Agglomeration of silica

One can distinguish the following steps in the agglomeration of silica dispersions with hydrolyzed aluminum (1) hydrolysis and multimerization of Al(III) to isopolycations (2) diffusion of these aluminum hydroxo complexes to the colloid surface and adsorption and (3) transport of suspended particles and collision resulting in certain instances in attachment of colloids. The transport of colloids to each other has been observed to proceed more slowly than all other steps under the given circumstances (3). It follows that the rate of agglomeration, — dN/dt, is obtained from the collision frequency, as determined solely by the trans-... 

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. ... 

Ergudenler A. and Ghaly A. E., Agglomeration of silica sand in a fluidised bed gasifier operating on wheat straw . Biomass and Bioenergy, 4(1993) 135-147. 

Aggregates and agglomerates of silica nanoparticles of A-400 are characterized by the textural porosity with a broad PSD (Figure 1.33). 

In the batch process there is formation of a single gel network following a fast percolation like mechanism which concerns the whole reactor content. The stirring breaks the gel in pieces keeping the original gel framework. In the semi-continuous process the gel network building is slow. It would result from the disordered agglomeration of silica particles on numerous gel embryos dispersed in the reactor. 

Figure 5. SEM micrograph of web-like agglomeration of silica nanoparticles.

The surface morphology of the sulfonated polysulfone membranes was investigated by scanning electron microscopy (SEM), as shown in Fig. 2.9. The composite membrane shows a relatively uniform distribution of silica particles in the polymer matrix (see Fig. 2.9c and d). However, agglomeration of silica particles with dimensions in the range of 20-50 nm is also observed. 

Figure 11.21 NMR images of a silicone rubber filled with silica obtained through two differing spatially resolved H echo techniques (a) is a spin-echo image and (b) is a gradient echo-derived image. Using both methods, agglomerations of silica filler particles (darker areas) are clearly visible. Reproduced with permission from [16] Copyright (1997) American Chemical Society.

The term polyethylene separator is somewhat misleading, since this separator consists mainly of agglomerates of precipitated silica, held within a network of extremely long-chained, ultrahigh-molecular weight polyethylene molecules. The raw materials, precipitated silica (Si02 — about 60 percent), ultrahigh-... 

The surface of silica is covered by a layer of acidic silanol and siloxane groups. This highly polar and hydrophilic character of the filler surface results in a low compatibihty with the rather apolar polymer. Besides, highly attractive forces between silica particles result in strong agglomeration forces. The formation of a hydrophobic shell around the silica particle by the sUica-sUane reaction prevents the formation of a filler-filler network by reduction of the specific surface energy [3]. 

In general, the rule of thumb is to add the diluting component to the abrasive component. There are, however, exceptions to this rule of thumb, so experience and the recommendations of the slurry manufacturer should be considered. In the case of silica-abrasive oxide-polishing slurries, this means slurry first, water second. Even when practiced in this manner, slurry diluted at the user site will typically have more agglomerates than slurry diluted to use-concentration by the slurry manufacturer. The primary reason... 

Fig. 7. Illustration of agglomeration mechanism of silica (after Hayashi et ai. Ref. [6]).

Silica occurs widely in Nature as quartz, often in large transparent crystals of characteristic shape but also in the translucent agglomerations of microscopic crystals known as chalcedony, which includes cherts and flint. Other natural crystalline varieties of Si02 include tridymite and cristobalite (opal is a semiprecious stone that consists of microcrystalline, hydrous cristobalite). All forms of silica involve three-dimensional networks of corner-linked Si04 tetrahedra. 

The consequence is an inhomogeneous distribution of additives in the individual polymer phases and thermodynamic nonequilibrium after mixing. The latter effect may cause demixing and re agglomeration of additives. A well-known example in this regard is the flocculation or reagglomeration of silica in S-SBR compounds [3]. 

For silica, the plasma-pyrrole coating clearly improves the compatibihty of the silica in the polymer blend. In contrast to this, the plasma-acetylene and plasma-thiophene treatments cause a high filler-filler interaction. This can be explained by the differences in compatibility between the plasma coatings and the polymers. In the case of PA-silica, the coating results in a higher compatibility towards EPDM in the blend, which could lead to an overconcentration of silica in the EPDM phase, with increased agglomeration of the filler. 

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