# Influence of High Temperature on the Crystal Structure of SrFe12O19 Nanoparticle

Authors: Znar Saeed; Bruska Azhdar
Journal: Journal of Nanomaterials
 (2022)

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## Abstract

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 SrFe12O19 nanoparticles were analyzed from field-emission scanning electron microscope and transmission electron microscope figures. Fourier-transform infrared spectroscopy confirmed 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 finding lattice parameters, densities, crystallite size, specific 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 specific surface area was found to be increased when nanoparticles were annealed at 1,000°C. In this study, influence of high temperature on the structure of SrFe12O19 nanoparticles was studied.


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## Body

## 1. Introduction

M-type hexagonal ferrite nanoparticles with the chemical structure of MFe12O19 (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–Fe2O3 is M-type SrFe12O19 [3]. Strontium ferrite (SrFe12O19), a hard magnetic material identified in the 1950s, has been intensively studied for use in microwave devices, permanent magnets, disk drivers, high-density magnetic memory media, and so on [4, 5].The synthesis method determines its homogeneity, particle size, and shape. SrFe12O19 can be synthesized by several procedures. The traditional way to synthesize SrFe12O19 using solid-state reaction has some inherent drawbacks such as coarser particle size, chemical substance inhomogeneity, and introduction to impurities during ball milling [6–8]. Different techniques are used to synthesize SrFe12O19 like the hydrothermal method [9–13], coprecipitation method [14–16], the salt melting method [17, 18], mechanical alloying [19–22], and sol–gel autocombustion method [23–28]. Among them, sol–gel autocombustion technique aroused interest because in a relatively short processing time nanoparticles with narrow size distribution were produced superior homogeneity, adjustable stoichiometry, high purity, phase-solid powders at a lower temperature, flexibility in manufacturing dense monoliths, thin films, or nanoparticles are only a few of the benefits of the sol–gel approach over other processes [29–31].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 SrFe12O19 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 SrFe12O19 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).


## 2. Materials and Methodology

### 2.1. Used Materials

Strontium nitrate Sr(NO3)2, iron nitrate nonahydrate Fe(NO3)3·9H2O, citric acid monohydrate C6H8O7·H2O, absolute ethanol alcohol (99.5% absolute; EMPARTA® ACS), and ammonia NH3 were purchased from Merck (Merck-KGaA, Darmstadt, Germany) and mixed with double-distilled water to make strontium hexaferrite nanoparticles.


### 2.2. Strontium Hexaferrites Preparation

The sol–gel autocombustion process was used to make powders of strontium hexaferrite with the chemical formula SrFe12O19. In the minimum amount of double-distilled water that is needed to dissolve the strontium nitrate Sr(NO3)2, iron nitrate nonahydrate Fe(NO3)3·9H2O, citric acid monohydrate C6H8O7·H2O. The calculated concentrations of Fe, Sr, and citric acid with ratios of metal nitrates to citric acid of 1 : 1 and iron nitrate to strontium nitrate of 12 : 1 [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.


### 2.3. Characterization

XRD (Panalytical Xpert Pro®, ul. Radzikowskiego, Kraków, Poland) monochromatized Cu-K radiation was used to identify crystalline phases, with the 2θ scanning angle ranging from 20° to 85° at a rate of 0.1°. Lattice parameters a and b, average crystallite size (Dav), cell volume (Vcell), crystal density (Dx), bulk density (Db), porosity percentage P (%), specific surface area (S), dislocation density (δ), and lattice strain (ε) were calculated as follows.The analytical method for noncubic crystals was used to calculate the lattice parametersa (Å), c (Å), and c/a (in hexagonal-shaped nanoparticles, a = b) [40].(1)Vcell=0.866∗a2∗c,(2)Dav=κλβcos⁡θ,(3)Dx=nMNAvcell,(4)Db=massvolume=masslength∗width∗height,(5)P=1−DbDx∗100,(6)S=6,000Dav∗Dx,(7)δ=1Dav2,(8)ε=β4∗tan⁡θ,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, NA 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.


## 2.1. Used Materials

Strontium nitrate Sr(NO3)2, iron nitrate nonahydrate Fe(NO3)3·9H2O, citric acid monohydrate C6H8O7·H2O, absolute ethanol alcohol (99.5% absolute; EMPARTA® ACS), and ammonia NH3 were purchased from Merck (Merck-KGaA, Darmstadt, Germany) and mixed with double-distilled water to make strontium hexaferrite nanoparticles.


## 2.2. Strontium Hexaferrites Preparation

The sol–gel autocombustion process was used to make powders of strontium hexaferrite with the chemical formula SrFe12O19. In the minimum amount of double-distilled water that is needed to dissolve the strontium nitrate Sr(NO3)2, iron nitrate nonahydrate Fe(NO3)3·9H2O, citric acid monohydrate C6H8O7·H2O. The calculated concentrations of Fe, Sr, and citric acid with ratios of metal nitrates to citric acid of 1 : 1 and iron nitrate to strontium nitrate of 12 : 1 [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.


## 2.3. Characterization

XRD (Panalytical Xpert Pro®, ul. Radzikowskiego, Kraków, Poland) monochromatized Cu-K radiation was used to identify crystalline phases, with the 2θ scanning angle ranging from 20° to 85° at a rate of 0.1°. Lattice parameters a and b, average crystallite size (Dav), cell volume (Vcell), crystal density (Dx), bulk density (Db), porosity percentage P (%), specific surface area (S), dislocation density (δ), and lattice strain (ε) were calculated as follows.The analytical method for noncubic crystals was used to calculate the lattice parametersa (Å), c (Å), and c/a (in hexagonal-shaped nanoparticles, a = b) [40].(1)Vcell=0.866∗a2∗c,(2)Dav=κλβcos⁡θ,(3)Dx=nMNAvcell,(4)Db=massvolume=masslength∗width∗height,(5)P=1−DbDx∗100,(6)S=6,000Dav∗Dx,(7)δ=1Dav2,(8)ε=β4∗tan⁡θ,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, NA 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.


## 3. Results and Discussion

### 3.1. X-Ray Diffraction

XRD pattern for strontium hexaferrite annealed at 1,000 and 1,300°C is shown in Figure1. The production of hexagonal SrFe12O19 crystallites with a space group of P63/mmc is obviously confirmed by diffraction peaks. This indicates that all samples are well crystallized into SrFe12O19. Highly purified SrFe12O19 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.Figure 1
XRD for strontium hexaferrite nanoparticles annealed at 1,000 and 1,300°C.Table 1
Calculated parameters from XRD diffraction data for strontium hexaferrite nanoparticles annealed at 1,000 and 1,300°C.
Annealed temperature (°C)Lattice parametera (Å)Lattice parameterc (Å)c/aVcell (nm3) ∗ 10−3Dav (nm)Dx (g/cm3)Db (g/cm3)P (%)SSA (m2/g)δ (nm−2)ε1,0005.86523.0833.935687.633.985.110.289434.560.000870.0031,3005.86523.0833.935687.0641.485.110.539028.310.000580.0031Bulk 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 m2/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 SrFe12O19 becomes better at lower annealing temperature [51].The XRD pattern with Rietveld refinement of the SrFe12O19 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 Rp (R-pattern factor), Rwp (R-weighted pattern factor), and χ2 (goodness of fit factor) obtained after refinement are shown in Table 2.Figure 2
Rietveld refinement for strontium hexaferrite nanoparticles annealed at 1,000 and 1,300°C.Table 2
TheR-pattern factor (Rp), the R-weighted pattern factor (Rwp), and the goodness of fit factor (χ2) from Rietveld refinement.
Annealed temperature (°C)RpRwpχ21,00023.6311.241,30024.432.71.28


### 3.2. Field-Emission Scanning Electron Microscope

At high temperatures surface properties were represented in FESEM figures, as shown in Figure3, 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 [56–58]. 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.Figure 3
FESEM images and particle size distribution for strontium hexaferrite nanoparticles annealed at 1,000 and 1,300°C.


### 3.3. Transmission Electron Microscope

The TEM micrographs of strontium hexaferrite nanoparticles annealed at 1,000 and 1,300°C for 5 hr are shown in Figure4. 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.Figure 4
TEM images and grain size distribution for strontium hexaferrite nanoparticles annealed at 1,000 and 1,300°C.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 SrFe12O19 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 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 CO2− 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 Fe3+–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].Figure 5
FTIR spectra of strontium hexaferrite nanoparticles annealed at 1,000 and 1,300°C.


### 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 Figure6. 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].Figure 6
TGA-DTG images for strontium hexaferrite nanoparticles annealed at 1,000 and 1,300°C.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 Figure7. 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].Figure 7
2D and 3D images, height profile and grain height distribution obtained from AFM for strontium hexaferrite nanoparticles annealed at 1,000 and 1,300°C.


### 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 Figure8. All samples concurrently detect the targeted Sr, Fe, and O elements [41]. The weight percentage of an Fe3+ element is increased from 59.2 to 60.4 and weight percentage of an Sr2+ element decreased from 16.3 to 16.2 when nanoparticles annealed at 1,000 and 1,300°C.Figure 8
EDS graphs for strontium hexaferrite nanoparticles annealed at 1,000 and 1,300°C.


## 3.1. X-Ray Diffraction

XRD pattern for strontium hexaferrite annealed at 1,000 and 1,300°C is shown in Figure1. The production of hexagonal SrFe12O19 crystallites with a space group of P63/mmc is obviously confirmed by diffraction peaks. This indicates that all samples are well crystallized into SrFe12O19. Highly purified SrFe12O19 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.Figure 1
XRD for strontium hexaferrite nanoparticles annealed at 1,000 and 1,300°C.Table 1
Calculated parameters from XRD diffraction data for strontium hexaferrite nanoparticles annealed at 1,000 and 1,300°C.
Annealed temperature (°C)Lattice parametera (Å)Lattice parameterc (Å)c/aVcell (nm3) ∗ 10−3Dav (nm)Dx (g/cm3)Db (g/cm3)P (%)SSA (m2/g)δ (nm−2)ε1,0005.86523.0833.935687.633.985.110.289434.560.000870.0031,3005.86523.0833.935687.0641.485.110.539028.310.000580.0031Bulk 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 m2/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 SrFe12O19 becomes better at lower annealing temperature [51].The XRD pattern with Rietveld refinement of the SrFe12O19 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 Rp (R-pattern factor), Rwp (R-weighted pattern factor), and χ2 (goodness of fit factor) obtained after refinement are shown in Table 2.Figure 2
Rietveld refinement for strontium hexaferrite nanoparticles annealed at 1,000 and 1,300°C.Table 2
TheR-pattern factor (Rp), the R-weighted pattern factor (Rwp), and the goodness of fit factor (χ2) from Rietveld refinement.
Annealed temperature (°C)RpRwpχ21,00023.6311.241,30024.432.71.28


## 3.2. Field-Emission Scanning Electron Microscope

At high temperatures surface properties were represented in FESEM figures, as shown in Figure3, 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 [56–58]. 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.Figure 3
FESEM images and particle size distribution for strontium hexaferrite nanoparticles annealed at 1,000 and 1,300°C.


## 3.3. Transmission Electron Microscope

The TEM micrographs of strontium hexaferrite nanoparticles annealed at 1,000 and 1,300°C for 5 hr are shown in Figure4. 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.Figure 4
TEM images and grain size distribution for strontium hexaferrite nanoparticles annealed at 1,000 and 1,300°C.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 SrFe12O19 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 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 CO2− 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 Fe3+–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].Figure 5
FTIR spectra of strontium hexaferrite nanoparticles annealed at 1,000 and 1,300°C.


## 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 Figure6. 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].Figure 6
TGA-DTG images for strontium hexaferrite nanoparticles annealed at 1,000 and 1,300°C.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 Figure7. 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].Figure 7
2D and 3D images, height profile and grain height distribution obtained from AFM for strontium hexaferrite nanoparticles annealed at 1,000 and 1,300°C.


## 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 Figure8. All samples concurrently detect the targeted Sr, Fe, and O elements [41]. The weight percentage of an Fe3+ element is increased from 59.2 to 60.4 and weight percentage of an Sr2+ element decreased from 16.3 to 16.2 when nanoparticles annealed at 1,000 and 1,300°C.Figure 8
EDS graphs for strontium hexaferrite nanoparticles annealed at 1,000 and 1,300°C.


## 4. 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.



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*Source: 5467020-2022-11-27.xml*