Dielectric and magnetic properties of Nickel ferrite ceramics using crystalline powders derived from DL alanine fuel in sol–gel auto-combustion

Abstract

Dielectric and magnetic properties of NiFe₂O₄ ceramics prepared with powders using DL-alanine fuel in the sol–gel auto combustion technique are studied. DL-alanine fuel yields crystalline as-burnt powders, and when used for ceramic processing yields varying microstructure at different sintering temperatures. The dielectric properties are influenced by the resulting microstructure and the magnetic properties show slight change in saturation magnetization M_s (~44 – 46 emu/g). The coercive fields, dielectric losses and dispersion are reduced considerably at higher sintering temperatures (1200–1300 °C). The influence of changing microstructure is analyzed through dielectric response, complex impedance analysis and electrical modulus spectroscopy in the frequency range (10⁻²–10⁷ Hz) to understand the interactions from the grain and grain boundary phases. Sintering at 1200 °C, is found to be optimum, yields lower losses & reduced dielectric dispersion, and high resistivity (3.4×10⁸ Ω cm).

Introduction

Nano-powders and ceramics of spinel ferrites find wide applications for magnetic recording media, magnetic memory devices, and as contrast agents in magnetic resonance imaging and magnetic drug delivery [1]. Nickel ferrite (NiFe₂O₄) is one of the most important soft ferrites with unique properties, exhibiting high electromagnetic performance, low coercivity, good mechanical hardness and chemical stability. These attractive properties are found useful for applications in microwave devices, memory devices, and transformer cores [2]. NiFe₂O₄ (NFO) has an inverse spinel structure in which the ferric ion occupies both tetrahedral (A-site) and octahedral (B-site) sites, and Ni²⁺ occupies the octahedral site (B-site) and the compound can be represented by the chemical formula as [Fe³⁺]_A[Ni²⁺·Fe³⁺]_BO₄ where A and B represent the tetrahedral and octahedral sites respectively [3]. The magnetic and dielectric properties have shown a strong dependence on the cation distribution. In ceramic form, the properties are greatly influenced by the chemical composition and microstructure, and are sensitive to the quality of the powders that are used [4]. The different stages: (i) powder synthesis and (ii) powder compaction followed by sintering at a high temperature demand individual attention and careful optimization during ceramic fabrication. Specific applications in the bio-medical field demand narrow particle size distribution and encapsulation of nano-sized powders, whereas for many electronic applications ferrite ceramics possessing high resistivity and good magnetic properties with low dielectric loss are desirable. Production of high quality powders with inexpensive methods, and testing ceramic properties with different powders is becoming equally important.

Several powder preparation techniques are available including conventional ball milling, and mechano-chemical synthesis, or high energy ball milling of inorganic oxide powders [5], and chemical methods such as co-precipitation [6], citrate precursor [7], hydrothermal [8], sol gel auto-combustion [9], reflux [10], electro spinning [11], sol gel [12] and reverse micelle [13] methods. Amongst these methods the sol–gel auto combustion technique is one of the most simplest and convenient, and yields high quality powders at a fast production rate with good chemical homogeneity and the preparation cost is low [14]. Sol gel auto-combustion involves a self-sustaining exothermic chemical reaction between the metal salts (oxidizer) and an organic fuel (reducing agent). Phase purity and particle size of the powders strongly depend on reaction temperature, the chemical nature of the fuel, and its concentration. The different fuels vary in their complexation ability, reducing power, the amount of generated gas [15], and the residual carbon content, and these factors are known to affect the particle size, crystallinity, stoichiometry and the final properties. Organic compounds like urea, glycine and citric acid have been commonly studied as reducing agents [14] to obtain high quality single phase nano-powders of varying size. Many other fuels including DL-alanine, hydrazine, acrylic acid, carbohydrazide, ethylene glycol and polyacrylic acid have also shown great promise [14], [16], [17], and necessitate post-annealing of the powders for improving the phase purity. Significant variations in the magnetic and dielectric properties have been reported, and variations in the ceramic properties are found to depend on microstructure, density, grain size and cation distribution [18], [19]. For example, NiFe₂O₄ ceramics prepared with powders obtained from different fuels (glycine, alanine, poly-mediated and sol gel synthesis) show significant differences in the microstructure and exhibit a strong dispersion in the dielectric properties with frequency [20]. Similarly, powders obtained from different techniques such as (i) soft mechano-chemical synthesis using different precursor powders [21], (ii) molten salt technique [22], (iii) use of different chelating/fuel agents [23], (iv) high-energy ball milling [24], (v) sol–gel method [25], and (vi) co-precipitation methods [26] exhibit varying dielectric characteristics. Likewise the compositionally modified Ni_0.5Zn_0.5Fe₂O₄ powders of varying size obtained by sol gel auto combustion method with different fuels when used for ceramic processing exhibit significant variations in the dielectric characteristics with strong dispersion in the low frequency region (<1 MHz) [4], [27], [28]. In comparison, NiFe₂O₄ ceramics prepared with powders obtained from the hydrothermal technique [18] and mixed oxide powders (NiO, and Fe₂O₃) sintered by microwave technique [29] have exhibited reduced dielectric losses and dispersion. In view of the developments in the powder production methods, and the availability of ultrafine NiFe₂O₄ powders from different routes, it is realized that the ceramic properties need to be examined in detail as the final microstructure affects the electrical and magnetic properties significantly.

In the present study properties of NiFe₂O₄ (NFO) ceramics prepared from the same starting powders derived from sol gel auto-combustion using DL-alanine fuel are analyzed, as there have been no reports on the utilization of these powders for ceramic processing, and their dielectric and magnetic properties have not been reported. DL-alanine fuel was preferred as it has been found to yield crystalline NiFe₂O₄ powders in the as-burnt stage with considerable control and stability over the particle size in comparison to powders obtained from other fuels [16]. Changes in the phase purity of the powders due to post-annealing, and microstructural evolution during ceramic sintering have been studied. Variations in electrical, magnetic and dielectric properties are correlated with changes in ceramic microstructure, and the influence of sintering conditions on the final properties are shown to be significant.