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Synthesis and characterization of nano crystalline BaFe12O19 powders by low temperature combustion
Materials Research Bulletin 38 (2003) 149±159
Synthesis and characterization of nano crystalline BaFe12O19 powders by low temperature combustion Jianguo Huang*, Hanrui Zhuang, WenLan Li Shanghai Institute of Ceramics, Chinese Academy of Science, Shanghai 200050, China Received 3 June 2002; accepted 27 September 2002
Abstract Nano crystalline BaFe12O19 powders have been prepared at a relatively low calcination temperature by a gel combustion technique using citric acid as a fuel/reductant and nitrates as oxidants. The effects of processing parameters, such as Ba/Fe ratio, citric acid/nitrates ratio, reaction temperature on the powder characteristics and magnetic properties of the resultant barium ferrites were investigated. By controlling the molar ratio of citric acid to metal nitrates, nano crystalline BaFe12O19 powders with different particle sizes have been obtained. Phase attributes, microstructures and magnetic properties of the powders were characterized using X-ray diffraction analysis, X-ray line-broadening technique, Fourier transform infrared spectroscopy measurements, transmission electron microscopy and vibrating sample magnetometer. The maximum saturation magnetization value and intrinsic coercivity value for the obtained barium hexaferrites are 59.36 emu/g and 5540 Oe. # 2002 Elsevier Science Ltd. All rights reserved. Keywords: Magnetic materials; Nanostructure; Magnetic properties; Microstructure
1. Introduction Barium hexaferrites have been extensively used as permanent magnets due to their unsurpassed low cost, relatively high coercivity, excellent chemical stability and corrosion resistance[1,2]. In order to get pure crystalline mono-domain particles of BaFe12O19, different synthesis techniques have been developed, such as aerosol pyrolysis [3], sol±gel technique [4], chemical coprecipitation [5], salt-melt technique [6], hydrothermal reaction [7], glass crystallization [8], etc. In this work, emphasis has been laid on the synthesis of nano crystalline BaFe12O19 powders by low temperature combustion (LCS). Self-propagating high-temperature synthesis (SHS) is a combustion technique that has been used to prepare a wide range of materials. In contrast to SHS, LCS synthesis, developed in recent years, has * Corresponding author. Tel.: 86-21-52412990; fax: 86-21-52413903. E-mail address: [email protected] (J. Huang).
0025-5408/02/$ ± see front matter # 2002 Elsevier Science Ltd. All rights reserved. PII: S 0 0 2 5 - 5 4 0 8 ( 0 2 ) 0 0 9 7 9 - 0
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many advantages. The LCS route is based on the gelling and subsequent combustion of an aqueous solution containing salts of the desired metals and some organic fuel, giving a voluminous and ¯uffy product with large surface area. LSC has been proved to be a novel, extremely facile, time-saving and energy-ef®cient route for the synthesis of ultra®ne powders [9,10]. In the present study, a metal nitrates±citric acid mixed solution has been used to prepare BaFe12O19. The effects of processing parameters, such as Ba/Fe ratio, citric acid/nitrates ratio (henceforth referred to as CA/NO3 ratio), reaction temperature on the powder characteristics and magnetic properties of the resultant barium ferrites were investigated. 2. Experimental procedure A.R. grade of citric acid (C6H8O7H2O), Ba(NO3)2 (99.5%) and Fe(NO3)39H2O (99.0%) were used as starting materials. In order to obtain a single phase of BaFe12O19, controlling the atomic ratio of Ba to Fe is very important. The metal nitrates were dissolved together in a minimum amount of deionized water to get a clear solution. An aqueous solution of citric acid was mixed with the nitrates solution, then ammonia solution was slowly added to adjust the pH at 7. The mixed solution above was allowed to evaporate on a hot plate with continuous stirring. As water evaporated, the solution became viscous and ®nally formed a very viscous brown gel. The obtained gel was placed in a furnace preheated to 250 8C. After several minutes, the gel automatically ignited and burnt with glowing ¯ints. The autoignition was completed within a minute, yielding the brown-colored ashes termed here as a precursor. The precursor was ®rst milled in a uncontaminated plastic container with an ethanol medium, then treated at 450 8C in air for 1 h to eliminate the carbonaceous residues. After that, it was calcined at different temperatures to ®nd out the lowest temperature for the formation of the desired phase. In order to characterize the precursors as well as their calcination products, X-ray diffraction, Fourier transform infrared spectroscopy measurements, thermal analysis, transmission electron microscope and magnetic measurements were applied. The X-ray diffractograms were recorded with an X-ray diffractometer with Cu Ka radiation (D/max 2550 V, Japan). The average crystallite size of the powders was measured by X-ray line-broadening technique employing the Scherrer formula using the pro®les of the (1 1 4) peak. The IR spectra in the range 400±4000 cm 1 were recorded using a Fourier transform infrared spectrometer (BioRad FTS185, USA). The samples were mixed with spectral grade KBr as the standard. The thermal analyses were carried out with a thermoanalyser (STA449C, Germany) which recorded differential scanning calorimetry, thermogravimetic analysis and mass spectra simultaneously. The heating rate was 10 8C/ min and the thermal analyses were carried out in static air. The Magnetic properties were measured at room temperature by a vibrating sample magnetometer (JDM-13, China) up to an applied ®eld of 1.8 kOe. The particle morphology was examined by a transmission electron microscope (JEM-200CX, Japan). 3. Results and discussion According to the concept of propellant chemistry [12], the total oxidizing valence should equal the reducing valence. Here C, H, Ba, and Fe are considered as reducing elements with corresponding
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Table 1 Compositions of the studied samples Sample no.
Ba2:Fe3:citric acid (molar ratio)
CA/NO3 (molar ratio)
A1 A2 A3 A4 A5 A6 A7
1:11.5:12.5 1:11.5:18.8 1:11.5:25.0 1:10.0:10.9 1:12.0:12.9 1:12.5:13.4 1:11.5:36.5
0.34 0.51 0.68 0.34 0.34 0.34 1.00
Fig. 1. DSC curves of the gels.
Fig. 2. TGA curves of the gels.
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Fig. 3. TG±MS plots of A3 gel.
Fig. 4. FT-IR spectra of the gels before and after combustion.
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valences of 4, 1, 2 and 3, respectively. O is regarded as an oxidizing element with valences of 2. Therefore, to form 1 mole of BaFe12O19, the total calculated valence of metal nitrates by arithmetic summation of oxidizing and reducing valences is 190, which means that metal nitrates are strong oxidizers. If the valence of the citric acid is adjusted to 190 in order to have the same value with the total valence of metal nitrates, allowing the ratio of oxidizing valence to reduce to 1, then the molar ratio of the mixture solution should be 1:12.0:10.56 of Ba2:Fe3:citric acid, theoretically. To guarantee the suf®cient completion of metal irons, super¯uous citric acid, relative to the theoretical amount calculated above, was added. The studied compositions are shown in Table 1. The pyrolysis processes of the dried gels with different CA/NO3 ratios were investigated by DSC (Fig. 1) and TG (Fig. 2). Three main weight-loss regions occur at 60±200, 200±220 and 220±420 8C. The DSC curves show endothermic peaks below 160 8C accompanied by small weight losses, corresponding to evaporation of water. There is an exothermic peak at about 204±220 8C. The vigorous combustion process corresponding to nitrates and citrate gel decomposition, yielding H2O, NH3, CO, CO2 and NO, generates the ions of H2O (m/e 18), NH3/OH (m/e 17), CO (m/e 28), CO2 (m/e 44) and NO (m/e 30), as proved by MS (Fig. 3). Previous work by Baythoun and Sale [13] has also recognized the existence of H2O, CO, CO2 and NO in the decomposition of metal±citrate precursor. The abrupt fall between 200 and 220 8C in the TG plot is another evidence of the main decomposition of the gel during the combustion. The weight loss at a lower rate between 220 and 420 8C is, presumably, due to the combustion of the remaining organic compound and the oxidation to CO2 of some carbonaceous residue. The total weight loss increases with the increase of citric acid content. Additional small weight loss associated with the decomposition of small amount of carbonates is observed above 420 8C, as further proved by FT-IR and XRD.
Fig. 5. FT-IR spectra of synthesized powders calcined at the indicated temperatures (8C).
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Citric acid with one hydroxyl and three carboxyl atoms is a multidentate ligand and complexes with many multivalent ions to form chelates. From the FT-IR spectrum of the powder precursors showed in Fig. 4, the characteristic bands of free carboxyl groups at about 1720 cm 1 are not found. The dried gel shows the characteristic bands of antisymmetrical and symmetrical stretching vibrations of COO at 1579±1627 and 1400 cm 1, the NO3 band at 1384 cm 1. The bands near 3600±3000 cm 1 assigned to the stretching vibrations of the hydrogen-boned OH groups. After the autoignition of the gel, bands of NO3 and carboxylate disappear in A1, while carboxylate bands can still be found at this step in A2 and A3. The new bands at 1431, 858 and 693 cm 1 assign to carbonate ions. At 700 8C, the band near 760 cm 1 indicates a small mount of BaFe2O4. At 800 8C, this band becomes very weak. At 850 8C, only the bands of BaFe12O19 can be observed, as shown in Fig. 5. Fig. 6 shows the XRD patterns of the A1 gel, the combustion derived precursor, and the calcined powders. The dry gel exhibits a highly amorphous nature (Fig. 6a). After autoignition of the gel the burnt powder is identi®ed as g-Fe2O3 (magnetite), a-Fe2O3 (hematite) and BaCO3 (Fig. 6b). After 1-h of calcination at 700 8C, the intensity of a-Fe2O3, g-Fe2O3 and BaCO3 decreases markedly, and BaFe12O19 forms accompanied by BaFe2O4 (Fig. 6c). After 1-h calcination at 800 8C, the major phase
Fig. 6. XRD patterns of (a) A1 dry gel, (b) burnt powder, and powders calcined at (c) 700 8C 1 h, (d) 800 8C 1 h, and (e) 850 8C 1 h.
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is BaFe12O19, only a small amount of residual a-Fe2O3 exists (Fig. 6d). After 1-h of calcination at 850 8C, the only phase retained is BaFe12O19, indicating the completion of the reaction (Fig. 6e). It has been previously reported that the formation of barium hexaferrite particles by the solid state reaction consists of two main steps, the decarbonization together with the formation of monoferrite, and the diffusion of Ba into the iron oxide [11]. BaCO3 Fe2 O3 ! BaFe2 O4 CO2 BaFe2 O4 5Fe2 O3 ! BaO 6Fe2 O3 The formation mechanism is similar in our experiment. First of all, nitrates and citric acid produce a gel, then the gel decomposes after autoignition and generates BaCO3 and Fe2O3. The reactions, as shown in the above equations, proceed during the subsequent heat treatment, enabling us to get the desired BaFe12O19. Although the stoichiometric Fe/Ba ratio should be 12 for BaFe12O19, it was noted that an excess of barium was necessary to ensure the formation of single phase BaFe12O19 [16±18]. Janasi et al. [16] produced barium ferrite powders by coprecipitation with Fe/Ba 11. The pure hexaferrite was obtained at Fe/Ba 10:5 in the work of Carp et al. [17]. They found that if working with a molar ratio Fe/Ba 12, constant small amounts of a-Fe2O3 impurity phase (9 11%) remained in the calcination residues even after an annealing at 900 8C for 30 h. Af¯eck et al. [18] prepared barium ferrite powders using SHS method with Fe/Ba 9:6 in the reaction. They suggested that the small amount of barium
Fig. 7. XRD patterns of powders calcined at 850 8C for 1 h at different Ba/Fe ratios.
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Fig. 8. TEM micrographs of the calcined powders with different CA/NO3 ratios: (a) sample A1, (b) sample A2 and (c) sample A3.
volatilized during the combustion and calcination process was responsible for the ``off stoichiometry'' [18]. In our study, the effect of Fe/Ba ratios from 10 to 12.5 on the synthesized powders was examined. Fig. 7 shows the XRD patterns for the powders synthesized at different Ba/Fe ratios. The calcination temperature and time were ®xed at 850 8C and 1 h. As expected, the Ba/Fe ratio plays an important role in the formation of BaFe12O19, single phase of BaFe12O19 has been obtained at a Fe/Ba ratio of 11.5. In contrast, a-Fe2O3 or BaFe2O4 is present at the Fe/Ba ratios of 10, 12 and 12.5. Fig. 8. shows the morphology of the calcined powders with different CA/NO3 ratios. It can be seen that, the average particle size decreases with increasing CA/NO3 ratio. During the combustion process, a large amount of gaseous material was evolved, the rich fuel style gave out much more gas than the poor fuel style, and the rich fuel style ®lled out its product with more space. Because the gas took out a great deal of caloric, the temperature in the rich fuel style was lower than that in the poor fuel style. Based on this viewpoint, the particle size can be controlled by adjusting the CA/NO3 ratio. As the CA/NO3 ratio increases, the amount of gaseous materials evolved also increases, resulting in lower mean agglomerative size. However, the decrease in average particle size is not signi®cant when the ratio is higher than 0.68, as can be seen in Fig. 9.
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Fig. 9. Dependence of average crystallite size of the calcined powders on the CA/NO3 ratio.
Hysteresis loops of samples A1, A2 and A3 at room temperature and a maximum applied ®eld of 18 kOe are shown in Fig. 10. The result shows that both the magnetisation and coercivity of A3 are higher than those of A1 and A2, which is relevant to the distinct difference of the three microstructures from each other. The differences between these samples are not only the particle size, but also the size distribution. Fig. 8a shows that the grain size of A1 is between 100 and 400 nm and has an uneven distribution; the grain size of A2 is between 60 and 150 nm, while the grain size of A3 has a narrower distribution between 80 and 120 nm. Previous studies showed that structural inhomogeneity could decrease the magnetization in ®ne particles [14]. The critical diameter for spherical single-domain barium ferrite particles is about 460 nm [15], so the particles of the samples A1, A2 and A3 are mainly
Fig. 10. Hysteresis loops at room temperature of the barium hexaferrite samples.
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single-domain. The low ss and coercivity of A1, in comparison with sample A2 and A3, could be a consequence of the wide particle size distribution. Moreover, evident agglomeration exists in A1 (Fig. 8a), which means that there is an important fraction of agglomerative particles over the critical diameter. The change from a single-domain structure to a multi-domain one led by agglomeration may result in the deterioration of magnetic properties. The most homogeneous grain size distribution and good dispersity of A3 guarantees best magnetic properties with the saturation magnetization value of 59.36 emu/g and intrinsic coercivity value of 5540 Oe. Obviously, magnetic properties are rather sensitive to the transformation of microstructures. The work of optimizing particle microstructures for the improvement of magnetic properties is still in progress. 4. Conclusions Nano crystalline BaFe12O19 powders have been prepared at a relatively low calcination temperature by a gel combustion technique. This route is based on the combustion of nitrate±citrate gels due to an exothermic redox reaction between nitrate and citrate ions. This method is time saving and easy to implement, while producing nano crystalline, compositionally homogeneous powders. The experiment shows that Fe/Ba ratios slightly lower than the stoichiometry lead to the production of pure BaFe12O19. The initial citric acid/nitrate ratio has a marked in¯uence on the combustion process and the grain size of the product. Maximum saturation magnetisation of 59.36 emu/g and a coercivity of 5540 Oe in A3 has been achieved. TEM observations reveal that the powder of A3 has a narrower distribution between 80 and 120 nm, with a majority of irregular grains and only a small amount of hexagonal grains. Acknowledgements Authors would like to thank Prof. Meiling Ruan for her assistance with TEM studies, Dr. Hui Xu for magnetic measurements, and Prof. Changwei Lu for the thermal analyses. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12]
H. Kojima, in: E.P. Wohlfarth (Ed.), Ferromagnetic Materials, vol. 3, North-Holland, New York, 1982 (Chapter 5). G. Pouillard, M.S. Alam, M.C. Trinel-Dufour, P. Perrot, J. Chem. Res. Symp. 5 (1981) 136. T. GonzaÂlez-CarrenÏo, M.P. Morales, C.J. Serna, Mater. Lett. 43 (2000) 97. W. Zhong, W.P. Ding, N. Zhang, J.M. Hong, Q.J. Yan, Y.W. Du, J. Magn. Magn. Mater. 168 (1997) 196. K. Haneda, C. Miyakawa, H. Kojima, J. Am. Ceram. Soc. 57 (1974) 354. R.H. Arendt, J. Solid State Chem. 8 (1973) 339. M.L. Wang, Z.W. Shih, J. Crystal Growth 114 (1991) 435. L. Rezlescu, E. Rezlescu, P.D. Popa, N. Rezlescu, J. Magn. Magn. Mater. 193 (1999) 288. M.D. Shaji Kumar, T.M. Srinivasan, P. Ramasamy, C. Subramaniam, Mater. Lett. 25 (1995) 171. Z. Zhong, P.K. Gallagher, J. Mater. Res. 10 (1995) 945. H.P. Steier, J. Requena, J.S. Moya, J. Mater. Res. 14 (1999) 3647. S.R. Jain, K.C. Adiga, V.R.P. Vemekeretc, Combust. Flame 40 (1981) 71.
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M.S.G. Baythoun, F.R. Sale, J. Mater. Sci. 17 (1982) 2757. K.V.P.M. Sha, Y. Koltypin, A. Gedanken, R. Prozorov, J. Balogh, J. Lendvai, I. Felner, J. Phys. Chem. B 101 (1997) 6409. L. Rezlescu, E. Rezlescu, P.D. Popa, N. Rezlescu, J. Magn. Magn. Mater. 193 (1997) 288. S.R. Janasi, M. Emura, F.J.G. Landgraf, D. Rodrigues, J. Magn. Magn. Mater. 238 (2002) 168. O. Carp, R. Barjega, E. Segal, M. Brezeanu, Thermochim. Acta 318 (1998) 57. L. Affleck, M.D. Aguas, I.P. Parkin et al., J. Mater. Chem. 10 (2000) 1925.
Synthesis and characterization of nano crystalline BaFe12O19 powders by low temperature combustion Jianguo Huang*, Hanrui Zhuang, WenLan Li Shanghai Institute of Ceramics, Chinese Academy of Science, Shanghai 200050, China Received 3 June 2002; accepted 27 September 2002
Abstract Nano crystalline BaFe12O19 powders have been prepared at a relatively low calcination temperature by a gel combustion technique using citric acid as a fuel/reductant and nitrates as oxidants. The effects of processing parameters, such as Ba/Fe ratio, citric acid/nitrates ratio, reaction temperature on the powder characteristics and magnetic properties of the resultant barium ferrites were investigated. By controlling the molar ratio of citric acid to metal nitrates, nano crystalline BaFe12O19 powders with different particle sizes have been obtained. Phase attributes, microstructures and magnetic properties of the powders were characterized using X-ray diffraction analysis, X-ray line-broadening technique, Fourier transform infrared spectroscopy measurements, transmission electron microscopy and vibrating sample magnetometer. The maximum saturation magnetization value and intrinsic coercivity value for the obtained barium hexaferrites are 59.36 emu/g and 5540 Oe. # 2002 Elsevier Science Ltd. All rights reserved. Keywords: Magnetic materials; Nanostructure; Magnetic properties; Microstructure
1. Introduction Barium hexaferrites have been extensively used as permanent magnets due to their unsurpassed low cost, relatively high coercivity, excellent chemical stability and corrosion resistance[1,2]. In order to get pure crystalline mono-domain particles of BaFe12O19, different synthesis techniques have been developed, such as aerosol pyrolysis [3], sol±gel technique [4], chemical coprecipitation [5], salt-melt technique [6], hydrothermal reaction [7], glass crystallization [8], etc. In this work, emphasis has been laid on the synthesis of nano crystalline BaFe12O19 powders by low temperature combustion (LCS). Self-propagating high-temperature synthesis (SHS) is a combustion technique that has been used to prepare a wide range of materials. In contrast to SHS, LCS synthesis, developed in recent years, has * Corresponding author. Tel.: 86-21-52412990; fax: 86-21-52413903. E-mail address: [email protected] (J. Huang).
0025-5408/02/$ ± see front matter # 2002 Elsevier Science Ltd. All rights reserved. PII: S 0 0 2 5 - 5 4 0 8 ( 0 2 ) 0 0 9 7 9 - 0
150
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many advantages. The LCS route is based on the gelling and subsequent combustion of an aqueous solution containing salts of the desired metals and some organic fuel, giving a voluminous and ¯uffy product with large surface area. LSC has been proved to be a novel, extremely facile, time-saving and energy-ef®cient route for the synthesis of ultra®ne powders [9,10]. In the present study, a metal nitrates±citric acid mixed solution has been used to prepare BaFe12O19. The effects of processing parameters, such as Ba/Fe ratio, citric acid/nitrates ratio (henceforth referred to as CA/NO3 ratio), reaction temperature on the powder characteristics and magnetic properties of the resultant barium ferrites were investigated. 2. Experimental procedure A.R. grade of citric acid (C6H8O7H2O), Ba(NO3)2 (99.5%) and Fe(NO3)39H2O (99.0%) were used as starting materials. In order to obtain a single phase of BaFe12O19, controlling the atomic ratio of Ba to Fe is very important. The metal nitrates were dissolved together in a minimum amount of deionized water to get a clear solution. An aqueous solution of citric acid was mixed with the nitrates solution, then ammonia solution was slowly added to adjust the pH at 7. The mixed solution above was allowed to evaporate on a hot plate with continuous stirring. As water evaporated, the solution became viscous and ®nally formed a very viscous brown gel. The obtained gel was placed in a furnace preheated to 250 8C. After several minutes, the gel automatically ignited and burnt with glowing ¯ints. The autoignition was completed within a minute, yielding the brown-colored ashes termed here as a precursor. The precursor was ®rst milled in a uncontaminated plastic container with an ethanol medium, then treated at 450 8C in air for 1 h to eliminate the carbonaceous residues. After that, it was calcined at different temperatures to ®nd out the lowest temperature for the formation of the desired phase. In order to characterize the precursors as well as their calcination products, X-ray diffraction, Fourier transform infrared spectroscopy measurements, thermal analysis, transmission electron microscope and magnetic measurements were applied. The X-ray diffractograms were recorded with an X-ray diffractometer with Cu Ka radiation (D/max 2550 V, Japan). The average crystallite size of the powders was measured by X-ray line-broadening technique employing the Scherrer formula using the pro®les of the (1 1 4) peak. The IR spectra in the range 400±4000 cm 1 were recorded using a Fourier transform infrared spectrometer (BioRad FTS185, USA). The samples were mixed with spectral grade KBr as the standard. The thermal analyses were carried out with a thermoanalyser (STA449C, Germany) which recorded differential scanning calorimetry, thermogravimetic analysis and mass spectra simultaneously. The heating rate was 10 8C/ min and the thermal analyses were carried out in static air. The Magnetic properties were measured at room temperature by a vibrating sample magnetometer (JDM-13, China) up to an applied ®eld of 1.8 kOe. The particle morphology was examined by a transmission electron microscope (JEM-200CX, Japan). 3. Results and discussion According to the concept of propellant chemistry [12], the total oxidizing valence should equal the reducing valence. Here C, H, Ba, and Fe are considered as reducing elements with corresponding
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Table 1 Compositions of the studied samples Sample no.
Ba2:Fe3:citric acid (molar ratio)
CA/NO3 (molar ratio)
A1 A2 A3 A4 A5 A6 A7
1:11.5:12.5 1:11.5:18.8 1:11.5:25.0 1:10.0:10.9 1:12.0:12.9 1:12.5:13.4 1:11.5:36.5
0.34 0.51 0.68 0.34 0.34 0.34 1.00
Fig. 1. DSC curves of the gels.
Fig. 2. TGA curves of the gels.
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Fig. 3. TG±MS plots of A3 gel.
Fig. 4. FT-IR spectra of the gels before and after combustion.
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valences of 4, 1, 2 and 3, respectively. O is regarded as an oxidizing element with valences of 2. Therefore, to form 1 mole of BaFe12O19, the total calculated valence of metal nitrates by arithmetic summation of oxidizing and reducing valences is 190, which means that metal nitrates are strong oxidizers. If the valence of the citric acid is adjusted to 190 in order to have the same value with the total valence of metal nitrates, allowing the ratio of oxidizing valence to reduce to 1, then the molar ratio of the mixture solution should be 1:12.0:10.56 of Ba2:Fe3:citric acid, theoretically. To guarantee the suf®cient completion of metal irons, super¯uous citric acid, relative to the theoretical amount calculated above, was added. The studied compositions are shown in Table 1. The pyrolysis processes of the dried gels with different CA/NO3 ratios were investigated by DSC (Fig. 1) and TG (Fig. 2). Three main weight-loss regions occur at 60±200, 200±220 and 220±420 8C. The DSC curves show endothermic peaks below 160 8C accompanied by small weight losses, corresponding to evaporation of water. There is an exothermic peak at about 204±220 8C. The vigorous combustion process corresponding to nitrates and citrate gel decomposition, yielding H2O, NH3, CO, CO2 and NO, generates the ions of H2O (m/e 18), NH3/OH (m/e 17), CO (m/e 28), CO2 (m/e 44) and NO (m/e 30), as proved by MS (Fig. 3). Previous work by Baythoun and Sale [13] has also recognized the existence of H2O, CO, CO2 and NO in the decomposition of metal±citrate precursor. The abrupt fall between 200 and 220 8C in the TG plot is another evidence of the main decomposition of the gel during the combustion. The weight loss at a lower rate between 220 and 420 8C is, presumably, due to the combustion of the remaining organic compound and the oxidation to CO2 of some carbonaceous residue. The total weight loss increases with the increase of citric acid content. Additional small weight loss associated with the decomposition of small amount of carbonates is observed above 420 8C, as further proved by FT-IR and XRD.
Fig. 5. FT-IR spectra of synthesized powders calcined at the indicated temperatures (8C).
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Citric acid with one hydroxyl and three carboxyl atoms is a multidentate ligand and complexes with many multivalent ions to form chelates. From the FT-IR spectrum of the powder precursors showed in Fig. 4, the characteristic bands of free carboxyl groups at about 1720 cm 1 are not found. The dried gel shows the characteristic bands of antisymmetrical and symmetrical stretching vibrations of COO at 1579±1627 and 1400 cm 1, the NO3 band at 1384 cm 1. The bands near 3600±3000 cm 1 assigned to the stretching vibrations of the hydrogen-boned OH groups. After the autoignition of the gel, bands of NO3 and carboxylate disappear in A1, while carboxylate bands can still be found at this step in A2 and A3. The new bands at 1431, 858 and 693 cm 1 assign to carbonate ions. At 700 8C, the band near 760 cm 1 indicates a small mount of BaFe2O4. At 800 8C, this band becomes very weak. At 850 8C, only the bands of BaFe12O19 can be observed, as shown in Fig. 5. Fig. 6 shows the XRD patterns of the A1 gel, the combustion derived precursor, and the calcined powders. The dry gel exhibits a highly amorphous nature (Fig. 6a). After autoignition of the gel the burnt powder is identi®ed as g-Fe2O3 (magnetite), a-Fe2O3 (hematite) and BaCO3 (Fig. 6b). After 1-h of calcination at 700 8C, the intensity of a-Fe2O3, g-Fe2O3 and BaCO3 decreases markedly, and BaFe12O19 forms accompanied by BaFe2O4 (Fig. 6c). After 1-h calcination at 800 8C, the major phase
Fig. 6. XRD patterns of (a) A1 dry gel, (b) burnt powder, and powders calcined at (c) 700 8C 1 h, (d) 800 8C 1 h, and (e) 850 8C 1 h.
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is BaFe12O19, only a small amount of residual a-Fe2O3 exists (Fig. 6d). After 1-h of calcination at 850 8C, the only phase retained is BaFe12O19, indicating the completion of the reaction (Fig. 6e). It has been previously reported that the formation of barium hexaferrite particles by the solid state reaction consists of two main steps, the decarbonization together with the formation of monoferrite, and the diffusion of Ba into the iron oxide [11]. BaCO3 Fe2 O3 ! BaFe2 O4 CO2 BaFe2 O4 5Fe2 O3 ! BaO 6Fe2 O3 The formation mechanism is similar in our experiment. First of all, nitrates and citric acid produce a gel, then the gel decomposes after autoignition and generates BaCO3 and Fe2O3. The reactions, as shown in the above equations, proceed during the subsequent heat treatment, enabling us to get the desired BaFe12O19. Although the stoichiometric Fe/Ba ratio should be 12 for BaFe12O19, it was noted that an excess of barium was necessary to ensure the formation of single phase BaFe12O19 [16±18]. Janasi et al. [16] produced barium ferrite powders by coprecipitation with Fe/Ba 11. The pure hexaferrite was obtained at Fe/Ba 10:5 in the work of Carp et al. [17]. They found that if working with a molar ratio Fe/Ba 12, constant small amounts of a-Fe2O3 impurity phase (9 11%) remained in the calcination residues even after an annealing at 900 8C for 30 h. Af¯eck et al. [18] prepared barium ferrite powders using SHS method with Fe/Ba 9:6 in the reaction. They suggested that the small amount of barium
Fig. 7. XRD patterns of powders calcined at 850 8C for 1 h at different Ba/Fe ratios.
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Fig. 8. TEM micrographs of the calcined powders with different CA/NO3 ratios: (a) sample A1, (b) sample A2 and (c) sample A3.
volatilized during the combustion and calcination process was responsible for the ``off stoichiometry'' [18]. In our study, the effect of Fe/Ba ratios from 10 to 12.5 on the synthesized powders was examined. Fig. 7 shows the XRD patterns for the powders synthesized at different Ba/Fe ratios. The calcination temperature and time were ®xed at 850 8C and 1 h. As expected, the Ba/Fe ratio plays an important role in the formation of BaFe12O19, single phase of BaFe12O19 has been obtained at a Fe/Ba ratio of 11.5. In contrast, a-Fe2O3 or BaFe2O4 is present at the Fe/Ba ratios of 10, 12 and 12.5. Fig. 8. shows the morphology of the calcined powders with different CA/NO3 ratios. It can be seen that, the average particle size decreases with increasing CA/NO3 ratio. During the combustion process, a large amount of gaseous material was evolved, the rich fuel style gave out much more gas than the poor fuel style, and the rich fuel style ®lled out its product with more space. Because the gas took out a great deal of caloric, the temperature in the rich fuel style was lower than that in the poor fuel style. Based on this viewpoint, the particle size can be controlled by adjusting the CA/NO3 ratio. As the CA/NO3 ratio increases, the amount of gaseous materials evolved also increases, resulting in lower mean agglomerative size. However, the decrease in average particle size is not signi®cant when the ratio is higher than 0.68, as can be seen in Fig. 9.
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Fig. 9. Dependence of average crystallite size of the calcined powders on the CA/NO3 ratio.
Hysteresis loops of samples A1, A2 and A3 at room temperature and a maximum applied ®eld of 18 kOe are shown in Fig. 10. The result shows that both the magnetisation and coercivity of A3 are higher than those of A1 and A2, which is relevant to the distinct difference of the three microstructures from each other. The differences between these samples are not only the particle size, but also the size distribution. Fig. 8a shows that the grain size of A1 is between 100 and 400 nm and has an uneven distribution; the grain size of A2 is between 60 and 150 nm, while the grain size of A3 has a narrower distribution between 80 and 120 nm. Previous studies showed that structural inhomogeneity could decrease the magnetization in ®ne particles [14]. The critical diameter for spherical single-domain barium ferrite particles is about 460 nm [15], so the particles of the samples A1, A2 and A3 are mainly
Fig. 10. Hysteresis loops at room temperature of the barium hexaferrite samples.
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single-domain. The low ss and coercivity of A1, in comparison with sample A2 and A3, could be a consequence of the wide particle size distribution. Moreover, evident agglomeration exists in A1 (Fig. 8a), which means that there is an important fraction of agglomerative particles over the critical diameter. The change from a single-domain structure to a multi-domain one led by agglomeration may result in the deterioration of magnetic properties. The most homogeneous grain size distribution and good dispersity of A3 guarantees best magnetic properties with the saturation magnetization value of 59.36 emu/g and intrinsic coercivity value of 5540 Oe. Obviously, magnetic properties are rather sensitive to the transformation of microstructures. The work of optimizing particle microstructures for the improvement of magnetic properties is still in progress. 4. Conclusions Nano crystalline BaFe12O19 powders have been prepared at a relatively low calcination temperature by a gel combustion technique. This route is based on the combustion of nitrate±citrate gels due to an exothermic redox reaction between nitrate and citrate ions. This method is time saving and easy to implement, while producing nano crystalline, compositionally homogeneous powders. The experiment shows that Fe/Ba ratios slightly lower than the stoichiometry lead to the production of pure BaFe12O19. The initial citric acid/nitrate ratio has a marked in¯uence on the combustion process and the grain size of the product. Maximum saturation magnetisation of 59.36 emu/g and a coercivity of 5540 Oe in A3 has been achieved. TEM observations reveal that the powder of A3 has a narrower distribution between 80 and 120 nm, with a majority of irregular grains and only a small amount of hexagonal grains. Acknowledgements Authors would like to thank Prof. Meiling Ruan for her assistance with TEM studies, Dr. Hui Xu for magnetic measurements, and Prof. Changwei Lu for the thermal analyses. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12]
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