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The effect of high-temperature aging for 4800 h at a temperature of 1123 K on the crystal structure and the conductivity of (ZrO2)0.90(Sc2O3)0.09(Yb2O3)0.01 and (ZrO2)0.90(Sc2O3)0.08(Yb2O3)0.02 single-crystal membranes were studied. Such membrane lifetime testing is critical to the operation of solid oxide fuel cells (SOFCs). The crystals were obtained by the method of directional crystallization of the melt in a cold crucible. The phase composition and structure of the membranes before and after aging were studied using X-ray diffraction and Raman spectroscopy. The conductivities of the samples were measured using the impedance spectroscopy technique. The (ZrO2)0.90(Sc2O3)0.09(Yb2O3)0.01 composition showed long-term conductivity stability (conductivity degradation not more than 4%). Long-term high-temperature aging of the (ZrO2)0.90(Sc2O3)0.08(Yb2O3)0.02 composition initiates the t″ → t′ phase transformation. In this case, a sharp decrease in conductivity of up to 55% was observed. The data obtained demonstrate a clear correlation between the specific conductivity and the change in the phase composition. The (ZrO2)0.90(Sc2O3)0.09(Yb2O3)0.01 composition can be considered a promising material for practical use as a solid electrolyte in SOFCs.

The mechanical properties, phase composition and luminescence of (ZrO2)1−x(Sm2O3)x (x = 0.02–0.06) crystals synthesized using directional melt crystallization were studied. The regularities of changes in the phase composition of the crystals depending on samaria concentration were analyzed. Optical spectroscopy showed that Sm ions were incorporated into the ZrO2 crystal lattice in the form of Sm3+. The microhardness of the crystals was shown to increase with Sm2O3 concentration and reached 12.45 GPa for (ZrO2)0.94(Sm2O3)0.06 crystals. The highest fracture toughness of 14.2 MPa∙m1/2 was observed for the crystals containing 3.7 mol.% Sm2O3. The experimental results were analyzed in order to understand the effect of phase composition on the mechanical properties of the crystals. The effect of ionic radii of stabilizing oxide cations (i.e., Y3+, Gd3+ and Sm3+) on the mechanical properties of the materials on the basis of partially stabilized zirconia was also discussed.

The effect of ceria doping of (ZrO2)1−x(Sm2O3)x crystals on their phase composition, microhardness and fracture toughness was studied. The (ZrO2)0.995−x(Sm2O3)x(CeO2)0.005 crystals (where x = 0.032, 0.037 and 0.04) were grown using directional melt crystallization in a cold crucible. The mechanical properties, such as microhardness and fracture toughness, were explored using Vickers indentation. It was shown that the (ZrO2)0.995−x(Sm2O3)x(CeO2)0.005 solid-solution crystals contained both Ce4+ and Ce3+ ions. Phase analysis data suggested that CeO2 doping increased the tetragonality degree of the transformable t phase and reduced the tetragonality degree of the non-transformable t’ phase as compared to the (ZrO2)1−x(Sm2O3)x crystals. As a result, the t→m phase transition triggered by the indentation-induced stress in the CeO2-doped crystals was more intense and covered greater regions. CeO2 doping of the solid solutions increased the fracture toughness of all the crystals studied, whereas the microhardness of the crystals changed only slightly. CeO2 doping of the (ZrO2)1−x(Sm2O3)x solid solutions in the experimental concentration range did not improve the high-temperature phase stability of the crystals and did not prevent high-temperature degradation of their fracture toughness.

This paper presents the results of studying the phase composition, luminescent characteristics, and ionic conductivity of ceramic scandium-stabilized solid solutions of zirconium dioxide containing 9 and 10 mol% Sc2O3. Ceramic samples were prepared by sintering powders obtained by grinding melted solid solutions of the same composition. A comparative analysis of the obtained data with similar characteristics of single crystals has been carried out. Differences in the phase composition of ceramics and initial single crystals were found. The effect of the structure and properties of grain boundaries on the ionic conductivity of ceramic samples is discussed. It is shown that the differences in the ionic conductivity of ceramic samples and crystals are mainly due to changes in the structure and phase composition.

In this work, we studied the phase composition, local structure and mechanical characteristics of ZrO2 crystals partially stabilized with Y2O3 and co-doped with Nd2O3, CeO2, Er2O3, Tb2O3 and Yb2O3. Crystals were grown by directional melt crystallization in a cold container. The phase composition and structure of crystals were studied by X-ray diffractometry and transmission electron microscopy. The study of the features of the incorporation of rare-earth cations with different ionic radii into the transformable (t) and nontransformable (t’) tetragonal phases was carried out by the method of selective laser spectroscopy and time-resolved spectroscopy. Mechanical characteristics such as microhardness and fracture toughness were studied by the indentation method. It is shown that the phase composition and structure of crystals at the same total concentration of doping oxides depends on the degree of substitution of Y3+ cations by rare-earth cations. Rare earth ions of the beginning of the lanthanide series predominantly occupy positions in the nontransformable tetragonal phase of crystals based on zirconium dioxide. Ions of the end of a series of lanthanides do not show selectivity when entering the transformable (t) phase and nontransformable (t’) phase. The study of the mechanical characteristics of the crystals showed that the values of fracture toughness increase with an increase in the ionic radius of the rare earth element of the co-doped oxide, while the values of the microhardness of the crystals slightly decrease.

The effect of long-term high-temperature annealing on the phase composition, local crystal structure, and oxygen-ion conductivity of SOFC membranes based on zirconium dioxide solid solutions was studied. Crystals with the composition of (ZrO2)0.99−x(Sc2O3)x(R2O3)0.01 (where x = 0.08–0.1; R-Yb, Y, Tb, Gd) were obtained by the method of directed melt crystallization in a cold crucible. The crystals were annealed in air at a temperature of 1000 °C for 400 h. The phase analysis of the crystals before and after annealing was studied by X-ray diffractometry and Raman spectroscopy. The study of the ionic conductivity of the crystals was carried out by the method of impedance spectroscopy in the temperature range 400–900 °C. It has been shown that when various rare earth cations (Yb, Y, Tb, and Gd) are used, the maximum conductivity is observed for the compositions (ZrO2)0.91(Sc2O3)0.08(Yb2O3)0.01, (ZrO2)0.89(Sc2O3)0.1(Y2O3)0.01, (ZrO2)0.90(Sc2O3)0.09(Tb2O3)0.01, and (ZrO2)0.89(Sc2O3)0.1(Gd2O3)0.01. At the same time, these crystals have a highly symmetrical pseudocubic structure, which is retained even after crystal annealing. At comparable concentrations of Sc2O3, the conductivity of crystals decreases with an increase in the ionic radius of the rare earth cation. The high-temperature degradation of the conductivity is also discussed depending on the type of rare earth oxide and the concentration of scandium oxide.

We studied the spectral-luminescent properties of (ZrO2)0.805(Y2O3)0.188(Pr2O3)0.007 and (ZrO2)0.802(Y2O3)0.195(Pr2O3)0.003 crystals grown by directional melt crystallization in a cold skull. Analysis of the absorption spectra of the crystals suggested the presence of Pr3+ and Pr4+ ions. Measurement of the relative intensities of the luminescence bands corresponding to the 3P0 → 3H4,5, 3P0 → 3F2,3,4, 3P1 → 3H5 and 1D2 → 3H4 optical transitions of the Pr3+ ions, and analysis of luminescence extinction kinetics for the 3P0 and 1D2 levels of the Pr3+ ions, suggests the presence of cross-relaxation (1D2 → 1G4) → (3H4 → 3F4) of the Pr3+ ions in the ZrO2-Y2O3-Pr2O3 crystals.

The phase composition and heat conductivity of (ZrO 2 ) 0.9 ( R 2 O 3 ) 0.1 solid solution single crystals have been studied, where R = (Gd, Yb, Sc, Y), (ZrO 2 ) 0.9 (Sc 2 O 3 ) 0.09 (Gd 2 O 3 ) 0.01 and (ZrO 2 ) 0.9 (Sc 2 O 3 ) 0.09 (Yb 2 O 3 ) 0.01 . Single crystals have been grown by directional melt crystallization in a cold skull. The phase composition of the crystals has been studied using X-ray diffraction and Raman spectroscopy. The heat conductivity of the crystals has been studied using the absolute steady-state technique of longitudinal heat flow in the 50–300 K range. We show that at a total stabilizing oxide concentration of 10 mol.% the phase composition of the crystals depends on the ionic radius of the stabilizing cation. The (ZrO 2 ) 0.9 (Sc 2 O 3 ) 0.1 crystals have the lowest heat conductivity in the 50–300 K range while the (ZrO 2 ) 0.9 (Gd 2 O 3 ) 0.1 solid solutions have the lowest heat conductivity at 300 K. Analysis of the experimental data suggests that the heat conductivity of the crystals depends mainly on the phase composition and ionic radius of the stabilizing cation. Phonon scattering caused by the difference in the weight of the co-doping oxide cation has a smaller effect on the heat conductivity.

The crystalline structure, ionic conductivity, and local structure of ZrO2-Gd2O3solid solution crystals have been studied for a wide range of compositions. The (ZrO2)1-х(Gd2O3)хcrystals (x = 0.03–0.33) have been grown by directional melt crystallization in cold crucible. The phase composition of the crystals has been studied using X-ray diffraction and transmission electron microscopy. The transport parameters have been studied using impedance spectroscopy in the 400–900 °С range. The local structure of the crystals has been studied by optical spectroscopy with Eu3+ ion probe. The maximum conductivity at 900 °С (0.047 S/cm) has been observed in the crystals containing 10 mol% Gd2O3. This composition is close to the cubic/tetragonal phase boundary. The compositions corresponding to the single-phase cubic region exhibit a decrease in the ionic conductivities with an increase in the Gd2O3 concentration. Studies of the local structure of the ZrO2-Gd2O3 system solid solutions have revealed specific features of the formation of optical centers which characterize the localization of oxygen vacancies in the lattice depending on the concentration of the stabilizing oxide. Comparison of the experimental values of the lattice parameter with those calculated using various models has shown that the best fit between these data is provided by the model of inequiprobable distribution of oxygen vacancies. We have discussed the correlation between the crystalline and local structures and the transport parameters of the crystals. Analysis of the results allows us to identify the Gd2O3 concentration ranges in which the ionic conductivity of the crystals is mainly determined either by the phase composition or by the regularity of oxygen vacancy localization in the crystal lattice.

Phase stability and transport properties of (ZrO2)0.91−x(Sc2O3)0.09(Yb2O3)x crystals (x = 0–0.01) have been studied before and after air annealing at 1000 °C for 400 h. The crystals have been grown by radio frequency (RF) heating in a cold crucible. The microstructure, phase composition, and electrical conductivity of the crystals have been studied using optical microscopy, X-ray diffraction, Raman spectroscopy, and impedance spectroscopy. Phase stability and degradation of ionic conductivity of the crystals upon long-term high-temperature heat treatment have been discussed. We show that the stabilization of ZrO2 co-doped with 9 mol.% Sc2O3 and 1 mol.% Yb2O3 provides transparent uniform crystals with the pseudocubic t″ phase structure having high phase stability. Crystals of this composition had the highest conductivity in the entire temperature range. Long-term high-temperature annealing of these crystals did not lead to conductivity degradation.