Functionalized Carbon Nanofiber Yarn

This application claims priority benefit from U.S. Provisional Application No. 63/402,185, filed Aug. 30, 2022, which is hereby incorporated by reference in its entirety.

The present disclosure generally relates to carbon nanofibers. Specifically, the present disclosure relates to carbon nanofiber yarn functionalization, fabrication, and applications of such modified carbon nanofiber yarns and yarn bundles.

Carbon nanofibers, interchangeable with carbon nanotubes (CNTs), are long and thin cylinders made of carbon that are arranged in a remarkable hexagonal lattice structure. Carbon nanofibers have a broad range of electronic, thermal, and structural superior properties, making them suitable for various applications, including, but not limited to, field emitters, conductors, transistors, energy storage, and fibers and fabrics with different strengths, biomedical applications, and industrial applications. However, carbon nanofibers are generally hydrophobic in that their surface repels water making them liquid-unfriendly with a strong tendency to agglomerate for the carbon nanofibers themselves, mainly due to the Van der Waals forces. Accordingly, carbon nanofibers may not perform as well where liquid absorption properties are desired, and alternative materials may be sought for such applications.

According to an aspect of the present disclosure, treated nanofiber yarn bundles with higher water absorption rates with retained yarn bundle structures are provided.

According to an aspect of the present disclosure, the treated nanofiber yarn bundles more than double or quadruple their water-absorption capacity than untreated or pristine nanofiber yarns or yarn bundles.

Through one or more of its various aspects, embodiments and/or specific features or sub-components of the present disclosure are intended to bring out one or more of the advantages as specifically described above and noted below.

Carbon nanofibers or CNTs are long tubes with small diameters typically measured in nanometers. They have a high aspect ratio of length vs. diameter in a range of generally above 1000:1 and are made up of one or more graphene sheets rolled up into a concentric structure. Each graphene sheet is regarded as a wall; a single-wall CNT (SWCNT) is made of a single graphene sheet; a double-wall CNT is made of two graphene sheets; a multi-wall CNT (MWCNT) has multiple graphene sheets.

Many CNT synthesis methods have been developed depending on product types, precursors, heating source, reaction time, temperature, and atmosphere. The most common methods include, but are not limited to, arc discharge, electrolysis, laser ablation, chemical vapor deposition (CVD), flame synthesis, mechano-thermal, etc. The well-known CVD methods have been classified into plasma-enhanced PE-CVD, aerosol CVD (AACVD), water-assisted WA-CVD, oxygen-assisted CVD, catalytic CVD, etc.

The CVD method utilizes acetylene (CH), ethylene (CH), or other hydrocarbons as a carbon source in a reaction chamber with a catalyst and a temperature ranging from 350 to 1,000° C. Depending on the actual reaction temperature, reaction time, selection of catalyst, catalyst density, and carbon sources, the resulting CNTs vary.

Within a CVD reaction chamber, a catalyst may be in a free-floating form within the reaction chamber during the synthesis (FC-CVD). The nanofibers are then collected and turned into yarns involving dispersing of CNTs in a liquid with a dispersant or surfactant, followed by extrusion and continuous spinning of the dispersed mixture into a conventional solution.

A variation of the above yarn spinning methods includes, but is not limited to, the use of superacid in the spinning solution and the use of polymer ethylene glycol for CNT dispersion.

An alternative and commonly deployed CVD synthesis method starts with a catalyst deposition on a substrate, e.g., a silicon wafer, by electron-beam (E-beam) deposition. The substrate may also comprise stainless steel or aluminum disposed on underlying silicon (Si) wafer or other ceramic substrates. Besides E-beam deposition, other catalyst deposition methods include, but are not limited to, sputtering, electrochemical methods, atomic layer deposition, laser-assisted CVD, and plasma-enhanced CVD. Exemplary catalysts include iron on a buffer layer of silicon dioxide or aluminum oxide over a substrate.

Once the CVD process is complete, as shown on the substrate, the result is a collection of individual nanofibers aligned vertically to each other with one end of the nanofibers attached to the substrate, like a forest, frequently referred to as a CNT forest. The van der Waals force and the entanglement among adjacent nanofibers are the main attributes that these CNTs are maintained in such a vertically aligned position, considering the small diameter of these individual CNTs from 0.3 nm up to 100 nm.

A CNT forest may eventually be spun into a CNT yarn, as described in WO 2007/015710 and incorporated herein.

Briefly, connect an attachment to the sidewall or near the sidewall of a CNT forest. Draw the attachment continuously in a direction away from the CNT forest with an angle in a range of about 90° to 5° between the drawing and the alignment directions of the nanofibers of the CNT forest to form a CNT sheet. Twist the CNT sheet about the axis of the draw direction to form a nanofiber yarn, which is then wound around a reel.

CNT yarns may be made by other methods well known in the literature. They may be commercially available from various suppliers.

As described above, carbon nanofibers incorporate carbon from various carbon source materials during the synthesis steps into one of the carbon allotropes, which include, but are not limited to, diamond, graphite, fullerenes, graphene, etc. From a molecular perspective, these materials are all entirely composed of carbon-carbon (C—C) bonds, with different orientations of these bonds in a specific material, indicating a drastically different material with distinguishable properties. Ramen spectroscopy is a commonly applied non-destructive tool well suited to characterize molecular interactions and bonds of carbon material. Its measured vibrational frequency is very sensitive to the orientation of the C—C bond and weights of the atoms at either end of the bond, indicative of one or more carbon allotropes.

As essentially the rolled-up graphene sheets, the nanofibers show the graphene characteristics on a Raman spectrum, the G band at 1590 cmplus a prominent 1340 cmD-band. The latter is a defect-induced feature attributed to dislocation defects. Amorphous graphite on the surface of nanofibers may also influence this D-band in the testing sample.

The intensity ratio of both D/G bands (ID/IG) quantitively reflects the defect status of the nanofibers; it also indicates the crystallinity. A smaller ID/IG value, compared to a large one, indicates better crystallinity or fewer defects. Changes in the Ip/IG value of a CNT material after a defined treatment correlate to structural or microstructural modification of CNT walls, which would appear as functional or property alterations of the CNT material.

Raman spectrometers are commercially available, from handheld to tabletop. The i-Raman Plus from B&W Tek was an exemplary spectrometer selected to scan the samples at power, collection time of 60,000 ms, and measure the G and D peak values at 1590 cm-1 and 1350 cm, respectively. The IG/ID ratios are calculated.

Carbon nanotubes are generally hydrophobic; their surface repels water making them liquid unfriendly with a strong tendency to agglomerate for CNTs themselves, mainly due to the Van der Waals forces. Studies by atomic force microscopy and the Wilhelmy method to measure contact angles between a single CNT and various liquids found that wettability was less favorable when the liquid became more polar. Surfactants are frequently applied to disperse CNTs in solutions to make CNT suspensions.

CNT yarns appear to inherit the same hydrophobic property. Insufficient contacts and interactions between individual CNT fibers or bundles of CNT fibers (CNT yarns) and other materials or matrix materials limit the uses and applications of CNT yarns.

As an exemplary instance, one hundred individual CNT yarns, about 15 μm in diameter each, made by drawing from CNT forests on supports and subsequent twisting and untwisting, were bundled together to form a single CNT bundle or a false-twisted CNT bundle, in which an individual CNT yarn within the CNT bundle is substantially parallel to a central axis of the CNT bundle. Bundles are further treated as described below to alter surface morphology and bundle functions for desired properties and applications.

Each nanofiber yarn bundle may have an equal length, e.g., 2 cm. The treated nanofiber yarn bundles are transferred into a silicone tube, which has an inner diameter of one-sixteenth inch ( 1/16″) or an inner diameter, in general, larger than the diameter of the nanofiber yarn bundle or a collection of the nanofiber yarn bundles. The initial weight was measured and recorded. One end of the nanofiber yarn bundles and the silicone tube were inserted into a water container with a constant one (1) cm deep water for three (3) consecutive days in a closed chamber to avoid water evaporation. The bottom of the container was at least ten times larger than the total cross-section of the silicon tubes to ensure a sufficient amount of water supply during the water (or liquid) absorption test. Each silicon tube with the nanofiber yarn bundles was weighted and subtracted by its initial weight to calculate the water absorption.

Untreated CNT yarns and CNT yarn bundles preserve their primitive surfaces and hydrophobic characteristics and are herein referred to as pristine yarns and yarn bundles.

CNT pristine yarn bundles may be treated with an agent or a group of agents with a strong oxidizing potential to alter the CNT material surface and structure or microstructure to endorse new properties and functions. The agent may be substantially pure, close to 100% purity, or have a percentage between 0-100%, depending on the chemical nature of the agent.

In accordance with the disclosure, an exemplary oxidizing agent may be a gas or at least two gases.

Exemplary strong oxidizing gases may include but are not limited to ozone and chlorine.

According to exemplary aspects, CNT yarn bundles may be placed in a sealed treatment chamber. Then the chamber may be filled with at least one selected oxidizing gas at a target flow rate for a predetermined period of time. Once the predetermined period is expired, the treatment chamber is purged with inert gas or gas mixture, such as argon and/or nitrogen.

In accordance with the disclosure, CNT treatment may include applying strong oxidizing agents and removing such agents upon achieving desired results.

Exemplary oxidizing agents may include but are not limited to strong chemical liquids, such as one or more strong acids. Moreover, certain acids may be mixed together in a volumetric ratio, such as a two-to-one volumetric ratio. In chemistry, a strong acid is an acid which ionizes its target or substrate completely in a water solution.

Exemplary strong acids include but are not limited to nitric acid, sulfuric acid, perchloric acid, nitic acid, perchloric acid, and combinations of nitric acid and sulfuric acid, sulfuric acid and potassium dichromate, and sulfuric acid and potassium permanganate.

For liquid-based treatment, according to an exemplary embodiment, CNT yarn bundles are soaked in intended solutions for a predetermined period until reaching the desired effects before being removed. Then, the treated yarn bundles are rinsed thoroughly with deionized water (DI water) and dried in the air, in a heated oven, or in a vacuum chamber.

is a flow chart illustrating a method of performing an acid treatment on a CNT yarn bundle in accordance with an exemplary embodiment.

In operation S, carbon nanotube forests were synthesized by standard chemical vapor deposition method on a silicon wafer using iron catalysts and acetylene gas as a carbon source. In operation S, CNT forests were drawn into CNT sheets. In operation S, the CNT sheets were twisted into CNT yarns with a predetermined diameter. The predetermined diameter may be 15 μm with standard deviation of 1 μm. In operation S, the CNT yarns are wounded into spools. In an example, a spool may have 100 loops of CNT yarns. In operation S, a spool of CNT yarns is cut open to form a CNT yarn bundle or more than one bundles, which may be referred to as a nanofiber yarn bundle or bundles.

In operation S, the CNT yarn bundles are treated with a mixture of strong acids. According to exemplary aspects, strong acids may include, without limitation, at least nitric acid (HNO) and sulfuric acid (HSO). For example, the mixture of strong acids may be formed of a 70% nitric acid mixed with a 98% sulfuric acid. Further, the nitric acid and the sulfuric acid may be mixed in a two-to-one volumetric ratio, respectively (i.e., two parts nitric acid and 1 part sulfuric acid). However, aspects of the present disclosure are not limited thereto, such that the mixture of strong acids may include, without limitation, a perchloric acid, a nitric acid, a sulfuric acid, a potassium dichromate, or a potassium permanganate. Although a mixture of acids is disclosed herein, aspects of the present disclosure are not limited thereto, such that a single acid solution may be utilized. In an example, the single acid solution may vary in its chemical concentration level.

In operation S, the CNT yarn bundles were submerged in the mixture of strong acids for a predetermined period of time. According to exemplary aspects, the predetermined period of time may be at least 30 minutes, 300 minutes, or less than 24 hours. However, aspects of the present disclosure are not limited thereto, such that the predetermined period of time may be any amount of time that results in a target increase of hydrophilicity for as short as 10 minutes. The target hydrophilicity may be at least double or quadruple of innate hydrophilicity of untreated or pristine CNT yarn bundles while maintaining the structural integrity of the CNT yarn bundles. The CNT yarn bundles may be submerged in the mixture of strong acids for the predetermined period of time with occasional stirring.

In operation S, the treated CNT yarn bundles are removed from the acid mixture, soaked in a water bath and rinsed with distilled water thoroughly.

is a flow chart illustrating a method of performing an ozone treatment on a CNT yarn bundle in accordance with an exemplary embodiment.

In operation S, carbon nanotube forests were synthesized by standard chemical vapor deposition method on a silicon wafer using iron catalysts and acetylene gas as a carbon source. In operation S, CNT forests were drawn into CNT sheets. In operation S, the CNT sheets were twisted into CNT yarns with a predetermined diameter. The predetermined diameter may be 15 μm with standard deviation of 1 μm. In operation S, the CNT yarns are wounded into spools. In an example, a spool may have 100 loops of CNT yarns. In operation S, a spool of CNT yarns is cut open to form a CNT yarn bundle or more than one bundles, which may be referred to as a nanofiber yarn bundle or bundles.

In operation S, the CNT yarn bundles are placed in a treatment chamber, and then the treatment chamber is sealed. In operation S, the treatment chamber is filled with a selected oxidizing gas. According to exemplary aspects, oxidizing gas may be formed of 20% ozone gas plus 80% nitrogen. However, aspects of the present disclosure are not limited thereto, such that other ozone percentage makeup oxidizing gas may be utilized. In operation S, the CNT yarn bundle may be exposed to the oxidizing gas for a predetermined period of time. In an example, the predetermined period of time may have a minimum value of 10 minutes, 30 minutes, 300 minutes, or a maximum value of 24 hours. However, aspects of the present disclosure are not limited thereto, such that the predetermined period of time may be any amount of time that results in a target increase of hydrophilicity. The target hydrophilicity may be at least double or quadruple of innate hydrophilicity of untreated or pristine CNT yarn bundles while maintaining the structural integrity of the CNT yarn bundles.

In operation S, upon expiration of the predetermined period of time (e.g., completion of ozone treatment), the treatment chamber is purged with inert gas. In an example, the inert gas may include, without limitation, nitrogen, argon and the like. In operation S, the treatment chamber is returned to the atmospheric environment, and the treated CNT yarn bundles are promptly removed from the treatment chamber and placed in storage, preferably in a sealed storage filled with inert gas.

One group of CNT yarn bundles is treated with a strong acid mixture for 300 minutes, as detailed in the description ofprovided above.

The other two groups of CNT yarn bundles are treated with ozone gas for 30 minutes and 300 minutes, respectively, as detailed in the description ofabove.

All three treated groups of CNT yarn bundles demonstrate significant changes in Ip/IG ratio, as shown in Table 1, and wettability in Table 2.

In Table 1, all treated groups with three samples per group show increases in G-peak value and D-peak value. After converting these peak values into ratios, the mean IG/Ip values decrease from 1.77 for the pristine CNT yarn bundles to 1.51 for the 30-min ozone-treated yarn bundles, 1.39 for the 300-min ozone-treated yarn bundles, and 1.30 for the acid-treated yarn bundles. As the treatment conditions get more aggressive, the IG/ID values drop more significant, indicating more structural changes in the CNT walls.

In Table 2, all CNT yarns demonstrate their water absorption before and after strong acid treatment and ozone treatment.presents the same results in a bar chart. The weight gain or water absorption rate, by percentage, increases from 1.47% to 2.68%, 4.32%, and 6.68% for the pristine group, 30-min ozone-treated group (Ozone L-Treated CNT Yarn Bundle), 300-min ozone-treated group (Ozone H-Treated CNT Yarn Bundle), and the acid-treated ozone group (Acid Treated CNT Yarn Bundle), respectively. The harsh treatment trend renders more water absorption into the treated nanofiber yarn bundles, leading to higher hydrophilicity. The CNT yarn water absorption capacity may be doubled or quadrupled with the most aggressive treatment, from a gain of 10 mg water per mg of CNT yarn bundles to 19 mg, 30 mg, and 47 mg for the pristine, 30-min ozone-treated, 300-min ozone-treated, acid-treated CNT yarn groups, respectively.

Increased hydrophilicity of carbon nanotubes will enable the nanostructures to retain more exogenous enzymatically active and non-active small molecules and larger molecules with a slower release property.