Method for preparing continuous p-n type aramid flame-retardant thermoelectric fiber based on gel state impregnation method

Abstract

This application provides a method for preparing continuous P-N type aramid flame-retardant thermoelectric fibers based on a gel-state impregnation method, belonging to the field of composite fibers. The preparation method includes: preparing gel-state aramid fibers through wet spinning; uniformly and firmly composited carboxylated carbon nanotubes onto the aramid fibers using a gel-state impregnation method; optimizing the conductive network by removing impurities with ethanol; subsequently, using a zoned selective impregnation process to dope the composite fibers with ferric chloride and oleylamine respectively to achieve P-type and N-type doping, realizing the continuous preparation of integrated P-N homojunctions on the same fiber. The aramid / carbon nanotube thermoelectric fibers prepared in this application possess excellent flame-retardant properties, high-temperature resistance, electrical conductivity, and thermoelectric properties, and achieve a continuous P-N structure on a single fiber, solving the problems of traditional thermoelectric fibers requiring physical splicing and having high contact resistance. It has broad application prospects in fields such as intelligent flame-retardant textiles and flexible wearable devices.

Description

Technical Field

[0001] This invention relates to the field of composite fiber technology, and specifically to a method for preparing continuous PN-type aramid flame-retardant thermoelectric fibers based on a gel impregnation method. Background Technology

[0002] Aramid fibers possess outstanding advantages such as ultra-high strength, excellent thermal stability, chemical corrosion resistance, and a high limiting oxygen index, making them a core substrate for high-end flame-retardant protective textiles and high-temperature resistant special industrial fabrics. However, pure aramid is an intrinsically insulating material with extremely poor electrical conductivity and lacks thermoelectric conversion properties, making it unsuitable for emerging smart textile fields such as flexible thermoelectric power generation, passive temperature sensing, and intelligent energy harvesting.

[0003] Carbon nanotubes (CNTs) possess ultra-high electrical conductivity, excellent carrier mobility, and controllable semiconductor conductivity, making them ideal functional fillers for fabricating flexible thermoelectric fibers. Combining carbon nanotubes with aramid fibers can simultaneously endow aramid fibers with multiple functions, including electrical conductivity, thermal conductivity, thermoelectricity, and flame retardancy.

[0004] Existing processes for preparing aramid / carbon nanotube composite fibers mainly include solution blending and surface coating of finished fibers. The blending method easily leads to severe agglomeration of carbon nanotubes in the spinning solution, disrupting the regular stacking structure of the aramid molecular chains, resulting in significant deterioration of the fiber's mechanical properties and poor conductivity. The surface coating method for dried finished fibers only allows carbon nanotubes to adhere to the fiber surface, resulting in weak interfacial bonding, easy detachment due to friction and bending, and poor modification durability. Furthermore, existing carbon nanotube-based thermoelectric fibers generally suffer from drawbacks such as limited doping methods, the need for splicing and assembly of PN heterojunctions, high interfacial contact resistance, discontinuous structures, and poor mechanical flexibility. Most PN thermoelectric fibers require physically stitching and binding separately prepared P-type fibers with N-type fibers, making it impossible to achieve a continuous, integrated PN structure from a single fiber, severely limiting flexible weaving and the integration of micro-thermoelectric devices.

[0005] In addition, existing thermoelectric fibers rarely combine flame retardancy and high temperature resistance, limiting their application in high-temperature industrial applications, fire protection, and extreme environment sensing scenarios.

[0006] Therefore, developing an integrated PN-type aramid / carbon nanotube thermoelectric fiber preparation method that is simple in process, has a continuous structure, controllable doping, and excellent flame retardant and heat-resistant properties has important scientific research value and engineering application prospects. Summary of the Invention

[0007] In view of the technical problems existing in the background art, this application provides a method for preparing continuous PN type aramid flame-retardant thermoelectric fibers based on gel impregnation method, which aims to solve the technical problems in the prior art, such as severe agglomeration of aramid / carbon nanotube composite fibers, weak bonding force, low conductivity, limited improvement of flame retardant performance, and the inability of thermoelectric fibers to achieve a single continuous PN homogeneous structure, single doping control, complex assembly, and high contact resistance.

[0008] This application leverages the rich active amide groups on the surface of gel-state aramid fibers and their loose and porous interior to achieve uniform and robust composite of carbon nanotubes. Mild ethanol cleaning is used to remove dispersants and optimize the conductive pathway. P-type doping is achieved using ferric chloride and N-type doping using oleylamine. A partitioned selective immersion process is used to continuously prepare integrated PN homojunctions on the same fiber without external splicing. At the same time, the excellent mechanical properties and ultra-high residual carbon flame retardant characteristics of aramid fibers are retained, achieving a multi-functional integrated system of flame retardancy, high temperature resistance, conductivity, and thermoelectricity.

[0009] This application provides a method for preparing continuous PN-type aramid flame-retardant thermoelectric fibers based on a gel impregnation method, comprising the following steps: S1, Aramid is dissolved in dimethyl sulfoxide to obtain aramid spinning solution; S2, the aramid spinning solution is extruded into a coagulation bath through a wet spinning process to obtain undried gel-state aramid fibers; S3, the undried gel-state aramid fiber is immersed in a carboxylated carbon nanotube aqueous dispersion; then, the immersed aramid / carbon nanotube composite fiber is soaked and washed in anhydrous ethanol and dried to obtain aramid / carbon nanotube composite fiber. S4, the aramid / carbon nanotube composite fiber is wound onto the surface of a plastic plate; using a layered liquid surface immersion method, the lower half of the plastic plate is immersed in a ferric chloride aqueous solution, dried and cleaned to form a P-type doped section; subsequently, the upper half of the plastic plate is immersed in an oleylamine solution to form an N-type doped section, dried, to obtain a single internally continuous, non-physically spliced, homogeneous PN-type aramid / carbon nanotube composite thermoelectric fiber.

[0010] Further, in step S4, the concentration of the ferric chloride aqueous solution is 1.2 ~ 2 mol / L.

[0011] Furthermore, in step S4, the mass concentration of the oleylamine solution is 5-10%.

[0012] Furthermore, in step S3, the mass concentration of the carboxylated carbon nanotube aqueous dispersion is 1%~10%, and the impregnation time is 0.5~3 hours.

[0013] Furthermore, in step S1, the mass concentration of the aramid spinning solution is 0.5% to 2%.

[0014] Furthermore, in step S4, the soaking time in the ferric chloride aqueous solution is 1 to 3 hours.

[0015] Furthermore, in step S4, the soaking time in the oleylamine solution is 1 to 3 hours.

[0016] Furthermore, in step S2, during wet spinning, the spinning speed is 1 ~ 10 mL / h, and the inner diameter of the spinning needle is 0.5 ~ 1 mm.

[0017] Furthermore, in step S2, the coagulation bath is deionized water, and the coagulation bath temperature is 20 ~ 30℃.

[0018] The beneficial effects of this application are as follows: (1) This application adopts a "gel-state impregnation and penetration" strategy. When the aramid fiber is in a gel state (not yet dried), it is impregnated in an aqueous dispersion of carboxylated carbon nanotubes. The surface of the gel-state aramid is rich in active amide groups and the interior is loose and porous. The electrostatic attraction and hydrogen bonding between the carboxylated carbon nanotubes and the protonated amide groups can better combine with the aramid fiber. The carbon residue of the resulting composite fiber is increased from 44.3% of pure aramid to 76.6%. It has excellent vertical combustion self-extinguishing properties and has multiple functions such as high temperature resistance, flame retardancy, electrical conductivity, and thermoelectricity.

[0019] (2) In this application, ferric chloride inorganic electron acceptor is used to realize hole-type P doping of carbon nanotubes; oleylamine organic electron donor is used to realize electron-type N doping. The doping method is simple, low cost, and has good doping uniformity, and the conductivity type of the fiber can be precisely controlled.

[0020] (3) This application uses a fixed plate winding + half-area liquid immersion method to accurately divide the P-doped region and N-doped region on the same fiber. The two regions have clear boundaries and no cross-contamination, forming an integrated continuous PN thermoelectric structure; solving the industry pain points of large contact resistance, loose structure and non-weaving caused by traditional physical splicing of P and N fibers.

[0021] (4) In this application, ethanol is used to simply soak the carbon nanotubes to gently remove the insulating dispersant on the surface, effectively opening up the conductive channels between the carbon nanotubes and significantly improving the conductivity of the fiber.

[0022] (5) The method of this application is free from high temperature and high pressure and high-risk reagents. It adopts wet spinning, normal pressure drying, normal temperature doping and water washing post-treatment. All processes are compatible with existing textile processing equipment and can continuously produce flexible thermoelectric fibers in batches. It is suitable for large-scale intelligent flame-retardant textile fabric industrial production.

[0023] The above description is only an overview of the technical solution of this application. In order to better understand the technical means of this application and to implement it in accordance with the contents of the specification, and to make the above and other objects, features and advantages of this application more obvious and understandable, the following are specific embodiments of this application. Attached Figure Description

[0024] To more clearly illustrate the technical solutions of this application, the accompanying drawings used in this application will be briefly described below. Obviously, the drawings described below are merely some embodiments of this application. For those skilled in the art, other drawings can be obtained from these drawings without any creative effort.

[0025] Figure 1 This is a schematic diagram of the process flow of steps S1-S3 in the method for preparing continuous PN-type aramid flame-retardant thermoelectric fibers based on the gel impregnation method of this application.

[0026] Figure 2 This is a schematic diagram of the process flow of step S4 in the method for preparing continuous PN-type aramid flame-retardant thermoelectric fibers based on the gel impregnation method of this application.

[0027] Figure 3 This is a surface morphology diagram of the aramid / carbon nanotube composite fiber prepared in Example 1.

[0028] Figure 4 Thermogravimetric curves of aramid and aramid / CNT fibers are shown.

[0029] Figure 5 Figures showing the vertical burning of aramid / CNT fibers and aramid fibers. Detailed Implementation

[0030] The embodiments of the technical solution of this application will now be described in detail with reference to the accompanying drawings. These embodiments are only used to more clearly illustrate the technical solution of this application and are therefore merely examples, and should not be used to limit the scope of protection of this application.

[0031] Unless otherwise defined, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this application pertains; the terminology used herein is for the purpose of describing particular embodiments only and is not intended to limit the application; the terms “comprising” and “having”, and any variations thereof, in the specification, claims, and foregoing description of the drawings are intended to cover non-exclusive inclusion.

[0032] In the description of the embodiments of this application, technical terms such as "first" and "second" are used only to distinguish different objects and should not be construed as indicating or implying relative importance or implicitly specifying the number, specific order, or primary and secondary relationship of the indicated technical features. In the description of the embodiments of this application, "multiple" means two or more, unless otherwise explicitly defined.

[0033] In this document, the term "embodiment" means that a particular feature, structure, or characteristic described in connection with an embodiment may be included in at least one embodiment of this application. The appearance of this phrase in various places throughout the specification does not necessarily refer to the same embodiment, nor is it a separate or alternative embodiment mutually exclusive with other embodiments. It will be explicitly and implicitly understood by those skilled in the art that the embodiments described herein can be combined with other embodiments.

[0034] To address the technical problems of severe agglomeration, weak bonding, low conductivity, and limited improvement in flame retardant performance of aramid / carbon nanotube composite fibers in existing technologies, this application provides a method for preparing continuous PN-type aramid flame-retardant thermoelectric fibers based on a gel-state impregnation method. This method leverages the rich active amide groups on the surface of gel-state aramid and its porous internal structure to achieve uniform and robust composite formation of carbon nanotubes. Mild ethanol cleaning is used to remove dispersants and optimize the conductive pathway. P-type doping is achieved using ferric chloride, and N-type doping is achieved using oleylamine. A zoned selective impregnation process is then used to continuously prepare integrated PN homojunctions on the same fiber without external splicing. This method retains the excellent mechanical properties and ultra-high residual carbon flame-retardant characteristics of aramid, achieving a multi-functional integrated system of flame retardancy, high-temperature resistance, conductivity, and thermoelectricity.

[0035] This application provides a method for preparing continuous PN-type aramid flame-retardant thermoelectric fibers based on a gel impregnation method, comprising the following steps: S1, Aramid is dissolved in dimethyl sulfoxide to obtain aramid spinning solution; The mass concentration of the aramid spinning solution is 0.5%~2%.

[0036] Specifically, aramid staple fibers are added to dimethyl sulfoxide (DMSO) and stirred at 25–60°C until completely dissolved to obtain an aramid spinning solution with a mass concentration of 0.5–2 wt%. The stirring speed is 300–1000 r / min and the stirring time is 72–120 h to ensure that the aramid is uniformly dispersed and without obvious agglomeration.

[0037] S2, the aramid spinning solution is extruded into a coagulation bath through a wet spinning process to obtain undried gel-state aramid fibers; In wet spinning, the spinning speed is 1 ~ 10 mL / h, and the inner diameter of the spinning needle is 0.5 ~ 1 mm.

[0038] The coagulation bath is made of deionized water, and the coagulation bath temperature is 20 ~ 30℃.

[0039] S3, the undried gel-state aramid fiber is immersed in a carboxylated carbon nanotube aqueous dispersion; then, the immersed aramid / carbon nanotube composite fiber is soaked and washed in anhydrous ethanol and dried to obtain aramid / carbon nanotube composite fiber. The carboxylated carbon nanotube aqueous dispersion has a mass concentration of 1% to 10% and an impregnation time of 0.5 to 3 hours.

[0040] S4, the aramid / carbon nanotube composite fiber is wound onto the surface of a plastic plate; using a layered liquid surface immersion method, the lower half of the plastic plate is immersed in a ferric chloride aqueous solution, dried and cleaned to form a P-type doped section; subsequently, the upper half of the plastic plate is immersed in an oleylamine solution to form an N-type doped section, dried, to obtain a single internally continuous, non-physically spliced, homogeneous PN-type aramid / carbon nanotube composite thermoelectric fiber.

[0041] The concentration of the ferric chloride aqueous solution is 1.2 ~ 2 mol / L, and the soaking time in the ferric chloride aqueous solution is 1 ~ 3 hours.

[0042] The mass concentration of the oleylamine solution is 5-10%, and the soaking time in the oleylamine solution is 1-3 hours.

[0043] The following are some specific embodiments. It should be noted that the embodiments described below are exemplary and are only used to explain this application, and should not be construed as limiting this application. Where specific techniques or conditions are not specified in the embodiments, they shall be performed in accordance with the techniques or conditions described in the literature in this field or according to the product instructions. Reagents or instruments whose manufacturers are not specified are all conventional products that can be obtained commercially.

[0044] I. Preparation Method Experimental Example 1 The thermoelectric properties of aramid / carbon nanotube composite fibers obtained by coating the surface of gel-state aramid fibers with carbon nanotube solution at different times were investigated, as follows: (1) Preparation of aramid solution: Add 0.5g of para-aramid short fiber to 50mL of dimethyl sulfoxide and stir to dissolve at room temperature to obtain an aramid spinning solution with a mass concentration of about 1%.

[0045] (2) Preparation of gel-state aramid fibers: The above aramid spinning solution is extruded through a spinneret into a deionized water coagulation bath for wet spinning. The coagulation bath temperature is 25℃, and undried gel-state aramid fibers are obtained.

[0046] (3) The undried gel aramid fibers were immersed in a 10% carboxylated carbon nanotube aqueous dispersion for 1 hour to obtain aramid / carbon nanotube composite fibers.

[0047] The thermoelectric properties of aramid / carbon nanotube composite fibers obtained by impregnation 1, 2, 3, 4 and 5 times were tested respectively, and the results are shown in Table 1.

[0048] Experimental Example 2 The thermoelectric properties of aramid / carbon nanotube composite fibers obtained by coating the surface of dry aramid fibers with carbon nanotube solution at different times were investigated, as follows: (1) Preparation of aramid solution: Add 0.5g of para-aramid short fiber to 50mL of dimethyl sulfoxide and stir to dissolve at room temperature to obtain an aramid spinning solution with a mass concentration of about 1%.

[0049] (2) Preparation of dry aramid fiber: The above aramid spinning solution is extruded through a spinneret into a deionized water coagulation bath for wet spinning. The coagulation bath temperature is 25℃. After drying, aramid fiber is obtained.

[0050] (3) The dry aramid fiber was immersed in a 10% carboxylated carbon nanotube aqueous dispersion for 1 hour to obtain aramid / carbon nanotube composite fiber.

[0051] The thermoelectric properties of aramid / carbon nanotube composite fibers obtained by impregnation 1, 2, 3, 4 and 5 times were tested respectively, and the results are shown in Table 1.

[0052] Table 1 As shown in the table above, compared with direct coating of dry aramid fibers, gel coating exhibits better conductivity and more uniform surface coating. The conductivity of the fiber reaches 137.7 S / cm after a single coating, and 159.7 S / cm after three coatings. This is likely due to the electrostatic attraction and hydrogen bonding between the protonated amide groups and carboxylated carbon nanotubes on the surface of gel-coated aramid fibers, allowing the carbon nanotubes to better bond with the aramid fibers. In contrast, dry aramid fibers have compact molecular chains and lack active amide groups, resulting in no significant interaction between carbon nanotubes and the fiber. This makes it difficult for carbon nanotubes to adsorb onto the aramid fiber surface, leading to poor conductivity even after multiple coatings. Even after five coatings, the conductivity only reaches 21.3 S / cm, far lower than that of the gel coating method. There is no significant difference in the Seebeck coefficient between the two coating methods, reflecting the p-type thermoelectric properties of carbon nanotubes in air, with a Seebeck coefficient of approximately 25 μV / K.

[0053] Experimental Example 3 The effect of FeCl3 concentration on the thermoelectric properties of aramid / CNT fibers was investigated, as follows: (1) Preparation of aramid solution: Add 0.5g of para-aramid short fiber to 50mL of dimethyl sulfoxide and stir to dissolve at room temperature to obtain an aramid spinning solution with a mass concentration of about 1%.

[0054] (2) Preparation of gel-state aramid fibers: The above aramid spinning solution is extruded through a spinneret into a deionized water coagulation bath for wet spinning. The coagulation bath temperature is 25℃, and undried gel-state aramid fibers are obtained.

[0055] (3) The undried gel aramid fibers were immersed in a 10% carboxylated carbon nanotube aqueous dispersion for 1 hour. Then, the immersed aramid / carbon nanotube composite fibers were soaked in anhydrous ethanol to remove the dispersant residue on the surface of the carbon nanotubes, eliminate the insulating coating layer, and construct a continuous conductive network. After cleaning, the fibers were dried under normal pressure to obtain aramid / carbon nanotube composite fibers.

[0056] (4) The obtained aramid / carbon nanotube composite fiber was completely immersed in ferric chloride aqueous solution and doped at a constant temperature (20℃) for 1 h; after doping, it was taken out and dried, rinsed with deionized water to remove residual salt impurities on the surface, and air-dried naturally; ferric chloride was used as an electron acceptor to realize the doping of carbon nanotube P-type holes and P-type thermoelectric fiber was prepared.

[0057] The properties of the P-type thermoelectric fibers prepared with ferric chloride aqueous solutions of 0.8 mol / L, 1.2 mol / L, 1.6 mol / L, 2.0 mol / L, and 2.4 mol / L are shown in Table 2.

[0058] Table 2 Experiments show that as the FeCl3 concentration increases from 0.8 mol / L to 1.6 mol / L, the fiber conductivity increases, reaching a maximum of 196 S / cm at 1.6 mol / L; with further increases in concentration, the conductivity decreases slightly. Simultaneously, the Seebeck coefficient of the fiber continuously increases with increasing doping concentration, reaching 39 μV / K at 1.6 mol / L, and then tends to stabilize. FeCl3 acts as an electron acceptor, and Fe... 3+ Exhibiting strong electron affinity, FeCl3 can extract electrons from the π-conjugated system of carbon nanotubes and inject a large number of holes, significantly increasing the carrier concentration and thus improving the fiber conductivity. Below 1.6 mol / L, the doping degree deepens with increasing concentration, and the conductivity continues to rise; above this concentration, excess FeCl3 tends to aggregate into ions inside the fiber, disrupting the continuity of the carbon nanotube conductive network and leading to a decrease in conductivity.

[0059] Furthermore, FeCl3 doping on the carbon nanotube surface constructs an interfacial barrier and induces an energy filtering effect, preferentially selecting high-energy charge carriers and thus improving the effective mass of charge carriers. This offsets the adverse effects of high charge carrier concentration on the Seebeck coefficient, causing the Seebeck coefficient to increase synchronously with increasing doping concentration and remain stable after saturation. The results show that 1.6 mol / L is the optimal doping concentration, under which the fiber achieves synergistic optimization of conductivity and Seebeck coefficient, with a power factor reaching 30.1 μWm. -1 K -2 .

[0060] Taking all factors into consideration, a FeCl3 concentration of 1.2 to 2 mol / L is more suitable.

[0061] Experiment Example 4 The effect of the mass fraction of oleylamine solution on the thermoelectric properties of aramid / CNT fibers was investigated, as follows: (1) Preparation of aramid solution: Add 0.5g of para-aramid short fiber to 50mL of dimethyl sulfoxide and stir to dissolve at room temperature to obtain an aramid spinning solution with a mass concentration of about 1%.

[0062] (2) Preparation of gel-state aramid fibers: The above aramid spinning solution is extruded through a spinneret into a deionized water coagulation bath for wet spinning. The coagulation bath temperature is 25℃, and undried gel-state aramid fibers are obtained.

[0063] (3) The undried gel aramid fibers were immersed in a 10% carboxylated carbon nanotube aqueous dispersion for 1 hour. Then, the immersed aramid / carbon nanotube composite fibers were soaked in anhydrous ethanol to remove the dispersant residue on the surface of the carbon nanotubes, eliminate the insulating coating layer, and construct a continuous conductive network. After cleaning, the fibers were dried under normal pressure to obtain aramid / carbon nanotube composite fibers.

[0064] (4) The obtained aramid / carbon nanotube composite fiber was completely immersed in oleylamine ethanol solution and doped at a constant temperature (20℃) for 1 hour; after doping, it was dried and washed with water to remove the free oleylamine on the surface; oleylamine was used as an electron donor to realize N-type electron doping of carbon nanotubes and N-type thermoelectric fiber was prepared.

[0065] The properties of the N-type thermoelectric fibers prepared with oleylamine ethanol solutions of 2.5%, 5.0%, 7.5%, 10.0%, and 12.5% ​​by mass are shown in Table 3.

[0066] Table 3 Experiments show that as the mass fraction of the oleylamine ethanol solution increases from 2.5% to 7.5%, the fiber conductivity rapidly increases, reaching a peak at 7.5% (455.7 S / cm). When the concentration continues to increase to 12.5%, the conductivity decreases slightly but remains at a high level. Simultaneously, oleylamine doping causes the fiber's Seebeck coefficient to change from positive to negative, indicating successful control over the carrier type from P-type to N-type. The absolute value of the Seebeck coefficient does not change significantly with increasing oleylamine concentration, remaining approximately around 21 μV / K.

[0067] As a typical electron donor, oleylamine can inject electrons into carbon nanotubes, transforming them from P-type doping to N-type doping, significantly increasing the electron carrier concentration and thus improving the fiber conductivity. At the same time, the interfacial barrier formed by oleylamine on the surface of carbon nanotubes during the doping process induces an energy filtering effect, preferentially selecting high-energy electrons to participate in conduction, improving the effective mass of carriers, and offsetting the adverse effect of increased carrier concentration on the Seebeck coefficient. Therefore, the Seebeck coefficient remains relatively stable during the doping process, achieving synergistic optimization of conductivity and N-type Seebeck coefficient.

[0068] Taking all factors into consideration, a mass fraction of 5-10% for the oleylamine solution is more suitable.

[0069] The thermoelectric properties of three fibers—undoped aramid, optimal P-type doped (FeCl3 concentration of 1.6 mol / L), and optimal N-type doped (oleylamine concentration of 7.5%)—are compared, as shown in Table 4 below.

[0070] Table 4 The comparison shows that, without doping, carbon nanotubes (CNTs) exhibit weak p-type conductivity due to oxygen adsorption in the environment, resulting in low fiber conductivity and power factor, with a power factor of only 10.7 μW. m - ¹ K - ², the Seebeck coefficient is positive.

[0071] After doping with FeCl3, FeCl3 acts as an electron acceptor, increasing the carrier concentration by injecting holes into the CNTs. Simultaneously, the interfacial barrier constructed by FeCl3 on the CNT surface induces an energy filtering effect, achieving a synergistic improvement in conductivity and Seebeck coefficient, thus increasing the fiber power factor to 30.1 μW. m - ¹ K - ².

[0072] After doping with oleylamine, oleylamine, as a typical electron-donating N-type dopant, can inject electrons into CNTs, changing the dominant charge carrier from holes to electrons. This achieves conduction type modulation from P-type to N-type, the Seebeck coefficient changes from positive to negative, and the power factor reaches 21.7 μW. m - ¹ K - ².

[0073] Compared to undoped fibers, both doping modifications improved the power factor of the fibers. Among them, P-type doped fibers exhibited superior overall thermoelectric performance, while N-type doped fibers successfully achieved a change in conductivity type, providing a matching N-type material for constructing flexible thermoelectric pairs.

[0074] Example 1 Please see Figures 1 to 2 As shown, this embodiment provides a method for preparing continuous PN-type aramid flame-retardant thermoelectric fibers based on a gel impregnation method, including the following steps: S1, Preparation of aramid solution: Add 0.5g of para-aramid short fiber to 50mL of dimethyl sulfoxide and stir to dissolve at room temperature to obtain an aramid spinning solution with a mass concentration of about 1%.

[0075] S2, Preparation of gelled aramid fibers: The above aramid spinning solution is extruded through a spinneret into a deionized water coagulation bath for wet spinning. The coagulation bath temperature is 25°C, and undried gelled aramid fibers are obtained.

[0076] S3, the obtained undried gelled aramid fibers were immersed in a 10% (w / w) aqueous dispersion of carboxylated carbon nanotubes for 1 hour each time, and the immersion was repeated 3 times. Subsequently, the immersed aramid / carbon nanotube composite fibers were soaked in anhydrous ethanol to remove residual dispersant from the carbon nanotube surface, eliminate the insulating coating layer, and construct a continuous conductive network. After cleaning, the fibers were dried under normal pressure to obtain the aramid / carbon nanotube composite fibers, the SEM image of which is shown below. Figure 3 As shown.

[0077] During the impregnation process, the carbon nanotubes are uniformly and firmly bonded to the aramid fiber by relying on the electrostatic attraction between the protonated amide groups on the surface of the gel-state aramid and the carboxylated carbon nanotubes, as well as the hydrogen bond interaction.

[0078] S4. Select an insulating plastic sheet with dimensions of 20 cm in length, 0.1 cm in width, and 0.5 cm in height. Wrap the aramid / carbon nanotube composite fiber evenly and tightly around the surface of the plastic sheet. Using a layered liquid surface immersion method, immerse the lower 0.25 cm of the plastic sheet in a 1.6 mol / L ferric chloride aqueous solution for 1 hour, then dry and clean to form a P-type doped section. Subsequently, immerse the upper 0.25 cm of the plastic sheet in a 7.5% oleylamine solution for 1 hour to form an N-type doped section. After drying, obtain a single, internally continuous, non-physically spliced, homogeneous, integrated continuous PN-type aramid flame-retardant thermoelectric fiber.

[0079] Surface morphology of aramid / carbon nanotube composite fibers ( Figure 3 As can be seen, carbon nanotubes are uniformly attached to the fiber surface, and there is no obvious agglomeration of carbon nanotubes.

[0080] Thermogravimetric analysis was performed on pure aramid fibers and the aramid / carbon nanotube composite fibers obtained in step S3 of Example 1. The results are as follows: Figure 4 As shown.

[0081] Tests show that the thermal decomposition temperature of pure aramid fiber is 525.3℃, with a residual carbon rate of 44.3%. After incorporating carbon nanotubes (CNTs), the thermal decomposition temperature of aramid / carbon nanotube fiber increases to 527.5℃, and the residual carbon rate significantly improves to 76.6%. Residual carbon rate is a key indicator for evaluating the flame-retardant performance of materials. A higher residual carbon rate indicates a more stable dense carbon layer formed at high temperatures, effectively blocking oxygen and heat transfer and inhibiting the release of combustible gases, thus significantly improving the flame-retardant performance of the material. The rigid aromatic structure of aramid itself endows the material with basic high-temperature resistance and flame-retardant properties, while the introduction of carbon nanotubes further enhances the synergistic effect of physical barrier and catalytic char formation, achieving a dual improvement in thermal stability and char formation ability of the composite fiber. This provides more reliable performance support for its application in high-temperature protection, flame-retardant fabrics, and other fields.

[0082] Flame retardant properties were tested on pure aramid fibers and the aramid / carbon nanotube composite fibers obtained in step S3 of Example 1. The results are as follows: Figure 5 As shown.

[0083] Specifically: When a 15cm long bundle of fibers (5 fibers) is placed vertically above an alcohol lamp flame, the pure aramid fiber shows rapid upward propagation of the flame along the fiber after contact with the flame, reaching a burning length of 7cm within 0.5s, indicating a fast heat release and combustible gas diffusion rate in the initial stage of combustion. In contrast, the flame in the aramid / carbon nanotube composite fiber is confined to the local area in contact with the flame and does not spread significantly upward, with a final burning length of only 1.5cm. This is attributed to the physical barrier structure formed by the carbon nanotube network inside the fiber, which effectively inhibits the transfer path of heat and combustible pyrolysis products.

[0084] Tests show that both pure aramid fibers and aramid / carbon nanotube composite fibers possess self-extinguishing properties upon removal from the flame. This is due to the inherent aromatic rigid structure of the aramid matrix, which allows for rapid formation of a char layer at high temperatures. Combined with the significant increase in residual carbon rate (44.3%→76.6%) in thermogravimetric analysis, it can be inferred that carbon nanotubes in aramid fibers simultaneously play a synergistic role in physical barrier and catalytic char formation: on the one hand, the high thermal conductivity and network structure of carbon nanotubes can evenly distribute heat, preventing flame spread caused by localized overheating; on the other hand, they can promote the dehydration and carbonization of the aramid matrix, forming a denser and more stable char layer on the fiber surface, blocking further intrusion of oxygen and heat, ultimately achieving effective suppression of flame spread, thus enabling the composite fiber to exhibit superior flame-retardant performance compared to pure aramid.

[0085] Example 2 The difference from Example 1 is that the concentration of the ferric chloride aqueous solution is 1.2 mol / L. Everything else is the same as in Example 1 and will not be repeated here.

[0086] Example 3 The difference from Example 1 is that the concentration of the ferric chloride aqueous solution is 2 mol / L. Everything else is the same as in Example 1 and will not be repeated here.

[0087] Example 4 The difference from Example 1 is that the oleylamine solution has a mass fraction of 5%. Everything else is the same as in Example 1 and will not be repeated here.

[0088] Example 5 The difference from Example 1 is that the mass fraction of the oleylamine solution is 10%. Everything else is the same as in Example 1 and will not be repeated here.

[0089] In summary, this application precisely divides the P-doped and N-doped regions on the same fiber using a fixed plate winding and half-area liquid immersion method. The boundaries between the two regions are clear and there is no cross-contamination, forming an integrated continuous PN thermoelectric structure. This solves the industry pain points of high contact resistance, loose structure, and inability to weave caused by the physical splicing of traditional P and N fibers. The resulting continuous PN-type aramid flame-retardant thermoelectric fiber has multiple functions including high temperature resistance, flame retardancy, electrical conductivity, and thermoelectricity.

[0090] It should be noted that this application is not limited to the above-described embodiments. The above embodiments are merely examples, and any embodiments with the same structure and effect as the technical concept within the scope of this application are included in the technical scope of this application. Furthermore, various modifications that can be conceived by those skilled in the art to the embodiments, and other ways of constructing by combining some of the constituent elements of the embodiments, without departing from the spirit of this application, are also included in the scope of this application.

Claims

1. A method for preparing continuous PN-type aramid flame-retardant thermoelectric fibers based on a gel impregnation method, characterized in that, Includes the following steps: S1, Aramid is dissolved in dimethyl sulfoxide to obtain aramid spinning solution; S2, the aramid spinning solution is extruded into a coagulation bath through a wet spinning process to obtain undried gel-state aramid fibers; S3, the undried gelled aramid fiber is impregnated in a carboxylated carbon nanotube aqueous dispersion; Subsequently, the impregnated aramid / carbon nanotube composite fibers were immersed in anhydrous ethanol for cleaning and then dried to obtain aramid / carbon nanotube composite fibers. S4, the aramid / carbon nanotube composite fiber is wound onto the surface of the plastic sheet; A layered liquid surface immersion method was adopted, in which the lower half of the plastic plate was immersed in ferric chloride aqueous solution, dried and cleaned to form a P-type doped section; then, the upper half of the plastic plate was immersed in oleylamine solution to form an N-type doped section, dried, and a single internally continuous, non-physically spliced, homogeneous PN-type aramid / carbon nanotube composite thermoelectric fiber was obtained.

2. The method for preparing continuous PN-type aramid flame-retardant thermoelectric fibers based on gel impregnation according to claim 1, characterized in that, In step S4, the concentration of the ferric chloride aqueous solution is 1.2 ~ 2 mol / L.

3. The method for preparing continuous PN-type aramid flame-retardant thermoelectric fibers based on gel impregnation according to claim 1, characterized in that, In step S4, the mass concentration of the oleylamine solution is 5-10%.

4. The method for preparing continuous PN-type aramid flame-retardant thermoelectric fibers based on gel impregnation according to claim 1, characterized in that, In step S3, the mass concentration of the carboxylated carbon nanotube aqueous dispersion is 1%~10%, and the impregnation time is 0.5~3 hours.

5. The method for preparing continuous PN-type aramid flame-retardant thermoelectric fibers based on gel impregnation according to claim 1, characterized in that, In step S1, the mass concentration of the aramid spinning solution is 0.5% to 2%.

6. The method for preparing continuous PN-type aramid flame-retardant thermoelectric fibers based on gel impregnation according to claim 1, characterized in that, In step S4, the soaking time in ferric chloride aqueous solution is 1 to 3 hours.

7. The method for preparing continuous PN-type aramid flame-retardant thermoelectric fibers based on gel impregnation according to claim 1, characterized in that, In step S4, the soaking time in the oleylamine solution is 1 to 3 hours.

8. The method for preparing continuous PN-type aramid flame-retardant thermoelectric fibers based on gel impregnation according to claim 1, characterized in that, In step S2, during wet spinning, the spinning speed is 1 ~ 10 mL / h, and the inner diameter of the spinning needle is 0.5 ~ 1 mm.

9. The method for preparing continuous PN-type aramid flame-retardant thermoelectric fibers based on gel impregnation according to claim 1, characterized in that, In step S2, the coagulation bath is deionized water, and the coagulation bath temperature is 20 ~ 30℃.

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