Agglomeration powder compaction

The success of compression agglomeration depends on the effective utilization and transmission ofthe applied external force and on the ability of the material to form and maintain interparticle bonds during pressure compaction (or consolidation) and decompression. Both these aspects are controlled in turn by the geometiy of the confined space, the nature of the apphed loads and the physical properties of the particulate material and of the confining walls. (See the section on Powder Mechanics and Powder Compaction.)... 

There have been many attempts to directly measure the mechanical properties of powders which have been agglomerated by compaction [85-90]. 

Formation of agglomerates by powder compaction may involve rearrangement of particles to increase their packing efficiency resulting in the enhancement of interparticle adhesion forces [89]. Furthermore, particle deformation at the point of contact between particles can greatly increase both the contact surface area and interparticle attraction [84]. 

Figure 10.10 Temperature dependence of sintered density for an agglomerated or as-received and agglomerate-free yttria-stabilized zirconia powder (Ih). Eliminating the agglomerates in the green body resulted in a powder compact that densified much more readily. " ...

Tor powder synthesis a large variety of methods is available, for example mixed-oxide solution precipitation or emulsion precipitation. Figure 8.11 shows as an example a (Y,Gd)202 powder prepared by three different precipitation techniques. The powders obtained ate different in particle morphology as well as in particle size. Powders prepared by the citrate and the oxalate precipitation techniques are strongly agglomerated [14], From powders derived in this way, compacts are formed. These powder compacts demonstrate total luminescent properties, but have still low light output due to the high porosity of about 50 vol.%. 

The major variables which determine sinterability and the sintered microstructure of a powder compact may be divided into two categories material variables and process variables (Table 1.1). The variables related to raw materials (material variables) include chemical composition of powder compact, powder size, powder shape, powder size distribution, degree of powder agglomeration, etc. These variables influence the powder compressibility and sinterability (densification and grain growth). In particular, for compacts containing more than two kinds of powders, the homogeneity of the... 

Consider a powder compact which consists of two kinds of agglomerates with different densities, p/, and p (ph > Pi)- If the relative densification rates of these agglomerates, (dp/dt)/p, are the same, what would happen Discuss whether this assumption is satisfied in real sintering. 

Consolidated structure The consolidated structure (i.e, powder compact) that forms after the liquid is removed from the suspension depends on the state of the dispersion. Well-dispersed suspensions tend to form consolidated structures with a higher particle packing density (lower total porosity) when compared to samples prepared from suspensions with flocs/agglomerates (Figure 10.26) (12). In addition, the samples prepared from well-dispersed suspensions tend to have a smaller average pore size and a narrower pore size distribution. 

The problem, shown in Fig. 6.1, is that liquids also seem to act as adhesives under certain conditions. For example, wet sand sticks together better than dry sand. If we wish to agglomerate powder, we add water and stir the powder to compact it. Similarly, the best way to make an adhesive Joint is to wet the... 

There are two problems of sintering which are not explained by the equations such as (9.26) above. The first is that sintering should proceed in proportion to time, but instead stops prematurely the second is that the expected shrinkage rate should be larger for smaller diameter particles but often is not. Both these problems are connected to the fact that powder compacts contain agglomerates, which get stronger and more numerous as the particles get finer. 

Fig. 8.4 Typical powder agglomerates detected in the powder electrodeposited at the Ni /Co = 0.67. (a) Agglomerate covered with fem-like dendrites, (b) Higher amount of dendritic particles around the agglomerates, (c) Compact agglomerate covered with small fem-like dendrites and the presence of cavities (Reprinted from Ref. [1] with kind permission from Springer)... 

With further decrease of the Ni /Co ions ratio in the solution (0.33, 83 at.% Co in the powder), compact agglomerates, typical for pure Co powder, could be clearly detected in the powder electrodeposit, as can be seen in Fig. 8.13a, together... 

Compare the densification of a homogeneously packed powder compact of 5 p.m single crystalline particles with the densification of a compact of 5 p.m agglomerates consisting of 0.5 (J m single crystalline particles. Assume the particles have the same chemical composition. 

Every system is constrained to some extent. For example, the sintering of a pure single-phase powder is constrained by inhomogeneities (such as agglomerates) present in the powder compact. However, the term constrained sintering is commonly taken to describe sintering in which the constraint is deliberately imposed and is a necessary feature of the system. An identical system in which the con-... 

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