Fabrication and characterization of Ni (1-x)CoxFe2O4 electromagnetic ceramic

Abstract

Ni-Co ferrites with the composition Ni1xCoxFe2O4 (x=0, 0.2, 0.4, 0.6, 0.8, 1) were synthesized by solid state reaction method. The starting materials for the preparation of the studied compositions were in the form of nano powder of ferrites (NiFe2O4 and CoFe2O4) of Inframat Advanced Materials, USA. The purity of materials was up to 99.99%. The NiFe2O4 and CoFe2O4 nano ceramic powders were mixed according to their molecular weight. Intimate mixing of materials was carried out using mortar and pestle and then ball milled for 10hrs and the slurry was dried and pressed into disc shaped sample. The disc shaped samples were pre-sintered at 8500C for 4hrs. Final sintering of the samples were carried out at 11000C,11250C,11500C,11750C,12000C and12500C for 4 hrsrespectively.

The phase identification and lattice parameter determination were performed on an X-ray diffraction patterns. The XRD patterns for all samples clearly showed their spinel phase and the formation of a spinel structure. The peaks (111), (220), (311), (222), (400), (511) and (440) correspond to the spinel phase. The lattice parameter was determined through the Nelson-Riley extrapolation method.The variation of lattice parameter as a function of Co2+ content was established. It was noticed that the lattice parameter increased with the Co content. This variation can be explained on the basis of an ionic size difference of the component ions. The Co2+ (0.745 Å) ions have a larger ionic radius than Ni2+ (0.69 Å) and Fe3+ions (0.645 Å).The bulk density (dB) and the X-ray density (dX) of the prepared samples werealso calculated.

The microstructural aspects were studied with a field emission scanning electron microscope (JEOL-JSM-7600F). Magnetic and electrical properties of ferrites highly depend on the microstructure. Between the grain size and porosity of microstructures, the grain size is more important parameter affecting the magnetic properties of a ferrite. Scanning electron micrographs of Ni1-xCoxFe2O4(x=0, 0.2, 0.4, 0.6, 0.8, 1) samplessintered at 11000C, 11250C, 11500 C, 11750C, 12000C and 12500C were measured. As seen from the micrographs, the average dimension of the particles is below 10 µm. Initial permeability as a function of frequency has been measured by an impedance analyzer. Phase transition temperature determined from DSC measurements and temperature dependence of initial permeability was found to display a good correlation. Curie temperature, Tc of all the samples were determined from the temperature dependence of permeability and compared with the peak temperature of DSC thermograph. Tc is found to decrease with increasing CoFe2O4substitution which ascribed due to weakening of JAB super-exchange interaction between tetrahedral and octahedral sites.

The magnetization measurement of the sample was conducted by a vibrating sample magnetometer (model VSM-02, Hirstlab, UK) at 80K, 200K and 300K as a function of field.The detail hysteresis parameters such as Ms, Mr, Hc determined from M-H loop measured at 80, 200 and 300K. Saturation magnetization, Ms measured at 80K was found much higher compared with data at 300K. Ms was found to increase with decreasing temperature and attained a maximum value corresponding to CoFe2O4 at any measurement temperature which may be attributed to higher magnetic moment of CoFe2O4 compared withNiFe2O4. A gradual magnetic hardening was observed to increase with increasing CoFe2O4 content with a maximum value of Hc at x=0.8.

The dielectric behavior is one of the most important characteristics of ferrites which distinctly depend on the preparation conditions, e.g. sintering time and temperature, type and quantity of additives. The dielectric constant (ε’) results are due to the heterogeneous structure of the material. The frequency dependence of the dielectric constant for all the samples were measured using an impedance analyzer. The change of dielectric properties was investigated varying modulating frequency (100 Hz to 10 MHz). All the samples showed similar behavior, i.e. dielectric constant decreased initially with increase in frequency and reached a constant value at higher frequency. After a certain increase in frequency all the samples exhibit a frequency-independent behavior.