Browsing by Author "Markova-Deneva I."
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Item AN EFFECT OF THE ELEMENTAL COMPOSITION ON THE ELECTROCHEMICAL BEHAVIOR OF ALLOYED (Co-Sn, Ni-Sn) NANOPOWDERS IN A Li-ION BATTERY(2020-01-01) Markova-Deneva I.; Petrov T.I.; Piskin M.B.; Zahariev I.Z.; Milanova V.L.Alloyed Co-Sn and Ni-Sn nanopowders are synthesized at a room temperature through a borohydride reduction with NaBH4 in a mixture of aqueous solutions of the relevant chloride salts (CoCl2.6H2O, NiCl2.6H2O and SnCl2.2H2O) at mass ratios of the metallic components chosen in accordance with the corresponding binary systems phase diagrams (Co:Sn = 35:65, Ni:Sn = 45:65). Intermetallic Co-Sn and Ni-Sn nanoparticles are obtained applying a template technique which involves the use of a carbon support such as graphite (CF) and a carbon powder (CP). As a result, carbon nanocomposites are prepared in-situ. The samples obtained are electrochemically tested as anodes in a three-electrode half cell (a Li-ion battery). Their electrochemical characteristics such as specific capacity, cyclability, and efficiency are evaluated. The investigated electrode materials based on the synthesized intermetallic Co-Sn and Ni-Sn nanoparticles and their carbon nanocomposites exhibit a different electrochemical behavior depending on their morphology, microstructure, their elemental and phase composition. Based on the capacity, the cyclability, and the efficiency observed it can be concluded that Co-Sn (Co:Sn = 35:65) and Ni-Sn (Ni:Sn = 45:55) nanoparticle’s powders and their carbon composites are promising electrode materials. CF and CP used as carriers in a template synthesis of intermetallic nanoparticles are suitable matrices for the preparation of nanocomposite electrode materials of improved electrochemical parameters. The resulting carbon nanocomposite materials based on Co-Sn and Ni-Sn nanoparticles exhibit more stable electrochemical characteristics compared to those of the synthesized Co-Sn and Ni-Sn alloys. They are characterized by a higher initial discharge capacity and better cycling stability after the 10th cycle and are suitable alternative of the graphite anodes in the Li-ion batteries. The investigation carried out verifies the effect of the elemental composition on the electrochemical behavior of the investigated intermetallic (Co-Sn, Ni-Sn) nanopowders.Item Raman spectroscopy investigation of magnetite nanoparticles in ferrofluids(2010-07-01) Slavov L.; Abrashev M.V.; Merodiiska T.; Gelev C.; Vandenberghe R.E.; Markova-Deneva I.; Nedkov I.Raman spectroscopy is used to investigate magnetite nanoparticles dispersed in two types of β-cyclodextrin suspensions. An approach is presented for characterization of the magnetic core in liquid surrounding at room temperature and atmospheric pressure. The effect of elevating laser power on the structural stability and chemical composition of magnetite in the ferrofluids is discussed. The data are compared with data from dry by-products from the fluids. Powder samples undergo total phase transition from magnetite to hematite at laser power of 1.95 mW. The same nanoparticles in the fluid undergo transformation at 9 mW, but no hematite positions appear throughout that investigation. The Raman spectra revealed that the main phase of the magnetic core in the fluids is magnetite. That is indicated by a strong and non-diminishing in intensity peak at 670 cm-1. A second phase is present at the nanoparticle's surface with Raman spectroscopy unveiling maghemite-like and small fractions of goethite-like structures. The Fourier transform infrared spectroscopy investigations confirm deviations in the surface structure and also point to the fact that the oxidation process starts at an early stage after formation of the nanoparticles. The analyses of the infrared data also show that β-cyclodextrin molecules retain their cyclic character and the coating does not affect the oxidation process once the particles are evicted from the fluids. A Mössbauer spectroscopy measurement on a ferrofluidic sample is also presented. © 2010 Elsevier B.V. All rights reserved.