Preformed particle gel

Preformed particle gels have been applied in about 2000 wells in China to reduce fluid channels in waterfloods and polymer floods (Liu et al 2006c). PPG treatment has been widely accepted and is seeing more and more use by operators because of its unique advantages over traditional in situ gel including... 

Controlled phase transformation of preformed particles can also result in enhanced size control and increased monodispersity [812], The sol-gel-phase transformation, known as the sol-gel process [809], is the best known example of... 

Modern Manufacturing Techniques. Manufacturing techniques for making bulk vitreous silica are for the most part improved variations of the historical processes. The main exception is the sol—gel process (see Sol-gel technology). All processes involve the fusion or viscous sintering of silica particles. The particles can be in the form of a loose powder or a porous preform. The powders can be made from natural quartz or from the decomposition of chemical precursors, such as silicon tetrachloride, and tetraethylorthosilicate (1 EOS). In some approaches, such as flame hydrolysis, the powder is produced and fused in a single step. The improvements made to these techniques deal mainly with the procedures used to prepare the powders, that is, to control purity and particle size, and the specific conditions under which the powders are consolidated. 

Using preformed sols instead of metal alkoxides as precursors is an attractive alternative in sol-gel preparation because recent advances in inorganic colloidal dispersions allow some control over the characteristics of the starting sols [11]. Often a colloidal suspension of sol particles is stabilized (i.e. prevented from flocculation) by pH adjustment. Thus, pH of the solution, which can be changed by the addition of either acid or base, is the single most important parameter in obtaining a gel from preformed sols. Other parameters that can impact on gel quality are the size and concentration of the starting sol particles. 

Using preformed sols instead of metal alkoxides as precursors is another effective way to change the pore structure, as recently demonstrated for the sol-gel preparation of zirconia [38], The two key parameters in this approach are the starting sol size and the intrinsic porosity of the sol particles. 

Finally, by using preformed sols instead of metal alkoxidcs as precursors, it is possible to use the sol particles as building blocks and form a gel around them, again in a single step. As discussed in Section 2.1.4.3.A, this approach offers control over the pore structure and the distribution of active components. For example, the use of a nonporous sol particle effectively prevents incorporation of other species within the particle itself. 

The electrophoretic sol-gel template method could overcome the pore size limitation to certain extent (down to a few tens of nanometers), but it is still limited by the size of the sol particles which were preformed prior to being subjected to the electric field. To address this problem, Miao et al. (2002) reported an electrochemical sol-gel template method in which the sol particles were generated within the pores of the AAO template membrane, as shown in Fig. 18.10. 

The process of macroscopic gelation that is sensed as a change in the rheological properties (a rapid increase in the steady-state shear viscosity) then involves the aggregation and crosslinking between these preformed gel particles to form an infinite network. 

The third type is a hybrid of the first two with poly-silicic acid chains joining preformed colloidal silica particles. Hybrid gels are formed by neutralization of mixtures of silicates and colloidal silica sols. 

Core-shell particles have attracted much research attention in recent years because of their great potential in the protection, modification, and functionalization of the core with suitable shell materials to achieve specific physical or chemical performances. For instance, the optical, electrical, thermal, mechanical, magnetic, and catalytic properties of core particles can be finely tuned by coating them with a thin mineral shell [73, 74]. Silica shells are produced by a variety of methods that can be divided into two groups (1) the layer-by-layer self-assembly of preformed silica nanoparticles on oppositely charged templates, and (2) seeded polycondensation techniques involving sol-gel precursors. The former method is outside the scope of this article and only the second method will be discussed. 

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