Biomass-based carbon aerogel-polyacrylonitrile fiber composite thermal insulation material and preparation method thereof

Abstract

The invention belongs to the field of aerogel composite materials, and particularly provides a polyacrylonitrile (PAN) and biomass-based carbon aerogel compounded light-weight high-strength thermal insulation material and a preparation method thereof. The material combines the low heat conduction characteristic of the biomass carbon aerogel and the reinforced mechanical property of the polyacrylonitrile fiber, the binding force of the fiber and the carbon aerogel is strong, the structure is stable, and the heat insulation performance is excellent. The PAN-based fibers and the carbon aerogel are carbonized synchronously, so that the technological process is simplified; a non-supercritical drying method is selected for drying, the technological process is simple, the energy consumption is low, and the cost is low; the composite carbon source CVD method realizes low-temperature, efficient and uniform deposition growth of porous carbon, enhances the interface bonding force and isolates a heat transfer path; the prepared biomass-based carbon aerogel-polyacrylonitrile fiber composite thermal insulation material has the advantages of low density, high specific surface area, low thermal conductivity and excellent mechanical property and thermal insulation property.

Description

Technical Field

[0001] This application relates to the field of composite materials technology, specifically to a lightweight, high-strength thermal insulation material based on a composite of polyacrylonitrile (PAN) and biomass-based carbon aerogel, and its preparation method. Background Technology

[0002] Traditional thermal insulation materials such as ceramic fiber and aluminosilicate fiber have a service temperature of 1000℃, while aerogel materials have even lower service temperatures (silicone-based aerogels melt and fail at 650℃, and metal aerogels have a service temperature of less than or equal to 1000℃). Carbon aerogel meets the needs of current high-temperature thermal insulation applications, with a service temperature of up to 1500℃ (oxygen-free).

[0003] Carbon aerogel products and their preparation processes still face multiple challenges in performance optimization and large-scale application. (1) The traditional sol-gel method requires supercritical drying to remove solvents to avoid structural collapse, but this process requires solvent replacement, which is cumbersome, involves complex equipment, consumes a lot of energy, and poses safety hazards. (2) In the preparation process of carbon aerogels, introducing gaseous carbon sources (such as through CVD technology) is a key post-processing step to regulate their microstructure, improve the degree of graphitization, and enhance mechanical properties and conductivity. A single carbon source is often used, but it has inherent defects: highly stable carbon sources (such as methane CH4) need to be effectively decomposed at extremely high temperatures (usually >1000°C), which requires high energy consumption and is not economical. Although highly reactive carbon sources (such as acetylene C2H2 and ethylene C2H) can decompose at lower temperatures and have a fast deposition rate, they are prone to excessive and rapid deposition at the inlet of aerogel pores, leading to a "pore blockage" effect. This prevents the gaseous carbon source from penetrating evenly into the material, resulting in uneven carbon layer distribution, dense on the outside and lacking in the inside, thus failing to optimize the overall performance. (3) The preparation of most carbon aerogels involves three stages: gelation, drying, and carbonization. The sol-gel method usually requires 3-4 weeks to complete the preparation, while carbonization, solvent replacement, and other steps further extend the production cycle.

[0004] Finding a balance between process simplification, structural design, and multifunctional integration in the preparation of carbon aerogel-related products to meet the diverse needs of aerospace, new energy storage, and building energy conservation is a problem that needs to be solved in recent years.

[0005] Existing patents mostly focus on inorganic materials, such as CN119241205A which mentions SiO2 aerogel composites with carbon fiber, or rely on complex processes, such as CN119968350A which mentions impregnation with alternating electric fields. Therefore, there is an urgent need for a composite thermal insulation material that is lightweight, high-strength, has a simple process, and is environmentally friendly. Summary of the Invention

[0006] This invention aims to overcome the shortcomings of the prior art and provide a highly efficient and energy-saving method for preparing carbon aerogel composite materials. It includes simultaneous carbonization and CVD reinforcement using mixed carbon sources, with the key being the application of a composite gaseous carbon source. This composition enables low-temperature, efficient, and uniform carbon deposition and infiltration, effectively avoiding pore blockage, and constructing a composite carbon layer on the carbon aerogel framework that possesses both high graphitization and excellent mechanical strength, thereby significantly improving the mechanical properties and structural thermal stability of the final product.

[0007] This invention provides a method for preparing a biomass-based carbon aerogel-polyacrylonitrile fiber composite thermal insulation material, comprising the following steps: (1) Preparation of biomass-based aerogel precursor: Dissolve cellulose in an alkaline solution and then add a catalyst to form a sol solution; Specifically, the biomass-based material is cellulose, and the cellulose mass fraction in the cellulose solution after dissolving in an alkaline solution is 5-25%, and the alkaline solution is 5-10 wt% NaOH.

[0008] Specifically, the cellulose is dissolved under stirring at -10~0℃ for 2~4 hours.

[0009] Specifically, the catalyst is epichlorohydrin, with an epichlorohydrin:cellulose solution mass ratio of 5~12%.

[0010] (2) In-situ composite of fiber-sol: The uniform sol obtained in step 1) is introduced into a mold containing PAN fiber felt, so that the sol can fully impregnate the fiber felt; Specifically, the impregnation is achieved by at least one of vacuum impregnation, mechanical vibration impregnation, or ultrasonic vibration impregnation. Preferably, the PAN fiber felt is placed in a sealed container and a vacuum is drawn in. The biomass-based aerogel solution is then drawn in using negative pressure to remove air bubbles from the system and to fully impregnate the PAN fiber felt.

[0011] (3) Drying: The fiber felt impregnated in step 2) is dried to obtain PAN fiber reinforced biomass-based aerogel; Specifically, the drying process involves freeze drying or atmospheric pressure drying.

[0012] Preferably, the impregnated fiber felt is transferred to -4~0℃ for pre-freezing for 10-12 hours, and then placed in a vacuum freeze dryer for 40-48 hours. The equipment parameters are set as follows: -10℃~-50℃, vacuum pressure ≤5Pa, so that the solvent crystallizes and sublimates to complete the drying.

[0013] (4) Synchronous gradient carbonization: PAN fiber felt and biomass-based aerosol are synchronously gradient carbonized under inert atmosphere conditions, with heating in three stages.

[0014] Specifically, the carbonization process is carried out in three stages with a temperature gradient: the first stage is 250-300℃, the second stage is 600-800℃, and the third stage is 800-900℃, with each stage held for 1-2 hours. Specifically, the inert atmosphere is argon or nitrogen, and the gas flow rate is 100~200 mL / min.

[0015] Specifically, the heating rate is 3~5℃ / min.

[0016] (5) Composite gas phase carbon source deposition: After the carbonization process is completed, the furnace temperature is adjusted to the optimized deposition temperature range (which is much lower than the temperature of methane alone), and a composite gas phase carbon source is introduced. Under this mixed atmosphere, deposition is carried out by CVD to generate porous carbon on the pore wall. The duration depends on the required carbon layer thickness.

[0017] Specifically, the deposition temperature range is 750-900℃.

[0018] Specifically, the gaseous carbon source includes the primary carbon source: methane; the secondary carbon source: acetylene; and the carrier gas: nitrogen or argon.

[0019] Specifically: the carbon source ratio is acetylene to methane flow rate ratio of 1:(4~10).

[0020] Specifically, the carbon source gas (excluding carrier gas) flow rate is 60~250mL / min, and the holding time is 0.5-3 hours.

[0021] (6) Post-treatment: Replace the gas source with an inert gas and reduce the temperature to room temperature to obtain biomass-based carbon aerogel-polyacrylonitrile fiber composite thermal insulation material.

[0022] Specifically, the inert atmosphere is argon or nitrogen, and the gas flow rate is 100~200 mL / min; the temperature is lowered to room temperature at a rate of 3~10℃ / min.

[0023] This invention also provides a biomass-based carbon aerogel-polyacrylonitrile fiber composite thermal insulation material prepared by the above preparation method, wherein the density of the biomass-based carbon aerogel-polyacrylonitrile fiber composite thermal insulation material is 120-180 kg / m³. 3 Thermal conductivity ≤0.030 W / m·K (room temperature, GB / T 10295-2008), ≤0.045 W / m·K (500℃, GB / T 10295-2008). Compressive strength ≥5 MPa (strain 10%). Temperature resistance: after heat treatment at 1500℃ for 20h, linear shrinkage ≤5%, mass loss ≤10%.

[0024] Compared with the prior art, the beneficial effects of the present invention are as follows: 1. This invention selects biomass-based materials as the carbon source for carbon aerogels and then uses PAN for fiber reinforcement to improve strength. Due to the inherent characteristics of biomass-based materials, the resulting gel precursor does not require the addition of crosslinking agents or initiators to promote the gelation of the precursor solution. In the gelation process, cellulose-based aerogels can automatically form a gel simply by dissolving cellulose.

[0025] 2. By simultaneously carbonizing PAN fibers and biomass-based aerogels, the PAN molecular chains are first induced to cyclize at low temperatures, such as 250-300℃, to form a high-temperature resistant skeleton. Further heating is then used to simultaneously carbonize the PAN fiber skeleton and biomass aerogel. High temperatures help increase the degree of carbonization, improve the specific surface area of ​​the carbon aerogel, and obtain smaller and more uniform pore sizes. During the carbonization process, the non-carbon elements in the PAN fibers gradually decrease, resulting in a thinner fiber diameter, increased density, and significantly increased strength and modulus. The in-situ bonded carbon aerogel detaches from the fibers, forming cavities. Once these cavities form, the thermal insulation performance will be affected. By combining this with a mixed gas source CVD method to grow nanoscale porous carbon on the pore walls, a carbon-carbon composite interface is formed between the PAN fibers and the carbon aerogel pores after carbonization. This fills the cavities formed by the detachment of the carbon aerogel from the PAN fibers during carbonization and strengthens the connection. Simultaneously, the deposited porous carbon further improves the thermal insulation performance.

[0026] 3. The preparation of this invention simplifies the process by simultaneously carbonizing PAN-based fibers and carbon aerogel; porous carbon is grown within the pores, enhancing interfacial bonding while isolating heat transfer paths; drying is performed using a non-supercritical drying method, resulting in a simple process with low energy consumption and low cost; the prepared biomass-based carbon aerogel-polyacrylonitrile fiber composite insulation material has low density, high specific surface area, low thermal conductivity, and excellent mechanical and insulation properties. Detailed Implementation

[0027] To make the objectives, technical solutions, and advantages of the embodiments of this application clearer, the technical solutions of the embodiments of this application will be clearly and completely described below in conjunction with the embodiments of this application. Obviously, the described embodiments are only some embodiments of this application, not all embodiments. Based on the embodiments of this application, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of this application.

[0028] Example 1 (1) Preparation of biomass-based aerogel precursor: Cellulose was dissolved in 5wt% NaOH alkaline solution at low temperature (0℃) for 4 hours by stirring. Epichlorohydrin (7% by mass of epichlorohydrin to cellulose solution) was added to obtain sol solution.

[0029] (2) In-situ composite of fiber-sol: Place the PAN fiber felt in a sealed container and evacuate it. Use negative pressure to draw in the sol solution obtained in step 1), remove the air bubbles in the system, fully impregnate the PAN fiber felt, let it stand for 4 hours to crosslink, and obtain wet gel composite fiber felt; heat at 60℃ for 48 hours to age.

[0030] (3) Drying: The impregnated fiber felt was transferred to -4℃ for pre-freezing for 12h, and then placed in a vacuum freeze dryer for 48h. The equipment parameters were set as follows: -40℃, vacuum pressure 5Pa, so that the solvent crystallized and sublimated to complete the drying, and PAN fiber-reinforced biomass-based aerogel was obtained.

[0031] (4) Synchronous gradient carbonization: PAN fiber reinforced biomass-based aerogel was synchronously carbonized in an argon atmosphere with a gas flow rate of 100 ml / min. The temperature was increased in three stages at a rate of 5 °C / min. The first stage was 300 °C, the second stage was 700 °C, and the third stage was 800 °C. Each stage was held for 2 hours.

[0032] (5) Composite gas phase carbon source deposition: Adjust the temperature to 850℃, replace the gas source with composite gas phase carbon source, methane 180mL / min, acetylene 30mL / min, argon 100mL / min and keep warm for 1 hour, and generate porous carbon on the pore wall by CVD method to enhance the bonding force between fiber and carbon aerogel.

[0033] (6) Post-treatment: The gas source was replaced with an inert gas (argon, flow rate 100 mL / min), and the temperature was reduced to room temperature at 3 °C / min to obtain biomass-based carbon aerogel-polyacrylonitrile fiber composite thermal insulation material.

[0034] Example 2 (1) Preparation of biomass-based aerogel precursor: Cellulose was dissolved in 5wt% NaOH alkaline solution at low temperature (0℃) for 4 hours by stirring. Epichlorohydrin (7% by mass of epichlorohydrin to cellulose solution) was added to obtain sol solution.

[0035] (2) In-situ composite of fiber-sol: Place PAN fiber felt in a container, pour in the sol solution obtained in step 1), sonicate for 30 min, let stand for 4 h for cross-linking, and obtain wet gel composite fiber felt; heat at 60℃ for 48 h for aging.

[0036] (3) Drying: The impregnated fiber felt was transferred to -4℃ for pre-freezing for 12h, and then placed in a vacuum freeze dryer for 48h. The equipment parameters were set as follows: -20℃, vacuum pressure 5Pa, so that the solvent crystallized and sublimated to complete the drying, and PAN fiber-reinforced biomass-based aerogel was obtained.

[0037] (4) Synchronous gradient carbonization: PAN fiber reinforced biomass-based aerogel was synchronously carbonized in an argon atmosphere with a gas flow rate of 100 ml / min. The temperature was increased in three stages at a rate of 5 °C / min. The first stage was 300 °C, the second stage was 700 °C, and the third stage was 800 °C. Each stage was held for 2 hours.

[0038] (5) Deposition of composite gas phase carbon source: Adjust the temperature to 900℃, replace the gas source with composite gas phase carbon source, methane 50mL / min, acetylene 10mL / min, argon 100mL / min and keep warm for 1 hour.

[0039] (6) Post-treatment: The gas source was replaced with an inert gas (argon, flow rate 100 mL / min), and the temperature was reduced to room temperature at 3 °C / min to obtain biomass-based carbon aerogel-polyacrylonitrile fiber composite thermal insulation material.

[0040] Example 3 (1) Preparation of biomass-based aerogel precursor: Cellulose was dissolved in 5wt% NaOH alkaline solution at low temperature (0℃) for 4 hours by stirring. Epichlorohydrin (7% by mass of epichlorohydrin to cellulose solution) was added to obtain sol solution.

[0041] (2) In-situ composite of fiber-sol: Place PAN fiber felt in a sealed container and evacuate it. Use negative pressure to draw in the uniform sol obtained in step 1), remove the air bubbles in the system, fully impregnate the PAN fiber felt, let it stand for 4 hours to crosslink, and obtain wet gel composite fiber felt; heat at 60℃ for 48 hours.

[0042] (3) Drying: The impregnated fiber felt was replaced with anhydrous ethanol every 12 hours for a total of 4 times. The alcohol gel felt was then transferred to an oven and heated in a gradient (60, 80, 100, 150, 200℃) for 2 hours at each temperature to complete the drying at normal pressure, thus obtaining PAN fiber-reinforced biomass-based aerogel.

[0043] (4) Synchronous gradient carbonization: PAN fiber reinforced biomass-based aerogel was synchronously carbonized in an argon atmosphere with a gas flow rate of 100 ml / min. The temperature was increased in three stages at a rate of 5 °C / min. The first stage was 300 °C, the second stage was 800 °C, and the third stage was 1200 °C. Each stage was held for 2 hours.

[0044] (5) Deposition of composite gas phase carbon source: Adjust the temperature to 850℃, replace the gas source with composite gas phase carbon source, methane 180mL / min, acetylene 30mL / min, argon 100mL / min and keep warm for 1 hour.

[0045] (6) Post-treatment: The gas source was replaced with an inert gas (argon, flow rate 100 mL / min), and the temperature was reduced to room temperature at 3 °C / min to obtain biomass-based carbon aerogel-polyacrylonitrile fiber composite thermal insulation material.

[0046] Comparative Example 1 In step (5), the gas source is changed to acetylene gas, the gas flow rate is 40 mL / min, and the temperature is maintained for 2 hours. Other steps are the same as in Example 1.

[0047] Comparative Example 2 In step (5), no composite carbon source gas is introduced; the other steps are the same as in Example 1.

[0048] Comparative Example 3 In step (5), the flow rate of the composite carbon source gas is 50 mL / min, the heat preservation time is 0.5 hours, and the other steps are the same as in Example 1.

[0049] Performance testing

[0050] Note: Compressive strength (MPa, strain 10%): tested according to GB / T 5486-2008 "Test Methods for Inorganic Rigid Thermal Insulation Products".

[0051] Vibration mass loss rate %: Tested according to the method in Appendix B of GB / T 34336-2017 "Nanoporous Aerogel Composite Thermal Insulation Products".

[0052] Inert atmosphere 1500℃ / 20H, linear shrinkage rate %, mass loss rate %: tested according to the method of GB / T 17430-2015 "Evaluation method for maximum service temperature of thermal insulation materials".

[0053] The thermal conductivity of Example 3 is slightly worse than that of Example 1 because, compared with gradient drying, the freeze-drying process is a crystallization sublimation, which avoids the surface tension when the internal solvent is heated and evaporated, and reduces the damage to the aerogel skeleton structure. As can be seen from Comparative Example 1, when only acetylene is introduced, the thermal conductivity, linear shrinkage rate at 1500℃, and mass loss rate of the aerogel increase, while the compressive strength decreases significantly. The main reason is that acetylene rapidly deposits on the material surface, blocking the nanoporous structure of the aerogel, preventing further deposition by subsequent gas sources, resulting in reduced thermal insulation performance and no significant enhancement of mechanical strength.

[0054] As can be seen from Comparative Example 2, even without the introduction of acetylene, the vibrational mass loss rate still increases significantly, and the thermal conductivity increases slightly. This is mainly because non-carbon elements gradually decrease during the carbonization process of PAN fibers, the fiber diameter becomes thinner, and the in-situ grown carbon aerogel detaches from the fibers, forming cavities and reducing the bonding force. Simultaneously, the composite carbon source CVD method grows nanoscale porous carbon in the pores of the PAN fibers and carbon aerogel after carbonization, forming a carbon-carbon composite interface that fills the cavities and enhances the connection. Furthermore, the deposited porous carbon further improves the thermal insulation performance.

[0055] As can be seen from Comparative Example 3, low composite gas flow rate and short insulation time will lead to insufficient gas phase deposition, and there will be no significant improvement in thermal insulation performance and mechanical properties. The above description is merely a specific embodiment of this application, enabling those skilled in the art to understand or implement this application. Various modifications to these embodiments will be readily apparent to those skilled in the art, and the general principles defined herein may be implemented in other embodiments without departing from the spirit or scope of this application. Therefore, this application is not to be limited to the embodiments shown herein, but is to be accorded the widest scope consistent with the principles and novel features claimed herein.

Claims

1. A method for preparing a biomass-based carbon aerogel-polyacrylonitrile fiber composite thermal insulation material, characterized in that, It includes the following steps: (1) Preparation of biomass-based aerogel precursor: Dissolve cellulose in an alkaline solution and then add a catalyst to form a sol solution; (2) In-situ composite of fiber-sol: The sol solution obtained in step (1) is introduced into a mold containing PAN fiber felt so that the sol can fully impregnate the fiber felt; (3) Drying: The fiber felt impregnated in step (2) is dried to obtain PAN fiber reinforced biomass-based aerogel; (4) Synchronous gradient carbonization: PAN fiber felt and biomass-based aerosol are carbonized by synchronous gradient heating under inert atmosphere conditions; (5) Composite gas phase carbon source deposition: After the carbonization process is completed, the furnace temperature is adjusted to the optimized deposition temperature, and a composite gas phase carbon source is introduced to generate porous carbon inside the material by CVD. (6) Post-treatment: Replace the gas source with an inert gas and reduce the temperature to room temperature to obtain biomass-based carbon aerogel-polyacrylonitrile fiber composite thermal insulation material.

2. The preparation method according to claim 1, characterized in that: In step (1), the biomass-based material is cellulose. The cellulose mass fraction in the cellulose solution after dissolving in the alkaline solution is 5~25%, and the alkaline solution is 5~10wt% NaOH.

3. The preparation method according to claim 1, characterized in that: In step (1), the cellulose is dissolved under the following conditions: stirring at -10~0℃ for 2~4 hours.

4. The preparation method according to claim 1, characterized in that: In step (1), the catalyst is epichlorohydrin, and the mass ratio of epichlorohydrin to cellulose solution is 5-12%.

5. The preparation method according to claim 1, characterized in that: In step (2), the impregnation is achieved by at least one of vacuum impregnation, mechanical vibration impregnation, or ultrasonic vibration impregnation.

6. The preparation method according to claim 1, characterized in that: In step (3), the drying process is either freeze drying or atmospheric pressure drying.

7. The preparation method according to claim 1, characterized in that: In step (3), the freeze drying process involves transferring the impregnated fiber felt to -4~0℃ for pre-freezing for 10-12 hours, and then placing it in a vacuum freeze dryer for 40-48 hours. The equipment parameters are set as follows: -10℃~-50℃, vacuum pressure ≤5Pa, so that the solvent crystallizes and sublimates to complete the drying process.

8. The preparation method according to claim 1, characterized in that: In step (4), the temperature gradient carbonization is carried out in three stages. The three stages are 250-300℃ in the first stage, 600-800℃ in the second stage, and 800-900℃ in the third stage. Each stage is held for 1-2 hours. The inert atmosphere is argon or nitrogen, and the gas flow rate is 100-200 mL / min.

9. The preparation method according to claim 1, characterized in that: In step (5), the gaseous carbon source flow rate is 60~250 mL / min, and the temperature is maintained for 2~4 hours.

10. A biomass-based carbon aerogel-polyacrylonitrile fiber composite thermal insulation material, characterized in that: The thermal insulation material prepared according to any one of claims 1-9 has a density of 120-180 kg / m³. 3 Thermal conductivity ≤0.030 W / m·K (room temperature, GB / T 10295-2008), ≤0.045 W / m·K (500℃, GB / T 10295-2008). Compressive strength ≥ 5 MPa; Temperature resistance: after heat treatment at 1500℃ for 20 hours, linear shrinkage ≤ 5% and mass loss ≤ 10%.

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