Electrical resistivities

In electrical conductors such as metals, the attraction between the outer eiectrons and the nucleus of the atom is weak the outer electrons can move readily and, since an electric current is essentially a flow of electrons, metals are good conductors of electricity. In electrical insulators (or dielectrics), electrons are strongly bonded to the nucleus and are not free to move.[ K l 

Electrically, graphite can be considered as a semi-metai, that is a conductor in the basal plane and an insulator normal to the basal plane. Its atomic structure is such that the highest-filled valence band overlaps the lowest-empty conduction band by approximately 36 meV and the delocalized fourth-valence electrons form a partially-filled conduction band between the basal planes where they can moved readily in a wave pattern as they respond to electric fields.P°l Consequently, the electrical resistivity of graphite parallel to the basal planes (ab directions) is low and the material is a relatively good conductor of electricity. 

In the c direction, the spacing between planes is comparatively large, and there is no comparable mechanism for the electrons to move from one plane to another, in other words, normal to the basal plane. As a result the electrical resistivity in that direction is high and the material is considered an electrical insulator. In some cases, it may be 10,000 times higher than in the ab directions.l °l Often quoted resistivity values are 3000 x 10 ohm.m in the c direction and 2.5 - 5.0 x 10 ohm.m in the ab directions. 

Incorporation of N into a-Th at concentrations in the range of 0.04 to 1 at% N increases the electrical resistivity at 4.2 K. The results of measurements as read from a plot can be represented by the equation R42K (in fiQ m) = 33-i-21.2x10 x (N atom fraction x = 0.0004 to 0.01). The alloys after water-quenching from 1000°C obey this equation at compositions up to Xn = 0.01, but not if they are furnace-cooled from 800°C, because under these conditions some of the N can precipitate out of solution. The latter alloys with x 0.004 show a resistance plateau at R4 2k=H5 fxQ-m [6]. 

Some of the values are taken from the figures given in the papers. 

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Figure C2.16.1. A nomogram comparing electrical resistivity of pure (intrinsic) and doped Si witli metals and insulators.

Selenium exhibits both photovoltaic action, where light is converted directly into electricity, and photoconductive action, where the electrical resistance decreases with increased illumination. These properties make selenium useful in the production of photocells and exposure meters for photographic use, as well as solar cells. Selenium is also able to convert a.c. electricity to d.c., and is extensively used in rectifiers. Below its melting point selenium is a p-type semiconductor and is finding many uses in electronic and solid-state applications. 

It is a white crystalline, brittle metal with a pinkish tinge. It occurs native. Bismuth is the most diamagnetic of all metals, and the thermal conductivity is lower than any metal, except mercury. It has a high electrical resistance, and has the highest Hall effect of any metal (i.e., greatest increase in electrical resistance when placed in a magnetic field). 

The electronic configuration for an element s ground state (Table 4.1) is a shorthand representation giving the number of electrons (superscript) found in each of the allowed sublevels (s, p, d, f) above a noble gas core (indicated by brackets). In addition, values for the thermal conductivity, the electrical resistance, and the coefficient of linear thermal expansion are included. 

Thermal Conductivity Detector One of the earliest gas chromatography detectors, which is still widely used, is based on the mobile phase s thermal conductivity (Figure 12.21). As the mobile phase exits the column, it passes over a tungsten-rhenium wire filament. The filament s electrical resistance depends on its temperature, which, in turn, depends on the thermal conductivity of the mobile phase. Because of its high thermal conductivity, helium is the mobile phase of choice when using a thermal conductivity detector (TCD). 

Surface conduction is monitored in most humidity sensors through the use of porous ceramics of MgCr204—Ti02 that adsorb water molecules which then dissociate and lower the electrical resistivity. 

Some nonhygroscopic materials such as metals, glass, and plastics, have the abiUty to capture water molecules within microscopic surface crevices, thus forming an invisible, noncontinuous surface film. The density of the film increases as the relative humidity increases. Thus, relative humidity must be held below the critical point at which metals may etch or at which the electrical resistance of insulating materials is significantly decreased. 

Spinel ferrites, isostmctural with the mineral spinel [1302-67-6] MgAl204, combine interesting soft magnetic properties with a relatively high electrical resistivity. The latter permits low eddy current losses in a-c appHcations, and based on this feature spinel ferrites have largely replaced the iron-based core materials in the r-f range. The main representatives are MnZn-ferrites (frequencies up to about 1 MH2) and NiZn-ferrites (frequencies 1 MHz). 

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