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1. Structure and properties of carbon fiber

    (1) Graphite-like lamellae and microcrystalline structure

    (2) Texture

     2. Influence of the preparation process of polyacrylonitrile carbon fiber on the structure of carbon fiber

    (1) Raw materials

    (2) Spinning

    (3) Oxidative carbonization

    3. Development status of carbon fiber

    (1) Performance and output

    (2) Stability and market

    First, let's talk about the relationship between the structure and performance of carbon fiber

    The so-called carbon fiber is the intersection of graphite or graphene and fiber, which is a fiber material with a graphite-like structure. To understand the structure of carbon fiber, let's start with graphene. We know that graphene is composed of carbon atoms, that is, it is all composed of carbon-carbon structures. There are three main ways to form carbon-carbon bonds. Because the carbon outer layer has 4 electrons, two s electrons, and two p electrons, when these 4 electrons are hybridized into all the same properties, it is horizontal sp3 hybridization. Each carbon atom is covalently bonded to the other 4 carbon atoms, which is the diamond structure, and the bond length is 0.154 nanometers. When a carbon atom is covalently bonded to three other carbon atoms, there is one p electron left to play soy sauce. The bond length is 0.142 nanometers. , like a giant football goal net, this is the structure of graphene. So some people say that graphene is a stronger material than diamond. In terms of tensile strength and modulus of a single layer, yes. Carbon fiber can be regarded as a material composed of graphene. The graphene is arranged in the axial direction of the fiber, which is called orientation; when it is completely parallel, the degree of orientation is 100%, which is of course impossible. When the carbon fiber is stressed in the axial direction, the mechanical properties of the graphene sheet come into play, which is what people call the theoretical strength of 180GPa. But how do so many layers of graphene line up along the fiber axis? The distance between graphenes is greater than 0.334 nanometers, and it is easy to slide, which is why graphite feels soft to us. Can be written as a pencil. Therefore, in addition to the parallel arrangement of graphene sheets along the fiber axis, the carbon fiber has a structure that prevents interlayer sliding, so it is different from the structure in natural graphite, so the carbon fiber is called a graphite-like structure. When the graphene sheets are rolled up, if they are cut in the axial direction, it is like a cut cabbage. At this time, the graphene sheets are not so easy to slide. If viewed from the cross section, it is better than the cabbage roll. If it is more messy, it will be more difficult to slide, so the carbon fiber is a kind of chaotic layer structure from the cross-section, and it is an approximately parallel structure from the axial direction;

    question:

    For sp2 hybridized carbon atoms, the extra p electrons should be interlayer conjugated. What effect does it have on carbon fibers? Thank you, teacher

     The extra p electrons are conjugated between layers, which shortens the carbon-carbon bond, increases the bond energy, conducts electricity, and turns black in color.

    Increased interlayer adhesion? Improve carbon fiber strength?

    Maybe not? The interlayer conjugation reduces the slip between the layers of cabbage and enhances the strength and modulus, while shortening the carbon-carbon bond increases the bond energy of the carbon atoms in the layer, right?

    It is difficult to conjugate between layers, I am talking about intra-layer

    Oh, so, in addition, the bond energy after sp2 hybridization is higher than that after sp3 hybridization, so the energy required to destroy the carbon-carbon bond of graphene is higher than that of diamond. Can you understand that? Mr. Lu ? thanks

    The sp2 hybrid bond length is shorter than that of sp3, so the bond energy is larger. The former can be considered as a double bond and the latter as a single bond

    Converting graphite to diamond requires heat absorption, so the thermal stability of graphite is high.

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Structure diagram of carbon fiber

    Therefore, from the cross-section, it is a kind of chaotic structure.

    So how is this turbulent structure formed?

    This is related to the raw materials and processing technology. When the state of the graphite layer stack is locally ordered, it can be detected by x-rays, which is the so-called crystallite, so the crystallite is not one by one, but a graphite-like crystal. The orderly region formed by the interpenetration of the lamellae can be understood by the arrangement direction of the microcrystals, and the arrangement direction of the graphite-like lamellae can be understood. You should have seen the TEM photos.

38.jpg

    The following is the axial direction, and the arrangement direction of the graphite-like layers can be seen

    The above is the direction of the section, which is a kind of zigzag structure

    This is because the graphite layer and microcrystalline structure involve many mass transfer processes from spinning to oxidative carbonization, resulting in inhomogeneity in the cross-sectional direction, which is the so-called skin-core structure, which can also be called texture. We want the carbon fiber to be uniform on the cross section---how to look uniform and non-uniform? Therefore, there are three main structural parameters that affect the mechanical properties of carbon fibers: (1) the degree of orientation of the graphite-like sheets, (2) the force between the graphite-like layers, and (3) defects, including skin-core structure and cracks. Only after understanding the relationship with structural properties can we understand the original intention of the carbon fiber preparation process. The preparation process of carbon fiber from raw materials to oxidation and carbonization is all about these three aspects. The one is the transmission electron microscope image, which needs to be sliced, and it is uneven locally. I mean the uniformity or the scanning electron microscope layer from the atomic level. No matter what, the degree of orientation, density, and crystallite parameters are the same. If one layer is peeled off, whether the bulk density is the same or not. The crystallite parameter refers to the size of the ordered area.

     D002 belongs to the category of crystallite parameters, which can be measured by x-ray. Do you want to quantify? Use electron diffraction to measure crystallite parameters at different points on the section. Or peel off the density of the test body layer by layer, the graphene on the carbon fiber is not so easy to stick, and some people do it, using the humoers method, oxidation peeling, peeling off to see the structure of graphene has become a method to analyze the structure of carbon fiber.

    Therefore, I do not agree with exaggerating the function of graphene. Graphene is just a molecule of graphite. In fact, carbon is very clear about the structure of graphite, and it is just borrowing the concept of graphene.

    The same is true for the graphite-like layers I talked about, so let's talk about the preparation process of polyacrylonitrile carbon fiber.

    the second part

    The polyacrylonitrile carbon fiber starts from the raw material and is all carried out around these three points.

    First, let's talk about raw materials

    Of course, the main component of the raw material is polyacrylonitrile, which needs to be prepared into a viscous solution with acrylonitrile raw material. This viscous solution will not break when it is extruded from the pores. This is the concept of viscosity. There are three main aspects that affect viscosity, polymer concentration, that is, solid content, temperature, and molecular weight of the polymer. The direct preparation of this viscous solution by polymerization is called a one-step method, or it can be polymerized into powder and redissolved, called a two-step method. Therefore, the first thing to control in polymerization is the molecular weight. It is hoped that the more uniform the molecular weight, the better. At present, it is mostly 100,000-150,000. The solid content is about 18%, and the solvent is generally DMSO. With the temperature, solids content, and molecular weight determined, a viscous solution is obtained. But for smooth polymerization, and for smooth subsequent spinning and oxidation, two other things need to be added. It is the initiator and comonomer. Initiators are free radicals that allow the reaction to proceed at low temperatures. The polycondensation of asphalt is also a free radical reaction. It needs to be above 350 degrees to generate enough free radicals. In short, the initiator is to generate free radicals, and the azo type is generally used. A disadvantage is that the initiator does not decompose completely after the polymerization is completed. As a result, there are continuous bubbles coming out, which affects the spinning. But there is no good way, everyone still uses it, relying on the defoaming process. Another thing to add is a comonomer. Due to the strong force of the nitrile group, it is not easy to straighten the polymer during the spinning process. Therefore, the force of this nitrile group should be weakened. The way to weaken it is to add bulky functional groups to the polymer chain. That's why it's called cohesion. Since comonomers also bring many concepts that are sequence distribution.

    It means that the comonomer is distributed evenly and unevenly on the polyacrylonitrile molecular chain. Another function of the comonomer is to control the rhythm of the subsequent non-melting reaction. These comonomers are difficult to convert to carbon. Therefore, the addition of these comonomers is also a contradiction. Adding less can not achieve those two effects. Adding too much is a defect, and you need to master the balance.

    Now into spinning. The viscous spinning dope is extruded from the spinneret holes, forming a thin stream of solution. How to become fiber. There are many ways to volatilize the solvent at high temperature, which is called dry spinning. It is also useful for civilian use, because the resulting fibers are too dense, which affects the subsequent process, and the shape is difficult to ensure round, generally dog-bone type, so it is not used. The solution is squeezed into water in a trickle, and the solvent is extracted by the water, and the solids are precipitated and fibers can also be formed. But the water solidified too quickly. So add a certain concentration of solvent to the water, this is the coagulation bath. The spinneret is stuffed in the coagulation bath, and the solution solidifies as soon as the trickle comes out, which is wet spinning. If the spinneret is raised, the coagulation bath is lifted out, and the thin stream passes through a section of air layer and then enters the coagulation bath, which is dry and wet spinning. The thin streams of solution formed in the coagulation bath are called spun fibers. This solidification process is extremely important. Maybe the shape is not round either. Therefore, the solvent concentration in the coagulation bath is generally relatively high.

    The temperature is high and low, and the solidification is slow at low temperature, but it is easy to cause the fiber cross section to be out of round. The reason for the non-circularity is that the solvent inside the thin stream of the fiber solution comes out quickly, and the water in the coagulation bath enters slowly, resulting in collapse. The slower the coagulation, the longer the time, so the length of the coagulation bath is also an important design parameter. This is the mass transfer process of solidification which can be described and calculated by Fick's second law. In addition to mass transfer, we remember that the degree of orientation is important. The degree of orientation of the graphite-like sheets produced in the future is closely related to the degree of orientation of the polymer along the fiber axis. This degree of orientation is the method of pulling, and the term is called drafting. It's like pulling a rubber band. It's just that the rubber band has no plasticity, only elasticity, so the position of the molecule goes back when the hand is released. The drafting of the coagulation bath depends on the speed ratio of the first guide roll to the spinneret. Although the orientation formed in the spinneret channel and the coagulation bath is not high, it has a great influence on the follow-up. To stretch is to give a force called tension. What does not necessarily have to be stretched is called drafting. A negative draft is also a draft. With constant draft and constant tension, a preliminary orientation is created. There are also extrusion swelling effects, uniformity between monofilaments, and many other factors. Then there are washing, hot water drafting and steam drafting. Similar to ramen noodles, one is to thin the fibers, and the other is to straighten the polymers. oriented along the fiber axis. The stretching factor should match the force between the polymers - how to judge it. Otherwise, it is only thinned, and the molecules are not straightened. The degree of polymer orientation can reach about 90% after being drawn by saturated steam. After obtaining a fiber with a high degree of orientation, the non-melting treatment is performed.

    The obtained strands are fired into carbon fibers. Need to be at 1300 degrees. After coming out, the carbon content can reach about 94%. The purpose of high temperature treatment is to remove non-carbon elements from the fibers. However, the high temperature cannot be directly applied, so that the fibers will melt or semi-melt. Therefore, it needs to go through a process called non-melting treatment. Because oxygen in the air needs to be used, the oxidation reaction occurs, so it is also called oxidation or pre-oxidation. Oxidation is a means, and non-melting is the purpose. In the range of 200-300 degrees, two main reactions occur in polyacrylonitrile molecules. One is that the nitrile group opens and connects with the adjacent nitrile group to form a pyridine ring, which is called a cyclization reaction, and the formed molecule is called a ladder molecule. The second reaction is an oxidative cross-linking reaction, and some oxygen-containing functional groups are formed on the ladder molecules. These functional groups are dehydrated to form a cross-linked structure. At this time, the whole molecule is cross-linked, that is, it becomes insoluble. Then it can be processed at high temperature, that is, carbonization. In the non-melting process, the linear polymer is preheated to de-orientate, so tension is applied. To avoid de-orientation of polyacrylonitrile molecules, tension is required throughout the process. Even so, the degree of orientation of the molecules after unmelting will be lower than that of the original yarn. The co-monomers just mentioned will work during this period. Therefore, some processes are to add two comonomers.

    What is the reason for the poor composite of carbon filaments? Mr. Lu? That's an interface problem

    Then there are low temperature carbonization and high temperature carbonization. That is, at 300-900 degrees, most of the non-carbon elements - coke are removed. Then carbonized at a high temperature of 1300-1500 degrees to obtain carbon fibers. Tension control is also required during this carbonization process. This process is the process of generating graphite-like lamellae, and the reason why it has a staggered structure is inseparable from the characteristics of polyacrylonitrile molecules. This is both a disadvantage and an advantage, and the advantages outweigh the disadvantages. The carbon content of carbon fiber after carbonization is about 94%---there are no functional groups on the surface, so the ability to bond with resin is poor. Therefore, surface treatment is required. There are many surface treatment methods. For epoxy resin, the most effective method is oxidation. Because anodization is the best method, anodization is generally used. After oxidation, functional groups grow on the surface of carbon fiber. It is easy to combine with epoxy resin.

    The above is the basic principle of carbon fiber production process.