Potential of Fibrous Materials with Nanoparticle-Based Ni/Cu Junctions for Wearable Temperature Sensors

Metal-based flexible thermocouples (TCs), such as thin-film TCs (TFTCs) and fiber-based TCs (FBTCs), have attracted considerable attention for wearable thermal sensing. While TFTCs offer high Seebeck coefficients (S), they suffer from low production efficiency and limited thermal comfort. FBTCs, although more breathable, often require complex structural designs and optimization of dual-material junctions. Here, we report an FBTC by constructing nanoparticle-engineered Ni/Cu junctions on Cu-coated PET fibrous materials via Ni electroplating. The thermoelectric (TE) performance of the fibrous material with nanoparticle-based Ni/Cu junctions is caused by suppression of the short-circuiting pathway, which is due to the porous architecture as well as the presence of Cu nanoparticles and Cu oxide nanoparticles. Furthermore, a simple linear prediction for voltage output is proposed by selecting a temperature difference of less than 58 °C. The Seebeck coefficient (S) of the fibrous material with nanoparticle-based Ni/Cu junctions depends on the mass ratio of Ni and Cu. The optimal sample with 8 min Ni electroplating exhibits an accurate temperature prediction by comparing a type K commercial TC, with a temperature prediction error of 0.22 °C. It also demonstrates a fast TE response, reaching 63 and 93 % of maximum voltage output in 2.5 and 4.2 s under a 5 °C gradient, respectively. Furthermore, the fibrous material with nanoparticle-based Ni/Cu junctions maintains stable output under mechanical deformation and after the washing cycle and exhibits excellent breathability. Due to feasible fabrication via the electroplating method and stable TE properties, the fibrous material with nanoparticle-based Ni/Cu junctions has great potential in large-scale production to meet requirements of smart temperature monitoring.