Microstructure And Thermoelectric Properties Of The M?Zn,Sn,In,Ge?-doping Bi_0.5Sb_1.5Te₃ Alloys Via Directional Solidification

Thermoelectric?TE?material,as a new environment-friendly energy conversion form,provide a direct and safety conversion of electricity and thermal energy.As the best TE material around room temperature,Bi₂Te₃-based alloys have a nice TE performance in refrigeration and power generation potentially.Bi_0.5Sb_1.5Te₃ is the most excellent p-type material and have a maximum figure of merit ZT near 1.Therefore,improving TE properties on the basis of Bi_0.5Sb_1.5Te₃ is of great significance to TE materials large-scale applications.In this paper,we choose the Bi_0.5Sb_1.5Te₃ alloy as the matrix,and the influence of element doping on the microstructure and thermoelectric transport performance were studied systematically by directional solidification technique.The Bi_0.5Sb_1.5Te₃ alloys doped with Zn,Sn and In elements were prepared by the high vacuum directional solidification furnace.The as-cast microstructure of Bi_0.5Sb_1.5Te₃ alloys is basically uniform,and solid-liquid interface remains flat and the composition is uniform at the rate of 3 ?m/s of the directional solidification of the alloy.In order to further adjust the alloy carrier concentration,an excess of Te element was added to Bi_0.5Sb_1.5Te₃ alloy and a series of alloys of Gex(Bi_0.5Sb_1.5Te₃)_1-x+8 wt.% Te?x = 0,0.005,0.01,0.05?were prepared.There exists obvious in as-cast microstructure,the microstructure composition distribution is uniform and the Te-rich phase of the as-cast structure disappears after the directional solidification at the rate of 3 ?m/s.The results shows that the elements of Zn and Sn are acceptor doped,increasing the hole concentration,improving the conductivity but reducing the Seebeck coefficient greatly of Bi_0.5Sb_1.5Te₃ alloy,the In element is opposite.In the alloys of Gex(Bi_0.5Sb_1.5Te₃)_1-x+8 wt.% Te?x = 0,0.005,0.01,0.05?,the addition of 8wt.% Te greatly reduces the alloy hole concentration,making the alloys get a lower carrier thermal conductivity and a higher Seebeck coefficient.he conductivity and intrinsic excitation temperature increase gradually with the increase of Ge content,and the Seebeck coefficient decreases gradually,mainly due to the addition of Ge element,which increases the hole concentration of the alloy.At the same time,the thermal conductivity of phonon decreases with the increase of element doping content,because Ge replaces the Bi or Sb position and forms substitutional solid solution,causing the changes of the crystal structure and the lattice distortion,enhancing the phonon scattering.In general,when x = 0.005,the alloy has a high power factor and the total thermal conductivity is relatively low,the TE performance is the best,the maximum value of ZT is 0.88 at 430 K,which improves the TE properties of the alloy of Bi_0.5Sb_1.5Te₃ in the wide temperature range.