Three-coordinated Silicon Research Articles

Fumed silica synthesis: influence of small molecules on the particle formation process

Burning silicon tetrachloride in an oxygen-hydrogen flame produces fumed silica. This process is known for at least 50 years [1-5], but some important details are still uncertain. We would like to study several starting steps of fumed silica synthesis on the way from molecules to products. To do this we have performed quantum-level simulations of protoparticle and primary particle formation, from silicon dioxide molecules. Additionally, we have simulated the behavior of silica clusters in the presence of small molecules like water and hydrochloric acid. The reaction of silicon dioxide molecules leads to a silica cluster, which is covered with chemicaly highly active sites of one-coordinated oxygen atoms and three-coordinated silicon atoms. These clusters interact together and produce silica bulk like quartz glass. Reaction with water terminates the silica particle surface and leads to a complicated structure of the particle surfaces. The hydroxyl shell protects the particle body against the increase in particle size, but leads to aggregate and agglomerate formation.

Character, mechanism of formation and transformation of point defects in type IV silica glass

In the temperature range 600–650°C, transformations of defects in quartz glass are observed. The dependence of a band at 163 nm on annealing temperature is measured. Changes of structure in this temperature range are observed for the SiH and SiOH centers. These are caused by the appearance of a structural relaxating process at ∼ 675°C, the character of which is not determined. It is shown that the band at 163 nm is caused by three-coordinated non-paramagnetic silicon Si 3 + (T 3 + center). The band at 248 nm is attributed to an oxygen vacancy SiSi. It is shown that at temperature < 650°C the formation of SiH centers (absorption band at 2254 cm −1) is due to the interaction of H 2 with oxygen vacancies. At temperatures > 650°C, the interaction of H 2 with T 3 + centers was achieved. The maximum of the band of the SiH centers thus formed is at 2264 cm −1. It was assumed that, in the type IV glass, the formation of SiOH is due to an interaction of H 2 with peroxy linkages. Changes of the fundamental absorption edge are correlated with the changes of concentration of OH groups from 1500 ppm to 2 ppm. In the region < 2 ppm, the linear dependence of these changes on OH concentration is violated.

Photochemical 1,2 Shift of the Phosphane Ligands in the Silanediyl Complex [(2‐Ph2PCH2C6H4)2SiCr(CO)5

An effective access to compounds with three-coordinate silicon is provided by the title reaction [Eq. (a)], which occurs in two successive steps. Compounds 1 and 3 were completely characterized, but compound 2 could only be detected by IR spectroscopy.

Topological and bonding defects in vitreous silica surfaces

A model structure for an annealed silica surface was obtained through the molecular dynamics simulation technique employing three-body interaction potentials. Nonbridging oxygen and edge-sharing tetrahedra were found to form on the oxygen-terminated surface with three-coordinated silicon, three-coordinated oxygen, and three-membered rings just below the outermost atoms. Four-membered rings were also created in relatively large concentrations during the surface relaxation. When considering the effect of removing periodic boundary conditions along the z direction, the concentration of larger rings, relative to the bulk, remained unchanged throughout the ∼11 Å simulated surface region. A strong physical association between three-membered rings and three-coordinated oxygen was found which may account for the D2 defect peak observed in Raman scattering.

Characterization of three E'-center variants in X- and γ-irradiated high purity a-SiO 2

Electron spin resonance (ESR) studies have been carried out on a suite of high purity amorphous silicas (a-SiO 2) following irradiation by 100 keV X-rays at 77 K or 60 Co γ-rays (∼1 MeV) near 300 K. Both high-OH (∼1200 ppm) and low-OH (< 5 ppm) samples were investigated. The existence of three E'-center variants is suggested on the bases of their phenomenological production and thermal bleaching behaviors. These variants, E α , E′ β , and E′ γ are then shown to differ one from another in aspects of their g matrices, suggesting structural differences among them. Finally, 29Si hyperfine structure ascribable to each of these variants is reported, demonstrating that each of the three consists of an unpaired electron in a dangling tetrahedral orbital of a single three-coordinated silicon. Production mechanisms and structural models for these three fundamental point defects are proposed.