous CoO and Fe2 O3 , which resulted in Fe2 O3 maintaining as amorphous phase. At 600 ◦ C, BaCO3 started to decompose and reacted with amorphous Fe2 O3 to form a BaM
Fig. 6. XRD patterns for the Co2 Z precursors calcined at various temperatures: (a) 500 ◦ C, (b) 600 ◦ C, (c) 700 ◦ C, (d) 850 ◦ C and (e) 1200 ◦ C.
phase (Fig. 6b). At 700 ◦ C, BaCO3 decomposed completely and only the BaM phase was observed (Fig. 6c). Wang et al. [7] prepared Co2 Z ferrite using a citrate precursor method. They found that a BaM phase was formed, with intermediate phases, ␣-Fe2 O3 and BaFe2 O4 , after calcination at 800 ◦ C. Co2 Z became the major phase with a small amount of Co2 Y phase retention at 1150 ◦ C. However, for the Co2 Z precursor prepared using chemical coprecipitation, the decomposition temperature of BaCO3 was lower than the crystallization temperature of ␣-Fe2 O3 from amorphous iron oxide, which may result from the smaller BaCO3 crystalline size. Consequently, BaFe12 O19 phase was obtained directly using the reaction between BaCO3 and amorphous iron oxide, with no ␣-Fe2 O3 and BaFe2 O4 formed as intermediates (Fig. 6c). This is supported by the finding observed by Pullar and Bhattacharya [16], who reported that BaM could be directly obtained using a sol–gel process, with no BaFe2 O4 formed as an intermediate. As the calcination temperature was raised from 700 to 850 ◦ C, the Co2 Y phase was predominant with a small amount of BaM phase (Fig. 6d). Pullar et al. [15] investigated the formation of Co2 Z ferrite prepared using a sol–gel technique and observed that CoFe2 O4 phase formed after calcining at 800 ◦ C. However, CoFe2 O4 phase was not observed after calcining Co2 Z precursor in this study. It may be due to the smaller BaCO3 crystallite which reacted with amorphous CoO and BaM to form Co2 Y before the crystallization of CoFe2 O4 phase. Fig. 6e shows the XRD pattern of the Co2 Z precursor after calcinations at 1200 ◦ C. The single Co2 Z phase was obtained through the reaction of the two previous formed BaM and Co2 Y phases, which compares well with the previous reports [15]. 3.3.3. BaM precursor Fig. 7 shows the XRD patterns for the BaM precursor calcined at various temperatures. After calcining at 550 ◦ C, the -Fe2 O3 was observed due to the crystallization of amorphous Fe2 O3 , which has not been reported in preparing BaM (Fig. 7a). The crystallite size of -Fe2 O3 estimated from Scherrer formula is around 10 nm. The single BaM phase was observed after calcinations at 600 ◦ C, with no BaFe2 O4 formed as an intermediate
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4. Conclusions
Fig. 7. XRD patterns for the BaM precursor calcined at various temperatures: (a) 550 ◦ C and (b) 600 ◦ C.
(Fig. 7b). The formation temperature of single BaM phase in this study was lower than the observations reported in previous studies [17–19]. This may be due to the reactants (nano-sized -Fe2 O3 and amorphous BaCO3 ) having more intimate contact, which decreased the average transport distance for surface and volume Ba2+ diffusion. Zhong et al. [20] investigated the formation mechanism of BaFe12 O19 prepared using a sol–gel technique and observed that if ␣-Fe2 O3 formed as the intermediate, a complete conversion of the reactants into single BaFe12 O19 phase required high temperature. However, Pullar et al. [17] investigated the formation of BaM ferrite prepared using a sol–gel technique and observed hematite formed at 600 ◦ C, but BaM was the major phase by 650 ◦ C and became a single phase at 750 ◦ C. This may be due to the existence of higher reactivity of BaO derived from Ba(NO3 )2 . Therefore, the beginning and complete formation of BaFe12 O19 phase for Co2 Z and BaM precursors occurred at a lower temperature than that for the Co2 Y precursor, which may originate from BaCO3 of smaller crystallite sizes and the lack of coarsened ␣-Fe2 O3 existing in the calcined samples. Therefore, the formation mechanisms of hexagonal ferrites for the precursors of BaM, Co2 Y and Co2 Z during calcinations can be summed up by Eqs. (1)–(3), respectively. Amorphous BaCO3 + amorphous Fe(OH)3 → -Fe2 O3 + amorphous BaCO3 → BaM
(1)
BaCO3 + amorphous Co(OH)2 + amorphous Fe(OH)3 → ␣-Fe2 O3 + BaCO3 + amorphous CoO → BaFe2 O4 + ␣-Fe2 O3 → BaFe2 O4 + BaM + Co2 Y → Co2 Y
(2)
BaCO3 + amorphous Co(OH)2 + amorphous Fe(OH)3 → amorphous Fe2 O3 + amorphous CoO + BaCO3 → BaM + Co2 Y → Co2 Z
(3)
(1) The as-precipitate BaM precursor was amorphous. The precursors of Co2 Z and Co2 Y were both a mixture of crystalline BaCO3 and amorphous phase. The crystallite size of the BaCO3 for the Co2 Z precursor (29 nm) was smaller than Co2 Y precursor (60 nm). (2) For Co2 Y precursor, the intermediate phase, BaM, was obtained through the reaction of the earlier formed BaFe2 O4 and ␣-Fe2 O3 . For BaM precursor, BaM phase was formed directly through the reaction of the preceding -Fe2 O3 and amorphous BaCO3. However, for the Co2 Z precursors, BaM phase was obtained directly from the BaCO3 and amorphous iron hydroxide reaction, with no ␣-Fe2 O3 and BaFe2 O4 formed as intermediates. (3) Co2 Z phase prepared was obtained through the reaction of the two previous formed BaM and Co2 Y phases. Acknowledgement The authors express thanks to the National Science Council of the Republic of China for financially supporting this project under contract number NSC-90-2216-E-006-073. References [1] B.D. Cullity, Introduction to Magnetic Materials, Addison-Wesley, Reading, MA, 1972, p. 575. [2] G.J. Jonker, H.P.J. Wijn, P.B. Braun, Philos. Tech. Rev. 18 (1956) 145. [3] H.I. Hsiang, Jpn. J. Appl. Phys. 41 (2002) 5137. [4] Y. Bai, J. Zhou, Z. Gui, L. Li, J. Magn. Magn. Mater. 278 (2004) 208. [5] O. Kimura, J. Am. Ceram. Soc. 78 (1995) 2857. [6] P.B. Braun, Philos. Res. Rep. 12 (1957) 491. [7] X. Wang, L. Li, Z. Gui, S. Shu, J. Zhou, Mater. Chem. Phys. 77 (2002) 248. [8] J. Huang, H. Zhuang, W. Li, J. Magn. Magn. Mater. 256 (2003) 390. [9] G. Xiong, G. Wei, X. Yang, L. Lu, X. Wang, J. Mater. Sci. 35 (2000) 931. [10] R.C. Pullar, S.G. Appleton, M.H. Stacey, M.D. Taylor, A.K. Bhattacharya, J. Magn. Magn. Mater. 186 (1998) 313. [11] H. Zhang, L. Li, J. Zhou, Z. Yue, Z. Ma, Z. Gui, J. Eur. Ceram. Soc. 21 (2001) 149. [12] W. Roos, J. Am. Ceram. Soc. 63 (1980) 601. [13] H.P. Steier, J. Requena, J.S. Moya, J. Mater. Res. 14 (1999) 3647. [14] H.I. Hsiang, F.S. Yen, Ceram. Int. 29 (2003) 1. [15] R.C. Pullar, M.H. Stacey, M.D. Taylor, A.K. Bhattacharya, Acta Mater. 49 (2001) 4241. [16] R.C. Pullar, A.K. Bhattacharya, Mater. Lett. 57 (2002) 537. [17] R.C. Pullar, M.D. Taylor, A.K. Bhattacharya, J. Eur. Ceram. Soc. 22 (2002) 2039. [18] S.E. Jacobo, C.D. Pascual, R.R. Clemente, M.A. Blesa, J. Mater. Sci. 32 (1997) 1025. [19] S.R. Janasi, M. Emura, F.J.G. Landgraf, D. Rodrigues, J. Magn. Magn. Mater. 238 (2002) 168. [20] W. Zhong, W. Ding, N. Zhang, J. Hong, Q. Yan, Y. Du, J. Magn. Magn. Mater. 168 (1997) 196.
Hexagonal ferrite powder synthesis using chemical coprecipitation Hsing-I Hsiang ∗ , Ren-Qian Yao Particulate Materials Research Center, Department of Resources Engineering, National Cheng Kung University, Tainan, Taiwan, ROC Received 12 April 2006; received in revised form 13 October 2006; accepted 19 February 2007
Abstract The formation mechanism of 3BaO·2CoO·12Fe2 O3 (Co2 Z), 2BaO·2CoO·6Fe2 O3 (Co2 Y) and BaO·6Fe2 O3 (BaM) powders were prepared using chemical coprecipitation methods in this study using X-ray diffraction (XRD), thermo-gravimetry (TG), differential thermal analysis (DTA) and Fourier transform infrared spectroscopy (FTIR). It was found that the BaM phase was formed directly through the reaction of the preceding -Fe2 O3 and amorphous BaCO3 for BaM precursor. For the Co2 Y precursor, the intermediate phase, BaM, was obtained through the reaction of the earlier formed BaFe2 O4 and ␣-Fe2 O3 . The Co2 Y phase was obtained through a BaM and BaFe2 O4 reaction. However, for the Co2 Z precursors, the BaM phase was obtained directly from the BaCO3 and amorphous iron hydroxide reaction, with no ␣-Fe2 O3 and BaFe2 O4 formed as intermediates. Co2 Z phase was obtained through the reaction of the two previous formed BaM and Co2 Y phases. © 2007 Elsevier B.V. All rights reserved. Keywords: Hexagonal ferrites; Chemical coprecipitation; Co2 Z; Co2 Y
1. Introduction Hexagonal ferrites, such as BaM, Co2 Y and Co2 Z are very attractive materials for magnetic recording media and high frequency circuits. BaM is an important hard magnetic material because of its large coercive force, which results from the large magnetic anisotropy [1]. However, Co2 Y and Co2 Z ferrites exhibit a higher dispersion frequency than that of nickel ferrites because of the magneto-plumbite magnetic anisotropy can be used in high frequency circuits [2]. Hsiang [3] and Bai et al. [4] observed that multilayer chip inductors made of Co2 Z and Co2 Y ferrites exhibited excellent magnetic properties. A single Co2 Z phase is difficult to produce using solid state reaction, requiring a temperature of at least 1200 ◦ C, due to its complicated structures [5]. The Co2 Z phase usually coexists with some of the M, Y, W and spinel phases [6]. The Co2 Y phase has been regarded as an intermediate phase during Co2 Z ferrite preparation [7]. Therefore, the formation temperature of a pure Co2 Y phase was lower than that of the Co2 Z phase. Bai et al. [4] prepared Co2 Y ferrite using a solid state reaction. They observed that Co2 Y became the major phase, accompanied with intermediate phases, BaFe2 O4 and BaM at 1000 ◦ C and that a single phase of Co2 Y was obtained at 1050 ◦ C. ∗
Corresponding author. Tel.: +886 6 2757575x62821; fax: +886 6 2380421. E-mail address: [email protected] (H.-I. Hsiang).
0254-0584/$ – see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.matchemphys.2007.02.030
Many wet chemical methods have been proposed for preparing hexagonal ferrite particles, such as chemically derived citrate precursor [8], stearic acid sol–gel [9], inorganic sol–gel method [10] and the self-propagation method [11] to decrease the synthesis temperature. The coprecipitation method is attractive for large-scale powder production because of good stochiometric control and homogeneous mixing on the atomic scale. However, reports on the formation mechanisms of Co2 Z, Co2 Y and BaM ferrites prepared using the coprecipitation method have not been made to the best of our knowledge. In this study, the formation mechanism of Co2 Z, Co2 Y and BaM powders prepared using chemical coprecipitation methods were investigated using XRD, TG, DTA and FTIR. 2. Experimental procedure 2.1. Preparation of powders 3BaO·2CoO·12Fe2 O3 , 2BaO·2CoO·6Fe2 O3 , BaO·6Fe2 O3 powders were prepared using coprecipitation methods. The starting materials were reagentgrade purity oxide powders (Ba(NO3 )2 , Co(NO3 )2 ·6H2 O, Fe(NO3 )2 ·9H2 O, NaOH and Na2 CO3 ). Desired amounts of Ba(NO3 )2 , Co(NO3 )2 ·6H2 O, Fe(NO3 )3 ·9H2 O were mixed to yield a clear aqueous solution. The mixed solution was added to an aqueous solution of NaOH/Na2 CO3 using vigorous stirring. The pH value reached around 13 after titration completion. After precipitation, the co-precipitated powders were filtered and washed with deionized water two times, followed by washing with 99.5 ethyl alcohol and filtering. The washed powders were dried at 80 ◦ C and then calcined at 500–1250 ◦ C for 2 h.
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Fig. 3. TG profiles for the as-precipitated precursors: (a) Co2 Z, (b) Co2 Y and (c) BaM. Fig. 1. XRD patterns of the as-precipitated: (a) BaM, (b) Co2 Z and (c) Co2 Y precursors.
2.2. Powder characterization The thermal properties of the samples were measured using DTA/TG (Netzsch STA 409C). Crystalline phase and chemical composition of the calcined specimens were characterized by an X-ray diffractometer with a Cu K␣ (Siemens D5000) and FTIR spectrometer (Bruker EQUINOX 55), respectively.
3. Results and discussion 3.1. Crystalline phase of the precursors Fig. 1 shows the XRD patterns of the as-precipitated BaM, Co2 Z and Co2 Y precursors. The as-precipitate BaM precursor was amorphous. The precursors of Co2 Z and Co2 Y were both a mixture of crystalline BaCO3 and amorphous phase. However, the crystallite size of the BaCO3 estimated using the Scherrer equation for the Co2 Z precursor (29 nm) was smaller than that for the Co2 Y precursor (60 nm). This result suggests that the crystallite size of BaCO3 may be related to the molar ratio of Co2+ /Ba2+ in the precursors, and a higher Co2+ /Ba2+ ratio lead to bigger crystallite sizes of BaCO3 . Fig. 2 shows the FTIR spectra of the as-precipitated BaM, Co2 Z and Co2 Y precursors. The main absorption bands at 693, 857, 1059 and 1454 cm−1 are attributed to the carbonate groups. The absorption bands at 1384, 1454 and 1749 cm−1 resulted
Fig. 2. FTIR spectra of the as-precipitated: (a) Co2 Y, (b) Co2 Z and (c) BaM precursors.
from the COO group. The absorption band at around 3400 cm−1 is associated with the OH group. These results reveal that the asprecipitate BaM, Co2 Z and Co2 Y precursors were all mixtures of BaCO3 and iron and cobalt hydroxides, which give good correlation with the crystalline phase observed in the XRD. 3.2. Thermal behavior of the precursors The TG profiles for the as-precipitated precursors are shown in Fig. 3. For the Co2 Y precursor, two distinct weight loss steps around 100–500 and 780–950 ◦ C were observed, which were attributed to the decomposition of hydroxides and barium carbonate, respectively. However, the weight loss for the Co2 Z and BaM precursors occurred gradually and finished around 710 and 650 ◦ C, respectively. The barium carbonate decomposition temperature for the Co2 Y precursor was higher than that for the Co2 Z and BaM precursors and may result from the larger BaCO3 crystallite size as suggested by the XRD results. Fig. 4 is the DTA diagram of the as-precipitated precursors. Roos investigated the reaction process of chemically precipitated barium ferrites using DTA and observed that a large exothermic peak occurred at 760 ◦ C due to the formation of a BaM phase [12]. The exothermic peak due to the crystallization of BaM phase for the Co2 Z and BaM precursors occurred around 710 ◦ C in the DTA curve. However, no exothermic peak was observed for the Co2 Y precursor.
Fig. 4. DTA diagram of the as-precipitated precursors: (a) Co2 Z, (b) BaM and (c) Co2 Y.
H.-I. Hsiang, R.-Q. Yao / Materials Chemistry and Physics 104 (2007) 1–4
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Fig. 5. XRD patterns for the Co2 Y precursors calcined at various temperatures: (a) 600 ◦ C, (b) 700 ◦ C and (c) 800 ◦ C.
3.3. Reaction processes of the precursors 3.3.1. Co2 Y precursor The XRD patterns for the Co2 Y precursors calcined at various temperatures are shown in Fig. 5. After calcining at 600 ◦ C, only BaCO3 phase was observed (Fig. 5a). Beyond BaCO3 , the new phase, ␣-Fe2 O3 due to the decomposition of iron hydroxide, the BaFe2 O4 phase peaks accompanied by a significant BaM phase were observed after calcinations at 700 ◦ C, as shown in Fig. 5b. The formation mechanism of the BaM solid state reaction has been shown to take place in two main steps: (1) the decomposition of BaCO3 accompanied with the formation of monoferrite and (2) the diffusion of Ba2+ into iron oxide [13]: BaCO3 + Fe2 O3 → BaFe2 O4 + CO2 BaFe2 O4 + 5Fe2 O3 → BaFe12 O19 Crystalline BaFe2 O4 was reported as an essential intermediate phase for the formation of BaM using a solid state reaction between BaCO3 and Fe2 O3 . In the case of the Co2 Y precursor, the ␣-Fe2 O3 phase was observed as an intermediate phase, which resulted from the conversion of amorphous iron hydroxide during calcination. It is well known that the phase transformation that occurs during calcination gives rise to transformed ␣-Fe2 O3 powder, which has undergone considerable aggregation and crystal growth [14]. The above characteristics are detrimental to the formation of nano-sized ␣-Fe2 O3 powder. Therefore, the transformed ␣-Fe2 O3 powder, which possesses larger crystallite size, then, reacted with BaCO3 to form BaFe2 O4 and BaFe12 O19 phases following the solid state reaction mechanism. At 1000 ◦ C, Co2 Y phase became the main phase, but the intermediate phase, BaFe2 O4 , still remained (Fig. 5c), which is supported by the study of Pullar et al. [15]. 3.3.2. Co2 Z precursor For the Co2 Z precursor calcined at 500 ◦ C, the iron hydroxide decomposed into amorphous Fe2 O3 (Fig. 6a). The crystallization of Fe2 O3 was inhibited due to the intimate mixture of amorphous CoO and Fe2 O3 , which resulted in Fe2 O3 maintaining as amorphous phase. At 600 ◦ C, BaCO3 started to decompose and reacted with amorphous Fe2 O3 to form a BaM
Fig. 6. XRD patterns for the Co2 Z precursors calcined at various temperatures: (a) 500 ◦ C, (b) 600 ◦ C, (c) 700 ◦ C, (d) 850 ◦ C and (e) 1200 ◦ C.
phase (Fig. 6b). At 700 ◦ C, BaCO3 decomposed completely and only the BaM phase was observed (Fig. 6c). Wang et al. [7] prepared Co2 Z ferrite using a citrate precursor method. They found that a BaM phase was formed, with intermediate phases, ␣-Fe2 O3 and BaFe2 O4 , after calcination at 800 ◦ C. Co2 Z became the major phase with a small amount of Co2 Y phase retention at 1150 ◦ C. However, for the Co2 Z precursor prepared using chemical coprecipitation, the decomposition temperature of BaCO3 was lower than the crystallization temperature of ␣-Fe2 O3 from amorphous iron oxide, which may result from the smaller BaCO3 crystalline size. Consequently, BaFe12 O19 phase was obtained directly using the reaction between BaCO3 and amorphous iron oxide, with no ␣-Fe2 O3 and BaFe2 O4 formed as intermediates (Fig. 6c). This is supported by the finding observed by Pullar and Bhattacharya [16], who reported that BaM could be directly obtained using a sol–gel process, with no BaFe2 O4 formed as an intermediate. As the calcination temperature was raised from 700 to 850 ◦ C, the Co2 Y phase was predominant with a small amount of BaM phase (Fig. 6d). Pullar et al. [15] investigated the formation of Co2 Z ferrite prepared using a sol–gel technique and observed that CoFe2 O4 phase formed after calcining at 800 ◦ C. However, CoFe2 O4 phase was not observed after calcining Co2 Z precursor in this study. It may be due to the smaller BaCO3 crystallite which reacted with amorphous CoO and BaM to form Co2 Y before the crystallization of CoFe2 O4 phase. Fig. 6e shows the XRD pattern of the Co2 Z precursor after calcinations at 1200 ◦ C. The single Co2 Z phase was obtained through the reaction of the two previous formed BaM and Co2 Y phases, which compares well with the previous reports [15]. 3.3.3. BaM precursor Fig. 7 shows the XRD patterns for the BaM precursor calcined at various temperatures. After calcining at 550 ◦ C, the -Fe2 O3 was observed due to the crystallization of amorphous Fe2 O3 , which has not been reported in preparing BaM (Fig. 7a). The crystallite size of -Fe2 O3 estimated from Scherrer formula is around 10 nm. The single BaM phase was observed after calcinations at 600 ◦ C, with no BaFe2 O4 formed as an intermediate
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H.-I. Hsiang, R.-Q. Yao / Materials Chemistry and Physics 104 (2007) 1–4
4. Conclusions
Fig. 7. XRD patterns for the BaM precursor calcined at various temperatures: (a) 550 ◦ C and (b) 600 ◦ C.
(Fig. 7b). The formation temperature of single BaM phase in this study was lower than the observations reported in previous studies [17–19]. This may be due to the reactants (nano-sized -Fe2 O3 and amorphous BaCO3 ) having more intimate contact, which decreased the average transport distance for surface and volume Ba2+ diffusion. Zhong et al. [20] investigated the formation mechanism of BaFe12 O19 prepared using a sol–gel technique and observed that if ␣-Fe2 O3 formed as the intermediate, a complete conversion of the reactants into single BaFe12 O19 phase required high temperature. However, Pullar et al. [17] investigated the formation of BaM ferrite prepared using a sol–gel technique and observed hematite formed at 600 ◦ C, but BaM was the major phase by 650 ◦ C and became a single phase at 750 ◦ C. This may be due to the existence of higher reactivity of BaO derived from Ba(NO3 )2 . Therefore, the beginning and complete formation of BaFe12 O19 phase for Co2 Z and BaM precursors occurred at a lower temperature than that for the Co2 Y precursor, which may originate from BaCO3 of smaller crystallite sizes and the lack of coarsened ␣-Fe2 O3 existing in the calcined samples. Therefore, the formation mechanisms of hexagonal ferrites for the precursors of BaM, Co2 Y and Co2 Z during calcinations can be summed up by Eqs. (1)–(3), respectively. Amorphous BaCO3 + amorphous Fe(OH)3 → -Fe2 O3 + amorphous BaCO3 → BaM
(1)
BaCO3 + amorphous Co(OH)2 + amorphous Fe(OH)3 → ␣-Fe2 O3 + BaCO3 + amorphous CoO → BaFe2 O4 + ␣-Fe2 O3 → BaFe2 O4 + BaM + Co2 Y → Co2 Y
(2)
BaCO3 + amorphous Co(OH)2 + amorphous Fe(OH)3 → amorphous Fe2 O3 + amorphous CoO + BaCO3 → BaM + Co2 Y → Co2 Z
(3)
(1) The as-precipitate BaM precursor was amorphous. The precursors of Co2 Z and Co2 Y were both a mixture of crystalline BaCO3 and amorphous phase. The crystallite size of the BaCO3 for the Co2 Z precursor (29 nm) was smaller than Co2 Y precursor (60 nm). (2) For Co2 Y precursor, the intermediate phase, BaM, was obtained through the reaction of the earlier formed BaFe2 O4 and ␣-Fe2 O3 . For BaM precursor, BaM phase was formed directly through the reaction of the preceding -Fe2 O3 and amorphous BaCO3. However, for the Co2 Z precursors, BaM phase was obtained directly from the BaCO3 and amorphous iron hydroxide reaction, with no ␣-Fe2 O3 and BaFe2 O4 formed as intermediates. (3) Co2 Z phase prepared was obtained through the reaction of the two previous formed BaM and Co2 Y phases. Acknowledgement The authors express thanks to the National Science Council of the Republic of China for financially supporting this project under contract number NSC-90-2216-E-006-073. References [1] B.D. Cullity, Introduction to Magnetic Materials, Addison-Wesley, Reading, MA, 1972, p. 575. [2] G.J. Jonker, H.P.J. Wijn, P.B. Braun, Philos. Tech. Rev. 18 (1956) 145. [3] H.I. Hsiang, Jpn. J. Appl. Phys. 41 (2002) 5137. [4] Y. Bai, J. Zhou, Z. Gui, L. Li, J. Magn. Magn. Mater. 278 (2004) 208. [5] O. Kimura, J. Am. Ceram. Soc. 78 (1995) 2857. [6] P.B. Braun, Philos. Res. Rep. 12 (1957) 491. [7] X. Wang, L. Li, Z. Gui, S. Shu, J. Zhou, Mater. Chem. Phys. 77 (2002) 248. [8] J. Huang, H. Zhuang, W. Li, J. Magn. Magn. Mater. 256 (2003) 390. [9] G. Xiong, G. Wei, X. Yang, L. Lu, X. Wang, J. Mater. Sci. 35 (2000) 931. [10] R.C. Pullar, S.G. Appleton, M.H. Stacey, M.D. Taylor, A.K. Bhattacharya, J. Magn. Magn. Mater. 186 (1998) 313. [11] H. Zhang, L. Li, J. Zhou, Z. Yue, Z. Ma, Z. Gui, J. Eur. Ceram. Soc. 21 (2001) 149. [12] W. Roos, J. Am. Ceram. Soc. 63 (1980) 601. [13] H.P. Steier, J. Requena, J.S. Moya, J. Mater. Res. 14 (1999) 3647. [14] H.I. Hsiang, F.S. Yen, Ceram. Int. 29 (2003) 1. [15] R.C. Pullar, M.H. Stacey, M.D. Taylor, A.K. Bhattacharya, Acta Mater. 49 (2001) 4241. [16] R.C. Pullar, A.K. Bhattacharya, Mater. Lett. 57 (2002) 537. [17] R.C. Pullar, M.D. Taylor, A.K. Bhattacharya, J. Eur. Ceram. Soc. 22 (2002) 2039. [18] S.E. Jacobo, C.D. Pascual, R.R. Clemente, M.A. Blesa, J. Mater. Sci. 32 (1997) 1025. [19] S.R. Janasi, M. Emura, F.J.G. Landgraf, D. Rodrigues, J. Magn. Magn. Mater. 238 (2002) 168. [20] W. Zhong, W. Ding, N. Zhang, J. Hong, Q. Yan, Y. Du, J. Magn. Magn. Mater. 168 (1997) 196.