• Anuj Upadhyay

      Articles written in Bulletin of Materials Science

    • Effect of isochronal annealing on phase transformation studies of iron oxide nanoparticles

      Anjali J Deotale R V Nandedkar A K Sinha Anuj Upadhyay Puspen Mondal A K Srivastava S K Deb

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      The effect of isochronal annealing on the phase transformation in iron oxide nanoparticles is reported in this work. Iron oxide nanoparticles were successfully synthesized using an ash supported technique followed by annealing for 2 h at various temperatures between 300 and 700° C. It was observed using X-ray diffraction (XRD) and transmission electron microscopy (TEM) that as-grown samples have mixed phases of crystalline haematite (α-Fe2O3) and a minor phase of either maghemite (𝛾-Fe2O3) or magnetite (Fe3O4). On annealing, the minor phase transforms gradually to haematite. The phase transformation is complete at annealing temperature of 442° C as confirmed by differential scanning calorimetric (DSC) analysis. The unresolved phases in XRD were further analysed and confirmed to be maghemite from the X-ray absorption near edge structure (XANES) studies. The magnetic measurements showed that at room temperature nano-𝛼-Fe2O3 is weak ferromagnetic, and its magnetization is larger than the bulk value. The mixed phase sample shows higher value of magnetization because of the presence of ferromagnetic 𝛾-Fe2O3 phase.

    • Pressure-induced variations of medium-range order in B$_2$O$_3$ glasses


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      Lead-borate glasses manifest unique structure with lead oxide composition owing to their dual role of modifier and former. The medium-range signature from the first sharp diffraction peak (FSDP) in lead-borates exhibits substantial correlations due to the presence of superstructural units. Pressure plays an important role in altering bonding characteristics and hence structure in materials. To understand pressure-induced changes in the medium-range order (MRO) in lead-borate glasses, we have carried out high pressure X-ray diffraction studies in the FSDP region of these glasses up to 25 GPa. Two clear peaks, $q_0$ and $q_1$ are observed in all the modified glasses, below and above the position of the FSDP in pure B$_2$O$_3$ glass. This shows that lead oxide alters the FSDP and causes changes in the network structure at the MRO. $q_1$ exhibits an increase under compression, resulting in a reduction in correlation lengths for all glass compositions.The initial steep decrease is accompanied by a lower rate of reduction at higher pressures. This shows that correlations arising from structural voids in the boron matrix exhibit similar decrease for various lead oxide compositions.

    • Compressibility studies of RE$_6$UO$_{12}$ at extreme conditions of pressure


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      Rare-earth uranates-RE$_6$UO$_{12}$ are synthesized by heating mixture of uranium oxide and rare-earth oxides in 1:6 ratio above 1273 K. These compounds stabilize in rhombohedral structure at ambient. High-pressure (HP) X-ray diffraction studies reveal that the compounds are stable at lower pressures, beyond which disorder is seen to originate and compound has a tendency to amporphize at very HPs. The a-axis of the lattice is found to be more rigid as compared to c-axis because of corner sharing polyhedra along a-axis. Anomalous compressibility behaviour is seen in Gd$_6$UO$_{12}$, where sharp decrease in the bulk modulus is observed. The behaviour is against the normal trend of compressibility in RE$_6$UO$_{12}$ compounds along rare-earth cation series.

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