Issue |
J. Chim. Phys.
Volume 82, 1985
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Page(s) | 76 - 82 | |
DOI | https://doi.org/10.1051/jcp/1985820076 | |
Published online | 29 May 2017 |
Influence de la pression d'oxygène et de la température sur le potentiel de sortie du tungstene polycristallin
1
Laboratoire d'adsorption et Réaction de Gaz sur Solides, ERA CNRS n° 368, ENSEEG, Domaine Universitaire — B.P. 75, 38402 St-Martin-D'Héres, France.
2
Enseignant à l'université de Grenoble I, France.
Des potentiels de surface stationnaires ont été mis en évidence et étudiés en fonction de la pression d'oxygène et de la température sur des filaments polycristallins de tungstène. Ils peuvent s'interpréter par l'addition (pouvant les amener à plus de 2,5 volts) d'une couche faiblement liée à l'oxygène fortement lié connu depuis longtemps. L'oxygène faiblement lié s'élimine, soit par simple évacuation, soit par chauffage vers 1 000 K sous la pression initiale d'adsorption.
Abstract
Surface potentials (i.e. here work function variations due to foreign species present on metal surface) were measured by a spacecharge limited diode made of two crossed tungsten wires, as described by the authors in a former paper. Stationary states dependent on oxygen pressure (between 10-8 and 10-6 torr) were reached after sufficient time (often several days). Other stationary states defined by their surface potentials have also been observed : by heating tungsten to 1 000-1 200 K under a stable oxygen pressure ; after cooling it back, still under the same pressure. Tentative application of the Clausius-Clapeyron formula at constant surface potential leads to an adsorption enthalpy of about – 105 kilojoules. This weakly bound oxygen is observed under the large exposures used here in addition to the strongly-bound one which is generally observed. This combined layer rises the surface potential above 2.5 volts; the weakly-bound oxygen is removed by pumping off at adsorption temperature or by heating the tungsten to 1 000 K under intial adsorption pressure.
© Paris : Société de Chimie Physique, 1985