Comparative Characterization of NiTi Filaments and NiTi Knit-Induced Air-Gap Effects on Heat Transfer in a Smart Textile System

This article presents a comparison of the different diameters and characterization of nickel–titanium (NiTi) alloy and the effect of an NiTi knit-induced air gap on heat transfer in a smart textile system. Weft-knitted fabric was produced using commercially available cold-worked NiTi wires (filaments), enabling active thermal insulation in protective garments by adjusting the air gap between two fabric layers in response to changes in environmental temperature. For this study, NiTi alloy filaments containing 54.8 wt. % nickel and 45.2 wt. % titanium with a diameter of 100 μm was used. The transition temperatures of the NiTi filaments were tailored for the intended application, with the austenite start temperature set at approximately 70 °C and the martensite start temperature set at around 20 °C. Various methods for determining transition temperatures were compared, including differential scanning calorimetry (DSC), dynamic mechanical analysis (DMA) and electrical resistance measurements (ERM), all of which yielded consistent results. Mechanical properties were evaluated through tensile tests conducted with an Instron dynamometer at room temperature (20 °C) and in a heated chamber at 100 °C. The stress-strain response of the annealed NiTi alloy exhibited the characteristic four-stage behaviour typical of shape memory materials. All obtained results were also compared with previously reported data for NiTi alloy filaments with a diameter of 200 μm, showing good agreement. In addition, the impact of the NiTi knit-induced air gap on heat transfer behaviour in thermal protective composites was investigated. This study confirms that 100 μm NiTi filaments can be effectively optimized for smart textile applications through controlled annealing, with heat treatment at 500 °C producing suitable transformation temperatures consistently validated using DSC, DMA and ERM methods. Additionally, integrating an annealed NiTi knit into a textile composite increases the air gap at elevated temperatures, thereby reducing heat transfer and significantly enhancing the thermal insulation and adaptive performance of the smart textile system.