Abstract

How to form a uniform fine-grained structure in n-type polycrystalline Bi₂Te₃ based alloys to stabilize and improve the thermoelectric properties is the burning issue to be solved. In the present work, equal channel angular extrusion (ECAE) method was used to extrude the cast ingot directly at various temperatures to obtain the BiTeSe polycrystalline bulk alloys with uniform microstructure. The initial lamellae coarse grains (hundreds microns to several millimeters) were efficiently and uniformly refined to a minimum of 4.07 μm at 703 K, which fully demonstrates the feasibility of grain refinement by ECAE for cast n-type BiTeSe ingot. The average grain size of the extruded billets rapidly increased from 5.35 to 29.55 μm with the increase of extrusion temperature from 703 to 823 K. Meanwhile, the behavior of abnormal grain growth became more evident as the extrusion temperature increased because the growth rate increased more rapidly than the rate of nucleation. As the extrusion temperature increased, the Vickers hardness decreased gradually due to grain coarsening. BiTeSe alloys extruded at 703 K achieved the maximum Vickers hardness of 59.8 HV_0.05, which was nearly three times higher than that of the initial cast ingot. All extruded BiTeSe billets possessed the much higher dimensionless figure of merit (ZT), compared with the initial cast ingot due to the simultaneously optimization of electrical and thermal transport performances caused by ECAE process. Finally, the specimen extruded at 823 K achieved the maximum ZT of 0.685 at 383 K.

Graphic Abstract

中文翻译：

---

等通道角挤压铸造n型Bi 2 Te 2.79 Se 0.21铸锭的组织细化及性能改进

摘要

如何在n型多晶Bi ₂ Te _3中形成均匀的细晶结构稳定和改善热电性能的基体合金是亟待解决的燃烧问题。在本工作中，采用等通道角挤压（ECAE）方法直接在各种温度下挤压铸锭，以获得具有均匀组织的BiTeSe多晶块状合金。初始薄片状粗晶粒（几百微米到几毫米）在703 K下被有效且均匀地精炼到最小4.07μm，这充分证明了ECAE对铸造n型BiTeSe铸锭进行晶粒细化的可行性。随着挤压温度从703 K升高到823 K，挤压坯料的平均晶粒尺寸从5.35μm迅速增加到29.55μm。随着挤压温度的升高，异常生长的行为变得更加明显，这是因为生长速率的增长快于成核速率。随着挤压温度的升高，维氏硬度由于晶粒粗化而逐渐降低。在703 K压力下挤出的BiTeSe合金的最大维氏硬度为59.8 HV_0.05，几乎是初始铸锭的三倍。与初始铸锭相比，所有挤压的BiTeSe坯料均具有更高的无量纲品质因数（ZT），这是因为同时优化了ECAE工艺导致的电和热传输性能。最终，在823 K下挤出的样品在383 K下获得的最大ZT为0.685。

图形摘要