Amarendra K. Singh

Dielectric properties of Mn-substituted Ni–Zn ferrites

The dielectric properties of MnxNi0.5−xZn0.5Fe2O4 ferrites with x varying from 0.05 to 0.4 synthesized by the citrate precursor method have been investigated as a function of frequency, temperature, composition, and sintering temperature. An increase in the dielectric constant is observed with the increase in Mn concentration except for x=0.3. Dispersion in the dielectric constant with frequency in the range of 100 Hz–1 MHz is observed. Resonance peaks were observed in tan δ∈ versus frequency curves for all samples. A shift in the resonance frequency towards higher frequency is observed with increase in temperature. The peak height also increases with increase in temperature. Possible mechanisms contributing to these processes have been discussed. From the temperature variation of the dielectric relaxation, activation energies for various samples have been calculated and compared with those obtained from dc resistivity.

Magnetic properties of Mn-substituted Ni–Zn ferrites

MnxNi0.5−xZn0.5Fe2O4 ferrites with x=0.05, 0.1, 0.2, 0.3, and 0.4, have been synthesized by the citrate precursor method and investigated for their magnetic properties. The initial relative permeability is observed to increase with increase in manganese concentration up to x=0.2 followed by a decrease, the maximum value being 632. An initial increase followed by a subsequent decrease of saturation magnetization with increase in x is observed. Curie temperature is observed to decrease continuously from 488 to 418 °C when x increases from 0.05 to 0.4. Frequency variation of complex initial permeability indicates that resonance peak due to domain wall oscillations is at a frequency above 13 MHz. Saturation magnetization is observed to increase by about 22% when the sintering temperature is increased from 1200 to 1400 °C. Initial permeability and B–H loops are observed to increase and constrict, respectively, with increase in sintering temperature.

Electrical and magnetic properties of Mn-Ni-Zn ferrites processed by citrate precursor method

Abstract MnxNi0.5−xZn0.5Fe2O4, (x=0.05, 0.1, 0.2, 0.3, 0.4) ferrites synthesized by the citrate precursor method are investigated in the present work. Compositional variation of AC resistivity is studied and values up to 103 times greater than those for samples prepared by the conventional ceramic method are observed. It is found that resistivity decreases with increase in Mn concentration except for x=0.3, where it shows a rise. Frequency variations of both dielectric constant and relative loss factor (rlf) are reported in the present work. It is observed that dispersion in dielectric constant is greatly influenced by the Mn concentration. Possible mechanisms contributing to these properties are discussed. Hysteresis properties are also reported.

Effect of cation distribution on the properties of Mn0.2ZnxNi0.8−xFe2O4

Abstract Mn 0.2 Zn x Ni 0.8− x Fe 2 O 4 ( x =0.2, 0.3, 0.4, 0.5, 0.6) are synthesized by the citrate precursor method. Effects of zinc substitution on DC resistivity, dielectric relaxation intensity, initial permeability, saturation magnetization and Curie temperature have been investigated. It is observed that resistivity increases with increase in zinc concentration up to x =0.5 and then decreases. The observed behaviour is explained in terms of hopping and site preference of ions in the lattice. The main contribution to dielectric relaxation intensity is observed to be due to space charge polarization. Initial permeability is observed to increase with increase in zinc concentration. Saturation magnetization increases up to x =0.4 and then starts decreasing. Canting effect is observed for higher zinc concentrations.

Dc resistivity of Mn-Ni-Zn ferrites

DC resistivity of MnxNi0.5-xZn0.5Fe2O4 with compositions x=0.05 to 0.4 prepared by the citrate precursor method has been investigated. It has been observed that resistivity decreases with increase in Mn concentration. The observed temperature variation curves show two linear regions. Activation energy corresponding to both regions has been calculated. Possible conduction mechanisms contributing to the processes have been discussed. Variation of resistivity with sintering temperature establishes a correlation between microstructure and conductivity.

Effect of manganese impurity on the conductivity, dielectric behavior and magnetic properties of Ni0.3MnxZn0.7-xFe2O4

Ni0.3MnxZn0.7-xFe2O4 samples, with x=0.1, 0.2, 0.3, 0.4, 0.5, have been prepared by the citrate precursor method and their dc resistivity, complex initial permeability, permittivity and saturation magnetization are studied. Initial increase followed by a decrease is observed in dc resistivity and saturation magnetization (maximum 57 emu/gm) with increase in x. Initial relative permeability is observed to decrease from 550 to 100 with increase in manganese content. A shift in dielectric relaxation peak towards higher frequencies is observed with increase in x. A slight variation of resistivity is observed with increasing applied field.

Modelling of Columnar-to-Equiaxed Transition and Inclusion Distribution in Continuously Cast Steel Billets

Solidification castings can exhibit a columnar or an equiaxed morphology or a combination of both. Since the relative proportions of these two components strongly influence the internal quality of cast product, the study of morphological transition from columnar to equiaxed structure (CET) becomes important. The transition also affects quality parameters like inclusion distribution in castings which has a significant bearing on the properties of cast products. In this work, a combined model for CET and inclusion distribution in continuously cast steel billets is presented. A conduction based transient thermal solidification model is employed in conjunction with Hunt’s criterion for CET to predict the evolution of melt temperature, the location of transition and area-fractions of columnar and equiaxed zones across the billet cross-section. A correlation between melt temperature and equiaxed nuclei density is proposed and incorporated in the model to account for a more realistic variation of CET with melt superheat. The model is compared with available experimental data and is used to explore the effect of process parameters on CET and determine the spatial distribution of non-metallic inclusions in the solidified billet.