Synthesis And Thermoelectric Properties Of P-type MnX(X=Si,Te)Compounds

Recently,energy crisis has become a great challenge for all countries in the world.With the rapid development of global industrialization and the continuous improvement of the living quality,the demands for energy has increased substantially,and the non-renewable fossil fuel resources have been continuesly consumed.Till2013,the remaining proven reserves of coal,oil and natural gas in the world are 891.5billion tons,238.2 billion tons and 186 tillion meters,respectively,and the total amount of converted standard coal is 1.2 tillion tons.According to the current world average mining intensity,global coal,oil and gas can be mined for 113 years,53years and 55 years,respectively.At the same time,the huge environmental pollution caused by the consumption of fossil fuels also threatens the survival and sustainable development of human beings.To fulfil the increasing requirements of the energy,clean energy such as hydropower,wind energy,solar energy and the others are being developed and utilized.The development of clean energy are playing an important role in world energy supply.Compared with other new energy conversion technologies,thermoelectric power generation technology can realize the direct conversion of thermal energy and electric energy through Seebeck effect,which is a kind of all-solid-state energy conversion mode.At the same time,thermoelectric power generation technology has the advantages of simple structure,no transmission parts,no chemical reaction,no fluid medium,high reliability,no noise,no vibration,no pollution and so on.Through Peltire effect,thermoelectric refrigeration,precise temperature control and microelectronic device cooling can be realized,which has been widely studied and paid more attention recently.Mn element belongs to transition metal element and its d electronic state may be spin-coupled with p or f electronic state,so Mn-based thermoelectric materials usually have high Seebeck coefficient.The compounds with Mn element have high potential thermoelectric properties,and the advantages of rich crustal reserves,low cost emphasize their value for commercial application.In this thesis,two kinds of thermoelectric materials,high manganese silicon and MnTe,were studied as the research objects,the synthesis methods and thermoelectric properties of the two compounds were optimized.High manganese silicon is a kind of medium temperature p type thermoelectric material with complex structure of chimney phase.In recent years,the optimization of high manganese silicon thermoelectric performance is mainly in the aspect of electrical performance optimization.Increasing carrier concentration by doping is the main way to optimize the electrical transport performance of high manganese silicon.However,for the preparation of high manganese silicon,the traditional processes such as melting,mechanical alloying and induction melting are still concentrated.The main problems of traditional melting method are that the diffusion rate of Si is slow,the peritectic reaction is difficult to complete during melt cooling,the second phase of MnSi is easily formed in the product,and the thermoelectric properties of high manganese silicon are degraded.For the mechanical alloying process,the problems are that the raw materials are lost in the milling process,the stoichiometric ratio is difficult to control,and the impurities in the ball milling tanks and balls are easy to be introduced.Induction smelting is one of the most efficient and relatively pure methods in the preparation of high manganese silicon.However,the high requirement of equipment and the high energy consumption limit the commercial application value of induction smelting.Aiming at the problems of preparation process and thermoelectric properties of high manganese silicon,the contents of this thesis are as follows:?1?Synthesis process:in view of the limitations of the preparation process,the self-propagating combustion synthesis method was applied to the preparation of high manganese silicon for the first time in this study.The main advantages of this method are as follows:?i?the preparation period is greatly shortened;?ii?accurate composition control,no second phase impurity.?2?Optimization of conductivity:for the optimization of electrical transport performance of high manganese silicon,it is mainly through doping Ge element,introducing positive electric center into the material,thus increasing the carrier concentration and optimizing the electrical transport performance of the material.?3?Optimization of thermal conductivity:the second phase composite high manganese silicon compound was prepared by the method of self-propagating combustion synthesis through stoichiometric ratio of raw materials using MnS and MnTe.This is mainly due to the formation energy and adiabatic combustion temperature of MnS and MnTe are lower than those of high manganese-silicon compounds,so the phase of MnS and MnTe is preferentially formed,thus avoiding the influence of S and Te on Si sites.After the second phase recombination,the grain size is refined and the lattice thermal conductivity is greatly reduced.The grain boundary plays an important role in the decrease of lattice thermal conductivity.At the same time,the composite of the second phase can optimize the mechanical properties of the materials at room temperature and high temperature to a certain extent.In the first part of the study,we mainly studied the high manganese silicon thermoelectric materials,and found that MnTe as the second phase composite in high manganese silicon thermoelectric properties will be optimized.MnTe itself has potentially high thermoelectric properties.Therefore,in the second part of the study,we mainly study the thermoelectric performance of MnTe itself and optimize its thermoelectric performance.The contents of the study are as follows:?1?The optimization of synthesis process:the feasibility of synthesis of MnTe compounds by high temperature self-propagating combustion was explored,and the synthesis mechanism was systematically studied.The relationship between thermoelectric properties and Mn content was studied.?2?The optimization of carrier concentration:in view of the extremely low intrinsic carrier concentration of MnTe,we use the Ag of+1 valence as the doping element to replace the Mn with+2 valence.The carrier concentration increased by an order of magnitude,the Seebeck coefficient at high temperature was not significantly decreased,and the final power factor was greatly increased.The lattice thermal conductivity can be optimized by using Ag₂Te as the second phase and the Te site se as the substitute,and the effect on the electrical transport performance is relatively small.Finally,the ZT value is further improved on the basis of Ag doping.