Nickel ferrite: combustion synthesis, characterization and magnetic properties

Transcription

1 Nickel ferrite: combustion synthesis, characterization and magnetic properties A. C. F. M. Costa 1, L. Gama 1, C. O. Paiva-Santos 2, M. R. Morelli 3 and R. H. G. A. Kiminami 3 1 Federal University of Campina Grande, Department of Materials Engineering, , Campina Grande PB, Brazil, Phone: anacristina@dema.ufcg.edu.br 2 Laboratório Computacional de Análises Cristalográficas e Cristalinas Depto de Físico- Química, IQ-UNESP, Araraquara SP, Brazil 3 Federal University of São Carlos, Department of Materials Engineering, São Carlos SP, Brazil, Phone: ruth@power.ufscar.br Keywords: Combustion synthesis, nickel ferrite, nanoparticles, magnetic properties. Abstract: Nanosized spinel nickel ferrite particles have attracted considerable attention and efforts continue to investigate them for their technological importance to the microwave industries, high speed digital tap or disk recording, repulsive suspension for use in levitated railway systems, ferrofluids, catalysis and magnetic refrigeration systems. Nanosize nickel ferrite powders (NiFe 2 O 4 ) have been prepared by combustion reaction using nitrates and urea as fuel. The resulting powders were characterized by X- ray diffraction (XRD), BET, and transmission electron microscopy (TEM). The results showed nanosize nickel ferrite powders with high specific surface area (55.21 m 2 /g). The powders showed extensive XRD line broadening and the crystallite size calculated from the XRD line broadening was 18.0 nm. The samples were uniaxially compacted by dry pressing, sintered at 1200 C/2h and characterized by bulk density, SEM and magnetic properties measurements. The samples showed uniform microstructures with grain size of 4.45 µm, maximum flux density of 0.18T, field coercive of the 488 A/m, and hysteresis loss of W/kg. Introduction Nanostructured materials exhibit unusual physical and chemical properties, significantly different from those of conventional bulk materials, due to their extremely small size or large specific surface area [1-3]. Nanosized spinel ferrite particles have attracted considerable attention and efforts continue to investigate them for their technological importance to the microwave industries, high speed digital tap or disk recording, repulsive suspension for use in levitated railway systems, ferrofluids, catalysis and magnetic refrigeration systems [4-6]. Nickel ferrite (NiFe 2 O 4 ) is a well-known spinel magnetic material. Its preparation by the classical solid state reactions requires a high calcinations temperature and hence induces the sintering and aggregation of particles [7]. To produce nanosized ferrite particles, some techniques such as chemical coprecipitation [8], hydrothermal synthesis [9, 10], hydrolysis of metal carboxylate in organic solvent [11], sol-gel [12], freeze drying [ 9], spray drying [13], citrate precursor [14] and aerosolization [15] have been developed. In particular, the synthesis by combustion reaction technique has been shown to have great potential in the preparation of the ferrites [16-18]. The process is quite simple since multiple steps are not involved. It is employed in the field of propellants and explosives, and involves an exothermic and self-sustaining chemical

2 reaction between the desired metals salts and a suitable organic fuel, usually urea [19, 20]. The resulting product is usually a product that is dry, crystalline, has high chemical homogeneity and purity, is inexpensive, and is agglomerated into a highly fluffy foam [21, 22]. This work report the prepared, characterization and magnetic properties of nickel ferrite nanoparticles (NiFe 2 O 4 ) by a single step solution combustion reaction using nitrates and urea as fuel Experimental Nanosize particle oxides of nickel ferrite with nominal composition NiFe 2 O 4 was prepared by combustion reaction using urea as fuel. The starting materials were iron nitrate, nickel nitrate and urea. Stoichiometric compositions of metal nitrate and urea were calculated using the total oxidizing and reducing valences of the components which serve as the numerical coefficients for the stoichiometric balance so that the equivalence ratio Φ c is unity and the energy released is maximum [23]. The batches were placed in a vitreous silica basin, homogenized and directly heated up in the hot plate to temperature around 480 o C until the ignition took place, producing a nickel ferrite in foam form. The ignition temperature was determined by an infrared pyrometer (Raytek, model MA2SC) and the ignition flame time was measured with digital chronometer (CONDOR). Powders and nickel compacted samples sintered at 1200 o C/2h were characterized by X-ray diffraction (Kristaloflex D5000, Cukα with a Ni filter, and scanning rate of 2 o 2θ /min, in a 2θ range of o ). The average crystallite sizes calculated from X-ray line broadening (d 311 ) using Scherrer s equation [24], the specific surface area and the average particle size were determined in N 2 gas with a Quantasorb Quantachrome (model Gemini Micromeritics) apparatus and the powder density using a helium pycnometer (Pycnometer Micromeritics, ACCUPYC 1330). The lattice parameters were calculated from the X-ray diffraction patterns. The bulk density was calculated by dimensions and weights measurements of the sintered pellets. B-H loop measurements were performed on toroids with primary and secondary windings of 26 SWG enameled copper wire, using a hysteresis loop tracer (TCH B). All measurements were carried out at room temperature. The maximum permeability (µ max ) was determined for the tangent the hysteresis B-H loop, where happens the displacements irreversible domain wall [25]. The microstructure of polished surfaces of the sintered sample was analyzed by using a scanning electron microscopy (Philips XL30 FEG) SEM. The morphology, size of the particles was observed using transmission electron microscopy (TEM). For the TEM studies the powders were supported on carbon-coated copper TEM grids and analyzed using a Philips EM420 transmission electron microscope at an accelerating voltage of 120 KV. Bright-field and dark-field imaging was performed to reveal the size and morphology of the nanopowders. Structure information was obtained using selected area diffraction patterns. Results and Discussion The combustion flame time and temperature of the synthesis by combustion reaction exerts an important effect on the final characteristics of powders. These parameters depend, mainly of the intrinsic characteristics of each system. For the system NiFe 2 O 4 the combustion flame time and temperature of reaction reached during the synthesis, whose obtained experimental values were 11.0 seconds and 622 o C (±2 o C), respectively. This show that the system led the lower temperature and lower combustion

3 time, avoided, this way, the pre-sintering and/or growth of particles. Table 1 shows the characteristics of the NiFe 2 O 4 powder (specific surface area, particle size, average agglomerate size, crystallite size, powders density and lattice parameters) prepared by combustion reaction. The average agglomerated size, specific surface area (BET) and the nanoparticle size calculated from BET was 8,13 µm, m 2 /g and 20.2 nm, respectively. Compared this value with the certain through Scherrer s equation (18.0 nm) it was observed that the values were close, indicating the efficiency of the synthesis process. The density of the powder was 4.99 g/cm 3 (92.9% of theoretical density). The theoretical density of the Ni-Zn ferrite utilized was 5.37 g/cm 3. These results can be attributed, mainly, to the combustion flame time and temperature of reaction during the synthesis. Table 1 Powder characteristics of NiFe 2 O 4 prepared by combustion reaction. Particle size* Crystallite [nm] size** [nm] Specific superficial area (BET) [m 2 /g] Average agglomerated size 50% [µm] Powder density [g/cm 3 ] (%TD) (92.9) *Calculated from specific superficial area **Calculated from Scherrer s equation [24] The X-ray diffraction patterns of the NiFe 2 O 4 nanopowder after synthesis and sintering at 1200 o C/2h are shown in Fig. 1. Through the diffractograms obtained for the powder, it was observed that the combustion flame temperature controlled during the combustion reaction (622 o C) was sufficiently high to the complete formation of the crystalline phase nickel ferrite with cubic spinel structure. The XRD pattern for the powder shows considerable line broadening, indicating the nanosize particle nature of nickel ferrite prepared by combustion reaction. The crystallite size calculated from X- ray line d (311) broadening using Scherrer s equation [24] was at 18.0 nm. The diffractogram pattern for the sample after sintering at 1200 o C/2h confirmed the presence of the single-phase cubic spinel of the nickel ferrite. -NiFe 2 O 4 Sinterized Powder Intensity (a.u.) θ (degrees) Figure 1 X-ray diffractogram patterns of NiFe 2 O 4 powder as prepared by combustion reaction and sample sintered at 1200 o C/2h in air. Fig. 2 presents the size, morphology and electron diffraction patterns of assynthesized nickel ferrite particles observed by TEM micrograph. The powders

4 appeared to be agglomerated. The electron diffraction clearly shows in both cases the rings belonging to the nickel ferrite phase and also the rings corresponding to spacing of o o around 2.97 A and 1.62 A of the nickel ferrite. The selected area aperture used in this study was enough to reveal all the corresponding rings for the spinel structure. The TEM results showed that the particles did not have a narrow size distribution. Thus it can be inferred that the nucleation occurred as a single event, and this resulted in a size distribution of nuclei. The particle size calculated by TEM micrograph was in the range of nm. This value of size particle obtained by combustion reaction is in range of the values calculated by TEM micrograph and reported for Chen [12] to NiFe2O 4 nanoparticles (5-30 nm) prepared by the sol-gel method. Figure 2 Transmission electron micrographs of NiFe 2 O 4 powder prepared by combustion reaction: (a) BF images with diffraction patterns (SADPs) showing rings that match d-spacing for the spinel structure, and (b) CDF images. The powder density was 4.99 g/cm 3 (92.9% of the theoretical value). The surface area of powder was m 2 /g and the particle size, calculated from of the surface area, was 18.0 nm. This result is in good agreement with the results obtained by TEM and Scherrer s equation (22-29 and 18.0 nm, respectively). The details of the microstructure of the samples of the nickel ferrite after sintering at 1200 o C/2h are presented in the electron micrographs (Fig. 3). The micrographs shows a homogeneous microstructure consist of grain approximately hexagonal with average grain size of the 4.45 µm and presence of second phase inter and intragranular. Was observed the presence of the pores inter and intragranular. The second phase observed probably is nickel oxide undetectable by XRD. Figure 3 Scanning electron micrographs of samples sintered at 1200 o C/2h. (a) x 5K and (b) x 20K.

5 Table 2 Grain size, density and magnetic properties of NiFe 2 O 4 sintered at 1200 o C/2h. Properties Hysteresis parameters (f=1khz; H=2000 A/m and e=4,0 mm) Average grain size [µm] Bulk density [g/cm 3 ] Relative density [%] Hc (A/m) Br (Τ) B max (Τ) P H (W/kg) The results of bulk density, relative density, average grain size and hysteresis measurements (coercive force H c, remanent flux density B r, maximum flux density B máx and hysteresis loss P H ) of the sample sintered at 1200 o C for 2h are presented in Table 2. The green density of the nickel ferrite sample was 3.25 g/cm 3 (60.5% theoretical density) indicating good efficiency during compaction. The bulk density of the pellets sintered at 1200 o C/2h was 5.25 g/cm 3, which is 97.7% of the theoretical density. The average grain size calculated from the image analyzing software MOCHA of Jandell was 4.45 µm. Fig. 4 shows the dependence of the flux density (B) with the applied magnetic field (H) from hysteresis loop for the sample sintered at 1200 o C/2h. Through hysteresis loop B-H was possible to observe the characteristic of soft magnetic material, firstly due the B-H loop deviates from rectangularity and secondly for the hyteresis loss of the sample (47.58 W/kg, Table 2). The microstructure constituted by average grain size 4.45 µm of the nickel ferrite phase and NiO secondary phase in the grain boundaries resulted in values of the maximum flux density B max, hysteresis loss and coercive force of the 0.18 T, W/kg and A/m, respectively. The B max value was slightly lower (0.18 T) than that for bulk materials (0.21 T [7,11]). The reduction in B max might be the incomplete crystallization of NiFe 2 O 4, which led to presence of the secondary phase probably of the NiO detectable by SEM (like inclusion in the grain boundaries) and undetectable by XRD. Another possible reason for the diminution in B max could be explained by the fact that the particles were so small that they might be superparamagnetic. The dependence of the nanoparticles size with superparamagnetic phenomenon was reported by Chen [12] and Albuquerque [26]. 0,2 0,1 B(Τ) 0, H (A/m) -0,1-0,2 Figure 4 Room temperature hysteresis loop B-H of nickel ferrite after sintering at 1200 o C/2h. Conclusion The synthesis by combustion reaction was favorable to obtain crystalline powders with nanosize particles nickel ferrite. The X-ray lines broadening and TEM confirmed the nature of the nanosize particle of the powders that was 18.0 and 22-29

6 nm, respectively. The powders nickel ferrite after sintering at 1200 o C/2h presented homogeneous microstructure consisted of the average grain size of the nickel ferrite of the 4.45 µm and second phase inter and intragranular. The flux density maximum hysteresis, hysteresis loss and field coercive measured were 0.18 Τ, W/kg and A/m, respectively. Acknowledgements The authors would like to thank the Brazilian Institutions, CAPES, CNPq and FAPESP for financial support. References 1. C. Hayashi, Phys. Today 40 (1987), p H. Gleiter, Progr. Mater. Sci. 33 (1989), p J. H. Fendler, Chem. Rev. 87 (1987), p T. Pannaparayil, R. Marande, S. Komarneni, S. G. Sankar, J. Appl. Phys. 64 (1988), p A. Goldman, in: Levenson (Ed.), Electronic Ceramics, Marcel Dekker, New York, 1988, p J. L. Dormann, D. Fiorani, Magnetic Properties of Fine Particles, North-Holland, Amsterdam, G. Economos, J. Am. Ceram. Soc. 42 (1959), p M. Tsuji, T. Kodama, T. Yoshida, Y. Kitayama, Y. Tamaura, J. Catalysis 164 (1996), p X. Yi, Q. Yitai, L. Jing, C. Zuyao, Y. Li, Mater. Sci. Eng. B34 (1995), p. L S. Komarneni, E. Fregeau, E. Breval, R. Roy, J. Am. Ceram. Soc. Commun. 71 (1988), p. C Y. Konishi, T. Kawamura, S. Asai, Ind. Eng. Chem. Res. 35 (1996), p D. H. Chen, X. R. He, Mater. Res. Bull. 36 (2001), p C. H. Marcilly, P. Country, B. Delmon, J. Am Ceram. Soc. 53 (1970), p S. Prasad, N. S. Gajbhiye, J. Alloys Comp. 265 (1998), p M. A. A. Elmasry, A. Gaber, E. M. H. Khater, Powder Technol. 90 (1997), p A. C. F. M. Costa, E. Tortella, M. R. Morelli, R. H. G. A. Kiminami, Mater. Sci. Forum (2003), p A. C. F. M. Costa, E. Tortella, M. R. Morelli, M. Kaufman, R. H. G. A. Kiminami, J. Mater. Sci. 37 (2002), p A. C. F. M. Costa, E. Tortella, M. R. Morelli, R. H. G. A. Kiminami, J. Metas. Nanocryst. Mater. 14 (2002), p R. H. G. A. Kiminami, J. KONA 19 (2001) p S. R. Jain, K. C. Adiga, V. Pai Verneker, Combust. Flame 40 (1981), p Y. Zhang and G. C. Stangle, J. Mater. Res., 9 [8] (1994), p S. T. Aruna, K. C. Patil, J. Mater. Syn.Proces. 4 [3] (1996), p S. R. Jain, K. C. Adiga, V. Pai Verneker, Combust. Flame 40 (1981), p H. Klung, and L. Alexander, in X-ray diffraction procedures, Wiley, New York, EUA, (1962), p R. W. Cahn, P. Haasen, (Ed.). Physical metallurgy. North-Holland: Physics Publishing, A. S. Albuquerque, J. D. Ardisson, W. A. A. Macedo, J. L. López, R. Paniago, A. I. C. persiano, J. Magn. Magn. Mater (2001), p