EFFECT OF ANNEALING OF CRYSTALS OF POTASSIUM CHLORIDE—POTASSIUM BROMIDE SOLID SOLUTIONS ON THE HEAT OF FORMATION, DIELECTRIC STRENGTH, AND MICROHARDNESS

A. N. KISLINA

(Presented by the Scientific Seminar on Dielectrics)

Data on the effect of annealing of crystals of potassium chloride—potassium bromide solid solutions on their heat of formation are based on isolated experiments by M. Popov [1] for low concentrations of solid solutions and by V. Fontell [2], as well as on more systematic investigations by V. Hovi [3] for potassium chloride—potassium bromide solid solutions of various concentrations.

According to the results of measurements of the heats of formation of crystals of potassium chloride—potassium bromide solid solutions obtained by V. Hovi, the effect of annealing manifests itself in fresh mixed crystals as an increase in the heat of formation. This is also consistent with Fontell’s observations.

A study of the results obtained by Hovi and Fontell shows that the heat of formation of annealed fresh mixed crystals, as a function of annealing time, at first increases rapidly and then slowly, approaching a state of saturation; for crystals aged for 8 months (with a potassium chloride content of more than 34%) the heat of formation, depending on annealing time, changes along a curve with a maximum. Thus, for mixed crystals having a sufficiently large value of the potassium chloride component and annealed for a prolonged time, the decrease in thermal energy in the process of natural aging exceeds the effect caused by annealing.

In the tables below we give the heats of formation according to Hovi’s data for mixed crystals with different times of natural aging and different annealing times.

Hovi assumes that heat treatment of mixed crystals accelerates the transformation of the initial disordered distribution of atoms into their ordered state, whereas at room temperature this process would require a longer time.

But, unfortunately, Hovi gives no explanation as to why this transformation into the ordered state at room temperature occurs with the liberation of heat, while at the annealing temperature it occurs with absorption.

The structure of a solid solution, determined by the arrangement of atoms and ions, determines the physical properties of solid solutions.

The study of the aging of solid solutions is based on measurements of physical properties sensitive to such aging. It is well known that hardness is a characteristic sensitive to the thermal aging of metal alloys. However, the literature contains no data indicating the effect of annealing on the microhardness and dielectric strength of mixed ionic crystals.

Table 1

Values of the heats of formation of equimolar mixed crystals potassium chloride–potassium bromide (2 weeks old), slowly crystallized from the melt, annealed at a temperature of 600°, and rapidly cooled.

We examined the effect of annealing temperature and time for single crystals of solid solutions potassium chloride–potassium bromide on their dielectric strength and microhardness.

Specimens for annealing were of the usual form—a spherical depression opposite a plane. Annealing was carried out in a muffle furnace with electric heating. The rate of increase and decrease of the temperature after annealing of the crystals did not exceed 1–1.5°C per minute. After annealing, the specimen was thoroughly wiped with alcohol, its thickness was measured, and then electrodes were applied by sputtering tin in vacuum.

Freshly annealed specimens were transparent, whereas in specimens stored in a dry atmosphere for 3 months a mosaic structure appeared. However, along cleavage planes such specimens split just as easily as transparent ones.

Table 2

Values of the heats of formation (cal/mol) of mixed crystals potassium chloride–potassium bromide (8 months old).

On 3–5 specimens taken from series of specimens prepared for dielectric-strength tests, microhardness values were determined.

In Fig. 1 the mean values of the dielectric strength of single crystals of solid solutions of potassium chloride in potassium bromide are compared for fresh crystals (curve 1), annealed at a temperature of 500° for 6 hours (curve 2), and annealed at a temperature of 300° for 40 hours (curve 3).

As is seen from Fig. 1, annealing of potassium chloride–potassium bromide specimens increases their dielectric strength, most substantially for specimens of intermediate component concentrations.

Figure 2 presents the dependence of the electrical strength of equimolar solid solutions of potassium chloride–potassium bromide on the annealing temperature. The annealing time was 6 hours. The figure plots the values of the electrical strength of all specimens broken down immediately after annealing. Crosses denote the arithmetic mean values of the electrical strength. The curves drawn through the maximum and arithmetic-mean values of the electrical strength have a similar form.

Fig. 1. Dependence of the electrical strength of single crystals of potassium chloride–potassium bromide solid solutions on chemical composition

The arithmetic mean values of the electrical strength have the same form. In this same Fig. 2 the arithmetic mean values of the electrical strength are plotted for specimens broken down 3 months after annealing.

Fig. 2. Dependence of the electrical strength of equimolar solid solutions of potassium chloride–potassium bromide on the annealing temperature (annealing time—6 hours).

\(o, x\)—electrical strength of specimens broken down immediately after annealing. \(\triangle\)—electrical strength (average) of specimens broken down 3 months after annealing

The electrical strength of specimens aged 3 months is considerably lower than the electrical strength of freshly annealed specimens. Moreover, the decrease in the electrical strength of specimens stored after annealing for 3 months is the greater, the higher their annealing temperature had been.

Figures 3 and 4 show the dependence of the dielectric strength on the annealing time of samples of the equimolar solid solution potassium chloride—potassium bromide. Annealing is carried out at temperatures of 500°C (Fig. 3) and 600°C (Fig. 4).

Similar dependences were obtained by us for the microhardness of single crystals of solid solutions potassium chloride—potassium bromide.

Fig. 3. Dependence of the dielectric strength of single crystals of equimolar solid solutions potassium chloride—potassium bromide on annealing time at a temperature of 500°C.

Fig. 4. Dependence of the dielectric strength of single crystals of equimolar solid solutions potassium chloride—potassium bromide on annealing time at a temperature of 600°C.

Figure 5 shows the microhardness of equimolar solid solutions potassium chloride—potassium bromide annealed for 6 hours at temperatures of 200, 300, 400, 500, and 600 degrees.

With increasing annealing temperature (for an annealing duration of 6 hours), the microhardness of equimolar solid solutions potassium chloride—potassium bromide increases.

Fig. 5. Dependence of the microhardness of single crystals of equimolar solid solutions potassium chloride—potassium bromide on the annealing temperature. Annealing time: 6 hours.

In Figs. 6 and 7 is shown the dependence of the microhardness of equimolar solid solutions potassium chloride—potassium bromide on the duration of annealing at a temperature of 600° (Fig. 7) and at a temperature of 500° (Fig. 6).

Fig. 6. Dependence of the microhardness of single crystals of equimolar solid solutions potassium chloride—potassium bromide on the annealing time.

The dependences indicated in Figs. 6 and 7 have the form of curves with a maximum. The maximum microhardness of equimolar solid solutions potas-

potassium chloride—potassium bromide corresponds to an annealing duration of 6–12 hours and exceeds the microhardness of unannealed samples of this composition by 30–40%.

Fig. 7. Dependence of the microhardness of equimolar solid solutions of potassium chloride—potassium bromide on annealing time at a temperature of 600°C.

Conclusions

Heat treatment of crystals of potassium chloride—potassium bromide solid solutions has an accelerating effect on their aging. In fresh crystals of solid solutions, the annealing effect manifests itself in an increase in the heat of formation, microhardness, and dielectric strength. At the same time, the heat of formation of mixed crystals as a function of annealing time at first increases rapidly and then slowly, approaching a state of saturation; the microhardness and dielectric strength of potassium chloride—potassium bromide crystals first increase, reaching a maximum value, and then decrease.

In mixed crystals of potassium chloride in potassium bromide of 8-month age, having a sufficiently high potassium chloride content (34% and higher), the decrease in the heat of formation due to their natural aging exceeds the effect caused by annealing. For the above-mentioned potassium chloride—potassium bromide crystals there exists a maximum value of the heat of formation. Thus, crystals annealed for 50 hours have a significantly greater heat of formation than crystals of the same composition but annealed for a longer time.

The decrease in the heat of formation of crystals of solid solutions as a result of natural aging will be the more significant, the longer the crystals are annealed.

In exactly the same way, the dielectric strength of annealed samples of potassium chloride—potassium bromide of equimolar composition, after storage at room temperature for 3 months, decreases significantly in comparison with freshly annealed samples, and this decrease is the more significant, the higher the annealing temperature. Thus, the dielectric strength of single crystals of potassium chloride—potassium bromide solid solutions, in contrast to the dielectric strength of pure potassium ...

single crystals depends on the temperature and duration of their annealing and, consequently, is a structurally sensitive property.

The regularities we have established in the variation of pulsed dielectric strength and microhardness during thermal and natural aging are similar in character. That is, just as during natural aging the microhardness and dielectric strength of single crystals of equimolar solid solutions of potassium chloride in potassium bromide change along a curve with a maximum [4], so too during thermal aging of fresh single crystals of this composition the microhardness and dielectric strength change along a curve with a maximum as a function of annealing time. The magnitude and position of the maximum of microhardness and dielectric strength during thermal aging evidently depend on the annealing temperature.

The change in the microhardness of equimolar solid solutions of potassium chloride—potassium bromide during annealing is analogous to the change in the hardness of metal alloys during thermal aging.

The established analogy in the change of dielectric strength and microhardness during heat treatment of single crystals of solid solutions of alkali-halide salts makes it possible to select the annealing duration of solid dielectrics according to an easily measured physical characteristic, namely microhardness.

REFERENCES

1. Popoff M. M., Choller W. Zs. f. Phys. Chem. A. 147, 302, 1930.
2. Fontell W. Soc. Scient. Fenn. Comm. Phys.-Math. X, 5, 1940.
3. Hovi V. Ann. Acad. Scien. Fenical, A, I. Phys.-Math., 55, 1—54, 1948.
4. Kistina A. N. Change in the dielectric strength and microhardness of single crystals of solid solutions KCl—KBr during natural aging (present collection).