At present, the flexible graphite industry in the United States, Japan, Germany and France is in a leading position. Japan's nuclear-grade ultra-low sulfur (S < 500ppm) and high-purity (S < 50ppm) products are leading the way. Among them, high-purity flexible graphite can only be produced in Japan. The world's flexible graphite production is up to 1.5~20,000 tons.
In the anode material of lithium ion battery, since graphite has excellent electric capacity, lithium ion batteries in the United States and Japan use modified graphite as an anode material, and have formed an industry.
Baptist silica ink, currently only Germany, the United States, Russia production. This product is a new material with high hardness and high mechanical strength, wear resistance, corrosion resistance and lubricity in a wide temperature range. Compared with silicon carbide products, the biggest feature is high yield and low price.
As the color TV update cycle is greatly shortened, the performance requirements for graphite tubes for color TV tubes are also changing rapidly. At present, foreign countries still attach great importance to the research and development of color tube graphite milk.
China's new flexible graphite industry is under construction and has achieved a number of flexible graphite processing technologies and equipment during the “Eighth Five-Year Plan†period, but it is still in the trial production stage. China's lithium-ion battery is still in the small test stage, but the structurally stable anode material has been developed with natural graphite. Its content density is up to 350mAh/g. China has been able to produce 5 kinds of color TV tube graphite milk through introduction and digestion. Silicon-impregnated graphite products are currently blank in China.
Development and application of flexible graphite and expanded graphite materials
Flexible graphite and expanded graphite are important graphite deep processing products. From the perspective of materials science, there is a big difference between expanded graphite and flexible graphite, which are two materials. Expanded graphite is a loose porous structural material with excellent barrier sealing properties. With the development of technology and new materials applications, these two new materials have significantly different application fields due to their different characteristics. This article will discuss the application status and development trends of these two different and closely related materials in the chemical industry and other fields.
Since the invention of flexible graphite manufacturing technology by American United Carbon Corporation in the 1960s, the flexible graphite industry has developed for nearly 40 years. As an excellent sealing material, flexible graphite has gradually become familiar to the engineering community, and some people have given it the title of “sealing kingâ€. Flexible graphite is processed from natural flake graphite. A key technique for the preparation is the preparation of natural flake graphite as a graphite intercalation compound. Industrially, concentrated sulfuric acid is commonly used as an interlayer intercalation, so it is also called acidified graphite (expandable graphite). However, acidified graphite is a residual graphite intercalation compound after the graphite is reacted with an acid, washed with water and dried, and most of the interlayer intercalation has escaped. When most of the graphite intercalation compound is heated rapidly, the interlaminar inserts are vaporized to form a high pressure, and the graphite particles are expanded in the C-axis direction. Very thin acidified graphite scales expand like a worm, and can be as long as 1~2cm, so it is called worm graphite or graphite worm, also known as expanded graphite. When the expanded graphite is pressed or rolled, the worms mesh with each other to be rolled into a graphite coil or sheet, which is a flexible graphite. Since the intercalation agent sulfuric acid escapes in most areas during heating, the soft graphite has the same chemical composition as flake graphite except for residual sulfur of about 1000 PPM, and is pure carbon.
2.1 Preparation of expandable graphite
At present, domestic expandable graphite production uses two processes: chemical and electrochemical. The two processes are the same except for the oxidation process, such as deacidification, water washing, dehydration, and drying. The quality of most manufacturers using chemical methods can meet the requirements of GB10698-89 "expandable graphite" standard, to meet the requirements of large flexible graphite sheet production materials and export supply standards. However, it is difficult to produce special low-volatility (<10%) and low-sulfur (<2%) products, and the production process is not enough. Strengthening technical management, carefully studying the intercalation process, mastering the relationship between process parameters and product performance, and producing stable expandable graphite is the key to improving the quality of subsequent products. The electrochemical method does not use other oxidants, and the natural flake graphite and the auxiliary anode together form an anode chamber and are immersed in the concentrated sulfuric acid electrolyte, and are oxidized by a direct current or a pulse current, and are taken out after being oxidized for a certain period of time, and are expanded graphite after being washed and dried. The biggest feature of this method is that it can control the degree of graphite reaction and the performance index of the product by adjusting the electrical parameters and reaction time, with low pollution, low cost, stable quality and excellent performance. Solving the stirring problem in the intercalation process, improving the efficiency and reducing the power consumption is an urgent problem to be solved by the method. After deacidification of the above two processes, the mass ratio of the sulfurized and adsorbed sulfuric acid of the graphite intercalation compound is still about 1:1, the consumption of the intercalation agent is large, and the washing water consumption and the sewage discharge amount are high. Most of the manufacturers have not solved the problem of wastewater treatment, are in a natural discharge state, and have serious environmental pollution, which will restrict the development of the industry.
2.2 Progress in processing methods of expanded graphite
2.2.1 Research on laser expansion method
(3) studied how to use laser to puff expandable graphite. The laser has the highest energy density, and the graphite has good absorption to the laser, and its absorption rate can reach more than 95%, which satisfies the requirement that the graphite expansion needs to be rapidly heated. The laser used in the literature is mid-infrared light with a wavelength of 10.6 microns. For graphite flakes with a thickness of only a few tens of microns, the penetration depth of infrared light is approximately equivalent to its wavelength. Therefore, it has a volume heating effect on graphite flakes. In the experiment, a domestic HGL-81 type CO2 gas laser was selected. The laser expansion device used in the experiment is shown in Figure 1-1.
Due to the high energy concentration of the laser and the way of heat transfer during the expansion process, the decomposition rate of the residual compound is greater than the expansion speed of the microcracks where it is located, and the generated large amount of gas causes the upper and lower surfaces of the crack to rapidly bend, causing severe expansion. In addition, the high energy of the laser increases the expansion of small residual compound islands inside the scale, preventing the suppression of the expansion of adjacent large islands. These two reasons are that the full laser expansion has a very high expansion factor.
2.2.2 Research on Microwave Puffing Methodology
A method for producing expanded graphite worms is reported in the patent literature, as shown in Figures 1-2. The puffing apparatus is characterized in that a certain thickness of expandable graphite particles (up to 10 mm thick) is laid flat on a conveyor belt. The expandable graphite is expanded after passing through a radiation zone. The flux density amounts in the radiation zone are at least 500 KW/m2. The radiation source can be an infrared light source, a microwave, or a laser beam.
2.2.3 Research on plasma expansion method
A method for expanding a fluorographite interlayer compound by Induction Coupled Plasma (ICP) is described in the literature. The experimental setup is shown in Figure 1-3. The fluorinated graphite intercalation compound is continuously injected into an argon plasma torch with a high temperature of up to 5000-8000 K, and an expanded graphite worm is obtained through a collecting system. Analysis of graphite worms by scanning electron microscopy and Raman Raman spectroscopy indicated that the expanded graphite produced by this method had a carbon nanotube structure. The article believes that this method of expansion may become a promising technology for the preparation of carbon nanostructures in the future.
2.3 Development and traditional application of flexible graphite
Due to the gas-solid two-phase structure of flexible graphite, it has a good sealing performance, and has a broad market as a sealing material. In particular, asbestos , a traditional sealing material, has been banned due to environmental pollution, providing opportunities for development of flexible graphite. Internationally, a number of new brands of high-grade flexible graphite have been developed according to different requirements of users. Nuclear power and fine chemicals require low-sulfur and high-purity flexible graphite. The general nuclear-grade flexible graphite has a sulfur content of less than 500 PPM; the nuclear island part has higher requirements. Petrochemical particular fine chemical, in order to prevent corrosion of the metal sealing surface, requiring inhibition flexible graphite type, and the like are generally added molybdate inhibitor in the flexible graphite. In addition, in order to stop production when the pipeline leaks, there is an injection type flexible graphite filler with temperature and pressure plugging. The production of domestic flexible graphite materials is mainly from six flexible graphite production paper and sheet production lines imported from Japan, the United States and Canada from the late 1980s to the mid-1990s (Shandong Binzhou Flexible Graphite Factory, Shandong Qingdao Heilongjiang Concrete Graphite Company, Inner Mongolia Linhao Flexible Graphite Company, Hubei Yichang Graphite Industry Company, Shandong Qingdao Advanced Sealing Material Co., Ltd.), the actual production capacity of each line is 100~150t, and the total output is 600~800t. The quality of the produced flexible graphite sheet, the overall level and the level of foreign 2, 3 grades. After 1992, the Shanxi coal technology, Tsinghua University, Zhejiang University and other units, equipment from abroad, combined with experience in production, design and manufacture of flexible graphite sheet width continuous production line 250 ~ 1000mm, the domestic equipment production capacity of about 300t. The quality of the plates is up to 1/3 of the total output, and most of them are only below the level 3 or 3. Production system function and equipment manufacturing accuracy are important factors affecting the quality of the board. The introduced flexible graphite production line has basically the same function, from expansion, feeding, calendering, shearing, to coiling, all continuously and automatically. The number of online monitoring points is small, and the lack of detection information feedback function restricts the improvement of the quality of the board. Another aspect of the development of flexible graphite sealing materials is the development of low-index flexible graphite. Japan has a flexible graphite with a minimum carbon content of 90%, which reduces costs and price. In fact, for some equipment, such as agricultural machinery, extensive chemical machinery, etc., it is not necessary to use high-grade flexible graphite. Low-grade flexible graphite, sealing performance is better than asbestos, can meet the technical requirements of the process, and the cost is low, so that in the low-end product market, there is a strong competitiveness, thus promoting the development of the industry. In addition, as a sealing material, flexible graphite has not been well solved in terms of sealing and leak rate quantitative testing from the user to the manufacturer in terms of technological innovation and testing equipment. Foreign countries attach great importance to technical research in this area. Germany has established standards in 1995, which stipulates testing techniques and equipment, requiring that the nitrogen leakage rate of flexible graphite sheets and composite sheets is less than 0.1 mg/m*s (DIN 28091). More work should be done in this area to improve the product quality and use level of the flexible graphite sealing materials in the country.
With the increasing application of flexible graphite products in modern engineering, the world's demand for flexible graphite materials is increasing year by year. Most of the foreign flexible graphite is used for seals such as cylinder head gaskets and intake and exhaust gaskets of automobile engines, followed by seals in petrochemical, chemical, power generation and other departments. Due to the controversy over the restrictions on asbestos in China, the application of flexible graphite in the seals of internal combustion engines is restricted. The main markets are still in the chemical, petrochemical and power generation industries. The main seals are packing rings (pressed directly with flexible graphite paper), wound gaskets, universal packing and asbestos products used in metal materials, all of which can be replaced by flexible graphite. Based on the superior properties of flexible graphite materials, the high ground of asbestos sealing materials largely means a huge potential market for flexible graphite materials. Flexible graphite materials are unlikely to completely replace asbestos sealing materials in the short term, and their potential market capacity is huge.
2.4 Extension of flexible graphite use
Flexible graphite is not only used for sealing, but also has excellent properties such as electrical conduction, thermal conductivity, lubrication, high and low temperature resistance, and corrosion resistance. For this reason, the use of flexible graphite has expanded over the years. Electrothermal materials made from their electrical conductivity and ease of processing can be used in some harsh conditions. Of course, due to its low strength, it must be fixed and have a support structure. An example that has been applied is a picture frame type electric heater; it has also been reported that flexible graphite paper is used for electromagnetic shielding. In the new energy system fuel cell, flexible graphite can be used as a current collector material, which not only conducts electricity well, but also resists corrosion of the electrolyte, and can conveniently suppress the complicated guiding groove system of the fuel gas and the oxidant gas. Using the excellent reflective properties of flexible graphite for thermal radiation conduction, thermal shielding (insulation) components for high temperature equipment can be fabricated. For radiative heat conduction (>850 °C), flexible graphite is an excellent thermal insulator, and its structural properties are stable, which has better shielding effect than metals such as tungsten and molybdenum. Graphite has long been used as a high-temperature lubricant, while flexible graphite foils are excellent in shape. It is used under high temperature mold conditions such as die forging, which is excellent in lubricity and can avoid lubrication dead spots, and the effect is very good. Other new uses are also being developed.
2.5 Application of expanded graphite in environmental protection and biomedicine
The expanded graphite obtained by high-temperature expansion has a rich pore structure and thus has excellent adsorption properties. Most environmentally friendly materials use the adsorption of pollutants to achieve pollution control purposes. Therefore, the porous structural material of expanded graphite can be used as an adsorbent material factory to develop and close pores, and the pore volume accounts for about 98%. The pore structure material is different, and it is a porous material with a mesoporous structure. Therefore, its adsorption characteristics are also different from that of activated carbon. Its adsorption performance in gas phase adsorption is far less than that of activated carbon; however, liquid phase adsorption is much larger than that of activated carbon. The macroporous structure facilitates the adsorption of organic and bioengineered macromolecular species. Due to the non-polarity of the graphite surface, the oil is hydrophobic in the liquid phase adsorption, and the oil is preferentially adsorbed. The experimental results show that lg expanded graphite can adsorb more than 80g of heavy oil. Therefore, it is a promising environmentally friendly material for removing surface oil pollution. In 1997, the offshore oil tanker in Fukuoka, Japan, leaked and tried to remove expanded graphite, which achieved good results. Due to its lipophilic and hydrophobic properties, it is also effective to remove trace oil contamination in water. In the wastewater treatment of chemical enterprises, environmental protection projects are often combined with various methods and materials. Expanded graphite is a good microbial carrier for the treatment of commonly used microorganisms (bacteria). Therefore, expanded graphite has a good application prospect in chemical water treatment, especially in the treatment of oily organic macromolecular contamination. And because of its chemical stability, regenerative recycling after pollution control is relatively easy. Expanded graphite has a wide application prospect in biomedical materials due to its adsorption properties to organic and biopolymer molecules. Tsinghua University and other developed the expansion of graphite to make medical dressings instead of gauze. After more than 300 experiments with white mice, rats, guinea pigs and military strength rabbits, it proved that it is non-toxic, has no side effects, no irritation to the wound, no black, and promotes heal. In the four hospitals of the First Military Medical University, the Department of Burns, 114 clinical trials were carried out. The effect is better than traditional gauze drainage, which is effective against infection, bacteriostasis and anti-inflammatory, and can replace 50-80% of gauze. This achievement has achieved the invention patent (ZL95115437.0) is being industrialized.
2.6 Other application prospects of expanded graphite
Expanded graphite has a large specific volume, a bulk volume of 100~300ml/g, stable chemical properties, high oxidation temperature (>400°C) and high safety. Therefore, it is an ideal thermal insulation (insulation) and sound insulation material. Some foreign high-end buildings, passenger ship wall materials, a layer of expanded graphite with a certain density to maintain insulation, sound insulation, and in the case of fire, to prevent the spread of fire. In foreign countries, expandable graphite has been added to the foam pad and the wall of the engine room. Once the fire expands rapidly, the expanded graphite takes up space and blocks the fire propagation passage. Although expanded graphite is used, the physical and chemical properties of expanded graphite are utilized. Expanded graphite is pulverized into fine powder, which has good scattering absorption characteristics for infrared waves. It is a good infrared shielding (stealth) material and plays an important role in the photoelectric countermeasures of modern warfare. The structure, properties and applications of expanded graphite are under research and development.
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