Microwave Induced Irradiation of Barium Ferrite Prepared by Sol-gel Auto-combustion

A. NURUDDIN1,2*, R. RIADY2 and SUYATMAN1

1Engineering Physics Research Group, Faculty of Industrial Technology, Institut Teknologi Bandung, Ganesha 10, Bandung

40132, Indonesia2Materials Engineering Study Program, Faculty of Mechanical

Engineering and Aeronautics, Institut Teknologi Bandung, Ganesha 10, Bandung 40132, Indonesia

*Corresponding author: email address of corresponding author

Abstract: Effect of microwave irradiation on the properties of barium ferrite (BaM) was investigated. Barium ferrite powder was synthesized via sol-gel auto-combustion route, and subjected to microwave radiation with various power and time exposures. The powder properties were examined by FTIR, DTA/TG, XRD, SEM, and VSM. It was shown that microwave irradiation enhanced the decomposition and effectively prepared primary precursors containing single phase barium ferrite of about 20.6 % weight. Microwave exposure alone was insufficient to produce single phase barium ferrite and completely remove secondary phase barium ferrite, further calcination at 900oC is required to fully attain single phase barium ferrite with average particle size of 180 nm. Saturation (Ms at 10kOe) and remanent (Mr) magnetization, and coercivity (Hc) of the powder are 67.4 emu/g, 40.1 emu/g, and 3.77 kOe, respectively. Relatively low coercivity is due to inhomogeneity of the powders.

Key words: Barium ferrite, Microwave irradiation, Sol-gel auto-combustion, Magnetic properties

1. INTRODUCTION

Barium ferrite (BaM) is a permanent magnet widely used for electromagnetic (EM) wave/radar absorber [1]. Sol-gel auto-combustion processes have been widely used to prepare barium ferrite powder [2]. Microwave heating was known to reduce processing time without sacrificing quality of the product. Exposing the gel during auto-combustion produced smaller grain than that treated by oven heating [3]. This report presents the effect of microwave irradiation on the crystalline structure, particle size and magnetic properties of barium ferrite.

2. EXPERIMENTAL METHOD

The starting materials are pure reagent of ferric nitrat (Fe(NO3)3. 9H2O), barium nitrat (Ba(NO3)2) as the main precursors, and citric acid (CA) acts as a chelating agent and fuel. The molar ratio of Ba : Fe : CA is 1 : 11.5 : 25 respectively. NH4OH was used to adjust the pH value of 3.4. The solution was thermally dehydrated to dryness and prior to microwave irradiation at various power at time exposures. The loose powder was heat treated at various temperatures calcination to reveal the evolution of crystalline formation. FTIR, XRD, SEM, and VSM were employed to characterize the BaM properties.

3. RESULTS AND DISCUSSION

Fig. 1 shows TG/DTA curve of dry gel heat treated from room temperature up to 900oC with heating rate of 10oC/minutes. It is clearly shown that there is an

Fig.1. TG/DTA curve of dry gel

exothermic reaction as indicated by high peak heat flow and significant sample weight loss. Sample weight loss of about 22% at temperature ≤ 320oC is primarily attributed to the evaporation of hydroxyl group. At this temperature ranges ferric nitrate and barium nitrate are thermally decomposed. In the presence of citric acid the decomposition of ferric nitrate can produce maghemit (-Fe2O3) [4]. Auto-combustion occures when the temperature is increased to about 384oC where exothermic reaction take place, corresponding to the decomposition of the remaining organic material from citric acid. This reaction ceases at 400oC accompanied by 32% weight-loss. Further temperature increases to 900oC there is weight gain corresponding to reaction of all intermediate phase precursors to form barium ferrite.

Fig.2. XRD patterns of microwave irradiated samples at power of (a) 385 watt, (b) 539 watt, and (c) 700 watt,

with BaM pad of 710 gr.

Fig.3. XRD patterns of microwave irradiated samples at power of 539 watt with BaM pad weight of (a) 710 gr.,

(b) 457 gr., (c) 215 gr., and (d) 97 gr.

Fig. 2 shows the XRD patterns of samples upon microwave exposures at different powers for 25 minutes. The BaM pad of 710 gram was placed underneath the sample during microwave irradiation. It shows that the microwave power of 385 watt is sufficient to decompose the chelated metal nitrates to form intermediate phase of -Fe2O3, BaCO3 and BaFe2O4. However, for samples irradiated at higher power the diffraction pattern remains the same. There seems that the microwave energy is insufficient to heat up the sample to provide precursors reaction to form BaFe12O19. In this condition, there is no barium ferrite is produced.

Fig. 3 shows that the XRD pattern of the sample exposed to microwave with different weight of BaM pads. Reducing the weight of BaM pad enhances XRD peak intensity, which means more microwave heat is absorbed by the sample than that by BaM pad. Further analysis shows that BaM pad can be utilized to fine tune microwave heat into the sample which assists precursor reactions to form single phase barium ferrite, as shown in Fig. 4 below.

Fig.4. Single phase barium ferrite content in the sample.

Single phase barium ferrite of about 20.6 % weight can be produced by placing 97 gram BaM pad underneath the sample. The concentration tends to decrease as the BaM pad weight was added. No single phase barium ferrite was produced for 710 gram BaM pad. Noted all samples were exposed to 539 watt microwave power for 25 minutes. In the current conditions, however, single phase barium ferrite could not be fully produced by merely exposing the microwave to sample. Instead, microwave irradiation is effective means for producing intermediate phase precursors (BaCO3, Fe2O3, BaFe2O4).

Fig.5. FTIR patterns of (a) dry gel, and (b) microwave irradiated sample.

Fig.5 shows FTIR pattern of dry gel and microwave irradiated sample for 25 minutes with power of 539 watt and BaM pad of 457 gram. In dry gel, the broad bands at 3000-3500 cm-1 are due to O-H stretching from citric

acid. Since there is no band at about 1650 cm -1 which is ascribed to H-O-H bending vibration, it indicates that sample is fully dry. Two bands centered at 1604 cm-1 and 1400 cm-1 correspond to COO- anti-symmetrical stretching and COO- symmetrical stretching modes, respectively [5]. These ligands chelate the metal ions to form complex coordination [6]. The peak at 1050 cm-1

remains unassigned. The band at 697 cm-1 is observed for CO3

2-, and the broad band appears between 450 and 600 cm-1 is characteristic of Fe-OH stretching.

20 40 60 80

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Fig.6. XRD patterns of microwave irradiated samples and calcinated at (a) 600oC, (b) 800oC, and (c) 900oC for 1 hr.

After microwave exposure at 539 watt for 25 minutes, O-H and COO- bands disappear, instead the band at 1435 cm-1, 768 cm-1 and 617 cm-1 appear corresponding to CO3

2-, BaFe2O4 and -Fe2O3, respectively. The bands at 578 and 428 cm-1 are ascribed to Fe-O [7-10].

Fig. 6 shows XRD patterns of microwave irradiated samples calcined at different temperatures. Calcination the sample at 600oC has no significant effect on the diffraction pattern. When the temperature calcinations is increased to 800oC single phase barium ferrite is form with the remaining BaFe2O4. This intermediate phase barium ferrite completely disappeared when the sample is calcined at 900oC. For comparison, the same result was obtained for the dry gel directly calcined with conventional oven at 900oC for 3 hours. Scherrer calculation of the XRD line broadening for the highest peak intensity provided average crystallite size of 42 nm. These samples were subjected to further characterizations to reveal the surface morphology and magnetic properties.

Fig.7. SEM images of sample calcined at 900oC.

Fig.3. VSM graph of BaM calcined at (a) 800oC, (b) 900oC.

Fig. 7 shows the SEM micrograph of barium ferrite powder calcined at 900oC. The morphology of particles is plate-like shape. The particle orientation is random with rather broad particle size distribution. The average particle size was measured as about 180 nm, which is larger than the average crystallite size. This suggests that each grain aggregate about five crystallites.

Fig. 8 shows the VSM magnetization hysteresis loop of the powder calcined at 800 and 900oC measured at room temperature. The shape of both curves are quiet similar, but higher calcinations temperature produces larger magnetization and coercivity. The saturation (Ms at 10kOe), remanent (Mr) magnetization, and coercivity (Hc) of the powder calcined at 900oC are 67.4 emu/g, 40.1 emu/g, and 3.77 kOe, respectively. For comaprison, the theoretical magnetization value calculated for single crystals of barium ferrite is 72 emu/g [11]. Relatively low coercivity is due to inhomogeneity of the powders.

4. CONCLUSIONS

Microwave irradiations have been effectively used for producing barium ferrite powder. Dry gel metal-nitrates

can be ignited and auto-combusted by moderate microwave power for relatively short time comparing to the conventional process. The gel fully decomposed into main precursors for preparing barium ferrite. However, microwave irradiation alone could not completely transform the precursors to produce single phase barium ferrite. Fast heat treatment has produced platelate-like shape and smaller grain size of barium ferrite with inhomogeneous distribution. This properties produce relatively high saturation magnetization but reduce the coercivity.

Acknowledgments This work is financially supported by ITB KK-Research Grant.

REFERENCES1. M.R. Meshram, N.K. Agrawal, B. Sinha, P.S. Misra, J.

Magn. Magn. Mater, 271 (2004) 207.2. L. Junliang, Z. Wei, G. Cuijing, Z. Yanwei, Journal of

Alloys and Compounds, 479 (2009) 863-869.3. D. Limin, H. Zhidong, W. Ze, Z. Xianyu, Key Eng. Mater.

368-372 (2008) 576.4. S.M. El-Sheikh, F.A. Harraz, and K.S. Abdel-Halim,

Nanotechnol. and Appl. 615 (2008) 005.5. Z. Yue, J. Zhou, L. Li, H. Zhang, Z. Gui, J. Magn. Magn.

Mater, 208 (2000) 55.6. J. Huang, H. Zhuang, W. Li, Mat. Res. Bull. 38 (1) (2003)

149.7 W.J. Lee, T.T.J. Fang, Mater. Sci. 30 (1995) 4349. 8. A. Mali, A. Ataie, Ceram. Int. 30 (2004) 1979. 9. D. Bahadur, S. Rajakumar, A.J. Kumar, Chem. Sci. 118

(2006) 15. 10 H.F. Yu, P.C. Liu, J. Alloys Comp. 416 (2006) 222.11. G. Benito, M.P. Morales, J. Requena, V. Raposoa, M.

Vazqueza, J.S. Moya, J. Magn. Magn. Mater. 234 (2001) 65.