Increasing the Seebeck Coefficient of Thermoelectric Calcium Cobaltite Ceramics through Incorporation of Rare-Earth Elements

Abstract

The demand for alternative sources of energy continues to grow, so it is essential to look for alternatives that are environmentally sustainable to minimize the environmental impact of energy generation. Projections suggest that renewables have the potential to make up over fifty percent of power generation by 2035. Studies have shown that in 2020 more than 62% of the energy produced in the US was lost, with most of this energy being in the form of waste heat. Waste heat can be utilized to improve the efficiency of power production or even power other devices. This waste heat can be recovered using thermoelectric generators, which are solid state devices comprised of P and N-type materials connected electrically in series and thermally in parallel. Such devices take advantage of the Seebeck effect to directly convert thermal energy into electrical energy. Thermoelectric generators have historically used toxic and expensive materials, such as lead telluride (PbTe) and bismuth telluride (Bi2Te3). Ceramic oxide materials have recently drawn significant attention as possible candidates to be used in thermoelectric generators. These materials have shown to be chemically stable in air at high temperatures and relatively cheap to be produced. However, the efficiencies of these materials have yet to be improved to be comparable to the efficiencies of well stablished materials. Single crystal calcium cobaltite Ca3Co4O9 has shown great potential as a candidate for thermoelectric applications. However, single crystal materials are difficult and relatively more expensive to fabricate when compared to polycrystalline ones. Efforts are focused on improving the performance of polycrystalline materials to make them a viable alternative for thermoelectric applications. The efficiency of a thermoelectric material is determined by the figure of merit ZT, which is defined by ZT=(S2σ/k)T, where S is the Seebeck coefficient, σ is the electrical conductivity, k is the thermal conductivity, and T is the temperature. Studies have shown that heterovalent substitution for Ca2+ in Ca3Co4O9 reduces the carrier concentration of the material, leading to an increase in Seebeck coefficient and decrease in thermal conductivity. These properties are directly related to the figure of merit ZT. This work focuses on improving the Seebeck coefficient of the ceramic oxide polycrystalline P-type material Ca3Co4O9. Results showed that single and dual doping of the material using rare-earth elements via stoichiometric substitution enhanced the Seebeck coefficient, power factor, thermal conductivity, and consequently the ZT value of Ca3Co4O9.