Ceramic suspensions concentrated polymer solutions

This section on concentrated suspensions discusses the rheological behavior of sj tems which are colloidally stable and colloidally unstable suspensions. For stable sj tems, the rheology of sterically stabilized and electrostatically stabilized systems wiU be considered. For sterically stabilized suspensions, a hard sphere (or hard particle) model has been successfid. Concentrated suspensions in some cases behave rheologically like concentrated polymer solutions. For this reason, a discussion of the viscosity of concentrated polymer solutions is discussed next before a discussion of concentrated ceramic suspensions. 

Oc is the geometric constant for the shape of the ceramic particles and [1 + (Lg/a f] is the volume fraction correction for the adsorbed layer on the ceramic particles. Again, this equation is only good for colloidally stable suspensions. Fleer et al. [19] verified this equation for cubic particles with polytvinyl alcohol) adsorbed at the surface. For polymer solution concentrations (i.e., p) that give essentially monolayer coverage of the particle surface, the value of [1 + (LJaf] is nearly constant for a wide range of ceramic powder concentrations (Le., d>c)-... 

Given the vast number of possible matrix-reinforcement combinations in composites and the relative inability of current theories to describe the viscosity of even the most compositionally simple suspensions and solutions, it is fruitless to attempt to describe the momentum transport properties of composite precursors in a general manner. There are, however, two topics that can be addressed here in an introductory fashion flow properties of matrix/reinforcement mixtures and flow of matrix precursor materials through the reinforcement. In both cases, we will concentrate on the flow of molten polymeric materials or precursors, since the vast majority of high-performance composites are polymer-based. Fnrthermore, the principles here are general, and they apply to the flnid-based processing of most metal-, ceramic-, and polymer-matrix composites. 

A schematic of the stoeoUthography technique is shown in Fig. 6.50. A laser beam is scanned on the surface of a highly concentrated suspension of ceramic particles in a monoma- solution, and the monoma- is cured to form a polymer layer that binds the particles together. When the first layCT is completed, the support platform is lowered by a depth equal to the laya- thickness, and the suspension flows ovct the polymerized layer. The lasCT is scanned over the new... 

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