Development of high-efficiency metal chalcogenide thermoelectric nanomaterials by nanostructure engineering

Abstract

The development of high performance thermoelectric materials, which can directly convert heat into electricity, is becoming an alternative to overcome the global energy shortage. The efficiency of thermoelectric materials is determined by the dimensionless figure-of-merit ZT=S2sT/k, where S is the Seebeck coefficient, s is the electrical conductivity, T is the absolute temperature, and k is the thermal conductivity includes the contribution from the electron (ke) and lattice (kL) components. High efficient thermoelectric materials can be extensively applied as alternative energy sources in many fields such as waste heat recycling, solid state power generation, and refrigeration. The challenge of developing high performance thermoelectric materials is how to enhance the ZT value by optimizing the conflict and interdependent parameters (S, s and k). Metal chalcogenides have been targeted in this thesis because they intrinsically have good electrical transport properties (high S and s) and low k. Among them, Bi2Te3, PbTe, and Cu2Se have the highest ZT in room temperature, intermediate (500-800 K) and higher temperature (~1000 K) range, respectively. To further enhance their thermoelectric performances, nanostructure engineering has been applied in this thesis. Cu2Se, Bi2Te3 and PbTe nanomaterials have been synthesized via facile and controllable solvothermal methods; their structures and thermoelectric properties were extensively investigated. Nanostructured b-phase Cu2Se materials were synthesized and sintered using spark plasma sintering process. The nano-sized grains were preserved after the sintering, leading to high density of small angle grain boundaries accommodated by defects, which significantly reduced the kL of as-prepared samples but did not affect the electrical transport properties, resulting an outstanding ZT of 1.82 at around 850 K. Via a controlled synthesis approach, Cu-deficient Cu2-xSe nanomaterials were obtained. The high degree of Cu deficiency was found to trigger the phase transition from b- to a-phase, leading to small amount of a-phase in the Cu1.95Se sample. The Cu deficiency was proved to harm the thermoelectric performance of Cu2-xSe nanomaterials via increasing the carrier concentration, and leading to a significantly reduced S. Tellurium was doped into Cu2Se nanomaterials to modify the electrical transport properties. The effects of Te doping to the Cu2Se nanomaterials were carefully studied, the Cu2Se0.99Te0.01 sample was found to have the highest S among all the Te-doped samples and the ZT of Cu2Se0.98Te0.02 has the highest peak ZT ~1.76. The developed nanostructure engineering was approved to be effective on Bi2Te3 and PbTe nanomaterials. Pure Bi2Te3 hexagonal nanoplates were synthesized and sintered. High density of small angle grain boundaries accommodated by defects were also found in the sintered Bi2Te3 nanomaterials, which significantly reduced the k and resulted in an improved ZT ~0.88 at 400 K. The Bi-doped PbTe nanocubes were obtained, and the doping of Bi was confirmed via multiple technologies. The high density of grain boundaries and the Bi dopant effectively reduced the k. Also, the Bi dopants improved the electrical transport properties of PbTe, finally leading to enhanced ZT. In this thesis, reliable, facile and controllable solvothermal methods were developed to obtain metal chalcogenides-based nanomaterials. By applying nanostructure engineering, the enhancement of thermoelectric performances for metal chalcogenides-based nanomaterials has been achieved.