Aggregated powder

Bircumshaw and Edwards [1029] showed that the rate of nickel formate decomposition was sensitive to reactant disposition, being relatively greater for the spread reactant, a—Time curves were sigmoid and obeyed the Prout—Tompkins equation [eqn. (9)] with values of E for spread and aggregated powder samples of 95 and 110 kJ mole-1, respectively. These values are somewhat smaller than those subsequently found [375]. The decreased rate observed for packed reactant was ascribed to an inhibiting effect of water vapour which was most pronounced during the early stages. 

There are many aggregated powders (clays, pigments, cements, etc.) that appear to give normal Type II adsorption isotherms, although their full adsorption-desorption isotherms exhibit Type H3 hysteresis. However, unlike Type IV isotherms, there is no plateau at high pjp°- These isotherms are now termed Type lib, as indicated in Figure 13.1. 

FIGURE 63.5 Cross section of a molded body obtained by compacting the material of Figure 63.4, showing bow tbe aggregate powder particles 7 have been crowded together into a relatively uniform structure with a micropore voids 9, lines 8 representing a portion of the mold wall. 

Porous mold n. A mold that is made up of bonded or sintered aggregates (powdered metal, pellets, etc.) in such a manner that the resulting mass contains numerous connected interstices or regular or irregular shape and size through which air may escape as the mold is filled. 

F. Babick, S. Gropp, U. Katzel, M. Vorbau, Dynamic light scattering of dispersed fumed silica aggregates. Powder Technol. 215-216, 39-45 (2012c). doi 10.1016/j.powtec.2011.10.064... 

In the case of powders formed by grinding and particles formed by aggregation, surface roughness can be so extreme that, curiously, it can be treated by mathematical geometry (see Mandelbrot, Ref. 102 also Ref. 103). We can... 

Sedimentation (qv) techniques, whether based on gravitational forces or centrifugation, derive the particle size from the measured travel rates of particles in a Hquid. Before the particle analysis is carried out, the sample is usually dispersed in a medium to break down granules, agglomerates, and aggregates. The dispersion process might involve a simple stirring of the powder into a Hquid, but the use of an ultrasonic dispersion is preferred. 

Two classes of grinding equipment are used to prepare dispersions. The first, the coUoid mill, does not effect a particle size reduction but does break down aggregates of fine particles. CoUoid mills are used for such powders as clays, precipitated whiting, etc. Sometimes these mills are used to process zinc oxide but for dipped mbber products that is not satisfactory. 

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