Tungsten contact potential

Contact-potential measurements usually indicate only a small surface dipole associated with chemisorbed hydrogen, and in the case of metals such as tungsten and nickel, the negative end of the dipole appears to be on the outside. While there is some uncertainty about the detailed interpretation of these measurements, they are more readily reconciled with covalent than with ionic bonding. 

For the value of the decrease of the work function of tungsten caused by its being covered with sodium atoms, Bosworth and Rideal (12) obtained Acontact-potential method. This value agrees well with the values of Table III. 

In fact, the oxidation of metals with very low work functions (alkali metals, alkaline earth metals) proceeds very quickly at room temperatures and more slowly at 83 K. If the work function of the metal has a medium-sized value, it is increased by oxygen, e.g., that of tungsten from 4.53 to 6.27 volts, or by 1.74 volts, according to the measurements of the contact potential by Bosworth and Rideal (12). In this case oxygen is adsorbed in the form of atoms. No oxidation was observed at low tem-... 

Bosworth and Rideal (67,68), too found with the contact-potential method an increase of the work function by 1.38 volts when nitrogen was adsorbed on tungsten at 90°K. Since the method described in section III,Id, was used, where a second tungsten filament is heated for the emission of thermionic electrons, it is to be expected that in this case, too, N atoms were formed, which reached the cold wire near the hot one and were adsorbed. 

It is, unfortunately, very difficult to compare the experimental data on the decrease of the heats of adsorption with the observed values for the changes in talkali metals on tungsten filaments, where sufficient reliable data are available. Contact potentials, measured when the gases are adsorbed on filaments, are less reliable. Most investigations concerning contact potentials on films have yielded values of surface potentials that are known for nearly complete films, but not for adsorbed layers with low 0 values. There are, moreover, rather serious deviations among the experimental values published by different authors (836). 

Films thus offer a means of preparing clean surfaces of relatively low melting metals. Once a strongly chemisorbed film has been formed, however, it cannot be removed. Where this is desirable one can sometimes use the metal as a coat on a tungsten wire. In this way Bosworth (24) determined the Ni-NiO contact potential as — 1.4K... 

Bosworth (7) determined the contact potential of a tungsten filament under the above conditions, deducing that 0 lay in the range 0.6-0.9. It was therefore concluded that for the conditions concerned, mechanism (b) was operative. 

When atoms of barium, caesium, potassium, thorium or similar metals are deposited on a surface of metallic tungsten, stable films may be built up varying in nominal thickness frpm a fraction of a monolayer to many monolayers. The new composite surface has contact potentials and thermionic or photoelectric work functions different from those of the clean metal. The movements of atoms in these films can therefore be followed by the variation in the thermionic or photoelectric currents, i, which alter as the fraction d of the surface covered alters. It is important to find how the current i depends on 0. 

Palmer (Proo. Poy. 8oc. A, cvi. 200,1924) in investigating the effect of various gases on the sensitiveness of a coherer or loose contact has noted that the potential required to break down the loose contact between platinum, tungsten and carbon films varies with the nature of the gas. 

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