Spherical green body, drying

TABLE 14.1 Spherical Green Body Drying Kinetics by Mass... 

In this section, we will discuss in detail the linked differential equations for mass and heat transfer which describe the drying of a spherical green body. This same analysis can also be used for plate and cylinder green bodies with corrections for the geometry. The equations for cylinder and plate drying are presented in Tables 14.3 and 14.4. 

Using these equations for heat conduction in the porous network emd the pseudo-steady state emalysis described for pore diffusion, the time to dry a spherical green body with pore heat conduction as the rate determining step is given by... 

Determine the total time to dry a 1.0 cm spherical green body with void fraction 40% formed from an aqueous Z1O2 paste with polymer at... 

TABLE 14.2 Evaporative Drying of Spherical Green Body, by Heat Transfer Controlling Steps... 

Determine the time to dry in 10% relative humidity air at 100°C a spherical green body of radius 10 cm with a void fi action of 0.4 filled with H2O. Assume that boundary layer mass or heat transfer is the rate limiting step. The temperature of the surface is the wet bulb temperature. This time is the maximum time for the constant rate period. Data mean particle size = 1.0 /am mean pore diameter = 0.1 /am. See Problem 14.1 for drying data on water and air. 

In the fabrication route used by Barringer and Bowen (89), powders with controlled characteristics are prepared and consolidated, by colloidal methods, into uniformly packed green bodies (e.g., similar to that shown in Fig. 1.17). An abbreviated flow diagram of the fabrication route is shown in Fig. 1.27b. Powders are prepared by controlled hydrolysis of titanium tetraisopropoxide (denoted powder I) or titanium tetraethoxide (powder II). The powder particles are nearly spherical and almost monosized. After washing, each powder is dispersed in a basic aqueous solution to produce a stable suspension. Consolidation of the suspension is accomplished by gravitational or centrifugal settling. The consolidated body is dried in a vacuum and sintered for —90 min at 800°C (for powder... 

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