Effect Of Microstructure And Properties Of Co To Ni-based Heusler Type Thermoelectric Alloy

Heusler alloys,one of thermolelectric materials which can convert heat directly to electricity,have the advantages of non-pollution,non-noise,abundant storage of required elements,ect.But its higher thermal conductivity as the thermoelectric materials leads to a generally lower the dimensionless figure of merit.based on the abundant resources of nickel and cobalt in the province,as well as many papers of study of Ni-based Heusler thermoelectric alloys,full-Heusler alloys of Ni₂-Mn-Sn and half-Heusler alloys of Ni-Mn-Sn were studied as the object of the research.The dimensionless figure of merit?ZT?of the alloy can be improved by enhance of the power factor?PF?and reduction of thermal conductivity of the alloyswhich be caused by substitution alloying and grain of refinement.Besides,the effects of alloying on the microstructure and thermoelectric properties of samples were investigated.The main results are as follows:Firstly,the XRD results show that full-Heusler alloys can be successfully prepared by arc melting-heat treatment.From the SEM?EDS and electron backscatter diffraction diagram of the samples,it can be found that the elements are evenly distributed after repeated melting.From the SEM and electron back scattered images,it can be known that the grains of alloy are larger,both of which include two phase structures?gray phase and black phase?,the black phase of those is rich Co and poor Sn,while the gray phase of those is matrix.Interestingly,the black phase volume gradually increases with the increase of Co content,which was caused by the Martensite phase transition.Secondly,from the data of the thermoelectric properties of the alloy,it can be seen that the alloying of the Co has reduced the thermal conductivity and electrical conductivity of the alloy,but the Seebeck coefficient of samples was increased.The best ZT of 7.88×10^-33 at 873K was attained in the full-Heusler alloy with the Ni₂₀Co₃₀Mn₃₉Sn₁₁.The improvement of the thermoelectric properties of samples is mainly due to the reduction of lattice symmetry and the increase of interface by the alloying of Co atoms,which hinder the vibration of lattice and carriers flow,reducing the thermal conductivity and increasing the Seebeck coefficient,and the process improves the thermoelectric properties of the alloys.Thirdly,the analysis of XRD of half-Heusler alloy shows that arc melting and heat-treatment combined with powder metallurgy can fabricate half-Heusler alloy of Co alloying.From the EDS and SEM images of the sample,it is known that the elements of the alloy are evenly distributed.Before ball milling,the particles of the sample,which are lamellar structure,are about 50?100 nm,the particles of the alloy become lager with the size of 100?300nm after hot-pressure sintering.In addition,microholes of a size of about 80 nm are distributed in the sample,which is conducive to scattering phonons and electrons and reducing the thermal conductivity of the alloy.Fourth,from the data of the thermoelectric properties of the sample,the thermal conductivity of those decreases with the increase of Co content,but the Seebeck coefficient increases.The best ZT of 13.13×10^-3at 873K was attained in the half-Heusler alloy of the Ni_0.8Co_0.2MnSn.The improvement of the thermoelectric properties of samples lies in the distortion of unit cell caused by the grain of refinement and nanometered defects or nanoparticles by ball milling,the size of the defect is similar to the average free path of the phonon,but is much smaller than the average free path of electrons?or holes?.Therefore,when the phonon moves in the lattice,the scattering probability increases,while the thermal conductivity decreases.