Modified silicon carbide fibers, methods of making and using the same

Abstract

The present application relates to modified silicon carbide fiber and its preparation method and application, the modified silicon carbide fiber is endowed with magnetic loss, combines the high-strength corrosion-resistant characteristics of geopolymer matrix, realizes the dual promotion of concrete mechanical properties and wave absorption efficiency. Replace carbon fiber in conventional wave-absorbing concrete with modified silicon carbide fiber, on the one hand, has good antioxidant performance at high temperature, greatly reduces carbon emission, has the advantages of energy saving and environmental protection, on the other hand, not only solves the shortcoming of performance reduction at high temperature, but also makes the metal magnetic loss and the dielectric loss of matrix form synergistic effect, greatly improves the wave absorption performance. The present application is suitable for green communication infrastructure, and has the characteristics of environmental protection, high durability and wide frequency wave absorption.

Description

Technical Field

[0001] This invention belongs to the field of functional geopolymer concrete technology, specifically relating to a modified silicon carbide fiber, its preparation method and application. The modified silicon carbide fiber is used to prepare geopolymer concrete with wave-absorbing properties. The geopolymer concrete with wave-absorbing properties is prepared by using modified silicon carbide fiber as the wave-absorbing material, and is suitable for engineering scenarios that require both electromagnetic absorption and green building requirements. Background Technology

[0002] Geopolymer concrete uses industrial solid waste (fly ash, slag) as raw materials, forming a three-dimensional network structure through an alkaline activation reaction. It boasts advantages such as low carbon footprint, environmental friendliness, high temperature resistance, and chemical corrosion resistance. However, existing geopolymer concrete has limited functionality and struggles to meet the electromagnetic shielding requirements of modern buildings. Traditional microwave-absorbing concrete often uses silicate cement as a matrix, resulting in high carbon emissions, and its microwave-absorbing fillers (such as ferrite) have high density and narrow frequency bands.

[0003] The prior art CN114105567A discloses a functional concrete incorporating slag, fly ash, and steel slag as microwave absorbing materials. This invention uses slag and fly ash to replace part of the cement as a gelling material, essentially increasing the concrete's strength through hydration. Its essence is consistent with cement-based materials. However, using steel slag as a microwave absorbing material not only increases the weight of the test blocks but also causes accelerated corrosion at pH < 3 or > 11, severely degrading its performance. The added polypropylene fibers only enhance the concrete's crack resistance and do not possess microwave absorbing properties themselves.

[0004] The existing technology CN109293372A discloses a method of adding iron powder, manganese iron ore powder and clay minerals to make microwave-absorbing ceramsite, which is then incorporated into concrete to give it microwave-absorbing properties. The cylinder pressure is around 13MPa, resulting in poor strength. In addition, under strong acid and alkali environments, the total mass of the microwave-absorbing ceramsite will be greatly reduced, which will also lead to a decrease in the strength of the block and a loss of microwave-absorbing properties. This is not suitable for some occasions with high requirements for mechanical properties or durability. Moreover, the addition of ceramsite increases the energy consumption in the ceramsite production process, resulting in relatively high costs. Summary of the Invention

[0005] In view of the shortcomings of the prior art, the technical problem to be solved by the present invention is to provide a modified silicon carbide fiber, its preparation method and application. The modified silicon carbide fiber is used in the preparation of geopolymer concrete with wave absorption properties. It has excellent wave absorption performance, high mechanical strength and convenient construction, thus solving the problem of difficulty in balancing wave absorption efficiency and mechanical properties in the prior art.

[0006] To achieve the above objectives, the technical solution of the present invention is as follows:

[0007] In a first aspect, the present invention provides a method for preparing modified silicon carbide fibers, the method comprising the following steps:

[0008] 1) Cut silicon carbide fibers and pretreat them with anhydrous ethanol to obtain dispersed silicon carbide fibers;

[0009] 2) The dispersed silicon carbide fibers pretreated in step 1) are placed in a mixed acid solution of hydrofluoric acid and nitric acid for acid etching to create grooves on the surface of the silicon carbide fibers. The control conditions for the acid etching process are: the silicon carbide fibers are completely immersed in the mixed acid solution and the reaction is carried out in a constant temperature water bath at 50~60℃ for 1.2~1.8h, with stirring every 12~16 minutes during the process.

[0010] After acid etching, the silicon carbide fiber is rinsed with deionized water until pH=6~7, and finally vacuum dried to obtain the acid-etched silicon carbide fiber.

[0011] 3) Using an MOCVD device, cobalt acetylacetonate and iron acetylacetonate powders are placed in the upstream evaporation zone, and the silicon carbide fibers after acid etching in step 2) are placed in the downstream deposition zone;

[0012] In the upstream evaporation zone, the temperature is controlled to rise to 180~210℃ over 2 hours. The material generated in the upstream evaporation zone is immediately transported to the midstream reaction zone via H2 carrier gas at a flow rate of 180~250mL / min.

[0013] In the midstream reaction zone, the temperature is controlled to rise to 600~800℃ within 2 hours and maintained for 1.8~2.2 hours;

[0014] The temperature in the downstream deposition area is controlled at 280~310℃, and modified silicon carbide fibers are formed and collected in the downstream deposition area.

[0015] Further, in step 1), the silicon carbide fiber is cut into micro segments with a length of 3-4 mm, and the diameter of the silicon carbide fiber monofilament is 8-10 μm.

[0016] In step 2), the nitric acid is concentrated nitric acid, and the volume ratio of hydrofluoric acid to concentrated nitric acid is 1:2.5 to 1:3. Hydrofluoric acid is commercially available.

[0017] Furthermore, in step 3), the ratio of the total mass of cobalt acetylacetone and iron acetylacetone powder to the mass of the acid-etched silicon carbide fiber is 8~12:20.

[0018] Furthermore, the depth of the trench is 1~2μm.

[0019] Furthermore, carbon nanotubes encapsulating nano-metal particles are grown on the surface of the modified silicon carbide fiber, with the inner walls of the carbon nanotube ends encapsulating nano-metal particles.

[0020] Secondly, the present invention provides a geopolymer concrete with wave-absorbing properties, wherein the modified silicon carbide fiber is added to the geopolymer concrete.

[0021] Further, the geopolymer concrete comprises, by weight: 180-270 parts fly ash, 90-180 parts slag, 1100-1110 parts coarse aggregate, 740-770 parts fine aggregate, 18-32 parts sodium hydroxide, 126-156 parts water glass, 58-74 parts water, and 5-15 parts modified silicon carbide fiber.

[0022] Thirdly, the preparation method of the aforementioned geopolymer concrete with wave-absorbing properties specifically involves the following process:

[0023] S1. Soak the coarse aggregate and fine aggregate in water for 1-3 hours, then take them out and wipe them dry to obtain pre-wet treated coarse aggregate and fine aggregate.

[0024] S2. Dissolve sodium hydroxide in water and let it stand to room temperature. Then add water glass to prepare an alkali activator. After that, add the pre-dispersed modified silicon carbide fiber.

[0025] S3. Mix the slag, fly ash, and fine aggregate treated by the pre-wetting method evenly, then slowly add a portion of the alkali activator with modified silicon carbide fiber, and continue stirring until the mixture becomes fine and uniform. Then add the coarse aggregate treated by the pre-wetting method, and slowly add the remaining alkali activator with modified silicon carbide fiber, and stir evenly again to obtain the geopolymer concrete slurry.

[0026] S4. Pour the geopolymer concrete slurry into the mold. After pouring, attach the hot air circulation curing hood to the surface of the geopolymer concrete slurry, ensuring that the edges of the hot air circulation curing hood are sealed to the mold. Then, perform three-stage gradient curing:

[0027] Initial stage: Apply hot air at 60~80℃ for 2 hours to the surface of the geopolymer concrete slurry to quickly form a highly dense shell in the surface 3~5cm area;

[0028] Intermediate stage: Stop the hot air and turn on the humidifier to raise the humidity of the maintenance space to 85%~92% within 30 minutes. Record the humidity data every hour thereafter and humidify for 10 hours to promote the slow growth of aluminosilicate gel.

[0029] Final stage: Transfer the geopolymer concrete after the intermediate stage treatment to the curing box and set up a microwave antenna above the curing box. Set the microwave power to 0.5~2W / g and achieve the balance of surface and internal humidity through microwave-assisted curing: Start microwave irradiation for 10~30 minutes every hour within 12 hours. During the irradiation process, control the surface temperature difference to ≤5℃ by scanning with a thermal imager.

[0030] Then, the treatment was adjusted to irradiate for 10-30 minutes every 2 hours within 24 hours, while covering the surface with plastic wrap to reduce excessive moisture evaporation.

[0031] Finally, remove the plastic wrap and irradiate the concrete with microwave power of 1W / g for 24 hours to allow the geopolymer concrete to naturally ventilate and balance the humidity, thus obtaining well-cured geopolymer concrete.

[0032] Compared with the prior art, the beneficial effects of the present invention are:

[0033] (1) This invention utilizes strong acid immersion etching of silicon carbide fibers to change surface activity, resulting in uniform and continuous grooves on the surface, which replaces the surface cleaning effect of conventional dilute nitric acid treatment. The grooves of a certain depth increase the deposition of nano-metal particles on silicon carbide fibers.

[0034] (2) This invention uses MOCVD equipment for silicon carbide fiber modification: ① Acid etching pretreatment: The fiber surface is etched with a mixed acid (HF:HNO3=1:3) to increase the specific surface area and improve the metal loading; ② Core-shell structure encapsulation: By controlling the deposition temperature (around 300℃) and H2 carrier gas flow rate (around 200 mL / min) through MOCVD, Fe and Co nanoparticles are introduced for use. The Fe and Co nanoparticles are encapsulated at the ends of carbon nanotubes (CNTs), forming a core-shell structure with carbon nanotubes as the shell and several nanoparticles as the core, which effectively avoids the oxidation and agglomeration of the iron and cobalt nanoparticles. This structure improves the stability and durability of the modified silicon carbide fiber, ensuring that it maintains good magnetic properties and microwave absorption effect during long-term use, and improving magnetic loss efficiency.

[0035] (3) This invention uses modified silicon carbide fiber as a microwave absorbing material and proposes for the first time to combine geopolymer concrete and modified silicon carbide fiber. The combination of the two can adapt to most harsh environments. Not only does the geopolymer concrete have strong high temperature resistance and oxidation resistance, but the modified silicon carbide fiber can still effectively resist acid and alkali after the outer layer peels off.

[0036] (4) This invention breaks through the traditional standardized process framework of "one-time mixing and room temperature curing" for polymer concrete and pioneers a gradient curing process. This process increases the compressive strength of the surface layer of polymer concrete by more than 20% and the flexural strength of the core by more than 15% (compared to polymer concrete cured by normal process), and reduces the overall shrinkage rate to below 0.03%. In the implementation of this gradient curing process, by precisely controlling the hot air temperature within the preset range and simultaneously optimizing the humidification time and environmental humidity parameters, the curing reaction can be ensured to be fully and thoroughly. In the end, not only can a better curing effect be achieved, but the mechanical properties of concrete can also be significantly enhanced.

[0037] (5) The geopolymer concrete of this invention does not use cement (it uses slag and fly ash as gelling materials). By optimizing the ratio of slag and fly ash, it reacts chemically with an alkali activator to prepare geopolymer concrete, generating a three-dimensional network distribution system. Its mechanical properties, high-temperature resistance, and corrosion resistance are all superior to ordinary silicate cement. The modified silicon carbide fiber used in this invention not only increases the crack resistance of the geopolymer, but also maintains good structural stability at high temperatures compared to other fibers, significantly improving the applicability of the material in extreme environments. In terms of splitting tensile strength, when the modified silicon carbide fiber content is 5 parts, the splitting tensile strength of the concrete increases by 22.45% compared to the case without silicon carbide material. In addition, it has excellent wave absorption properties and is resistant to strong acids and alkalis, and can be used in more extreme environments while maintaining stable performance. Attached Figure Description

[0038] Figure 1 This is an electron microscope image of the modified silicon carbide fiber obtained in Example 1 at 5.00 KX.

[0039] Figure 2 This is an electron microscope image of the modified silicon carbide fiber obtained in Example 1 at 1.00 KX. Detailed Implementation

[0040] The present invention is further illustrated below with reference to embodiments and accompanying drawings. It should be noted that the embodiments are for explanation only and not for limiting the scope of protection of this application. Operations for which experimental conditions are not explicitly specified in the embodiments are assumed to follow industry-standard practices or technical specifications provided by suppliers.

[0041] Unless otherwise specified, all terms used in this application are those generally understood by those skilled in the art. Reagents and raw materials required for the experiments were procured through conventional market channels, and their usage was in accordance with industry standard operating procedures or product technical manuals. Furthermore, any technical solutions and equivalent materials similar to the principles of this invention can be applied in practical implementation. The examples and materials listed in this specification are merely illustrative, and percentages not explicitly stated refer to mass fractions.

[0042] The method for preparing modified silicon carbide fibers in this invention includes the following steps:

[0043] 1) Cut silicon carbide fibers and pretreat them with anhydrous ethanol to obtain dispersed silicon carbide fibers;

[0044] 2) The dispersed silicon carbide fibers pretreated in step 1) are placed in a mixed acid solution of hydrofluoric acid and nitric acid for acid etching to create continuous grooves on the surface of the silicon carbide fibers. The control conditions for the acid etching process are: the silicon carbide fibers are completely immersed in the mixed acid solution and the reaction is carried out in a constant temperature water bath at 50~60℃ for 1.2~1.8h, with stirring every 12~16 minutes during the process.

[0045] After acid etching, the silicon carbide fiber is rinsed with deionized water until pH=6~7, and finally vacuum dried to obtain the acid-etched silicon carbide fiber.

[0046] 3) Using MOCVD (organic metal chemical vapor deposition) equipment, cobalt acetylacetone and iron acetylacetone powders are placed in the upstream evaporation zone. The temperature in the upstream evaporation zone is controlled to rise to 180~210℃ in 2 hours. The material generated in the upstream evaporation zone is immediately transported to the midstream reaction zone by H2 carrier gas.

[0047] In the midstream reaction zone, the temperature is controlled to rise to 600~800℃ within 2 hours and maintained for 1.8~2.2 hours;

[0048] The silicon carbide fibers after acid etching in step 2) are placed in the downstream deposition area, and the temperature of the downstream deposition area is controlled at 280~310℃. Modified silicon carbide fibers are formed and collected in the downstream deposition area.

[0049] The modified silicon carbide fiber obtained above is used in the preparation of geopolymer concrete with wave-absorbing properties. The geopolymer concrete comprises, by weight, 180-270 parts of fly ash, 90-180 parts of slag, 1100-1110 parts of coarse aggregate, 740-770 parts of fine aggregate, 18-32 parts of sodium hydroxide, 126-156 parts of water glass, 58-74 parts of water, and 5-15 parts of modified silicon carbide fiber.

[0050] The fly ash is Class I fly ash, with a particle size of 10~20μm and a density of 2.55 g / cm³. 3 ;

[0051] The slag is S95 slag with a specific surface area of ​​≥400 m². 2 / kg, density 2.84 g / cm³ 3 ;

[0052] The sodium hydroxide has a purity of ≥97%.

[0053] The water glass has a modulus of 3.26, a Baumé degree of 40, a sodium oxide content of 8.53%wt, a silica content of 26.98%wt, and a solid content of 35.5%wt.

[0054] The raw material for the modified silicon carbide fiber is Zerafber S20 silicon carbide fiber.

[0055] The silicon carbide fiber monofilament has a diameter of 10 μm, a length of 3 mm, and a resistivity of 10. 3 Ω·cm.

[0056] The preparation process of the aforementioned geopolymer concrete with wave-absorbing properties is as follows:

[0057] S1. Soak the coarse aggregate and fine aggregate in water for 1-3 hours, then remove and dry them to obtain pre-wet treated coarse aggregate and fine aggregate; pre-disperse the modified silicon carbide fiber with anhydrous ethanol.

[0058] S2. Dissolve sodium hydroxide in water and let it stand to room temperature. Then add water glass to prepare an alkali activator and add pre-dispersed modified silicon carbide fiber.

[0059] S3. Mix the slag, fly ash, and fine aggregate treated by the pre-wetting method evenly, then slowly add a portion of the alkali activator with modified silicon carbide fiber, and continue stirring until the mixture becomes fine and uniform. Then add the coarse aggregate treated by the pre-wetting method, and slowly add the remaining alkali activator with modified silicon carbide fiber, and stir evenly again to obtain the geopolymer concrete slurry.

[0060] S4. Pour the geopolymer concrete slurry into the mold. After pouring, attach the hot air circulation curing hood to the surface of the geopolymer concrete slurry, ensuring that the edges of the hot air circulation curing hood are sealed to the mold. Then, perform three-stage gradient curing:

[0061] Initial stage: Apply hot air at 60~80℃ for 2 hours to the surface of the geopolymer concrete slurry to quickly form a highly dense shell in the surface 3~5cm area;

[0062] Intermediate stage: Stop the hot air and turn on the humidifier to raise the humidity of the maintenance space to 85%~92% within 30 minutes. Record the humidity data every hour thereafter, humidify for 10 hours, and maintain the core temperature at 30~40℃ to promote the slow growth of aluminosilicate gel.

[0063] Final stage: Transfer the geopolymer concrete after the intermediate stage treatment to the curing box and set up a microwave antenna above the curing box. Set the microwave power to 0.5~2W / g and achieve the balance of surface and internal humidity through microwave-assisted curing: Start microwave irradiation for 10~30 minutes every hour within 12 hours. During the irradiation process, control the surface temperature difference to ≤5℃ by scanning with a thermal imager.

[0064] Then, the treatment was adjusted to irradiate for 10-30 minutes every 2 hours within 24 hours, while covering the surface with plastic wrap to reduce excessive moisture evaporation.

[0065] Finally, remove the plastic wrap and irradiate the concrete with microwave power of 1W / g for 24 hours to allow the geopolymer concrete to naturally ventilate and balance the humidity, thus obtaining well-cured geopolymer concrete.

[0066] In this invention, preferably, the microwave power and the irradiation time for different stages are set as follows: 0.5W / g, 30min per irradiation; 1W / g, 20min per irradiation; 2W / g, 10min per irradiation.

[0067] In the preparation of modified silicon carbide fibers, during the anhydrous ethanol treatment, the mass ratio of silicon carbide fibers to anhydrous ethanol is 1:0.02~0.05.

[0068] In this invention, cobalt acetylacetonate and iron acetylacetonate are selected as two organometallic sources, which can provide both carbon and metal. Silicon carbide fibers are heat-resistant (800℃), and the temperature in the upstream evaporation zone is set at around 200℃ to generate nano-metal particles, allowing the carrier gas to carry an appropriate amount of metal. Cobalt and iron are chosen because their reaction temperatures are similar, ensuring activity at this temperature. Too much organometallic source can lead to agglomeration, while too little will result in poor magnetization.

[0069] This invention also provides a method for preparing related materials, comprising: placing raw blast furnace slag in a drying oven and drying it at 105±5℃ to constant weight (time ≥24 hours) to remove free moisture and volatile substances. After drying, cooling to room temperature, and then preliminarily crushing it to particles with a particle size ≤5 mm using a jaw crusher. The crushed slag is then fed into a dry magnetic separator with a magnetic field strength set to 0.8~1.2 T to remove ferromagnetic impurities (such as metal fragments and incompletely reacted iron oxides), yielding slag for later use.

[0070] In one preferred embodiment, the fly ash is screened, the screening comprising the following steps:

[0071] The raw fly ash was dried in a drying oven at 80±5℃ for 12 hours to reduce the moisture content to ≤1%. According to GB / T1596-2017, an air classifier was used, with the airflow velocity controlled at 8~12 m / s, to separate particles of different sizes: primary sorting: removing coarse particles with a diameter >45 μm (≤12%); secondary sorting: collecting fine ash with a diameter ≤20 μm (≥70%). In this embodiment, particles with a diameter of 10~20 μm and a density of 2.55 g / cm³ were used. 3 fly ash.

[0072] This invention relates to a geopolymer concrete with microwave absorption properties, exhibiting significantly improved microwave absorption and mechanical properties, making it a multifunctional concrete with excellent overall performance. The geopolymer concrete is cured using a gradient curing process (key parameters: hot air temperature 60-80℃ (insufficient densification below 60℃; surface cracking above 80℃); core humidity around 90% (gel growth rate decreases by 40% when humidity is <85%); microwave power 0.5-2W / g (excessive power leads to localized overheating, while insufficient power results in ineffective humidity balance)), comprehensively enhancing its mechanical properties. The geopolymer concrete possesses physical characteristics such as high temperature resistance, high compressive strength, long service life, good fluidity, and good corrosion resistance. The addition of modified silicon carbide fibers inhibits early microcrack formation, and their disordered three-dimensional distribution improves the mechanical properties of the geopolymer concrete. Furthermore, the introduction of nano-metal particles endows the modified silicon carbide fibers with magnetic loss function, achieving a synergistic effect between the magnetic loss of the metal and the dielectric loss of the matrix (silicon carbide fibers themselves are not magnetic), greatly enhancing the microwave absorption performance. Modified silicon carbide fiber exhibits superior heat resistance and oxidation resistance compared to traditional mainstream microwave absorbing material, carbon fiber (which is commonly used in microwave absorbing concrete; however, carbon fiber has poor high-temperature resistance and environmental resistance, with strength loss reaching 80%–90% or even complete failure at 600–650℃). It also demonstrates excellent oxidation resistance under high-temperature conditions and is not easily oxidized or corroded in harsh chemical environments, maintaining long-term stability and service life.

[0073] The geopolymer concrete of this invention can be directly prepared at room temperature. During subsequent curing, a three-stage gradient curing process can be preferred, which can significantly improve its mechanical and wave-absorbing properties. The geopolymer concrete of this invention exhibits excellent mechanical properties, high early strength, and outstanding performance in repair work and temporary construction. It can meet the mechanical performance requirements of most situations and shows minimal performance loss and quality degradation under acidic and alkaline environments.

[0074] Example 1:

[0075] This embodiment prepares modified silicon carbide fibers, and the specific preparation process is as follows:

[0076] 1) Cut silicon carbide fibers into micro segments with a length of 3 mm. Add the cut silicon carbide fibers to anhydrous ethanol and ultrasonically clean for 30 min. The amount of anhydrous ethanol added should be enough to cover the silicon carbide fibers. Then put them into a drying oven and dry at 90°C for 24 h to obtain dispersed silicon carbide fibers.

[0077] 2) Mix 10 mL of 40% HF and 30 mL of 68% HNO3 in a container and stir until homogeneous to obtain a mixed acid solution. Place the pretreated dispersed silicon carbide fibers from step 1) into the mixed acid solution of hydrofluoric acid and nitric acid for acid etching, ensuring that the silicon carbide fibers are completely submerged. Place the container in a constant temperature water bath and control the reaction conditions as follows: temperature 50℃, time 1.5 h, with gentle stirring every 15 minutes during the reaction.

[0078] After acid etching, the silicon carbide fibers are rinsed with deionized water until the pH reaches 6-7, and finally vacuum dried to obtain silicon carbide fibers with grooves 1-2 μm deep. If the acid etching time is too long, it will damage the silicon carbide fibers and affect their mechanical properties.

[0079] 3) Load 5g of cobalt acetylacetone and 5g of iron acetylacetone powder into a quartz boat and place it in the upstream evaporation zone (upstream evaporation zone, heated to 200℃ in 2h). Place 20g of acid-etched silicon carbide fiber in a slightly larger quartz boat and place it in the downstream deposition zone (downstream deposition zone, 300℃).

[0080] Open the H2 valve, maintain the flow rate at 200 mL / min, control the furnace pressure at 100 Pa, and raise the temperature to 200 °C in the upstream evaporation zone over 2 hours. The material produced in the upstream evaporation zone is immediately transported to the midstream reaction zone via the H2 carrier gas. The material produced in the upstream evaporation zone mainly consists of nano-metal particles Fe and Co, as well as organic carbon sources (such as methane), which are transported to the midstream reaction zone with the H2 carrier gas.

[0081] In the midstream reaction zone, the temperature is controlled to rise to 700℃ within 2 hours and maintained at 700℃ for 2 hours to form a composite form of iron and cobalt encapsulated by carbon nanotubes (CNTs), denoted as CNT@FeCo. CNT@FeCo is then transported to the downstream deposition zone under the action of H2 carrier gas.

[0082] The temperature in the downstream deposition zone is controlled at 300℃, where black modified silicon carbide fibers are formed and collected. The modified silicon carbide fibers contain a large number of carbon nanotubes coated on the surface of the silicon carbide fiber matrix, and the inner walls of the carbon nanotube ends are coated with nano-metal particles of iron and cobalt.

[0083] Figure 1 and Figure 2 The image shows an electron microscope image of the modified silicon carbide fiber obtained in this embodiment. As can be seen from the image, a large number of carbon nanotubes are wrapped on the surface of the silicon carbide fiber matrix. Elemental analysis shows that the inner walls of the carbon nanotube ends are wrapped with nano-metal particles of iron and cobalt.

[0084] Example 2:

[0085] In this embodiment, the modified silicon carbide fiber prepared in Example 1 is used to prepare geopolymer concrete with wave-absorbing properties.

[0086] The composition and content of the basic formula (excluding silicon carbide fiber) for geopolymer concrete are as follows:

[0087] Raw material weight

[0088] Slag: 126 portions

[0089] Fly ash: 234 parts

[0090] Sand (fine aggregate - standard sand): 770 parts

[0091] Stone (coarse aggregate): 1110 parts

[0092] Water: 74 parts

[0093] 32 parts of sodium hydroxide

[0094] 126 portions of water glass.

[0095] In this embodiment, the amount of modified silicon carbide fiber added is 5 parts.

[0096] The preparation process is as follows:

[0097] S1. Soak the coarse aggregate and standard sand (fine aggregate) in water for 2 hours, then take them out and wipe them dry to obtain pre-wet treated coarse aggregate and fine aggregate.

[0098] S2. Dissolve sodium hydroxide in water, allow the solution to stand at room temperature, then add water glass to prepare an alkali activator, followed by the addition of pre-dispersed modified silicon carbide fibers. The pre-dispersion method here involves ultrasonic dispersion with anhydrous ethanol followed by drying; the amount of anhydrous ethanol added is 0.03 times the mass of the modified silicon carbide fibers.

[0099] S3. The slag, fly ash, and pre-wetted fine aggregate are pre-mixed evenly. In this embodiment, mixing takes 5 minutes. Then, a portion of the alkali activator with modified silicon carbide fibers is slowly added, and mixing continues until the mixture becomes fine and uniform. Next, the pre-wetted coarse aggregate is added, followed by the remaining alkali activator with modified silicon carbide fibers. The mixture is stirred again for 5 minutes, and its fluidity is observed to obtain the geopolymer concrete slurry. In this embodiment, the alkali activator with modified silicon carbide fibers is added in two separate portions.

[0100] S4. Quickly pour the well-mixed geopolymer concrete slurry into the mold, and then quickly transfer the concrete slurry with the mold to an environment with appropriate relative humidity and temperature for curing.

[0101] The maintenance method in this embodiment is a three-stage gradient maintenance (total time 72 hours):

[0102] In the initial stage (0-2 hours), hot air circulation at 70°C is applied to the surface to quickly form a highly dense shell in the surface 3-5cm area (utilizing the rapid migration effect of heat-driven alkali ions); that is, after the geopolymer concrete is poured, the hot air circulation curing cover is fastened to the surface of the geopolymer concrete slurry to ensure that the edge of the cover is sealed with the mold, 70°C, hot air system for 2 hours.

[0103] During the intermediate stage (2-12 hours), stop the hot air treatment and turn on the humidifier to raise the humidity of the curing space to 90% within 30 minutes. Thereafter, record the humidity data every hour and humidify for 10 hours. Maintain the core temperature at 35°C and increase the humidity to 90% to promote the slow growth of aluminosilicate gel.

[0104] Final stage (12-72 hours): Transfer the geopolymer concrete treated in the intermediate stage to a curing chamber, and set a microwave antenna above the curing chamber at a distance of 30cm from the top of the chamber. Set the microwave power to 1W / g to achieve moisture balance between the surface and interior through microwave-assisted curing.

[0105] 12~24 hours: Microwave irradiation is started for 20 minutes every hour. During the irradiation process, a thermal imager is used to scan and ensure that the surface temperature is uniform (temperature difference ≤5℃).

[0106] 24~48 hours: Adjust to irradiation for 20 minutes every 2 hours, and cover the surface with plastic wrap at the same time (to reduce excessive moisture evaporation).

[0107] 48~72 hours: Remove the plastic wrap and irradiate continuously for 24 hours at a microwave power of 1W / g, allowing the geopolymer concrete to naturally ventilate and balance the humidity.

[0108] Example 3:

[0109] This embodiment is the same as that in embodiment 2, except that conventional curing process is used in this embodiment. The well-mixed concrete slurry is quickly poured into the mold, and the concrete slurry with the mold is quickly transferred to a curing box with appropriate relative humidity and temperature for curing. After the concrete slurry solidifies and forms, it is demolded and the mold is repaired. After demolding, the sample is placed in the curing box for curing. After curing, the geopolymer concrete test block is obtained.

[0110] Example 4:

[0111] The parts of this embodiment are the same as those in embodiment 3, except that the amount of modified silicon carbide fiber added in this embodiment is 10 parts.

[0112] Example 5:

[0113] The parts of this embodiment are the same as those in embodiment 3, except that the amount of modified silicon carbide fiber added in this embodiment is 15 parts.

[0114] Example 6:

[0115] The steps in this embodiment are the same as in embodiment 3, except that the basic formula in this embodiment is:

[0116] Raw material weight

[0117] Slag: 126 portions

[0118] Fly ash: 234 parts

[0119] Sand (fine aggregate - standard sand): 770 parts

[0120] Pebbles: 1110 portions

[0121] Water: 65 parts

[0122] 24 parts of sodium hydroxide

[0123] 143 portions of water glass.

[0124] Example 7:

[0125] The steps in this embodiment are the same as in embodiment 3, except that the basic formula in this embodiment is:

[0126] Raw material weight

[0127] Slag: 126 portions

[0128] Fly ash: 234 parts

[0129] Sand (fine aggregate - standard sand): 770 parts

[0130] Pebbles: 1110 portions

[0131] Water: 58 portions

[0132] 18 parts of sodium hydroxide

[0133] 156 portions of water glass.

[0134] Example 8:

[0135] The steps in this embodiment are the same as in embodiment 3, except that the basic formula in this embodiment is:

[0136] Raw material weight

[0137] Slag: 90 parts

[0138] Fly ash: 270 parts

[0139] Sand (fine aggregate - standard sand): 770 parts

[0140] Pebbles: 1110 portions

[0141] Water: 32 parts

[0142] 74 parts of sodium hydroxide

[0143] 126 portions of water glass.

[0144] Example 9:

[0145] The steps in this embodiment are the same as in embodiment 8, except that the basic formula in this embodiment is:

[0146] Raw material weight

[0147] Slag: 90 parts

[0148] Fly ash: 270 parts

[0149] Sand (fine aggregate - standard sand): 770 parts

[0150] Pebbles: 1110 portions

[0151] Water: 65 parts

[0152] 24 parts of sodium hydroxide

[0153] 143 portions of water glass.

[0154] Example 10:

[0155] The steps in this embodiment are the same as in embodiment 8, except that the basic formula in this embodiment is:

[0156] Raw material weight

[0157] Slag: 90 parts

[0158] Fly ash: 270 parts

[0159] Sand (fine aggregate - standard sand): 770 parts

[0160] Pebbles: 1110 portions

[0161] Water: 58 portions

[0162] 18 parts of sodium hydroxide

[0163] 156 portions of water glass.

[0164] Comparative Example 1:

[0165] The basic formulation of the geopolymer concrete in this comparative example is the same as that in Example 3. The difference is that 5 parts of unmodified silicon carbide fibers of the same size are added to this comparative example.

[0166] Comparative Example 2:

[0167] The basic formula of the geopolymer concrete in this comparative example is the same as that in Comparative Example 1. The difference is that 10 parts of unmodified silicon carbide fibers of the same size are added in this comparative example.

[0168] Comparative Example 3:

[0169] The basic formula of the geopolymer concrete in this comparative example is the same as that in Comparative Example 1. The difference is that 15 parts of unmodified silicon carbide fibers of the same size are added in this comparative example.

[0170] Comparative Example 4:

[0171] The basic formula of the geopolymer concrete in this comparative example is the same as that in Example 3, except that silicon carbide fiber is not added in this comparative example.

[0172] The specific proportions in the above embodiments and comparative examples are shown in Table 1.

[0173] Table 1. Example and Comparative Example Proportions (by weight)

[0174]

[0175] The concrete samples obtained from the above embodiments and comparative examples were subjected to relevant performance tests, and the specific test results are shown in Table 2.

[0176] Compression test:

[0177] The tests were conducted on a universal concrete press in the Civil Engineering Materials Laboratory of the School of Civil Engineering, in accordance with the "Standard for Test Methods of Mechanical Properties of Ordinary Concrete" (GB / T 50081-2002).

[0178] =

[0179] In the formula:

[0180] F cc —Compressive strength of concrete test block (MPa);

[0181] F—Specimen failure load (N);

[0182] A—Compression bearing area of ​​the specimen (mm²) 2 ).

[0183] The experimental steps are as follows:

[0184] Step 1: Clean the surface of the specimen and the upper and lower pressure plates of the testing machine to ensure there are no impurities interfering;

[0185] Step 2: Using the molded side as the bearing surface, accurately center the specimen on the lower pressure plate of the testing machine.

[0186] Step 3: Start the testing machine and adjust the upper and lower pressure plates to make even contact with the specimen;

[0187] Step 4: Apply load continuously at a constant rate of 0.5 MPa / s until the specimen fails.

[0188] Slump and slump spread tests:

[0189] Slump tests can evaluate the workability of concrete, particularly its fluidity and plasticity. They also determine the homogeneity and stability of the concrete mix, ensuring good workability during construction to meet specific requirements.

[0190] The slump and slump spread tests should be conducted according to the following steps:

[0191] 1. Wet the inner wall and bottom plate of the slump cone to ensure there are no residual water stains. Fix the bottom plate to the horizontal platform and place the slump cone vertically in the center.

[0192] 2. Take representative concrete samples and fill them into the cylinder in three layers, each layer being approximately 1 / 3 of the cylinder height (about 70-80mm). First layer: Use a shovel to evenly spread the sample to the predetermined height, avoiding aggregate accumulation. Tamping should be done in a spiral direction from the outside to the center, with each tamping stroke evenly distributed across the cross-section. When tamping the concrete at the edge of the cylinder, the tamping rod can be slightly tilted. When tamping the bottom layer, the tamping rod should penetrate the entire depth; when tamping the second and top layers, the tamping rod should penetrate the current layer to the surface of the next layer. When pouring the top layer, the concrete should be poured to above the cylinder opening. During tamping, if the concrete settles below the cylinder opening, it should be added immediately. After tamping the top layer, scrape off excess concrete and smooth it with a trowel.

[0193] 3. After removing the concrete from the bottom plate of the slump cone, lift the slump cone vertically and steadily. The lifting process of the slump cone should be completed within 5-10 seconds. The process from the start of loading to lifting the slump cone should be uninterrupted and should be completed within 150 seconds.

[0194] 4. After lifting the slump cone, measure the height difference between the cone height and the highest point of the slumped concrete specimen. This is the slump value of the concrete mixture. If the concrete collapses or shears on one side after the slump cone is lifted, a new sample should be taken and measured again. If the above phenomenon still occurs in the second test, it indicates that the workability of the concrete is poor and should be recorded for future reference.

[0195] 5. Observe the cohesiveness and water retention of the collapsed wet concrete specimens. The cohesiveness is checked by gently tapping the side of the collapsed concrete cone with a tamping rod. If the cone gradually sinks, it indicates good cohesiveness; if the cone collapses, partially cracks, or exhibits high segregation, it indicates poor cohesiveness. Water retention is judged by whether thin slurry is released from the concrete mixture. If no thin slurry or only a small amount of thin slurry is released from the bottom after lifting the slump cone, it indicates good water retention of the concrete mixture.

[0196] Wave absorption performance test:

[0197] The absorption performance of the test block was determined using a vector network analyzer and the waveguide method. The test steps are as follows:

[0198] Step 1: The sample needs to be cut into a rectangle that matches the waveguide cavity; in the 8~12GHz frequency band, the length and width of the cut are 22.82×10.82mm, and the thickness is 2mm.

[0199] Step Two: Check if the device is in normal working condition, and confirm that the power supply, data cable, and other connections are correct. Calibrate the device, performing the necessary calibration steps according to the device manual.

[0200] Step 3: Apply conductive adhesive evenly to the inner cavity of the waveguide and precisely embed the sample into the waveguide interface; apply pressure using a waveguide clamp to ensure seamless contact between the sample and the waveguide wall (air gap ≤ 0.05 mm).

[0201] Step 4: Based on the sample material and experimental requirements, input the relevant parameters on the equipment's control interface, such as sample thickness, initial frequency, and test time.

[0202] Step 5: Start the equipment and begin the measurement process.

[0203] Table 2

[0204]

[0205] As shown in Table 2, within the range of the geopolymer concrete formulation given in this application, adjusting the replacement rate of slag and fly ash, the amount of modified silicon carbide fiber, and the amounts of sodium hydroxide and water glass all affect the compressive strength, microwave absorption performance, and workability of the concrete. As the amount of modified silicon carbide fiber increases from 5 parts to 15 parts, the 28-day strength increases from 60 MPa to 65 MPa, demonstrating that the modification treatment can strengthen the bond between the fiber and the matrix, significantly improving the strength. The groups with reduced sodium hydroxide and water glass content (Examples 6-7, 9-10) have a 28-day strength of only 41-47 MPa, far lower than Examples 2-5 with high alkali content, indicating that sufficient alkali content is a necessary condition to ensure sufficient geopolymer reaction and the formation of a high-strength matrix. However, the microwave absorption performance of these examples still remains at a high level. The compressive strength of Example 2 at all ages was 13 MPa higher than that of Example 3, and the 28-day strength reached 73 MPa, far exceeding the 60 MPa of Example 3, proving that gradient curing under the same conditions can significantly improve the upper limit of the final strength of concrete.

[0206] A comparison between silicon carbide fiber and modified silicon carbide fiber shows that silicon carbide fiber has effective wave absorption performance, with a maximum of -14dB, while modified silicon carbide fiber has a significantly improved reflectivity compared to unmodified silicon carbide fiber, reaching a maximum of -25dB.

[0207] Meanwhile, hydrochloric acid loss tests showed that the concrete with modified silicon carbide fibers was effectively resistant to acids and alkalis. After 18 months of soaking in 1% H2SO4, the strength decreased by only 1.2%, while in 3% H2SO4, the strength decreased by only 4.5% with a 3% mass loss. In contrast, when using unmodified silicon carbide fibers, the strength decreased by more than 10% after 18 months of soaking in 1% H2SO4, and the mass loss was severe.

[0208] Under high temperature conditions of 600℃, the high temperature resistance of the geopolymer concrete in the test examples and comparative examples was tested. The geopolymer concrete in the examples of this application can still maintain a high original strength (more than 60%, or even more than 80%). Under the same conditions, the high temperature resistance of the geopolymer concrete in the comparative examples is significantly worse than that of the examples.

[0209] The above description is merely a preferred embodiment of the present invention and is not intended to limit the present invention. Although the present invention has been described in detail with reference to the foregoing embodiments, those skilled in the art can still modify the technical solutions described in the foregoing embodiments or make equivalent substitutions for some of the technical features. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the protection scope of the present invention.

[0210] Any aspects not covered in this invention are applicable to existing technologies.

Claims

1. A method for preparing modified silicon carbide fibers, characterized in that, The method for preparing the modified silicon carbide fiber includes the following steps: 1) Cut silicon carbide fibers and pretreat them with anhydrous ethanol to obtain dispersed silicon carbide fibers; 2) The dispersed silicon carbide fibers pretreated in step 1) are placed in a mixed acid solution of hydrofluoric acid and nitric acid for acid etching to create grooves on the surface of the silicon carbide fibers. The control conditions for the acid etching process are: the silicon carbide fibers are completely immersed in the mixed acid solution and the reaction is carried out in a constant temperature water bath at 50~60℃ for 1.2~1.8h, with stirring every 12~16 minutes during the process. After acid etching, the silicon carbide fiber is rinsed with deionized water until pH=6~7, and finally vacuum dried to obtain the acid-etched silicon carbide fiber. 3) Using an MOCVD device, cobalt acetylacetonate and iron acetylacetonate powders are placed in the upstream evaporation zone, and the silicon carbide fibers after acid etching in step 2) are placed in the downstream deposition zone; In the upstream evaporation zone, the temperature is controlled to rise to 180~210℃ over 2 hours. The material generated in the upstream evaporation zone is immediately transported to the midstream reaction zone via H2 carrier gas at a flow rate of 180~250mL / min. In the midstream reaction zone, the temperature is controlled to rise to 600~800℃ within 2 hours and maintained for 1.8~2.2 hours; The temperature in the downstream deposition area is controlled at 280~310℃, and modified silicon carbide fibers are formed and collected in the downstream deposition area.

2. The method for preparing modified silicon carbide fiber according to claim 1, characterized in that, In step 1), silicon carbide fibers are cut into micro segments with a length of 3-4 mm, and the diameter of the silicon carbide fiber monofilament is 8-10 μm; In step 2), the nitric acid is concentrated nitric acid, and the volume ratio of hydrofluoric acid to concentrated nitric acid is 1:2.5~1:

3.

3. The method for preparing modified silicon carbide fiber according to claim 1, characterized in that, In step 3), the ratio of the total mass of cobalt acetylacetone and iron acetylacetone powder to the mass of the acid-etched silicon carbide fiber is 8~12:

20.

4. The method for preparing modified silicon carbide fiber according to claim 1, characterized in that, The depth of the trench is 1~2μm.

5. A modified silicon carbide fiber obtained by the preparation method according to any one of claims 1-4, characterized in that, The modified silicon carbide fiber has carbon nanotubes encapsulating nano-metal particles grown on its surface, with the inner walls of the carbon nanotube ends encapsulating nano-metal particles.

6. A geopolymer concrete with wave-absorbing properties, characterized in that, The modified silicon carbide fiber as described in claim 5 is added to the geopolymer concrete.

7. The geopolymer concrete with wave-absorbing properties according to claim 6, characterized in that, The geopolymer concrete comprises, by weight, 180-270 parts fly ash, 90-180 parts slag, 1100-1110 parts coarse aggregate, 740-770 parts fine aggregate, 18-32 parts sodium hydroxide, 126-156 parts water glass, 58-74 parts water, and 5-15 parts modified silicon carbide fiber.

8. The geopolymer concrete with wave-absorbing properties according to claim 7, characterized in that, The fly ash is Class I fly ash, with a particle size of 10~20μm and a density of 2.55 g / cm³. 3 ; The slag is S95 slag with a specific surface area of ​​≥400 m². 2 / kg, density 2.84 g / cm³ 3 ; The sodium hydroxide has a purity of ≥97%. The water glass has a modulus of 3.26, a Baumé degree of 40, a sodium oxide content of 8.53%wt, a silica content of 26.98%wt, and a solid content of 35.5%wt. The raw material for the modified silicon carbide fiber is Zerafber S20 silicon carbide fiber.

9. A method for preparing geopolymer concrete with wave-absorbing properties as described in claim 7 or 8, characterized in that, The preparation process of the aforementioned geopolymer concrete with wave-absorbing properties is as follows: S1. Soak the coarse aggregate and fine aggregate in water for 1-3 hours, then take them out and wipe them dry to obtain pre-wet treated coarse aggregate and fine aggregate. S2. Dissolve sodium hydroxide in water and let it stand to room temperature. Then add water glass to prepare an alkali activator. After that, add the pre-dispersed modified silicon carbide fiber. S3. Mix the slag, fly ash, and fine aggregate treated by the pre-wetting method evenly, then slowly add a portion of the alkali activator with modified silicon carbide fiber, and continue stirring until the mixture becomes fine and uniform. Then add the coarse aggregate treated by the pre-wetting method, and slowly add the remaining alkali activator with modified silicon carbide fiber, and stir evenly again to obtain the geopolymer concrete slurry. S4. Pour the geopolymer concrete slurry into the mold. After pouring, attach the hot air circulation curing hood to the surface of the geopolymer concrete slurry, ensuring that the edges of the hot air circulation curing hood are sealed to the mold. Then, perform three-stage gradient curing: Initial stage: Apply hot air at 60~80℃ for 2 hours to the surface of the geopolymer concrete slurry to quickly form a highly dense shell in the surface 3~5cm area; Intermediate stage: Stop the hot air and turn on the humidifier to raise the humidity of the maintenance space to 85%~92% within 30 minutes. Record the humidity data every hour thereafter and humidify for 10 hours to promote the slow growth of aluminosilicate gel. Final stage: Transfer the geopolymer concrete after the intermediate stage treatment to the curing box and set up a microwave antenna above the curing box. Set the microwave power to 0.5~2W / g and achieve the balance of surface and internal humidity through microwave-assisted curing: Start microwave irradiation for 10~30 minutes every hour within 12 hours. During the irradiation process, control the surface temperature difference to ≤5℃ by scanning with a thermal imager. Then, the treatment was adjusted to irradiate for 10-30 minutes every 2 hours within 24 hours, while covering the surface with plastic wrap. Finally, remove the plastic wrap and irradiate the concrete with microwave power of 1W / g for 24 hours to allow the geopolymer concrete to naturally ventilate and balance the humidity, thus obtaining well-cured geopolymer concrete.

Images