Solution Phase Synthesis And Property Study Of Thermoelectric Nanomaterials

How to release the energy crisis has been one of the most important problems for the 21^st century.Thermoelectric(TE) devices that can directly generate electric power by thermal energy and vice versa,have been considered as an efficient approach for improving the fuel efficiency and relieving the world's energy crisis.Nonetheless,the central issue in TE area is still to enhance the TE figure of merit(ZT) of the materials. Theoretical calculations indicate that the TE performance will be remarkably enhanced in low dimensional systems due to the sharper density and enhanced phonon scattering,which have been proved by studying the TE properties of superlattice films, bulk materials with embedded nanocrystals,nanowires and so on.However,it is still underdeveloped on the effective routes,especially the solution phase methods,for the synthesis of TE nanomaterials,therefore,it is still challenging works to obtain various promising TE nanostructures,including core/sheU heterostructure nanowires,large scale single crystalline nanotubes and so on,which will undoubtedly limit the advance of the TE area.This dissertation focuses on developing the solution phase synthesis of TE nanostructures and aims to solve some challenging topics in TE areas.The main contents in the dissertation are presented as follows:In chapter 1,we introduced the history and research progresses of TE materials, as well as the potential applications.The main contents include:the dominating factors and the possible routes for improving TE performance,the state-of-art bulk TE materials and the most important advances and synthetic routes for the TE nanostructures.According to the general analyses,several open questions are proposed at the end of this chapter.In chapter 2,we carried out the selective synthesis of PbTe hierarchical flower-like crystals,one-dimensional nanowires and size controllable nanocubes through a facile solution phase method using non-toxic ethylene glycol as solvent.On the basis of the research on the influence of NaOH concentration,surfactant and the systematic analysis on the shape evolution of the time-dependent products,we proposed a plausible growth mechanism of the products with different morphologies.In chapter 3,we developed a facile solvothermal route for the general synthesis ofâ…¤-â…¥alloy hexagonal platelets.Based on various control experiments,it can be concluded that the organic additive acts as the stabilizing agent and the NaOH plays a critical role during the formation of hexagonal platelets.The formation mechanism can be confirmed as the oriented attachment aider the systematic analyses on the time-dependent products.This work may provide a new rationale pertaining to the design of the solution synthesis of nanoarchitectures for materials possessing similar intrinsic crystal symmetry and will contribute to the development on exploring the novel synthetic routes for nanostructures.In chapter 4,we obtained Te/Bi and Te/Bi2Te3 core/shell heterostructure nanowires,for the first time,through the epitaxial growth involved in an ethylene glycol mediated facile solution phase method.The epitaxial relationship between Te and Bi or Te and Bi₂Te₃ can be described as:Te(100)//Bi or Bi₂Te₃(100), Te<100>//Bi or Bi₂Te₃<100>.The largely enhanced Seebeck coefficient and greatly lowered thermal conductivity of the heterostructure nanowire composites strongly contribute to the improvement of the thermoelectric performances,while the electrical conductivity of the nanowire composites greatly reduced compared with that of bulk Bi,where much effort should be devoted to enhance the quality of the bulk composite. This facile and general strategy provides an alternative route to synthesize 1D core/shell heterostructures and could be extended to other material systems with low lattice mismatch.In chapter 5,by introducing the nanoscale Kirkendall effect into an ethylene glycol mediated facile solution phase method,we successfully obtained PbSe and NiSe₂ hollow spheres,and Bi2Te3 hollow tubular nanostructures.It is confirmed that the Kirkendall effect could be the involved formation mechanism during the formation of these hollow structure based on the observation of the time-dependent products.The optical band gap of the PbSe and the magnetic property of the NiSe₂ hollow spheres both exhibit notable size effect.In addition,the Seebeck coefficient and thermal conductivity of the Bi₂Te₃ nanotube composites are greatly optimized, which indicates that the one dimensional tubular structure may possess excellent TE performance.In chapter 6,we investigated the influence of the extemal magnetic field distribution on the morphology of the ferromagnetic Ni crystal during the growth based on a facile solution phase method and carried out the controllable synthesis of three dimensional urchin-like crystals and one dimensional chain-like nanowires. Through the analysis of the shape evolution process of the intermediate products,we proposed the formation process of the urchin-like Ni nanostructure.It is indicated that the magnetic properties are quite dependent on the microstructure of the products. This work figures out the effect of non-uniform magnetic field on the growth process of the ferromagnetic metal and may benefit the advance of the magnetic-field assisted synthetic approaches.In chapter 7,we developed an optimized sol-gel process based on Pechini method which successfully lowered the crystallization temperature of the YBa₂Cu₃O₇ phase.According to the chemical reaction involved in this sol-gel process,it could be extended to other material systems.Furthermore,we obtained the highly ordered YBa₂Cu₃O₇ nanowire arrays through the sol-gel template route with the multi-step evacuation perfusion process in order to enhance the filling efficiency.The superconducting transition temperature(T_c) of the YBa₂Cu₃O₇ nanowires annealed in oxygen is about 92 K.