Faculty Publications

Permanent URI for this communityhttps://idr.nitk.ac.in/handle/123456789/18736

Publications by NITK Faculty

Browse

Author: search.filters.author.Ajith, K.M.Subject: search.filters.subject.Defects

Search Results

Now showing 1 - 3 of 3
  • No Thumbnail Available
    Item
    Temperature dependent structural properties and bending rigidity of pristine and defective hexagonal boron nitride
    (Institute of Physics Publishing custserv@iop.org, 2015) Thomas, S.; Ajith, K.M.; Chandra, S.; Valsakumar, M.C.
    Structural and thermodynamical properties of monolayer pristine and defective boron nitride sheets (h-BN) have been investigated in a wide temperature range by carrying out atomistic simulations using a tuned Tersoff-type inter-atomic empirical potential. The temperature dependence of lattice parameter, radial distribution function, specific heat at constant volume, linear thermal expansion coefficient and the height correlation function of the thermally excited ripples on pristine as well as defective h-BN sheet have been investigated. Specific heat shows considerable increase beyond the Dulong-Petit limit at high temperatures, which is interpreted as a signature of strong anharmonicity present in h-BN. Analysis of the height fluctuations, ?h2?, shows that the bending rigidity and variance of height fluctuations are strongly temperature dependent and this is explained using the continuum theory of membranes. A detailed study of the height-height correlation function shows deviation from the prediction of harmonic theory of membranes as a consequence of the strong anharmonicity in h-BN. It is also seen that the variance of the height fluctuations increases with defect concentration. © 2015 IOP Publishing Ltd.
  • No Thumbnail Available
    Item
    Empirical potential influence and effect of temperature on the mechanical properties of pristine and defective hexagonal boron nitride
    (Institute of Physics Publishing helen.craven@iop.org, 2017) Thomas, S.; Ajith, K.M.; Valsakumar, M.C.
    The major objective of this work is to present results of a classical molecular dynamics study to investigate the effect of changing the cut-off distance in the empirical potential on the stress-strain relation and also the temperature dependent Young's modulus of pristine and defective hexagonal boron nitride. As the temperature increases, the computed Young's modulus shows a significant decrease along both the armchair and zigzag directions. The computed Young's modulus shows a trend in keeping with the structural anisotropy of h-BN. The variation of Young's modulus with system size is elucidated. The observed mechanical strength of h-BN is significantly affected by the vacancy and Stone-Wales type defects. The computed room temperature Young's modulus of pristine h-BN is 755 GPa and 769 GPa respectively along the armchair and zigzag directions. The decrease of Young's modulus with increase in temperature has been analyzed and the results show that the system with zigzag edge shows a higher value of Young's modulus in comparison to that with armchair edge. As the temperature increases, the computed stiffness decreases and the system with zigzag edge possesses a higher value of stiffness as compared to the armchair counterpart and this behaviour is consistent with the variation of Young's modulus. The defect analysis shows that presence of vacancy type defects leads to a higher Young's modulus, in the studied range with different percentage of defect concentration, in comparison with Stone-Wales defect. The variations in the peak position of the computed radial distribution function reveals the changes in the structural features of systems with zigzag and armchair edges in the presence of applied stress. © 2017 IOP Publishing Ltd.
  • No Thumbnail Available
    Item
    Optical and mechanical studies on free standing amorphous anodic porous alumina formed in oxalic and sulphuric acid
    (Springer Verlag service@springer.de, 2018) Ramana Reddy, P.R.; Ajith, K.M.; Udayashankar, N.K.
    Anodic porous alumina (APA) membranes with a uniform pore arrangement typically serve as an ideal template for formation of different types of nanostructured materials. In the present work, APA membranes were synthesized using two-step anodization in 0.3 M of oxalic and 0.3 M of sulphuric acid under the anodization potential of 40 V and 20 V respectively, at 8 °C. Alumina nanowires (ANW) were synthesized by the chemical etching of the APA membranes using phosphoric acid solution. The optical absorbance and reflectance measurements of APA membranes were performed on a spectrometer in the wavelength range of 200–600 nm. The band-gap energy (3.7 and 4.3 eV for oxalic and sulphuric acid) of APA membranes was determined from UV–visible absorption data. The photoluminescence (PL) investigations have revealed the presence of F and F+ defect centers, which could be attributes to oxygen vacancy-related defect centers in oxalic (483 and 466 nm) and sulphuric (423 and 421 nm) alumina. The mechanical properties of amorphous APA membranes were investigated by micro- and nanoindentation techniques. The results indicate that highest hardness (7.70 GPA) and Young’s modulus (138.80 GPA) for sulphuric alumina compared with oxalic alumina. In sulphuric alumina, more number of ANW were observed compared with oxalic alumina. © 2018, Springer-Verlag GmbH Germany, part of Springer Nature.