[ Date: 2018-08-20 13:23:55 Author: Popularity: ]
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Why can carbon make up the hardest material - diamond in the world, and also very soft material - graphite? 

Why can graphite conduct electricity while diamond cannot? 

Why do carbon fibers have high strength? 

Are you confused about carbonization and graphitization? 

This article tries to popularize the basic science of carbon materials. 

Carbon materials will be understood in terms of atomic, crystal and microscopic scales.

Atomic level

Matter is made up of atoms. Atoms are made up of nuclei and extra nuclear electrons, which revolve around the nucleus, just like the earth revolves around the sun. The nucleus is made up of protons and neutrons, which need not be understood. You just need to 

know that according to the number of protons, they are divided into different elements. For example, hydrogen is the proton number 1, helium is the proton number 2, so carbon element ranks sixth on the periodic table of elements, and there are six protons in its nucleus, and the number of electrons outside the nucleus is the same as that of protons, so there are six electrons outside the nucleus of carbon atom. The whole weight of an atom is concentrated in the nucleus, and the weight of an electron is almost neglected. But don't underestimate the extra-nuclear electron, which is the key to determining how an atom makes up a substance. Different physical and chemical properties originate from it.


Extra nuclear electrons rotate around the nucleus regularly. They move in orbits at different distances from the nuc

leus. Just as the sun has eight planets (Pluto has been expelled), planets have different orbits depending

The diagram of extra nuclear electrons

 on how far they are from the sun. But unlike the planetary orbits of the solar system, the orbits outside the nucleus can have two electrons in the nearest orbit, and eight electrons in the farther orbit, which we will not say, because we only care about carbon. So we know that carbon atoms are composed of carbon nuclei and six extra nuclear electrons, and there are two electrons in the innermost layer of carbon atoms and four electrons in the subouter layer. See the diagram.


diamond structure

But it means you can put eight electrons in it.  If it is not full, then it will be instable. How should we do? Sharing electrons with the surrounding atoms stabilizes the bond, which is called a covalent bond. It takes a lot of energy to break the covalent bond in order to separate the two electrons. One carbon atom forms a covalent bond with four carbon atoms around it. The four covalent bonds repel each other and form the largest angle in space. The angle between the two covalent bonds is 109 degrees. So we often see the atomic structure of diamond as shown in Figure 2. You can see carbon atoms as small balls. Covalent bonds are rigid rods connecting the balls. If you want to move any of the balls, it will involve the movement of the surrounding balls. So unless you break the rigid rods connecting the balls, you will not be able to move easily. Ball, that's why diamonds are so hard. You see, the properties of any material in materials science are intrinsically related. In addition, the external electrons of carbon atoms form covalent bonds with the surrounding carbon atoms. The electrons are fixed between the two nuclei and cannot move freely, even if the applied voltage is not enough, then no current can be formed, that is, diamond does not conduct electricity.

So why is there a hexagonal plane structure like graphite? Let's look at the picture of the hexagonal structure of graphite first.


This is the principle of minimum energy. Why do rocks roll down on mountains and objects on tables fall down? It is because everything will spontaneously transform from high energy to low energy, and then release the remaining energy. The object on the stone river table on the mountain has high energy, so it falls down spontaneously, and the remaining energy will be transformed into a pit on the ground. The two adjacent carbon atoms in graphite structure are close, so the energy is low. Don't ask me why. You just need to remember that graphite, a carbon atom, and forms covalent bonds with three other carbon atoms in a plane. The angle between each two covalent bonds is 120 degrees, so the energy of the structure is the lowest. Some people have said that carbon atoms in diamond structure can also be close to distance, ah, we cannot do it, because two carbon atoms near will be repulsed by other atoms. Is it very intertwined? Why can the graphite structure carbon atoms get close? I suggest you draw two kinds of structure diagrams on paper. The covalent bond angle of graphite carbon atoms is 120 degrees. There is enough space to get close. The covalent bond angle of diamond is 109 degrees. There is no space left. Children with poor spatial sense are going to hit the wall. Don't worry. You just need to remember that graphite is the most stable structure. That's another problem. So diamond has high energy, it can spontaneously change to graphite structure. Sell the diamonds collected at home. In case it comes out as graphite one day, it's not cost-effective. This worry is superfluous. Why don't the coins in my pocket fall on the floor? Coins are not low energy in my pocket on the ground. That's because they have to jump from the bottom of my pocket to the pocket to fall out. If I jump up and down and give them energy, maybe they will fall out. Similarly, although diamond is unstable, it exists for a long time, which is called metastable state. Only by giving energy can it spontaneously transform from diamond to graphite structure. If you put diamond into high temperature, it will gradually change into graphite. If you don't believe it, try it, but don't ask me to pay for it.

Let's look at the structure of graphite. One carbon atom forms covalent bonds with three carbon atoms around it. The remaining electrons are free above or below the plane. Each carbon atom has an extra free electron, which is the graphene structure. When the carbon atoms are stable, the bond to free electrons is small, and the voltage free electrons will move in a very fast direction. That's why graphene belongs to superconducting materials with very good conductivity and almost no resistance. That's why graphene conducts electricity. In addition, a layer of electrons on the carbon hexagonal plane can absorb visible light, which can explain why graphite is black. When light shines on graphite, it is absorbed, that is black.

After the atomic scale, let's look at the crystal scale. What is crystal? In materials science, the periodic arrangement of atoms according to certain rules is the crystal structure. Look at the diamond structure, it is a carbon atom and four carbon atoms around the structure of a regular cube periodic arrangement, look at the graphite structure, are carbon atoms and three carbon atoms around the hexagonal grid, and then periodic repetition, this is graphite crystal. Graphite materials have such six-element mesh planes stacked one by one, and the two six-element mesh planes are stacked by the mutual attraction of free electrons. This binding force is very weak, so it is very easy to slip under external force. This is why graphite can be used as an additive to lubricating oil, because two planes can slide freely and lubricate. This is also why pencils use graphite. When pencils are scratched on paper, some of the net planes are rubbed. Come down and stick to the paper.

Why can carbon materials resist corrosion? You can see that coal has been sold underground for hundreds of millions of years, and the soil is not corroded in such a complex environment. Look at the old wood pole surface burned black buried in the ground, because the surface wood burned charcoal to prevent wood pole rotting in the soil. These are all due to the corrosion resistance of carbon materials. Because carbon has high binding activity with other elements, that is to say, it is difficult to react at room temperature, so it can resist corrosion. So why does carbon react so strongly with oxygen? Coal burns easily and graphite oxidizes easily. According to the principle of combustion, when carbon atoms combine with oxygen atoms, it is necessary to interrupt the original combination of carbon atoms and carbon atoms, and then form a bond with oxygen atoms. Because breaking the bond between carbon atoms and carbon atoms requires less energy than the bond between carbon atoms and oxygen atoms. That is to say, when carbon atoms combine with oxygen atoms, the released energy will interrupt the combination of carbon atoms and carbon atoms around them, so that the combination of carbon atoms and oxygen atoms will continue to combine, and the combination of carbon atoms and carbon atoms will continue to be interrupted, which is combustion.

When it comes to crystal structure, there must be an irregular arrangement of atoms. This is amorphous. Like amorphous carbon, it is amorphous. In fact, these are relative concepts. There is no perfect crystal in the world. Even in a single crystal, there will be atoms arranged irregularly. Amorphous carbon means that the arrangement of carbon atoms is disorderly and almost irregular, and there are still unsaturated electrons remaining in carbon atoms, that is to say, no stable combination is formed. In this way, amorphous carbon can easily react with oxygen, because oxygen atoms first combine with carbon atoms that do not form a combined structure, and then the combustion underneath proceeds spontaneously.

Let's look at the micro level. As mentioned above, the atoms of a single crystal are arranged regularly. If the whole object is arranged regularly, it is a single crystal. But such objects are rare. Most of them are small grains of single crystal composed of atoms. Small grains can be synthesized again. There is not necessarily a clear boundary between grains. It may be that the grain boundaries between grains are just a little disorderly, and the carbon atoms between them are covalently bonded. In terms of graphite composition, carbon atoms form a six-element mesh, and then the six-element mesh is superimposed to form graphite grains. Graphite grains may be staggered with adjacent graphite grains, or the lattice direction may be staggered to form graphite objects. It is very difficult to pull the six-element mesh apart by applying external forces, because the covalent bonds between carbon atoms and carbon atoms are very strong. But if there is irregular arrangement or lack of carbon atoms somewhere, the bonding force in these places is relatively weak, and cracks will propagate along such places. If observed by a microscope, the broken ports of graphite basically propagate along the grain boundaries between grains.

Finally, if carbon fiber is a perfect carbon hexagonal plane single crystal, then the tensile strength will reach 800 GPa. What is the concept? The strength of T700 carbon fibers is 4900 MPa, or 4.9 GPa, which is only about one-200 of the theoretical strength. Because the interior of carbon fibers belongs to disordered graphite structure, that is to say, there is graphite six-element grid structure, but the grid plane is relatively disordered, there are defects, which seriously limit the strength of carbon fibers. In addition, the orientation of the six-element mesh is not completely parallel to the length direction of the fiber, which also limits the strength of carbon fibers.