HPHT Synthesis And Performance Characterization Of Doping Type-Ⅰ Clathrates Thermoelectric Materials

With the development of society,the gradual increase in energy use,combined with the low conversion efficiency,has resulted in the depletion of traditional energy sources.Meanwhile,excessive emission of greenhouse and waste gases has caused irreparable damage to the ecological environment.Therefore,energy and environmental problems have become the bottleneck of economic sustainable and healthy development.Developing new energy and improving the utilization of energy become the main topics of common concern in the world.Thermoelectric(TE)materials,which can convert heat directly into electricity,have been identified as one of the most important promising materials and have important role in some crucial fields.TE materials can improve the conversion efficiency of materials,which provide new ideas for alleviating environmental pollution and energy shortage.Clathrates that possess own “phonon glass and electron crystal” characteristic have been extensively examined as prospective TE materials in medium and high temperature regions.Due to the unique structures and properties of clathrates,TE performance could be improved effectively by many ways,which has highlighted the unique advantages in the TE performance of materials.Type-I clathrates,have received considerable attention as novel materials for medium and high-temperature applications,especially the Ge-based clathrates with competitive ZT values.A number of signature results have been achieved: the Ba8Ga16Ge30 single crystal prepared by Czochralski method obtained the highest thermoelectric figure of merit ZT(1.35@900K);Blake studied the transport properties of the clathrates using density functional calculations in the generalized gradient approximation and estimated that ZT values of 1.7@800K may be possible for optimally Sr8Ga16Ge30 or Ba8In16Sn30.Hou prepared Ba8Ga16Ge30 clathrate crystals and had a good ZT value 0.93@850K and could reach 1.3 as extrapolated to 1000 K.There are many methods to synthesize TE materials,such as Flux growth method,Czochralski methods and Spark plasma sintering,and so on.In recent years,ultrahigh pressure preparation technology represented by High Pressure and High Temperature(HPHT)synthesis has provided new means and methods that can control material composition and microstructure to optimize material performance in larger scale,which has shown unique advantages in the study of TE materials.HPHT method with unique advantages has been used to synthesis bulk TE material.This method is a rapid synthesis process which can convert the element mixture directly into a block material,and can maintain the high pressure characteristics to ambient conditions.Beyond that,a considerable number of grain boundaries and lattice defects can be formed during the HPHT process,which play a significant role in scattering phonons then reducing thermal conductivity.In this paper,a series of clathrates materials are prepared by HPHT method.The influence and mechanism of pressure,element substitution and doping concentration on the structure and thermoelectric properties of the type-I clathrates are systematically explored.The main research results are as follows:1.The influence of framework element replacement on thermoelectric properties of Si-based clathrates materials Ba8 Cux Gey Si46-x-y(x=0,6;y=0,20).The Polycrystalline bulk materials Ba8 Cux Gey Si46-x-y are successfully synthesized by HPHT method under 3.5GPa.XRD and Rietveld refinement confirm that the synthesized specimens are type-I clathrate.The samples prepared by HPHT are composed of disordered grains,which are rich in grain boundaries,and introduces many multi-type and multi-scale lattice defects.As the framework element increase,the crystal boundary and defects of specimen are increased,which can effectively scatter phonons and reduce thermal conductivity.In addition to phonon scattering,the bonding of filling atoms and frame elements also reduces the thermal conductivity.With the increase of temperature,the Seebeck coefficient and electrical resistivity of the specimen are increased,and the thermal conductivity is first decreased then increased.The substitution of framework elements increase the Seebeck coefficient and electrical resistivity,but decrease the thermal conductivity.A minimum thermal conductivity of 1.06Wm-1K-1@520K is obtained,and a good ZT value of 0.25@720K is achieved for Ba8Cu6Ge20Si20.Those results show that the TE property of the materials can be improved by the substitution of framework elements.2.The effects of Al elements on the structure and electrical properties of typeI clathrates Ba8 Alx Si46-x(x=0,10,16).The clathrate compounds Ba8 Alx Si46-x are successfully synthesized using the HPHT and high pressure sintering method under 3.5GPa,which changes the microstructure of the specimen.With the increase of Al content,the XRD peaks of specimen shifts to the small angle,which leads to an increase in lattice constants.The bond lengths of Ba8Si46 prepared by HPHT show the tendency: M2-M2<M2-M3<M1-M3<M3-M3,while that of Ba8Al10Si36 shows a different trend.The Ba ions in dodecahedron have spherical thermal ellipsoids while that in tetrakaidecahedron display anisotropic thermal ellipsoids.The Si-Si bond angles,ranging from 105.85(14)° to 123.22(12)°,imply bonding is similar to sp3 hybridization in the diamond structure of Si and Ge.The Seebeck coefficient and Power factor are greatly enhanced by Al substitution,and increase with the increasing temperature.3.The effect of Eu elements on the thermoelectric properties of the clathrates Ba8-x Eux Cu6Si40(x=0,1,2).Bulk clathrate materials Ba8-x Eux Cu6Si40 are successfully fabricated via the HPHT method,which takes just an hour to synthesize clathrates.The samples prepared by HPHT are composed of disordered grains,which are rich in grain boundaries,and introduces many multi-type and multi-scale lattice defects,such as grain boundary,dislocation,nano-structure,amorphous region,lattice distortion,etc.The increased Eu content results in the enlarged polyhedral grain and reduced defect density,which can lower scattering of the phonons and thus has important influence on the thermal conductivity.As the temperature increase,the Seebeck coefficient and electrical resistivity increase,but thermal conductivity decrease.With the increase of Eu content,both Seebeck coefficient and Power Factor decrease while thermal conductivity increase.A minimum thermal conductivity of 1.26Wm-1K-1@720K and the maximum ZT value of 0.16@720K are achieved for Ba8Cu6Si40.4.The effect of In elements on the thermoelectric properties of the clathrates Ba8Ga16 Inx Ge30-x(x=0.5,1.0,1.5).Bulk materials Ba8Ga16 Inx Ge30-x are successfully prepared by HPHT and high pressure sintering method under 3.5GPa,and the crystal structure has been confirmed by X-ray diffraction and Rietveld refinement.The grain boundaries and defects generated by high pressure can effectively scatter phonons of different frequencies,which reduce the lattice thermal conductivity.As the In content increase,both Seebeck coefficient and electrical resistivity increase,and also change the microstructure of specimen.With the increase of temperature,the electrical resistivity and Seebeck coefficient first increase and then decrease,and the thermal conductivity changes in the opposite.The substitution of In enlarges the carrier concentration,which inhibits the excitation of the bipolar effect and moves the extreme value of thermoelectric properties to higher temperature.The minimum thermal conductivity of 0.84Wm-1K-1 @623K and the maximum ZT value of 0.52@773K are achieved for Ba8Ga16In1.5Ge28.5.The lower thermal conductivity can be attributed to the scattering of full-spectrum of phonons by the multi-scale microstructure prepared by HPHT.In summary,HPHT method is used to synthesis high-performance bulk TE material.We can effectively control the band structure,microstructure and TE transmission mechanism through the systemic optimization of alternative framework elements,elements doping,and pressure et al parameters to optimize the TE performance of materials.HPHT method can prepare many TE materials with different compositions and doping levels in relatively short time,and provide a broader platform for the synthesis of TE materials.