Faculty Publications
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Item Microstructure and corrosion behavior of laser processed NiTi alloy(Elsevier Ltd, 2015) Marattukalam, J.J.; Singh, A.K.; Datta, S.; Das, M.; Balla, V.K.; Bontha, S.; Kalpathy, S.K.Abstract Laser Engineered Net Shaping (LENS™), a commercially available additive manufacturing technology, has been used to fabricate dense equiatomic NiTi alloy components. The primary aim of this work is to study the effect of laser power and scan speed on microstructure, phase constituents, hardness and corrosion behavior of laser processed NiTi alloy. The results showed retention of large amount of high-temperature austenite phase at room temperature due to high cooling rates associated with laser processing. The high amount of austenite in these samples increased the hardness. The grain size and corrosion resistance were found to increase with laser power. The surface energy of NiTi alloy, calculated using contact angles, decreased from 61 mN/m to 56 mN/m with increase in laser energy density from 20 J/mm2 to 80 J/mm2. The decrease in surface energy shifted the corrosion potentials to nobler direction and decreased the corrosion current. Under present experimental conditions the laser power found to have strong influence on microstructure, phase constituents and corrosion resistance of NiTi alloy. © 2015 Elsevier B.V.Item Some new observations on the structural and phase evolution of nickel titanate nanofibers(Elsevier Ltd, 2017) Kumar, B.S.; Shanmugharaj, A.M.; Kalpathy, S.K.; Anandhan, S.In this study, we report for the first time the synthesis of nickel titanate (NTO) nanofibers containing a mixture of ilmenite and spinel phases of NTO, at an atypical low temperature. Precursor nanofibers produced by sol-gel electrospinning were calcined at three different temperatures to produce the NTO nanofibers. Thermal analysis along with X-ray photoelectron spectroscopy confirmed the formation of non-crystalline stable phases of TiN and Ti-O-N that restrained the formation of ilmenite NTO, and the Ni-rich environment pushed the Ti atoms to tetrahedral sites to form a defective spinel structure. The crystallite size of spinel NTO was observed to increase as a function of the calcination temperature above 700 °C, as the activation energy for coalescence and growth of spinel NTO was favorable. NTO nanofibers obtained above the calcination temperature of 700 °C exhibited new band gap energy around 2.5 eV in Tauc plot. Oxygen vacancies in these ceramic nanofibers decreased as the calcination temperature was increased. A hypsochromic shift of 20 nm in the photoluminescence spectra suggested that the material had a Ni2+ rich NTO (spinel). © 2017 Elsevier Ltd and Techna Group S.r.l.Item Synergism of fictitious forces on nickel cobaltite nanofibers: Electrospinning forces revisited(Royal Society of Chemistry, 2018) Kumar, B.; Kalpathy, S.K.; Anandhan, S.Randomly oriented nanofibers of nickel cobaltite (NCO) were fabricated using sol-gel electrospinning followed by calcination. The precursor fibers were collected on rotating disc (RDI) and rotating drum (RDR) collectors. Variable fictitious forces produce continuous deflection at each fiber landing position on the RDI collector, which subjects the nanofibers to non-bundling. On the other hand, in the case of the RDR collector, the fictitious forces act just at the surface, and these forces merely cause slip of the fibers along the rotational axis of the RDR. This slip along with the retained Columbic charges on the surface of the fibers produces fiber bundling, which affects the morphological and structural properties of the NCO nanofibers obtained by calcining the precursor fibers. The use of the RDI collector in sol-gel electrospinning is a simple and optimal method of fabricating precursor nanofibers, which yields non-agglomerated and impurity-free inorganic nanofibers. © the Owner Societies 2018.Item Effect of heat treatment on microstructure, corrosion, and shape memory characteristics of laser deposited NiTi alloy(Elsevier Ltd, 2018) Marattukalam, J.J.; Balla, V.K.; Das, M.; Bontha, S.; Kalpathy, S.K.The aim of this work is to study the effect of heat treatment on the microstructure, phase transformations, shape memory characteristics and corrosion behaviour of laser deposited equiatomic NiTi alloy. Dense samples of NiTi alloy were fabricated using Laser Engineered Net Shaping (LENS™) with two different laser energy densities by varying the scan speed and laser power. These samples were annealed for 30 min at 500 °C and 1000 °C in flowing argon, followed by furnace-cooling to room temperature. The resulting microstructures and properties were compared with the corresponding as-deposited samples. Microstructural analysis after heat treatment showed needle-shape martensite in the samples processed at lower laser energy density of 20 J/mm2, and lenticular or plate-like martensite in the samples processed at 80 J/mm2. The XRD results revealed relatively high concentration of martensite (B19?) in heat-treated NiTi alloy compared to as-processed samples. Furthermore, the heat treatment decreased the forward and reverse transformation temperatures of NiTi alloy from 80 – 95 °C to 20–40 °C, presumably due to annihilation of thermally induced defects. Interestingly, the samples annealed at 500 °C showed a measurable increase of 1–2% in the shape memory recovery, from the net recovery of 8% exhibited by the as-processed NiTi alloy. The corrosion resistance of laser-processed NiTi alloy decreased upon annealing. © 2018 Elsevier B.V.Item Pyrolysis-controlled synthesis and magnetic properties of sol–gel electrospun nickel cobaltite nanostructures(Springer New York LLC barbara.b.bertram@gsk.com, 2018) Kumar, B.S.; Dhanasekhar, C.; Adyam, A.; Kalpathy, S.K.; Anandhan, S.Nickel cobaltite (NCO) is a binary transition-metal oxide, which is extensively used as an electrocatalyst and magnetic material. NCO nanofibers and NCO/graphene composite exhibit high electrochemical reactivity due to the directional bridging of NCO particles. This makes NCO a promising candidate electrode material for use in supercapacitors and batteries. Besides, NCO is also a promising magnetic material due to its unique structural composition, where the cations are seated in octahedral sites surrounded by oxygen vacancies. In the present work, a simple and reliable method was discovered for tuning the morphological and structural changes of nickel cobaltite (NCO) nanoparticles, which were reshaped along the NCO nanofibers, by controlling the pyrolysis soaking time. As the pyrolysis soaking time increases, NCO transforms from inverse spinel to normal spinel; and the morphology of NCO nanoparticles changes from spherical to rod-like. These changes were validated by the hypsochromic peak shifts in Raman, and FTIR spectroscopies. The magnetic measurements reveal changes in the shape of the hysteresis loop, which are explained on the basis of structural and morphological changes in the nanostructure. The net magnetisation increases and coercivity decreases, with an increase in pyrolysis soaking time. These changes in magnetic parameters are attributed to structural changes caused by the formation of oxygen vacancies, and surface effects due to switching in morphology of the NCO nanoparticle. [Figure not available: see fulltext.]. © 2018, Springer Science+Business Media, LLC, part of Springer Nature.Item A Mechanistic Study on the Structure Formation of NiCo2O4 Nanofibers Decorated with In Situ Formed Graphene-Like Structures(Springer New York LLC barbara.b.bertram@gsk.com, 2018) Kumar, B.; Gudla, V.C.; Ambat, R.; Kalpathy, S.K.; Anandhan, S.Nickel cobaltite (NCO) nanofibers were synthesized using poly(styrene-co-acrylonitrile) (SAN) as the polymeric binder through sol–gel assisted electrospinning. Defect-free precursor nanofiber mats were pyrolyzed at 773 K at three different pyrolysis soaking times t = 2, 4, and 6 h. The SAN present in the precursor nanofibers caused morphological changes in the NCO nanofibers during their thermochemical degradation. Consequently, fractal aggregates of NCO nanoparticles were formed along the length of the nanofibers. X-ray photoelectron spectroscopy (XPS) revealed both + 2 and + 3 oxidation states for Ni and Co, with spinel crystal defects due to oxygen rich atmosphere. XPS, high-resolution transmission microscopy, and optical analysis showed graphene-like structures embedded within the NCO nanofibers. With increase in pyrolysis soaking time, the morphology of the NCO particles markedly changed from spherical to rod-like. We propose a mechanism for the morphological change of NCO nanoparticles on the basis of crystallite splitting accompanied by particle splitting and reordering. © 2018, Springer Science+Business Media, LLC, part of Springer Nature.Item Polymorph nickel titanate nanofibers as bifunctional electrocatalysts towards hydrogen and oxygen evolution reactions(Royal Society of Chemistry, 2019) Kumar, B.; Tarafder, K.; Shetty, A.R.; Hegde, A.C.; Gudla, V.C.; Ambat, R.; Kalpathy, S.K.; Anandhan, S.Producing pure H2 and O2 to sustain the renewable energy sources with minimal environmental damage is a key objective of photo/electrochemical water-splitting research. Metallic Ni-based electrocatalysts are expensive and eco-hazardous. This has rendered the replacement or reduction of Ni content in Ni-based electrocatalysts a decisive criterion in the development of bifunctional electrocatalytic materials. In the current study, spinel/ilmenite composite nickel titanate (NTO) nanofibers were synthesised using sol-gel assisted electrospinning followed by pyrolysis at different soaking temperatures (viz., 773, 973, and 1173 K). The presence of a defective spinel NTO phase (SNTO) distributed uniformly along the nanofibers was confirmed by X-ray photoelectron and Raman spectroscopy. The electron micrographs revealed the morphological change of NTO nanofibers from a mosaic to bamboo structure with an increase in pyrolysis soaking temperature. The electrocatalytic activity of NTO nanofibers obtained at different pyrolysis soaking temperatures for alkaline water-splitting was studied. The highly defective SNTO manifests properties similar to metallic Ni and favours H2 evolution through the hydrogen evolution reaction (HER) by adsorbing more H+ ions on active sites. In contrast, the ilmenite NTO favours O2 discharge. These results are explained based on the morphology of the NTO nanofibers. The mosaic structure which has higher porosity and greater SNTO content shows excellent HER performance. In contrast, the large bamboo structured NTO nanofibers which have lesser porosity and SNTO content cage the bigger (OH)ads ions at their catalytic sites to facilitate OER performance. 2019 © The Royal Society of Chemistry.Item Graphene nanoclusters embedded nickel cobaltite nanofibers as multifunctional electrocatalyst for glucose sensing and water-splitting applications(Elsevier Ltd, 2019) Kumar, B.S.; Gudla, V.C.; Ambat, R.; Kalpathy, S.K.; Anandhan, S.Nickel cobaltite (NCO) attains the apex of Sabatier-type volcano plot for electrochemical reaction compared to simple oxides due to synergetic effect of mixed transition metal cations. The combination of high surface area, aspect ratio, and porosity of electrospun NCO nanofibers (NCO-NF) enhance their electrocatalytic performance by improved electron mobility and more active sites. In the present study, NCO-NF fabricated using poly (styrene-co-acrylonitrile) (SAN) as a sacrificial polymer, were embellished with graphene nanoclusters (GNC), which augment the electrocatalytic performance of the NCO-NF. The in situ formed GNC along the NCO-NF are result of the interaction between the polar functional groups of the polymer, and the cations of precursor salts during the calcination of precursor nanofibers. The GNC/NCO-NF with least crystallite size and high aspect ratio having porous NCO nanoparticles and in situ grown GNC were developed using sol-gel electrospinning process assisted by calcination of precursor nanofibers. This simple, eco-friendly, and economical synthesis route with unique structure chemistry of SAN to form GNC and the presence of dual cations (Ni and Co) provides enhanced performance and multifunctionality to GNC/NCO-NF electrodes for electrocatalytic applications, such as biosensors and water-splitting. In the present study, the modified electrodes (GNC/NCO-NF/graphite electrode) exhibited excellent non-enzymatic glucose detection over a wide range of concentration with a lower limit of 1.2 ?M and sensitivity of 1827.5 ?A mM?1 mg?1 in 0.1 M NaOH. Further, the modified electrodes were also tuned for H2O2 detection to aid enzymatic glucose sensing. When examined for bifunctional water-splitting in 1 M NaOH, the electrode reached an onset potential of ?0.537 V and 0.735 V against reversible hydrogen reference electrode and a Tafel slope of 37.6 mV·dec?1 and 67.0 mV·dec?1 for hydrogen and oxygen evolution reactions, respectively. The results prove that GNC/NCO-NF are promising candidates as multifunctional electrocatalyst. © 2019 Elsevier Ltd and Techna Group S.r.l.Item Magnetic behavior of polymorph composite nickel titanate nanofibers(Royal Society of Chemistry, 2021) Kumar, B.; Dhanasekhar, C.; Anandhan, S.; Kalpathy, S.K.Polymorph (spinel/ilmenite) composite nanofibers of nickel titanate (NTO) were prepared by a sol-gel assisted electrospinning process followed by pyrolysis using the styrene-acrylonitrile copolymer as a precursor at three different pyrolysis soaking temperatures (i.e. T= 773, 973, and 1173 K). The magnetic behavior of these composite NTO nanofibers was studied under isothermal and non-isothermal conditions in the temperature range of 20-300 K. The magnetic parameters such as coercivity (Hc), remanence (Mr), and saturation magnetisation (Ms) were found to be strongly dependent onT. The highestHcandMrwere observed for NTO nanofibers developed at 973 K, which have a mosaic structured morphology with spinel and ilmenite NTO crystallite sizes of ~39 nm and ~24 nm, respectively. On the other hand, the highestMsand switching field distribution were observed for mosaic structured NTO nanofibers having smaller crystallites (~13 nm and 24 nm for spinel and ilmenite NTO, respectively, with high inter-particle distance and high porosity) developed at 773 K, which are also rich in spinel NTO content. The correlation between the variation in magnetic behavior and structural/morphological features of NTO nanofibers is useful for NTO-based soft magnetic and multiferroic applications. © The Royal Society of Chemistry and the Centre National de la Recherche Scientifique 2021.