Труды КНЦ вып.9 (ХИМИЯ И МАТЕРИАЛОВЕДЕНИЕ вып. 1/2018(9) часть 1)

Introduction Molten lanthanide fluorides have been investigated for possible use in the nuclear fuel industry in two different roles: as models for the pyrochemical processing, reprocessing and separation of the heavier (and usually more radioactive) actinide systems; and as models, or potential components in, cooling/heating/heat transport systems. For this purposes several candidate systems with two or more components have been selected and investigated including almost all lanthanides. However, this investigation has not been systematic, even though a lot of data has been gathered over a long period of time. When considering only systems with the most frequently investigated LaF 3 , such systems are e.g. LiF + LaF 3 , NaF + LaF 3 , KF + LaF 3 , RbF + LaF 3 , LiF + NaF + LaF 3 and others [1-6]. The majority of these research reports are focused on phase equilibria, and other properties are investigated only occasionally, such as NMR, EXAFS of XPS [5, 7, 8 ]. The investigation of basic physico-chemical properties (like density, surface tension, viscosity, electrical and thermal conductivity) has been more or less random, even when these properties are important for potential applications. This work is related to the systems like LiF — CaF 2 — LnF 3 . Literature overview of physico-chemical properties of LiF / CaF 2 -LnF 3 binary systems has showed that there are no data for e.g. density, viscosity or other physico-chemical properties even for binary systems. Almost no experimental data for ternary systems are accessible, as well. The aim of this work is to investigate systems of (LiF — CaF 2 )eut. — LnF 3 (Ln = La, Gd, Nd, and Sm) in terms of primary crystallisation temperature measurements and volume properties analysis in the temperature range up to 1273 K in order to get some systematic data suitable for comparison. Experimental Chemicals The following chemicals were used: LiF (99,9 %, Sigma-Aldrich), CaF 2 (99 %, Merk), LaF 3 (99,9 %, ChemPur), SmF 3 (99,9 %, Chempur), NdF 3 (99,9 %, Chempur) and GdF 3 (99,9 %, Chempur). LiF was dried at 773 K during 4 hours and CaF 2 was dried at 773 K during 4 hours. All chemicals were handled inside high purity argon atmosphere (99,9990 %, Messer Tatragas) in a glove box (water content < 10 ppm). Experimental methods Phase equilibrium The phase equilibria of the investigated system were determined by the means of a thermal analysis method. Detailed measuring procedure was published several times and can be found in e.g. [9-17]. All samples were homogenized and placed in a platinum crucible in Glove box under inert atmosphere (Ar - Messer, 99.999 % purity). Homogenized sample (ca 7 g) in a platinum crucible was transferred into the preheated furnace at 353 K under dried argon atmosphere (Ar - Messer, 99,996 % purity). The experiments were done in tightly closed vertical resistance furnace with water cooling. Density The Archimedean method was used for determination of the density of investigated system. A platinum vessel suspended on a platinum wire with a diameter of d/m = 0,3 x 10-3, attached to the bottom of an electronic balance unit, was used as the measuring body. The dependence of the vessel volume on temperature was determined by calibration using molten NaCl and KF. The temperature was measured using a Pt-Pt10Rh thermocouple. The precision in the temperature measurement was ± 2 K. A PC with the LabView software environment was used for controlling the measuring device and for evaluation of the experimental data. The experimental device and procedure have been described in more detail elsewhere [18, 19]. Results Phase equilibrium Eutectic composition of the systems LiF — CaF 2 was reported several times with the following coordinates: 19,5 mol % CaF 2 Teut. = 1042 K [20] and 21,7 mol % CaF 2 Teut. = 1041,5 K [21]. In this work the composition of 21 mol % CaF 2 was used and the measured temperature of primary crystallisation was as high as 1036 K. For all investigated systems (LiF — CaF 2 )eut. — LnF 3 (Ln = La, Sm, Gd, and Nd) temperatures of primary crystallisation were measured in the temperature accessible concentration range up to x(LaF 3 ) = 0,35, x(SmF 3 ) = 0,55, x(GdF 3 ) = 0,3 and x(NdF 3 ) = 0,4, respectively, because of relatively high melting temperatures of pure salts (LaF3=1766 K, SmF 3 1572 K, GdF 3 1509 K, and NdF 3 1649 K [22]). All the quasi-binary systems are cross sections of the ternary systems LiF — CaF 2 — LnF 3 . In all cases only schematic phase diagram in the measured concentration range is presented as the real calculation of curves of the primary crystallisation cannot be performed. The substantial objection for this calculation is the absence of fusion enthalpies data of particular components and the second complication arise from the existence of new unidentified phases formed in the systems. All schematic phase diagrams are shown at Figures 1-4. 418