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We will just give a brief look at
some especially important or useful non-metallic
conductors: |
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Conducting Polymers |
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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. |
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The resistivity can be
exceedingly low. e.g. for
Iodine (I) doped poly-acethylene
(pAc) we may have.
r £ 6,7
mWcm. |
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Or in other words: If you divide by the
density for some figure of merit, it
beats everything else, since
{r/density} (pAc) > {r/density} (Na)! |
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More typical, however, are resistivities around (10 ....
1000) mWcm. |
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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.
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The conductivity is strongly dependent on "doping"
(in the % range!) with ions, and on many other parameters, the
link gives an example. |
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So do not confuse this with the doping of semiconductors,
where we typically add far less than 1 % of a dopant! |
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A new object of hot contemporary research are now
semiconducting polymers which have been
discovered about 10 years ago. |
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Transparent conductors |
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Indium
Tin Oxide (ITO) (including some
variations) is the only really usable transparent conductor with reasonable
conductivity (around 1 Wcm)! It consists of
SnO2 doped with In2O3. |
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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). |
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ITO is one example of conducting oxides,
others are TiO, NiO, or ZnO. The field is growing rapidly
and known as "TCO" = Transparent
Conducting Oxides |
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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. |
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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. |
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Specialities: Intermetallics, Silicides, Nitrides
etc. |
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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: |
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| Silicide |
MoSi2 |
TaSi2 |
TiSi2 |
CoSi2 |
NiSi2 |
PtSi |
Pd2Si |
| r (mWcm) |
40 ...100 |
38...50 |
13..16 |
10...18 |
» 50 |
28...35 |
30...35 |
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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. |
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Some more examples of
special conductors which find uses out
there: |
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| Material |
HfN |
TiN |
TiC |
TiB2 |
C (Graphite) |
| r (mWcm) |
30...100 |
40...150 |
ca. 100 |
6 ...10 |
1000 |
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Why do we
need those "exotic" materials?. There are two general
reasons: |
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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. |
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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. |
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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!). |
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© H. Föll (Advanced Materials B, part 1 - script)
