Synthesis and characterization of nano-sized pure and Al-doped lithium ferrite having high value of dielectric constant

Abstract

Fine particles of Al-doped lithium ferrite of general formula Li_0.5Al_xFe_2.5−xO₄ (0.0 ≤ x ≤ 0.4, in step of 0.1) were synthesized by the citrate gel auto combustion method and characterized using XRD, dielectric and impedance spectroscopy. X-ray diffraction studies confirmed that all the samples exhibit single-phase cubic spinel structure. The average particle size of as obtained samples has been found in the range of 38–41 nm. Dielectric permittivity, ac conductivity and complex impedance of all the samples were evaluated as a function of frequency and composition at room temperature. The variation of dielectric properties ɛ′, ɛ″, tan δ and ac conductivity (σ_ac), with frequency reveals that the dispersion is due to Maxwell–Wagner type of interfacial polarization in general and the hopping of charge between Fe⁺² and Fe⁺³. Interestingly, Al-doped nano-crystalline lithium ferrite samples exhibit a very large value for dielectric constant of the order of 10⁵. The impedance spectroscopy technique has been used to study the effect of grain and grain boundary on the electrical properties of Li_0.5Al_xFe_2.5−xO₄ ferrites. The analysis of data shows two semicircles corresponding to the grain and grain boundary volume suggesting that grain boundary is mainly involved in conduction mechanism. Magnetic properties have been measured by vibrating sample magnetometer (VSM) for all samples at room temperature.

Introduction

Ferrites form a very good class of electrical materials because of their high resistivity and low loss behaviour, and hence have vast technological applications over a wide range of frequencies [1]. Ferrites assume special significance in the field of electronics and telecommunication industry because of their novel electrical properties which makes them useful in radiofrequency circuits, high quality filters, rod antennas, transformer cores, read/write heads for high digital tapes and other devices [2], [3]. Hence it is important to study their dielectric behaviour at different frequencies. The dielectric properties of ferrites are dependent on several factors, such as method of preparation, heat treatment, sintering conditions, chemical composition, cation distribution and crystallite size [4].

The preparation of ferrites in the nano range had got considerable attention, because the size of the ferrite particles determine whether they are going to behave as super paramagnetic, single domain or multi-domain materials [5]. Many synthesis techniques are available for the nano particle synthesis, however the thermal decomposition of the citrate gel shows good results. This method is simple, does not require high temperature and prolonged heating and gives very fine particles [6]. When a citrate gel is heated on a hot plate, burn itself because of exothermic nature, thus converting the precursor mixture directly into final products. The self-propagating route shows many advantages, like inexpensive, short preparation time, low heating and simplicity. Therefore we have prepared an Al-doped lithium ferrite by this method. Lithium and substituted lithium ferrites are of great importance in the microwave and memory core applications owing to their high Curie temperature, high saturation magnetization, and excellent hysteresis loop properties and less sensitive to stress [7], [8], [9]. In addition lithium ferrites do not contain costly ingredients [10]. Lithium ferrites behave as an n-type semiconductor based on the inverse spinel structure being characterized by high electrical resistivity (10⁵–10⁶ Ω cm) and high Curie temperature (640–680 °C), and are therefore used for gas sensing applications [11].

At low temperature the compound adopts an inverse structure in which the Li⁺¹ and 3/5 of Fe³⁺ ions occupy B-sites in an ordered fashion, while the remaining Fe³⁺ ions occupy A-sites and the space group is P₄332. At temperature above 735–755 °C, the octahedral B-site occupancy becomes disordered and the solid is indexed according to the space group Fd3m [12]. Synthesis of structurally stabilized lithium ferrite with and without flux is still a hot topic of research in the area of magnetic materials. Several multi-valence atoms can be combined along with lithium to alter properties of the ferrite suitable for a specific use. Substitution with a trivalent cation, to maintain charge neutrality is one of the most important choices. Such mixed lithium ferrites are unique, since they do not contain any divalent cation.

Al³⁺ substituted lithium ferrites have attracted the attention of several workers. Miles [13], Caster et al. [14], Waxwell and Pickert [15] Schulkes and Blasse [16], Maria et al. [17], Widataulah et al. [8], and George et al. [18] have studied Li–Al ferrites for their electrical and magnetic properties. Jung et al. [19] studied ion distribution in a series of Al³⁺ doped lithium ferrite. By carefully analyzing the powder X-ray diffractograms, they found that the Al³⁺ substitute some of the Fe³⁺ in the tetrahedral as well as octahedral site of the spinel structure. Nanoti et al. [20] studied the electrical and magnetic properties of these ferrites synthesized by ceramic technique. The wider use of lithium ferrites particularly in the microwave devices is restricted because of the difficulties experienced in sintering the material at high temperatures employed to achieve high densities. The high temperature used for sintering (>1000 °C) lead to irreversible loss of lithium and oxygen resulting in a lower saturation magnetization due to the formation of α-Fe₂O₃ or Fe₃O₄. Furthermore the reduction of Fe³⁺ to Fe ²⁺ due to the formation of Fe₃O₄ results in an increased conductivity. It is therefore important to try a process not requiring high calcination temperature to induce solid-state reaction.

In the present work we have reported the preparation of Li_0.5Al_xFe_2.5−xO₄ (0.0 ≤ x ≤ 0.4) ferrite by citrate gel auto combustion method, in which the pH of the solution was maintained at 6. The powders were characterized by X-ray diffraction (XRD). Electrical properties of the polycrystalline material were studied as a function of frequency and composition at room temperature using dielectric and impedance spectroscopy. The effect of Al³⁺ substitution on magnetic properties of lithium ferrites have been evaluated by M–H curves at room temperature.