DETERMINATION OF GAS-TRANSPORT COEFFICIENTS IN CERAMIC BODIES DURING THERMOLYSIS OF ORGANIC ADDITIVES

In many ceramic processing routes organic additions are made which are later removed from the particle assembly before firing. These include residues from the manufacture of chemically derived powders, pressing aids or lubricants used in compaction processes, dispersants and bind ers added to casting suspensions, and organic vehicles which convey ce ramic particles in plastic forming processes such as injection mouldin g. In many cases, large organic molecules are required to move in the pore structure of ultrafine powder assemblies so that a range of flow regimes, e.g. Knudsen, intermediate, molecular, and viscous, have to b e considered. The quantitative evaluation of gas flow in such systems is used to evaluate the pressure that develops during ceramic processi ng and the viability of heating schedules. This review describes the p rocedures that can be used for the determination of transport coeffici ents in such systems and highlights the ways in which simplification c an be achieved by judicious approximation. The effects of molecular di mensions and pore size are explored for typical ceramic processing sit uations. (C) 1996 The Institute of Materials and ASM International.