A Cu–Ni-based alloy with a high power factor is a commercially utilized metallic thermocouple material. However, the high thermal conductivity has been a major limitation to achieving thermoelectric performance in semiconductor materials. Herein, this work presents a 76.1% reduced thermal conductivity (∼7.7 W m–1 K–1) in Cu70Ni30, which is one of the lowest reported values in the literature. Such suppression of thermal conductivity can be attributed to the varied frequency phonon scattering by the interfacial potential barrier, built from micron-scale defects formed via sintering melt-spun ribbons. However, the defects simultaneously reduce the charge carrier concentration, mobility, and thus the electrical conductivity. The lowest thermal conductivity leads to the highest zT and ZT avg in the sample sintered at 673 K under 15 MPa. The values are 0.24 (@573 K) and 0.15 (323–573 K), respectively, which are 130.3 and 140.0% higher than the values of the pristine counterpart. Our work demonstrates that improved thermoelectric performance in Cu–Ni-based alloys can be obtained by creating various interfacial defects even at micron scales, which paves the way to suppress thermal conductivity largely in metallic thermoelectric materials via melt-spinning (MS) and spark plasma sintering (SPS) synthesis.

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