Designing n-type zintl phases for thermoelectric power generation applications

The advancement of technology and a growing global population have increased energy demand, with over 60% of generated energy lost as waste heat, highlighting inefficiencies. Recovering waste heat is crucial for sustainable energy solutions. Thermoelectric materials, which convert heat into electricity, are of significant interest due to their silent operation, lack of moving parts, and scalable solid-state design. Thermoelectric generators use temperature gradients for power generation, and their efficiency is measured by the dimensionless figure of merit, zT (zT = α²σT/( κlat + κe)), where α is the Seebeck coefficient, σ is electrical conductivity, κlat is lattice thermal conductivity, κe is electronic thermal conductivity, and T is temperature. Achieving optimal zT is challenging due to the interdependence of these properties. Certain ternary and binary metal phosphides, like GaP, NaSnP, and SrLiP, show promising potential as thermoelectric materials due to their superior electronic properties and low thermal conductivities, outperforming PbTe and Bi2Te3. Zintl phase thermoelectric materials, known for their tunable electronic transport properties, complex crystal structures, and low thermal conductivities, also exhibit high thermoelectric efficiencies. Notably, Mg3Sb2-based Zintl phases are particularly interesting for their excellent zT values as n-type materials, highlighting their potential in advancing thermoelectric technology. In this thesis, the synthesis and characterization of binary and ternary phosphides have been explored. These phosphides were synthesized through solid-state synthesis or high-energy ball milling (also known as mechanochemical synthesis), followed by spark plasma sintering to achieve densification for measurement of physical properties. Efforts were made to identify the most efficient synthesis routes for ternary phosphides. Despite rigorous efforts, our exploration encountered significant obstacles owing to the high air sensitivity of the ternary phases and substantial resistivity characteristic of the binary phases. These properties rendered the measurement of both electrical and thermal conductivities unfeasible. Subsequent investigations focused on the design and determination of electrical and thermal transport properties of innovative and efficient n-type Mg3Sb2-based Zintl phases, such as CaMg2Sb2 and CaMg2Bi2. This was achieved through band structure engineering, employing doping strategies with tellurium and lanthanum (Ca0.8La0.2Mg2.1Sb2, Ca0.9La0.1Mg2.1Sb1.95Te0.05, Ca0.8La0.2Mg2.1Bi2 and Ca0.9La0.1Mg2.1Bi1.95Te0.05). These samples were successfully synthesized using high-energy ball milling and sintered through the spark plasma sintering technique. With the help of microstructural characterization and WDX analysis determination of effective doping has been made. Transport measurements were conducted to determine the thermoelectric performance of doped and undoped samples between 300-700 K. After introducing La and Te doping, resistivity and thermal conductivity values had a decrease, Seebeck coefficient values shifted from positive to negative across the entire temperature range studied. Additionally, the observation of negative Hall mobility values indicated that the electrical conductivity in these materials is predominantly due to the movement of free electrons, suggesting that effective doping strategies have indeed increased the carrier concentrations. Also, the calculations made to determine the main contribution of thermal conductivity values revealed that the main contribution is coming from the lattice thermal conductivity part. In conclusion, the La and Te doped Mg3Sb2-based samples, specifically CaMg2Sb2 and CaMg2Bi2, demonstrate potential as efficient n-type thermoelectric materials. By increasing the electrical conductivity values and achieve a promising zT value these findings suggest their viability for incorporation into complete Zintl phase thermoelectric modules, offering promising pathways for enhancing thermoelectric technology's effectiveness and sustainability.