Defect engineering for more reliable and efficient PbTe-based thermoelectric materials

Male, James Patrick

doi:https://doi.org/10.21985/n2-ht4g-3d36

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Defect engineering for more reliable and efficient PbTe-based thermoelectric materials

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Thermoelectric devices utilize semiconducting n-type and p-type thermoelectric materials to convert heat into electricity. Despite their promise for deep space power generation or waste heat recovery, most high-performing thermoelectric materials reported in literature are absent in practical applications - partially due to inconsistent synthesis and poor mechanical performance. This work addresses such shortcomings and introduces new strategies to improve efficiency in the classic PbTe system by engineering point defects and dislocations. In examinations of poorly performing n-type PbTe in the literature, charged intrinsic vacancy point defects are found to suppress doping and lower performance far below optimized levels. A new strategy involving thermodynamic control of phase equilibrium is introduced as a viable means to tune doping efficiency and achieve consistently high performance by controlling intrinsic defects. Similar strategies are also used to enhance dopant solubility in the p-type PbTe analogue, which improves both carrier doping and electronic band convergence. Defect engineering is then used to understand poor (and historically misunderstood) mechanical performance in PbTe. Embrittlement is explained using measurements of semiconducting, elastic, and mechanical properties and advanced microscopy. Typical p-type doping strategies are found to cause embrittling defect-dislocation interactions through high intrinsic defect concentrations or clustering of mobile p-type dopants. Further, some n-type dopants are identified as embrittling defects for the first time due to their highly strained interstitial configurations. Temperature-dependent neutron diffraction experiments confirm that high internal strain and high defect concentrations result from embrittling defects, while also indicating that some stain annealing may be possible at high temperatures in PbTe-based powders. Similar concepts are extended to newer rare earth telluride thermoelectric systems under consideration for power generation on future NASA missions. These compounds simultaneously show elastic stiffening and degrading mechanical performance with increasing cation vacancy concentration. The combined work in this thesis illustrates the advantages and consequences of excessive defect engineering and the necessity to consider both mechanical and thermoelectric performance when synthesizing thermoelectric materials for practical applications.

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