Powder mechanics

Another technique is to change the particle size distribution. There are, however, disadvantages. If segregation is occurring by the sifting mechanism, the particles must be almost identical in size before sifting is prevented. Alternatively, the mean particle size can be reduced below 100 p.m, but this size reduction (qv) increases the probabiUty of segregation by the too fine powder mechanisms. 

Compaction Rate Processes The performance of compac tion techniques is controlled by the ability of the particulate phase to uniformly transmit stress and the relationship between apphed stress and the compaction and strength characteristics of the final compacted particulate phase. The general area of study relating compac tion and stress transmission is referred to as powder mechanics (Brown Richards, Principles of Powder Mechanics, Pergamon Press Ltd., Oxford, 1970). 

Powder Mechanics Measurements As opposed to fluids, powders may withstand applied shear stress similar to a bulk solid due to interparticle friction. As the applied shear stress is increased, the powder will reach a maximum sustainable shear stress T, at which point it yields or flows. This limit of shear stress T increases with increasing applied normal load O, with the functional relationship being referred to as a yield locus. A well-known example is the Mohr-Coulomb yield locus, or... 

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.)... 

R. L. Brown and J. C. Richards, Principles of Powder Mechanics, Pergamon Press, Oxford, 1966. 

In the theory of elasticity and plasticity, as well as in fluid mechanics, tensile stress is chosen to be positive and compressive stress is chosen to be negative. In soil and powder mechanics, compressive stress predominates, therefore, it is customary to define compressive stress as positive. 

In soil mechanics, the pressures lie in a range up to and over 2 MPa, whereas in powder mechanics they are usually below 0.1 MPa. For this range of pressure, the Coulomb criterion is generally not applicable. Only cohesionless solids exhibit Coulomb yield behavior at low pressures. 

Figure 1. Schematic indication of the distinction between densifying and nondensifying mechanisms during the sintering of ceramic powders. Mechanism 1 Surface diffusion 2 vapor transport 3 lattice diffusion 4 grain boundary diffusion.

Amorphous powder formed by mechanical alloying for 510 h at intensity 5 was further milled at intensity 7 for 25 h. The typical broad diffuse maximum of the amorphous state disappeared, and the characteristic intensity distribution of the quasicrystalline phase showed up in the X-ray diffraction pattern. Therefore, additional milling at higher intensity led to an amorphous-to-quasicrystal transition. Amorphization can also be achieved for crystalline starting powder mechanically alloyed for 206 h at intensity 9 by a further milling for 433 h at intensity 3 [3.108]. 

Enstad, G.G. 2001. Segregation of powders Mechanisms, process and counteraction. In Handbook of Conveying and Handling of Particulate Solids (A. Levi and K. Kalman, eds), pp. 589-603. Elsevier, Amsterdam. 

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