S.K. Pradhan

Microstructure and positron annihilation studies of mechanosynthesized CdFe2O4

Abstract Nanocrystalline non-stoichiometric cadmium ferrite (CdFe2O4) has been synthesized by high-energy ball milling the mixture (1:1 mol%) of CdO and α-Fe2O3 at room temperature. Formation of nanocrystalline CdFe2O4as normal spinel structure has been noticed after ball milling the mixture for 5 h. The structural and microstructural evolution of CdFe2O4 caused by milling is investigated by X-ray powder diffraction. The relative phase abundances, particle sizes, r.m.s. strains, lattice parameter changes of different phases have been estimated employing Rietveld powder structure refinement analysis of XRD data. It appears that solid state diffusion of nanocrystalline CdO into α-Fe2O3 particles results in formation of nanocrystalline CdFe2O4 spinel. XRD data analysis indicates that the ferrite prepared by ball milling is a non-stoichiometric Cd-riched phase. Positron annihilation lifetime measurements on the milled samples indicate rise of defect density up to 5 h milling time and supports the formation of non-stoichiometric CdFe2O4 phase.

Single step synthesis of ZnS quantum dots and their microstructure characterization and electrical transport below room temperature

Low dimensional cubic phase ZnS quantum dots (QDs) are formed by mechanical alloying the stoichiometric mixture of Zn and S powders at room temperature. During milling process the primary mixed phase ZnS is formed at about 3.5 h of milling and strain less single phase (cubic) ZnS QDs are formed with ~4.5 nm in size after 20 h of milling. Detailed microstructure study has been done by both Rietveld analysis of x-ray diffraction pattern and high resolution transmission electron microscope images. Dc resistivity decreases with increasing temperature which can be explained by three-dimensional hopping conduction mechanisms. Observed negative magnetoconductivity has been analyzed by wave function shrinkage model. Alternating current conductivity can be described by the correlated barrier hopping conduction mechanism. Analysis of complex impedance indicates that the grain boundary resistance is found to be dominating over the grain resistance. Relaxation behavior has been explained by the analysis of the electric modulus.