[en] It has been computed the potential-energy curve of π² SiH, as a function of the silicon hydrogen bond distance, by means of ab initio HFR-MO-LCAD-SCF-CI method. It has been investigated, in particular, the influence of different basis sets on the computed structural and vibrational properties of above-quoted molecule

No abstract available

[en] The transpiration method was used to study the reaction of SiO₂ (cristobalite) with water vapor. Measurements were made between T = (1073 and 1728) K with water vapor contents from (0.05 to 0.6) by volume fraction in argon. From the results, the thermodynamic parameters for Si(OH)₄(g) and SiO(OH)₂(g) were determined. At T = (1073 to 1373) K, Si(OH)₄ is the primary vapor species and second law measurements lead to Δ_f H ^.(1200 K) = (-1344.6 ± 2.7) kJ . mol^-1 and S ^.(1200 K) = (544.4 ± 2.1) J . mol^-1 . K^-1. Third law measurements lead to Δ_f H ^.(298 K) (-1351.3 ± 1.7) kJ . mol^-1. These are in very good agreement with previous measurements and previous ab initio calculations. At T = (1673 and 1728) K, there is evidence of a second vapor species, SiO(OH)₂(g). By subtracting the extrapolated Si(OH)₄(g) pressure, it was possible to do a third law analysis and obtain a Δ_f H ^.(298 K) for SiO(OH)₂(g) of (-836 ± 40) kJ . mol^-1. Previous measurements and ab initio calculations are within this range

[en] Infrared reflection spectroscopy (IRS) has been used to identify the Si-O vibrational mode and confirm previous assignments of Si-OH, and Si-OD vibrational modes in porous amorphous silica implanted with heavy ions and with H⁺ and D⁺ ions. The Si-O stretching mode (approx. 1015 cm^-1) is produced by the damage cascade and is seen in all implanted bulk silicas as well as in porous silica. Implantation of porous silica with H⁺ and D⁺ ions produces bands at approx. 985 cm^-1 and approx. 960 cm-¹, respectively. The position of all three bands is consistent with O, OH, and OD mass considerations. Implantation of D⁺ ions into porous silica containing molecular water and OH^- groups results in D-H exchange. The Si-OH and Si-OD vibrations are also seen in the bulk fused silica at low H/D fluences. These results suggest that intrinsic E'-type defects in bulk silica and dangling Si bonds at internal surface sites

[en] A new gamma camera based on hydrogenated amorphous silicon (a-Si:H) pixel arrays to be used in nuclear medicine is introduced. Various performance characteristics of a-Si:H image rs are reviewed and compared with those of currently used equipment. An important component in the a-Si:H imager is the scintillator screen. A new approach for fabrication of high resolution CsI(T 1) scintillator layers, appropriate for coupling with a-Si:H arrays, are presented. Using Monte Carlo Simulation, the performance of the new a-Si:H based gamma camera is evaluated

[en] We constructed regular fractal SiOH atomic clusters which pending bonds are saturated with OH molecules. We calculated the binding energies of these clusters as well as for sp² hybridization as for sp³ hybridizations. The result are the following: for the two hybridizations, the total binding energies have a linear dependence on the size of the fractal cluster, which comes directly from the scaling law of the fractal characteristic of the building of the cluster. We related by a scaling law, the number of electronic bonds and the total bonding energy

[en] Document available in extended abstract form only. One of the most important factor to understand and predict the sorption of cations on clay surfaces is the protonation state of the surface sites, which is difficult to determine experimentally. Indeed, titration provides a global measure only; it does not probe the status of different silanol (Si-OH) and aluminol sites (Al-OH and Al-OH₂) present along the edges of clay sheets in the presence of water. A novel method has been recently designed to quantify the acidity of chemical species. This method allows to estimate pKa values from First Principles Molecular Dynamics by thermodynamic integration. We have applied it to the case of pyrophyllite clay edge sites. These calculations will allow us to subsequently perform classical Molecular Dynamic simulations with realistic surface structures (protonated/deprotonated sites) for natural clays in the presence of water. After presenting the method, we show its application to the (010) edge of pyrophyllite. We find that the most acidic group is Si-OH while the least acidic one is Al-OH, which never deprotonates in water because of its high pKa value (22.1). We further provide a microscopic understanding of the solvation structure and reactivity of the edges of neutral clays. In particular we address the tendency to deprotonation of the different reactive groups on the (010) face of pyrophyllite, showing the important role of solvation and its rearrangements after deprotonation. Finally, we compare our results to the one predicted by the empiric method MUSIC and the estimate from deprotonation energies in the vacuum, confirming the important role of solvation in both the protonated and deprotonated states. (authors)

[en] In present paper the dependence of the displacement cross sections of the different species of atoms in the a-Si:H structure, with the energy of the secondary electrons generated by the X-rays of the typical energies using in medical imaging applications, was calculated using the Mott-McKinley-Feshbach approach. It was verified that for electron energies higher than 1.52 keV it is possible the occurrence of hydrogen atoms displacements, while for the silicon atoms the threshold energy is 126 keV. These results were compared with those obtained for similar detectors but developed with crystalline silicon. With the use of the mathematical simulation of the radiation transport in the matter, the energy spectrum of the secondary electrons was calculated in order to estimate the number of atomic displacements, which take place in the semiconducting amorphous device in working regime. The spatial distribution of the dpa in the detectors volume, as well as its behavior with the depth in the work region are presented and discussed in the text. (Author)

[en] SiO₂-TiO₂ mixed hydroxide gels were prepared and the apparent adsorption capacity for Na⁺ (1.30-1.60 mmol/g dry sample) determined by pH titration. The effect of solution pH on the sorption ability of the binary gels was studied. It is found that: the sorption mechanism depends upon the pH, and the two most important factors are the nature of both counter ions in bulk solution and the surface of silica-titania gels, the best pH range for the sorption of uranyl ions is 4-7. The thermodynamic functions were evaluated, suggesting that the H/UO₂²⁺ reaction is chiefly attributed to physical adsorption rather than chemical one, and the opposite is the case of the UO₂/H⁺ reaction. (author) 22 refs.; 5 figs.; 3 tabs

No abstract available