Low-Temperature Crystallization of Barium Ferrite Nanoparticles by a Sodium Citrate-Aided Synthetic Process

International Journal of Modern Physics B Vol. 22, Nos. 18 & 19 (2008) 3144-3152 © World Scientific Publishing Company

LOW TEMPERATURE CRYSTALLIZATION OF BARIUM FERRITE NANO-PARTICLES VIA CO-PRECIPITATION METHOD USING

DIETHYLENE GLYCOL

M. MONTAZERI-POUR

School of Metallurgy and Materials Engineering, University of Tehran, Tehran, Iran,

montazeri.pour@ gmail. com

A. ATAIE

School of Metallurgy and Materials Engineering, University of Tehran, Tehran, Iran,

[email protected]

Received 20 June 2008

Nano-crystalline particles of barium ferrite have been prepared by co-precipitation route using aqueous and non-aqueous solutions of iron and barium chlorides with a Fe/Ba molar ratio of 11. Water and a mixture of diethylene glycol and water with volume ratio of 3:2 were used as solvents in the process. Co-precipitated powders were annealed at various temperatures for 1 h. Phase composition of the samples was evaluated by XRD while their morphology was studied by TEM and SEM techniques. The XRD results showed that the single phase barium ferrite obtained at 750°C when diethylene glycol/water mixture was used as a solvent. This temperature increased to 900°C when the starting materials dissolved in water. Nano-size particles of barium ferrite with mean particle size of almost 50 and 80 nm were observed in the SEM micrographs of the samples synthesized in diethylene glycol/water solution after annealing at 750°C and 800°C for 1 h, respectively. The corresponding mean crystallite size measured by TEM for sample annealed at 800°C was 40 nm.

Keywords: Permanent magnets; nano-particles; co-precipitation; X-ray diffraction.

1. Introduction

Barium ferrite magnetic material (BaFei20i9) has significant potential for applications such as permanent magnets and microwave absorbing coatings, because of the adequate combination of low cost, high Curie temperature (450°C), high coercivity, chemical stability and resistance to corrosion. Fine BaFe^O^ (BaF) particles also exhibit suitable properties for perpendicular magnetic recording media.1

The fabrication of bulk BaF requires powders with outstanding characteristics in terms of particle size, monodispersity, morphology, composition uniformity, purity and magnetic properties.2 Finer starting powders exhibit a superior sintering behavior,

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Low Temperature Crystallization of Barium Ferrite Nano-Particles 3145

resulting in lower sintering temperatures and denser ceramics even without any sintering aids.3 It should be noted that powder particle characteristics are influenced significantly by the synthesis method.

The conventional ceramic method, which involves the annealing of iron oxide and barium carbonate mixtures at around 1200°C induces sintering and aggregation in the particles. Furthermore, the milling process to reduce the particle size yields non-homogeneous mixtures and causes lattice strains in the material.4

Therefore, in order to improve the material properties, several non-conventional routes including co-precipitation,5 hydrothermal,6 self propagating high temperature synthesis7 and glass crystallization8 have also been employed to synthesize barium ferrite. Among these methods, co-precipitation is the most attractive due to simple operation and ease of mass production. However, it often is not easy to synthesize monodispersed nano-particles using this technique because of the aggregation of resultant particles.

The use of a mixed solvent is a new approach in nano-materials synthesis and processing. Diethylene glycol (0(CH2CH2OH)2) is a polar solvent which has reducing ability. It is miscible with water at any ratio, and the addition of diethylene glycol to water can easily change the physicochemical properties.9 Recently, diethylene glycol has been widely used in nano-materials synthesis as solvent and chelating agent by solvothermal method and/or polyol-mediated synthesis.10"11

The objective of the present study is to investigate the influence of using 60 Vol.% diethylene glycol as a co-solvent along with water and comparing it with water alone on the phase constitution, crystallite size and morphology of Ba-ferrite particles processed by co-precipitation method.

2. Experimental

FeCl3.6H20 (Merck, > 99%) and BaCl2.2H20 (Merck, > 99%) with a Fe/Ba molar ratio of 11 were dissolved in water and a mixture of water and diethylene glycol with volume ratio of 2:3. Two prepared solutions were co-precipitated by addition of NaOH with OH7C1" molar ratio of 2 at room temperature.

The co-precipitated samples were washed by distilled water and dried at 70°C for 15 h. The dried powders were then annealed at various temperatures for 1 h in air to obtain barium ferrite magnetic phase. The samples synthesized in diethylene glycol/water and water solutions are named for brevity VVDEG" and VVW" in this manuscript, respectively.

The phase identification of the specimens was performed using X-ray diffraction (XRD) on a Philips PW3170 X-ray diffractometer using Co-Ka radiation. The mean crystallite size of BaFei20i9 powders annealed at different temperatures was calculated from the X-ray peak broadening of the (114) diffraction peak (the most intense XRD peak) using the classical Scherrer equation:12

3146 M. Montazeri-Pour & A. Ataie

Where D is the average crystallite size in nm, X the radiation wavelength (0.17889 nm for Co-Ka), p the corrected half width, and 0 the diffraction peak angle.

CamScan MV2300 scanning electron microscopy was used to characterize the particles morphology.

3. Results and Discussion

Fig. 1 shows the X-ray powder diffraction patterns for the co-precipitated precursors. It appears that the powders are amorphous with some poorly crystallized material. The wide peaks correspond to the barium carbonate.

As is well known, precursors after chemical co-precipitation with OH" ions consist of micron/submicron agglomerates of amorphous hydroxide or oxide-hydroxide nano-particles. S.E. Jacobo et al recognized mixture of crystalline Ba(OH)2.2H20 and amorphous Fe203 in the precursor obtained by chemical co-precipitation.5

Formation of BaC03 is probably due to the loss of water from the amorphous precursors during the drying according to the following reaction:

Ba(OH)2.xH20 + C02 - • BaC03 + (x+l)H20

It means that since the samples are exposed to air for several hours during the drying, BaC03 crystallized from the Ba-precursor and C02 absorbed from the air.

However, the wider and less distinctive of BaC03 peaks related to the DEG precursor than the W precursor show the finer and more amorphous nature of powders obtained from the diethylene glycol solution.

BaCG3

40 SO 60

Degree (26)

Fig. 1. X-ray diffraction patterns for the precursors dried at 70°C (a) W precursor, (b) DEG precursor.

It should be noted that formation of barium ferrite normally involves the decomposition of BaC03, which determines the minimum temperature for M-ferrite

Low Temperature Crystallization of Barium Ferrite Nano-Particles 3147

formation. On the other hand, since the bonding of amorphous nano-particles in precursor is rather weak, it is easier to break these bonds through heating during annealing treatment. Therefore, the DEG precursor is more suitable for low temperature formation of BaF than other.

XRD patterns of the water processed sample (W) after annealing at various temperatures are shown in Fig. 2. At 750°C, BaF phase became the major phase, but some un-reacted intermediate phases like barium monoferrite (BaFe204) and hematite (Fe203) still observed.

The formation mechanism of the BaF solid state reaction has been shown to take place in two main steps:13

(1) the decomposition of BaC03 accompanied with the formation of monoferrite

BaC03 +Fe203 - • BaFe204 +C02

2+; (2) the diffusion of Ba into iron oxide

BaFe204 +5Fe203 - • BaFei2Oi9

Remaining of secondary phase impurities up to temperature 850°C indicates the poor reactivity of the hard agglomerated precursor co-precipitated from the water solution that retarded the diffusion of Ba2+ and following BaF formation during annealing. At 900°C, the second step of BaF formation was accomplished and single phase barium ferrite was obtained.

S3

BaFei2Oi< BaFei04 Fe 2 0 3

I T i fmmmvrmm

luJJL^uJLJNL^ 750°O

800 «C

W L I X ^ W X J ^ 50 «C

JKAJNAX^JU^AJ^JJ^^ 900 "C

20 30 40 50 60 70 80

Degree (28)

Fig. 2. X-ray diffraction patterns for W sample after annealing at various temperatures for 1 h.

3148 M. Montazeri-Pour & A. Ataie

Analysis of the XRD patterns of DEG precursor annealed at 750 and 800°C revealed the formation of single phase barium ferrite (Fig. 3).

BaFciaQis •

75©«C

20 30 40 50 60 70 80 Degree (.29)

Fig. 3. X-ray diffraction patterns for DEG sample after annealing at 750°C and 800°C for lh.

In the case of the annealed W sample, Fe203 phase was observed as an intermediate phase at 750°C, which resulted from the conversion of amorphous iron hydroxide during annealing. It is well known that the phase transformation that occurs during annealing gives rise to transformed Fe203 powder, which has undergone considerable aggregation and coarsening.14 The above characteristic is detrimental to the complete conversion of the reactants into single BaFe^O^ phase. Therefore, the completion of the formation of BaFei20i9 phase for W samples occurred at a higher temperature than DEG sample, which may originate from the coarsened Fe203 existing in the annealed W samples.

Table 1 summarizes the mean crystallite size of the synthesized powders under various synthesis conditions.

Owing to the existence of coarsened Fe203 and BaFe204 in the annealed W sample, the crystallite size of the formed BaF for the W sample was larger than that of DEG sample.

It is found that the crystallite size of single phase BaF obtained for W precursor annealed at 900°C is about two orders of magnitude higher than that of DEG precursor annealed at 750°C.

It is also observed that the difference between crystallite size at temperatures 750 and 800°C for DEG precursor in comparison with W precursor is rather high. This phenomenon could be explained by some poorly crystallized phase present in DEG sample at 750°C (See Fig. 3). It means that this phase can promote the growth of other crystals at high temperature 800°C accelerately.

Low Temperature Crystallization of Barium Ferrite Nano-Particles 3149

Table 1. Mean crystallite size of the synthesized powders under various synthesis conditions.

Sample Temperature (°C) Crystallite size (nm)

750 38

800 40 W

850 46

900 49

750 26 DEG

800 35

The typical transmission electron micrograph for the DEG sample annealed at 800°C is shown in fig. 4. Hexagonal platelet shape of barium ferrite crystallites with mean size of 40 nm, which is approximately consistent with size obtained from XRD pattern using the Scherrer formula, can be seen in this image.

Fig. 4. TEM micrograph of the DEG sample annealed at 800°C for 1 h.

Figure 5 shows the SEM micrograph of the W sample after annealing at 800°C. It is observed that the particles with mean diameter of 160 nm and thickness of 50 nm are grown in many directions to give an elongated branched hexagonal structure.

SEM micrographs of the DEG sample after annealing at 750 and 800°C exhibit a hexagonal platelet shape of barium ferrite (Fig. 6). The mean particle size of DEG sample increases from almost 50 nm to 80 nm, by increasing the annealing temperature from 750 to 800°C.

3150 M. Montazeri-Pour & A. Ataie

Fig. 5. SEM micrograph of the W sample annealed at 800°C for 1 h.

Fig. 6. SEM micrograph of the DEG sample annealed for 1 h (a) at 750°C and (b) at 800°C.

However, behavior of a precursor in the annealing step reflects the nature of that precursor from view points of phase composition and state of its particles such as size and monodispersity.

It is assumed that the solution environment can remarkably influence on the formation of the precursor particles in co-precipitation method. The nucleation and growth of crystals are strongly dependent on the properties of the solvent. The different particle, crystallite size and morphology of BaF formed were due to the different physical

Low Temperature Crystallization of Barium Ferrite Nano-Particles 3151

and chemical properties of diethylene glycol and water, such as boiling point, dielectric constant, viscosity, polarity and surface tension.

The high dielectric constant and low viscosity led to the high solubility and diffusion rate of ions in the aqueous solution, as a result, better crystallized particles with larger size in the solution is produced. On the other hand, in the case of using high viscosity solvents such as diethylene glycol, because of the low diffusion rate of ions in the solvent, crystalline growth was suppressed, and led to the formation of finer particles.15

4. Conclusion

(1) Nano-size particles of barium ferrite have been synthesized at a relatively low temperature of 750°C by a co-precipitation method using diethylene glycol/water mixture solution of iron and barium chlorides with Fe/Ba molar ratio of 11. (2) XRD analysis showed that the single phase barium ferrite with mean crystallite size of 26 nm and 49 nm could be obtained by using diethylene glycol/water mixture and water as solvents after annealing at 750°C and 900°C, respectively. The mean crystallite size estimated by XRD also confirmed by that obtained from TEM for sample synthesized at 800°C using diethylene glycol/water mixture. (3) SEM micrographs revealed that nano-particles with mean diameter sizes of 50 nm and 80 nm could be obtained by using non-aqueous solution after annealing at 750°C and 800°C, respectively, These sizes were lower than that of obtained by using aqueous solution after annealing at 800°C. (4) It is believed that diethylene glycol physicochemical properties such as low dielectric constant and high viscosity are probably responsible for the low temperature formation of single phase BaF nano-particles by using diethylene glycol/water mixture solutions.

Acknowledgments

The financial support of this work by the Iranian Nanotechnology Initiative is gratefully acknowledged.

References

1. H. Kojima, Ferromagnetic Materials: A Handbook on Properties of Magnetically Ordered Substances, vol. 3, ed. Wohlfarth (North-Holland, Amsterdam, 1982), p. 305.

2. A. Goldman, Modern Ferrite Technology (Springer, Pittsburgh, 2006). 3. D. Lisjak, M. Drofenik, J. Eur. Ceram. Soc. 26, 3681 (2006). 4. H. Stablin, Ferromagnetic Materials: A Handbook on Properties of Magnetically Ordered

Substances, vol. 3, ed. Wohlfarth (North-Holland, Amsterdam, 1982), p. 441. 5. S. E. Jacobo, C. D. Pascual, R. Clemente and M. A. Blesa, J. Mater. Sci. 32, 1025 (1997). 6. G. V. Duong, R. S. Turtelli, B. D. Thuan, D. V. Linh, N. Hanh and R. Groessinger, J. Non-

Cry st. Solids 353, 811 (2007). 7. R. Nikkhah-Moshaie, A. Ataie and S.A. Seyyed Ebrahimi, J. Alloys Compd. 429, 324 (2007). 8. R. Muller, C. Ulbrich, W. Schuppel, H. Steinmetz and E. Steinbei, J. Eur. Ceram. Soc. 19,

1547 (1999). 9. I. M. Smallwood, Handbook of organic solvent properties (Edward Arnold, New York, 1996).

10. H. Zhang and L. Wang, Mater. Lett. 61, 1667 (2007). 11. Y. Yu, L. Ma, W. Huanga, J. Li, P. Wong and J. C. Yu, J. Solid State Chem. 178, 1488 (2005).

3152 M. Montazeri-Pour & A. Ataie

12. B. D. Cullity, Elements of X-ray Diffraction (Addison-Wesley, Massachusetts, 1978). 13. G. Benito, M. P. Morales, J. Requena, V. Raposo, M. Vazquez and J. S. Moya, /. Magn. Magn.

Mater. 234, 65 (2001). 14. H. Hsiang and F. S. Yen, Ceram. Int. 29, 1 (2003). 15. S. Yin, M. Shinozaki and T. Sato, /. Lumin. 126, 427 (2007).