A number of synthesis methods are being used to produce the spinel ferrites, however a standard co-precipitation technique and double sintering method were followed in the presented work. Fine particles of Mg_x-Zn_1-x-Fe₂O₄ were co-precipitated, ball milled and sintered to investigate the effect of the ferrite composition on its properties. The AR grade chemicals including hexa hydrated manganese chloride, hexa hydrated zinc chloride, hexa hydrated ferric chloride (FeCl₃.6H₂O), sodium hydroxide (NaOH) and acetone (CH₃COCH₅) were imported from Sigma- Aldrich (Germany). All the chemicals and reagents, used in the conducted research work, were of high purity. To obtain the desired ferrite composition, the salts of stoichiometric amounts of manganese chloride, zinc chloride and ferric chloride were dissolved in distilled water and subjected to magnetic stirring. The precipitant agent (NaOH) was used to attain the desired pH of the mixture. Additionally, NaOH also worked to accelerate the reaction and to limit the formation of the other non-crystalline phases.

Different samples of Mg_x-Zn_1-x-Fe₂O₄ ferrite were synthesized by varying the ‘x’ value in the composition (0.1, 0.3, 0.7, 0.75, and 0.9). The beaker containing dark brown precipitates was immersed in a pre-heated water bath. After 90 min of heat at constant temperature of 90 ^°C, the precipitates were settled down to the bottom of the beaker. These precipitates were filtered and washed several times with distilled water and acetone, in order to remove the sodium and chloride ions. The final product was dried for 4 h in an electric oven at heating temperature of 60 ^°C. The fully dried samples were grinded to obtain the desired particle size. At this end, PM400 Planetary type ball mill was used to grind the ferrite particles, as shown schematically in Figure 1. Fifty balls of four grams each were used to convert the ferrite particles into fine power. Thereafter, sintering of the precursors was carried out using double sintering technique. The source materials were initially mixed thoroughly and pre-fired twice at 600 °C. The prepared samples having different compositions were finally heated up to 800 °C for nearly 12 h and then allowed to cool down to the room temperature under ambient conditions. The sintered samples were size reduced in a ball mill operated at 35 rpm. The ratio of the steel balls to the powder was maintained as 10:1 for effective grinding [21]. As the method for grinding is so called wet method, toluene was used as a grinding medium during ball milling. The samples were collected at periodic intervals for further analysis and characterization.

Figure 1:

Schematic diagram of the planetary ball milling system.

Mg_x-Zn_1-x-Fe₂O₄ nanoparticles were successfully synthesized using co-precipitation technique. The concentration effect on crystal structure, crystal size, volume of the unit cell, density of the unit cell, lattice parameters and microstructures of the produced spinel ferrites was elaborated from XRD spectra and SEM micrographs. XRD spectra of Mg_x-Zn_1-x-Fe₂O₄ powders are shown in Figure 2. These spectra confirmed the formation of ferrite phases in the identified crystal structures. Sharp diffraction peaks, consistent with the face centered cubic spinel structure, were noticed in XRD patterns of the investigated samples. The lattice parameter ‘a’ was determined using the relation ‘a=d²(h² + k² + l²)^1/2ʹ whereas X-ray density of the ferrite samples was determined using the formula ‘D₌8M/Na^3ʹ. In density relation, ‘M’ is the molecular weight, ‘N’ is the Avogadro’s number and ’a’ is the lattice constant of the sample. The crystal size of the ferrite samples was determined by considering the most intense diffraction peaks in XRD spectra. In this study, the Debye-Scherrer formula (T=0.9 λ/βCosθ) was used to calculate the crystal size of the tested samples. The volume of the unit cell of the synthesized samples was calculated using the respective lattice constant values. A relationship between the volume and the lattice constant existed in the form ‘V=a^3ʹ.

Figure 2:

XRD patterns of pure Fe₂O₃, MgO and ZnO.

In Figure 2, XRD patterns of MgO, ZnO and α-Fe₂O₃ showed only the individual reflections of the starting phase of ZnO, MgO and α-Fe₂O₃. However, in Mg_x-Zn_1-x-Fe₂O₄, the major ferrite phase was seen along with a small amount of α-Fe₂O₃ phase, which remained unreacted [22]. In Figure 3, the broad width of the peaks revealed the growth of the ferrite phase with very small particle size. The intensity ratios of the individual reflections were in accordance with the stoichiometric composition of the mixture. The particle size reduced down sharply with an increase in peak width. The content of ZnO decreased quickly and became zero with an increase in milling time and concentration [23], whereas the α-Fe₂O₃ content exhibited slow decrease over milling time.

Figure 3:

XRD patterns of calcined and uncalcined samples of Mg_x Zn_1-xFe₂O₄ ferrite.

Peaks at 2θ of 30.55°, 35.98°, 37.52°, 43.58°, 57.66° and 63.32° were referred to XRD diffraction planes (220), (311), (222), (400), (333) and (440), respectively. These peaks confirmed the formation of cubic spinel nickel copper ferrite when compared with JCPDS card 070–2674. XRD results of calcined sample of Mg_x Zn_1-x Fe₂O₄ confirmed the ferrite phase formation with the Bragg’s angles of 31.9°, 35.7°, 36.3°, 54.2° and 56.8°. Then sample was calcined at temperature of 600 ^°C for one hour in a furnace for comparative phase transition studies. Five out of seven peaks revealed 71 % purity of calcined Mg_0.1Zn_0.9Fe₂O₄ ferrite and 43 % purity of uncalcined ferrite. The peaks at 35.7°, 36.4° and 62.8°, as observed in Mg_xZn_1-xFe₂O₄ at x=0.3, 0.7, 0.75, 0.9, revealed that the ferrite phase dominates after calcination. The maximum purity of the sample was measured about 57 % while uncalcined sample revealed relatively lower purity of 43 %. It was due to accumulation of the energy during ball milling through repeated fragmentation and rewelding of the small grains, which led to formation of Mg–Zn ferrite phase. This phenomenon can be manifested during contraction or expansion of the lattice parameters of the above discussed phases [24].

Other phases have also been presented with the Bragg’s angles of 24.2°, 33.3°, 41.0°, 49.5°, 62.6° and 64.1°, which were removed by increasing the milling time [25]. Due to high-energy impact, lattice parameter of MgO, ZnO and Fe₂O₃ phases came down to nanometric order within a very short duration of milling. It indicates that contamination in the sample, produced by the ball milling, was heterogeneous in nature [26]. All Mg_xZn_1-xFe₂O₄ structures, obtained with concentrations of 0.1, 0.3, 0.7, 0.75, 0.9, exhibited FCC geometry [27]. The crystal size of the ferrite samples was determined by considering the most intense diffraction peak (311) in XRD patterns. In this study, the Debye-Scherrer formula was used to calculate the crystal size of the tested samples. The average crystal sizes were measured as 34, 54, 49, 40 and 45 nm whereas the average lattice parameter remained as 0.8 nm for all concentrations, as shown in Figure 4. The X-ray density was found in the range of 5.3–4.6 g/cm³.

Figure 4:

Volume, lattice parameter, density and crystal size vs concentration of the Mg_XZn_1-xFe₂O₃ of calcined (red) and uncalcined (black).

A decreasing trend in crystallite size of the synthesized ferrite particles was found with an increase in concentration from 0.3 to 0.9. On increasing the value of ‘x’, Mg²⁺ ions in the sample and crystal size were also increased [28]. An increase in Mg²⁺ ions could be attributed to the ionic size difference, since the unit cell had to expand when substituted by ions with large ionic sizes. Zn²⁺ ions had larger ionic radius (0.82 Å) than Mg²⁺ ions (0.66 Å), which when substituted reside on A-site and displaced small Fe³⁺ ions from A-site to B-site. MgO and ZnO were cold welded during mechanical milling and finally the crystal size was further reduced due to heating at 600 ^°C. The atoms were diffused and replaced by the other atoms, which have smaller ionic radii like Mg²⁺. This replacement resulted in a decrease in crystal size [28].

An increasing trend has been found in lattice parameter of ionic crystals of uncalcined samples with an increase in concentration from 0.1 to 0.7. On the other hand, a sudden decrease in lattice parameter has been observed between 0.7 and 1. This initial increment and then decrease of the crystallite size value is attributed to the well-known fracturing (reduction) and re-welding (increment) processes that happen usually in high energy ball milling. Thermal heat treatment at 600 ^°C for one hour grew the ferrite phase with a very high value of lattice parameter [29].

The X-ray density was decreased with an increase in concentration. This decrease in density was attributed to a continuous reduction in α-Fe₂O₃ phase content at a moderate rate during the course of milling. X-ray density of normal spinel was relatively larger than uncalcined sample and reduced slowly in the course of calcination; trend line also explained decreasing behavior. X-ray density of uncalcined samples showed a continuous behavior between concentrations of 0.1 and 0.3. Such variations in X-ray density with increasing concentration clearly indicate that if the lattice parameter increases the X-ray density will decrease [30]. The volume of these newly grown ionic crystals in the calcined sample increases with a rise in concentration from 0.1 to 0.7. After value of 0.7 lattice parameter was suddenly dropped down and this initial reduction and then increment in the particle size is attributed to the well-known fracturing (reduction) and re-welding (increment) process that happens usually in high energy ball milling. When the sample was placed in the furnace at 600 ^°C, the ferrite phase grew and the value of the lattice parameter decreased. The plots in Figure 4 show that in the uncalcined sample, the ferrite phase grows with a very high value of lattice parameter and then drops due to ball milling [31].

Calcined ferrite samples were also analyzed by SEM technique for evaluation of their microstructures and grain size. Figure 5 shows the SEM micrographs of calcined samples: (a) Mg_0.1 Zn_0.9Fe₂O_4., (b) Mg_0.3Zn_0.7Fe₂O₄, (c) Mg_0.7 Zn_0.3Fe₂O₄, and (d) Mg_0.9 Zn_0.1Fe₂O₄. It can be observed that all the samples exhibit different grain sizes but similar grain shape. The grains are homogenously distributed. The grain size was calculated by the line intercept method and found in the range of 0.5–2 mµ, depending on the contents into the spinel lattice [32]. Moreover, the grain size was found larger and homogenous for the ferrites, substituted with MgO and ZnO. These results are in line with XRD data where the lattice parameter values increased with increasing MgO and ZnO contents and produced larger unit cell volumes for the substituted samples. Similar types of results have been reported in literature for rare earth substituted spinel ferrites [32, 33].

Figure 5:

SEM images of calcined samples: (a) Mg_0.1 Zn_0.9Fe₂O₄, (b) Mg_0.3Zn_0.7Fe₂O₄, (c) Mg_0.7 Zn_0.3Fe₂O₄, and (d) Mg_0.9 Zn_0.1Fe₂O₄ with scale bar of 2 µm.

The fine powders of Mg_x-Zn_1-x-Fe₂O₄ ferrite were produced, ball milled and characterized for the effect of composition on their morphological and structural properties. XRD patterns revealed the formation of impurity free single phase spinel structures of Mg_x-Zn_1-x-Fe₂O₄. The lattice parameter increased with an increase in ‘x’ content in the ferrite composition. The lattice parameters of the calcined and uncalcined ferrite samples exhibited an initial increase upto a certain value and thereafter suddenly dropped down due to fracturing and re-welding processes. Overall, the lattice parameter ‘a’ varied from 0.80 to 0.85 nm, crystallite size from 34 to 80 nm, unit cell volume from 0.528 × 10⁻²¹ to 0.62 × 10⁻²¹ cm³ and X-ray density from 5.8 to 4.5 g/cm³. The grain size was found larger and homogenous for the ferrites substituted with MgO and ZnO. X-ray density of normal spinel was relatively larger than uncalcined sample, which was reduced slowly in the course of calcination. X-ray density of uncalcined samples showed a continuous behavior between concentrations of 0.1 and 0.3. These findings suggested that the synthesized ferrites can potentially be used as core material in high frequency devices.