Powder process

Initially in ceramic powder processing, particle surfaces are created tliat increase tlie surface energy of tlie system. During shape fomiing, surface/interface energy and interiiarticle forces are controlled witli surface active additives. 

Ultimately, the surface energy is used to produce a cohesive body during sintering. As such, surface energy, which is also referred to as surface tension, y, is obviously very important in ceramic powder processing. Surface tension causes liquids to fonn spherical drops, and allows solids to preferentially adsorb atoms to lower tire free energy of tire system. Also, surface tension creates pressure differences and chemical potential differences across curved surfaces tlrat cause matter to move. 

Fabrication technologies for ah electronic ceramic materials have the same basic process steps, regardless of the appHcation powder preparation, powder processing, green forming, and densiftcation. 

A fundamental requirement in powder processing is characterization of the as-received powders (10—12). Many powder suppHers provide information on tap and pour densities, particle size distributions, specific surface areas, and chemical analyses. Characterization data provided by suppHers should be checked and further augmented where possible with in-house characterization. Uniaxial characterization compaction behavior, in particular, is easily measured and provides data on the nature of the agglomerates in a powder (13,14). 

S. Natansohn and V. Sarin in H. Hausner, G. Messing, and S. Hirano, eds.. Ceramic Powder Processing Science, Deutsche Keramische GeseUschaft e.V. Knfn, Germany, 1989, p. 433. 

D. R. Mouta and co-workeis, Mayards Analysis of the Final Design of the Improved Black Powder Process, Vols. 1—2, Rpt. J6329, Illinois Institute of Technology, Chicago, 1963. 

Glass-ceramic articles can be fabricated by means of either bulk or powder processing methods. Both methods begin with melting a glass of the desired composition. 

Preceramic polymer precursors (45,68) can be used to make ceramic composites from polymer ceramic mixtures that transform to the desired material when heated. Preceramic polymers have been used to produce oxide ceramics and are of considerable interest in nonoxide ceramic powder processing. Low ceramic yields and incomplete burnout currently limit the use of preceramic polymers in ceramics processing. 

The dry powder process has several additional advantages over the wet process. For example, much less waste of enamel occurs because the dry over-spray is airborne and recycled in a closed system. No-pidde ground coats have broadened the apphcation of both wet-process and dry-process systems. These enamels are appHed over cleaned-only metal. Thus the problems of disposing of pickling acid wastes containing iron sulfates and nickel wastes are eliminated (see Metal surface treatments) (7). 

Yates, B., 1981. Systems engineering in powder processing. In POWTECH HI, Institution of Chemical Engineers. Symposium Series No. 63, Rugby Institution of Chemical Engineers, D4/Z/1-D4/Z/16. 

In all metal spraying processes the particles emerge from the nozzle in a conical stream, and although the particles near the centre are molten, those at the periphery have solidified. In the powder process there are in addition solid particles which have not melted. The solid particles tend to become entrapped in the coating, making it porous. The effect is more pronounced in the powder process owing to the larger number of solid particles present. 

Williams, B. E., Stiglich, J., and Kaplan, R, CVD Coated Powder Composites, Part I Powder Processing and Characterization, Ultramet, in Proc. IMS Ann. Meeting, New Orleans, LA (Feb. 1991)... 

Enomoto N, Akagi JI, Z-l N (1996) Sonochemical powder processing of iron hydroxides. Ultrason Sonochem 3 S97-S103... 

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