ON THE RELATIONSHIP BETWEEN THE ELECTRIC STRENGTH OF LIQUID DIELECTRICS AND THEIR PHYSICOCHEMICAL PROPERTIES

A. A. VOROB’EV and A. F. KALGANOV

It has been shown experimentally [1, 2] that the electric strength of solid crystalline dielectrics regularly increases with an increase in the stability of the chemical compound. The electric strength of single crystals of alkali-halide salts increases with an increase in the heat of their formation [1]. The latter, for ionic crystals, is a measure of their chemical stability. For crystals of solid solutions of alkali-halide salts, the electric strength increases with a decrease in the heat absorbed in their formation [2]. It is known that the stability of solid solutions, as equilibrium systems at different temperatures, depends on the thermal effect of their formation. Isomorphous mixtures and solid solutions formed from their components with a positive thermal effect remain stable when the temperature is lowered. Conversely, solid solutions formed with appreciable absorption of heat are unstable substances at low temperatures and tend to decompose.

The observed regularity corresponds to the generally accepted energetic concept, according to which any destruction of a substance occurs when the energy required for this is released.

In light of this concept, it is natural to expect a dependence of electric strength on physicochemical properties also for liquid dielectrics. The consequence of electrical breakdown of a solid dielectric is its destruction—the formation of a channel that ceases to be filled with matter in the solid state. In this case all the bonds of the solid substance as a chemical compound may be destroyed.

The final stage of the electrical breakdown of liquid dielectrics is the formation, between the electrodes, of a channel filled with dielectric vapors. The evaporation of the liquid during breakdown is associated with the rupture of intermolecular bonds without destruction of the molecule. In this case one should expect an increase in the electric strength of a liquid dielectric with an increase in the boiling temperature of the liquid, the heat of evaporation of the liquid, the surface tension, density, polarization forces, and other quantities characterizing intermolecular bonds. It should be noted that a decrease in the boiling temperature of a liquid with decreasing pressure should not affect the breakdown voltage, since in the presence of enormous surface-tension forces a change in the external pressure will not substantially change the bonding forces between molecules. Indeed, in works [3] and [4] it is shown that, for degassed toluene and xylene, the electric strength does not depend on the external pressure.

If another, energetically less probable process takes place—the rupture of chemical bonds—one should expect an increase in the electric strength of liquids with an increase in the thermodynamic (isobaric) potential, the bond energy of atoms in the molecule, and other quantities characterizing the stability of the chemical compound.

A comparison of the electric strength of liquids with their physicochemical properties can provide substantial data for judging the nature of breakdown phenomena in liquid dielectrics and scientific grounds for selecting materials with high electric strength.

However, until recently, comparing the electric strength of liquid dielectrics with their physicochemical characteristics has not been meaningful, since the measured breakdown voltage of liquids was determined not so much by their chemical nature as by various secondary phenomena accompanying breakdown. Therefore, the electric strength of almost all liquid dielectrics of technical purity proved to be the same and did not exceed several hundred kV/cm [5].

Recently, higher values (\(>1\ \mathrm{MV/cm}\)) have been obtained for the measured electric strength of liquids, approaching the electric strength of solid dielectrics [6, 7]; moreover, the values of electric strength for each individual liquid differed from one another and were specific for each liquid. Thus, Crowe, Bratt, and Shcherbakh [6] obtained, for the electric strength of certain organic liquids—aliphatic hydrocarbons—a linear dependence on density.

Fig. 1.

The high values obtained for the electric strength of liquids give grounds to believe that, in these experiments, breakdown was determined not by accidental or secondary causes, but was associated with the nature of the liquid itself. Comparing electric strength only with the density of the liquid, as Crowe and others did, did not yet provide a complete picture of the influence of the physicochemical properties of the dielectric on its strength; therefore, using the data of Crowe and others, we compared the electric strength of aliphatic hydrocarbons with other physicochemical properties of the indicated liquids. Figure 1 presents the relationships we constructed for the electric-

…of the electric strength of liquid hydrocarbons on the heat of vaporization \(Q_{\mathrm{vap}}\), boiling point \(t_{\mathrm{boil}}\), surface tension \(\gamma\), viscosity \(\eta\), molecular weight \(M\), and thermodynamic potential \(\Delta Z\). There is also given the dependence of electric strength on density \(\rho\), obtained by Crowe and others.

As can be seen from Fig. 1, an increase in electric strength is observed with an increase in the above-mentioned parameters, except for the isobaric potential \(\Delta Z\). With an increase in the isobaric potential, the electric strength decreases. It is also known that the strengths of the carbon–carbon \(C—C\) bonds and of the carbon–hydrogen \(C—H\) bond for organic compounds are, on average, constant [8].

It may be supposed that, as a consequence of the foregoing, the quantities characterizing the bond strength of the molecules of a given homologous series of organic liquids do not determine the electric strength of the latter.

On the contrary, with an increase in the quantities characterizing intermolecular bonds, the electric strength increases. It should therefore be assumed that in this case electric breakdown is connected with the overcoming of intermolecular forces.

In the general case one may also put forward the proposition that the breakdown process begins with the destruction of intermolecular bonds, which are weaker than intramolecular ones, and then, as the energy concentration in the discharge channel increases, destruction of intramolecular bonds may also occur.

It is interesting to note that the dependence of the electric strength \(E_{\mathrm{p}}\) on the heat of vaporization \(Q_{\mathrm{vap}}\) and density \(\rho\), which characterize intermolecular bonds to the greatest extent, is close to linear, whereas the other curves have a nonuniform slope.

The data on the physicochemical characteristics of liquid hydrocarbons were taken from works [9, 10, and 11].

Conclusions

1. It has been established that the electric strength of a homologous series of saturated liquid hydrocarbons decreases with an increase in the thermodynamic (isobaric) potential \(\Delta Z\), which characterizes the bond strength of the chemical compound.

2. It has been established that the electric strength of these same hydrocarbons increases with the growth of quantities characterizing intermolecular bonds.

3. On the basis of what has been said above, it may be supposed that electric breakdown of the investigated liquids is connected with the overcoming of intermolecular forces.

In conclusion, the authors express their gratitude to Professor B. V. Tronov and Associate Professor A. S. Naumova for discussion of the topic and critical remarks.

REFERENCES

1. A. A. Vorob’ev and E. K. Zavadovskaya, ZhFKh, 28, 1910, 1954.
2. A. A. Vorob’ev, E. K. Zavadovskaya, and A. M. Trubitsyn, DAN SSSR, 100, 1065, 1955.
3. A. F. Walter and O. V. Chelyustkina, ZhTF, 6, 1900, 1936.
4. A. F. Walter and L. D. Ing, ZhTF, 1, 745, 1931.
5. F. F. Bolkenshtein, Breakdown of Liquid Dielectrics. ONTI, 1934.
6. R. W. Crowe, J. K. Bragg, A. H. Sharbaugh, J. Appl. Phys., 25, 392, 1954.
7. W. D. Edwards, J. Chem. Phys., 20, 753, 1952.
8. V. A. Kireev, Course of Physical Chemistry. Goskhimizdat, 1951.
9. Physicochemical Properties of Individual Hydrocarbons, Issue III. Gostoptekhizdat, 1951.
10. Chemist’s Handbook, Vol. I. Goskhimizdat, 1951.
11. R. D. Obolentsev, Physical Constants of Hydrocarbons, Liquid Fuels, and Oils. Gostoptekhizdat, 1953.