Thermoelectric conversion materials enable direct conversion of thermal energy and electrical energy, and have important applications in aerospace special power/heat flow management, waste heat/cogeneration and portable refrigeration. Thermoelectric performance is derived from the dimensionless figure of merit (ZT=S2σT/κ) For characterization, high conversion efficiency needs to increase the power factor S2σ of the material as much as possible and reduce the thermal conductivity κ as much as possible. Recently, several types of environmentally friendly new thermoelectric materials such as SnSe and SnTe have been advanced manufacturing by the Ningbo Institute of Materials Technology and Engineering, Chinese Academy of Sciences The team of optoelectronic functional materials and devices is closely integrated with theory and experiment, and has conducted a series of research work on the regulation of thermoelectric properties.
In 2014, the new thermoelectric material SnSe reported by Nature magazine has a very low thermal conductivity (0.3W -1K-1With the highest ZT value (2.6), the follow-up investigators found that the thermoelectric properties of SnSe materials changed greatly and their repeatability was poor. Based on this, the team developed a horizontal gas phase method to prepare high-quality SnSe singles. Crystal, and carried out relevant measurement work on its intrinsic carrier transport, phonon transport and phase transition. It was found that the intrinsic thermal conductivity of SnSe single crystal at room temperature is 2.0W -1K-1, reduced to 0.55W at 773K -1K-1This study proposes the variation of the carrier concentration, mobility, effective mass and deformation potential constants of SnSe before and after phase transformation with temperature, and describes the SnSe intrinsic carrier transport using the single parabolic energy band model. Behavior, thus indicating SnSe's performance control ideas and reasonable prediction of SnSe optimized thermoelectric performance. Related research results published in ACS Energy Lett.
Energy band engineering is an effective method to regulate the electrical properties of thermoelectric materials. In the previous work, the team has optimized the thermoelectric properties of SnTe through two energy band engineering mechanisms: 'degeneration by valence band' and 'resonance energy level'. The co-doping of a lower concentration of In with appropriate amounts of Mg, Mn, Cd, or Hg will enable the synergistic effect of the two energy-band control mechanisms, enabling the overall improvement of the SnTe's thermoelectric performance in the larger temperature range. The team used hot pressing. The In-Hg co-doped SnTe samples were prepared and it was confirmed that the synergistic effect of the two control mechanisms significantly improved the Seebeck coefficient. Further study found that after the temperature rises, the energy banding and resonance energy level will gradually change from synergy to competition. Further enriched the design connotation of thermoelectric material energy band engineering. Related research results were published on J. Materiomics.
Structural design is an effective way to control the phonon transport of materials, especially in layered thermoelectric materials. The team inserted a small amount of Na between layers of MoS2 to achieve a significant reduction in the lattice thermal conductivity. Studies suggest that Na intercalation MoS 2The decrease of the lattice thermal conductivity is mainly reflected in two aspects. One is the decrease of the phonon vibration frequency, and the other is the increase of the local phonon frequency branch. Na intercalation MoS 2Afterwards, the non-harmonic interactions of the low-frequency branch phonons are enhanced, and more phonon scattering channels are added, which reduces the phonon lifetime by 1-2 orders. This study provides in-depth control of the thermal conductivity of similar thermoelectric materials. Understanding and feasible ideas, relevant research results were published on J. Phys. Chem. C.
The study was funded by the National Natural Science Foundation of China, the National Key R&D Program, the Zhejiang Outstanding Youth Fund, the Natural Science Foundation of Zhejiang Province, and the Ningbo Science and Technology Innovation Team.
Figure 1. SnSe single crystal preparation and thermoelectric value
Figure 2. Increased Seebeck coefficient in In&Hg co-doped SnTe
Figure 3. Lattice thermal conductivity of Na intercalated MoS2
