Ultra-fast Fabrication And Thermoelectric Performance Of Half-Heusler Alloy And Cu₂Se Compound Based On SHS Technique

Large increase in global energy demands along increasing awareness in environmental pollution has made the development of renewable energy sources of tremendous importance.Along this line,the thermoelectric energy conversion technology has attracted wide attention given its ability to realize a direct and reversible conversion between thermal energy and electric energy using,respectively,the Seebeck effect and Peltier effect of the material.Half-Heusler?HH?alloys have drawn a lot of attention due to their nontoxic elements and excellent thermoelectric and mechanical properties.Very good mechanical properties and thermal stability are also highly desirable for high temperature power generation.So far,HH alloys have been mainly synthesized either by arc melting,by induction melting,by levitating melting,or by long time solid-state reactions.However,all the above synthesis processes are highly time and energy consuming and strongly rely on expensive equipment,which seriously restricts large-scale fabrication and application of HH materials.Therefore,it is crucial to develop a facile fabrication method for HH materials that shortens the synthesis period and precisely controls the composition at low cost.Self-propagation high-temperature synthesis?SHS?,which is also referred to as the combustion synthesis?CS?,is a novel process for rapid preparation of materials by using the energy released during the chemical reaction.The SHS technique has the advantages of short reaction time and energy saving,simple process,accurate composition control and low cost.In this work,we apply the SHS process to a preparation of ZrNiSn,an important member of the half-Heusler family of thermoelectric materials.The SHS process of ternary ZrNiSn includes a series of SHS reactions and mass transfers initiated by fusion of Ni and Sn and culminating in a synthesis of a single-phase ternary ZrNiSn compound in a very short time.We describe the relevant thermodynamic and kinetic processes,phase transformations,and we correlate them with the microstructural evolution.The products prepared by SHS technique are uncompact.The densification of the products after SHS are achieved by spark plasma sintering.The figure of merit ZT of the ZrNiSn alloys prepared by SHS-SPS reached 0.67 at 870 K,which is not only competitive but actually a somewhat higher that the figure of merit of ZrNiSn prepared by the traditional methods.The thermoelectric properties of ZrNiSn are optimized based on SHS-SPS technique.Sb doped ZrNiSn_1-xSb_x and Cu filled interstitial sites ZrNiCu_xSn are synthesized by SHS-SPS.The carrier concentration is increased by Sb doping,which lead to the increasing of power factor.The ZT value of ZrNiSn_0.99Sb_0.01.01 reached 0.67 at870 K.The occupation of Cu in the interstitial sites increased carrier concentration and enhanced phonon scattering,which lead to the increasing of power factor and decreasing of lattice thermal conductivity.The ZT value of ZrNiCu_0.01Sn reached 0.8 at 900 K,which is 15%increasement compared with the pristine ZrNiSn.Although SHS technique has many advantages that traditional preparation methods do not have,the biggest disadvantage is that products obtained are uncompacted.It is still necessary to get fully dense bulk materials by method such as SPS.Therefore,we have improved on the SHS technique by incorporating an in-situ quick pressing?QP?in order to achieve a fully dense bulk material in just one step.This new technique called“self-propagating high-temperature synthesis combined with quick pressing”?SHS-QP?technique is a simple and economical method to fabricate ceramics.It not only retains the advantages of SHS technology,such as low cost and simple process,but it also overcomes the disadvantage of the uncompact products,which greatly saves the cost.Herein,high thermoelectric performance ZrNiSn bulks were synthesized by one step SHS-QP.The correlation among SHS process,QP process and the critical pressure,is systematically investigated.The plastic flow mechanism is the dominant densification mechanism for ZrNiSn in the SHS-QP process.The SHS-QP prepared sample contains a lot of nanostructures,which lead to the reduction in lattice thermal conductivity.The ZT value of the sample prepared by SHS-QP reaches 0.7 at 900 K,which is higher than the samples prepared by the traditional methods.The SHS-QP technique is also extended to Cu₂Se thermoelectric material with lower combustion temperature and slower combustion wave propagation velocity.Fully dense single-phase Cu₂Se bulk material was prepared by SHS-QP for the first time.Due to the ultra-fast SHS process,the composition of Cu₂Se can be precisely controlled.The homogeneity of the SHS-QP prepared Cu₂Se is very good.The ion migration of Cu₂Se as a fast ion conductor during the SPS process has been overcome.Due to the high temperature gradient and high pressure during SHS-QP process,the bulk sample prepared by SHS-QP has preferential orientation.Along perpendicular to pressure direction the sample has a preferential orientation on the?0l0?plane,while along parallel to pressure direction the sample has a preferential orientation on the?410?plane.The ZT value of Cu₂Se sample fabricated under the condition;t_s=0s,P=100 MPa and t_h=10s,and cut parallel to the pressure direction reaches 1.5 at 900 K,which is higher than the ZT value of 1.2 at 900 K reported for Cu₂Se sample prepared by the traditional melt-annealing combined with SPS sintering method.