Iranian Journal of Materials Science & Engineering Vol. 17, No. 1, March 2020 45 1. INTRODUCTION One of the factors contributing to environ- mental pollution is the emission of greenhouse gases from fossil fuel consumption. The amount of these gases is increasing by the growth of in- dustries and cars. According to industrialized na- tion’s agreement, members are required to reduce the amount of greenhouse gases to control the air pollution. Therefore, changes in energy produc- tion and cleaner energy technologies is felt more than ever. These changes must be capable to re- duce the fossil fuel consumption and greenhouse gases and, as a result, reduce environmental pol- lutions. One of these technologies that can reduce the environmental pollutions is Concentrated So- lar Power (CSP) plants joint with Thermal Energy Storage (TES) technology. The most important use of TES is to overcome the mismatch between energy production and consumption especially in Thermochemical Heat Storage Properties of Mechanically Activated Co 3 O 4 -5 wt. % Al 2 O 3 and Co 3 O 4 -5 wt. % Y 2 O 3 Composite Powders A. Hasanvand, M. Pourabdoli * and A. Ghaderi Hamidi * mpourabdoli@hut.ac.ir Received: January 2019 Revised: April 2019 Accepted: August 2019 Advanced & Energy Storage Materials Lab., Department of Metallurgy and Materials Engineering, Hamedan University of Technology, Hamedan, Iran. DOI: 10.22068/ijmse.17.1.45 Abstract: The main problem with cobalt oxide as a thermochemical heat storage material is its slow re-oxidation kinet- ics. In addition, redox (reduction and oxidation) behavior of as-received Co 3 O 4 is degraded with increasing the number of redox cycles. To overcome this drawback, Al 2 O 3 and Y 2 O 3 were added to Co 3 O 4 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 Co 3 O 4 -5 wt.% Al 2 O 3 and Co 3 O 4 -5 wt. % Y 2 O 3 composites was investigated using thermogravimetry method. The composite powder 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 Co 3 O 4 - 5wt. % Al 2 O 3 in comparison with as-received Co 3 O 4 . Although, the alumina-containing samples, activated in short time showed the better redox kinetics than samples activated for longer times. It was found that increasing the activation time to more than 8 h for alumina-containing samples reduces the redox kinetics due to a decrease in the positive effect of Al 2 O 3 in controlling the particle size growth and sintering. In the case of Co 3 O 4 -5wt. % Y 2 O 3 , an increase in activation time generally reduced the redox kinetics. As a result, redox reactions in a 16 h-activated Co 3 O 4 -5wt. % Y 2 O 3 composite sample was completely stopped. In addition, results showed that weak performance of Co 3 O 4 -5 wt. % Y 2 O 3 is related to intensive sintering and growth of cobalt oxide particles during redox reactions. Keywords: Cobalt oxide, Redox reactions, Reversibility, Thermochemical heat storage, Mechanical activation. technologies such as CSP plants that use the solar energy as a clean energy source. Solar energy is the most abundant source of renewable energy on the earth [1-5]. A description of the TES based on the chemical reduction and oxidation (redox) re- actions, thermochemical energy storage (TCES), is derived from the following equation: (1) Reactants ↔ Products + ΔH Equation (1) represents a balance between two or more reactive factors on one side, and products and enthalpy on the other side. In recent years, research interests in the feld of TCES system have increased many folds due to its possibility to achieving a high energy density and capability of working at higher temperatures compared to other storage methods. The operating temperature range of the reversible reactions associated with proper redox pairs is between 400 and 1250 °C, which is RESEARCH PAPER [ DOI: 10.22068/ijmse.17.1.45 ] [ Downloaded from ijmse.iust.ac.ir on 2021-11-28 ] 1 / 11