Development and understanding of solvothermal synthesized copper chalcogenide thermoelectric materials

Abstract

Thermoelectric materials, with the capacity of direct energy conversion between heat and electricity, are attracting ever-growing research interest. Traditional thermoelectric materials, such as Bi2Te3 and PbTe, contain earth-rare or highly toxic element. Hence, recent studies in thermoelectrics are focusing developing new eco-friendly and low-cost thermoelectric materials.Phonon-liquid electron-crystal Cu2X-based thermoelectric are attracting extensive research interest due to relatively high performance. Solvothermal-synthesized highly crystallized Cu2X-based thermoelectric are acquiring attention due to low lattice thermal conductivity which should be mainly attributed to the nano-sized grains induced dense grain boundaries.Triggered by the idea that point defects can effectively scatter phonons in traditional thermoelectric materials, leading to low lattice thermal conductivity and enhance the thermoelectric performance, in this thesis, the influence of Ag doping on thermoelectric performance of Cu2Se were firstly studied. The abnormally enhanced lattice thermal conductivity is different from traditional understanding which offers new understanding on this material system. The additional point defects deriving from heavy element doping might have dramatic influence on superionicity of Cu2X-based thermoelectric material which can subsequently deteriorate the lattice thermal conductivity. Meanwhile, over-doping Cu2Se with additional Ag can introduce additional CuAgSe secondary phase, which can effectively reduce the lattice thermal conductivity of Cu2Se leading to enhanced thermoelectric performance.However, one key issue of the synthesized Cu2Se is the existence of common impurities, such as Cu, Cu2O and Cu3Se2. Although, these impurities have limited influence on thermoelectric performance of as-prepared Cu2Se, they are supposed leading to uncontrollable material synthesis and engineering processes as well as further application. For these reasons, we tuned the Cu/Se precursor ratios and successfully synthesized high-purity superionic cubic phase of Cu2Se which can maintain stable at room-temperature. This superionic cubic phase is generally believed being stable only above ~400 K. Thus, this material preparation idea can enlighten researchers interested in further structure and application study of this Cu2Se phase. Moreover, after spark plasma sintering, the sintered Cu2-xSe pellet exhibits high porosity with low lattice thermal conductivity, which implies potentially high thermoelectric performance with proper carrier concentration optimization.Other than Cu2Se-based thermoelectric materials, Cu2S-based thermoelectric materials are also attractive due to its low-cost and reported similar performance to Cu2Se-based ones. However, limited by the complex room-temperature phases and crystal structures of Cu2S-based thermoelectric materials, successfully synthesis of Cu2S-based thermoelectric materials, especially solvothermal method, has been rarely reported. Here, we successfully synthesized high-purity Cu2S-based thermoelectric materials through solvothermal method. Although, the synthesized Cu2S powders are composed of multiple phases at room-temperature, they can finally transfer into single-phase superionic cubic Cu2S at high temperature. The thermoelectric performance of as-prepared Cu2S can be easily tuned via controlling the reaction kinetic conditions, which can subsequently control the room-temperature phase contents. As the room-temperature phases have different vacancy levels, the changed phase contents subsequently changed the average Cu vacancy levels and leading to different Cu vacancy levels in the single phase high-temperature Cu2-xS. The modified Cu vacancy levels can subsequently tune the carrier concentration and finally enhance the thermoelectric performance via carrier concentration optimization.These studies here demonstrate both successful synthesis and effectiveness of material engineering strategies on thermoelectric performance of Cu2X-based thermoelectric materials. In the future, with further material-engineering, especially secondary phase hybridization, the thermoelectric performance and stability of Cu2X-based thermoelectric materials might be further enhanced. Enhanced performance and stability can further boost the corresponding applicational studies.