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2.2.3 Non-Metalic Conductors

We will just give a brief look at some especially important or useful non-metallic conductors:

Conducting Polymers

That polymers, usually associated with insulators, can be very good conductors was a quite unexpected discovery some 20 years ago (Noble prize 2001). They always need some "doping" with ionic components, however.

  • The resistivity can be exceedingly low. e.g. for Iodine (I) doped poly-acethylene (pAc) we may have. 

\[\rho\le 6.7 \mu\Omega\text{cm}\]

  • Or in other words: If you divide by the density for some figure of merit, it beats everything else, since

\[\left\{\frac{\rho}{\text{density}}\right\}(\text{pAc})\lt\left\{\frac{\rho}{\text{density}}\right\}(Na)!\]

  • More typical, however, are resistivities around (10 .... 1000) μcm.

The conduction mechanism is along –C=C–C=C–C= chains, it is not yet totally clear. In fact, the first question is why this kind of chain is not generally highly conducting. Use the link for the answer.

  • The conductivity is strongly dependent on "doping" (in the % range!) with ions, and on many other parameters, the link gives an example.
  • So do not confuse this with the doping of semiconductors, where we typically add far less than 1 % of a dopant!

A new object of hot contemporary research are now semiconducting polymers which have been discovered about 10 years ago.

Transparent Conductors

Indium Tin Oxide (ITO) (including some variations) is the only really usable transparent  conductor with reasonable conductivity (around cm)! It consists of SnO2 doped with In2O3.

  • ITO is technically very important, especially for: 
    - flat panel displays, e.g. LCDs .
    - solar cells.
    - research (e.g. for the electrical measurements of light-induced phenomena).
  • ITO is one example of conducting oxides, others are TiONiO, or ZnO. The field is growing rapidly and known as "TCO" = Transparent Conducting Oxides

If you can find a transparent conductor much better than ITO (which leaves a lot to be desired), you may not get the Nobel prize, but you will become a rich person rather quickly.

  • Since In is rare, and the demand is exploding since the advent of LCDs, you also would be a rich person of you invested in In some years ago.

Ionic Conductors

Solid Ionic conductors are the materials behind "Ionics", including key technologies and products like

  • Primary batteries.
  • Rechargeable (secondary) batteries.
  • Fuel cells.
  • Sensors.
  • High temperature processes, especially smelting, refining, reduction of raw materials (e.g. Al-production).
  • There is an extra module devoted to the Li ion battery. This is important for you if you are interested in driving an affordable a car in 20 years or so.
  • They are also on occasion the unwanted materials causing problems, e.g. in corrosion or in the degradation of dielectrics.
  • See Chapter 2.4 for some details about ionic conductors.

Specialties: Intermetallics, Silicides, Nitrides etc.

Silicides, i.e. metal - silicon compounds, are important for microelectronics (ME) technology, but also in some more mundane applications, e.g. in heating elements. Some resistivity examples for silicides:

Silicide MoSi2 TaSi2 TiSi2 CoSi2 NiSi2 PtSi Pd2Si
ρ (μcm) 40... 100 38...50 13..16 10...18  50 28...35 30...35
  •  It looks like the winner is CoSi2. Yes, but it is difficult to handle and was only introduced more recently, like NiSi2. In the earlier days (and at present) the other silicides given above were (and still are) used.

Some more examples of special conductors which find uses out there:

Material HfN TiN TiC TiB2 C (Graphite)
ρ (μcm) 30...100 40...150 ca. 100 6 ...10 1000

Superconductors

Superconductors are in a class of their own. All kinds of materials may become superconducting at low temperatures, and there are neither general rules telling you if a material will become superconducting, nor at which temperature.

  • There will be an advanced module some time in the future.

Why do we need those "exotic" materials?. There are two general reasons:

  1. Because, if just one specific requirement exists for your application that is not met by common materials, you simply have no choice. For example, if you need a conductor usable at 3000 K - you take graphite. No other choice. It's as simple as that.
  2. Because many requirements must be met simultaneously. Consider e.g. Al for integrated circuits - there are plenty of important requirements; see the link. Since no material meets all of many requirements, an optimization process for finding an optimum material is needed.
  • Al won the race for chip metallization for many years, but now is crowded out by Cu, because in some figure of merit the importance of low resistivity in the list of requirements is much larger now than it was in the past. It essentially overwhelms almost all other concerns (if there would not be an almost, we would have Ag!).