Fumed silica dispersion

By electron microscopy the size of the primary particles in the aggregates is estimated to be about 10 nm. Particle size measurements using a nanosizer show the size of aggregates dispersed in a well wetting solvent to be in the range of 100 nm. Laser diffraction of fumed silica dispersed in air provides sizes of agglomerates larger than 5 pm. 

It is evident that for a profound understanding of the rheological properties of fumed silica dispersion at rest and under shear conditions, detailed information about the nature and strength of the interparticulate interaction forces is indispensable. 

Cabosil 2-5 Fumed silica Disperse with high shear. Use polar additives to improve thixotropy. 

In the case of the fumed-silica dispersion (cf. Figure 14.1(b)) a strong dependency of the RRs on the pH value can be noticed, but the influence of the ionic strength is not significant. Whilst the RR for a salt-free dispersion at a pH value of 4.1 is negligible it is very high at a pH value of 10.1. 

The dependency of the RRs on the pH value had been measured with slurries containing a constant total addition of ions (0.065 M) and shows the characteristic behaviour as presented in Figure 14.2. The RRs with the alkali-silicate sol as well as wifli die Stober-sol decrease and the RRs with the fumed silica dispersion increase towards higher pH values. With respect to the similar dependencies between the zeta potential... 

Figure 14.5 Cryo-TEM images (a) Stober-, (b) alkali silicate-sols and fumed silica-dispersions at (c) pH 4 and (d) pH 11.

Silica-based slurries are widely used not only to pohsh Si02 but also other materials, for example (poly)silicon and copper. As hsted and characterized in Section 14.2, the three main types of Si02 abrasive formulations commonly used are Stober- and alkali-silicate sol and fumed-silica dispersions (Choi et al., 2004a,b,c Estel et al., 2010). 

A polymer electrolyte with acceptable conductivity, mechanical properties and electrochemical stability has yet to be developed and commercialized on a large scale. The main issues which are still to be resolved for a completely successful operation of these materials are the reactivity of their interface with the lithium metal electrode and the decay of their conductivity at temperatures below 70 °C. Croce et al. found an effective approach for reaching both of these goals by dispersing low particle size ceramic powders in the polymer electrolyte bulk. They claimed that this new nanocomposite polymer electrolytes had a very stable lithium electrode interface and an enhanced ionic conductivity at low temperature. combined with good mechanical properties. Fan et al. has also developed a new type of composite electrolyte by dispersing fumed silica into low to moderate molecular weight PEO. 

This property is responsible for the fact that silica gels are mainly applied for adsorption while fumed silicas are used in rheological applications. In these the presence of highly dispersed small particles is important. The applications of silica will be discussed in detail furtheron. 

Fumed silica A-200 (Pilot plant at the Institute of Surface Chemistry, Kalush, Ukraine specific surface area Ascorbic acid (vitamin C) and all-rac-a-Tocopheryl acetate (vitamin E acetate) were used as adsorbates. Folin-Ciocalteu s phenol reagent (Merck) was used to measure the total polyphenolic index. Silica samples with different degree of surface silylation were obtained using gas-phase chemical modification of highly disperse silica (A-200) surface by trimethylchlorosilane.6... 

The bare vulcanized silicone pol)mier, a polydimethylsiloxane network, shows only a low mechanical strength. Adding 30 wt. % of fumed silica by mixing and dispersing increases the elongation and stress at break by a factor larger than 10 (Fig. 8). 

Fumed silica is a highly dispersed silicon dioxide of large industrial importance and a wide spectrum of applications. Due to its production in a flame process fumed silica exhibits a smooth and nonporous particle surface. Additionally to its high surface area fumed silica bears isolated and statistically distributed surface silanol groups that render this product hydrophilic. A most important technical reaction, therefore, is the silylation and hydrophobization of the hydrophilic surface. 

Fumed silica acts as a highly reinforcing filler in silicone elastomers. Its activity results fi-om its highly dispersed particle structure, high surface area and surface energy. To better understand the interplay of these properties first studies on gas adsorption of hexamethylsiloxane on hydrophilic and silylated silica have been conducted. The shape of the adsorption isotherm revels the existence of low- and high-energy adsorption sites, the latter qualitatively seem to be related to reinforcement of the silicone elastomer. Further quantitative studies in this field are needed. 

Interlayer dielectric (ILD) CMP typically uses a fumed silica slurry dispersed in an aqueous medium at a pH near 11 Fumed silica is a widely adapted abrasive for ILD CMP because of its inexpensive price, high purity, and colloidal stability. However, fumed silica is difficult to disperse in an aqueous system, and it is difficult to control powder processing because of the large specific surface area of 90 15 mVg, making it very reactive. ILD CMP slurry was prepared at pH 11 to accelerate the chemical attack on the deposited PETEOS film on the wafer surface. But silica particles dispersed in aqueous media are partially dissolved at pH 11. Consequently the removal rate decreased and microscratches were generated on the wafer surface due to agglomeration of silica particles as surface potentials decreased. ... 

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