Nanosize nickel ferrite particles synthesized by combustion reaction: evaluation of two synthesization routes

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1 Nanosize nickel ferrite particles synthesized by combustion reaction: evaluation of two synthesization routes M. A.. Ramalho 1, A. C.. M. Costa 1, L. Gama 1, R. H. G. A. Kiminami 2, E. P. Hernandez 3, D. R. Cornejo 4, S. M. Rezende 3 1 ederal University of Campina Grande, DEMa/UCG, Technology Sciences Center Laboratory of Ceramic Materials Synthesis, Caixa Postal 134, Campina Grande, 5897-, PB, Brazil 2 USCar - Department of Materials Engineering, SP, Brazil 3 UPE - Department undamental Physics, PE, Brazil 4 USP - Institute of Physics, SP, Brazil 1 anacristina@dema.ufcg.edu.br 2 ruth@power.ufscar.br Keywords: Combustion Synthesis; Nickel errite; Synthesization Routes, Nanoparticles. Abstract: Ultrafine magnetic nickel ferrite particles have a significant potential for use in many applications such as magnetic recording media, ferrofluids, microwaves, catalysis and radar-absorbing coatings [1, 2]. Nickel ferrite powders with a nominal Nie 2 O 4 composition were synthesized by combustion reaction and an evaluation was made of the effect of two different conditions of synthesis on the nanostructural and magnetic characteristics of the resulting powders. Two synthesization routes were studied. The first, NB, involved the preparation of the powder using a Pyrex beaker heated directly on a hot plate at 48ºC until self-ignition occurred. By the second route, NC, the powder was obtained under the same synthesization condition as the NB route, but a vitreous silica basin was used. The resulting powders were characterized by X-ray diffraction (XRD), nitrogen adsorption by BET and scanning electron microscopy (SEM). The first route, NB, proved more favorable to obtain powders with high surface area and, hence, smaller crystalline sizes (5.7 nm) and a superparamagnetic behavior. The NC route confirmed the feasibility of obtaining powders with a crystalline size of 18. nm and a magnetic behavior. Saturation magnetization was emu/g and the coercivity field was Oe for powders obtained by the NC route. 1. Introduction Over the last decade, novel and interesting magnetic properties have been reported for nanocrystalline spinel ferrites. Nanosized spinel ferrite particles have attracted considerable attention and their unique properties have driven continued efforts to investigate them thanks to their technological importance to the microwave industry, high-speed digital tape or disk recording, magnetic refrigeration systems and ferrofluids [1-3]. These materials are widely utilized in electric and electronic devices and in catalysis [4], since they exhibit exceptional physical and chemical properties owing to their small crystallite sizes [5]. Oxide ceramic powders have conventionally been produced by the solid state reaction method [6,7], which requires heating at high temperatures for long periods of time. In addition to high energy consumption, the production rate is slow. Large-scale applications of ferrites with small particles have led to the development of various chemical methods which include hydrothermal, co-precipitation, freeze drying, precursor, sol gel and combustion reaction for the preparation of stoichiometric and chemically pure spinel ferrites [8-13]. Among these methods of chemical synthesis, combustion reaction synthesis stands out as an alternative and

2 promising method for after-ceramics obtained on a nanometric scale. This synthesization method is also known as auto-propagating synthesis [14]. The combustion reaction process offers the advantages of simplicity, rapid preparation of the reagent to the final product, the preclusion of subsequent intermediary stages of calcination, and low consumption of energy during synthesization [15, 16]. Moreover, the nonconventional method of combustion reaction synthesizes highly pure and chemically homogeneous powders, usually leading to products with the desired structures and composition due to their excellent homogeneity aided by the salts solubility in water. The purpose of this work was therefore to prepare nickel ferrite by combustion reaction, using two synthesization routes and characterizing the structural, morphologic and magnetic properties of the powders resulting from the two routes investigated here. 2. Experimental Nanosize particle oxides of nickel ferrite with a nominal Nie 2 O 4 composition were prepared by combustion reaction using urea as fuel. The materials used were iron nitrate e(no 3 ) 3.9H 2 O (Merck), nickel nitrate Ni(NO 3 ) 2.6H 2 O (Merck) and urea CO(NH 2 ) 2 (Synth). 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 [17, 18]. Two synthesization routes were studied. The first, NB, involved the preparation of the powder using a Pyrex beaker heated directly on a hot plate at 48ºC until self-ignition occurred. By the second route, NC, the powder was obtained under the same synthesization conditions as the NB route, but a vitreous silica basin was used. The ignition temperature was determined by an infrared pyrometer (Raytek, model MA2SC) and the ignition flame time was measured with a digital chronometer (CONDOR). The nickel ferrite powder was characterized by X-ray diffraction (Shimadsu 6, Cukα with a Ni filter, using a scan rate of 2 o 2θ /min, in a 2θ range of 2-6 o ). The average crystallite size was calculated from X-ray line broadening (d 311 ) using Scherrer s equation [19] and the lattice parameters were calculated from the X-ray diffraction patterns using routine DICVOL91 for Windows, available in the programs package of ullproff [2]. The specific surface area and average particle size were determined in N 2 gas with a Micromeritics model Gemini 237 device. The morphology of the powders was analyzed by scanning electron microscopy (SEM Philips XL3 EG). Magnetization measurements were taken at room temperature using an alternative gradient magnetometer (AGM). 3. Results The combustion flame time and temperature of the synthesis by combustion reaction strongly affect the powders final characteristics. Therefore, several important parameters must be evaluated during the combustion. The experimentally determined temperature and combustion flame time of the two routes were 562ºC and 12 seconds and 622ºC and 11 seconds, respectively, for the NB (Pyrex beaker) and NC (vitreous silica basin) routes. The different conditions of synthesis (Pyrex beaker and vitreous silica basin) resulted in specific changes in the reaction s kinetics and exothermic nature. The combustion synthesis route using a Pyrex beaker (NB) provided heat from the external source (plate at 48ºC) and produced a lower combustion flame temperature than that of the synthesization route using a vitreous silica basin, which absorbed more heat from the external source and resulted in a higher combustion flame temperature. Diniz et al. (24) [21] reported the same finding in their study of the different conditions of combustion synthesis to obtain samarium-doped Ni- Zn ferrites.

3 The X-ray diffractogram in ig.1 indicates that the two routes led to the direct formation of a single crystalline cubic phase of the nickel ferrite s inverse spinel. In addition, the lines of the X-ray diffraction spectrum of the powder produced by the NB route revealed a considerable increase in the reflections peak widths, indicating a reduction in the particle size and the degree of crystallinity. The average crystallite sizes calculated from X-ray line broadening, using the (d 311 ) peaks of the basal reflection and Scherrer s equation [19], were 5.7 and 18. nm, respectively, for the powders produced by the NB and NC routes. Intensity (a.u.) Nie 2 O 4 NB Intensity (a.u.) Nie 2 O 4 NC (2θ degree) (2θ degree) igure 1 X-ray powder diffraction patterns of the powders prepared by combustion reaction using different routes: NB; NC. Table 1 shows the characteristics of the powders (specific superficial area BET, particle size calculated by BET, and crystallite size calculated by DRX) prepared by combustion reaction using the NB and NC routes. The specific superficial area (BET) and size of the nanoparticles calculated from BET were m 2 /g and nm, respectively. As indicated in this table, the powder produced by the NB route presented a six-fold higher specific superficial area than that obtained by the NC route. This difference was attributed to the lower combustion flame temperature reached during the synthesis in which the Pyrex beaker was used. Table 1 Characteristics of Nie 2 O 4 powders prepared by combustion reaction Synthesization routes Specific superficial area Particle size* Crystallite size** [nm] (BET) [m 2 /g] [nm] NB NC *rom specific superficial area. **rom Scherrer s equation [19]. ig. 2 shows the morphology of the powders resulting from the synthesis by the NB and NC routes, indicating that the nanoparticles tended to be agglomerated. As can be seen, both routes led to the formation of soft agglomerates (easy deagglomeration) with an irregular morphology and fairly fine, porous particles. igure 2 Morphology of the powders prepared by combustion reaction by the two routes: NB, and NC

4 ig. 3 and 4 illustrate the characteristic hysteresis loop when the magnetization of the ferrite powders obtained by the NB and NC routes was measured as a function of the applied field at room temperature. igure 3 Room temperature hysteresis loops of synthesized Nie 2 O 4 ferrite powders obtained by the NB route; Zoom view of the plot showing the vicinity of the origin Sigma (emu/g) NC Ms=39.14 emu/g Sigma (emu/g) NC Mr= 4.2 emu/g Applied ield (Oe) Applied ield (Oe) igure 4 Room temperature hysteresis loops of synthesized Nie 2 O 4 ferrite powders obtained by the NC route; Zoom view of the plot showing the vicinity of the origin. According to igs. 3a and 4a, the powders obtained by the NB and NC routes, with particle sizes of and 2.2 nm, presented saturation magnetization values of and emu/g, respectively, with very narrow hysteresis curves, indicating the behavior of soft magnetic materials. However, the powders obtained by the NB route displayed a reduced hysteresis loop and, hence, a lower coercive field, indicating that the material was softer, which is more advantageous for high frequency transformer applications. The value of saturation magnetization (33.18 emu/g) for Nie 2 O 4 system resultant of NB route obtained by combustion reaction method was similar to the results of the saturation magnetization (33 emu/g) reported by Kinemuchi et al. [22] who the Nie 2 O 4 particles synthesized by pulsed wire discharge. However, the values of coercitivy field (46.36 Oe) obtained by NB route was inferior to the value of coercitivy field (7 Oe) reported by Kinemuchi et al. [22]. The values in such a way for coercitivy field (76.65 Oe) and saturation magnetization (39.14 emu/g) of Nie 2 O 4 system obtained by NC route had been superior to the values for this parameters reported for Kinemuchi et al. [22]. Table 2 lists the values of saturation magnetization (M s ), remanent magnetization (M r ), coercivity (H c ) and the squareness factor obtained by the two routes. It can be observed that the powders obtained by the NC route possessed a saturation magnetization of 15.2% and a coercive field of 39.5% and were therefore higher than those of the powders obtained by the NB route. This confirms that the NB route leads to powders whose characteristics lower coercive field and

5 slightly lower saturation magnetization render them more suitable for use as soft magnetic material. The M s of 15.2% results from the small particle size of this powder, for the smaller the particle size the smaller the domain area needed for spin alignment as a function of the applied magnetic field. The low coercive field, H c, confirms the material s soft ferromagnetic nature. Table 2 Magnetic parameters of powders prepared by reaction combustion using the NB and NC routes Routes M s (emu/g) M r (emu/g) H c (Oe) Squareness factor NB NC ig. 4a and b shows the MR spectra for the ferrite powders obtained by the two routes. In these experiments, when the magnetic field was parallel to the disk, only one signal was observed for the Nie 2 O 4 ferrite phase, indicating the material s single-phase characteristic. The measurements of the resonance fields were H R = koe and H R = koe, respectively, for the powders obtained by the NB and NC routes. On the other hand, the broad width of the Γ line revealed by the spectra, Γ NB = 638 Oe and Γ NC = 761 Oe, is characteristic of a material with high magnetic inhomogeneities. urther investigations of the magnetic properties are ongoing to elucidate the effective role of interparticle interactions in these samples NB 6 4 NC Signal (arb. units) Signal (arb. units) , 1,5 2, 2,5 3, 3,5 4, 4,5 Applied field (koe) 2 4 Applied field (koe) igure 4 Room temperature parallel MR spectra of ferrite powders obtained by two routes: NB, and NC Conclusions The results of this study confirm that the different synthesization routes employed to initiate and sustain the combustion reaction in the synthesis of nickel ferrite affect the characteristics of the resulting powders. The two routes resulted in nanosized powders of nickel ferrite. The combustion reaction performed in the vitreous silica basin (NC) produced highly crystalline powders with a crystallite size of 18. nm, surface area of the m 2 /g and particle size of 2.2 nm. The reaction carried out in the Pyrex beaker (NB) produced nickel ferrite powders with lower crystallinity, a large surface area of m 2 /g, particle size of nm and crystallite size of 5.7 nm. Both routes led to a very narrow hysteresis loop that introduced a behavior characteristic of soft magnetic materials. However, the powders obtained by the NC route showed a higher saturation magnetization, 15.2%, and coercive field, 39.5%, than did the powders obtained by the NB route. Therefore, the NB route led

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