Thermoelectric materials and applied physics
经过上百年的发展, 人们清楚地认识到热电材料难以取得较高热电优值 ZT值的原因在于材料的 Seebeck系数 S、电阻率 r和电子热导率 ke 三个热电参数间的强耦合关系, 即它们都与载流子浓度 n密切相关, 很难通过单独操控某一参数提升 ZT值. 目前, 热电性能优化策略大致包括两个方面, 即电学性能的提升及热学性质的优化. 1) 电学性质方面, 通过能带工程可实现 Seebeck系数S和电阻率 r的解耦, 从而有效提升功率因子 PF, 其中能带收敛、态密度共振、能带各向异性以及能带嵌套是最为典型的能带工程策略. 此外, 界面工程, 例如能量过滤效应和调制掺杂也可显著优化电学性质. 2)热学性质方面, 通常采用降低声子弛豫时间或声子群速度的方法以实现唯一相对独立的参数—晶格热导率 kl的最小化, 包括引入晶格缺陷 (点缺陷、纳米相、位错、晶界等)及晶格软化; 另外, 寻找具有本征低晶格热导率的新型热电材料也是一种可行方式, 可通过在具有复杂晶体结构、强晶格非谐性、类液态行为或低声速的材料中筛选出具有本征低晶格热导率的化合物.
可以根据热电材料的适用温区范围对热电材料进行分类. 1) 300—550 K近室温区热电材料.目前达到商业化程度的近室温区材料为 Bi2Te3基热电材料, 其中 p型 Bi0.5Sb1.5Te3材料最高ZT值可达~1.8. 近年来, n型 Mg3(Sb, Bi)2热电材料在 300—700 K具有优异的热电性能 (ZT峰值可达 ~1.8). 2) 550—950 K中温区热电材料 . 包括 PbQ (Q = S, Se, Te), SnTe, GeTe, PbTe-AgSbTe2(LAST)及 GeTe-AgSbTe2(TAGS)等合金体系. 近年来, 研究人员相继发现了更多高性能中温区热电材料, 如方钴矿、黝铜矿、BiCuSeO、类液态材料等, 这些中温区热电材料往往都具有较高的热电性能. 3) 950 K以上的高温热电材料. SiGe合金自 20世纪 50年代末被发现以来, 便成为在高温区工作发电的主要热电材料, 目前最高 ZT值在 1173 K下可达 1.5. 此外, half-Heusler合金展现出了卓越的高温热电性能, 其 p型 FeNbSb基热电材料在 1200 K时 ZT值可达 1.6. 目前不同温区的热电材料诸如 Bi2Te3, Mg3Sb2, SnSe, PbTe, PbS, CoSb3, BiCuSeO, SnTe, GeTe, Cu2Se, half-Heusler 及SiGe合金等, 不少热电材料的ZT值均已超过1.5, 甚至超过2.0.
鉴于热电材料领域关键物理科学问题研究的紧迫性, 受《物理学报》编辑部委托, 我们邀请了国内部分活跃在该领域前沿的中青年专家撰稿, 较为全面、深入地探讨了该领域最新研究成果以及基础物理科学问题. 本次专题包括五个方面的内容. 1) MAX及其衍生 MXene相碳化物热电性能调控. 主要综述了近些年 MAX相及其衍生 MXene相材料在制备技术和热电性能的发展现状, 并针对 MXene相材料的特性提出了一些改善热电性能的可行性方案, 展望了 MAX相以及 MXene材料在未来的发展方向和前景. 2) 通过 Cu插层协同优化 SnSe2层内和层外的热电性能. 基于SnSe2材料特殊的层状结构, 引入额外的 Cu可稳定存在于范德瓦耳斯间隙, 被包围在由层间 Se所形成的四面体中心位置, 协同优化了两个方向的载流子浓度和 载流子迁移率, 从而证实了 SnSe2作为层状热电材料的发展潜力. 3) 高性能 Bi2Te3基热电薄膜的可控生长. 利用磁控溅射法制备了一系列 n型 Bi2Te3基薄膜, 研究衬底温度和工作压强对薄膜生长模式的影响规律, 通过溅射参数精确调控薄膜的形貌、结构和生长取向, 制备出层状生长的高质量致密薄膜, 克服了 n型 Bi2Te3基薄膜材料难以匹配 p型 Bi2Te3基薄膜材料的困难. 4) 二维共价键子结构 Zintl相热电材料. 主要综述了性能突出的 CaAl2Si2结构 1-2-2型、原胞内原子较多本征低热导率的 9–4+x–9型、具有天然空位而本征热导率极低的 2-1-2型、以及电性能相对较好的 ZrBeSi结构 1-1-1型 Zintl相的研究进展. 5) 黄铜矿 CuGaTe2热电性能优化. Ni原子可有效替代 CuGaTe2材料中 Cu的位置, 并引起载流浓度下降和迁移率提升, 掺杂后费米能级附近态密度的提升是 Seebeck系数显著增强的主因, 最终ZT值在 873 K可达 1.26, 因此证实磁性元素掺杂是提升热电性能的有效手段. 以上五个方面的热电材料研究, 从不同材料、不同视角探讨了热电材料的最新进展、问题、现状以及展望. 希望本专题能为国内热电材料及应用物理领域的学术交流做一些贡献, 进一步促进该研究领域的发展.
2021, 70 (20): 206501.
doi: 10.7498/aps.70.20211050
Abstract +
Thermoelectric materials, a kind of new energy material, can directly convert heat energy into electric energy, and vice versa, without needing any other energy conversion. However, the present development status of thermoelectric materials severely restricts their engineering applications in thermoelectric devices. Improving the thermoelectric performances of existing thermoelectric materials and exploring new thermoelectric materials with excellent performance are eternal research topics in thermoelectricity field. In recent years, the MAX phases and their derived MXene phases have gradually received the attention of researchers due to their unique microstructures and properties. The crystal structure of MAX phases is comprised of Mn+1Xn structural units and the single atomic plane of A stacked alternately. The two-dimensional MXene phase derived can be prepared after the atoms in the A-layer of MAX have been etched. The MAX phases and their derived MXene phases have both metal feature and ceramic feature, and also have good thermal conductivity and electric conductivity, and they are anticipated to be the promising thermoelectric materials. In this paper, the present development status of the preparation technology and the thermoelectric properties of MAX phases and MXene are reviewed. Finally, some feasible schemes to improve the thermoelectric properties of MAX and its derived MXene phase materials are proposed, and the development direction and prospect of MAX phases and MXene are prospected as well.
2021, 70 (20): 207101.
doi: 10.7498/aps.70.20211165
Abstract +
Thermoelectric material is a new type of functional material that can realize the direct conversion between heat energy and electric energy. It has received a lot of attention because it has wide practical applications. However, the applications of thermoelectric devices are limited by their low conversion efficiencies. The conversion efficiency is determined mainly by the thermoelectric properties of the material. In this work, a compound of CuGaTe2 chalcopyrite is selected as a research object, and a series of Ni-doped samples Cu1–xNixGaTe2 (x = 0–0.75%) is synthesized by the vacuum melting method. The temperature dependent thermal and electrical properties for Cu1–xNixGaTe2 (x = 0–0.75%) compounds are investigated. The results show that the Ni atom can effectively replace the Cu atom of the material, and thus leading the carrier concentration to decrease slightly and inducing the mobility to increase. At the same time, the Seebeck coefficient increases significantly after Ni doping: on the one hand, the increase is due to the decrease of the carrier concentration of the sample; on the other hand, the effective increase of the density of states near the Fermi level plays an important role in increasing Seebeck coefficient. Then, the thermal conductivity decreases effectively due to the enhancement of point defect scattering caused by Ni doping, and the minimum lattice thermal conductivity is reduced by ~30% in comparison with the matrix lattice thermal conductivity. Finally, the maximum ZT value for Cu0.095Ni0.005GaTe2 sample (ZT = 1.26 at 873 K) is obtained to be ~56% larger than that for CuGaTe2. This work indicates that the doping magnetic element Ni at Cu site is also one of the effective ways to improve the thermoelectric properties of CuGaTe2 materials.
2021, 70 (20): 208401.
doi: 10.7498/aps.70.20211444
Abstract +
SnSe, a layered material with intrinsic low thermal conductivity, is reported to have excellent thermoelectric properties. SnSe2 has a similar structure to SnSe, but the SnSe2 has a low electrical transport, resulting in a poor thermoelectric performance, and the intrinsic SnSe2 has a maximum ZT value of only ~ 0.09 at 773 K. In this work, SnSe1.98Br0.02-y%Cu (y = 0, 0.50, 0.75, 1.0) bulk materials are synthesized by the melting method combined with spark plasma sintering (SPS) based on the carrier concentration improved through Br doping. In the SnSe2 materials with van der Waals chemical bonding between layers, the synergistic effects of intercalating Cu on the thermoelectric properties are investigated. On the one hand, the extra Cu not only provides additional electrons but also can be embedded stably in the van der Waals gap and form an intercalated structure, which is beneficial to the charge transfer in or out of the layers, and thus synergistically improving the carrier concentration and carrier mobility. On the other hand, owing to the dynamic Cu doping, the increase of carrier concentration compensates for the decrease of carrier mobility caused by carrier-carrier scattering, which maintains the high electrical transport properties at high temperature. The present results show that at room temperature, the power factors along the parallel and perpendicular to the SPS (//P and ⊥P) sintering directions increase from ~0.65 and ~0.98 µW·cm–1·K–2 for intrinsic SnSe2 to ~10 and ~19 μW·cm–1·K–2 for SnSe1.98Br0.02-0.75%Cu samples, respectively. Finally, at 773 K, the maximum ZT value of ~0.8 is achieved along the ⊥P direction. This study proves that the SnSe2 greatly promises to become an excellent thermoelectric material.
2021, 70 (20): 207303.
doi: 10.7498/aps.70.20211090
Abstract +
Bi2Te3-based alloys have been long regarded as the materials chosen for room temperature thermoelectric (TE) applications. With superior TE performances, Bi2Te3-based bulk materials have been commercially used to fabricate TE devices already. However, bulk materials are less suitable for the requirements for applications of flexible or thin film TE devices, and therefore the thin film materials with advanced TE properties are highly demanded. Comparing with bulk materials and P-type Bi2Te3-based thin films, the TE properties of N-type Bi2Te3-based thin films have been relatively poor so far and need further improving for practical applications. In this study, a series of N-type Bi2Te3–xSex thin films is prepared via magnetron sputtering method, and their structures can be precisely controlled by adjusting the sputtering conditions. Preferential layered growth of the Bi2Te3–xSex thin films along the (00l) direction is achieved by adjusting the substrate temperature and working pressure. Superior electrical conductivity over 105 S/m is achieved by virtue of high in-plane mobility. combining the advanced Seebeck coefficient of Bi2Te3-based material with superior electrical conductivity of highly oriented Bi2Te3–
Research progress of two-dimensional covalent bond substructure Zintl phase thermoelectric materials
2021, 70 (20): 207304.
doi: 10.7498/aps.70.20211010
Abstract +
Thermoelectric materials can realize the direct conversion between thermal energy and electrical energy, and thus having important applications in semiconductor refrigeration and heat recovery. Zintl phase is composed of highly electronegative cations and anions, which accords with the concept of “phonon glass, electron crystal” (PGEC). Thermoelectric properties of Zintl phase have attracted extensive interest, among which the two-dimensional (2D) covalent bond structure featured Zintl phases have received more attention for their outstanding electrical properties. In this review, Zintl phase materials with two-dimensional covalent bond substructures are reviewed, including 1-2-2-type, 9–4+x–9-type, 2-1-2-type and 1-1-1-type Zintl phase. The 1-2-2-type Zintl phase is currently the most widely studied and best-performing Zintl material. It is worth mentioning that the maximum ZT value for the Mg3Sb2-based n-type Zintl material with the CaAl2Si2 structure has been reported to reach 1.85, and the average ZT value near room temperature area also reaches 1.4. The 9–4+x–9-type Zintl material with a mass of atoms in unit cell contributes to lower thermal conductivity thus relatively high ZT value. The 2-1-2-type Zintl material has extremely low thermal conductivity due to the intrinsic vacancies, which has been developing in recent years. The 1-1-1-type Zintl material with the same ZrBeSi structure as the 2-1-2-type Zintl material, shows better electrical transport performance. In sum, this review summarizes the recent progress and optimization methods of those typical Zintl phases above. Meanwhile, the future optimization and development of Zintl phase with two-dimensional covalent bond substructures are also prospected.
