Flexible thermoelectric films based on interconnected magnetic nanowire networks

Recently, there has been increasing interest in the fabrication of flexible thermoelectric devices capable of cooling or recovering waste heat from hot surfaces with complex geometries. This paper reviews recent developments on three-dimensional networks of interconnected ferromagnetic nanowires, which offer new perspectives for the fabrication of flexible thermoelectric modules. The nanowire arrays are fabricated by direct electrodeposition into the crossed nanopores of polymeric templates. This low-cost, easy and reliable method allows control over the geometry, composition and morphology of the nanowire array. Here we report measured thermoelectric characteristics as a function of temperature and magnetic field of nanowire networks formed from pure metals (Co, Fe, Ni), alloys (NiCo, NiFe and NiCr) and FM/Cu multilayers (with FM = Co, Co50Ni50 and Ni80Fe20). Homogeneous nanowire arrays have high thermoelectric power factors, almost as high as their bulk constituents, and allow for positive and negative Seebeck coefficient values. These high thermoelectric power factors are essentially maintained in multilayer nanowires which also exhibit high magnetic modulability of electrical resistivity and Seebeck coefficient. This has been exploited in newly designed flexible thermoelectric switches that allow switching from an ‘off’ state with zero thermoelectric output voltage to an ‘on’ state that can be easily measured by applying or removing a magnetic field. Overall, these results are a first step towards the development of flexible thermoelectric modules that use waste heat to power thermally activated sensors and logic devices.