Energy shortage is becoming increasingly prominent. On the other hand, every year more than 60% of the global energy consumption eventually dissipates into the environment in the form of waste heat^[1,2]. With the advantages of no moving parts and no emission, thermoelectric (TE) power generation can convert heat directly into electricity based on the Seebeck effect, which can be applied in the field of waste heat recovery^[3,4,5,6]. The theoretical conversion efficiency limit of TE devices mainly depends on the performance of TE materials, which is usually presented by the dimensionless figure of merit zT (zT=S²sT/k, where S, s, T, and k are the Seebeck coefficient, electrical conductivity, absolute temperature, and thermal conductivity, respectively). In the low and medium temperature range (room temperature to 300 ℃), the most commonly used TE materials are bismuth telluride (BT) based alloys with the zT of 1.0-1.5^[7,8,9,10].

The TE device is usually composed of multiple TE couples connected electrically in series and thermally in parallel. Each TE couple owns one P-type and one N-type element, connected with the electrodes. The interface between the electrode and the TE material is usually the weakest part of the TE device^[11,12,13]. With the working temperature increasing or the service time prolonging, the inter-diffusion at the interface causes changes of the microstructure and chemical composition on both sides, which affects the interfacial strength, the interfacial resistivity and the material’s TE properties. Meanwhile, the continuously increasing interfacial resistivity between the electrode and the TE material causes additional energy loss, which deteriorates the service performance of the TE device^[14,15,16]. To suppress the interfacial diffusion and improve the interfacial stability, a barrier layer is required between the electrode and TE material^[17,18].

Various efforts have been made to study the barrier layer and improve the interfacial stability of bismuth telluride TE devices. Currently, Ni is the most widely used barrier layer material for BT devices^[19]. Chen, et al.^[20] found Ni barrier layer can significantly reduce 32% of the interfacial resistivity by effectively suppressing the diffusion of Te and Sb on the border of Bi₂Te_2.5Se_0.5 and solder. Chen, et al.^[21] observed different NiTe_x phases generated at P-type Ni/(Bi_0.25Sb_0.75)₂Te₃ and N-type Ni/Bi₂(Te_0.9Se_0.1)₃ interfaces. At the temperature lower than 200 ℃, Ni/BT elements usually present low interfacial resistivities (<10 μΩ·cm²). However, Liu, et al.^[22] observed accelerated inter-diffusion between Ni and BT materials at the temperature higher than 200 ℃, which caused abnormal increase of the interfacial resistivity. To suppress the high temperature inter-diffusion, great efforts have been carried out to search for an improved barrier layer, including Ni-based alloys^[23,24,25] and other binary^{[26,27,28,29]} or ternary alloys^[30]. Even though some of these selected barrier layer materials show effective resistance on interfacial diffusion, unfortunately still no candidate can synchronously provide low interfacial resistivity and high stability.

The traditional way to develop barrier layer for TE device is essentially a serial try-and-error searching work with heavy workload and low efficiency. In this study, an efficient screening method based on high-throughput strategy^[31] was employed, and Fe was swiftly selected as the preferred barrier layer material for P-type Bi_0.5Sb_1.5Te₃ (P-BT) from 6 candidates (Cr, Co, Ti, Fe, Zr, Al). Fe/P-BT TE elements were then prepared and aged. The evolution of the interfacial microstructure was analyzed, and the interfacial resistivity was investigated. Finally, life prediction of the Fe/P-BT interface was made based on the diffusion kinetics.

The commercial Bi_0.5Sb_1.5Te₃ ingot (Ferrotec) was crushed and ground into fine powder. Fe (99.9+%, 0.68 mm), Co (99.998%, 0.68 mm), Cr (99.99%, 0.25 mm), Ti (99.9%, 106 μm), Al (99.9%, 75-150 μm), Zr (99.9%, 150 μm) were selected as the barrier layer candidates. Each of the barrier layer candidates taking up about 1wt%, were added into the P-BT powders and mixed manually for uniform dispersion. Then the mixed powder was loaded into a graphite mold, and co-sintered in a spark plasma sintering system (SPS-5000) at 420 ℃ under a pressure of 60 MPa and held for 10 min to afford densified samples containing corresponding micro-interfaces.

To fabricate TE elements, an appropriate amount of P-BT powder (~2.2 g) and Fe powder (~0.1 g) were loaded into a graphite mold as two layers and sintered by spark plasma sintering. The sintered samples were cut into rectangular Fe/P-BT TE elements with a cross- sectional area of 3 mm×3 mm. The aging samples were sealed in quartz tubes under vacuum, and then placed in a furnace for accelerated isothermal aging. The accelerated aging temperatures were set to be 300, 325, and 350 ℃, respectively. The interfacial microstructure was characterized with a field emission scanning electron microscopy (SEM, Zeiss Spura 55, Carl Zeiss SMT, Germany). The chemical composition around the interface was analyzed using an Energy Dispersive Spectrometer (OXFORD, X-MaxN, Oxfordshire, UK). The interfacial resistivity was measured with the four-probe method reported in ref.[32]. All data of the diffusion layer thickness and the interfacial resistivity in this study are the average of three parallel samples.

By co-sintering the mixed powders, various microinterfaces between P-BT matrix and 6 kinds of barrier layer candidate particles were integrated into one single sample, as shown in Fig. 1. The sample was then aged at 300 ℃ for 2 d. Fig. 2 exhibits the surface mappings of the 6 aged micro-interfaces.

After being aged at 300 ℃ for 2 d, obvious diffusion layers appear at the Fe/P-BT and Co/P-BT interfaces. The diffusion layer thickness of the Co/P-BT interface is thicker than that of Fe/P-BT. The inter-diffusion at the other interfaces can be almost ignored. From this perspective, Fe should be the most suitable barrier layer material in the above 6 candidates. Meanwhile, Fe is cheap and owns a matched thermal expansion coefficient with P-BT matrix. As a result, Fe was screened out as the preferred barrier layer material for P-BT.

Fe/P-BT TE elements with a cross-sectional area of 3 mm×3 mm were prepared. Microstructures and chemical compositions at the interface before and after aging were studied. As shown in Fig. 3(a), the sintered Fe/P-BT interface before aging is well bonded, but the interfacial structure is complicated. There is a clear intermediate region between the Fe barrier layer and the P-BT matrix. We confirmed that the darker part of the intermediate region adjacent to Fe is Fe₃₆Sb₂₃Te₄₁ from the line scanning of the interface shown in Fig. 3(b). According to ref. [33], it is the ternary Fe-Sb-Te phase compound, which is the diffusion layer resulting from the diffusion of some Sb and Te atoms in the interfacial P-BT matrix into the Fe barrier layer. The brighter part of the intermediate region adjacent to the P-BT matrix is Bi₂₀Sb₂₅Te₅₅. This is most likely a transitional zone with the same structure of the P-BT matrix and increased Bi content, which is transformed from the interfacial P-BT matrix when part of its internal Sb and Te atoms move out into the Fe barrier layer. Then the morphology of the Fe/P-BT interface after aging was investigated. As a typical example, Fig. 4 demonstrates the evolution of the Fe-Sb-Te diffusion layer and the Bi-rich transitional zone at the Fe/P-BT interface during aging at 275 and 350 ℃. With the aging time prolonging and the temperature increasing, the Fe-Sb-Te diffusion layer gradually grows, and its composition remains basically unchanged. While for the transitional zone, the situation is more complicated. With the aging time prolonging and the temperature increasing, the transitional zone grows thicker, but its thickness becomes increasingly uneven. Finally, the continuity of the transitional zone is destroyed. After being aged at 350 ℃ for 9 d(Fig. 4(a-d)), it transforms from a continuous layer into a number of zones dispersing as far as 20 μm inside the interfacial P-BT matrix. It’s worth mentioning that of the composition of the transitional zone after aging can still be expressed as Bi_xSb_45-xTe₅₅(x>15) despite the great changes of its morphology.

Based on the evolution of the Fe/P-BT interfacial microstructure during accelerated aging, we studied the corresponding diffusion kinetics. Fig. 5(a) shows the relationship between the diffusion layer thickness and the square root of the aging time at different temperatures. They are linearly related, indicating that the evolution of the diffusion layer is controlled by diffusion process.

By linearly fitting the diffusion layer thickness with the square root of the aging time, its growth rates (D) at 300, 325, and 350 ℃ are obtained respectively. As shown in Fig. 5(b), according to the linear fitting of lnD with 1000/T from the transformed Arrhenius formula^[34]:

where D₀ is the growth constant, Q is the growth activation energy of the diffusion layer, T is the Calvin temperature, and R is the gas constant. The growth activation energy of the diffusion layer was figured out to be 199.6 kJ/mol in the temperature range from 300 to 350 ℃.

At the same time, we also measured the interfacial resistivity of the Fe/P-BT elements using the four-probe method. It was found that the initial interfacial resistivity was lower than 1 μΩ·cm². This result indicates that good contacts form during sintering between the Fe barrier layer, the Fe-Sb-Te diffusion layer, the transitional zone and the P-BT matrix. Fig. 6 presents the variation of the interfacial resistivity with aging time at 350 ℃. During aging, the interfacial resistivity increases slowly and shows a tendency of saturation. After being aged for 16 d at 350 ℃, it is still below 10 μΩ·cm². This implies that the essence of the interfacial contacts remains unchanged during aging.

For an intact interfacial structure, during short-term aging, the formation of the interfacial diffusion layers may change the essence of the interfacial contact and result in dramatic variation of the interfacial resistivity. In addition, the growth of the interfacial diffusion layers introduces additional resistance. These are the main contributors for the increase of the interfacial resistivity. Furthermore, excessive growth of the interfacial diffusion layers may lead to interfacial structural defects like micro-cracks and holes, which are the main reasons of some abnormal variations of the interfacial resistivity during long-term aging.

As the essence of the interfacial contacts remains unchanged during aging, and according to Fig. 4(d), the micro-structure at the Fe/P-BT interface remains intact, we can affirm that the interfacial resistivity is mainly related to the growth of the interfacial diffusion layers. So the evolution of the interfacial resistivity of the Fe/P-BT interface during long-term aging can be analyzed based on the growth kinetics of the diffusion layer in 2.3. Assuming that the Fe/P-BT interface fails when the interfacial resistivity exceeds 10 μΩ·cm². Then its service life at 350 ℃ is not less than 16 d, and the corresponding diffusion layer thickness is not less than 18.8 µm. Based on the results of the diffusion kinetics in Table 1, the service life of the Fe/P-BT interface at 300 ℃ is longer than 428 d. Considering that the possible hot side working temperature of bismuth telluride TE devices (250-275 ℃) should be evidently lower than 300 ℃, the practical service life of the Fe/P-BT interface can be extended significantly. Based on the above results, it is concluded that Fe is a promising barrier layer material of the P-BT TE devices for high temperature power generation.

In this study, Fe was screened out from 6 candidates as the barrier layer material for Bi_0.5Sb_1.5Te₃. Fe/P-BT thermoelectric elements were fabricated by one-step spark plasma sintering and aged at 300, 325 and 350 ℃ to evaluate the high temperature interfacial stability. It is found that during sintering, a ternary phase diffusion layer of Fe₃₆Sb₂₃Te₄₁ forms adjacent to Fe barrier layer, while a transitional zone of Bi_xSb_45-xTe₅₅ (x>15) forms adjacent to the P-BT matrix. During accelerated aging, the composition of the Fe-Sb-Te diffusion layer remains basically unchanged. Its thickness increases linearly with the square root of the aging time. The composition of the Bi-rich transitional zone is still expressed as Bi_xSb_45-xTe₅₅ (x>15), but its morphology changes greatly. After being aged for 9 d at 350 ℃, the separated transitional zones stretched nearly 20 μm into the P-BT matrix. The interfacial resistivity was lower than 1 μΩ·cm² and increased slowly during the aging process. Life prediction based on the diffusion kinetics of the Fe/P-BT interface shows that the Fe/P-BT TE element has a promising application at the temperature below 300 ℃.