Mass Production of Cu_2-xSe Nanoparticle for Thermoelectric Bulk Materials with Nanosized Grains via High Concentration Metal Complex Precursor

Electrical energy consumption is closely related to the growth of economic activity. Energy from fossil fuels causes air pollution, climate change, and global warming due to the release of harmful substances during combustion, so it must be replaced with renewable energy as a result. In accordance with this trend, thermoelectric (TE) technology using waste heat is attracting a lot of attention as an eco-friendly mechanism with high efficiency, wide application temperature range, and high reliability without moving parts. Currently, the medium and high temperature (400-750K) waste heat generated in the industry is being dissipated in a high proportion of more than 60% due to the limit of energy efficiency. Therefore, Cu_2-xSe, which has recently been attracting attention among various thermoelectric materials, is a copper-based chalcogenide and has low toxicity and cost-effectiveness. In the high-temperature superion phase above 400K, Cu ions are very disordered and mobile, resulting in very low k_L and high zT values, so the TE efficiency is high in the medium and high-temperature regions. Furthermore, to maximize TE characteristics, a technique for further reducing thermal conductivity by increasing phonon scattering with nano-sized particles through a powder metallurgy process is attracting attention.<br/>Although various synthetic methods including hydrothermal, solvothermal, and thermolysis to prepare Cu_2-xSe nanoparticles have been developed, they require suitable conditions such as relatively low precursor concentration, high temperature, high vacuum, many numbers of reactants, special experimental arrangements to perform the synthetic procedures. This causes low yield and difficulties in process optimization and commercialization. According to homogeneous nucleation and growth theory, the critical nuclei radius and critical energy to generate specific nuclei depend on a precursor concentration in the reaction medium. In general, the nucleation rate is faster at high precursor concentrations, which means that smaller nanoparticles can be synthesized in large quantities. However, the actual synthesis process using high concentration precursors causes problems in which particles are uneven in size and increase in size. Therefore, a new methodology is needed to solve this problem.<br/>In this study, a solution-based mass-production technology of Cu_2-xSe nanoparticles was successfully developed using a high-concentration metal complex precursor and ultrasonic energy. This approach can prevent a particle agglomeration by introducing complex molecules and instantaneous uniform energy supply and recovery via ultrasound. Using this facile and cost-effective strategy, Cu_2-xSe nanoparticles with an average diameter of 150nm were successfully synthesized at room temperature and atmospheric pressure. In addition, by adjusting the Cu/Se precursor ratio in the starting reaction solution, it was possible to easily control the alpha and beta phase fractions of Cu_2-xSe, which greatly contribute to the thermoelectric properties. Then, according to process variables, the physicochemical properties of synthesized nanoparticles were systematically investigated by various analysis methods. After the Cu_2-xSe powder of each composition was sintered using Spark Plasma Sintering (SPS), a sample having a nano-sized grain was obtained. Then, the physicochemical properties of the samples prepared according to the Cu/Se composition were systematically analyzed, and the correlation between the composition and the thermoelectric properties was identified through various analysis methods.