Annealing of Strontium Titanate Based Thermoelectric Materials by Graphite: Mechanistic Analysis by Spectroscopic and Chromatographic Techniques

Strontium titanate (SrTiO₃) based materials are promising for high-temperature thermoelectric applications. In order to enhance their performance, annealing is usually required and carried out under various atmospheres. Annealing with graphite is quite effective, but the mechanism is not yet clear. In this work, we use IR spectroscopy and gas chromatography (GC) to monitor the chemical environment under the annealing conditions (1350 °C for 8 h under 16.9 mL/min N₂ with graphite) and quantify the various gases evolved in the process. It is shown that reducing agents, H₂ and CO (concentrations peaked at ca. 0.4–0.5 %), are generated from graphite in the annealing process. H₂ is produced in carbon gasification reaction, which also generates CO. Additional CO is produced from incomplete combustion of carbon. In the annealing of a La-doped SrTiO₃-based ceramic with graphite, higher levels of H₂O and CO₂ are detected, which is resulted from the reduction of the ceramic by H₂ and CO, respectively. About 67 % of the oxygen vacancies were created by CO reduction while about 33 % by H₂ reduction. The conclusions are well supported by direct weight loss measurements with a difference of less than 6 %.