Effects of Various Sintering Conditions on the Structural and Magnetic Properties of Zinc

Zinc ferrite (ZnFe2O4) nanoparticles have attracted the attention of researchers because of their unique chemical properties and particle-size-dependent magnetic properties. High-surface-area spinel ferrites have numerous technical applications in areas such as high-density information storage, ferrofluids, and catalysis. The coprecipitation technique is the preferred method for preparing nanoscale ZnFe2O4 because it results in a small crystals with a uniform size distribution. In this study, the synthesis was conducted in solution for 48 h, followed by sintering at 1000°C for 1.5–3.5 h. The smallest crystals (70.58 nm) were found in the sample sintered for 2.5 h. Fourier transform infrared spectroscopy (FTIR) functional-group analysis confirmed the presence of Fe–O and Zn–O bonds of cations in octahedral and tetrahedral sites. The ZnFe2O4 sintered for 3.5 h was superparamagnetic. The characterization results suggest that the obtained ZnFe2O4 could be used as a nanofluid in heat exchangers.


Introduction
Studies on nanomaterials have become prevalent for several reasons, such as the need to produce new smallerscale materials to reduce the cost and increase information transmission and storage speeds. Another reason is that nanomaterials exhibit enhanced properties compared with conventional materials, which provides an opportunity for new technological applications. Theoretically, a nanomaterial is an element with a grain size of one per million meters 1 . Numerous authors have explored various aspects of nanomaterials, such as the possibility of controlling their size, defect concentration, solution concentration, atom interaction, and synthesis process 2 .
ZnFe 2 O 4 is a critical technology material commonly used as a photocatalyst, a sorbent for desulfurizing hot coal gas, and a magnetic material in devices such as radio-frequency cores, sensors, and magnetic resonance imaging devices. ZnFe 2 O 4 adopts the cubic spinel structure. Cations occupy one-eighth of the interstitial tetrahedral (A) and one-half of the octahedral (B) sites 3 .
Among numerous spinel ferrites, ZnFe 2 O 4 nanoparticles have attracted particular attention because of their unique chemical properties and thermal stability, as well as their particle-size-dependent magnetic properties 4  The coprecipitation technique is the most appropriate approach for synthesizing ZnFe 2 O 4 because it results in small crystals with a uniform distribution, is simple, and does not require a calcination step. On the basis of the theory underlying coprecipitation, several parameters (e.g., the counterions, ionic strength, pH, and precipitation temperature) can influence the structure and magnetic properties of the resultant ferrite 3 . A previous investigation found that the chemical composition of ZnFe 2 O 4 is influenced by the Zn/ Fe molar ratio, pH, sintering temperature, and sintering duration 6 . This previous study also showed that a Zn/Fe ratio  Sintering is the compaction process of powders at high temperatures below the melting point until there is a change in microstructure such as a reduction in the number and size of pores, grain growth, shrinkage, or increased density 7 . In the present work, sintered ZnFe 2 O 4 is characterized using several methods: phase identification by X-ray diffraction (XRD) analysis, morphological analysis by scanning electron microscopy (SEM), and elemental composition analysis by energy-dispersive X-ray spectroscopy in conjunction with SEM (SEM-EDX). The functional groups are analyzed by Fourier transform infrared (FTIR) spectroscopy, and the magnetic properties are analyzed on the basis of hysteresis curves recorded at room temperature using a vibrating sample magnetometer (VSM) 2 . The objective of this research was to characterize the fine structure and properties of ZnFe 2 O 4 as a potential sensor material.

Method
The coprecipitation synthesis method was used in the present work. In the first step, ZnO and Fe 2 O 3 (2 g each) were dissolved into 40 mL of ethylene glycol using a magnetic stirrer at room temperature; the resultant solution was stirred for 48 h at 200 rpm to ensure homogeneity. To neutralize the acidic solution, the obtained solution was titrated with NaOH until its pH was 12. The solution was then heated at 70-80°C to form a gel. The sedimented material was washed three times with 1500 mL distilled water. The gel was then dried in an oven at 1100°C until it turned into a dry powder. The dried gel was crushed for 1 h and then sintered at 1000°C for 1.5, 2.5, or 3.5 h. The sintered powder was then characterized by XRD (PANalytical) with Cu Kα (λ = 1.54 Å) radiation to determine its phase and crystal size. The morphology was characterized by SEM (Phenom). The functional groups were characterized by FTIR analysis (IRPrestige21), and the magnetic properties were characterized using an Oxford 1.2H VSM. Figure 1 shows SEM images of the ZnFe 2 O 4 synthesized using the coprecipitation method and sintered at 1000°C for 1.5, 2.5, and 3.5 h. The micrographs show that the materials tend to have cubic crystallites. Figure 1a and 1c show changes in the material structure, with greater agglomeration than observed in Figure 1b. The sintering time at 1000 °C strongly influenced the change in morphology. The heat treatment induces bonding between among particles and increases the strength of the obtained product 8 . The sintering process strongly affects the formation of the crystalline phase of the material. The phase fraction formed generally depends on the sintering time and temperature 9 .   compared with those specified in the ICDD PDF (22-1012) reference pattern and were concluded to correspond to a single-phase cubic spinel-structured material. No additional peaks were observed in the patterns, indicating that the synthesized product was free of impurities 10 . The samples exhibited similar particle sizes (smaller than 90 nm) irrespective of the sintering duration (1.5, 2.5, or 3.5 h). The sample sintered for 2.5 h exhibited the smallest grain size of 70.58 nm; thus, the sintering duration strongly influenced the grain size 11 . The particles shrank with increasing sintering time, resulting in the gradual removal of water fom the crystal lattice 12 . The samples sintered for 1.5, 2.5, and 3.5 h exhibited particle sizes of 84.72, 70.58, and 84.72 nm, respectively, and main peak heights of 836.72, 767.80, 609.98 counts in their XRD patterns, respectively (Table 1). These results are attributed to an increase in the growth activity of nanoparticles with increasing temperature because of the influence of heat during the synthesis process 4 . Particle size depends on numerous factors, including the synthesis conditions, sintering temperature, time, and the rate of heating and cooling 13 . The lattice parameter of the synthesized nanoparticles was greater than that of bulk ZnFe 2 O 4 (a = 8.441 Å) (JCPDS No. . This sample is an example of ZnFe 2 O 4 nanoparticles with a mixture of normal and inverse spinel structures. The substitution of several Fe 3+ cations for Zn 2+ cations, whose ionic radius (0.74 Å) is larger than that of Fe 3+ (0.64 Å), results in an expansion of the spinel lattice, with a concomitant increase in the corresponding lattice parameter 4 .

Results and Discussion
In the normal AB 2 O 4 spinel structure, A is generally a divalent cation that occupies tetrahedral sites, whereas B is a trivalent cation that occupies octahedral sites. Bulk ZnFe 2 O 4 is a completely normal spinel structure with Zn 2+ ions at the tetrahedral (A) sites and Fe 3+ ions at the octahedral (B) sites. In fact, when ZnFe 2 O 4 is prepared at the nanoscale, the high surface energy associated with the small particle size leads to the distribution of mixed cations, where Zn 2+ ions and Fe 3+ ions are distributed among A and B sites, giving rise to mixed spinel structures. This difference is attributed to cation distribution defects 14,15 . Figure 3 displays the FTIR spectra of ZnFe 2 O 4 in the wavenumber region 4000-400 cm −1 . The functional groups of ZnFe 2 O 4 sintered for various times are similar except in the case of the sample sintered for 2.5 h, whose spectrum shows additional peaks in the 0-500 cm −1 range. The main peaks of ZnFe 2 O 4 are observed in the range 400-600 cm −1 , representing Fe-O and Zn-O bonds 10,16,17 involved in metal-oxygen vibration stretching in octahedral and tetrahedral sites 18,19 . The strong absorption peak at ~1508 cm −1 corresponds to the C=C stretching vibration 20 and indicates the presence of surface hydroxyl groups 19 or moisture 18 17,18,20 . A decrease in intensity of the lower-frequency peaks might be caused by the formation of a H bridge on the precursor, which subsequently disappears upon thermal treatment [22][23][24] .
The hysteresis curve of ZnFe 2 O 4 sintered at 1000°C for 1.5, and 2.5 h indicates a nonmagnetic material; no hysteresis curve with an apparent magnetic saturation (M s ) or coercivity (H c ) was observed (Figure 4). Table 2 shows that the sample  sintered for 3.5 h exhibits an M s value of 52 emu/g and a H c of 0.0708 T. An S-shape hysteresis curve 25 and a H c value close to zero indicates a superparamagnetic material 10 . Such an increase in the H c value can be achieved with single-domain crystallites because of the magnetization process due to spin-orbit rotation without domain wall movement 26 . These results indicate that the sintering duration greatly influenced the magnetic properties of the ZnFe 2 O 4 . The longer sintering duration of 3.5 h for ZnFe 2 O 4 caused spin-orbit coupling with various hyperfine structures 27 and superexchange interactions 28 between the spin electron in ZnO and Fe 2 O 3 . The magnetization of spinnel ferrite is given by super-exchnage interaction of side A and B. In bulk zinc ferrites, Zn 2+ ions of zero moment and Fe 3+ ions fill A and B sites respectively. Few Fe 3+ ions may occur A site and has possibility of AB interaction. In the probability distribution of magnetic hyperfine fields, the superparamagnetic behaviour at 3.5 h samples because there is an occupancy of some Fe 3+ ions migrates from B to A side. Therefore, it will disturb the antiparallel ordering of B site and enhance the saturation magnetization and magnetic moment 29 .
At room temperature, the magnetic properties arise due to the formation of superparamagnetic domains. The absence of hysteresis, immeasurable magnetic remenance, coercivity, and the non-attainment of saturation indicate the presence of superparamagnetic behaviour 30 . Figure 5 displays the calculated magnetization data in charts that compare particle size (D m ) with the M s , D m with the remanence ratio (M r /M s ), and D m with the H c . The results show that, on the basis of the average particle size obtained from the SEM images, longer sintering times led to smaller particles and more substantial M s values. These results are attributed to a decrease in cationic disorder, which in turn decreased the D m 26 . Similarly, a smaller D m led to greater H c . This variation is related to a favorable particle size as a result of the cation distribution between sites 31 . The relationship between the D m and the M r /M s ratio, which was inversely proportional to the particle size, is consistent with the relationship between the D m and the M s . The substantial magnetization values show that the cation distribution changed from normal spinel type to mixed spinel type 28 .

Conclusion
The morphological identification and SEM-EDX elemental analysis of ZnFe 2 O 4 synthesized via the coprecipitation method showed that the sample sintered for 2.5 h exhibited less agglomeration than the samples sintered for 1.5 or 3.5 h. The elemental composition indicated that the samples were free of impurities, and the XRD phase identification showed that the sample sintered for 2.5 h crystallized in the cubic spinel structure with the smallest particle size (70.58 nm) among the investigated samples. The FTIR functional-group characterization confirmed the presence of vibrations due to Fe-O and Zn-O bonds of cations in the octahedral and tetrahedral sites in the 400-600 cm −1 wavenumber range. The peak at 3400 cm −1 indicated the presence of O-H groups of water steam due to sintering of the ZnFe 2 O 4 material.
The hysteresis curves showed that ZnFe 2 O 4 sintered for 3.5 h was superparamagnetic, with M s = 52 emu/g and H c = 0.0708 T, whereas the samples sintered for 1.5 and 2.5 h were nonmagnetic. According to these results, the sintering duration did not influence the magnetic properties of ZnFe 2 O 4 . The magnetization value in this research showed that the cation distribution changed from normal spinel type to mixed spinel type.

Acknowledgments
Our sincere thanks to the RISTEKBRIN for the research funding for PP.