Developing high-efficiency ternary thermoelectric phosphides in the systems Ca-Ag-P and Ca-Cu-P

Since the discovery of the thermoelectric (TE) effect by Seebeck, Peltier, and Thomson in the XIXth century, thermoelectricity has attracted significant attention for both fundamental science and industrial applications. Nowadays, this interest has been rejuvenated by the ever-growing worldwide energy demand and the concomitant environmental concerns tied to the emission of pernicious greenhouse gases. Among renewable energy sources, thermoelectricity stands out for not only waste-heat harvesting but also for being replaced by commercial refrigerators. Considering that about two-thirds of the energy produced is lost as waste heat, this versatile technology is quite useful as it converts heat into electricity and vice-versa. The thermoelectric efficiency of a given material is quantified through the dimensionless thermoelectric figure of merit, zT, defined as zT= α2σT/(κl+κe) where α, σ, κl, κe and T correspond to the Seebeck coefficient, electrical conductivity, lattice thermal conductivity, electronic thermal conductivity, and temperature, respectively. The higher the zT value at a given temperature, the higher the thermoelectric efficiency of the module. The challenge to overcome in thermoelectricity is thus clearly material: a good thermoelectric material should possess low electrical resistivity to minimize heat losses through Joule effect, high thermopower to maximize the thermoelectric effect, and low thermal conductivity to maintain a temperature gradient large enough between either side of the thermocouple. Ternary phosphides (CaAgP and CaCuP) are stable and can exhibit promising thermoelectric properties. To date, these materials have remained largely unexplored, but recent theoretical studies demonstrate their high predicted thermoelectric efficiencies. In this thesis study, the electrical and thermal transport properties of p-type CaAgP and CaCuP materials were investigated through Zn, Na, Sn, nano-boron (nB), and La doping. CaAgP and CaCuP powders have been synthesized by high-energy ball milling and were sintered by spark plasma sintering technique. The dopants were introduced to the materials by applying the same synthesis processes. Additionally, annealing procedures were performed on certain samples following the sintering process. Detailed chemical characterization of these polycrystalline materials has been carried out. Thermoelectric examinations of the materials obtained in the desired purity and composition have been completed. The mechanical properties of thermoelectrically promising samples were measured. The measurements of the dielectric constant of CaAgP and CaCuP, and thermoelectric modeling were also conducted by the Korean partner. The intrinsically doped CaAg0.90P sample achieved the highest zT value of approximately 0.33 at 823 K, with additional dopants failing to further enhance zT values in the CaAgP compound. Notably, the sample doped with 0.05 at.% nano boron demonstrated the maximum hardness within the Ca-Ag-P system. In the CaCuP system, the zT value improved significantly, increasing from 0.37 to 0.45 at 823 K for the Zn-doped and annealed Ca1.05Cu0.95P-Zn0.05 sample. Additionally, the inclusion of nano boron and zinc not only enhanced zT values but also improved the mechanical properties of the CaCuP and CaAg0.90P microstructures. These findings underscore the importance of detailed experimental and computational studies on ternary phosphides, which can substantially contribute to the development of new thermoelectric materials and their applications.