Research On The Thermoelectric Properties,mechanical Properties,as Well As The Thermal Stability Of Mg₂IV?IV=Si,Ge,Sn? Based Solid Solutions

Thermoelectric?TE?power generation is a kind of technology capable of directly converting waste industrial heat?such as from exhaust pipes of cars and trucks or industrial kilns and furnaces?into usable electricity,with the advantages of exceptionally high reliability,absence of any moving parts,and a silent operation.Thus,TE power generation shows a great potential for large scale applications among other new green energy technologies.TE materials play a key role in the TE power generation process,which contributes directly to the energy conversion efficiency.Mg2IV?IV=Si,Ge,Sn?based compounds are materials intended for TE power generation in the temperature range of300-800 K.The attribute of these compounds is not only a good TE performance,but also the fact that the chemical elements are plentiful,inexpensive and environmentally friendly.However,before coming into large scale applications,there are lots of key issues to deal in terms of Mg₂IV?IV=Si,Ge,Sn?based compounds.Generally speaking,there are four main unsolved problems,which related with preparation techniques,TE performance,mechanical properties,thermal stability,and so on.Take the solid solution Mg₂Si_0.3Sn_0.7.7 as an example,it should be single phase due to the Mg2Si-Mg2Sn quasi-binary phase diagram,however,many researchers have reported different phase composition results when prepared by different methods,and some of them are even opposite.Also,many researchers have observed conduction band convergence in the Mg₂Si_1-xSn_x and Mg₂Ge_1-xSn_x?0?x?1?binary systems,but whether it is the same case in the Mg₂Si_1-x-yGe_xSn_y?0?x,y,x+y?1?ternary system,and what are the relations between the phase composition,the electronic band structure,and the lattice thermal conductivity with the Si/Ge/Sn ratio,are still open questions for all researchers.By the way,engineers in TE generators found that Mg₂IV?IV=Si,Ge,Sn?based compounds are very easy to form cracks during the machining processes?like cutting and polishing?when fabricating modules,and the cause is the high brittleness.However,they have not put forward any good plans to toughen these materials without the deterioration of TE performance by now.In addition,researchers have pointed out that Mg2IV?IV=Si,Ge,Sn?based compounds share poor stability in the early 1970s,they are not appropriate for application in air at high temperatures.How to solve this problem still remains unknown in years past.These issues mentioned above are the necessary prerequisites for the TE power generation of Mg₂IV?IV=Si,Ge,Sn?based materials.Based on the practical application,we have taken a complete and systematic research,covering from the field of sample preparation,TE&mechanical properties optimization,to oxidation prevention,aiming to a full scale enhancement of the performance of Mg₂IV?IV=Si,Ge,Sn?based materials.Details about the content and the results of our research are listed as below:Taken the example of Sb doped Mg₂Si_0.3Sn_0.7,dependence of the phase composition and the microstructure on preparation temperature was studied via a two step solid reaction method followed by quenching,with the help of Mg2Si-Mg2Sn quasi-binary phase diagram.Due to the multiple peritectic reactions in Mg₂Si_1-xSn_x(the similar case in Mg₂Ge_1-xSn_x and Mg2Si1-x-yGexSny),which generating products with different compositions and phases?i.e.solid and liquid?,modulation on the composition,morphology,and content of in-situ nanostructures can be realized with heat-treatment around these peritectic temperatures.This is a good news for controllable preparation of in-situ nanocomposites.Agglomerates of Mg₂Si rich nano-precipitates with the averaged size of 40 nm appeared when treated at900 K?the similar case when treated at 837 K?.However,the precipitates changed to be Mg2Sn phase when treated at 1130 K.Though sharing the same size with Mg2Si rich nano-precipitates,Mg₂Sn rich nano-precipitates dispersed more homogeneously without any agglomeration.And single phase was obtained when treated at 1080 K.The lattice thermal conductivity decreased dramatically due to the well distributed Mg2Sn rich nano-precipitates,and a maximum ZT of about 1.20 was achieved at around 750 K.Moreover,the averaged value of the dimensionless figure of merit ZT_AVG reached about 0.9in the range of 300-800 K,about 15%higher than the value composed of Mg₂Si rich nano-agglomerates.Then,based on the research of Mg₂Si_1-xGe_x,Mg₂Si_1-xSn_x and Mg₂Ge_1-xSn_x?0?x?1?binary systems,a newly Mg₂Si_1-x-yGe_xSn_y?0?x,y,x+y?1?ternary system has been developed,and it was observed that the immiscible gap in the Mg2Si1-xSnx binary system could be narrowed with the introduction of minor Mg₂Ge.Beyond this,the dependence of the electronic band structure of Mg₂Si_0.3-xGe_xSn_0.7 and Mg₂Si_0.3Ge_ySn_0.7-y?0?x,y?0.05?ternary solid solutions on the composition and temperature,is explained by a simple linear model,and the lattice thermal conductivity of solid solutions with different Si/Ge/Sn ratio is predicted by the Adachi model,together with the well explanation of Callaway model.The experimental results show excellent consistency with the calculations which suggests that these calculations might be suitable to describe the electronic band structure and the lattice thermal conductivity of other solid solutions.Due to the similar physical properties of Mg₂Si and Mg₂Ge,Mg_2.16(Si_0.3-xGe_xSn_0.7)_0.98Sb_0.02?0?x?0.05?basically shared the same value with Mg_2.16(Si_0.3Sn_0.7)_0.98Sb_0.02.However,for the Sb doped solid solutions Mg_2.16(Si_0.3Ge_ySn_0.7-y)_0.98Sb_0.02?0?y?0.05?,the energy offset between the light conduction band and the heavy conduction band at higher temperatures?500-800 K?was decreased with the increasing Ge content,thus making a contribution to the conduction band degeneracy,and enhancing the power factor in turn.Meanwhile,mass fluctuation and strain field scattering processes were enhanced when Ge substitutes for Sn in Mg_2.16(Si_0.3Ge_ySn_0.7-y)_0.98Sb_0.02?0?y?0.05?due to the large discrepancy between the mass and size of Ge and Sn atoms,and the lattice thermal conductivity was decreased as a consequence.Thus,the TE performance was improved,with the figure of merit ZT larger than 1.45 at about 750 K and the averaged ZT value between 0.9 to 1.0 in the range of300-800 K,which is one of the best results in the Sb doped Mg₂IV?IV=Si,Ge,Sn?systems with single phase.Introducing finely distributed Mg₂Sn rich in-situ nanostructures by quenching at 1130 K made a contributiong to the lattice thermal conductivity reduction,but so was the power factor,resulting by the siginificant Mg loss at such high temperature.And the peak ZT value was slightly reduced at last.Furthermore,on account of the brittleness,SiC and CaSO₄ additives with excellent toughness as well as high strength were incorporated in Mg_2.16(Si_0.3Sn_0.7)_0.98Sb_0.02.02 and Mg_2.16(Si_0.3Ge_0.05Sn_0.65)_0.98Sb_0.02 in an attempt to improve the mechanical properties without any notable TE performance degradation.The effects of the phase fraction and morphology?i.e.SiC nano-powders,SiC nano-wires,and CaSO₄ whiskers?of the additives on the TE&mechanical properties of the composites have been characterized in detail.It was observed that SiC nano-powders were dispersed uniformly on the grain boundaries of the matrix,while SiC nano-wires and CaSO₄ whiskers were embedded randomly in the matrix.Due to the pinning effect,fiber bridging and fiber pull-out mechanisms,significant enhancements in the mechanical properties were observed,as the fracture toughness and the compressive strength of the composite with 0.8 at.%SiC nano-powders or nano-wires were improved by about 50%and 30%,respectively.The toughening effect of CaSO₄ whiskers was inferior to that of SiC nano-wires on the same incorporation level,mainly attributing to the morphology?the former is much longer than the latter?.However,due to the much larger incorporation limit,CaSO4 whiskers also produced very good results in optimizing the mechanical properties.By the way,the TE properties of the composites changed little due to the small amount of additives,with a maximum ZT value of¹.20 at 750 K.And the toughness basically showed no changes by forming Mg₂Sn rich nanostructures.Comparing with the aforementioned results,it suggests that nano-toughnening works only if the nanostructure itself shares reasonable toughness.Last but most important,the thermal stability of Mg₂IV?IV=Si,Ge,Sn?based materials was thoroughly studied in simulation of the practical service condition,i.e.by changing the annealing temperature,the annealing time,the annealing atmosphere and preventive coatings.The results showed that,oxidation was inevitable when the materials were operated in air without any protection measures.And things have not got better in vacuum,as Mg evaporation and the formation of?-SnSb alloy were observed in this case.But when sprayed with BN coatings and worked in air,the aforementioned difficulties were successfully solved.This simple technique is a good news for the TE application.In addition,the working temperature of Mg₂IV?IV=Si,Ge,Sn?based materials is suggested to be below 773 K,otherwise,phase segregation will occur due to the peritectic reaction.And,for composites,such as those with SiC nano-wires and CaSO₄ fibers,the smaller the discrepancy between the coefficient of thermal expansion of the additives and the matrix,the better the thermal stability.