Thermodynamics and vaporization of the Sm2O3–ZrO2 system studied by Knudsen effusion mass spectrometry
Introduction
Systems containing zirconia and rare earth oxides are traditionally considered as a base for highly refractory ceramics. At present, yttria-stabilized zirconia (YSZ) is the most widely used material as a thermal barrier coating for the protection of gas turbine and diesel engines from high-temperature corrosion [1,2] and as a material for moulds and cores for casting gas turbine engine blades [[3], [4], [5]]. However, because of the limitations of the YSZ application as the refractory ceramics related to the high-temperature phase instability and the high-temperature deterioration of physicochemical properties [[6], [7], [8]], it is necessary to search for novel materials with higher phase stability and lower thermal conductivity.
Among a wide range of approaches to search a substitution for YSZ in high temperature technologies, the most direct and evident method is the use of different rare earth oxides instead of or in addition to yttria [7,9,10]. Recently, a wide range of rare earth oxides has been proposed to increase the thermal stability of refractory materials, including lanthana, ceria, samaria, gadolinia, ytterbia, and others [[11], [12], [13], [14], [15]]. Further development of this approach consists in increasing the rare earth oxide content in materials as compared with the traditionally used composition of YSZ (3–5 mol % Y2O3) to obtain rare earth zirconates or even compositions with higher rare earth oxide contents [16,17]. The zirconates of the following lanthanoids were synthesized and studied for potential application as refractory materials: lanthanum [18,19], samarium [[20], [21], [22]], europium [23], ytterbium [24], dysprosium [25], and others.
It is believed that the usage of the systems under consideration will increase the range of exploitation temperatures of novel refractory materials. However, the rise of exploitation temperatures of the ceramics based on zirconia and rare earth oxides might lead to selective vaporization of lanthanoid oxides, which are more volatile than zirconia from the materials, or to the problem of high-temperature phase instability in the condensed phase. Therefore, it is important to study vaporization processes and thermodynamic properties of the systems containing zirconia and rare earth oxides, namely the Sm2O3–ZrO2 system for the first time, which is the main purpose of the present investigation.
It should also be noted that the combination of zirconia and samaria is valuable for understanding the processes in the nuclear industry because it is promising as the cladding material for nuclear fuels [26] and for the immobilization of nuclear waste because of the formation of solid solutions with actinoids and resistance against ionizing radiation [[27], [28], [29]]. Moreover, samarium zirconate alongside zirconates of other rare earth elements was examined for humidity sensing, with samarium zirconate possessing higher sensing properties and stability in the corrosive environment [30]. Thus, the study of vaporization processes and thermodynamic properties of ceramics based on the Sm2O3–ZrO2 system by the Knudsen effusion mass spectrometric method (KEMS) is a relevant and important topic for the consideration of the possible applications of the materials based on zirconia and rare earth oxides.
The phase diagram of the Sm2O3–ZrO2 system was examined in experimental studies [[31], [32], [33], [34]] and optimized by using thermodynamic modeling approaches [34,35]. In the system under consideration, phases based on the polymorphic modifications of Sm2O3 and ZrO2 and the pyrochlore phase corresponding to the Sm2Zr2O7 compound were identified. At the temperature of 2450 K, the following phase equilibria were observed [34,35]. In the concentration range close to pure samaria, the solid solution based on high temperature hexagonal H–Sm2O3 modification existed in the system under the study. In the concentration range of 0.83–0.92 mol fraction Sm2O3, the solid solution based on the high-temperature cubic X–Sm2O3 modification was stable. There was a two-phase region of equilibrium between the X-cubic solid solution and the solid solution based on the fluorite modification of zirconia (F–ZrO2) in the range of 0.60–0.83 mol fraction Sm2O3 at the temperature of 2450 K. Then, the homogeneous fluorite solid solution existed in a wide concentration range of 0.02–0.60 mol fraction Sm2O3. Close to pure ZrO2, the solid solution based on the tetragonal ZrO2 modification was stable [34,35].
Vaporization of zirconia-based ceramics is known [36] to proceed with dissociation following the transition into the gaseous phase of pure oxides. Therefore, it is reasonable to briefly consider the available information on the vaporization processes of Sm2O3 and ZrO2.
Vaporization of Sm2O3 was studied for the first time by KEMS in our previous experiments [37,38]. Sm2O3 was demonstrated to vaporize with dissociation into gaseous samarium monoxide, atomic samarium, and atomic oxygen according to Equations (1), (2) [37,38]:Sm2O3 (c) = 2 SmO (g) + O (g),Sm2O3 (c) = 2 Sm (g) + 3 O (g),where (c) and (g) correspond to condensed and gaseous phases, respectively.
Vaporization of ZrO2 was studied repeatedly including the KEMS method as reviewed in Refs. [39,40]. Vapor over ZrO2 was demonstrated to consist of ZrO, ZrO2, and atomic oxygen, and to be formed according to Equations (3), (4), with the dominant process being the dissociative vaporization:ZrO2 (c) = ZrO (g) + О (g),ZrO2 (c) = ZrO2 (g).
Vaporization processes of the Sm2O3–ZrO2 system were not studied so far. However, the following systems based on zirconia and rare earth oxides were examined earlier by the KEMS method: Y2O3–ZrO2 [40,41], La2O3–ZrO2 [42], CeO2–ZrO2 [43], Yb2O3–ZrO2 [44], and Lu2O3–ZrO2 [45]. Moreover, the vaporization processes in the samaria-containing systems (Sm2O3–Y2O3 and Sm2O3-HfO2) have recently been investigated [38]. Consequently, there is a clear gap in the scientific literature regarding vaporization and thermodynamics of the Sm2O3–ZrO2 system that needs to be filled in the present study.
Thermodynamic properties obtained earlier in the Sm2O3–ZrO2 system corresponded mostly to the Sm2Zr2O7 compound. The review of the thermodynamic data found before 2007 was summarized by Wang et al. [34]. Later, the Sm2Zr2O7 heat capacities were obtained from many studies [[46], [47], [48]] in the overall temperature range 0–1400 K. Thermodynamic properties of the pyrochlore-fluorite phase transition in Sm2Zr2O7, thermal expansion coefficient, and site occupancies data in this compound up to the temperature of 2423 K were determined by differential thermal analyses and synchrotron X-ray diffraction analysis, respectively [49]. Electrical conductivity of the pyrochlore solid solution in the Sm2O3–ZrO2 system was obtained as a function of composition and temperature of synthesis in Refs. [[50], [51], [52]].
Thus, to study the Sm2O3–ZrO2 system by KEMS, the following objectives were achieved in the present research:
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synthesis of samples in the Sm2O3–ZrO2 system;
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identification of the vapor species over the samples in the system under the study;
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determination of partial pressures of the vapor species over the samples, the component activities, and excess Gibbs energies in the Sm2O3–ZrO2 system at high temperatures;
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evaluation of the correlations between the thermodynamic properties obtained and the structure of the solid solutions in the system under consideration by using the statistical thermodynamic approach Generalized Lattice Theory of Associated Solutions (GLTAS).
Section snippets
The sample synthesis and characterization
The samples in the Sm2O3–ZrO2 system were synthesized by the solid-state method based on pure Sm2O3 and ZrO2 using a high-energy ball mill. The purity of oxides was analyzed by the X-ray fluorescence method (S8 Tiger wavelength spectrometer with rhodium anode, Bruker AXS, Karlsruhe, Germany), Table 1. It is demonstrated that the concentration of the main substance was 0.999 mol fraction in Sm2O3 and 0.985 mol fraction in ZrO2. The main impurity in ZrO2 was HfO2 with a mole fraction of 0.014.
Results
The SmO+ and Sm+ ions were identified in the mass spectra of vapor over the sample in the Sm2O3–ZrO2 system starting from the temperature value of 2200 K. To determine molecular precursors of these ions, their appearance energies were measured by the vanishing ion current method [53,54]. The SmO+ and Sm+ appearance energies corresponded to the ionization energies of the SmO and Sm vapor species, which were equal to 5.0 eV and 5.2 eV [63], respectively, within the margin of the standard
The KEMS results
In assumption that at the temperature of 2452 K, a continuous solid solution existed in the concentration range under the study in the Sm2O3–ZrO2 system, it is possible to determine the activities of the second component ZrO2 and the excess Gibbs energies by using the data obtained. Firstly, it is necessary to approximate the Sm2O3 activities using the Redlich–Kister equation [64] for the component activities:where B and C are adjustable coefficients.
Conclusions
Ceramics based on the Sm2O3–ZrO2 system was studied by the Knudsen effusion mass spectrometric method. Vapor over the samples consisted of SmO, Sm, and O in the temperature range 2200–2540 K, which corresponds to the vapor composition over pure Sm2O3. The partial pressures of the vapor species mentioned, the component activities, and the excess Gibbs energies were obtained at the temperature of 2452 K in a wide concentration range of 0.23–0.73 Sm2O3 mole fraction. The values of the
Funding
This study was supported by the Russian Foundation for Basic Research [grant number 19-03-00721].
CRediT authorship contribution statement
Viktor A. Vorozhtcov: Investigation, Writing – original draft, preparation, Data curation. Valentina L. Stolyarova: Conceptualization, Supervision, Project administration, Funding acquisition. Andrey L. Shilov: Formal analysis, Data curation, Writing – review & editing. Sergey I. Lopatin: Investigation, Resources. Sergey M. Shugurov: Investigation, Resources. Fedor N. Karachevtsev: Investigation, Resources.
Declaration of competing interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
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