Microscale Carbon Fibers Research Articles

An investigation of self-sensing and mechanical properties of smart engineered cementitious composites reinforced with functional materials

Abstract In this study, hybrid functional fillers were used to create an electrical network that is used to develop self-damage sensing ability within conventional cementitious mixtures. This electrical network was used to examine the self-sensing properties of cementitious composites under different loading scenarios such as compression, splitting tensile, and cyclic loading for three ages, those are 7, 14, and 28. With the help of a single type of functional filler or fiber, the self-sensing capabilities of the majority of previous works are demonstrated. This study incorporates two types of functional fillers for sustainability and low cost: micro-scale carbon fibers (CF) and waste iron (Ir) powder in the form of microparticles. The purpose of the current work is to fill up the subject’s gap using two different types of functional fillers as a hybrid form. Three hybrid proportions of a micro-scale CF (vol% of mixture) and waste iron powder (Ir) (wt% of cementitious materials) are utilized. These are (0.33, 15), (0.67, 10), and (1.00, 5), respectively. Unlike carbon-based materials, polyvinyl alcohol is used as a mechanical reinforcing fiber 2% by volume of the cementitious mixture. Additionally, a control combination without any fillers has been created. The electrical resistivity of the fabricated samples was monitored during various loading applications for every second to confirm their self-sensing capabilities. Regarding the fractional change in electrical resistivity, the self-sensing behavior was superior for mixes containing high dosages either of waste iron powder or CF in the same hybrid matrix. Quite the contrary to that, the piezoresistivity was modest in the middle hybrid ratio.

The synergistic effects of micro-and nano-fillers on the properties of polyamide 11/poly (lactic acid) blend

This study focused on investigating the effects of micro-scale carbon fibers (CF) and nano-scale graphene nanoplatelets (GNP) on the morphological, rheological, mechanical, thermo-mechanical and thermal properties of polyamide 11/poly (lactic acid) (PA11/PLA) blend. A twin-screw extruder and an injection molding machine were utilized to produce the samples. The amount of GNPs was varied up to 5 wt% for a fixed CF fraction of 20 wt%, and the composites were characterized. SEM micrographs of hybrid composites revealed stronger interfacial interactions between the matrix and CF and improved dispersion of GNPs. The pseudo solid-like behavior transition and high modulus detected in the rheological study with the addition of GNP confirmed the strong matrix-filler interactions in hybrid composites. The synergistic effects of multi-scale fillers have been observed in mechanical and thermo-mechanical properties. In comparison to 20CF, hybrid composites’ tensile strength and modulus increased by 8%–4.8% and 4.5%–17%, respectively, by adding 0.5–5 wt% GNP. Dynamic mechanical analysis revealed that the tan δ peak shifted to higher temperatures in low GNP containing hybrid composites, indicating an increase in Tg. Also, differential scanning calorimetry confirmed the Tg increment and synergistic effect of fillers on crystallization. The thermogravimetric analysis demonstrated that hybrid composites exhibited enhanced thermal stability. As a consequence, these hybrid composites offer an inventive viewpoint for environmentally friendly composites in the automobile industry thanks to their enhanced mechanical and thermal characteristics.

Aligned microfibers interweaved with highly porous carbon nanofibers: A Novel electrode for high-power vanadium redox flow batteries

In this work, we propose and develop a novel electrode made of aligned microscale carbon fibers interweaved with highly porous carbon nanofibers for vanadium redox flow batteries. Such a unique combination not only endows the electrode with a high permeability due to the reduced tortuosity and large macropores, but also an ultra-large specific surface area for redox reactions. As a result, a vanadium redox flow battery equipped with the newly-developed electrode is able to achieve an energy efficiency of 79.3% at the current density of 400 mA cm−2, which is 9.9% higher than that with pure porous carbon nanofiber electrodes, and 14.1% higher than that with pure aligned fiber electrodes. More remarkably, the battery is capable of delivering a peak power density of as high as ∼1.9 W cm−2 and a limiting current density of ∼5000 mA cm−2. These results demonstrate that our strategy of interweaving porous carbon nanofibers between aligned microfibers effectively addresses the contradiction between permeability and surface area of porous electrodes, opening a new platform to fabricate high-performance electrodes for vanadium redox flow batteries.

Gravity driven ice-templated oriental arrangement of functional carbon fibers for high in-plane thermal conductivity

Polymer-based thermal conductivity composites composed with nanoscale fillers are promising thermal conductors to dissipate accumulated heat of integrated circuits in electronic devices, while interfacial thermal resistance (ITR) and disordered conductive pathways caused by the size effect of fillers limit their heat transfer performance. Herein, gravitational-driven ice-templated oriental arrangement of ionic liquid (IL) modified microscale carbon fibers (CFs) were conducted to improve thermal conductivity via both weakening phonons scattering effect and forming horizontal orientation heat transfer pathways in composites. The cation-π interaction between CFs and the hydrogen bonding between CFs and epoxy resin induced by IL effectively reduced ITR in composites. As a result, the composite including 22.3 vol% CFs reached a maximum in-plane thermal conductivity of 7.98 W m−1 K−1. The mechanical properties of the composite were also developed. This insight provides a strategy for the construction of high-performance composites with potential applications in thermal management materials.

The effect of nanoparticle conglomeration on the overall conductivity of nanocomposites

This study provides a multi-step analytical model to investigate the synergistic effects of carbon black nanoparticles and micro-scale carbon fibers on the electrical conductivity of the polymer matrix multi-scale nanocomposites. In the first step, the homogenized electrical conductivity of the nanoparticle-polymer nanocomposites containing the nanoparticle agglomeration is calculated using the percolation like electrical network model. The second step presents the details of Mori-Tanaka micromechanical model to predict the effective electrical conductivity of the multi-scale nanocomposites composed of the micro-fibers embedded in the nanoparticle-enriched polymer as the matrix phase. The influences of different parameters such as the nanoparticle agglomerate diameter to tunneling distance ratio, fiber aspect ratio, intrinsic electrical conductivity, volume fraction, and polymer potential barrier height on the electrical resistivity of the multi-scale nanocomposites are investigated. Based on the comparative studies, the model predictions are in good agreement with the available experimental results. It is found that the electrical resistivity decreases with the addition of multi-scale fillers. The results also suggest that the tunneling distance between the agglomerates plays a dominant role in the electrical conductivity of the multi-scale nanocomposites.

A novel electrode formed with electrospun nano- and micro-scale carbon fibers for aqueous redox flow batteries

This work reports on a novel electrode prepared with electrospun nano- and micro-scale carbon fibers for aqueous redox flow batteries. Larger fibers, ~10 μm in diameter, form larger pores to provide pathways for electrolyte flow, while smaller fibers, ~1 μm in diameter, increase active surface area for redox reactions. Brunauer-Emmett-Teller and pressure drop tests show that the specific surface area of the prepared dual-diameter electrode is doubled as compared with the large-fiber electrode, while the estimated permeability is enhanced by 1.4 times as opposed to the small-fiber electrode. The application of the dual-diameter electrodes to a vanadium redox flow battery allows the battery to achieve an energy efficiency of 84.78% at the current density of 100 mA cm−2, which is 13.57% higher than that with small-fiber electrodes, and 3.91% higher than that with large-fiber electrodes. Even at a high current density of 200 mA cm−2, the battery with the prepared electrode can still maintain the energy efficiency of 74.18%, which is 5.5% higher than that with large-fiber electrodes, and the battery with small-fiber electrodes cannot be operated at such high current density. This dual-diameter fibrous structure provides inspirations for the future electrode design in aqueous redox flow batteries.

Anisotropic thermal conductivities and structure in lignin-based microscale carbon fibers

It is very important to know whether the thermal conductivity of carbon fibers in the directions of fiber axis (axial thermal conductivity, κa) and fiber radius (radial thermal conductivity, κr) are anisotropic. Relevant measurement is strongly hindered by microsize of carbon fibers in the radial direction. In this work, a novel method by combining frequency domain energy transport state-resolved Raman and transient electrothermal techniques is developed to overcome this drawback to achieve thermal conductivity anisotropy study of lignin-based microscale carbon fibers. Four fibers are characterized and the difference of κa among them is very small, and κa is around 1.8 W m−1 K−1 while the difference of κr is very large. The κr varies from 0.11 to 8.0 W m−1 K−1, revealing strong structure anisotropy and radial structure variation. The thermal conductivity variation against temperature also shows very different behavior. κa features a reduction of more than one order of magnitude from room temperature to 10.4 K while κr shows very little change from room temperature to 77 K. For the same carbon fiber, there is also a large difference of κr at different axial positions. Detailed Raman study of the axial and radial structures uncovers very strong structure anisotropy and explains the observed anisotropic thermal conductivities.

Synergistic improvement of electrical and thermal conductivities of carbon-based nanocomposites and its prediction by Mori-Tanaka scheme with interfacial resistances

Carbon-based nanomaterials have widely been used to enhance the thermal and electrical properties of polymer composites with the aid of miniaturization of electronic devices that require fast heat dissipation and electromagnetic interference shielding at a desired level. A combination of different carbon nanofillers can lead to a strong synergistic effect by creating the electron hopping pathway and conductive network. Here, we fabricate the hybrid nanocomposites composed of two-dimensional graphite nanoplatelets, one-dimensional microscale carbon fibers, and one-dimensional nanoscale multiwalled carbon nanotubes. The synergistic improvement of electrical and thermal conductivities of the hybrid nanocomposites is achieved by a three-dimensional network formation among the microscale and nanoscale fillers. A Mori-Tanaka averaging scheme with perfect and imperfect interfacial resistances under uni-directional and random fiber orientations is presented to predict the synergistic improvement of thermal conductivity of the nanocomposites. A transient temperature response in the nanocomposites is investigated to evaluate heat dissipation capability in which heat transfer rate is enhanced in the case of hybrid nanocomposites. Finite element simulation results are in a good agreement with the transient temperature response experimentally obtained.

Modeling of thermomechanical properties of polymeric hybrid nanocomposites

This article reports modeling of effective thermomechanical properties of multifunctional carbon nanotube (CNT)‐reinforced hybrid polymeric composites (hereafter, it is referred as “fuzzy fiber composite”). The novel constructional feature of fuzzy fiber composite (FFC) is that nanoscale CNTs are radially grown on the circumferential surfaces of microscale carbon fibers. Several micromechanical models were developed to predict the effective thermomechanical properties of FFC. The waviness of CNTs is intrinsic to many manufacturing processes and it influences thermomechanical behavior of two‐phase CNT‐reinforced composites. Therefore, an endeavor was also made to investigate the effect of wavy CNTs on the effective thermomechanical properties of FFC. The proposed modeling approach was applied to a heat exchanger made of FFC to determine its effective thermal conductivities. The findings of our study suggest that FFC containing nano‐ and micro‐scale fillers show improved thermomechanical properties and are promising next‐generation polymeric composites for advanced structural applications. POLYM. COMPOS., 39:4148–4164, 2018. © 2017 Society of Plastics Engineers

Thermal conductivity and annealing effect on structure of lignin-based microscale carbon fibers

This work reports on systematic investigation of the structure and thermal conductivity of lignin-based carbon fibers (CF) at the microscale. The lignin-based CF is produced by melt-spinning pyrolytic lignin derived from red oak. The 0 K-limit phonon scattering mean free path uncovers a characteristic structure size of ∼1.2 nm, which agrees well with the crystallite size by X-ray scattering (0.9 and 1.3 nm) and the cluster size by Raman spectroscopy (2.31 nm). The thermal conductivity of as-prepared CFs is determined at ∼1.83 W/m·K at room temperature. The thermal reffusivity of CFs shows little change from room temperature down to 10 K, uncovering the existence of extensive defects and grain boundaries which dominate phonon scattering. The localized thermal conductivity of CFs is increased by more than ten-fold after being annealed at ∼2800 K, to a level of 24 W/m·K. Our microscale Raman scanning from less annealed to highly annealed regions shows one-fold increase of the cluster size: from 1.83 nm to 4 nm. This directly confirms structure improvement by annealing. The inverse of the thermal conductivity is found linearly proportional to the annealing temperature in the range of 1000–2800 K.

Multiscale modeling of regularly staggered carbon fibers embedded in nano-reinforced composites

Abstract This article deals with the multiscale modeling of stress transfer characteristics of nano-reinforced polymer composite reinforced with regularly staggered carbon fibers. The distinctive feature of construction of nano-reinforced composite is such that the microscale carbon fibers are packed in hexagonal array in the carbon nanotube reinforced polymer matrix (CNRP). We considered three different cases of CNRP, in which carbon nanotubes (CNTs) are: (i) aligned along the direction of carbon fiber, (ii) aligned radially to the axis of carbon fiber, and (iii) randomly dispersed. Accordingly, multiscale models were developed. First, molecular dynamics (MD) simulations and then Mori-Tanaka technique were used to estimate the effective elastic properties of CNRP. Second, a micromechanical three-phase shear lag model was developed considering the staggering effect of microscale fibers and the application of radial loads on the cylindrical representative volume element (RVE) of nano-reinforced composite. Our results reveal that the stress transfer characteristics of the nano-reinforced composite are significantly improved by controlling the CNT morphology, particularly, when they are randomly dispersed around the microscale fiber. The results from the developed shear lag model were also validated with the finite element shear lag simulations and found to be in good agreement.

Advanced in situ multi-scale characterization of hardness of carbon-fiber-reinforced plastic

In situ multi-scale characterization of hardness of carbon-fiber-reinforced plastic (CFRP) is demonstrated by a traditional hardness tester, instrumented indentation tester and atomic-force-microscope (AFM)-based nanoindentation. In particular, due to the large residual indentation and nonuniform distribution of the microscale carbon fibers, the Vickers hardness could not be calculated by the traditional hardness tester. In addition, the clear residual microindentation could not be formed on the CFRP by instrumented indentation tester because of the large tip half angle of the Berkovich indenter. Therefore, an efficient technique for characterizing the true nanoscale hardness of CFRP was proposed and evaluated. The local hardness of the carbon fibers or plastic matrix on the nanoscale did not vary with nanoindentation location. The Vickers hardnesses of the carbon fiber and plastic matrix determined by AFM-based nanoindentation were 340 ± 30 and 40 ± 2 kgf/mm2, respectively.

Development of pulsed laser-assisted thermal relaxation technique for thermal characterization of microscale wires

A transient technique is developed to measure the thermal diffusivity of one-dimensional microscale wires. In this technique, the thin wire is suspended over two copper electrodes. Upon fast (nanosecond) pulsed laser irradiation, the wire’s temperature will quickly increase to a high level and then decrease gradually. Such temperature decay can be used to determine the sample’s thermal diffusivity. To probe this temperature evolution, a dc is fed through the wire to sensor its voltage variation, from which the thermal diffusivity can be extracted. A 25.4 μm thin Pt wire is characterized to verify this technique. Sound agreement is obtained between the measured data and reference value. Applying this pulsed laser-assisted thermal relaxation technique, the thermal diffusivity of multiwall carbon nanotube bundles and microscale carbon fibers is measured. Detailed analysis is conducted to study the effect of the wire embedded in the paste/base on the final measurement result.

Directional pyrolytic growth of microscale carbon fibers on electrochemically pretreated polyacrylonitrile-based carbon microfibers

Vapor-grown carbon microfibers were synthesized pyrolytically on electrochemically pretreated polyacrylonitrile-based carbon microfibers (PAN-CFs) without introducing any catalyst. Cyclohexane was used as the carbon source for the growth of fibers in a thermal chemical vapor deposition process (CVD). Scanning electron microscopy (SEM) and X-ray photoelectron spectroscopy (XPS) were used for the characterization of the fibers. Microscale carbon fibers with about 1 to 2 µm in diameter were formed at 1123 K, while carbon microfibers of about 0.3 µm in diameter were synthesized at 1048 K. A pretreatment at negative potential or sequentially at both positive and negative potentials was found to be a prerequisite for the growth of carbon microfibers in the CVD process. Without electrochemical pretreatment or when only a positive potential was applied, the growth of microscale carbon fibers did not occur.

Thermal Characterization of Micro/Nanoscale Wires/Tubes Using Pulsed Laser-assisted Thermal Relaxation

ABSTRACTA novel transient technique is developed to measure the thermal diffusivity of one-dimensional microscale wires. In this technique, the thin wire is suspended over two copper electrodes. Upon fast (nanosecond) pulsed laser irradiation, the wire's temperature will quickly increase to a high level and then decrease gradually. Such temperature decay can be used to determine sample's thermal diffusivity. To probe this temperature evolution, a dc current is fed through the wire to sensor its voltage variation, from which the thermal diffusivity can be extracted. A 25.4-μ;m thin Pt wire is characterized to verify this technique. Sound agreement is obtained between the measured data and reference value. Applying this pulsed laser-assisted thermal relaxation technique, the thermal diffusivity of multi-wall carbon nanotube bundles and microscale carbon fibers is measured. Detailed analysis is conducted to study the effect of the wire embedded in the paste/base on the final measurement result.