Aggregation, colloidal silicas

Preparation of Silica Solid Microspheres by Hydrolysis of Tetraethyl Ortho Silicate (TEOS) and Silica Porous Microspheres by Spray Drying Aggregated Colloidal Silica... 

Figure 19.13 shown the mesopore size distribution obtained nitrogen absorption-desorption of an aggregated colloidal silica slurry. 

Silica sols are often called colloidal silicas, although other amorphous forms also exhibit colloidal properties owing to high surface areas. Sols are stable dispersions of amorphous siUca particles in a Hquid, almost always water. Commercial products contain siUca particles having diameters of about 3—100 nm, specific surface areas of 50—270 m /g, and siUca contents of 15—50 wt %. These contain small (<1 wt%) amounts of stabilizers, most commonly sodium ions. The discrete particles are prevented from aggregating by mutually repulsive negative charges. 

Amorphous Silica The term amorphous silica refers to aggregate of smaU particles with high specific surface area. They lack crystal structure and do not form a sharp x-ray diffraction pattern. They are known in several forms such as colloidal silica, precipitated silica, silica gels, and fumed sdica. The surface of such amorphous silica may contain silanol (SiOH) groups or can be anhydrous. 

There are many similarities between oxide CMP and poly-Si CMP. The main difference between the two processes is that poly-Si CMP slurries contain less abrasives and are, in general, more chemically active. Therefore, the poly-Si CMP process is by nature very sensitive to the polishing temperature. Temperature has a direct effect on removal rate, topography removal, and defect density (pitting and voids). Most poly-Si CMP slurries use colloidal silica that is less likely to form large aggregates than the fumed silica. 

This article presents the research on amorphous colloidal silica aggregation. 

Kobayashi, M. et al., Aggregation and charging of colloidal silica particles Effect of particle size, Langmuir, 21, 5761, 2005. 

Findlay, A.D., Thompson. D.W., and Tipping, E., The aggregation of silica and haematite particles dispersed in natural water samples. Colloids Surf. A, 118, 97, 1996. 

Phan, T.N.T., et al., Adsoption of zinc on colloidal silica, triple layer modelization and aggregation data. Colloids Surf. A, 244, 131, 2004. 

Silica sols lose their stability by aggregation of the colloidal particles. Colloidal silica particles can be linked together or aggregate by gelation, coagulation or flocculation, or coacervation. 

The behavior of colloidal silica is based on its morphology and structure and on the chemistry of its surface. To fully characterize colloidal silicas one must pleasure not only the particle size, particle size distribution, surface area, and degree of aggregation, but also the structural and surface properties. 

Although field-flow fractionation was first developed in the 1960s (II), the first major study of colloidal silica by FFF was not reported until 1978 (4). At that time it was shown that the subtechnique of flow FFF could be used to fractionate colloidal silica down to a particle size of 0.01 pm (see Figure 2). The fractionation was verified by electron microscopy. Size distribution curves were obtained under different experimental conditions and shown to be consistent with one another. The effects of particle aggregation were examined. 

Ludox Colloidal Silica. Ludox colloidal silicas (DuPont) were among the first silica materials studied by FFF. In the first paper describing the applicability of FFF to colloidal silica (4), Ludox and related silicas were fractionated by flow FFF the fractionation was verified by transmission electron microscopy (TEM). The theory behind the fractionation and the acquisition of size distribution data was developed, and evidence of aggregation was examined. An example of the fractionation of four colloidal silicas from that study is shown in Figure 2. The primary drawback of this earlier work was the lengthy runs, in some cases requiring over 10 h. Most of the experimental runs on Ludox described in this section were completed in less than 10 min. 

Frolov, Shabanova, and co-workers (37-39) studied the transition of a sol into a gel and the aggregate stability of colloidal silica. Their aim was to develop a technology for the production of highly-concentrated silica sols and to use them as binders, catalyst supports, polymer fillers, adsorbents, and so forth. Kinetic studies were made of polycondensation and gel formation in aqueous solutions of silicic acids. At the stage of particle growth, poly condensation proceeds in the diffusion-kinetic region. With changes in pH, temperature, concentration, and the nature of electrolytes,... 

Figure 5.16 Combined small-angle light and x-ray scattering results for a colloidal aggregate of silica particles. (From Schaefer et al.31)...

Figure 5.23. Light scattering data from aggregates of colloidal silica demonstrate convergence to power-law scattering behaviort J.

IR spectra of suspensions of amorphous and crystalline colloidal silica aggregates were obtained by Mintova et al. [868]. These spectra revealed the presence of small (nano-sized) siHcate units possibly containing D5R subunits. A complete transformation to crystalline particles of MEL-type zeoHte was observed after an additional hydrothermal treatment of the aged precursor suspensions at 363 K for 68 h. 

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