Modulation Of Impurity Phase To Optimize The Thermoelectric Properties Of AgSbTe₂ Based Materials

In recent decades,the use of fossil fuels can cause many environmental problems.Fossil fuels are non-renewable resources and much of their energy is wasted in the form of heat.As a result,there is an urgent need for clean energy.Thermoelectric materials are environmentally friendly materials that can directly convert thermal energy into electrical energy.As a medium or low temperature thermoelectric material,AgSbTe₂ has attracted the attention of researchers due to its low lattice thermal conductivity.The low lattice thermal conductivity of AgSbTe₂ is due to the strong bond anharmonicity of the Sb-Te bond and the natural nanostructure.However,AgSbTe₂ slowly decomposes into Ag₂Te and Sb₂Te₃ at 630 K.It has been shown that the no impurity phase of AgSbTe₂ is difficult to obtain.Although this decomposition is very slow at room temperature due to kinetic,it will inevitably experience high temperature during preparation or application.Therefore,the thermodynamic stability of AgSbTe₂ is a serious issue.In addition,the low electrical conductivity of AgSbTe₂ is a major problem because its carrier concentration is always in the range of 10¹⁸to 10¹⁹ cm^-3.The lattice thermal conductivity of AgSbTe₂ can be further reduced.In view of the above defects,we did the following work:1.We found that the formation of solid solutions is a very effective way to suppress the main impurity phase Ag₂Te in AgSbTe₂.We have successfully suppressed the formation of the impurity phase Ag₂Te by Mg doping.From the XRD and DSC curves,we can find that the diffraction and endothermic peaks of Ag₂Te disappear when the concentration of Mg is greater than or equal to 2%.In addition,SEM also shows that the Ag₂Te particles in the AgSbTe₂ matrix disappear with Mg doping.The electronegativity difference between Mg and Te is greater than that between Ag,Sb,and Te,so it can be assumed that Mg Te forms preferentially and combines with Ag₂Te to form the solid solution,thus inhibiting the formation of Ag₂Te.The change in band structures shows that Mg doping makes more valence bands involved in electrical transport,thus increasing the carrier concentration.Near the Fermi energy level,the flattening of the valence band and the increase in DOS also lead to an increase in the Seebeck at room temperature.The TEM image shows the Mg doping producing dislocations and grain boundaries.Finally,the ZT_max value of 1.31 was achieved at 523 K for the AgSb_0.98Mg_0.02Te₂ sample.2.Co-doping of Mg and Ti can improve the thermoelectric properties of AgSbTe₂.Due to the low electrical conductivity of the samples,we doped the samples with Mg and Ti.From XRD,the doping of Mg and Ti can suppress the formation of impurity phases.Crystallite size,dislocation density,and microstrain obtained from XRD data analysis.It can be found that the doping of Ti leads to an increase in the average crystallite size,a decrease in dislocation density and a decrease in microstress in samples.This is consistent with SEM.These changes,which result in a weaker scattering of carriers,increase the relaxation time,increase the carrier mobility and finally increase the electrical conductivity.These changes weaken phonon scattering,but the point defects caused by Mg and Ti doping compensate for this.Finally,the ZT_max value（1.45）was achieved at 523 K for the AgSb_0.96Mg_0.02Ti_0.02Te₂sample.3.Mn doping can modulate nanoparticles and improve the thermoelectric properties of AgSbTe₂.Although the impurity phase Ag₂Te has a strong negative effect on electrical properties and thermodynamic stability,it has a strong scattering effect on phonons because it is a particle or even a nanoparticle.We suppress part of the Ag₂Te impurity phase by Mn doping.As can be seen from the SEM,the Mn doping can reduce the size and number of Ag₂Te particles.At the same time,the Mn doping increases the configurational entropy change of samples.Nanoparticles,configurational entropy change and edge dislocations can strengthen medium or high frequency phonons scattering,resulting in decreasing lattice thermal conductivity in medium and high temperature.Finally,the ZT_max value（1.28）was achieved at 573 K for the AgSb_0.98Mn_0.02Te₂ sample.