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Sonochemical assisted synthesis of SrFe12O19 nanoparticles
Ultrasonics Sonochemistry 29 (2016) 470–475
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Ultrasonics Sonochemistry journal homepage: www.elsevier.com/locate/ultson
Sonochemical assisted synthesis of SrFe12O19 nanoparticles R.L. Palomino a, A.M. Bolarín Miró a, F.N. Tenorio a, F. Sánchez De Jesús a,⇑, C.A. Cortés Escobedo b, S. Ammar c a
Área Académica de Ciencias de la Tierra y Materiales, UAEH, Carr. Pachuca-Tulancingo Km. 4.5, C.P. 42184 Pachuca, Hidalgo, Mexico Centro de Investigación e Innovación Tecnológica del Instituto Politécnico Nacional, Cerrada Cecati s/n Col Sta. Catarina, C.P. 02250 Azcapotzalco, D.F., Mexico c ITODYS, UMR-CNRS 7086, Université de Paris-Diderot, 75205 Cedex 13 Paris, France b
a r t i c l e
i n f o
Article history: Received 9 July 2015 Received in revised form 25 October 2015 Accepted 30 October 2015 Available online 31 October 2015 Keywords: Strontium hexaferrite Sonochemical synthesis Sonochemistry Hexaferrite nanoparticles SrFe12O19
a b s t r a c t We present the synthesis of M-type strontium hexaferrite by sonochemistry and annealing. The effects of the sonication time and thermal energy on the crystal structure and magnetic properties of the obtained powders are presented. Strontium hexagonal ferrite (SrFe12O19) was successfully prepared by the ultrasonic cavitation (sonochemistry) of a complexed polyol solution of metallic acetates and diethylene glycol. The obtained materials were subsequently annealed at temperatures from 300 to 900 °C. X-ray diffraction analysis shows that the sonochemical process yields an amorphous phase containing Fe3+, Fe2+ and Sr2+ ions. This amorphous phase transforms into an intermediate phase of maghemite (c-Fe2O3) at 300 °C. At 500 °C, the intermediate species is converted to hematite (a-Fe2O3) by a topotactic transition. The final product of strontium hexaferrite (SrFe12O19) is generated at 800 °C. The obtained strontium hexaferrite shows a magnetization of 62.3 emu/g, which is consistent with pure hexaferrite obtained by other methods, and a coercivity of 6.25 kOe, which is higher than expected for this hexaferrite. The powder morphology is composed of aggregates of rounded particles with an average particle size of 60 nm. Ó 2015 Elsevier B.V. All rights reserved.
1. Introduction M-type hexagonal ferrites have been used extensively as permanent magnets and high-density magnetic recording media, among other interesting applications in recent decades; they are currently used in microwave devices at frequencies in the gigahertz range [1,2]. Although many other hard magnetic materials have been developed, the hexaferrite performance/cost ratio remains extremely favorable. Strontium hexaferrites are ferromagnetic materials with the formula MFe12O19, where M is the divalent Sr ion [3]. The magnetic properties of strontium hexaferrite depend on its structure and particle and crystallite sizes, which depend on the synthesis method used. This material possesses a high magneto-crystalline anisotropy (MCA), which occurs when the induced magnetization has a preferred orientation within the crystal structure. Because of their magnetization (60 emu/g) and high coercivities (5.5 kOe) [3], strontium hexaferrites are commonly used as permanent magnetic materials in modern devices [4], such as loud speakers, permanent magnetic motors, coil instruments (galvanometer, voltmeter and ammeter) and microphones [7], as ⇑ Corresponding author. E-mail address: [email protected] (F. Sánchez De Jesús). http://dx.doi.org/10.1016/j.ultsonch.2015.10.023 1350-4177/Ó 2015 Elsevier B.V. All rights reserved.
well as being used to induce magnetic hyperthermia [5] and to generate hybrid photocatalysts [6]. SrFe12O19 has excellent characteristics because of its structure. As a typical magnetoplumbite type of hexagonal ferrite [8], Sr-hexaferrite is built from smaller units: a cubic block S with a spinel-type structure and a hexagonal block R that contains the Sr ions. The hexagonal Sr-ferrite has 24 magnetic Fe3+ ions per unit cell, which are distributed on five crystallographic sites [9]: three octahedral sites, 12 k, 2a and 4f2; one tetrahedral site, 4f1; and one trigonal bi-pyramidal site, 2b [10]. The chemical function has 12 Fe3+ ions, four of which have spins in the downward direction at 4f1 (2 Fe3+) and 4f2 (2 Fe3+), whereas the other 8 Fe3+ ions have spins in the upward direction at 12 k (6 Fe3+), 2a (1 Fe3+) and 2b (1 Fe3+). Because of its complicated crystal structure, the synthesis of Sr-ferrite represents a major challenge regardless of the selected method, particularly when a nanoparticle is the target of the study [11]. Because of the interest in these materials, nanostructured hexaferrites have been obtained using different methods. The conventional and oldest one is by calcination and sintering of a mixture of oxides or carbonates in a furnace at 1300 °C [12], but this technique produces large particles and consumes extensive energy. There are other synthesis methods that involve wet chemistry:
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co-precipitation [13], hydrothermal synthesis [14], mechanosynthesis [3,15] and sol–gel [16], among others. Recent studies have focused on different approaches to enhance the magnetic properties: substituting the cation positions in this type of structure [17–19], and synthesizing nanoparticles [20,21], which is ideal for magnetic performance. However, it is difficult to obtain ultrafine and monodisperse particles by commercial ceramic methods because they involve high temperatures (>1300 °C) that produce unavoidable particle growth. The use of ultrasound to produce nanoparticles has been a research topic of great interest in recent years [22–31]. In sonochemistry, molecules undergo a chemical reaction caused by the application of ultrasound radiation (20 kHz–10 MHz) [22]. This radiation induces the phenomenon of acoustic cavitation, which consists of the formation, growth, and implosive collapse of air bubbles in an irradiated liquid due to the relaxation and compression of a liquid solvent. These bubbles implode and cause shockwaves, which structurally modify the ultrasound-irradiated items [23]. Cavitation can produce a temperature of 5000 K over the course of a nanosecond, large cooling rates (1011 Ks1) and pressures greater than 1800 kPa [24]. The sonochemical route has emerged as a potential alternative to synthesize ferrites of nanometric size [25–28]. Recently, Junliang et al. [29] synthesized hexaferrite via ultrasonic irradiation, which was used to assist a co-precipitation method after calcination at 800 °C. Others studies have focused on demonstrating the influences of ultrasonic irradiation variables (power and time) on the shape and size of the synthesized ferrite nanoparticles [30–32]. Different studies have verified the ability of ultrasounds to induce chemical reactions, particularly through the formation of reactive radicals. In addition, the use of ultrasound is eco-friendly because all solvents can be reused, and it is more economic than other synthesis routes [22–28]. According to some authors [30–33], the synthesis of Srhexaferrite by ultrasonicating a polyol solution can be represented as follows: a) The salts are completely dissolved in diethylene glycol (DEG) with a controlled quantity of water, and the hydrolysis of iron acetate in an aqueous medium ensues, according to Eq. (1): DEG
ðC2 H3 O2 Þ2 Sr þ 12ðC2 H3 O2 Þ2 Fe þ H2 O ! Srþ2 þ2
þ 12Fe
þ 26ðC2 H3 O2 Þ þ H þ OH þ
ð1Þ
Water is added to promote the hydrolysis of the metals, which is necessary to synthesize the oxides. b) After that, when the solution is sonicated, a strong oxidant (H2O2) it is generated via the cavitation effect [24,30]. Then, the oxidant H2O2 is suggested to initiate the oxidation of Fe2+ to Fe3+ [32]. c) Finally, annealing promotes the reaction between the metallic ions and H2O2 to yield strontium hexaferrite, according to the following reaction: ;D
Sr2þ þ 12Fe2þ þ 19H2 O2 ! SrFe12 O19 þ 19H2 O
ð2Þ
This work aims to produce SrFe12O19 nanoparticles using sonochemistry and a subsequent annealing at low temperature. This method is proposed as a simple, economical and eco-friendly synthesis for this type of nanomaterial. Moreover, the reaction mechanism behind Sr-hexaferrite nanoparticle formation was studied using the sonochemical-assisted method. 2. Experimental To obtain SrFe12O19, stoichiometric amounts of (C2H3O2)2Sr (purity 99.995% Sigma Aldrich) and (C2H3O2)2Fe (purity 99.8%
471
Fig. 1. Schematic representation of the sonochemical setup.
Sigma Aldrich), according to Eq. (2), were dissolved in 50 mL of a solution of diethylene glycol (DEG, C4H10O3, purity 99.8% Sigma Aldrich) with water (weigh ratio of DEG:water was 2:1) for 30 min under mechanical stirring in air atmosphere. A solution without sonication was studied as a control experiment, as Sr2+ and Fe2+ ions were completely dissolved for 30 min of stirring time, solution was stirred for 5 h until observation of particles in suspension. The solutions were subjected to different sonication times from 10 min to 3 h using an Ultrasonic Homogenizer Q700 sonicator. Fig. 1 shows the schematic diagram of the sonochemical experiment setup. A water bath was used to control the reaction temperature. The solution temperature was keep at 25 °C during irradiation. After the sonochemical step, the powders were washed multiple times by forming a suspension in ethanol and centrifugation at 16,000 rpm (three times) for 15 min. Then, the powders were dried at 80 °C in air. At this point the powders were named ‘‘as obtained.” After that, the powders were annealed at different temperatures from 300 to 900 °C in air. The obtained powders were characterized by X-ray diffraction (XRD) using an Inel Equinox 2000 diffractometer with Co Ka1 (k = 1.7890100 Å) radiation. The diffraction pattern was collected over a 2h interval of 20–80° with increments of 0.02 (2h). Rietveld refinements were performed on the X-ray diffraction patterns to obtain the percentages of different phases, crystallite sizes and microstrains of the powders. This method considers all of the collected information in a diffraction pattern and uses a least-squares approach to refine the theoretical line profile until it matches the measured profile [34]. Crystallographic data were obtained from the Crystallography Open Database (COD) [35] Scanning electron microscopy (SEM) using a JEOL-100-CX II was used to determine the morphologies and qualitatively characterize the particle sizes. The stabilities of the synthesized powders were measured by studying their thermal behaviors in a differential scanning calorimeter and a thermogravimetric analyzer (DSC/TGA 851e Mettler-Toledo). The experiments were performed with a heating rate of 10 Kmin1 using an air flow of 6 103 m3s1. Magnetization studies were performed at room temperature using a MicroSense EV7 vibrating sample magnetometer (VSM) with a maximum field of ±18 kOe.
3. Results and discussion Fig. 2 shows the X-ray diffraction patterns of the powders obtained by applying 10 min of sonication and annealing at temperatures from 300 to 900 °C. Also, the XRD pattern corresponding to the powder obtained without applying sonication
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Fig. 2. X-ray diffraction (XRD) patterns of the powders that were obtained using the sonochemistry process for 10 min and heat-treated at different temperatures.
Fig. 3. X-ray diffraction (XRD) patterns of the powders that were obtained using the sonochemistry process for 1 h and heat-treated at different temperatures.
and mechanically stirred for 5 h is included. As it is observed in the XRD pattern of the ‘‘sonication time = 0 min” curve, reflection peaks corresponding to goethite, a-FeOOH (COD #2211652, Pbnm), are detected. This iron (III) oxide hydroxide is generated when Fe (OH)2 (formed by the hydrolysis of iron acetate) is oxidized with oxygen from air atmosphere introduced during the mechanical stirring. Besides, in the XRD pattern of the ‘‘as obtained” powder (without heat treatment), there is only a broad peak, which is associated with a non-crystalline structure (amorphous). Therefore, the formation of strontium hexaferrite SrFe12O19 did not occur during the sonochemical process, as shown by the absence of the corresponding SrFe12O19 diffraction peaks in the X-ray diffraction pattern. As was expected, sonochemistry does not carry the reaction to completion Eq. (2), which corresponds to the formation of strontium hexaferrite; thus, an annealing process is required. The XRD patterns of the annealed powders are shown in Fig. 2. As can be observed, when the powder is annealed at 300 °C the peaks that correspond to maghemite, c-Fe2O3 (COD 9012692, P43-21:2), are detected. Additionally, there is substantial peak broadening, which is associated with nanosized particles. Increasing the annealing temperature produces new phases at 800 °C: c-Fe2O3 (COD #9012692, P43-21:2) and Fe2Sr2O5 (COD 2002240, Ibm2), and at 900 °C: FeO (COD 9002673, Fd-3 m). Despite the annealing treatment at high temperatures, no peaks that are associated with strontium hexaferrite are detected using XRD. Therefore, under these experimental conditions, hexaferrite cannot be synthesized. When the irradiation time is increased to 1 h, the results change, as shown in Fig. 3, which presents the XRD patterns of the powders that were annealed at temperatures from 300 to 900 °C. Similar to the previous experimental conditions, the powder named ‘‘as obtained” consists of an amorphous phase. According to some authors [25,30], sonochemistry or ultrasonic irradiation of water produces the free radicals H and OH that can combine to produce H2O2, which is a strong oxidant. Thus, it is suggested that the produced oxidizing agent initiates the partial oxidation of Fe2+ as follows:
(c-Fe2O3), which is generated by a crystallization process, as follows:
2Fe2þ þ H2 O2 ! 2Fe3þ þ 2OH
ð3Þ
This reaction is spontaneous because its redox potential (DE°) is positive. Therefore, the obtained amorphous powder using sonochemistry consists of a mixture of amorphous oxides: Fe3+, Fe2+ and Sr2+. When the ‘‘as obtained” powder is annealed at 300 °C, as can be observed in Fig. 3, the amorphous phase transforms to maghemite
D
Fe2þ þ Fe3þ þ 6OH ! cFe2 O3 þ 3H2 O "
ð4Þ
If the annealing temperature is increased (>300 °C), the c-Fe2O3 transforms spontaneously to hematite, a-Fe2O3 (COD 9015964, R-3c:H), due to the standard Gibbs free energy of the reaction is negative (14.4 kJ mol1) [37]. The transformation of maghemite to hematite is considered to be a topotactic transformation, and it is favored at high temperatures [38]. For temperatures up to 700 °C, strontium hexaferrite, SrFe12O19 (COD 1008364, P63/mmc), forms and is accompanied by small quantities of the unreacted hematite phase, a-Fe2O3. The synthesis of strontium hexaferrite is completed at 800 °C and is given by the following: D
6Fe2 O3 þ SrO ! SrFe12 O19
ð5Þ
Table 1 shows the Rietveld refinement results for the volume percentage of each phase, the crystallite size and the microstrain, all of which were obtained from the XRD pattern analyses. These results confirm that a nanocrystalline material was obtained using sonochemistry. Furthermore, these results support the proposed mechanism and quantify the phases that are present. The XRD patterns in Fig. 3 show no peaks corresponding to strontium compounds because the Sr:Fe atomic ratio was 1:12, and the SrO volume was much lower than the magnetite/maghemite volume. This means that the SrO reflections are beyond the detection limit of the instrument. To determine the effect of sonication time, an additional experiment that increased this parameter to 3 h was performed. Fig. 4 shows the XRD patterns of the powders that were synthesized by sonochemistry for 3 h and annealed at temperatures from 300 to 900 °C. An interesting difference was found; the ‘‘as obtained” powder shows crystalline maghemite (c-Fe2O3) together with hematite (a-Fe2O3). These species exhibit a remarkable peak broadening, which is associated with the nanosize of the particles. The average crystallite size, which was calculated by Rietveld refinement, is approximately 5 nm and 60 nm for maghemite and hematite, respectively, and the microstrain is 0.0164 and 0.0002 nm for maghemite and hematite, respectively. Therefore, increasing the sonication time promotes crystallization of the amorphous phases, a result that is in agreement with other authors [27,32]. It is also likely that extended sonication decreases the
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Table 1 Data from the Rietveld refinements of the XRD patterns of the powders, which were obtained using the sonochemistry process for 1 h and annealed at different temperatures. Heat treatment (°C)
Phases
# COD
300
P43-21:2, c-Fe2O3 Fm-3m, SrO P43-21:2, c-Fe2O3 R-3c:H, Fe2O3 Fm-3m, SrO P63/mmc, SrFe12O19 R-3c:H, Fe2O3 P63/mmc, SrFe12O19 P63/mmc, SrFe12O19
9,012,692 1,011,328 9,005,813 9,000,139 1,011,328 1,008,856 9,015,964 1,008,856 1,008,856
500
700 800 900 *
% Vol.
Crystallite size (Å)
Microstrain (adim) 4.07 105
100
330
*
*
48.16 51.84
175 53
*
*
84.6 15.4 100 100
507 433 570 595
*
0.0028 3 107 *
0.0013 0.0004 0.0017 0.0018
Beyond detection limit.
Fig. 4. X-ray diffraction (XRD) patterns of the powders that were obtained using the sonochemistry process for 3 h and heat-treated at different temperatures.
nuclei size because the crystallite size of the ‘‘as obtained” powder is smaller than those obtained with shorter sonication times. The XRD patterns of the annealed powders sonicated for 3 h confirm the proposed mechanism for the sonochemical reaction. To confirm the reaction’s mechanism, a thermal analysis of the ‘‘as obtained” powder sonicated for 1 h was performed. The TGA curve of the ‘‘as obtained” sample is shown in Fig. 5. It indicates three stages of weight loss, one down to 100 °C, another between 170 and 315 °C and the last stage starts at 550 °C. The initial weight loss up to a temperature of 100 °C could be ascribed to the evaporation of the water of hydration or the loss of –OH groups on the surface of iron oxide. The second stage displays an abrupt weight loss that is attributed to the crystallization of the amorphous material obtained from the sonochemical process. Here, H2O is liberated as a result of maghemite formation, as is shown in Eq. (4). The final stage of weight loss may be assigned to the Sr-hexaferrite synthesis and is completed at 642 °C because there is no significant weight change above this temperature. Moreover, Fig. 6 depicts the DSC curve from 30 °C to 700 °C for the ‘‘as obtained” powder sonicated for 1 h. At 98.9 °C, an endothermic peak is observed, which is ascribed to the removal of weakly bound groups (H2O or –OH). After that, two sharp exothermic peaks are observed: the first peak at 243 °C
Fig. 5. TGA curve of the ‘‘as obtained” powder, which was synthesized by sonochemistry for 1 h (without annealing).
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Fig. 6. DSC curve of the ‘‘as obtained” powder, which was synthesized by sonochemistry for 1 h (without annealing).
Fig. 7. Magnetic hysteresis loops of the powders, which were synthesized by sonochemistry for 1 h and annealed at different temperatures.
corresponds to the formation/crystallization of c-Fe2O3 nanoparticles, and the second peak, which appears between 290 and 320 °C, corresponds to the topotactic transformation of c-Fe2O3 to a-Fe2O3 [38]. Finally, a broad peak at approximately 622 °C corresponded to the Sr-hexaferrite formation. These results confirm the previously proposed reactions for the formation of SrFe12O19 by heat treatment-assisted sonochemistry. Fig. 7 presents the magnetic hysteresis loops for the powders that were synthesized by sonochemistry for 1 h and annealed at different temperatures. As observed in Fig. 7, the ‘‘as obtained” powder shows a very small value of magnetization (1 emu/g), which is associated with the magnetic behavior of an amorphous phase. The powders annealed at 300 °C show a higher value of specific magnetization, 33.37 emu/g at 18 kOe, which represents the magnetic behavior of the c-Fe2O3 nanoparticles [36] and the SrO (diamagnetic). When the temperature was increased to 500 °C, a reduction in specific magnetization was detected with respect to that observed for the sample treated at 300 °C. This result is associated with the partial transformation of c-Fe2O3 to hematite (a-Fe2O3). At 700 °C, the material exhibits a radically different magnetic behavior compared with those annealed at low temperatures. This material presents a high coercivity of 5.57 kOe and a strong specific magnetization (51 emu/g at
Fig. 8. SEM micrograph of the powder, which was synthesized using sonochemistry for 1 h and annealed at 800 °C for 1 h.
18 kOe), which is similar to the behavior of Sr-hexaferrite. However, its specific magnetization is lower than the values typically reported for nanostructured Sr-hexaferrites, because this compound is diluted by hematite (a-Fe2O3), as confirmed by the XRD results in Fig. 3. Therefore, at this temperature, the hexaferrite synthesis is not yet complete and considerable amounts of a-Fe2O3 are present (15.4% in vol. according to the Rietveld refinement shown in Table 1). Moreover, it is observed in Fig. 7 that the powder annealed at 800 °C presents a typical specific saturation magnetization for Sr-hexaferrite (62.3 emu/g at 18 kOe) and a high coercivity value (6.25 kOe), which is greater than the reported values for this type of hexaferrite [1]. When the temperature is increased to 900 °C, a slight increase in the specific magnetization is produced, but it is not relevant because by this point the Sr-hexaferrite had been completely formed. Furthermore, the coercivity increase confirms the nanosize of the particles and the single domain behavior. Finally, Fig. 8 shows the micrograph that corresponds to the hexaferrite powder, which was synthesized using ultrasound for 1 h and annealed at 800 °C. The powder morphology is composed of aggregates of rounded particles; it is possible to qualitatively infer an average particle size of 60 nm. Furthermore, particle size is one of the most influential parameters capable of modifying the coercivity of magnetic materials, and its effect depends on the transition from the single-domain state to the multi-domain state. The critical diameter of the particle that separates the magnetic states (single- and multi-domain) for SrFe12O19 is 367.22 nm [10]. Therefore, the strontium hexaferrites obtained with this method are expected to exhibit single-domain behavior. 4. Conclusions Crystalline hexaferrite nanoparticles were successfully synthesized by ultrasonicating a polyol solution for 1 h and then annealing the product at 800 °C. These results confirm that sonochemistry is a convenient method to prepare strontium hexaferrite nanoparticles by decreasing the sintering temperature, compared with the temperatures commonly required for solid-state techniques. A mechanism for the synthesis of Sr-hexaferrite nanoparticles is proposed: during the sonochemical process (1 h), a mixture of amorphous oxides of Fe3+, Fe2+ and Sr2+ is obtained; the partial oxidation of Fe2+ to Fe3+ is due to the formation of H2O2. At approximately 300 °C, maghemite (c-Fe2O3) crystallization occurs. When the temperature is increased, a-Fe2O3 is formed through a topotactic transition. At 700 °C, Fe2+ is completely oxidized to Fe3+, and a-Fe2O3 is obtained, which reacts with SrO to partially
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form hexaferrite. The hexaferrite formation is completed at 800 °C. The obtained hexaferrite nanoparticles have a higher specific magnetization (62 emu/g at 18 kOe) and a larger coercive field (6.25 kOe) than the values reported by other methods. This consistency is indicative of the single-domain nature of the obtained powder and its particle size, which was confirmed by electron microscopy. For high sonication time (up to 3 h), the ultrasonic cavitation promotes the crystallization of amorphous phases and also decreases the nuclei size because the crystallite size of the ‘‘as obtained” powder is smaller than those obtained using shorter sonication times. References [1] R.C. Pullar, Hexagonal ferrites: a review of the synthesis, properties and applications of hexaferrite ceramics, Prog. Mater. Sci. 57 (2012) 1191–1334. [2] R. Valenzuela, Novel applications of ferrites, Phys. Res. Int. 2012 (2012) 1–9. Article ID 591839. [3] A.M. Bolarín Miró, F. Sánchez De Jesús, C.A. Cortés Escobedo, S. Díaz De la Torre, R. Valenzuela, Synthesis of M-type SrFe12O19 by mechanosynthesis assisted by spark plasma sintering, J. Alloys Compd. 643 (2015) 226–230. [4] J.R. Liu, R.Y. Hong, W.G. Feng, D. Badami, Y.Q. Wang, Large-scale production of strontium ferrite by molten-salt-assisted coprecipitation, Powder Technol. 262 (2014) 142–149. [5] A.U. Rashid, P. Southerm, J.A. Darr, S. Awan, S. Manzoor, Strontium hexaferrite (SrFe12O19) based composites for hyperthermia applications, J. Magn. Magn. Mater. 344 (2013) 134–139. [6] A. Abd Aziz, Y. Harng Yau, G. Li Puma, C. Fischer, S. Ibrahim, S. Pichiah, Highly efficient magnetically separable TiO2-graphene oxide supported SrFe12O19 for direct sunlight-driven photoactivity, Chem. Eng. J. 235 (2014) 264–274. [7] S. Shakoor, M. Naeem Ashiq, M. Aslam Malana, A. Mahmood, M. Farooq Warsi, M. Najam-ul-Haq, N. Karamat, Electrical, dielectric and magnetic characterization of Bi–Cr substituted M-type strontium hexaferrite nanomaterials, J. Magn. Magn. Mater. 362 (2014) 110–114. [8] Y.F. Xu, Y.Q. Ma, S.T. Xu, F.L. Zan, G.H. Zheng, Z.X. Dai, Effects of vacancy and exchange-coupling between grains on magnetic properties of SrFe12O19 and aFe2O3 composites, Mater. Res. Bull. 27 (2014) 13–18. [9] P. Jihoon, H. Yang-Ki, K. Seong-Gon, K. Sungho, S.I. Laalitha, L. Jaejin, A. SungYong, Maximum energy product at elevated temperatures for hexagonal strontium ferrite (SrFe12O19) magnet, J. Magn. Magn. Mater. 355 (2014) 1–6. [10] B.K. Rai, S.R. Mishra, V.V. Nguyen, J.P. Liu, Synthesis and characterization of high coercivity rare-earth ion doped Sr0.9RE0.1Fe10Al2O19 (RE: Y, La, Ce, Pr, Nd, Sm, and Gd), J. Alloys Compd. 550 (2013) 198–203. [11] Z. Zhipeng, W. Zhanyong, W. Xiuting, W. Xiaorui, Z. Jingshi, D. Fengkai, X. Jiayue, Differences in the structure and magnetic properties of Sr1x REx Fe12 O19 (RE: Pr and Dy) ferrites by microwave-assisted synthesis method, J. Alloys Compd. 610 (2014) 264–270. [12] K. Samikannu, J. Sinnappan, S. Mannarswamy, T. Cinnasamy, K. Thirunavukarasu, Synthesis and magnetic properties of conventional and microwave calcined strontium hexaferrite, Mater. Sci. Appl. 2 (2011) 638–642. [13] M.M. Rashad, I.A. Ibrahim, A novel approach for synthesis of M-type hexaferrites nanopowders via the co-precipitation method, J. Mater. Sci.: Mater. Electron 22 (2011) 1796–1803. [14] M. Jean, V. Nachbaur, J. Bran, J.M. Le Breton, Synthesis and characterization of SrFe12O19 powder obtained by hydrothermal process, J. Alloys Compd. 496 (2010) 306–312. [15] F. Sánchez De Jesús, A.M. Bolarín Miró, C.A. Cortés Escobedo, R. Valenzuela, S. Ammar, Mechanosynthesis, crystal structure and magnetic characterization of M-type SrFe12O19, Ceram. Int. 40 (2014) 4033–4038.
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Contents lists available at ScienceDirect
Ultrasonics Sonochemistry journal homepage: www.elsevier.com/locate/ultson
Sonochemical assisted synthesis of SrFe12O19 nanoparticles R.L. Palomino a, A.M. Bolarín Miró a, F.N. Tenorio a, F. Sánchez De Jesús a,⇑, C.A. Cortés Escobedo b, S. Ammar c a
Área Académica de Ciencias de la Tierra y Materiales, UAEH, Carr. Pachuca-Tulancingo Km. 4.5, C.P. 42184 Pachuca, Hidalgo, Mexico Centro de Investigación e Innovación Tecnológica del Instituto Politécnico Nacional, Cerrada Cecati s/n Col Sta. Catarina, C.P. 02250 Azcapotzalco, D.F., Mexico c ITODYS, UMR-CNRS 7086, Université de Paris-Diderot, 75205 Cedex 13 Paris, France b
a r t i c l e
i n f o
Article history: Received 9 July 2015 Received in revised form 25 October 2015 Accepted 30 October 2015 Available online 31 October 2015 Keywords: Strontium hexaferrite Sonochemical synthesis Sonochemistry Hexaferrite nanoparticles SrFe12O19
a b s t r a c t We present the synthesis of M-type strontium hexaferrite by sonochemistry and annealing. The effects of the sonication time and thermal energy on the crystal structure and magnetic properties of the obtained powders are presented. Strontium hexagonal ferrite (SrFe12O19) was successfully prepared by the ultrasonic cavitation (sonochemistry) of a complexed polyol solution of metallic acetates and diethylene glycol. The obtained materials were subsequently annealed at temperatures from 300 to 900 °C. X-ray diffraction analysis shows that the sonochemical process yields an amorphous phase containing Fe3+, Fe2+ and Sr2+ ions. This amorphous phase transforms into an intermediate phase of maghemite (c-Fe2O3) at 300 °C. At 500 °C, the intermediate species is converted to hematite (a-Fe2O3) by a topotactic transition. The final product of strontium hexaferrite (SrFe12O19) is generated at 800 °C. The obtained strontium hexaferrite shows a magnetization of 62.3 emu/g, which is consistent with pure hexaferrite obtained by other methods, and a coercivity of 6.25 kOe, which is higher than expected for this hexaferrite. The powder morphology is composed of aggregates of rounded particles with an average particle size of 60 nm. Ó 2015 Elsevier B.V. All rights reserved.
1. Introduction M-type hexagonal ferrites have been used extensively as permanent magnets and high-density magnetic recording media, among other interesting applications in recent decades; they are currently used in microwave devices at frequencies in the gigahertz range [1,2]. Although many other hard magnetic materials have been developed, the hexaferrite performance/cost ratio remains extremely favorable. Strontium hexaferrites are ferromagnetic materials with the formula MFe12O19, where M is the divalent Sr ion [3]. The magnetic properties of strontium hexaferrite depend on its structure and particle and crystallite sizes, which depend on the synthesis method used. This material possesses a high magneto-crystalline anisotropy (MCA), which occurs when the induced magnetization has a preferred orientation within the crystal structure. Because of their magnetization (60 emu/g) and high coercivities (5.5 kOe) [3], strontium hexaferrites are commonly used as permanent magnetic materials in modern devices [4], such as loud speakers, permanent magnetic motors, coil instruments (galvanometer, voltmeter and ammeter) and microphones [7], as ⇑ Corresponding author. E-mail address: [email protected] (F. Sánchez De Jesús). http://dx.doi.org/10.1016/j.ultsonch.2015.10.023 1350-4177/Ó 2015 Elsevier B.V. All rights reserved.
well as being used to induce magnetic hyperthermia [5] and to generate hybrid photocatalysts [6]. SrFe12O19 has excellent characteristics because of its structure. As a typical magnetoplumbite type of hexagonal ferrite [8], Sr-hexaferrite is built from smaller units: a cubic block S with a spinel-type structure and a hexagonal block R that contains the Sr ions. The hexagonal Sr-ferrite has 24 magnetic Fe3+ ions per unit cell, which are distributed on five crystallographic sites [9]: three octahedral sites, 12 k, 2a and 4f2; one tetrahedral site, 4f1; and one trigonal bi-pyramidal site, 2b [10]. The chemical function has 12 Fe3+ ions, four of which have spins in the downward direction at 4f1 (2 Fe3+) and 4f2 (2 Fe3+), whereas the other 8 Fe3+ ions have spins in the upward direction at 12 k (6 Fe3+), 2a (1 Fe3+) and 2b (1 Fe3+). Because of its complicated crystal structure, the synthesis of Sr-ferrite represents a major challenge regardless of the selected method, particularly when a nanoparticle is the target of the study [11]. Because of the interest in these materials, nanostructured hexaferrites have been obtained using different methods. The conventional and oldest one is by calcination and sintering of a mixture of oxides or carbonates in a furnace at 1300 °C [12], but this technique produces large particles and consumes extensive energy. There are other synthesis methods that involve wet chemistry:
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co-precipitation [13], hydrothermal synthesis [14], mechanosynthesis [3,15] and sol–gel [16], among others. Recent studies have focused on different approaches to enhance the magnetic properties: substituting the cation positions in this type of structure [17–19], and synthesizing nanoparticles [20,21], which is ideal for magnetic performance. However, it is difficult to obtain ultrafine and monodisperse particles by commercial ceramic methods because they involve high temperatures (>1300 °C) that produce unavoidable particle growth. The use of ultrasound to produce nanoparticles has been a research topic of great interest in recent years [22–31]. In sonochemistry, molecules undergo a chemical reaction caused by the application of ultrasound radiation (20 kHz–10 MHz) [22]. This radiation induces the phenomenon of acoustic cavitation, which consists of the formation, growth, and implosive collapse of air bubbles in an irradiated liquid due to the relaxation and compression of a liquid solvent. These bubbles implode and cause shockwaves, which structurally modify the ultrasound-irradiated items [23]. Cavitation can produce a temperature of 5000 K over the course of a nanosecond, large cooling rates (1011 Ks1) and pressures greater than 1800 kPa [24]. The sonochemical route has emerged as a potential alternative to synthesize ferrites of nanometric size [25–28]. Recently, Junliang et al. [29] synthesized hexaferrite via ultrasonic irradiation, which was used to assist a co-precipitation method after calcination at 800 °C. Others studies have focused on demonstrating the influences of ultrasonic irradiation variables (power and time) on the shape and size of the synthesized ferrite nanoparticles [30–32]. Different studies have verified the ability of ultrasounds to induce chemical reactions, particularly through the formation of reactive radicals. In addition, the use of ultrasound is eco-friendly because all solvents can be reused, and it is more economic than other synthesis routes [22–28]. According to some authors [30–33], the synthesis of Srhexaferrite by ultrasonicating a polyol solution can be represented as follows: a) The salts are completely dissolved in diethylene glycol (DEG) with a controlled quantity of water, and the hydrolysis of iron acetate in an aqueous medium ensues, according to Eq. (1): DEG
ðC2 H3 O2 Þ2 Sr þ 12ðC2 H3 O2 Þ2 Fe þ H2 O ! Srþ2 þ2
þ 12Fe
þ 26ðC2 H3 O2 Þ þ H þ OH þ
ð1Þ
Water is added to promote the hydrolysis of the metals, which is necessary to synthesize the oxides. b) After that, when the solution is sonicated, a strong oxidant (H2O2) it is generated via the cavitation effect [24,30]. Then, the oxidant H2O2 is suggested to initiate the oxidation of Fe2+ to Fe3+ [32]. c) Finally, annealing promotes the reaction between the metallic ions and H2O2 to yield strontium hexaferrite, according to the following reaction: ;D
Sr2þ þ 12Fe2þ þ 19H2 O2 ! SrFe12 O19 þ 19H2 O
ð2Þ
This work aims to produce SrFe12O19 nanoparticles using sonochemistry and a subsequent annealing at low temperature. This method is proposed as a simple, economical and eco-friendly synthesis for this type of nanomaterial. Moreover, the reaction mechanism behind Sr-hexaferrite nanoparticle formation was studied using the sonochemical-assisted method. 2. Experimental To obtain SrFe12O19, stoichiometric amounts of (C2H3O2)2Sr (purity 99.995% Sigma Aldrich) and (C2H3O2)2Fe (purity 99.8%
471
Fig. 1. Schematic representation of the sonochemical setup.
Sigma Aldrich), according to Eq. (2), were dissolved in 50 mL of a solution of diethylene glycol (DEG, C4H10O3, purity 99.8% Sigma Aldrich) with water (weigh ratio of DEG:water was 2:1) for 30 min under mechanical stirring in air atmosphere. A solution without sonication was studied as a control experiment, as Sr2+ and Fe2+ ions were completely dissolved for 30 min of stirring time, solution was stirred for 5 h until observation of particles in suspension. The solutions were subjected to different sonication times from 10 min to 3 h using an Ultrasonic Homogenizer Q700 sonicator. Fig. 1 shows the schematic diagram of the sonochemical experiment setup. A water bath was used to control the reaction temperature. The solution temperature was keep at 25 °C during irradiation. After the sonochemical step, the powders were washed multiple times by forming a suspension in ethanol and centrifugation at 16,000 rpm (three times) for 15 min. Then, the powders were dried at 80 °C in air. At this point the powders were named ‘‘as obtained.” After that, the powders were annealed at different temperatures from 300 to 900 °C in air. The obtained powders were characterized by X-ray diffraction (XRD) using an Inel Equinox 2000 diffractometer with Co Ka1 (k = 1.7890100 Å) radiation. The diffraction pattern was collected over a 2h interval of 20–80° with increments of 0.02 (2h). Rietveld refinements were performed on the X-ray diffraction patterns to obtain the percentages of different phases, crystallite sizes and microstrains of the powders. This method considers all of the collected information in a diffraction pattern and uses a least-squares approach to refine the theoretical line profile until it matches the measured profile [34]. Crystallographic data were obtained from the Crystallography Open Database (COD) [35] Scanning electron microscopy (SEM) using a JEOL-100-CX II was used to determine the morphologies and qualitatively characterize the particle sizes. The stabilities of the synthesized powders were measured by studying their thermal behaviors in a differential scanning calorimeter and a thermogravimetric analyzer (DSC/TGA 851e Mettler-Toledo). The experiments were performed with a heating rate of 10 Kmin1 using an air flow of 6 103 m3s1. Magnetization studies were performed at room temperature using a MicroSense EV7 vibrating sample magnetometer (VSM) with a maximum field of ±18 kOe.
3. Results and discussion Fig. 2 shows the X-ray diffraction patterns of the powders obtained by applying 10 min of sonication and annealing at temperatures from 300 to 900 °C. Also, the XRD pattern corresponding to the powder obtained without applying sonication
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Fig. 2. X-ray diffraction (XRD) patterns of the powders that were obtained using the sonochemistry process for 10 min and heat-treated at different temperatures.
Fig. 3. X-ray diffraction (XRD) patterns of the powders that were obtained using the sonochemistry process for 1 h and heat-treated at different temperatures.
and mechanically stirred for 5 h is included. As it is observed in the XRD pattern of the ‘‘sonication time = 0 min” curve, reflection peaks corresponding to goethite, a-FeOOH (COD #2211652, Pbnm), are detected. This iron (III) oxide hydroxide is generated when Fe (OH)2 (formed by the hydrolysis of iron acetate) is oxidized with oxygen from air atmosphere introduced during the mechanical stirring. Besides, in the XRD pattern of the ‘‘as obtained” powder (without heat treatment), there is only a broad peak, which is associated with a non-crystalline structure (amorphous). Therefore, the formation of strontium hexaferrite SrFe12O19 did not occur during the sonochemical process, as shown by the absence of the corresponding SrFe12O19 diffraction peaks in the X-ray diffraction pattern. As was expected, sonochemistry does not carry the reaction to completion Eq. (2), which corresponds to the formation of strontium hexaferrite; thus, an annealing process is required. The XRD patterns of the annealed powders are shown in Fig. 2. As can be observed, when the powder is annealed at 300 °C the peaks that correspond to maghemite, c-Fe2O3 (COD 9012692, P43-21:2), are detected. Additionally, there is substantial peak broadening, which is associated with nanosized particles. Increasing the annealing temperature produces new phases at 800 °C: c-Fe2O3 (COD #9012692, P43-21:2) and Fe2Sr2O5 (COD 2002240, Ibm2), and at 900 °C: FeO (COD 9002673, Fd-3 m). Despite the annealing treatment at high temperatures, no peaks that are associated with strontium hexaferrite are detected using XRD. Therefore, under these experimental conditions, hexaferrite cannot be synthesized. When the irradiation time is increased to 1 h, the results change, as shown in Fig. 3, which presents the XRD patterns of the powders that were annealed at temperatures from 300 to 900 °C. Similar to the previous experimental conditions, the powder named ‘‘as obtained” consists of an amorphous phase. According to some authors [25,30], sonochemistry or ultrasonic irradiation of water produces the free radicals H and OH that can combine to produce H2O2, which is a strong oxidant. Thus, it is suggested that the produced oxidizing agent initiates the partial oxidation of Fe2+ as follows:
(c-Fe2O3), which is generated by a crystallization process, as follows:
2Fe2þ þ H2 O2 ! 2Fe3þ þ 2OH
ð3Þ
This reaction is spontaneous because its redox potential (DE°) is positive. Therefore, the obtained amorphous powder using sonochemistry consists of a mixture of amorphous oxides: Fe3+, Fe2+ and Sr2+. When the ‘‘as obtained” powder is annealed at 300 °C, as can be observed in Fig. 3, the amorphous phase transforms to maghemite
D
Fe2þ þ Fe3þ þ 6OH ! cFe2 O3 þ 3H2 O "
ð4Þ
If the annealing temperature is increased (>300 °C), the c-Fe2O3 transforms spontaneously to hematite, a-Fe2O3 (COD 9015964, R-3c:H), due to the standard Gibbs free energy of the reaction is negative (14.4 kJ mol1) [37]. The transformation of maghemite to hematite is considered to be a topotactic transformation, and it is favored at high temperatures [38]. For temperatures up to 700 °C, strontium hexaferrite, SrFe12O19 (COD 1008364, P63/mmc), forms and is accompanied by small quantities of the unreacted hematite phase, a-Fe2O3. The synthesis of strontium hexaferrite is completed at 800 °C and is given by the following: D
6Fe2 O3 þ SrO ! SrFe12 O19
ð5Þ
Table 1 shows the Rietveld refinement results for the volume percentage of each phase, the crystallite size and the microstrain, all of which were obtained from the XRD pattern analyses. These results confirm that a nanocrystalline material was obtained using sonochemistry. Furthermore, these results support the proposed mechanism and quantify the phases that are present. The XRD patterns in Fig. 3 show no peaks corresponding to strontium compounds because the Sr:Fe atomic ratio was 1:12, and the SrO volume was much lower than the magnetite/maghemite volume. This means that the SrO reflections are beyond the detection limit of the instrument. To determine the effect of sonication time, an additional experiment that increased this parameter to 3 h was performed. Fig. 4 shows the XRD patterns of the powders that were synthesized by sonochemistry for 3 h and annealed at temperatures from 300 to 900 °C. An interesting difference was found; the ‘‘as obtained” powder shows crystalline maghemite (c-Fe2O3) together with hematite (a-Fe2O3). These species exhibit a remarkable peak broadening, which is associated with the nanosize of the particles. The average crystallite size, which was calculated by Rietveld refinement, is approximately 5 nm and 60 nm for maghemite and hematite, respectively, and the microstrain is 0.0164 and 0.0002 nm for maghemite and hematite, respectively. Therefore, increasing the sonication time promotes crystallization of the amorphous phases, a result that is in agreement with other authors [27,32]. It is also likely that extended sonication decreases the
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Table 1 Data from the Rietveld refinements of the XRD patterns of the powders, which were obtained using the sonochemistry process for 1 h and annealed at different temperatures. Heat treatment (°C)
Phases
# COD
300
P43-21:2, c-Fe2O3 Fm-3m, SrO P43-21:2, c-Fe2O3 R-3c:H, Fe2O3 Fm-3m, SrO P63/mmc, SrFe12O19 R-3c:H, Fe2O3 P63/mmc, SrFe12O19 P63/mmc, SrFe12O19
9,012,692 1,011,328 9,005,813 9,000,139 1,011,328 1,008,856 9,015,964 1,008,856 1,008,856
500
700 800 900 *
% Vol.
Crystallite size (Å)
Microstrain (adim) 4.07 105
100
330
*
*
48.16 51.84
175 53
*
*
84.6 15.4 100 100
507 433 570 595
*
0.0028 3 107 *
0.0013 0.0004 0.0017 0.0018
Beyond detection limit.
Fig. 4. X-ray diffraction (XRD) patterns of the powders that were obtained using the sonochemistry process for 3 h and heat-treated at different temperatures.
nuclei size because the crystallite size of the ‘‘as obtained” powder is smaller than those obtained with shorter sonication times. The XRD patterns of the annealed powders sonicated for 3 h confirm the proposed mechanism for the sonochemical reaction. To confirm the reaction’s mechanism, a thermal analysis of the ‘‘as obtained” powder sonicated for 1 h was performed. The TGA curve of the ‘‘as obtained” sample is shown in Fig. 5. It indicates three stages of weight loss, one down to 100 °C, another between 170 and 315 °C and the last stage starts at 550 °C. The initial weight loss up to a temperature of 100 °C could be ascribed to the evaporation of the water of hydration or the loss of –OH groups on the surface of iron oxide. The second stage displays an abrupt weight loss that is attributed to the crystallization of the amorphous material obtained from the sonochemical process. Here, H2O is liberated as a result of maghemite formation, as is shown in Eq. (4). The final stage of weight loss may be assigned to the Sr-hexaferrite synthesis and is completed at 642 °C because there is no significant weight change above this temperature. Moreover, Fig. 6 depicts the DSC curve from 30 °C to 700 °C for the ‘‘as obtained” powder sonicated for 1 h. At 98.9 °C, an endothermic peak is observed, which is ascribed to the removal of weakly bound groups (H2O or –OH). After that, two sharp exothermic peaks are observed: the first peak at 243 °C
Fig. 5. TGA curve of the ‘‘as obtained” powder, which was synthesized by sonochemistry for 1 h (without annealing).
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Fig. 6. DSC curve of the ‘‘as obtained” powder, which was synthesized by sonochemistry for 1 h (without annealing).
Fig. 7. Magnetic hysteresis loops of the powders, which were synthesized by sonochemistry for 1 h and annealed at different temperatures.
corresponds to the formation/crystallization of c-Fe2O3 nanoparticles, and the second peak, which appears between 290 and 320 °C, corresponds to the topotactic transformation of c-Fe2O3 to a-Fe2O3 [38]. Finally, a broad peak at approximately 622 °C corresponded to the Sr-hexaferrite formation. These results confirm the previously proposed reactions for the formation of SrFe12O19 by heat treatment-assisted sonochemistry. Fig. 7 presents the magnetic hysteresis loops for the powders that were synthesized by sonochemistry for 1 h and annealed at different temperatures. As observed in Fig. 7, the ‘‘as obtained” powder shows a very small value of magnetization (1 emu/g), which is associated with the magnetic behavior of an amorphous phase. The powders annealed at 300 °C show a higher value of specific magnetization, 33.37 emu/g at 18 kOe, which represents the magnetic behavior of the c-Fe2O3 nanoparticles [36] and the SrO (diamagnetic). When the temperature was increased to 500 °C, a reduction in specific magnetization was detected with respect to that observed for the sample treated at 300 °C. This result is associated with the partial transformation of c-Fe2O3 to hematite (a-Fe2O3). At 700 °C, the material exhibits a radically different magnetic behavior compared with those annealed at low temperatures. This material presents a high coercivity of 5.57 kOe and a strong specific magnetization (51 emu/g at
Fig. 8. SEM micrograph of the powder, which was synthesized using sonochemistry for 1 h and annealed at 800 °C for 1 h.
18 kOe), which is similar to the behavior of Sr-hexaferrite. However, its specific magnetization is lower than the values typically reported for nanostructured Sr-hexaferrites, because this compound is diluted by hematite (a-Fe2O3), as confirmed by the XRD results in Fig. 3. Therefore, at this temperature, the hexaferrite synthesis is not yet complete and considerable amounts of a-Fe2O3 are present (15.4% in vol. according to the Rietveld refinement shown in Table 1). Moreover, it is observed in Fig. 7 that the powder annealed at 800 °C presents a typical specific saturation magnetization for Sr-hexaferrite (62.3 emu/g at 18 kOe) and a high coercivity value (6.25 kOe), which is greater than the reported values for this type of hexaferrite [1]. When the temperature is increased to 900 °C, a slight increase in the specific magnetization is produced, but it is not relevant because by this point the Sr-hexaferrite had been completely formed. Furthermore, the coercivity increase confirms the nanosize of the particles and the single domain behavior. Finally, Fig. 8 shows the micrograph that corresponds to the hexaferrite powder, which was synthesized using ultrasound for 1 h and annealed at 800 °C. The powder morphology is composed of aggregates of rounded particles; it is possible to qualitatively infer an average particle size of 60 nm. Furthermore, particle size is one of the most influential parameters capable of modifying the coercivity of magnetic materials, and its effect depends on the transition from the single-domain state to the multi-domain state. The critical diameter of the particle that separates the magnetic states (single- and multi-domain) for SrFe12O19 is 367.22 nm [10]. Therefore, the strontium hexaferrites obtained with this method are expected to exhibit single-domain behavior. 4. Conclusions Crystalline hexaferrite nanoparticles were successfully synthesized by ultrasonicating a polyol solution for 1 h and then annealing the product at 800 °C. These results confirm that sonochemistry is a convenient method to prepare strontium hexaferrite nanoparticles by decreasing the sintering temperature, compared with the temperatures commonly required for solid-state techniques. A mechanism for the synthesis of Sr-hexaferrite nanoparticles is proposed: during the sonochemical process (1 h), a mixture of amorphous oxides of Fe3+, Fe2+ and Sr2+ is obtained; the partial oxidation of Fe2+ to Fe3+ is due to the formation of H2O2. At approximately 300 °C, maghemite (c-Fe2O3) crystallization occurs. When the temperature is increased, a-Fe2O3 is formed through a topotactic transition. At 700 °C, Fe2+ is completely oxidized to Fe3+, and a-Fe2O3 is obtained, which reacts with SrO to partially
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form hexaferrite. The hexaferrite formation is completed at 800 °C. The obtained hexaferrite nanoparticles have a higher specific magnetization (62 emu/g at 18 kOe) and a larger coercive field (6.25 kOe) than the values reported by other methods. This consistency is indicative of the single-domain nature of the obtained powder and its particle size, which was confirmed by electron microscopy. For high sonication time (up to 3 h), the ultrasonic cavitation promotes the crystallization of amorphous phases and also decreases the nuclei size because the crystallite size of the ‘‘as obtained” powder is smaller than those obtained using shorter sonication times. References [1] R.C. Pullar, Hexagonal ferrites: a review of the synthesis, properties and applications of hexaferrite ceramics, Prog. Mater. Sci. 57 (2012) 1191–1334. [2] R. Valenzuela, Novel applications of ferrites, Phys. Res. Int. 2012 (2012) 1–9. Article ID 591839. [3] A.M. Bolarín Miró, F. Sánchez De Jesús, C.A. Cortés Escobedo, S. Díaz De la Torre, R. Valenzuela, Synthesis of M-type SrFe12O19 by mechanosynthesis assisted by spark plasma sintering, J. Alloys Compd. 643 (2015) 226–230. [4] J.R. Liu, R.Y. Hong, W.G. Feng, D. Badami, Y.Q. Wang, Large-scale production of strontium ferrite by molten-salt-assisted coprecipitation, Powder Technol. 262 (2014) 142–149. [5] A.U. Rashid, P. Southerm, J.A. Darr, S. Awan, S. Manzoor, Strontium hexaferrite (SrFe12O19) based composites for hyperthermia applications, J. Magn. Magn. Mater. 344 (2013) 134–139. [6] A. Abd Aziz, Y. Harng Yau, G. Li Puma, C. Fischer, S. Ibrahim, S. Pichiah, Highly efficient magnetically separable TiO2-graphene oxide supported SrFe12O19 for direct sunlight-driven photoactivity, Chem. Eng. J. 235 (2014) 264–274. [7] S. Shakoor, M. Naeem Ashiq, M. Aslam Malana, A. Mahmood, M. Farooq Warsi, M. Najam-ul-Haq, N. Karamat, Electrical, dielectric and magnetic characterization of Bi–Cr substituted M-type strontium hexaferrite nanomaterials, J. Magn. Magn. Mater. 362 (2014) 110–114. [8] Y.F. Xu, Y.Q. Ma, S.T. Xu, F.L. Zan, G.H. Zheng, Z.X. Dai, Effects of vacancy and exchange-coupling between grains on magnetic properties of SrFe12O19 and aFe2O3 composites, Mater. Res. Bull. 27 (2014) 13–18. [9] P. Jihoon, H. Yang-Ki, K. Seong-Gon, K. Sungho, S.I. Laalitha, L. Jaejin, A. SungYong, Maximum energy product at elevated temperatures for hexagonal strontium ferrite (SrFe12O19) magnet, J. Magn. Magn. Mater. 355 (2014) 1–6. [10] B.K. Rai, S.R. Mishra, V.V. Nguyen, J.P. Liu, Synthesis and characterization of high coercivity rare-earth ion doped Sr0.9RE0.1Fe10Al2O19 (RE: Y, La, Ce, Pr, Nd, Sm, and Gd), J. Alloys Compd. 550 (2013) 198–203. [11] Z. Zhipeng, W. Zhanyong, W. Xiuting, W. Xiaorui, Z. Jingshi, D. Fengkai, X. Jiayue, Differences in the structure and magnetic properties of Sr1x REx Fe12 O19 (RE: Pr and Dy) ferrites by microwave-assisted synthesis method, J. Alloys Compd. 610 (2014) 264–270. [12] K. Samikannu, J. Sinnappan, S. Mannarswamy, T. Cinnasamy, K. Thirunavukarasu, Synthesis and magnetic properties of conventional and microwave calcined strontium hexaferrite, Mater. Sci. Appl. 2 (2011) 638–642. [13] M.M. Rashad, I.A. Ibrahim, A novel approach for synthesis of M-type hexaferrites nanopowders via the co-precipitation method, J. Mater. Sci.: Mater. Electron 22 (2011) 1796–1803. [14] M. Jean, V. Nachbaur, J. Bran, J.M. Le Breton, Synthesis and characterization of SrFe12O19 powder obtained by hydrothermal process, J. Alloys Compd. 496 (2010) 306–312. [15] F. Sánchez De Jesús, A.M. Bolarín Miró, C.A. Cortés Escobedo, R. Valenzuela, S. Ammar, Mechanosynthesis, crystal structure and magnetic characterization of M-type SrFe12O19, Ceram. Int. 40 (2014) 4033–4038.
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