R. W. Crowe

Electric Strength and Molecular Structure of Saturated Hydrocarbon Liquids

A number of investigators have reported that the apparent electric strengths of straight‐chain hydrocarbon liquids increase in a regular manner with increasing molecular chain length. In a recent publication we have presented evidence that this phenomenon is an illustration of a kind of Paschens law for liquids. An observed linear dependence of the electric strengths of such hydrocarbons upon density suggested that chain length variation merely provided a means of changing the electron mean free path in the liquid.In the present paper we describe the development of an improved technique for measuring electric strengths of liquids, and the application of this technique to a series of pure straight‐chain and branched‐chain liquid alkanes. As had been observed previously, the strengths of the straight‐chain members of the series exhibit a linear relationship with density. The introduction of branches into the hydrocarbon chain, however, results in a definite decrease in strength. Extension of the measuremen...

Electric Strength of Saturated Hydrocarbon Gases

An attempt has been made to correlate the sparking potentials of nonattaching polyatomic gases with molecular structure. The investigation has included the normal alkanes containing from one to six carbon atoms, their branched chain isomers, cyclopentane, and cyclohexane. For a given electrode separation and pressure, the electric strengths of the normal alkanes increase with increasing chain length. Using a Townsend‐like criterion for breakdown it has been possible, with certain assumptions, to calculate from the measurements relative cross sections for electron scattering by these molecules. This scattering cross section is proportional to the number of carbon‐hydrogen bonds in the molecule, and independent of the number of carbon‐carbon bonds. It is independent of molecular shape, i.e., of branching or cyclization.

Formative time lags in the electric breakdown of liquid hydrocarbons

The physical processes operative in the development of electrical breakdown in a dielectric are often reflected in the time interval between the application of a d.c. voltage and the appearance of the spark. This time may include both the time required for a free electron to appear near the cathode (statistical time lag) and the time required for the spark to develop after such an electron begins to multiply by collision ionization (formative time lag). It is the latter that provides the most information about the breakdown process.

A Semiempirical Expression for the First Townsend Coefficient of Molecular Gases

The general utility of an approximate expression for the first Townsend coefficient of a gas, derived several years ago by Druyvesteyn and Penning, has been tested by introducing into the theoretical treatment somewhat more realistic assumptions regarding the relevant scattering cross sections of the gas molecule. It has been demonstrated by numerical calculations that the functional dependence of the Townsend α upon the electric field and the gas pressure is, within certain limits, insensitive to the assumed energy dependence of the elastic and inelastic cross sections. The functional form of the inelastic cross section does, however, influence the way in which the ionization energy of the gas molecule enters in the exponential part of the expression.

Cathode effects in the dielectric breakdown study of liquids

The electron emission characteristics of the surface of the cathode used in the study of the electric breakdown of insulators are believed to have a fundamental bearing on electric strengths observed. It has been suggested that the formation of a negative space charge is the mechanism through which this influence is exerted1.

A New Type of Disorder Affecting the Dielectric Properties of Long‐Chain Esters

The dielectric properties of a series of long‐chain esters have been investigated in the solid state. Like other long‐chain molecules containing 20 or more carbon atoms, most of the ester molecules showed evidence of hindered molecular rotation about their long axes in the crystal lattice. It was also found that higher dielectric constants resulted when the samples were rapidly frozen than when they were allowed to solidify gradually, the effect being more pronounced as the chain length was increased. In order to explain this behavior, it has been assumed that the esters may crystallize in an ordered lattice in which the dipoles are in a plane, or in a longitudinally disordered lattice in which some of the chains are reversed. In the latter case, the dipoles may or may not form two planes depending upon the position of the polar group in the chain. It has also been assumed that the degree of longitudinal disorder increases with increase in freezing rate. Once frozen in, however, it remains fixed until the...

Influence of Molecular Structure upon the Electric Strengths of Liquid Hydrocarbons

The electric strengths of straight‐chain liquid alkanes are known to increase with increasing molecular chain length. The introduction of branches in the hydrocarbon chain, on the other hand, results in a reduction in strength. In an effort to gain a clearer understanding of this behavior, we have extended our investigations to include hydrocarbons of more complex molecular structure. In the present paper we present the results of a study of the influence of molecular structure upon the electric strengths of benzene and a series of alkyl benzenes. The strengths of those members in which the alkyl group is unbranched bear a nearly linear relationship to the molecular weight, not unlike that observed for the n alkanes. Furthermore, branching of the alkyl side chain again results in a decrease of electric strength. An attempt is made to interpret the results in terms of a simple model.

On the Electric Strengths of Aliphatic Hydrocarbons

Paschens similarity law, which expresses a certain relationship between the breakdown voltage of a gas, the gas pressure, and the distance between electrodes, is known to fail at high pressures. This is owing, at least in part, to a change in the way in which electrons dissipate energy to the gas molecules. At high enough pressure, the electron transfers energy principally to molecular vibrations. In condensed phases this vibrational barrier is dominant, and one should expect a new kind of similarity law to hold under certain controlled conditions. The liquid aliphatic hydrocarbons provide an illustration of such a law. The electric strengths of nine hydrocarbons of various densities were measured and found to depend linearly upon density. It has also been found that the temperature dependence of the electric strength of a liquid aliphatic hydrocarbon can be accounted for by the change in density of the liquid due to thermal expansion.

Electric strength and molecular structure of saturated gaseous hydrocarbons

While many intensive investigations have been made on a few gases (e.g. the noble gases, hydrogen, nitrogen and air) in an effort to elucidate the mechanism of spark breakdown, little effort has been made to correlate the sparking potential with molecular structure (la). We have attempted to do this by studying a series of closely related saturated hydrocarbons from C 1 through C 6 , including straight chain, branched and cyclic compounds. We have measured also the so-called “corona starting voltage” for these gases placed in an uniform field between glass dielectrics. This is of interest because it represents gas breakdown between electrodes of markedly higher work function than the metals, and also because of its importance in the field of electrical insulation.

Sparking Potential and Molecular Structure of Unsaturated Hydrocarbon Gases

In earlier experiments, we have shown that it is possible to relate sparking potentials of saturated hydrocarbon gases to certain molecular properties. From measurements of electric breakdown in a series of these gases, it has been found that the effective molecular cross section for electron scattering is proportional only to the number of carbon‐hydrogen bonds in the molecule and is independent of their disposition. The lack of a contribution by the carbon‐carbon bonds has been attributed to a shielding effect.In the present paper, we describe and discuss the results of a similar investigation of breakdown in unsaturated hydrocarbon gases. It was found that, for a series of olefin gases, the cross section associated with the carbon‐carbon double bond is greater than three times that of the carbon‐hydrogen bond, while that of the carbon‐carbon single bond is essentially zero as it was in the alkane series. An attempt is made to interpret this observation in terms of (1) a decrease in the shielding effect...