Synthesis of Cobalt Based Nanoferrites and Study of Their Structural and Conduction Properties

Abstract

Synthesis of Cobalt Based Nanoferrites and Study of Their Structural and Conduction Properties Crystal structure and cation distribution at particular sites in crystal lattice play the primary role in determining the properties of nanocrystalline transition metal oxide materials. Spinel ferrites are a class of compounds with general formula MFe2O4 (M = Mn, Co, Ni, Zn, Mg, etc.). The main focus of this research work was the synthesis of nanocrystalline CoFe2O4 ferrite with different dopant elements like Zn, Mn and Cd with different ratio by weight. The study of cation distribution in doped Co nanocrystalline ferrites and dependence of structural and electrical properties on dopant cations and distribution of these cations in certain interstitial sites in entire crystal structure, was made. All compositions were synthesized and characterized under same conditions so that a comparative analysis could be done. In the first experiment nanocrystalline ferrite particles of Co1-xZnxFe2O4 (x= 0.0 to 1.0 with step of 0.2) were synthesized by co-precipitation synthesis technique. In the case of CoZnFe2O4 relative concentration of Co and Zn with their particular site occupancy plays a crucial role in deciding the ultimate material properties. Samples synthesized at the reaction temperature of 70°C were sintered for 3 hours at 600°C. For the selection of sintering temperature to have maximum crystallinity, thermal analysis was done through differential scanning calorimetry (DSC) and thermogravimetry analysis (TGA) techniques. The FCC spinel structure of the synthesized particles was confirmed by X-ray diffraction (XRD) patterns. Lattice constants obtained were in the range 8.36(1) to 8.44(1) Å. The crystallite sizes calculated from the most intense peak (311) via the Scherrer equation, were found in the range of 10 nm to 35 nm. XANES spectroscopy was used at Fe, Co and Zn K-edges to examine the cation distribution in the crystal structure. Dependence of electrical transport properties on shift in crystal structure due to the successive replacement of Co by Zn in CoFe2O4 was examined. The dc electrical conduction measurements were taken as a function of temperature ranging from 313 K to 700 K. Activation energy values (0.518-0.537 eV) indicated the polaron hopping conduction mechanism. The ac electrical transport properties were studied by measuring the dielectric constant, dielectric loss tangent (tan δ) and ac conductivity as a function of frequency. A regular shift in electrical properties is observed depending upon the cation distribution. Jonscher power law and Maxwell- Wagner two layer models were employed to investigate the conduction phenomenon. Manganese substituted cobalt ferrites are promising materials for stress and torsion sensor applications. Effects of Mn doping on the crystal structure and change in electrical transport properties with the shift of cation distribution in CoFe2O4were studied. Co1-xMnxFe2O4(x= 0.0 to 1.0 with step of 0.2) nanocrystallite particles with stoichiometric proportion were synthesized via co-precipitation method at 70°C of reaction temperature. The crystalline phase of FCC spinel structure, with lattice constants in the range 8.36(1) to 8.46(8) Å, were confirmed by XRD patterns. Space group was found to be Fd3m. The crystallite sizes were found to be in the range from 16 nm to 35 nm. X-ray absorption fine structure (XAFS) spectrometry is an elemental specific technique and is sensitive to the local crystal structure. X-ray absorption near- edge structure (XANES) spectroscopy is a prevailing tool for the structural study of metal oxide materials. XANES spectroscopy is used at Fe K-edges to investigate the cation distribution in the crystal structure. DC electrical resistivity measurements were done at different temperatures by means of two-probe method from 370 K to 700 K. AC electrical properties were also analyzed. Results are explained in terms of polaron hopping model under the effects of cation distribution. Nyquist plots were done to determine the equivalent circuits for grains and grain boundary conduction mechanism. Co contents were also replaced by Cd successively in CoFe2O4 nanocrystalline particles. FCC spinel structure with space group Fd3m of Co1-xCdxFe2O4(x= 0.0 to 1.0 with step of 0.2) crystal was confirmed by XRD patterns. XANES spectroscopy was used at Fe and Co K- edges to find the distribution of cations. The lattice constants were in the range from 8.36(1) Å to 8.60(1) Å. The variation in dielectric properties such as dielectric constant, dielectric loss tangent (tan δ) and ac conductivity (σac) have been observed as a function of composition and frequency. Conduction mechanism was correlated with cation type and hopping lengths for charge carriers. The results were discussed in terms of the polaron hopping model under the effects of cation distribution. A systematic replacement of Co, in CoFe2O4 ferrites by Zn, Mn and Cd is done. Structural and electrical properties are correlated and explained. Deep structural investigation by XAFS is rarely reported. Electrical properties could be controlled by structure (cation distribution and site occupancy by specific element) for the desired useful applications. Mn substituted Co ferrites are useful for relatively low resistance demanding applications while Cd substituted Co ferrites are suggested for high resistance requiring applications.