Relaxation Phenomena In Glasses Research Articles

Effect of composition and temperature on the second harmonic generation in silver phosphate glasses

We herein employ nonlinear laser imaging microscopy to explicitly study the dynamics of second harmonic generation (SHG) in silver iodide phosphate glasses. While glasses of this family have gained extensive scientific attention over the years due to their superior conducting properties, considerably less attention has been paid to their unique nonlinear optical characteristics. In the present study, firstly, it is demonstrated that SHG signal intensity is enhanced upon increasing silver content due to the random formation of silver microstructures within the glass network. Secondly, the SHG temperature dynamics were explored near the glass transition temperature (Tg) regime, where significant glass relaxation phenomena occur. It is found that heating towards the Tg improves the SHG efficiency, whereas above Tg, the capacity of glasses to generate second harmonic radiation is drastically suppressed. The novel findings of this work are considered important in terms of the potential employment of these glasses for the realization of advanced photonic applications like optical-switches and wavelength conversion devices.

Putative role of attractive and repulsive forces in the glass transition

At a given temperature a balance between repulsive and attractive molecular forces determines liquid density. As temperature is lowered, attractive forces increase, but eventually saturate and asymptote to a near fixed value. At saturation, the attractive/repulsive force balance stabilizes the liquid density, which thereafter becomes effectively temperature independent. Configurational entropy also saturates, but at a much lower temperature. Once entropy begins to saturate, it converges to zero at absolute zero. There is no second order phase transition nor is there a divergent temperature above absolute zero predicted for glass relaxation phenomena. Using a phenomenological argument, it is shown that the relaxation time for volume relaxation varies inversely with configurational entropy. Stoichiometric electron density is proposed as a metric for repulsive force strength, which was determined at Tg and averaged 0.61±0.03 mol/cc for 15 polymers that contain oxygen and 0.53±0.02 mol/cc for 7 hydrocarbon polymers. Qualitatively, similar polymer liquids that pack to higher electron densities at a given temperature are expected to experience a glass transition earlier as temperature is lowered. For certain polymer types, the glass transition appears to be an isoelectronic state.

Mechanical Relaxation of Metallic Glasses: An Overview of Experimental Data and Theoretical Models

Relaxation phenomena in glasses are a subject of utmost interest, as they are deeply connected with their structure and dynamics. From a theoretical point of view, mechanical relaxation allows one to get insight into the different atomic-scale processes taking place in the glassy state. Focusing on their possible applications, relaxation behavior influences the mechanical properties of metallic glasses. This paper reviews the present knowledge on mechanical relaxation of metallic glasses. The features of primary and secondary relaxations are reviewed. Experimental data in the time and frequency domain is presented, as well as the different models used to describe the measured relaxation spectra. Extended attention is paid to dynamic mechanical analysis, as it is the most important technique allowing one to access the mechanical relaxation behavior. Finally, the relevance of the relaxation behavior in the mechanical properties of metallic glasses is discussed.

Relaxation phenomena in glass technology

Investigations of relaxation phenomena in glasses are reviewed. The role of such phenomena in the formation and heat-treatment as well as in other technological processes is shown. The characteristics of the surface layer are indicated from the standpoint of the physics of mesomechanics and the general structural features of inorganic glasses. A number of operational reliability factors for glass articles are examined.

Measurement and Calculation of Residual Stress in Axi-Symmetric Glass Sample Using Structural Relaxation Theory

Consistent model including structural relaxation is necessary for correct glass stress calculation in numerical computations of glass forming processes. Calculation of glass relaxation phenomena is often done by combining independent empirical formula on stress relaxation (for a simple temperature regime) and independent model of time-temperature dependence on Tool fictive temperature, Tf. Another approach was developed and verified here. Tool-Narayanaswamy and Moynihan/Mazurin relaxation model was adopted. Relaxation was obtained not from empirical model, but from calculated time-temperature dependence of Tool fictive temperature (Tf), dynamic viscosity, heat capacity and coefficient of thermal expansion. Heat conduction in a glass probe of known temperature history, structural relaxation and stress calculation were used in one computation scheme using Matlab. The stress of glass probe was measured using polarized light (Senarmont method). Rather high stress computation accuracy was obtained in comparison to stress experimental results. The applied model approach is being to be extended for application in commercial finite element codes for modeling of glass forming processes.

Theory of relaxation phenomena in glasses and doped semiconductors at low temperatures

Low-temperature relaxation properties of dielectric glass and doped disordered semiconductors, also called Fermi glasses, are studied. The isomorphism in the description of both types of material is established. In particular, it is proved that the flip-flop relaxation mechanism, which is a characteristic of dielectric glasses, is also specific for Fermi glasses. The low-temperatures behavior for the thermal conductivity, electrical conductivity and nuclear relaxation in the glassy systems considered is investigated. The relation to experiment is analyzed, which allows us to clarify the key role of the flip-flop processes in the formation of the universal properties of disordered dielectrics. This assertion is still a point of much controversy.

Transitions between metastable states in silica clusters

Relaxation phenomena in glasses can be related to jump processes between different minima of the potential energy in the configuration space. These transitions play a key role in the low temperature regime, giving rise to tunneling systems responsible for the anomalous specific heat and thermal conductivity in disordered solids with respect to crystals. By using a recently developed numerical algorithm, we study the potential energy landscape of silica clusters, taking as a starting point the location of first order saddle points. This allows us to find a great number of adjacent minima. We analyze the degree of cooperativity of these transitions and the connection of physical properties with the topology of the configuration space. We also identify two-level systems (pairs of minima constituting a tunneling system) and calculate the quantum mechanical ground state splitting by means of the WKB approximation.

A theory for a certain crossover in relaxation phenomena in glasses

It is shown that the simplified mode coupling theory for supercooled liquid dynamics allows for a scenario where a first component of the system is within an ideal glass state while the second component exhibits a crossover from an ideal glass state to a state of almost liquid behaviour. The dynamics near the crossover is governed by two relevant control parameters and it can be described by a scaling law. There appear fractal spectra. Under certain simplifying assumptions or symmetry conditions the crossover becomes an ergodic to non-ergodic transition, where the Edwards-Anderson parameter of the second component changes continuously from zero to a non-zero value.

Direct dipole induced energy transfer: A microscopic model for glass relaxation

A microscopic model for glass relaxation phenomena is presented based on direct energy transfer from ‘‘tight’’ donor sites to ‘‘loose’’ acceptor sites. The rate of energy transfer, induced by the stress energy field from the donor W, is proportional to 1/R6. Summing over donor sites with different acceptor environments, gives the Williams–Watt form for relaxation vs time with β=0.5. More generally, β is the dimensionality of the structure divided by the order of the donor–acceptor energy transfer mechanism and thermorheological simplicity follows from one relaxation mechanism. Other properties of glass relaxation phenomena including the temperature and fictive temperature dependence of relaxation times are readily understood.

Relaxation phenomena in glass

A summary is given of the current state of the knowledge of dielectric and mechanical relaxation phenomena in glass. Four areas are indicated where further investigations are desirable.

RELAXATION PHENOMENA IN GLASS

An Abstract is given of the current state of the knowledge of dielectric and mechanical relaxation phenomena in glass.

Relaxation phenomena in glass

Relaxation phenomena in glass