Abstract:

Objective Current temperature-sensing fibers often suffer from insufficient responsiveness to temperature fluctuations, poor mechanical durability, and inadequate structural stability in practical wearable scenarios. Notably, bismuth sulfide (Bi₂S₃) possesses a high thermoelectric coefficient, carbon nanotubes (CNTs) offer excellent electrical conductivity and prominent mechanical reinforcement effects, while polyvinylidene fluoride (PVDF) features superior flexible fiber-forming capabilities. The composite integration of these three components is expected to achieve synergistic performance complementarity. Therefore, Bi₂S₃/CNT/PVDF composite fibers were fabricated to meet the urgent demand for wearable temperature sensors with high sensitivity, reliable flexibility, and robust stability.

Method Bi₂S₃ nanorods were synthesised hydrothermally, and then different masses of Bi₂S₃ and CNT were added at mass ratios of 1∶1, 2∶1, 3∶1, 4∶1 and 8∶1 into a mixture containing 6.36% CNT, 11% PVDF and a 2∶1(V/V) acetone/DMF solution. The mixture was wet-spun at room temperature and 65% RH into a coagulation bath, washed and dried to obtain continuous fibers. The optimal ratio of 15.2% Bi₂S₃/6.36% CNT was identified by varying the Bi₂S₃ content from 5.0% to 32.4%.

Results The performance of Bi₂S₃/CNT/PVDF composite fibers varied non-monotonically with Bi₂S₃ content. SEM images showed that the 5.0% Bi₂S₃/CNT/PVDF composite fibers exhibited smooth surfaces and uniformly dispersed particles. When Bi₂S₃ content was increased to 10.7% and 15.2%, the composite fibers exhibited local agglomeration that roughened the surface. However, enhanced CNT-Bi₂S₃ interfacial contact was achieved, the three-dimensional scaffold remained intact, and an alternating "semiconductor node-metal highway" structure was formed. When Bi₂S₃ content was further increased to the range of 18.4%-32.4%, agglomeration intensified, producing structural fractures and uneven component distribution that destroy the integrity of the composite fibers. DSC confirmed that Bi₂S₃ did not shift the PVDF melting peak, guaranteeing thermal stability. I-U curves and resistance statistics consistently indicated that the resistance of the composite fibers first decreased and then increased with rising Bi₂S₃ content, reaching a minimum of 51.16 kΩ in the 15.2% Bi₂S₃/CNT/PVDF composite fibers, where conductivity was highest. This was attributed to optimized percolation; excess doping introduced lattice defects and carrier saturation, reducing conductivity. Temperature-sensing tests confirmed that the 15.2% Bi₂S₃/CNT/PVDF composite fibers exhibited excellent temperature sensitivity in the range of 25-60 ℃, with a negative temperature coefficient of resistance. For composite fibers with a Bi₂S₃ mass fraction below 15.2%, CNTs enhanced carrier transport by constructing a three-dimensional conductive network. As a narrow band-gap n-type semiconductor, Bi₂S₃ showed an exponential increase in intrinsic carrier concentration with increasing temperature, which reduced the resistance at the junctions of Bi₂S₃ nanorods. The CNT network promptly transmitted the local junction resistance changes to the entire fiber, resulting in synchronous current variations. In contrast, composite fibers with a Bi₂S₃ mass fraction above 15.2% underwent phase separation induced by the plasticizing effect of the PVDF matrix. This phase separation disrupted the conductive pathways, leading to unstable temperature responses. Additionally, the temperature sensor device fabricated by sewing Bi₂S₃/CNT/PVDF temperature-sensing fibers onto elastic fabric in an S-shape did not rupture even under 75% stretching, and exhibited excellent stretch-resistant sensing performance. Consequently, the 15.2% Bi₂S₃/CNT/PVDF composite fibers achieved an optimal balance between sensing performance and structural stability, providing a reliable material platform for high-performance wearable temperature sensors.

Conclusion Through wet-spinning, Bi₂S₃/CNT/PVDF ternary composite temperature-sensing fibers were successfully prepared. The 15.2% Bi₂S₃/CNT/PVDF composite fibers demonstrate exceptional overall performance. The three-dimensional conductive network built by CNTs and the narrow-bandgap semiconducting nature of Bi₂S₃ act synergistically, effectively overcoming the traditional trade-off between sensitivity and mechanical flexibility in flexible temperature sensors. Benefiting from their excellent temperature response, good flexibility and textile-process compatibility, these fiber sensors show broad prospects in smart medical monitoring, adaptive thermal-control garments and flexible electronic devices, offering a new material system and design concept for next-generation high-performance wearable temperature-sensing platforms.

Key words: textile material, bismuth sulfide, carbon nanotube, wet spinning, temperature-sensing fiber, synergistic effect, temperature sensor, smart textiles