Pore shrinkage

During pressure sintering, interiDarticle compressive stress, approximated by the externally applied stress and nonnalized by the relative density of the compact p, supplements the surface tension driving force for pore shrinkage ... 

The atmosphere is also important in sintering. Gas trapped in closed pores will limit pore shrinkage unless the gas is soluble in the grain boundary and can diffuse from the pore. Alumina doped with MgO can be sintered to essentially zero porosity in hydrogen or oxygen atmospheres, which are soluble, but not in air, which contains insoluble nitrogen. The density of oxides sintered in air is commonly less than 98% and often only 92-96%. The sintering atmosphere is also important in that it may influence the sublimation or the stoichiometry of the principal particles or dopants. 

Since pore shrinkage is driven by a decrease in interfacial energy, it may be expected on general principles that the capillary force, Y, must correspond to... 

Differential pore shrinkage and grain growth in heterogeneous material)... 

Figure 10.16 Effect of pore coordination on pore shrinkage for a system for which the dihedral angle o = tt/2. (a) Intersection of four grains around a pore, (h) Intersection of six grains that is, = 120 Note that to maintain the equilibrium dihedral angle, the surfaces of the grains surrounding the pore have to be convex, (c) Same system with = 60. Here the pore surface has to be concave in order to maintain the equilibrium dihedral angle.

Mukai and co-workers [105] used Keggin-type heteropolyacids immobilized in the network structure of resorcinol-formaldehyde carbon gels as catalysts for the synthesis of methyl tert-butyl ether from methyl alcohol and tert-butyl alcohol. Large amounts of 12-tungstophosphoric and 12-molybdophosphoric acids were immobilized into the support by two methods, pore shrinkage and the ship-in-the-bottle method, which are essentially impregnation methods. The authors reported that these catalysts showed activity in the reaction studied and could be of practical utility as solid acid catalysts in various reactions. 

De Jonghe, L. C. Rahaman, M. L. (1984). Pore shrinkage and sintering stress. Journal of the American Ceramic Society, 67, C-214-15. 

Ti02 (anatase) Pore shrinkage 20 % (SEM) Crystallite size 6-12 nm (TEM) Photocatalyst Ti02 (anatase) Solar cell... 

Since pore shrinkage oceurs at all of the 14 surfaces of the grain, the rate of pore volume change dVp/dt is expressed as... 

Figure 5.4. Schematic of pore shrinkage during sintering ° (a) just before the isoiation of the pore, (b) shrinkage stage, and (c) finai state.

EFFECT OF GRAIN BOUNDARY ENERGY ON PORE SHRINKAGE... 

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