Preparation And Thermoelectric Properties Of Na,Na/M（M=Mg,Ga,） Co-doped β-Zn₄Sb₃

At present,the major energy source consumption structure of human beings is still coal,oil and natural gas.However,majority of the energy is emitted to nature in the form of waste heat,resulting in inefficient energy conversion,in addition to a wide range of pollution when using traditional non-renewable fossil energy sources.Therefore,the demand of developing low-carbon energy makes thermoelectric materials get attention.As a newly developed energy material,thermoelectric materials can achieve the mutual tranormation between waste heat and electricity by the directional movement of the internal carrier of the material.And thermoelectric power generation has the benefits of small size,no noise,no pollution,and high stability.Among many thermoelectric materials,β-Zn₄Sb₃ has a typical“phonon-glass and electron-crystal”structure,which is of great interest because of its low thermal conductivity,abundant reserves and environmental friendliness.However,the common preparation methods require high equipment requirements,and the prepared samples have more cracks and poor mechanical properties,which limit the application ofβ-Zn₄Sb₃ in practical thermoelectric devices.In this work,β-Zn₄Sb₃ thermoelectric materials were prepared by a flux method combined with melting slow cooling process,and they were investigated the effects of different elemental co-doped（Na/Mg,Na/Ga）on the crystal structure,thermoelectric properties,thermal stability and oxidation resistance.The main conclusions were obtained as follows:1.The Na/Mg co-dopedβ-Zn₄Sb₃ crystals with high crystallinity were prepared by the NaCl flux method.The XRD results reveal that the diffraction peaks of all samples belong to theβ-Zn₄Sb₃ phase and there is no second phase.The substitution of Naand Mg elements at the Zn site improves the power factor of the samples.The PF value of the sample with x=0.1 could reach～2.06×10^–3 W·m^–1·K^–2 at 630 K,and the thermal conductivity as low as 0.78 W·m^–1·K^–1 at 630 K.Finally,the sample with x=0.1 reaches a maximum ZT value～1.58 at 690 K,which increases by 12%compared with the maximum ZT value（～1.41 at 690 K）of the Nadoped sample.This result indicates that Na/Mg co-doped is one of the useful strategies to enhance the thermoelectric properties ofβ-Zn₄Sb₃crystal.2.Theβ-Zn₄Sb₃ thermoelectric materials were prepared according to the stoichiometric ratio Zn:Sb:NaCl:Ga=4.1:3:7:x（x=0,0.08,0.12,0.16,0.2）.With the increase of Ga doping concentration,the carrier concentration ofβ-Zn₄Sb₃samples varies from 1.003×10²⁰～1.862×10²⁰ cm^-3at room temperature,and the lowest 1.003×10²⁰ cm^-3 for the Zn_4.1Sb₃（NaCl）₇Ga_0.12 sample.Suitable ratio of Naand Ga double Zn positions in substitutionβ-Zn₄Sb₃ could optimize the carrier concentration and increase the PF value of the sample,the maximum PF value of Zn_4.1Sb₃（NaCl）₇Ga_0.12 sample is about 2.28×10^-3W·m^–1·K^–2 at 690 K.Although,the doping of Ga did not reduce the thermal conductivity of the material,the Seebeck coefficient was significantly improved by optimizing the carrier concentration,resulting in higher PF and ZT values.Finally,the Zn_4.1Sb₃（NaCl）₇Ga_0.12 sample obtained the maximum ZT value of 1.73 at 690 K,which is about 22%higher than that of the without Ga-doped sample（1.42 at 690 K）.To further improve the ZT value ofβ-Zn₄Sb₃ for commercial application,it is necessary to further reduce its thermal conductivity.3.Based on the conclusions in Chapter 4,the Zn_4.1Sb₃（NaCl）₇Ga_0.12 with the best thermoelectric performances was selected for the research.It was treated with four heat treatments of different durations（12 h,24 h,48 h,and 72 h）under a high temperature oxidation environment at 573 K to study its thermal stability and oxidation resistance ability.The XRD results indicate that the major diffraction peaks of the samples still matched the standard card ofβ-Zn₄Sb₃（PDF#34-1013）after the high temperature oxidation heat treatment,and its diffraction intensity enhanced with increasing time of heat treatment,but the presence of the secondary phase Zn was detected in the heat-treated sample.The TG-DSC curves show no significant mass loss below 810 K for the Zn_4.1Sb₃（NaCl）₇Ga_0.12sample,and the Na/Ga co-doped sample increased the phase transition temperature of theβ-phase toγ-phase transition of Zn₄Sb₃.The Hall test indicates that the high-temperature oxidation treatment enhanced the carrier concentration of the sample,and the proper high-temperature oxidation was beneficial to improve the electrical transport properties of the sample.At 300 K,the ZT values of the heat-treated samples remained constant and were in the range of 0.2-0.28.However,the ZT values of the heat-treated samples decreased with increasing test temperature.Especially the fourth heat-treated sample had a low ZT value of 1.13,which was 34%lower compared to the untreated sample（1.73 at 690K）.