Isothermal sintering

Fig. 3.5.8 Schematic and NMR image of C4F8 gas at 80 kPa in a hybrid phantom containing Vycor glass, a nanoparticulate AI2O3 powder, a nanoparticulate ZnO powder and sintered ceramics made from each of these powders. Dashed boxes indicate the regions of interest (ROIs) from which the isotherms in Figure 3.5.9 were extracted. Adapted from Ref. [21].

Truly isothermal operation of a tubular reactor may not be feasible in practice because of large enthalpies of reaction or poor heat transfer characteristics. Nor is it always desirable, as, for example, in the case of a reversible exothermic reaction (see Sect. 3.2.4). In an exothermic catalytic reaction, it may be necessary to provide adequate means for heat transfer to prevent the development of local hot-spots on which coking may occur and reduce the catalyst activity. An excessive temperature rise may also cause the catalyst particles to sinter, thereby reducing their surface area and causing an irreversible decrease in catalytic activity. 

Fig. 12. IR spectra of NO adsorbed at 300 K on NiO samples sintered at progressively higher temperatures (the spectra correspond to 6 0.5) and volumetric [number of adsorbed moles of NO per gram of NiO (1.5 m2 g 1)] and spectroscopic (integrated IR intensity) isotherms. The two sets of data have been normalized to coincide at Pno = 16 Torr [adapted from Escalona Platero et al. (285) with permission of Elsevier Science Publishers].

Problem 1 assumes that the BaTiOs powder is monosized. This is not actually the case, the powder used has a geometric mean size of 0.71 jum and a geometric standard deviation of 1.6. Determine the isothermal shrinkage at 1200°C of this BaTiOs sample during both initial and intermediate stage sintering. 

From elements by sintering at 753 773 K under 40-50 bar hydrogen pressure black powder, stable in air nonmetal-lic pressure-composition isotherms show two plateaus of... 

This well accepted method [28] has been used extensively in the characterization of M41S materials [11-12,14]. From the application of this method to MCM-41, it has been concluded that this material contains no significant amounts of microporosity. This is the main evidence presented so far in order to conclude that MCM-41 is exclusively mesoporous. As it happens with any good method its limitations need to be considered in order to avoid misinformation. In the case of the a method the choice of the reference isotherm is crucial. All the reference silicas should be nonporous in order to allow a reliable analysis of MCM-41. Unfortunately, we observed that most of them have a steep rise in their N2 adsorption isotherms at 77 K at low relative pressures and BET surface areas varying from 40 [29] to 400 mVg [30], For this reason, our sample of MCM-41 was heat-treated so as to sinter the silica particles and thus obtain a nonporous silica (BET surface area 1.5 mVg) and as similar as possible to our MCM-41. The N2 adsorption isotherms for a reference silica [29] and our sintered MCM-41 are shown in Figure 7. 

For the ACs the data are representative of the samples after heat-treatment at all three temperatures since during their fabrication these materials have already been treated at temperatures in excess of 850°C. However, for the alumina and clay samples the surface areas and pore volumes are shown after treatment at each temperature as these materials undergo various phase transitions that lead to sintering of the samples and shifts in their relative pore size distributions with heat-treatment. The particle size was determined from the corresponding MIP curve for the powder raw material. The Sbet in the case of microporous ACs should be considered as an apparent surface area due to the micropore filling mechanism associated with these materials [15]. The external area and micropore volumes were calculated from the slope and intercept of the t-plots of the corresponding isotherms. The total pore volume was taken as the amount of gas adsorbed at a relative pressure of 0.96 on the desorption isotherm, equivalent to a pore diameter of 50 nm. The mesopore volume was calculated from the difference in the total pore volume and the micropore volume. 

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