Ceramic powder processing drying

Dogan and Hausner [5.1] presented a survey of the applications of freeze drying in ceramic powder processing, the three main objectivesof which have been pursued ... 

Dogan, F., Hausner, H. The role of freeze-drying in ceramic powder processing. Ceram. Trans., 1 (Ceram. Powder Sci., PT.A), p. 127-134, 1988. Reprinted with permission of the American Ceramic Society, Post Office Box 6136, Westerville, Ohio 43086-6136. Copyright 1988 by The American Ceramic Society. All rights reserved. 

Reetz, T., Haase, I. The Influence of freezing process on the properties of freeze-dried powders. Ceram. Powder Process. Sci.,Proc. Int. Conf.,p. 641-648,1988. Edited by Hausner, H., Messing, G. L., Hirano S. 

Thermal decomposition also plays an important role in the conventional ceramic powder process, which makes use of starting materials such as metal carbonates that decompose during calcining, exposing reactive surfaces, which in turn aid in phase formation and chemical homogenization. Thermal decomposition is an important final step in solution preparation techniques. Precipitated hydroxides, carbonates, or oxalates are decomposed to oxides. Freeze-drying yields dried salts such as sulfates, which must be thermally decomposed to oxides. Similarly, many solution techniques use nitrates, which are decomposed as part of the process or in a separate step. 

F Dogan and H Hausner. The Role of Freeze-Drying in Ceramic Powder Processing Ceramic Transactions, Vol. 1 (Ceramic Powder Science II) (GL Messing, ER Fuller Jr., and H Hausner, eds.), Westerville, OH American Ceramic Society, 1988, pp. 127-134. 

Forming additives or processing aids (2,33—37) are commonly used to render ceramic powders more processible. Binders and plasticizers (qv) are typically added to improve or aid dry powder and plastic forming, whereas deflocculants, surfactants (qv), and antifoams are commonly used in slurry processing. 

Tape can be cast on a stainless steel table or belt, glass plate, or a Mylar, Teflon, or ceUulose acetate film carrier. The tape should adhere to the carrier sufficiently to prevent curling, but should be easily removable. In a continuous casting process, the tape is dried by air flowing 1—2 m/min counter to the casting direction. A typical dry green tape contains approximately 35 vol % organics, 50% ceramic powder, and 15% porosity. 

Fosbel [Foseco Belret] A ceramic welding process for repairing refractories. A dry mixture of a refractory and a metallic powder is projected, in a stream of oxygen, onto the surface to be repaired. Oxidation of the metal produces the necessary high temperature. Developed by Foseco, UK, and Belret, Belgium, hence the name. 

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. 

In the wet process a slip carrying the ceramic powder is laid down, by screenprinting for example, onto a temporary carrier such as a glass tile. The process can be repeated to build up the required thickness of the dielectric onto which the electrodes are screen-printed. The next dielectric layer is then laid down and the process repeated. The mutlilayer structure is diced as described above, and the individual chips removed from the tile for the subsequent stages, as for the dry process. 

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