Fe/Ni/Ti-based Electrode Barrier Materials For Medium And High Temperature Thermoelectric Devices Of Half-Heusler Compounds

As a new type of thermoelectric materials,half-Heusler compounds(HH),which has excellent electrical and mechanical properties,good thermal stability,relatively inexpensive and environmentally friendly components and many other advantages,is regarded as a promising candidate for the large-scale commercial thermoelectric power generation device applications in the intermediate temperature.In recent years,HH thermoelectric materials have been extensively studied and made some inspiring progress.The ZT values of the N-type and P-type HH thermoelectric materials have exceeded 1.0,and the theoretical calculation value of the thermoelectric device conversion efficiency has reached 15%.However,the research on the preparation of thermoelectric devices is insufficient.Besides,the stability of the device electrode connector has an important impact on the thermoelectric device conversion efficiency.In this work,The N-type Hf0.25Zr0.75 Ni Sn0.99Sb0.01 and P-type Fe Nb0.88Hf0.12 Sb HH thermoelectric materials were prepared by arc melting,solid-phase reaction and spark plasma sintering(SPS).The ZT values of the two HH thermoelectric materials were close to 1 at 800 K.On these basis,pure metal Fe/Ni/Ti and Fe Ni,Ti Ni alloys were used as barrier materials to prepare the connections of barrier/HH by a two step SPS technique,and the difussion interfaces,contact resistance and isothermal aging were studied in detail.The main results are shown below.(1)The diffusion layer thickness of the Fe,Ni,and Fe Ni/N,P-HH connections are less than 50 ?m,and the contact resistivity are 16~18 ???cm2,but the quality of the connection is poor.The fracture failure is occurred in the SPS procedure for Fe,Ni,and Fe Ni/P-HH connections,and in the isothermal aging procedure for Fe,Ni,and Fe Ni/N-HH connections.Fe/Ni-Sn/Sb intermetallic compounds may have formed in the difussion interfaces,allotropic transformation or solid-state phase transformation are existed for these compounds below the temperature of 873 K,which cause a sudden change of volume,resulting the fracture failure.(2)The connection of Ti as a barrier layer with N or P-HH is firm,but the diffusion of Ti / N-HH interface is drastic with Ti2 Ni,Ti3Sn and(Ti,Zr)Sn as reaction products.The diffusion layer thickness after post-aging(192 h)is about 500 ?m,and the contact resistivity is about 18 ???cm2.These results indicated that pure metal Ti is not suitable to use as N-HH barrier material.Ti/P-HH connected firmly,no cracks.The interfacial diffusion is slow even in long time aging.The thickness of diffusion layer is about 8~12 ?m.The growth rate of the diffusion layer gradually decreases with the aging time,and the average growth rate of the diffusion layer is about 0.04 ?m/h.The products of the interfacial reaction are ?(Ti,Nb)and Ti3 Sb.The contact resistivity of the interface increases with the increase of the aging time,the maximum RASC(192h)= 0.86 ???cm2.The results indicated that Ti is a reliable barrier material for P-HH.(4)10at.% Ni mixed in Ti barrier can significantly inhibited the diffusion reaction for Ti/N-HH.The diffusion layer thickness was 50 ?m which is decreased by 90%.After the isothermal aging,the contact resistivity is 11 ???cm2,which decreases by about 30%.The products of the diffusion layer are Ti3 Sn,(Hf,Ti,Zr)Sn,Ti2 Ni,Ti Ni,Ti2 Sn.When the Ti Ni is used as the P-HH barrier material,the interface diffusion layer thickness is about 10 ?m and the contact resistivity is about 1 ???cm2,which are close to Ti/P-HH.The product of the diffusion layer are Nb4Sb3?(Fe(HT2)Ni)and Fe Ti.Further more,Ti Ni has higher temperature oxidation resistance,stronger mechanical properties than Ti,which is more suitable for high temperature N/P-HH thermoelectric device as electrode barrier material.The rate of diffusion of the interface is slow,which is about 0.125 ?m/h for the aging time of 24 h and 0.04 ?m/h for longer.The thickness of layer after 48 h aging tends to be stable,which effectively reducing the contact resistivity with the aging time changes and help to improve the stability of the device.