Finally, the capsule was closed with Swagelok type of tight seal,then taken out of the glove box and placed into an electric furnace that had already been heated up to the test temperature in a range of 300–600◦C. The capsule was kept at the constant and uniform temperature for 24 h. An example of the temperature versus time is shown in Fig. 3. T4, T5 and T6 are the capsule temperatures which were measured by thermocouples 4, 5 and 6 at different positions as denoted in Fig. 1.