Self-propagating High-temperature Synthesis And Thermoelectric Performance Of N-type CoSb₃-based And P-type SnTe Intermediate Temperature Materials

With the rapid development of industrialization nowadays,the demands of energy is increasing drastically.However,the energy utilization rate is still not high,most of which is wasted as exhaust heat.The emission of the exhaust heat not only cause a loss of the energy,but can also bring about the green house effects.It is very significant to recycle the waste heat for thermoelectric power generation,especially the heat of mid-temperature in the range of 600⁹20 K.However,before the large-scal application for mid-temperature thermoelectrics,following problems should be solved:?1?the option of proper n/p type thermoelectric?TE?materials,?2?the development of suitable synthesis method for mass production.Focusing on the demands of the waste heat recyle proposed above,both n-type CoSb₃-based skutterudites and p-type SnTe compounds were rapidly synthesized via a Self-propagating High-temperature Synthesis coupled with Plasma Activated Sintering?SHS-PAS?technique and the TE performance were optimized.The formation mechanism of both the CoSb₃ and SnTe phase and the microstructure evolution are studied during the SHS processing.The TE properties are studied by adjusting the compositions,band structures and microstructures combined with the characteristics of SHS-PAS processing.The main contents and results are listed as follow.In the study of n type CoSb₃ based compounds,thermodynamic and kinetics of the SHS reaction were investiagated.The ignition temperature,adiabatic termperature,and the propagation speed of the combustion wave in the synthesis of CoSb₃ are 723 K,861 K,and 1.25 mms^-1,respectively.DSC and SHS quenching experiment were employed to study the phase formation mechanism and the microstructure evolution.The results show that a small amount of CoSb is formed by solid state reaction at the beginning,then the SHS reaction takes place when achieving the ignition temperature and releases intense heat which melts the Sb powders in the front of combustion wave,making both the Co particles and CoSb phase surrounded by molten Sb,followed by precipitating CoSb₃ phase rapidly.Based on the study of CoSb₃ SHS reaction,n tpye CoSb_3-xTe_x and In_xCo₄Sb₁₂compounds were synthesized.The results show that doping with Te and In can remarkably enhance the power factor and reduce the thermal conductivity which improve the TE performance markedly.The maximum ZT values of CoSb_2.85Te_0.15and In_0.45Co₄Sb₁₂ are 0.98 at 820 K and 1.0 at 660 K.respectively.PAS sintering technique is employed here to research the feature of CoSb₃ SHS reaction under the condition of electric current and pressure field,and the CoSb₃based compounds were fabricated directly by One-Step-PAS?OS-PAS?processing.The results show that local high temperature caused by the electric current can rapidly melt Sb,and the Co particles will be coated by melting Sb following by the precipitation of CoSb₃.The application of current may increase the rate of nucleation and refine the grains by lowering the thermodynamic barrier during the formation of a particular phase.Besides,when applying an electrical current during the sintering process,the temperature profile is often inhomogeneous due to the non-uniform distribution of the current passing through the powdered compact,Combined with the point defects introduced by Te substitution,hierarchically structured materials are synthesized by OS-PAS,which expected to impede the flow of phonons over a very broad range of frequencies and result in exceptionally low lattice thermal conductivities.Using this synthesis technique,we obtained high performance bulk Te-doped skutterudites with the maximum thermoelectric figure of merit ZT of 1.1 at 820 K for the composition CoSb_2.875Te_0.125.Such high ZT values rival those obtained from singly-filled skutterudites.SnTe compounds were prepared by SHS and the adiabatic temperature,the combustion temperature,the propagation speed of the combustion wave and the product melting percentage are 1079 K?1073 K?20 mm/s and 37.6%,respectivity.TheSHSreactionofSnTeisfitforthemechanismof dissolution-reaction-precipitation.As the temperature reaches the Sn melting point,Te particles would be coated by the liquid Sn and begin to dissolve in melting Sn.At the temperature of 549 K,Sn and Te react rapidly to form SnTe and release an intense heat which leads to a rapid increase in temperature,making the reaction self-sustaining until the whole reaction completed.In and Cd doped SnTe were prepared by SHS-PAS.The resonance level introduced by In atom can significantly enhances the Seebeck coefficient???and makes the power factor increased markedly in the whole temperature range,resulting in an effective improvement of the average ZT value of p-type SnTe.Cd doping modifies the electronic band structures,leading to an enhancement of?in high temperature,and the maximum of ZT value are increased.Coupled with each advantage of In doping and Cd doping in SnTe compound,In_0.01Cd_0.02Sn_0.97Te was fabricated and obtained a maximum ZT of 1.12 at 920 K,almost a 77%enhancement over the intrinsic SnTe.Meanwhile,the average ZT value in the range of 600⁹20 K is also optimized,reaching 0.86,which represents a 117%improvement compared to undoped SnTe.Based on the ultra-fast and non-equilibrium features of SHS process,we apply phase segregation technique the first to prepare in-stu nanostructured SnTe based compounds,which effectively adjusts both the microstructure and the carrier density of SnTe-based materials.Doping SnTe by In enhances the ZT valure of the annealed sample with x=0.005 recorded the highest ZT=0.92 which represents about 12%enhancement compared to the figure of merit of samples unannealed.This study offers a new approach to optimize the thermoelectric properties of SnTe.