Thermo-Analytical and SEM Studies on Li1+ Substituted Ni-Zn Ferrites

Abstract

Ni-Zn Ferrites systems exhibit remarkable magnetic properties and finds applications in devices such as inductor cores, magnetic heads converters electromagnetic wave absorbers etc. coupled with low production cost. The soft chemistry approaches make it possible to synthesize homogenous dense and smaller particle size ferrites at relatively low temperatures. Lithium oxalate solution was synthesized by adding hot solution of oxalic acid with lithium carbonate1, was filtered and cooled, when lithium oxalate was precipitated and then dried. Ni-Zn-Fe oxalate complexes were synthesized by co-precipitation technique using oxalate precursor2. Lithium oxalate was mixed to the solid solution of Ni-Zn-Fe oxalate complex for 2 to 3 hours. Thus solid solution of oxalate complex having general composition Ni0.32Zn0.68-2xLixFe2+x(C2O4)3, nH2O with x=0.00, 0.025, 0.05, 0.075, 0.10, 0.15 & 0.20 was obtained by method suggested by Wickham3.
STA studies were carried out on above synthesized oxalate complex in order to determine the ferritization temperature. The temperature at which ferrite phase is formed and to fix the number of water molecules in oxalate complex for proposing the formulae of oxalate complex. The oxalate complexes were decomposed at their respective ferritization temperatures as revealed from thermograms (TG). STA studies revealed formation of single spinel phase ferrites in the temperature range 196 –370oC. Li1+ plays a significant role in lowering of ferritization
temperature. Increasing Li1+ contents leads to the lowering of ferritization temperature. This has been attributed to earlier initiation of decomposition of Li1+ in the oxalate complex.
Characterization of samples was done by XRD techniques. XRD’s of oxalate complex decomposed at 350oC for 3 hours and sintered at 1050oC for 24 hours were recorded. XRD’s showed formation of a single spinal phase at lower temperature. Lattice parameter showed a decreasing trend with increasing Li1+ content. This has been attributed to ionic sizes of cations involved. Tetrahedral bond length, tetrahedral radii, tetrahedral edge, unshared octahedral edge decrease with increase in Li1+ content. Octahedral bond length, octahedral site radii, shared octahedral edge increase with increase in Li1+ content.
Density measurement on sintered samples sintered at 1050oC for 24 hours were carried and by using xylene medium exhibit close resemblance. All compositions showed density values which are 97% of the theoretical values. Thus high density ferrites are obtained at low temperature lying in the range of 196 -370oC. High density plays a crucial role in enhancement of magnetic properties, such as, initial permeability, saturation magnetization, Coercive force. The increase in Li1+ content leads to lowering of density values.
SEM studies were carried out on sintered samples sintered at 1050oC for 24 hours in order to measure the grain size. SEM studies revealed formation of fully developed, close and compact grain structure. The grain size of Li1+ Ni-Zn ferrite is larger than that of Ni-Zn ferrites. This
suggests Li1+ favors grain growth.