「經典妙文」熱電材料新星—硒化物

硒化物具有較低的成本、較為豐富的儲量和優異的熱電性能，使得它成為了熱電材料圈的明日之星。清華大學李敬鋒教授研究團隊首次對硒化物的熱電性能進行了綜合評述，該文章發表在Journal of Materiomics第4卷第4期，題目為Low-cost and environmentally benign selenides as promising thermoelectric materials。

Wei T-R, Wu C-F, Li F, Li J-F. Low-cost and environmentally benign selenides as promising thermoelectric materials. Journal of Materiomics 2018; 4 (4): 304-320. https://doi.org/10.1016/j.jmat.2018.07.001

內容梳理

硒化物材料包含的晶體結構，帶隙，導電類型（載流子或離子）和相結構（單相或多晶型物）範圍很寬。本綜述中，介紹了二元和多元硒化物的最新進展,包括傳統PbSe ,類液體狀Cu2Se、層狀SnSe 、類金剛石和無序的多元化合物。

首先，討論了二元化合物PbSe，它可作為PbTe的低成本替代品，但是通常PbSe的熱電性能較差，可以採用成分優化，電子傳導優化和降低熱傳導的方式改善。以實現高性能。

Fig. 3. (a) Electrical resistivity and Seebeck coefficient of nominally stoichiometric PbSe synthesized by MA and SPS; (b) Hall coefficients and Hall carrier concentration (inset), (c) zT and (d) average zT for PbSe + x% Pb.

然後，介紹了無鉛硒化物，包括類液態聲子的超離子Cu2Se，Bi2Se3和準一維In4Se3。Cu2Se具有無序結構，類液體行為，相變和高zT值。Cu2Q（Q = Se，S，Te）材料將「聲子玻璃，電子晶體（PGEC）」的概念擴展到「聲子液體，電子晶體（PLEC）」，為探索熱電材料開闢了新途徑。

Fig. 6. (a) Unit cell of Cu2Se at high temperatures (β-phase) with a cubic anti-fluorite structure. (b) Projected plane representation of the crystal structure along the cubic direction. The arrows indicate that the Cu ions can freely travel among the interstitial sites. (c) The high-temperature specific heat capacity of Cu2Se. The theoretical value (Dulong–Petit) for the high-temperature specific heat at constant volume Cv is 3NkB in a solid crystal. The lowest Cv theoretical value in a liquid is 2NkB. (d) zT of the low-temperature (α) and high-temperature (β) phases in Cu2-xSe.

Bi2Se3和In4Se3具有獨特的電子能帶結構。Bi2Se3是具有拓撲保護表面的三維拓撲絕緣體（3D-TI），如圖8所示。In4Se3具有較低的維度，支持準一維In-鏈[見圖9（a）]，納米棒結構和非對稱色散。 可通過利用硒缺乏來減少帶隙能量。

Fig. 8. (a) Crystal structure and (b) band structure of Bi2Se3; (c) energy and momentum dependence of local density of states on [111] surface, where the surface state can be clearly seen around Γ point with red lines.

Fig. 9. (a) Crystal structure of In4Se3; (b) generalized electron susceptibility along the X-U-X line; (c) zT of single-crystal In4Se3.

層狀化合物SnSe具有高zT值，低導熱率和多價帶。未來的研究應著重於識別缺陷的類型，數量和分布，以便在缺陷與傳輸行為之間建立更準確的關係，並最終制定控制這些缺陷的策略。

Fig. 11. Energy-barrier scattering of carriers at grain boundaries. (a) Schematic depiction of this model. Carriers are trapped at grain boundaries, resulting a sharp rise of hole density therein and bending the valence band, forming energy barriers for charge transport; (b) energy barrier scattering in mechanically alloyed SnSe samples; (c) temperature dependence of mobility in polycrystalline SnSe. Carriers are subjected to a mixed scattering process by acoustic phonons and energy barriers at grain boundaries.

接下來，討論了三元類金剛石硒化物和無序三元銅或銀基硒化物。

Fig. 13. Optical, electrical and thermal properties of Cu3SbSe4. (a) Direct band gap determined by optical and electrical measurements, (b) calculated band structure and density of states spectrum, (c) Pisarenko plot indicating an increased in DOS effective mass and (d) lattice thermal conductivity as a function of S fraction of Cu3SbSe4-Cu3SbS4 solid solutions.

Fig. 14. (a) Crystal structure and (b) ultralow thermal conductivity of Cu3SbSe3; (c) calculated atomic displacement parameter (ADP) of Cu, Sb and Se atoms along different directions, showing liquid behavior of Cu1z projection; (d) effect of lone pair and the bond angle on thermal conductivity of Cu3SbSe3 and similar ternary Cu-, Ag-based chalcogenides; (e) electrical properties and zT of Cu3SbSe3.

總結和展望

硒化物表現出優異的熱電性能，是碲化物的綠色且低成本的替代品。由於硒位於S和Te之間，因此硒化物的傳輸性能介於同結構的硫化物和碲化物，這為通過與S或Te合金化來調整性能提供了很大空間。如果硒化物與硫化物或碲化物的對應物不具有相同結構，很可能會發現相變或沉澱，這將帶來一些有趣的現象。由於組成，鍵合和晶體結構的多樣性，硒化物是探索有關微帶結構，大聲子非諧性和異常運輸行為的新物理和化學機理的理想對象。反過來，這些有趣的現象對於硒化物的高效率至關重要，並可以指導性能優化和材料開發。

還應注意高溫下熱電硒化物的材料質量。由於硒相對較低的熔融和沸騰溫度，許多硒化物易發生化學揮發，並因此導致偏離化學計量，劣化或變形，特別是當測量或工作溫度接近化合物的熔點時，經常出現「異常」數據（通常突然降低的熱導率導致高zT值），這應謹慎對待，建議在加熱和冷卻循環中進行測量以評估樣品的穩定性。

如今，通過結合材料基因組的概念和高通量計算，可以快速篩選潛在的高性能硒化物。在獲得對傳輸性質和潛在機理的深入了解之後，可以整合各種策略來合理優化硒化物的性能。

亮點

• Earth-abundant and low-toxicity selenides have been emerging as promising thermoelectric materials.
• Recent advances on thermoelectric selenides are presented focusing on unique features and structure-property relationship.
• Challenges and design strategies are proposed for further development of thermoelectric selenides.

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