Pressing ceramics, powdered

The thimble element is clamped between housing and ceramic bushings. To provide a potential-free signal the measuring electrode needs to be electrically insulated from the housing by an insulation layer. The planar element is held by a pressed ceramic powder, which also does the insulation. 

Uniaxial pressing is the method most widely used to impart shape to ceramic powders (24). Binders, lubricants, and other additives are often incorporated into ceramic powders prior to pressing to provide strength and assist in particle compaction (25). Simple geometries such as rectangular... 

Binder selection depends on the ceramic powder, the size of the part, how it is formed, and the green density and strength requited. Binder concentration is deterrnined by these variables and the particle size, size distribution, and surface area of the ceramic powder. Three percent binder, based on dry weight, generally works for dry pressing and extmsion. 

When you squeeze snow to make a snowball, you are hot-pressing a ceramic. Hot-pressing of powders is one of several standard sintering methods used to form ceramics which require methods appropriate to their special properties. 

Early tests [37] utilized a cell design similar to that of early MCFC experiments. The assembled cell, machined from graphite blocks, is shown as Fig. 24. The electrodes and current collectors were machined from graphite and dense carbon, respectively. The electrolyte was a mixture of 63% Na2S, 37% Li2S, believed to melt near 850 °C the melting point after several days of operation was below 700 °C, probably because of polysulfide formation. The electrolyte was immobilized in a matrix of MgO, the whole formed by hot-pressing a mixture of electrolyte and ceramic powders. 

The powder-forming processes are similar in many ways to those nsed for powder metallurgy described in the previons section. For example, pressing is a common method for processing ceramics however, ceramic powders can be pressed in either dry or wet form. In wet form, they can also be extended, just like metals, and cast in a variety of process variations. The nominal forming pressures and shear rates associated with some of these processing methods are snmmarized in Table 7.3. Yon may want to refer back to this table when each of the varions processing techniques is described in more detail. 

Considering a mass of ceramic powder about to be molded or pressed into shape, the forces necessary and the speeds possible are determined by mechanical properties of the diy powder, paste, or suspension. For any material, the elastic moduli for tension (Young s modulus), shear, and bulk compression are the mechanical properties of interest. These mechanical properties are schematically shown in Figure 12.1 with their defining equations. These moduli are mechanical characteristics of elastic materials in general and are applicable at relatively low applied forces for ceramic powders. At higher applied forces, nonlinear behavior results, comprising the flow of the ceramic powder particles over one another, plastic deformation of the particles, and rupture of... 

The pressing of dry ceramic powders follows the sequence shown schematically in Figure 12.33 ... 

FIGURE 12.34 Schematic of snap-throu buckle of a particle arch during pressing of a ceramic powder. The force exerted by the particles on the wall in the vertical direction is a result of the coefficient of pressure at rest. 

Data from D. Bortzmeyer, Dry Pressing of Ceramic Powders, NKV Summer School, September 1991, Pfetten, The Netherlands. 

The Kawakita compaction equation is another equation which is often used for ceramic powder pressing. It can be derived by considering that compaction is similar to packing by tapping, where the compaction pressure, P, is directly substituted for the number of taps, N, in the analysis in Section 13.5.1. The Kawakita equation is a special case, where the value of m in the Weibul distribution function for tapping is 1. In the Kawakita equation, the compaction, C, defined as the relative reduction in volume is given by [72]... 

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