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Why can pencil lead conduct electricity?

2020-12-28 10:26

Description

We know that what kind of properties a substance has is determined by its structure. Diamond does not conduct electricity, so how does graphite conduct electricity? This is about the structure of graphite. As mentioned before, graphite is composed of layers of carbon atoms, and this single layer of carbon atoms is actually what we often hear about "graphene".


The carbon atoms in graphene have a very special structure. The outermost layer of carbon has 4 electrons, so its stable structure should form four covalent bonds, but the carbon atoms in the carbon atom layer structure we see seem to be There are only three bonds. In fact, this kind of carbon atom layer also contains a bond, which is the π bond.

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The carbon atoms are arranged in the same plane in the form of a six-membered ring. Adjacent carbon atoms in the same plane are interconnected in the form of δ bonds, and each carbon atom will form three δ bonds, so that graphene can have good mechanical properties. The remaining valence electrons around the carbon atom will form a large conjugated π bond perpendicular to the carbon plane. The electrons in the large π bond can be regarded as a whole, and the electrons can be free and automatic in the entire π bond, so the electrons can be in the entire graphene Free movement in the plane, which ensures that graphene has excellent electrical conductivity, and electrical conductivity and thermal conductivity are related, so the electrical and thermal conductivity of graphite are very good, precisely because graphene has such excellent properties, Countless scientists are now studying graphene and want to use graphene to make superconducting materials.


Band structure in graphene


Since graphite is composed of layers of this kind of graphene, graphite will naturally have conductivity, but the conductivity of graphite is not particularly good. We just said that the conductivity of graphene depends on the layer of carbon atoms. With the large π bond between, electrons can freely shuttle in the single-layer carbon atom layer, but the movement of electrons between carbon atom layers is hindered. The carbon atom layer in graphite is not a large whole, but is composed of countless tiny fragmented carbon atom layers. Therefore, in the process of electron flow, different carbon atom layers need to be continuously crossed, so it shows a certain resistance.