Structural relaxations of glass-forming alkali-borate and alkali-tellurite in the GHz frequency range
Masnik, John Edward
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https://hdl.handle.net/2142/22917
Description
Title
Structural relaxations of glass-forming alkali-borate and alkali-tellurite in the GHz frequency range
Author(s)
Masnik, John Edward
Issue Date
1996
Doctoral Committee Chair(s)
Kieffer, John
Department of Study
Materials Science and Engineering
Discipline
Materials Science and Engineering
Degree Granting Institution
University of Illinois at Urbana-Champaign
Degree Name
Ph.D.
Degree Level
Dissertation
Keyword(s)
Engineering, Materials Science
Language
eng
Abstract
The Brillouin light scattering technique was used to determine the high-frequency complex mechanical modulus of a series of strong glass-forming alkali-borate and fragile glass-forming alkali-tellurite systems. Lithium, sodium, and potassium, were added to the systems to determine their influence on the structure and degree of networking in these melts.
The storage modulus (real part of the mechanical modulus) measures the structural integrity of the liquid and increases with the degree of networking in the structure. The glass transition is marked by a discontinuity in the temperature dependence of the moduli, which results in a change of slope. The rate of decrease of the modulus above T$\sb{\rm g}$ is commensurate with the fragility of the liquid.
The loss modulus (imaginary part of the mechanical modulus), on the other hand, reflects the dissipation of energy due to aperiodic motion of the structural constituents. By obtaining the loss modulus as a function of temperature, the Brillouin technique allows one to uncover the individual mechanisms which underly viscous dissipation. Strong liquids were found to be characterized by the presence of several different relaxation mechanisms, whereas fragile liquids are described by only a single one. This suggests that fragile liquids are structurally more uniform (or less complex) than strong ones.
In the high-temperature limit, viscosity data from Brillouin measurements and rotating cylinder viscometry were found to agree with each other. Because no mechanical energy is imparted to change the structure, however, Brillouin scattering is the preferable technique for studying structural changes upon glass formation.
This data can be described with a modified Maxwell model, which assumes a temperature dependence of static and relaxational moduli, based on the relative volume fractions of the structure found in two different thermodynamic states: one rigid, the other one visco-elastic. By fitting the mechanical modulus with this model, we obtained the relaxation time constants and activation energies characteristic of the structural mechanisms. The values obtained by experiments varying temperature are not, however, the same as those obtained by a frequency probe since thermo-rheological simplicity does not apply.
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