Influences Of SPS Process On The Microstructures And Properties Of Cunimn Thermoelectric Alloys

Thermoelectric materials can achieve the direct conversion between the heat and electricity and have the advantage of energy-saving,light-weight and convenience,no mechanical transmission,which provides the solutions for the energy crisis.The CuNiMn thermoelectric alloys have the high power factor.However,they also have the high thermal conductivity,which limits the improvement of ZT.In this paper,melt-spinning,ball milling and Spark Plasma Sintering?SPS?process was used to prepare CuNiMn thermoelectric alloys.The influences of the SPS parameters on the densification,microstructures,hardness and thermoelectric properties of CuNiMn thermoelectric alloys were studied.Meanwhile,the thermal conductivity was reduced by introducing a fine grain structure and defects to scatter phonons.The results showed that when the sintering pressure was constant,with the increase of the sintering temperature,the densification of CuNiMn thermoelectric alloys first increased and then decreased,and reached the maximum at 700°C.When the sintering pressure was 70 MPa,the densification and hardness reached the maximum of 92.53%and 250.86 HV respectively at the sintering temperature of 700?.When the sintering temperature exceeded 700?,the grains grew and agglomerated,and the voids in the tissue increased.When the sintering pressure was 70 MPa and the sintering temperature was 700?,the electrical conductivity of sample reached a maximum of 2.351×10⁶S/m at 323K.With the increase in the sintering temperature from 700?to 800?,the defects increased and the carrier filtering enhanced,leading to an increase in seebeck coefficient from 50.61?V/K to 60.66?V/K.Based on the above research,the influences of the sintering pressure on densification,microstructure,hardness and electrical transport properties of CuNiMn thermoelectric alloy were studied when the sintering temperature was constant.the results showed that the densification of CuNiMn thermoelectric alloy increased with the increase of sintering pressure at the sintering temperature of 600?.However,the densification decreased with the increase of pressure at the sintering temperature of700?,800?and 900?.This can be ascribed to the formation of thin layers with a certain thickness on the surface of the sample,which reduces the pulse current inside the sample.When the sintering temperature was 800?and the sintering pressure was65 MPa,the densification of the sample reached the maximum of 92.08%and the hardness reached the maximum of 319.6 HV.When the sintering temperature was800?,the conductivity decreased with the increase of sintering pressure.When the sintering pressure was 65 MPa,the sample conductivity reached the maximum of1.80×10⁶ S/m at 323 K.When the sintering pressure increased,the density of defects in the sample increased,which can effectively scatter low-energy carriers and result in an increase in the seebeck coefficient.In a word,the sample had the maximum of power factor(5.22 mWm^-1K^-2)and the best electrical transport performance at the sintering temperature of 800?and the pressure of 70 MPa.The combination of the melt-spinning and the SPS process can effectively reduce the grain size of the CuNiMn thermoelectric alloys,and the defects with various scales formed in the sample can effectively scatter phonons,thereby reducing the lattice thermal conductivity.Compared with the arc-melted ingot samples,the thermal conductivity of the samples prepared by the melt-spinning combined with SPS process can be reduced by about 20%.The ZT reached 0.127,when the sintering temperature was 800?and sintering pressure was 65MPa.