Modification And Mechanism Of N-type Mg₃Sb₂ Based Thermoelectric Materials

n-type Mg₃Sb₂-based thermoelectric material has low lattice thermal conductivity due to its complex crystal structure,and is a medium-temperature thermoelectric material with great development potential.However,the high concentration of Mg vacancies leads to low carrier mobility,and the power factor and thermoelectric figure of merit need to be improved.In this paper,the carrier scattering is manipulated to increase the carrier mobility of Mg₃Sb₂-based alloys to increase the power factor.Porous structure is constructed to enhance phonon scattering to further reduce the thermal conductivity and increase the thermoelectric merit.Advanced measurement methods such as X-ray diffraction,neutron diffraction,spherical aberration correction transmission electron microscope,Hall effect measurement system,Seebeck coefficient/conductivity measurement system and Laser Flash Apparatus,combined with calculations such as DFT calculation and Debye-Callaway model are conducted to systematically study the influence of element doping(Mn and B),porous structure construction,grain boundary and energy band control on the structure and thermoelectric transport characteristics.The research results showed that Mn doping significantly improves the carrier mobility and power factor of n-type Mg₃Sb₂-based thermoelectric materials.Mn doping suppresses the Mg vacancy concentration,reduces the ionized impurity scattering center,and changes the carrier scattering mechanism from ionized impurity scattering to a mixed mechanism of ionized impurity scattering and phonon scattering.At the same time,the Bi nanoprecipitation phase,the point defects introduced by Mn doping and nano-sized crystal grains constitute a multi-level microstructure,which has a strong effect on phonon scattering,and the thermal conductivity of the lattice is significantly reduced.Finally,the thermoelectric figure of merit is greatly improved.When Mn doping amount is 0.05,ZT reaches the maximum value 1.85 at 723 K;when Mn doping amount is 0.025,the average thermoelectric figure of merit for the alloy between 300 K and 723 K is up to 1.25.The Mn-doped Mg₃Sb₂-based porous thermoelectric material was prepared by adjusting the displacement of the indenter during the SPS sintering process.The power factor decreases slightly after the porous structure is constructed,but the density is reduced,the phonon scattering is enhanced,and the thermal conductivity is significantly reduced.When the porosity is 20%,the room temperature thermal conductivity is as low as 0.63 W m^-1 K^-1.Combined with the Debye-Callaway model analysis,it is confirmed that the porous structure plays an important role in enhancing phonon scattering.With the increase of porosity,ZT value first increases and then decreases.When the porosity is 10%,the low temperature ZT value is as high as 0.88at 323 K.The porous sample with a porosity of 10%showed no change in thermoelectric properties after three consecutive cycles of testing,showing good stability.It was found that B doping can increase the conductivity of n-type Mg₃Sb₂-based thermoelectric materials.The mechanism is that B doping reduces the polarity of the Mg-Sb covalent bond,thereby reducing carrier scattering and improving carrier mobility.Doping as a donor at the same time increases the carrier concentration.The point defects introduced by B doping reduce the lattice thermal conductivity and increase the thermoelectric figure of merit;in addition,the B doping also increases the hardness of the material.Based on boron doping,the sintering temperature is increased,thus the grain size is increased,and the grain boundary scattering is reduced,leading to greatly improved carrier mobility and power factor in the low temperature range.The ZT value increases significantly with the sintering temperature.The average ZT value is 1.4 bewtween 300 K and 773 K.In addition,increasing the amount of Bi can adjust the band structure,reducing the band gap and effective mass,and increasing the carrier concentration and mobility correspondingly,which significantly improves the conductivity and power factor at low temperatures.When Bi doping amount is 1.49,the room temperature ZT value reaches 0.76,and the average ZT value between 300 K and 470 K is 1.12,which is higher than that of commercial Bi₂Te₃ thermoelectric materials.