Sintering effect, with small particles

Lubricants protect die and punch surfaces from wear and bum-out of the compact during sintering without objectionable effects or residues. They must have small particle size, and overcome the main share of friction generated between tool surfaces and powder particles during compaction and ejection. They must mix easily with the powder, and must not excessively impede powder flow (see Lubrication and lubricants). 

Fig. 9. Influence of local temperature T and varying catalyst structure on the CO oxidation effectiveness factor . Each Pt/y-Al203 catalyst was reconstructed by packing of y-Al203 particles of two different sizes with small-to-large particles number 16. Small particles size d = 1 pm was kept constant, while large particles size <72 and particles fractional overlap (level of sintering) were varied. Gas concentrations the same as in Fig. 8 (Koci et al., 2007a).

Because of the potential importance for industrial-scale catalysis, we decided to check (i) whether an influence of a semiconductor support on a metal catalyst was present also if the metal is not spread as a thin layer on the semiconductor surface but rather exists in form of small particles mixed intimately with a powder of the semiconductor, and (ii) whether a doping effect was present even then. To this end the nitrates of nickel, zinc (zinc oxide is a well-characterized n-type semiconductor) and of the doping element gallium (for increased n-type doping) or lithium (for decreased n-type character) were dissolved in water, mixed, heated to dryness, and decomposed at 250°-300°C. The oxide mixtures were then pelleted and sintered 4 hr at 800° in order to establish the disorder equilibrium of the doped zinc oxide. The ratio Ni/ZnO was 1 8 and the eventual doping amounted to 0.2 at % (75). 

Sintering with small amounts of melt involving substantial densification is possible when the solid particles are dissolved at the points of contact as a result, the particles come closer together, being attracted by the effect of surface forces acting in the melt surface. This phenomenon requires a certain solubility of the solid phase in the melt (so-called sintering with a reactive liquid) ... 

Fig. 5 shows the SEM images of Fe-oxide(urea), Al-Fe-oxide(urea) and Cr-Fe-oxide(urea) before the first reduction and after the third oxidation. Al and Cr cations are effective additives for redox performance of iron oxides, as described earlier. The SEM images of the samples before the first reduction showed that all the samples consisted of small particles with uniform size and the particles sizes of AI-Fe-oxide(urea) (Fig. 5 (b)) and Cr-Fe-oxide(urea) (Fig. 5 (c)) were smaller than that of Fe-oxide(urea) (Fig. 5 (a)), suggesting that the addition of Al and Cr cations into iron oxides divides the oxides into small particles. In the SEM image of the Fe-oxide(urea) after the third oxidation (Fig. 5 (a)), larger particles with different size were observed. Therefore, the iron species in Fe-oxide(urea) were sintered during redox cycles. On the other hand, the SEM images of... 

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