Study On The Barrier Layer Materials And The Fabrication Of Half-Heusler Thermoelectric Devices

In the presence of temperature differences,thermoelectric devices convert thermal energy into electrical energy through Seebeck coefficient.A complete thermoelectric device is composed of p-type and n-type thermoelectric legs connected in series in parallel by electrodes.Half Heusler?HH?compound is a new kind of thermoelectric semiconductor material.It has high ZT value in the middle and high temperature range,good mechanical properties,high thermal stability and low cost.It is considered that the thermoelectric material can be used to make medium and high temperature thermoelectric devices.Including Hf_0.25Zr_0.75NiSn_0.99Sb_0.01 and FeNb_0.88Hf_0.12Sb are typical of n-type and p-type HH thermoelectric materials,both of them ZT values are close to 1.0 in 900 K..There are still many problems in using HH thermoelectric materials to make thermoelectric devices.One of the main problems is to achieve mechanical,electrical and chemical stability of the connection between the thermoelectric material and the electrode material.Because the conversion efficiency of a thermoelectric device depends not only on the ZT value of the thermoelectric material,but also on the structural factors of the device,such as the quality of the thermoelectric material and the electrode connection.Therefore,this paper studies the preparation of thermoelectric devices with HH as the thermoelectric material,including the selection and practical effect of barrier layer materials and the preparation process and performance of thermoelectric devices.?1?As a barrier layer material,Ag is reliably connected with n-type and p-type HH thermoelectric materials.There is no obvious element diffusion phenomenon at the interface.After aging for 96 h at 873 K,both thermoelectric materials and Ag layers are cracked.It was found that Ag basically did not react with HH thermoelectric materials.No certain compound was formed at the interface to make the connection between thermoelectric materials and barrier layer materials.Under high temperature environment,the interface cracked due to the difference in thermal expansion coefficient.?2?A small amount of Ga was added to the Ag to obtain a reliable connection.However,in the process of high temperature aging,it was found that the AgGa layer appeared to melt and deform,indicating that AgGa was not suitable as the barrier layer material for middle and high temperature thermoelectric devices.Reliable connection with HH was obtained by adding a small amount of Al into Ag,but after 192 h of aging,the diffusion thickness of Al in N-HH thermoelectric material reached 150?m.Ni in N-HH thermoelectric material would react with Al in AgAl to form NiAl intermetallic compound,and the diffusion layer became thicker with the extension of aging.After aging,the diffusion thickness of Al in P-HH thermoelectric material reaches 500?m,and the diffusion layer products are intermetallic compounds of FeAl and Fe₃Nb.Continuous reaction between barrier layer material and thermoelectric material will destroy the performance of thermoelectric material,so AgAl is not suitable for the barrier layer material of HH thermoelectric material.?3?The thermal expansion coefficient of the barrier layer material was adjusted by adding Ni with an atomic content of 10%in the Ag.After 873 K annealing for 192 h,the electrode material and the thermoelectric material remained intact.A small amount of Ni at the p-type HH interface was enriched at the interface and formed N i₃Nb and FeNi₃compound with Nb and Fe.The diffusion layer thickness remained at about 20?m after 192h.After aging for 192h at 873K,a very thin intermetallic compound layer was formed at the n-hh junction surface,the thickness of which was less than 1 m.the main composition of the compound is NiSn alloy.Through the four point probe method measured the interfacial contact resistance rate of P-HH/Ag_0.9Ni_0.1.1 is 0.4116??·cm²,N-HH/Ag_0.9Ni_0.1 interfacial contact resistance rate is less than 1??·cm².As a barrier layer material,Ag_0.9Ni_0.1 has thin interfacial layer thickness,low contact resistance and reliable mechanical strength.In conclusion,Ag_0.9Ni_0.1 can be used as an appropriate barrier layer for HH thermoelectric materials.?4?HH thermoelectric devices were prepared by two-step SPS sintering method using Ag_0.9Ni_0.1 as barrier layer material.The thermoelectric conversion efficiency of 7.33%,the output power of 2.24 W when the hot side is 600?,cold side is 28.8?.The power density per unit area is 1.12 W/cm².In order to verify the stability of the device,after repeated tests for 5 times,it was found that the efficiency of the device could be maintained between 7.22%and 7.52%.