Search Results (22)

High-entropy oxides, HEOs, represent a relatively new class of ceramic materials characterized by the incorporation of multiple cations, typically four or more, into a single-phase crystal structure. This extensive compositional flexibility allows for the introduction of specific chemical elements into a crystal lattice that would normally be unable to accommodate them, making it difficult to predict a priori their properties and crystal structures. Consequently, studying the phase stability of these single-phase materials presents significant challenges. This work examines the key parameters commonly employed to predict the stabilization of HEOs and introduces a unified framework for analyzing their stability. The proposed approach incorporates a normalized configurational entropy per mole of atoms and the relative volume occupied by cations into the mean atomic size deviation. By combining these parameters, the approach enables, as a first approximation, the identification of compositional ranges that favor the formation of single-phase and multi-phase HEO compounds with rock salt, spinel, fluorite, pyrochlore, and perovskite structures. Full article

Rare-earth tantalates (RETaO₄) are considered as a type of emerging thermal barrier coating materials applied to the hot components of gas turbines and aerospace engines due to their excellent thermal stability, high-temperature fracture toughness, corrosion resistance and extremely low thermal conductivity. However, the relatively low hardness and thermal expansion coefficients may limit their service lifetime in a harsh engine environment. To address the current limitation of rare-earth tantalates and further optimize the mechanical and thermal properties, the defective fluorite-structured Y₂Zr₂O₇ (YZ) was introduced as a second phase into the YTaO₄ (YT) matrix to form YT_1−x–YZ_x (x = 0, 0.25, 0.5, 0.75, 1) composite ceramics in this work. The mechanical and thermal properties of YT_1−x–YZ_x composite ceramics are significantly improved compared to pure-phase YTaO₄ ceramics. The Vickers hardness of YT_1−x–YZ_x (x = 0.25, 0.5, 0.75) composite ceramics is 9.1~11.3 GPa, which are 2~2.5 times higher than that of YTaO₄ (4.5 GPa). Among them, YT_0.75–YZ_0.25 exhibits a maximum fracture toughness (3.7 ± 0.5 MPa·m^1/2), achieving a 23% improvement compared to YTaO₄ (3.0 ± 0.23 MPa·m^1/2) and a 118% improvement compared to Y₂Zr₂O₇ (1.73 ± 0.28 MPa·m^1/2). The enhancement is attributed to the combined effect of the intrinsic strengthening of the second phase, as well as the residual stress and grain refinement caused by the introduction of a second phase. Additionally, the thermal expansion coefficients of YT_1−x–YZ_x composite ceramics at 1673 K range from 10.3 × 10⁻⁶ K⁻¹ to 11.0 × 10⁻⁶ K⁻¹, which is also higher than that of YTaO₄ (10.0 × 10⁻⁶ K⁻¹). Consequently, the superior mechanical and thermal properties indicate that YT–YZ composite ceramics possess promising application prospects for thermal barrier coatings. Full article

Multiphase fluoride polycrystalline eutectics pRF₃ × qMF₂ forming in the MF₂–RF₃ (M = Ca, Sr, Ba; R = La–Nd) binary systems were synthesized by the directional crystallization technique from a melt. The phase composition, morphology, and temperature dependences of fluorine ionic conductivity in fabricated composites were studied in detail. The pRF₃ × qMF₂ (p and q are the mole percentages of components) eutectic composites consist of both extremely saturated fluorite-type structure M_1−xR_xF_2+x solid solutions and the tysonite-type R_1−yM_yF_3−y ones. Microsized growth blocks with a fine lamellar structure are typical for synthesized composites. The thinnest (from 3 μm) and longest lamellae are observed in the 68LaF₃ × 32BaF₂ composition. The ionic conductivity values of pRF₃ × qMF₂ composites are determined by the phase composition, practically do not depend on their morphological features, and reach 10⁻³–10⁻² S/cm at 500 K (with an ion transport activation enthalpy of about 0.5–0.6 eV). Crystallized eutectics are superior to any single-phase M_1−xR_xF_2+x solid solutions and ball-milling R_1−yM_yF_3−y nanoceramics in terms of ion-conducting properties. These fluoride materials represent an alternative to widely applied tysonite-type ceramic composites in various electrochemical devices and require further in-depth studies. Full article

Synthesis and study of materials based on bismuth cerates and titanates were carried out. Complex oxides Bi_1.6Y_0.4Ti₂O₇ were synthesized by the citrate route; Bi₂Ce₂O₇ and Bi_1.6Y_0.4Ce₂O₇—by the Pechini method. The structural characteristics of materials after conventional sintering at 500–1300 °C were studied. It is demonstrated that the formation of a pure pyrochlore phase, Bi_1.6Y_0.4Ti₂O₇, occurs after high-temperature calcination. Complex oxides Bi₂Ce₂O₇ and Bi_1.6Y_0.4Ce₂O₇ have a pyrochlore structure formed at low temperatures. Yttrium doping of bismuth cerate lowers the formation temperature of the pyrochlore phase. As a result of calcination at high temperatures, the pyrochlore phase transforms into the CeO₂-like fluorite phase enriched by bismuth oxide. The influence of radiation-thermal sintering (RTS) conditions using e-beams was studied as well. In this case, dense ceramics are formed even at sufficiently low temperatures and short processing times. The transport characteristics of the obtained materials were studied. It has been shown that bismuth cerates have high oxygen conductivity. Conclusions are drawn about the oxygen diffusion mechanism for these systems. The materials studied are promising for use as oxygen-conducting layers in composite membranes. Full article

Five kinds of multi-component entropy-stabilized oxide ceramics were prepared by a solid-state reaction method for thermal barrier coatings, namely La_0.125Y_0.125Yb_0.125Gd_0.125Zr_0.5O_1.75 (LaYYbGdZr), Y_0.125Yb_0.125Gd_0.125Ta_0.125Zr_0.5O_1.875 (YYbGdTaZr), La_0.1Y_0.1Yb_0.1Gd_0.1Ta_0.1Zr_0.5O_1.85 (LaYYbGdTaZr), Y_0.125Yb_0.125Gd_0.125Ta_0.125Hf_0.25Zr_0.25O_1.875 (YYbGdTaHfZr), and La_0.1Y_0.1Yb_0.1Gd_0.1Ta_0.1Hf_0.25Zr_0.25O_1.85 (LaYYbGdTaHfZr). Many properties of the materials were studied, such as their microscopic morphology, crystal structure, thermophysical properties, and ablation resistance. The results show that the oxide ceramics synthesized in this paper have a uniform single-phase defect fluorite structure, and can still maintain this structure after high-temperature treatment at 1500 °C. The YYbGdTaHfZr coatings had the lowest thermal conductivity (0.61~0.89 W·m^–1·K^–1), which was much lower than that of YSZ. The ceramic blocks also exhibited excellent thermal expansion properties. The thermal expansion coefficient of LaYYbGdTaZr could reach 11.09 × 10⁻⁶ K⁻¹ (1400 °C), which was slightly higher than that of 8YSZ (11.0 × 10⁻⁶ K⁻¹). The antioxidant ablation results proved that the YYbGdTaHfZr coating showed the best heat-insulating property. All the results showed that the YYbGdTaHfZr coating is a promising thermal barrier coating. Full article

One of the most promising applications of high entropy ceramics is their use as high temperature protective materials. Due to the additional entropic stabilization of the crystal structure, four- and five-element high entropy ceramics exhibit enhanced thermal and mechanical properties. For these applications, one of the most promising high entropy protective oxides are ZrO₂- and HfO₂-based protective HEOs. In this article, we study the HfO₂–ZrO₂–Y₂O₃–CeO₂ equimolar system with the addition of MgO as a fifth component. We found that the HfZrCeY(Mg)O system maintains a single FCC crystalline structure up to the MgO concentration = 31.9 mol.%. Additionally, we determined that an addition of MgO at the close-to-equimolar HfZrCeY(Mg)O composition enhances the thermal properties of HEO, but reduces the mechanical properties such as hardness and resistance to crack formation. The minimum weight loss at the heating from RT up to 1450 °C was measured for the close-to-equimolar HfZrCeY(Mg)O composition at 18.4 mol.% MgO. The hardness of such composition was around 18 GPa. Due to the combination of these properties, the synthesized coating can be used as a protective material for high temperature applications, such as the protection of turbine parts. Full article

Porous high-entropy ceramics are a new alternative material for thermal insulation. Their better stability and low thermal conductivity are due to lattice distortion and unique pore structures. In this work, rare-earth-zirconate ((La_0.25Eu_0.25Gd_0.25Yb_0.25)₂(Zr_0.75Ce_0.25)₂O₇) porous high-entropy ceramics were fabricated by a tert-butyl alcohol (TBA)-based gel-casting method. The regulation of pore structures was realized through changing different initial solid loadings. The XRD, HRTEM, and SAED results showed that the porous high-entropy ceramics had a single fluorite phase without impurity phases, exhibiting high porosity (67.1–81.5%), relatively high compressive strength (1.02–6.45 MPa) and low thermal conductivity (0.0642–0.1213 W/(m·K)) at room temperature. Porous high-entropy ceramics with 81.5% porosity demonstrated excellent thermal properties, showing a thermal conductivity of 0.0642 W/(m·K) at room temperature and 0.1467 W/(m·K) at 1200 °C. The unique pore structure with a micron size contributed to their excellent thermal insulating performance. The present work provides the prospect that rare-earth-zirconate porous high-entropy ceramics with tailored pore structures are expected to be thermal insulation materials. Full article

Tm₂(Ti_2−xTm_x)O_7−x/2 (x = 0, 0.1, 0.18, 0.28, 0.74) solid electrolytes have been investigated as potential electrolyte materials for solid oxygen fuel cells (SOFCs), operating in the medium temperature range (600–700 °C). The design of new oxygen-conducting materials is of importance for their possible utilization in the solid oxide fuel cells. The oxygen–ion conductivity of the Tm₂(Ti_2−xTm_x)O_7−x/2 (x = 0, 0.1, 0.18, 0.28, 0.74) “stuffed” pyrochlores ceramics was investigated by electrochemical impedance spectroscopy (two-probe AC) in dry and wet air. The synthesis of precursors via co-precipitation and the precipitate decomposition temperature have been shown to be of key importance for obtaining dense and highly conductive ceramics. At ~770 °C, the highest total conductivity, ~3.16 × 10⁻³ S/cm, is offered by Tm₂Ti₂O₇. The conductivity of the fluorite-like solid solution Tm₂(Ti_2−xTm_x)O_7−x/2 (x = 0.74) is an order of magnitude lower. However, for the first time a proton contribution of ~5 × 10⁻⁵ S/cm at 600 °C has been found in Tm₂(Ti_2−xTm_x)O_7−x/2 (x = 0.74) fluorite. Until now, compositions with proton conductivity were not known for the intermediate and heavy rare earth titanates Ln₂(Ti_2−xLn_x)O_7−x/2 (Ln = Ho − Lu) systems. The use of X-ray diffraction (structural analysis with Rietveld refinement), optical spectroscopy and dielectric permittivity data allowed us to follow structural disordering in the solid solution series with increasing thulium oxide content. High and low cooling rates have been shown to have different effects on the properties of the ceramics. Slow cooling initiates’ growth of fluorite nanodomains in a pyrochlore matrix. The fabrication of such nanostructured dense composites is a promising direction in the synthesis of highly conductive solid electrolytes for SOFCs. We assume that high-temperature firing of nanophase precursors helps to obtain lightly doped “stuffed” pyrochlores, which also provide the high oxygen–ion conductivity. Full article

It is confirmed that Fluorite-structured Entropy-Stabilized Oxides (F-ESO) can be obtained with multicomponent (5) equimolar systems based on cerium, zirconium, and other rare earth elements, selected according to the predictor already proposed by the authors. Indeed, in the present study, three different samples owning a standard deviation (SD in the following) of their cationic radii greater than the threshold value (i.e., SD > 0.095 with cationic radii measured in Å) needed to ensure the formation of the single-phase fluorite structure, were prepared via co-precipitation method. After a calcination step at 1500 °C for 1 h, the entropy-driven transition from multiple phases to single-phase fluorite-like structure has been actually confirmed. Thus, with the aim of defining the temperature at which such entropy-driven transition occurred, and identifying possible relation between such temperature and the actual value of SD, the phase evolution of all the prepared samples as a function of temperature (ranging from 800 °C to 1300 °C) was analyzed by in situ High Temperature X-ray Diffraction. An apparent inverse correlation between the standard deviation and the entropy-driven transition temperature has been identified, i.e., the higher the former, the lower the latter. These results, based on the conducted basic structural analysis, provide further support to the SD-based empirical predictor developed by the authors, suggesting that high values of SD could bring additional contribution to the overall entropy of the system, other than the configurational one. Thus, this SD-driven entropy contribution directly increases with the increasing of the standard deviation of the cationic radii of a given F-ESO. Full article

Since the beginning of the use of nuclear energy, humans have been faced with the problem of radionuclide disposal. At present, a large amount of waste is stored in pools or dry tanks at reactor sites. With the development of the nuclear power generation industry worldwide, the high storage cost (including building, maintaining, and operating storage pools) is overwhelming and serious, and urgent radionuclide disposal problems have become increasingly difficult. Safe and economical strategies are urgently needed for long-term storage and disposal of nuclear waste, which has become among the core issues in the utilization of nuclear energy. Pyrochlore ceramics are able to immobilize a variety of radionuclides and have excellent irradiation stability, so they have received extensive attention as hosts of radionuclides waste. This review summarizes the structure, composition, synthesis process, properties, and irradiation stability of pyrochlore ceramics, focusing on the ion irradiation effect of pyrochlore. In general, the cation radii ratio r_A/r_B is a key parameter related to various properties of pyrochlores. Zirconate pyrochlore is more easily transformed from pyrochlore to defective fluorite, and leads to better irradiation resistance. Full article

Lanthanide tungstates and molybdates are promising materials for hydrogen separation membranes due to their high protonic conductivity. A promising approach to fabricating ceramics based on these materials is radiation thermal sintering. The current work aims at studying the effect of radiation thermal sintering on the structural morphological and transport properties of (Nd,Ln)_5.5(W,Mo)O_11.25–δ as promising materials for hydrogen separation membranes. The defect fluorite structure was shown to be preserved during radiation thermal sintering at 1100 °C. The presence of protons in hydrated samples was confirmed by TGA. According to four-electrode studies and the isotope exchange of oxygen with C¹⁸O₂, the samples demonstrate a high proton conductivity and oxygen mobility. Residual porosity (up to 29%) observed for these samples can be dealt with during membrane preparation by adding sintering aids and/or metal alloys nanoparticles. Hence, sintering by e-beams can be applied to the manufacturing of hydrogen separation membranes based on these materials. Full article

The samples of ceramics (Y,Ce,Mg,Sr)O_2-δ; 10YCe3Mg5Sr; 10SmCe3Mg5Sr were obtained by the method of standard solid-state reaction. According to X-ray analysis, all systems have the fluorite cubic structure, but in the (Y,Ce,Mg,Sr)O_2-δ system, the presence of the second orthorhombic phase was observed. The microstructure of powders and synthesized tablets were characterized by electron-microscopy. The average particle size of the powder was shown to be 23.88 nm; 22.32 nm; and 13.4 nm for the compositions (Y,Ce,Mg,Sr)O_2-δ; 10YCe3Mg5Sr; and 10SmCe3Mg5Sr, respectively. After the sintering at temperature 1450 °C; the grain size increased to 5.1 μm; 3.95 μm; 5.07 μm for (Y,Ce,Mg,Sr)O_2-δ; 10YCe5Mg5Sr; and 10SmCe5Mg5Sr, respectively. The ionic conduction of the obtained samples was defined by ac impedance spectroscopy. The activation energy was then calculated. The 10SmCe5Mg5Sr system was found to have the highest electrical conductivity, reaching 20.5 mS/cm at a temperature of 700 °C. The activation energy was 0.62 eV in the temperature range of 600–800 °C. The results of the density, microhardness, and crack resistance measurements of the solid solutions under investigation were obtained. Full article

A novel class of high-entropy pyrochlore ceramics (HEPCs) with multiple heavy and light rare-earth elements at the A site were successfully synthesized via solid-state reaction. Both the XRD patterns and Raman spectroscopy demonstrated the single pyrochlore structure feature of seven kinds of HEPCs. Electron microscopic images revealed the typical morphology and the homogeneous distribution of all rare-earth elements. It can be concluded that the significance of configuration entropy in the HEPC system has promoted the tervalent lanthanide nuclides to form a single pyrochlore structure. This work is expected to provide guidance for the further design of high-entropy pyrochlore/fluorite ceramics. Full article

Diopside-based glass-ceramics are successfully produced by recycling blast furnace slag and fluorite tailing with the addition of supplementary elements such as TiO₂, Fe₂O₃ and Cr₂O₃ as nucleation agents, using a conventional melting method. The effects of various nucleating agents on the phase components and structure of the prepared glass-ceramics were evaluated by a differential scanning calorimeter, X-ray diffraction and scanning electron microscope–energy disperse spectrometer methods to determine the optimal dosage of nucleating agents. The results show that, in the preparation of diopside-based glass-ceramics, the suitable percentages of blast furnace and fluorite tailing are 55% and 45%, and the recommended composite nucleating agents consist of 1.5% Cr₂O₃, 2% TiO₂ and 3% Fe₂O₃. Heat treatment was conducted at a nucleation temperature of 720 °C and a crystallization temperature of 920 °C, and the nucleation and crystallization durations were 1.0 h and 1.5 h, respectively. Under the abovementioned parameters, the obtained diopside-based glass-ceramics displayed a Vickers hardness of 7.12 GPa, density of 2.95 g·cm⁻³, water absorption of 0.02%, acid resistance of 0.23% and alkali resistance of 0.02%. Full article

Ceramics samples with the nominal composition [(ZrO₂)_0.95(Y₂O₃)_0.05]_1-x[PrO_y]_x and praseodymia contents of x = 0.05–0.15 were prepared by the direct firing of compacted 5YSZ + PrO_y mixtures at 1450–1550 °C for 1–9 h and characterized for prospective applicability in reversible solid oxide cells. XRD and SEM/EDS analysis revealed that the dissolution of praseodymium oxide in 5YSZ occurs via the formation of pyrochlore-type Pr₂Zr₂O₇ intermediate. Increasing PrO_y additions results in a larger fraction of low-conducting pyrochlore phase and larger porosity, which limit the total electrical conductivity to 2.0–4.6 S/m at 900 °C and 0.28–0.68 S/m at 700 °C in air. A longer time and higher temperature of firing promotes the phase and microstructural homogenization of the ceramics but with comparatively low effect on density and conductivity. High-temperature processing leads to the prevailing 3+ oxidation state of praseodymium cations in fluorite and pyrochlore structures. The fraction of Pr⁴⁺ at 600–1000 °C in air is ≤2% and is nearly independent of temperature. 5YSZ ceramics with praseodymia additions remain predominantly oxygen ionic conductors, with p-type electronic contribution increasing with Pr content but not exceeding 2% for x = 0.15 at 700–900 °C. The average thermal expansion coefficients of prepared ceramics are in the range of 10.4–10.7 ppm/K. Full article