Research Of High Temperature Thermoelectric Materials And Devices For Space Nuclear Power Applications

Static thermoelectric conversion mode is one of the best choices for the conversion of thermal energy to electric energy in space nuclear power system.SiGe and Half Heusler thermoelectric materials can show excellent thermoelectric performance,mechanical performance and thermal stability at high temperature.And SiGe thermoelectric device is the most mature and reliable thermoelectric device successfully applied in space nuclear power supply.Considering the application background of space nuclear power,thermoelectric materials will face neutron and gamma ray irradiation,which may affect the performance of thermoelectric materials,The output power and conversion efficiency of SiGe thermoelectric device are obviously affected by the thermoelectric properties of the thermocouple arm material,the connection of cold and hot electrodes and the geometric structure layout.In view of the problems and challenges faced by the application of high temperature thermoelectric materials and devices in space nuclear power supply,this paper studies the neutron and gamma radiation effects of n-and p-type thermoelectric materials SiGe and Half-Heusler,explores the brazing connection between SiGe and W cool electrode,diffusion connection between SiGe and SiMo hot electrode,optimizes the geometric structure layout of SiGe devices,and also studies the performance improvement of p-type SiGe thermoelectric material.The main work and conclusions of this paper are as follows:(1)The effects of irradiation conditions of neutron average energy 1.25 MeV and flux 1×1013 n·cm-2?1×1015n·cm-2 on the properties of thermoelectric materials SiGe and Half-Heusler were studied.The results show that the carrier mobility of n-and p-type SiGe alloy decreases significantly after neutron irradiation,the electrical conductivity decreases by 3-4 orders of magnitude,and the thermoelectric properties of n-type SiGe material degrade more obviously than p-type SiGe material,but the thermoelectric properties of n-and p-type SiGe alloy recover basically after heat treatment at 1273 K and 635 K for 2 h,respectively.At the same time,the properties of n-type Half-Heusler materials have no obvious change under neutron irradiation,while the thermoelectric properties of p-type Half-Heusler materials have decreased by about 13.8%?18.9%,and after 773 K-2 h and 873 K-12 h heat treatment can recover the properties as un-irradiated.In addition,the effects of gamma ray with average energy of 1.25 MeV and dosage of 1×104 Gy(Si)on SiGe thermoelectric materials have been studied,and the results show that the properties of n-type p-type SiGe materials have no obvious degradation Change.(2)The brazing of SiGe and W electrode at the cold end with Ag72-Cu28 solder and Ti contact layer at 1078 K-10 min was studied.It is found that there are no obvious cracks and holes in the brazed joint interface,and the interface is well connected.When the thickness of Ti contact layer is 1.05 ?m and 2.12 ?m,the interface resistivity of n-type SiGe/W solder joint is about 6.35 ??·cm2 and 12.6 ??·cm2 respectively,and that of p-type SiGe/W solder joint is about 17.8 ??·cm2 and 7.8??·cm2 respectively.The maximum interfacial shear strength of n-type and p-type brazed joints is 5 MPa and 6.5 MPa respectively under the Ti contact layer of 2.12 ?m thickness.After 773 K-7 d heat aging,cracks or holes appear on the brazing interface.The interface resistivity and shear strength of n-type brazing in the thickness of two Ti contact layers increase to 17.8 ??·cm2,31.3 ??·cm2 and 8 MPa,12.2 MPa respectively;the interface resistivity and shear strength of p-type brazing in the thickness of two Ti contact layers increase to 25.8 ??·cm2,19.7 ??·cm2 and 14 MPa,16.6 MPa respectively.(3)The diffusion bonding of the thermoelectric material SiGe and the hot electrode SiMo at different temperatures,pressures and times was studied.It is found that the interface of diffusion joint has obvious boundary,no crack,hole and other defects,and the interface is well connected.Among various diffusion bonding processes,n-type SiGe/SiMo has low interface resistivity of 43.7 ??·cm2?46.2 ??·cm2 under 15 MPa welding pressure,and the maximum interface shear strength is about 23.3 MPa;p-type SiGe/SiMo has low interface resistivity of 20.3 ???·cm2?43.1 ??·cm2 under 1423 K welding temperature,and the maximum interface shear strength is about 18 MPa.After 673 K-7 d thermal aging treatment,the interfacial shear strength of n-type and p-type SiGe/SiMo diffusion joint was obviously enhanced;after 1273 K-7 d thermal aging,the interfacial shear strength decreased to 12.7 MPa and 13.3 MPa.(4)Based on ANSYS finite element,the influence of the geometry layout of the device on the output performance and conversion efficiency of the thermoelectric device is analyzed.The results show that the output power and conversion efficiency of the thermoelectric device decrease with the increase of the thickness of the hot electrode SiMo and the distance between the thermocouple arms.Combined with the feasibility of the experimental operation,the minimum size of the thermocouple arm of the ? type thermoelectric device is 10 mm×10 mm×15 mm,the thickness of the hot electrode SiMo is 5 mm,and the distance between the thermocouple arms is 3 mm.The SiGe device with four thermocouple arms was fabricated successfully,and the maximum output power was 0.54 W under the temperature difference of 632 K.(5)The thermoelectric properties of p-type SiGe were improved by preparing p-type SiGe-x%SiMo composite thermoelectric material.The results show that the p-type SiGe-20%SiMo has the highest power factor of 2.9 mW-1·m-1·K-2@1073 K and the best thermoelectric value of 0.79@1073 K.By ANSYS finite element analysis of SiGe-20%SiMo alloy as p-type thermocouple arm shows that the maximum output power density and conversion efficiency of thermoelectric devices are 3.65 W·cm-2 and 8.53%respectively,which is higher than that of SiGe alloy as p-type thermocouple arm by 3.25 W·cm-2 and 8.09%respectively.