Although new barrier layers that meet the CTE matching criterion, including Fe, Co-P, FeCrNi, and Ni-based alloy 27, 28, 29, 30, 31, have been continuously explored in recent years, they have not been demonstrated to exhibit sufficiently low contact resistivity or the ability to ensure long-term device stability above 473 K. However, when such a device is operated above 473 K, the Ni will continue to react with the BiTe to form brittle Ni-Te compounds, leading to an increase in both interface resistivity and CTE mismatch until cracking and failure occur, especially in the n-type joint 24, 25, 26. Ni has been widely used as a barrier layer for BiTe-based devices because of its similar CTE value 22, 23. To satisfy the above-mentioned requirements, it has long been assumed that the following standards need to be met between the barrier layer and the TE material: coefficient of thermal expansion (CTE) matching to reduce interfacial stress, a certain degree of reaction at their interface to facilitate strong bonding, and work function matching to ensure low levels of contact resistivity and parasitic loss 16, 17, 18, 19.īismuth telluride (BiTe) exhibits chemical stability and excellent low-temperature TE performance 20, 21 and is currently the most reliable choice for low-grade waste heat recovery using TE technology. Unfortunately, barrier layer design remains a challenge since the interface between the barrier layer and the TE material should exhibit both high strength and low resistivity while ensuring long-term device stability at the service temperature, and the current lack of adequate barrier layers is a bottleneck limiting the application of TE devices in waste heat recovery 13, 14, 15. In the last two decades, significant breakthroughs have been made in improving the performance of TE materials 10, 11, 12. To prevent performance degradation or device failure caused by mutual diffusion between the TE materials and the electrodes during connection and service, it is essential to introduce a diffusion barrier layer on the surface of the TE materials 9. The conversion efficiency ( η) of TE devices is related to the dimensionless figure of merit ( ZT) of the constituent TE materials and the connection quality 6, 7, 8. The most common configuration of these TE devices is to connect p- and n-type TE legs with electrodes electrically in series and thermally in parallel 5. Additionally, there is great promise for the use of TE devices in low-grade waste heat recovery since more than 60% of the energy generated through burning fossil fuels is discharged into the environment as waste heat, about half of which is low-grade 4, for which there remains a lack of effective recycling methods. Thermoelectric (TE) devices that can convert thermal energy into electrical energy have been widely used in maintenance-free power supply systems for deep space exploration and other extreme environments 1, 2, 3. Highly competitive conversion efficiency of 6.2% and power density of 0.51 W cm −2 are achieved for a module with leg length of 2 mm at the hot-side temperature of 523 K, and no degradation is observed following operation for 360 h, a record for stable service at this temperature, paving the way for its application in low-grade waste heat recovery. A titanium barrier layer with loose structure is optimized, in which the low Young’s modulus and particle sliding synergistically alleviates interfacial stress, while the TiTe 2 reactant enables metallurgical bonding and ohmic contact between the barrier layer and the thermoelectric material, leading to a desirable interface characterized by high-thermostability, high-strength, and low-resistivity. Here we propose a new design principle of barrier layers beyond the thermal expansion matching criterion. The lack of desirable diffusion barrier layers currently prohibits the long-term stable service of bismuth telluride thermoelectric devices in low-grade waste heat recovery.
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