Iranian Journal of Materials Science and Engineering

The main problem of cobalt oxide as a thermochemical heat storage material is its slow re-oxidation kinetics. In addition, redox (reduction and oxidation) behavior of as-received Co₃O₄ is degraded with increasing the number of redox cycles. To overcome this drawback, Al₂O₃ and Y₂O₃ were added to Co₃O₄ and  effect of mechanical activation time (2, 4, 8, and 16 h) on the redox behavior (weight change value/rate, redox reversibility, reduction and re-oxidation values, and particle morphologies) of Co₃O₄-5 wt.% Al₂O₃ and Co₃O₄-5 wt. % Y₂O₃ composites was investigated using thermogravimetry method. The composites were studied by SEM, EDS, and X-ray map analyses before and after redox reactions. Results showed that increasing the mechanical activation time improves the redox kinetics of Co₃O₄-5wt. % Al₂O₃ in comparison with as-received Co₃O₄. Although, the alumina-containing samples, activated in short time showed the better redox kinetics than samples activated in long time. It was found that increasing the activation time to more than 8 h for alumina-containing samples reduces the redox kinetics due to decreasing the positive effect of Al₂O₃ in controlling the particle size growth and sintering. In the case of Co₃O₄-5wt. % Y₂O₃, an increase in activation time generally reduced the redox kinetics. As a result, redox reactions in a 16 h-activated Co₃O₄-5wt.% Y₂O₃ composite was completely stopped. In addition, results showed that weak performance of Co₃O₄-5 wt. % Y₂O₃ is related to intensive sintering and growth of cobalt oxide particles during redox reactions