Factors Affecting the Synthesis and Formation of Single-Phase Barium Hexaferrite by a Technique of Oxalate Precursor

Problem statement: Barium hexaferrite (BaFe 12O19), is of great importance as permanent magnets, particularly for magnetic recording as wel l as in microwave devices. Approach: The aim of this study was to synthesize Stoichiometric and sin gle-phase barium hexaferrite through a technique of oxalate precursor. Effects of different Fe /Ba mole ratio and annealing temperature on the partic le size, microstructure and magnetic properties of the resulting barium hexaferrite powders has been studied and reported in the presented research. The ann aling temperature was controlled from 9001200°C, while the Fe /Ba was controlled from 12-8.57. Results: The resultant powders were investigated by Differential Thermal Analyzer (DTA) , X-Ray Diffractometer (XRD), Scanning Electron Microscopy (SEM) and Vibrating Sample Magn etometer (VSM). Single phase of well crystalline BaFe 12O19 was first obtained at Fe /Ba mole ratio of 9.23 and 8.57 at annealing temperature 1100°C. Moreover, at annealing temperat ure 1200°C the single phase BaFe 12O19 appeared at all different Fe/Ba mole ratio. The SEM results showed that the grains were regular hexagonal platelets. In addition, maximum saturation magnetiz ation (70.25 emu g ) was observed at mole ratio 10 and annealing temperature 1200°C. However, it wa s found that the coercivety of the synthesized BaFe12O19 samples were lower than the theoretical values. Conclusion: The barium hexaferrite was synthesized at annealing temperature 1200°C with a single phase using oxalate as precursor route.


INTRODUCTION
Permanent magnet materials are essential in devices for storing energy in a static magnetic field. Ferrite is a class of ceramic materials with useful electromagnetic properties and the electromagnetic properties for ferrite materials is affected by operating conditions such as temperature, pressure, field strength, frequency and time. Ferrites play an important role in the field of electronics industry because they are relatively inexpensive, more stable and easily manufactured. Ferrites are widely used in microwave devices, permanent magnets, high density magnetic and magneto-optic recording media and telecommunications devices. Hexagonal ferrites, MFe 12 O 19 , (where M = Ba, Sr and/or Pb) are a group of magnetic compounds, which all have high resistivity, magneto-crystalline anisotropy and saturation magnetization, low dielectric losses and are thermally stable well above their Curie temperature (Kojimi and Wohlfarth, 1982;Smit and Wijn, 1959). The hexagonal M-type hard ferrites have attracted much attention as the most widely used permanent magnets, which account for about 90% of the annual production of the permanent magnets due to the good combination of high magnetic properties, chemical stability and low cost. Moreover, M-type hexaferrites have widely used in telecommunication, magnetic recording media, magneto-optics and microwave devices (Kojima, 1982;Smit and Wijn, 1959;Ogasawara and Oliveira, 2000). As a result of its specific magnetic properties, barium hexaferrite and its derivatives can be used for permanent magnets, magnetic recording media and microwave applications . BaFe 12 O 19 (BaM) and its derivatives are currently magnetic material with great scientific and technological interest, due to its relatively high curie temperature, high coercive force and high magnetic anisotropy field as well as an excellent chemical stability and corrosion resistivity (Kojimi and Wohlfarth, 1982). Barium hexaferrite (BaFe 12 O 19 ) has a complex hexagonal unit cell and belonging to the magnetoplumbite structures (Richerson, 1992). Magnetoplumbite are of the type A 2+ O1.6B 2 3+ O 3 . The arrangement of the 12 Fe 3+ ions in the unit cell is as follows: Two ions in the tetrahedral sites (four nearest O 2− neighbors), nine ions in the octahedral sites (six nearest O 2− neighbors) and one ion in the hexagonal site (five nearest O 2− neighbors). Materials of this type have a strong uniaxial magnetic direction, making as permanent magnets. The reported theoretical calculated coercive force, saturation magnetization and curie temperature values for pure and single domain barium hexaferrite was 6700 Oe, 72 emu g −1 and 450°C, respectively (Miller and Drillon, 2002;Pillai et al., 1993). It is difficult to obtain ultrafine and monodispersed particles by the conmmercial ceramic method (solid-state reaction) which involves the firing of stoichiometric mixture of barium carbonate and iron oxide at high temperatures (about 1200°C) (Cabaoas and Gonzalez-Calbet, 1993). In this respect, several low-temperatures chemical methods were investigated for the formation of ultrafine BaFe 12 O 19 particles. These methods comprised coprecipitation Jacobo et al., 1997;Matutes-Aquino et al., 2000), hydrothermal (Wang et al., 1993;Liu et al., 1999;Mishra et al., 2004), sol-gel (Surig et al., 1996;Zhong et al., 1997;Garcia et al., 2001;Jacobo et al., 1997), microemulsion (Pillai et al., 1993), oxalate precursor (Sankaranarayanan and Khan, 1996), glass crystallization (El-Hilo et al., 1994), sonochemical (Shafi and Gedanken, 1999) and mechano-chemical activation (Abe and Narita, 1997). The oxalate precursor technique was found to be more suitable for synthesis of barium ferrite with single phase powder. In addition, this technique needs relatively low-processing temperature to produce homogenous microstructure with narrow size distribution and uniform shape. In this study, the oxalate precursor technique was used to synthesize nanocrystalline barium ferrite with high saturation magnetization. Effects of Fe 3+ /Ba 2+ mole ratios and the annealing temperature on the synthesis of ferrite powders were investigated. The annealing temperature was controlled from 900-1200°C, while Fe 3+ /Ba 2+ mole ratios were controlled from 12-8.57.

Preparation:
The oxalate precursor method was applied for the preparation of Barium hexaferrite (BaFe 12 O 19 ). Chemically grade ferric chloride (FeCl 3 -6H 2 O), barium chloride (BaCl 2 .H 2 O) and oxalic acid as source of organic were used as starting materials. A series of ferric chloride and barium chloride solution with various Fe 3+ /Ba 2+ molar ratios of (12, 10.9, 10, 9.23 and 8.57) and containing equivalent amount of oxalic acid were prepared. The mixtures of barium chloride and ferric chloride solution firstly prepared and then stirred for 15 min on a hot-plate magnetic stirrer, followed by addition of an aqueous solution, which was evaporated to 80°C with constant stirring until dry and then dried in a dryer at 100°C overnight. The dried powders obtained as barium ferrite precursors.
Measurements: Differential Thermal Analyzer (DTA) analysis of various un-annealed precursors was carried out. The rate of heating was kept at 10°C min −1 between room temperature and 1000°C. The measurements were carried out in a current of argon atmosphere.
For the formation of the barium ferrite phase, the dry precursors were annealed at the rate of 10°C min −1 in static air atmosphere up to different temperatures (900-1200°C and maintained at the temperature for annealed time (2 h). The crystalline phases presented in the different annealed samples were identified by XRD on a Brucker axis D8 diffractometer using Cu-Kα (λ = 1.5406) radiation and secondary monochromator in the range 2Ø from 10-80°. The ferrites particles morphologies were observed by Scanning Electron Microscope (SEM, JSM-5400).
The magnetic properties of the ferrites were measured at room temperature using a Vibrating Sample Magnetometer (VSM; 9600-1 LDJ, USA) in a maximum applied field of 10 kOe. From the obtained hysteresis loops, the saturation Magnetization (M s ), remanence Magnetization (M r ) and Coercivity (H c ) were determined. Figure 1 shows the Differential Thermal Analysis (DTA) plot (a and b) of the synthesized mixture of barium-iron oxalates precursors at two different Fe 3+ /Ba 2+ mole ratios 12 and 8.57 respectively. It can be seen in peaks I-IV that an endothermic reaction occurred at around (95.11, 158.52, 182.45 and 232.15°C) which corresponds to the dehydration of iron and barium oxalates. This is consistent with earlier findings suggesting that, two different crystals hydrate types namely MeC 2 O 4 .2H 2 O and MeC 2 O 4 .3H 2 O (Me = metal ion) (Shafi and Gedanken, 1999;Abe and Narita, 1997;Hessien, 2008). Thereafter, peaks V, VI and VII (448.02, 493.88 and 586.56°C) were significantly related to the anhydrous oxalate mixture decomposition into both metal oxide and gases (CO 2 and CO). Figure 1b shows that the VI peak has higher intensity in the case of Fe 3+ /Ba 2+ mole ratio 8.57 as compared with mole ratio 12. This is most likely due to the increase of barium oxalate amount. Peak VIII in the plot at (956.90°C), showed the initial step to form BaFe 12 O 19 , Moreover, the intensity and sharpness of peak VIII in (plot b) was also increased, indicating that the stability of the formed barium ferrite will be increased with increasing content of barium oxalate ratio. Therefore, the DTA results indicate that barium ferrite cannot form before 956.90°C (Carp et al., 1998).  XRD analysis was carried out in this study to investigate the effect of Fe 3+ /Ba 2+ mole ratios of the powders thermally treated at different temperatures (900-1200°C) for 2 h and the results are presented in Fig. 3-6. The results in Fig. 3 indicate that the thermal calcination of barium-iron oxalate precursor at 900°C has not yielded barium ferrite phase BaFe 12 O 19 in any case. Instead iron oxide (Fe 2 O 3 ) phase has appeared clearly in all the Fe 3+ /Ba 2+ ratio. These results confirm the DTA results, which showed no sign of BaFe 12 O 19 formation at 900°C. Figure 4 shows XRD patterns at annealing temperature 1000°C, formation of barium ferrite was observed for all mole ratios.   Figure 7 shows the effect of various mole ratios on the crystalline size of the obtained powders. It can be observed that increasing the annealing temperatures helps significantly agglomeration of the particles and grains growth during calcination course. This leads to the increase of grain size and formation of single phase barium hexaferrite powders. Figure 8 displays SEM micrographs of BaFe 12 O 19 powders obtained from oxalate precursors with Fe 3+ /Ba 2+ mole ratio of 9.23 and annealed for 2 h.

RESULTS
Clearly, it appears that increasing calcination temperature (900-1200°C) has a substantial effect on the microstructure of synthesized BaFe 12 O 19 powders. In Fig. 8a, fine precipitated particles, with random grain orientation. This confirms the previous results of XRD and DTA, which showed no sign of BaFe 12 O 19 growth at 900°C. However, as the annealing temperatures increased to 1000°C Fig. 8b, individual particles possess a plate-like hexagonal shape containing a fewer numbers of spherical small particles. At annealing temperature 1100°C (Fig. 8c), the ferrite powders showed uniform coarse structure with a wellclear hexagonal shape which is in line with XRD patterns in Fig. 2, for where pure single crystal peaks of barium ferrite was very evident. The grains were then started to distort again at 1200°C (Fig. 8d), which may lead to agglomeration of the particles at more higher annealing temperatures. Figure 9a-e presented the SEM micrographs of synthesized BaFe 12 O 19 powders obtained from oxalate precursors and annealed at 1000°C for 2 h. Effect of changing Fe 3+ /Ba 2+ mole ratios on the microstructure was observed. In Fig. 9a and b, very fine particles of hematite powder started to agglomerate, where the mole ratios of Fe 3+ /Ba 2+ were 12 and 10.9 respectively. In addition, a few large crystal particles were formed, indicating that these ratios of the composition were insufficient for the complete formation of the structure. As the Ba 2+ ion concentration increased in the composition of the samples (Fig. 9c-e), uniform and coarse structure with clear homogeneous microstructure become more pronounced. Moreover, a well-clear crystalline micro-structure containing a fewer numbers of spherical small particles can be seen in these SEM micrographs. As the annealing temperature increased to 1100°C (Fig. 10a-e), the produced powders of BaFe 12 O 19 possessed very well-defined plate-like hexagonal shape. Table 1 and Fig. 11-14 present the magnetic properties of the synthesized barium ferrite powders, which were obtained at room temperature under an applied field of 10 kOe. The results showed that the saturation magnetization of the produced powders increased by increasing the temperatures. Figure 11 display the effect of annealing temperature on the hystersis loop of BaFe 12 O 19 powders obtained from oxalate precursors at Fe 3+ /Ba 2+ mole ratio 10. This is likely due to the presence of single domain of BaFe 12 O 19 particles. In line with SEM results, the change in magnetic properties can be attributed to the presence of well crystalline BaFe 12 O 19 microstructures, as the annealing temperature of the powders was increasing gradually to reach optimum conditions.  Hc ( Figure 12 showed that at annealing temperature 1000°C decreasing the Fe 3+ /Ba 2+ mole ratios from 12-8.57 increased the saturation magnetization of the formed BaFe 12 O 19 particles from 33.01-44.8 emu g −1 . This is mainly due to increasing formation of well crystalline BaFe 12 O 19 powders and decrease of the presence of non-magnetic species of Fe 2 O 3 as the mole ratio percentage went up. These results are in substantial agreement with the previous XRD and SEM results, which was shown in Fig. 4 and 8. However, the effect of Fe 3+ /Ba 2+ mole ratio on the M-H hysteresis loop of synthesized BaFe 12 O 19 powders obtained at 1100°C and 1200°C was less significant in Fig. 13 and 14. This suggests that the Fe 2 O 3 particles diminished dramatically at temperature 1100°C and 1200°C, which lend support to the XRD and SEM results corresponding to these temperatures in Fig. 5 and 10, where the crystallinity of BaFe 12 O 19 powders were very evident.

CONCLUSION
The structural and magnetic properties of newly prepared barium hexaferrite powders were studied in a comparative way. The results from DTA, XRD, SEM and VSM studies can be summarized as follows: • Differential Thermal Analysis (DTA) plots of the synthesized mixture of barium-iron oxalates precursors showed that the initial step to form BaFe 12 O 19 started at (956.90°C) • Single phase of well crystalline BaFe 12 O 19 was first obtained at Fe 3+ /Ba 2+ mole ratio of 9.23 and 8.57 at annealing temperature 1100°C while at annealing temperature 1200°C the single phase BaFe 12 O 19 appeared at all different Fe 3+ /Ba 2+ mole ratio • The morphology of the particles at 1000 and 1100°C are hexagonal platelet crystal. By increasing the temperature up to 1200°C, grains have coalesced to form larger grains • The oxalate precursor route has proven to produce pure barium ferrite powders with good magnetic properties with maximum saturation magnetization value of (70.25 emu g −1 ) and coercivity force (451.3 Oe) • Regarding the particles size, it can be seen that, the minimum particle size appeared at (1000°C) and the maximum size was found at (1200°C), which most likely explained by the formation of the single phase of barium hexaferrite (BaFe 12 O 19 )