## 3 Results and Discussion
While analyzing the shape of isotherms, one can conclude that the surface of MgO was covered with MgF\({}_{2}\), because the shape of the isotherms measured for the bare MgF\({}_{2}\) and impregnated samples was similar, whereas that for bare MgO clearly differed from them. X-ray diffraction patterns (Figure 2) show that in the MgF\({}_{2}\)/MgO system calcined at 500 \({}^{\circ}\)C, no mixed phase was formed. The studied samples contained only separate phases of MgF\({}_{2}\) and MgO. The MgF\({}_{2}\)/MgO samples were employed as a filler for the preparation of electrolyte-containing pellets in the form of a eutectic mixture consisting of 31.54 wt.% LiCl, 23.47 wt.% KCl, and 44.99 wt.% RbCl. Earlier studies indicated that the most advantageous properties are encountered in the case of pellets containing 40 wt.% of the filler and 60 wt.% of the electrolyte [5]. Thus, we applied the above  
Figure 1: Nitrogen adsorption–desorption isotherms for MgF\({}_{2}\), 10 MF, 25 MF, and MgO calcined at 500 \({}^{\circ}\)C.  
Figure 2: X-ray diffraction patterns of MgO, 10 MF, 25 MF, and MgF\({}_{2}\) samples heated at 500 \({}^{\circ}\)C.  
filler/electrolyte proportion while studying the electrolyte absorption, mechanical strength, unfilled spaces (voids) in the pellets, and the state of the pellets after cooling. The results of these studies are presented in Tables 2 and 3. The electrolyte absorption was determined based on the change in the thickness of pellets (%) at the pressure 60 kPa and temperature of 400 \({}^{\circ}\)C. The smaller the changes of the above parameter, the more stable the behavior of the electrolyte-containing pellet. From this point of view, the most advantageous was the behavior of bare MgO, in which case the higher the temperature of the initial calcination resulted in a greater stability of the pellets. The introduction of MgF\({}_{2}\) to MgO deteriorated the absorption properties, although the absorption ability of the filler 25 MF calcined at 500 \({}^{\circ}\)C was close to that of bare MgO. A particularly large deterioration of absorbability occurred in the MgF\({}_{2}\)/MgO fillers calcined at higher temperatures (600 and 700 \({}^{\circ}\)C). Regarding the mechanical strength, the addition of MgF\({}_{2}\) clearly improved this parameter (almost twofold). For all the studied samples, a reduction in the mechanical strength was observed when the filler calcination temperature increased. The volumes of unfilled spaces (V%) were similar for all the fillers, however, bare MgO behaved a bit better. \begin{table}
\begin{tabular}{c c c c c c c c c} \hline \hline
**Calcination Temperature** & \multicolumn{3}{c}{**MgO**} & \multicolumn{3}{c}{**10 MF**} & \multicolumn{3}{c}{**25 MF**} \\ \cline{2-10} **of the Electrolyte** & \multicolumn{3}{c}{**S, \%**} & **MS, N** & **V,\%** & **S,\%** & **MS, N** & **V,\%** & **S,\%** & **MS, N** & **V,\%** \\ \hline
500 & \(7.6\pm 0.3\) & \(4.12\pm 0.08\) & \(47.8\pm 0.4\) & \(12.7\pm 0.4\) & \(6.49\pm 0.08\) & \(49.1\pm 0.4\) & \(7.0\pm 0.4\) & \(6.02\pm 0.08\) & \(48.8\pm 0.4\) \\
600 & \(7.0\pm 0.2\) & \(3.74\pm 0.08\) & \(43.1\pm 0.4\) & \(14.9\pm 0.4\) & \(5.83\pm 0.08\) & \(47.6\pm 0.4\) & \(12.7\pm 0.5\) & \(5.89\pm 0.08\) & \(47.8\pm 0.4\) \\
700 & \(6.2\pm 0.2\) & \(3.82\pm 0.08\) & \(30.0\pm 0.4\) & \(20.4\pm 0.5\) & \(5.43\pm 0.08\) & \(47.5\pm 0.4\) & \(70.3\pm 1.1\) & \(5.92\pm 0.08\) & \(41.6\pm 0.4\) \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Results of the measurements of dimensional stability (**S**), mechanical strength (**MS**), and the unfilled spaces (**V**) in the pellets, when using MgO, 10 MF, and 25 MF as immobilizing agents. The immobilizing agent content in a pellet was 40 wt.%.