Effects of carbon nanotubes on electrical contact resistance of a conductive Velcro system under low frequency vibration
Introduction
Carbon nanotubes (CNTs) have been acknowledged to enhance traditional material performance because of their superior performance in strength, elastic modulus, and wear-resistance. Their physical characteristics such as chemical stability and electrical and thermal conductivity are also highly valued [1], [2].
It has been reported that CNTs can be used to form metal/polymer composites, leading to the enhancement of mechanical characteristics such as tensile strength and elastic modulus. Specifically, a new structural composite material utilizes CNTs as reinforcement for a polymer creating a light-weight, high strength and super-elastic structural member. The member has been utilized for its bullet proof capabilities [3], [4] and as an artificial muscle [5], [6]. Furthermore, if CNTs are cast into a composite material, in which the original material was a non-conductor, the composite material is newly endowed with conductivity in addition to enhanced mechanical properties [7], [8]. The effects of CNTs are further revealed in the fiber structure of CNTs [9], [10], [11], [12], [13].
A distinctive characteristic of CNTs is that their aspect ratio is much larger than that of any other material, leading to higher specific surface area. This high specific surface area of CNTs enhances the percolation for conductivity of composite materials. For example, the electrical percolation between the CNTs and the polymer in composite material is a vital factor to establish the effective electrical path between them [14], [15], [16]. The application of CNTs to electrical connectors could solve a persistent problem in the automotive industry known as fretting. In the electrical connectors of automobiles, the low-frequency and small amplitude vibrational environment can increase the possibility of fretting wear [17], establishing large electrical contact resistance, which – in turn – can generate heat and oxidation [18], [19]. These factors contribute to the shortening of the connectors’ life and can induce unexpected short-circuits, possibly causing malfunction or explosions and loss of human life [20]. The adverse effects of fretting on electrical contact resistance have long been observed. To overcome the shortcomings of electrical connectors in terms of electrical contact resistance, a continuous effort has been made to utilize CNTs at the interface of the connector [21], [22], [23].
Velcro fasteners, traditionally known as fabric fasteners, have expanded their applicability to the entire area of engineering joining structures because of their easy fastening and detachable properties [24], [25], [26]. Specifically, the application of conductive Velcro fasteners in a dynamic loading environment such as the battery assembly of an automotive vehicle is noticeable [27]. However, the mechanical behavior of Velcro fasteners is complex because of the random alignment of hooks and loops. Moreover, their frictional behavior also shows non-compliance with the Amontons–Coulomb law; i.e., they exhibit nonlinear behavior of friction force against applied force, and fluctuations of the frictional forces in a dynamic motion, which requires more understanding of the system [28]. Furthermore, the conductive Velcro fastener has an intrinsically large electrical contact resistance since the electrical conduction is dependent upon the actual microscopic contact spots between the fiber-like loops and hooks [29], [30], [31], [32]. Recently, a study of contact resistance in the conductive Velcro system has shown that a certain range of low frequency at a low load amplitude causes higher electrical contact resistance and the wear behavior after a long exposure to dynamic loading under a specific frequency at a low load amplitude has been confirmed [33].
Motivated by the characteristics of CNTs and the issues of conductive Velcro, we consider that the application of CNT to the interface of the Velcro connection may be a possible way to enhance the performance of conductive Velcro, specifically reducing the electrical contact resistance of the fasteners under a low-frequency vibrating environment. For this reason, the electrical contact resistance according to electrical loads, frequencies and amplitudes of vibration for several types of CNTs with different aspect ratios is investigated. As a result of the analysis, tribological evidences affecting the electrical contact behavior of the system will be explained.
Section snippets
CNT and conductive Velcro
Carbon nanotubes (CNTs), as an additive material to conductive Velcro systems to diminish the electrical contact resistance, are prepared by considering the ranges of the aspect ratio of the CNTs. The selection of this range of aspect ratio could be critical because the contact geometry at the fiber and hook interface is affected by the CNT size and length. Table 1 shows several CNTs, including single-wall and multi-wall CNT (SWNTs and MWNTs, respectively) with various aspect ratios. Typical
Results and discussion
The characteristics of the electrical contact resistance for conductive Velcro fasteners under low frequency vibrational loading are reported and discussed here. Firstly, CNT formation on conductive Velcro interfaces according to the CNT deposition methods is investigated. Using the outcomes from the proposed deposition methods, the most appropriate method is suggested. Secondly, electrical contact resistances of conductive Velcro systems with different CNT aspect ratios are determined
Conclusion
The effect of CNTs on the electrical contact resistance of conductive Velcro connectors, especially under low frequency vibration was experimentally investigated. Among the various types of CNTs, the conductive Velcro connector with the MWNTs having a small aspect ratio maintained a constant electrical contact resistance regardless of frequency and load amplitude, and had a considerably smaller electrical contact resistance than the Velcro without CNTs. A nearly linear I–V curve, independent of
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