Crystalline silicas properties

Reliability of airflow measurements m assessing ventilation systems performance Cobalt - a review of properties, use and levels of exposure A review of respirable crystalline silica - Exposure and control... 

Antiblock additives are often incorporated into packaging films to prevent them sticking together. Their effectiveness relates directly to the roughness they impart on a film surface while minimising loss in optical properties. They are often used in association with slip additives such as erucamide. Common antiblock additives include crystalline silica, diatomaceous earth and talc (magnesium silicate). 

Adsorption and Catalytic Properties of Palladium Supported by Silica, Alumina, Magnesia, and Amorphous and Crystalline Silica-Aluminas... 

New plants are successfully operating with clean and efficient process technologies. This is the most significant development made possible by the discovery, in 1983, of titanium-containing crystalline silicas and their unique catalytic properties, especially in selective oxidation reactions with H202 as the oxidant. 

These important results have stimulated many research workers in universities and industrial research laboratories in the world to investigate the particular state of aggregation and coordination that Tilv assumes when forced into framework positions of hydrophobic crystalline silicas. Researchers are also engaged in the search for other compounds containing titanium and silicon oxides with Tilv in the same coordination and environment, on the assumption that similar catalytic properties would be obtained. Relevant discoveries have been made, and additional valuable information has been obtained on this new class of materials and on their catalytic performance in many different reactions. 

The unusual properties of titanium silicalites have been attributed to the presence of Tiiv in framework positions of the Si02 lattice. It is important to realize that there is a limit to the extent of substitution the exact value is still under discussion, but is certainly not more than a few percent. Very likely, the structure of crystalline silica is not stable at higher degrees of substitution. This suggestion is consistent with theoretical predictions of lack of substitution these predictions referred mainly to high degrees of substitution, which have not been observed. 

The first discovered member of the group of crystalline microporous materials made of oxides of titanium and silicon is titanium silicalite-1 (TS-1). TS-1 has attracted much interest for its unique catalytic properties it is also of interest by virtue of the proposal that Tiiv assumes tetrahedral coordination in substituting for SiIV in framework positions of crystalline silica, as stated above. To clarify this point, many detailed studies of the TS-1 structure have been carried out. An outcome of the work was the discovery of new crystalline microporous titanium silicates. 

In summary, the properties of these mesoporous amorphous materials do not duplicate those of the crystalline silicas, but present a useful addition to the array of titanium catalysts useful for oxidation reactions. 

Much work has been done to characterize surface and catalytic properties of Si02—A1203 and other mixed-oxide systems. It is well known from these studies that Si02—A1203 possesses Lewis as well as Br nsted acid sites that are interconvertible. Different models for the site structures have been proposed. Tanabe 20) has recently reviewed these attempts to characterize the surfaces of silica-alumina mixed oxides as well as those of crystalline silica-aluminas. 

In 1978, the same year that the structure of ZSM-5 was first described, Flanigen and her co-workers reported the synthesis, structure and properties of a new hydrophobic crystalline silica molecular sieve (Flanigen et al., 1978). The new material, named Silicalite (now generally called Silicalite-I), has a remarkably similar channel structure to that of ZSM-5 but contains no aluminium. It was pointed out by the Union Carbide scientists that, unlike the aluminium-containing zeolites, Silicalite has no cation exchange properties and consequently exhibits a low affinity for water. In addition, it was reported to be unreactive to most acids (but not HF) and stable in air to over 1100°C. 

Fubini B. et al., Relationship between surface properties and cellular responses to crystalline silica Studies with heat-treated cristobalite, Chem. Res. Toxicol., 12, 737, 1999. 

Sand consists of mainly crystalline silica (quartz) and its aqueous solubility is negligible. As a result, it does not participate in the reaction that forms CBPCs. However, because sand is made of hard particles, it improves mechanical properties of CBPC products, especially the toughness. Being low cost filler, it can also be used in a large percentage of CBPC products. 

Typical Properties Typical physical properties for commercial amorphous silica hydrogels are shown in Table 4. Chemical purity is high typically >99.5% Si02, <0.1% AI2O3, <0.01% Fc203, with only traces of alkali and alkaline earth oxides on a volatile free basis. By definition, amorphous sihca is noncrystalline it contains no crystalline silica forms (i.e., quartz, ciistobahte, tridymite). 

The potential of Eq. (1) with parameters determined in Refs. [10, 11] was thoroughly tested in computer simulations of silica polymorphs. In Ref. [10], the structural parameters and bulk modulus of cc-quartz, a-cristobalite, coesite, and stishovite obtained from molecular dynamics computer simulations were found to be in good agreement with the experimental data. The a to / structural phase transition of quartz at 850 K ha.s also been successfully reproduced [12]. The vibrational properties computed with the same potential for these four polymorphs of crystalline silica only approximately reproduce the experimental data [9]. Even better results were reported in Ref. [5] where parameters of the two-body potential Eq. (1) were taken from Ref. [11]. It was found that the calculated static structures of silica polymorphs are in excellent agreement with experiments. In particular, with the pressure - volume equation of state for a -quartz, cristobalite, and stishovite, the pressure-induced amorphization transformation in a -quartz and the thermally induced a — j3 transformation in cristobalite are well reproduced by the model. However, the calculated vibrational spectra were only in fair agreement with experiments. 

Silicalite is a microporous crystalline silica molecular sieve with remarkable hydrophobic properties ( 1) and has been considered to offer practical applications in the clean-up of water contaminated with hydrocarbons and the separation of ethanol from dilute fermentation aqueous solutions (2 ii> 2) Many studies have been reported on the properties of adsorption and diffusion of gases in silicalite (e.g., 6, 8, , HI) However, despite the many potential applica-... 

PHYSICAL PROPERTIES finely divided gray powder containing less than 1% crystalline silica odorless insoluble in water mixture with water, sand, and stones is fluid or plastic-like MP (NA) BP (NA) SG (much greater than 1) VD (NA) VP (0 mmHg at 20°C). 

D.I. Mendeleyev was among the first to suggest polymer structure of silica. Exactly that, in his view, was the reason for non-volatility and and refractory properties of the substances with composition (SiO ). X-ray diffraction studies showed that crystalline silica is a 3D interlaced polymer of [SiOj tetrahedrons (Figure 2.43). 

When antiblocking agents are incorporated into PE films, other important properties of the polymer are also affected. These include an increase in stiffness, a decrease in the coefficient of friction, and an increase in haze. Interaction effects with processing aids can also result, especially with fluorocarbon elastomers that are added to prevent melt fracture in the blowing process of films. Worker exposure to dust generated by these additives can be hazardous if impurities such as crystalline silica or asbestos are present. 

To obtain an accurate estimate of thermodynamic properties for crystalline silica polymorphs, one needs an accurate description of the phonon density of states. Given the complexity of the problem, this is tractable only with several assumptions. For example, it is often assumed that the calculated phonon spectra are not strongly dependent on temperature. Certainly this will be a satisfactory assumption in the absence of any thermal expansion, and any changes in the interatomic potentials as a function of temperature. In this case, the force constants, and consequently the dynamical properties like phonon frequencies and density of states, will be independent of temperature. 

When the application was extended to simulation of crystalline silica and silicate, however, we encounter the fact that these interatomic potentials does not necessarily reproduce important structural properties such as bulk moduli, elastic constants or sometimes even dynamical stability of the crystalline symmetry. Thus some efforts have been made to incorporate crystal structures and their elastic properties into the process of empirical determination of the interatomic potentials by means of a static energy minimization technique [5,6, 7, 8, 9],... 

We have now to look if such bonding parameters can be used to predict solution properties. Looking at some simple ionic species displaying a wide range of pKa values, we have selected the oxy-ions gathered in table 3. Calculations of charge distributions in the fully deprotonated ion have been done with %5 (0) = 0, as found in crystalline silica, %.s X ) = 0.25 and %. (X ) = 0.15. For the hardnesses standard covalent radii are used. The pKa values have been taken from [111], with the exception of the value... 

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