• Sunday, August 16, 2009
  • On Going Research
  • Fragility index determination of Ge40S60 chalcogenide glass
    To determine the value of the fragility index, m, it is necessary to choose an expression for the relaxation time. For a glass, the relaxation time, τ, depends on the temperature and on some order parameters defining the structure of the glass . Among the different expressions describing these variations that we may find  in the literature for τ, we use here the Tool-Narayanaswamy-Moynihan (TNM) expression which is given by:
    where x (0≤  x ≤ 1) is the non-linearity or Narayanaswamy param
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  • Wednesday, June 20, 2007
  • Mean-coordination number dependence of the fragility in Ge-Se-In glass forming liquids
  • Published at:Physica. B, Condensed matter, 2007, vol. 389, no2, pp. 275-280
  • Differential scanning calorimetry measurements have been performed on elemental Se as well as on GexSe94ÀxIn6 (x ¼ 4, 8, and 11 at%) and on GeySe88ÀyIn12 (y ¼ 5, 7, and 9 at%) chalcogenide glasses. From the cooling rate dependence of the fictive temperature,the apparent activation energies, Dh*, and the fragility indices, m, as defined in the strong–fragile glass-forming liquid concept, are determined. It is found that, in Ge–Se–In system, there is an evolution from strong (m ¼ 67) to fragile (m ¼ 116) glass-forming liquids. The dependence of ‘m’ on the mean-coordination number, Z, is also obtained. This dependence is rationalized by assuming that, in this glassy alloy system, there is a tendency for the formation of In2Se3 clusters.
    r 2006 Elsevier B.V. All rights reserved
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  • Tuesday, May 1, 2007
  • The composition dependence of the optical band gap in Ge–Se–In thin films
  • Published at:Optical Materials, Volume 29, Issue 9, May 2007, Pages 1143-1147,http://dx.doi.org/10.1016/j.optmat.2006.05.003
  • Bulk glasses with the chemical composition Gey Se94−yIn6 (8  y  30 at%) have been prepared from high purity constituent elements. Fragments of the bulk glasses are used to deposit thin films by vacuum thermal evaporation. The optical band gaps (Eg s) of the as-deposited films have been measured. The allowed optical transition is found to be indirect. The relation of Eg to the covalent coordination number, Z, is demonstrated by varying the composition parameter y of the thin films. A maximum in the compositional dependence of Eg is attained at Z = 2.63. The cohesive energies (CE) of the investigated samples have also been calculated using the method suggested by the chemical bond approach. It is found that the composition dependence of the CE also possesses a peak at Z = 2.63.
    A plausible explanation based on the bonding considerations between the constituents has been provided for the understanding of the Eg-Z and CE-Z dependences for these Ge–Se&ndas
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Ghassan Muhammad Saffarini, Professor of experimental physics, An-Najah National University
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