PLASTIC CONDUCTORS

The electrons in the outer shell of an atom ("valency" electrons) are involved in chemical reactions, for example by the atom shar-ing its valency electrons with the atom of another element to form a molecule of a chemical compound.

The outer shell has room for a maximum of 8 electrons. When all 8 places are occupied, as is the case in argon or neon, the atom is very stable and does not enter into chemical combinations. To the contrary, when there is only one electron, as is the case in cop-per, this electron is only very loosely held in the atom by the nucleus, and is liable to drift away and become a free electron. The flow of free electrons constitutes an electric current; a conductor, then, is a material that has free electrons. Metals have only a few valency electrons; that is why they are conductors.

That is also the reason why plastics are insulators. Plastics are polymers, i.e., chains of the same chemical structure linked to each other and repeated over and over again. The outer electrons of each such basic structure are needed to provide the link to the next identical structure; there are none left to drift around in the material as free electrons.

The same is true of other insulators that are not necessarily polymers or plastics, but simply materials that have no free electrons. For example, each silicon atom has four electrons in its outer shell, but the shell gets filled up to 8 by sharing some more electrons with the neighboring atoms in a silicon crystal as shown in the figure. At room temperatures, silicon is an insulator: when a voltage (electrical pressure) is put across the crystal, there are no free electrons to flow as an electric current.

There is, however, a way to put in some free electrons. If some impurities are added in the form of an element that has five electrons in its outer shell, such as arsenic, the atoms of this "donor" element can be put into the crystalline structure as shown, but each such cuckoo's egg has now swindled in an extra electron, and that electron is free. The resulting silicon "doped" with arsenic is one of many possible "semiconductors," and the difference from ordinary conductors such as metals is that its conductivity can be controlled in various ways, for example by shining light on it, or by joining two wafers of different semiconductors into a transistor which can act as an amplifier. From here the story continues to the electronics revolution, but we branch off to another road.

An ordinary metallic conductor has some shortcomings, for ex-ample, it is heavy, and its conductivity is not changeable. But in 1977 it was found that some plastics could be doped in the manner of semiconductors. One of the early applications of conducting polymers were the windows and eye glasses that turn darker (more opaque) under more intense light.

Since then, many new conducting polymers have been developed, and many applications for them have been found, though the most alluring remain uncertain. For example, lithium polymer laminate batteries have been used for calculators and other low-power devices. But the light-weight high-power ear bat-tery still remains only a promise for the future. Also, since such batteries can be charged and &charged faster than the present ones, these batteries provide (as yet unfulfilled) hope for the storage of electric energy for utilities, now done indirectly by pumped storage and other devices.

In general, the great advantage of conducting polymers is their low weight. To screen the field of a cable, it has to be surrounded with an insulator, such as rubber, and put in a (grounded) metal sheath. That is both heavy and expensive. Surrounding the cable with a plastic as insulator with a conductive polymer on the outside is both light and cheap.

An application that has no competitor is the bonding of PVC pipes. An admixture of conductive polymers will make them a con-ductor, and the bell-spigot joints of adjoining pipes can then be in-duction-heated, which will make them flow and seal.

[More: "The promise of conductive plastics," EPRI J., (Electric Power Res. Inst., Palo Alto), July-Aug. 1991.]