Stress during calcination

The development of stress during calcination is shown in Fig. 8.20 for boehmite membranes calcined at 600°C (thickness after calcination is 5 pm). Curve c in Fig. 8.20 represents the curve which is corrected for support effects (see the preceding section on this subject). Three heating and cooling cycles are shown. During the first heating the Al-hydroxide particles of the gel are transformed to boehmite and subsequently to (hydrated) y-aluminium oxide particles and the shape of the first peak of curve c differs from the subsequent peaks. The maximum tensile stress calculated from the deflection amounts about 30 MPa. 

The enhancement of the surface area of ccria-based materials related to the surfactant effect that reduces the surface tension inside the pores by decreasing capillary stress during drying and calcination processes. Better thermal stability is related to the structural arrangement and the morphology of the inorganic-organic... 

The total stress developed during calcination and sintering is the sum of ... 

The latter has been indeed reported in a smdy by Schwidder et al. [88] where the activity for fast SCR was found to survive hydrothermal stress and impact of SO2 much better than that for standard SCR (Fig. 7.12). From the observation that fast SCR was effectively catalyzed by a sample containing just 0.2 wt% Fe, according to UV-Vis and EPR spectroscopy almost exclusively as isolated sites and that more iron, be it as isolated, oligomeric, or particulate species, did not result in significant improvement, it was concluded that fast SCR is catalyzed by a sub-entity of the isolated sites. In recent operando EPR studies, Fe sites in P and y positions remaining in the 2+ state during calcination but being oxidized to Fe " in presence of NO2 have been identified as candidates [104]. 

Fig. 8.20. Deflection (stress) versus time diagram during the cyclic heat treatment (calcination) of boehmite membranes converted to y-alumina at 600°C. Heating and cooling rates were 60°C/h. From Kumar [13]. Curve a (dotted) blank run, support only curve b actual run with supported membrane curve c deflection of b corrected for support effects.

From the preceding sections it has become clear that tensile stresses developing during the drying of the lyogel and subsequent calcination are important causes for defect formation by cracking. A tentative scheme to account for a number of data emerges but many details are unknown. Nevertheless some trends are qualitatively predictable. 

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