Synthesis and Characterization of New Zintl Phases for Thermoelectrics

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PART 1: NON-TECHNICAL SUMMARY Thermoelectric materials convert a temperature gradient into electricity and could impact energy conversion from solar or current energy sources with significant waste heat available. The conversion efficiency of a material requires that it have good electrical conductivity like a metal and low thermal conductivity like an insulator. The primary challenge is decoupling these interrelated properties, a task that a type of compound known as Zintl phases are well suited for. These compounds' unique and complex structures provide materials where thermal conductivity is low and therefore it is possible to focus on tuning the electrical transport properties. This project, supported by the Solid State and Materials Chemistry program within the Division of Materials Research, is focused on the synthesis of new Zintl phase compounds containing transition metals and rare earth elements. This project identifies new and existing crystal structures with proposed low thermal conductivity and provides a road map for enhancing the electronic properties for efficient energy conversion. This research impacts technology for our future energy needs and specifically the utilization of waste heat conversion into electricity. Next generation scientists receive fundamental training in materials synthesis and development of structure-property correlations. Undergraduate and graduate students are provided hands-on training and their scientific and communication skills are developed through workshops and individualized mentoring. Undergraduate laboratory and classroom curriculum development is also incorporated in the project. PART 2: TECHNICAL SUMMARY This proposal is focused on synthesis of new thermoelectric materials described as Zintl phases and development of structure-property correlations toward efficient energy conversion technologies. Thermoelectricity is based on three important transport parameters: the Seebeck coefficient, electrical resistivity, and thermal conductivity. The efficiency of thermoelectric materials is described by the unitless figure of merit zT, which is dependent upon all three of these interdependent properties. Zintl phases are considered a subgroup of intermetallics where metal cations and anions or polyanions form complex structures and are semiconductors. The foundational idea of Zintl to employ electron counting to understand complex solid state structure provides insight into bonding and allows for systematic optimization of properties. Therefore, Zintl phases are prime candidates for thermoelectric applications as they are semiconductors with large tunability because they contain both ionic and covalent bonding. Solid solutions of iso- and alio-valent elements provide a means to change carrier concentration and therefore the electronic structure to tune properties. Within this project, supported by the Solid State and Materials Chemistry program within the Division of Materials Research, specific structure types that are expected to have low thermal conductivity are targeted and the application of Zintl counting rules is tested in order to tune the electronic properties. Goals include both further optimization of Zintl structures and the synthesis of new compounds with structures that are predicted to give rise to low thermal conductivity and efficient thermoelectric energy conversion. The project focuses on mixed cations and light element containing phases that have not yet been investigated. New compounds are synthesized as single crystals and high purity powders via flux and metallurgical routes and fully dense pellets will be prepared via spark plasma sintering. Students and next generation scientists are provided hands-on training and their scientific and communication skills are developed through workshops and individualized mentoring. The research will be presented at national and international meetings and the findings published in peer-review journals.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.