Preparation And Optimization Of New Thermoelectric MgAgSb And Cu₂Se

In recent years,thermoelectric materials show promise in solving energy problems,which attracte great attention from reserchers.However,there are coupling relationships between the Seebeck coefficient,electrical conductivity and thermal conductivity of thermoelectric materials,which make it difficult to enhance thermoelectric performance.We investigate MgAgSb and Cu₂Se with intrinsic low thermal conductivity.Furthermore,thermoelectric properties are improved by optimizing preparation and compositing nano materials,which remarkably improve the electrical and thermal properties and greatly promote the practical development of these two novel thermoelectric materials.The novel near-temperature thermoelectric material MgAgSb has attracted much attention since 2014.However,the difficulty in preparation slows the research progress of improving the thermoelectric performance of MgAgSb.In this paper,the preparation of pure phase and optimization of thermoelectric properties of MgAgSb are carried out.Due to the volatility of Mg element,the impurity Sb and Ag₃Sb appear in the MgAgSb matrix,which result in a significant deterioration in thermoelectric properties.Adopting solid-phase reaction method,we obtain pure MgAgSb by adjusting the elements ratio and controling the holding time and temperature of spark plasma sintering?SPS?.In addition,the carrier mobility is effectively enhanced by compositing the ultra-high mobility MWCNT.Meanwhile,more interfaces are introduced to enhance the scattering of mid-long-wave phonons,which reduces the low-temperature thermal conductivity,and significantly improves the thermoelectric performance of MgAgSb.The details are as follows:?1?We confirm the conditions for preparing pure MgAgSb by investigating the element ratio of raw materials,the sintering temperature and holding time of SPS,which are as follows:element ratio of Mg:Ag:Sb=1:0.97:0.99,and the samples keep in SPS at 723 K for 10 min.Under this condition,there are no impurities such as Ag₃Sb and Sb in the matrix MgAgSb and the power factor reaches a maximum value,stabilizing at 17?Wcm^-1K^-2.The matrix MgAgSb has the lowest lattice thermal conductivity 0.45 Wm^-1K^-1at 375 K.Finally,a maximum ZT value of 0.85 is obtained at this temperature,which is more than 150 K lower than the maximum ZT value temperature?525 K?in previous reports.The development of the near-room temperature thermoelectric material MgAgSb has been greatly advanced.?2?As we all know,MWCNT is a nanomaterial with excellent electrical transport property.The carrier mobility could be remarkably enhanced by compositing MWCNT in the MgAgSb matrix.When 0.1wt%MWCNT is combined,the room temperature carrer mobility increases from 39 cm^-2V^-1S^-1 of the pure phase MgAgSb to 55 cm^-2V^-1S^-1 of MgAgSb/0.1 wt%MWCNT.The room temperature conductivity is improved by 1.3 times and the room temperature thermal conductivity is reduced to 0.85 of the pure phase.Finally,the room temperature ZT value is enhanced from 0.55 of the pure phase MgAgSb to 0.83,which is higher than that in most of the previous reportes.In addition,the maximum ZT value of MgAgSb with 0.1wt%MWCNT is 30%higher than that of the pure phase,reaching 1.05.The thermoelectric performance of MgAgSb at low temperature is significantly enhanced by compositing MWCNT.As a"phonon liquid,electron crystal"material,Cu₂Se is regarded as one of the most promising thermoelectric materials.In the high-temperature phase?-Cu₂Se,liquid-like Cu⁺can freely migrate to scatter phonons in the sub-lattice provided by Se atoms,resulting in a relatively low thermal conductivity and excellent electrical transport properties.Under the non-equilibrium condition of high-speed ball milling,introducing the nano-SiC into Cu₂Se could enhance the liquid-like behavior of Cu⁺.There are a mass of defects including twins and stress fields at the border of Cu₂Se and SiC,which can be found by high-resolution TEM.These defects effectively facilitate the escape of Cu from the crystal lattice,which is in turn to enhance the liquid-like behavior of Cu⁺and improve the thermoelectric properties of Cu₂Se.In Cu₂Se/0.05 wt%SiC sample,the conductivity at 873 K is increased nearly 7 times and reaches 73439 S/m.Meanwhile,the introduced nano-SiC provides more interfaces to scatter the mid-long range phonons,effectively reducing the thermal conductivity from 0.81 Wm^-1K^-1 of Cu₂Se to 0.55 Wm^-1K^-1 of Cu₂Se/0.05wt%SiC.The thermoelectric performance of Cu₂Se has been significantly improved by enhancing the liquid-like behavior of Cu⁺and the ZT value has been improved from 0.7 of the pure phase at 873 K to 2.0...