Electrical Conductivity of Molten ZnCl₂ at Temperature as High as 1421 K

1 Introduction

The electrical conductivity of most molten salts is measured in the narrow temperature ranges significantly below their boiling points.

The electrical conductivity of molten ZnCl₂ is also well studied at temperatures below the boiling point (t_m(ZnCl₂)=1004 K [1]) [2–6] and remains almost unknown at higher temperatures. For the first time in the present work, the electrical conductivity of molten ZnCl₂ has been determined at temperature reached 1421 K.

2 Experimental

An analytically grade ZnCl₂ was carefully dehydrated by the long-term (about 20 h) gradual heating to 570 K under reduced pressure (∼1 Pa). Before testing, in addition it was purified by double distillation in a vacuum. Because of high sensitivity to moisture, the chloride being studied was loaded into the cell in a dry glove box under the dry nitrogen atmosphere because of its high sensitivity to moisture.

A special cell has been developed for the conductivity measurements of melts above the boiling temperature when the salt vapor pressure reaches several MPa. The cell was a capillary type entirely made of quartz and equipped with graphite electrodes. It was calibrated against molten ZnCl₂ [4], and the cell constant was obtained as 40.1 cm^–1. The loaded and sealed cell was heated in the electric furnace fitted with a metallic block. The temperature was registered with accuracy ±1 K with Pt/Pt-Rh (10 % Rh) thermocouple. The melt resistance was measured by an AC-bridge at the input frequency of 10 kHz. In more details, the cell and the measuring procedure are described elsewhere [6, 7].

3 Results and Discussion

In the present work, the electrical conductivity of molten ZnCl₂ was measured in the temperature range from 558 to 1421 K. In our previous work [6], the conductivity of molten ZnCl₂ was studied at lower temperatures below 1328 K. Experimental points obtained in [6] and in the present study as well as at heating and cooling of the melt are well fitted by a single line. Combining these data and approximating them with a polynomial, we get the general equation for the electrical conductivity of molten ZnCl₂ in the range of 558–1421 K:

ln(κ) = 0.61373 + 878.82/T − 7.8434 ⋅ 105/T2− 1.3218 ⋅ 109/T3, S/cm; T, K.

The determination coefficient R²>0.999; the standard deviation of ln(κ)=0.005. The overall error in κ determination does not exceed 2 % in the whole temperature range.

The results are shown in Figure 1 along with the most “high-temperature” literature data [5]. There is a good agreement between our data and the data of Grantham and Yosim [5]. The maximum temperature in their work is 1136 K. Thus, we have expanded the studied temperature range on 285 K that is far above the ZnCl₂ boiling point. At the maximum temperature the vapor pressure of the salt reaches the order of 3 MPa.

Figure 1

Specific conductivity (κ) polytherms of molten ZnCl₂ and its activation energy (E_A).

Because of its network structure [8], the ZnCl₂ is proned to overcooling. In our case, the overcooling amounted to 33 K. Any break was not observed when passing through the melting point (T_m ∼ 591 K [1]). The specific conductivity of the molten ZnCl₂ rose almost by four orders with the increasing temperature from 558 to 1421 K (from 0.000363 to 1.453 S/cm). Meanwhile, the conductivity activation energy decreased from ∼117 (558) to ∼17 kJ mol^–1 (1421 K).

According to the spectral data [8] near the melting point, the molten zinc dichloride consists of the “ZnCl_4/2” tetrahedra bounded by bridged halides. As the temperature increases the network structure breaks up to smaller fragments. The apex-bridged “ZnCl_4/2” tends to become edge bridged and, in addition, a large number of terminal halide bonds are formed. Consequently, the initial fast increasing of conductivity (Fig. 1) is a result of step by step breakdown of the initial network structure of the molten zinc chloride accompanied with quick viscosity decreasing [2, 3] and rising the ions amount and ions mobility. In the studied temperature range, the electrical conductivity of molten ZnCl₂ changed from values typical for molecular strongly associated liquid to values peculiar for ionic liquids (for comparison at 1300 K κ(ZnCl₂)=1.26, and κ(SrCl₂)=2.58 S/cm [2]).

It is known that the electrical conductivity of molten salts passes through a maximum with increasing temperature [5]. At the maximum, the formal conductivity activation energy is equal to zero. In our case, the order of 150 degrees remains to the maximum (estimated using Grantham and Yosim data [5]).