• N Lakshmi

      Articles written in Bulletin of Materials Science

    • Ion transport in some solid state proton conducting composites studied from volta cell e.m.f. and complex impedance spectroscopy

      N Lakshmi S Chandra

      More Details Abstract Fulltext PDF

      Proton conducting composites of heteropolyacid hydrates (phosphomolybdic acid H3PMo12O40.𝑛H2O,PMA; phosphotungstic acid H3PW12O40.𝑛H2O, PTA) and salt hydrate like NiCl2.6H2O were prepared with insulating Al2O3 as dispersoid. The ionic conductivity peaks at two concentrations of Al2O3 indicating two percolation thresholds for proton conduction. Two separate experiments were carried out to check the existence of such percolation thresholds viz. the volta battery experiment involving the measurement of e.m.f. of an electrochemical cell with composites of different compositions used as electrolyte and the composition vs conductivity measured by the complex impedance spectroscopy. The presence of two maxima has been attributed to two different percolation thresholds for the two possible mobile protonic species 𝐻+ (H3O+) and OH arising from the hydrates.

    • Hyperfine field distributions in disordered Mn2CoSn and Mn2NiSn Heusler alloys

      N Lakshmi Anil Pandey K Venugopalan

      More Details Abstract Fulltext PDF

      Heusler alloys, Mn2CoSn and Mn2NiSn, were prepared and characterized by X-ray studies. Mössbauer studies using Sn-119 were carried out to investigate the hyperfine fields present at the Sn site in these alloys. The hyperfine field distribution in these alloys as well as X-ray studies point to the chemical disorder present in both alloys. Co-existence of a paramagnetic portion along with the magnetic hyperfine part was observed in Mn2CoSn even at low temperatures, while this was not found in Mn2NiSn spectra. Hyperfine fields at Sn site were calculated using Blandin and Campbell model and compared with the experimental results.

    • Study of dependence of magnetic properties on seeding temperature in fine particles of Cu0.25Co0.25Zn0.5Fe2O4

      Hina Bhargava V D Sudheesh Snehal Jani N Lakshmi K Venugopalan

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      Using a combination of X-ray diffraction, room temperature/low temperature Mössbauer studies and d.c. magnetization, the structural and magnetic properties of nano-sized Cu0.25Co0.25Zn0.5Fe2O4, prepared using the coprecipitation method at different seeding temperatures, have been studied. Formation of 𝛼-Fe2O3 in these samples is observed to be very sensitive to the seeding temperature and is totally suppressed at a seeding temperature of 333 K. These results are explained in terms of the rate of coprecipitation of the different components as a function of temperature. Since 𝛼-Fe2O3 is antiferromagnetic, the presence and quantity of this impurity is also observed to deteriorate the overall magnetic properties of the spinel phase.

    • Size-dependent magnetic properties in Cu0.25Co0.25Zn0.5Fe2O4

      Hina Bhargava V D Sudheesh J Nehra Varkey Sebastian N Lakshmi K Venugopalan V R Reddy Ajay Gupta

      More Details Abstract Fulltext PDF

      Cu0.25Co0.25Zn0.5Fe2O4 nanoparticles were prepared by a co-precipitation method and the size was varied by varying annealing temperatures. The cation distribution, estimated using Mössbauer spectroscopy, shows that it is the same for all samples, irrespective of the size. The variation of Curie temperature and saturation magnetization as a function of particle size, studied using d.c. magnetization, clearly indicates that these are a result of finite size scaling effects. The superparamagnetic size limit is estimated to be 8 nm.

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      Chemical Sciences 2020

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