Fine particles experimental results

Most heat transfer studies were carried out under ambient conditions and with fine particles in small experimental sets. Though exhaustive reviews have been presented (e.g., Grace, 1986 Glicksman, 1988 Leckner, 1990), the data obtained at Tsing Hua University (Bi and Jin, 1989, 1990 Bai and Jin, 1992) will be the focus of this discussion and will be compared to the results of other researchers. 

The powders produced from the above method are very fine. TEM observation in Figure 3.29 shows that the typical powder is composed of equiaxed particles and has a size in the range of 50 to 80 nm. Experimental results reveal that the chemical composition of the powders depends weakly on processing conditions. The main chemical compositions of the powders are 50% wt of Si, 28% wt of C, 16% wt of N, while there is a small amount of O and H, 4% wt and 2% wt respectively. Selected area electron diffraction (SAED) also reveals the existence of a partial crystalline phase in the amorphous structure, this being a unique material feature which cannot normally be obtained using other manufacturing techniques. This feature makes it possible for the powders to be used as good functional materials, such as radar wave absorption materials. 

Thus, experimental studies relating dissolution rates to defect concentrations are needed. This has been done recently in two different types of experiments where the importance of line defects has been measured. Casey et al. (1988) and Holdren et al. (1988) have measured the rate of dissolution of rutile and labradorite powders that were shocked with an explosive charge to induce a high density of dislocations (> 1011 cm-2). Concurrently, in order to avoid artifacts due to fine particles, Schott et al. (1989) have performed with the rotating disc apparatus dissolution runs of single crystals of calcite in which dislocations were induced in a constant-strain apparatus. Examples of the results are shown in Fig. 15. Surprisingly, it can be seen that dislocations have only a small effect on the dissolution rates the dissolution rates of samples with over 6 order of magnitude in dislocation density differ only by a factor less than 3. 

However, moderately fine powders of the copper-modified alloys, when properly heat treated, do not suffer the described aging loss of Hc nor do compacts made from them (Cremer et al. 1982, R.M.W. Strnat and Luo 1982). The reason is that their coercivity is caused by a homogeneous intra-granular precipitate that impedes the movement of domain walls wherever they are located. So, Hc does not strongly depend on the surface condition of these particles. If, however, the precipitate is dissolved by a homogenization heat-treatment, these alloys act just like SmCo5. This is also illustrated in fig. 14, showing early experimental results on a Sm-Co-Cu-Fe alloy near the 1-5 composition. It is equally true of the newer 2-17 alloys, which usually contain other minor elements, such as Zr, in addition to Cu. 

The average contact time carmot be predicted, but it would be expected to depend on the particle type and size, the gas velocity, the size and arrangement of the tubes, and perhaps the bed dimensions. However, the theory does help explain some of the experimental results, and it is a guide in the development of empirical correlations. Only a few studies are reviewed here, to show the major trends and typical coefficients for fine solids. 

Brownian adhesion occurs at very small dimensions, from O.l-lOOOnm, where thermal diffusion is dominant. Because of the small sizes. Brownian adhesion is difficult to observe experimentally, but it can be seen with small dispersed particles such as colloids and blood cells, for example (see Chapters 10 and 12). One result from this theory is that fine particles will stick together to form doublets whose number depends on the interparticle adhesion. Consider a number of fine particles immersed in dispersing fluid and undergoing Brownian motion in a box as in Fig. 7.7(a). With zero adhesion, no doublets should form. However, adhesion eauses doublets to develop, as in Fig. 7.7. 

Sada et.al.(10,15) considered the case where the reaction was finite and presented numerical solutions(An approximate solution for this case was obtained previously(4) j.Sada et.al.(11) considered simultaneous absorption of two gases and presented numerical analysis and experimental data.They(16) have interpreted also their experimental results on dilute sulphur dioxide absorption into aqueous slurries of sparingly soluble fine rectant particles in terms of a "two-reaction plane" model.Sada et.al.(17, 18) considered also other interesting examples and proposed a model on the basis of above discussed theory as well as incorporating the possible solid surface reaction.In this case,the... 

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