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Fragility, melt/glass homogenization, self-organization in chalcogenide alloy systems

Gunasekera, Kapila
http://rave.ohiolink.edu/etdc/view?acc_num=ucin1382372615

Abstract Details

2013, PhD, University of Cincinnati, Engineering and Applied Science: Electrical Engineering.
We report on fragility and observation of the three elastic phases in the following chalcogenides: GexSe100-x, GexSbxSe100-2x and GexSixTe100-2x. In each case, experimental evidence shows a strong correlation between the three elastic phases and the variation of melt fragility index, m(x. In general, m(x) is high (> 25) in the flexible and stressed-rigid phase but decrease remarkably in the Intermediate Phase (IP) to show a global minimum (< 20). These observations correlate the melt structure (view above Tg ) to the glass structure (view below Tg) and suggest that the strong (low fragility) character of melts in the IP compositions is due to presence of adaptability and extended range structural correlations in the rigid but unstressed networks formed in the melt, features that they share with the (self-organized) networks formed in corresponding glasses at T < Tg. Comprehensive Raman scattering, calorimetric glass transitions, non-reversing enthalpy of relaxation at Tg, complex specific heat (Cp*), and volumetric measurements on the GexSbxSe100-2x ternary were undertaken in the 0 < x < 22% range. These data provide evidence of a rather well defined reversibility window, volumetric window and fragility window in the IP compositions, 14.9% < x < 17.5% range. A curious local minimum of fragility, m(x), was observed in the flexible phase, and may represent presence of quasi-tetrahedral 4-fold coordinated Sb structural motifs in melts but not in corresponding glasses. Elastic phases of GexSixTe100-2x ternary glasses are established by measurements of non-reversing enthalpy of relaxation at Tg, and volumetric measurements. Fragility of corresponding melts are established from Cp* measurements. The flexible phase extends in the 6% < x < 7.5% range, intermediate phase in the 7.5%< x<9% range , and stressed-rigid phase in the 9% < x < 12% range. Glasses at x > 12% are found to be chemically phase separated. A global minimum in molar volumes of glasses-the volumetric window coincides with the reversibility window, and confirms the space filing nature of networks formed in the IP. Non-aging in a Telluride chalcogenide system is observed for the first time in the present study. Retention loss of the amorphous phase, which can be attributed to physical aging in glasses, is a key reliability issue in phase change memory devices which could be resolved by resorting to glassy compositions inside intermediate phases where physical again is found to be minimal. Most significantly, the physics underlying slow homogenization of chalcogenide alloy melts/glasses is addressed in each of the three systems. Global minimum of fragility for IP melts is responsible for inhibiting melt mixing at high temperatures. For that reason, special care was taken to synthesize homogeneous melts/glasses in each case by reacting starting materials at high temperatures and FT-Raman profiling melts to ascertain their homogeneity. Physical properties of chalcogenide melts/glasses are found to vary systematically as their heterogeneity is steadily lowered by prolonged melt reaction. These results are key to establishing the intrinsic physical behavior of chalcogenides glasses in compositional studies both at a basic level and for real world applications.
Punit Boolchand, Ph.D. (Committee Chair)
Wayne J. Bresser, Ph.D. (Committee Member)
Matthieu Micoulaut, Ph.D. (Committee Member)
Chong Ahn, Ph.D. (Committee Member)
Marc Cahay, Ph.D. (Committee Member)
Peter Kosel, Ph.D. (Committee Member)
97 p.
Materials Science
Raman Scattering; Rigidity Theory; Elastic Phases; Intermediate Phase; Slow Homogenization; Chalcogenides

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Citations

  • Gunasekera, K. (2013). Fragility, melt/glass homogenization, self-organization in chalcogenide alloy systems [Doctoral dissertation, University of Cincinnati]. OhioLINK Electronic Theses and Dissertations Center. http://rave.ohiolink.edu/etdc/view?acc_num=ucin1382372615

    APA Style (7th edition)

  • Gunasekera, Kapila. Fragility, melt/glass homogenization, self-organization in chalcogenide alloy systems. 2013. University of Cincinnati, Doctoral dissertation. OhioLINK Electronic Theses and Dissertations Center, http://rave.ohiolink.edu/etdc/view?acc_num=ucin1382372615.

    MLA Style (8th edition)

  • Gunasekera, Kapila. "Fragility, melt/glass homogenization, self-organization in chalcogenide alloy systems." Doctoral dissertation, University of Cincinnati, 2013. http://rave.ohiolink.edu/etdc/view?acc_num=ucin1382372615

    Chicago Manual of Style (17th edition)

ucin1382372615
437
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This open access ETD is published by University of Cincinnati and OhioLINK.