Shen Qing
(School of Material Science & Engineering, Shaanxi University of Science & Technology, Xi'an 710021, China)

Received Feb.22, 2007.

Abstract Ultrafine substituted M-type barium ferrite BaTiCoFe₁₀O₁₉ powders were synthesized successfully by sol-gel method. The hydroxide precursor particles were formed in gel solution containing ethanol and water at a ratio of 1:1 and NH₄OH as precipitation agent. The effects of molten salt Na₂CO₃ on formation of BaTiCoFe₁₀O₁₉ nano-powders were studied. The precursors were calcined at 900^oC for 2h. XRD analysis indicated single phase substituted M-type barium ferrites were formed with and without molten salt. Molten salt Na₂CO₃ at 5-15M% resulted in obviously decrease of particle size of powders, which decreased with increasing amount of molten salt. The particle sizes of 32.2-48.5nm closing to the theoretical single-domain size and good crystallinity of particles ensure perfect microwave absorbing properties for the substituted M-type barium ferrites. SEM analysis indicated that powders were platelike particles with smaller aspect ratio in morphology which were most suitable for application of microwave absorption materials.
Keyword sol-gel method, preheat treatment, molten salt，BaTiCoFe₁₀O₁₉, powder

1. INTRODUCTION
Substituted M-typed barium hexaferrite Ba(TiCo)_xFe_10-2xO₁₉ exhibits a high saturation magnetization and a high coercivity because of the relatively high magnetocrystalline anisotropy field^[1] and it has been explored for many technological demanding and challenging application, such as for high-density type perpendicular recording media and microwave materials^[2-4]. The properties of Ti-Co-substituted barium ferrite are largely dependent on the processing routes used for fabrication, multidomain ferrite, increase in magnetic and absorbing properties as particle size decrease and crystallinity increase. Therefore, the preparation of a barium ferrite powder with refined particle size, narrow particle-size distribution, minimal particle agglomeration, and high crystallinity have been received considerable attentions. Many processing routes have been devised, including hydrothermal process^[5] microemulsion technique^[6] et al.
    Sol-gel method have the advantages of low cost and simple technological process and is an useful method to prepare ultrafine Co-Ti-substituted Ba-ferrite powders with single-domain perfect crystallography, narrow size distribution and excellent magnetic properties and perfect absorbing ability. Many technological factors influence the character of powders prepared with these processing routes. In which, molten-salt method was effective for decreasing the particle size and agglomeration of powders, preheat treatment have been verified to be effective in decreasing particle size of powders. The objectives of the present works are to study the effects of molten-salt and preheat treatment on the formation and the character of the Co-Ti-substituted M-type Ba-ferrite powders.

2. EXPERIMENTAL   PROCEDURE
2.1 Preparation of hydroxide Precursor
The ferric chloride hexahydrate and barium chloride dihydrate and cobalt chlorite hexahydrate were dissolved in the solution containing ethanol and water at a ratio of 1:1. Ttitanium propoxide was dissolved in anhydrous ethanol and stabilized with a little acetylacetone. Then two solutions were mixed and stirred for 0.5h. In the mixed solution, the molar concentration were 0.005M for Ba²⁺, Co²⁺, and Ti²⁺ cations, respectively, and was 0.05M for Fe³⁺ cation. Then, ammonia water was dropwisely slowly added into the resultant solutions at room temperature and constant stirring condition until pH>9. After the precipitation was completed, the precipitate slurry was filtrated, washed with anhydrous ethanol until its pH≈7. As-washed precursor was divided in to four parts. Anhydrous sodium carbonate at the content of 0M%, 5M%, 10M%, and 15M% were added respectively with stirring and were dried at 95^oC for 2-5h and put in chamber respectively.
2.2  Heat Treatment and powder characterization
Three portions of as-dried precursor containing 0N% Na₂CO₃ was preheated at 250^oC, 300^oC and 350 ^oC for 0.5--1h respectively. All the precursors were heated at a heating rate of 20^oC/min and calcined for 2h at 900^oC in air, respectively. The cooling was performed at a slow rate in furnace. The calcined powders were soaked in water at 80^oC for 4h and rinsed repeatedly to wash off NaCl and then dried at 95^oC for 1h. The phase identification of the calcined Ti-Co-substituted Ba- ferrite powders were conducted at room temperature using X-Ray diffractometer (XRD, CuK_α1, l=0.15406nm, Model No. D/Max-2200PC, Rigaku, Japan). The phase and particle sizes of powders were determined with the Jade5 analysis software carried with X-Ray diffractometer. The particles morphology and particles agglomeration of the powder were analyzed using scanning electron microscopy (SEM, Model No: JXM-6700F, Japan).

3. RESULTS AND DISCUSSION

Figure 1 XRD patterns of BaTiCoFe₁₀O₁₉ powders with different amouts of molten salt added

To investigate the effects of molten salt on the particle size and agglomeration of powders, molten salt NaCO₃ at the contents of 0N%, 5N%, 10N%, and 15N% were added to hydroxides, respectively. As-added precursors were dried and calcined at 900^oC for 2h. The XRD patterns of the powders were illustrated in Figure 1, which indicated that BaTiCoFe₁₀O₁₉ is the only detectable phase. Particle sizes determined with strong peak(114) at 2q≈34.12^o were 48.5nm, 37.6nm, 34.3nm, and 32.2nm for NaCO₃ content of 0M%, 5M%, 10M%, and 15M% respectively, as shown in Figure 2, which indicate that molten salt NaCO₃ can remarkably decrease the particle size of BaTiCoFe₁₀O₁₉ powders and particle size decreased as increasing molten salt content.

Figure 2 The relation of particle size of BaTiCoFe₁₀O₁₉ powders with amount of molten salt

    The SEM photographs were shown in Figure 3 and indicated that particles of powder were hexagonal platelets with 20-40nm of thickness and 20-300nm of width. The average particle size determined with average value of width and thickness was in a range of 20-130nm that decreased with the increase in NaCO₃ content. The agglomerations of powders were less. These indicated that molten salt method is effective in reducing the agglomeration and particle size of powders.  The effects of molten salt in the decrease of particle size can be contributed to inhibition of NaCO₃ on the growth of BaTiCoFe₁₀O₁₉ crystallite. The effect of  molten salt in decrease of agglomeration can be contributed to the separation effect of washing away NaCO₃ between particles.

    To investigate the effect of preheat treatment on the formation of powders, the hydroxide precursors without adding molten salt were preheated at 250^oC, 300^oC, 350^oC for 50min respectively, followed by calcined at 900^oC for 2h. As-calcined powders were characterized with XRD and SEM at room temperature respectively. The XRD patterns of the powders as shown in Figure 4 indicated that BaTiCoFe₁₀O₁₉ is the only detectable phase and the particle sizes of powders were 37.6nm, 36.5nm and 39.9nm for preheat temperature of 250^oC, 300^oC and 350^oC respectively as shown in Figure 5. Compared with particle size (48.5nm) of powder without preheat treatment, preheat treatment also result in decrease in particle size of BaTiCoFe₁₀O₁₉ powders. The saturation density of nucleation is higher and the size of nucleation is smaller for the lower temperature of nucleation. Thus the preheat treatment results in the formation of larger number and smaller size of  g-Fe₂O₃ crystallite, which reacted with BaO, TiO₂ and CoO at high temperature and induce the formation of larger number and smaller size of BaTiCoFe₁₀O₁₉ particles. However, the particle size at preheat temperature of 250^oC was larger than that of 300^oC, which may be due to stable g-Fe₂O₃ nucleation was not formed at lower temperature of 250^oC. The SEM photograph of BaTiCoFe₁₀O₁₉ powder preheated at 300^oC for 50min as shown in Figure 6 indicated that the particles were hexagonal platelets with 15-40nm of thickness and 15-250nm of width and 15-120nm of average particle size and agglomeration was less.

Figure 4 XRD patterns of BaTiCoFe₁₀O₁₉ powders preheated at different temperatures

Figure 5 Relation of particle size of BaTiCoFe₁₀O₁₉ powders with preheat treatment temperature

Figure 6 SEM photograph of BaTiCoFe₁₀O₁₉ powder preheated at 300^oC

4. CONCLUSION
Ultrafine Ti-Co-substituted Ba-ferrite powders have been successfully synthesized with sol-gel method. Molten salt and preheat treatment can remarkably decreased the particle size of powders. The agglomeration of powders can be decreased by washing the powders after calcining.

Acknowledgements This work was supported by Natural Science Foundation of Shaanxi University of Science and Technology under the grant No:ZX05-18

REFERENCES
[1] Li Yinyuan, Li Guodong. Physics of ferrite, Beijing, Science Press, 1978, 325.
[2] Bursik J, Simis Z, Stichauer L et al. J. Magn. Magn. Mat., 1996, 157-158(2), 311-312.
[3] Willias J M, Adetunji J, Gregori M. J. Magn. Magn. Mat., 2000, 220(2), 124-128.
[4] Kreisel J, Vincent H, Taeest F. J. Magn. magn. Mat., 2001, 224, 17.
[5] D'Arrigo Maria Cristina, Leonelli Cristina, Pellacani Gian Carlo. J. Am. Ceram. Soc., 1998, 81(11), 3041.
[6] Pankov V. J. Mat. Sci. Eng., 1997, A224, 101.

Sol-gel法替代M-型BaTCoFe₁₀O₁₉粉体的制备
沈清
（陕西科技大学材料学院712081）
摘要  应用sol-gel法成功地合成了超细替代M-型BaTiCoFe₁₀O₁₉磁粉。先驱体氢氧化物在醇：水比=1：1的溶胶溶液中以共沉淀法制得。研究了熔盐Na₂CO₃和预热处理对粉体的粒子尺寸和粘连度的影响。X-射线衍射分析(XRD)分析表明添加5-15N%的Na₂CO₃的分别在900^oC烧成2h后得到了纯的BaTiCoFe₁₀O₁₉粉体。粉体的粒子尺寸随熔盐Na₂CO₃的添加量增大而减小。250-350^oC预热处理也使粉体的粒子尺寸显著减小。SEM分析表明粉体为六角片状，粒子尺寸与XRD分析结果一致。
关键词  sol-gel法，工艺，BaTiCoFe₁₀O₁₉粉体，粒子尺寸

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