Systematic Study on the Synthesis and Magnetism Properties of Manganese Ferrite MnFe 2 O 4 by an Oxidation Roasting Process

: A low-cost and high-efﬁciency solid reaction method has been reported as an effective technology to synthesize manganese ferrite MnFe 2 O 4 with a spinel crystal structure. This work clariﬁed the underlying reason for the inﬂuence mechanism of SiO 2 and Al 2 O 3 on the synthesis of MnFe 2 O 4 . Synthetic MnFe 2 O 4 polyhedral microparticles with a saturated magnetization of 71.19 emu/g, a ratio of saturation magnetization to residual magnetization (Ms/Mr) of 0.062 and a coercivity (Hc) of 6.50 Oe were successfully obtained at an oxidization roasting temperature of 1100 ◦ C for 60 min. The experimental results indicate that the tetrahedral Mn 2+ ions and octahedral Mn 3+ ions in the crystal structure of manganese ferrite MnFe 2 O 4 were replaced by tetrahedral Si 2+ ions and octahedral Al 3+ ions from (Mn 2+ ) x (Fe 2+ ) y (Si 2+ ) 1 − x − y [Fe 3+ ] 2 O 4 and (Mn 2+ )[Fe 3+ ] 2 − x [Al 3+ ] x O 4 , respectively. In addition, hercynite Fe x Mn 1 − x Al 2 O 4 with a spinel crystal structure and olivine Mn x Fe 2 − x SiO 4 with an orthorhombic crystal structure were partially formed in the synthesis of manganese ferrite MnFe 2 O 4 , in which some Fe 2+ ions were easily replaced by Mn 2+ ions to form stable hercynite MnAl 2 O 4 and olivine Mn 2 SiO 4 in these crystal structures. The current research work provides comprehensive insights for synthesizing manganese ferrite MnFe 2 O 4 and continuously advances its technical progress.

By comparison with the above synthesis methods, hydrometallurgy and combined methods are reported for synthesizing high-purity manganese ferrites.However, the abovementioned methods have high cost, high energy consumption and complex technology, which seriously restrict their development.Solid-state synthesis has been widely considered a cost-effective way to synthesize manganese ferrite [18,28,29].As we all know, magnetism is an important index for characterizing manganese ferrite, and the purity of raw materials and preparation conditions will directly affect its magnetism.As studied by Lin et al. [15], MnFe 2 O 4 with different morphologies was successfully synthesized by a solvothermal method using cetyltrimethylammonium bromide as the raw material, and the minimum coercivity was 11.9 Oe.Rashad et al. [23] successfully synthesized MnFe 2 O 4 with a saturation magnetization of 27.24 emu/g by acid leaching with low-grade manganese ore at pH 10, a calcination temperature of 1000 • C and a calcination time of 120 min.Ahmed et al. [24] found that manganese ferrite with a maximum saturation magnetization of 62 emu/g was successfully synthesized by sintering for 2 h at 1300 • C using mill scale and fine particles of manganese ore sinter as the source of ferromanganese spinel.Ding et al. [25] showed that the maximum saturation magnetization of MnFe 2 O 4 samples formed after annealing at 600~700 • C increased from 54 emu/g to 79 emu/g.Reddy et al. [22] performed spark plasma sintering (SPS) of the hydrothermally pretreated material to obtain a soft magnetic material MnFe 2 O 4 with a maximum saturation magnetization of 82.5 emu/g.To date, much research has been carried out on the preparation of MnFe 2 O 4 with the solid-phase reaction method [18,30,31], and the experiments have proven that the magnetic properties of manganese ferrite are closely related to the structure and morphology of the crystal [18][19][20][21].Since the influence of SiO 2 as well as Al 2 O 3 on the asynchronous reaction behavior and product properties of ferromanganese oxides in the solid-phase process is not clear, the study of silicon and aluminum elements is of great significance in elucidating their impact on the synthesis of manganese ferrite, providing theoretical support for the solid-state reaction method of preparing manganese ferrite from ferromanganese ore.The clarification of the interfacial reaction process and the control of the interfacial reaction products is one of the urgent problems in this field.
In this paper, high-purity manganese ferrite MnFe 2 O 4 was prepared by a solid-reaction method in which the evolution of its phase transition, magnetic properties and morphological characteristics were investigated by scanning electron microscopy (SEM), energy dispersive X-ray spectroscopy (EDS), X-ray diffraction (XRD), vibrating sample magnetometer (VSM) and Mössbauer spectroscopy (MBS) analyses.Some impurity components have a certain impact on the synthesis process of MnFe 2 O 4 .This work clarified the underlying reason for the influence mechanism of SiO 2 and Al 2 O 3 on the synthesis of MnFe 2 O 4 in SiO 2 -MnO-Fe 2 O 3 systems and Al 2 O 3 -MnO-Fe 2 O 3 systems.Therefore, the phase formation and synthesis mechanism of SiO 2 -MnO-Fe 2 O 3 systems and Al 2 O 3 -MnO-Fe 2 O 3 systems were also investigated by X-ray diffraction (XRD) and scanning electron microscopy (SEM) and energy dispersive X-ray spectroscopy (EDS) analysis.Based on these findings, the current research work provides comprehensive insights for synthesizing manganese ferrite MnFe 2 O 4 and continuously advances its technical progress.

Raw Materials
The purity of the MnO, Fe 2 O 3 , SiO 2 and Al 2 O 3 raw materials used in this study was 99.9%, and the reagents were not further purified during the experiment.The gas used was compressed air with a purity of 99.9% or higher.The average particle sizes of the Fe 2 O 3 , MnO, SiO 2 and Al 2 O 3 purity reagent powders were 56.2 µm, 35.2 µm, 30.6 µm and 26.4 µm, respectively, using a Malvern laser particle size analyzer (Mastersizer 2000, Shanghai, China).

Synthesis Procedure
In this paper, three kinds of samples were used to assess oxidative competition behavior and synthesis mechanism in the synthesis process of manganese ferrite MnFe 2 O 4 .Next, the prepared powder sample was mixed evenly by the ball star mixer for 2 h.Then, the mixed samples with diameters of 15 mm and heights of 20 mm were placed into cylindrical molds with briquette equipment operated at a pressure of 15 MPa.The briquettes were placed on a corundum substrate (size: 60 mm × 60 mm) and roasted in air atmosphere to synthesize MnFe 2 O 4 .The oxidization roasting temperature was also measured by a Pt-Rh thermocouple and controlled using a digital temperature controller (accuracy ± 1 • C).After the equipment reached the set temperature during roasting, the samples placed in a square corundum crucible were placed in the constant temperature zone of the furnace chamber, and at the same time, a mass flow rate of 2.0 dm 3 /min was passed into the reaction equipment in air atmosphere.Then, following a certain time of oxidative roasting at the set temperature, the roasted product was placed in the cooling apparatus and cooled to room temperature at a cooling rate of 22 • C/min for subsequent characterization tests.In the same process as above, different mass fractions of SiO 2 and Al 2 O 3 were added to the MnO-Fe 2 O 3 system with a molar ratio of 1:1 by oxidative roasting, and the products were later used for characterization tests.Other detailed descriptions of the above equipment were reported in our previous studies [31].

Characterization Methods
The samples were cooled after oxidative roasting in a muffle furnace and ground with 200-mesh (<0.074 µm) fine particles.X-ray diffraction (XRD) was performed on the samples using RIGAKU D/Max 2500 with a scanning angle of 5  /min.The scanning results were analyzed using X'Pert HighScore Plus software 5.2 to assess the phase transformation and microstructure of the specimens.
The microscopic morphological characteristics of the intermediate products and the synthesized manganese ferrite particles were observed by scanning electron microscopy (SEM) and energy dispersive X-ray spectroscopy (EDS), and the interfacial reaction behaviors during the synthesis process were analyzed by point scanning and line scanning.The oxidization reaction behavior and formation mechanism of MnO and Fe 2 O 3 powder were analyzed by a diffusion coupling model.
The magnetic properties of the synthesized manganese ferrite (MnFe 2 O 4 ) particles were examined at room temperature using a JDAW-2000 C&D vibrating sample magnetometer (VSM).Since different magnetic materials have different magnetization properties, in considering the magnetic properties of the materials for comparison, the magnetic properties of the reaction samples will be compared by saturation magnetization intensity.
To further determine the occupancy of trivalent ferric ions in manganese ferrite, the experimental samples were analyzed by Mössbauer spectroscopy at room temperature.In the study of magnetic properties of solids, Mössbauer spectroscopy can be used to determine the magnetic ordering temperature, type of magnetic ordering and whether the ferromagnetic solid is antiferromagnetic or ferromagnetic.And this device can analyze the population of magnetic ions between various sublattices and study magnetic or spin structures.In the study of microcrystals, Mössbauer spectroscopy can provide information on the relaxation process of magnetic microcrystals, the number of magnetic anisotropy constants, the size and distribution of microcrystals and other aspects.The diffraction peaks of Fe 2 O 3 were not detected with increasing oxidization roasting temperature from 1000 • C to 1100 • C. Nevertheless, the diffraction peak intensity of Mn x Fe 3−x O 4 was enhanced and approached the standard diffraction peak of (2 2 0), which indicated that the purity and crystallinity of the synthesized MnFe 2 O 4 increased with increasing roasting temperature.As observed in Figure 1b, the characteristic diffraction peak intensities of Fe x O 4 and Fe 2 O 3 were detected when the oxidization roasting temperature was below 1000 • C, while the intensity of the characteristic peak of MnFe 2 O 4 showed an initial increasing trend followed by stabilization with increasing roasting temperature.In addition, the 2θ value of the diffraction peak (3 1 1) of Mn x Fe 3−x O 4 (1 ≤ x ≤ 2) was 35.132 • .These results demonstrated that the higher oxidization roasting temperature facilitated the synthesis of manganese ferrite, improved its crystal structure and optimized the atomic distribution in the crystal structure of spinel MnFe 2 O 4 .Figure 1c    Figure 2 illustrates the scanning electron micrographs of the cross-sectional layers of the synthesized manganese ferrite samples at 900 • C and 1100 • C for 60 min.Figure 2a clearly shows that the synthesized ferromanganese oxides have a regular polyhedral structure at a roasting temperature of 900 • C. From the overall appearance morphology, it can be seen that the polyhedral particles have obvious accumulation and gaps of different sizes exist.However, the sample particles have obvious boundaries.The corresponding energy spectrum scan lines reveal that the Mn, Fe and O contents in the sample are basically constant at a ratio of 1:1:3, indicating a relatively uniform distribution of the products.This result precisely coincides with the characteristic XRD peak at 900 • C in Figure 1.The cross-sectional SEM images of the sample roasted at 1100 • C for 60 min are shown in Figure 2b,c.The particles in the sample exhibit a flat spherical shape, and the gaps between the particles are relatively uniform with obvious boundaries.The energy spectrum analysis revealed that the average contents of integrated Mn, Fe and O were 13.67%, 26.51% and 59.83%, respectively, which were close to the theoretical values of the atomic ratio of pure MnFe 2 O 4 .The Line 2 scan observed that the atomic ratios and contents of Mn, Fe and O were constant on individual particles, indicating that the distribution of these elements in the sample product was uniform.The experimental results do not correspond to the theoretical values due to the rapid oxidation rate of Mn 2+ , and the related reasons can be further determined comprehensively by magnetic analysis.2θ=33.162°◆

Magnetic Transformation
The magnetization hysteresis loops of the prepared MnxFe 3−x O 4 products at 900-1200 • C were investigated with VSM, as illustrated in Figure 3.The saturation magnetization intensity Ms of the samples at different temperatures increased from 16.03 emu/g to 72.04 emu/g with increasing roasting temperature.The saturation magnetization intensity was 16.03 emu/g at 900 • C. The explanation for this result combined with Figure 3 shows that a large amount of MnFeO 3 could be generated from MnO and Fe 2 O 3 at lower temperature, which leads to the decline of magnetism.Researchers have found that the saturation magnetization strength is proportional to the grain size of the magnetic material [24,32,33], and increasing the temperature will benefit the growth of Mn x Fe 3−x O 4 [18,24].In addition, the saturation magnetic strength of MnFe 2 O 4 involves the distribution of cations on the A or B sites of the spinel crystal structure sites.Therefore, the variation pattern of Ms of manganese ferrite as affected by temperature is also attributed to the variation in Mn 2+ or Fe 2+ (Fe 3+ ) concentration in the Mn x Fe 3−x O 4 sample.The coercivity (Hc) and the ratio of saturation magnetization intensity (Ms) to residual magnetization intensity (Mr) at room temperature and oxidation roasting temperature are shown in Figure 3.The hysteresis lines of the roasted samples show a narrow hysteresis behavior of the ferromagnetic material.As the roasting temperature increased from 900 • C to 1200 • C, the coercivity of the samples decreased from 30.3 Oe to 3.2 Oe, and Ms/Mr decreased from 0.121 to 0.051.The saturation magnetization intensity of the pure MnFe 2 O 4 sample at 1100 • C was 71.8 emu/g [34] and 71.19 emu/g in this study.The crystalline particles of the samples are more complete, and the magnetic properties are closer to those of the pure MnFe 2 O 4 sample, which has better magnetic properties.
which leads to the decline of magnetism.Researchers have found that the saturation magnetization strength is proportional to the grain size of the magnetic material [24,32,33], and increasing the temperature will benefit the growth of MnxFe3−xO4 [18,24].In addition, the saturation magnetic strength of MnFe2O4 involves the distribution of cations on the A or B sites of the spinel crystal structure sites.Therefore, the variation pattern of Ms of manganese ferrite as affected by temperature is also attributed to the variation in Mn 2+ or Fe 2+ (Fe 3+ ) concentration in the MnxFe3−xO4 sample.The coercivity (Hc) and the ratio of saturation magnetization intensity (Ms) to residual magnetization intensity (Mr) at room temperature and oxidation roasting temperature are shown in Figure 3.The hysteresis lines of the roasted samples show a narrow hysteresis behavior of the ferromagnetic material.As the roasting temperature increased from 900 °C to 1200 °C, the coercivity of the samples decreased from 30.3 Oe to 3.2 Oe, and Ms/Mr decreased from 0.121 to 0.051.The saturation magnetization intensity of the pure MnFe2O4 sample at 1100 °C was 71.8 emu/g [34] and 71.19 emu/g in this study.The crystalline particles of the samples are more complete, and the magnetic properties are closer to those of the pure MnFe2O4 sample, which has better magnetic properties.

Oxidative Competition Behavior of MnO-Fe2O3 System in Oxidization Roasting Process
X-ray photoelectron spectroscopy (XPS) was used to detect and assess the chemical states of Fe and Mn to clarify the oxidization behavior and consolidation mechanism of manganese ferrite MnFe2O4.The XPS spectra in the Fe 2P regions and Mn 2P regions are illustrated in Figure 4a,b.XPS data are systematically analyzed and evaluated through XPS database XPS Peak software 4.2.In the preparation process of MnFe2O4, the valuable X-ray photoelectron spectroscopy (XPS) was used to detect and assess the chemical states of Fe and Mn to clarify the oxidization behavior and consolidation mechanism of manganese ferrite MnFe 2 O 4 .The XPS spectra in the Fe 2P regions and Mn 2P regions are illustrated in Figure 4a,b.XPS data are systematically analyzed and evaluated through XPS database XPS Peak software 4.2.In the preparation process of MnFe 2 O 4 , the valuable metal ions could occupy the tetra-coordination and octa-coordination in the spinel crystal structure, resulting in different valence states of Fe cations (Fe 2+ and Fe 3+ ) and Mn cations (Mn 2+ and Mn 3+ ) for different binding energies of the Fe 2p electrons and Mn 2p electrons.In addition, the binding energies of the Fe 2p 3/2 peaks for FeO, Fe 3 O 4 and Fe 2 O 3 appeared at 709.8 eV, 710.6 eV and 711.6 eV, respectively.The binding energies of the Mn 2p 3/2 peaks for MnO, Mn 3 O 4 and Mn 2 O 3 appeared at 640.0 eV, 641.2 eV and 642.2 eV, respectively [34,35].It was found that the intergrade area of Fe2p 3/2 for FeO at a binding energy of 709.8 eV gradually decreased and that of Fe 3 O 4 at 710.6 eV also decreased, while that of Fe 2 O 3 at 711.6 eV gradually increased with increasing oxidization roasting temperature from 900 • C to 1000 • C. In the meantime, the intergrade area of Mn2p 3/2 for MnO at a binding energy of 709.8 eV and Mn 3 O 4 at a binding energy of 641.2 eV changed slightly, and that of Mn 2 O 3 at a binding energy of 642.2 eV instantly decreased.These results showed that the higher temperature could effectively enhance the oxidation of Fe 2+ ions in composite oxides to Fe 3+ ions and improve the dissociation of Mn 2 O 3 in the crystal spinel structure of MnFe 2 O 4 .
at 709.8 eV, 710.6 eV and 711.6 eV, respectively.The binding energies of the Mn 2p3/2 peaks for MnO, Mn3O4 and Mn2O3 appeared at 640.0 eV, 641.2 eV and 642.2 eV, respectively [34,35].It was found that the intergrade area of Fe2p3/2 for FeO at a binding energy of 709.8 eV gradually decreased and that of Fe3O4 at 710.6 eV also decreased, while that of Fe2O3 at 711.6 eV gradually increased with increasing oxidization roasting temperature from 900 °C to 1000 °C.In the meantime, the intergrade area of Mn2p3/2 for MnO at a binding energy of 709.8 eV and Mn3O4 at a binding energy of 641.2 eV changed slightly, and that of Mn2O3 at a binding energy of 642.2 eV instantly decreased.These results showed that the higher temperature could effectively enhance the oxidation of Fe 2+ ions in composite oxides to Fe 3+ ions and improve the dissociation of Mn2O3 in the crystal spinel structure of MnFe2O4.  Figure 5a illustrates the element ratios on the reaction surface.In theory, the atomic ratios of Fe, Mn and O should be 14.29% 28.57% and 57.14%, respectively, in the crystal structure of MnFe2O4.The error between the experimental data and the theoretical calculation value is small.However, there are differences in the occupancy of metal ions in the crystal structure of spinel manganese ferrite, which demonstrated that the vacancy oxygen in the crystal structure of MnxFe3−xO4 was gradually filled to form lattice oxygen in the crystal structure of MnFe2O4.The higher oxidization roasting temperature helps to adjust the atomic ordering of the spinel structure.Figure 5b illustrates the Mössbauer spectrum of the MnO-Fe2O3 system at different temperatures.The Mössbauer spectrum of each sample is clearly composed of asymmetric magnetic fine splitting hexagonal peaks.Moreover, each sample is superimposed with two different sets of characteristic diffraction peaks of spinel-type manganese ferrite, representing the Mössbauer spectra of the tetrahedral site (A-site) and octahedral site (B-site) [36,37].Based on the characteristic that the oxygen atoms in the A-site are closer to the metal ions (≈0.67 Å) than in the B-site (≈0.72 Å), both the values of the homogeneous anomeric shift of Fe in the B-site are larger than those in the A-site.It is evident that the percentage of ortho-spinel increases in the shift from the Asite to the B-site as the roasting temperature of the sample is transformed from 900 °C to 1000 °C, which demonstrated that the single bridging oxygen of tetra-coordination and hexa-coordination in composite oxides were transformed into the double bridging oxygen Figure 5a illustrates the element ratios on the reaction surface.In theory, the atomic ratios of Fe, Mn and O should be 14.29% 28.57% and 57.14%, respectively, in the crystal structure of MnFe 2 O 4 .The error between the experimental data and the theoretical calculation value is small.However, there are differences in the occupancy of metal ions in the crystal structure of spinel manganese ferrite, which demonstrated that the vacancy oxygen in the crystal structure of Mn x Fe 3−x O 4 was gradually filled to form lattice oxygen in the crystal structure of MnFe 2 O 4 .The higher oxidization roasting temperature helps to adjust the atomic ordering of the spinel structure.Figure 5b illustrates the Mössbauer spectrum of the MnO-Fe 2 O 3 system at different temperatures.The Mössbauer spectrum of each sample is clearly composed of asymmetric magnetic fine splitting hexagonal peaks.Moreover, each sample is superimposed with two different sets of characteristic diffraction peaks of spinel-type manganese ferrite, representing the Mössbauer spectra of the tetrahedral site (A-site) and octahedral site (B-site) [36,37].Based on the characteristic that the oxygen atoms in the A-site are closer to the metal ions (≈0.67 Å) than in the B-site (≈0.72 Å), both the values of the homogeneous anomeric shift of Fe in the B-site are larger than those in the A-site.It is evident that the percentage of ortho-spinel increases in the shift from the A-site to the B-site as the roasting temperature of the sample is transformed from 900 • C to 1000 • C, which demonstrated that the single bridging oxygen of tetra-coordination and hexa-coordination in composite oxides were transformed into the double bridging oxygen of hexa-coordination in the crystal spinel structure of MnFe 2 O 4 under the action of high temperature.C to 1100 • C. SiO 2 exists mainly as free SiO 2 and a very small amount of manganese-iron olivine phase (Mn x Fe 1−x ) 2 SiO 4 , corresponding to the card information (PDF#65-0466, 2θ = 26.636• ) in this temperature range.This demonstrated that the influence of SiO 2 on the oxidative roasting of the MnO-Fe 2 O 3 system for the synthesis of MnFe 2 O 4 is small when the roasting temperature is within 900-1100 • C. As the temperature increases to 1200 • C, the intensity of the characteristic peaks of SiO 2 and MnFe 2 O 4 slightly decreases, while (Mn x Fe 1−x ) 2 SiO 4 is relatively enhanced, indicating that the excessive temperature will force SiO 2 to react with some of the Fe and Mn oxides to form a small amount of manganese-iron olivine phase (Mn x Fe 1−x ) 2 SiO 4 .XRD patterns of SiO 2 with different contents were roasted at 1100 • C for 120 min in the MnO-Fe 2 O 3 system.The intensity of the characteristic diffraction peaks of SiO 2 and (Mn x Fe 1−x ) 2 SiO 4 in the product increased with increasing SiO 2 content, the intensity of the characteristic peak of MnFe 2 O 4 decreased and then increased and the main characteristic peak shifted from 2θ = 34.936• to 2θ = 35.022• .It can be inferred that the formation of (Mn y Fe x Si 1−x−y )Fe 2 O 4 is related to both temperature and SiO 2 content.The formation of (Mn y Fe x Si 1−x−y )Fe 2 O 4 mainly originated from Mn x Fe 3−x O 4 , in which the tetrahedral Fe 2+ ions in the crystal structure of Mn x Fe 3−x O 4 were replaced by tetrahedral Si 2+ ions in the crystal structure of silicates to form (Mn y Fe x Si 1−x−y )Fe 2 O 4 at higher oxidation roasting temperatures.Next, the tetrahedral Fe 2+ ions and Si 2+ ions in the crystal structure of (Fe x Si 1−x )Fe 2 O 4 were replaced by tetrahedral Mn 2+ and Fe 2+ ions to form (Mn y Fe x Si 1−x−y )Fe 2 O 4 (0 < x + y < 1) at higher oxidization roasting temperatures.The replaced Mn 2+ ions and Fe 2+ ions were combined with the replaced Si 2+ ions to form Mn x Fe 2−x SiO 4 (0 ≤ x ≤ 2) by the oxidation roasting process.In summary, higher oxidization roasting temperatures are recommended for improving the recrystallization process of MnFe 2 O 4 .
Figure 7 shows the scanning electron micrographs of the cross-sectional layers of the MnO-Fe 2 O 3 -SiO 2 sample when roasted at 1100 • C for 120 min.The sample cross-section was found to have an irregular polyhedral structure and pores of different sizes, whereas there were obvious boundaries between individual particles and obvious precipitates at the edges of individual particles.Figure 7a presents the results of the line-scan energy spectrum, which shows that the content of Mn, Fe and O in the precipitates is close to 1:2:4, while the content of Si is almost nonexistent.This shows that Si cannot enter the crystal lattice of MnFe 2 O 4 and that SiO 2 does not react with the MnO-Fe 2 O 3 system under the experimental conditions.The reason for the presence of free SiO 2 in Figure 5 was further verified.The results of Figure 7b were obtained by energy spectrum scanning of points A and B. The apparent Si to O atomic ratio was approximately 1:2, which again verified that SiO 2 exists in the free state and that its morphology is obviously different from that of manganese ferrite.Combined with the presence of Si in the XRD spectrum of Figure 6, it can be inferred that free SiO 2 and (Mn x Fe 1−x ) 2 SiO 4 are mainly attached to the surface layer of the product and have similar polyhedral structures.The presence of SiO 2 mainly affects the microstructure of manganese ferrite compared with the SEM image of the MnO-Fe 2 O 3 system.
Intensity (A.U.) 12 24 36 48 60 72 84 Figure 7 shows the scanning electron micrographs of the cross-sectional layers of the MnO-Fe2O3-SiO2 sample when roasted at 1100 °C for 120 min.The sample cross-section was found to have an irregular polyhedral structure and pores of different sizes, whereas there were obvious boundaries between individual particles and obvious precipitates at the edges of individual particles.Figure 7a presents the results of the line-scan energy spectrum, which shows that the content of Mn, Fe and O in the precipitates is close to 1:2:4, while the content of Si is almost nonexistent.This shows that Si cannot enter the crystal lattice of MnFe2O4 and that SiO2 does not react with the MnO-Fe2O3 system under the experimental conditions.The reason for the presence of free SiO2 in Figure 5 was further verified.The results of Figure 7b were obtained by energy spectrum scanning of points A and B. The apparent Si to O atomic ratio was approximately 1:2, which again verified that SiO2 exists in the free state and that its morphology is obviously different from that of manganese ferrite.Combined with the presence of Si in the XRD spectrum of Figure 6, it can be inferred that free SiO2 and (MnxFe1−x)2SiO4 are mainly attached to the surface layer of the product and have similar polyhedral structures.The presence of SiO2 mainly affects the microstructure of manganese ferrite compared with the SEM image of the MnO-Fe2O3 system.Figure 9 displays the SEM image of the cross-sectional layers of the sample roasted at 1100 °C with a mass content of 6% Al2O3.It can be clearly seen that the synthesized sample presents a regular polyhedral structure with a relatively uniform distribution.There are obvious boundaries between the particles, and the morphology of individual particles is mainly flat and spherical.The results obtained by the energy spectral line scan of the physical phase are shown in Figure 9b, which illustrates that the O, Fe and Mn contents are basically stable and the contents are close to 4:2:1, indicating that the composition of the synthesized sample is relatively pure.Combined with the analysis in Figure 9b, only a very small amount of Mny(Fe3−y−zAlz)O4 was present, which further proves the slow solid-phase reaction of Al2O3 with manganese and iron oxides.From Figure 9a, it is found that the contents of Fe and Mn at different positions are relatively evenly distrib-

Oxidative Competition Behavior and Formation Mechanism in Synthesis Process of MnFe2O4
Based on the abovementioned results and analysis, the schematic diagram for preparation is illustrated in Figure 10a during the oxidative roasting process.In summary, the three stages were roughly divided for synthesizing manganese ferrite MnFe2O4 in Equations (1)- (7).In the initial stages (Equations ( 1)-( 3)), magnetite (Fe)x[Fe]2O4 (0.878 ≤ x ≤ 1) was easily oxidized to hematite, which contained both hexagonal and cubic crystal structures.The existence of magnetite (Fe)x[Fe]2O4 and maghemite γ-Fe2O3 have a spinel-type crystal structure with a face-center cubic (FCC) configuration, in which both of them combine with MnO to form (MnxFe1−x)Fe2O4 (0 ≤ x ≤ 1) with a similar crystal structure [38,39].Primarily because the tetrahedral Fe 2+ ions in magnetite were partially replaced by octahedral Fe 3+ ions, the octahedral coordination occupied by the Fe 2+ ions generated vacancies, providing an advantageous environment for the Mn 2+ ions to form (MnxFe1−x)Fe2O4 (0 ≤ x ≤ 1) during the oxidization roasting process of converting from magnetite to maghemite.In the second stage (Equations ( 4)-( 6)), the replaced Fe 2+ ions were directly oxidized to Fe 3+ ions, while the other Fe 2+ ions combined with Mn 2+ ions to form an interme-  Based on the abovementioned results and analysis, the schematic diagram for preparation is illustrated in Figure 10a during the oxidative roasting process.In summary, the three stages were roughly divided for synthesizing manganese ferrite MnFe 2 O 4 in Equations ( 1)- (7).In the initial stages (Equations ( 1)-( 3)), magnetite (Fe) x [Fe] 2 O 4 (0.878 ≤ x ≤ 1) was easily oxidized to hematite, which contained both hexagonal and cubic crystal structures.The existence of magnetite (Fe) x [Fe] 2 O 4 and maghemite γ-Fe 2 O 3 have a spinel-type crystal structure with a face-center cubic (FCC) configuration, in which both of them combine with MnO to form (Mn x Fe 1−x )Fe 2 O 4 (0 ≤ x ≤ 1) with a similar crystal structure [38,39].Primarily because the tetrahedral Fe 2+ ions in magnetite were partially replaced by octahedral Fe 3+ ions, the octahedral coordination occupied by the Fe 2+ ions generated vacancies, providing an advan-

3 . 1 .
Figure1displays the XRD patterns of the MnO-Fe 2 O 3 system oxidized and roasted at 900 to 1200 • C for 60 min.As shown in Figure1a, the characteristic diffraction peaks

Figure 1 .Figure 1 .
Figure 1.XRD patterns of the synthetic MnFe2O4 samples at a temperature from 900 to 1200 °C with the atomic ratio of MnO: Fe2O3 = 1:1 for 60 min in air atmosphere and the crystal structure of spinel MnFe2O4, (a) the oxidization roasting temperature of 900 °C to 1200 °C, (b) Partial enlargement of Figure with two-theta range from 33.5° to 36.5°,(c) Partial enlargement of Figure with two-theta range from 32.5° to 36.7°,(c) Crystal structure of spinel MnFe2O4.

Figure 1 .Figure 2 .
Figure 1.XRD patterns of the synthetic MnFe2O4 samples at a temperature from 900 to 1200 °C with the atomic ratio of MnO: Fe2O3 = 1:1 for 60 min in air atmosphere and the crystal structure of spinel MnFe2O4, (a) the oxidization roasting temperature of 900 °C to 1200 °C, (b) Partial enlargement of Figure with two-theta range from 33.5° to 36.5°,(c) Partial enlargement of Figure with two-theta range from 32.5° to 36.7°,(c) Crystal structure of spinel MnFe2O4.

Figure 2 .
Figure 2. SEM-EDS micrograph of the surface layer in the section of the synthetic manganese ferrite samples in air atmosphere, for 60 min (a) Samples roasted at 900 • C (b) Samples roasted at 1000 • C and Samples roasted at 1100 • C. (c) The atomic ratios and contents of Mn, Fe and O on individual particles, A and B.

Figure 3 .
Figure 3. Magnetic hysteresis loops of MnFe2O4 samples roasted at temperatures in the range of 900 °C-1200 °C for 60 min in an air atmosphere.

Figure 3 .
Figure 3. Magnetic hysteresis loops of MnFe 2 O 4 samples roasted at temperatures in the range of 900 • C-1200 • C for 60 min in an air atmosphere.3.1.3.Oxidative Competition Behavior of MnO-Fe 2 O 3 System in Oxidization Roasting Process

Figure 4 .
Figure 4. XPS spectra of the synthetic MnFe2O4 with (a) Spectra in Fe2p region and (b) Spectra in Mn2p region.

Figure 4 .
Figure 4. XPS spectra of the synthetic MnFe 2 O 4 with (a) Spectra in Fe2p region and (b) Spectra in Mn2p region.

Crystals 2023 ,Figure 5 .
Figure 5. Element ratios on the surface (a) and Mössbauer spectrum (b) at different oxidation roasting temperature for 30 min in air atmosphere, A-tetrahedral site and Boctahedral site.3.2.Discussion on the Oxidization Behavior of MnO-Fe2O3-SiO2 System Figure 6 illustrates the XRD patterns of the MnO-Fe2O3-SiO2 system at temperatures from 900 to 1200 °C with different SiO2 contents.The XRD patterns at different roasting temperatures from 900 °C to 1200 °C are shown in Figure 6a.The main phases detected

Figure 5 .
Figure 5. Element ratios on the surface (a) and Mössbauer spectrum (b) at different oxidation roasting temperature for 30 min in air atmosphere, A-tetrahedral site and B-octahedral site.

3. 2 .Figure 6
Figure6illustrates the XRD patterns of the MnO-Fe 2 O 3 -SiO 2 system at temperatures from 900 to 1200 • C with different SiO 2 contents.The XRD patterns at different roasting temperatures from 900 • C to 1200 • C are shown in Figure6a.The main phases detected are Mn 2 O 3 , Fe 2 O 3 , SiO 2 and a small amount of Mn x Fe 3−x O 4 at 900 • C, indicating that Mn 2 O 3 and Fe 2 O 3 are poorly synthesized Mn x Fe 3−x O 4 at lower temperatures.It can be seen from Figure6a,b that the diffraction peaks of Mn 2 O 3 and Fe 2 O 3 disappear, the intensity of the characteristic peak of Mn x Fe 3−x O 4 constantly increases and the main diffraction peak approaches the main characteristic peak of MnFe 2 O 4 (PDF#10-0319) with 2θ = 34.981• as the roasting temperature increases from 900 • C to 1100 • C. SiO 2 exists mainly as free SiO 2 and a very small amount of manganese-iron olivine phase (Mn x Fe 1−x ) 2 SiO 4 , corresponding to the card information (PDF#65-0466, 2θ = 26.636• ) in this temperature range.This demonstrated that the influence of SiO 2 on the oxidative roasting of the MnO-Fe 2 O 3 system for the synthesis of MnFe 2 O 4 is small when the roasting temperature is within 900-1100 • C. As the temperature increases to 1200 • C, the intensity of the characteristic peaks of SiO 2 and MnFe 2 O 4 slightly decreases, while (Mn x Fe 1−x ) 2 SiO 4 is relatively enhanced, indicating that the excessive temperature will force SiO 2 to react with some of the Fe and Mn oxides to form a small amount of manganese-iron olivine phase (Mn x Fe 1−x ) 2 SiO 4 .XRD patterns of SiO 2 with different contents were roasted at 1100 • C for 120 min in the MnO-Fe 2 O 3 system.The intensity of the characteristic diffraction peaks of SiO 2 and (Mn x Fe 1−x ) 2 SiO 4 in the product increased with increasing SiO 2 content, the intensity of the characteristic peak of MnFe 2 O 4 decreased and then increased and the main characteristic peak shifted from 2θ = 34.936• to 2θ = 35.022• .It can be inferred that the formation of (Mn y Fe x Si 1−x−y )Fe 2 O 4 is related to both temperature and SiO 2 content.The formation of (Mn y Fe x Si 1−x−y )Fe 2 O 4 mainly originated from Mn x Fe 3−x O 4 , in which the tetrahedral Fe 2+ ions in the crystal structure of Mn x Fe 3−x O 4 were replaced by tetrahedral Si 2+ ions in the crystal structure of silicates to form (Mn y Fe x Si 1−x−y )Fe 2 O 4 at higher oxidation roasting temperatures.Next, the tetrahedral Fe 2+ ions and Si 2+ ions in the crystal structure of (Fe x Si 1−x )Fe 2 O 4 were replaced by tetrahedral Mn 2+ and Fe 2+ ions to form (Mn y Fe x Si 1−x−y )Fe 2 O 4 (0 < x + y < 1) at higher oxidization roasting temperatures.The replaced Mn 2+ ions and Fe 2+ ions were combined with the replaced Si 2+ ions to form Mn x Fe 2−x SiO 4 (0 ≤ x ≤ 2) by the oxidation roasting process.In summary, higher oxidization roasting temperatures are recommended for improving the recrystallization process of MnFe 2 O 4 .Figure7shows the scanning electron micrographs of the cross-sectional layers of the MnO-Fe 2 O 3 -SiO 2 sample when roasted at 1100 • C for 120 min.The sample cross-section was found to have an irregular polyhedral structure and pores of different sizes, whereas there were obvious boundaries between individual particles and obvious precipitates at the edges of individual particles.Figure7apresents the results of the line-scan energy spectrum, which shows that the content of Mn, Fe and O in the precipitates is close to 1:2:4, while the content of Si is almost nonexistent.This shows that Si cannot enter the crystal lattice of MnFe 2 O 4 and that SiO 2 does not react with the MnO-Fe 2 O 3 system under the experimental conditions.The reason for the presence of free SiO 2 in Figure5was further verified.The results of Figure7bwere obtained by energy spectrum scanning of points A and B. The apparent Si to O atomic ratio was approximately 1:2, which again verified that SiO 2 exists in the free state and that its morphology is obviously different from that of manganese ferrite.Combined with the presence of Si in the XRD spectrum of Figure6, it can be inferred that free SiO 2 and (Mn x Fe 1−x ) 2 SiO 4 are mainly attached to the surface layer of the product and have similar polyhedral structures.The presence of SiO 2 mainly affects the microstructure of manganese ferrite compared with the SEM image of the MnO-Fe 2 O 3 system.

Figure 8
Figure 8 illustrates the XRD patterns of the MnO-Fe2O3-Al2O3 system at 900~1200 °C and at 1100 °C with different Al2O3 contents.Figure 8a shows the XRD patterns at different roasting temperatures from 900 °C to 1200 °C.The main phases detected are Mn2O3, Fe2O3 and a small amount of MnxFe3−xO4 at 900 °C, indicating that MnxFe3−xO4 is not easily synthesized by Mn2O3 and Fe2O3 at low temperatures.Al was present in the form of Al2O3, and other than a small amount of Al2O3 characteristic peaks, no other forms of characteristic peaks were obviously identified.As the roasting temperature increases from 900 °C to 1100 °C, the diffraction peaks of Mn2O3 and Fe2O3 disappear, and the intensity of the characteristic peak of MnxFe3−xO4 increases continuously.The intensity of the characteristic peak of MnxFe3−xO4 increases continuously and moves toward the main characteristic peak of MnFe2O4 (PDF#10-0319) at 2θ = 34.981°.When the temperature increases from 1000 °C to 1100 °C, the diffraction peak of MnFe2O4 shifts from 2θ = 34.921° to 2θ = 35.176°.At the same time, the presence of spinel phases Mny(Fe3−y−zAlz)O4 and FexO4 were detected, however in small amounts.The above results indicate, on one hand, that some MnxFe3−xO4 was

Figure 7 .
Figure 7. SEM-EDS micrograph of the surface layer in the section of the synthetic manganese ferrite samples in air atmosphere, (a) Samples roasted at 1100 • C for 60 min, (b) Samples roasted at 1100 • C for 120 min.

3. 3 . 16 34
Figure 8 illustrates the XRD patterns of the MnO-Fe 2 O 3 -Al 2 O 3 system at 900~1200 • C and at 1100 • C with different Al 2 O 3 contents.Figure 8a shows the XRD patterns at different roasting temperatures from 900 • C to 1200 • C. The main phases detected are Mn 2 O 3 , Fe 2 O 3 and a small amount of Mn x Fe 3−x O 4 at 900 • C, indicating that Mn x Fe 3−x O 4 is not easily Mny(Fe3-y-zAlz)O4(0≤y,z≤1)

Figure 8 .
Figure 8. XRD patterns of MnFe2O4 samples calcined in MnO-Fe2O3-Al2O3 system in air atmosphere; (a) XRD patterns of MnFe2O4 samples with a MnO to Fe2O3 ratio of 1:1 and the Al2O3 mass fraction of 6% calcined at 900~1200 °C for 60 min; (b) XRD patterns of different Al2O3 contents roasted at 1100 °C for 60 min.

Figure 8 .
Figure 8. XRD patterns of MnFe 2 O 4 samples calcined in MnO-Fe 2 O 3 -Al 2 O 3 system in air atmosphere; (a) XRD patterns of MnFe 2 O 4 samples with a MnO to Fe 2 O 3 ratio of 1:1 and the Al 2 O 3 mass fraction of 6% calcined at 900~1200 • C for 60 min; (b) XRD patterns of different Al 2 O 3 contents roasted at 1100 • C for 60 min.

Figure 9 4 、 4 AFigure 9 .
Figure 9 displays the SEM image of the cross-sectional layers of the sample roasted at 1100 • C with a mass content of 6% Al 2 O 3 .It can be clearly seen that the synthesized sample presents a regular polyhedral structure with a relatively uniform distribution.There are obvious boundaries between the particles, and the morphology of individual particles is mainly flat and spherical.The results obtained by the energy spectral line scan of the physical phase are shown in Figure 9b, which illustrates that the O, Fe and Mn contents are basically stable and the contents are close to 4:2:1, indicating that the composition of the synthesized sample is relatively pure.Combined with the analysis in Figure 9b, only a very small amount of Mn y (Fe 3−y−z Al z )O 4 was present, which further proves the slow solid-phase reaction of Al 2 O 3 with manganese and iron oxides.From Figure 9a, it is found that the contents of Fe and Mn at different positions are relatively evenly distributed, showing an overall ratio of 2:1.There is an obvious gradient of Al, which indicates that Al 2 O 3 enters the spinel-type manganese ferrite more slowly.In summary, the presence of a low content of Al 2 O 3 in the MnO-Fe 2 O 3 system has less effect on manganese ferrite, but a synthesis temperature above 1100 • C and Al 2 O 3 content above 6% will promote the conversion of Mn x Fe 3−x O 4 to Mn y (Fe 3−y−z Al z )O 4 .This led to a decrease in the amount of manganese ferrite produced, which in turn affected the purity of the product.Crystals 2023, 13, x FOR PEER REVIEW 12 of 16

Figure 9 .
Figure 9. SEM-EDS micrograph of the surface layer in the section of the synthetic manganese ferrite samples in air atmosphere, (a) Samples roasted at 1100 • C for 60 min, (b) Samples roasted at 1100 for 120 min.

3. 4 .
Oxidative Competition Behavior and Formation Mechanism in Synthesis Process of MnFe 2 O 4 Sample A was Fe 2 O 3 -MnO powder used to obtain optimized process parameters for this technology.Sample B was a mixture of pure SiO 2 , MnO and Fe 2 O 3 powders used to clarify the effect of SiO 2 on the synthesis mechanism of MnFe 2 O 4 .Finally, sample C was a mixture of pure MnO, Fe 2 O 3 and Al 2 O 3 powders used to elucidate the effect of Al 2 O 3 on the synthesis mechanism of MnFe 2 O 4 .
displays the crystal structure of MnFe 2 O 4 , which is able to verify the transformation of Mn x Fe 3−x O 4 to MnFe 2 O 4 in the mixture with increasing roasting temperature.In the MnFe 2 O 4 mixed spinel structure, the metal cations Mn 2+ and Fe 3+ are distributed in the A (tetrahedral) and The above analysis demonstrates that MnFe 2 O 4 cannot be stabilized at lower stabilization temperatures.However, with increasing roasting temperature, the spinel manganese ferrite MnFe 2 O 4 can exist stably.In addition, PDF standard card (MnFe 2 O 4 , PDF#74-2403, cubic, Fd-3m (2 2 7), 8.511 Å × 8.511 Å × 8.511 Å) and then the XRD Rietveld refinement indicated that the synthesized MnFe 2 O 4 had a cubic structure, and the cell parameter was 8.505 Å × 8.505 Å × 8.505 Å, which was close to the abovementioned PDF standard card for MnFe 2 O 4 .