Carbon nanotube wires for high-temperature performance
Introduction
Ever since the implementation of electricity on a large scale in the XIX century, there has been an incessant search and development of conductive materials for a plethora of applications and, over the years, the technical demands have become much more sophisticated than Edison’s first transmission line made of copper rods wrapped in jute in 1882 [1]. A hundred years later, the discovery of carbon nanotubes (CNTs) [2] has breathed a fresh air into the field of electricity. This novel type of material was found to by-pass limitations of classical wiring by exhibiting ballistic conduction of electrons and holes [3], lack of electron migration [4] or suppressed skin effect [5] and it has been proven that CNTs may outperform copper by three orders of magnitude in terms of current density [6]. However, to have a successful transformation from Cu to CNT, it has to be possible to replicate the properties of individual CNTs into a macroscopic material. Assembling CNTs into wires has been found to be a potential way of doing this. The current main-stream strategies to prepare wires are based on wet-spinning from hazardous super acid solution [7] or laborious polymer-surfactant coagulation systems [8]. Alternatively, wires can be drawn ex situ by applying mechanical force, but this approach requires a CNT array synthesized before [9].
The wires used herein, were obtained by the direct-spinning method from a CVD furnace [10], [11] that obviates aforementioned complicated processing methodologies and synthesizes CNT wires of arbitrary length in a single step. Our goal was to evaluate the electrical behavior of CNT wires in high-temperature regime as compared with copper of similar dimensions. Electrical resistance of the wiring was monitored on-line as the temperature was constantly ramped up until failure of the fiber was registered. Furthermore, we performed quasi-isothermal treatments, wherein the temperature increase was executed in discrete steps. The samples were kept at respective temperatures for 1 h in between the temperature rise. The motivation was to prove the electrical durability of CNT wires at high temperatures for a prolonged time. Finally, we investigated whether the operational window may be enlarged by enclosing the CNTs in an oxygen impermeable matrix so as to hamper oxidation of CNTs at high temperatures.
Classical wiring is subject to regular failure in foundries, glass, ceramics factories and similar high temperature environments because of oxidation and their relatively low melting point (e.g. Cu: 1083 °C [12]), whereas the limit of carbon lies at its triple point at 3727 °C [13]. Theory has proved that CNTs can retain their cylindrical shape in vacuum in these conditions [14]. However, experimental measurements were found troublesome due to the melting of metal nanoparticles from the synthesis stage [15], what contributed to the observed difference in estimated sublimation temperature reported by various groups being of 300 °C or more [16], [17], [18]. Other factors that can interfere with the failure temperature of CNTs wires are the average diameter of nanotubes [19], [20] and the prevalent type of CNTs present, in which the thermal stability order is as follows: multi-wall > double wall ≫ single-wall CNTs [17].
The results of this study reveal that the use of CNT wires (CNWs) may be beneficent and offer appreciable electrical properties whilst attaining good thermal stability at elevated temperatures when SiC coating is applied. In order to demonstrate the practical implications of these findings, we created a CNW-based electrothermal heating film, coated it with SiC and tested its performance. The CNW film heater showed excellent stability at 700 °C for a prolonged time, what is a 300 °C improvement to its operational window [21].
Section snippets
Materials
Methane (or ethanol, where indicated) as carbon source, ferrocene as catalyst and thiophene as promoter were subjected to Catalytic Vapor Deposition (CVD) in a vertical reactor kept at 1200 °C in hydrogen atmosphere. The formed aerogel made up of multi-wall CNTs was drawn continuously out from the reactor and transferred onto a fast spinning winder generating CNWs on transparent cellulose acetate sheets [10], [11]. They were subsequently cut with a razor blade into 15 cm long wires (Supplementary
Material characterization
We started by performing Raman spectroscopy on as-made samples from methane and ethanol to assess what is their composition (Fig. 3). The samples proved to have high D/G ratios, 0.97 and 0.67, respectively, what is indicative of high degree of disorder and the presence of carbonaceous impurities [23]. In most circumstances that is unwanted, but our motivation was to examine such samples to establish the base level of performance for regular CNTs. In this case, any increment in the degree of
Conclusions
We developed CNWs and monitored in situ their electrical properties when they were subjected to high temperature conditions for the first time. The study has shown that the type of CNTs present in the material and packing density of thereof can have a significant effect on the thermal stability of CNWs. Furthermore, a comparison between two heating profile, which we employed, show that kinetics of CNW oxidation plays an important role and slow heating rates or prolonged heating times are
Acknowledgments
We acknowledge the European Research Council (under the Seventh Framework Program FP7/2007-2013, ERC grant agreement no. 259061), the Royal Society and the Gates Cambridge Scholarship for the financial support.
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