Surface Electrical Conductivity of Fresh Cleavages of Mica Crystals

M. S. MESHCHIK

Method

In studying the electrical properties of micas, we discovered the phenomenon of unusually high surface electrical conductivity of fresh cleavages of mica crystals. With time the latter decreases irreversibly, gradually assuming a more or less stable value.

To study the electrical conductivity, galvanometric and electrometric circuits were used. The sensitivity of the circuits in electrical conductivity was \(10^{-12} \div 10^{-13}\ \Omega^{-1}\). Three types of electrodes were used.

Fig. 1. Ring-shaped metallic polished electrodes. 1, 2—electrodes, 3—specimen.

Fig. 2. Vacuum bell jar with electrodes and a device for cleaving the crystal. 1—electrodes, 2—electromagnetic hammer for cleaving the crystal.

1. Platinum plates 15 mm wide, pressed by means of screws against the surface of the crystal under study. The distance between the electrodes was 5–10 mm; the insulation was amber.

2. Ring-shaped metallic polished electrodes 16.0–17.5 mm in diameter, against which a flat plate with a mica specimen placed on it was pressed by a screw. The central electrode was mounted on quartz.

3. When mica crystals were cleaved in a controlled medium, mercury electrodes were placed under a sealed glass bell jar; their cross-section is shown in Fig. 1.

A rectangular Plexiglas box was divided by a wedge-shaped partition into two compartments, insulated from one another, filled with mercury, which contacted the surfaces of the crystal being cleaved.

In the middle part of the partition, narrow wedges are glued on, preventing the mica sheets from being pressed against the partition.

A pre-split rectangular mica crystal is placed on the edge of the partition and, with the aid of the electromagnetic device shown in Fig. 2, is split up to the end clip. The current was used to calculate the specific surface electrical conductivity. The value of the conductivity measured in this way with plate electrodes was approximate, since the scattering of the field between the electrodes was not taken into account (the error amounted to 5–10%).

Such a systematic error does not affect the nature of the regularities in the change of surface electrical conductivity.

Decrease in the electrical conductivity of fresh cleavages of mica crystals (natural decay)

After mica crystals were split, electrodes were immediately placed on the fresh surfaces. At known time intervals a voltage was applied to the electrodes at the moment when the current was read from the instrument (approximately from 2 to 10 sec). The experiments showed that the surface electrical conductivity of fresh cleavages of mica crystals decreases quite regularly with time.

The decrease (decay) of electrical conductivity under natural conditions occurs especially rapidly in the initial period; then it slows somewhat, and the electrical conductivity, gradually decreasing, reaches a “stable” value sometimes after 2–3 days.

Table 1

Values of the initial and “stable” electrical conductivity for different micas

Table 1 gives the values of the initial and “stable” surface electrical conductivity for some micas under ordinary room conditions.

It is evident that the initial electrical conductivity for muscovite crystals is several times lower than for phlogopite crystals and, under room conditions, decreases by tens of times.

The experiments showed that the initial and “stable” electrical conductivity of plates cleaved from one and the same crystal under identical conditions changes practically little from plate to plate.

The two surfaces newly formed when a crystal is split possess the same electrical conductivity under equal testing conditions.

and electrically they are similar (mirror-symmetric).

The decrease in electrical conductivity depends substantially on the circumstances of cleavage of the crystal and on the conditions under which it is studied (humidity, air pressure and temperature, applied voltage).

Typical curves of the natural irreversible decrease of surface electrical conductivity for muscovite and phlogopite crystals at a temperature of 20°C, a pressure of 725 mm Hg, and humidity of 40–70% are presented in the graphs of Fig. 3 in coordinates of lg conductivity versus time. It is evident from the graphs that the electrical conductivity of the specimens decreases with time according to a combined exponential law.

Several seconds after splitting, the decrease of conductivity \(\sigma\) with time \(t\) can be represented with sufficient accuracy in the following form:

\[ \sigma = \sigma_1 e^{-\alpha_1 t}, \tag{1} \]

where \(\alpha_1\) is a constant depending on the conditions of splitting and on the circumstances of the decrease. Process (1), in the case of natural decrease, ends after a time ranging from several seconds to an hour or more, depending on the air humidity and other circumstances. Then the electrical conductivity decreases considerably more slowly; its change is also described by the law

\[ \sigma = \sigma_2 e^{-\alpha_2 t} \tag{2} \]

\(\alpha_2\) is less than \(\alpha_1\). Process (2) sometimes lasts tens of hours. Approximate characteristics of the decrease of conductivity in various micas under room conditions are presented in Table 2.

Fig. 3. Dependence of the electrical conductivity of fresh cleavages of mica crystals on time (natural decrease).

Table 2

Mean characteristics of the “natural decrease” of the conductivity of fresh cleavages of mica crystals at \(T = 20^\circ\)C

Ohm’s Law for Irreversible Surface Electrical Conductivity

Figure 4 shows the dependence of electrical conductivity on field strength for muscovite and phlogopite crystals.

It is evident from the graphs that the currents on freshly cleaved surfaces of mica crystals cease to obey Ohm’s law even at comparatively low field strengths.

For phlogopite crystals, Ohm’s law is satisfied only up to fields with a limiting strength of the order of 500 V/cm; for muscovite, up to 1200 V/cm.

At stronger fields, deviations from Ohm’s law are observed, consistent with the Poole [1] relation.

With the same degree of accuracy, the formula of Ya. I. Frenkel [2] is also satisfied in the investigated range of fields. At fields greater than those indicated in the graphs of Fig. 5, the conductivity began to decrease.

Subsequently, all investigations of the irreversible decrease in surface electrical conductivity were carried out only at fields for which Ohm’s law was satisfied.

Fig. 4. Dependence of the surface electrical conductivity of fresh cleavages of mica crystals on field strength.
1—muscovite, 2—phlogopite.

Fig. 5. Change in surface electrical conductivity under the action of a continuously applied field.
1 — \(E = 300\) V/cm, 2 — \(E = 600\) V/cm

Decrease in Electrical Conductivity of Fresh Cleavages under a Continuously Applied Voltage

(decay under voltage)

If a voltage is continuously applied to the electrodes, the surface electrical conductivity decreases with time considerably faster than in natural decay. The phenomenon of high-voltage polarization for currents different from stable ones was practically not observed.

The decay of conductivity under voltage for phlogopite crystals is shown in Fig. 5.

The temporal decay under voltage proceeds considerably faster than the natural one; moreover, the first and second parts of the decay curve usually have the form

two exponentials with a slope that decreases slowly as the conductivity decreases.

Often this change in the slope of the curves during the decay period is so small that, over short intervals of observation time, the decay curve in semilogarithmic coordinates appears as a broken straight line.

The break usually occurs several minutes after the beginning of the decay. The values of typical characteristics of the decay of conductivity under voltage at \(T = 20^\circ\mathrm{C}\), \(P = 725\) mm Hg for different micas are given in Table 3, from which it is seen that the electrical conductivity of muscovites decreases faster than the electrical conductivity of phlogopites, by approximately a factor of two. With an increase in the field strength, the decay of electrical conductivity is accelerated within the known limits.

Table 3

Average characteristics of the conductivity decay of fresh cleavages of mica crystals under a constantly applied field, \(T = 20^\circ\mathrm{C}\)

Decay of the Electrical Conductivity of Fresh Cleavages of Crystals at Different Temperatures

It has been established that, as the temperature of the mica crystal is increased, the decay of electrical conductivity is accelerated; the slope of the semilogarithmic decay curve increases strongly.

At crystal temperatures close to \(100^\circ\mathrm{C}\), irreversible phenomena on the surface are not observed at all.

The graphs of the decay of the electrical conductivity of muscovite crystals under a constantly applied voltage for different temperatures are presented in Fig. 6.

From them it is seen that the coefficient \(\alpha_2\) in the exponent of the decay law depends strongly on the temperature of the crystal.

Table 4 gives the values of the coefficient \(\alpha_2\) for two muscovite specimens at different decay temperatures.

Using the data of Table 4, the graphs of Fig. 7 were constructed, showing the existence of a linear relation between \(\lg \alpha_2\) and \(\frac{1}{T}\).

Fig. 6. Decay curves of the surface conductivity of fresh cleavages of mica crystals at different temperatures, °K.

1 — 303°, 2 — 315°, 3 — 328°, 4 — 358°

Fig. 7. Dependence of the characteristic decay rate of conductivity—$\alpha_2$—on the temperature $T$ for 2 samples of muscovite crystals.

Table 4

Dependence of the exponent on temperature

Effect of Air Humidity on the Conductivity of Fresh Cleavages of Mica Crystals

The experiments were carried out under a vacuum bell jar. Humidity was set by standard sulfuric-acid solutions, with exposure for no less than 12 hours. The conductivity of fresh surfaces of mica crystals also depends strongly on humidity, changing by several orders of magnitude with the latter. In this case there is a dependence between relative humidity and $\lg \sigma$ that is close to linear (Fig. 8).

Fig. 8. Dependence of the surface electrical conductivity of fresh cleavages on the relative humidity of the air at $T = 20^\circ\mathrm{C}$.

1 — initial conductivity,
2 — conductivity 2 hours after splitting.

The time decrease of conductivity is accelerated somewhat as the humidity is reduced. At low humidities this effect becomes practically unnoticeable.

In air dried with sulfuric acid, surface conductivity was not detected within the sensitivity of the apparatus ($10^{-12}\ \Omega^{-1}$).

Additional Experimental Facts on Irreversible Conductivity

In order to clarify the nature of the above-described irreversible electrical phenomena observed on the surface of freshly cleaved mica crystals, we carried out several additional experiments, the description of which is given below.

Experiment 1. As is known, a mica crystal consists of elementary sheets, fused together, with a thickness of $10\ \text{\AA}$.

It could be assumed that the above-noted phenomena also occur on all internal cleavage planes not exposed by splitting.

To test this assumption, in a thick mica crystal two grooves about $1\ \mathrm{mm}$ deep were drilled, separated from one another by a distance equal to the distance between the electrodes used in studying surface currents.

Mercury was poured into the grooves, against which the metal electrodes used in the study of surface conductivity were pressed.

At the same time, measures were taken to ensure reliable contact between the mercury and the mica. The electrical conductivity measured in this way along the internal cleavage planes of the mica crystal proved to be several times smaller than the surface conductivity, although an enormous number of internal cleavage planes of the crystal ($10^6$) participated in the conduction.

No irreversible phenomena were detected. The current decreased with time according to a law close to hyperbolic, but when the voltage was removed the conductivity was restored again, i.e., the usual phenomenon of high-voltage polarization was observed.

Consequently, the irreversible electrical phenomena arising on fresh surfaces of mica crystals are due only to the process of splitting.

Experiment 2. If a freshly cleaved mica plate is taken and one half of its surface is washed with distilled water, leaving the other half untouched, and it is then slightly dried, removing the remnants of visible moisture, the following can be observed:

1) the electrical conductivity of the washed part of the surface is close to the “stable” value.
2) the electrical conductivity of the unwashed part of the surface is close to the electrical conductivity of a fresh cleavage. This experiment leads us to the conclusion that the unstable elevated surface electrical conductivity of fresh cleavages of mica crystals can be eliminated by washing their surface.

Experiment 3. If the surface conductivity of a mica crystal is brought to a stable value and then a drop of distilled water is placed on the surface of the plate between the electrodes, the conductivity either does not change or receives an insignificant increase. The introduction of a drop of water onto the surface of a plate whose conductivity had not decreased to the stable value, however, causes a considerable increase in conductivity.

Experiment 4. Heating from below a mica plate clamped between electrodes causes a rapid decrease in its electrical conductivity. When the plate is cooled, the electrical conductivity is partially restored.

Such phenomena are not observed if the plate has previously been dried at a temperature of about 100°C for 30–60 minutes. If drying lasted several minutes, then at first the value of the conductivity is insignificant, but then it increases with time over the course of 1–2 minutes and after this begins to decrease as usual.

Experiment 5. If, at the moment of splitting a mica crystal, a drop of a liquid that does not yield electrolytes—for example, benzene—is introduced into the crack, then no irreversible surface conductivity is observed.

Thus, experiments 3, 4, and 5 lead to the conclusion that a very essential condition for the occurrence of surface electrical conductivity of fresh cleavages of mica crystals is the presence of an electrolytic aqueous medium on their surface. For the surface electrical conductivity of non-fresh cleavages of mica crystals this was convincingly proved in the experiments of Gokhberg [3], Semenov and Chirkov [4,5].

Influence of the Pressure of Air and Water Vapor on the Conductivity of Fresh Cleavages of Mica Crystals

Mica crystals were split at atmospheric pressure, in moist air. Then the pressure was lowered stepwise to 10–30 mm Hg, with a hold at each step of 15, 30 sec. As long as the pressure remained unchanged, the conductivity remained practically constant, but decreased when the pressure was lowered.

The author has data on the dependence of conductivity on pressure, from which it is seen that the conductivity decreases with decreasing pressure of moist air. When air is admitted, the conductivity was restored, but not

reached its initial value. The picture of the phenomenon was repeated upon repeated reductions of the pressure. Since in air dehydrated with sulfuric acid and calcium chloride the conductivity of fresh cleavages was not detected, the change in conductivity with pressure can be attributed only to a change in the partial pressure of water vapor in the air. As is seen from Fig. 9, between $\lg$ and the quantity reciprocal to the air pressure there exists a linear relation over a fairly wide range.

Fig. 9. Dependence of the initial conductivity on the pressure of humid air. $E = 60\%$, $T = 20^\circ\mathrm{C}$.
1—2—3—4—sequence of air evacuations after splitting the crystal.

The experiments described above convincingly show that the surface electrical conductivity of fresh cleavages of mica crystals is associated with the adsorption of water vapor from the surrounding air. Holding a split mica crystal for 20–30 minutes at an air pressure of 10 mm Hg ($P_{\mathrm{H_2O}} < 0.1$ mm Hg) eliminates the phenomena described above on its surface. This can be explained by the fact that evacuation causes an irreversible decrease in the adsorption activity of fresh surfaces of mica crystals.

Dependence of the “stable” surface electrical conductivity on temperature

A mica crystal that has spent a long time in humid air exhibits a certain surface electrical conductivity, dependent on its temperature and on the humidity of the air. This electrical conductivity, in contrast to the electrical conductivity of fresh cleavages, we call “stable.” To clarify the nature of the “stable” electrical conductivity, its change with temperature was studied. The experiment was arranged as follows.

Electrodes heated to a definite temperature were placed on a mica specimen. When the specimen assumed the temperature of the electrodes, a voltage was applied to it. At the initial moment the conductivity was maximal. Then it rapidly decreased and, after approximately a minute,

reached the sensitivity limit of the apparatus \((5 \cdot 10^{-15}\ \Omega^{-1})\). The investigations were carried out in the temperature interval \(18\text{--}85^\circ\mathrm{C}\). The upper temperature limit was set by the sensitivity of the apparatus.

The results of measurements for two muscovite specimens are presented in Fig. 10 as plots in the coordinates: logarithm of conductivity—reciprocal temperature. It is seen that a linear dependence exists between these quantities.

Discussion of the experimental results and their theoretical interpretation

The regularities described above can be explained on the basis of the idea that a conducting water film arises on the surface of freshly cleaved mica crystals.

Its formation is undoubtedly connected with the structural features of mica crystals.

The first ideas about the structure of mica crystals were put forward by G. V. Wulff [6]. Through the work of Wulff, and subsequently of Mauguin, Pauling, Jackson, and West [7], the structure of mica crystals was studied rather fully.

Mica crystals consist of double silicon–oxygen tetrahedral layers joined to one another through potassium ions. In the potassium layer, the bonding of the sheets is due to the interaction forces of potassium ions and oxygen, considerably smaller than the forces acting between ions in the layers themselves.

Upon cleavage of the crystal, its division takes place along the lattice of potassium ions. The latter are distributed, on average, equally between the newly formed surfaces and are boundary particles.

Fig. 10. Dependence of the “stable” surface electrical conductivity of muscovite on temperature.

Fig. 11. Dependence of the initial electrical conductivity on the partial pressure of water vapor.

Our special experiments on measuring the electrification of the surfaces of mica crystals upon their splitting show that, among the checkerboard distribution of ions on the crystal surface, there occur individual

groups of electric charges of one sign, retained for some time on the surface by the internal field of the lattice.

After cleavage of a mica crystal, on the newly formed electrically active surfaces there begins a multilayer adsorption of water molecules from the surrounding air. The thickness of the film increases layer by layer until evaporation of molecules from the layer is balanced by their adsorption.

All kinds of molecules present in the air may adhere to the charges on the surface of the crystal, but only water molecules, having the greatest dipole moment, will be most firmly bound to the surface of the crystal.

Therefore the adsorbed molecules will more or less rapidly be replaced by water molecules. The binding energy of each subsequent adsorbed layer decreases, and evaporation from it is facilitated.

At the same time, despite the multilayer character of the film, over a wide range of pressure variation the Langmuir adsorption equation is well satisfied (Fig. 11).

The results [3, 4, 5] for the “stable” conductivity also satisfy the Langmuir equation, although the authors themselves do not indicate this.

The equilibrium between evaporation and condensation is determined, at a given temperature and humidity, by the adsorption energy of water molecules [8]. The latter decreases with time as a result of neutralization of the surface electric charges [9, 10], which also leads to a thinning of the adsorbed film.

In the light of these assumptions, the established regularities in the change of the electrical conductivity of fresh cleavages of mica crystals describe the kinetics of sorption processes on crystal surfaces and the influence of various factors on it. Quite definite judgments about the mechanism of these processes will be possible only after carrying out a number of additional experiments.

Conclusion

As a result of the investigation carried out, general regularities have been established for the irreversible surface electrical conductivity of fresh cleavages of mica crystals, which apparently make it possible to form certain ideas about the processes of change in the sorption activity of the surfaces of freshly cleaved mica crystals.

The results obtained also provide a number of experimental foundations for the theory of cleavage of mica crystals.

The fact that fresh cleavages of mica crystals possess increased electrical conductivity must constantly be taken into account also in the practice of using them as insulators.

To eliminate the harmful influence of fresh cleavages of mica crystals on the insulating properties of mica plates, the latter must either be kept in air for 2–3 days, or dried at a temperature of about 100° for 0.5–1 hour, or the surface of the crystal must be washed with water or alcohol.

The same result is achieved by wiping the surface with cotton wool moistened with benzene.

All these types of treatment remove from the crystal surface the conducting electrolytic film and eliminate the irreversible electrical processes caused by it.

In conclusion I express my gratitude to B. V. Deryagin for participating in the discussion of the results of this work and for a number of recommendations, and to I. A. Parfianovich for advice in several experiments.

References

1. F. F. Vol’kenshtein. Electrical Conductivity of Semiconductors, OGIZ, Moscow–Leningrad, 1947.
2. Ya. I. Frenkel. ZhETF, 8, 12, 1292, 1938.
3. B. M. Gokhberg. ZhETF, 1, 275, 1931.
4. E. N. Semenov, N. M. Chirkov. DAN SSSR, new series, 1, 1, 37, 1946.
5. H. M. Chirkov. ZhFKh, XXI, 11, 1947.
6. G. V. Vul’f. Proceedings of the Institute of Applied Mineralogy, Issue 25, 1926.
7. E. K. Lashev. Mica, Promstroiizdat, 1948.
8. Ya. I. Frenkel. Kinetic Theory of Liquids, Academy of Sciences of the USSR, 1945.
9. B. V. Deryagin and N. A. Krotova. UFN, XXXVI, issue 3, 1948.
10. B. V. Deryagin and N. A. Krotova. Adhesion, Academy of Sciences of the USSR, 1949.

Irkutsk, State University