1.Hubei Key Laboratory of Theory and Application of Advanced Materials Mechanics, Wuhan University of Technology, Wuhan 430070, China 2.State Key Laboratory of Advanced Technology for Materials Synthesis and Processing, Wuhan University of Technology, Wuhan 430070, China
Fund Project:Project supported by the National Natural Science Foundation of China (Grant Nos. 51772231, 51972253) and the Fundamental Research Fund for the Central Universities, China (Grant Nos. 2020IB001, 2020IB013)
Received Date:29 November 2020
Accepted Date:17 March 2021
Available Online:07 June 2021
Published Online:05 August 2021
Abstract:Lead-free chalcogenide SnTe has a similar crystal structure and energy band structure to high performance thermoelectric material PbTe, which has been widely concerned in recent years. However, due to its low Seebeck coefficient, high intrinsic Sn vacancy concentration and high thermal conductivity, its intrinsic thermoelectric performance is poor. In this study, Mn-In-Cu co-doping SnTe-based thermoelectric materials are prepared by hot pressing sintering at high-temperature and high-pressure. Indium (In) doping brings the resonant level in SnTe and increases the density of states which greatly improves Seebeck coefficient at room temperature; the Seebeck coefficient of Sn1.04In0.01Te(Cu2Te)0.05 reaches 70 μV·K–1 at room temperature. With adding manganese (Mn), the Seebeck coefficient at room temperature is well preserved, indicating that Mn doping has little effect on the resonant level brought by In doping. In addition, due to the band convergence brought by Mn doping, the high temperature Seebeck coefficient of the material is improved, the maximum Seebeck coefficient reaches 215 μV·K–1 for the sample with 17% Mn doping amount at 873 K. Owing to the combination of band convergence and resonant level, the Seebeck coefficient of the whole temperature range of the material increases, the power factor of the material is also greatly optimized, and all samples have a power factor of more than 1.0 mW·m–1·K–2 at room temperature. On the other hand, the point defects brought by Mn alloying and the interstitial defects introduced by copper (Cu) enhance the phonon scattering and effectively reduce the lattice thermal conductivity of the material, the lattice thermal conductivity decreases to 0.68 W·m–1·K–1 at 873 K. The electrical and thermal properties of the materials are optimized simultaneously under the combination of various strategies, the peak zT ≈ 1.45 is obtained at 873 K in the p-type Sn0.89Mn0.15In0.01Te(Cu2Te)0.05 sample and the average zT of 300–873 K reaches 0.76. In the process of multi-strategy coordinated regulation of SnTe-based thermoelectric materials, the excellent properties of single strategy can be well maintained, which provides a possibility for further improving the performance of SnTe-based thermoelectric materials. Keywords:SnTe/ high-temperature and high-pressure/ doping/ thermoelectric materials
为了进一步表征样品的相组成, 对Sn0.89Mn0.15In0.01Te(Cu2Te)0.05样品进行了扫描电子显微镜分析. 如图2所示, Sn, Te和In元素在基体内分布均匀, 在二次电子成像图中存在微米级别的浅灰色区域, 元素分布情况显示为Cu元素富集, Sn元素缺少, 表明存在Cu2Te的杂相, 与XRD结果一致. 此外, 二次电子成像图中还能观察到黑斑的存在, 对应着少量Mn的富集, 这可能是由于Mn元素掺杂过多后开始出现MnTe杂相. 图 2 Sn0.89Mn0.15In0.01Te(Cu2Te)0.05样品的扫描电子显微镜图像 Figure2. Scanning electron microscope images of the Sn0.89Mn0.15In0.01Te(Cu2Te)0.05 sample.
图3为Sn1.04–xMnxIn0.01Te(Cu2Te)0.05 (x = 0—0.17)样品电性能随温度的变化曲线. 从图3(a)可以看出, 所有样品的电导率均随温度的增加而降低, 表现为典型的简并半导体行为. 当x < 0.06时, 样品的电导率随掺杂量有所增加, 这与文献报道结果类似[23], 可能是Mn掺杂导致SnTe中可溶性Cu2Te的量减少造成的. 当x ≥ 0.06时, 样品的电导率随着Mn含量的增加而逐渐降低, 这主要是由样品的载流子迁移率大幅降低造成的[28,29], 如图3(b)所示. 迁移率的降低可能是由于Mn掺杂后点缺陷增加, 导致散射增强以及价带收敛带来的载流子有效质量的增加(详细讨论见下文). 随着Mn掺杂量的增加, 载流子浓度先降低后升高. 载流子浓度先降低可能是因为Mn掺杂后降低了Cu2Te在基体中的溶解度, 使得部分作为电子受体的Cu1+减少[17,23]. 而随着Mn掺杂量的继续增加, 载流子浓度上升可能是因为Mn取代Sn后引入了更多的Sn空位[28-30]. 图 3 Sn1.04–xMnxIn0.01Te(Cu2Te)0.05(x = 0—0.17)样品的(a) 电导率随温度的变化, (b) 室温下载流子浓度和迁移率随x的变化, (c) Seebeck系数随温度的变化, (d) 室温下Seebeck系数与载流子浓度关系以及和相关研究的对比图[11,17,30-32], (e) 室温下有效质量对比图, (f) 功率因子随温度的变化 Figure3. Sn1.04–xMnxIn0.01Te(Cu2Te)0.05 (x = 0–0.17) samples: (a) Electrical conductivities as a function of temperature; (b) carrier concentration and mobility as a function of x at room temperature; (c) Seebeck coefficients as a function of temperature; (d) the relationship between Seebeck coefficient and carrier concentration at room temperature and comparison with the correlation studies[11,17,30-32]; (e) effective mass comparison at room temperature; (f) power factor as a function of temperature.