"research carbon" (X): organic to inorganic conversion-carbon fiber is how to "burn"?

The specific strength (strength/density) and specific modulus (modulus/density) of carbon fibers (note: the higher the modulus of the material, the stronger the deformation resistance) are metal materials, from aluminum alloys to alloy steels, titanium alloys, etc. 5 to 15 times. It is precisely because of its light weight, strong and not easy to deform, carbon fiber has become the most critical material for the development of satellites, aerospace vehicles, fighters and advanced military equipment. At the same time, it is also used in civil aircraft, clean energy, transportation, Civil construction and ski poles, tennis rackets, fishing rods, rowing, audio equipment and other advanced cultural and sports products have broad applications. Figure 1 is an overview of the development of carbon fiber applications in the past decade. It is estimated that an Airbus A380 aircraft will use about 35 tons of CF, and a Boeing B787 aircraft will also use about 25 tons of carbon fiber. China will put into production the domestic large aircraft C919 in 2013. It is said that the amount of composite materials will account for 15%. Of course, carbon fiber is also an essential material.

1 Development of carbon fiber application in recent ten years

In our impression, charcoal is both brittle and hard things, how can we make charcoal become both as thin and soft as silk and strong than steel? In fact, it is quite simple to say, and with wood fired into charcoal and other manufacturing of most carbon materials, carbon fiber is also made of organic matter "burned. As early as 1879, Edison used cotton fiber to burn carbon wire and used it as a filament to invent the electric light.

So far, a solvent that can dissolve solid carbon has not been found, and it is necessary to melt the carbon at 100 atmospheric pressure and at a high temperature above 3800 ° C., so it is impossible to manufacture it directly from elemental carbon by melting or solvent spinning of general synthetic fibers. Fibrous carbon can be obtained by "burning" or "long" methods. If the organic fiber is directly heated in the atmosphere, it will burn. Of course, it cannot be refined into carbon fiber. The "burning" here refers to heating it in the isolated air, removing the non-carbon atoms, and forming only carbon fiber while maintaining the same shape. "Long" is the gas containing carbon monoxide or hydrocarbons isolated from the air heating, so that they are pyrolyzed to form carbon atoms, under the action of the catalyst by the carbon atoms grow into carbon-containing filaments, but the latter method can only get nano-carbon tubes, nano-carbon fibers or gas phase growth carbon fibers and other non-continuous short filaments. Under certain conditions, almost all organic fibers can be "burned" into carbon fibers, but it has been found that only polyacrylonitrile fiber (acrylic), asphalt fiber and rayon three kinds of raw materials are more suitable, and polyacrylonitrile fiber is the most ideal, almost 90% of the world's commercially available carbon fibers are made from its precursor. The three raw materials mentioned earlier, in addition to the raw silk manufacturing process is quite different, the "burning" process is similar, here only polyacrylonitrile fiber for the raw silk to be introduced. 2 is a photograph of a polyacrylonitrile precursor (white), a carbon fiber (black), and a tennis racket and a golf club made.

Fig.2 Polyacrylonitrile precursor (white), carbon fiber (black), tennis racket and golf club

Pure acrylonitrile fiber has a tight internal structure and poor clothing performance, so other ingredients should be added to improve elasticity, feel and dyeing. It can be made into a variety of artificial wool, wool, blankets, fur, puffed yarn and so on. Carbon fiber, of course, can not use this civilian acrylic yarn as raw material, it must use a special raw silk.

First of all, the content of acrylonitrile in the spinning dope of the carbon fiber should be greater than 98%, otherwise the fiber will be too soft, but the remaining 2% of the copolymer component is essential. They include components capable of promoting cyanocyclization, which will be described later, penetration of oxygen into the interior of the filament, drawing of the tow, and the like. The density of the fiber should be as high as possible, the structure should be relatively uniform, the difference between the skin layer and the core layer should be small, the surface of the fiber should be smooth, and the internal defects of the fiber should be few. The compactness of the fibers can be improved by drawing. After drawing, the tow must also be oilized to protect it from damage during post-processing.

Organic raw silk through solid-phase carbonization into inorganic carbon, some of the inherent characteristics of raw silk will be left to the final products, like ordinary organisms, there is a "raw" and "nurturing" relationship between the two. Without good quality and well-structured embryos, even good cultivation cannot produce a healthy body. On the other hand, even if there is a good embryo, but the acquired cultivation conditions are too poor, it is also difficult to get a good body. In the subsequent processing, the polyacrylonitrile fiber will inherit the basic structure formed during spinning. To obtain high-strength carbon fiber, it must have high-quality, ideal structure of the precursor, and then in the subsequent "burning" In the process, the non-carbon atoms are removed while maintaining the original structure intact, and new cracks and defects are not generated on the tow as much as possible during the processing.

In the "burning" system, if it is directly heated in an inert gas, the carbon backbone of the polyacrylonitrile fiber will be decomposed due to the rapid heat release of the reaction, and the complete carbon fiber will not be obtained. If the first heating in the air, the so-called "pre-oxidation", the oxygen in the air can promote the cyano group in polyacrylonitrile crosslinking, cyclization, the formation of heat-stable ladder molecular structure. Figure 3 is the "burning" process of polyacrylonitrile precursor and its structural transformation in different processes. The chemical structure of the pre-oxidized filament is shown in Figure 3. In this ladder structure, a bond break will not reduce the molecular weight of the whole polymer like a linear polymer, even if there are several bond breaks, as long as it does not occur in the same ring at the same time, the molecular weight of the polymer will not decrease; only when two bonds on both sides of a ring are broken at the same time, the whole molecule will break, which is usually very unlikely. If the cyano crosslinking is incomplete, the remaining cyano group will be released as HCN or NH3 upon heating, which will reduce the carbon yield. However, excessive oxidation not only deteriorates the orientation of the polyacrylonitrile fiber molecules, but also reduces the carbon yield due to excessive formation of CO2 and CO during carbonization. When the temperature of the pre-oxidation is 200-300°C, the time is only 10-100 minutes. The fibers must also be tensioned during preoxidation to keep the polyacrylonitrile backbone aligned along the fiber axis.

Fig.3 Burning process and structure transformation of polyacrylonitrile precursor

After the pre-oxidation of the original silk, it can be under the protection of inert gas by solid-phase carbonization, thermal release in addition to nitrogen, oxygen, hydrogen and other non-carbon atoms, and finally form inorganic carbon fiber. When carbonization is used in two stages, the temperature of the former stage is 650-1000 ℃, and the latter stage is 1200-1400 ℃. At this time, the strength of carbon fiber is the highest. With the increase of temperature, HCN, NH3, N2, CO2 and H2O are released. At the same time, the carbon atoms of sp3 hybrid orbital in organic polyacrylonitrile are gradually converted into carbon atoms of sp2 hybrid orbital. The carbon ring composed of sp3 hybrid orbital is also continuously expanded. Short trapezoidal molecules are combined with each other, and the condensed carbon ring is continuously extended. After heat treatment at about 3000 ℃, the final graphite fiber will form an inorganic graphite-like stack composed almost entirely of carbon planes, and the modulus of the graphite fiber reaches the highest value. The formation of a carbon web in the fiber with increasing temperature is shown in Figure 4.

Fig.4 Formation of carbon web in fiber with increasing treatment temperature

By about 1400 DEG C "burned" over the carbon fiber, the vast majority of carbon has been converted into along the fiber axis parallel to the arrangement of the carbon network, and these carbon network is one of the strongest valence bonds in nature, SP2 hybrid orbital carbon atoms form C = C covalent bond composition, so there is good rigidity and strength. When fully converted to a graphite layered structure and ideally perfectly parallel to the fiber axis, its theoretical intrinsic strength and modulus can reach 184GPa and 1020GPa, respectively. However, the tensile strength of T1000 carbon fiber with the best performance is only 7.02GPa, which is only 3.9% of the theoretical value, which is mainly due to incomplete structure, impurities and surface defects. Obviously, there is still much room for improving the tensile strength of CF. To this end, it is necessary to control the structural transformation of polyacrylonitrile organic fibers to inorganic carbon fibers, in which the integrity of the ideal structure of the precursor is very important. Impurities and defects in the precursor must be eliminated as much as possible, the fiber structure is uniform and dense, and the orientation is regular and intact. When removing the thermal transformation of non-carbon atoms, the carbon atoms are not separated as much as possible, and the carbon structure formed is more intact.

The manufacture of carbon fiber looks very simple, just a "burn" word, but it is very complicated to implement. It includes a series of fine chemical processes such as polymerization, spinning, pre-oxidation and carbonization of polyacrylonitrile. Its production process, equipment manufacturing and on-site management are very strict, and it must not be sloppy. Because of this, its price is still relatively expensive, depending on the performance and brand, the price per kilogram can reach hundreds to thousands of yuan. So far, three Japanese companies alone account for 75% of the world's total output of less than 50000 tons, while Toray accounts for more than 35%. Although there is a small amount of production in China, it is still in the stage of vigorous research and development. It is hoped that large aircraft with domestic carbon fiber as the main structural material will come out as soon as possible!

This article is from Wang Maozhang Science Net blog.
Link address: http://blog.sciencenet.cn/blog-526286-428746.html