In ﬂ uence of High Temperature on the Crystal Structure of SrFe 12 O 19 Nanoparticle

In this study, strontium hexaferrite nanoparticles synthesized by sol – gel autocombustion method and the effect of high annealing temperature on produced nanoparticle have been studied. Powders were annealed at 1,000 and 1,300 ° C. Structural properties of synthesized powders were analyzed by X-ray diffraction (XRD). Hexagonal structure and shape of SrFe 12 O 19 nanoparticles were analyzed from ﬁ eld-emission scanning electron microscope and transmission electron microscope ﬁ gures. Fourier-transform infrared spectroscopy con ﬁ rmed the formation of an M-type hexagonal structure. The thermal behavior of nanoparticles has been investigated by thermogravimetric analysis (TGA and DTG). Height measurement of nanoparticles with 2D and 3D images of the surface obtained from atomic force microscopy. XRD pattern of strontium hexaferrite nanoparticle was used for determining the crystal structure and examining the effect of high annealing temperature on powders by ﬁ nding lattice parameters, densities, crystallite size, speci ﬁ c surface area, and strain. Crystallite size, strain, and bulk density decreased when produced nanoparticles annealed at 1,000 ° C with decreased porosity when nanoparticles were annealed at 1,300 ° C. The speci ﬁ c surface area was found to be increased when nanoparticles were annealed at 1,000 ° C. In this study, in ﬂ uence of high temperature on the structure of SrFe 12 O 19 nanoparticles was studied.


Introduction
M-type hexagonal ferrite nanoparticles with the chemical structure of MFe 12 O 19 (M = Ba, Sr, and Pb) have risen as composite materials of significant scientific and technological significance due to their high coercive force, thermal diffusivity, and powerful magnetic anisotropic field [1,2]. The dominant composition in the iron-rich part of the pseudo-binary system SrO-Fe 2 O 3 is M-type SrFe 12 O 19 [3]. Strontium ferrite (SrFe 12 O 19 ), a hard magnetic material identified in the 1950s, has been intensively studied for use in microwave devices, permanent magnets, disk drivers, highdensity magnetic memory media, and so on [4,5].
Ferrites are classified according to their crystal structure and properties. The crystal structure of material controls various properties [32]. Different kinds of ferrites have various structures ranging from simple to complex [33]. It had been noted that M-type strontium ferrite is stable till 1,448°C [1]. Strontium hexaferrite has exceptional chemical stability and corrosion resistance, as well as reasonably high electrical resistivity, magnetic anisotropy, and Curie temperature values [34]. The magnetic characteristics of M-type ferrites result from interactions between metallic ions holding certain locations relative to the oxygen ions in its hexagonal crystalline structure. The M-type ferrites crystallize in a hexagonal form with space group p63/mmc [35]. With these attributes for scientific and industrial applications in the sectors of telecommunications, the recording industry, magneto-optical, and microwave devices, micron to submicron strontium hexaferrite powders are promising prospects [34]. Many experts have focused on the effect of temperature on the crystal structure. Roohani et al. [36] determined crystal structure of strontium hexaferrite annealed at different temperatures and found that crystallite size increases when annealed temperature increases. Wong et al. [37] prepared strontium hexaferrite by sol-gel autocombustion prepared at 800 and 1,000°C which a mixture of hematite and strontium hexaferrite was observed when annealed at 800°C while no other impurities were detected when annealed at 1,000°C.
In this study, the effect of high temperature on SrFe 12 O 19 nanoparticles that annealed at 1,000 and 1,300°C has been discussed in detail. The novelty of this research lies in examing the relationship between annealing temperature and produced SrFe 12 O 19 nanoparticles. Thus one can determine different applications depending on the resulting nanostructures. An entire procedure for determining various parameters from X-ray diffraction (XRD) is described. X-ray crystallography is a tool used for identifying the structure of a crystal powder as well as lattice parameters of the unit cell, densities from XRD patterns. Surface structure was studied by field-emission scanning electron microscope (FESEM), transmission electron microscope (TEM), and atomic force microscopy (AFM). Structural studies were tested by Fourier-transform infrared spectroscopy (FTIR) and the thermal stability of synthesized nanoparticle was tested by differential thermogravimetric analysis (TGA-DTG).  [38]. The mixtures were sonicated and temperature was kept at 50°C, then gradually ammonia was added to increase the pH of the solution to 7. Increased pH leads to the development of a gel network. The 3D network structure is complete at pH values of 6 and 7. Gels made from solutions with a pH greater than 7 impede the process of spontaneous igniting [39]. To obtain viscous gel from solution the temperature of the solution gradually increased to 100°C with continuous stirring. The fluffy powder was obtained when the viscous gel is put in an oven heated at 200°C. The obtained fluffy powder was grinded and annealed for 5 hr at (1,000 and 1,300°C) with a heating rate of 5°C/min.
The analytical method for noncubic crystals was used to calculate the lattice parameters a (Å), c (Å), and c/a (in hexagonal-shaped nanoparticles, a = b) [40].
where a and c are the unit cell's lattice parameters, k is the shape factor 0.94, λ is the incident radiation, β is the full width half maximum, θ is the peak angle in radians, n is the number of atoms per unit cell, M is the molecular weight of the material, N A is Avogadro's number, and "mass" indicates the total amount of mass fitted to the sample holder to be tested by XRD [28]. Morphological studies were tested by FESEM, TEM, and AFM. Structural studies were examined by FTIR and the thermal stability of synthesized powders was tested by TGA-DTG.

Results and Discussion
3.1. X-Ray Diffraction. XRD pattern for strontium hexaferrite annealed at 1,000 and 1,300°C is shown in Figure 1. The production of hexagonal SrFe 12 O 19 crystallites with a space group of P63/mmc is obviously confirmed by diffraction peaks. This indicates that all samples are well crystallized into SrFe 12 O 19 . Highly purified SrFe 12 O 19 nanoparticles formed without detecting any other additional phases [41]. The intensity peaks of strontium hexaferrite annealed at 1,000°C are higher than that of strontium hexaferrite peaks that annealed at 1,300°C. To understand the effect of temperature on prepared strontium hexaferrite nanoparticles, we found 11 parameters from the XRD pattern. Initially, the lattice parameters a (Å), c (Å), and c/a for noncubic crystals were found analytically [40]. When lattice parameters are established, the next nine parameters can be determined. From calculations in Table 1, the lattice parameters remain unchanged. This means that the high annealing temperature does not effect on the system's lattice parameters [42]. The average crystallite size increased when nanoparticles were annealed at 1,300°C compared to annealed samples at 1,000°C, which is found to be 41.48 and 33.98 nm for nanoparticles annealed at 1,300 and 1,000°C. When the annealing temperature increased, the average crystallite size increased [43,44]. Bulk density increased when the sample was annealed at a higher temperature, that is bulk density depends upon temperature.
Bulk density calculated from Equation (4) was found to be increased when annealed at a higher temperature of 1,300°C because of the crystal formation phase and the dispersion of the compound atoms when the annealing temperature increased [45]. Porosity depends on bulk density. Porosity decreases when bulk density increases. That is the porosity of strontium hexaferrite decreased when powders were annealed at 1,300°C. To improve absorption capacity per unit mass and increase volume/area ratio, specific surface area found which found to be higher when strontium hexaferrite nanoparticles were annealed at 1,000°C. The specific surface area was found to be 34.56 and 28.31 m 2 /g for nanoparticles annealed at 1,000 and 1,300°C [46]. A measure of the number of dislocations in a unit volume of a crystalline material is known as the dislocation density. Less number of dislocations exhibit high mechanical strength [47]. As reported from Equation (7), the dislocation density is inversely proportional to crystallite size [48], dislocation density is found to be lower when nanoparticles are annealed at 1,300°C, as shown in Table 1, in accordance obtaining a bigger grain size. Lattice strain was found to be decreased when nanoparticles were annealed at 1,000°C with lower crystallite size synthesized. Thus a higher dislocation density leads to lower strain rate sensitivity [49,50]. The intensity of the X-ray peaks decreases as powders annealed at higher temperature. This shows that the crystallinity of SrFe 12 O 19 becomes better at lower annealing temperature [51].
The XRD pattern with Rietveld refinement of the SrFe 12 O 19 nanoparticles is shown in Figure 2. The refinements were carried out by using Foolproof software [52]. Rietveld refinement offers a structural model that has an approximation for the original structure. The space group of P63/mmc is used for Rietveld refinement of XRD patterns for strontium hexaferrite which originates from hexagonal structure [53,54]. The parameters R p (R-pattern factor), R wp (R-weighted pattern factor), and χ 2 (goodness of fit factor) obtained after refinement are shown in Table 2.

Field-Emission Scanning Electron
Microscope. At high temperatures surface properties were represented in FESEM figures, as shown in Figure 3, to evaluate the changes in powders. The micrographs reveal that strontium hexaferrite annealed at 1,000°C as in Figure 3 shows the presence of hexaferrite indicating a proper c-axis orientation of nanoparticle. Nanoparticles annealed at 1,000°C possess a uniform hexagonal shape with narrower particle size distribution hence narrow size distribution leads to lower porosity [55]. When particles are annealed at 1,300°C the nanoparticles illustrate an increase in the average grain sizes with calcination temperature due to the fusion of smaller grains into larger grains [56][57][58]. Particle size distribution was shown in Figure 3 for nanoparticles annealed at 1,000 and 1,300°C. Mean particle size was found to be 195.33 and 1,395.07 nm with standard deviation of 74.31 and 679.3 nm for strontium hexaferrite when annealed at 1,000 and 1,300°C. These results reveal that at higher temperature of 1,300°C particle size increased with wider size distribution compared to smaller size and narrower particle size distribution when annealed at 1,000°C. The microstructure of the materials is seen using FESEM [59]. Particle size measured from FESEM might be in the form of single crystal or agglomeration of several crystals [60,61] therefore the results show bigger particle sizes as compared to XRD crystallite size, which was found to be 33.98 and 41.48 nm for nanoparticles, annealed at 1,000 and 1,300°C.

Transmission Electron
Microscope. The TEM micrographs of strontium hexaferrite nanoparticles annealed at 1,000 and 1,300°C for 5 hr are shown in Figure 4. As can be seen, most of the grains shown up as hexagonal platelets having sharp grain boundaries and hollows can be seen between the particles, leads in the formation of some agglomerations as shown in Figure 4 for nanoparticles annealed at 1,000°C. TEM images of strontium hexaferrite nanoparticles annealed at 1,300°C can be seen in Figure 4. The hexagonal shape of these particles varies with elevated temperatures near to the melting point of the substance and shows a more irregular hexagonal shape due to the merging of adjacent nanoparticles when the surface diffusion increased at a higher annealing temperature of 1,300°C [62]. No boundaries observed nanoparticles might aggregate at higher temperatures [63]. Primary particle size distribution was found by TEM micrographs as shown in Figure 4. Main primary particle size was found to be 62.15 and 70.9 nm with standard deviations of 25.93 and 41.14 for strontium hexaferrite nanoparticles annealed at 1,000 and 1,300°C. The results clarify that primary nanoparticle size increases when nanoparticles annealed at 1,300°C. Crystallite size from XRD is a measurement of coherent diffracting domain size. Due to the existence of polycrystalline aggregates, the crystallite size of the particles is typically different from the particle size [64].
3.4. Fourier-Transform Infrared Spectroscopy. FTIR spectra act as a useful technique for structural characterization. A spectrum of SrFe 12 O 19 nanoparticles that calcined at 1,000 and 1,300°C in the range of 400-4000 cm −1 is illustrated in Figure 5. The vibration observed around 3,434 cm −1 attributed to O-H stretching vibration of adsorbed water     Journal of Nanomaterials molecules, which was found to be low when annealed at 1,000°C. Absorption band at 2,925 and 2,859 cm −1 relates to C-H stretching vibration with symmetrical and asymmetrical stretching vibrations of CO 2− group at 1617 cm −1 . Observed peaks for all the samples in the range of 1,100-1,500 cm −1 correspond to metal-oxygen-metal (Fe-O-Fe) bonds [65].
The tetrahedral and octahedral Fe 3+ -O stretching vibrations conform to the frequency absorption bands at 599 and 554 cm −1 , and the characteristic peaks around 444 cm −1 are attributed to the Sr-O stretching vibration band [66], which ensures the hexaferrite structure and peak concentration corresponding to strontium hexaferrite are found to be increased when annealed at 1,000°C [67].
3.5. Differential Thermogravimetric Analysis. To investigate the thermal stability and the weight loss of the prepared nanoparticles samples were characterized by TGA-DTG as shown in Figure 6. The TGA curve indicates that from 100 to 200°C there is a rapid weight loss of about 30% for the nanoparticles annealed at 1,000°C this weight loss decreased   to 10% for temperatures from 100 to 350°C when powders annealed at 1,300°C which can be attributed to the dehydration of the reactants [68]. Gradually weight gain was observed of ∼15.6% and 6.3% for powders annealed at 1,000 and 1,300°C that is due to the oxidation processes. DTG conversion curve for temperature less than 150°C is due to the dehydration process and moisture leaves the sample with insignificant change in the mass [69].
3.6. Atomic Force Microscopy. AFM images display the surface morphology of the nanopowders are synthesized under different annealing temperatures as shown in Figure 7. Gwyddion (open-source software) is used for AFM color rendering. Height profiles, 2D images, and 3D images of AFM results were extracted from Gwyddion software [70]. Nanopowders annealed at 1,000°C grew into sheet grains with a uniform size which agrees with FESEM images. The thickness of surface roughness concluded from height profile as shown in Figure 7 is around 2-5 nm when nanopowders annealed at 1,000°C which is lower than powders annealed at 1,300°C that is from 2 to 13 nm and with some much greater nanoparticles. Grain height distribution as obtained from height profile with mean grain height of nanoparticle at surface to be as 2.22 and 4.91 nm for nanoparticles annealed at 1,000 and 1,300°C. This might be due to that at a higher temperature of 1,300°C the nanoparticles grow bigger as a result of fusing smaller nanoparticles with bigger ones so the flatness of the surface becomes worse. Upon results, the morphology and crystal structure of strontium hexaferrite nanoparticles can be controlled by annealing temperature [71].
3.7. Energy-Dispersive Spectroscopy. Energy-dispersive spectroscopy (EDS), EDX, or EDXA, commonly known as energy-dispersive X-ray spectroscopy, is an effective method that enables the user to examine the elemental composition of a sample. EDS spectrum for strontium hexaferrite nanoparticles annealed at 1,000 and 1,300°C shown in Figure 8. All samples concurrently detect the targeted Sr, Fe, and O elements [41]. The weight percentage of an Fe 3+ element is increased from 59.2 to 60.4 and weight percentage of an Sr 2+ element decreased from 16.3 to 16.2 when nanoparticles annealed at 1,000 and 1,300°C.

Conclusion
The sol-gel autocombustion method was used to produce strontium hexaferrite annealed at 1,000 and 1,300°C. To study how the annealing temperature affects the structure morphology and surface of synthesized nanoparticles. Crystallite size increases from 33.98 nm at 1,000°C to 40.48 nm at 1,300°C. Bulk density increased for powders annealed at 1,300°C which leads to a porosity percentage decrease from 94 to 90 at 1,000 and 1,300°C. Specific surface area increased due to the smaller crystallite size of nanoparticles when annealed at 1,000°C. Hexaferite shape of nanoparticles is obvious that nanoparticles grow bigger when annealed at 1,300°C. The grain boundary of nanoparticles is seen when annealed at 1,000°C, while at 1,300°C of annealing temperature the grain boundary disappears. FTIR intensity peak corresponds to strontium hexaferrite is higher with lower intensity peak correspond to adsorbed water molecule when annealed at 1,000°C. Uniform size of sheeted grains was obtained when nanoparticles annealed the thickness of surface roughness concluded from height measurement is around 2-5 nm when nanopowders annealed at 1,000°C, while for powders annealed at 1,300°C was 2-13 nm.

Additional Points
Research Highlights. (i) Synthesis M-type strontium hexaferrite nanoparticles by sol-gel autocombustion method. (ii) Study the effect of high annealing temperature on crystal structure and morphology of SrFe 12 O 19 nanoparticles. (iii) Crystal size bulk density decreased when powders annealed at 1,000°C. (iv) Specific surface area increased for nanoparticles annealed at 1,000°C. (v) Narrower particle size distribution from FESEM and TEM when nanoparticles annealed at 1,000°C. (vi) FTIR intensity peak corresponds to strontium hexaferrite is higher with lower intensity peak correspond to adsorbed water molecule when annealed at 1,000°C. (vii) Uniform size of sheeted grains obtained when nanoparticles annealed at 1,000°C. The thickness of surface roughness concluded from height measurement is around 2-5 nm when nanoparticles annealed at 1,000°C, while for powders annealed at 1,300°C was 2-13 nm.  Journal of Nanomaterials