Crystalline–Amorphous Ni/NiCoCrMoTiAl/Ni Sandwich Diffusion Barrier Layers with High Thermal Stability for Bi ₂ Te ₃ Thermoelectric Modules

High thermal stability of Bi₂Te₃-based thermoelectric devices is critically limited by interfacial degradation. Here, we demonstrate a gradient Ni-based composite film (crystalline Ni/amorphous NiCoCrMoTiAl alloy/crystalline Ni), fabricated via magnetron sputtering with power-gradient modulation, as an effective barrier layer of Bi₂Te₃ thermoelectric modules. Among them, the inner-layer Ni enhances bonding strength via chemical bonding, while the outer-layer Ni facilitates the formation of a sound welding interface through its wetting property; the high-entropy amorphous intermediate layer eliminates grain boundaries and diffusion pathways, thereby suppressing interfacial reactions among Bi₂Te₃, electrodes, and solders. The design combines low-temperature deposition (preserving thermoelectric properties), high film density (minimizing interface resistance), and strong chemical bonding (enhancing adhesion). It enables us to achieve a shear strength of 11.9 MPa and interface resistance of 4.25 μΩ·cm² for p-type Bi₂Te₃, a shear strength of 10.1 MPa and interface resistance of 5.45 μΩ·cm² for n-type Bi₂Te₃. This approach provides a scalable solution for long-term thermoelectric modules.