Grain, shrinkage

FIGURE 24.15 Coalescence of three TJs due to grain shrinkage and associated increase of material in the pocket. 

The properties of the transition zone vary with time due to hydration of cement grains, shrinkage and eventual corrosion processes. 

Because densification occurs via tire shrinkage of tliennodynamically unstable pores, densification and microstmcture development can be assessed on tire basis of tire dihedral angle, 0, fonned as a result of tire surface energy balance between tire two solid-vapour and one solid-solid interface at tire pore-grain boundary intersection [, 78, 79 and 80],... 

Com symps used in ice cream and fro2en desserts are generally 36- or 42-DE acid-converted symps. The symp serves primarily to provide maximum flexibiUty in adjusting flavor, texture, body, and smoothness. It also aids in grain control and in the modification of meltdown and shrinkage characteristics of the fro2en product. 

Clays composed of mixtures of clay minerals having from 20—50% of unsorted fine-grain nonclay materials are most satisfactory. Large amounts of iron, alkaHes, and alkaline earths, either in the clay minerals or as other constituents, cause too much shrinkage and greatiy reduce the vitrification range thus, a clay with a substantial amount of calcareous material is not desirable. Face bricks, which are of superior quaHty, are made from similar materials but it is even more desirable to avoid these detrimental components (see Building materials, survey). 

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. 

The first term in brackets is the rate at which grains of area A and sides N accumulate in area space due to the growth or shrinkage of grains as described by Eq. 15.33. The second is the corresponding rate of accumulation of such grains due... 

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