Densification of powders

Parrott, E.L. Densification of powders by concavo-convex roller compactor. J. Pharm. Sci. 1981, 70 (30), 288-291. 

Because of their strong chemical bonds, bulk ceramics are most efficiently fabricated by means of densification of powders. The fabrication process involves two main stages (1) consolidation of the powder to form a porous, shaped article (the green body), also referred to as forming, and (2) heating of the shaped powder form to produce a dense article, referred to as firing or sintering. The final product commonly consists of a relatively dense polycrystal with some residual porosity (Fig. 1). The microstructure, which... 

B.8 W. Herrmann, Das Verdichten von Pulvem zwischen zwei Walzen (The densification of powders between two rollers), Verlag Chemie GmbH, Weinheim, FR Germany (1973). 

Das Verdichten von Pulvern zwischen zwei Walzen (The densification of powders between two rollers)... 

The preparational route for these luminescent ceramics follows the known ceramic technologies [12,13]. They are generated by thermally induced diffusion densification of powder aggregates below their melting temperature, which is referred to as sintering. As a consequence of this preparational route the properties of such ceramic scintillators are not only determined by the host lattice and the dopants (like the luminescent activator), but additionally by the processing technique. 

SLS processes are more complex than most other RP-technologies. Because of its pressureless characteristic, the only driving force for densification of powder is a reduction in total surface area [96]. So there is a high influence of the initial density of the powder layer on the resulting part density and contour accuracy depending on the properties of the used powder materials, for example grain size distribution. Therefore, surface properties are not quite as good as with SLA. 

Rybakov KI, Semenov VE (1996) Densification of powder materials in non-uniform temperature fields. Philos Mag A-Phys Condens Matter Struct Defects Mech Prop 73 295-307... 

The final densification of powder compacts is strongly affected by the sintering atmosphere because the atmospheric gas is entrapped within pores as they are isolated. In the case of a fast diffusing gas, full densification is possible, but this is impossible in a slowly diffusing or inert gas unless a high external pressure is applied. 

Celik [1] has reviewed the literature concerning the densification of powders during compaction. Here only some of the highlights are discussed. 

At least 19 different equations have been discussed in the literature in order to explain the densification of powders under compaction. At present, no universally accepted equation exists although some of the discussed equations are more popular than others. Further details regarding these models can be found in Celiks discussion. 

Alumina is a useful, abundant, and simple oxide refractory. It is substantially ionic in bond character. The powder is compacted and sintered in the temperature range of 1200°C-1800°C. Alumina parts are also made by the application of heat and pressure at the same time (hot pressing). Densification of powder compacts by these two methods brings about shrinkage. The void spaces between the particles decrease in size on achieving 100% theoretical density, they are eliminated. Linear shrinkage ranges from 5% to 20%. 

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