Comportamento em fluência sob variação de temperatura e condutividade elétrica em solicitação cíclica de filamentos de nanotubos de carbono

Abstract

The popularization of portable electronic devices intensifies the pursuit of increasingly flexible, adaptable, and energetically self-sufficient systems. The structural rigidity of traditional electronics and the need for power supply from conventional batteries represent barriers to technological advancement. In this scenario, nanomaterials, especially carbon-based ones, prove to be promising options for developing increasingly compact flexible devices. Moreover, these materials are capable of harvesting energy from environmental sources, such as light, thermal, mechanical, etc., and converting it into usable energy, making them self-sufficient. Regarding mechanical energy harvesting, carbon nanotube yarns (CNTYs) have shown potential as an alternative for this application. Besides their excellent mechanical and electrical properties, these nanotubes in the form of macroscopic yarns also exhibit the ability to convert mechanical energy into electrical energy, enabling their use as sensors, actuators, artificial muscles, among others. Although there are high expectations surrounding these materials, they are still at the frontier of knowledge and require further studies for a better understanding of their characteristics and responses, especially in adverse and extreme environments, such as dynamic loading and temperature variations. In this context, the present project aimed to evaluate CNTYs under static and dynamic conditions, as well as to assess their electrical response under dynamic loading. For this, CNTYs were produced by the forest spinning method, manufactured with 5 layers of nanotube films with a width of 55 mm, twisted 2000 times per meter, and densified with acetone. During the twisting process, the yarns were tensioned with a 5 g load. The nanotubes were characterized by microscopy and Raman spectroscopy. Subsequently, they were evaluated by tensile and creep mechanical tests, where the creep was based on the percentage of the breaking load obtained in the tensile tests (50, 70, and 80% YBL). Furthermore, these tests were performed with temperature variation (25 and 50°C). After the mechanical tests, filament fracture was evaluated by scanning electron microscopy, seeking to understand the mechanism undergone by the yarns. Additionally, the electrical conductivity of the yarns was evaluated in situ during defined fatigue cycles, to understand the influence of CNTY stretching under cyclic tensile/relaxation loads on their electrical conductivity. The production resulted in dense, uniform yarns with good reproducibility, having an average diameter of 89.69 ± 3.47 µm, an average helix angle of 28.56 ± 1.87º, and an average maximum tensile load of 1.65 ± 0.5 N. Their failure mechanism showed mechanical densification preceding rupture. Initially, there is an elimination of voids in the structure, generating greater interaction between them. Under creep, the yarns tested at 50ºC presented shorter failure times compared to those at 25ºC, attributed to the thermal agitation caused by the high temperature. Strain increased with higher load percentages for both temperatures. Regarding electrical behavior, the cycles of mechanical loading provided improvements in the yarn’s electrical conductivity due to the possible approximation of nanotubes and elimination of voids in the structure.