Super-elastic carbon aerogel composites and methods of making the same

Abstract

This invention discloses a superelastic carbon aerogel composite material and its preparation method. The preparation method includes the following steps: mixing substance A with deionized water and sonicating to obtain a dispersion; mixing the dispersion with an aqueous solution of graphene oxide; preparing a liquid to be dried using a freeze-drying method, a hydrothermal method, or a method of adding ethylenediamine during hydrothermal treatment; directionally freezing the liquid to be dried for 10-15 hours to obtain a wood fiber gel; immersing the wood fiber gel in a SiBCN precursor solution for 30-60 minutes under vacuum; removing the wood fiber gel and drying it; and holding the wood fiber gel at 250-350℃ for 4-5 hours under an inert gas environment, followed by holding it at 950-1050℃ for 2-3 hours to obtain the superelastic carbon aerogel composite material. The superelastic carbon aerogel composite material of this invention has a pressure more than 4 times that of the uncomposite SiBCN under deformation, and a rebound rate of more than 85% when the deformation is 10%.

Description

Technical Field

[0001] This invention belongs to the field of preparation technology of superelastic inorganic materials, specifically relating to a superelastic carbon aerogel composite material and its preparation method. Background Technology

[0002] Carbon aerogel is a porous carbon material with a unique three-dimensional network structure and properties. It was first obtained by high-temperature treatment of resorcinol-formaldehyde organic aerogel, exhibiting low thermal conductivity and high-temperature stability, as well as ultra-high porosity and ultra-high specific surface area. It has significant potential applications in thermal insulation, energy storage, catalyst support, and sound insulation. In recent years, the brittleness of carbon aerogel has been gradually overcome. Researchers have used a method of impregnating fibrous carbon aerogel with SiBCN precursors to significantly increase the pressure of carbon aerogel, demonstrating excellent compressibility.

[0003] Although the brittleness problem of carbon aerogels has been largely solved, when fibrous carbon aerogels are subjected to significant compressive deformation, their shape is difficult to fully recover to its initial form. In other words, under high compressive strain, fibrous carbon aerogels are prone to large permanent deformations, which is also detrimental to the structural stability in practical applications. Furthermore, the strength of such fibrous carbon aerogels is generally low, making them difficult to apply in real-world scenarios. Therefore, finding a way to maintain both superelasticity and high load-bearing capacity in fibrous carbon aerogels under high compressive strain is of great significance for their practical applications. Summary of the Invention

[0004] In view of the shortcomings of the prior art, the purpose of this invention is to provide a method for preparing a superelastic carbon aerogel composite material.

[0005] Another objective of this invention is to provide a superelastic carbon aerogel composite material obtained by the above preparation method, which can solve the technical problems of existing fiber carbon aerogels having low strength, being unable to maintain superelasticity and high load-bearing capacity under high compressive strain, and failing to meet the needs of practical applications.

[0006] The objective of this invention is achieved through the following technical solution.

[0007] A method for preparing a superelastic carbon aerogel composite material includes the following steps:

[0008] 1) Mix substance A with deionized water and sonicate to obtain a dispersion, wherein substance A is cellulose gel or oxidized cellulose gel, and the content of substance A in the dispersion is 0.6-1.0 wt%.

[0009] In step 1), the duration of the ultrasound is 30-60 seconds, and the power of the ultrasound is 1000-1500W.

[0010] 2) Mix the dispersion and the graphene oxide aqueous solution, and use the freeze-drying method, hydrothermal method or the method of adding ethylenediamine during the hydrothermal process to obtain the liquid to be dried. Directional freeze the liquid to be dried for 10-15 hours to obtain wood fiber gel. In this case, the ratio of substance A in the dispersion to graphene oxide in the graphene oxide aqueous solution is (3-6):(4-8) by mass fraction.

[0011] In step 2), the graphene oxide aqueous solution is a mixture of graphene oxide and water, and the concentration of graphene oxide in the graphene oxide aqueous solution is 6-8 mg / mL.

[0012] In step 2), the freeze-drying method is as follows: sonicate at 1000-1300W for 25-35 minutes until gel-like, heat in a water bath at 85-95℃ for 50-70 minutes, and cool to room temperature to obtain the liquid to be dried.

[0013] In step 2), the hydrothermal method is as follows: ultrasonically crush at a power of 130-150W for 25-35 minutes, react at 110-130℃ for 5-7 hours, dialyze in a 20wt% ethanol aqueous solution until it floats in the ethanol aqueous solution, and take it out to obtain the liquid to be dried.

[0014] In step 2), the method of adding ethylenediamine during the hydrothermal process is as follows: ultrasonically crush the sample at a power of 130-150W for 25-35 minutes, add ethylenediamine under stirring conditions to adjust the pH to 9.5-10.5, react at 110-130℃ for 5-7 hours, dialyze in a 20wt% ethanol aqueous solution until it floats in the ethanol aqueous solution, and take it out to obtain the liquid to be dried.

[0015] In step 2), the directional freezing involves placing the liquid to be dried into a mold with both the top and bottom surfaces open, so that the bottom surface of the mold is in contact with a plane at -180 to -210°C.

[0016] In step 2), the directional freezing is achieved by a directional freezing device, which includes a rectangular copper plate and the mold. The bottom surface of the mold is placed on the rectangular copper plate, and both ends of the rectangular copper plate are bent downwards at 90 degrees and immersed in liquid nitrogen.

[0017] 3) Under vacuum, immerse the wood fiber gel obtained in step 2) in the SiBCN precursor solution for 30-60 min, remove the wood fiber gel, dry it, and keep it at 250-350℃ for 4-5 h in an inert gas environment, and then keep it at 950-1050℃ for 2-3 h to obtain a superelastic carbon aerogel composite material. The SiBCN precursor solution is a mixture of polyborosilane and water, and the ratio of the mass fraction of polyborosilane to the volume fraction of the SiBCN precursor solution is 1:10. The mass fraction is in g and the volume fraction is in mL.

[0018] In step 3), the drying time is 20-24 hours and the drying temperature is 75-85°C.

[0019] In step 3), the heating rate to 250℃~350℃ is 2~4℃ / min, and the heating rate to 950~1050℃ is 4~6℃ / min.

[0020] In step 3), after the wood fiber gel is removed and before drying, an absorbent paper is placed under the wood fiber gel for 5 to 10 minutes and the absorbent paper is replaced every 1 minute.

[0021] The superelastic carbon aerogel composite material obtained by the above preparation method.

[0022] Compared with the prior art, the present invention has the following beneficial effects:

[0023] (1) This invention uses a dispersion as a carbon aerogel precursor and wood fiber gel as a preform. The pyrolysis and carbonization of the preform are performed simultaneously with the pyrolysis and sintering of the SiBCN precursor to obtain an aerogel-SiBCN composite material, thus preparing a three-dimensional macroscopically shaped superelastic carbon aerogel composite material. The superelastic carbon aerogel composite material possesses excellent high-temperature resistance and oxidation resistance, as well as superior mechanical properties, making it suitable for use in the thermal protection system of high-dynamic near-space vehicles. This not only solves the aerodynamic heating problem but also achieves lightweighting of the vehicle structure.

[0024] (2) Directional freeze drying makes the cellulose and graphene oxide in the cellulose gel more tightly bound, forming a unique columnar pore structure. This gives the composite aerogel compressive resilience in all three directions, and also results in higher strength.

[0025] (3) The strength of the superelastic carbon aerogel composite material prepared by the present invention is greatly improved. The pressure during deformation is more than 4 times that of the uncomposite SiBCN, and the rebound rate is more than 85% when the deformation is 10%. Attached Figure Description

[0026] Figure 1 SEM images of the hyperelastic carbon aerogel composite material prepared in Example 1 at different magnifications;

[0027] Figure 2 The compression resilience of the superelastic carbon aerogel composite material prepared in Example 1, the carbon aerogel composite material prepared in Comparative Example 1, the carbon aerogel composite material prepared in Example 3, and the superelastic carbon aerogel composite material prepared in Example 4 at a deformation of 10% is shown.

[0028] Figure 3 This is a SEM image of the internal structure of the carbon aerogel composite material obtained in Example 2 of the present invention;

[0029] Figure 4 This is a SEM image of the internal structure of the carbon aerogel composite material obtained in Example 3 of the present invention;

[0030] Figure 5 This is a SEM image of the internal structure of the superelastic carbon aerogel composite material prepared in Example 4 of the present invention. Detailed Implementation

[0031] The technical solution of the present invention will be further described below with reference to specific embodiments.

[0032] The raw materials in the following examples were sourced from the following locations:

[0033] The cellulose gel was added via a 3.33 wt% aqueous solution of cellulose gel, purchased from Zhongshan Nanofiber New Material Co., Ltd.

[0034] Sodium nitrate (NaNO3, analytical grade), concentrated sulfuric acid (H2SO4, analytical grade), and potassium permanganate (KMnO4, analytical grade) were purchased from Tianjin Damao Reagent Co., Ltd.

[0035] Hydrogen peroxide (H2O2, analytical grade) was purchased from Tianjin Jiangtian Chemical Technology Co., Ltd.

[0036] The graphite powder has a purity of ≥99.95% and is supplied by Shanghai Aladdin Reagent Co., Ltd.

[0037] Ethylenediamine (EDA, analytical grade) was purchased from Tianjin Xiens Biochemical Technology Co., Ltd.

[0038] SIBCN was purchased from the Institute of Chemistry, Chinese Academy of Sciences.

[0039] The directional freezing device includes a rectangular copper plate and a mold. The bottom of the mold rests on the rectangular copper plate, and both ends of the rectangular copper plate are bent downwards at 90 degrees and immersed in liquid nitrogen. Directional freezing involves placing the liquid to be dried into the mold, which has an open top and bottom, so that the bottom of the mold is in contact with a plane at -200°C.

[0040] The following examples illustrate the preparation of graphene oxide (GO) using a modified Hummer method, specifically as follows: 10g of graphite powder and 5g of NaNO3 were mixed and placed in an ice-water bath. 250mL of H2SO4 was slowly added, followed by the addition of 40g of KMnO4 over 40 minutes, with continuous stirring. After adding KMnO4, stirring continued for 3 hours. The temperature was increased to 35°C at a rate of 1°C / min and maintained at this temperature for 3 hours. Then, 500mL of deionized water was added, and the solution was heated to 90°C at a rate of 3°C / min and held for 1.5 hours. Next, another 500mL of deionized water was added and stirred for 30 minutes. H2O2 (analytical grade) was added dropwise until the solution color changed from dark brown to yellow. Finally, the solution was placed in a dialysis bag and dialyzed with deionized water, changing the deionized water every 12 hours until the solution reached neutrality. The solution was then freeze-dried to obtain graphene oxide.

[0041] The oxidized cellulose gel in the following examples was prepared using the TEMPO oxidative cellulose method, specifically as follows: Cellulose gel and deionized water were mixed and sonicated at 1200W for 30 seconds to obtain a mixture, wherein the cellulose gel content in the mixture was 0.8 wt%. Under continuous stirring, 0.025 g of TEMPO catalyst (purchased from Shanghai Maclean Biochemical Technology Co., Ltd.) and 0.25 g of NaBr were added to 300 mL of the mixture, followed by 20 mL of 4 mmol / L NaClO aqueous solution. The pH was maintained at 10.5 by adding 5 mL of 0.5 mol / L NaOH aqueous solution, and the mixture was stirred for 12 h. After the reaction was complete, the mixture was centrifuged at 12000×g for 20 minutes. The lower layer of oxidized cellulose gel was then washed with an appropriate amount of deionized water until neutral (pH = 7) to obtain the oxidized cellulose gel.

[0042] Example 1

[0043] A method for preparing a superelastic carbon aerogel composite material includes the following steps:

[0044] 1) Mix substance A with deionized water and sonicate at 1200W for 30s to obtain a dispersion, wherein substance A is cellulose gel and the dispersion contains 0.8wt% substance A.

[0045] 2) Mix the dispersion and the graphene oxide aqueous solution, sonicate at 1200W for 30 minutes until a gel is formed, heat in a 90℃ water bath for 60 minutes, and cool to room temperature (20-25℃) to obtain the liquid to be dried. Directionally freeze the liquid to be dried for 12 hours using a directional freezing device to obtain wood fiber gel. The mass ratio of substance A in the dispersion to graphene oxide in the graphene oxide aqueous solution is 6:8. The graphene oxide aqueous solution is a mixture of graphene oxide and water, and the concentration of graphene oxide in the graphene oxide aqueous solution is 8 mg / mL.

[0046] 3) Under vacuum, immerse the wood fiber gel obtained in step 2) in the SiBCN precursor solution for 30 min. Remove the wood fiber gel and place absorbent paper under it for 8 min, replacing the paper every 1 min to remove excess SiBCN precursor solution. Dry in an oven at 80℃ for 24 h. Place the wood fiber gel in a tube furnace and heat to 300℃ at 3℃ / min under argon atmosphere, holding at 300℃ for 4 h (argon flow rate 40 mL / min). Then heat to 1000℃ at 5℃ / min and hold at 1000℃ for 2 h to obtain a superelastic carbon aerogel composite material. The SiBCN precursor solution is a mixture of polyborosilane and water, with a mass ratio of polyborosilane to volume of SiBCN precursor solution of 1:10. Mass parts are in g, and volume parts are in mL.

[0047] like Figure 1 As shown, a layer of polyborosilane (SiBCN) is attached to the surface of the superelastic carbon aerogel composite material, and it is uniformly distributed in a dotted pattern on the surface of the superelastic carbon aerogel composite material.

[0048] Comparative Example 1

[0049] A method for preparing a carbon aerogel composite material includes the following steps: placing the wood fiber gel obtained in step 2) of Example 1 into a tube furnace and heating it to 300°C at a rate of 3°C / min under an argon atmosphere (argon flow rate of 40 mL / min) and holding it at 300°C for 4 h; then heating it to 1000°C at a rate of 5°C / min and holding it at 1000°C for 2 h to obtain the carbon aerogel composite material. The SiBCN precursor solution is a mixture of polyborosilane and water, and the ratio of the mass fraction of polyborosilane to the volume fraction of the SiBCN precursor solution is 1:10. The mass fraction is expressed in g, and the volume fraction is expressed in ml.

[0050] like Figure 2As shown in the figure, compared with Comparative Example 1, the resilience of Example 1 decreased slightly after SiBCN was added, but the resilience was still above 85% when the deformation was 10%. After SiBCN was added, the pressure required for the superelastic carbon aerogel composite material prepared in Example 1 to achieve a deformation of 10% was more than 4 times that of the aerogel composite material prepared in Comparative Example 1.

[0051] Example 2 (Comparative)

[0052] A method for preparing a carbon aerogel composite material includes the following steps:

[0053] 1) Mix substance A with deionized water and sonicate at 1200W for 30s to obtain a dispersion, wherein substance A is oxidized cellulose gel and the dispersion contains 0.8wt% substance A.

[0054] 2) Mix the dispersion and the graphene oxide aqueous solution, sonicate at 1200W for 30 minutes until a gel is formed, heat in a 90℃ water bath for 60 minutes, and cool to room temperature (20-25℃) to obtain the liquid to be dried. Directionally freeze the liquid to be dried for 12 hours using a directional freezing device to obtain wood fiber gel. The mass ratio of substance A in the dispersion to graphene oxide in the graphene oxide aqueous solution is 6:8. The graphene oxide aqueous solution is a mixture of graphene oxide and water, and the concentration of graphene oxide in the graphene oxide aqueous solution is 8 mg / mL.

[0055] 3) Under vacuum, immerse the wood fiber gel obtained in step 2) in the SiBCN precursor solution for 30 min. Remove the wood fiber gel and place absorbent paper under it for 8 min, replacing the paper every 1 min to remove excess SiBCN precursor solution. Dry in an oven at 80℃ for 24 h. Place the wood fiber gel in a tube furnace and heat to 300℃ at 3℃ / min under argon atmosphere (argon flow rate 40 mL / min), then heat to 1000℃ at 5℃ / min and hold at 1000℃ for 2 h to obtain carbon aerogel composite material. The SiBCN precursor solution is a mixture of polyborosilane and water, and the mass ratio of polyborosilane to the volume ratio of the SiBCN precursor solution is 1:3. The mass ratio is in g and the volume ratio is in mL.

[0056] like Figure 3 As shown, when the SiBCN precursor solution is impregnated, SiBCN fills the pores between the cellulose sheets. Due to the high concentration of the precursor, SiBCN will experience certain stress during the pyrolysis process, and the strength of the cellulose matrix is ​​not sufficient to support this stress, which will cause cracks. That is, the cracks generated during the precursor pyrolysis process will penetrate and destroy the cellulose sheets.

[0057] Example 3 (Comparative)

[0058] A method for preparing a carbon aerogel composite material includes the following steps:

[0059] 1) Mix substance A with deionized water and sonicate at 1200W for 30s to obtain a dispersion, wherein substance A is oxidized cellulose gel and the dispersion contains 0.8wt% substance A.

[0060] 2) The dispersion and graphene oxide aqueous solution were mixed and ultrasonically disrupted for 30 min using a cell disruptor at 130 W. The mixture was then placed in a hydrothermal reactor and reacted at 120 °C for 6 h. The mixture was dialyzed in a 20 wt% ethanol aqueous solution until it floated to the surface. The resulting liquid was then removed and oriented to be dried. The liquid was then oriented to be frozen for 12 h using a directional freezing device to obtain wood fiber gel. The mass ratio of substance A in the dispersion to graphene oxide in the graphene oxide aqueous solution was 6:8. The graphene oxide aqueous solution was a mixture of graphene oxide and water, with a graphene oxide concentration of 8 mg / mL.

[0061] 3) Under vacuum, immerse the wood fiber gel obtained in step 2) in the SiBCN precursor solution for 30 min. Remove the wood fiber gel and place absorbent paper under it for 6 min, replacing the paper every 1 min to remove excess SiBCN precursor solution. Dry in an oven at 75℃ for 23 h. Place the wood fiber gel in a tube furnace and heat to 300℃ at 3℃ / min under argon atmosphere (argon flow rate 40 mL / min), then heat to 1000℃ at 5℃ / min and hold at 1000℃ for 2 h to obtain carbon aerogel composite material. The SiBCN precursor solution is a mixture of polyborosilane and water, and the mass fraction of polyborosilane to the volume fraction of the SiBCN precursor solution is 1:10. The mass fraction is in g and the volume fraction is in mL.

[0062] like Figure 4 As shown, the internal structure of the carbon aerogel composite material obtained in Example 3 is relatively chaotic, with irregular pore structure and uneven pore size. In some areas, there is a regularly arranged layered structure, but there are layers in other directions that intersect each other, resulting in poor macroscopic compression and resilience performance.

[0063] Example 4

[0064] A method for preparing a superelastic carbon aerogel composite material includes the following steps:

[0065] 1) Mix substance A with deionized water and sonicate at 1200W for 30s to obtain a dispersion, wherein substance A is oxidized cellulose gel and the dispersion contains 0.8wt% substance A.

[0066] 2) The dispersion and graphene oxide aqueous solution were mixed and ultrasonically disrupted for 30 min using a cell disruptor at 130 W. Ethylenediamine was added under stirring to adjust the pH to 10. The mixture was then placed in a hydrothermal reactor and reacted at 120 °C for 6 h. After the reaction was complete, the mixture was dialyzed in a 20 wt% ethanol aqueous solution until it floated to the surface. The resulting liquid was then removed and directionally frozen for 12 h using a directional freezing device to obtain wood fiber gel. The ratio of substance A in the dispersion to graphene oxide in the graphene oxide aqueous solution was 6:8 by mass. The graphene oxide aqueous solution was a mixture of graphene oxide and water, with a graphene oxide concentration of 8 mg / mL.

[0067] 3) Under vacuum, immerse the wood fiber gel obtained in step 2) in the SiBCN precursor solution for 30 min. Remove the wood fiber gel and place absorbent paper under it for 8 min, replacing the paper every 1 min to remove excess SiBCN precursor solution. Dry in an oven at 80℃ for 24 h. Place the wood fiber gel in a tube furnace and heat to 300℃ at 3℃ / min under argon atmosphere, holding at 300℃ for 4 h (argon flow rate 40 mL / min). Then heat to 1000℃ at 5℃ / min and hold at 1000℃ for 2 h to obtain a superelastic carbon aerogel composite material. The SiBCN precursor solution is a mixture of polyborosilane and water, with a mass ratio of polyborosilane to volume of SiBCN precursor solution of 1:10. Mass parts are in g, and volume parts are in mL.

[0068] like Figure 5 As shown, the pore size distribution is relatively uniform, and the layers exhibit a stacked structure of spherical pores. Ethylenediamine binds the graphene oxide more tightly, resulting in a more uniformly distributed layered structure. The presence of pores in the layers is due to the carboxyl groups of cellulose oxide penetrating the layered structure during the graphene oxide bonding process.

[0069] Comparative Example 2

[0070] A method for preparing wood fiber gel includes the following steps:

[0071] 1) Mix oxidized cellulose gel and deionized water, and sonicate at 1200W for 30s to obtain a dispersion, wherein the oxidized cellulose gel in the dispersion is 0.8wt%;

[0072] 2) Mix the dispersion and the graphene oxide aqueous solution, sonicate at 1200W for 30 minutes until gel-like, heat in a 90℃ water bath for 1 hour, cool to room temperature (20-25℃) to obtain the liquid to be dried, and freeze-dry the liquid to be dried in a directional freezing device for 12 hours to obtain wood fiber gel. The concentration of graphene oxide in the graphene oxide aqueous solution is 4 mg / mL, and the ratio of cellulose oxide gel in the dispersion to graphene oxide in the graphene oxide aqueous solution is 3:4 by mass fraction.

[0073] The wood fiber gel obtained in Comparative Example 2 has a loose structure and cannot form a bulk structure.

[0074] Depend on Figure 2 It can be seen that the pressure and rebound rate in Example 3 were not very satisfactory. However, the pressure and rebound rate of Example 4, which added ethylenediamine, were significantly improved, with the rebound rate reaching more than 85%, and the pressure was only half that of Example 1.

[0075] The present invention has been described above by way of example. It should be noted that any simple modifications, alterations or other equivalent substitutions that can be made by those skilled in the art without creative effort without departing from the core of the present invention fall within the protection scope of the present invention.

Claims

1. The application of a superelastic carbon aerogel composite material in improving resilience, characterized in that, The preparation method of the superelastic carbon aerogel composite material includes the following steps: 1) Mix substance A with deionized water and sonicate to obtain a dispersion, wherein substance A is a cellulose gel or oxidized cellulose gel, and the content of substance A in the dispersion is 0.6~1.0 wt%; 2) Mix the dispersion and the graphene oxide aqueous solution, and use the freeze-drying method to obtain the liquid to be dried. Directionally freeze the liquid to be dried for 10-15 hours to obtain wood fiber gel. The ratio of substance A in the dispersion to graphene oxide in the graphene oxide aqueous solution is (3-6):(4-8) by mass. The freeze-drying method is as follows: sonicate at 1000-1300W for 25-35 minutes until gel-like, heat in a water bath at 85-95℃ for 50-70 minutes, and cool to room temperature to obtain the liquid to be dried. The directional freezing method is to place the liquid to be dried into a mold with both the top and bottom surfaces open, so that the bottom surface of the mold is in contact with a plane at -180~-210℃. In step 2), the graphene oxide aqueous solution is a mixture of graphene oxide and water, and the concentration of graphene oxide in the graphene oxide aqueous solution is 6-8 mg / mL. 3) Under vacuum, immerse the wood fiber gel obtained in step 2) in the SiBCN precursor solution for 30-60 min, remove the wood fiber gel, dry it, and keep it at 250-350℃ for 4-5 h in an inert gas environment, and then keep it at 950-1050℃ for 2-3 h to obtain a superelastic carbon aerogel composite material. The SiBCN precursor solution is a mixture of polyborosilane and water, and the ratio of the mass fraction of polyborosilane to the volume fraction of the SiBCN precursor solution is 1:

10. The mass fraction is in g and the volume fraction is in mL.

2. The application according to claim 1, characterized in that, In step 1), the duration of the ultrasound is 30-60 seconds, and the power of the ultrasound is 1000-1500W.

3. The application according to claim 2, characterized in that, The directional freezing is achieved by a directional freezing device, which includes a rectangular copper plate and a mold. The bottom surface of the mold is placed on the rectangular copper plate, and both ends of the rectangular copper plate are bent downwards at 90 degrees and immersed in liquid nitrogen.

4. The application according to claim 1, characterized in that, In step 3), the drying time is 20-24 hours and the drying temperature is 75-85°C.

5. The application according to claim 1, characterized in that, In step 3), the heating rate to 250℃~350℃ is 2~4℃ / min, and the heating rate to 950~1050℃ is 4~6℃ / min.

6. The application according to claim 1, characterized in that, In step 3), after removing the wood fiber gel and before drying, place an absorbent paper under the wood fiber gel for 5-10 minutes and replace the absorbent paper every 1 minute.

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