Agglomeration silica fume

Fig. 7.85 Schematic of a fluidized bed agglomerator for dry silica fume (courtesy Norchem Concrete Products, Fort Pierce, FL, USA).

The level of ultrasound that is necessary to disperse aggregated particles in a liquid can also be used as a measure of agglomerate strength of, for example, granulated silica fume in a somewhat modified test arrangement [12.3]. 

Some examples will be presented in more detail. They are selected such that, with the exception of natural agglomeration (used, for example, for the densification of silica fume. Section 6.7.3), at least one type of the available agglomeration processes is demonstrated. 

In many geographical areas the humidity is very low during winter. Since the gas used for the fluidization of silica fume (Section 6.7.1, Fig. 6.7-9) is ambient air, little, if any, adsorption of moisture takes place. This, in turn, reduces the enhancement of bonding due to adsorption layers, which takes place during other seasons of the year and results in little or no agglomeration. 

The use of the agglomeration factor allows us to give a number to a particular treatment that aims at reducing agglomeration or aggregation (see the silica fume example in Section 10.2.4.6). 

Measuring the PSD of silica fume can be challenging due to its extreme fineness and high SSA (BET = 21.36 mVg). Such fine powders can agglomerate and strong forces are needed to break them apart. Figure 10.12 shows the PSD evaluation of silica fume using different dispersants, ultrasonic... 

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. 

By Eq. 2 and D = 2.5 from DLA we estimate the apparent density of the aggregate to be 700 g 1 . This is the density found experimentally, when powdered fumed silica is pressed to a solid disc. > = 2.1 from RLCA then gives an agglomerate density of about 20 g 1 - close to the bulk density of the freshly produced fluffy product. 

The tensile strength of cross-linked polysiloxane elastomers is low, but can be markedly improved by reinforcement with a filler. The material of choice is fumed silica with high surface area, which can increase the strength by a factor of 20. It is thought that the silica particles agglomerate to form a three-dimensional network within the siloxane, greatly reinforcing the stmcture. 

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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