Introduction

Nickel–zinc ferrite is an important magnetic ceramic with spinel-type structure and centered face cubic arrangement. Ni–Zn ferrites are formed by eight AB2O4 type units, in which A is a 2+ cation located in the tetrahedral interstitium and B is a 3+ cation located in the octahedral interstices of the network [1]. This material has wide application, ranging from high-density information storage devices, cores of transformers, microwave devices [2], NEM/MEMS [3], magnetic fluids [4], to miniature transformers, multilayer chip inductors (MCIs), high-frequency chips [5], pigments [6], thin films [5, 7], gas sensors [8], and others [9, 10]. Parameters such as composition, grain size, amount of dopants, impurities, production method, and heating conditions are sensitive in relation to the microstructural properties of Ni–Zn ferrites. The stoichiometry and processing methods are critical factors that determine the physical and magnetic properties of this material [10, 11].

Several methods have been used to obtain ferrite with improved homogeneity and high purity. Among the synthesis methods, the polymeric precursor method (Pechini method) has emerged as a promising alternative to consume less energy and be less aggressive to the environment compared to other conventional techniques such as solid state reactions [12, 13]. This method was developed to obtain powders of oxides of various compositions, thus presenting homogeneous composition and high purity. Calcination is aimed at removing organic compounds and achievement of the desired ceramic phase [14]. Calcination treatment at about 500 °C results in the breakdown of polymeric chains, promoting the oxidation of cations, thereby giving crystallites of the desired oxides [15]. Calcination treatment has been little investigated in materials microtechnology. However, the rise of nanotechnology in ceramics has led to the search for treatment techniques that maintain the integrity of particles (powders) and their microstructure [16].

Many researchers have reported the heating alternative to improve the properties of materials, among them microwave energy. Comparing conventional treatment and microwave energy, in the conventional treatment, energy is transferred to the material through convection, conduction, and radiation. This means that there is a temperature gradient between the surface and the inner part of the material [17]. In the heat treatment by microwave energy, heating is volumetric, where microwaves interact with the material, and part of the energy is transmitted, part is reflected and some is absorbed by the material. The phenomenon that occurs in the microwave heating is the molecular friction of the material dipoles, resulting in mechanical stress that is manifested as heat within the material [18, 19]. In many cases, processing materials using microwaves have shown several advantages when compared to conventional processes [20]. These advantages include precise control of the volumetric heating, faster heating rates, low energy consumption, increased speed and performance of chemical reactions, and improved properties of the processed materials [21]. Studies have used microwave technology for processing different synthesized materials [17–19, 22, 23]; however, little has been reported in relation to calcination of ferrites using microwave energy at relatively low temperatures and times.

This study aimed to assess calcination by microwave energy at temperature of 500 °C for 30 min of ferrite Ni0.75Zn0.25Fe2O4 obtained by the method of synthesis of polymeric precursors. The morphological and microstructural characteristics of this material were analyzed.

Experimental

Ferrite powders Ni0.75Zn0.25Fe2O4 were synthesized by the polymeric precursor method. Monohydrate citric acid (C6H8O7.H2O) was added to distilled water at temperature of about 70 °C in a 3:1 proportion (AC:Metal), also adding the following metal nitrates: iron nitrate (Fe(NO3)3·9H2O), nickel nitrate (Ni(NO3)2·6H2O) and zinc nitrate (Zn(NO3)2·6H2O), and ethylene glycol (C2H6O) to promote polymerization. The solution was submitted to a final temperature of 100 °C under conditions of constant agitation and heating to form the polymeric resin. The resin was pre-calcined at temperature of 350 °C for 2 h with a heating rate of 10 °C min−1 for pyrolysis of the organic material, and then calcined in Fortelab microwave oven with controller FE50RPN at temperatures of 500, 650, 800, and 950 °C for 30 min in a heating rate of 10 °C min−1 in air atmosphere.

The structural characterization was performed by X-ray diffraction (XRD) using a Shimadzu X-ray diffractometer XRD 7000, Cu Ka radiation (λ = 0.154056 nm) operating with copper target tube at a voltage of 40 kV and 30 mA of current with scanning angles of 10° < 2θ < 70°. The thermal analysis (TG/DSC) was performed using a thermogravimetric analyzer Model Netzsch STA 449F with mass of about 15 mg under synthetic air flow, heating rate of 10 °C min−1 up to temperature of 950 °C. Absorption spectroscopy in the Fourier transform infrared (FTIR) region was performed on an ABB Bomem infrared spectrophotometer model MB104 resolution of 4 cm−1 and range of 2 cm−1, obtained in the spectral range of 4,000–400 cm−1 using Ni–Zn ferrite samples in the form of tablets dispersed in KBr. Morphological characterization was performed using field-emission gun scanning electron microscope FEG-SEM Jeol JSM 6330F with the aid of Energy-dispersive X-ray spectroscope (EDS) model Swift ED3000.

Results and discussion

The X-ray diffraction data of ferrite Ni0.75Zn0.25Fe2O4 are shown in Fig. 1 for samples calcined at 500, 650, 800, and 950 °C for 30 min in microwave oven. X-ray diffraction data were used (JCPDS 08-0234 standard file) for the identification of phases corresponding to the crystal structure of ferrite spinel. The average size of crystallites Dhkl (δ) was determined using X-ray diffraction data by basal planes (220), (311), (511), and (440) calculated by the Scherrer equation [24] and microdeformation and by the Williamson–Hall method (Eq. 1).

$$ \beta cos\theta = \frac{0, 9\lambda }{\delta } + \, ( 2\varepsilon ) \, sin\,\theta , $$
(1)

where λ is the radiation wavelength of the material used in the diffractometer (Cu Kα = 0.154056 nm), θ is the equivalent Bragg angle (diffraction angle), β is the width at half height of a diffraction peak (FWHM), δ is the size of the crystallites, and ε is the network microdeformation. The lattice parameter was calculated using the UnitCell - 97 software [25] through refinement from XRD data. The density of the ferrite obtained by XRD was calculated using the following equation [26–28]:

$$ D_{\text{x}} ({\text{Ferrite}}) = - \frac{8M}{N \times a^{3}}, $$
(2)

where M corresponds to the molecular weight of ferrite compound, N is the Avogadro’s number (6.023 × 1023 mol−1), and a 3 represents the volume of the unit cell. Theoretical density of 5.30 g cm−3 was used as standard JCPDF 08-0234 [29].

Fig. 1
figure 1

X-ray diffraction of ferrite Ni0.75Zn0.25Fe2O4 calcined in microwave oven at different temperatures

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Figure 1 shows a single phase of Ni–Zn ferrite spinel, with orderly structure and well-defined peaks according to standard JCPDS 08-0234. It shows peaks of greater intensity d311 in plane (311), with secondary hematite phases (α-Fe2O3) at temperature of 950 °C. The ferrite spinel phase was obtained at 500 °C for 30 min, thus demonstrating the removal of organic compounds and crystallization of the material as shown by thermal analysis (Fig. 2). Increasing the temperature resulted in a greater crystallinity of the material. It was found that calcination by microwave energy provided a rapid stabilization and crystallization step of agreeing with Al-Alas (2013). The degree of interaction of microwaves with the material is related to the dielectric loss of the material, which makes it capable of absorbing microwave radiation. When the polarization of the material occurs in an alternating field of the energy is released as heat [30].

Fig. 2
figure 2

Thermal analysis (TG/DSC) of ferrite Ni0.75Zn0.25Fe2O4

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Mouallem-Bahout et al. [31] synthesized Ni–Zn ferrite by the citrate precursor method and obtained single phase at temperature of 650 °C for 900 min, showing an average crystallite size between 20 and 90 nm and lattice parameter of 8.44 Å. Saba et al. [32] obtained ferrite Ni0.5Zn0.5Fe2O4 with secondary phases by the electrodeposition method by anodizing with calcination at 800 °C for 240 min with average crystallite size of 39 nm and lattice parameter of 8.40 Å. Rahimi et al. [13] produced ferrite Ni0.9Zn0.1Fe2O4 by Sol–gel method calcined at 500 °C for 60 min, with single phase with average crystallite size of 30 nm and lattice parameter of 8.41 Å.

The literature reports calcination in a temperature range from 450 to 900 °C for different times according to Table 1.

Table 1 Ni–Zn ferrite calcined at different temperatures and times
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The ferrite calcined by microwave energy showed crystallite size and lattice parameter smaller, which indicates that the occurrence of phenomena that can crystallographic ordering is related to microwave irradiation results in improved stability and crystallinity [30]. The formation of secondary phases is undesirable for Ni–Zn ferrites, since these phases modify the microstructure of the material, significantly changing its properties. The presence of secondary phases such as hematite (α-Fe2O3) is associated with the flow of oxygen in the oven atmosphere [33, 34]. Kingery et al. [35] reported the need to control oxygen in the oven atmosphere, since calcination in ambient atmosphere and even in vacuum, favors the formation of the hematite phase, explained by the high percentage of oxygen in the calcination atmosphere. The oxygen partial pressure influences the formation of additional phases, more likely to occur during cooling.

It is understood that the most stable state for nickel Ni2+ and iron Fe2+ ions is in the form of NiO and Fe2O3 oxides, respectively, and increased calcination temperature and the presence of oxygen in the atmosphere tend to provide a change from state 2+ to 3+ for both Ni2+ and Fe2+ ions. Fe2+ and Fe3+ ions have energy to occupy the tetrahedral and octahedral sites, respectively, since Ni2+ and Zn2+ ions have preference to occupy octahedral and tetrahedral sites, respectively. In the calcination step, the oxygen in the atmosphere causes changes from Ni2+ to Ni3+, and from Fe2+ to Fe3+. The addition of Zn2+ in the spinel structure of AB2O4 causes excess of Fe2+, displacing these cations from position A to position B. Excess of Fe 2+ leads to their migration out of the spinel network, which deviate forming the second phase hematite (α-Fe2O3) [36]. The nickel ions (ionic radius 0.69 Å) have preferences for octahedral positions, which dispute the position with iron ions (ionic radius 0.55 Å) and for being smaller, diffuses into the structure with relative ease, taking free positions which should be occupied by nickel ions [37].

A determining factor in the formation of additional phases is the cooling step. Increasing the oxygen partial pressure in cooling favors the formation of more hematite and less spinel. There is variation in the Gibbs free energy during ferrite formation (Eq. 3), which considering the activity of solids as individual, the reaction energy becomes a function of the square root of the oxygen activity, which can be changed by gas partial pressure [38, 39]. Thus, it can be described as

$$ \varDelta G \, = \varDelta G^{0} {-} \, RT \, {\text{ln}} \, \left( {P_{\text{O2}} } \right)^{0,5} $$
(3)

The low oxygen partial pressure favors the formation of the Ni–Zn ferrite spinel phase, where there is a hematite/espinel phase relationship in the cooling step, which is intensified at high partial oxygen pressure in the atmosphere [40]. The reaction of Zn with oxygen and the relationship between the partial Zn and O2 pressures can be understood by the following expression (Eq. 4) [39]:

$$ ZnO \, = \, Zn_{\text{(g)}} + { 1/2 }O_{2\,\text{(g)}}\quad{\text{and}}\quad P_{\text{Zn}} = \frac{K}{{\sqrt {P_{{\rm O}2} }}} $$
(4)

Table 2 shows the crystallographic standards of ferrite Ni0.75Zn0.25 calcined by microwave energy at temperatures of 500, 650, 800, and 950 °C. The Ni–Zn ferrite presents theoretical values of lattice parameters, unit cell volume and calculated density of 8.39 Å, 592.49 Å3, and 5.30 gcm−3, respectively [41]. The values obtained for ferrite Ni0.75Zn0.25Fe2O4 calcined at 500 °C for 30 min agree with the theoretical values reported in the literature [32, 42, 43].

Table 2 Crystallographic standard of ferrite Ni0.75Zn0.25Fe2O4 calcined by microwave energy (MO) at different temperatures
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Table 2 shows a tendency of increasing the average crystallite size with increasing calcination temperature. The lattice parameters, unit cell volume, and estimated density showed values close to standard values [41]. The variation of the lattice and microdeformation parameter is associated with the distortion of the crystalline network of ferrite Ni0.75Zn0.25, probably due to the size of the ionic radii shown by the material.

It was observed that with increasing temperature, the lattice parameter, the network microdeformation and the cell volume decrease, and the material density increase, a result that agrees with Saba et al. [32] Zhang et al. [43] and Mouallem-Bahout et al. [31], who calcined Ni–Zn ferrite at 800 °C for 240 min, 900 °C for 120 min, and 650 °C for 900 min, respectively.

Figure 2 shows the thermal analysis of Ni–Zn ferrite precursor powders. From the TG curve analysis, it was observed that the samples showed initial mass loss between 28 and 235 °C for ferrite Ni0.75Zn0.25, which is assigned to the evaporation of volatile material, in particular evaporation of water coming from the dissolution of citric acid into water in the process of obtaining metal citrates, in addition to adsorbed gases [44]. A significant mass loss was observed from 254 °C to 411.86 °C, with a percentage of 76.75 % associated with the combustion of organic compounds and to the decomposition of nitrate wastes, which subsequently enabled the formation of oxides and crystallization of the material. Compounds CO, CO2, H2, N, and N2 may volatilize with greater mass loss. Due to the absence of other peaks up to temperature of 1,000 °C, the material was thermally stable, showing high thermal stability of Ni–Zn ferrites. Reducing the oxygen partial pressure leads to state change (reduction of ions) Fe3+ → Fe2+, as well as to the removal of oxygen from the anionic interstices of the crystalline network, according to expression [35, 45]:

$$ \frac{2}{3}Fe^{3 + } + \frac{1}{3}Ni^{3 + } + \frac{3}{8}V_{c} + \frac{1}{2}O_{0}^{2 - } = \frac{2}{3}Fe^{2 + } + \frac{1}{3}Ni^{2 + } + \frac{1}{4}O_{2} , $$
(5)

where V c and O 0 correspond to the cation and oxygen ion vacancy, respectively. This reaction configures the reduction, like the oxygen moles come out from the spinel structure in addition to the corresponding absence of the cation vacancy. Thus, the reaction results in mass loss, while the opposite reaction causes mass gain. The high mass loss is characteristic of the material obtained by the polymeric precursor method and can be explained by the oxidation–reduction reaction in relation to the nickel/zinc proportion [6, 45]. The reaction of Eq. 6 confirms the reduction, in which oxygen moles leave the spinel structure along with the absence corresponding to the cation vacancy, which depends on stoichiometry and features a higher mass loss of ferrite Ni0.75Zn0.25.

From the DSC curve, it is possible to observe loss of energy of ferrite Ni0.25Zn0.75, which showed an exothermic peak of 14.57 mW mg−1 at 346.28 °C attributable to the combustion of organic material, in which there is a release of energy, and therefore the oxidation of Ni2+ ions that remain in the spinel structure. These exothermic peaks are attributed to the release of energy in the combustion of organic compounds, accompanied by the release of CO2 and NO, and hence the phase transformation, in which this event is assigned the enthalpy variation (ΔH) of the system and crystallization of the cubic spinel phase.

The enthalpy variation in the system can be comprised in the Ni–Zn ferrite system as the energy required for transforming from amorphous phase to crystalline phase, which is in agreement with the TG results and according to Jiang et al. [46]. In the microwave energy heating, a temperature gradient occurs because the interior of the particle is at high temperature, and its outer surface is at low temperature, resulting in deformation and internal fusion, which provides a fast thermal decomposition [47].

Figure 3 shows the infrared spectra of ferrite Ni0.75Zn0.25 calcined by microwave energy at 500 and 650 °C for 30 min. The figure illustrates the evolution of absorption bands according to the calcination temperature.

Fig. 3
figure 3

Infrared spectra of ferrite Ni0.75Zn0.25 calcined at a and 500 b 650 °C for 30 min

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The band 2928 cm−1 is related to the hydroxyl group (OH) on carboxylic acids (citric acid), with stretch (medium and high intensity) of OH attached to H. The absorption band 2357 cm −1 corresponds to the axial deformation of CO2 molecules derived from the oven atmosphere in the calcination process and the environmental exposure. The bands 1358 cm−1 are assigned to C=O stretching of the carboxyl group (CO2) [45, 46].

The bands in the infrared between 1,000 and 400 cm−1 are attributed to intrinsic vibrations of the ions in the crystal structure. The absorption bands corresponding to ν1 (tetrahedral site) and ν2 (octahedral site) of the crystalline lattice are in the range from 600 to 400 cm−1 [31]. For the spinel structure of Ni–Zn ferrite, the stretch is observed in the range from 600 to 400 cm−1 [27], which corresponds to the intrinsic vibrations of the metal (Mtetra → O) in the tetrahedral site and in the range from 450 to 385 cm−1, corresponding to vibrations (Mocta → O). Due to the fact that the link length in the tetrahedral site is shorter, the vibration in the tetrahedral sites is more intense than in octahedral sites [48, 49]. Through the infrared spectra, it could be seen the removal of most of the organic compounds and subsequent formation of the ferrite phase, confirmed by X-ray diffraction (Fig. 1).

Figure 4 shows the influence of microwave heat treatment on the material morphology, because the effect of microwave heating is volumetric, and from the heat generated by molecular excitation, the surface of samples calcined at 500–650 °C by microwave presents roughness [42]. The excitation acts on the microwave properties magnetotransport, which causes excitation of the material, due to the interaction of the material with radiation which depends on the amplitude of the magnetoresistance oscillations induced in the sense of linear polarization of the microwaves. The electronic polarization is sensitive to the relative orientation between the polarized microwave linear shape and the amplitude of the magnetoresistance oscillations. The oscillation amplitude is influenced by the fact that the electric field of microwave linear polarization, Eω where ω = 2πf is parallel or perpendicular to the electric field, E. Studies have shown the effect of biasing in sensitivity to radiation-induced magnetoresistance oscillations. The microwave excitation may possibly is related to the function of the microwave frequency, f, as well as the orientation of the magnetic field [50]. At all temperatures, the formation of irregular agglomerates with a wide distribution of particle sizes is observed, indicating the presence of interparticle pores and longitudinal plates. The micrographs show agglomerated, porous, and non-dense particles of friable characteristics (easy deagglomeration). Sample calcined at 500 °C for 30 min showed irregular morphology with large clusters of nano-sized particles below 100 nm (Fig. 4a), with results similar to Jebeli Moeen et al. [6] and Kambale et al. [26], who used calcination times of 1,000 °C for 60 min and 700 °C for 120 min, respectively.

Fig. 4
figure 4

Micrograph of ferrite Ni0.75Zn0.25Fe2O4 calcined by microwave energy at a 500 °C for 30 min, b 650 °C c 800 °C d 950 °C

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The micrograph of ferrite Ni0.75Zn0.25Fe2O4 calcined at 950 °C (Fig. 4d) indicated the occurrence of coalescence of thermal diffusion caused by the high surface area of particles together with the calcination temperature and the microwave heating mechanism. Microwave heating is different from conventional heating in the oven, because its thermal heat energy is generated from the interaction between microwaves and the material directly at the molecular level through dipolar interactions, which lowers the activation energy of calcination, which promotes ionic diffusion. The presence of pores can be justified as a consequence of the crystallization by rapid heating microwave [51]. Aspects of pre-sintering the ferrites treated by microwaves were observed, which is due to the heating effect concentrated in the neck region of the two particles interconnecting them (Fig. 4d), being reinforced by ionic mass transport in the neck region, as observed by Oghbaei and Mirzaee [17]. The calcination by microwave energy uses heat generated by the vibration of the electronic polarization caused by microwaves of 2.45 GHz at high frequency. The heat supplied is related to the dielectric loss, which is generally proportional to the permittivity of the material. The heat generated by the microwaves, the energy of electromagnetic wave is moved within the particles where there is a conversion into kinetic energy of electrons, ions, molecules, and crosslinked, wherein the material temperature increases. The heating by microwave energy does not rely on heat conduction, the particle is heated from the inside by the electronic polarization and interfacial polarization atomic polarization, the heat being transferred to the surface of the particles [47]. Based on the micrographs, it was observed that all Ni–Zn ferrite samples showed significant porosity degree, thus varying material density, with values close to the theoretical value of 5.30 gcm−3, as shown in Table 2.

Figure 5 shows the mapping of chemical elements that formed the ferrite Ni0.75Zn0.25 through the edge Kα, where there is a high dispersion of Fe, Ni, and Zn ions, showing homogeneity of the diffusion process of ions, justified by ferrite phase obtained by XRD (Fig. 1).

Fig. 5
figure 5

Mapping of element through EDS of ferrite Ni0.75Zn0.25 calcined at 500 °C for 30 min

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The analysis of the mapping of elements in the edge Kα by EDS shows the dispersion of ions that form Ni–Zn ferrite in the material treated by microwave at 500 °C for 30 min.

Table 3 shows the elemental composition of Ni–Zn ferrite calcined at 500 °C obtained by EDS.

Table 3 Elemental composition predicted and obtained by semi-quantitative analysis by EDS of Ni–Zn ferrite calcined at 500 °C/30 min
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Note that the sample showed chemical composition different from the expected ratio by the stoichiometric calculation. The disparity between the values of chemical elements may be explained by the measurement accuracy of Fe, Ni, Zn, and O, since the EDS technique is of semi-quantitative nature, which shows an error variation from 2 to 3 %, thus being unable to provide an accurate estimate of oxygen [52, 53]. The quantitative analysis may be impaired by the relationship of two different cations in the same position, with Ni/Fe and Zn/Fe ratio reproducing different magnitudes of backscattered signals and influenced by different ion sizes.

Conclusions

The rapid microwave calcination is efficient for obtaining ferrite Ni0.75Zn0.25Fe2O4 at 500 °C for 30 min, obtaining single stage at low temperature and short time when compared to other calcination techniques. The values of crystallographic standards approached the theoretical values of Ni–Zn ferrite. FEG-SEM with the aid of EDS demonstrated that ferrite Ni0.75Zn0.25Fe2O4 showed vast particle distribution with good chemical homogeneity. The results demonstrated that microwave calcination is a promising technique for obtaining Ni–Zn ferrite, considering benefits such as time reduction, crystallization at low temperatures, and great effect on the material microstructure.