Joon-Hyung Byun

State of the Art of Carbon Nanotube Fibers: Opportunities and Challenges

The superb mechanical and physical properties of individual carbon nanotubes (CNTs) have provided the impetus for researchers in developing high-performance continuous fibers based upon CNTs. The reported high specific strength, specific stiffness and electrical conductivity of CNT fibers demonstrate the potential of their wide application in many fields. In this review paper, we assess the state of the art advances in CNT-based continuous fibers in terms of their fabrication methods, characterization and modeling of mechanical and physical properties, and applications. The opportunities and challenges in CNT fiber research are also discussed.

Stretchable Wire-Shaped Asymmetric Supercapacitors Based on Pristine and MnO2 Coated Carbon Nanotube Fibers

While the emerging wire-shaped supercapacitors (WSS) have been demonstrated as promising energy storage devices to be implemented in smart textiles, challenges in achieving the combination of both high mechanical stretchability and excellent electrochemical performance still exist. Here, an asymmetric configuration is applied to the WSS, extending the potential window from 0.8 to 1.5 V, achieving tripled energy density and doubled power density compared to its asymmetric counterpart while accomplishing stretchability of up to 100% through the prestrainning-then-buckling approach. The stretchable asymmetric WSS constituted of MnO2/CNT hybrid fiber positive electrode, aerogel CNT fiber negative electrode and KOH-PVA electrolyte possesses a high specific capacitance of around 157.53 μF cm(-1) at 50 mV s(-1) and a high energy density varying from 17.26 to 46.59 nWh cm(-1) with the corresponding power density changing from 7.63 to 61.55 μW cm(-1). Remarkably, a cyclic tensile strain of up to 100% exerts negligible effects on the electrochemical performance of the stretchable asymmetric WSS. Moreover, after 10,000 galvanostatic charge-discharge cycles, the specific capacitance retains over 99%, demonstrating a long cyclic stability.

Graphene‐Based Fibers: A Review

Motivated by their unique structure and excellent properties, significant progress has been made in recent years in the development of graphene-based fibers (GBFs). Potential applications of GBFs can be found, for instance, in conducting wires, energy storage and conversion devices, actuators, field emitters, solid-phase microextraction, springs, and catalysis. In contrast to graphene-based aerogels (GBAs) and membranes (GBMs), GBFs demonstrate remarkable mechanical and electrical properties and can be bent, knotted, or woven into flexible electronic textiles. In this review, the state-of-the-art of GBFs is summarized, focusing on their synthesis, performance, and applications. Future directions of GBF research are also proposed.

The analytical characterization of 2-D braided textile composites

An analytical model based on the unit cell was developed for the prediction of the geometric characteristics and three-dimensional (3-D) engineering constants of 2-D braided textile composites. The crimp yarn angle and the fiber-volume fraction were obtained from the geometric model. The elastic model utilizes coordinate transformation and the averaging of stiffness and compliance constants on the basis of the volume fraction of each reinforcement and matrix material. Seven different fabric architectures have been fabricated and tested by tensile loading to verify the model. The classical thin laminate theory has also been applied to the braided composites in order to compare the predictions with the averaging method. Although the two analytical approaches are well correlated with experimental results, the averaging method is more accurate when the braider-yarn angle is small or when the bundle size of axial yarns is much larger than that of the braider yarns. Parametric studies have been conducted to investigate the effects of the braider-yarn angle and the axial-yarn content to the elastic properties of the composites. The results are demonstrated in the form of property maps for selected moduli and Poissons ratios.

Laminated Ultrathin Chemical Vapor Deposition Graphene Films Based Stretchable and Transparent High-Rate Supercapacitor

Due to their exceptional flexibility and transparency, CVD graphene films have been regarded as an ideal replacement of indium tin oxide for transparent electrodes, especially in applications where electronic devices may be subjected to large tensile strain. However, the search for a desirable combination of stretchability and electrochemical performance of such devices remains a huge challenge. Here, we demonstrate the implementation of a laminated ultrathin CVD graphene film as a stretchable and transparent electrode for supercapacitors. Transferred and buckled on PDMS substrates by a prestraininig-then-buckling strategy, the four-layer graphene film maintained its outstanding quality, as evidenced by Raman spectra. Optical transmittance of up to 72.9% at a wavelength of 550 nm and stretchability of 40% were achieved. As the tensile strain increased up to 40%, the specific capacitance showed no degradation and even increased slightly. Furthermore, the supercapacitor demonstrated excellent frequency capability with small time constants under stretching.

Omnidirectionally Stretchable High-Performance Supercapacitor Based on Isotropic Buckled Carbon Nanotube Films.

The emergence of stretchable electronic devices has attracted intensive attention. However, most of the existing stretchable electronic devices can generally be stretched only in one specific direction and show limited specific capacitance and energy density. Here, we report a stretchable isotropic buckled carbon nanotube (CNT) film, which is used as electrodes for supercapacitors with low sheet resistance, high omnidirectional stretchability, and electro-mechanical stability under repeated stretching. After acid treatment of the CNT film followed by electrochemical deposition of polyaniline (PANI), the resulting isotropic buckled acid treated CNT@PANI electrode exhibits high specific capacitance of 1147.12 mF cm(-2) at 10 mV s(-1). The supercapacitor possesses high energy density from 31.56 to 50.98 μWh cm(-2) and corresponding power density changing from 2.294 to 28.404 mW cm(-2) at the scan rate from 10 to 200 mV s(-1). Also, the supercapacitor can sustain an omnidirectional strain of 200%, which is twice the maximum strain of biaxially stretchable supercapacitors based on CNT assemblies reported in the literature. Moreover, the capacitive performance is even enhanced to 1160.43-1230.61 mF cm(-2) during uniaxial, biaxial, and omnidirectional elongations.

Process-microstructure relationships of 2-step and 4-step braided composites

This paper presents a comprehensive study of three dimensional 2-step and 4-step braided composites. The analysis includes the geometric model of unit cells, identification of key process parameters, limiting geometries of yarn jamming, and microstructural characteristics (i.e. yarn orientation and fiber volume fraction). Since several types of unit cells are identified in the thickness and width directions of 2-step and 4-step braided composites, characterization of microstructures is based upon the macro-cell, which occupies the entire cross-section and the unit pitch length of the sample. Finally, processing windows are identified for assessing the effect of various geometric and process parameters on the attainable fiber volume fraction of the composites.

Damage monitoring in fiber-reinforced composites under fatigue loading using carbon nanotube networks

The formation of carbon nanotube networks around the structural reinforcement in fiber composites has enabled in situ monitoring of matrix damage accumulation. Real-time monitoring of damage development under fatigue loading was studied. The electrical response of the fatigue specimens change synchronously with the applied fatigue loading and enable a quantitative measure of the damage state. The fatigue response of the nanotube network was examined and the damage accumulation validated using microscopic technique. Various damage stages in composite cross-ply laminates under fatigue loading can be clearly detected by adopting the quantitative parameter, damaged resistance change. The sensitivity of the technique to the onset and accumulation of damage may enable future life prediction methodologies.

Analytical Characterization of Two-Step Braided Composites

The fabric geometric models based on lamination analogy and stiffness averaging method are utilized to numerically characterize a 2-step braided composite. The model predictions were found to be in good agreement with results of mechanical tests performed in this study. The architecture of this material is investigated by identifying the geometric and braiding process parameters which include the linear density ratio between axial and braider yarns, the pitch length of braider yarn, the aspect ratio of axial yarn, and the aspect ratio of braider yarn. Parametric study showed a wide range of variability in the elastic constants of the composites. Results of the elastic properties are presented in the form of performance maps.

Mode I delamination of a three-dimensional fabric composite

Finite element analysis was conducted on the double cantilever beam speci men (DCB) of carbon-epoxy (T300/3501-6) orthogonal interlocked fabric composites. The Mode I strain energy release rate, GI, was evaluated to investigate the influence of through-the-thickness (z-axis) fibers on crack driving force as a function of crack length. The finite element analysis of the DCB specimens showed good agreement with ex perimental results for both compliance and strain energy release rate. The presence of z- axis fibers sharply reduced the strain energy release rate in comparison to the traditional two dimensional (2-D) laminated material. A parametric study revealed that increasing the z-axis fiber stiffness reduces GI. Progressive debonding of the z-axis reinforcement was also modelled. The Mode I strain energy release rate increased as the fiber debonded but was significantly less than the 2-D laminated material in all cases investigated. A model for the prediction of critical load is proposed for Mode I interlaminar fracture of 3-D orthogonal interlocked fabric composites. Fundamental inputs to the model are the z-axis fiber properties, fiber architecture, fiber volume fraction and the Mode I critical strain energy release rate for the 2-D laminate.