024

parameters of each materials we used are shown in Table 2.

The results of calculation are obtained as the stress distribution in the cell components. Using these results and the mechanical properties of 8YSZ sheets which we measured''", we roughly estimated the failure probability of the 20 cm square 8 YSZ sheets in the cooling cycle. It can be calculated from the equation (1) using Weibull parameter such as 'shape parameter'; m and 'scale parameter'; ยง. Because the main cause of failure in 8YSZ sheet should be surface crack from the fractography.

Failure Probability=J^ ^j dA (l)

<Results & Discussion>

The results of the calculation are shown in table 3 and figure 2. The table 3 involves five cases and shows the maximum stress in the 8YSZ sheet and the failure probability.

In table 3, the difference of case 1 and case 2 shows that the lower constraint temperature ,the smaller the stress occurred and failure probability. But as shown in case 3, even if glass sealant is used and the constraint temperature is lowered, if the glass has a much different TEC such as glass 1, the maximum stress and the failure probability becomes very large. And in the case4 using glass which has near value ofTEC to the other components such as glass 2, the failure probability becomes lower than the case 2. This indicates that constraint temperature and the correspondence ofTEC between glass sealant and the other components are essential to the thermal stress.

Table 3 Maximum Stress and Failure Probability of 8YSZ
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