Sonochemical Synthesis and Characterization of Manganese Ferrite

Oct 30, 2013 - M is the divalent (Mn2+, Fe2+, Co2+, Ni2+, Zn2+ etc.) and Fe is the trivalent (Fe3+) metal cation occupying the face-centered cubic (FC...
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Sonochemical Synthesis and Characterization of Manganese Ferrite Nanoparticles Partha Pratim Goswami,† Hanif A. Choudhury,‡ Sankar Chakma,† and Vijayanand S. Moholkar*,†,‡ †

Department of Chemical Engineering and ‡Center for Energy, Indian Institute of Technology Guwahati, Guwahati−781039, Assam, India S Supporting Information *

ABSTRACT: This paper reports sonochemical synthesis and characterization of Mn−ferrite nanoparticles using acetates precursors. Mn−ferrite synthesis requires external calcination of oxide precursors formed by sonication. pH does not play a dominant role in the synthesis. Collisions between metal oxide particles induced by shock waves generated by transient cavitation are unable to cross the activation energy barrier for the formation of ferrite. The calcination temperature is a significant parameter that influences the magnetic properties of ferrites. The size, coercivity, and saturation magnetization of ferrite particles increases with the calcination temperature. Ferrites formed at calcination temperatures of 650, 750, and 850 °C show ferromagnetic behavior with easy axis magnetization. Calcination at 950 °C leads to the formation of rods with grain growth that introduces large shape anisotropy. The magnetization curve for rods does not reach saturation, indicating paramagnetic behavior. The cause leading to this effect is nonalignment of the easy axis of magnetization with the direction of the applied magnetic field, resulting in hard axis magnetization.

1. INTRODUCTION Metal ferrites are among the most researched materials in modern day materials science, because of their potential applications in electronic gadgets, such as ignition systems, generators, vending machines, medical implants, watches, inductor core, transformer circuits, magnetic sensors, recording equipment, telecommunications, magnetic fluids and magnetic resonance imaging, microwave absorbers, and similar other applications.1−3 A major virtue of metal ferrites is high electrical resistivity, which prevents the induction of eddy currents and the resultant loss of energy. This makes ferrites the most useful materials for use in very-high-frequency fields. Ferrites also exhibit high permeability and stability in terms of temperature of operation and longevity. With reduction in particle size, the structural and magnetic properties are further enhanced. Thus, the development of new synthesis techniques for transitionmetal ferrite nanoparticles with less-expensive precursors and less hazardous and energy-intensive chemistry has been an area that has witnessed intense research activity. Spinel ferrites have a general formula (MδFe1−δ)[M1−δFe1+δ]O4, where δ is the cation distribution factor. This factor illustrates the fraction of tetrahedral (A) sites occupied by divalent metal (M2+) cations. The round and square brackets in the general formula denote the tetrahedral (A) and octahedral (B) interstitial sites. M is the divalent (Mn2+, Fe2+, Co2+, Ni2+, Zn2+ etc.) and Fe is the trivalent (Fe3+) metal cation occupying the face-centered cubic (FCC) lattice formed by O2− anions. If δ = 1, the resulting compound is called “normal” spinel, where all divalent (M) cations occupy tetrahedral (A) sites and all trivalent (Fe3+) cations occupy octahedral (B) sites. In this case, the compound can be represented by the formula (M)[Fe3+Fe3+]O4. For δ = 0, the compound is called “inverse” spinel, i.e., (Fe3+)[MFe3+]O4, in which the divalent cations occupy the B sites and the trivalent cations are equally divided among the A and leftover B © 2013 American Chemical Society

sites. For greater details on structural description of ferrites and their properties, we refer the reader to state-of-the-art reviews by Neel et al.,4 Mathew and Jiang,5 and Pullar.6 Among the transition-metal ferrite family, the most researched ferrites are zinc ferrite, cobalt ferrite, and manganese ferrite. The conventional techniques for synthesizing these ferrites are thermal treatment,7 reverse micelles,8 coprecipitation synthesis,9 microemulsion,10 sol−gel autocombustion,11 and mechanochemical,12 etc. A relatively new route for the ferrite synthesis is the sonochemical route, in which ferrites are synthesized using ultrasound irradiation (or sonication) of the reaction mixture.13−19 In our earlier papers, we have dealt with the issue of mechanistic aspects or physical mechanism of sonochemical route of zinc ferrite synthesis.20,21 In these papers, we have attempted to highlight the links between chemistry of ferrite synthesis and physics of ultrasound and cavitation bubbles by coupling experimental results with numerical simulations of cavitation bubble dynamics. In the sonochemical route, the metal oxides are synthesized by hydrolysis of the metal salt (usually acetate). The particles of metal oxide undergo in situ calcinations in reaction mixture itself, because of highly energetic collisions between them induced by shockwaves. These particles may also undergo in situ microcalcinations in the thin liquid shell surrounding the bubble where the temperature reaches high values during the transient collapse of the cavitation bubble.22−24 Taking inspiration from our earlier studies, in this paper, we have explored the sonochemical route for the synthesis of Mn− ferrite. Literature on the sonochemical, ultrasound-assisted Received: Revised: Accepted: Published: 17848

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synthesis of Mn−ferrite is limited.25−28 Our earlier papers have clearly demonstrated the mechanistic features of sonochemical synthesis route of zinc ferrite.20,21 However, whether the collisions between metal oxide particles are capable of generating conditions suitable and sufficient for formation of ferrite also depends on the pair of metal oxides involved. Every chemical reaction has its own activation energy and intrinsic kinetics. Therefore, the extent of influence of ultrasound and cavitation may also differ from system to system. We have addressed this hypothesis in the present work with sonochemical synthesis of Mn−ferrite. In addition, we have also carried out the magnetization tests of the ferrite particles synthesized through the sonochemical route.

of main experiments, which were devised based on the results of preliminary experiments. Sonication of the Reaction Mixture. A high-grade titanium alloy ultrasonic probe (diameter of 13 mm) driven at a frequency of 20 kHz (Model VCX500, 20 kHz, 500 W), using a microprocessor-controlled unit, was used to sonicate the medium. The probe was operated at 20% amplitude with a theoretical power dissipation of 100 W. However, the actual power delivered to the system depends on the acoustic impedance of the system.29 As the reaction proceeds, the composition of the liquid medium in which the sonicator probe is immersed changes. This causes a change in the acoustic impedance of the system, due to which the power delivered by the sonicator probe to the system also varies giving rise to artifacts (this effect is clearly demonstrated in our earlier paper30). This processor had facilities of automatic frequency tuning and amplitude compensation, which ensures constant power delivery to the reaction system during sonication, irrespective of the changes occurring in the medium. The actual ultrasound energy dissipation in the medium was determined using the calorimetric method.31,32 The acoustic pressure amplitude generated by the sonicator probe was found to be 1.5 bar. Synthesis of the Ferrites. The chemistry of synthesis of Mn−ferrite from metal acetate precursors under ultrasound irradiation can be described as follows.33 Initially, the hydrolysis of metal acetates generates their respective hydroxides and oxides:

2. MATERIALS AND METHODS 2.1. Materials. The following chemicals were used for the synthesis of Mn−ferrite particles: manganese acetate dihydrate (MnAc2·2H2O, AR grade, Merck), iron(II) acetate (FeAc2, AR grade, Sigma−Aldrich), sodium hydroxide pellets (NaOH, Merck), hydrochloric acid (HCl, Merck). All chemicals were used as received without any further purification. All reaction mixtures were prepared using Millipore water from Milli-Q Synthesis unit (Model Elix 3, Millipore, USA). 2.2. Methods. A schematic diagram of the experimental setup is shown in Figure 1. All experiments were conducted in a

Fe(CH3COO)2 + 2H 2O → Fe(OH)2 + 2CH3COOH Mn(CH3COO)2 + 2H 2O → MnO + 2CH3COOH + 2H 2O

It should be specifically noted that the direct formation of MnO from its acetate has also been observed by Bellin et al.34 This reaction essentially occurs in the thin liquid shell surrounding the bubble (or the bubble interface) during transient collapse, where the temperature inside the bubbles reaches extreme values (∼5000 K), and the temperature in the thin liquid shell can reach up to 600 K.35 To support the formation of MnO from its acetate, we have also analyzed the chemical equilibrium by Gibbs energy minimization for the Mn−acetate and water system for a temperature of 600 K in the Supporting Information provided with the manuscript. The ferrous hydroxide gets further oxidized to Fe3O4 during sonication. There are two alternate and parallel routes for the oxidation of Fe(OH)2 to Fe3O4, as described below: (1) It is well-known that sonolysis of water produces •OH radicals due to dissociation of water vapor entrapped in the bubble during transient collapse of cavitation bubbles. These radicals can recombine to generate H2O2 in situ.36

Figure 1. Schematic diagram of experimental setup for sonochemical synthesis of Mn−ferrite. Legend: (1) sonic processor, (2) transducer, (3) reaction mixture, (4) magnetic stirrer with hot plate, and (5) stand.

25-mL beaker made of borosilicate glass. The total reaction mixture volume in each experiment was 20 mL, with desired concentration of manganese acetate and iron(II) acetate. The sonication of the reaction mixture was carried out for 30 min in each experiment using a sonicator probe driven by a microprocessor-controlled unit. The temperature of the reaction mixture was controlled either using a refrigerated circulator (Jeotech, Model RW 0525G) for experiments at temperature lower than ambient or a magnetic hot plate (Remi Equipments, Ltd., Model 1-MLH) for experiments at elevated temperature. The details of the experimental conditions are given in Table S1 in the Supporting Information. The experiments have been performed in four protocols, out of which the first three protocols were the preliminary experiments in which we assessed the relative influence of different parameters on ferrite synthesis. The fourth protocol comprised

H 2O → H• + •OH •

OH + •OH → H 2O2

(2) Alternatively, direct thermal decomposition of hydroxide particles can also occur in the thin liquid shell surrounding the cavitation bubble, in which the temperature reaches moderately high values (473−573 K) at the 17849

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the reaction mixture in all protocols was 4.5. The pH of the solution was adjusted to the desired value by dropwise addition of either 0.1 N NaOH or 0.1 N HCl. After 30 min of sonication, the blackish solid precipitate was obtained as product. The precipitate was separated by centrifugation for 20 min at 6000 rpm (Hermile, Model Z300). The solid product was then dried in a hot air oven for overnight at 373 K and characterized using powder X-ray diffraction (XRD) (Bruker, Model Avance-D8) to assess the formation of ferrite. Depending on the result of XRD analysis, the sample was further calcined for 6 h at different calcination temperatures as mentioned in Table S1 in the Supporting Information. Characterization of Product. The structural characteristics of the solid powder obtained in all protocols was performed using a powder XRD device (Bruker, Model Avance-D8) with monochromatic Cu Kα (λ = 1.5406 Å) radiation operated at 40 kV and 40 mA in the range from 10° to 70°. The mean crystallite particle size (DXRD) of the product was calculated from the most intense peak observed at (311) using the Debye−Scherrer equation:42,43

moment of transient collapse of the cavitation bubble.35,37−39 Comparing the two routes for the formation of oxide from hydroxide, the latter route is likely to dominate, as indicated by results of simulations of cavitation bubble dynamics in our earlier study.21 The bulk temperature of reaction mixture in the experiments is 343 K. At this high temperature, large amount of water vapor enters the cavitation bubble during the expansion phase due to high vapor pressure. Consequently, significant fraction of this vapor gets entrapped in the bubble during transient collapse and cushions the collapse. Thus, the peak temperature attained in the bubble at the moment of collapse is drastically reduced to ∼1200−1500 K. At this temperature, the extent of formation of •OH radicals from dissociation of water (and their subsequent recombination to H2O2) is very less. Therefore, the formation of Fe3O4 through a H2O2-induced oxidation route is negligible. The thermal decomposition of Fe(OH)2 in the thin liquid shell surrounding the bubble can produce Fe3O4, and this is the dominant route for formation of Fe3O4. To support this, the equilibrium calculations (Gibbs energy minimization) of thermal dissociation of Fe(OH)2 have been provided in the Supporting Information.40 The transient motion of cavitation bubbles results in the formation of high-intensity shock waves. The metal oxide particles get drifted in these waves at very high velocities and collide with each other. The energy released during collision can induce reaction between the metal oxides leading to the formation of ferrite. This phenomenon can be termed as microcalcination induced by ultrasound. An experimental evidence of ultrasound- and cavitation-driven collisions of solid particles and energy release thereupon has been given by Doktycz and Suslick.41 They demonstrated melting and fusion of zinc particles in n-decane as a medium (at 288 K) under ultrasound irradiation with localized temperatures exceeding 2800 K. The reaction between two metal oxides (MnO and Fe3O4) to form Mn−ferrite is represented as

DXRD =

0.9λ β cos θ

where λ is the X-ray wavelength, β the FWHM of the relevant diffraction peak, and θ the Bragg angle. The morphology of the ferrite particles was determined using field-emission scanning electron microscopy (FE-SEM) (Model SIGMA VP, Carl Zeiss Microscopy GmbH, Germany). The magnetic characteristics of the ferrite particles were determined using vibrating sample magnetometry (VSM) (Lakeshor, Model 7410).

3. RESULTS The results have been presented in two sections, viz. experimental results in four protocols (Table S1 in the Supporting Information), followed by results on the magnetic properties of the ferrites. The XRD spectra of the solid crystalline phase products obtained in protocol 4 are given in Figure 2. The XRD spectra of the product obtained in protocols 1−3 are given in Figures S.1−S.9 in the Supporting Information. The XRD results of protocols 1(c), 2(c), and 3(c) showed weak characteristic peaks for crystallographic plane (311), indicating initiation of formation of ferrites. This trend was consistent for all three pH values from acidic (pH 4) to alkaline (pH 8) range. This result showed that sonication alone was not sufficient for complete synthesis of Mn−ferrites, and an external calcination was necessary. Moreover, this result also demonstrated that pH of the reaction mixture had an insignificant role in the chemistry of ferrite synthesis in comparison to the temperature of the reaction mixture. Therefore, in the next set of experiments, we fixed the temperature of reaction mixture to 343 K only, and, moreover, we preferred a neutral pH of reaction mixture of 7. In other Xray diffractograms for protocols 1−3, no characteristic spinel peak was seen, indicating the absence of ferrite phase. The diffraction peaks corresponding to the characteristic crystallographic planes of the spinel structure of ferrites [(112), (220), (311), (400), (422), (440)] can be seen in Figure 2 for protocol 4 in which external calcination was applied. These Miller indices indicate the single-phase MnFe2O4 with FCC.7 The other peaks were also observed which are marked with an

[0]

2Fe3O4 + 3MnO → 3MnFe2O4

However, whether the collisions between metal oxide particles will induce reaction depends on two factors; first, the energy release during collision and, second, the activation energy for the reaction. The reaction would occur only if the former exceeds the latter. Quantification of activation energy of ferrite formation as well as the temperature and energy release during inter−particle collision is beyond the scope of instruments used in this study. However, based on simulations of cavitation bubble dynamics reported in our previous study, we can conjecture that the intensity of shock waves would also be reduced at bulk temperature of 343 K in the present study, and hence, the condition for formation of ferrite as mentioned above may not be reached. In a typical experiment, stoichiometric quantities (as determined by the reaction scheme presented above) of manganese acetate dihydrate (0.692 g or 0.2 M) and iron(II) acetate (1.39 g or 0.4 M) were dissolved in 20 mL of Millipore water. The stoichiometric molar ratio of manganese acetate to iron(II) acetate was 1:2. The solution was sonicated for 30 min in a pulse mode (20 on 5 s off) cycle to avoid rise of temperature of reaction mixture. Thus, the actual sonication time was 24 min. During the reaction, the temperature of the solution was maintained at a predetermined value (mentioned in Table S1 in the Supporting Information). The initial pH of 17850

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Figure 2. X-ray diffraction (XRD) spectra of Mn−ferrite obtained in protocol 4: (A) without external calcination and (B) with external calcination at different temperatures.

Figure 3. Field-emission scanning electron microscopy (FE-SEM) images of Mn−ferrite formed with external calcination at different temperatures in protocol 4: (A) protocol 4(b), 923 K; (B) protocol 4(c), 1023 K; (C) protocol 4(d), 1123 K; and (D) protocol 4(e), 1223 K. Scale bars are as shown.

asterisk (*) and a phi symbol (Φ). The asterisk (*) indicates the presence of the hematite phase (α-Fe2O3) in the product,44 whereas the phi symbol (Φ) indicates the presence of Mn2O3.45 The average crystal sizes of the particles in these protocols, as calculated from the Debye−Scherrer formula, are given in Table 1. The FE-SEM micrographs of the solid particles formed in these protocols are given in Figure 3. It could be seen that the ferrite particles have a spherical shape, more or less, with a narrow size distribution. The particle size increases with calcination temperature. Some agglomeration seen could be attributed to high surface activity at the nanometer size range. For ferrites calcined at 1223 K, however, growth of the particles was seen with the formation of rods with μmn the micrometer size range. Dong et al.45 have also reported that, at higher temperature (1173 K), the particles start to recrystallize and grow in size. The results of the magnetization experiments are given in Table 1, which lists saturation magnetization, magnetic remanence, and coercivity of the ferrite particles obtained in different experiments of protocol 4. The magnetization curves (M−H) of the particles are given in Figure 4. It could be seen that all three magnetization factors show increasing trend with the calcination temperatures and the particle size. Moreover, the ferrite formed in protocol 4(d) (with a high calcination temperature of 1223 K, causing recrystallization of the ferrite phase) does not show saturation magnetization, at least in the range of magnetic field applied during the tests.

general description on M−H curves for material with uniaxial anisotropy with magnetic field applied along either the hard or easy axis can be found in ref 46. An introduction to the general topic of magnetic anisotropy can also be found in ref 47. 4.2. Comparative Evaluation of Different Protocols. In our previous paper,16 we had outlined the role played by calcination in the chemistry of formation of ferrites. For the zinc ferrite, the in situ calcination generated by highly energetic collisions between oxide particles is sufficient for the reaction between metal oxides. However, as seen from the results of present study, for Mn−ferrite, the in situ calcination is not sufficient. This is indicated by results of protocols 1−3. Variation in reaction temperature is seen to have marginal influence on the reaction chemistry, as mild characteristic peaks of spinel are seen at elevated temperature of 343 K. The pH of the reactions mixture (that would alter the dissociation behavior of acetates) also does not seem to affect. With application of external calcination at higher temperature and prolonged period, the metal oxides react to form ferrites. We attribute this result to the intrinsic kinetics (activation energy) of the formation of Mn−ferrites. The sonophysical effect of energetic collisions between metal oxide particles is not sufficiently intense (at least at the frequency and power levels applied in the present study) to introduce in situ calcination. 4.3. Magnetic Properties of As-Synthesized Mn− Ferrite. Figure 4 shows the magnetic hysteresis loops (or the M−H curves) of ferrite particles obtained with different calcination temperatures. The M−H curve shows hysteresis loop for particles synthesized with protocols 4(a), 4(b), and

4. DISCUSSION 4.1. Preamble: Magnetization Processes. For readers who are less conversant with the topic of magnetization, a

Table 1. Summary of Magnetic Properties of As-Synthesized Mn−ferrite calcination temperature (K)

particle size (nm)

coercivity (Oe)

magnetic saturation, Ms (emu/g)

magnetic remanence, Mr (emu/g)

remanence ratio, Ra

923 1023 1123 1223

34 42 46

1000 1200 1400 100

15 18 22

7.5 10 12.5

0.5 0.55 0.57

a

R = Mr/Ms. 17851

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Figure 4. Room-temperature M−H curves for Mn−ferrite synthesized in protocol 4: (A) protocol 4(b), 923 K; (B) protocol 4(c), 1023 K; (C) protocol 4(d), 1123 K; and (D) protocol 4(e), 1223 K.

Table 2. Comparison of the Properties of As-Synthesized Mn−Ferrite with Literature Valuesa method of synthesis this study ultrasonic wave-assisted ball milling (ref 27) ultrasound-assisted coprecipitation (ref 26) thermal treatment (ref 7) coprecipitation (ref 9) one-pot (ref 50) coprecipitation (ref 51) combustion route (ref 52) ceramic technique (ref 53) a

particle size (nm)

coercivity (Oe)

magnetic saturation, Ms (emu/g)

magnetic remanence, Mr (emu/g)

remanence ratio, R

34−46 20

1000−1400 NA

15−22 (Tcalc =923−1123 K) 29.44−74.405

7.5−12.5 NA

0.5−0.57 NA

42

NA

69 ± 1 (Treact = 355 K)

NA

3.06−15.78 (Tcalc = 996−1146 K) 16.36 (Tcalc = 873 K) 56 (Treact = 423 K) 2.046−26.48 (Tcalc = 873−1473 K) 80 (Treact = 623 K) 62 (Tcalc = 1173−1573 K)

NA 8 38 0.23−0.62 50 ∼10−45

12−22 26.53 35−50 112−139 30 ∼7000

910−1470 NA NA 66.61−17.85 82 ∼700−1500

NA NA 0.49 0.67 0.11−0.02 0.625 0.16−0.73

Table legend: NA = not available, Tcalc = calcination temperature, and Treact = temperature of reaction mixture during sonication. bR = Mr/Ms.

and 4C with the theoretical curves46 reveals that the particles undergo easy direction magnetization. By correlating this behavior with particle shape, one can easily perceive that, because of the almost-spherical shape at lowest calcination temperature, there will be six easy axes of magnetization, leading to the lowest coercivity. As the calcination temperature increases, the particles grow and their shape deviates to a greater extent from spherical, leading to a rise in coercivity. But still, the rise in coercivity for calcination temperatures of 923− 1123 K is rather moderate (∼40%). The SEM image of the sample calcined at 1223 K confirms a drastic change in the size and shape of the particles, i.e., the

4(c), and the saturation magnetization and the coercivity value increases as the calcination temperature increases up to 1123 K. This essentially means that the value of Ku, as defined by the equation f = K u cos 2 θ − MsH cos θ

increases with calcination temperature. According to Herzer,41 the coercivity, the saturation magnetization, and the field required for the saturation depends strongly on the particle size (which, in turn, is a function of the heat-treatment temperature) which influences the magnetic exchange interaction.48,49 Comparing the shapes of the M−H curves in Figures 4A, 4B, 17852

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coercivity, and low remanence ratio, which make them more versatile for application in electronic gadgets. We believe that results of this preliminary study could form useful guidelines for further research in the area of sonochemical manganese ferrite synthesis.

spherical particles agglomerate and grow preferentially in one direction (i.e., length) to form rodlike structures. However, it is also clear from the SEM image that these rods are randomly distributed in the mixtures and their length axes do not coincide. The conversion of spherical particles to rodlike shape creates large shape anisotropy with easy direction of magnetization being along the length axis of the rod, and the hard direction of magnetization along the radial axis, i.e., perpendicular to the length axis.44 Even in completely random distribution of the ferrites rods, it could be possible that some of these rods have their length axes in the same direction as the applied external magnetic field. These ferrite rods can be easily tune with the magnetic field; as a result, the M−H curve depicted in Figure 4D shows a small loop close to the origin with coercivity of only 100 Oe. However, for remaining ferrite rods, the length axis has an angle θ > 0° with the direction of the magnetic field, and, in extreme situations, the easy axis of magnetization (or the length axis) and the direction of magnetic field could be perpendicular to each other. This situation resembles the theoretical M−H curve for hard axis magnetization. In this case, the applied magnetic field must have very high strength (beyond the strength of VSM instrument used in this study) to tune all domains of the ferrite rods along the easy axis, i.e., parallel to the direction of applied magnetic field, to reach saturation. This essentially results in a linear M−H curve up to the maximum strength of the applied field (20 000 Oe), resembling Figure 4D. The magnetic properties of the Mn−ferrite obtained by sonochemical synthesis have been compared with those obtained from conventional methods in Table 2. It could be seen that the sonochemical synthesized particle have rather uncommon combination of small particle size (30−50 nm), high coercivity (1000−1400 Oe), and small remanence ratio (0.5). Because of these combinations, the sonochemicaly prepared particles are likely to be useful for more diverse applications in electronic and electrical gadgets.

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ASSOCIATED CONTENT

S Supporting Information *

The following Supporting Information has been provided with the manuscript: (1) X-ray diffractograms of solid product obtained in protocols 1, 2, and 3; (2) experimental protocols and observation (Table S1); and (3) FACTSAGE results. This material is available free of charge via the Internet at http:// pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*Tel.: +91 361 258 2258. Fax: +91 361 258 2291. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS One of the authors (V.S.M.) gratefully acknowledges stimulating discussions and inputs from Prof. M. Sivakumar (University of Nottingham, Malaysia) regarding sonochemical synthesis of ferrites. Authors sincerely acknowledge the Central Instruments Facility (CIF), IIT Guwahati for FESEM and VSM analyses to characterize the materials. The authors are also grateful to anonymous referees of this paper for their meticulous assessment and constructive criticism.

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REFERENCES

(1) Snelling, E. C. Soft Ferrites: Properties and Applications, 2nd Edition; Butterworth Publishing: London, 1989. (2) Jiles, D. C. Introduction to Magnetism and Magnetic Materials, 2nd Edition; Chapman and Hall: London, 1991. (3) Willard, M. A.; Kurihara, L. K.; Carpenter, E. E.; Calvin, S.; Harris, V. G. Chemically prepared magnetic nanoparticles. Int. Mater. Rev. 2004, 49, 125. (4) Neel, L. Magnetic properties of ferrites: Ferrimagnetism and antiferromagnetism. Ann. Phys. Paris 1948, 3, 137. (5) Mathew, D. S.; Jiang, R. S. An overview of the structure and magnetism of spinel ferrite nanoparticles and their synthesis in microemulsion. Chem. Eng. J. 2007, 129, 51. (6) Pullar, R. C. Hexagonal ferrites: A review of the synthesis, properties and applications of hexaferrite ceramics. Prog. Mater. Sci. 2012, 57, 1191. (7) Naseri, M. G.; Saion, E. B.; Ahangar, H. A.; Hashim, M.; Shaari, A. H. Synthesis and characterization of manganese ferrite nanoparticles by thermal treatment method. J. Magn. Magn. Mater. 2011, 323, 1745. (8) Rondinone, A. J.; Liu, C. Determination of magnetic anisotropy distribution and anisotropy constant of manganese spinel ferrite nanoparticles. J. Phys. Chem. B 2001, 105, 7967. (9) Elahi, I.; Zahira, R.; Mehmood, K.; Jamil, A.; Amin, N. Coprecipitation synthesis, physical and magnetic properties of manganese ferrite powder. Afr. J. Pure Appl. Chem. 2012, 6, 1. (10) Yener, D. O.; Giesche, H. Synthesis of pure and manganese-, nickel-, and zinc doped ferrite particles in water-in-oil microemulsions. J. Am. Ceram. Soc. 2001, 84, 1987. (11) Winiarska, K.; Szczygieł, I.; Klimkiewicz, R. Manganese−zinc ferrite synthesis by the sol−gel autocombustion method. Effect of the precursor on the ferrite’s catalytic properties. Ind. Eng. Chem. Res. 2013, 52, 353.

5. CONCLUSION In this work, we have demonstrated the synthesis of Mn−ferrite with sonochemical methods. Unlike the zinc ferrite, the Mn− ferrite synthesis requires external calcination of the metal oxide precursors, which are formed during sonication from hydrolysis of metal acetates with further oxidation of hydroxides. Quite interestingly, the pH of reaction mixture does not play a dominant role in the chemistry of ferrite synthesis. The collisions between metal oxide particles in the present situation are unable to cross the activation energy barrier, leading to the formation of ferrite. The particle size and coercivity, as well as saturation magnetization, of ferrite particles increases with calcination temperature. Calcination at very high temperatures (1223 K) causes elongation of ferrite particles due to recrystallization and grain growth, leading to the formation of rodlike structures. This introduces large shape anisotropy. This also generates an easy axis of magnetization along the length of the ferrite rods. However, these rods are randomly distributed in the solid samples. The length axis of all rods is not aligned parallel to the direction of applied magnetic field. As a result, the magnetization process is not along the easy axis: the magnetization results in hard axis magnetization. The M−H curve for the ferrite rod does not reach saturation, even for the highest strength of the applied magnetic field. Sonochemically synthesized ferrite particles are found to have unique combination of properties of nanometer size range, moderate 17853

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dx.doi.org/10.1021/ie401919x | Ind. Eng. Chem. Res. 2013, 52, 17848−17855

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NOTE ADDED IN PROOF Following publication of a previous version of this paper as a Just Accepted Manuscript (JAM), the authors removed the majority of Section 4.1, Figure 5, and Appendix A, given extensive overlap with published materials (see refs 46 and 47).

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dx.doi.org/10.1021/ie401919x | Ind. Eng. Chem. Res. 2013, 52, 17848−17855