Interpretation of silica-disk-tests including the effect of limited mass transfer at the surface

The concentration of water penetrated into silica depends strongly on the stress state in the surface region. In the water diffusion zone hydroxyl water is generated by the water/silica reaction that is at temperatures <450°C a first-order reaction. When [2S]=[2 SiOH] is the concentration of the immovable hydroxyl, the reaction equation reads ≡Si-O-Si≡ +H$_2$O ↔ [2 SiOH] When evaluating the deformation measurements on water-infiltrated silica disks, the question remained unanswered as to why the tests can lead to increased diffusion constants or whether these are accidental (possibly dependent on the special glass composition). The result of this Report is: • The activation volume for stress affected hydroxyl S is ΔV≅58 cm$^3$/mol, • the mass transfer coefficient of water into silica is h/√D = 0.266/√h, • due to the mass transfer coefficient, the effective diffusivity isincreased by a factor of roughly 1.6

Mass-transfer coefficient for water at silica surfaces: Effect of surface dissolution?

In earlier evaluations of literature results, it was found that the mass transfer when storing silica in liquid water was significantly greater than in tests in a steam atmosphere under saturation vapor pressure. In the literature, different glass qualities were compared and it is conceivable that small material differences may lead to the observed results. On the other hand, it can be shown qualitatively that there is also the possibility of mass removal through dissolution, which is different for liquid water and saturated steam. This is the subject to be addressed in this report

Mass transfer coefficient for silica in liquid water at 200°C

In previous reports on the mass transfer coefficient of quartz glass in water, we were able to determine that this is significantly larger in liquid water than in saturated water vapor as the surrounding medium. From literature results by Zouine et al. (2007) a lower limit value of the time parameter h/√D in eq.(4) of >10$^{-6}$ h$^{-1/2}$, could be deduced for 200°C and liquid water. One explanation for this result could be, among other things, an increased roughness due to the dissolution of the surface layers of the glass in water than is to be expected for water vapour

Interpretation of irregular water profiles at silica surfaces

In the present Report, we will discuss the irregular water distributions at silica surfaces as were reported by the measured data from Zouine et al. [1], especially those at room temperature. The question was whether the deviations from the usually found erfc-shaped water pro- files can be understood by competing diffusion and surface dissolution and whether back-diffusion after water or air storage, may affect the type of water profile. We will come to the conclusion that the irregular distributions of the measured data by Zouine et al. [1], especially those at room tem- perature, can be understood by back-diffusion after water or air storage, respectively

Distribution of equilibrium constant k and hydroxyl S in silica surface layers

The concentration of water penetrated into silica depends strongly on the stress state in the surface region. In the water diffusion zone hydroxyl water is generated by the water/silica reaction that is at temperatures <450°C a first-order re action. When [2S ]=[2 SiOH] is the concentration of the im movable hydroxyl, the reaction equation reads ≡Si-O-Si≡ +H$_2$O ↔ [2 SiOH] For this reaction the equilibrium constant is strongly affected by stresses, internal swelling stresses and externally applied stresses. Since no activation volumes for this first- order reaction are available in literature, we will show here the principal influence by a parameter study. As the main results we concluded: 1) With increasing reaction volume ∆V, the concentration of the hydroxyl S(z) at the surface is significantly reduced and slightly increased inside. 2) The shape of the S-distribution deviates strongly from the profile of the total water concentration, C$_w$(z). 3) The product of the "effective layer thickness" and the level of the S- concentration, S(0)×z$_{eff}$, is largely independent of the assumed reaction volume

Evaluation of water profiles at silica surfaces at 23°C, 100°C and 200°C by Zouine et al

The reaction equation for the reaction between water and silica reads ≡Si-O-Si≡ +H$_2$O ↔ [2 SiOH] For this reaction the equilibrium constant, k, is affected by internal swelling stresses. We investigated the behaviour of erfc-shaped distributions of the total water amount in earlier Reports. The present report is intended to evaluate more complicated water profiles. Water concentrations at silica surfaces were measured by Zouine et al. Some of their data showed deviations from the expected erfc-shape. The associated datasets were used to demonstrate the procedure of determining the local equilibrium constant and the distributions of the hydroxyl and molecular water species on irregular water distributions. In an Appendix we discuss what could be the reason for the deviations in the distribution of the total water C$_w$ from the erfc-shape. The occurrence of back-diffusion after aging is assumed

A reason for the occurrence of mass-transfer coefficients at silica surfaces

At silica surface exposed to a water vapour environment, there is a monotonously increasing water concentration observed that calls for a limited mass transfer from the humid environment to the surface. Such behaviour is characterized by a “mass transfer coefficient". We have demonstrated in several studies that silica glass surfaces may present a barrier to the diffusion of water into silica. We could interpret the experimental diffusion results of Oehler and Tomozawa (2004) and those of Helmich and Rauch (1993). Such a surface layer formed by diffusion and water reaction with the surface material, was studied by Mahadevan and Garofalini (2008). Compressive hydrostatic stresses in the surface layers reach a maximum value of σ$_{sw,h}$ = − 2.8GPa. Due to such high compressive stresses, the water diffusivity must be strongly reduced within this thin layer and, consequently, also the mass transfer coefficient. These effects will be considered in detail in this report

Electronic structure of fully epitaxial Co2TiSn thin films

In this article we report on the properties of thin films of the full Heusler compound Co2TiSn prepared by DC magnetron co-sputtering. Fully epitaxial, stoichiometric films were obtained by deposition on MgO (001) substrates at substrate temperatures above 600{\deg}C. The films are well ordered in the L21 structure, and the Curie temperature exceeds slightly the bulk value. They show a significant, isotropic magnetoresistance and the resistivity becomes strongly anomalous in the paramagnetic state. The films are weakly ferrimagnetic, with nearly 1 \mu_B on the Co atoms, and a small antiparallel Ti moment, in agreement with theoretical expectations. From comparison of x-ray absorption spectra on the Co L3/L2 edges, including circular and linear magnetic dichroism, with ab initio calculations of the x-ray absorption and circular dichroism spectra we infer that the electronic structure of Co2TiSn has essentially non-localized character. Spectral features that have not been explained in detail before, are explained here in terms of the final state band structure.Comment: 11 pages, 8 figure