Stress formation in membranes 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. 

In the first cycle the shape of the heating period (oxide formation) is different from the cooling period (thermal mismatch between layer and support). After the first cycle, the shape of the subsequent peaks are identical and all processes seem to be reversible. 

The maximum stress is obtained after each cycle and so no stress relaxation occurs. Note that the deflection measurements start with a dried membrane which already shows a certain deflection which is equivalent to a tensile stress level of 30 0 MPa. It is not clear at the moment whether it is allowed to sum up these two contributions or that the drying stress relaxes during heating and is replaced by stresses originating in the phase transformation/thermal mismatch processes. In any case when summing up is allowed the final stress in the y-alumina after cooling down is not greater than 30 MPa in the other case it is zero. 

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