Visco-Elastic Relaxations in Alkali Borates and Alkali Silicates
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ABSTRACT The Brillouin light scattering technique was used to study short time relaxations in glass forming oxide melts. The line shape analysis of Brillouin spectra yields a complex mechanical modulus. The temperature dependence of this modulus provides insight into the structural disintegration above the glass transition, as well as the thermally activated mechanisms which facilitate the momentum transport in the process of viscous dissipation. A series of binary and ternary alkali borates and alkali silicates have been investigated. The results show fundamental differences in the way alkali cations affect the network structure in borates as compared to silicates. While these cations move relatively freely within the rigid silicate network, their motions are strongly coupled to the network relaxation in borates. The activation energies for cation motion are in good agreement with those found by other methods.
INTRODUCTION The successful design of glassy materials that exhibit high ionic conductivities, for application as electrolytes, depends on a thorough understanding of the relationship between a structure and its dynamic behavior.1 ,2 High ionic conductivity is typically achieved when combining small cations with network forming elements that assemble into a backbone structure with large openings. 3 On the other hand, larger network openings reduce the rigidity of this backbone. In a mechanically soft network the coupling between the motion of the mobile cations and that of the network elements is more pronounced, which causes an increase in the collision rate and the dissipative momentum transfer. The correlations between structural relaxations and the motion of highly mobile cations are still poorly understood. Dielectric relaxation spectroscopy is by far the most common technique for the investigation of fast ion conductors. 4 Mechanical actuation can be realized up to MHz frequencies by means of ultrasonic transducers. At GHz frequencies, inelastic light scattering provides a unique way to probe the propagation and dissipation of acoustic phonons. Being close to the first Brillouin zone boundary (or equivalent construct for amorphous substances), this frequency range is crucial for diffusive jumps of charged species, due to the proximity between acoustic and optical branches in the dispersion relation. In this paper we report our results on the determination of the complex mechanical modulus in alkali silicates and borates, using Brillouin scattering.
THEORETICAL BACKGROUND Light scatters as a result of the local change in the dielectric constant due to density fluctuations, and therefore acts as a gauge capable of evidencing the propagation and attenuation of acoustic phonons. Elastically scattered light produces a central Rayleigh line. Sustainable density fluctuations, propagating with the velocity of sound, vo, give rise to Brillouin scattering 203 Mat. Res. Soc. Symp. Proc. Vol. 321. @1994 Materials Research Society
at frequencies that are shifted with respect to that of the incident light by t2-
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