Ondrej Gedeon

Microanalysis of Glass Containing Alkali Ions

Abstract. The common problems connected with alkali ion migration during EPMA were studied on glasses containing nearly all possible alkali ions (Na, K, Rb, Cs). Binary silica glasses were prepared by melting from a very pure batch in Pt crucible. The glasses were carefully polished using alcohol to prevent surface corrosion by water and they were stored in vacuum. The specimens were coated with carbon layers approximately 30-nm thick and exposed to a 50- keV electron beam of 100 μm diameter. It was found that all alkali ions migrate under the electron beam, but the rate of the migration depends on the current density. The decay curves (characteristic X-ray intensity versus time) are similar in shape in all cases. The decay curve shows two transport regimes, the first being linear-like, the second being the exponential-like. The first transport regime busts into the rapid alkali migration after a time known as the incubation period. The period is in general longer for the larger-alkali ions size. It was found that even large rubidium and caesium ions migrate inside the glass with the same mechanism as sodium and potassium ions. While for K, Rb, and Cs ions the incubation periods were observed under the suitable experimental conditions, binary glass containing Na exhibits no observable incubation period. Except for the binary Na2O + SiO2 glass, the suitable experimental conditions for reliable quantitative EPMA can be found.

Low energy and low dose electron irradiation of potassium–lime–silicate glass investigated by XPS. I. Surface composition

Abstract X-ray induced photoelectron spectroscopy has been used to study the influence of low-energy electron beam on the pristine potassium–lime–silicate glass surface prepared by fracturing in situ under ultrahigh vacuum. Relatively low-energy electron beam of 1600 eV with low-electron beam current density of 0.02–0.22 A/m2 and low-electron dose of 29–5200 C/m2 was used. The expected modified near-surface region thickness is in this case comparable with the mean sampling depth of the analytical tool. Therefore, possible changes and modifications due to electron irradiation could be recorded with high sensitivity. The freshly fractured glass surface was found to be significantly enriched with potassium, and slightly with calcium. As a consequence of the lowest electron dose irradiation used, the potassium signal substantially increased by a factor 1.24 relative to the value found for the fresh surface. For higher doses used, the potassium signal continuously increased with the dose to a maximum and decreased thereafter. This variation was accompanied with the qualitative opposite behaviour of calcium signal. The concentrations of the other elements present in the glass, oxygen and silicon, varied only slightly with the electron dose. They can be considered to be constant within experimental uncertainty. In agreement with experimental results, a model assuming mobility of only two most mobile cations, potassium and calcium, was suggested. The models assuming one layer and two layers on the bulk were developed. Their results reproduce well experimental findings: (i) the formation of a potassium-rich surface layer, and (ii) the opposite-like signal variation of calcium in comparison with potassium.

Molecular dynamic modelling of potassium transport in a potassium-silicate glass irradiated by fast electrons

Abstract The binary potassium-silicate glass has been prepared from an ionic liquid by molecular dynamics (MD) using 900 ions interacting via Born–Mayer type pair potentials. The standard MD procedure was adapted so that an electric field and/or momentum transfer, corresponding to the elastic scattering of primary electrons, could be applied to a particular set of ions. Results have shown that the electric field itself should cause dielectric breakdown to drive the ions out of their sites. If the electric field is accompanied by momentum transfer from the primary electron, then potassium ions can be released from their sites and can be further driven by the macroscopic electric field, created by trapped electrons in the glass. The depth and width of the potassium potential well has been estimated by studying the motion of ions in a high electric field.

Mixed alkali effect in glass irradiated by 50 keV electron beam

Abstract A series of three sodium–potassium-silicate glasses with different alkali contents were prepared. The glasses were exposed to a 50 keV electron bombardment of defocused beams of various current densities. Decay curves (alkali X-ray intensity versus time) have been measured and analysed to obtain incubation periods and maximum transport rates. Obtained data have been compared with those known for binary alkali-silicate glass. The results confirm the influence of the mixed alkali effect on the transport behaviour of alkali ions under the electron bombardment.

Structural Changes in Alkali Depleted Alkali-Silicate Glass – MD Study

Alkali-silicate glass irradiated with electrons of the medium energy becomes to be fully depleted of alkali ions after some time. The chemical changes are expected to be accompanied with significant changes of glass structure. The changes in structure of 15K2O•85SiO2 glass were simulated by ab-initio MD. Experimental conditions were imitated by the following procedure. First, glass was prepared by the numerical cooling from the high-temperature melt. Second, alkali ions were removed. Third, silica structure was relaxed at temperatures 300 K and 1000 K. Structural changes are characterized in terms PP RDF’s and coordination numbers. It was found peroxide bonds were created from the dangling bonds, coming from non-bridging oxygen.

Voronoi polyhedra analysis of MD simulated silicate glasses

The relationship between the volume and the surface of Voronoi polyhedra (VP) appeared to be an effective tool for the characterization of the structure of MD simulated silicate glasses. This feature was demonstrated on the series of binary potassium-silicate glasses with the alkali content ranging from 5 to 20 mol%. Moreover, the temperature dependence of the slope of the above dependence, when expressed for various types of central atoms (Si, O and K in our case) was studied. Each type of central atom is characterised by its own iron line linearly relating the volume of VP to its properly powered surface, so that new geometrical constrains to the VP was found.

Potassium Migration in Silica Glass During Electron Beam Irradiation

Potassium glass was irradiated by 10, 30 and 50 keVelectron beams of various diameters and the time decay of potassium x-radiation was measured. These measurements were complemented by SEM observation of the etched cross sections of exposed areas and potassium depth concentration profiles in order to observe structural changes in the glass. Whilst the temperature increase remained low, the length of the incubation period was inversely proportional to the specimen current density and a possible explanation of this phenomenon was suggested.

Analysis of alkali-silicate glasses by electron probe analysis

Abstract The brief review of our previous papers concerning the bombardment of alkali-silicate glass by electron beam is presented. Phenomena connected with alkali ions migration are summarized and explained. The paper contributes to a better understanding of glass structure as well as to the achievement of optimum analytical conditions and consequent results in glass containing alkalis.

Experimental Verification of Theoretical Models Simulating the Temperature Increase in EPMA of Glass

The temperature field in glass exposed to electron beams of different diameters and deposited energy was calculated. A cylindrical shape of the excited volume was chosen for the heat source, having a diameter equal to the diameter of the beam and a height equal to the maximum range of electrons calculated with the Kanaya and Okayama formula. The grid method for the numerical calculation of the partial differential equation for the heat transport was used. The calculated temperature field was verified by means of specially prepared glass-ceramic samples that crystallize over a suitable temperature range. Scanning electron images of the cross-sections of the exposed areas have given satisfactory results, in good agreement with calculated temperatures. It has been found that at normal conditions of EPMA when the absorbed current does not exceed 10 nA the maximum temperature in glass remains far under the transformation interval of the glass and cannot be the main reason for the rapid escape of alkali ions from the excited volume.

Electron Beam Induced Migration of Alkaline Ions in Silica Glass

Sodium and potassium silica glasses were irradiated with a 50keV electron beam and the migration of alkalis was studied on cross-sectioned specimens. Alkaline ions moved from the grounded (i.e. electrically connected with the ground) surface deeper into the sample even at low temperatures due to the electric field and at about half the electron range colloidal particles with a diameter in the range from 10 to 200 nm were created. Near the surface alkali depleted layers with modified structure were formed.