Now showing 1 - 10 of 16
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    Microstructural transitions in camphene-based freeze casted ceria: effect of primary building blocks
    (01-01-2021)
    Mukkavilli, Raghunath Sharma
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    Papakollu, Kousik
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    Porous ceria for high temperature catalytic applications demands structural integrity concomitant with sinter resistance and improved gas permeability. The current state of the art hinges on complex synthesis methodologies which are not only expensive but also lack flexibility in pore tailorability. Hence, the development of porous scaffolds through low-cost processes without compromising on the functionality is in order. Herein, we demonstrate porous ceria with an open porosity of 88% developed through camphene assisted freeze casting for the first time. Microstructural evolution with different building blocks–micrometre-sized particles and short fibres were also studied. Preliminary catalytic activity obtained via temperature programmed reduction exemplified similar profiles showing no effect of the initial building blocks on the activity.
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    Machine learning application for prediction of sapphire crystals defects
    (01-01-2020)
    Klunnikova, Yulia Vladimirovna
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    Anikeev, Maxim Vladimirovich
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    Filimonov, Alexey Vladimirovich
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    We investigate the impact of different numbers of positive and negative examples on machine learning for sapphire crystals defects prediction. We obtain the models of crystal growth parameters influence on the sapphire crystal growth. For example, these models allow predicting the defects that occur due to local overcooling of crucible walls in the thermal node leading to the accelerated crystal growth. We also develop the prediction models for obtaining the crystal weight, blocks, cracks, bubbles formation, and total defect characteristics. The models were trained on all data sets and later tested for generalization on testing sets, which did not overlap the training set. During training and testing, we find the recall and precision of prediction, and analyze the correlation among the features. The results have shown that the precision of the neural network method for predicting defects formed by local overcooling of the crucible reached 0.94.
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    Low frequency dielectric behavior and AC conductivity of polymer derived SiC(O)/HfCxN1-x ceramic nanocomposites
    (15-02-2021)
    Awin, Eranezhuth Wasan
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    Sridar, Soumya
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    Kousaalya, Adhimoolam Bakthavachalam
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    Vendra, S. S.Lokesh
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    Koroleva, Ekaterina
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    Filimonov, Alexey
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    Vakhrushev, Sergey
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    The dielectric behavior of spark plasma sintered SiC(O)/HfCxN1-x nanocomposites synthesized through polymer derived ceramic route was investigated in the frequency range of 1 kHz to 1 MHz at room temperature. The nanostructural features revealed HfCxN1-x nanocrystals encapsulated in a nanometric thin layer of carbon dispersed uniformly in a SiC(O) matrix with segregated free carbon. The nanocomposites exhibited colossal permittivity values in the order of 103 at 1 kHz which reduced to 646 at 1 MHz. The interfacial polarization mechanism existing between complex nanostructural interfaces and the percolation of HfCxN1-x nanocrystals are believed to be responsible for the high permittivity values observed in the measured frequency range. The AC conductivity exemplified a frequency independent behavior at lower frequencies while at higher frequencies, the conductivity exhibited frequency dependence, indicating the existence of hopping type mechanism.
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    TiNb2O7-Keratin derived carbon nanocomposites as novel anode materials for high-capacity lithium-ion batteries
    (01-06-2021)
    Thiyagarajan, Ganesh Babu
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    Shanmugam, Vasu
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    Wilhelm, Michael
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    Mathur, Sanjay
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    Moodakare, Sahana B.
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    TiNb2O7/carbon nanocomposites synthesized through a simple, surfactant assisted precursor route is reported as a promising alternative anode material for lithium-ion batteries (LIBs). The carbon component of the nanocomposites is derived from an inexpensive and sustainable keratin rich biological source. The reinforcement of carbon in TiNb2O7 facilitated the formation of non-stoichiometric (Ti0.712Nb0.288)O2 crystalline phase, in addition to the stoichiometric TiNb2O7 phase. It also yielded a high specific surface area (~90 ​m2 ​g−1) and reduced crystallite size (~4 ​nm). Electrochemical results exemplified high reversible capacity of 356 mAh g−1 at 0.1 ​C and remarkable rate capability of ~26 mAh g−1 at ultra-high current rate of 32C. TiNb2O7/carbon nanocomposites also demonstrated remarkable cyclic stability with large capacity retention of 85% even after 50 cycles at 1 ​C. The experimental data attests the potential of TiNb2O7/keratin derived carbon nanocomposites as economically and environmentally viable promising anode material for LIBs.
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    Design and development of a miniaturized multiaxial test setup for in situ x-ray diffraction experiments
    (01-01-2021)
    Bhaskar, Lalith Kumar
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    Kumar, Gobind
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    Srinivasan, Nedunchezhian
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    In this work, a novel stand-alone multi-axial loading test setup was developed to test miniature samples under uniaxial tension, uniaxial compression, in-plane biaxial tension, and biaxial compression stress states. Good agreement in stress-strain responses was observed between the uniaxial experiments carried out using the miniature sample geometry in the custom-built setup and the uniaxial standard geometry in a universal testing machine. With regard to biaxial experiments, the full-field strain captured using digital image correlation for the biaxial specimens revealed strain homogeneity in the central gage section of the sample. Furthermore, the in situ capability of the setup was demonstrated by integrating it with a commercial laboratory x-ray diffractometer, and good agreement was found between the calculated stress values from the load sensor and the stress obtained using x-ray diffraction.
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    Critical role of cationic local stresses on the stabilization of entropy-stabilized transition metal oxides
    (01-05-2020)
    Bhaskar, Lalith K.
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    Nallathambi, Varatharaja
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    Entropy-stabilized transition metal oxides ([MgNiCoCuZn]O) (ESO) in recent years have received considerable attention owing to their unique functional properties. Solution combustion and solid state syntheses resulted in crystallites varying from 5-15 nm to 3-5 μm respectively. Phase stability studies showed that all the systems containing Cu2+ ions in the ESO lattice segregated upon slow cooling in the furnace. It was only when ESO was quenched in air from 1000°C the lattice stabilized to a single phase. Experiments concomitant with molecular dynamics (MD) simulations demonstrated that the local stress fields around the cations played a critical role in stabilizing the single phase. The local stress fields are a result of Jahn-Teller distortion induced by the Cu2+ ions in the lattice. It is clearly established that in the absence of the minimization of the local stress fields around the Cu2+ ions, segregation leading to the formation of a multi-phase material is imminent for this particular composition.
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    Disordered mesoporous polymer derived N-doped TiO2/Si-O-C-N nanocomposites with nanoscaled heterojunctions towards enhanced adsorption and harnessing of visible light
    (01-04-2020)
    Awin, Eranezhuth Wasan
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    Lale, Abhijeet
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    Bernard, Samuel
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    The mesoporous N-doped TiO2/Si-O-C-N ceramic nanocomposites has been revealed to be a potential candidate towards visible light photocatalytic degradation of organic dyes. The polymer-derived ceramic route was implemented to prepare uniformly distributed in-situ crystallized N-doped TiO2 nanocrystals in a mesoporous amorphous siliconoxycarbonitride matrix. This chemical approach assisted by the hard template pathway resulted in a high surface area (186 m2/g) nanocomposite exhibiting predominantly mesoporous structure with an average pore size of 11 nm. The two-step process involved pyrolysis of the polyhydridomethyilsiloxane impregnated in CMK3 (hard template) under argon generating SiOC-C composites and functionalizing it with titanium n-tetrabutoxide to be pyrolyzed under ammonia to form the titled nanocomposite. Interestingly, pyrolysis in a reactive ammonia atmosphere resulted in the incorporation of nitrogen in the titania lattice while decomposing the template. The Si-O-C-N support on which N-doped TiO2 exhibited superior adsorption of organic dye molecules and photocatalytically active in the visible wavelength. The nanoscaled heterojunctions reduced the recombination rate and the presence of superoxide anions/hydroxyl radicals was found to be responsible for the dye degradation.
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    Ultra-low thermal expansion behavior of spark plasma sintered Sn-doped ZrMo2O8
    (01-07-2020)
    Ramamurthy, Jayanth R.
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    Sn-doped ZrMo2O8 exhibits a coefficient of thermal expansion that is tunable based on the amount of Sn present in the Zr lattice sites. The current study focuses on the facile sintering of this material using spark plasma sintering (SPS) process. Powder particles of the material were produced by co-precipitation, and the formation of the desired cubic phase was confirmed using X-ray diffraction. The powder particles were sintered using SPS at different temperatures to identify the optimum sintering conditions. The extremely high rate of heating during SPS process enables complete sintering while retarding the decomposition reactions. It was noticed that attaining dense phase pure samples is strongly dependent on the sintering conditions. The results from the dilatometry carried out on the sintered pellets resulted in ultra-low thermal expansion coefficients in the order of 10−7/°C for the samples sintered at 500°C. Comparisons with other commercially available low coefficient of thermal expansion materials highlight the advantage of the current work.
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    The role of brass texture on the deformation response of 7075-T651 aluminum alloy under equi-biaxial tension
    (22-04-2021)
    Srinivasan, Nedunchezhian
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    Velmurugan, R.
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    Bhaskar, Lalith Kumar
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    Singh, Satish Kumar
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    Pant, Bhanu
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    Cruciform specimen is utilized to study the influence of equi-biaxial stress state on the plastic strain response of AA7075-T651 alloy until fracture. The failure ductility as a function of stress state is evaluated using non-contact digital image correlation technique. The distinct biaxial deformation behavior in contrast to uniaxial response is attributed to strong crystallographic texture developed during prior hot-rolling reduction. The weakening of initial brass texture upon equi-biaxial deformation is also evident from the orientation distribution analysis. Increased activity of grain boundary assisted cracking due to increased stress triaxiality due to biaxial loading is evident, when compared to uniaxial stress state.
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    Temperature dependent conductivity and broadband dielectric response of precursor-derived Nb2O5
    (01-05-2020)
    Nanda, Gokul
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    Awin, Eranezhuth Wasan
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    Gasyak, Tatiana
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    Koroleva, Ekaterina
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    Filimonov, Alexey
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    Vakhrushev, Sergey
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    Sujith, Ravindran
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    Different polymorphs of niobium pentoxide (Nb2O5) were synthesized using niobium ethoxide as a precursor by varying the pyrolyzing temperature. The room temperature X-ray diffractograms revealed the irreversible phase evolution from amorphous to pseudohexagonal (823 K) to orthorhombic (1023 K) and to monoclinic crystal structure (1223 K). While phase evolution was also confirmed by thermogravimetry and dilatometry, Raman spectroscopy clearly suggested complete elimination of free carbon in the pyrolysed ceramics. The sintering conditions were optimized to produce a highly dense (>95%) thermodynamically stable monoclinic Nb2O5. The electrical properties of stable monoclinic Nb2O5 sample were thoroughly studied. The monoclinic Nb2O5 was found to have a dielectric constant of around 28 with a dielectric loss of 0.008 at room temperature and at 100 kHz. At low measurement frequencies, an anomalous increase in the effective dielectric permittivity with increasing temperature was observed. Large values of the ε’ are associated with polarization due to the accumulation of free electrons at the grain boundaries. An analysis of the dispersion curves of Nb2O5 revealed that two relaxation processes are responsible for the observed anomalies, and the temperature dependencies of their parameters (dielectric strength, relaxation time and spectrum broadening parameter) were determined. The low-frequency process (relaxation time τ0 ~ 0.45 s), which makes the largest contribution to the dielectric constant, was apparently due to the inhomogeneous conductivity of ceramics. It was revealed that the DC conductivity of ceramics has thermoactivation character with activation energy of about 660 meV and was determined by the oxygen vacancies.