Brajesh Pandey

A facile, single-step synthesis of flowery shaped, pure/lithium-doped 3D iron oxides

The shape-dependent surface properties of iron oxides are being paid increasing attention for their many advanced and synergistic applications. The present investigation deals with the preparation of pure and lithiated 3D iron oxide through a simple and single-step synthesis route. The nano-hierarchical flowers were synthesized by adopting a semi-aqueous ligated system. Here, the reagent played a double role for ligation as well as for precipitation. In the absence of lithium, goethite and ferrihydrite phases were formed, whereas formation of a mixture of hematite and ferrihydrite was observed in its presence, confirming participation of Li in phase transformation of goethite/ferrihydrite to hematite. With the progress of time, flowery shaped nanoparticles developed. Mossbauer spectroscopy revealed Li ion-induced formation of an α-Fe2O3 phase. Single-phase hematite was formed on annealing at 500 °C. The Li-doped iron oxide sample has high surface area and has a sharp distribution peak centered at 19.13 nm, showing homogeneity of the pores. On calcination of the sample at 400 °C, the surface area decreased; however, pore size distribution remained unchanged, which was an unusual trend. The annealed sample (500 °C) possessed bimodal (small and large) mesopore distribution. The fluoride adsorption behaviour and magnetic properties of the as-synthesized and annealed Li-doped samples are discussed. Magnetic properties of the samples suggest that incorporation of Li resulted in an increase of coercivity due to stabilization of the domain. The unique surface behaviour of the present samples can be further examined for other high end applications. The present synthesis strategy has the advantage of producing shape-controlled hierarchical materials with tunable surface properties, which promises the further development of other functional materials.

Mössbauer and positron annihilation studies in plastically deformed Fe72-xAl28Tix (x = 0, 2, 9) alloys

Abstract Positron lifetime measurements in deformed Fe–Al–Ti alloys, obtained by filing the homogenized ingots show that monovacancies are created during the filing process. This is in contrast with known results on deformation due to rolling where dislocations have been reported as the major defect type. These results together with those from Mossbauer spectroscopy suggest that the vacancies are dominantly created due to aluminum atoms being displaced to the nearby sites where they replace iron atoms. Annealing at 900°C for extended periods caused defect concentration to greatly reduce. Positron lifetime spectroscopy has been successfully used to detect agglomeration of vacancies to form di- or tri-vacancies for the first time. Addition of titanium is found to facilitate fast removal of defects during controlled heat treatments.

Phase changes in Fe72−x Al28Cr x (x = 0,2,4,6) alloys due to mechanical strain

Fe72−xAl28Crx (x = 0,2,4,6) are made by arc melting a mixture of constituent elements in stoichiometric proportion, in argon atmosphere. The ingots so obtained are filed to make powder samples thereby giving them substantial mechanical deformation. It is observed that as-powdered samples show hyperfine field distribution typical of α-phase, where the atoms are randomly distributed on the available sites. Annealing at 900°C for 60 h leads to preferential occupation of lattice sites by the atoms and this results in better defined groups of hyperfine magnetic field (HMF) which can be associated with specific configuration in the neighbourhood of probe iron atoms. The average HMF is found to decrease sharply with increasing Cr concentration even though the net chromium concentration remains low (≤6 at%). The results show that cold working on samples is very important in changing the atomic ordering and must be taken into account if properties of equilibrium phases are probed.

Mössbauer studies of nano phase Ce–Fe oxide composites

Chemical co-precipitation method was used to synthesize nano-structured α-Fe2O3-CeO2 composite by calcination of the goethite–cerium hydroxide precursor. It was observed that the precursor contained goethite matrix doped with cerium. Calcination of the precursor at 400°C showed the formation of nanosize hematite. Mossbauer spectra show the presence of a paramagnetic component in the precursor but not in the samples calcined at 400°C to 800°C temperatures. Our study shows that Ce precipitated as CeO2 and stuck on the surface of hematite particles. The precipitation of Ce as CeO2 is independent of the concentration of Ce in the Ce–Fe–O composite.