Flow Stresses during Drying

These equations assume that the green body is isotropic (i.e., = 

Using an analogy [18] with the thermal stress, liquid flow stress can be determined from a replacement of 

The liquid expansion coefficient, a, for the thermal expansion coefficient, a-p 

Thermal difiiisivity, a, with the liquid flow difiusivity, Df. 

For an infinite plate of thickness 2xq, the normal stress a-(x) at a position X in the green body depends on the volume fraction difference between that point, / , and the average value, / , in the green bocty. This gives the strain at that point and fixes the net local stress at [28] 

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Consequently due to the flow limit of the ceramic body - the stresses resulting from the shaping process cannot be completely relieved. The same applies to subsequent relaxation of the body some amount of residual pressure will always remain behind. This phenomenon is also called the ceramic memory of the extrusion body. Nevertheless, it is very likely that the induced stresses will ease later on during drying and firing. 

The volume shrinkage of the gel during drying induces an increase of its stiffness. At a given time, the solid network is no more compliant and the meniscus recedes in the pores. At this moment, the stress is maximum since the curvature radius corresponds to that of the shrunk pore (assumed cylindrical). Associated to evaporation, the liquid flows from the core of e gel to the surface. This flow is hindered by the solid arms of the gel. A gel is badly permeable because the size of the pores lies mainly within the range 0.2-10 nm indicating that a gel is a mesoporous material. According to the Darcy s law, the liquid flow, /, is related to permeability, D, by the relation ... 

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