Effects of Various Sintering Conditions on the Structural and Magnetic Properties of Zinc Ferrite (ZnFe₂O₄)

Abstract

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.

Keywords:
Ferrite; Coprecipitation; Hysteresis; Sintering

1. Introduction

Studies on nanomaterials have become prevalent for several reasons, such as the need to produce new smaller-scale 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 meters¹. Numerous authors have explored various aspects of nanomaterials, such as the possibility of controlling their size, defect concentration, solution concentration, atom interaction, and synthesis process².

ZnFe₂O₄ 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. ZnFe₂O₄ adopts the cubic spinel structure. Cations occupy one-eighth of the interstitial tetrahedral (A) and one-half of the octahedral (B) sites³.

Among numerous spinel ferrites, ZnFe₂O₄ nanoparticles have attracted particular attention because of their unique chemical properties and thermal stability, as well as their particle-size-dependent magnetic properties⁴. Most spinel ferrites have a Curie temperature greater than room temperature, whereas ZnFe₂O₄ does not. Thus, ZnFe₂O₄ is compatible with most applications that require operating temperatures as high as 80°C⁵.

Bulk ZnFe₂O₄ crystallizes in the standard spinel structure with diamagnetic Zn²⁺ ions in the tetrahedral sites and magnetic Fe³⁺ ions in the octahedral sites. As a result of super antiferromagnetic interactions among octahedrally coordinated Fe³⁺ ions, bulk ZnFe₂O₄ becomes antiferromagnetic at its Néel temperature (T_N = 10 K). However, when prepared at the nanometer scale, the structure of ZnFe₂O₄ is substantially different, with Fe³⁺ and Zn²⁺ cations occupying both octahedral and tetrahedral sites. As a result, nanocrystalline ZnFe₂O₄ exhibits ferromagnetic properties³.

The coprecipitation technique is the most appropriate approach for synthesizing ZnFe₂O₄ 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 ferrite³. A previous investigation found that the chemical composition of ZnFe₂O₄ is influenced by the Zn/Fe molar ratio, pH, sintering temperature, and sintering duration⁶. This previous study also showed that a Zn/Fe ratio of 1:2 results in main diffraction peaks of ZnFe₂O₄ as well as secondary peaks of Fe₂O₃ and ZnO. Varying the pH of the reaction solution resulted in different intensities of the main peak and in different amounts of precipitate.

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 density⁷. In the present work, sintered ZnFe₂O₄ 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 ZnFe₂O₄ as a potential sensor material.

2. Method

The coprecipitation synthesis method was used in the present work. In the first step, ZnO and Fe₂O₃ (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.

3. Results and Discussion

Figure 1 shows SEM images of the ZnFe₂O₄ 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 11c 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 product⁸. 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 temperature⁹.

Figure 1
SEM morphology of ZnFe₂O₄ sintered for (a) 1.5 h, (b) 2.5 h and (c) 3.5 h; all images are at 100,000× magnification.

Figure 2 shows the XRD patterns of ZnFe₂O₄ sintered at 1.5–3.5 h. The XRD patterns show diffraction peaks corresponding to the (220), (311), (400), (422), (511), and (440) planes of the spinel structure. The observed peaks were 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 impurities¹⁰. 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 size¹¹. The particles shrank with increasing sintering time, resulting in the gradual removal of water fom the crystal lattice¹². 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 process⁴. Particle size depends on numerous factors, including the synthesis conditions, sintering temperature, time, and the rate of heating and cooling¹³. The lattice parameter of the synthesized nanoparticles was greater than that of bulk ZnFe₂O₄ (a = 8.441 Å) (JCPDS No. 22-1012). This sample is an example of ZnFe₂O₄ nanoparticles with a mixture of normal and inverse spinel structures. The substitution of several Fe³⁺ cations for Zn²⁺ cations, whose ionic radius (0.74 Å) is larger than that of Fe³⁺ (0.64 Å), results in an expansion of the spinel lattice, with a concomitant increase in the corresponding lattice parameter⁴.

Figure 2
ZnFe₂O₄ phase identification of samples sintered for various durations.

Table 1
ZnFe₂O₄ Crystal Characteristics Obtained from XRD Analysis

Sintering Duration	2 𝛉 (degree)	Height (cts)	d-Spacing [Å]	FWHM ( $\beta$ )	Crystal Size (nm)
1.5	35.28	836.72	2.54	0.0984	84.72
2.5	35.25	767.80	2.54	0.1181	70.58
3.5	35.28	609.98	2.54	0.0984	84.72

In the normal AB₂O₄ spinel structure, A is generally a divalent cation that occupies tetrahedral sites, whereas B is a trivalent cation that occupies octahedral sites. Bulk ZnFe₂O₄ is a completely normal spinel structure with Zn²⁺ ions at the tetrahedral (A) sites and Fe³⁺ ions at the octahedral (B) sites. In fact, when ZnFe₂O₄ is prepared at the nanoscale, the high surface energy associated with the small particle size leads to the distribution of mixed cations, where Zn²⁺ ions and Fe³⁺ 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 ZnFe₂O₄ in the wavenumber region 4000–400 cm⁻¹. The functional groups of ZnFe₂O₄ 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 ZnFe₂O₄ 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 vibration²⁰ and indicates the presence of surface hydroxyl groups¹⁹ or moisture¹⁸. The peak at ~2300 cm⁻¹ represents stretching vibrations of ether groups with C=O and N–H or O–H²¹ either the peak corresponding to the O–H (H₂O bending mode) group indicates the conversion of water to steam in the ZnFe₂O₄ nanoparticles^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^22-24.

Figure 3
ZnFe₂O₄ functional-group identification of samples sintered for different durations.

The hysteresis curve of ZnFe₂O₄ 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 curve²⁵ and a H_c value close to zero indicates a superparamagnetic material¹⁰. 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 movement²⁶. These results indicate that the sintering duration greatly influenced the magnetic properties of the ZnFe₂O₄. The longer sintering duration of 3.5 h for ZnFe₂O₄ caused spin–orbit coupling with various hyperfine structures²⁷ and superexchange interactions²⁸ between the spin electron in ZnO and Fe₂O₃. The magnetization of spinnel ferrite is given by super-exchnage interaction of side A and B. In bulk zinc ferrites, Zn²⁺ ions of zero moment and Fe³⁺ ions fill A and B sites respectively. Few Fe³⁺ 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 Fe³⁺ ions migrates from B to A side. Therefore, it will disturb the antiparallel ordering of B site and enhance the saturation magnetization and magnetic moment²⁹. 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 behaviour³⁰.

Figure 4
Hysteresis VSM curves for ZnFe₂O₄ sintered for (a) 1.5 h, (b) 2.5 h, and (c) 3.5 h.

Table 2
Magnetic Properties of ZnFe₂O₄

Sintering Time	Magnetic Saturation/ $M_s$ (emu/g)	Magnetic Remanence/ $M_r$ (emu/g)	Remanence Ratio /Mr/Ms	Coercivity /Hc
				T	Oe
1.5 h	1.12	−0.06	−0.054	0	0
2.5 h	1.15	−0.04	−0.035	0	0
3.5 h	52.52	14.42	0.028	0.0708	708

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_m²⁶. 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 sites³¹. 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 type²⁸.

Figure 5
Correlation of the particle size of ZnFe₂O₄ with its (a) magnetic saturation, (b) remanence ratio, and (c) coercivity.

4. Conclusion

The morphological identification and SEM-EDX elemental analysis of ZnFe₂O₄ 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 ZnFe₂O₄ material.

The hysteresis curves showed that ZnFe₂O₄ 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 ZnFe₂O₄. The magnetization value in this research showed that the cation distribution changed from normal spinel type to mixed spinel type.

5. Acknowledgments

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

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Publication Dates

• Publication in this collection
  15 Jan 2021
• Date of issue
  2021

History

• Received
  07 July 2020
• Reviewed
  22 Oct 2020
• Accepted
  29 Nov 2020