Why is graphite anisotropic in its electrical conductivity?

The electrical conductivity

When is a substance electrically conductive?

A substance is electrically conductive if it contains freely moving charged particles.

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The conductivity of metals

Why do metals conduct electricity? If atoms only have 1, 2 or 3 valence electrons, neither ion nor atomic bonds can form between 2 atoms of this element.

Ionic bonds cannot develop because one atom would have to give up its valence electrons (this works fine with 1 - 3 valence electrons) and the other atom would have to accept these valence electrons in order to form a noble gas configuration with its own valence electrons. This is not possible; there are not enough electrons.

Atomic bonds cannot form because 2 atoms use their valence electrons together to fill the valence shell with 8 electrons. There aren't enough electrons for that either.

So the metal bond is formed. With it, all valence electrons are freely movable in the entire crystal, they are, so to speak, shared by all atoms. This state is called “electron gas”, although it has nothing to do with the gaseous state of matter.

Fig. 1: Power line in metals

What happens with the power line in metals? When a voltage is applied, the electrons slowly migrate through the crystal (and of course through other metallic objects such as copper wires). The situation is shown in Figure 1.

The most important properties of metallic conductors are

  • The power line does not cause the conductor to decompose
  • The conductor has a negative temperature coefficient, i.e. when the temperature increases, the electrical conductivity decreases.

Why do some metals conduct electricity better than others? This question can be answered with the band model derived from quantum mechanics - not on my website.

Examples of metallic conductors:

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The conductivity of graphite

Why does graphite conduct electricity?


Fig. 2: Power line in graphite

The graphite crystal is made up of layers. Each layer consists of an infinite number of six-membered rings made of carbon atoms. Each carbon atom uses 3 of its valence electrons to form bonds with the neighboring atoms. The remaining, “fourth” valence electrons form a system of delocalized molecular orbitals, which means that they can move freely in the entire layer, and current conduction can take place. Figure 2 shows a situation in which 2 electrodes (plus and minus poles) are placed parallel to the layers. Electricity will flow.


Fig. 3: There is no current flow in graphite perpendicular to the layers.

Graphite therefore only conducts the electrical current within the layers. Perpendicular to the layers (see picture 3) it is an insulator. This phenomenon is called anisotropy.

In commercially available graphite, a great number of tiny crystals are randomly arranged so that layers of individual crystals that are tilted against each other touch each other. Power is conducted in all directions.

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The conductivity of salts and other ionic compounds

Why do ionic compounds conduct electricity? Ionic compounds (for example salts, acids, oxides and hydroxides) are substances that can dissociate into ions. Positively charged ions are called cations, negatively charged hot anions.


Fig. 4: There is no current flow in ion crystals.

In the solid state, ionic compounds form crystals. In these crystals, the ions are not free to move, but are located in a crystal lattice in fixed places (Fig. 4). So there can be no current flow.


Figure 5: In the liquid state, electricity flows in ionic compounds.

In the liquid (molten) state, the ions can move freely. If a voltage is applied, the cations migrate to the cathode, the anions to the anode and discharge there (Fig. 5). Electricity flows. The same thing happens in the dissolved state.

What happens when current flows in ionic compounds? When a voltage is applied, the ions migrate: cations to the cathode, anions to the anode.

The most important properties of ion conductors are:

  • When a current flows, the ionic compound decomposes. As soon as all ions have migrated to one of the electrodes, there are no more freely moving ions (of course!) And no more current flows.
  • Ionic conductors have a positive temperature coefficient, i.e. when the temperature increases, the electrical conductivity increases. The reason is the decreasing viscosity of the water. The conductivity only decreases again at high temperatures.

Why do metals conduct electricity better than ionic compounds? The charge carriers in metals are electrons, in ion conductors they are ions. Ions are much larger than electrons and therefore much less mobile.

Examples of ion conductors:

Applications of power conduction in solutions of ionic compounds:

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semiconductor

You will soon be able to read more about the conductivity of semiconductors here.

Examples of semiconductors:

  • Iodine - a very weak semiconductor

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special cases

And then of course there are also substances whose conductivity does not fit into one of the diagrams above.

Examples:

  • Molecular switches: It is not a substance that is conductive, only the individual molecules. One tries to use such molecules as transistors as switches.

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The unit of conductivity

The unit of conductivity (also called conductance) is Siemens, abbreviation S. The following applies for the conversion:

1 S = 1 =

The conductivity is the reciprocal of the resistance.

The conductivity L depends on the length l and the cross-sectional area q of the conductor through which it flows, and of course also on the material from which the conductor is made. The following applies:

L = κ ⋅

Here κ (kappa) is the specific conductivity. It is different for each substance and depends on the temperature and, in the case of solutions, also on the concentration. The unit of the specific conductivity κ is 1 / ohm ⋅ cm.

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How big is the conductivity?

The conductivity of different substances is very different. It extends over many powers of ten. Figure 6 gives an approximate overview of the order of magnitude of the conductivities of individual substance classes.

Figure 6: Electrical conductivity scale

 

 

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