Electric strenght and molecular properties of gaseous dielectrics

A fundamental matter bearing on the use of dielectric gases is the delineation of the possible correlations between their molecular properties and dielectric strengths, under comparable conditions of electrode surface, electrode geometry, electrode separation, etc. Based on the criticism that a much larger number of dielectric gases be employed in the explorations of the foregoing type, it was decided to investigate whether some more general correlations could be established between the dielectric strength of gases and their molecular properties. The object of the present paper is to examine aspects of this matter with particular attention to the fact that a large number of gases will be considered in order to arrive at some general conclusions. It would be ideal to calculate the electron capture cross sections for the various gases (starting, e.g., from their molecular structure) and attempt to correlate them with the dielectric strength of gases, with an aim to formulate a quantitative measure of their "electronegativity" predictable from electron attachment cross sections, and, thence the molecular structure. It is believed, however, that a calculation of electron attachment cross-sections cannot be carried out, especially for multi-atomic gases possessing usefully high dielectric strengths. Here, we adopt an empirical, qualitative approach to the problem in which it is concluded that the electron attachment cross sections of gases would be proportional to their molecular volumes. However, for complex gases, molecular volumes are not easily calculated since one does not have data on the bonding arrangement, bond lengths, and the covalent radii of the constituent atoms of multiatomic dielectric gases of interest. As an approximation, however, one may take the molecular weight of a gas as an index of its molecular volume since a gas containing numerous and bulky atoms would tend to have both a large molecular weight and volume; the reverse would, of course, be true- - for a gas possessing fewer atoms, especially those of lower atomic weights.