CN110237783B - Method for improving oxidation resistance of carbon aerogel and modified carbon aerogel prepared by method - Google Patents
Method for improving oxidation resistance of carbon aerogel and modified carbon aerogel prepared by method Download PDFInfo
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- CN110237783B CN110237783B CN201910711840.1A CN201910711840A CN110237783B CN 110237783 B CN110237783 B CN 110237783B CN 201910711840 A CN201910711840 A CN 201910711840A CN 110237783 B CN110237783 B CN 110237783B
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Abstract
The invention relates to a method for improving the oxidation resistance of carbon aerogel and modified carbon aerogel prepared by the method. The method comprises the following steps: embedding the carbon aerogel by using mixed powder, performing high-temperature treatment in an inert atmosphere, and preparing a first coating on the surface of the carbon aerogel, wherein the mixed powder comprises 55-85% of silicon powder, 5-25% of carbon powder and 5-20% of alumina powder in percentage by mass, and the particle sizes of the silicon powder, the carbon powder and the alumina powder are 300-500 meshes; and preparing a second coating on the basis of the first coating by using trichloromethylsilane as a reaction gas, argon as a carrier gas and hydrogen and argon as diluent gases through a chemical vapor deposition method, so that a gradient oxidation-resistant coating is prepared on the surface of the carbon aerogel to improve the oxidation resistance of the carbon aerogel. The carbon aerogel with excellent oxidation resistance and protected by the gradient coating is prepared by the method, and the oxidation resistance of the carbon aerogel is improved by more than 30% compared with that of untreated carbon aerogel.
Description
Technical Field
The invention belongs to the technical field of carbon aerogel materials, and particularly relates to a method for improving oxidation resistance of carbon aerogel and modified carbon aerogel prepared by the method.
Background
The development of aerospace technology puts higher demands on high-temperature-resistant thermal protection materials, and high-efficiency and light-weight high-temperature-resistant thermal insulation materials are the key of various thermal protection systems. The working temperature of the scramjet engine is extremely high, and the requirement on heat insulation materials is particularly strict: the material applied to the engine heat insulation has good high temperature (more than or equal to 2000 ℃) stability, excellent heat insulation performance, lower density and good process performance, and obtains reliable heat insulation effect with smaller thickness and lighter weight.
Aerogel material is the best heat insulation material at present, and is a nano porous solid material with a network structure formed by mutually accumulating colloidal particles or high polymer molecules, and because the pore diameter (50 nm) of the nano porous solid material is smaller than the mean free path (70 nm) of air molecules, no air convection exists in the pores of the aerogel, and the nano porous solid material has extremely low gaseous heat conduction; meanwhile, the aerogel has extremely high porosity, the volume ratio of solids is very low, and the solid heat conduction is also very low, so the aerogel is an ideal high-efficiency light heat-insulating material and has wide application prospect in the field of heat insulation. The most mature technology in the currently available aerogel materials is silica aerogel, but the application temperature does not exceed 800 ℃, and the silica aerogel cannot be used at higher temperature because the nano structure is destroyed. The carbon aerogel has the best high-temperature resistance (more than or equal to 2000 ℃) in various aerogels, and has irreplaceable superiority in the field of thermal protection of high-Mach number aircrafts.
In practical application, the high-temperature aerobic environment causes serious oxidation phenomenon of the carbon aerogel material. The long-term temperature resistance of the carbon aerogel material is influenced, and therefore, the method has important significance in researching methods for improving the oxidation resistance of the carbon aerogel.
Carbon aerogels (chinese patent applications CN1891622A and CN101468795A) are obtained by further drying and carbonizing phenol and aldehyde by a sol-gel method, but their application is limited by poor oxidation resistance.
Disclosure of Invention
The invention aims to provide a method for improving the oxidation resistance of carbon aerogel and modified carbon aerogel prepared by the method, so as to solve the problem of poor oxidation resistance of the existing carbon aerogel.
In order to achieve the above object, the present invention provides in a first aspect a method for improving oxidation resistance of a carbon aerogel, the method comprising the steps of:
(1) embedding carbon aerogel by using mixed powder and performing high-temperature treatment under the protection of inert atmosphere, and preparing a first coating on the surface of the carbon aerogel, wherein the mixed powder comprises 55-85% of silicon powder, 5-25% of carbon powder and 5-20% of alumina powder in percentage by mass, and the particle sizes of the silicon powder, the carbon powder and the alumina powder are 300-500 meshes; and
(2) and (2) preparing a second coating on the basis of the first coating prepared in the step (1) by using trichloromethylsilane as a reaction gas, argon as a carrier gas and hydrogen and argon as diluent gases through a chemical vapor deposition method, so that a gradient oxidation-resistant coating consisting of the first coating and the second coating is prepared on the surface of the carbon aerogel to improve the oxidation resistance of the carbon aerogel.
Preferably, the mixed powder further comprises 0-5% by mass of silica powder, and the particle size of the silica powder is 300-500 meshes.
Preferably, the mixed powder further comprises 0-5% by mass of boron oxide powder, and the particle size of the boron oxide powder is 300-500 meshes.
Preferably, the mixed powder comprises, by mass, 55-85% of silicon powder, 5-20% of carbon powder, 5-15% of alumina powder, 0-5% of silica powder and 0-5% of boron oxide powder.
Preferably, the inert atmosphere is an argon atmosphere or a nitrogen atmosphere.
Preferably, the temperature of the high-temperature treatment is 1300-1600 ℃, and the time of the high-temperature treatment is 30-60 min.
Preferably, in the process of preparing the second coating layer on the basis of the first coating layer prepared in the step (1) by a chemical vapor deposition method, the flow rate of trichloromethylsilane is 60-200 g/h, and the flow rate of hydrogen is 0.2-1.5 m3The flow rate of the argon is 0.5-2 m3/h。
Preferably, in the step (2), the temperature of the chemical vapor deposition is 950 to 1100 ℃, and the time of the chemical vapor deposition is 20 to 100 hours.
Preferably, in the step (2), trichloromethylsilane is introduced into the chemical vapor deposition furnace by bubbling to produce the second coating layer on the basis of the first coating layer produced in the step (1) by chemical vapor deposition.
The present invention provides in a second aspect a modified carbon aerogel produced by the method of the first aspect of the invention, the modified carbon aerogel comprising a carbon aerogel and the gradient antioxidant coating formed on the surface of the carbon aerogel.
Compared with the prior art, the invention at least has the following beneficial effects:
(1) the method of the invention improves the oxidation resistance of the carbon aerogel by preparing the gradient oxidation resistant coating on the surface of the carbon aerogel for the first time, and overcomes the technical prejudice that the temperature resistance and the oxidation resistance of the carbon aerogel are certainly influenced because the powder raw material enters the nano porous structure of the carbon aerogel and the nano structure of the carbon aerogel is damaged in the coating preparation process because the carbon aerogel is generally considered to be a nano porous solid material.
(2) The mixed powder in the method comprises 55-85% of silicon powder, 5-25% of carbon powder and 5-20% of alumina powder by mass percentage, the particle sizes of the silicon powder, the carbon powder and the alumina powder are 300-500 meshes, the first layer of coating is prepared on the surface of the carbon aerogel by adopting mixed powder with reasonable composition, reasonable proportion and reasonable particle size through an embedding method, the mixed powder can be effectively prevented from entering the internal nano structure of the carbon aerogel, and a second coating (SiC coating) is prepared on the basis of the first coating by means of chemical vapour deposition, thereby preparing a gradient oxidation resistant coating consisting of the first layer coating and the second layer coating, the combined action of the first coating and the second coating obviously improves the oxidation resistance of the carbon aerogel, and the modified carbon aerogel with excellent oxidation resistance is prepared.
(3) According to the invention, through a muffle furnace high-temperature aerobic environment examination test, the oxidation resistance of the carbon aerogel (modified carbon aerogel) with the gradient oxidation resistant coating is improved by more than 30% compared with that of untreated carbon aerogel under the same condition.
Detailed Description
In order to make the objects, technical solutions and advantages of the present invention clearer, the technical solutions of the present invention will be clearly and completely described below with reference to the embodiments of the present invention. All other embodiments, which can be obtained by a person skilled in the art without any inventive step based on the embodiments of the present invention, are within the scope of the present invention.
The present invention provides in a first aspect a method of improving the oxidation resistance of a carbon aerogel, said method comprising the steps of:
(1) embedding carbon aerogel (carbon aerogel) and performing high-temperature treatment under the protection of inert atmosphere by using mixed powder, and preparing a first coating (a first anti-oxidation coating) on the surface of the carbon aerogel, wherein the mixed powder comprises 55-85% (such as 55%, 60%, 65%, 70%, 75%, 80% or 85%) of silicon powder (Si powder), 5-25% (such as 5%, 10%, 15%, 20% or 25%) of carbon powder (C powder) and alumina powder (Al powder) in percentage by mass2O3Powder) 5-20% (e.g. 5%, 10%, 15% or 20%), the particle size of the silicon powder, the carbon powder and the alumina powder is 300-500 mesh (25-48 μm); specifically, for example, Si powder, C powder and Al powder with high purity (. gtoreq.99.5%) are mixed2O3Ball-milling the powders in a ball mill for 3-6 h, and sieving with a 300-mesh and 500-mesh sieve to obtain the Si powder and the C powder with the particle sizes of 300-500 meshesAnd said Al2O3Powder, and then, according to the mass ratio of the several kinds of powder, Si: c: al (Al)2O355-85%: 5-25%: 5-20 percent of the mixture is mixed, the uniform powder is poured into a graphite crucible, the carbon aerogel small blocks are embedded, and high-temperature treatment is carried out in the protection of inert atmosphere of a vacuum furnace to prepare the first coating.
(2) And (2) preparing a second coating (a second anti-oxidation coating) on the basis of the first coating prepared in the step (1) by using trichloromethylsilane as a reaction gas, argon as a carrier gas and hydrogen and argon as diluent gases through a chemical vapor deposition method, thereby preparing a gradient anti-oxidation coating consisting of the first coating and the second coating on the surface of the carbon aerogel so as to improve the anti-oxidation performance of the carbon aerogel.
Although there are many reports in the prior art on the preparation of an oxidation resistant coating on a C/C composite material to improve the oxidation resistance of the composite material, however, in the process of preparing the oxidation resistant coatings, the powder raw materials can enter the interior of the C/C composite materials and the like, the carbon aerogel material has a structure different from that of a C/C composite material and the like, and the carbon aerogel is a nano-porous solid material, so that if an anti-oxidation coating is prepared on the carbon aerogel, the powder raw material enters into the nano-porous structure of the carbon aerogel in the coating preparation process to cause the damage of the nano-structure of the carbon aerogel, thereby being inevitably not beneficial to improving the long-term temperature resistance and oxidation resistance of the carbon aerogel, this is why no reports are found in the prior art relating to the preparation of an antioxidant coating on the surface of a carbon aerogel material; the method of the invention improves the oxidation resistance of the carbon aerogel by preparing the gradient oxidation resistant coating different from the existing oxidation resistant coating on the surface of the carbon aerogel for the first time, and overcomes the technical prejudice that the carbon aerogel is generally considered to be a nano porous solid material, and the powder raw material enters the nano porous structure of the carbon aerogel to cause the damage of the nano structure of the carbon aerogel in the preparation process of the coating, thereby being certainly not beneficial to improving the temperature resistance and the oxidation resistance of the carbon aerogel.
The mixed powder in the method comprises 55-85% of silicon powder, 5-25% of carbon powder and 5-20% of alumina powder by mass percentage, the particle sizes of the silicon powder, the carbon powder and the alumina powder are 300-500 meshes, the mixed powder with reasonable composition, reasonable proportion and reasonable particle size is adopted to prepare a first layer of coating on the surface of the carbon aerogel through an embedding method, so that the oxidation resistance of the carbon aerogel is improved to a certain extent by the first layer of coating, simultaneously micron-sized mixed powder can be effectively prevented from entering the internal nano structure of the carbon aerogel, a second layer of coating (SiC layer) is prepared on the basis of the first layer of coating through a chemical vapor deposition method, a gradient oxidation resistance coating consisting of the first layer of coating and the second layer of coating is prepared through the embedding method and the vapor deposition method, and the combined action of the first layer of coating and the second layer of coating obviously improves the oxidation resistance of the carbon aerogel, the modified carbon aerogel with excellent oxidation resistance is prepared; in addition, the inventor finds that when the particle size of each component powder contained in the mixed powder is not 300-500 meshes, the modified carbon aerogel with excellent oxidation resistance is not favorably prepared. According to the invention, through a muffle furnace high-temperature aerobic environment examination test, the oxidation resistance of the carbon aerogel (modified carbon aerogel) with the gradient oxidation resistant coating is improved by more than 30% compared with that of untreated carbon aerogel under the same condition.
According to some preferred embodiments, the mixed powder further comprises 0 to 5% (e.g., 0%, 1%, 2%, 3%, 4%, or 5%) by mass, preferably 1 to 5% by mass, of a silica powder (SiO)2Powder), the particle diameter of the silicon dioxide powder is 300-500 meshes.
According to some preferred embodiments, the mixed powder further comprises 0 to 5% (e.g., 0%, 1%, 2%, 3%, 4%, or 5%) by mass, preferably 1 to 5% by mass, of boron oxide powder (B)2O3Powder), the particle diameter of the boron oxide powder is 300-500 meshes.
According to some preferred embodiments, the mixed powder comprises, by mass, 55 to 85% (e.g., 55%, 60%, 65%, 70%, 75%, 80%, or 85%) of silicon powder, and 5 to 20% (e.g., 5%, 10%, 15%, or 20%) of carbon powder) And 5-15% (e.g., 5%, 10%, or 15%) alumina powder, 0-5% (e.g., 0%, 1%, 2%, 3%, 4%, or 5%) silica powder, and 0-5% (e.g., 0%, 1%, 2%, 3%, 4%, or 5%) boron oxide powder. The inventor finds out through a large number of experiments that the preferable components and mixture ratio of the mixed powder are Si: c: al (Al)2O3：SiO2：B2O355-85%: 5-20%: 5-15%: 0-5%: 0-5%, so as to further avoid micron-sized mixed powder from entering the internal nano structure of the carbon aerogel, and further ensure that the carbon aerogel (modified carbon aerogel) with more excellent oxidation resistance and a gradient oxidation resistant coating is prepared.
According to some preferred embodiments, the mixed powder comprises, by mass, 55-85% of silicon powder, 5-20% of carbon powder, 5-15% of alumina powder, 1-5% of silica powder and 1-5% of boron oxide powder.
According to some preferred embodiments, the inert atmosphere is an argon atmosphere or a nitrogen atmosphere.
According to some preferred embodiments, the temperature of the high temperature treatment is 1300 to 1600 ℃ (e.g., 1300 ℃, 1350 ℃, 1400 ℃, 1450 ℃, 1500 ℃, 1550 ℃ or 1600 ℃), and the time of the high temperature treatment is 30 to 60min (e.g., 30, 35, 40, 45, 50, 55 or 60 min). Through a large number of tests and grope, the inventor finds that the carbon aerogel is preferably subjected to high-temperature treatment for 30-60 min and more preferably for 30-40 min in an inert atmosphere at 1300-1600 ℃, so that the nano structure of the carbon aerogel can be further ensured not to be damaged in the embedding process, and the carbon aerogel with the gradient antioxidant coating and excellent antioxidant performance can be further ensured to be prepared.
According to some specific embodiments, step (1) is: si powder, C powder and Al powder with high purity (more than or equal to 99.5 percent)2O3Powder, SiO2Powder, B2O3Powder and other powder are ball-milled in a ball mill for 3-6 hours, and are sieved by a 300-500-mesh sieve, and then the powder is prepared by the following steps of: c: al (Al)2O3：SiO2：B2O355-85%: 5-20%: 5-15%: 0-5%: 0-5%, pouring the uniform powder into a graphite crucible, embedding the small carbon aerogel blocks, and carrying out high-temperature treatment at 1300-1600 ℃ for 30min in an argon atmosphere to prepare the first coating.
According to some preferred embodiments, the trichloromethylsilane has a flow rate of 60 to 200g/h (e.g., 60, 70, 80, 90, 100, 110, 120, 130, 140, 150, 160, 170, 180, 190, or 200g/h) and the hydrogen has a flow rate of 0.2 to 1.5 m/h in the preparation of the second coating layer on the basis of the first coating layer prepared in step (1) by chemical vapor deposition3H (e.g., 0.2, 0.3, 0.4, 0.5, 0.6, 0.7, 0.8, 0.9, 1.0, 1.1, 1.2, 1.3, 1.4, or 1.5m3H), and/or the flow rate of the argon is 0.5-2 m3H (e.g., 0.5, 0.6, 0.7, 0.8, 0.9, 1.0, 1.1, 1.2, 1.3, 1.4, 1.5, 1.6, 1.7, 1.8, 1.9, or 2.0 m)3H). The invention preferably controls the flow of trichloromethylsilane to be 60-200 g/h and the flow of hydrogen to be 0.2-1.5 m3The flow rate of argon is controlled to be 0.5-2 m3And h, further controlling the concentration of the reaction gas to realize the control of the SiC coating (silicon carbide coating).
According to some preferred embodiments, in the step (2), the temperature of the chemical vapor deposition is 950 to 1100 ℃ (such as 950 ℃, 1000 ℃, 1050 ℃ or 1100 ℃), and the time of the chemical vapor deposition is 20 to 100h (such as 20, 30, 40, 50, 60, 70, 80, 90 or 100 h).
According to some preferred embodiments, in the step (2), trichloromethylsilane is introduced into the chemical vapor deposition furnace by bubbling to produce the second coating layer on the basis of the first coating layer produced in the step (1) by chemical vapor deposition.
According to some specific embodiments, step (2) is: trichloromethylsilane is used as a raw material (reaction gas), the flow rate of the trichloromethylsilane is 60-200 g/h, argon is used as a carrier gas, hydrogen and argon are used as diluent gases, and the flow rates of the hydrogen and the argon are 0.2-1.5 m respectively3A sum of 0.5 to 2m3H, carrying the trichloromethylsilane by bubblingAnd (3) putting the carbon aerogel into a reaction chamber, and depositing for 20-100 h at 950-1100 ℃ in a chemical vapor deposition furnace, namely preparing a second coating on the basis of the first coating of the carbon aerogel.
According to some preferred embodiments, the thickness of the first layer of coating is 30 to 50 μm, and the thickness of the second layer of coating is 10 to 30 μm. In the invention, preferably, the thickness of the first layer of coating is 30-50 μm, and the thickness of the second layer of coating is 10-30 μm, so that the thickness of the first layer of coating is thicker than that of the SiC coating, the first layer of coating can protect the carbon aerogel and prevent the SiC coating from entering the interior of the first layer of coating, and the thickness of the second layer of SiC coating is thinner, mainly for enhancing the oxidation resistance of the first layer of coating.
The present invention provides in a second aspect a modified carbon aerogel produced by the method of the first aspect of the invention, the modified carbon aerogel comprising a carbon aerogel and the gradient antioxidant coating formed on the surface of the carbon aerogel.
The invention will be further illustrated by way of example, but the scope of protection is not limited to these examples.
Example 1
Firstly, high-purity (more than or equal to 99.5 percent) Si powder, C powder and Al powder2O3Powder, SiO2Powder, B2O3Ball-milling powder in a ball mill for 6 hours, sieving the powder by a 300-mesh sieve, and then mixing the powder with Si: c: al (Al)2O3：SiO2：B2O355%: 20%: 15%: 5%: 5 percent of the mixture is uniformly mixed, the uniform powder is poured into a graphite crucible, the carbon aerogel small blocks are embedded, and the mixture is treated at the high temperature of 1300 ℃ for 30min in the argon atmosphere to prepare a first layer of coating with the thickness of 30 mu m.
② trichloromethylsilane with a flow rate of 70g/h, argon as carrier gas, hydrogen and argon as diluent gas, wherein the flow rates of the hydrogen and the argon are respectively 0.2m3H and 0.5m3The trichloromethylsilane is brought into the reaction chamber by bubbling and deposited for 50h in a chemical vapor deposition furnace at 950 ℃, namelyPreparing a second coating with the thickness of 15 mu m on the basis of the first coating prepared on the surface of the carbon aerogel to prepare the carbon aerogel (modified carbon aerogel) with the gradient anti-oxidation coating.
In the embodiment, a muffle furnace high-temperature aerobic environment examination test shows that the oxidation resistance of the modified carbon aerogel in the embodiment is 32% higher than that of untreated carbon aerogel at 1000 ℃ for 30 min.
Example 2
Firstly, high-purity (more than or equal to 99.5 percent) Si powder, C powder and Al powder2O3Ball-milling powder for 5 hours in a ball mill, sieving the powder by a 300-mesh sieve, and then mixing the powder with Si: c: al (Al)2O365%: 20%: mixing 15% of the mixture evenly, pouring the even powder into a graphite crucible, embedding the carbon aerogel small blocks, and processing the carbon aerogel small blocks at the high temperature of 1400 ℃ for 30min in the argon atmosphere to prepare a first layer of coating with the thickness of 45 mu m.
② trichloromethylsilane with a flow rate of 100g/h, argon as carrier gas, hydrogen and argon as diluent gas, wherein the flow rates of the hydrogen and the argon are respectively 1.5m3H and 1m3And/h, introducing trichloromethylsilane into the reaction chamber in a bubbling mode, and depositing for 20h in a chemical vapor deposition furnace at 1100 ℃, namely preparing a second coating with the thickness of 25 mu m on the basis of the first coating prepared on the surface of the carbon aerogel to prepare the carbon aerogel with the gradient antioxidant coating (modified carbon aerogel).
In the embodiment, a muffle furnace high-temperature aerobic environment examination test shows that the oxidation resistance of the modified carbon aerogel in the embodiment is 35% higher than that of untreated carbon aerogel at 1000 ℃ for 30 min.
Example 3
Firstly, high-purity (more than or equal to 99.5 percent) Si powder, C powder and Al powder2O3Powder, B2O3Ball-milling powder for 5 hours in a ball mill, sieving the powder by a 300-mesh sieve, and then mixing the powder with Si: c: al (Al)2O3： B2O360%: 20%: 15%: 5 percent of the mixture is uniformly mixed, the uniform powder is poured into a graphite crucible, the carbon aerogel small blocks are embedded,high temperature treatment at 1400 ℃ for 30min in argon atmosphere to prepare a first coating with a thickness of 45 μm.
② trichloromethylsilane with a flow rate of 100g/h, argon as carrier gas, hydrogen and argon as diluent gas, wherein the flow rates of the hydrogen and the argon are respectively 1.5m3H and 1m3And/h, introducing trichloromethylsilane into the reaction chamber in a bubbling mode, and depositing for 20h in a chemical vapor deposition furnace at 1100 ℃, namely preparing a second coating (SiC coating) with the thickness of 25 mu m on the basis of the first coating prepared on the surface of the carbon aerogel to prepare the carbon aerogel (modified carbon aerogel) with the gradient oxidation-resistant coating.
In the embodiment, a muffle furnace high-temperature aerobic environment examination test shows that the oxidation resistance of the modified carbon aerogel in the embodiment is 36% higher than that of untreated carbon aerogel at 1000 ℃ for 30 min.
Example 4
Example 4 is essentially the same as example 2, except that:
in the first step, Si powder, C powder and Al powder with high purity (not less than 99.5 percent) are added2O3Ball-milling powder for 5 hours in a ball mill, sieving the powder by a 300-mesh sieve, and then mixing the powder with Si: c: al (Al)2O365%: 20%: mixing 15% of the powder uniformly, pouring the uniform powder into a graphite crucible, embedding the carbon aerogel small blocks, and treating at the high temperature of 1000 ℃ for 60min in the argon atmosphere to prepare the first coating.
Example 5
Example 5 is essentially the same as example 2, except that:
in the first step, Si powder, C powder and Al powder with high purity (not less than 99.5 percent) are added2O3Ball-milling powder for 5 hours in a ball mill, sieving the powder by a 300-mesh sieve, and then mixing the powder with Si: c: al (Al)2O365%: 20%: mixing 15% of the powder uniformly, pouring the uniform powder into a graphite crucible, embedding the carbon aerogel small blocks, and treating at 1800 ℃ for 20min in an argon atmosphere to prepare a first layer of coating.
Comparative example 1
The oxidation weight loss rate of the untreated carbon aerogel is shown in Table 1 after a muffle furnace high-temperature aerobic environment examination test is carried out at 1000 ℃ for 30 min.
Comparative example 2
Comparative example 2 is substantially the same as example 1 except that: step two is not included.
Comparative example 3
Comparative example 3 is substantially the same as example 3 except that: the method does not comprise the step I, and the SiC coating is directly prepared on the surface of the carbon aerogel.
Comparative example 4
Comparative example 4 is substantially the same as example 1 except that:
in the first step, Si powder, C powder and Al powder with high purity (not less than 99.5 percent) are added2O3Powder, SiO2Powder, B2O3Ball-milling powder in a ball mill for 6 hours, sieving the powder by a 300-mesh sieve, and then mixing the powder with Si: c: al (Al)2O3：SiO2：B2O350%: 20%: 15%: 8%: and 7 percent of the mixture is uniformly mixed, the uniform powder is poured into a graphite crucible, the carbon aerogel small blocks are embedded, and the mixture is treated at the high temperature of 1300 ℃ for 30min in the argon atmosphere to prepare the first coating.
Comparative example 5
Comparative example 5 is substantially the same as example 2 except that:
in the first step, Si powder, C powder and Al powder with high purity (not less than 99.5 percent) are added2O3Ball-milling powder for 5 hours in a ball mill, sieving the powder by a 300-mesh sieve, and then mixing the powder with Si: c: al (Al)2O350%: 25%: mixing 25% of the mixture evenly, pouring the even powder into a graphite crucible, embedding the carbon aerogel small blocks, and treating the carbon aerogel small blocks at a high temperature of 1400 ℃ for 30min in an argon atmosphere to prepare a first layer of coating.
From the results in table 1, it can be seen that when the first coating layer is prepared only on the surface of the carbon aerogel, the oxidation weight loss rate is 50%, which is only 15% higher than the oxidation resistance of the untreated carbon aerogel, and when the second coating layer (SiC coating layer) is prepared only on the surface of the carbon aerogel, the oxidation weight loss rate is 42%, which is only 23% higher than the oxidation resistance of the untreated carbon aerogel, while the oxidation resistance of the modified carbon aerogel with the gradient oxidation-resistant coating layer prepared by the present invention is more than 30% higher than the oxidation resistance of the untreated carbon aerogel. The reason why the first coating or the second coating is independently prepared on the surface of the carbon aerogel to improve the oxidation resistance of the carbon aerogel is not obvious is that although the first coating prepared by the invention can effectively prevent micron-sized mixed powder from entering the internal nano structure of the carbon aerogel and can improve the oxidation resistance of the carbon aerogel to a certain extent, the oxidation resistance of the carbon aerogel needs to be enhanced by the second coating; while the oxidation resistance of the carbon aerogel can be improved by directly adopting the SiC coating on the surface of the carbon aerogel, two problems exist: 1. The surface coating has limited density, and oxygen easily enters the carbon aerogel through the microscopic defects of the coating to cause oxidation; 2. the pure SiC coating can also penetrate into the carbon aerogel, so that the density is increased, and a certain pore structure in the carbon aerogel can be filled with SiC and damaged; therefore, the second coating is directly prepared on the surface of the carbon aerogel, and the oxidation resistance of the carbon aerogel is not obviously improved.
Finally, it should be noted that: the above examples are only intended to illustrate the technical solution of the present invention, but not to limit it; although the present invention has been described in detail with reference to the foregoing embodiments, it will be understood by those of ordinary skill in the art that: the technical solutions described in the foregoing embodiments may still be modified, or some technical features may be equivalently replaced; and such modifications or substitutions do not depart from the spirit and scope of the corresponding technical solutions of the embodiments of the present invention.
Claims (6)
1. A method for improving the oxidation resistance of a carbon aerogel, comprising the steps of:
(1) embedding carbon aerogel by using mixed powder and performing high-temperature treatment under the protection of inert atmosphere, and preparing a first coating on the surface of the carbon aerogel, wherein the mixed powder comprises 55-85% of silicon powder, 5-20% of carbon powder, 5-15% of alumina powder, 1-5% of silicon dioxide powder and 1-5% of boron oxide powder in percentage by mass; the particle sizes of the silicon powder, the carbon powder, the alumina powder, the boron oxide powder and the silicon dioxide powder are 300-500 meshes; the temperature of the high-temperature treatment is 1300-1600 ℃, and the time of the high-temperature treatment is 30-60 min;
(2) and (2) preparing a second coating on the basis of the first coating prepared in the step (1) by using trichloromethylsilane as a reaction gas, argon as a carrier gas and hydrogen and argon as diluent gases through a chemical vapor deposition method, so that a gradient oxidation-resistant coating consisting of the first coating and the second coating is prepared on the surface of the carbon aerogel to improve the oxidation resistance of the carbon aerogel.
2. The method of claim 1, wherein:
the inert atmosphere is argon atmosphere or nitrogen atmosphere.
3. The method of claim 1, wherein:
in the process of preparing the second coating layer on the basis of the first coating layer prepared in the step (1) by a chemical vapor deposition method, the flow rate of trichloromethylsilane is 60-200 g/h, and the flow rate of hydrogen is 0.2-1.5 m3The flow rate of the argon is 0.5-2 m3/h。
4. The method of claim 1, wherein:
in the step (2), the temperature of the chemical vapor deposition is 950 to 1100 ℃, and the time of the chemical vapor deposition is 20 to 100 hours.
5. The method of claim 1, wherein:
in the step (2), trichloromethylsilane is introduced into the chemical vapor deposition furnace by bubbling to produce the second coating layer on the basis of the first coating layer produced in the step (1) by chemical vapor deposition.
6. The modified carbon aerogel produced by the method of any of claims 1 to 5, wherein the modified carbon aerogel comprises a carbon aerogel and the gradient antioxidant coating formed on the surface of the carbon aerogel.
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