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Graphite materials have a variety of excellent properties, but in oxygen-containing high temperature environments, graphite materials are easy to oxidize and corrode, limiting their high-temperature properties, increasing production costs and resource consumption and waste of graphite materials. Therefore, the antioxidant research of graphite materials has become a subject of general concern.
the root cause of graphite oxidation problems
the crystal structure of graphite materials, as shown in Figure 1, shows that the carbon mesh plane of graphite crystals is spatially limited and ends at the crystal boundary, misalmed, or surface . There is always a certain number of edge carbon atoms in the carbon mesh plane of graphite crystals, which have unsaturated chemical bonds and free π electrons compared to the inner carbon atoms, which are easy to come into contact with oxygen atoms and prone to chemical reactions at temperatures of up to 400 degrees Celsius. The oxidation of the edge carbon atoms reduces the carbon mesh plane, and starting with the internal activated carbon atoms, the surrounding carbon atoms break the bonds one by one, producing CO2 or CO, which eventually creates corrosion pits on the carbon mesh plane.
Figure 1: Crystal structure of graphite
The essence of graphite oxide is that oxygen spreads near the surface of the material, and because of the diffusion of molecules, oxygen molecules are adsorbed at the active point of graphite material, and oxygen and carbon react to produce carbon oxides.
the way carbon is oxidized varies under different conditions.
Graphite Antioxidant Method
From the graphite material oxidation weight loss can be seen from the graphite material oxidation weight loss of the main methods are two main methods: add antioxidants to the graphite-based body to reduce the oxidation rate of the material and apply an antioxidant coating on the outer surface of the graphite material, so that the graphite material and oxidizing gas isolation, prevent the gas from entering the material, improve the antioxidant properties of the graphite material. The specific technical methods are: solution impregnation method, substation correction method and surface coating antioxidant coating.
solution impregnation method
solution impregnation method is an effective way to improve the high temperature antioxidant properties of graphite materials, the method is to use appropriate impregnation agent preparation solution to impregnate graphite products, and then after heat treatment, so that seeping into the graphite material The internal impregnation agent is transformed into a high temperature antioxidant, filling the internal pores of the graphite material and covering the graphite surface, thus isolating the oxidizing gas and blocking the gas from entering the inside of the graphite material, delaying the oxidation reaction and improving the high temperature antioxidant properties of the graphite material.
: simple process operation, low cost, not easy to fall off and not easy to produce micro-cracks and other advantages, and basically maintain the performance of graphite materials.
: The impregnation agent used in the impregnation method has the disadvantage of volatile at high temperatures (above 800 degrees C) and is therefore limited to oxidation protection below 800 degrees C.
substation modifier
substation modifier is an internal modified method that adds oxidative inhibitors inside the graphite substate to improve the antioxidant properties of the material. The selection of additives should meet the following requirements:
(1) additives and graphite substate materials should have good chemical compatibness;
(2) have low oxygen, moisture penetration capacity;
(3) can not have catalytic effect on oxidation reaction;
(4) can not affect the original mechanical and thermal properties of graphite-based materials, can effectively inhibit the oxidation of graphite materials.
In practice, the commonly used additives include Al, B, Si, Ti, Zr, Mo, Hf, Cr's oxides, carbides, nitrides and borates, which have high melting point, low volatility, high hardness and so on.
: can greatly improve the high temperature antioxidant properties of graphite materials, the effect is obvious.
: The addition of oxidative inhibitors tends to reduce the mechanical and thermal properties of the material.
surface coating method
high temperature antioxidant coating is the most effective and important means of natural graphite antioxidant, it can greatly improve the temperature of graphite materials in the oxidation environment. Its basic function is to isolate the substation material from the oxidizing environment.
A complete antioxidant coating design should take into account the factors
shield antioxidant old difficulties have made new progress
A complete coating system should include an oxygen blocking layer, a sealing layer, and a bonding layer.
types of antioxidant
can be divided into single and multi-layer coatings, depending on the form of the coating. The multi-layer coating includes: double-layer antioxidant coating system, multi-layer antioxidant coating system. According to its preparation raw materials can be divided into: glass coating, ceramic coating, metal coating and so on.
glass coating
e.g. boron silicate glass coating, the process is simple and inexpensive, but the glass coating is volatile at high temperatures and consumes too much in the antioxidant process.
ceramic coating
the most extensive antioxidant system studied is mainly the silicone series. SiC is usually used as a preferred material for the preparation of antioxidant coatings. However, there are still some differences between graphite and SiC thermal expansion coefficients, which can lead to certain thermal stresses between the coating and the graphite substate at high temperatures. Residual thermal stress can cause the coating to crack or even fall off, preventing the coating from providing continuous and effective antioxidant protection for the substation material. One solution is to prepare a composite coating with a certain concentration gradient on the surface of the material.
metal coating
metal coating is mainly the use of some high melting point of metal (e.g. molybdenum, chromium, nickel, etc.) to prepare antioxidant coating, so that the material to obtain a certain degree of antioxidant properties.
Preparation of antioxidant coating
The current method of preparing antioxidant coating is mainly envelining conversion method, liquid phase impregnation method, CVD method, plasma spray method, liquid phase melt seepage method, sol-gel method, electron beam spray method and so on.
pulation method
the process principle of the enculation method is to bury the graphite material in the already mixed enculated powder, and then put the graphite crucible in a high-temperature sintering furnace in a vacuum or inert gas atmosphere for heating. As the sintering temperature increases, the buried powder in direct contact with graphite begins to form a coating of a certain thickness on the surface of the substation in contrast to the liquid phase or gas of the graphite material.
: the preparation process is simple, and the product size changes less before and after the burial.
: some ceramic melting point is higher, preparation needs a higher temperature. The process principle of
slurry method
is to mix the raw materials used for the preparation of the coating with adhesives and solvents into the slurry, adjust the parameters of each slurry until the experimental requirements are met, the prepared slurry is painted on the surface of the substing material or impregnated the substation in the slurry, after high temperature sintering, the coating is made on the surface of the substing.
: simple process, easy to operate.
: the coating tightness is poor, the binding strength of the coating and substation is poor. the
solgel gel method
gel method is the metal organic salts or metal inorganic salts into a uniform solution, at low temperature after hydrolyzing, condensation and other chemical reactions, from the sol to the gel, and then at a lower temperature the gel heat treatment and synthesis of glass, ceramics and other coatings.
: the equipment requirements are not high, can be coated on the surface of large substates.
disadvantages: coating materials are highly demanding, require a certain viscosity and fluidity, are evenly cured on the substrate surface, and can be chemically and physically combined with each other.
Chemical vapor deposition method
The principle of chemical vapor deposition is to first vaporize the coating material into steam, which enters the reactor and adheres to the substrate due to the effect of pressure difference.
: the resulting coating density is high, the coating thickness is uniform.
Disadvantages: The process is complex, the experimental period is long, when depositing the inner hole wall of irregular aperture, due to the temperature difference between the inside and outside of the hole, the steam of the preparation coating is deposited in the outer layer of the hole and then closed, thus making the inner layer of the coating deposition resulting in poor density, uneven coating and poor integrity.
Plasma Spraying method
using the high temperature of 800 degrees C-1500 degrees C generated by the arc plasma spray gun, any substance that does not decompose or sublimate when melted is sprayed on the solid substrate surface after melting at high temperature through the input device.
: gas-solid opposite should be pollution-free, treatment time is relatively short, through continuous adjustment of raw material composition, to achieve the composition gradient transition.
: the process is complex, plasma production requires a certain vacuum environment, equipment is complex.