Microlevel Processes

Compaction Microlevel Processes Compaction is a forming process controlled by mechanical properties of the feed in relationship to applied stresses and strains. Microlevel processes are controlled by particle properties such as friction, hardness, size, shape, surface energy, and elastic modulus (Fig. 21-86). The performance of compaction techniques is controlled by the ability of the particulate phase to uniformly transmit stress, and the relationship between applied stress and the compaction and strength characteristics of the final compacted particulate phase. 

These microlevel processes of compaction control the final flaw and density distribution throughout the compact, whether it is a roll... 

Compaction is a forming process controlled by mechanical properties of the feed in relationship to applied stresses and strains. Microlevel processes are controlled by particle properties such as friction, hardness, size, shape, surface energy, and elastic modulus. 

Figure 5 The microlevel processes of compressive agglomeration, or compaction. These processes combined control compact strength, hardness, and porosity.

These microlevel processes of compaction control the final flaw and density distribution throughout the compact, whether it is a roll pressed, extruded, or tab-leted product, and as such, control compact strength, hardness, and dissolution behavior. Compaction processes will not be discussed further here, with the remainder of the chapter focusing on wet granulation, agitative processes. For further discussion regarding compaction, see Chapter 6 and Refs. 1, 2, and 10. 

To achieve the goal of required performance, durability, and cost of plate materials, one approach is improvement of the control of the composition and microstructure of materials, particularly the composite, in the material designing and manufacturing process. For example, in the direction of development of thermoplastics-based composite plate, CEA (Le Ripault Center) and Atofina (Total Group) have jointly worked on an irmovative "microcomposite" material [33]. The small powders of the graphite platelet filler and the PVDF matrix were mixed homogeneously by the dispersion method. The filler and matrix had a certain ratio at the microlevel in the powder according to the optimized properties requirements. The microcomposite powders were thermocompressed into the composite plate. 

Strength, brittleness, and solvent permeability properties are limited because of lack of control of the ceramic composition on a macro- and microlevel. Even small particle sizes are large compared with the molecular level. There have been a number of attempts to produce uniform ceramic powders including the sol-gel synthesis in which processing involves a stable liquid medium, coprecipitation in which two or more ions are precipitated simultaneously. More recently, Carraher and Xu have used the thermal degradation of metal containing polymers to deposit metal atoms and oxides on a molecular level. 

We recall that our wave equation represents a long wave approximation to the behavior of a structured media (atomic lattice, periodically layered composite, bar of finite thickness), and does not contain information about the processes at small scales which are effectively homogenized out. When the model at the microlevel is nonlinear, one expects essential interaction between different scales which in turn complicates any universal homogenization procedure. In this case, the macro model is often formulated on the basis of some phenomenological constitutive hypotheses nonlinear elasticity with nonconvex energy is a theory of this type. 

It is interesting to note that calculations of turbulent flows during fast chemical reactions, predicted that the chemical reaction rate constant influences the effective diffusion coefficient and accelerates micromixing, due to an increase of the local reactant concentration gradients [13]. The dependence of the lower boundaries of the reaction front macrostructure formation, in particular, the plane and the torch front, which characterise different scales of liquid flow mixing, on the values of the chemical reaction constants is experimental evidence of the correlation between the kinetic and diffusive parameters of the process. At the same time, one can suppose that the formation of the characteristic reaction front macrostructures is defined by the mixing at the macro- and microlevels. 

Many models have been proposed (117) to explain the electrical conductivity of mixtures composed of conductive and insulating materials. Percolation concentration is the most interesting of all of these models. Several parameters, such as filler distribution, filler shape, filler/matrix interactions, and processing technique, can infiuence the percolation concentration. Among these models, the statistical percolation model (118) uses finite regular arrays of points and bonds (between the points) to estimate percolation concentration. The thermodynamic model (119) emphasizes the importance of interfacial interactions at the boimdary between individual filler particles and the polymeric host in the network formation. The most promising ones are the structure-oriented models, which explain condnctivity on the basis of factors determined from the microlevel stmctin-e of the as-produced mixtures (120). 

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