Abstract
The toxicity of arsenic in drinking water is hazardous for human health. Different strategies are used for arsenic removal from drinking water. Nanoparticles with higher adsorption capacities are useful for arsenic remediation. In the current study, magnesium ferrite nanoparticles were synthesised by three different methods followed by their characterisation XRD, SEM, and EDX. The SEM morphology and the porosity of magnesium ferrite nanoparticles were best in case of auto‐combustion method. These particles had an average particle size of about 20–50 nm with spherical shape. These particles showed efficient remediation of arsenic up to 96% within 0.5 h. However, the co‐precipitation and sol‐gel‐based nanoparticles showed arsenic remediation upto85 and 87% at 0.5‐h time point. Moreover, the minimum inhibitory concentration of nanoparticles against two strains E.coli and Pseudomonas aeruginosa was found to be4.0 mg/L of these nanoparticles. However, the sol‐gel‐based nanoparticles showed efficient anti‐microbial activity against E.coli at 4.0 and 8.0 mg/L against Pseudomonas aeruginosa. The co‐precipitation‐based nanoparticles were least efficient both for arsenic remediation and anti‐microbial purposes. Thus, the synthesised auto‐combustion‐based nanoparticles are multifunctional in nature.
Inspec keywords: nanoparticles, sol‐gel processing, nanofabrication, porosity, nanomagnetics, magnesium compounds, antibacterial activity, X‐ray chemical analysis, ferrites, scanning electron microscopy, X‐ray diffraction, particle size, magnetic particles, surface morphology, precipitation (physical chemistry)
Other keywords: drinking water, arsenic removal, arsenic remediation, magnesium ferrite nanoparticles, SEM morphology, auto‐combustion method, sol‐gel‐based nanoparticles, co‐precipitation‐based nanoparticles, higher adsorption capacity, particle size, XRD, SEM, EDX, porosity, spherical shape, Escherichia coli, Pseudomonas aeruginosa, anti‐microbial activity, time 0.5 hour, size 20.0 nm to 50.0 nm, MgFe2 O4
1 Introduction
Spinel type magnesium ferrite is a binary oxide with general chemical formula of M+2 Fe2 O4 [1]. They crystallise into spinel type cubic structure and are soft magnetic, semi conducting material. Moreover the cationic distribution in the crystal lattice is sensitive to heat treatment due to high diffusibility of Mg+2 ions [2]. Magnesium ferrite has lot of applications in transformers, Ferro fluids, and magnet cores of coil [3]. They have photoelectrical properties and are used as semiconductors, and have applications in sensors, catalysts, and adsorption. They are used to decompose and reduce different toxic dyes including methylene blue, methyl orange etc. [4]. Due to their spinel structure, they absorb visible light and act as photo catalyst. The MgFe2 O4 is effective for the adsorption of SO2. If they can be used as a photo catalyst as well as an adsorbing surface, it can make them highly effective in cleaning water sources by both the degradation of contaminants, as well as by removing other unwanted substances from the environment [5, 6].
Magnesium ferrite nanoparticles are synthesised using different techniques and methods. Chemical methods including co‐precipitation, hydrothermal synthesis, micelle synthesis, and solid‐state reactions have been reported in literature to produce large quantities of nanoparticles [7]. These process routes are economical; however, the size of nanoparticles was compromised due to agglomeration. Alternatively, sol‐gel is another method for synthesising nanoparticles at moderate conditions, having good homogeneity, mono‐dispersity, and higher purity [8, 9].
In the present study, magnesium ferrite nanoparticles were synthesised using three different methods like co‐precipitation, sol‐gel, and auto‐combustion methods. We have employed urea for the first time in the synthesis of magnesium ferrite particles via sol‐gel method. The structures and the properties were analysed in detail using different physical approaches. Out of three methods, auto‐combustion method was the most optimal. In this study, we tested the nanoparticles for their anti‐microbial activity and arsenic remediation.
2 Materials &methods
2.1 Reagents &solutions
All the reagents (chemicals) used in this work were of high quality. All solutions used in this work were prepared in ultrapure water and in autoclaved glassware/ lab ware to ensure the work carried out was free of all the contaminations.
2.2 Magnesium ferrite nanoparticles synthesis
Magnesium ferrite (MgFe2 O4) nanoparticles were synthesised using co‐precipitation process, sol‐gel, and sol‐gel‐auto combustion method.
2.2.1 Co‐precipitation method
In the co‐precipitation method, chloride salts of iron and calcium were used as precursors. 0.4 M Iron (III) chloride hexa‐hydrate (FeCl3. 6H2 O) was used with combination of 0.2 M magnesium chloride hexa‐hydrate (MgCl2. 6H2O). The salts were dissolved in 100 mL water. Ammonia water was added drop‐wise till the pH of the solution reached 11–13. Then, the solution was heated up to 95°C for 2 h. Then, the solution was cooled to room temperature and as a result black precipitate was obtained. The precipitate was washed 10 to 20 times till the pH drop again to neutral (pH = 7). The solution was dried in oven at 120°C for 12 to 24 h. This was followed by several washings with deionised water to remove impurities and again dried in oven. The obtained powder was then calcined in furnace at 700°C for 3 h.
2.2.2 Auto‐combustion method
In the auto‐combustion method, nitrate salts of iron and magnesium were used as precursors. 1 M Fe(NO3)3. 9H2 O, 0.5 M Mg(NO3)2. 6H2 O, and 1.5 M EDTA were dissolved in 100 mL ethylene glycol. Ammonia water was added drop‐wise till the pH reach to basic. The solution was heated up to 80 to 100°C with continuous stirring till it was auto‐ignited. The powder was then washed several times with deionised water in order to remove impurities of unreacted salts. The resulted dried powder was then calcined in furnace at 700°C for 3 h.
2.2.3 Sol‐gel method
Nitrate salts were also used in this method. 1 M Fe(NO3)3. 9H2 O, 0.5 M Mg(NO3)2. 6H2 O and 1.5 M citric acid were dissolved in 100 mL of water. Then, 7 mL ethylene glycol was added to the solution. Urea was added to the solution in excess amount till the pH of the solution become basic. The solution was heated from 80 to 100°C with continuous stirring till a gel was obtained. The gel was then dried in oven at 120°C for 24 h. The powder obtained was then calcined in furnace at 700°C for 3 h.
2.3 Magnesium ferrite nanoparticles characterisation
2.3.1 Scanning electron microscopy imaging and EDX
Electron microscopy imaging was performed in SEM. A Si wafer doped with B was used as a substrate for imaging in SEM. A volume of 5 μg sample was placed in the middle of a clean wafer and install in the microscope for imaging. All SEM imaging was performed in JEOL (JSM5910, Japan) at CRL, University of Peshawar. EDX spectra were collected for the samples that were imaged in SEM.
2.3.2 X‐Ray diffraction
XRD was performed for magnesium ferrite nanoparticles to identify the phases. Nanoparticles (powder) was placed on a clean glass slide and scraped off onto a zero background Si holder. The phases were identified by XRD in an X‐ray diffractometer (X'Pert, PANlytic, Netherlands) in reflection mode.
2.3.3 Determination of the minimum inhibitory concentration (MIC) by growth
Overnight cultures of E.coli and Pseudomonas aeruginosa sub‐cultured into LB starting an O.D.600 nm of 0.1. 5 ml of the bacterial culture was placed in the test tube with different amount of Calcium‐magnesium ferrite nanoparticles ranging from 8–64 mg/L. At least three independent preparations of each con nanoparticle concentration were tested. After 14 h incubation in a humid chamber at 37°C, the optical density (OD600) was measured using the Specord (Analytik Jena AG) spectrophotometer. The MIC for growth was defined as the lowest concentration of NPs, which inhibited bacterial growth. Growth was compared to negative control i.e. plain LB without nanoparticles. Moreover, the growth inhibition was verified by spotting on plain LB plates in dilution series.
2.3.4 Remediation experiment
The remediation of arsenic was performed with magnesium ferrite nanoparticles. 10,000 ppb of arsenic solution was used as initial concentration of arsenic. The nanoparticles were used in concentration of 1 g/L.
The nanoparticles were mixed with 10,000 ppb of arsenic solution at pH 7 and were incubated at room temperature within shaking incubator. The incubation time was ranging from 0.5 to 8 h. After incubation, the solution was filtered for the removal of nanoparticles. Spectrophotometric method was used to determine the filtrate final concentration. 0.5 mL of the concentrated sulphuric acid (H2 SO4) was added to 1 ml of the filtrate followed by 2 ml of 2% potassium iodate with shaking. Furthermore, 0.5 mL of hydrogen peroxide was added with gentle mixing. Finally, 2 mL of chloroform was added. The reaction was left for 15 min till the pink colour develops in the organic layer. The organic layer was isolated into a new tube followed by its absorbance determination at 515 nm. The absorbance of the post‐treated sample was compared with the standard curve for the determination of concentration of arsenic as described previously [10]. The remediation efficiency was also determined as mentioned previously [10]
3 Results
3.1 X‐Ray diffraction
The room temperature XRD patterns were collected by X‐ray diffractometer with a step size of 0.01° in the 2θ angle range of 20–80°. The XRD patterns of all the synthesised MgFe2 O4 nanoparticles are shown in Fig. 1. The diffractograms of the MgFe2 O4 nanoparticles synthesised through co‐precipitation reveal the specific reflections (220), (311), (400), (511), (440) of Fd‐3m (FCC cubic spinel). The absence of the extra peaks ensures the formation of single‐phase spinel structure (JCPDS card no: 17‐0464)[11, 12]. However, XRD patterns of magnesium nanoferrites prepared by sol gel (13) as well as auto‐combustion routes indicate some secondary reflections like (220), (012), (−210), (003) belongs to Fe3 O4 and Fe2 O3. The average crystallite sizes (Scherrer's equation) of nanoparticles prepared by co‐precipitation, sol‐gel, and auto‐combustion technique were 22.06, 50.07, and33.35 nm, respectively.
Fig. 1.
XRD patterns of MgFe2 O4
3.2 Lattice constant
Table 1 shows the values of lattice constants of different magnesium ferrite nanoparticles synthesised by three different routes. The lattice constants were calculated by using the standard equation for cubic structures as described previously [13].
Table 1.
Lattice constants of Magnesium ferrite nanoparticles synthesized by three different processes
| Sr.No | Synthesis method | Lattice constant |
|---|---|---|
| 1 | co‐precipitation | 8. 37A0 |
| 2 | sol‐gel | 8.39A0 |
| 3 | auto‐combustion | 8.40A0 |
A slightly larger lattice parameter values in case of nanoparticles synthesised by the sol‐gel and auto‐combustion methods may be due to the presence of small percentages of secondary phases.
3.3 Bulk density and porosity
Table 2 shows X‐ray densities of the samples were calculated by the analysis of X‐ray diffraction data using the relation [13].
where M is the molecular weight, N is the Avogadro's number and a 3 is the cell volume.
Table 2.
Showed the X‐ray density, bulk density and the % porosity of the synthesised nanoparticles
| Sr.No | Synthesis Method | D x gcm −3 | D x gcm −3 | %porosity |
|---|---|---|---|---|
| 1 | co‐precipitation | 4.53 | 2.14 | 52 |
| 2 | sol‐gel | 4.50 | 1.99 | 58 |
| 3 | auto‐combustion | 4.47 | 1.66 | 62 |
The bulk densities (DB ) were measured by applying Archimedes principle using toluene according to the formula
where Ws represents the weight of the specimen in the air, Wt is the apparent weight loss in toluene and pt stands for the density of toluene (0.857 g/cc). The percentage porosity of the samples was determined by using the following formula
3.4 Scanning electron microscopy (SEM) analysis
The surface and grain morphology of the prepared spinel magnesium ferrite nanoparticles by three different approaches was studied by scanning electron microscopy. The SEM micrograph of magnesium ferrite nanoparticles prepared by co‐precipitation method is shown in Fig. 2 at low magnification micrograph, the particles seem to have even size distribution with few agglomerates. At high magnification micrograph, the particles appeared spherical in shape but with variable sizes. Some agglomerates have been observed probably due to the magnetic interactions within the particles. Magnesium ferrite nanoparticles prepared by sol‐gel method are shown in Fig. 2. High amount of agglomeration can be observed at both low and high magnification. The particles look like highly embedded with excess amount of unreacted salts. Therefore, sol‐gel method is not suitable for the synthesis of magnesium ferrite particles. Clusters could be seen at low magnification in case of auto‐combustion method. While at high magnification, the particles could be seen clearly. They are almost mono‐dispersed. The particles appeared evenly distributed. The particle size ranges from 20 to 60 nm. Thus, auto combustion method yielded the best morphology of magnesium ferrite nanoparticles among the three methods.
Fig. 2.
SEM micrographs of Magnesium ferrite nanoparticles. Left panel indicates low magnification whereas right panel indicates high magnification
3.5 Energy dispersive spectroscopic analysis
The EDX graph of magnesium ferrite nanoparticles by co‐precipitation method is shown in Fig. 3. The graph clearly shows the elemental composition of magnesium ferrite nanoparticles. The atomic peaks of iron (Fe), magnesium (Mg), and oxygen (O) are highly prominent. Small peaks of carbon (C) and chloride (Cl) were found, which could be due to the impurities from sample holder of the instrument. The EDX graph of magnesium ferrite nanoparticles by sol‐gel method displays the elemental composition of magnesium ferrite nanoparticles is shown in Fig. 3. Prominent peaks of magnesium (Mg), iron (Fe), and oxygen (O) were observed. Small peaks of carbon (C), nitrogen (N), and chloride (Cl) were also found, which could be due to the impurities from sample holder of the instrument and the unreacted starting material. The EDX graph of magnesium ferrite nanoparticles by auto‐combustion method is shown in Fig. 3. The graph displays prominent peaks of magnesium, oxygen, and iron validating the composition of magnesium ferrite nanoparticles.
Fig. 3.
EDX of Magnesium ferrite nanoparticles
3.6 Growth inhibition by magnesium ferrite nanoparticles
Incubating them with the bacteria in LB broth tested the antibacterial activity of the magnesium ferrite nanoparticles. This difference was observed between the negative control (plain LB) and LB with different concentration of nanoparticles. This difference in growth was determined by checking the O.D. of the LB at 600 nm with a spectrophotometer. Two gram negative bacteria E.coli and Pseudomonas aeruginosa, a multidrug resistant bacterium were used. Both the strains were incubated for 14 h. 4 mg/L concentration of magnesium ferrite nanoparticles showed growth inhibition in case of E.coli and Pseudomonas aeruginosa in the case of nanoparticles synthesised by auto‐combustion method. However, strong inhibition in growth was observed at 8 mg/L in the auto‐combustion and sol‐gel‐based nanoparticles. The co‐precipitation method showed weak growth inhibition as compared to the other two as shown in Fig. 4.
Fig. 4.
Comparativeanti‐microbial activity of Magnesium ferrite nanoparticles against E.coli (a) and Pseudomonas aeruginosa (b)
The absorbance of the bacteria directly correlates with its growth. The sol‐gel and auto‐combustion‐based nanoparticles showed significant growth inhibition of E.coli starting from 4.0 mg/L amount of nanoparticles (Fig. 5 a). However, significant antibacterial activity was observed at 8.0 mg/L amount of co‐precipitation‐based nanoparticles. Comparison of anti‐bacterial activity against E.coli showed significant difference between auto‐combustion and co‐precipitation‐based nanoparticles (Fig. 5b). However, no significant difference in the anti‐bacterial effect was observed during the comparison of sol‐gel and auto‐combustion (Fig. 5c). The sol‐gel‐based nanoparticles also showed significant difference in anti‐microbial activity as opposed to co‐precipitation ones (Fig. 5d).
Fig. 5.
Antibacterial activity of Magnesium ferrite nnaoparticles against E.coli
(a) Graph showing anti‐microbial activity of magnesium ferrite nanoparticles against E.coli. Nanoparticles synthesised via so‐gel and auto‐combustion method showed reduction in bacterial growth indicated by the reduction in the optical density observed at 600 nm at 4.0 mg/L, (b) Bar graph showing significant difference in anti‐bacterial activity between nanoparticles synthesised by co‐precipitation and auto‐combustion method, (c) Graph showing no significant difference in anti‐bacterial activity between nanoparticles synthesised by sol‐gel and auto‐combustion method, (d) Bar graph showing significant anti‐microbial activity of sol‐gel‐based nanoparticles as opposed to the ones synthesised by co‐precipitation method. Data (A‐D) represent mean ± SD of three independent experiments. P ‐values (*P < 0.05) were obtained by using Student's t‐test
However, in the case of multi‐drug resistant Pseudomonas aeruginosa, we observed that the auto‐combustion‐based nanoparticles were the most efficient starting from 4.0 mg/L amount. However the sol‐gel and co‐precipitation‐based nanoparticles anti‐microbial activity started from 8.0 and 16.0 mg/L amount of nanoparticles (Fig. 6 a). The auto‐combustion‐based nanoparticles show significantly increased anti‐bacterial activity at different concentrations ranging from 4.0 to 32 mg/L as compared to the co‐precipitation‐based ones (Fig. 6b). A significant difference in anti‐bacterial activity was observed at 4.0 mg/L in comparison with nanoparticles synthesised via sol‐gel method (Fig. 6c). The sol‐gel‐based nanoparticles showed strong reduction in bacterial growth at 16 and 32 mg/L concentration of nanoparticles as opposed to the co‐precipitation‐based ones (Fig. 6d). Based on this data, we can conclude that auto‐combustion‐based nanoparticles displayed strong anti‐microbial activity against both the Gram‐negative bacteria.
Fig. 6.
Antibacterial activity of Magnesium ferrite nnaoparticles against P.aeruginosa
(a) Graph showing anti‐microbial activity of magnesium ferrite nanoparticles against Pseudomonas aeruginosa at different amounts. The auto‐combustion‐based nanoparticles showed significant reduction in bacterial growth that can be depicted by the decrease in optical density at 4.0 mg/L, (b) Bar graph showing significant difference in anti‐bacterial activity between nanoparticles synthesised by co‐precipitation and auto‐combustion method at different concentrations, (c) Graph showing significant difference in bacterial growth when treated with n nanoparticles synthesised by auto‐combustion method, (d) Bar graph showing significant anti‐microbial activity of sol‐gel‐based nanoparticles at 16 and 32 mg/L as opposed to the ones synthesised by co‐precipitation method. Data (A‐D) represent mean ± SD of three independent experiments. P ‐values (*P < 0.05) were obtained by using Student's t‐test
3.7 Remediation of arsenic through magnesium ferrite nanoparticles
The magnesium ferrite nanoparticles were also tested for arsenic remediation. 0.5 g/L amount of nanoparticles were incubated with 10,000 ppb of arsenic for different time intervals. All the particles showed significant reduction in the amount of arsenic as opposed to the pre‐treated sample (Fig. 7 a). The arsenic remediation efficiency was calculated as demonstrated previously (10). However, the auto‐combustion‐based nanoparticles showed significantly efficient arsenic remediation starting from 0.5 to 6 h as opposed to the co‐precipitation‐based nanoparticles (Fig. 7b). However at 8 h, we did not observe any significant difference (Fig. 7b). In comparison with the sol‐gel‐based synthesised nanoparticles, the auto‐combustion ones at 0.5–2 h and we observed a slight but significant difference at 8 h time point (Fig. 7c). The nanoparticles‐based nanoparticles showed efficient and significant increase in arsenic remediation at all time points except at 6 h (Fig. 7c). Hence, we can conclude that auto‐combustion‐based nanoparticles are well suited for the removal of arsenic from water.
Fig. 7.
Comparison of arsenic remediation efficiency of Magnesium ferrite nanoparticles
(a) Bar graph showing removal of arsenic from water (pH 7) when treated with magnesium ferrite nanoparticles synthesised by three different methods at 0.5 g/L concentration. All the nanoparticles showed efficient removal of arsenic within 30 min, (b) Bar graph showing significantly increased arsenic remediation efficiency by nanoparticles synthesised via auto‐combustion method as opposed to the co‐precipitation method, (c) Bar graph showing significantly increased arsenic remediation efficiency by nanoparticles synthesised via auto‐combustion method as opposed to the sol‐gel method, (d) Bar graph showing significantly increased arsenic remediation efficiency by nanoparticles synthesised via sol‐gel method as opposed to the co‐precipitation method. Data (A‐D) represent mean ± SD of three independent experiments. P ‐values (*P < 0.05) were obtained by using Student's t‐test
4 Discussion
Arsenic is posing serious health hazards worldwide. Its’ spinel ferrites are generally represented by a formula MFe2 O4. where M could be any divalent metal cations such as calcium, magnesium etc [14]. Magnesium being biocompatible posed it as a suitable candidate for arsenic remediation in combination with iron. In the current study, magnesium ferrite nanoparticles were synthesised using a bottom‐up approach through three different chemical routes i.e. co‐precipitation, sol‐gel, and auto‐combustion methods. These nanoparticles were characterised by different analysis. The XRD analysis showed that the co‐precipitation route showed exclusively magnesium ferrite nanoparticle synthesis; however, secondary phases of iron oxide were observed in the other two routes. The observed difference may be due to the presence of unreacted or undissolved salts. The SEM, EDX analysis also reflected different proportions of the magnesium and iron ratios. This might be due to different synthesis approaches employed. This might be due to the presence of unreacted salts or incomplete incorporation of the respective metal particles. The size and shape of the particles apart from their chemical nature are critical determinants of adsorption capacity. The bulk density and porosity reflected that the auto‐combustion‐based nanoparticles offered better surface to volume ratio due to small particle size, low bulk density indicated their high dispersion and the high porosity refers to high adsorption capacity. As expected, these nanoparticles showed efficient arsenic. We propose that this high efficiency of arsenic remediation is probably due to the relative high porosity of these nanoparticles as compared to the ones synthesised via the other two approaches. The small size of auto‐combustion‐based nanoparticles gives them an advantage to efficiently disrupt the bacterial cell wall.
5 Conclusion
The magnesium ferrite nanoparticles synthesised by auto‐combustion method can significantly remove arsenic from contaminated water at pH 7.0. These particles also showed strong anti‐microbial activity.
6 Acknowledgments
This work was supported by the higher education commission grant No. 20‐3777/R&D/HEC/14 awarded to Aamira Tariq.
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