**1. Introduction**

Materials are the basis of the evolution of human civilization. The pursuit of the ultimate properties of materials, such as super strength and super toughness, has strongly promoted the development of human culture. In 1895, Konstantin Tsiokovsky, a Soviet scientist, put forward building a "sky castle" at the top of a giant tower, which later evolved into the concept of "space elevator". By connecting the earth and the space station with a cable, people can achieve space sightseeing and transport items to the space station [1]. However, the biggest challenge of this concept is finding light and strong cable that can overcome its gravity. A variety of nanostructures can be composed of single carbon elements, such as fullerenes (0D), carbon nanotubes (1D), and graphene (2D). Carbon nanotubes (CNTs) are cylinders rolled from single or multi-layer graphene sheets. Single-walled carbon nanotubes (SWCNTs) are cylinders rolled from a singlelayer graphene sheet, while double-walled carbon nanotubes (DWCNTs) and multi-walled carbon nanotubes (MWCNTs) are composed of two and multiple layers of rolled graphene sheets, respectively. As one of the strongest chemical bonds in nature [2,3], the in-plane σ covalent bond of graphene formed by sp2 hybridization endows CNTs with extremely high axial Young's modulus (~1.1 TPa) and tensile strength (~120 GPa). Theoretical calculations have shown that CNTs are the most probable material to help mechanical materials achieve a breakthrough and even realize the "space elevator" dream [1]. However, CNTs with

**Citation:** Zhu, Y.; Yue, H.; Aslam, M.J.; Bai, Y.; Zhu, Z.; Wei, F. Controllable Preparation and Strengthening Strategies towards High-Strength Carbon Nanotube Fibers. *Nanomaterials* **2022**, *12*, 3478. https://doi.org/10.3390/ nano12193478

Academic Editor: Muralidharan Paramsothy

Received: 16 September 2022 Accepted: 30 September 2022 Published: 5 October 2022

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**Copyright:** © 2022 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/).

extremely excellent mechanical properties are nanoscale solids, and practical applications require macro-scale materials. It is the prerequisite for CNTs to play a significant role in practical applications that they can maintain excellent mechanical properties after being assembled from a single nanoscale unit to a macroscopic aggregate.

In recent years, rapid progress has been made in the preparation and mechanical properties optimization of carbon nanotube fibers (CNTFs). Different spinning methods of CNTFs have been put forward and improved upon, and CNTFs with a tensile strength comparable to carbon fibers (CFs) have been prepared [4–7]. However, their mechanical properties are still far lower than single CNTs [7], which also shows an unsatisfactory phenomenon of property transfer across scales. Theoretical calculations and experimental results show that the tensile strength of CNTs with a nanoscale diameter can exceed 100 GPa [8,9]. CNT bundles with a diameter of 10–100 nm can have a tensile strength of up to 80 GPa [10]. CNTFs with a diameter of more than 1 μm, as a representative of macro assemblies of CNTs, have a maximum tensile strength of only 9.6 GPa [4], which is far lower than the intrinsic mechanical strength of CNTs. The reasons for such cross-scale tensile strength transfer are mainly due to defect accumulation and the lack of ideal tube– tube interactions during CNT assembly. Defects can have a fatal effect on the strength of CNTs [11,12]. With the increase in fiber size, defects also accumulate across scales. As shown in Figure 1b, the improvement of strength for CFs and CNTFs is closely related to the reduction in defect size [13,14]. For CFs, the tensile strength was increased from about 1 GPa to 10 GPa when the defect size was reduced from the micron to the nano scale. For CNTs, due to the fewer defects in structure compared with CFs, less attention was paid to their precise structural control, especially defects, resulting in their tensile strength having long been at a lower level. Until the 2000s, a series of achievements were made in the prepration of defect-free CNTs, which have shown extraodinary tensile strength performance both at the single-tube and bundle levels [9,15]. Therefore, the preparation of ideal solids such as defect-free or defectless CNTs is the basis for preparing CNTFs with high tensile strength. At the same time, many studies have shown that the mechanical properties of CNTFs can be significantly influenced by the tube–tube interactions involving orientation, length, and density. It is of great significance to regulate the tube–tube interactions and precisely control the atomic defects for the improvement of CNTs' mechanical tensile strength [4,16–18].

**Figure 1.** (**a**) Comparison of mechanical performance between CNTs and some high−performance materials. Reproduced with permission from [15]. Copyright 2020, American Association for the Advancement of Science. (**b**) Evolution of the tensile strength of CNTs and CFs at different defect sizes. The data are collected in [4,7,10,15].

In the past few decades, CNTFs have attracted extensive attention from academia and industry, and there are many reviews on the development and application of CNTFs [19–23]. However, few reviews have focused on the controllable preparation and strength optimization of CNTFs at different scales, which will be specifically highlighted in this review. First, the intrinsic mechanism of the excellent mechanical properties of CNTs and the preparation of CNTs are introduced. Then, the development and characteristics of techniques for fabricating CNTFs at different scales are discussed. Furthermore, we introduce the tensile strength of CNTFs from the nanoscale to the microscale, showing the recent development in the transfer of tensile properties across scales. Then, we analyze strategies for the strength optimization of CNTFs at different scales, particularly the aspects of defect and tube–tube interaction control. Finally, we provide an outlook for the practical applications of CNTFs with high mechanical strength. As a result, we aim to provide insights and prospects for the controllable preparation and performance optimization of macro CNTFs with high tensile strength in the future.
