Creep and electrical properties of carbon nanotube yarns for long-term applications

Abstract

One of the primary challenges to turning CNTs into commercial applications is the feasibility to manufacture macroscopic structures that mirror the extraordinary properties of individual CNTs. CNT yarns (CNTY) are an alternative to overcome this challenge. However, most yarn-like structures are exposed to dynamic systems, like creep and/or fatigue, and limited studies are published predicting their behavior. Therefore, the current study aimed to assess the behavior of CNTYs under creep situations and establish a relationship with their electrical properties. Five layers of CNTs were drawn from the MWCNT forest, placed one on top of the other, and scrolled into a 250 mm yarn, that was later twisted to 2000 twists m⁻¹. Afterward, these CNTYs were densified with acetone. Creep experiments were performed with a sustained load of 50, 70, and 80 % of the YBL. The yarn behavior seen during creep experiments was unexpected, which, although partial breakage of the yarn was seen, it sustained the creep load much for longer periods. The mechanism proposed for the phenomenon includes CNT alignment and void elimination at first, followed by partial CNT sliding, with breakage of van der Waals forces. As the creep load continues, CNTs are able to rebind to their neighbors and recover their strength before total failure. Mechanical gripping due to friction is also occurring between nanotubes which favors a strength increase. This behavior indicates that there is a regeneration of van der Waals forces occurring among the CNTs, which recovers the strength of the yarn under constant load, increasing creep lifespan. The time to failure obtained under creep load is about 9, 10, and 11 days to failure for CNTYs at 50, 70 and 80 % of YBL, respectively. Raman results showed that I_D/I_G ratio remained the same after mechanical loads, which indicates that no structural modification is seen in the CNTs when static and/or dynamic mechanical systems are imposed on the yarn. The electrical conductivity of a CNTY is improved after the mechanical tests, increasing by 5.9 and 6.7 % after static tensile and creep, respectively. The improvement of electrical conductivity is attributed to densification, which leads to an enhancement of the intertubes contact area among the nanotubes within the yarn.