Synthesis,microstructures And Thermoelectric Properties Of ?-FeSi₂-based Materials

With the increasingly environmental pollution and the aggravation of the energy crisis,it is urgent to explore and utilize green and new energy sources.On basis of the Seebeck and Peltier effect,thermoelectric materials can directly convert heat into electricity and vice versa.Due to the advantage of being free of moving parts,no greenhouse gasses emissions and high reliability,TE technology shows the potential to be used in many applications such as waste heat harvesting,radioisotope TE power generation,and micro-refrigeration.?-Fe Si2 is one of the most promising candidates for RTG applications because of its non-toxic earth-abundant components,strong oxidation resistance and good thermal stability.However,the real application of?-FeSi₂is still limited by its tedious synthetic processes and low z T.The rapid condensation techniques provide efficient routes to synthesizing the single?phase,but the internal reaction mechanisms are still unclear.In addition,although many studies have been foucsed on optimizing the electrical and thermal transports of?-FeSi₂,most of which just briefly report the z T values.There is a lack of systematic analysis of the phase structures,microstructures,electrical and thermal transport mechanisms.Moreover,the z Ts for the doped FeSi₂ systems are still too low due to the limited number and the low doping content or poor carrier donation efficiency of the dopants.Based on the above,this paper firstly studies the reaction processes and mechanisms for quickly preparing?-FeSi₂material under the rapid condensation techniques.Secondly,for the first time,a series of heavy elemnts doped or alloyed FeSi₂ compounds have been prepared.Their phase structures,microstructures,thermoelectric properties,electrical and thermal transport mechanisms are systematically studied.Furthermore,the high-temperature stability of the optimized n and p type FeSi₂materials are evaluated.Finally,we manufacture the FeSi₂ device elements and invesitigate their interface structures,together with a preliminary screening of the interface barrier layer materials.This paper has achieved the following innovative results:1.Under the rapid condensation(Arc-melting)technique,the as-melted peritectic structure,which consist of?-FeSi_2+?and?-Fe Si phase,is homogeneous and size-fine.The?and?phase are completely transformed into the?phase without any residual secondary phases after annealing at 1173 K for just 48 h.The phase transformation from?and?phase to?phase is complicated though the slow condensation prapeation methods,which is finished by the following reactions:(1)?+???;(2)???+Si,?+Si??.However,this process can be accomplished only with the reaction(1)by means of the rapid condensation techniques,and the reaction process can be well explained with the Ginstling-Brounshtein-Habert kinetic model.The fine size of the peritectic phase and the simplification of the solid phase reaction during the annealing process are the main reasons for quickly preparing?-FeSi₂material under the rapid condensation techniques.2.The effect of Co-doping on the TE properties of n type FeSi₂are investigated.The carier concentration,electrical conductivity and power factor can be significantly enhanced by substituting Fe with Co.A maximum z T value of 0.26 is achieved for Fe_0.94Co_0.06Si₂at 900 K.Besides,a unique two-step annealing process(pre-annealing at 1423 K for 20 min;re-annealing at 1173 K for 48 h)is adopted to further alloy the heavy element Ru at Fe sites.Ru exhibits an inhomogenous distribution in?phase,which is attributed to the different Ru solution contents in?-Fe Si and?-FeSi₂₊?before the formation of?-FeSi₂ and the slow diffusion behavior of Ru during the re-annealing process.Ru-alloying scarcely modifies the electrical transport properties while it obviously reduces the lattice thermal concuctivity by inducing strong lattice distortions and mass fluctuations to enhance the phonon scaterring.A maximun z T of 0.33 at 900 K for Fe_0.89Ru_0.05Co_0.06Si₂is achieved,increased by 27%compared with the pristine Fe_0.94Co_0.06Si₂.3.The microstructures,elemental distributions and TE properties of the n type Fe_1-xIr_xSi₂compounds are studied.For the prestine FeSi₂ and Fe_0.84Ir_0.16Si₂,pure phases with no elemental agglomeration are obtained.When the Ir-doping concentration 0<x<0.16,Ir atoms exhibit multi-gradient distributions inside the?phase.Ir-doping can significantly optimize the electrical transport and suppress the lattice thermal conductivity.The average power factor of Fe_0.84Ir_0.16Si₂ at 300?900 K can reach 14.4?W cm^-1 K^-2.Besides,Fe_0.84Ir_0.16Si₂ has the lowest?_L values of 2.1 W m^-1K^-1at 300 K and 1.6 W m^-1K^-1at 1000 K,decreases by 89%and 71%respectively as compared with that for the prestine FeSi₂.A maximum z T value of 0.62 is obtaind at 1000 K for Fe_0.84Ir_0.16Si₂.More importantly,the average z T value of Fe_0.84Ir_0.16Si₂at 300?900 K is as high as 0.36.4.The effect of Al-and Mn-doping on the electrical transport of p type FeSi₂are investigated.Al has a much shallower defect acceptor transition level in?-FeSi₂ than Mn,leading to the higher carrier donation efficiency of Al than Mn.The carrier concentration for FeSi_2-xAl_xis quite close to the optimal value predicted by the single parabolic band(SPB)model.Consequencely,FeSi_2-xAl_xexhibite high electrical conductivity and power factors in the entire temperature range.Both highest z T values of 0.18 are obtained for FeSi_1.96Al_0.04and Fe_0.92Mn_0.08Si₂ at 900 K,but the FeSi_1.96Al_0.04 demonstrates higher TE performance than the Fe_0.92Mn_0.08Si₂ in the middle-low temperature range.We further alloy heavy element Os at Fe sites and prepare a series of Fe_1-zOs_zSi_1.96Al_0.04 compounds.Os-alloying has little effect on the electrical transport properties,while can significantly reduce the lattice thermal conductivity.The lattice thermal conductivity of Fe_0.80Os_0.20Si_1.96Al_0.04 at 300 K and 900 K is52%and 41%lower than that of FeSi_1.96Al_0.04,respectively.The highest z T value is 0.35 for Fe_0.80Os_0.20Si_1.96Al_0.04 at 850 K,and the average z T value at 400?900 K reaches 0.24,which both double that of the pristine FeSi_1.96Al_0.04.5.The prestine FeSi₂,Fe_0.84Ir_0.16Si₂and Fe_0.80Os_0.20Si₂Al_0.04 all show excellent oxidation resistance and good thermal stability at 1173 K in atmospheric condition.Fe_0.94Co_0.06Si₂ and FeSi_1.96Al_0.04 are slected as n and p type materials to prepare thermoelectric elements.When there is no interfical barrier layer material,Co and Al will diffuse into the p and n type materials respectively,leading to an exceptionally high interface contact resistivity(6.1�10³??cm²).Several kinds of interfical barrier layer materials are added between the n-type Fe_0.94Co_0.06Si₂ and p-type FeSi_1.96Al_0.04.Ni,Ti and Ti Al₃ barrier layer materials are completely damaged duo to the severe diffusion or chemical reactions with FeSi₂ components.Although no serious diffusion phenomenon occur between Nb and FeSi₂,Nb is totally oxidized at high temperature,leading to the collapses of the devices elements.Besides,MoSi₂,WSi₂ and Ti B₂cannot be sintered into dense blocks under the sintering temperature of FeSi₂.Fe_0.94Co_0.06Si₂/CoSi₂/FeSi_1.96Al_0.04element is prepared by adopting CoSi₂ as interfical barrier layer material.Only a weak diffusion occur between CoSi₂ and FeSi₂.The devices element remain intact without significant oxidation after aging in air at 1173 K.Moreover,no further diffusion occurr between the barrier layer material(CoSi₂)and FeSi₂.Therefore,CoSi₂ is very promising to be used as barrier layer material for the FeSi₂ devices element.