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Studies on characterization, microstructures and magnetic properties of nano-size barium hexa-ferrite prepared through a hydrothermal precipitation–calcination route
Materials Chemistry and Physics 86 (2004) 132–136
Studies on characterization, microstructures and magnetic properties of nano-size barium hexa-ferrite prepared through a hydrothermal precipitation–calcination route D. Mishra a , S. Anand a,∗ , R.K. Panda b , R.P. Das a b
a Regional Research Laboratory, Bhubaneswar 751013, Orissa, India Materials Science Division, Department of Chemistry, Berhampur University, Berhampur 760007, Orissa, India
Received 17 July 2003; received in revised form 21 January 2004; accepted 17 February 2004
Abstract An attempt was made to prepare nano-size barium hexa-ferrite particles following a hydrothermal precipitation–calcination route using barium and iron nitrate solutions. During hydrothermal treatment at 180 ◦ C (2 h precipitation time) barium carbonate and hematite phases were formed. This precursor was calcined at 800, 1000 and 1200 ◦ C to determine the conditions for obtaining barium hexa-ferrite. The characterization studies on calcined products revealed that up to 800 ◦ C, the major crystalline phases (barium carbonate and hematite) of the precursor were retained. At 1000 ◦ C, formation of barium hexa-ferrite started and at 1200 ◦ C, though most of the major peaks of X-ray diffractogram corresponded to barium hexa-ferrite, a number of peaks corresponding to hematite were also present. Some low intensity peaks for barium carbonate were observed. The average particle size was 40 nm. Saturation magnetization, remanence magnetization and coercivity were found to be 40.0, 21.6 emu g−1 and 2.87 kOe, respectively. The values obtained both for coercivity and magnetization for the present sample were lower than the reported bulk values which could be due to the fact that the sample prepared through the present technique was not mono-phasic. © 2004 Elsevier B.V. All rights reserved. Keywords: Barium hexa-ferrite; Nano-size; Urea; Hydrothermal treatment; Calcination; Magnetic properties
1. Introduction Barium hexa-ferrite (BaFe12 O19 ) having hexagonal symmetry belongs to the magneto-plumbite group of oxides and is classified as hard ferrite due to its high saturation magnetization and high intrinsic coercivity. The major areas in which barium hexa-ferrite finds applications include perpendicular high density recording media, microwave devices, in electric generators and in many magnetically operated devices such as magnetic leviations, telephone ringers and receivers, etc. [1–3]. In recent years, the hydrothermal technique of preparation for hexa-ferrite appears to be quite attractive among all the processes [4–12]. The particle size, morphology and magnetic properties of the hydrothermally prepared barium hexa-ferrites are strongly dependent on the preparation conditions and on the nature of precursors used [4]. All the above cited reports mentioned the direct synthesis of this material under high temperature and pressure conditions from mixtures of various starting materials ∗ Corresponding author. Tel.: +91-674-581750; fax: +91-674-581750. E-mail address: s p [email protected] (S. Anand).
0254-0584/$ – see front matter © 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.matchemphys.2004.02.017
such as: Ba(NO3 )3 and Fe(NO3 )3 ·9H2 O in the presence of NaOH [5,7,10], from FeOOH and Ba(OH)2 [5,6,9,11], from ␣-Fe2 O3 and Ba(OH)2 [5,8,11] and from FeCl3 and Ba(OH)2 mixtures [12]. In our earlier investigations on preparation of barium aluminates and Mn-doped barium aluminates, it was observed that it was possible to obtain nano-size particles by employing a combination of hydrothermal precipitation–calcination technique [13,14]. The hydrothermally prepared hematite particles and Mn–Zn ferrite samples exhibited unusual magnetic properties [15–18]. With a view to prepare nano-size barium hexa-ferrite particles and to study their magnetic properties, the present investigations were taken up. 2. Experimental details The barium hexa-ferrite sample was prepared from barium and iron nitrate solutions following hydrothermal precipitation–calcination techniques. For hydrothermal precipitation the calculated amount of Ba and Fe3+ nitrate stock solutions were mixed thoroughly according to their
D. Mishra et al. / Materials Chemistry and Physics 86 (2004) 132–136
stoichiometric requirement and then urea was added in the urea/metal molar ratio of 2. The contents were then taken inside a PARR 2 l capacity reactor for hydrothermal precipitation at 180 ◦ C. After keeping the contents for 2 h at 180 ◦ C, the autoclave was cooled. The precipitated slurry was filtered, washed to remove anions and then dried at 100 ◦ C for 24 h in an air oven. Calcination of the air-dried sample was done at 800, 1000 and 1200 ◦ C inside a muffle furnace. The X-ray diffractograms of the powder samples were taken using a Philips PW 1710 X-ray diffractometer. The diffracted X-ray intensities were recorded as a function of diffraction angle 2θ using nickel-filtered copper target (Cu K␣ radiation with wavelength = 1.5404 Å). The Fourier transform infrared spectroscopy studies of the samples were done using a Perkin-Elmer P-500 spectrophotometer in the range 400–4000 cm−1 . Samples of 0.2 mg each were mixed thoroughly with 20 mg spectroscopy grade KBr (E-MERCK India Ltd.) and were then made into transparent pellets by using a 10 mm stainless steel die inside a pelletizer. The thermogravimetric and differential thermal analysis studies (TG–DTA) were carried out using a Stanton Thermal Analyzer STA-740 instrument. The weighed sample (10 mg) was taken in a small alumina sample holder and heated up to 1300 ◦ C in normal non-isothermal conditions at a heating rate of 15 K min−1 using ␣-Al2 O3 as reference material. The microstructure of the sample was studied using a transmission electron microscopy (TEM, JEOL, JEM-200CX Model). The magnetic measurements were carried out at room temperature with a vibrating sample magnetometer (PARR, Model 4500). The field at the actual position was calibrated using a solid nickel cylindrical sample. The maximum applied field of 15 kOe has been used to evaluate the magnetic parameters.
3. Results and discussion 3.1. Phase characterization by powder X-ray diffraction The X-ray powder diffraction patterns of the hydrothermally prepared precursors (at 180 ◦ C, 2 h) and the products obtained after calcination at 800, 1000 and 1200 ◦ C are shown in Fig. 1. The XRD pattern of the precursor (Fig. 1a) shows the presence of both hematite (␣-Fe2 O3 ) and orthorhombic barium carbonate. These two phases were confirmed from their characteristic peaks at 2θ = 24.17◦ , 28.06◦ , 34.97◦ and 42.2◦ for BaCO3 [19a] and at 2θ = 33.39◦ , 35.90◦ , 49.62◦ and 54.13◦ for hematite [19b]. The rest of the peaks having relative intensities between 5 and 10% are either of BaCO3 or of Fe2 O3 . The XRD pattern of the sample calcined at 800 ◦ C (Fig. 1b) also shows BaCO3 and Fe2 O3 to be the major crystalline phases present. When the precursor was calcined at 1000 ◦ C (Fig. 1c), peaks corresponding to BaFe12 O19 [19c] are observed at 2θ = 30.13◦ , 31.96◦ , 33.88◦ , 36.86◦ , 40.60◦ , 42.17◦ , 56.34◦ , 62.82◦ , and
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Fig. 1. (a) XRD patterns of hydrothermally prepared precursor. Precursors calcined at (b) 800 ◦ C for 2 h, (c) 1000 ◦ C for 2 h and (d) 1200 ◦ C for 2 h.
at 72.39◦ confirming formation of hexa-ferrite phase at this temperature. The peaks for barium carbonate and ␣-Fe2 O3 are also observed at this temperature (Fig. 1c). Two prominent peak corresponding to BaO are also identified at 2θ = 28.20◦ and 54.83◦ with slight shifts [19d] but other high intensity peaks for BaO were not present. The XRD pattern of the sample calcined at 1200 ◦ C is shown in Fig. 1d. All the major peaks of this compound are corresponding to the standard BaFe12 O19 [19c], with disappearance of major peaks of BaCO3 . Some low intensity peaks for barium carbonate are observed. Presence of hematite is still detected in this sample with appearance of a number of peaks corresponding to this compound. Due to complexities of the patterns many times relative intensities did not match. 3.1.1. Sequence of reactions during formation of barium hexa-ferrite During hydrothermal treatment at 180 ◦ C: Ba(NO3 )2 + NH2 –CO–NH2 + 2H2 O → BaCO3 (c) + 2NH4 NO3
(1)
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2Fe(NO3 )3 + 3NH2 –CO–NH2 + 6H2 O → Fe2 O3 (c) + 6NH4 NO3 + CO2
(2)
During calcination at 800, 1000, 1200 ◦ C: at 800 ◦ C, the phases of barium carbonate and hematite are retained and formation of barium hexa-ferrite starts at 1000 ◦ C. At 1200 ◦ C, though most of the peaks correspond to barium hexa-ferrite but peaks corresponding to hematite are still identified. Only one prominent peak corresponding to barium oxide was identified. BaCO3 (c) + 6Fe2 O3 (c) → BaFe12 O19 + CO2
(3)
3.2. TG–DTA trace of the hydrothermally prepared precursor The TG–DTA trace of the hydrothermally prepared barium hexa-ferrite precursor obtained under nitrogen atmosphere and at a heating rate of 15 K min−1 is shown in Fig. 2. The TGA trace shows no perceptible weight loss up to 500 ◦ C, indicating the absence of any hydrated oxides of Fe or Ba and this result agrees with the XRD observation that the initial compound obtained on hydrothermal treatment is a mixture of hematite and barium carbonate. In the 500–800 ◦ C and 900–1050 ◦ C region, weight losses of 1–2 and 4.5% are observed from the TGA trace. The latter weight loss is because of the decomposition of BaCO3 and the value obtained is close to the theoretical (4.86%) weight loss expected from the decomposition of BaCO3 +6Fe2 O3 mixture as given by Eq. (3). The TGA shows no further weight loss above 1100 ◦ C, confirming the formation of the stable oxide BaFe12 O19 . The DTA of hydrothermal precursor (Fig. 2) shows only one endo-peak at 800 ◦ C, corresponding to the polymorphic ␣– phase transformation of BaCO3 [20–22]. An interesting observation is that the DTA shows no peaks corresponding to the decomposition of BaCO3 , instead the DTA shows some minor modulations in the 950–1250 ◦ C region. This may due to either the small weight fraction (17.02%) of BaCO3 in the BaCO3 + 6Fe2 O3 or due to the
Fig. 3. (a) FT-IR of hydrothermally prepared precursor at 180 ◦ C. (b) Precursor calcined at 1200 ◦ C.
polycrystalline nature of barium carbonate causing a wide range of decomposition temperature [23]. 3.3. FT-IR analysis The XRD and TG–DTA results of the hydrothermally prepared precursor showed that initially a mixture of BaCO3 and ␣-Fe2 O3 were precipitated and calcination of this precursor at 1200 ◦ C resulted in the formation of BaFe12 O19 . In order to further confirm these phases, the Fourier transform infrared spectra (FT-IR) of the hydrothermally prepared sample and the sample obtained after calcination at 1200 ◦ C for 2 h were recorded in the 400–4000 cm−1 region (Fig. 3). The FT-IR of the hydrothermally prepared (180 ◦ C, 2 h) sample (Fig. 3a) shows no peaks in the 1200–4000 cm−1
Fig. 2. TG–DTA of the hydrothermally prepared precursor sample.
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Fig. 4. TEM image of the 1200 ◦ C calcined hydrothermally prepared precursor calcined at 1200 ◦ C for 2 h. Arrows show hematite phase and encircled particles are of barium hexa-ferrite.
region, indicating the absence of any hydroxides or hydrated oxides of Ba and Fe. Carbonates in the sample are confirmed by two weak and sharp peaks at 1060 and 860 cm−1 [24,25]; the other two characteristic peaks for BaCO3 at 1445 and 710 cm−1 are not visible in the spectrum. The peaks for ␣-Fe2 O3 are also not prominent and appeared as a broad band between 700 and 400 cm−1 , with very minor humps at 632, 593, 500 and 480 cm−1 . These peaks have been identified as the vibrations related to O2− in the Fe2 O3 lattice having symmetries Eg (a), Eu (a), A1g and Eu (p1 ), respectively [26–28]. The FT-IR spectrum of the sample calcined at ≈1200 ◦ C (Fig. 3b) showed only one broad and strong peak centered at ≈600 cm−1 and very weak satellite peaks at 432 and 402 cm−1 . These peaks are assigned by Belloto et al. [29] as Fe–O vibrations for BaFe12 O19 having E1u symmetry. As in case of barium hexa-aluminates the majority of infrared active modes of vibrations for Ba and Fe in BaFe12 O19 appear below 400 cm−1 which could not be recorded in the present case. 3.4. TEM studies A TEM image of the hydrothermally precipitated sample calcined at 1200 ◦ C is shown in Fig. 4. Three different types of particles are clearly visible from the micrograph, although the XRD of the sample showed the presence of only barium hexa-ferrite and hematite. The dark patches (indicated by arrows in Fig. 4) appearing in this micrograph may be due to the surface layers of hematite on BaFe12 O19 . The later phase is seen in this micrographs as cubic particles having ≈40 nm size. The appearance of needle-shaped particles may be due to under grown barium hexa-ferrite particles or of trace amounts of BaO.
4. Magnetic studies Magnetic parameters were determined for the sample calcined at 1200 ◦ C using a vibrating sample magnetometer at room temperature at a maximum applied field of 15 kOe. Saturation magnetization, remanence magnetization and coercivity were determined from the hystersis loop and were found to be 40.0, 21.6 emu g−1 and 2.87 kOe, respectively. Values for both coercivity and saturation magnetization are found to be lower when as compared to reported bulk values [30]. This could be due to presence of minor amounts of phases other than barium hexa-ferrite. 5. Conclusions The attempts made for preparation of nano-size barium hexa-ferrite particles through a hydrothermal precipitation route have been reported. The precipitation of both barium and iron was stoichiometric during hydrothermal precipitation at 180 ◦ C and the phases formed were barium carbonate and hematite. This precursor was calcined at 800, 1000 and 1200 ◦ C. It was observed from the XRD pattern of the sample calcined at 1200 ◦ C, that almost all the lines corresponded to barium hexa-ferrite with some line corresponding to hematite. Saturation magnetization, remanence magnetization and coercivity were 40.0, 21.6 emu g−1 and 2.87 kOe, respectively. Values for both coercivity and magnetization were lower than the reported bulk values due to presence of minor amounts of phases other than barium hexa-ferrite. Acknowledgements The authors are thankful to Dr. Vibhuti N. Misra, Director, Regional Research Laboratory, Bhubaneswar,
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for his kind permission to publish this paper. Help provided by Dr. S.C. Das, Head, Hydrometallurgy Department and Shri K. Sanjay, scientist is acknowledged. The support provided by Dr. S.K. Date, Ms. S.D. Kulkarni and Mr. Chanda of National Chemical Laboratory, Pune during XRD and VSM measurements is thankfully acknowledged.
References [1] W.F. Kladnig, M. Zenger, Modern Ferrites: Technologies and Products, United Nations International Development Organization, New York, 1992. [2] B. Viswanathan, Ferrite Materials: Science and Technology, Norosa Publishing House, Delhi, 1990. [3] D. Bahadur, Bull. Mater. Sci. 5 (1992) 432. [4] A. Ataie, M.R. Priamoon, I.R. Harris, C.B. Ponton, J. Mater. Sci. 30 (1995) 5600. [5] M. Kiyama, Bull. Chem. Soc. Jpn. 49 (1976) 1855. [6] D. Barb, L. Diamandescu, A. Rusi, D. Mihalla, M. Morariu, V. Toedorescu, J. Mater. Sci. 21 (1986) 1118. [7] E. Sada, H. Kumazawa, H.M. Cho, Ind. Eng. Chem. Res. 30 (1993) 1319. [8] M.L. Wang, J.W. Shih, J. Cryst. Growth 116 (1992) 483. [9] H. Kumazawa, H.M. Cho, E. Sada, J. Mater. Sci. 28 (1993) 5247. [10] T. Takada, Y. Ikeda, H. Yoshinaga, Y. Bandoo, in: Proceedings of the International Conference on Ferrites, University Park Press, Tokyo, Japan, 1970, p. 275. [11] C.H. Lin, W. Shih, T.S. Chin, M.L. Wang, Y.C. Yu, IEEE Trans. Magn. Magn. 26 (1990) 15. [12] M. Yoshimura, N. Koudera, T. Noma, S. Somiya, J. Ceram. Soc. Jpn. 97 (1989) 16. [13] D. Mishra, S. Anand, R.K. Panda, R.P. Das, J. Am. Ceram. Soc. 85 (2000) 437. [14] D. Mishra, S. Anand, R.K. Panda, R.P. Das, Mater. Phys. Chem. 82 (2003) 892–896.
[15] C. Rath, K.K. Sahoo, S. Anand, S.K. Date, N.C. Mishra, R.P. Das, J. Magn. Magn. Mater. 202 (1999) 77. [16] C. Rath, K.K. Sahoo, S. Anand, N.C. Mishra, R.P. Das, Appl. Phys. Lett. 75 (1999) 4171. [17] K.K. Sahoo, C. Rath, S. Anand, N.C. Mishra, R.P. Das, J. Colloid Interf. Sci. 185 (1997) 402. [18] C. Rath, S. Anand, R.P. Das, K.K. Sahoo, S.D. Kulkarni, S.K. Date, N.C. Mishra, J. Appl. Phys. 91 (2002) 2211. [19] (a) Joint Committee on Powder Diffraction Standard (JCPDS), International Center for Diffraction Data, JCPDS File No. 5–378, Swathmore, PA; (b) Joint Committee on Powder Diffraction Standard (JCPDS), International Center for Diffraction Data, JCPDS File No. 13–534, Swathmore, PA; (c) Joint Committee on Powder Diffraction Standard (JCPDS), International Center for Diffraction Data, JCPDS File No. 7–276, Swathmore, PA; (d) Joint Committee on Powder Diffraction Standard (JCPDS), International Center for Diffraction Data, JCPDS File No. 1–746, Swathmore, PA. [20] W.J. Smothers, I. Chiang, Differential Thermal Analysis Theory and Practice, Chemical Publishing Company, New York, 1958. [21] R.B. Fahime, M.I. Jaki, G.A.M. Hussien, Powder Technol. 33 (1982) 161. [22] M.I. Zaki, G.A.M. Hussien, R.B. Fahim, J. Mater. Sci. Lett. 4 (1985) 517. [23] R.C. Mackenzie, Differential Thermal Analysis, Fundamental Aspects, vol. 1, Academic Press, London, 1970. [24] M. Nayak, T.N.R. Kutty, Mater. Res. Bull. 31 (1996) 227. [25] D.E. Chasan, G. Norwitz, Appl. Spectrosc. 25 (1971) 226. [26] K. Nakamoto, Infrared and Raman Spectra of Inorganic and Coordination Compounds, Wiley, New York, 1963. [27] V.C. Farmer, The Infrared Spectra of Minerals, Mineralogical Society Monograph 4, London, 1974, p. 240. [28] L.C. Afremow, J.E. Vandeber, J. Paint Technol. 38 (1966) 169. [29] M. Belloto, G. Busca, C. Cristiani, G. Groppi, J. Solid State Chem. 117 (1995) 8. [30] V.F. Belov, T.A. Khimich, M.N. Shipko, I.H. Zheludev, E.V. Kormev, N.S. Ovanesyan, Sov. Phys. JETP 37 (1973) 1089.
Studies on characterization, microstructures and magnetic properties of nano-size barium hexa-ferrite prepared through a hydrothermal precipitation–calcination route D. Mishra a , S. Anand a,∗ , R.K. Panda b , R.P. Das a b
a Regional Research Laboratory, Bhubaneswar 751013, Orissa, India Materials Science Division, Department of Chemistry, Berhampur University, Berhampur 760007, Orissa, India
Received 17 July 2003; received in revised form 21 January 2004; accepted 17 February 2004
Abstract An attempt was made to prepare nano-size barium hexa-ferrite particles following a hydrothermal precipitation–calcination route using barium and iron nitrate solutions. During hydrothermal treatment at 180 ◦ C (2 h precipitation time) barium carbonate and hematite phases were formed. This precursor was calcined at 800, 1000 and 1200 ◦ C to determine the conditions for obtaining barium hexa-ferrite. The characterization studies on calcined products revealed that up to 800 ◦ C, the major crystalline phases (barium carbonate and hematite) of the precursor were retained. At 1000 ◦ C, formation of barium hexa-ferrite started and at 1200 ◦ C, though most of the major peaks of X-ray diffractogram corresponded to barium hexa-ferrite, a number of peaks corresponding to hematite were also present. Some low intensity peaks for barium carbonate were observed. The average particle size was 40 nm. Saturation magnetization, remanence magnetization and coercivity were found to be 40.0, 21.6 emu g−1 and 2.87 kOe, respectively. The values obtained both for coercivity and magnetization for the present sample were lower than the reported bulk values which could be due to the fact that the sample prepared through the present technique was not mono-phasic. © 2004 Elsevier B.V. All rights reserved. Keywords: Barium hexa-ferrite; Nano-size; Urea; Hydrothermal treatment; Calcination; Magnetic properties
1. Introduction Barium hexa-ferrite (BaFe12 O19 ) having hexagonal symmetry belongs to the magneto-plumbite group of oxides and is classified as hard ferrite due to its high saturation magnetization and high intrinsic coercivity. The major areas in which barium hexa-ferrite finds applications include perpendicular high density recording media, microwave devices, in electric generators and in many magnetically operated devices such as magnetic leviations, telephone ringers and receivers, etc. [1–3]. In recent years, the hydrothermal technique of preparation for hexa-ferrite appears to be quite attractive among all the processes [4–12]. The particle size, morphology and magnetic properties of the hydrothermally prepared barium hexa-ferrites are strongly dependent on the preparation conditions and on the nature of precursors used [4]. All the above cited reports mentioned the direct synthesis of this material under high temperature and pressure conditions from mixtures of various starting materials ∗ Corresponding author. Tel.: +91-674-581750; fax: +91-674-581750. E-mail address: s p [email protected] (S. Anand).
0254-0584/$ – see front matter © 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.matchemphys.2004.02.017
such as: Ba(NO3 )3 and Fe(NO3 )3 ·9H2 O in the presence of NaOH [5,7,10], from FeOOH and Ba(OH)2 [5,6,9,11], from ␣-Fe2 O3 and Ba(OH)2 [5,8,11] and from FeCl3 and Ba(OH)2 mixtures [12]. In our earlier investigations on preparation of barium aluminates and Mn-doped barium aluminates, it was observed that it was possible to obtain nano-size particles by employing a combination of hydrothermal precipitation–calcination technique [13,14]. The hydrothermally prepared hematite particles and Mn–Zn ferrite samples exhibited unusual magnetic properties [15–18]. With a view to prepare nano-size barium hexa-ferrite particles and to study their magnetic properties, the present investigations were taken up. 2. Experimental details The barium hexa-ferrite sample was prepared from barium and iron nitrate solutions following hydrothermal precipitation–calcination techniques. For hydrothermal precipitation the calculated amount of Ba and Fe3+ nitrate stock solutions were mixed thoroughly according to their
D. Mishra et al. / Materials Chemistry and Physics 86 (2004) 132–136
stoichiometric requirement and then urea was added in the urea/metal molar ratio of 2. The contents were then taken inside a PARR 2 l capacity reactor for hydrothermal precipitation at 180 ◦ C. After keeping the contents for 2 h at 180 ◦ C, the autoclave was cooled. The precipitated slurry was filtered, washed to remove anions and then dried at 100 ◦ C for 24 h in an air oven. Calcination of the air-dried sample was done at 800, 1000 and 1200 ◦ C inside a muffle furnace. The X-ray diffractograms of the powder samples were taken using a Philips PW 1710 X-ray diffractometer. The diffracted X-ray intensities were recorded as a function of diffraction angle 2θ using nickel-filtered copper target (Cu K␣ radiation with wavelength = 1.5404 Å). The Fourier transform infrared spectroscopy studies of the samples were done using a Perkin-Elmer P-500 spectrophotometer in the range 400–4000 cm−1 . Samples of 0.2 mg each were mixed thoroughly with 20 mg spectroscopy grade KBr (E-MERCK India Ltd.) and were then made into transparent pellets by using a 10 mm stainless steel die inside a pelletizer. The thermogravimetric and differential thermal analysis studies (TG–DTA) were carried out using a Stanton Thermal Analyzer STA-740 instrument. The weighed sample (10 mg) was taken in a small alumina sample holder and heated up to 1300 ◦ C in normal non-isothermal conditions at a heating rate of 15 K min−1 using ␣-Al2 O3 as reference material. The microstructure of the sample was studied using a transmission electron microscopy (TEM, JEOL, JEM-200CX Model). The magnetic measurements were carried out at room temperature with a vibrating sample magnetometer (PARR, Model 4500). The field at the actual position was calibrated using a solid nickel cylindrical sample. The maximum applied field of 15 kOe has been used to evaluate the magnetic parameters.
3. Results and discussion 3.1. Phase characterization by powder X-ray diffraction The X-ray powder diffraction patterns of the hydrothermally prepared precursors (at 180 ◦ C, 2 h) and the products obtained after calcination at 800, 1000 and 1200 ◦ C are shown in Fig. 1. The XRD pattern of the precursor (Fig. 1a) shows the presence of both hematite (␣-Fe2 O3 ) and orthorhombic barium carbonate. These two phases were confirmed from their characteristic peaks at 2θ = 24.17◦ , 28.06◦ , 34.97◦ and 42.2◦ for BaCO3 [19a] and at 2θ = 33.39◦ , 35.90◦ , 49.62◦ and 54.13◦ for hematite [19b]. The rest of the peaks having relative intensities between 5 and 10% are either of BaCO3 or of Fe2 O3 . The XRD pattern of the sample calcined at 800 ◦ C (Fig. 1b) also shows BaCO3 and Fe2 O3 to be the major crystalline phases present. When the precursor was calcined at 1000 ◦ C (Fig. 1c), peaks corresponding to BaFe12 O19 [19c] are observed at 2θ = 30.13◦ , 31.96◦ , 33.88◦ , 36.86◦ , 40.60◦ , 42.17◦ , 56.34◦ , 62.82◦ , and
133
Fig. 1. (a) XRD patterns of hydrothermally prepared precursor. Precursors calcined at (b) 800 ◦ C for 2 h, (c) 1000 ◦ C for 2 h and (d) 1200 ◦ C for 2 h.
at 72.39◦ confirming formation of hexa-ferrite phase at this temperature. The peaks for barium carbonate and ␣-Fe2 O3 are also observed at this temperature (Fig. 1c). Two prominent peak corresponding to BaO are also identified at 2θ = 28.20◦ and 54.83◦ with slight shifts [19d] but other high intensity peaks for BaO were not present. The XRD pattern of the sample calcined at 1200 ◦ C is shown in Fig. 1d. All the major peaks of this compound are corresponding to the standard BaFe12 O19 [19c], with disappearance of major peaks of BaCO3 . Some low intensity peaks for barium carbonate are observed. Presence of hematite is still detected in this sample with appearance of a number of peaks corresponding to this compound. Due to complexities of the patterns many times relative intensities did not match. 3.1.1. Sequence of reactions during formation of barium hexa-ferrite During hydrothermal treatment at 180 ◦ C: Ba(NO3 )2 + NH2 –CO–NH2 + 2H2 O → BaCO3 (c) + 2NH4 NO3
(1)
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2Fe(NO3 )3 + 3NH2 –CO–NH2 + 6H2 O → Fe2 O3 (c) + 6NH4 NO3 + CO2
(2)
During calcination at 800, 1000, 1200 ◦ C: at 800 ◦ C, the phases of barium carbonate and hematite are retained and formation of barium hexa-ferrite starts at 1000 ◦ C. At 1200 ◦ C, though most of the peaks correspond to barium hexa-ferrite but peaks corresponding to hematite are still identified. Only one prominent peak corresponding to barium oxide was identified. BaCO3 (c) + 6Fe2 O3 (c) → BaFe12 O19 + CO2
(3)
3.2. TG–DTA trace of the hydrothermally prepared precursor The TG–DTA trace of the hydrothermally prepared barium hexa-ferrite precursor obtained under nitrogen atmosphere and at a heating rate of 15 K min−1 is shown in Fig. 2. The TGA trace shows no perceptible weight loss up to 500 ◦ C, indicating the absence of any hydrated oxides of Fe or Ba and this result agrees with the XRD observation that the initial compound obtained on hydrothermal treatment is a mixture of hematite and barium carbonate. In the 500–800 ◦ C and 900–1050 ◦ C region, weight losses of 1–2 and 4.5% are observed from the TGA trace. The latter weight loss is because of the decomposition of BaCO3 and the value obtained is close to the theoretical (4.86%) weight loss expected from the decomposition of BaCO3 +6Fe2 O3 mixture as given by Eq. (3). The TGA shows no further weight loss above 1100 ◦ C, confirming the formation of the stable oxide BaFe12 O19 . The DTA of hydrothermal precursor (Fig. 2) shows only one endo-peak at 800 ◦ C, corresponding to the polymorphic ␣– phase transformation of BaCO3 [20–22]. An interesting observation is that the DTA shows no peaks corresponding to the decomposition of BaCO3 , instead the DTA shows some minor modulations in the 950–1250 ◦ C region. This may due to either the small weight fraction (17.02%) of BaCO3 in the BaCO3 + 6Fe2 O3 or due to the
Fig. 3. (a) FT-IR of hydrothermally prepared precursor at 180 ◦ C. (b) Precursor calcined at 1200 ◦ C.
polycrystalline nature of barium carbonate causing a wide range of decomposition temperature [23]. 3.3. FT-IR analysis The XRD and TG–DTA results of the hydrothermally prepared precursor showed that initially a mixture of BaCO3 and ␣-Fe2 O3 were precipitated and calcination of this precursor at 1200 ◦ C resulted in the formation of BaFe12 O19 . In order to further confirm these phases, the Fourier transform infrared spectra (FT-IR) of the hydrothermally prepared sample and the sample obtained after calcination at 1200 ◦ C for 2 h were recorded in the 400–4000 cm−1 region (Fig. 3). The FT-IR of the hydrothermally prepared (180 ◦ C, 2 h) sample (Fig. 3a) shows no peaks in the 1200–4000 cm−1
Fig. 2. TG–DTA of the hydrothermally prepared precursor sample.
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Fig. 4. TEM image of the 1200 ◦ C calcined hydrothermally prepared precursor calcined at 1200 ◦ C for 2 h. Arrows show hematite phase and encircled particles are of barium hexa-ferrite.
region, indicating the absence of any hydroxides or hydrated oxides of Ba and Fe. Carbonates in the sample are confirmed by two weak and sharp peaks at 1060 and 860 cm−1 [24,25]; the other two characteristic peaks for BaCO3 at 1445 and 710 cm−1 are not visible in the spectrum. The peaks for ␣-Fe2 O3 are also not prominent and appeared as a broad band between 700 and 400 cm−1 , with very minor humps at 632, 593, 500 and 480 cm−1 . These peaks have been identified as the vibrations related to O2− in the Fe2 O3 lattice having symmetries Eg (a), Eu (a), A1g and Eu (p1 ), respectively [26–28]. The FT-IR spectrum of the sample calcined at ≈1200 ◦ C (Fig. 3b) showed only one broad and strong peak centered at ≈600 cm−1 and very weak satellite peaks at 432 and 402 cm−1 . These peaks are assigned by Belloto et al. [29] as Fe–O vibrations for BaFe12 O19 having E1u symmetry. As in case of barium hexa-aluminates the majority of infrared active modes of vibrations for Ba and Fe in BaFe12 O19 appear below 400 cm−1 which could not be recorded in the present case. 3.4. TEM studies A TEM image of the hydrothermally precipitated sample calcined at 1200 ◦ C is shown in Fig. 4. Three different types of particles are clearly visible from the micrograph, although the XRD of the sample showed the presence of only barium hexa-ferrite and hematite. The dark patches (indicated by arrows in Fig. 4) appearing in this micrograph may be due to the surface layers of hematite on BaFe12 O19 . The later phase is seen in this micrographs as cubic particles having ≈40 nm size. The appearance of needle-shaped particles may be due to under grown barium hexa-ferrite particles or of trace amounts of BaO.
4. Magnetic studies Magnetic parameters were determined for the sample calcined at 1200 ◦ C using a vibrating sample magnetometer at room temperature at a maximum applied field of 15 kOe. Saturation magnetization, remanence magnetization and coercivity were determined from the hystersis loop and were found to be 40.0, 21.6 emu g−1 and 2.87 kOe, respectively. Values for both coercivity and saturation magnetization are found to be lower when as compared to reported bulk values [30]. This could be due to presence of minor amounts of phases other than barium hexa-ferrite. 5. Conclusions The attempts made for preparation of nano-size barium hexa-ferrite particles through a hydrothermal precipitation route have been reported. The precipitation of both barium and iron was stoichiometric during hydrothermal precipitation at 180 ◦ C and the phases formed were barium carbonate and hematite. This precursor was calcined at 800, 1000 and 1200 ◦ C. It was observed from the XRD pattern of the sample calcined at 1200 ◦ C, that almost all the lines corresponded to barium hexa-ferrite with some line corresponding to hematite. Saturation magnetization, remanence magnetization and coercivity were 40.0, 21.6 emu g−1 and 2.87 kOe, respectively. Values for both coercivity and magnetization were lower than the reported bulk values due to presence of minor amounts of phases other than barium hexa-ferrite. Acknowledgements The authors are thankful to Dr. Vibhuti N. Misra, Director, Regional Research Laboratory, Bhubaneswar,
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for his kind permission to publish this paper. Help provided by Dr. S.C. Das, Head, Hydrometallurgy Department and Shri K. Sanjay, scientist is acknowledged. The support provided by Dr. S.K. Date, Ms. S.D. Kulkarni and Mr. Chanda of National Chemical Laboratory, Pune during XRD and VSM measurements is thankfully acknowledged.
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