PULSED ELECTRICAL CONDUCTIVITY AND DIELECTRIC STRENGTH OF SOLID SOLUTIONS OF THE KCl—KBr SYSTEM

A. V. BARANOV, T. K. KOCHERBAEV

(Presented by Doctor of Technical Sciences G. A. Vorob'ev)

The components of the KCl—KBr system at room temperature are completely mutually soluble in all compositions [1—3]. An X-ray study showed that the critical temperature of mutual solution of the components of this system in the solid state lies below \(-90^\circ\) [1]. According to theoretical calculations, this temperature is about \(-50^\circ\text{C}\) [2]. The mutual-solubility curve of the components of the KCl—KBr system, obtained on the basis of semiempirical calculations, is asymmetric with respect to the equimolar composition, and the maximum in this curve corresponds to a composition of 42 mol% KBr in KCl [3]. Such asymmetry of the curve was explained by the fact that Br ions are more readily replaced by Cl ions than vice versa.

Fig. 1. Dependences of dielectric strength (curve 1) and current density (curve 2) on the composition of solid solutions of the KCl—KBr system.

X-ray studies showed that the greatest density of lattice defects corresponds to compositions between 30—40 mol% KBr in KCl [4].

The relative maximum of the intensity of energy absorption by \(F\)-centers is also between 30–40 mol% KBr in KCl [5].

We have investigated the dependence of the electrical strength of solid solutions \(K(Cl, Br)\) on their composition [6]. The electrical strength has a maximum value at 30–40 mol% KBr in KCl, while the lowest electrical strength corresponds to the composition 18 mol% KCl in KBr (Fig. 1, 1). The development of an electron avalanche, leading to electrical breakdown of a solid dielectric, depends on the intensity of impact ionization produced by electrons. Defects of the crystal lattice in solid dielectrics impede the development of an electron avalanche by trapping and scattering electrons; the path length between two ionizing collisions of an electron, other conditions being equal, is greater in solid solutions than in a perfect crystal. Therefore, although in solid solutions the bond between ions is weaker than in pure crystals, breakdown of solid solutions with completely mutually soluble components requires an increased electric field.

Below we discuss the question of the conductivity of solid solutions \(K(Cl, Br)\) in a strong pulsed electric field. Single crystals of the solid solutions were grown from the melt by the Kyropoulos method. KCl and KBr of “ChDA” grade served as starting materials.

The chemical composition of the solid solutions was determined by potentiometric titration with an accuracy of 1–2%. The specimens were prepared by the method of [7], and the current in these specimens was measured at a field strength of \(1.5 \cdot 10^{6}\) V/cm by the method of [8]. The results obtained showed that the conductivities of solid solutions \(K(Cl, Br)\), depending on their composition, also vary along an asymmetric curve (Fig. 1, 2), with a relative maximum and minimum, respectively, at 15 mol% KBr in KCl and 19 mol% KCl in KBr. The conductivity of solid solutions enriched in KCl is greater than the conductivity of solid solutions rich in KBr, except for solid solutions with a small content (up to 10 mol%) of one component in the other. For solid solutions rich in KCl and for solid solutions rich in KBr, the strength of the applied field was, respectively, 70% and 85–95% of the voltage corresponding to a 90% probability of their electrical breakdown.

Fig. 2. Oscillograms of voltage and currents: \(a\) — applied pulsed voltage; \(b\) — current in the first pulse; \(v\) — current in the second pulse (currents were measured in a specimen of solid solution containing 10 mol% KBr in KCl).

Thus, although the strength of the applied field for solid solutions with a high KCl content is relatively small, their conductivity is greater than the conductivity of solid solutions enriched in KBr. Evidently, for solid solutions rich in KCl, point defects of the crystal lattice affect their conductivity. Since the density of point defects is greater in compositions rich in KCl, and the mobility of vacancies of positive ions is greater in comparison with the mobility—

…by the density of vacancies of negative ions, it may be assumed that in these solid solutions, when a voltage pulse is applied, a positive charge is formed more rapidly in the near-cathode regions, increasing the emission of electrons from the cathode.

A comparison of the dependences of the electric strength \((E_{\text{br}})\) and the current density on the composition of the solid solutions shows that \(E_{\text{br}}\) of the solid solutions is not determined by their conductivity in prebreakdown fields. Measurements of the conductivity of fresh pure KCl and KBr crystals and of their solid solutions under several successive pulses of the same voltage show that a large current is observed only in the first pulse; in the second pulse the current amplitude decreases by a factor of 3–4. Figure 2 shows characteristic current oscillograms obtained when the first and second voltage pulses were applied to specimens of a solid solution of composition 10 mol% KBr in KCl.

After high-temperature annealing (artificial aging at \(650^\circ\text{C}\) for 10 h), current is also observed in subsequent pulses, and the current amplitude decreases slowly from pulse to pulse. The curve for the current density shown in Fig. 1, 2 was plotted from the values of the current amplitudes obtained for unannealed specimens upon application of the first pulse. The current amplitude observed upon application of the first pulse in unannealed specimens is greater than in annealed ones. This is analogous to the fact [9] that in AgCl crystals, after quenching, a strong increase in ionic conductivity was observed.

The slow decrease in current amplitude in annealed specimens, observed when successive voltage pulses are applied, shows that in a strong electric field in crystalline dielectrics the main current carriers are electrons.

In unannealed crystals, when such voltage pulses are applied, the capture of electrons by traps leads to the formation of a negative space charge, which retards the emission of electrons from the cathode. Therefore, for unannealed specimens, already at the second pulse the current amplitude decreases significantly. In annealed specimens the number of traps due to lattice defects is smaller [10] than in unannealed ones, and in the case of annealed specimens, when several successive voltage pulses are applied, current is observed in almost all pulses, and the current amplitude decreases very slowly from pulse to pulse.

Conclusions

1. The conductivities of solid solutions of compositions rich in KCl in a strong pulsed electric field are relatively greater than those of compositions enriched in KBr, with the exception of compositions with a small content of one component in the other.

2. A comparison of the dependences of electric strength and current density on the composition of the solid solutions of this system shows that \(E_{\text{br}}\) of the solid solutions is not determined by their conductivity in prebreakdown fields.

3. The character of the decrease in current amplitude observed when several successive voltage pulses are applied to unannealed and annealed specimens of KCl and KBr crystals and their solid solutions shows the possibility of the formation of a negative space charge due to electrons captured by traps.

4. The slow decrease in current amplitude from pulse to pulse in annealed specimens shows that, in a strong pulsed electric field in crystalline dielectrics, the main charge carriers are electrons emitted from the cathode.

References

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