Unusual Conductors

To analyze a transient in a distributed-parameter line, a traveling-wave theory is explained for both single- and multiconductor systems. A method to introduce velocity difference and attenuation in the multiconductor system is described together with field test results. Impedance and admittance formulas of unusual conductors, such as finite-length and vertical conductors, are also explained. 

Metals also possess unusually high thermal conductivity, as anyone who has drunk hot coffee from a tin cup can testify. It is noteworthy that among metals the best electrical conductors are also the best thermal conductors. This is a clue that these two properties are somehow related and, again, the electron configuration proves to be responsible. 

Tin oxide, Sn02, has unusual physical properties. It is a good electrical conductor. It is highly transparent to the visible and highly reflective to the infrared spectrum. It is deposited extensively by CVD mostly for optical applications. Its characteristics and properties are summarized in Table 11.6. 

The most unusual and interesting feature of these polymers is their capacity to switch between insulating and conducting (or semiconducting) states. All other materials, with the only additional exception of some intercalation compounds, are normally found only as conductors or semiconductors or insulators, without the facility to switch between these states. 

Liquid helium exhibits some unusual characteristics when supercooled. First, it is the only element that will not turn into a solid by just using pressure. Heat must be removed as the pressure is increased, but helium will freeze at -272.2°C, which is the lowest temperature scientists have ever achieved. Second, it is an excellent conductor of heat. As a supercold liquid, it will move toward heat—even flow up the sides and over the top of a container. 

There is a large, growing family of ionic solids in which certain ions exhibit unusually rapid transport. These materials have come to be known as fast ion conductors (FICs). In some cases, the rapid ion transport is accompanied by appreciable electronic conduction as well. There is tremendous interest in the science and technology of fast ion conductors in view of their potential use as electrodes or electrolyte materials in electrochemical energy conversion devices (Hagenmuller van Gool, 1978 Chandra, 1981 Goodenough, 1984). 

If a compound contains two ions of different valency it may become an electronic conductor. NiO is not an electronic conductor, because, if electrons move from one Ni2+ ion to another, a pair Ni2+ -j- Ni2+ is changed into a pair Ni3+ + Ni+, in which both ions have unusual, unstable valencies. Now suppose that part of the Ni2+ ions are substituted by Ni3+ ions. Then an electron can travel from a Ni2+ ion to a Ni3+ ion, forming a Ni3+ ion from the first and changing the second into a Ni2+ ion... 

Arsenic exhibits allotropy, which is characteristic of non-metals the usual, more stable, metallic form resembles the typical metals in appearance and in being a fairly good conductor of electricity. Under atmospheric pressure it begins to volatilise at about 450° C. and passes into a vapour containing complex molecules, As4, which at higher temperatures dissociate to As2 this complexity is not unusual in non-metals. The yellow allotrope, which is stable at low temperatures, resembles white phosphorus in being soluble in carbon disulphide—a property which emphasises the non-metallic character of this variety. The reactivity of the allotropes, as in the case of phosphorus, differs considerably. 

Dry photocopiers use an unusual property of selenium, the group 6A element below sulfur in the periodic table. Selenium is a photoconductor, a substance that is a poor electrical conductor when dark but whose conductivity increases (by a factor of 1000) when exposed to light. When the light is removed, the conductivity again drops. 

It has been reported (4,5) that solid electrolyte sensors using stabilized zirconia can detect reducible gases in ambient atmosphere by making use of an anomalous EMF which is unusually larger than is expected from the Nernst equation. However, these sensors should be operated in a temperature range above ca. 300°C mainly because the ionic conductivity of stabilized zirconia is too small at lower temperatures. On the other hand, solid state proton conductors such as antimonic acid (6,1), zirconium phosphate (8), and dodecamolybdo-phosphoric acid (9) are known to exhibit relatively high protonic conductivities at room temperature. We recently found that the electrochemical cell using these proton conductors could detect... 

Metals feel cool to the touch compared with other materials because they are very good conductors of heat. How can we explain this unusual thermal conductivity ... 

A few works have also been published reporting unusual valence states of metal ions in lattices of such oxide semiconductors as ln203, ZnO, Sn02 [44, 55-68]. These compounds attract researchers attention because they are very perspective materials like thin films and ceramics for constructing new chemical sensors [55], as well as highly conductive thermo- and chemically stable n-type conductors (ln203). 

Carbon nanotubes are cylindrical carbon molecules with properties that make them potentially useful in extremely small scale electronic and mechanical applications. They exhibit unusual strength and unique electrical properties, and are efficient thermal conductors. 

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