The Study On Novel Synthesize Technique And Thermoelectric Performance Of SnSe-based Materials

Nowadays,energy shortage has becomed a critical factor to our economic development,and it also significant to the national security.Therefore,the development and utilization of new energy and energy conversion technology is indeed urgent.Thermoelectric materials are capable of realizing the conversion between heat and electricity through Seebeck and Peltier effect.Recently,the SnSe-based thermoelectric materials have drew public attention due to it's high thermoelectric performance,the single-crystal p-type SnSe prepared by Bridgman method obtains highest ZT value of2.6 at 923 K along the b-axis,dominating the bulk thermoelectric materials so far.However,the long preparation time and the poor mechanical properties seriously suppress the application of single-crystal SnSe.Therefore,developing an ultra-fast preparative technique for SnSe materials with excellent mechanical and thermoelectric properties is of great concern for the large-scale application.This study focusing on the SnSe-based materials.Aimed at solving the problems of long preparation time and poor mechanical properties for single crystal SnSe.The Self-propagating High-temperature Synthesis combined with Quick Pressing?SHS-QP?and Plasma Activated Sintering?SHS-PAS?are first time to be employed for the synthesis of SnSe thermoelectric materials,and developing an ultra-fast and low cost preparative technique for SnSe-based compounds with excellent thermoelectric and mechanical properties.The phase transformation and reaction mechanism during SHS process are studied.The thermodynamic and kinetic parameters governing the SHS reaction,as well as the role of pre-synthesized SnSe,added to the starting mixture of elemental Sn and Se powders,are investigated.Effect of the sample containing pre-synthesized SnSe and composite with other compounds and dopant on thermoelectric and mechanical properties were studied.The thermoelectric and mechanical properties of SnSe prepared by SHS-QP also been studied.A series of Sn-Bi-Se ternary compounds were synthesized by accident when attempting to prepared the intercalation structure of SnSe and Bi₂Se₃ by SHS.The main contents and results of this study are summarized as follow.For the SnSe-based compounds,the possibility of synthesis by using SHS technique is explored,and the results show that the SnSe material can be synthesized through the SHS technique,and the whole preparation of SHS-PAS process is within 1h.The SHS process of SnSe is a violent exothermic reaction.The combustion temperature and propagating velocity of SnSe during SHS are 1156 K and 12 mm/s respectively.The pre-synthesized SnSe plays a role of diluent,which can effectively control the SHS process.With the increasing content of pre-synthesized SnSe,the combustion temperature and propagation velocity decrease.In addition,the effect of pre-synthesized SnSe on the thermoelectric and mechanical properties is investigated.The maximum ZT value perpendicular to the press direction is 0.44 at 775 K for pure SnSe and 0.50 for sample with 30%pre-synthesized SnSe,which increases 14%.And the maximum ZT value parallel to the press direction is 0.34 at 775 K for pure SnSe and 0.42 for sample with 30%pre-synthesized SnSe,which increases 25%.The hardness discreteness of samples with 30%pre-synthesized SnSe turns smaller,the compressive strength,flexural strength and fracture toughness are 74.7 MPa,40.6 MPa and 0.32 MPam^1/2 respectively,representing an increase of 11%,15.4%and 17.6%over that of sample without incorporating of pre-synthesized SnSe.Concerning the fact that the SnSe compound has low intrinsic carrier concentration and low electrical conductivity.A series of Na doped SnSe composite with TiTe₂ have been synthesized by the SHS-PAS technique.The influence of Na doping and high-conductivity second phase TiTe₂ on carrier concentration,thermoelectric properties and mechanical properties are systematically studied.The results show that the carrier concentration of SnSe matrix increases from 2.8×10¹77 cm^-3to 3.17×10¹99 cm^-3 when Na content is 1%,the maximum ZT value of 0.6 is achieved at775 K,much higher than that of undoped SnSe?ZT=0.44?.The incorporating of TiTe₂improved room temperature electrical properties,and the uniform distribution of TiTe₂particles at grain boundaries increases the phonon scattering,which can effectively reduce the thermal conductivity.The maximum ZT value of 0.7 is achieved at 775 K for Sn_0.99Na_0.01Se with 0.5%TiTe₂,representing about 60%improvement compared with the pure SnSe.Moreover,the hardness,compressive strength,bending strength and fracture toughness of sample composited with TiTe₂ showing an enhancement of18%,56%,43%and 47%toward pure SnSe respectively.In this study,the Self-propagating High-temperature Synthesis combined with Quick Pressing?SHS-QP?technology was applied to thermoelectric materials for the first time,and SnSe bulk thermoelectric materials were successfully prepared,the dense bulk of SnSe can be successfully prepared in one step within 1 min.The ultrafast preparation of SnSe bulk thermoelectric material is achieved with this energy-saving technique.ANSYS finite element simulation results show that the temperature difference and residual stress between the internal and surface of SnSe bulk are too large to obtain a bulk without flaw while the adding of pre-synthesized SnSe can soften the temperature difference and distribute the stress uniformly,which is benefit to get a intact product.The thermoelectric and mechanical properties of SnSe prepared by SHS-QP are studied.In comparison with the SHS-PAS technique,the thermal conductivity of samples prepared by SHS-QP method shows a decrease of 24%and 12%in the directions perpendicular and parallel to the QP pressure respectively.The maximum ZT values of samples prepared by SHS-QP technique can reach 0.30???and 0.26???at 775 K respectively.Although compare with the SHS-PAS technique,the SHS-QP technique do not make advances in improving the thermoelectric performance and mechanical properties,this work provide a new method for ultra-fast preparation of thermoelectric materials and also give theoretical guidance to the exploration of SHS-QP technology for other systems with high heat release.A series of Sn-Bi-Se compounds were prepared by accident when attempting to intercalate the SnSe into the Bi₂Se₃ by using SHS technique.The crystal structure,band structure,valence state,magnetic and thermoelectric transport properties were studied.We found that,Sn_3-1.5yBi_2+ySe₆?-0.54?y?0.4?belongs to orthonormal system?62space group Pnma?.And the band structure calculation revealed that,the bottom of conduction band is located at X point,and the top of the valence band lies between Y and?point,and the band gap is 0.17 eV.And the valence band is mainly constituted by the Se element,while the Sn,Bi and Se make a joint contribution to the conduction band.The valence states of Sn,Bi and Se are+2,+3 and-2 respectively,and the Sn_3-1.5yBi_2+ySe₆ compounds is free of magnetism.The Debye temperature is about 188K,the weak and anharmonic bonds between atoms make this compound possess an intrinsic low thermal conductivity?similar to SnSe?.The maximum ZT value of Sn₃Bi₂Se₆ compound is about 0.12 at 400K,for the Sn₂Bi₂Se₅ compound,that value is about 0.14 at 475K.The intrinsic ZT value of these compounds are still too low,but the thermal conductivities are also very lowto some traditional thermoelectric materials,which lays the foundation for further optimization of its thermoelectric properties.