Batch Up Fabrication And Interfacial Behavior Of P-Type NbFeSb-Based Half-Heusler Materials

The half-Heusler compound?half-Heusler?,which has been widely studied in recent years,is generally regarded by researchers as one of the most high-temperature thermoelectric materials with large-scale commercial applications at present,because it has many excellent properties,such as electrical properties,thermal stability and mechanical properties,etc.After a lot of experimental and theoretical research by the researchers over the past few years,the P-type half-Heusler compound has a thermoelectric figure of merit?zT?of 1.5,and it has the conditions to be assembled into a thermoelectric device for application.So it is urgent to explore the process of preparing bulk thermoelectric materials and to find matching electrode materials that form smaller contact resistance and contact thermal resistance.This article takes NbFeSb-based half-Heusler alloy as the research object,mainly studies the preparation process of bulk thermoelectric materials and the causes of its performance attenuation,and selects suitable metals as electrodes to obtain smaller contact resistance,and studies the electrode junction at high temperature Stability.The following main results are obtained:1)Compared with small pieces of material,the preparation of large pieces of NbFeSb-based half-Heusler thermoelectric materials,the more important process changes are to extend the sintering time of the thermoelectric material and the annealing time of the sintered samples.In this way,bulk NbFeSb-based half-Heusler thermoelectric materials with high density and substantially no impurity phases can be obtained.But at the same time,it will cause the growth of grains and reduce the scattering of phonons at the grain boundaries,resulting in a decline in the thermoelectric performance of the bulk NbFeSb-based half-Heusler thermoelectric material throughout the test temperature range.At 1123 K,the thermoelectric figure of merit zT decreases by about 10%,reaching about 1.0.2)Nb_0.8Ti_0.2FeSb/Ti thermoelectric junction During SPS sintering,an iron-depleted layer and a Ti_0.9Fe_0.1 layer were formed at the interface.The work function of Nb_0.8Ti_0.2FeSb is lower than that of Ti0.9Fe0.1 intermediate layer,they form ohmic contact.The contact resistivity is about 0.15??·cm^-2.This is smaller than the contact resistance of the recently reported Nb_0.8Ti_0.2FeSb/Mo thermoelectric junction,and it is expected that half-Heusler thermoelectric devices will achieve higher conversion efficiency.Moreover,further research found that field emission is the main carrier transport mechanism in this interface layer.After a long-term aging treatment of 973 K,the Ti_0.9Fe_0.1 layer and the iron-depleted layer gradually expanded.After 25 days of aging,the contact resistivity reached 1.9??·cm^-2.The thickness of the interface layer increases linearly with the square root of the aging time,which indicates that the growth of the interface layer is determined by the interdiffusion of Fe and Ti atoms.3)After calculating the formation energy of point defects,the gap iron in the half-Heusler structure has the lowest formation energy,which is the most likely stable defect in the structure.Experimental studies have found that the maximum interstitial iron concentration in NbFeSb is about 6%.Continue to increase the Fe element will lead to the production of heterogeneous Fe₂Nb and Fe elemental.Pure NbFeSb shows the typical performance of p-type non-degenerate semiconductors.The introduction of interstitial iron atoms in the NbFeSb system can reduce the lattice thermal conductivity,but it has little effect on the electrical properties of the material.