Effect of electron irradiation on Na-K silicate glass investigated using X-ray photoelectron spectroscopy and pattern recognition method

Alkali silicate glasses and melts play an important role in material science. Electron interaction with glasses is important for radioactive waste deposition, where electrons of various energies lead to irreversible changes. These changes are caused mainly by ionization and ballistic interaction of electrons with atoms, introducing structural disorder, changes in atomic composition and chemical state, accompanied by alkali ions diffusion. The Na-K silicate glass (5Na₂O · 10K₂O · 85SiO₂), pristine and electron irradiated (doses from 25 C m⁻² to 20 236 C m⁻²) are investigated using X-ray photoelectron spectroscopy (XPS) and the pattern recognition (PR) and fitting procedures. Changes of composition and chemical state of atoms dependent on electron dose are analyzed. At low doses (100-300 C m⁻²), decrease followed by increase of O and Si concentrations was observed. Surface segregation, probable desorption, and in-bulk diffusion of K and Na ions (doses of about 50 C m⁻² and 2000 C m⁻², respectively) were observed. This was accompanied by changes in the chemical state of K atom, where with an electron dose increasing content of elemental K form accompanied by decreasing potassium peroxide form were observed. No difference in chemical state of Si and O atoms was visible under electron irradiation dose to 20 236 C m⁻², within the sensitivity of the applied method.     

Changes in surface morphology of silicate glass induced by fast electron irradiation

Binary potassium-silicate glass was irradiated with a defocused electron beam. During the irradiation the alkali ions migrate from the surface into the depth and alkali ions depleted layer is created near the surface. Such changes in the chemical composition are also accompanied with changes of the glass structure and finally result in the volume changes of the irradiated glass. This was directly studied using atomic force microscope (AFM). A series of exposures with energy of electrons of 7–50 keV and with different doses were performed. For low doses the irradiated glass is continuously depressed with the increasing dose, indicating this way the structural changes leading to the volume compaction. It is suggested the compaction is caused by the relaxation processes of the silica subnetwork. A further increase of the electron dose causes a formation of the small bump inside the center of the depression. The bump arises with the dose and finally exceeds the surrounding surface. It is suggested that the expansion is connected with the migration of alkali ions and the formation of Si–O–O–Si bonds which result in the formation of new rings with new space requirements.     

XPS and nuclear analysis of compositional changes occurring in glass on electron beam irradiation

X-ray photoelectron spectrometry and nuclear microanalysis have been utilized to study the compositional changes occurring in a commercial soda-lime-silica glass after electron beam irradiation. Samples were irradiated with electron energies between 2.5 and 4.5 keV and current densities of 1.2 and 4.0 μA/cm².After electron bombardment a reduction in the Na and O surface concentration appears. Na has been observed to migrate towards the inside of the sample. It accumulates at depths comparable to the maximum electron ranges. The accumulation rate has been observed to depend on the beam power.For a beam energy of 4.5 keV the amount of Na accumulated at depth is nearly twice its surface depletion, supporting the hypothesis that the irradiation influences the glass composition at depths greater than the maximum electron ranges.     

Thermal stability of a low Tg phosphate glass investigated by DSC, XRD and solid state NMR

The thermal stability of a low Tg phosphate glass (Tg = 339 °C), formulated in the ZnO-Na₂O-P₂O₅ system, is investigated in this contribution by Differential Scanning Calorimetry (DSC), X-Ray diffraction (XRD) and solid state Nuclear Magnetic Resonance (NMR). DSC measurement indicates a very important T_x − T_g value (197 °C) for the investigated composition compared to other low Tg phosphate glasses found in literature. XRD and 1D ³¹P solid state NMR were used to monitor the isothermal crystallization process occurring at 60 °C above Tg (400 °C). The mixture of phases formed after crystallization cannot be identified by XRD but 2D MAS-NMR experiment provides valuable information about the composition and the structure of the unknown sample.     

Investigation of metal–GaN and metal–AlGaN contacts by XPS depth profiles and by electrical measurements

Metal/GaN Schottky contacts have been studied by X-ray photoelectron spectroscopy (XPS). Au/GaN, Pt/GaN, Pd/GaN are sharp while Ti/GaN is diffuse with the following composition, starting from the surface: Ti+TiN, Ti+Ti_xGa_yN, Ti+Ti_xGa_yN+Ga, GaN+Ga. Au/AlGaN and Ni/AlGaN contacts are much broader than Au/GaN: Al and Ga are found more than 100 Å away from the interface. Schottky barrier height was measured for the Au/GaN, Pd/GaN, Pt/GaN, Au/AlGaN and Ni/AlGaN contacts.     

Rapid quenching and glass formation in chalcogenide glasses: Preparation and properties of Ge–As–Te glasses over an extended composition ranges

In Ge–As–Te system, the glass forming region determined by normal melt quenching method has two regions (GFR I and GFR II) separated by few compositions gap. With a simple laboratory built twin roller apparatus, we have succeeded in preparing Ge_7.5As_xTe_92.5−x glasses over extended composition ranges. A distinct change in T_g is observed at x = 40, exactly at which the separation of the glass forming regions occur indicating the changes in the connectivity and the rigidity of the structural network. The maximum observed in glass transition (T_g) at x = 55 corresponding to the average coordination number (Z_av) = 2.70 is an evidence for the shift of the rigidity percolation threshold (RPT) from Z_av = 2.40 as predicted by the recent theories. The glass forming tendency (K_gl) and ΔT (=T_c−T_g) is low for the glasses in the GFR I and high for the glasses in the GFR II.     

Coordination and optical attenuation of TiO2-SiO2 glass by electron energy loss spectroscopy

Electron energy loss spectroscopy (EELS) techniques have been applied to investigate both the coordination and optical attenuation of TiO₂-SiO₂ glass. The coordination was determined from the electron energy loss near edge fine structure (ELNES) of Ti L_2,3-edges, and the results show that Ti ions substitute for Si ions in the tetrahedral coordinated sites. The optical attenuation of the glass was obtained from low energy loss of EELS data through Kramers-Kronig analysis, and the results show that Ti-doped silica has an absorption edge near 4.0 eV.     

Effect of glass composition on Judd–Ofelt parameters and radiative decay rates of Er in fluoride phosphate and phosphate glasses

For fluoride phosphate and phosphate glasses, Judd–Ofelt Ω_t parameters, spontaneous emission probabilities, radiative lifetimes and branching ratios of several Er³⁺ transitions were determined from the absorption spectra by the Judd–Ofelt treatment. The compositional changes of Ω₂ and Ω₆ are attributed to changes in the bonding between Er³⁺ and surrounding ligand groups. The phosphate groups play a special role. They show electron donation from the 2p orbitals of the oxygen ions, as well π-electron donation from the resonating P–O π-bond. Larger electron donation from the ligand anions leads to larger Ω₂, but smaller Ω₆. By contrast, larger π-electron donation from the phosphate groups increases Ω₆. The changes of Ω₂ with increasing phosphate content and with variation of modifier ions are ascribed to changes in the content and polarizability of the oxygen ions, leading to changes in the electron donation from the ligands at the rare earth (RE) sites. In the case of Pb²⁺, changes in the degree of asymmetry at the RE sites could also be important. The smaller Ω₆ parameter of phosphate glasses compared with fluoride phosphate glasses is caused by larger σ-electron donation due to larger oxygen content and by smaller π-electron donation due to smaller delocalization of the π-bonds. The changes of Ω₆ with phosphate content, in the case of fluoride phosphate glasses and with variation of modifier ions, are dominated by changes in the π-electron donation.     

Properties and microstructure of silica glass incorporated with tributyl phosphate by sol–gel method

An organic phosphate species tributyl phosphate (TBP) was incorporated into sol–gel-derived glass matrix. TBP could be directly added to the hydrolyzed silica source from tetraethylorthosilicate (TEOS) and immobilized in silica glass matrix. TBP was stably immobilized in silica glass matrix even in the case where the weight ratio of TBP to silica was unity, and where the volume fraction of the glass sample occupied by TBP moiety was as large as 69%. The glass sample showed an appearance of hard glassy solid even at such a large fraction of TBP which is an organic solvent in the neat state at room temperature. The FT-IR spectrum showed that TBP was immobilized in silica glass in an intact state without chemical bonding with the siloxane network. The Vickers hardness was large enough even at higher weight ratios of TBP to silica to be measured as data indicating that the immobilized TBP molecules could play a promotive role in forming the siloxane bonding. The wide-angle X-ray scattering experiments revealed that the siloxane bonding was expanded by TBP molecules entrapped in the siloxane network. Furthermore, TBP molecules are dispersed in the siloxane network in the molecular scale.     

Synthesis and characterization of a micro scale zinc oxide–PVA composite by ultrasound irradiation and the effect of composite on the crystal growth of zinc oxide

Micro scale zinc oxide-polyvinyl alcohol (ZnO–PVA) composite has been synthesized by ultrasound irradiation. The properties of the as-prepared ZnO–PVA composite material are characterized by X-ray diffraction (XRD), thermo gravimetric analysis (TGA), transmission electron microscopy (TEM), and diffuse reflection spectroscopy (DRS). A band gap of 3.25 eV is estimated from DRS measurements. The controlled crystal growth of zinc oxide has been studied by using the as-prepared micro scale ZnO–PVA composite as seeds for the crystal growth of ZnO.     

Comment on ‘Modification of vacuum-ultraviolet absorption of SiOH groups in SiO2 glass with temperature, F2 laser irradiation, and H-D isotope exchange’ by K. Kajihara et al. [J. Non-Cryst. Solids 2307 (2006)]

The modification of SiOH groups in SiO₂ by F₂ laser irradiation studied in the paper by Kajihara et al. [K. Kajihara, M. Hirano, L. Skuja, H. Hosono, J. Non-Cryst Solids 352 (2006) 2307] is discussed on the base of ab initio calculations. The calculated energy barrier of transformation of an oxygen tricluster (bridging oxygen forming the third covalent bond with an hydrogen atom) into a silanol group and the frequency of OH stretching vibration in the tricluster suggest that the triclusters can be products of F₂ laser irradiation of ‘wet’ silica glass.