Study On The Preparation And Thermoelectric Properties Of P-type Mg₂（Si,Sn）-based Compounds

With the influence of the global energy crisis and environmental degradation,the researchers from all over the world are working hard to explore efficient,clean and environment-friendly energies such as hydroelectric power and solar battery which have been used widely in life.Thermoelectric materials,developing rapidly in recent years,are a kind of environmental friendly semiconductors which can realize the reversible conversion between heat energy and electric energy.The devices made by thermoelectric materials can be used for thermoelectric power generation and refrigeration.The thermoelectric devices have some advantages such as small volume,easy to maintain,safe and reliable,no noise,clean,environmental protection and so on,which have been widely applied in the fields of aerospace,microelectronics and medical devices.For high efficient thermoelectric devices,both n-type and p-type legs are needed to maximize power output for practical application.Thermoelectric materials Mg2(Si,Sn)-based compounds have received considerable attention due to their non-toxicity,environmental friendliness,the abundance of their constituent elements in the earth’s crust.They are a kind of thermoelectric materials having important application prospect in the field of medium temperature.At present,the research of n-type Mg2(Si,Sn)materials have been quite mature,and the thermoelectric properties have been well optimized.By appropriate doping,the maximum thermoelectric dimensionless value ZT has achieved 1.55 in n-type Mg2(Si,Sn)-based compounds.However,the ZT values for p-type Mg2(Si,Sn)-based materials mostly remain below unity and the maximum ZT value has not exceeded 0.5.Therefore,the research on p-type Mg2(Si,Sn)-based thermoelectric materials has important scientific significance and application value.As for the tough problem that the compounds containing Mg element are easily volatilized and oxidized in the preparation,we use a home-made melt spinning equipment,assembled inside the glove box with protective atmosphere,combined with spark plasma sintering technology to prepare samples.Successfully,the single-phase,nanostructured,homogeneous and p-type Mg2(Si,Sn)-based thermoelectric materials with precise control of Mg content are obtained in a short time.In this paper,Ag and Li elements are used respectively to dope the Mg2Si0.4Sn0.6 in order to change the intrinsic n-type Mg2Si0.4Sn0.6 into p-type.Also,we study the influence of different doping elements on thermoelectric properties of p-type Mg2Si0.4Sn0.6 compounds and obtain the optimal components and optimized thermoelectric property.We explore internal relations of the melt spinning technology on the structure,morphology and product superiority in improving thermoelectric performance.Besides,we deeply study the internal connections among microstructures and the performance of thermoelectrics.The specific contents and results of the research are as follows:(1)Combined with spark plasma sintering technology,a home-made melt spinning device placed in the atmosphere of Ar gas protection is used to prepared a series of p-type Mg2-xAgxSi0.4Sn0.6(x = 0.00,0.01,0.02,0.05,0.07)compounds in a short time.This experimental method not only overcomes the problem existed in the preparation of Mg2(Si,Sn),but also greatly saves the time of preparation and one sample just need 2~3 hours to synthesize.The phase analysis of XRD shows that the prepared samples have only single phase,and no MgO or other impurity phases,which demonstrate the validity of our experimental method.Through the analysis of the thermoelectric properties,the electric conductivity of the compounds increases with the increased content of Ag element,while the Seebeck coefficient shows the contrary trend.With the combined role of electric conductivity and Seebeck,the power factor PF value increase with the increasing Ag content.The sample Mg1.95Ag0.05Si0.4Sn0.6 posses the largest PF value ~1.03 mW K-2m-1@690K and relative low thermal conductivity.Finally,the peak ZT~0.45@690K is achieved in the Mg1.95Ag0.05Si0.4Sn0.6 sample.(2)Based on the successful use of melt spinning method,we continue to use the “melt spinning combined with spark plasma sintering technology ” method to prepare a series of Li-doped p-type Mg2(1-x)Li2xSi0.4Sn0.6(x= 0.00,0.01,0.03,0.07,0.09)compounds quickly.Observing the SEM images of ribbon,we find that the morphology of two sides has huge differences because of the different cooling rates during the quenching process for two surfaces.In detail,there exists little grain structure on the contact surface of the ribbon,while much nano-and microscaled grains can be found on the free face of the ribbon.For the sample compacted by SPS,the density is pretty high,observed from SEM image of the fracture surface.TEM images show that the presence of Sn-rich nano precipitates in the sample,which is beneficial for the enhancement of the role to scatter the phonons with medium to long wavelength and the reduction of thermal conductivity.Furthermore,the results of the electron energy loss spectroscopy demonstrate the successful incorporation of the Li element.With the combined action of the enhanced conductivity and lower thermal conductivity,the ZT value of the sample reaches the peak value of ~0.58 when the Li doping content reaches 7%,an improvement by 15% as compared to the record value to date.