Method for preparing carbon nanofiber membrane through plasma-assisted temperature programming

Abstract

The invention discloses a preparation method of a carbon nanofiber membrane based on a synergistic effect of plasma pretreatment and three-temperature-zone programmed heating, and relates to the technical field of preparation of high-performance carbon fiber materials. The method takes a polyacrylonitrile (PAN)-based nanofiber non-woven fabric prepared by electrostatic spinning as a precursor, and sequentially comprises the following steps: firstly, carrying out surface activation and cleaning treatment on the precursor by adopting oxygen, air or Ar / O2 mixed gas low-temperature plasma; then three-stage programmed heating pre-oxidation treatment is carried out in the air atmosphere, the target temperatures of the three stages are 220-240 DEG C, 245-265 DEG C and 280-300 DEG C respectively, and gradual uniform cyclization of PAN molecules is achieved by controlling the heating rate and the heat preservation time of each stage; and finally, carrying out two-stage heating carbonization treatment in an inert atmosphere to obtain a final product. According to the method, through the synergistic effect of plasma pretreatment and multi-stage precise temperature control, surface impurities are effectively removed, active sites are introduced, the problems of non-uniform initial pre-oxidation reaction and'skin-core effect 'are solved, the process time is remarkably shortened, the tensile strength of the obtained carbon nanofiber membrane is larger than or equal to 185 MPa, the performance consistency is good, and the method is suitable for continuous large-scale production.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of high-performance carbon fiber material preparation, and particularly relates to a nanocarbon fiber film preparation method based on the synergistic effect of plasma pretreatment and three-temperature zone programmed heating. BACKGROUND

[0002] Polyacrylonitrile (PAN) based carbon fiber is a kind of high-performance fiber material with excellent mechanical properties, thermal stability and chemical stability, which plays an irreplaceable role in the fields of aerospace, energy equipment and new functional materials. With the rapid development of fiber preparation technologies such as electrospinning, PAN-based nanofiber membranes have become a research hotspot due to their unique fiber structure (fine fiber diameter, high porosity, and large specific surface area), and have shown broad prospects in applications such as filtration and separation, energy storage device electrodes, electromagnetic shielding, flexible sensors, and composite reinforcement.

[0003] Such materials are usually prepared by electrospinning PAN-based nanofiber non-woven fabric as a precursor, which is converted through pre-oxidation and carbonization treatment. However, compared with traditional PAN-based carbon fiber tows, non-woven fabrics face more severe technical challenges during heat treatment, and the existing process has the following outstanding problems:

[0004] Poor pre-oxidation reaction uniformity: due to the significant increase in the specific surface area of nanofibers, the heat and mass transfer behavior during pre-oxidation is more complex. The traditional single heating mode temperature control strategy is difficult to match the reaction kinetics of the gradual cyclization of PAN molecular chains, which easily leads to excessive oxidation of the fiber surface and insufficient cyclization of the interior, resulting in a significant "skin-core effect". This not only reduces the mechanical properties of the final carbon fiber, but also leads to poor batch consistency of product performance.

[0005] Insufficient structural morphology stability: nanofiber membranes are prone to overall shrinkage, curling, or even local fusion or collapse during heat treatment. This is due to the lack of continuous axial constraint in the form of non-woven fabric and the insufficient strength of the fiber connection points. In the existing technology, directly using the tensioning method for filaments easily leads to tearing or stress concentration of nanofiber membranes, and lack of tension control cannot inhibit thermal shrinkage, making it difficult to balance the form integrity and performance optimization.

[0006] Residual solvents and surface impurities affect the reaction process: PAN nanofiber membranes obtained by electrospinning often contain a certain amount of organic solvents (such as DMF, DMSO, etc.) or adsorbed environmental impurities. These substances will volatilize or decompose at an early stage of pre-oxidation temperature rise, which may cause local micro-zone reaction out of control, produce bubbles or defects, thereby increasing the structural defect density in the final carbon fiber membrane.

[0007] High energy consumption and low efficiency: To avoid excessive heat release, the existing pre-oxidation process often uses slow heating or long time holding strategy, with total processing time up to several hours, which not only leads to low production efficiency, but also causes high energy consumption, making it difficult to meet the economic requirements of large-scale production.

[0008] Performance improvement faces bottleneck: At present, most of the nanocarbon fiber membranes prepared by electrospinning combined with heat treatment have a tensile strength of 150-180 MPa. Although some studies can improve the strength to more than 180 MPa by optimizing the precursor or process parameters, it is still difficult to achieve high-performance products with stable strength exceeding 185 MPa, which restricts its application in high-strength demand fields.

[0009] It is difficult to ensure consistency during industrialization amplification: The excellent performance obtained in small-scale laboratory preparation is often difficult to reproduce when scaled up to continuous production. The core difficulty lies in the difficulty of accurately ensuring the consistency of the reaction conditions, the uniformity of the temperature field distribution, and the stability of the tension control during the pre-oxidation starting stage in the amplification process.

[0010] In view of the above problems, existing research attempts to improve the precursor formula, introduce additives or optimize the heat treatment process, but most of them fail to systematically solve the contradiction between "reaction uniformity", "morphology stability" and "process efficiency". Therefore, it is of great significance to develop a new high-efficiency heat treatment method that can coordinate and control the pre-oxidation reaction process, inhibit structural defects, and be suitable for the morphology of nanofiber membranes, for promoting the preparation and application of high-performance nanocarbon fiber membranes. SUMMARY

[0011] To solve the problems of uneven reaction initiation, significant difference between skin and core structure, residual solvents and surface contamination, long pre-oxidation time and high energy consumption, heat release control difficulty and poor consistency during industrialization amplification of PAN-based nanofiber membranes in the prior art, the present application proposes a nanocarbon fiber membrane preparation method based on plasma pretreatment and three-temperature zone programmed temperature control.

[0012] The method effectively removes residual solvents and impurities and introduces oxygen-containing active functional groups by plasma surface activation and cleaning treatment of the PAN nanofiber non-woven fabric prepared by electrospinning before pre-oxidation, and then combines a multi-stage programmed temperature pre-oxidation process to realize the synergistic regulation of surface chemical modification and bulk reaction kinetics, significantly improve the pre-oxidation uniformity, inhibit the formation of structural defects, shorten the process time, and finally obtain a nanocarbon fiber membrane with excellent mechanical properties and high batch consistency.

[0013] One of the purposes of the present application is to provide a nanocarbon fiber membrane preparation method combining plasma pretreatment and three-temperature zone programmed temperature pre-oxidation.

[0014] The nanocarbon fiber membrane preparation method provided by the present application comprises the following steps:

[0015] (a) precursor preparation

[0016] An electrospun PAN-based nanofiber non-woven fabric is provided. The fiber diameter ranges from 80 to 500 nm, preferably from 100 to 300 nm. The film thickness is 10-200 μm, preferably 20-100 μm.

[0017] (b) plasma pretreatment of the non-woven fabric

[0018] The plasma pretreatment uses low-temperature plasma technology, the treatment gas is oxygen, air or Ar / O2 mixed gas, preferably oxygen; and / or the power of the plasma treatment is 50-200 W, preferably 80-150 W, more preferably 100-120 W; and / or the time of the plasma treatment is 30-300 s, preferably 60-180 s, more preferably 90-120 s.

[0019] (c) pre-oxidation

[0020] The multi-stage programmed temperature pre-oxidation process comprises:

[0021] First stage: heating to 220-240℃, preferably 225-235℃, at a heating rate of 1-6℃ / min, preferably 2-4℃ / min, holding for 10-40min, preferably 15-25min; and / or,

[0022] Second stage: heating to 245-265℃, preferably 250-255℃, at a heating rate of 1-5℃ / min, preferably 2-3℃ / min, holding for 20-60min, preferably 30-40min; and / or,

[0023] Third stage: temperature is raised to 280-300°C, preferably 285-290°C, at a rate of 0.5-3°C / min, preferably 1-2°C / min, and is kept for 30-120 min, preferably 60-90 min;

[0024] The total time of the pre-oxidation treatment is 90-180 min, preferably 120-150 min.

[0025] (d) the pre-oxidation treated film is subjected to a two-stage temperature raising carbonization treatment under inert atmosphere, first stage: temperature is raised to 600-800°C at a rate of 5-8°C / min, and is kept for 30-60 min; and / or,

[0026] Second stage: temperature is raised to 1000-1300°C at a rate of 3-5°C / min, and is kept for 30-120 min.

[0027] (e) cooling and product forming

[0028] The second object of the present application is to provide a nanocarbon fiber film prepared by the above method, which has a tensile strength ≥185 MPa, reaches 185-202 MPa, a fiber diameter of 100-300 nm, a diameter uniformity (in terms of coefficient of variation) ≤10%, a tensile strength variation coefficient ≤5%, and a carbon yield of 35-50%.

[0029] Compared with the prior art, the present application has the following significant differences and advantages:

[0030] Firstly, the deep synergy of plasma surface activation and programmed temperature kinetics is realized: active sites are introduced at the molecular level through plasma pretreatment, combined with three-stage precise temperature control, which fundamentally solves the technical problems of uneven pre-oxidation reaction initiation and skin-core structure difference;

[0031] Performance improvement breakthrough: the tensile strength of the obtained nanocarbon fiber film reaches 185-202 MPa, which is more than 80% higher than that of the traditional method (90-120 MPa), and the tensile strength variation coefficient is ≤5%, realizing the unity of high strength and high consistency;

[0032] Process efficiency is significantly improved: the total pre-oxidation time is controlled to be 90-180 min, preferably 120-150 min, which is shortened by about 40% compared with the traditional process, and the energy consumption is reduced by more than 35%, while avoiding fiber damage caused by excessive heat release;

[0033] Suitability and reliability are enhanced: the process is specially designed for nanofiber film morphology, and the plasma treatment is mild and uniform, avoiding the problem of fiber strength degradation caused by chemical treatment, and is more suitable for industrialized production. DETAILED DESCRIPTION

[0034] The application will be further described in conjunction with specific examples. It is necessary to point out here that the following examples are only used to illustrate the technical solutions of the application and cannot be understood as a limitation on the protection scope of the application. Non-essential improvements and adjustments to the application made by those skilled in the art based on the content of the application still belong to the protection scope of the application.

[0035] The experimental methods used in the following examples and comparative examples are conventional methods in the art unless otherwise specified; the reagents, materials, etc. used are commercially available unless otherwise specified.

[0036] Example 1

[0037] 10 g of polyacrylonitrile (PAN, Mw = 150,000) was weighed into 90 g of DMF and magnetically stirred for 12 h to obtain a 10 wt% uniform spinning solution. An PAN nanofiber membrane with a thickness of about 60 μm and a fiber diameter of about 280 nm was prepared by electrospinning technology under the conditions of a liquid supply rate of 1.0 mL / h, a voltage of 20 kV, a receiving distance of 15 cm, a temperature of 25 °C, and a humidity of 40%.

[0038] The obtained fiber membrane was subjected to oxygen plasma pretreatment at a power of 100 W for 90 s. Subsequently, the sample was placed in a pre-oxidation furnace and subjected to a three-stage programmed temperature treatment in an air atmosphere:

[0039] First stage: heating to 230 °C at a rate of 3 °C / min and maintaining for 20 min;

[0040] Second stage: heating to 250 °C at a rate of 2 °C / min and maintaining for 35 min;

[0041] Third stage: heating to 285 °C at a rate of 1.5 °C / min and maintaining for 75 min.

[0042] After the pre-oxidation was completed, the sample was transferred to a carbonization furnace and subjected to two-stage carbonization in a nitrogen atmosphere: first stage, heating to 800 °C at a rate of 5 °C / min and maintaining for 60 min; second stage, heating to 1200 °C at a rate of 5 °C / min and maintaining for 90 min. After cooling in the furnace, a nanocarbon fiber membrane was obtained.

[0043] Performance test results: fiber diameter 155 nm, diameter uniformity (in terms of coefficient of variation) 8%, carbon yield 46%, tensile strength 188 MPa measured by GB / T 24218.3 test method. The membrane body is flat, without defects such as curling and fusion.

[0044] Example 2

[0045] The precursor preparation method is the same as in Example 1.

[0046] The plasma pretreatment condition was adjusted as follows: power 120 W, treatment time 120 s, and treatment gas air.

[0047] The three-stage programmed temperature raising condition of pre-oxidation was adjusted as follows:

[0048] First stage: raising temperature to 235°C at a rate of 2°C / min and maintaining for 25 min;

[0049] Second stage: raising temperature to 255°C at a rate of 2.5°C / min and maintaining for 40 min;

[0050] Third stage: raising temperature to 290°C at a rate of 1°C / min and maintaining for 80 min.

[0051] The carbonization condition was the same as in Example 1.

[0052] The performance test results were as follows: fiber diameter 150 nm, diameter uniformity (in terms of coefficient of variation) 7%, carbon yield 48%, and tensile strength 202 MPa measured by using the test method of GB / T 24218.3. It was shown that moderate plasma condition strengthening and temperature zone adjustment could further improve the performance.

[0053]

Example 3

[0054] The precursor preparation method was the same as in Example 1.

[0055] The plasma pretreatment condition was as follows: power 80 W, treatment time 180 s, and treatment gas Ar / O2 mixed gas (volume ratio 1:1).

[0056] The three-stage programmed temperature raising condition of pre-oxidation was as follows:

[0057] First stage: raising temperature to 225°C at a rate of 4°C / min and maintaining for 15 min;

[0058] Second stage: raising temperature to 260°C at a rate of 3°C / min and maintaining for 30 min;

[0059] Third stage: raising temperature to 280°C at a rate of 2°C / min and maintaining for 60 min.

[0060] The carbonization condition was the same as in Example 1.

[0061] The performance test results were as follows: fiber diameter 165 nm, diameter uniformity (in terms of coefficient of variation) 9%, carbon yield 42%, and tensile strength 182 MPa measured by using the test method of GB / T 24218.3. It was shown that under the relatively mild plasma condition and the faster temperature raising rate, carbon fiber membranes with excellent performance could still be obtained.

[0062]

Example 4

[0063] The precursor preparation method was the same as in Example 1.

[0064] The obtained fiber membrane was subjected to atmospheric air plasma pretreatment: power 150 W, treatment time 180 s.

[0065] Subsequently, the sample was placed in a pre-oxidation furnace and subjected to a three-stage programmed temperature treatment under an air atmosphere:

[0066] First stage: temperature rise to 230°C at 3°C / min, holding for 20 min;

[0067] Second stage: temperature rise to 250°C at 2°C / min, holding for 35 min;

[0068] Third stage: temperature rise to 285°C at 1.5°C / min, holding for 75 min.

[0069] After the pre-oxidation was completed, the sample was transferred to a carbonization furnace and subjected to two-stage carbonization under a nitrogen atmosphere: first stage temperature rise to 800°C at 5°C / min, holding for 60 min; second stage temperature rise to 1200°C at 5°C / min, holding for 90 min. After furnace cooling, a nanocarbon fiber membrane was obtained.

[0070] Performance test results: fiber diameter 158 nm, diameter uniformity (in terms of coefficient of variation) 9%, carbon yield 44%, tensile strength 185 MPa measured using GB / T 24218.3 test method. The membrane body was flat, without defects such as curling and fusion. The results show that atmospheric plasma treatment is also effective, expanding the process applicability.

[0071]

Example 5

[0072] The precursor preparation method was the same as in Example 1.

[0073] The obtained fiber membrane was subjected to oxygen plasma pretreatment: power 100 W, treatment time 90 s.

[0074] Subsequently, the sample was placed in a pre-oxidation furnace and subjected to a three-stage programmed temperature treatment under an air atmosphere:

[0075] First stage: temperature rise to 230°C at 3°C / min, holding for 20 min;

[0076] Second stage: temperature rise to 250°C at 2°C / min, holding for 35 min;

[0077] Third stage: temperature rise to 285°C at 1.5°C / min, holding for 75 min.

[0078] After the pre-oxidation, the sample was transferred to a carbonization furnace and subjected to two-stage carbonization under nitrogen atmosphere: the first stage was to increase the temperature to 800℃ at a rate of 5℃ / min and keep it for 60 min; the second stage was to increase the temperature to 1200℃ at a rate of 5℃ / min and keep it for 90 min. After cooling down with the furnace, a nanometer carbon fiber film was obtained.

[0079] Performance test results: fiber diameter 154 nm, diameter uniformity (in terms of coefficient of variation) 8%, carbon yield 45.5%, tensile strength 186 MPa measured by GB / T 24218.3 test method. The film body is flat and has no defects. Independent repeated experiments of three batches, the performance data deviation is less than 5%, indicating that the process of the application has excellent reproducibility and product consistency.

[0080]

Comparative Example 1

[0081] The precursor preparation method is the same as that of Example 1.

[0082] The plasma pretreatment step was omitted and the pre-oxidation was directly performed. The pre-oxidation three-stage temperature increase conditions were exactly the same as those of Example 1.

[0083] The carbonization conditions were the same as those of Example 1.

[0084] Performance test results: fiber diameter 175 nm, diameter uniformity (in terms of coefficient of variation) 15%, carbon yield 38%, tensile strength 125 MPa measured by GB / T 24218.3 test method. There are a small amount of micropores in the local film body. The results show that the lack of plasma pretreatment leads to the volatilization of residual solvents or impurities during the temperature increase process to produce defects, and the surface reactivity is insufficient, which leads to the decrease of pre-oxidation uniformity, and the performance is significantly worse than that of Example 1.

[0085]

Comparative Example 2

[0086] The precursor preparation method is the same as that of Example 1.

[0087] The plasma pretreatment conditions were exactly the same as those of Example 1.

[0088] Instead of using three-temperature-zone programmed temperature increase, the pre-oxidation process was to increase the temperature from room temperature to 285℃ at a single rate of 2℃ / min, and keep it at 285℃ for 130 min (the total time is close to that of Example 1).

[0089] The carbonization conditions were the same as those of Example 1.

[0090] Performance test results: the fiber diameter distribution is extremely uneven (120-350 nm), the diameter uniformity is 22%, the carbon yield is 35%, the tensile strength is 105 MPa measured by using the test method of GB / T 24218.3. The film body has obvious "skin-core structure" difference. The results show that the single heating mode cannot match the reaction kinetics of the gradual cyclization of PAN, leading to uneven oxidation inside and outside, and although the plasma pretreatment is used, the structural defects cannot be overcome.

[0091] Comparative Example 3

[0092] The precursor preparation method is the same as that in Example 1.

[0093] The plasma pretreatment step is omitted, and the pre-oxidation adopts the traditional single-temperature zone programmed heating process in Comparative Example 2.

[0094] The carbonization conditions are the same as those in Example 1.

[0095] Performance test results: the obtained carbon fiber film has poor quality, the fibers are seriously fused, the structure is collapsed, the carbon yield is only 30%, the tensile strength is less than 50 MPa, the structure is seriously damaged, and the mechanical properties cannot be effectively tested. This comparison proves the necessity of the synergistic effect of plasma pretreatment and three-temperature zone programmed heating, and one cannot be dispensed with the other.

[0096] Comparative Example 4

[0097] The precursor preparation method is the same as that in Example 1.

[0098] The obtained fiber film is subjected to oxygen plasma pretreatment: power 100 W, treatment time 90 s.

[0099] Subsequently, the sample is placed in a pre-oxidation furnace and subjected to three-stage programmed heating treatment in an air atmosphere:

[0100] First stage: heating to 230°C at 3°C / min, and keeping for 20 min;

[0101] Second stage: heating to 250°C at 2°C / min, and keeping for 35 min;

[0102] Third stage: heating to 285°C at 1.5°C / min, and keeping for 75 min.

[0103] After the pre-oxidation is completed, the sample is transferred to a carbonization furnace and subjected to carbonization treatment in a nitrogen atmosphere: heating directly to 1200°C at a heating rate of 5°C / min, and keeping for 150 min. After cooling in the furnace, a nanometer carbon fiber film is obtained.

[0104] Performance test results: fiber diameter 160 nm, diameter uniformity (in terms of coefficient of variation) 10%, carbon yield 43%, tensile strength 175 MPa measured by GB / T 24218.3 test method. The mechanical properties of the film body are lower than those of Example 1. The results show that single heating rate carbonization is not conducive to the ordered formation of the microcrystalline structure compared to two-stage programmed heating carbonization, resulting in a decrease in the mechanical properties of the final product, proving the necessity of the two-stage carbonization process.

Claims

1. A method for preparing carbon nanofiber membranes based on the synergistic effect of plasma pretreatment and three-zone programmed temperature rise, characterized in that, The method includes the following steps: (a) Provides a polyacrylonitrile-based nanofiber nonwoven fabric prepared by electrospinning; (b) Plasma pretreatment of the nonwoven fabric; (c) Perform a three-stage programmed temperature rise pre-oxidation treatment on the nonwoven fabric after plasma pretreatment; (d) The pre-oxidized film is carbonized under an inert atmosphere; (e) Cooling to obtain the carbon nanofiber membrane.

2. The method according to claim 1, characterized in that: In step (a), the fiber diameter of the polyacrylonitrile-based nanofiber nonwoven fabric is 80-500 nm, preferably 100-300 nm; and / or, The thickness of the polyacrylonitrile-based nanofiber nonwoven fabric is 10-200 μm, preferably 20-100 μm.

3. The method according to claim 1, characterized in that: In step (b), the plasma pretreatment employs low-temperature plasma technology; and / or, the gas used for plasma pretreatment is oxygen, air, or an Ar / O2 mixture, preferably oxygen; and / or, the power of the plasma treatment is 50-200W, preferably 80-150W, more preferably 100-120W; and / or, the time of the plasma treatment is 30-300s, preferably 60-180s, more preferably 90-120s.

4. The method according to claim 1, characterized in that: In step (c), the three-stage programmed temperature pre-oxidation treatment includes: First stage: Heat to 220-240℃, preferably 225-235℃, at a heating rate of 1-6℃ / min, preferably 2-4℃ / min, and hold for 10-40 min, preferably 15-25 min; and / or, Second stage: Heat to 245-265℃, preferably 250-255℃, at a heating rate of 1-5℃ / min, preferably 2-3℃ / min, and hold for 20-60 min, preferably 30-40 min; and / or, The third stage: heat up to 280-300℃, preferably 285-290℃, at a heating rate of 0.5-3℃ / min, preferably 1-2℃ / min, and hold for 30-120min, preferably 60-90min.

5. The method according to claim 4, characterized in that: The total time for the pre-oxidation treatment is 90-180 min, preferably 120-150 min.

6. The method according to claim 1, characterized in that: In step (d), the carbonization process is a two-stage heating carbonization process; and / or, the carbonization process is carried out under a nitrogen or argon atmosphere; and / or, the carbonization process includes: First stage: Increase the temperature to 600-800℃ at a heating rate of 5-8℃ / min, and hold for 30-60 min; and / or, Second stage: Heat to 1000-1300℃ at a heating rate of 3-5℃ / min, and hold for 30-120min.

7. A carbon nanofiber membrane prepared by the method of any one of claims 1 to 6.

8. The carbon nanofiber membrane according to claim 7, characterized in that: The tensile strength of the carbon nanofiber membrane is ≥185MPa; and / or the fiber diameter of the carbon nanofiber membrane is 100-300nm; and / or the diameter uniformity of the carbon nanofiber membrane is ≤10%; and / or the coefficient of variation of the tensile strength of the carbon nanofiber membrane is ≤5%; and / or the carbon yield of the carbon nanofiber membrane is 35-50%.

9. The application of the carbon nanofiber membrane according to claim 7 or 8 in filtration and separation, electromagnetic shielding of electrodes for energy storage devices, flexible sensors, or composite material reinforcements.