CHANGES IN THE ELECTRICAL STRENGTH AND MICROHARDNESS OF SINGLE CRYSTALS OF KCl—KBr SOLID SOLUTIONS DURING NATURAL AGING

A. N. Kislina

(Presented by Professor Doctor A. A. Vorob’ev)

S. F. Zhemchuzhnyi and F. Rambach [1] experimentally established the fact that the heat of formation changes with time for single crystals of equimolar solid solutions KBr—NaBr and KJ—NaJ. V. Hovi [2] traced the change in the heat of formation of equimolar solid solutions KCl—KBr in the course of natural aging.

I. B. Vrzhesnevsky [3] noted a change with time in the flow pressure of KCl—KBr and KJ—KBr alloys.

A change in microhardness and electrical strength during natural aging has been established for certain solid solutions of the systems KCl—KBr and KBr—NaBr [4]. A more systematic study has been made of the change in electrical strength and microhardness for equimolar solid solutions of the systems KCl—KBr, KJ—KBr, and NaCl—NaBr [5].

It is of interest to trace the change in the electrical strength and microhardness of single crystals of solid solutions with different component concentrations, formed with the absorption of different amounts of heat, since the aging rate of solid solutions depends on their heat of formation.

For such a study we chose solid solutions of the KCl—KBr system. The choice of this system may be explained by the fact that KBr—KCl single crystals are sufficiently stable and nonhygroscopic. In addition, the available experimental data on the change in electrical strength of KCl—KBr solid solutions as a function of chemical composition are contradictory. Thus, in the curve of the dependence of the electrical strength of KCl—KBr crystals on chemical composition, according to Hippel’s data [6], a maximum is indicated (Fig. 1), whereas according to A. M. Trubitsyn’s data [7] there is a minimum.

We determined the impulse electrical strength (front duration ≃ 7 μsec) of freshly grown single crystals of solid solutions of the KCl—KBr system and then of single crystals of this system that had been stored for a prolonged time at room temperature in a dry atmosphere. The shape of the specimens for breakdown was “well—against plane.” The electrodes were applied by spraying tin in va-

to the beaker. Breakdown of the specimens was carried out in dried transformer oil.

From each series of specimens prepared for breakdown, 3–5 specimens were taken, and their microhardness was measured. The method for measuring microhardness has been described previously [4].

In Fig. 2, changes in the electrical strength (arithmetic mean values) of single crystals of solid solutions of the KCl—KBr system are compared for fresh crystals and for crystals after 3, 6, 12–14 months and after three years of storage from the time of growth.

In the course of the dependence of electrical strength on chemical composition for fresh specimens of KCl—KBr solid solutions, a minimum is observed in the region of approximately equal component contents. The values of the electrical strength of single crystals of solid solutions of composition 30–80 mol% KBr in KCl after 3 months of storage are arranged, as a function of chemical composition, along a curve with a maximum falling approximately in the region of medium concentrations.

Fig. 1. Dependence of the electrical strength of single crystals of KCl—KBr solid solutions on chemical composition (according to H. Hippel’s data)

After 6 months of storage of KCl—KBr solid-solution crystals containing up to 30 mol% of solute in the solvent, their electrical strength is practically equal to the electrical strength of fresh specimens; while for the region of medium concentrations of the components in the solid solution the values of electrical strength are arranged along a curve with a maximum; however, this curve lies below the curve corresponding to the electrical strength of solid-solution single crystals of 3-month age.

Fig. 2. Dependence of the electrical strength of single crystals of KCl—KBr solid solutions on chemical composition

1. X—\(E_{\text{br. av.}}\) of freshly grown crystals
2. ○—\(E_{\text{br. av.}}\) after 3 months of storage of the crystals
3. □—\(E_{\text{br. av.}}\) after 6 months of storage of the crystals.
4. △—\(E_{\text{br. av.}}\) after 12–14 months of storage of the crystal.
5. ▲—\(E_{\text{br. av.}}\) after 3 years of storage of the crystals

The electrical strength of crystals containing 2.5 and 5 mol% KBr in KCl exceeds the electrical strength of KCl crystals after 3 years of storage.

As is clear from the foregoing, in the process of natural aging of KCl—KBr solid-solution crystals, their electrical strength changes more significantly for crystals of equimolar composition and for compositions close to it in component concentration. Therefore we attempted to follow the change in the electrical strength of KCl—KBr solid-solution crystals of various concentrations during aging. The experimental results are presented in the form of graphs of the dependence of the electrical strength of solid-solution crystals on their storage time (Figs. 3–9).

Fig. 3. Change in the electrical strength of single crystals of solid solutions containing 90 mol% KCl and 10 mol% KBr during aging.
Axes in the figure: vertical—electrical strength, MV/cm; horizontal—storage time of the crystals, months.

The microhardness of KCl—KBr solid-solution single crystals of various concentrations changes during their aging in a manner similar to the electrical strength.

As confirmation we present several figures showing the change in the microhardness of solid-solution single crystals containing 10 mol% (Fig. 11), 20 mol% (Fig. 12), and 40 mol% (Fig. 10) KBr in KCl during their aging.

For a more visual comparison of the aging of KCl—KBr solid-solution single crystals of various concentrations, we give Table 1, in which we indicate the mean microhardness values of the above-mentioned crystals, fresh and after aging at room temperature.

Table 1

Mean microhardness values for KCl—KBr solid-solution single crystals, fresh and after a certain period of storage

Analyzing the results obtained, one may note the following effect of natural aging of solid-solution single crystals

Fig. 4. Change in the electrical strength of single crystals of solid solutions of 80 mol % KCl and 20 mol % KBr during aging.

Fig. 5. Change in the electrical strength of single crystals of solid solutions of 30 mol % KBr and 70 mol % KCl during aging.

Fig. 6. Change in the electrical strength of single crystals of solid solutions of 60 mol % KCl and 40 mol % KBr during aging.

Fig. 7. Change in the electrical strength of single crystals of solid solutions of 60 mol % KBr and 40 mol % KCl during aging.

Fig. 8. Change in the electrical strength of single crystals of solid solutions of 70 mol % KBr and 30 mol % KCl during aging

Fig. 9. Change in the electrical strength of single crystals of solid solutions of 80 mol % KBr and 20 mol % KCl during aging

Fig. 10. Change in the microhardness of single crystals of solid solutions of 90 mol % KBr and 10 mol % KCl during aging.

Vertical axis: Microhardness, in kg/mm².
Horizontal axis: Storage time of the specimens, in months.

Fig. 11. Change in the microhardness of single crystals of solid solutions of 90 mol % KCl and 10 mol % KBr during aging.

Vertical axis: Microhardness, kg/mm².
Horizontal axis: Storage time of the specimens, in months.

Fig. 12. Change in the microhardness of single crystals of solid solutions of 20 mol % KBr and 80 mol % KCl during aging.

Vertical axis: Microhardness, in kg/mm².
Horizontal axis: Storage time, in months.

KCl—KBr depends on the concentrations of the components in the solid solution and, consequently, on their heat of formation. The heat of formation of KCl—KBr solid solutions varies along a curve with a maximum, depending on the chemical composition [1, 2].

The lower the concentration of the dissolved substance in the solvent of the solid solution and, consequently, the lower its heat of formation, the later the stage of crystal aging at which an increase in microhardness and electrical strength is observed.

For solid solutions containing 30–70 mol% of dissolved substance in the solvent, the electrical strength and microhardness change during crystal aging along a curve with a maximum, just as for equimolar solid solutions.

For KCl—KBr solid solutions containing up to 20 mol% of dissolved substance in the solvent, the electrical strength and microhardness increase upon more prolonged storage of the crystals (beginning with 6 months of storage of the crystals and later, whereas for the equimolar composition and compositions close to it this begins after 2–3 months). In addition, for these solid solutions we did not observe decreases in electrical strength and microhardness during their storage for up to 12–14 months and even up to 2–3 years.

Single crystals of KCl—KBr solid solutions of all concentrations may, at a certain stage of their aging, attain an electrical strength higher than the electrical strength of the pure components KCl and KBr.

On the basis of the foregoing, it is possible to explain the deviation, obtained by Hippel, from the dome-shaped “electrical strength—composition” curve of the values of electrical strength for KCl—KBr solid solutions containing more than 17 mol% of dissolved substance in the solvent (which Hippel did not do), by the different degree of aging of the solid-solution crystals.

This, probably, is also the reason for the different experimental values of the electrical strength of KCl—KBr solid-solution crystals obtained by Hippel and later by A. M. Trubitsyn.

The character of the change in the microhardness of single crystals of KCl—KBr solid solutions lying in the region of intermediate concentrations of the components during their aging is analogous to the change in hardness of metal alloys during artificial aging; while for KCl—KBr solid solutions with a content of the second component of 20 mol% or less, an analogy with the natural aging of metal alloys is observed.

For metal alloys, the aging effect also depends on the percentage content of the components. Alloys in which the component content corresponds to the limiting solubility at high temperature show maximum strengthening during heat treatment [8].

According to present-day views, the increase in hardness of metal alloys during natural aging is not connected with decomposition of the solid solution. During natural aging, preparatory processes for phase separation occur; such separation can take place only at high temperatures that provide a sufficient rate of atomic displacements (diffusion).

During artificial aging, only at temperatures above 200° and with holding times that do not ensure the attainment of maximum strength, precipitation of a phase from the solid solution is in fact observed and is detected by numerous methods of physicochemical analysis.

The maximum strength and hardness of metal alloys is observed in the first stages of their aging and is explained by the appearance, during the aging process, of special zones coherently connected with the lattice of the solid solution and oriented parallel to the \([100]\) plane. The extent of these zones is several tens of angstroms, and their thickness is 2–3 atomic layers.

For crystals of solid solutions of alkali-halide salts, this question, connected with changes in the crystal structure during aging, requires special investigation (for example, by X-ray diffraction).

The analogy we have established in the change of electrical strength and microhardness during aging of single crystals of solid solutions of the KCl—KBr system must also be checked for other dielectrics, for this will make it possible to monitor the state of the insulator structure during aging by means of an easily measured quantity, namely microhardness.

REFERENCES

1. S. F. Zhemchuzhnyi and F. Rambakh. Heat of formation of solid solutions. Izv. SPBPI, 12, 349, 1909.

2. V. Hovi. Heat of formation of solid solutions KCl—KBr. Ann Acad Scient. Fenn. A. I., 55, 1, 1948.

3. I. B. Rozenskii. The simplest case of a hardness diagram. ZhRFKhO, X, 43, 1364, 1911.

4. A. N. Kislina. Change in the electrical strength, microhardness, flow pressure, and heat of formation of solid solutions of certain alkali-halide salts during their aging. Izv. Tomsk. politekhn. in-ta, vol. 91, 137, 1956.

5. A. N. Kislina. Change in the electrical strength and microhardness of equimolar solid solutions of alkali-halide salts during their aging. Annotations of completed scientific-research works, issue 1. Electrophysics, Tomsk, TPI, 1957.

6. A. V. Hippel. Electrical strength of alkali-halide crystals and the influence on it of the formation of mixed crystals and extraneous impurities. Zs. f. Physik, 88, 358, 1934.

7. A. M. Trubitsyn. Electrical strength of single crystals of solid solutions of alkali-halide salts with a substituted haloid. Izv. Tomsk. politekhn. in-ta, vol. 91, 119.

8. A. P. Gulyaev. Physical Metallurgy. GIOP, 1956.

Errata