Solid Solution Tetrelides and Pnictides for Thermoelectric Applications

Recent major advancements in thermoelectric material performance center around the development and understanding of band structure engineering techniques and phonon scattering mechanisms. Solid solution materials have the potential to access these major strategies simultaneously within a single system. In this thesis, solid solutions of tetrels (C, Si, Ge, Sn, and Pb) and pnictogens (N, P, As, Sb, and Bi) as thermoelectric materials are explored. Electronic structures are examined to understand established materials and propose band engineering strategies. New synthesis approaches for established materials are designed while established methods are utilized to synthesize novel solid solutions. Thermoelectric properties are measured and discussed in terms of the underlying chemistry of the materials. Future work is proposed for the systems studied where improvements can be suggested. Chapter 1 discusses the various principles and strategies underlying the design and application of thermoelectric materials with a focus on solid solution materials. An overview of current high-performance materials and the principles which provide their status is presented. Finally, the classes of materials which are experimentally studied are discussed to provide background and motivation for the research conducted. Chapter 2 reviews the principles and practices for the experimental methods and instrumentation utilized throughout the course of this study. The first solid solution material focused on in Chapter 3 is the tetrelide Mg2Si0.3Sn0.67Bi0.03; a high performance, nontoxic, and inexpensive thermoelectric material. A scaled-up reaction process was developed providing the first steps towards large scale applications. Large, condensed pieces of material were pressed on a scale which had not been achieved previously. Statistical analysis of measured thermoelectric properties is performed on the material using samples cut at various positions and orientations. Over 1 kg of material was prepared which displayed a zTmax above 1.2 reliably. These methods are used to assure a consistent quality of the process and material which is the first step towards establishing device applications. Pnictide-tetrel chalcopyrite solid solutions are investigated in Chapters 4 and 5, with ZnGe1-xSnxP2 explored in the former, while ZnSnP2-yAsy and ZnGe1-xSnxP2-yAsy are explored in the latter. A robust synthesis method for end members and solid solutions was developed using ball milling techniques followed by hot pressing. Successful synthesis and full miscibility of end members and solid solutions are confirmed with powder X-ray diffraction followed by Rietveld refinements. The synthesis method is primarily discussed in Chapter 4 which is further developed for synthesis of higher order solid solutions in Chapter 5. The methods developed provide a useful tool for low temperature synthesis of solid solutions with differently melting and difficult to synthesize end members. Structural investigations conducted on resulting ZnGe1-xSnxP2 (x = 0, 0.25, 0.5, 0.75, and 1), show a tendency for tetragonality (c/(2a)=1) which maintained high Seebeck coefficients for the Sn rich and equal substituted members. Electronic structure calculations with Boltzmann transport analysis and experimental lattice thermal conductivities were used to predict thermoelectric performance. Doping ZnSnP2 with p-type carriers was predicted to give zT = 1 at 0.002 carriers per formula unit and 900 K (such as with ZnSn0.998-In0.002P2), and 1.3 at 0.007 carriers per formula unit. Measured thermoelectric performance was most improved by decreased thermal conductivity due to alloy phonon scattering at equal Ge and Sn substitution (x = 0.5) while maintaining a large Seebeck coefficient. The end members displayed thermal conductivity of 4.4 W m-1 K-1 and 2.5 W m-1 K-1 for Ge and Sn respectively which decreased to 1.8 W m-1 K-1 for x = 0.5 at 875 K. Improvements from zT = 3.9∙10-4 and 2.0∙10-3 for Ge and Sn end members respectively were achieved to zT = 5.5∙10-3 for x = 0.5 at 800 K while increased thermal stability allowed greater performance at higher temperatures. Chapter 5 focuses on improving the carrier concentration of ZnSnP2 and ZnGe1-xSnxP2 by substitution of As for P. The first half of the chapter explores ZnSnP2-yAsy substitutions (y = 0, 0.5, 1, 1.5, and 2) where full miscibility of the solid solutions is achieved. The measured electrical conductivity shows exponential increase with As substitution from 0.03 S cm-1 for ZnSnP2 to 10.3 S cm-1 for ZnSnAs2 at 715 K. Band gaps as calculated from the activation energies showed a steady decrease with increasing As concentration from 1.4 eV for ZnSnP2 to 0.7 eV for ZnSnAs2. The Seebeck coefficient decreases significantly with As substitution from nearly 1000 μV K-1 for the P end member to -100 μV K-1 for the As end member at 650 K. Indications of bipolar conductivity are seen starting with the ZnSnP0.5As1.5 member which decreases down to 100 μV K-1 at 650 K. Thermal conductivity is decreased due to alloy phonon scattering with y = 1 and y = 0.5 showing the lowest values of 1.4 W m-1 K-1 at 825 K. Figure of merit values are increased at lower temperatures when compared to the ZnGe1-xSn¬xP2 series due to increased electrical conductivity, with y = 1 reaching zT = 2.1∙10-3 and y = 2 reaching 2.8∙10-3 at 700 K. The ZnSnP2-yAsy series displayed lower thermal stability and therefore overall lower figures of merit were found. The higher order quinary solid solutions ZnGe1-xSnxP2-yAsy (x = 0.5, 0.75, and y = 0, 0.5, 1, 1.5, and 2) are also studied in Chapter 5. Successful synthesis and structural refinements of the solid solutions were performed with a preference for tetragonality again observed. The alloy phonon scattering effect shows additive behavior which decreased the thermal conductivity further to 0.8 W m-1 K-1 at 775 K for x = 0.75, y = 1 to within the glasslike regime. Transport properties for the x = 0.5 (y = 0, 0.5, 1.5, and 2) series were measured which showed significant improvements compared to properties obtained for quaternary series. Large Seebeck coefficients were maintained despite exponential increase of electrical conductivities with increasing As substitution displaying characteristics similar to high entropy alloys. For ZnGe0.5Sn0.5P0.5As1.5 electrical conductivity increases from 0.02 S cm-1 to 2 S cm-1 while Seebeck coefficient also increases from 500 μV K-1 to 575 μV K-1 between 325 K and 775 K. The resulting thermoelectric performance of ZnGe0.5Sn0.5P0.5As1.5, zT = 0.038, is increased by more than 30-fold of the highest performing end member ZnSnAs2 with greater thermal stability. The final solid solutions explored are the pnictides Ca11Sb10-xBix and Ca11Sb10-yAsy series in Chapter 6. A direct liquid solid synthesis method is performed which succeeds for many attempted samples while some contained elemental impurities. Single crystals of Ca11Sb10-xBix were obtained and structures solved which display coloring substitution effects. A correlation parameter using electronic structure calculations was developed which predicted the substitution effects well. The highest thermoelectric performance was found for Ca11Sb10, with zT = 0.093 at 1000 K, which showed improvement compared to other literature studies of the compound. Evidence of intrastructural suppression of bipolar conductivity is observed resulting in simultaneous increase in Seebeck coefficient and electrical conductivity with increasing temperatures. Bi substitution tended to increase electrical conductivity while decreasing the Seebeck coefficient due to increasing bipolar conductivity. Low thermal conductivity values were measured for all samples with the lowest Ca11Sb10 displayed phonon glass electron crystal like behavior of 0.6 W m-1 K-1 to 0.7 W m-1 K-1 at 300 K and 1050 K respectively