Method for preparing carbon nanotube fiber gradient doped

Abstract

The application discloses a preparation method of a carbon nanotube fiber gradient doping method, characterized in that a position-dependent conductivity gradient distribution is constructed on the axial direction of a single carbon nanotube fiber through a segmented acid treatment mode, and the electrical performance of the fiber is simultaneously synergistically enhanced; the method comprises the following steps: S1, segmented mask forming treatment and protection areas; S2, differential acid treatment, time, concentration or temperature are controlled; S3, in-situ reaction of the acid treatment to form graphite carbon shell-iron oxide core nanospheres, and bridging of carbon nanotube bundles to form a conductive path; S4, axial conductivity measurement, and calculation of gradient characteristic parameters; and S5, if the target is not reached, the mask area or the treatment parameters are closed-loop optimized and adjusted, and are fed back to S1 iteration until the gradient conductivity fiber is output after reaching the standard. The application provides the preparation method of the carbon nanotube fiber gradient doping method, so as to solve the problems that in the prior art, it is difficult to accurately construct the conductivity gradient distribution on the axial direction of the single carbon nanotube fiber, the microstructure controllability is poor, and the closed-loop optimization mechanism is lacked.

Description

Technical Field

[0001] This invention relates to the fields of chemical engineering, new materials, and nanomaterial preparation, and particularly to a method for preparing carbon nanotube fibers by gradient doping. Background Technology

[0002] Carbon nanotube fibers, as a novel one-dimensional macroscopic carbon-based material, combine the excellent mechanical and electrical properties of carbon nanotubes with the flexibility and weavability of fiber materials, showing broad application prospects in fields such as smart fabrics, structural health monitoring, flexible electronic devices, and aerospace. In many application scenarios, carbon nanotube fibers not only need to have excellent overall electrical conductivity, but also require their electrical properties to exhibit a controllable gradient change along the axial direction to meet the special requirements of graded functional materials, flexible sensing and actuation devices.

[0003] Floating catalytic chemical vapor deposition is the mainstream method for preparing continuous carbon nanotube fibers. However, carbon nanotube fibers prepared directly by this method contain a large amount of amorphous carbon and residual iron-based catalysts between the internal carbon nanotube bundles. These impurities not only reduce the overall electrical conductivity of the fiber, but also make the electrical conductivity of the fiber uniform along the axial direction, making it difficult to achieve a gradient distribution.

[0004] In existing technologies, post-processing modifications of carbon nanotube fibers, such as acid treatment, are widely used to remove impurities and improve electrical conductivity. However, these methods typically involve uniform treatment of the entire fiber, resulting in fibers with uniform overall properties, failing to create a performance gradient along the axial direction of individual fibers. Although some studies have attempted to regulate fiber properties by changing processing conditions, how to achieve directional regulation at the microstructural level and establish a preparation method that can precisely control gradient distribution and perform closed-loop optimization remains a pressing technical challenge in this field.

[0005] Therefore, developing a method for preparing carbon nanotube fibers with gradient doping that can precisely construct conductivity gradient distribution along the axial direction of a single carbon nanotube fiber, achieve controllable transformation of microstructure, and dynamically adjust process parameters through closed-loop optimization has significant technical value and practical implications. Summary of the Invention

[0006] This invention provides a method for preparing gradient doped carbon nanotube fibers to solve the problems in the prior art, such as difficulty in accurately constructing conductivity gradient distribution along the axial direction of a single carbon nanotube fiber, poor controllability of microstructure, and lack of closed-loop optimization mechanism.

[0007] To solve the above-mentioned technical problems, the present invention is implemented as follows: This invention provides a method for preparing gradient-doped carbon nanotube fibers. Through segmented acid treatment, a position-dependent conductivity gradient distribution is constructed along the axial direction of a single carbon nanotube fiber, simultaneously achieving synergistic enhancement of the fiber's electrical properties. The method includes at least a segmented masking module, an acid treatment control module, a microstructure transformation module, a performance gradient evaluation module, and a closed-loop optimization module. The method includes at least the following steps: S1: Carbon nanotube fibers are applied with removable masks in different regions along the axial direction through the segmented mask module to form a spatial distribution pattern of at least two processing regions and a protection region. S2: The segmented masked carbon nanotube fibers are placed in the acid treatment control module for acid treatment. By controlling at least one parameter among the acid treatment time, acid concentration or treatment temperature of different treatment areas, a differentiated degree of acid treatment is formed in the fiber axis. S3: During the acid treatment process, the iron-based catalyst remaining in the carbon nanotube fiber and the amorphous carbon consumed in situ are reacted in an acidic oxidation environment through the microstructure transformation module to form graphite carbon shell-iron oxide core nanospheres in situ. The nanospheres are bridged between carbon nanotube bundles to form conductive pathways. S4: Input the processed fiber into the performance gradient evaluation module, measure the conductivity at at least three different locations along the fiber axis, and calculate the conductivity gradient distribution characteristic parameters; S5: If the conductivity gradient distribution characteristic parameter does not reach the preset target value, the closed-loop optimization module dynamically adjusts at least one parameter among the mask region distribution of the segmented mask module, the treatment time gradient, the treatment concentration gradient, or the treatment temperature gradient of the acid treatment control module according to the deviation between the current characteristic parameter and the target value, and feeds back the adjusted parameter to S1 to form a closed-loop iteration until the performance gradient evaluation module reaches the target; if the performance gradient evaluation module reaches the target, it outputs carbon nanotube fibers with gradient conductivity distribution along the axial direction.

[0008] Optionally, the segmented mask module includes at least a mask material and a region precision control submodule; the mask material is selected from one or more of photoresist, paraffin wax, and peelable polymer; the region precision control submodule is used to control the boundary precision between adjacent processing areas and protection areas, and the boundary precision is not less than ±1 mm.

[0009] Optionally, the carbon nanotube fibers are prepared by floating catalytic chemical vapor deposition, wherein the iron-sulfur molar ratio of the catalyst is 1:2 to 1:5; the acidic solution in the acid treatment control module is one or more of hydrochloric acid, sulfuric acid, and nitric acid; the time of the acid treatment control module is 0.5 to 2 hours, and the temperature is 20 to 80°C.

[0010] Optionally, the differentiated acid treatment degree is achieved by any of the following methods: (a) setting a time gradient for mask removal along the fiber axis so that different regions have different acid treatment durations; (b) setting different concentrations of acid solution for zoned treatment along the fiber axis; (c) setting different treatment temperature gradients along the fiber axis.

[0011] Optionally, the microstructure transformation module includes a nanosphere formation condition monitoring submodule and a nanosphere distribution density regulation submodule; the nanosphere formation condition monitoring submodule is used to monitor the reaction process of iron-based catalyst and amorphous carbon under acidic oxidation environment; the nanosphere distribution density regulation submodule adjusts the distribution density and size of the graphite carbon shell-iron oxide core nanospheres along the fiber axis by controlling the degree of acid treatment.

[0012] Optionally, the performance gradient evaluation module includes a conductivity testing submodule and a gradient feature extraction submodule; the conductivity testing submodule uses a four-probe method to test the conductivity at at least three different locations along the fiber axis; the gradient feature extraction submodule is used to calculate at least one feature parameter among the rate of change of conductivity along the axial direction, gradient coefficient, or distribution function.

[0013] Optionally, the closed-loop optimization module includes a target gradient setting submodule and a parameter iteration optimization submodule; the target gradient setting submodule is used to preset the type of conductivity distribution curve along the axial direction, the distribution curve type including linear gradient, step gradient or specific function curve gradient; the parameter iteration optimization submodule adjusts the processing parameters based on the deviation between the current gradient feature parameters and the target gradient feature parameters using a proportional-integral-derivative control algorithm or a machine learning optimization algorithm.

[0014] Optionally, before or after the acid treatment described in S2, a heat treatment step is further included on the carbon nanotube fibers; the heat treatment is carried out in an inert atmosphere at a temperature of 600–1000°C for 0.5–2 hours.

[0015] Optionally, the carbon nanotube fibers have a gradient of electrical conductivity along the axial direction, and the conductivity of the acid-treated region is increased by no less than 80% compared to the untreated region.

[0016] Optionally, the carbon nanotube fibers form graphite carbon shell-iron oxide core nanospheres in situ within the acid-treated region. The nanospheres have a diameter of 50–200 nm and are distributed among the carbon nanotube bundles to form a conductive bridging network.

[0017] This invention introduces a segmented mask module, enabling precise definition of the treatment and protection regions along the axial direction of a single fiber, providing a spatial basis for constructing a gradient distribution. An acid treatment control module differentiates treatment parameters, creating an axial gradient of acid treatment intensity to drive microstructure transformation. A microstructure transformation module utilizes the existing iron-based catalyst and amorphous carbon within the fiber to generate graphite carbon shell-iron oxide core nanospheres in situ under acidic oxidation. These nanospheres bridge the carbon nanotube bundles, forming new conductive pathways and synergistically enhancing the conductivity of the treatment region. The introduction of a performance gradient evaluation module and a closed-loop optimization module creates a closed-loop iterative system from carbon nanotube fiber preparation to performance evaluation and final optimization. This system dynamically adjusts process parameters according to the preset target gradient, ensuring the final product precisely meets design requirements. This invention solves the problems of existing technologies, such as the inability to construct precise conductivity gradients on single fibers, uncontrollable microstructure, and lack of self-optimization capabilities in the process. Attached Figure Description

[0018] To more clearly illustrate the technical solutions of the embodiments of the present invention, the drawings used in the description of the embodiments of the present invention will be briefly introduced below. Obviously, the drawings described below are only some embodiments of the present invention. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0019] Figure 1 A flowchart illustrating the preparation method of gradient doping of carbon nanotube fibers provided in an embodiment of the present invention; Figure 2 A diagram illustrating the composition of the method for preparing gradient doped carbon nanotube fibers according to an embodiment of the present invention; Figure 3 This is a graph showing the distribution of fiber axial conductivity under different gradient processing modes in the embodiments of the present invention; Figure 4 This is a scanning electron microscope image of graphite carbon shell-iron oxide core nanospheres formed in situ inside the acid-treated region in an embodiment of the present invention.

[0020] Explanation of reference numerals in the attached figures: 10. Segmented Mask Module; 20. Acid Treatment Control Module; 30. Microstructure Transformation Module; 40. Performance Gradient Evaluation Module; 50. Closed-Loop Optimization Module. Detailed Implementation

[0021] The technical solutions of the present invention will now be clearly and completely described with reference to the accompanying drawings. Obviously, the described embodiments are only some, not all, of the embodiments of the present invention. All other embodiments obtained by those skilled in the art based on the embodiments of the present invention without inventive step are within the scope of protection of the present invention.

[0022] It should be understood that the phrase "one embodiment" or "an embodiment" throughout the specification means that a specific feature, structure, or characteristic related to the embodiment is included in at least one embodiment of the invention. Therefore, "in one embodiment" or "in an embodiment" appearing throughout the specification do not necessarily refer to the same embodiment. Furthermore, these specific features, structures, or characteristics can be combined in any suitable manner in one or more embodiments.

[0023] See Figures 1 to 4 This invention provides a method for preparing gradient-doped carbon nanotube fibers, addressing the technical problems in the prior art of constructing a conductivity gradient distribution along the axial direction of a single carbon nanotube fiber and the failure to effectively utilize internal impurities during post-processing to achieve synergistic performance enhancement. This method constructs a position-dependent conductivity gradient distribution along the axial direction of a single carbon nanotube fiber through segmented acid treatment, simultaneously achieving synergistic enhancement of the fiber's electrical properties. The method includes at least a segmented mask module (10), an acid treatment control module (20), a microstructure transformation module (30), a performance gradient evaluation module (40), and a closed-loop optimization module (50). The method includes at least the following steps: S1: Carbon nanotube fibers are applied to different regions along the axial direction through the segmented mask module (10) to form a spatial distribution pattern of at least two processing regions and a protection region; S2: The segmented masked carbon nanotube fibers are placed in the acid treatment control module (20) for acid treatment. By controlling at least one parameter of the acid treatment time, acid concentration or treatment temperature in different treatment areas, a differentiated degree of acid treatment is formed in the fiber axis. S3: During the acid treatment process, the iron-based catalyst remaining in the carbon nanotube fiber and the amorphous carbon consumed in situ are reacted in an acidic oxidation environment through the microstructure transformation module (30) to form graphite carbon shell-iron oxide core nanospheres in situ. The nanospheres are bridged between carbon nanotube bundles to form a conductive path. S4: Input the processed fiber into the performance gradient evaluation module (40), measure the conductivity at at least three different locations along the fiber axis, and calculate the conductivity gradient distribution characteristic parameters; S5: If the conductivity gradient distribution characteristic parameter does not reach the preset target value, the closed-loop optimization module (50) dynamically adjusts at least one parameter of the mask region distribution of the segmented mask module (10), the treatment time gradient, the treatment concentration gradient, or the treatment temperature gradient of the acid treatment control module (20) according to the deviation between the current characteristic parameter and the target value, and feeds back the adjusted parameter to S1 to form a closed-loop iteration until the performance gradient evaluation module (40) reaches the target; if the performance gradient evaluation module (40) reaches the target, it outputs carbon nanotube fibers with gradient conductivity distribution along the axial direction.

[0024] In this invention, a segmented mask module (10) precisely defines the treatment and protection regions along the axial direction of a single carbon nanotube fiber. For example, photoresist is used as the mask material, and the boundary accuracy of adjacent regions is controlled within ±0.5 mm by a region precision control submodule, laying the foundation for constructing a precise gradient distribution. The segmented masked fiber is then fed into an acid treatment control module (20). In this module, by controlling the acid treatment time gradient of different regions, for example, treating one end of the fiber for 2 hours, the other end for 0.5 hours, and the treatment time of the middle region changing linearly, a differentiated degree of acid treatment is formed along the fiber axial direction.

[0025] The microstructure transformation module (30) plays a crucial role in the acid treatment process. The carbon nanotube fibers prepared by floating catalytic chemical vapor deposition contain residual iron-based catalysts (iron-sulfur molar ratio of 1:2 to 1:5) and amorphous carbon. In an acidic oxidizing environment (such as a mixed solution of hydrochloric acid, sulfuric acid, and nitric acid), the iron-based catalyst is oxidized to form iron oxides, while some of the amorphous carbon is consumed in situ. The surface of the iron oxide nanoparticles generated by the reaction catalyzes the formation of a graphite carbon shell, ultimately forming graphite carbon shell-iron oxide core nanospheres with a diameter of 50–200 nm in situ. These nanospheres bridge adjacent carbon nanotube bundles, constructing additional conductive pathways, resulting in an increase in conductivity of the treated area of ​​at least 80% compared to the untreated area. Through the nanosphere distribution density control submodule, the degree of acid treatment can be precisely controlled, thereby adjusting the distribution density and size of the nanospheres along the fiber axis to achieve a gradient change in conductivity.

[0026] The processed fiber is input into the performance gradient evaluation module (40). This module uses a four-probe method to test the conductivity at at least three different locations along the fiber axis, and calculates characteristic parameters such as the rate of change of conductivity along the axis, gradient coefficient, or distribution function through the gradient feature extraction submodule. For example, it can calculate the rate of change of conductivity from one end of the fiber to the other, or fit a linear, step-like, or specific function curve of conductivity as a function of axial distance.

[0027] The closed-loop optimization module (50) is the core for ensuring process accuracy and consistency. The target gradient setting submodule allows users to preset the type of conductivity distribution curve along the axial direction, such as a linear gradient. The parameter iteration optimization submodule compares the gradient characteristic parameters obtained from the current test with the target parameters and calculates the deviation. Based on this deviation, a proportional-integral-derivative control algorithm is used to dynamically adjust one or more parameters in the segmented mask module (10), the treatment time gradient, the treatment concentration gradient, or the treatment temperature gradient in the acid treatment control module (20). The adjusted parameters are fed back to S1 to start a new round of preparation and evaluation, forming an efficient closed-loop iteration process until carbon nanotube fibers that fully meet the preset gradient requirements are output.

[0028] Optionally, the segmented mask module (10) includes at least a mask material (11) and a region precision control submodule (12); the mask material is selected from one or more of photoresist, paraffin wax, and peelable polymer; the region precision control submodule (12) is used to control the boundary precision between adjacent processing areas and protection areas, and the boundary precision is not less than ±1 mm.

[0029] In this invention, the boundary accuracy is controlled within ±1 mm to ensure that the transition region width of the gradient region is controllable. If the boundary accuracy is too low, it will lead to unexpected cross-contamination between the processing area and the protection area or an excessively wide transition region, making it impossible to achieve a precise gradient distribution. High-precision mask materials such as photoresist, combined with microfabrication technology, can achieve higher boundary accuracy, providing a guarantee for constructing complex and accurate gradient distributions.

[0030] Optionally, the carbon nanotube fibers are prepared by floating catalytic chemical vapor deposition, wherein the iron-sulfur molar ratio of the catalyst is 1:2 to 1:5; the acidic solution in the acid treatment control module (20) is one or more of hydrochloric acid, sulfuric acid, and nitric acid; the time of the acid treatment control module (20) is 0.5 to 2 hours, and the temperature is 20 to 80°C.

[0031] In this invention, controlling the iron-sulfur molar ratio during carbon nanotube fiber preparation within the range of 1:2 to 1:5 ensures that sufficient and moderately active iron-based catalyst remains within the fibers. This is fundamental for the subsequent in-situ formation of nanospheres and the construction of conductive pathways. The duration, temperature, and type of acid treatment collectively determine the intensity of the acid treatment. Too short a time or too low a temperature results in incomplete reaction and limited improvement in conductivity; too long a time or too high a temperature may excessively corrode the carbon nanotubes, impairing the fiber's mechanical properties. The preferred range is 0.5–2 hours and 20–80°C, within which effective control of the reaction process can be achieved.

[0032] It should be noted that the preferred iron-sulfur molar ratio of the catalyst is 1:3.

[0033] Optionally, the differentiated acid treatment degree is achieved by any of the following methods: (a) setting a time gradient for mask removal along the fiber axis so that different regions have different acid treatment durations; (b) setting different concentrations of acid solution for zoned treatment along the fiber axis; (c) setting different treatment temperature gradients along the fiber axis.

[0034] In this invention, these three methods can be used individually or in combination. Method (a) achieves a treatment gradient by gradually removing the mask at different time points, resulting in varying total exposure times of different regions of the fiber to the acid. Method (b) directly forms a treatment intensity gradient by applying acid solutions of different concentrations at different locations along the fiber axis using microfluidics or similar technologies. Method (c) establishes a temperature gradient along the fiber axis through localized heating or cooling, utilizing the exponential effect of temperature on the reaction rate to achieve precise gradient control. Those skilled in the art can select the appropriate method based on the type and precision requirements of the target gradient.

[0035] Optionally, the microstructure transformation module (30) includes a nanosphere formation condition monitoring submodule (31) and a nanosphere distribution density regulation submodule (32); the nanosphere formation condition monitoring submodule (31) is used to monitor the reaction process of iron-based catalyst and amorphous carbon under acidic oxidation environment; the nanosphere distribution density regulation submodule (32) adjusts the distribution density and size of the graphite carbon shell-iron oxide core nanospheres along the fiber axis by controlling the degree of acid treatment.

[0036] In this invention, the nanosphere formation condition monitoring submodule (31) can indirectly determine the reaction progress by monitoring changes in pH, redox potential, or fiber conductivity of the acid treatment solution in real time. It can issue a signal when the reaction rate slows down or is completed. The nanosphere distribution density control submodule (32) precisely adjusts the distribution density and size of the nanospheres along the fiber axis by controlling parameters such as acid treatment time, temperature, and concentration, based on a preset gradient target. For example, a higher treatment degree results in a greater nanosphere distribution density and potentially larger size, leading to a higher conductivity. Through the synergy of these two submodules, precise control from macroscopic treatment parameters to microscopic structural evolution is achieved.

[0037] Optionally, the performance gradient evaluation module (40) includes a conductivity testing submodule (41) and a gradient feature extraction submodule (42); the conductivity testing submodule (41) uses a four-probe method to test the conductivity at at least three different locations along the fiber axis; the gradient feature extraction submodule (42) is used to calculate at least one feature parameter among the rate of change of conductivity along the axial direction, gradient coefficient, or distribution function.

[0038] In this invention, the four-probe method can accurately eliminate the influence of contact resistance and is the preferred method for measuring the conductivity of one-dimensional fiber materials. At least three test locations are used to fit the gradient curve. The gradient feature extraction submodule (42) transforms discrete test point data into continuous gradient feature parameters. For example, the rate of change describes how quickly the conductivity changes with distance; the gradient coefficient can be a dimensionless value used to quantify the steepness of the gradient; and the distribution function can fully describe the distribution law of conductivity along the axial direction, providing accurate input for closed-loop optimization.

[0039] Optionally, the closed-loop optimization module (50) includes a target gradient setting submodule (51) and a parameter iteration optimization submodule (52); the target gradient setting submodule (51) is used to preset the type of conductivity distribution curve along the axial direction, the type of distribution curve includes linear gradient, step gradient or specific function curve gradient; the parameter iteration optimization submodule (52) adjusts the processing parameters based on the deviation between the current gradient feature parameters and the target gradient feature parameters using a proportional-integral-derivative control algorithm or a machine learning optimization algorithm.

[0040] In this invention, the target gradient setting submodule (51) can preset any form of conductivity distribution curve according to the requirements of downstream applications. The parameter iteration optimization submodule (52) is the core of realizing intelligent manufacturing. When a deviation occurs, the PID control algorithm can calculate the adjustment amount according to the proportional, integral and derivative values ​​of the deviation, and quickly and stably converge the system to the target value. For more complex nonlinear systems, machine learning optimization algorithms (such as Bayesian optimization, genetic algorithms, etc.) can be used to quickly explore the optimal combination of process parameters based on historical data, and achieve more efficient closed-loop optimization.

[0041] Optionally, before or after the acid treatment described in S2, a heat treatment step is further included on the carbon nanotube fibers; the heat treatment is carried out in an inert atmosphere at a temperature of 600–1000°C for 0.5–2 hours.

[0042] In this invention, the heat treatment step can serve as a supplement to or pretreatment of the acid treatment. Heat treatment prior to acid treatment can remove some of the amorphous carbon in the fibers or pre-agglomerate the iron-based catalyst, altering its reactivity. Heat treatment after acid treatment can repair surface defects on the carbon nanotubes that may have been introduced by the acid treatment, further graphitize the carbon shell, and improve the stability of the conductive pathway. An inert atmosphere (such as argon or nitrogen) can prevent the carbon nanotubes from oxidizing at high temperatures.

[0043] Optionally, the carbon nanotube fibers have a gradient of electrical conductivity along the axial direction, and the conductivity of the acid-treated region is increased by no less than 80% compared to the untreated region.

[0044] In this invention, the carbon nanotube fibers prepared by the above method exhibit significantly improved conductivity in their acid-treated regions due to the in-situ formation of a conductive bridging network composed of graphite carbon shells and iron oxide core nanospheres. An improvement of at least 80% is an effective lower limit to ensure a clear gradient and meet functional application requirements. Higher improvements (e.g., over 150%) can be achieved by optimizing the processing parameters.

[0045] Optionally, the carbon nanotube fibers form graphite carbon shell-iron oxide core nanospheres in situ within the acid-treated region. The nanospheres have a diameter of 50–200 nm and are distributed among the carbon nanotube bundles to form a conductive bridging network.

[0046] In this invention, this unique microstructure is key evidence for the synergistic enhancement of electrical conductivity achieved by the technical solution. The nanospheres, with sizes ranging from 50 to 200 nm, effectively bridge adjacent carbon nanotube bundles. Their graphite-carbon shells ensure good conductivity, while the iron oxide cores provide mechanical support and a good interfacial bond with the carbon nanotubes. The resulting three-dimensional conductive network significantly increases the effective conductive pathways within the fibers, thereby substantially improving the conductivity of the treated area.

[0047] It should be noted that, in this document, the terms "comprising," "including," or any other variations thereof are intended to cover non-exclusive inclusion, such that a process, method, article, or apparatus that comprises a list of elements includes not only those elements but also other elements not expressly listed, or elements inherent to such a process, method, article, or apparatus. Unless otherwise specified, an element defined by the phrase "comprising one..." does not exclude the presence of other identical elements in the process, method, article, or apparatus that includes that element.

[0048] Through the above description of the embodiments, those skilled in the art can clearly understand that the methods of the above embodiments can be implemented by means of software plus necessary general-purpose hardware platforms. Of course, they can also be implemented by hardware, but in many cases the former is a better implementation method. Based on this understanding, the technical solution of the present invention, or the part that contributes to the prior art, can be embodied in the form of a software product. This computer software product is stored in a storage medium (such as ROM / RAM, magnetic disk, optical disk) and includes several instructions to cause a terminal (which may be a mobile phone, computer, server, air conditioner, or network device, etc.) to execute the methods described in the various embodiments of the present invention.

[0049] The embodiments of the present invention have been described above with reference to the accompanying drawings. However, the present invention is not limited to the specific embodiments described above. The specific embodiments described above are merely illustrative and not restrictive. Those skilled in the art can make many other forms under the guidance of the present invention without departing from the spirit and scope of the claims, and all of these forms are within the protection scope of the present invention.

[0050] Example 1 This embodiment aims to verify the microstructure evolution and electrical property changes of carbon nanotube fibers (CNTFs) prepared by floating catalytic chemical vapor deposition under different iron-sulfur ratios under acid treatment, heat treatment, and a combination of both treatments, and to reveal the mechanism by which the in-situ graphite carbon shell-iron oxide core nanospheres formed during acid treatment enhance electrical conductivity.

[0051] Experimental preparation: Carbon nanotube fibers were prepared using a floating catalytic chemical vapor deposition (FCVD) method. The reactor was a three-zone fluidized bed reactor, with each zone set at 1200 °C and the injection port temperature at 500–520 °C. The total carrier gas flow rate was 1400 sccm (800 sccm for hydrogen and 600 sccm for argon). Ferrocene powder was dissolved in anhydrous ethanol, and different masses of thiophene were added to adjust the iron-sulfur molar ratio. Deionized water was also added to prepare the catalyst solution. The solution was injected into the reactor at a rate of 150 μL / min using a syringe pump. The resulting carbon nanotube aerogel was collected as fibers using a fiber drawing device. The iron-sulfur molar ratios used were 1:0, 1:1, 1:2, 1:3, 1:5, and 1:10.

[0052] Post-processing: Acid treatment: The dried carbon nanotube fibers were immersed in 10 mL of hydrochloric acid (36%-38%) and sonicated for 1 hour, then washed with deionized water and dried.

[0053] Heat treatment: The acid-treated or untreated carbon nanotube fibers are placed in a tube furnace and heat-treated at 800°C for 1 hour under a nitrogen atmosphere. Before heating, the tubes are purged with inert gas for 10 minutes to remove air.

[0054] Composite treatment: The carbon nanotube fibers are first acid-treated and then heat-treated.

[0055] Characterization methods: The fiber morphology was observed using scanning electron microscopy (SEM); the elemental distribution was characterized using energy dispersive spectroscopy (EDS); the carbon structure was analyzed using Raman spectroscopy (Raman, 532 nm excitation laser); and the conductivity at multiple different locations along the fiber axis was tested using the four-probe method.

[0056] Experimental results: (1) Effect of acid treatment on conductivity: The conductivity of all iron-sulfur ratio samples improved after acid treatment. The sample with an iron-sulfur ratio of 1:2 showed the largest decrease, with a conductivity increase of 90.8%. The sample with an iron-sulfur ratio of 1:3 showed a large number of uniformly distributed spherical particles after acid treatment. EDS analysis showed that these particles were rich in Fe, O, and C elements, forming graphite carbon shell-iron oxide core nanospheres (GC-FeOx NS). These nanospheres grew in situ between carbon nanotube bundles, acting as conductive bridging structures, compensating for the decrease in fiber density caused by the consumption of amorphous carbon, and significantly reducing the inter-tube contact resistance.

[0057] (2) Effect of heat treatment on conductivity: The effect of heat treatment on conductivity is dependent on the iron-sulfur ratio. After heat treatment, the fiber structure of the sulfur-free system (1:0) becomes significantly denser, and the conductivity increases significantly. The medium sulfur content (1:1–1:5) sample has a more compact structure, and the change in conductivity is controlled by the heat treatment temperature and time. The high sulfur content (1:10) sample has a higher original density, and the increase in conductivity after heat treatment is limited.

[0058] (3) Effect of composite treatment on conductivity: The composite process of acid treatment followed by heat treatment showed a synergistic enhancement effect under most iron-sulfur ratio conditions. The conductivity of the sample with an iron-sulfur ratio of 1:3 was further improved after composite treatment, and the microstructure showed that GC-FeOx NS had a more stable structure, increased graphitization degree, and enhanced conductive bridging effect after heat treatment.

[0059] (4) Microstructure evolution mechanism: Under acidic oxidation, the iron-based catalyst remaining in the carbon nanotube fibers reacts with the amorphous carbon consumed in situ, forming graphite carbon shell-iron oxide core nanospheres in situ. This process includes two stages: first, strong acid consumes amorphous carbon, exposing a clean carbon nanotube surface and reducing contact resistance; second, the iron-based catalyst is oxidized in the acidic environment, catalyzing the surrounding remaining amorphous carbon to undergo structural reorganization, forming core-shell structured nanospheres with conductive graphite shells, bridging the carbon nanotube bundles and forming additional conductive pathways. The iron-sulfur ratio has a decisive influence on this conversion process: when the iron-sulfur ratio is 1:3, the iron catalyst particle size is moderate, neither easily dissolved in acid nor easily over-aggregated, which is most conducive to the efficient formation of nanospheres; while excessively high or low sulfur content is not conducive to the formation of this structure.

[0060] in conclusion: This embodiment verifies the technical effectiveness of the microstructure transformation module in the gradient doping preparation method for carbon nanotube fibers described in this invention. Through the in-situ reaction of an iron-based catalyst with amorphous carbon during acid treatment, graphite carbon shell-iron oxide core nanospheres were successfully formed inside the fiber, realizing the transformation of impurities into a conductive reinforcing structure. Experimental results show that the nanosphere formation efficiency is highest when the iron-sulfur ratio is 1:3, the conductivity is significantly improved after acid treatment, and the performance is further enhanced after composite treatment. This embodiment fully demonstrates the significant progress of this invention in achieving synergistic enhancement of the electrical properties of carbon nanotube fibers.

Claims

1. A method for preparing carbon nanotube fibers with gradient doping, characterized in that, A position-dependent conductivity gradient distribution is constructed along the axial direction of a single carbon nanotube fiber through segmented acid treatment, simultaneously achieving synergistic enhancement of the fiber's electrical properties. The method includes at least a segmented mask module (10), an acid treatment control module (20), a microstructure transformation module (30), a performance gradient evaluation module (40), and a closed-loop optimization module (50). The method includes at least the following steps: S1: Carbon nanotube fibers are applied to different regions along the axial direction through the segmented mask module (10) to form a spatial distribution pattern of at least two processing regions and a protection region; S2: The segmented masked carbon nanotube fibers are placed in the acid treatment control module (20) for acid treatment. By controlling at least one parameter of the acid treatment time, acid concentration or treatment temperature in different treatment areas, a differentiated degree of acid treatment is formed in the fiber axis. S3: During the acid treatment process, the iron-based catalyst remaining in the carbon nanotube fiber and the amorphous carbon consumed in situ are reacted in an acidic oxidation environment through the microstructure transformation module (30) to form graphite carbon shell-iron oxide core nanospheres in situ. The nanospheres are bridged between carbon nanotube bundles to form a conductive path. S4: Input the processed fiber into the performance gradient evaluation module (40), measure the conductivity at at least three different locations along the fiber axis, and calculate the conductivity gradient distribution characteristic parameters; S5: If the conductivity gradient distribution characteristic parameter does not reach the preset target value, the closed-loop optimization module (50) dynamically adjusts at least one parameter of the mask region distribution of the segmented mask module (10), the treatment time gradient, the treatment concentration gradient, or the treatment temperature gradient of the acid treatment control module (20) according to the deviation between the current characteristic parameter and the target value, and feeds back the adjusted parameter to S1 to form a closed-loop iteration until the performance gradient evaluation module (40) reaches the target; if the performance gradient evaluation module (40) reaches the target, it outputs carbon nanotube fibers with gradient conductivity distribution along the axial direction.

2. The method for preparing carbon nanotube fibers with gradient doping according to claim 1, characterized in that, The segmented mask module (10) includes at least a mask material (11) and a region precision control submodule (12); the mask material is selected from one or more of photoresist, paraffin wax, and peelable polymer; the region precision control submodule (12) is used to control the boundary precision between adjacent processing areas and protection areas, and the boundary precision is not less than ±1 mm.

3. The method for preparing carbon nanotube fibers with gradient doping according to claim 1, characterized in that, The carbon nanotube fibers are prepared by floating catalytic chemical vapor deposition, wherein the iron-sulfur molar ratio of the catalyst is 1:2 to 1:5; the acid solution in the acid treatment control module (20) is one or more of hydrochloric acid, sulfuric acid, and nitric acid; the time of the acid treatment control module (20) is 0.5 to 2 hours, and the temperature is 20 to 80°C.

4. The method for preparing carbon nanotube fibers with gradient doping according to claim 1, characterized in that, The differentiated acid treatment degree is achieved by any of the following methods: (a) setting a time gradient for mask removal along the fiber axis so that different regions have different acid treatment durations; (b) setting different concentrations of acid solution for zoned treatment along the fiber axis; (c) setting different treatment temperature gradients along the fiber axis.

5. The method for preparing carbon nanotube fibers with gradient doping according to claim 1, characterized in that, The microstructure transformation module (30) includes a nanosphere formation condition monitoring submodule (31) and a nanosphere distribution density regulation submodule (32). The nanosphere formation condition monitoring submodule (31) is used to monitor the reaction process of iron-based catalyst and amorphous carbon under acidic oxidation environment. The nanosphere distribution density regulation submodule (32) adjusts the distribution density and size of the graphite carbon shell-iron oxide core nanospheres along the fiber axis by controlling the degree of acid treatment.

6. The method for preparing carbon nanotube fibers with gradient doping according to claim 1, characterized in that, The performance gradient evaluation module (40) includes a conductivity testing submodule (41) and a gradient feature extraction submodule (42); the conductivity testing submodule (41) uses a four-probe method to test the conductivity at at least three different locations along the fiber axis; the gradient feature extraction submodule (42) is used to calculate at least one feature parameter among the rate of change of conductivity along the axial direction, gradient coefficient, or distribution function.

7. The method for preparing carbon nanotube fibers with gradient doping according to claim 1, characterized in that, The closed-loop optimization module (50) includes a target gradient setting submodule (51) and a parameter iteration optimization submodule (52). The target gradient setting submodule (51) is used to preset the type of conductivity distribution curve along the axial direction. The distribution curve type includes linear gradient, step gradient or specific function curve gradient. The parameter iteration optimization submodule (52) adjusts the processing parameters based on the deviation between the current gradient feature parameters and the target gradient feature parameters using a proportional-integral-derivative control algorithm or a machine learning optimization algorithm.

8. The method for preparing carbon nanotube fibers with gradient doping according to claim 1, characterized in that, Before or after the acid treatment described in S2, the process further includes a heat treatment step on the carbon nanotube fibers; the heat treatment is carried out in an inert atmosphere at a temperature of 600–1000°C for 0.5–2 hours.

9. The method for preparing carbon nanotube fibers with gradient doping according to claim 1, characterized in that, The carbon nanotube fibers have a gradient of electrical conductivity along the axial direction, and the conductivity of the acid-treated region is increased by no less than 80% compared with the untreated region.

10. The method for preparing carbon nanotube fibers with gradient doping according to claim 9, characterized in that, The carbon nanotube fibers form graphite carbon shell-iron oxide core nanospheres in situ within the acid-treated region. The nanospheres have a diameter of 50–200 nm and are distributed among the carbon nanotube bundles to form a conductive bridging network.

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