Electrical Conductivity of Magnesium Oxide/Molten Carbonate Eutectic Coexisting System

2.1 Sample Preparation

Chemically pure-grade Li₂CO₃, Na₂CO₃, and K₂CO₃ (“Vekton,” St. Petersburg, Russian Federation) were preliminarily dehydrated under vacuum under stepwise increase of temperature and melted in argon atmosphere. The carbonate eutectic mixture Li₂CO₃ (43.5)–Na₂CO₃(31.5)–K₂CO₃(25) was prepared by fusion of the weighted portions of the prepared salts. The thoroughly mixed salt mixtures were dried under vacuum at 473 K for 1 h to remove traces of adsorbed water. After drying, the mixtures were heated up to 1073 K in an atmosphere of carbon dioxide. The melting point of the carbonate eutectic (670 K) determined by the DSC method (STA 449C Jupiter Thermal Analyzer; NETZSCH, Germany) agrees well with the literature data [9].

Analytically pure-grade MgO powder (“Chimreaktivsnab,” Ufa, Russian Federation) was used as the solid phase. The average crystallite size (coherent scattering region) of the MgO powder was estimated using the Scherrer formula [10] and half-widths of diffraction peaks obtained on a Shimadzu XRD-7000 diffractometer (Japan). Certified silicon powder was the standard that indicated the instrumental contribution to the peak width. Specific surface area of MgO powders was determined by gas absorption using a SORBI instrument (Meta, Russia). The morphology of MgO powder investigated by SEM (TESCAN MIRA 3 LMU; TESCAN, Czech Republic) is shown in Figure 1. The image reveals that the MgO powder consists of particles smaller than 100 nm. The average powder crystallite size was 95 nm. The specific surface area was 9.35 ± 0.07 m²/g. The lack of any sedimentation of finely dispersed powder in molten Li₂CO₃–Na₂CO₃–K₂CO₃ eutectic in a course of measurements was shown earlier [8].

Figure 1:

The morphology of MgO powder.

Samples were tested for possible impurities using infrared (IR) absorption spectroscopy on a Tensor 27 FT-IR spectrometer (Bruker, Germany). The IR spectrum of MgO (Fig. 2) showed a strong broadband in the range of 400–700 cm⁻¹ that corresponded to Mg–O vibrations. Vibrational characteristic bands of adsorbed H₂O or hydroxyls were not observed.

Figure 2:

The IR spectrum of MgO.

The volume fraction of the solid phase, φ, was calculated as follows:

where w_l, d_l, w_s, and d_s are weight of liquid phase, density of liquid phase, weight of solid phase, and density of solid phase, respectively. The density of MgO and the density of the melt were calculated according to the equations given in [9], [11]. The average values of the MgO volume fraction for the temperature interval under study were used.

2.2 Electrical Conductivity Measuring Technique

AC impedance was measured under Ar atmosphere with the impedance meter Z-1500J (Elins, Russia) in a frequency range 20 Hz–1.5 MHz using platinum (Pt) electrodes. The melt resistance, which was defined via the impedance diagrams, was used to calculate the melt electrical conductivity. The cell layout and the measurement procedures were the same as those in our previous study [8]. The electrical conductivities of the electrolytes under study were calculated taking into consideration the temperature dependence of the cell constant, which values were (1.0–1.4) ⋅ 10⁻² m⁻¹ within the temperature range of 673–823 K.

The salt composition and compositions of frozen melts after high-temperature experiments were determined using XRD method. Certification of the samples for the presence of possible impurities was carried out by IR absorption spectroscopy.

3.1 Temperature and Composition Dependence of the Electrical Conductivity

The electrical conductivity values of the ternary carbonate eutectic melt obtained in our earlier work [8] agree well with the literature data [9] in the temperature range 673–800 K. The temperature dependences of the electrical conductivity (ln(s) vs. 1/T) of the MgO/molten (Li₂CO₃–Na₂CO₃–K₂CO₃)_euc coexisting systems for various MgO content are shown in Figure 3. In all cases, the conductivity decreases with an increase of the solid content. Thus, the conductivity reduces at the order of magnitude when volume fraction of MgO is equal to 40 vol. %.

Figure 3:

Temperature dependence of the electric conductivity for MgO/(Li₂CO₃–Na₂CO₃–K₂CO₃)_eut systems with different MgO volume concentrations (ϕ).

3.2 Variation of Apparent Activation Energy ΔE_a With MgO Content

The measured conductivity showed Arrhenius-type temperature dependence. The electrical conductivity as a function of temperature can be approximated by linear equations as follows:

Figure 4:

Variation of the activation energy of the electrical conductivity of the liquid phase with the MgO content.

Figure 5:

Isotherm of specific electroconductivity of molten carbonate eutectic Li₂CO₃–Na₂CO₃–K₂CO₃, thickened with MgO, normalised by the specific conductivity of molten phase (T = 773 K).

The coefficients of these equations for various solid contents are given in Table 1. Therefore, the apparent activation energy of the electrical conductivity ΔE_a was calculated by the Arrhenius equation ΔE_a = −B ⋅ R. Here R is the universal gas constant. The variations of ΔE_a with the MgO content are shown in Figure 4. The increase of ΔE_a began at ca. 30 vol. % of solid phase content. These results indicate that the liquid phase is greatly perturbed by the solid phase in this region.

3.3 Implementation of Maxwell Model

Figure 5 demonstrates variations of the electrical conductivity with solid content for MgO/molten (Li₂CO₃–Na₂CO₃–K₂CO₃)_euc coexisting system at 773 K. Beginning with the 20 vol. % of the MgO content, the experimental values of conductivity were less than those calculated according to (1). So the specific conductivity of those systems could not be described using the Maxwell model [7]. Consequently, the change in conductivity of the carbonate electrolyte when adding a finely dispersed MgO should be associated not only with the reduction of the charge carrier concentration but also with a certain kind of interaction between the components.

According to X-ray diffraction results for frozen fusions after the experiment, no chemical reactions in this system occurred, and the only phases of magnesium oxide and lithium, sodium, and potassium carbonates were found. The chemical inertness of MgO in (Li₂CO₃–Na₂CO₃–K₂CO₃)_eut melt was demonstrated using in situ Raman spectroscopy [12].

Raman spectra [12] indicated that the symmetric stretching mode of carbonate is slightly perturbed by the solid phase. These peculiarities can be associated with the solvation process of the carbonate ions on the particles of the solid phase. Hence, the anions adsorbed on nanopowder-particle surfaces formed solvate shells that should considerably affect the physicochemical properties of the high-temperature disperse system. In particular, such kind of interaction between solid and liquid phases should have an appreciable effect on the electrical conductivity in the suspensions with relatively low oxide phase contents. The coagulation of the solid particles and their subsequent sedimentation were expected to be suppressed in the systems of similar type.