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Сведения об авторе Кротов Владимир Евгеньевич
доктор химических наук, Институт высокотемпературной электрохимии УрО РАН, г. Екатеринбург, Россия [email protected]
Krotov Vladimir Evgenjevich
Dr. Sc.(Chemistry), Institute of High-Temperature Electrochemistry of the Ural Branch of the RAS, Yekaterinburg, Russia [email protected]
DOI: 10.25702/KSC.2307-5252.2018.9.1.417-421 УДК 621.3.035.45
СРАВНЕНИЕ ФАЗОВОГО РАВНОВЕСИЯ И ОБЪЕМНЫХ СВОЙСТВ
ВЫБРАННЫХ ФТОРИДНЫХ РАСПЛАВНЕННЫХ СИСТЕМ НА ОСНОВЕ ЛАНТАНОИДОВ
Б. Кубикова, Ж. Млинарикова, М. Бока, М. Симурда, Е. Миксикова, З. Нетриова, И. Маскова, В. Гурисова
Аннотация
С помощью термального анализа и измерений плотности исследованы системы (LiF-CaF2)eut. — LnF3 (Ln = La, Sm, Gd, and Nd). Вычислены объемные свойства. Нетипичное поведение наблюдалось во всех случаях, когда молярные объемы уменьшались с первоначальными добавками LnF3 до 1 мол. %. Дальнейшие добавки LnF3 приводили к увеличению молярных объемов. В случае системы GdF3 аномальное поведение молярного объема наблюдалось при более 1 мол. % GdF3, при более низких температурах молярный объем выше. Ключевые слова:
расплавленная соль, температура первичной кристаллизации, плотность, молярный объем, парциальный молярный объем, лантаноиды.
COMPARISON OF PHASE EQUILIBRIUM AND VOLUME PROPERTIES OF SELECTED FLUORIDE MOLTEN SYSTEMS BASED ON LANTHANIDES
B. Kubikova, J. Mlynarikova, M. Boca, M. Simurda, E. Miksikova, Z. Netriova, I. Mackova, V. Gurisova,
Institute of Inorganic Chemistry, Slovak Academy of Sciences, Bratislava, Slovakia Abstract
Systems (LiF-CaF2)eut. — LnF3 (Ln = La, Sm, Gd, and Nd) were investigated by means of thermal analysis and density measurements. Consequently, volume properties were calculated. Unusual behaviour was observed in all cases, when molar volumes decrease with initial LnF3 additions up to 1 mol% of LnF3. Further LnF3 additions result in molar volumes increase. In the case of GdF3 system anomalous molar volume behaviour was observed over 1 mol% of GdF3 molar volume is higher at lower temperatures. Keywords:
molten salt, temperature of primary crystallisation, density, molar volume, partial molar volume, lanthanides.
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 LaF3, such systems are e.g. LiF + LaF3, NaF + LaF3, KF + LaF3, RbF + LaF3, LiF + NaF + LaF3 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 — CaF2 — LnF3. Literature overview of physico-chemical properties of LiF / CaF2-LnF3 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 — CaF2)eut. — LnF3 (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), CaF2 (99 %, Merk), LaF3 (99,9 %, ChemPur), SmF3 (99,9 %, Chempur), NdF3 (99,9 %, Chempur) and GdF3 (99,9 %, Chempur). LiF was dried at 773 K during 4 hours and CaF2 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 — CaF2 was reported several times with the following coordinates: 19,5 mol % CaF2 Teut = 1042 K [20] and 21,7 mol % CaF2 Teut. = 1041,5 K [21]. In this work the composition of 21 mol % CaF2 was used and the measured temperature of primary crystallisation was as high as 1036 K.
For all investigated systems (LiF — CaF2)eut. — LnF3 (Ln = La, Sm, Gd, and Nd) temperatures of primary crystallisation were measured in the temperature accessible concentration range up to x(LaF3) = 0,35, x(SmF3) = 0,55, x(GdF3) = 0,3 and x(NdF3) = 0,4, respectively, because of relatively high melting temperatures of pure salts (LaF3=1766 K, SmF3 1572 K, GdF3 1509 K, and NdF3 1649 K [22]). All the quasi-binary systems are cross sections of the ternary systems LiF — CaF2 — LnF3. 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.
1050 HMO
1010 1000
C
- c
□ n
" A A aA a A A A A
A
0.00 0.05 0.10 0.15 0.20 0.25 0.30 0.35 0.40 *(L«F,)
Fig. 1. Schematic phase diagram of the (LiF — CaF2)eut — LaF3 molten system
Fig. 2. Schematic phase diagram of the (LiF CaF2)eut — SmF3 molten system
Fig. 3. Schematic phase diagram of the (LiF — CaF2)eut — GdF3 molten system
Fig. 4. Schematic phase diagram of the (LiF CaF2)eut — NdF3 molten system
Based on the schematic phase diagrams it is clear to see that investigated systems with exception of (LiF — CaF2)eut — SmF3 are not simple eutectic ones. Eutectic temperatures vary between approx. 989 K for system with LaF3, 935 K for system with SmF3, 925 K for GdF3 and 965 K for system with NdF3. Based on the previously reported data concerning the presence of peritectic point in the LiF — CaF2 — LaF3 molten system [1] and formation of some ternary phases like Cao,65Lao,35F2,35 and Cao,9Lao,iF2,i, the formation of ternary fluorides was expected in the (LiF — CaF2)eut — LaF3. However XRD patterns of solidified samples from thermal analysis showed no other crystalline phase except the initial ones. Information about XRD analysis and further information about the phase diagram could be found in our published work [23].
In the quasi-binary system (LiF — CaF2)eut. — SmF3 the formation of other phases fields was observed as expected based on results of CaF2 — SmF3 system. In order to estimate whether some new phases could be formed XRD patterns of the solidified samples were recorded. The intensity of the additional diffractions changes with the increasing content of SmF3 indicating the formation of some new component or even more components. Final complication arises from the shift of some diffractions indicating the formation of solid solutions. Measured experimental data of primary crystallisation temperatures in the (LiF — CaF2)eut — GdF3 indicate the existence of simple binary system; however it is probably not the case and these data represent similar situation as in the system above i. e. the formation of solid solution fields. This increased tendency to form additional phases is visible also from analysis of XRD patterns of the solidified samples.
The presence of GdF3 was not observed as the most probably it reacts with either LiF or CaF2 or both. The formation of other phases (unidentified) is also highly probable as some unidentified diffractions were observed in solidified samples and their intensity change with concentration variation. Solid-solid phase transformation of pure GdF3 was reported at temperature 1172 K [24] or at 1310 K [22].
System (LiF — CaF2)eut. — NdF3 is more similar to the system (LiF — CaF2)eut. — SmF3 as to the gadolinium analogue. It is evident that Nd system shows similar features like Sm system; only initial phases in solidified samples were identified (together with some NdOF impurities) and the formation of any other known ternary fluorides was observed, in contrast to Gd system. However, the presence of unidentified diffractions indicates to the formation of some new phases. Detail analysis of obtained results are summarized in [25].
Volume properties
Some more information about the system behaviour and properties can be deduced from volume properties. These properties are calculated from experimental density data.
Molar volume dependence on composition in binary systems is usually monotonic, but occasionally the presence of local minimum can occur. This is the case for all investigated systems in this work. Molar volume of (LiF — CaF2)eut. — LnF3 (Ln = La, Sm, Gd, and Nd) initially decreases with small addition of LnF3 up to 1 mol %.
With further addition of LnF3 the molar volume increses. A ca. x(LaF3) = 0.04 the value of the molar volume is comparable with that of the pure solvent.
With further SmF3 addition (up to 50 mol %) molar volume increases up to ca 50 % higher value in comparison to the initial molar volume.
With further addition of GdF3 to (LiF — CaF2)eut molar volume increases like in samarium analogous system, however, the values of molar volume decreases with temperature at particular concentrations. This is significantly different behaviour from the previous system. Reason for these observations is unknown and might arise from the structural properties of gadolinium salt.
Analogous behaviour of volume properties was observed for (LiF — CaF2)eut. — NdF3. These are, however, more close to samarium system (without anomalous behaviour of molar volume) like in gadolinium system (i. e. values of molar volume decreases with temperature at certain concentration). Molar volume shows local minimum at low NdF3 concentrations, like in Sm and Gd systems. When comparing values of all three systems molar volume of Nd system at e.g. 15 mol % at 1123 K is lowest in the series Nd < Sm < Gd what obeys the elements position in the periodic table (but does not obey the sequence of molar volumes of tri-fluorides in solid state that is Gd < Nd < Sm). More information about the volume properties could be founf in [23, 25].
Conclusions
Temperatures of primary crystallisation of the systems (LiF — CaF2)eut. — LnF3 (Ln = Sm, Gd, and Nd) were measured in order to provide, at least, scheme of phase diagrams. Based on XRD patterns of solidified samples it can be expected that some new phases are formed in all investigated systems. The number of these phases, however, cannot be predicted. In spite of the effort, several procedures to obtain suitable single crystals for their structural characterisation have failed.
Volume properties analysis revealed unusual behaviour of investigated systems, in some aspect even anomalous. Small additions of LnF3 up to 1 mol % to (LiF—CaF2)eut. result in decrease of molar volume. This has consequence to the partial molar volumes of LnF3 thus having very small even negative values at higher temperatures. Such a relatively large volume contraction, when expressed through the compressibility parameter, may reach values that are bigger (in absolute value) that molar volume of pure component.
Samarium and neodymium systems show similar properties in phase analysis, as well as in volume properties while gadolinium system shows different behaviour. The reason for this macroscopic behaviour could arise from the structural and electronic properties of the central atoms. The main difference between the studied three systems is that Nd3+ and Sm3+ have similar electronic properties in the sense of only partially occupied f orbitals, while Gd+3 has f orbitals fully half filled. These different microscopic properties might result also to the different macroscopic ones. In order to prove this hypothesis more extended experiments are required including also other lanthanides.
Acknowledgement
This work was supported by the Science and Technology Assistance Agency under contract No. APVV-0460-10 and by the Slovak Grant Agency VEGA 2/0116/14. This publication is the result of the project implementation: "Applied research of technology of thermal plasma processes", ITMS code 26240220070, supported by the Research & Development Operational Programme funded by the ERDF.
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DOI: 10.25702/KSC.2307-5252.2018.9.1.421-425 УДК 621.793.6
ИССЛЕДОВАНИЕ ДЕГРАДАЦИОННЫХ ПРОЦЕССОВ КЕРАМИКИ НА ОСНОВЕ ОКСИДА МАГНИЯ, ОКСИДА АЛЮМИНИЯ И НИТРИДА КРЕМНИЯ В РАСПЛАВЕ ХЛОРИДОВ ЛИТИЯ И КАЛИЯ
Е. В. Никитина1,2, Н. А. Казаковцева1, Е. А. Никоненко2, А. С. Молодых2, Е. С. Филатов1,2
1 Институт высокотемпературной электрохимии УрО РАН, г. Екатеринбург, Россия
2 Уральский федеральный университет им. первого Президента России Б. Н. Ельцина, г.Екатеринбург, Россия
Аннотация
Рассматривается деградация керамических материалов на основе оксида магния, оксида алюминия и нитрида кремния в расплаве хлоридов лития и калия в диапазоне температур 470650 °С. Изучено взаимодействие керамических материалов с расплавом солей галогенидов щелочных металлов, содержащим добавки трихлоридов урана, церия и неодима. Использованы гравиметрический, химико-аналитический, микрорентгеноспектральный и рентгенофазовый методы анализа. Ключевые слова:
оксид магния, оксид алюминия, деградация керамики, нитрид кремния.
RESEARCH OF DEGRADATION PROCESSES OF CERAMICS ON THE BASIS OF MAGNESIUM OXIDE, ALUMINUM OXIDE AND SILICON NITRIDE IN THE MELTING OF CHLORIDES OF LITHIUM AND POTASSIUM
E. V. Nikitina12, N. A. Kazakovtseva1, E. A. Nikonenko2, A. S. Molodih2, E. S. Filatov1'2
11nstitute of High-Temperature Electrochemistry of the Ural Branch of the RAS, Yekaterinburg, Russia 2 Ural Federal University Named after the First President of Russia B. N. Yeltsin, Yekaterinburg, Russia
