Recently, Qiu Pengfei, an associate researcher at the Shanghai Institute of Ceramics, Chinese Academy of Sciences, researcher Shi Xun, Chen Lidong and G. Jeffrey Snyder, professor of Northwestern University, and Jürgen Janek, professor of Giessen University, Germany, have analyzed the mobile ions in liquid-like thermoelectric materials. The mechanism of migration and precipitation under the action of field, combined with theory and experiment to propose the thermodynamic stability limit criterion for the precipitation of 'liquid-like' ions from the material, and the corresponding experimental characterization methods and techniques are given. On this basis, The introduction of 'ion barrier-electron conduction' interface can significantly improve the service stability of liquid-like thermoelectric materials under strong electric field or large temperature difference. This research is of great significance for the practical application of liquid-like thermoelectric materials. Related research results are published in Nature Communications, DOI:10.1038/s41467-018-05248-8, the equipment and some measurement results independently developed by the research team were published in the Journal of Inorganic Materials (Vol.32, 2017, 1337- 1344), and apply for Chinese invention patents.
Thermoelectric energy conversion technology uses the Seebeck and Peltier effects of semiconductor materials to directly convert thermal energy and electrical energy. It has important and broad application prospects in the fields of industrial waste heat and automobile exhaust gas thermal power generation. However, subject to control Due to the long-range order of the structure, the lattice thermal conductivity of conventional crystalline compound thermoelectric materials has a minimum limit (minimum lattice thermal conductivity), which limits the space for continuous optimization of thermoelectric performance. Beginning in 2012, the thermoelectric team led by Chen Lidong and Shi Xun proposed to introduce ions with 'liquid-like' characteristics into solid materials to reduce thermal conductivity and optimize thermoelectric performance, successfully breaking the lattice thermal conductivity in solid glass or crystalline state. Restrictions on materials have led to the discovery of a new class of high-performance (ZT~2.0@1000 K) liquid thermoelectric material systems with the characteristics of 'phononic liquid-electronic crystals' (Nat. Mater. 2012, Adv. Mater. 2013&2014& 2015&2017, Energ. Environ. Sci. 2014&2017, npj Asia Mater. 2015, etc., has become a hot spot in the field of thermoelectric materials in recent years. However, these types of liquid heat Metal cations with 'liquid-like' characteristics in materials (such as Cu2-δSe, Ag9GaSe6, Zn4Sb3, etc.) are prone to long-term migration and precipitation under the action of electric field or temperature field, resulting in poor service stability and limited practical applications. By studying the migration process and physical mechanism of ions in liquid-like thermoelectric materials, and improving their service stability, it is the key to the application of new high-performance liquid thermoelectric materials.
The team found that under the action of the external field, metal cations (such as Cu, Ag, Zn) in the liquid-like thermoelectric material will migrate long-range from one end of the sample to the other and produce an ion concentration gradient. However, only the metal at high concentration When the cation chemical potential is equal to or higher than the chemical potential of the corresponding metal element, the metal cation will be precipitated from the material into a metal element, which will lead to decomposition of the material. Therefore, each type of liquid thermoelectric material has a thermodynamic stability limit, only when the external field The effect is strong enough, so that when the material exceeds this limit, ion precipitation and material decomposition will occur. Otherwise, the liquid-like thermoelectric material will be similar to the traditional crystalline thermoelectric compound, maintaining good stability and thermoelectric properties under the action of the external field. Deriving the electrochemical formula, the team found that the specific value of this thermodynamic limit can be given by the maximum applied voltage (ie, the threshold voltage) that the material can withstand without decomposition. The threshold voltage is a characteristic parameter that is independent of the material size, only It is related to the chemical composition of the material and the ambient temperature.
In order to experimentally prove the existence of the thermodynamic stability limit of liquid-like thermoelectric materials, the team independently built an instrument for quantitatively characterizing the service stability of liquid thermoelectric materials. In a constant temperature environment and a given temperature difference environment, the relative resistance and relative Seebeck were respectively used. The change of the coefficient is used as the evaluation parameter. The critical voltage of a series of Cu2-δ(S,Se) liquid thermoelectric materials has been successfully measured, and its value range is 0.02-0.12V. Under the constant temperature environment, the amount of Cu loss increases or When the ambient temperature increases, the critical voltage of Cu2-δ(S,Se) material increases gradually, and its value agrees with the theoretical prediction, indicating that the metal cations with 'liquid state' in the material are more difficult to precipitate. Under the given temperature difference environment The critical voltage of the Cu2-δ(S,Se) material is also related to the heat flow direction inside the material. When the heat flow direction is the same as the current direction, the material has a smaller critical voltage, indicating that the metal cation in the material is more likely to be precipitated. When the direction of heat flow is opposite to the direction of current, the material has an enhanced threshold voltage and material stability increases significantly.
Based on an in-depth understanding of the mechanism of ion migration and precipitation, the team proposed to introduce an 'ion barrier-electron conduction' interface in a liquid-like thermoelectric material to effectively suppress the precipitation of metal cations with 'liquid-like' characteristics and improve the liquid state. Thermoelectric material service stability. Because metal cations cannot pass the 'ion barrier-electron conduction' interface, the external field will be shared by the various types of liquid thermoelectric materials blocked by the 'ion barrier-electron conduction' interface. In turn, the material as a whole can remain stable under a stronger electric field or a larger temperature difference. Meanwhile, the 'ion barrier-electron conduction' interface does not affect the free transmission of electrons/holes, so the multi-stage material is obtained high. At the same time of service stability, the intrinsic excellent thermoelectric performance will remain. This strategy has been successfully verified in multi-segment Cu1.97S materials connected by conductive carbon layers. This work not only provides practical applications for liquid-like thermoelectric materials. The possibility also provides a new idea for improving the service stability of other electronic/ion mixed conductors.
The research work has received funding and support from the National Key Research and Development Special Project, the National Natural Science Foundation of China, and the Chinese Academy of Sciences Youth Innovation Promotion Association.
(a) Working environment of liquid thermoelectric materials; (b) ordinary liquid hydroelectric materials under high current and (c) metal Cu precipitation of liquid-like thermoelectric materials with 'ion barrier-electron conduction' interface
Physical and chemical processes for ion migration and precipitation in liquid-like thermoelectric materials
(a) Critical current and critical voltage of Cu1.97S samples of different lengths; (b) Critical voltage of Cu2-dS samples with different stoichiometric ratios; (c) Critical current of Cu1.97S samples in a given temperature difference environment; d) Critical voltage of Cu1.97S in different temperature difference and heat flow direction
The principle of improving the stability of service using the 'ion barrier-electron conduction' interface (a, b); (c) constant temperature environment and (d) experimental results in a given temperature difference environment
