Formation of Nanoscale Mg ( x ) Fe ( 1 − x ) O ( x = 0 . 1 , 0 . 2 , 0 . 4 ) Structure by Solution Combustion : Effect of Fuel to Oxidizer Ratio

Mg(x)Fe(1−x)O (magnesiowustite) nanopowder samples synthesized by solution-combustion method and fuel to oxidizer ratio (Ψ = 1, 1.25) are used as a control parameter to investigate how particle size and morphology vary with Ψ. The method is inexpensive and efficient for synthesis of oxide nanoparticles. The average crystallite size of Mg(x)Fe(1−x)O nanoparticles was estimated from the full-width-half maximum of the X-ray diffraction peaks of powders using Debye-Scherrer’s formula; the average crystallite size varies from 16 nm to 51 nm. From X-ray diffraction analysis, it was observed that Mg(x)Fe(1−x)O nanoparticles have cubic structure. The particle size measured by particle size analyzer ranges from 37.7 nm to 73 nm which is in the order of XRD results. Thermal analysis was done by thermal gravimetric-differential thermal analyzer. The particle size and morphology of the synthesized powder were examined by transmission electron microscope and scanning electron microscope. The crystal size and particle size were compared with some of the most recently published research works by XRD and TEM. FTIR conforms formation of the Mg(x)Fe(1−x)O.


Introduction
Magnetic Nanoparticles (NPs) have many unique magnetic properties such as super paramagnetic, high coactivity, low Curie temperature, high magnetic susceptibility, and so forth.Magnetic NPs are of great interest for researchers from a broad range of disciplines [1][2][3][4][5], microwaves devices, magnetic recording media, fabrication of radio frequency coils, transformer cores, chock coils, noise filters, and recording, heading, and rod antennas [6][7][8] and also are very important in heterogeneous catalysis, adsorption, and sensors [9,10].Especially, magnetic Ferro fluids and data storage are the applied researches that have led to the integration of magnetic NPs in a myriad of commercial applications.
In the present work, Solution Combustion Synthesis technique (SCS) has been used and is known as one of the effective and economic method [18].Solution Combustion synthesis (CS) [19] has emerged as an important technique for the synthesis and processing of advanced ceramics (structural and functional), catalysts, composites, alloys, and intermetallic and nanomaterial.The method exploits selfsustaining solid flame combustion reaction for the internal development of very short period [20].Fuel to oxidizer ratio [21] is very crucial and effective amount in this method [22], it can change the properties of the Nanomaterial as the reaction temperature depends on fuel to oxidizer ratio.It is well recognized that the fuel is an important component for the preparation of oxides by SCS.However, there are not many studies that have been found in the literature correlating the microstructure and effect of grain size with the magnetic properties of Mg (x) Fe (1−x) O powders.

Experimental
The Mg (x) Fe (1−x) O powders were prepared by using combustion synthesis in solution under different molar compositions of fuel (glycine, NH 2 CH 2 COOH) and fuel to oxidizer ratio Ψ = 1 and Ψ = 1.25.The iron nitrate (Fe(NO 3 ) 3 • 9H 2 O) and magnesium nitrate (Mg(NO 3 ) 3 • 6H 2 O) were dissolved in a beaker with sufficient deionized water with glycine in appropriate amount and the solution was placed under thermal stirring with the temperature of 60 • C; at this temperature, different molar amounts of the fuel were added.The precursor solutions were maintained under thermal stirring for few minutes to ensure homogeneity, subsequently these solutions were placed in a hot plate.As the temperature reached 100 • C, water started to boil and evaporate from the solution which increased solution viscosity substantially.The obtained precursor is annealed for 2 h at 300 • C. The decomposition reactions of starting compositions are (1) The overall combustion reaction in air may be written as The crystal phase of the synthesized powders were determined by X-ray diffraction (XRD, Bruker D8 Advance, Germany) using Cu Kα as radiation source (40 kV, step size 0.02, scan rate 0.5 min −1 ).The particle sizes were measured by nanoparticle size analyzer (SZ-100 Nanoparticle, Horiba, Japan).The thermal decomposition behaviors of the precursors were investigated by thermogravimetric analysis (TGA/DTA, SII EXSTAR 6300R, Japan).Fourier transform infrared spectroscopy was used as well (FTIR, PERKIN ELMER, India).The particles size and morphology of the synthesized powder were examined by scanning electron microscope (SEM, S-3400N-hitachi-Japan), transmission electron microscope (TEM, JEM-2100, Jeol).

Result and Discussion
3.1.Crystal Analysis.The crystalline phases present in the different samples were identified by X-ray diffraction (XRD) on a Bruker axis D8 diffractometer using Cu-Kα (λ = 1.5406Å) radiation and secondary monochromator in the range 2θ from 20 • C to 80 • C. The crystallite size of magnesiwuestite present was investigated based on X-ray diffraction line broadening and calculated by using full-width at halfmaximum (FWHM) of the XRD all peaks by making use of the Debye-Scherrer formula [32], then the average was extracted where D is the crystallite size, λ is the wavelength of Cu-Kα radiation, θ is a diffraction angle, and β is full-width at halfmaximum (FWHM) of the XRD all peaks.Figure 1 shows the XRD pattern of the synthesized nanopowder; the graphs show different Ψ values.It is clear cut that a spinel structure of magnesiwuestite which is described in (JCPDS 89-4924) is detected in all samples.Seven peaks centered at 2θ = 30.28According to JCPDS 89-4924, the obtained phase has a cubic structure.The existence of a peak around the diffraction angle (2θ) equal to 35.66 • C corresponding to (3 1 1) plane confirms the formation of spinel ferrites.A careful analysis of XRD patterns helps to determine their respective planes and face-centered cubic structure of these ferrites.Another method used to calculate particle size is Williamson-Hall equation [33]: where β is full-width at half-maximum (FWHM) of the XRD all peaks, λ is the wavelength of Cu-Kα radiation (constant), k is Scherrer constant, D is the crystallite size, ε is the micro strain, and θ is the Bragg angle.Using a linear line to this plot, the slope gives the strain and the intercept gives the particle size (kλ/D).Based on the diffraction peak positioned at different degrees, the crystallite size is estimated by plotting crystallite size versus peak positions and extrapolation of the straight line by linearly fitting the points (Figure 2).
Table 1 Shows crystallite size (D) (nm) with Williamson-Hall equation, XRD, and particle size analyzer (SZ-100).Particle size of Mg (x) Fe (1−x) O is done by particle size analyzer (SZ-100).Small amount of sample is dissolved in ethanol then sonicated to form clear solution.The particle sizes which were estimated by particle size analyzer (SZ-100) are greater than the particle sizes which were estimated by XRD with Dybe-Scherrer's formula.3 illustrates the thermogravimetric curve of the Mg (x) Fe (1−x) O powder precursor after calcination at 300 • C. Thermal analysis of Mg (x) Fe (1−x) O samples was carried out in air atmosphere.It is well known that materials with a cubic crystal structure are prone to grow into a spherical shape [34][35][36] to minimize the surface tension.Thermogravimetric method measures the mass of a sample as the temperature increasing.This method is useful for determining sample purity of water, carbonate, and organic content and for studying decomposition reaction.TG curve shows consistent weight loss as it is illustrated in Figure 3.Among the samples which are placed in    TG-DTA after calcination, less weight loss and impurity were observed.The net weight loss is illustrated in Table 2.

Fourier Transform Infrared Spectroscopy Analysis.
The FTIR spectra measurement of fresh Mg (x) Fe (1−x) O Nanoparticles in the range of 4000-450 cm −1 are shown in Figure 4.
A broad absorption spectrum is seen around 3430 cm −1 which is a characteristic stretching vibration of hydroxylate group (O-H).Peaks localized at 1560 cm −1 and 1411 cm −1 are assigned to asymmetrical and symmetrical stretching vibration of carboxylate (O-C-O), respectively.IR spectrum of magnesiwuestite is found to exhibit two major bands in the range of 450-700 cm −1 .The bands in the region 450 to 700 cm −1 are attributed to the fundamental vibrations of the ions of the crystal lattice.For the analysis of such spectra, it is necessary to consider the vibrational spectrum of periodic structures.The 1400 cm −1 (broad) characterizes the formation of surface sulfate species with S=O bond.The high frequency band is around 1561 cm −1 .Wave number (cm −1 )   Mg (x) Fe (1−x) O was observed to have high porous structure which was shown in Figure 5.

Conclusion
The nanostructured Mg (x) Fe (1−x) O powder samples were synthesized by simple eco-friendly solution-combustion with economically and environmentally beneficial method.The average crystallite size of Mg (x) Fe (1−x) O nanoparticles was estimated from the full-width-half maximum of the Xray diffraction peaks and particle size analyzer and the results are in the same order.It was observed from XRD pattern that Mg (x) Fe (1−x) O nanoparticles have cubic structure.Thermal analysis was done by Thermogravimetric-Differential thermal analyzer which shows weight loss percentage.FTIR conforms formation of the Mg (x) Fe (1−x) O.The results reported in this study could be applied for establishing a simple method for the preparation of Mg (x) Fe (1−x) O nanopowders.The synthesis process is economically and environmentally beneficial, inexpensive, and efficient method when compared to other solid state reactions.It was observed that Mg (x) Fe (1−x) O has porous structure as it is shown in Figure 5 and has finer dispersion with spherical morphology of particles as illustrated in Figure 6.

Table 2 :
Net weight loss illustration.
graph for six samples and EDS also shows the percentage of element in the powders.It can be seen in Figure6that the particles are quite well dispersed and not much agglomerations are present.The percentage of element is shown in the EDS table.Comparison table for crystallite and particle size from XRD and TEM characterizations of present and previous papers are given in Table3.

Table 3 :
Comparison of particle size determined from TEM.