Study on the structure-property relationship for high-performance carbon nanotube fibers

Abstract

Assembly of microscopic carbon nanotube (CNT) with excellent properties into macroscopic carbon nanotube fiber (CNTF) is very important for the application of CNTs to the industry. However, properties of CNTF is significantly poorer than those of individual CNTs, because the properties of CNTF are mainly determined by the interaction between CNTs, not by the nature of sp$^2$ C–C bonding within CNTs. Therefore, it is necessary to understand the relationship between the structure and properties of CNTFs, and to assemble individual CNTs into an aligned and densified structure for high-performance CNTF.

In chapter 1, I discuss the relationship between the structure and properties of CNTFs. Fracture of CNTFs is mainly determined by slippage between CNTs, not by breakage of sp$^2$ C–C bonding, and conduction is determined by hopping between CNTs as well as electron transfer within CNTs

it means that individual CNTs should be assembled into an ideal structure, maximizing friction and minimizing contact resistance and junction resistance. I investigate the synthesis methods of CNTF (e.g. wet spinning and direct spinning), identify the problems of each method, and propose the solutions.

In chapter 2 (wet spinning), I report that small amount of oxygen incorporated into CNTs during the purification process greatly increases their solubility in chlorosulfonic acid (CSA). Using as-purchased high aspect ratio CNTs, the optimal purification process is established to increase the solubility of CNTs in CSA, and spin densified and aligned CNTFs with high specific tensile strength (0.84 N tex$^{−1}$) and electrical conductivity (1.4 MS m$^{−1}$) from the liquid crystal dope with high concentration of CNTs in CSA. The knowledge obtained here may guide development of a method to dissolve extremely high aspect ratio CNTs at high concentration and thereby to enable fabrication of CNTFs with ultimate properties.

In chapter 3 (direct spinning), I demonstrate the feasibility of a highly efficient and potentially-continuous spinning method based on protonation principle of CNTs in CSA to fabricate high-performance CNTF. The spinning method consists of consecutive steps of swelling of CNTFs in CSA, stretching, and coagulation. The swelling of CNTFs makes even distribution of the CNT bundles, which are the subunits of CNTFs, and thereby promotes the removal of the interstitial voids during subsequent coagulation. The stretching straightens tortuous CNT bundles, and thereby facilitates dense packing of aligned bundles along the fiber axis that are evenly distributed by swelling. The processing time is < 1 min from synthesis of CNTs to fabrication of highly densified and aligned CNTFs. CNTFs that are fabricated by the developed spinning method are ultra-lightweight, strong (specific tensile strength = 4.08 ± 0.25 N tex$^{−1}$), stiff (specific tensile modulus = 187.5 ± 7.4 N tex$^{−1}$), electrically conductive (2,270 Sm$^2$ kg$^{−1}$), and highly flexible (knot efficiency = 48 ± 15%).

In chapter 4, I report that spinning method developed in Chapter 3 is also well applied to ultra-thick CNTFs with a high linear density of ~ 6 tex (g km$^{-1}$) which can possibly meet industrial-scale productivity, and they must be stretched to stretching ratio (RS) > 20% to simultaneously achieve high degrees of densification and alignment. Highly-tortuous CNT bundles in the high linear-density CNTF leads to incremental improvement of structure and properties. The stretching is only effective at RS > 20 %, and both tensile strength and electrical conductivity gradually increase as RS increases, to 27- and 8.7-times at the maximum RS = 100 %. I believe that this study on the influence of RS on the structure and properties of high linear-density CNTFs provide a new opportunity for designing the industrial process of the post-treatment to commercialize CNTFs.