Investigation On Some Environmental-friendly Thermoelectric Silicides And Oxides

Thermoelectric(TE) materials are functional materials that can convert heat energy to electricity directly and visas verse.They show great potential in practical applications providing a safe,reliable and full-solid-state way of power generation and refrigeration.Most of the state-of-the-art TE materials are however alloys composed of high cost and toxic elements,such as Te,Sb,Co,Pb,etc.,which constrains the development of TE applications due to the environmental and economic issues with them.Higher manganese silicides(HMS), one of the most promising TE silicides,have attracted increasing interest for their low cost of raw materials,environmental friendliness and thermal and chemical stabilities.Most of the research on HMS have being carried out in Russia and Japan,where a maximum dimensionless figure of merit of 0.7 was reported and a conversion efficiency of 8%-12%of a HMS-based silicide TE module was achieved.However,China is just on the starting line for the research of HMS TE materials and devices.Based on the concept of developing environmental-friendly TE materials,detailed investigation was performed on HMS and oxide materials.Focuses have been made in several aspects from property tuning,nanostructuring to module simulation.Aiming at improving the TE performance,HMS composites with secondary phases were prepared through in situ melting or ex situ mechanical mixing combined with hot-pressing densification process.Composites of CeSi₂/HMS,PbTe(Ag₂Te)/HMS and SiGe(Ge)/HMS were synthesized and their phase structures,microstructure and TE properties were investigated.It was found that the TE performance of CeSi₂/HMS and PbTe(Ag₂Te)/HMS was degraded due to the physical transport characteristics of the secondary phases and their inhomogeneous distribution in the matrix.In the composite SiGe(Ge)/HMS,although inhomogeneity of SiGe precipitations was observed,the figure of merit was improved due to increased power factor and reduced lattice thermal conductivity by partial substitution of Ge for Si.The ZT value was improved by 25%-50%in SiGe(Ge)/HMS composites.Moreover, the microstructure of the inherent MnSi layers in SiGe(Ge)/HMS was significantly influenced by addition of Ge,which may also affect the transport properties of the material.Nanostructuring is an effective way to improve the TE performance by quantum effect in many material systems.HMS powders with different grain sizes and phase constitution were obtained by high-energy ball milling.The mechanochemical decomposition of HMS was for the first time discovered in this work,and the effects of ball milling parameters on the decomposition rate of HMS and their grain sizes were studied in detail.It was found that by hot-pressing the single phase HMS nanopowders,the MnSi phase was obtained in the pellet. In situ energy dispersive x-ray diffraction revealed some phase transformations under high temperature,on which the grain size effect was important.Compared to the micropowders, the HMS nanopowders showed much lower temperature and faster rate of phase transformation.High pressure could either depress or accelerate the phase transformation of nanopowders in different temperature ranges.It was understood that the phase transformation of HMS should be attributed to the inter-evolution of several incommensurate phases,which is closely related to the total internal energy of the system and the thermodynamic features of Mn-Si system.Intrinsically,the highly anisotropic crystal structures of HMS and their decreased stability under nanoscale dimension were supposed to introduce mechanochemical decomposition and inter-evolution of HMS.The presence of metallic MnSi phase during the phase transformation neglected the possibility of using ball-milled HMS nanopowders as TE materials.Perovskite-type La_1-xSr_xCoO₃ oxides were synthesized by solid state reaction and sol-gel method combined with hot-pressing,and the effects of temperature and Sr-doping on the TE properties were explored.It was found that the undoped LaCoO₃ showed an extremely high Seebeck coefficient of 555μV/K at 300K,which however decreased quickly with increasing temperature.As Sr-doping was introduced,the Seebeck coefficient showed also sudden decrease,and a good TE performance was not expected due to reduced power factors.The maximum figure of merit was obtained in sol-gel prepared La_0.9Sr_0.1CoO₃ with a ZT value of only 0.046 at 300K,which is even not considerable for TE use.The increase of carrier concentration by Sr-doping and the spin state transition of Co³⁺ at different temperatures were attributed to the change of electrical transport properties of La_1-xSr_xCoO₃.Optimization of size parameters is usually needed before building TE devices to obtain best performances.Using the well-known TE theories and Mathematica programming software,the optimized size parameter A_p/A_n,heat flow Q,maximum power density P_max and efficiencyη_max were simulated for a simple TE module based on P-type HMS and N-type Mg₂(Si,Sn).The results show that the module is able to produce a maximum efficiency of 8.29%in ideal contact condition when the temperatures of the cold and hot side maintain 298K and 898K,respectively.The size parameter A_p/A_n for obtaining P_max andη_max should be 1.97 and 1.89,respectively.Contact simulation revealed that when the electrical and thermal conductivity of the contact layer is comparable or better than the P- and N-type materials,the performance of the TE module shows no significant decline,while it is the opposite case when the electrical and thermal conductivity of the contact layer decrease simultaneously by the magnitude of 10¹-10².