SiO2 Molecules Research Articles

Reinforcing Porous Silica with Carbon Nanotubes to Enhance Mechanical Performance

In this study, porous silica (SiO2) film is reinforced with carbon nanotubes (CNTs), which gets homogeneously dispersed in porous SiO 2 gel by ultrasonic stirring. The size of SiO2 molecules is the same as that of nanotubes, so a fine CNTs/SiO2 network is formed. Nanoindenters used to test the mechanical properties of the resulting specimens show a marked enhancement in the properties as a function of filler content. Interfacial bonding and the distribution of CNTs also strongly affected the mechanical performance. Although a high concentration of CNTs improves the mechanical characteristics of the matrix, it also causes agglomeration such that the mechanical performance at the surface was disproportionate. Fourier transform infrared spectroscopy (FTIR) analysis indicates that there is little chemical reaction occurring between CNTs and SiO2. The results suggest that CNTs are ineffective reinforcements.

Silicon-nitride-based nanoceramic materials

Based on the principle of maximum bond energy, a structural model is proposed for Si3N4 + 60 wt % MoSi2 + 5 wt % Y2O3 + 1.5 wt % Al2O3 ceramics. In this model, Si3N4 clusters form a star-like structure with columnar pores 0.45 and 1.5 nm in size. The smaller pores are filled with MoSi2 molecules, and the larger pores are filled with Y2O3 and Al2O3 clusters. The solubility of MoSi2 molecules in silicon nitride is 70 wt %, and those of Y2O3 and Al2O3 are 5.45 wt %. The energies of intercluster interaction in two mutually perpendicular directions are 3.2 and 5.5 eV, which is comparable to bond energies in structural metallic materials. The lattice parameters of molybdenum disilicide are determined by x-ray diffraction. The formation of MoO3 and SiO2 molecules during thermal cycling has an insignificant effect on the structure of the material.

Simulating aggregation and reaction: New Hounslow DPB and four‐parameter summary

AbstractA new formulation is developed of the popular Discretized Population Balance due to Hounslow and co‐workers. This new formulation is simpler and permits some variations in the method; three variations are put forward and tested. It also allows more flexibility in the choice of discretization, so that detail can be added at small sizes. The method is applied to the case of free‐molecular aggregation of SiO2 molecules in flames. The SiO2 is in equilibrium with SiO, which is assumed not to participate in aggregation; SiO2 molecules are, therefore, continuously created during aggregation. This source term complicates the resulting particle‐size distributions compared to the self‐preserving size distribution. Four parameters are used to summarize the PSDs, and their rates of change are found by approximations. This summarized model, a quasi‐self‐preserving size distribution, matches the detailed simulations very well. The cause of the stiffness of the differential equations is shown to be collisions between small and large particles. © 2004 American Institute of Chemical Engineers AIChE J, 50:578–588, 2004

Structure simulation of silica glasses: approach to CVD

We describe a new way to simulate the structure of vitreous silica. Using animproved semi-empirical code, we simulate the condensation of SiO2molecules (typically 27 SiO2 molecules) into aggregates, stopping thecomputation at the first local total energy minimum. The structure of theseaggregates appears to be very disordered with a distribution of coordinationnumber, angle and bond length. It is representative of small particlesobtained in any chemical vapour deposition process.

Formation Processes of Silica Nanotubes through a Surfactant-Assisted Templating Mechanism in Laurylamine Hydrochloride/Tetraethoxysilane System

The formation processes of the silica nanotubes by a surfactant-assisted templating mechanism were elucidated in a laurylamine hydrochloride (LAHC)/tetraethoxysilane (TEOS) system by measuring the evolution of the shape and size of surfactant/silicate molecular assemblies with small-angle X-ray scattering, together with TEM, SEM, nitrogen adsorption isotherm, and 29Si NMR. The formation processes in dilute LAHC solution at pH 4.5 are elucidated as follows. Partially hydrolyzed TEOS penetrates into finely divided bilayerlike surfactant assemblies and converts the assemblies to globular aggregates. Then, the polymerization of the hydrolyzed TEOS proceeds on the surface of the aggregates, converting the aggregates to cylindrical form due to the geometric matching between the occupied area of a surfactant molecule and that of four SiO2 molecules at the surface of the cylinder. Then, the cylinders connect with each other at the cylinders spherical end cap because of the relative instability at the end caps due...

Mechanism of Secondary Ion Emission from Silicon Dioxide Bombarded with Argon Ions

SiO2 powder which includes oxygen of both mass numbers 16 and 18 was Ar+-bombarded in a high vacuum and in 16 O2 gas at various pressures. Dependence of the secondary-ion currents on the oxygen pressure was used to consider the secondary ion formation mechanism, and the total current for SiO+ was separated into three parts corresponding to the following mechanisms: (I) atomic-bond breaking in SiO2 molecules, (II) ionization of SiO* (SiO in an excited, neutral state) formed by bond breaking through an interaction with oxygen, and (III) combination of Si* formed by bond breaking with gas-phase oxygen. The first mechanism was proven to be independent of the amount of adsorbed oxygen on the sample surface. SiO+ formed by the third mechanism increased proportionally with the oxygen pressure. Interaction of SiO* with oxygen in the second mechanism did not show a simple relationship with the pressure.

Infrared absorption and x-ray photoelectron spectroscopy studies of the anodic oxidation of plasma silicon nitride

Silicon oxide films have been grown by anodic oxidation of plasma enhanced chemically vapor deposited silicon nitride films onto silicon substrates. Infrared absorption and nuclear reaction analysis (NRA) of the films reveals that during the anodic oxidation the nitrogen of the silicon nitride films is lost to electrolyte/oxide interface. These analyses combined with x-ray photoelectron spectroscopy analysis show that the final oxide structure is formed by an outer silicon dioxide layer followed by the inner layer of the previously deposited silicon nitride. In the case of thicker oxides the silicon dioxide layer is followed by a region made of a mixture of SiO2 and SiNx molecules. The elementary reactions as well as the ionic transport through the interface is discussed.

Charge-transfer dipole moments at the Si–SiO2 interface

A first-order model has been developed to calculate the magnitude of the dipole moment at the Si–SiO2 interface resulting from partial charge transfer that takes place upon the formation of interface bonds. The charge transfer occurs because of the difference in electronegativity between the two species across the interface, namely silicon atoms and SiO2 molecules. This approach is similar to that introduced to describe the modification of band lineup at the Si–SiO2 interface by means of an intralayer of H or Cs. The charge-transfer estimation uses the principle of electronegativity equalization, and results obtained for (100) and (111) silicon substrates indicate that the magnitude of the interface dipole is orientation dependent. Dipole moments at the Si–SiO2 and gate–SiO2 interfaces should be included in the definition of the flatband voltage VFB of metal-oxide-semiconductor structures. The metal-semiconductor work function difference φms determined from capacitance-voltage measurements on (100) and (111) silicon oxidized in dry oxygen and metallized with Al agree well in magnitude and sign with the predictions of this model. Other types of interface dipoles and their processing dependence will be examined.

New data on the solubility of amorphous silica in organic compound-water solutions and a new model for the thermodynamic behavior of aqueous silica in aqueous complex solutions

Experimental solubilities of amorphous silica in several aqueous electrolyte solutions and in aqueous solutions of organic compounds, and theoretical considerations concerning cavity formation, electrostriction collapse, ion solvation, and long- and short-range interaction of the solvated ions with one another(1) permit the calculation of the partial excess free energies and the activity coefficients of aqueous silica. It is shown that, in the case of non-dissociated aqueous organic solutions, the variation of log m (SiO2) with the reciprocal of the dielectric constant of the solution is described by a single linear equation independent of the nature of the organic compound. For aqueous electrolyte solutions, a specific linear relationship between log m (SiO2) and the reciprocal of the dielectric constant occurs for each electrolyte. The success of the equation in reproducing the experimental solubilities of amorphous silica in aqueous solutions of electrolytes and organic compounds supports previous evidence indicating a polar charge distribution in the solvated SiO2 molecule. Our data permit the calculation of the effective local charge of dissolved SiO2 molecules and of the short-range interaction parameters between SiO2 and various ions. The proposed equation of state can be used to calculate the affinity of reactions among SiO2 minerals and complex aqueous solutions.

Charge-Transfer Dipoles at the Si-SiO2 Interface and the Metal-Semiconductor Worfunction Difference In Mos Devices.

AbstractThe magnitude of the dipole moment at the Si-SiO2 interface resulting from partial charge transfer that takes place upon the formation of interface bonds has been calculated. The charge transfer occurs because of the difference in electronegativity between silicon atoms and SiO2 molecules which are present across the interface. Results obtained for (100) and (111) silicon substrates indicate that the magnitude of the interface dipole moment is dependent on substrate orientation and the interface chemistry. Dipole moments at the Si-SiO2 and gate-SiO2 interfaces should be included in the definition of the flatband voltage VFB of MOS structures. CV-based measurements of the metal-semiconductor workfunction difference φms on (100) and (111) silicon oxidized in dry oxygen and metallized with Al agree with the predictions of this model. Other types of interface dipoles and their processing dependence are briefly discussed.

Application of molecular models to electronic structure calculations of defects in oxide crystals

AbstractAb initio and INDO methods are applied to investigate the interaction between several SiO2 molecules and for electronic structure calculations of idealized β‐cristobalite within the framework of the cluster model. Stoichiometric fragments are chosen for simulating perfect and imperfect crystal structures of SiO2 by means of combinations of interacting structural units OSiO. The boundary conditions for such clusters are imposed in the form of the electrostatic fields in the rest of the crystal. The results of the electronic structure calculations are only slightly dependent on size and shape of the chosen clusters and agree quite well with experimental data.

SILICA GLASS: A BENT-BOND MODEL

from distorted hexagonal rings. These grains are joined together by rings other than 6-membered, probably mostly 7- and 5-membered, but 8- and 4-membered rings would not seem impossible. In the example shown in Figure 1, the tetrahedra represent Si atoms and the bent links 0 atoms. The model was built mostly from 6membered rings, but contains several 5- and 7-membered rings. When the Si-O bonds in the cristobalite lattice bend and rotate to the extents indicated, long-range order ceases and the structure melts. Relatively few bonds need to be broken, but some are. These are rejoined in a largely random way, and require the migration of some SiO2 molecules in order to preserve electrical neutrality. On the basis of Eyring's significant structure theory of liquids,4 the holes in the liquid which reflect the structure of the solid are those extra positions in the neighborhood of each Si and 0 atom about their equilibrium positions in the solid which can be filled by the atoms when the bonds are bent and rotated to form the liquid. These can be considered to provide solidlike degrees of freedom and are to be associated with 6-membered rings. The gaslike degrees of freedom are used by the migrating SiO2 molecules which result from formation of 5-membered, 7-membered, and possibly other kinds, but not 6-membered rings. The bent and rotated