Thermodynamics and vaporization of the Sm₂O₃–ZrO₂ system studied by Knudsen effusion mass spectrometry

Highlights

• Vapor composition and partial vapor pressures were determined over the Sm₂O₃–ZrO₂ system.

• Concentration dependences of the component activities were obtained at 2452 K.

• Thermodynamic properties in Sm₂O₃–ZrO₂ system corresponded to subregular solution model.

• Correlation between thermodynamic properties and calculated number of bonds was shown.

• The results were compared with the data for the Sm₂O₃–Y₂O₃ and Sm₂O₃–HfO₂ systems.

Abstract

The purpose of the present study was to examine the thermodynamic properties and vaporization processes in the Sm₂O₃–ZrO₂ system by the Knudsen effusion mass spectrometric method (KEMS). The samples in the system under consideration were synthesized by the solid-state method using a high-energy ball mill. Vapor over the Sm₂O₃–ZrO₂ system consisted of SmO, Sm, and O in the temperature range 2200–2540 K. Concentration dependences of the SmO and Sm partial vapor pressures, the component activities, and excess Gibbs energies were obtained at a temperature of 2452 K in the compositional range of 0.23–0.73 Sm₂O₃ mole fraction. These data evidenced negative deviations from the ideal behavior and a possibility to use the subregular solution model to describe the concentration dependences of the thermodynamic properties in the Sm₂O₃–ZrO₂ system. Correlations were found between the observed changes in the thermodynamic behavior of the system mentioned and the relative number of bonds when the second coordination sphere was taken into consideration in the condensed phase calculated by the Generalized Lattice Theory of Associated Solutions.

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 Sm₂Zr₂O_7, 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:

• synthesis of samples in the Sm₂O₃–ZrO₂ system;

• identification of the vapor species over the samples in the system under the study;

• 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;

• 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).