Green body uniformity

In all these isostatic pressing methods, the pressure is applied uniformly to the surface of the green body because the rubber mold deforms to follow the compaction of the powder. In addition there is little or no wall friction between the powder and the rubber mold. As a result, the force balance given by A t = 0 for (ylindrical coordinates gives, for the radial component. 

To assess the uniformity of the microstructure in a green body, the variance, is used [91]. 

Scherer [32] has determined that the capillary pressure in the pores of a viscous gel network is not uniform during drying in the constant rate period. This capillary pressure distribution is emalogous to the solids volume fraction distribution in particulate green bodies, which is not uniform during the constant rate period. 

SEM photographs of the surfaces of the sintered samples are shown in Fig. 4 (A). The surface microstructure reveals uniform and fine grain growth about 2-3 pm. No pores were observed on the surface of the sample, but there were some pores from the fracture surface of the sample, as shown in Fig. 4 (C). The sintered density is 6.6 g/cm, which is over 95% of the theoretical value. The residual pores may be partly attributed to the agglomerates in the source powders that lowered the sinterability of the green bodies. It can also be seen that there are no great changes between the pellets sintered in air before and after heat treatment at 750°C for 5h in H2. This result is in aecordance with that of XRD. It confirms that the samples are chemically stable in H2 atmosphere at least below 750°C. 

Today, the best method of overcoming the problem of uneven densities in a green body is isostatic pressing. In this compaction process, the pressure is applied by a fluid that is pressurized and acts uniformly from all sides on dry particulate solids that are enclosed in a flexible container ( mold ) immersed in the fluid [B.13a]. This results in the most uniform consolidation possible. Of course, there is still a density gradient across the part, the center is always less compacted than the surface, but the gradient is uniform and, typically, does not cause distortion during sintering. 

Mechanical consolidation with dies or molds is still the most widely used green body forming technology, especially in ceramic industries. Because of the particle-wall of die and particle-particle frictions, pressure cannot be transmitted uniformly... 

Surface area of a powder increases geometrically with decreasing particle size, so that the volume fraction of the outermost layer of ions on the surface increase significantly, which has a significant effect on properties of the powder. With the development of nanotechnology, it is readily to synthesize powders with nanosized particles (1-100 nm). Therefore, characterization of surface properties becomes more and more important. Specifically for ceramics or transparent ceramics, the consolidation of fine ceramic powders with liquid suspensions to produce more uniform green bodies has been shown to play an important role in the fabrication ceramics, especially when special or complex structures are required. Because the quality of microstructure of the consolidated body is determined by the dispersion behavior of the powder and the interaction between the particles in the suspension, which is closely related to the surface properties of the particles, controlling the physical and chemical properties of particles is a critical to ceramics fabrication. 

Various additives, generally with very low concentrations, have been used to control the characteristics of the feed materials, in order to achieve desired shapes and control the packing uniformity of the green body of ceramics. For some methods, such as tape casting and injection molding, the use of suitable additives is especially important. The additives can be either organic or inorganic materials. 

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