C. Martinet

Soda-lime silicate glass under hydrostatic pressure and indentation: a micro-Raman study

Raman micro-spectroscopy is used to analyse the plastic behaviour of window glass (a soda-lime silicate glass) under high hydrostatic pressure and Vickers indentation. We show pressure-induced irreversible structural changes, notably an increase of Q(2) species at the expense of Q(3). For the first time, a very accurate [Formula: see text] calibration curve has been established. Local density variations of a Vickers indented window glass have been characterized by micro-Raman mapping using a high spatial resolution device. The effects of glass depolymerization on indentation and hydrostatic compression are discussed. Differences between window glass and pure SiO(2) glass behaviour under high stresses are also highlighted and analysed at a local scale.

Density hardening plasticity and mechanical ageing of silica glass under pressure: a Raman spectroscopic study

In addition to a flow, plastic deformation of structural glasses (in particular amorphous silica) is characterized by a permanent densification. Raman spectroscopic estimators are shown to give a full account of the plastic behavior of silica under pressure. While the permanent densification of silica has been widely discussed in terms of amorphous–amorphous transition, from a plasticity point of view, the evolution of the residual densification with the maximum pressure of a pressure cycle can be discussed as a density hardening phenomenon. In the framework of such a mechanical ageing effect, we propose that the glass structure could be labeled with the maximum pressure experienced by the glass and that the saturation of densification could be associated with the densest packing of tetrahedra only linked by their vertices.

Permanent densification of compressed silica glass: a Raman-density calibration curve

Raman scattering experiments have been carried out to study persistent densification in SiO(2) glass following hydrostatic compression at room temperature. A new relationship linking selective Raman parameters to the degree of densification in the glass has been developed here. This approach will allow quantification of the residual densification in silica following microindentation experiments, with the goal being the development of a constitutive law for amorphous silica.

Progressive transformations of silica glass upon densification.

The elastic and plastic behaviors of silica glasses densified at various maximum pressure reached (12 GPa, 15 GPa, 19 GPa, and 22 GPa), were analyzed using in situ Raman and Brillouin spectroscopies. The elastic anomaly was observed to progressively vanish up to a maximum pressure reached of 12 GPa, beyond which it is completely suppressed. Above the elastic anomaly the mechanical behavior of silica glass, as derived from Brillouin measurements, is interpreted in terms of pressure induced transformation of low density amorphous silica into high density amorphous silica.

Structural studies of germanium doped silica glasses: the role of the fictive temperature

Abstract We studied the influence of germanium atom introduction into the silica network and the influence of heat treatements on the glass structure by doing Raman and infra-red spectra of vitreous germanium doped silica (5% wt GeO2, 95% wt SiO2). Germanium incorporates into the network by substituting Si atoms and consequently, we observed band shifts and broadening of defect peaks D1 and D2. The peak positions are directly correlated with the structural state characterized by the glass fictive temperature TF of the different samples.

Radial distribution of the fictive temperature in pure silica optical fibers by micro-Raman spectroscopy

Micro-Raman spectra are performed at different radial positions in order to determine the fictive temperature distribution in pure silica optical fibers. As-received and annealed optical fibers are compared and Raman results reveal a large decrease in the fictive temperature after annealing. The fictive temperature gradient between the core and the edge of the optical fiber can be eliminated after annealing and the spectroscopic decoupling between stress and the fictive temperature variation is discussed. The micro-Raman is a good probe in determining the spatial distribution of the fictive temperature in the micrometer length scale.

Correlation between boson peak and anomalous elastic behavior in GeO2 glass: An in situ Raman scattering study under high-pressure

We present low-frequency Raman scattering of pure GeO(2) glass under pressure up to 4 GPa, corresponding to an elastic transformation. Intensity variation and frequency shift of the boson peak are analysed and compared to the Debye model. The decrease of the boson peak intensity scaled by the Debye energy is correlated to the elastic anomalous properties under pressure up to 1.5 GPa, and interpreted as an elastic homogenisation process at the local scale. We emphasize similarities between a-GeO(2) and a-SiO(2) behavior under pressure, and compare our results to other experiments, numerical studies, and predictions of several models concerning amorphous systems.

In situ Brillouin study of sodium alumino silicate glasses under pressure.

The in situ elastic and plastic behaviors of sodium aluminosilicate glasses with different degrees of depolymerization were analyzed using Brillouin spectroscopy. The observed elastic anomaly progressively vanished with depolymerization. The densification process appears to be different from that observed in pure silica glass. In the plastic regime of densified glasses hysteresis loops were observed and related to modification of the local silicon environment facilitated by the addition of sodium.

Permanently densified SiO2 glasses: a structural approach.

Densified silica can be obtained by different pressure and temperature paths and for different stress conditions, hydrostatic or including shear. The density is usually the macroscopic parameter used to characterize the different compressed silica samples. The aim of our present study is to compare structural modifications for silica glass, densified from several routes. For this, densified silica glasses are prepared from cold and high temperature (up to 1020 °C) compressions. The different densified glasses obtained in our study are characterized by micro-Raman spectroscopy. Intertetrahedral angles from the main band relative to the bending mode decrease and their values are larger for densified samples from high temperature compression than those samples from cold compression. The relative amount of 3-membered rings deduced from the D2 line area increases as a function of density for cold compression. The temperature increase during the compression process induces a decrease of the 3 fold ring population. Moreover, 3 fold rings are more deformed and stressed for densified samples at room temperature at the expense of those densified at high temperature. Temperature plays a main role in the reorganization structure during the densification and leads to obtaining a more relaxed structure with lower stresses than glasses densified from cold compression. The role of hydrostatic or non-hydrostatic applied stresses on the glass structure is discussed. From the Sen and Thorpe central force model, intertetrahedral angle average value and their distribution are estimated.

A-thermal elastic behavior of silicate glasses.

Depending on the composition of silicate glasses, their elastic moduli can increase or decrease as function of the temperature. Studying the Brillouin frequency shift of these glasses versus temperature allows the a-thermal composition corresponding to an intermediate glass to be determined. In an intermediate glass, the elastic moduli are independent of the temperature over a large temperature range. For sodium alumino-silicate glasses, the a-thermal composition is close to the albite glass (NaAlSi3O8). The structural origin of this property is studied by in situ high temperature Raman scattering. The structure of the intermediate albite glass and of silica are compared at different temperatures between room temperature and 600 °C. When the temperature increases, it is shown that the high frequency shift of the main band at 440 cm(-1) in silica is a consequence of the cristobalite-like alpha-beta transformation of 6-membered rings. This effect is stronger in silica than bond elongation (anharmonic effects). As a consequence, the elastic moduli of silica increase as the temperature increases. In the albite glass, the substitution of 25% of Si(4+) ions by Al(3+) and Na(+) ions decreases the proportion of SiO2 6-membered rings responsible for the silica anomaly. The effects of the silica anomaly balance the anharmonicity in albite glass and give rise to an intermediate a-thermal glass. Different networks, formers or modifiers, can be added to produce different a-thermal glasses with useful mechanical or chemical properties.