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Microstructure and hysteresis curves of the barium hexaferrite from co-precipitation by organic agent
Magma=21190=C Mamatha=Venkatachala=BG
Journal of Magnetism and Magnetic Materials 217 (2000) 147}154
Microstructure and hysteresis curves of the barium hexaferrite from co-precipitation by organic agent T. Ogasawara*, M.A.S. Oliveira Department of Metallurgical and Materials Engineering of the COPPE, Institute Alberto Luiz Coimbra of Graduate Courses of Engineering, Federal University of Rio de Janeiro, P.O. Box 68505-Ilha do FundaJ o ZIP CODE 21945-970 Rio de Janeiro-RJ, Brazil Received 1 September 1999; received in revised form 23 February 2000
Abstract This work correlates the magnetic hysteresis curves to the microstructure of the sintered polycrystalline barium hexaferrite discs produced from co-precipitated barium and iron citrates. Citric acid was used as the organic precipitating agent. Thermogravimetric and di!erential thermal analyses were performed on the co-precipitated product in order to guide its calcination into barium hexaferrite crystals, which was con"rmed by means of the X-ray #uorescence and X-ray di!raction. After that, the hexaferrite powder was pressed in a steel die and submitted to "ring in air at various temperatures. The "nal ceramic pieces were characterized by means of scanning electron microscopy and magnetic hysteresis grapher. The obtained results indicate a strong e!ect of the "ring temperature on the microstructure, which in turn a!ected the magnetic hysteresis curve. 2000 Elsevier Science B.V. All rights reserved. PACS: 75.60.!d; 75.60.Ej; 75.50.Ww Keywords: Barium hexaferrite; Citrate process; Hard ferrites; Magnetic hysteresis loop; Magnetic ceramics
1. Introduction The application of barium hexaferrite (BaFe O ), both pure and doped, remains im portant presently [1}4] due to its low production cost while providing excellent magnetic properties. Citric acid has already been tested successfully [5,6] in the production of the hard ferrite. This chemical method of synthesis, di!erent from the traditional powder mixing for producing ceramics, presented the advantages of providing a material with small, well-formed particles with good mag* Corresponding author. Tel.: #55-21-2807443; fax: #5521-2906626. E-mail address: [email protected] (T. Ogasawara).
netic properties. Iron and barium citrates were coprecipitated from the starting aqueous solution, "ltered, dried, and calcined to give barium hexaferrite and ground. This powder was then processed by uniaxial pressing into discs, followed by their sintering in air. The resulting magnetic properties were correlated to the microstructure of the sintered ceramic pieces, which in turn depended on the sintering temperature.
2. Materials and methods The synthesis of the barium hexaferrite has been performed using the following chemicals: Merck ferric nitrate (99% purity), Reagen barium nitrate
0304-8853/00/$ - see front matter 2000 Elsevier Science B.V. All rights reserved. PII: S 0 3 0 4 - 8 8 5 3 ( 0 0 ) 0 0 0 8 0 - 9
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(99% purity), Merck (99% purity) citric acid monohydrate, concentrated Reagen ammonium hydroxide, Vetec absolute (99.9% purity) ethanol, Vetec re-crystallized iodine, and Vetec magnesium scraps. The co-precipitated powder was characterized by the following techniques: (a) TGA-DTA analysis using Shimadzu instruments (TGA-50H and DTA-5OH); (b) scanning electron microscopy (SEM) carried out through a Zeiss DSM 940A module microscope, operated at 20}25 kV; (c) chemical analysis by means of a Philips Model PW 2400 X-ray #uorescence spectrometer; (d) phase analysis using a Philips PW 1130/60 X-ray di!ractometer. The characterization of the sintered disc was performed by: (e) SEM (microstructure determination); (f) density measurement through a 25 ml capacity mercury picnometer and an analytical balance (Analytical Plus Electronics Balances Model AP25); and (g) determination of the magnetic hysteresis curves at 60 Hz using Walker Scienti"c Inc., AMH-40H Module hysteresis graph. Using distilled and de-ionized water, the following aqueous solutions were prepared in order to produce the hexaferrite powders: (a) reddish-brown and translucent solution of ferric nitrate monohydrate; (b) colorless and translucent solution of the barium nitrate; and (c) colorless and translucent solution of citric acid monohydrate. In these preparations, the solid reagents were put inside erlenmeyers and hermetically sealed with a rubber stopper before weighing. The three #asks were then evacuated and kept under super-dry nitrogen gas. Distilled and de-ionized water was added to each of the #asks, under agitation, through syringes and needles up to complete dissolution of the solids. The three solutions were then transferred and mixed with each other (in the 1Ba : 12Fe : 13 citric acid molar ratio, corresponding to the BaFe O stoichiometry) in a magnetically agi tated glass globe, previously kept under super-dry nitrogen gas too. The resultant mixture, brown colored, was heated up to 803C under intense stirring in order to fully reach the reaction end. This heating step was carried out under re#ux so as to keep the inert atmosphere and to make possible later additions of the volatile ammonium hydroxide. Ammonium concentrated solution was progressively added to the mixed solution referred above in
order to raise its pH (which was around 5) up to a value close to 10, which made the precipitation of the desired organo-metallic compound possible in the following. Ethanol, previously dried [7], was then added, drop by drop under intensive stirring, to the mixed reacting solution to promote the precipitation of the strong yellow colored barium and iron citrate. (The ethanol was dried through its treatment with iodine crystals and magnesium scraps, heated up under re#ux for some hours.) After precipitation of the yellow solid, the excess solution resulting from the mixture and ethanol addition was removed with the help of a rotary evaporator up to a high solid/liquid ratio. The solid was then "ltered and washed with dried ethanol in order to eliminate any residual moisture. In the following, it was dried in an oven at 1003C to give rise to a brown}yellow colored solid. The next step was the thermal decomposition of the iron}barium citrate and elimination of the organic precursor (citrate). For that purpose, the dry mixed citrate was subjected to a heating at a rate of 103C/min up to 5003C, where it remained during 24 h, in air; the resultant powder presented a grayish color, in an amorphous mass, and the iron, barium and oxygen had not formed a crystalline array [6]. A second heating in air, at a rate of 103C/min, increased the temperature of the material to 8003C, where it was maintained during 6 h for achieving the crystalline barium hexaferrite. The result was an extremely "ne very intensely gray colored powder, already presenting magnetic characteristics, detected through a magnet which immediately attracted the powder particles. The hexaferrite powder was then dry pressed in a steel die with 1000 kg cm\ pressure [8], in order to give cylindrical discs of 1.5 cm diameter and 0.5 mm height (no binder was used). The sintering of the barium hexaferrite discs was carried out in air, with a heating rate of 103C/min before reaching the burning temperature (12003C, 12503C, 13003C, 13503C and 14003C), where it remained during 4 h.
3. Results and discussion During the heating of the citrates of iron and barium, about 84.6% of the initial mass of the
Magma=21190=CM=VVC=BG
T. Ogasawara, M.A.S. Oliveira / Journal of Magnetism and Magnetic Materials 217 (2000) 147}154
Fig. 1. Plot of the thermogravimetric analysis of the barium and iron citrate co-precipitate during heating in air.
Fig. 2. Plot of the di!erential thermal analysis for the barium and iron citrate co-precipitate during heating.
sample was lost at a temperature lower than 4003C, occurring with two distinct decomposition reactions of the citrates: one at 1983C and the other at 3983C (see the weight losses in Fig. 1). The di!erential thermal analysis showed exothermic reaction peaks at 1983C and at 3983C (see Fig. 2). These two exothermic reactions following the iron and barium citrate decomposition are attributed to the conversion of the iron and barium cations to their oxides of the iron (in the "rst peak) and the barium oxide (in the second peak). Although some citrate organic radical may be converted to carbon dioxide, the desired result is to eliminate it as a citrate vapor instead of carbon dioxide (which may combine with the barium cation to generate a quite stable barium carbonate, thereby delaying the barium hexaferrite formation). Ariaca et al. and Medarde et al. [9,10] found the formation of c-Fe O at a temperature
149
Fig. 3. X-ray di!raction patterns for the barium hexaferrite powder formed by calcination of the co-precipitated barium and iron citrate.
just above 1503C during the "rst step of the decomposition of the iron}barium citrate. This behavior was expected in the present work because of the careful handling of the precursors solutions in a dry box, avoiding any chance of the formation of barium carbonate during co-precipitation. The "nal decomposition of these citrates was found to occur at temperatures of about 420}4503C [6,9}12]. Therefore, the second peak at 3983C in Fig. 2 corresponds to the conversion of the barium cation to its oxide and the simultaneous formation of the amorphous barium hexaferrite. Sandaranarayanan and Khan [11] point out that the crystallization of the barium hexaferrite begins around 5503C and that a fully crystallized product is obtained through thermal treatment of the material in the range of 700}9003C. In the present work, the calcination process was ended at 8003C and the X-ray di!raction pattern (Fig. 3) allowed the identi"cation of the product as being the desired barium hexaferrite. Table 1 gives its elemental composition as determined by X-ray #uorescence analysis while Table 2 presents the corresponding composition as oxides (BaO and Fe O ). In Table 3 it is seen that the majority of the calcined mass was retained in the 270 mesh screen after rough crushing. Furthermore, about 96.5% of the total crushed powder mass was (!200 mesh, #270 mesh), with an average agglomerate size of
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Table 1 Theoretical and actual elemental stoichiometric composition of the barium hexaferrite powder generated in the calcination of the citrate coprecipitate Elements
Theoretical stoichiometric chemical composition (%)
Actual composition (%)
Ba Fe O C Other metals
12.356 60.294 27.350 * *
11.452 59.793 * Undetected Traces
Table 2 Theoretical and actual stoichiometric composition of the barium hexaferrite powder in terms of oxide components Oxides
Theoretical stoichiometric composition (%)
Actual stoichiometric composition (%)
BaO Fe O Other oxides
13.795 86.205 *
12.109 87.761 Traces
60 lm. This implies the need for using an e!ective method of dispersing the agglomerated particles before the shape-forming process. In the present work, that improvement was not carried out. Figs. 4(a)}(d) are scanning electron micrographs of agglomerated barium hexaferrite powder as obtained from calcination of the mixed citrate. It is seen that calcination led to the aggregation of what was before very "ne co-precipitated particles. The individual particles in the aggregates of Figs. 4(c) and (d) are smaller than 0.25 lm. In Fig. 4(b), which is a 150% ampli"cation of the picture in Fig. 4(a), one can see the size of the individual particles in the aggregate as being of about 1 lm. The setup of a size distribution during calcination process of the iron}barium citrate is a natural trend involving the nucleation of particles followed by the growth of the larger particles at the expense of smaller ones. The scanning electron micrograph in Fig. 5(a) shows that the bonding between individual par-
Table 3 Agglomerate size distribution of the calcined barium hexaferrite powder, as calcined and deagglomerated by rough grinding Screen (Tyler mesh)
Diameter (lm)
Retained mass (g)
Retained mass (%)
150 200 270 325 400
105 74 53 44 37
0.0 45.06 48.86 3.01 0.40
0.0 46.30 50.20 3.09 0.41
ticles in the barium hexaferrite sintered at 12003C for 4 h does not di!er much from that one observed in the calcined powder (Fig. 4). The grain size is about 1 lm. Only 45% of the theoretical density was achieved in the sintered disc. Fig. 5(b) presents the corresponding magnetic hysteresis curve to which are associated the following values of the relevant magnetic properties: BH "0.06943 MOe;
H "2.285 kOe; B "0.5602 kG. The low values of BH and H result from a low degree of sintering.
The scanning electron micrograph of the barium hexaferrite sintered at 12503C revealed that the sintering of the particles is still incipient, giving a morphology similar to that of the original particles prior to burning (the particle size is about 1.5 lm), which agrees with the density of the sintered disc equal to 50% of the theoretical value. To that corresponds a magnetic hysteresis curve having associated to it the following values of the relevant magnetic properties: BH "
0.117718 MOe; H "4.358 kOe; B "0.6944 kG. The magnetic properties are still not good, re#ecting an unsatisfactory microstructure. Fig. 6(a) shows the scanning electron micrograph of the disc of the barium hexaferrite sintered at 13003C, noting that much evolution occurred in the sintering of the particles, giving rise to a morphology of round-shaped and grown particles (about 3 lm), which is consistent with a density of sintered discs equal to 75% of the theoretical value. To that corresponds the magnetic hysteresis curve shown in Fig. 6(b), to which are associated the following values of the relevant magnetic properties: BH "0.4022 MOe; H "4.379 kOe; B "
1.346 kG. The magnetic properties are already
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151
Fig. 4. Scanning electron micrographs of the aggregated barium hexaferrite powder as produced by calcination of the barium}iron citrate co-precipitate. (a) to (d) refer to the same sample; (a), (c) and (d) are 3 di!erent regions in the sample, while (b) is a 150% ampli"cation of the picture in (a).
almost good, re#ecting the improved microstructure. Fig. 7(a) shows the scanning electron micrograph of the barium hexaferrite disc sintered at 13503C, noting that the sintering of the particles progressed more, giving rise to a morphology of recrystallized and grown grains (about 5 lm), which is consistent with a density of the sintered disc equal to 90% of the theoretical value. To that corresponds the mag-
netic hysteresis curve shown in Fig. 7(b), to which are associated the following values of the relevant magnetic properties: BH "0.5877 MOe;
H "4.340 kOe; B "1.644 kG. The magnetic pro perties are good this time, re#ecting the good microstructure. A "ne-grained, well-sintered ceramic piece, with nearly theoretical density should be found in a hard ferrite in order to achieve the desired coercive force.
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Fig. 5. (a) Scanning electron micrograph of the surface of the barium hexaferrite disc sintered at 12003C; and (b) curve of the magnetic hysteresis of the barium hexaferrite disc sintered at 12003C.
Fig. 6. (a) Scanning electron micrograph of the surface of the barium hexaferrite disc sintered at 13003C; and (b) magnetic hysteresis curve of the barium hexaferrite disc sintered at 13003C.
From this point of view, the hysteresis curve of Fig. 7(b), while good, is not excellent (the hysteresis loop should be &square-shape'). The quality limitation comes from the microstructure. In Fig. 7(a) one can see that: (a) the 10% porosity is apparent as intergranular pores; (b) the grains are too large (about 5 lm, instead of smaller than 2.5 lm as they should
be). Both factors, (a) and (b) referred to above, decrease the coercive force. This unhappy result may be corrected using a more e$cient grinding of the calcined powder followed by spray granulation. A denser green compact should convert to denser sintered ceramic piece at a burning temperature lower than 13503C.
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153
Fig. 7. (a) Scanning electron micrograph of the barium hexaferrite disc sintered at 13503C, and (b) magnetic hysteresis curve of the barium hexaferrite discs sintered at 13503C.
Of course, hot-pressing would provide high-density sinter with controlled grain size, performing the burning at the lowest temperature possible.
4. Conclusions (a) It is e!ectively possible to get co-precipitated barium and iron citrates of good quality for the production of barium hexaferrite powder in the subsequent calcination at 8003C (after elimination of the organics at 5003C), if the preparation and handling of the aqueous solutions of the interesting salts is carried out under super-dry nitrogen gas and use is made of anhydrous ethanol in the precipitation of the citrate; the need for more e!ective grinding of the calcined powder is recognized. (b) The sintering temperature, in fact, has a determining in#uence on the microstructure of the sintered disc of the barium hexaferrite and, consequently, on the magnetic properties of the "nal ceramics. The temperature of 13503C has been revealed as being the best for sintering of the barium hexaferrite in the range explored in the present work. (c) Hot pressing of a compacted powder appears as an important key for producing small grain-
sized, well-sintered, highly densi"ed ceramic pieces of barium hexaferrite capable of providing high coercive force.
Acknowledgements The authors give thanks to CNPq, CAPES, PADCT, UFF, FUJB and FAPERJ for the support to the present work.
References [1] X. Sui, J.A. Bain, M.H. Kryder, US5863661 Patent Issued January 26, 1999, Carnegie Mellon University, Pittsburgh, PA, Applicant, 1999. [2] G.C. Stangle, K.R. Venkatachari, S.P. Ostrander, W.A. Schulze, J.D. Pietras, US5660772 Patent Issued August 26, 1997, Alfred University, Alfred, NY, Applicant, 1997. [3] J. Kreisel, G. Lucazeau, H. Vincent, J. Raman Spectrosc. 30 (2) (1999) 115. [4] S. Thongmee, T. Osotchan, P. Winotai, I.M. Tang, Int. J. Mod. Phys. B 12 (27}28) (1998) 2847. [5] V.K. Sankaranarayanan, Q.A. Pankhust, D.P.E. Dickson, C.E. Johnson, J. Magn. Magn. Mater. 125 (1993) 199. [6] W. Zhong, W. Ding, Y. Jiang, N. Zhang, J. Zhang, Y. Du, Q. Yan, J. Am. Soc. 80 (12) (1997) 3258.
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[7] D.D. Perrin, W.C.F. Armarego, Puri"cation of Laboratory Chemicals, 3rd edition, Pergamon Press, Oxford, UK, 1988, p. 60. [8] G. Turilli, A. Paoluzi, M. Lucenti, J. Magn. Magn. Mater. 97 (1991) 338. [9] R. Ardiaca, M. Medarde, X. Obradors, M. Vallet, M. Pernet, J. RodrmH guez, J. Fontcuberta, J. Phys. Colloque C8 (Suppl. 12) (1988) 1848.
[10] M. Medarde, J. RodrmH guez, M. Vallet, M. Pernet, X. Obradors, J. Pannetier, Physica B 156&157 (1998) 36. [11] V.K. Sandaranarayanan, D.C. Khan, J. Magn. Magn. Mater. 153 (1996) 337. [12] R. Ardiaca, R. Ramos, A. IsalgueH , J. RodrmH guez, X. Obradors, M. Pernet, M. Vallet, IEEE Trans. Magn. MAG-23 (1) (1987) 22Ref}in}table.
Journal of Magnetism and Magnetic Materials 217 (2000) 147}154
Microstructure and hysteresis curves of the barium hexaferrite from co-precipitation by organic agent T. Ogasawara*, M.A.S. Oliveira Department of Metallurgical and Materials Engineering of the COPPE, Institute Alberto Luiz Coimbra of Graduate Courses of Engineering, Federal University of Rio de Janeiro, P.O. Box 68505-Ilha do FundaJ o ZIP CODE 21945-970 Rio de Janeiro-RJ, Brazil Received 1 September 1999; received in revised form 23 February 2000
Abstract This work correlates the magnetic hysteresis curves to the microstructure of the sintered polycrystalline barium hexaferrite discs produced from co-precipitated barium and iron citrates. Citric acid was used as the organic precipitating agent. Thermogravimetric and di!erential thermal analyses were performed on the co-precipitated product in order to guide its calcination into barium hexaferrite crystals, which was con"rmed by means of the X-ray #uorescence and X-ray di!raction. After that, the hexaferrite powder was pressed in a steel die and submitted to "ring in air at various temperatures. The "nal ceramic pieces were characterized by means of scanning electron microscopy and magnetic hysteresis grapher. The obtained results indicate a strong e!ect of the "ring temperature on the microstructure, which in turn a!ected the magnetic hysteresis curve. 2000 Elsevier Science B.V. All rights reserved. PACS: 75.60.!d; 75.60.Ej; 75.50.Ww Keywords: Barium hexaferrite; Citrate process; Hard ferrites; Magnetic hysteresis loop; Magnetic ceramics
1. Introduction The application of barium hexaferrite (BaFe O ), both pure and doped, remains im portant presently [1}4] due to its low production cost while providing excellent magnetic properties. Citric acid has already been tested successfully [5,6] in the production of the hard ferrite. This chemical method of synthesis, di!erent from the traditional powder mixing for producing ceramics, presented the advantages of providing a material with small, well-formed particles with good mag* Corresponding author. Tel.: #55-21-2807443; fax: #5521-2906626. E-mail address: [email protected] (T. Ogasawara).
netic properties. Iron and barium citrates were coprecipitated from the starting aqueous solution, "ltered, dried, and calcined to give barium hexaferrite and ground. This powder was then processed by uniaxial pressing into discs, followed by their sintering in air. The resulting magnetic properties were correlated to the microstructure of the sintered ceramic pieces, which in turn depended on the sintering temperature.
2. Materials and methods The synthesis of the barium hexaferrite has been performed using the following chemicals: Merck ferric nitrate (99% purity), Reagen barium nitrate
0304-8853/00/$ - see front matter 2000 Elsevier Science B.V. All rights reserved. PII: S 0 3 0 4 - 8 8 5 3 ( 0 0 ) 0 0 0 8 0 - 9
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(99% purity), Merck (99% purity) citric acid monohydrate, concentrated Reagen ammonium hydroxide, Vetec absolute (99.9% purity) ethanol, Vetec re-crystallized iodine, and Vetec magnesium scraps. The co-precipitated powder was characterized by the following techniques: (a) TGA-DTA analysis using Shimadzu instruments (TGA-50H and DTA-5OH); (b) scanning electron microscopy (SEM) carried out through a Zeiss DSM 940A module microscope, operated at 20}25 kV; (c) chemical analysis by means of a Philips Model PW 2400 X-ray #uorescence spectrometer; (d) phase analysis using a Philips PW 1130/60 X-ray di!ractometer. The characterization of the sintered disc was performed by: (e) SEM (microstructure determination); (f) density measurement through a 25 ml capacity mercury picnometer and an analytical balance (Analytical Plus Electronics Balances Model AP25); and (g) determination of the magnetic hysteresis curves at 60 Hz using Walker Scienti"c Inc., AMH-40H Module hysteresis graph. Using distilled and de-ionized water, the following aqueous solutions were prepared in order to produce the hexaferrite powders: (a) reddish-brown and translucent solution of ferric nitrate monohydrate; (b) colorless and translucent solution of the barium nitrate; and (c) colorless and translucent solution of citric acid monohydrate. In these preparations, the solid reagents were put inside erlenmeyers and hermetically sealed with a rubber stopper before weighing. The three #asks were then evacuated and kept under super-dry nitrogen gas. Distilled and de-ionized water was added to each of the #asks, under agitation, through syringes and needles up to complete dissolution of the solids. The three solutions were then transferred and mixed with each other (in the 1Ba : 12Fe : 13 citric acid molar ratio, corresponding to the BaFe O stoichiometry) in a magnetically agi tated glass globe, previously kept under super-dry nitrogen gas too. The resultant mixture, brown colored, was heated up to 803C under intense stirring in order to fully reach the reaction end. This heating step was carried out under re#ux so as to keep the inert atmosphere and to make possible later additions of the volatile ammonium hydroxide. Ammonium concentrated solution was progressively added to the mixed solution referred above in
order to raise its pH (which was around 5) up to a value close to 10, which made the precipitation of the desired organo-metallic compound possible in the following. Ethanol, previously dried [7], was then added, drop by drop under intensive stirring, to the mixed reacting solution to promote the precipitation of the strong yellow colored barium and iron citrate. (The ethanol was dried through its treatment with iodine crystals and magnesium scraps, heated up under re#ux for some hours.) After precipitation of the yellow solid, the excess solution resulting from the mixture and ethanol addition was removed with the help of a rotary evaporator up to a high solid/liquid ratio. The solid was then "ltered and washed with dried ethanol in order to eliminate any residual moisture. In the following, it was dried in an oven at 1003C to give rise to a brown}yellow colored solid. The next step was the thermal decomposition of the iron}barium citrate and elimination of the organic precursor (citrate). For that purpose, the dry mixed citrate was subjected to a heating at a rate of 103C/min up to 5003C, where it remained during 24 h, in air; the resultant powder presented a grayish color, in an amorphous mass, and the iron, barium and oxygen had not formed a crystalline array [6]. A second heating in air, at a rate of 103C/min, increased the temperature of the material to 8003C, where it was maintained during 6 h for achieving the crystalline barium hexaferrite. The result was an extremely "ne very intensely gray colored powder, already presenting magnetic characteristics, detected through a magnet which immediately attracted the powder particles. The hexaferrite powder was then dry pressed in a steel die with 1000 kg cm\ pressure [8], in order to give cylindrical discs of 1.5 cm diameter and 0.5 mm height (no binder was used). The sintering of the barium hexaferrite discs was carried out in air, with a heating rate of 103C/min before reaching the burning temperature (12003C, 12503C, 13003C, 13503C and 14003C), where it remained during 4 h.
3. Results and discussion During the heating of the citrates of iron and barium, about 84.6% of the initial mass of the
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T. Ogasawara, M.A.S. Oliveira / Journal of Magnetism and Magnetic Materials 217 (2000) 147}154
Fig. 1. Plot of the thermogravimetric analysis of the barium and iron citrate co-precipitate during heating in air.
Fig. 2. Plot of the di!erential thermal analysis for the barium and iron citrate co-precipitate during heating.
sample was lost at a temperature lower than 4003C, occurring with two distinct decomposition reactions of the citrates: one at 1983C and the other at 3983C (see the weight losses in Fig. 1). The di!erential thermal analysis showed exothermic reaction peaks at 1983C and at 3983C (see Fig. 2). These two exothermic reactions following the iron and barium citrate decomposition are attributed to the conversion of the iron and barium cations to their oxides of the iron (in the "rst peak) and the barium oxide (in the second peak). Although some citrate organic radical may be converted to carbon dioxide, the desired result is to eliminate it as a citrate vapor instead of carbon dioxide (which may combine with the barium cation to generate a quite stable barium carbonate, thereby delaying the barium hexaferrite formation). Ariaca et al. and Medarde et al. [9,10] found the formation of c-Fe O at a temperature
149
Fig. 3. X-ray di!raction patterns for the barium hexaferrite powder formed by calcination of the co-precipitated barium and iron citrate.
just above 1503C during the "rst step of the decomposition of the iron}barium citrate. This behavior was expected in the present work because of the careful handling of the precursors solutions in a dry box, avoiding any chance of the formation of barium carbonate during co-precipitation. The "nal decomposition of these citrates was found to occur at temperatures of about 420}4503C [6,9}12]. Therefore, the second peak at 3983C in Fig. 2 corresponds to the conversion of the barium cation to its oxide and the simultaneous formation of the amorphous barium hexaferrite. Sandaranarayanan and Khan [11] point out that the crystallization of the barium hexaferrite begins around 5503C and that a fully crystallized product is obtained through thermal treatment of the material in the range of 700}9003C. In the present work, the calcination process was ended at 8003C and the X-ray di!raction pattern (Fig. 3) allowed the identi"cation of the product as being the desired barium hexaferrite. Table 1 gives its elemental composition as determined by X-ray #uorescence analysis while Table 2 presents the corresponding composition as oxides (BaO and Fe O ). In Table 3 it is seen that the majority of the calcined mass was retained in the 270 mesh screen after rough crushing. Furthermore, about 96.5% of the total crushed powder mass was (!200 mesh, #270 mesh), with an average agglomerate size of
Magma=21190=CM=VVC=BG
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Table 1 Theoretical and actual elemental stoichiometric composition of the barium hexaferrite powder generated in the calcination of the citrate coprecipitate Elements
Theoretical stoichiometric chemical composition (%)
Actual composition (%)
Ba Fe O C Other metals
12.356 60.294 27.350 * *
11.452 59.793 * Undetected Traces
Table 2 Theoretical and actual stoichiometric composition of the barium hexaferrite powder in terms of oxide components Oxides
Theoretical stoichiometric composition (%)
Actual stoichiometric composition (%)
BaO Fe O Other oxides
13.795 86.205 *
12.109 87.761 Traces
60 lm. This implies the need for using an e!ective method of dispersing the agglomerated particles before the shape-forming process. In the present work, that improvement was not carried out. Figs. 4(a)}(d) are scanning electron micrographs of agglomerated barium hexaferrite powder as obtained from calcination of the mixed citrate. It is seen that calcination led to the aggregation of what was before very "ne co-precipitated particles. The individual particles in the aggregates of Figs. 4(c) and (d) are smaller than 0.25 lm. In Fig. 4(b), which is a 150% ampli"cation of the picture in Fig. 4(a), one can see the size of the individual particles in the aggregate as being of about 1 lm. The setup of a size distribution during calcination process of the iron}barium citrate is a natural trend involving the nucleation of particles followed by the growth of the larger particles at the expense of smaller ones. The scanning electron micrograph in Fig. 5(a) shows that the bonding between individual par-
Table 3 Agglomerate size distribution of the calcined barium hexaferrite powder, as calcined and deagglomerated by rough grinding Screen (Tyler mesh)
Diameter (lm)
Retained mass (g)
Retained mass (%)
150 200 270 325 400
105 74 53 44 37
0.0 45.06 48.86 3.01 0.40
0.0 46.30 50.20 3.09 0.41
ticles in the barium hexaferrite sintered at 12003C for 4 h does not di!er much from that one observed in the calcined powder (Fig. 4). The grain size is about 1 lm. Only 45% of the theoretical density was achieved in the sintered disc. Fig. 5(b) presents the corresponding magnetic hysteresis curve to which are associated the following values of the relevant magnetic properties: BH "0.06943 MOe;
H "2.285 kOe; B "0.5602 kG. The low values of BH and H result from a low degree of sintering.
The scanning electron micrograph of the barium hexaferrite sintered at 12503C revealed that the sintering of the particles is still incipient, giving a morphology similar to that of the original particles prior to burning (the particle size is about 1.5 lm), which agrees with the density of the sintered disc equal to 50% of the theoretical value. To that corresponds a magnetic hysteresis curve having associated to it the following values of the relevant magnetic properties: BH "
0.117718 MOe; H "4.358 kOe; B "0.6944 kG. The magnetic properties are still not good, re#ecting an unsatisfactory microstructure. Fig. 6(a) shows the scanning electron micrograph of the disc of the barium hexaferrite sintered at 13003C, noting that much evolution occurred in the sintering of the particles, giving rise to a morphology of round-shaped and grown particles (about 3 lm), which is consistent with a density of sintered discs equal to 75% of the theoretical value. To that corresponds the magnetic hysteresis curve shown in Fig. 6(b), to which are associated the following values of the relevant magnetic properties: BH "0.4022 MOe; H "4.379 kOe; B "
1.346 kG. The magnetic properties are already
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151
Fig. 4. Scanning electron micrographs of the aggregated barium hexaferrite powder as produced by calcination of the barium}iron citrate co-precipitate. (a) to (d) refer to the same sample; (a), (c) and (d) are 3 di!erent regions in the sample, while (b) is a 150% ampli"cation of the picture in (a).
almost good, re#ecting the improved microstructure. Fig. 7(a) shows the scanning electron micrograph of the barium hexaferrite disc sintered at 13503C, noting that the sintering of the particles progressed more, giving rise to a morphology of recrystallized and grown grains (about 5 lm), which is consistent with a density of the sintered disc equal to 90% of the theoretical value. To that corresponds the mag-
netic hysteresis curve shown in Fig. 7(b), to which are associated the following values of the relevant magnetic properties: BH "0.5877 MOe;
H "4.340 kOe; B "1.644 kG. The magnetic pro perties are good this time, re#ecting the good microstructure. A "ne-grained, well-sintered ceramic piece, with nearly theoretical density should be found in a hard ferrite in order to achieve the desired coercive force.
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Fig. 5. (a) Scanning electron micrograph of the surface of the barium hexaferrite disc sintered at 12003C; and (b) curve of the magnetic hysteresis of the barium hexaferrite disc sintered at 12003C.
Fig. 6. (a) Scanning electron micrograph of the surface of the barium hexaferrite disc sintered at 13003C; and (b) magnetic hysteresis curve of the barium hexaferrite disc sintered at 13003C.
From this point of view, the hysteresis curve of Fig. 7(b), while good, is not excellent (the hysteresis loop should be &square-shape'). The quality limitation comes from the microstructure. In Fig. 7(a) one can see that: (a) the 10% porosity is apparent as intergranular pores; (b) the grains are too large (about 5 lm, instead of smaller than 2.5 lm as they should
be). Both factors, (a) and (b) referred to above, decrease the coercive force. This unhappy result may be corrected using a more e$cient grinding of the calcined powder followed by spray granulation. A denser green compact should convert to denser sintered ceramic piece at a burning temperature lower than 13503C.
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Fig. 7. (a) Scanning electron micrograph of the barium hexaferrite disc sintered at 13503C, and (b) magnetic hysteresis curve of the barium hexaferrite discs sintered at 13503C.
Of course, hot-pressing would provide high-density sinter with controlled grain size, performing the burning at the lowest temperature possible.
4. Conclusions (a) It is e!ectively possible to get co-precipitated barium and iron citrates of good quality for the production of barium hexaferrite powder in the subsequent calcination at 8003C (after elimination of the organics at 5003C), if the preparation and handling of the aqueous solutions of the interesting salts is carried out under super-dry nitrogen gas and use is made of anhydrous ethanol in the precipitation of the citrate; the need for more e!ective grinding of the calcined powder is recognized. (b) The sintering temperature, in fact, has a determining in#uence on the microstructure of the sintered disc of the barium hexaferrite and, consequently, on the magnetic properties of the "nal ceramics. The temperature of 13503C has been revealed as being the best for sintering of the barium hexaferrite in the range explored in the present work. (c) Hot pressing of a compacted powder appears as an important key for producing small grain-
sized, well-sintered, highly densi"ed ceramic pieces of barium hexaferrite capable of providing high coercive force.
Acknowledgements The authors give thanks to CNPq, CAPES, PADCT, UFF, FUJB and FAPERJ for the support to the present work.
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