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Synthesis of Strontium Ferrite Ultrafine Particles Using Microemulsion Processing
Journal of Colloid and Interface Science 236, 41–46 (2001) doi:10.1006/jcis.2000.7389, available online at http://www.idealibrary.com on
Synthesis of Strontium Ferrite Ultrafine Particles Using Microemulsion Processing Dong-Hwang Chen1 and Yuh-Yuh Chen Department of Chemical Engineering, National Cheng Kung University, Tainan, Taiwan 701, Republic of China Received February 1, 2000; accepted December 18, 2000
formation of ultrafine particles. They not only act as microreactors for processing reactions but also inhibit the excess aggregation of particles because the surfactants could adsorb on the particle surface when the particle size approaches that of the water pool. As a result, the particles obtained in such a medium are generally very fine and monodispersed (3, 7–9). Since the platinum-group metal ultrafine particles were successfully synthesized in w/o microemulsions by Boutonnet et al. (10), the microemulsion technique has been widely used to prepare ultrafine particles of numerous materials, including metals (10–14), metal sulfides and selenides (15–19), metal borides (20), metal carbonates (21), metal halides (22), metal oxides and hydroxides (23–28), and organic polymers (29). Recently, some mixed-metal oxides have also been synthesized by microemulsion processing, including YBa2 Cu3 O7−x , Y3 Fe5 O12 , LaNiO3 , La2 CuO4 , BaPbO3 , Mn0.45 Zn0.55 Fe2 O4 , and particularly BaFe12 O19 (30–37). Strontium ferrite (SrFe12 O19 ) possesses high coercivity due to its relatively high magnetocrystalline anisotropy factor and this property makes it an attractive material for use in permanent magnet applications. However, its production in w/o microemuslions has not been reported until now. The possible reason is the high solubility of strontium hydroxide in water, which results in the difficulty in stoichiometric control. Recently, Fang et al. prepared fine strontium ferrite powders in ethanol-in-oil microemuslions (38). The difficulty in stoichiometric control was reduced, but the calcination temperature for the formation of pure SrFe12 O19 must be above 900◦ C. Also, although they claimed the particle size was 50–80 nm based on the surface area analysis, the SEM photographs indicated the product was sintered seriously. The size of the aggregated particles was 0.1–2 µm. In this study, the synthesis of strontium ferrite in a water-in-oil microemulsion of water/CTAB/ n-butanol/isooctane was investigated. The magnetic property of the final product, which was quite from that observed by Fang et al. (38), was also described.
The strontium ferrite ultrafine particles have been prepared using the microemulsion processing. The mixed hydroxide precursor was obtained via the coprecipitation of Sr2+ and Fe3+ in a water-in-oil microemulsion of water/CTAB/n-butanol/isooctane. According to the investigation on the thermochemical properties by TGA/DTA and the phase analysis by XRD, it was shown that the precursor could yield pure strontium ferrite after calcination at 700◦ C for 5 h while using an appropriate molar ratio of Sr/Fe in microemulsions. From TEM measurement, the diameters of the precursor and calcined particles were 3.8 ± 0.7 and 50–100 nm, respectively. The magnetic properties characterized by a SQUID magnetometer showed that the saturation magnetization, remanent magnetization, coercivity, and squareness ratio were 55 emu/g, 28 emu/g, 492 Oe, and 0.51, respectively. The magnetization was also observed to increase with the decrease of temperature at 5–400 K. Compared with those reported earlier, the quite low coercivity implies the potential application of final product in the high-density perpendicular recording media. °C 2001 Academic Press Key Words: strontium ferrite; ultrafine particles; microemulsion.
INTRODUCTION
Recently, the ultrafine particles in the size range of 1–100 nm have received increasing attention because they exhibit unusual physical and chemical properties different from those of larger particles of the same materials due to their extremely small size and large specific surface area (1–3). As have been known, some magnetic properties such as saturation magnetization, remanent magnetization, and coercitivity depend strongly on the particle size and microstructure of the materials. Therefore, it is interesting and important to develop techniques by which the size, shape, and chemical homogeneity of particles can be well controlled. Water-in-oil (w/o) microemulsion solutions are transparent, isotropic liquid media with nanosized water droplets that are dispersed in a continuous oil phase and stabilized by surfactant molecules at the water/oil interface (4–6). These surfactantcovered water pools offer a unique microenvironment for the
MATERIALS AND METHODS
Materials The cationic surfactant cetyltrimethylamonium bromide (CTAB) and iron(III) nitrate nonhydrate were obtained from
1 To whom correspondence should be addressed. Fax: 886-6-2344496. E-mail: [email protected].
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0021-9797/01 $35.00
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CHEN AND CHEN
TABLE 1 Compositions of the Microemulsion System Used for the Synthesis of Precursor
Aqueous phase Surfactant Cosurfactant Oil phase
Microemulsion I
Microemulsion II
Wt%
0.01 M Sr(NO3 )2 + 0.08–0.11 M Fe(NO3 )3 CTAB n-Butanol Isooctane
1.0 M NaOH
22
CTAB n-Butanol Isooctane
21 15 42
Acros Organics (Belgium). Strontium nitrate was the product of Janssen (Beerse). Isooctane, n-butanol, chloroform, methanol, and ethanol were the HPLC-grade reagents supplied by TEDIA (Fairfield). All reagents were used without further purification. The water used throughout this work was the reagent-grade water produced by Milli-Q SP Ultra-Pure-Water Purification System of Nihon Millipore Ltd., Tokyo. Synthesis of Strontium Ferrite Ultrafine Particles The microemulsion system used in this study consisted of CTAB as the surfactant, n-butanol as the cosurfactant, isooctane as the continuous oil phase, and an aqueous solution as the dispersed phase. The composition was indicated in Table 1. Two microemulsions (I and II) with identical compositions but different aqueous phases were prepared. The aqueous phase in microemulsion I was a mixture of strontium nitrate and ferric nitrate solution in the molar ratio of 1/8–1/11. Although a molar ratio of 1/12 should be sufficient according to stoichiometry, an excess of strontium nitrate was necessary because strontium hydroxide is partially soluble in water (39). The appropriate molar ratio will be discussed in the next section. The aqueous phase in microemulsion II was the precipitation agent, NaOH, with a sufficient concentration for the precipitation of Sr2+ and Fe3+ ions. When these two microemulsions were mixed, the mixed hydroxide precipitate appeared within the nanosized aqueous droplets of the microemulsion. After being centrifuged at 15,000 rpm for 10 min, the mixed hydroxide precipitate was washed several times with a mixture of chloroform and methanol (1 : 1) and then vacuum-dried at 70◦ C. According to elemental analysis on carbon, the residual surfactant (CTAB) was below 1%. Thermogravimetric analysis (TGA) and differential thermal analysis (DTA) were done on the dried precursor in air with a heating rate of 10◦ C/min on the Shimadzu TA-50WSI TGA and DTA instruments, respectively. Based on the results of these analyses, which will be discussed in the next section, the dried precursor was calcined in air under appropriate conditions (typically, at 700◦ C for 5 h) to ensure complete conversion of the mixed hydroxide to strontium ferrite. Characterization The sizes of the precursor particles and the calcined particles were determined by TEM using a Hitachi Model HF-2000
field emission transmission electron microscope with a resolution of 0.1 nm. For the precursor particles, the microemulsion solution was sonicated for above 1 min prior to depositing it onto a Formvar-covered copper grid. Over more than 200 particles from different parts of the grid were used to estimate the average diameter and size distribution of the precursor particles. For the calcined particles, the sample was ultrasonically dispersed in ethanol for several minutes prior to depositing it onto a Formvar-covered copper grid. For the irregularly shaped particles, the diameters refer to the longest dimensions. The phase analysis of the calcined particles was done by powder XRD on a Rigaku D/max III.V X-ray diffractometer using CuK α radiation (λ = 0.1542 nm). The magnetic measurements for the calcined particles were carried out using a SQUID magnetometer (MPMS7, Quantum Design). RESULTS AND DISCUSSION
Size and Thermochemical Behavior of Precursor As stated above, since strontium hydroxide is partially soluble in water, an appropriate molar ratio of strontium nitrate to ferric nitrate is important for the synthesis of strontium ferrite. The appropriate molar ratio of Sr/Fe may vary with the production method used. In this study, from the phase analysis of the calcined particles by XRD, which will be shown later, it was found that pure SrFe12 O19 could be obtained when the molar ratio of Sr/Fe was 1/8. The typical transmission electron micrograph and the size distribution for the resultant precursor particles are shown in Fig. 1. The particles essentially are monodispersed with an average diameter of 3.8 nm and a standard deviation of 0.7 nm. This reveals that the surfactant monolayer indeed provides a barrier restricting the growth of the hydroxide particles and hinders coagulation of particles. The typical TGA and DTA curves for the dried precursor obtained when the molar ratio of Sr/Fe was 1/8 are indicated in Fig. 2. The TGA curve showed a continuous weight loss from room temperature to about 700◦ C. The slight weight loss below 100◦ C was due to the loss of residual solvent and water in the precursor. The weight loss over the temperature from 100 to about 700◦ C was associated with the burning of the residual surfactant and the decomposition of the mixed hydroxides into their oxides. Theoretically, the decomposition of the dried hydroxides (Sr(OH)2 and Fe(OH)3 ) into their oxides (SrO and Fe2 O3 ) at a molar ratio of Sr/Fe = 1/12 by calcination in air would lead to a weight loss of 24.4%. This is in agreement with the TGA curve. Furthermore, the TGA curve revealed that the decomposition of hydroxides proceeded up to about 700◦ C. Thus, the mixture calcined below 700◦ C might contain oxides and hydroxides. Also, only strontium ferrite was present at above 700◦ C because no significant weight change was observed. This could be confirmed by the XRD analysis described later. The DTA curve indicated an endothermic peak at 98◦ C and two exothermic peaks at 237 and 686◦ C. The endothermic peak was due to the vaporization of residual solvent and water. The first exothermic peak was
SYNTHESIS OF STRONTIUM FERRITE ULTRAFINE PARTICLES
43
FIG. 1. Transmission electron micrograph and size distribution of precursor particles.
related to the burning of residual surfactant. This can be confirmed by the phenomenon that the DTA curve exhibited only one broad endothermic peak around 98◦ C and one exothermic peak around 700◦ C for the precursor obtained from the coprecipitation of Sr2+ and Fe3+ in an aqueous solution. Because no
significant weight loss occurred within the temperature range, the second exothermic peak should be due to a phase transition, i.e. the crystallization process of strontium ferrite, instead of chemical change. Phase Analyses and Size of Calcined Particles
FIG. 2. Thermogravimetric and differential thermal analyses of precursor particles.
To further confirm the above result, the precursor was calcined at a heating rate of 2◦ C/min to 600 or 700◦ C for 5 h and then characterized by XRD. The XRD patterns for the calcined particles are shown in Fig. 3. It was found that most of the particles calcined at 600◦ C for 5 h remained amorphous except that a small amount of α-Fe2 O3 was formed. However, the XRD pattern for the particles calcined at 700◦ C for 5 h showed all the characteristic peaks for strontium ferrite, marked by their indices. No other phases were detected. This reveals that the precursor can be converted completely into pure SrFe12 O19 after being calcined at 700◦ C for 5 h. This result was consistent with that from thermal analyses. Calcining the precursor obtained at a molar ratio of Sr/Fe = 1/11 under different temperatures (600–950◦ C) and times (5– 12 h), the XRD patterns of the calcined particles obtained are shown in Fig. 4. It was obviously seen that the phase of α-Fe2 O3 could not removed completely, even calcined at a higher temperature or for a longer time, indicating the amount of strontium nitrate was not sufficient at a molar ratio of Sr/Fe = 1/11. This indicates that an appropriate molar ratio of Sr/Fe is indeed quite important. Other possible molar ratios of Sr/Fe between 1/11
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CHEN AND CHEN
FIG. 3. X-ray diffraction pattern of calcined particles. (Sr/Fe = 1/8; M : SrFe12 O19 , F : α-Fe2 O3 ).
FIG. 5. Magnetization versus magnetic field for strontium ferrite ultrafine particles at 25◦ C.
Magnetic Properties of Strontium Ferrite Ultrafine Particles and 1/8 were not tested in this study. However, it can be recognized that a molar ratio of Sr/Fe = 1/8 is sufficient for the synthesis of pure strontium ferrite. According to the transmission electron micrograph of the strontium ferrite calcined at 700◦ C for 5 h, it was observed that these particles were slightly agglomerated due to the removal of protective agent. From the discrete particles, their diameters could be roughly estimated to be 50–100 nm. The larger average diameter than that of precursor particles implies that the growth of the particles has taken place during calcination.
The magnetization versus magnetic field plots (M–H loop) at 25◦ C for the resultant strontium ferrite is shown in Fig. 5 and the magnetic parameters determined from the loop are indicated in Table 2. The saturation magnetization (Ms = 55.0 emu/g) was lower than the theoretical one (67.7 emu/g), but comparable to those observed in other preparation methods (50–60 emu/g) (39). The reduction in M s might be due to the decrease in particle size and the accompanied increase in surface area. It is known that the energy of a magnetic particle in an external field is proportional to its size via the number of magnetic molecules in a single magnetic domain. When this energy becomes comparable to thermal energy, thermal fluctuations will significantly reduce the total magnetic moment at a given field (40). It is also known that a spin-canting phenomenon can be induced near the surface by the thermal gradient between surface and bulk (41, 42). This phenomenon is more significant for the ultrafine particles due to their large surface-to-volume ratio. Therefore, it is reasonable that the magnetization of ultrafine particles is usually smaller than that of the corresponding bulk materials. Another possible reason for the diminution in Ms might be the incomplete crystallization of SrFe12 O19 particles, which led to amorphous impurities undetectable by XRD. In Fig. 5, it is noticed that the remanent magnetization (28 emu/g) remained at 51% of saturation magnetization but the coercivity (492 Oe) was unexpectedly low compared with those reported earlier and the theoretical limit (7500 Oe) (38, 39, 43).
TABLE 2 Magnetic Properties of Strontium Ferrite Ultrafine Particles
FIG. 4. X-ray diffraction pattern of calcined particles. (Sr/Fe = 1/11; M : SrFe12 O19 , F : α-Fe2 O3 ).
Saturation magnetization, Ms Remanent magnetization, Mr Coercivity, Hc Squareness ratio, Sr (Mr /Ms )
55.0 emu/g 28.0 emu/g 492 Oe 0.51
SYNTHESIS OF STRONTIUM FERRITE ULTRAFINE PARTICLES
45
and also might depend on the morphology and microstructure of the final product. The appropriately low coercivity implies that the strontium ferrite produced in this study has a potential application in the high-density perpendicular recording media. ACKNOWLEDGMENT This work was performed under the auspices of the National Science Council of the Republic of China, under Contract NSC 88-2214-E006-017, to which the authors express their thanks.
REFERENCES
FIG. 6. Temperature dependence of the magnetization for strontium ferrite ultrafine particles.
Since the saturation magnetization reached 81% of the theoretical one, such a low coercivity should not be the result of only a small amount of amorphous impurities undetectable by XRD. It might be mainly due to the fact that the particles were so small that they were close to the superparamagnetic state. Since the magnetic properties were dependent on the morphology of particles, the variation in the shape anisotropy might be another reason for the reduction in Hc as observed from TEM. In addition, it is noted that the two sides of the loop become almost vertical within −28 to 28 emu/g, which is typical behavior for the uniaxial materials. This implies that the reduction in Hc also depended on the microstructure of the final product. The temperature dependence of the magnetization for the resultant strontium ferrite ultrafine particles at an applied field of 50 kOe is shown in Fig. 6. In the temperature range of 5 to 400 K, the magnetization increased with the decrease of temperature. This is typical behavior for ferrimagnetic materials and can be reasonably considered as a result of the decrease in thermal energy.
1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15. 16. 17. 18. 19. 20. 21. 22.
SUMMARY
The strontium ferrite ultrafine particles have been successfully synthesized using w/o microemulsions as the reaction medium to produce ultrafine precursor particles. From the measurements by TEM, the diameters of the precursor and calcined particles were 3.8 ± 0.7 nm and 50–100 nm, respectively. The investigation of the thermochemical properties showed that the precursor could yield strontium ferrite at a calcination temperature above 700◦ C. Phase analysis of the final product by XRD confirmed the formation of pure strontium ferrite and revealed that the appropriate molar ratio of Sr/Fe in microemulsions was about 1/8. The microemulsion-derived strontium ferrite ultrafine particles showed a saturation magnetization of 55 emu/g, a remanent magnetization of 28 emu/g, a coercivity of 492 Oe, and a squareness ratio of 0.51. They all reflected the nature of ultrafine particles
23. 24. 25. 26. 27. 28. 29. 30. 31. 32. 33.
Hayashi, C., Phys. Today. 40, 44 (1987). Gleiter, H., Prog. Mater. Sci. 33, 223 (1989). Fendler, J. H., Chem. Rev. 87, 877 (1987). Luisi, P. L., and Magid, L. J., CRC Crit. Rev. Biochem. 20, 409 (1986). Pileni, M. P., “Structure and Reactivity in Reverse Micelles.” Elsevier, Amsterdam, 1989. Paul, B. K., and Moulik, S. P., J. Dispersion Sci. Technol. 18, 301 (1997). Osseo-Asare, K., and Arriagada, F. J., Ceram. Trans. 12, 3 (1990). Pileni, M. P., J. Phys. Chem. 97, 6961 (1993). Pillai, V., Kumar, P., Hou, M. J., Ayyub, P., and Shah, D. O., Adv. Colloid Interface Sci. 55, 241 (1995). Boutonnet, M., Kizling, J., Stenius, P., and Maire, G., Colloids Surf. 5, 209 (1982). Kurihara, K., Kizling, J., Stenius, P., and Fendler, J. H., J. Am. Chem. Soc. 105, 2574 (1983). Petit, C., Lixon, P., and Pileni, M.-P., J. Phys. Chem. 97, 12974 (1993). Qi, L., Ma, J., and Shen, J., J. Colloid Interface Sci. 186, 498 (1997). Chen, D. H., Yeh, J. J., and Huang, T. C., J. Colloid Interface Sci. 215, 159 (1999). Lianos, P., and Thomas, J. K., J. Colloid Interface Sci. 117, 505 (1987). Kortan, A. R., Hull, R., Opila, R. L., Bawendi, M. G., Steigerwald, M. L., Carroll, P. J., and Brus, L. E., J. Am. Chem. Soc. 112, 1327 (1990). Ward, A. J. I., O’Sullivan, E. C., Rang, J.-C., Nedeljkovic, J., and Patel, R. C., J. Colloid Interface Sci. 161, 316 (1993). Hirai, T., Shiojiri, S., and Komasawa, I., J. Chem. Eng. Jpn. 27, 590 (1994). Haram, S. K., Mahadeshwar, A. R., and Dixit, S. G., J. Phys. Chem. 100, 5868 (1996). Nagy, J., Colloids Surf. 35, 201 (1989). Kandori, K., Kon-No, K., and Kitahara, A., J. Colloid Interface Sci. 122, 78 (1988). Dvolaitzky, M., Ober, R., Taupin, C., Anthore, R., Auvray, X., Petipas, C., and Williams, C., J. Dispersion Sci. Technol. 4, 29 (1983). Osseo-Asare, K., and Arriagada, F. J., Colloids Surf. 50, 321 (1990). Joselevich, E., and Willner, I., J. Phys. Chem. 98, 7628 (1994). Chhabra, V., Lal, M., Maitra, A. N., and Ayyub, P., Colloid Polym. Sci. 273, 939 (1995). Chang, C.-L., and Fogler, H. S., AIChE J. 42, 3153 (1996). Liz, L., L´opez-Quintela, M. A., Mira, J., and Rivas, J., J. Mater. Sci. 29, 3797 (1994). Singhal, M., Chhabra, V., Kang, P., and Shah, D. O., Mater. Res. Bull. 32, 239 (1977). Antonietti, M., Basten, R., and Lohmann, S., Macromol. Chem. Phys. 196, 441 (1995). Ayyub, P., Maitra, A. N., and Shah, D. O., Physica C 168, 571 (1990). Pillai, V., Kumar, P., Multani, M. S., and Shah, D. O., Colloids Surf. A 80, 69 (1993). Vaqueiro, P., L´opez-Quintela, M. A., and Rivas, J., J. Mater. Chem. 7, 501 (1997). Wang, J., Chong, P. F., Ng, S. C., and Gan, L. M., Mater. Lett. 30, 217 (1997).
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34. Gan, L. M., Zhang, L. H., Chan, H. S. O., Chew, C. H., and Loo, B. H., J. Mater. Sci. 31, 1071 (1996). 35. Chen, M. S., Shen, Z. X., Liu, X. Y., and Wang, J., J. Mater. Res. 15, 483 (2000). 36. Rawlinson, D. A., and Sermon, P. A., J. Phys. IV 7(C1), 755 (1997). 37. Chhabra, V., Lal, M., Maitra, A. N., and Ayyub, P., J. Mater. Res. 10, 2689 (1995). 38. Fang, J., Wang, J., Gan, L.-M., Ng, S.-C., Ding, J., and Liu, X., J. Am. Ceram. Soc. 83, 1049 (2000).
39. Choy, J. H., Han, Y. S., and Song, S. W., Mater. Lett. 19, 257 (1994). 40. Shafi, K. V. P., Gedanken, A., Prozorov, R., and Balogh, J., Chem. Mater. 10, 3445 (1998). 41. Kurisu, S., Ido, T., and Yokoyama, H., IEEE Trans. Mag. 23, 3137 (1987). 42. Batlle, X., Obradors, X., Medarde, M., Carvajal, J. R., Pernet, M., and Regi, M. V., J. Magn. Magn. Mater. 124, 228 (1993). 43. Pankov, V. V., Pernet, M., Germiand, P., and Mollard, P., J. Magn. Magn. Mater. 120, 69 (1993).
Synthesis of Strontium Ferrite Ultrafine Particles Using Microemulsion Processing Dong-Hwang Chen1 and Yuh-Yuh Chen Department of Chemical Engineering, National Cheng Kung University, Tainan, Taiwan 701, Republic of China Received February 1, 2000; accepted December 18, 2000
formation of ultrafine particles. They not only act as microreactors for processing reactions but also inhibit the excess aggregation of particles because the surfactants could adsorb on the particle surface when the particle size approaches that of the water pool. As a result, the particles obtained in such a medium are generally very fine and monodispersed (3, 7–9). Since the platinum-group metal ultrafine particles were successfully synthesized in w/o microemulsions by Boutonnet et al. (10), the microemulsion technique has been widely used to prepare ultrafine particles of numerous materials, including metals (10–14), metal sulfides and selenides (15–19), metal borides (20), metal carbonates (21), metal halides (22), metal oxides and hydroxides (23–28), and organic polymers (29). Recently, some mixed-metal oxides have also been synthesized by microemulsion processing, including YBa2 Cu3 O7−x , Y3 Fe5 O12 , LaNiO3 , La2 CuO4 , BaPbO3 , Mn0.45 Zn0.55 Fe2 O4 , and particularly BaFe12 O19 (30–37). Strontium ferrite (SrFe12 O19 ) possesses high coercivity due to its relatively high magnetocrystalline anisotropy factor and this property makes it an attractive material for use in permanent magnet applications. However, its production in w/o microemuslions has not been reported until now. The possible reason is the high solubility of strontium hydroxide in water, which results in the difficulty in stoichiometric control. Recently, Fang et al. prepared fine strontium ferrite powders in ethanol-in-oil microemuslions (38). The difficulty in stoichiometric control was reduced, but the calcination temperature for the formation of pure SrFe12 O19 must be above 900◦ C. Also, although they claimed the particle size was 50–80 nm based on the surface area analysis, the SEM photographs indicated the product was sintered seriously. The size of the aggregated particles was 0.1–2 µm. In this study, the synthesis of strontium ferrite in a water-in-oil microemulsion of water/CTAB/ n-butanol/isooctane was investigated. The magnetic property of the final product, which was quite from that observed by Fang et al. (38), was also described.
The strontium ferrite ultrafine particles have been prepared using the microemulsion processing. The mixed hydroxide precursor was obtained via the coprecipitation of Sr2+ and Fe3+ in a water-in-oil microemulsion of water/CTAB/n-butanol/isooctane. According to the investigation on the thermochemical properties by TGA/DTA and the phase analysis by XRD, it was shown that the precursor could yield pure strontium ferrite after calcination at 700◦ C for 5 h while using an appropriate molar ratio of Sr/Fe in microemulsions. From TEM measurement, the diameters of the precursor and calcined particles were 3.8 ± 0.7 and 50–100 nm, respectively. The magnetic properties characterized by a SQUID magnetometer showed that the saturation magnetization, remanent magnetization, coercivity, and squareness ratio were 55 emu/g, 28 emu/g, 492 Oe, and 0.51, respectively. The magnetization was also observed to increase with the decrease of temperature at 5–400 K. Compared with those reported earlier, the quite low coercivity implies the potential application of final product in the high-density perpendicular recording media. °C 2001 Academic Press Key Words: strontium ferrite; ultrafine particles; microemulsion.
INTRODUCTION
Recently, the ultrafine particles in the size range of 1–100 nm have received increasing attention because they exhibit unusual physical and chemical properties different from those of larger particles of the same materials due to their extremely small size and large specific surface area (1–3). As have been known, some magnetic properties such as saturation magnetization, remanent magnetization, and coercitivity depend strongly on the particle size and microstructure of the materials. Therefore, it is interesting and important to develop techniques by which the size, shape, and chemical homogeneity of particles can be well controlled. Water-in-oil (w/o) microemulsion solutions are transparent, isotropic liquid media with nanosized water droplets that are dispersed in a continuous oil phase and stabilized by surfactant molecules at the water/oil interface (4–6). These surfactantcovered water pools offer a unique microenvironment for the
MATERIALS AND METHODS
Materials The cationic surfactant cetyltrimethylamonium bromide (CTAB) and iron(III) nitrate nonhydrate were obtained from
1 To whom correspondence should be addressed. Fax: 886-6-2344496. E-mail: [email protected].
41
0021-9797/01 $35.00
C 2001 by Academic Press Copyright ° All rights of reproduction in any form reserved.
42
CHEN AND CHEN
TABLE 1 Compositions of the Microemulsion System Used for the Synthesis of Precursor
Aqueous phase Surfactant Cosurfactant Oil phase
Microemulsion I
Microemulsion II
Wt%
0.01 M Sr(NO3 )2 + 0.08–0.11 M Fe(NO3 )3 CTAB n-Butanol Isooctane
1.0 M NaOH
22
CTAB n-Butanol Isooctane
21 15 42
Acros Organics (Belgium). Strontium nitrate was the product of Janssen (Beerse). Isooctane, n-butanol, chloroform, methanol, and ethanol were the HPLC-grade reagents supplied by TEDIA (Fairfield). All reagents were used without further purification. The water used throughout this work was the reagent-grade water produced by Milli-Q SP Ultra-Pure-Water Purification System of Nihon Millipore Ltd., Tokyo. Synthesis of Strontium Ferrite Ultrafine Particles The microemulsion system used in this study consisted of CTAB as the surfactant, n-butanol as the cosurfactant, isooctane as the continuous oil phase, and an aqueous solution as the dispersed phase. The composition was indicated in Table 1. Two microemulsions (I and II) with identical compositions but different aqueous phases were prepared. The aqueous phase in microemulsion I was a mixture of strontium nitrate and ferric nitrate solution in the molar ratio of 1/8–1/11. Although a molar ratio of 1/12 should be sufficient according to stoichiometry, an excess of strontium nitrate was necessary because strontium hydroxide is partially soluble in water (39). The appropriate molar ratio will be discussed in the next section. The aqueous phase in microemulsion II was the precipitation agent, NaOH, with a sufficient concentration for the precipitation of Sr2+ and Fe3+ ions. When these two microemulsions were mixed, the mixed hydroxide precipitate appeared within the nanosized aqueous droplets of the microemulsion. After being centrifuged at 15,000 rpm for 10 min, the mixed hydroxide precipitate was washed several times with a mixture of chloroform and methanol (1 : 1) and then vacuum-dried at 70◦ C. According to elemental analysis on carbon, the residual surfactant (CTAB) was below 1%. Thermogravimetric analysis (TGA) and differential thermal analysis (DTA) were done on the dried precursor in air with a heating rate of 10◦ C/min on the Shimadzu TA-50WSI TGA and DTA instruments, respectively. Based on the results of these analyses, which will be discussed in the next section, the dried precursor was calcined in air under appropriate conditions (typically, at 700◦ C for 5 h) to ensure complete conversion of the mixed hydroxide to strontium ferrite. Characterization The sizes of the precursor particles and the calcined particles were determined by TEM using a Hitachi Model HF-2000
field emission transmission electron microscope with a resolution of 0.1 nm. For the precursor particles, the microemulsion solution was sonicated for above 1 min prior to depositing it onto a Formvar-covered copper grid. Over more than 200 particles from different parts of the grid were used to estimate the average diameter and size distribution of the precursor particles. For the calcined particles, the sample was ultrasonically dispersed in ethanol for several minutes prior to depositing it onto a Formvar-covered copper grid. For the irregularly shaped particles, the diameters refer to the longest dimensions. The phase analysis of the calcined particles was done by powder XRD on a Rigaku D/max III.V X-ray diffractometer using CuK α radiation (λ = 0.1542 nm). The magnetic measurements for the calcined particles were carried out using a SQUID magnetometer (MPMS7, Quantum Design). RESULTS AND DISCUSSION
Size and Thermochemical Behavior of Precursor As stated above, since strontium hydroxide is partially soluble in water, an appropriate molar ratio of strontium nitrate to ferric nitrate is important for the synthesis of strontium ferrite. The appropriate molar ratio of Sr/Fe may vary with the production method used. In this study, from the phase analysis of the calcined particles by XRD, which will be shown later, it was found that pure SrFe12 O19 could be obtained when the molar ratio of Sr/Fe was 1/8. The typical transmission electron micrograph and the size distribution for the resultant precursor particles are shown in Fig. 1. The particles essentially are monodispersed with an average diameter of 3.8 nm and a standard deviation of 0.7 nm. This reveals that the surfactant monolayer indeed provides a barrier restricting the growth of the hydroxide particles and hinders coagulation of particles. The typical TGA and DTA curves for the dried precursor obtained when the molar ratio of Sr/Fe was 1/8 are indicated in Fig. 2. The TGA curve showed a continuous weight loss from room temperature to about 700◦ C. The slight weight loss below 100◦ C was due to the loss of residual solvent and water in the precursor. The weight loss over the temperature from 100 to about 700◦ C was associated with the burning of the residual surfactant and the decomposition of the mixed hydroxides into their oxides. Theoretically, the decomposition of the dried hydroxides (Sr(OH)2 and Fe(OH)3 ) into their oxides (SrO and Fe2 O3 ) at a molar ratio of Sr/Fe = 1/12 by calcination in air would lead to a weight loss of 24.4%. This is in agreement with the TGA curve. Furthermore, the TGA curve revealed that the decomposition of hydroxides proceeded up to about 700◦ C. Thus, the mixture calcined below 700◦ C might contain oxides and hydroxides. Also, only strontium ferrite was present at above 700◦ C because no significant weight change was observed. This could be confirmed by the XRD analysis described later. The DTA curve indicated an endothermic peak at 98◦ C and two exothermic peaks at 237 and 686◦ C. The endothermic peak was due to the vaporization of residual solvent and water. The first exothermic peak was
SYNTHESIS OF STRONTIUM FERRITE ULTRAFINE PARTICLES
43
FIG. 1. Transmission electron micrograph and size distribution of precursor particles.
related to the burning of residual surfactant. This can be confirmed by the phenomenon that the DTA curve exhibited only one broad endothermic peak around 98◦ C and one exothermic peak around 700◦ C for the precursor obtained from the coprecipitation of Sr2+ and Fe3+ in an aqueous solution. Because no
significant weight loss occurred within the temperature range, the second exothermic peak should be due to a phase transition, i.e. the crystallization process of strontium ferrite, instead of chemical change. Phase Analyses and Size of Calcined Particles
FIG. 2. Thermogravimetric and differential thermal analyses of precursor particles.
To further confirm the above result, the precursor was calcined at a heating rate of 2◦ C/min to 600 or 700◦ C for 5 h and then characterized by XRD. The XRD patterns for the calcined particles are shown in Fig. 3. It was found that most of the particles calcined at 600◦ C for 5 h remained amorphous except that a small amount of α-Fe2 O3 was formed. However, the XRD pattern for the particles calcined at 700◦ C for 5 h showed all the characteristic peaks for strontium ferrite, marked by their indices. No other phases were detected. This reveals that the precursor can be converted completely into pure SrFe12 O19 after being calcined at 700◦ C for 5 h. This result was consistent with that from thermal analyses. Calcining the precursor obtained at a molar ratio of Sr/Fe = 1/11 under different temperatures (600–950◦ C) and times (5– 12 h), the XRD patterns of the calcined particles obtained are shown in Fig. 4. It was obviously seen that the phase of α-Fe2 O3 could not removed completely, even calcined at a higher temperature or for a longer time, indicating the amount of strontium nitrate was not sufficient at a molar ratio of Sr/Fe = 1/11. This indicates that an appropriate molar ratio of Sr/Fe is indeed quite important. Other possible molar ratios of Sr/Fe between 1/11
44
CHEN AND CHEN
FIG. 3. X-ray diffraction pattern of calcined particles. (Sr/Fe = 1/8; M : SrFe12 O19 , F : α-Fe2 O3 ).
FIG. 5. Magnetization versus magnetic field for strontium ferrite ultrafine particles at 25◦ C.
Magnetic Properties of Strontium Ferrite Ultrafine Particles and 1/8 were not tested in this study. However, it can be recognized that a molar ratio of Sr/Fe = 1/8 is sufficient for the synthesis of pure strontium ferrite. According to the transmission electron micrograph of the strontium ferrite calcined at 700◦ C for 5 h, it was observed that these particles were slightly agglomerated due to the removal of protective agent. From the discrete particles, their diameters could be roughly estimated to be 50–100 nm. The larger average diameter than that of precursor particles implies that the growth of the particles has taken place during calcination.
The magnetization versus magnetic field plots (M–H loop) at 25◦ C for the resultant strontium ferrite is shown in Fig. 5 and the magnetic parameters determined from the loop are indicated in Table 2. The saturation magnetization (Ms = 55.0 emu/g) was lower than the theoretical one (67.7 emu/g), but comparable to those observed in other preparation methods (50–60 emu/g) (39). The reduction in M s might be due to the decrease in particle size and the accompanied increase in surface area. It is known that the energy of a magnetic particle in an external field is proportional to its size via the number of magnetic molecules in a single magnetic domain. When this energy becomes comparable to thermal energy, thermal fluctuations will significantly reduce the total magnetic moment at a given field (40). It is also known that a spin-canting phenomenon can be induced near the surface by the thermal gradient between surface and bulk (41, 42). This phenomenon is more significant for the ultrafine particles due to their large surface-to-volume ratio. Therefore, it is reasonable that the magnetization of ultrafine particles is usually smaller than that of the corresponding bulk materials. Another possible reason for the diminution in Ms might be the incomplete crystallization of SrFe12 O19 particles, which led to amorphous impurities undetectable by XRD. In Fig. 5, it is noticed that the remanent magnetization (28 emu/g) remained at 51% of saturation magnetization but the coercivity (492 Oe) was unexpectedly low compared with those reported earlier and the theoretical limit (7500 Oe) (38, 39, 43).
TABLE 2 Magnetic Properties of Strontium Ferrite Ultrafine Particles
FIG. 4. X-ray diffraction pattern of calcined particles. (Sr/Fe = 1/11; M : SrFe12 O19 , F : α-Fe2 O3 ).
Saturation magnetization, Ms Remanent magnetization, Mr Coercivity, Hc Squareness ratio, Sr (Mr /Ms )
55.0 emu/g 28.0 emu/g 492 Oe 0.51
SYNTHESIS OF STRONTIUM FERRITE ULTRAFINE PARTICLES
45
and also might depend on the morphology and microstructure of the final product. The appropriately low coercivity implies that the strontium ferrite produced in this study has a potential application in the high-density perpendicular recording media. ACKNOWLEDGMENT This work was performed under the auspices of the National Science Council of the Republic of China, under Contract NSC 88-2214-E006-017, to which the authors express their thanks.
REFERENCES
FIG. 6. Temperature dependence of the magnetization for strontium ferrite ultrafine particles.
Since the saturation magnetization reached 81% of the theoretical one, such a low coercivity should not be the result of only a small amount of amorphous impurities undetectable by XRD. It might be mainly due to the fact that the particles were so small that they were close to the superparamagnetic state. Since the magnetic properties were dependent on the morphology of particles, the variation in the shape anisotropy might be another reason for the reduction in Hc as observed from TEM. In addition, it is noted that the two sides of the loop become almost vertical within −28 to 28 emu/g, which is typical behavior for the uniaxial materials. This implies that the reduction in Hc also depended on the microstructure of the final product. The temperature dependence of the magnetization for the resultant strontium ferrite ultrafine particles at an applied field of 50 kOe is shown in Fig. 6. In the temperature range of 5 to 400 K, the magnetization increased with the decrease of temperature. This is typical behavior for ferrimagnetic materials and can be reasonably considered as a result of the decrease in thermal energy.
1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15. 16. 17. 18. 19. 20. 21. 22.
SUMMARY
The strontium ferrite ultrafine particles have been successfully synthesized using w/o microemulsions as the reaction medium to produce ultrafine precursor particles. From the measurements by TEM, the diameters of the precursor and calcined particles were 3.8 ± 0.7 nm and 50–100 nm, respectively. The investigation of the thermochemical properties showed that the precursor could yield strontium ferrite at a calcination temperature above 700◦ C. Phase analysis of the final product by XRD confirmed the formation of pure strontium ferrite and revealed that the appropriate molar ratio of Sr/Fe in microemulsions was about 1/8. The microemulsion-derived strontium ferrite ultrafine particles showed a saturation magnetization of 55 emu/g, a remanent magnetization of 28 emu/g, a coercivity of 492 Oe, and a squareness ratio of 0.51. They all reflected the nature of ultrafine particles
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