The first part of this review addresses mechanical, physical, and some chemical methods (thermal decomposition, dynamic method, solution combustion synthesis, and sonochemical synthesis) for producing nanocrystalline and fine-grained ZrO2-based powders. Mechanical methods (high-energy grinding in planetary and ball mills in dry and liquid environments) are used in the synthesis of ZrO2 powders and analysis of ZrO2 phase transformations, in the hydrothermal synthesis of ZrO2 powders in acidic and alkaline environments, and for the deagglomeration of powders produced by other methods. Physical methods (plasma processing, reactive magnetron sputtering, and chemical vapor deposition) are employed when the requirements for powders are prioritized over production costs. They are used in the development of catalysts, sorbents, and coatings. Chemical methods provide control over the formation of primary particles with specific morphology, size, and surface area. Thermal decomposition produces primary particles shaped as spheres, nanorods, and hollow ZrO2 microspheres with customizable shell structures. Dynamic methods, involving the detonation of high-energy materials or explosives, are promising for the synthesis of nanosized ceramic oxide powders with narrow particle size distributions. Solution combustion synthesis is based on the propagation of self-sustaining exothermic reactions in aqueous or sol–gel environments. Sonochemical synthesis relies on acoustic cavitation. The synthesized powders are applied in the design of photocatalysts, optical materials, forensic materials for fingerprint detection, sensors, biological markers, etc. There is no universal synthesis method that would meet the diverse requirements for all ZrO2-based materials. The selection of a method to synthesize the starting powders depends on the requirements for properties of the resulting composites.

The influence of heat treatment temperature of the starting powders, ranging from 400 to 1450°C, on the consolidation of zirconia-toughened alumina (ZTA) composites was studied. In these composites, particles of a ZrO2 (Y2O3, CeO2) solid solution with high fracture toughness are dispersed in an Al2O3 matrix. The powders for developing the ZTA composites (wt.

The low-temperature phase stability of materials in the ZrO2—Y2O3—CeO2 system with compositions, % (mol.): 97ZrO2—3Y2O3; 95ZrO2—3Y2O3—2CeO2; 92,5ZrO2—2,5Y2O3—5CeO2; 90ZrO2—2Y2O3—8CeO2; 88ZrO2—12CeO2 was studied. Treatment of materials in hydrothermal conditions under an increased holding time (14 hours) was used. The starting powders were produced by the hydrothermal synthesis in an alkaline medium and heat-treated at 850 °C. The samples were sintered at 1350 oC. The materials properties were investigated by the X-ray phase analysis and electron microscopy. The aging stability of ceramics was determined by the degree of phase transformation T-ZrO2 → M-ZrO2 under experimental conditions. Porous microstructures were formed in the samples, which differ in the size distribution of both grains and pores. A characteristic feature is the presence of various amounts of fine-grained fragments with a regular microstructure and the formation of both rounded and elongated grains.The phase transformation T-ZrO2 → M-ZrO2 leads to an increase of the samples porosity. This, in turn, contributes to the intensification of the ceramics aging. After 14 h the phase transformation T-ZrO2 → M-ZrO2 was found in four samples. In the sample 97ZrO2—3Y2O3, 46% of M-ZrO2 was formed; in the 95ZrO2—3Y2O3—2CeO2 sample, 48% of M-ZrO2 was formed; in the 92,5ZrO2—2,5Y2O3—5CeO2 sample, 39% of M-ZrO2 was formed. In the 90ZrO2—2Y2O3–8CeO2 sample ≈1% of M-ZrO2 appeared, and in the 88ZrO2—12CeO2 sample M-ZrO2 was not identified. Formation features of the solid solution during the doping of zirconia with yttrium oxide and cerium oxide, the amount of cerium oxide in a ZrO2-based solid solution, the phase transformation F-ZrO2 → T-ZrO2 during the sintering and the formation of a homogeneous microstructure contribute to increasing the low-temperature phase stability of samples both 90ZrO2—2Y2O3—8CeO2 and 88ZrO2—12CeO2 composition. During the microstructural design of ceramics in the ZrO2—Y2O3—CeO2 system with increased low-temperature phase stability, it is necessary to establish such a ratio of Y2O3 and CeO2 in the solid solution based on ZrO2 that would provide the necessary strength behavior according to the ceramics use. Keywords: ZrO2—Y2O3—CeO2 system, ZrO2-based solid solution, M-ZrO2 phase, aging, low-temperature phase stability.

The low-temperature phase stability of 97 mol.% ZrO 2 –3 mol.% Y 2 O 3 , 95 mol.% ZrO 2 –3 mol.% Y 2 O 3 –2 mol.% CeO 2 , 92.5 mol.% ZrO 2 –2.5 mol.% Y 2 O 3 –5 mol.% CeO 2 , 90 mol.% ZrO 2 –2 mol.% Y 2 O 3 –8 mol.% CeO 2 , and 88 mol.% ZrO 2 –12 mol.% CeO 2 materials in the ZrO 2 –Y 2 O 3 –CeO 2 system was studied. The phase stability was determined through accelerated aging in hydrothermal conditions for 7 h and 14 h. The evaluation criterion was the amount of the M-ZrO 2 phase that formed in the samples when aged in hydrothermal conditions. The properties of the materials were analyzed by X-ray diffraction and electron microscopy. The T-ZrO 2 → M-ZrO 2 phase transformation occurred to varying degrees in all samples except for the 88 mol.% ZrO 2 –12 mol.% CeO 2 sample after the first and second aging cycles. The smallest amount of M-ZrO 2 formed in the 90 mol.% ZrO 2 –2 mol.% Y 2 O 3 –8 mol.% CeO 2 sample. After both aging cycles, the fracture patterns for the 90 mol.% ZrO 2 –2 mol.% Y 2 O 3 –8 mol.% CeO 2 and 88 mol.% ZrO 2 –12 mol.% CeO 2 samples did not change significantly. With the complex stabilization of zirconia by yttria and ceria, the T-ZrO 2 → M-ZrO 2 phase transformation was controlled in the aging process by the number of oxygen vacancies resulting from the presence of yttria and by the stresses induced by the presence of ceria in the solid solutions. The number of oxygen vacancies decreased as ceria content in the ZrO 2 -based solid solutions increased, slowing down the rate of water diffusion and enhancing the low-temperature phase stability in the ZrO 2 –Y 2 O 3 –CeO 2 materials. The effectiveness of using the 90 mol.% ZrO 2 –2 mol.% Y 2 O 3 –8 mol.% CeO 2 and 88 mol.% ZrO 2 –12 mol.% CeO 2 composites for the microstructural design of medical materials with increased resistance to low-temperature degradation in humid environments was shown.

The thermal fatigue life of zirconia-based complex composite ceramics doped with a mixture of rare earth oxides was studied. Two concentrates of rare earth oxides were chosen (wt.%): 1) cerium- subgroup concentrate of composition 62.4 CeO2, 13.5 La2O3, 10.9 Nd2O3, 3.9 Pr6O11, 0.92 Sm2O3, 1.2 Gd2O3, 0.24 Eu2O3, 2.66 ZrO2, 1.2 Al2O3, 1.7 SiO2, and 1.38 other oxides (light concentrate (LC)) and 2) yttrium-subgroup concentrate of composition 13.3 Y2O3, 1.22 Tb4O7, 33.2 Dy2O3, 8.9 Ho2O3, 21.8 Er2O3, 1.86 Tm2O3, 12.5 Yb2O3, 0.57 Lu2O3, and 6.65 other oxides (heavy concentrate (HC)). Two-layer metal/ceramic thermal-barrier coatings (TBCs) were deposited on gas turbine engine blades by electron-beam physical vapor deposition (EB-PVD) in one process cycle. The properties of ZrO2–LC and ZrO2–HC TBC ceramic top coats were compared to those of a standard yttria-stabilized zirconia layer (ZrO2–Y2O3). The thermal fatigue experiment was performed by heating the samples to 1100°C in a muffle furnace for 5 min, holding them at this temperature for 50 min, and cooling in water for 5 min. The standard ZrO2–Y2O3 layer withstood 138 thermal cycles, while the ZrO2–LC and ZrO2–HC layers withstood 161 thermal cycles. The porous microstructure of the ceramic layers developed during thermal cycling was found to depend on laminar microstructures acquired by the layers in the EB-PVD process. The number of spherical pores in the ZrO2–LC and ZrO2–HC layers was much higher than in the ZrO2–Y2O3 layer. This increased their thermal fatigue life by 16% compared to the standard coating. An integrated approach to the choice of the ceramic top coat composition based on ZrO2 solid solutions doped with natural rare earth oxide concentrates and of the technique for their deposition, as well as improvement in the coating architecture, will promote cost-effective TBCs with the properties required.

The low-temperature phase stability of 97 mol.

The work compares ceramic materials based on ZrO 2 co-stabilized with 8 mol. % of CeO 2 and 2 mol. % of Y 2O3 (8Ce2YSZ) and stabilized with 3 or 8 mol. % of Y 2O3 (3YSZ and 8YSZ). Both hydrothermal and co-precipitation approaches were used for powder preparation. The formation of a tetragonal structure with 6–8 % residual porosity and enhanced biaxial bending strength was observed for the 8Ce2YSZ samples. The 8YSZ and 3YSZ samples have cubic and tetragonal/monoclinic phases, respectively, with about 1% porosity and smaller strength values. The specific electrical conductivity of 8Ce2YSZ and 8YSZ is 1.1·10-3, 4·10-3, 1.2· 10 −2 S/cm and 5.2 · 10 −3 , 2.7· 10 −2 , 9.3· 10 −2 S/cm at 600, 700, 800 °C, respectively. The 8Ce2YSZ material developed is promising to be used in SOFC carrier anode layer due to higher mechanical strength comparing with 8YSZ. Moreover, the presence of cerium may improve the efficiency of anode electrode due to additional electronic conductivity, and better branching of the anode reaction zone.

The properties of nanocrystalline powders of compositions (mol.%) 97 ZrO2–Y2O3, 95 ZrO2–3 Y2O3–2 CeO2, 92.5 ZrO2–2.5 Y2O3–5 CeO2, 90 ZrO2–2 Y2O3–8 CeO2, and 88 ZrO2–12 CeO2 were studied. The powders were produced by hydrothermal synthesis in an alkaline environment from a coprecipitated hydroxide mixture with a residual moisture of 15–20%. The powder properties were determined by X-ray diffraction (XRD), electron microscopy, BET, and petrography. Metastable F-ZrO2 was found to form in the hydrothermally synthesized powders. According to XRD, the F-ZrO2 → T-ZrO2 phase transformation began at 700°C and finished at 850–1000°C. The crystal optical characteristics of the powders indicate that the F-ZrO2 → T-ZrO2 phase transformation started at 400°C. The variations in F-ZrO2 and T-ZrO2 unit cell volumes are associated with lattice distortions under the action of different mechanisms in costabilization of the zirconia-based solid solution and with the ratio of Y2O3 and CeO2 in the solid solution. The tetragonality of the powders increases in the ZrO2 costabilization. The transformation strengthening mechanism for ceramics based on ZrO2 (Y2O3, CeO2) solid solutions becomes more effective with the formation of T-ZrO2, whose capability to the T-ZrO2 → M-ZrO2 phase transformation increases. The morphology of the powders varies topologically continuously, and the sizes of their primary particles hardly increase up to 1150°C. The variation in the specific surface area (from 153 to 2 m2/g) of the powders is determined by the F-ZrO2 → T-ZrO2 phase transformation and their sintering activity above 1000°C.

Solid oxide fuel cells (SOFC) are among the most promising technologies for the electricity generation due to their high efficiency, reliability, flexibility in fuel selection, absence of valuable platinum group metal catalysts, safety and environmental friendliness.Typically, the SOFC is built on the basis of its anode, which is actually also its carrier. This is due to the researchers wish to minimize the ohmic resistance of the electrolyte layer via its thinning that is extremely critical for reducing SOFC operating temperature. In this regard, the anode must be strong enough both to make it easier to handle when making the whole cell and to ensure its stable operation. In addition to the carrier function, the anode shall provide sites for reacting gaseous fuel with oxygen ions, which are delivered through the electrolyte, and supplying the fuel gas components to the reaction sites and removing the fuel oxidation reaction products to the outside.The work deals with the comparative study of ceramic materials based on ZrO2, co-stabilized with CeO2 and Y2O3, and stabilized with Y2O3to be used in producing the SOFC anode, and for further structural optimization for future SOFCs.8Ce2YSZ ceramic samples made by hydrothermal synthesis (with two different modes of drying precipitation) have tetragonal phase and 6—8% residual porosity. The 8Ce2YSZ samples, showed the biaxial bending strength — 542 MPa and 486 MPa, respectively. The 8YSZ and 3YSZ samples have cubic phase with a strength of 181 MPa and tetragonal phase with a strength of 577 MPa, respectively at 1% porosity.The specific electrical conductivity of 8Ce2YSZ and 8YSZ is 1,1•10-3, 4•10-3 S/cm, 1,2•10-2 S/cm and 5,2•10-3, 2,7•10-2 S/cm, 9,3•10-2 S/cm at 600, 700, 800 °C, respectively. Keywords: solid oxide fuel cell, electrolyte, anode, zirconium dioxide, mechanical strength, ionic conductivity.

The use of compositionally complex ZrO2-based ceramics doped with a mixture of yttrium-subgroup rare-earth metal (REM) oxides for the deposition of thermal barrier coatings (TBCs) was studied. For research, a heavy concentrate (HC) of yttrium-subgroup REM oxides, consisting of (wt.%) 13.3 Y2O3, 1.22 Tb4O7, 33.2 Dy2O3, 8.9 Ho2O3, 21.8 Er2O3, 1.86 Tm2O3, 12.5 Yb2O3, 0.57 Lu2O3, and 6.65 other oxides (including 3.2 Al2O3), and Y2O3 and M-ZrO2 powders were chosen. The targets for depositing electron-beam ceramic TBC layers—both standard and compositionally complex ones—were made of ceramic mixtures including (wt.%) M-ZrO2–7 Y2O3 and 90 M-ZrO2–10 HC. The properties of the compositionally complex and standard yttria-stabilized ZrO2-based ceramic layers in electron-beam TBCs applied in one process cycle were compared. Two-layer metal/ceramic TBCs were deposited onto model blades by directional crystallization from the ZhS-26VI alloy employing an UE-174 industrial electron-beam installation (ELTECHMACH, Vinnytsia). The ceramic M-ZrO2–7 Y2O3 topcoat was denoted as YSZ and 90 M-ZrO2–10 HC as HCSZ. The MZP-6 alloy (nickel–chromium–aluminum–yttrium) was used to form an inner heat-resistant bond coat. This resulted in rough dense glassy coatings differing by color: light-gray YSZ and dark-gray HCSZ. The coatings were 90–95 μm thick on the blade back side and 90 μm thick on the pressure side. Both coatings included the F-ZrO2 phase. Feather-like microstructures emerged in the coatings. The YSZ topcoat contained two types of dense structures, represented by columns and branched formations, and the HCSZ layer was of irregular microstructure with wide feather-like formations growth together. The laminar microstructure of the ceramic topcoat was due to the process features peculiar to electron-beam deposition. The YSZ topcoat had the following microhardness: 3884 MPa on the back side and 6052 MPa on the pressure side. The HCSZ topcoat had much lower microhardness: 1381 MPa on the back side and 1679 MPa on the pressure side. The compositionally complex coating withstood 161 thermal cycles and the standard coating 138 thermal cycles. Previous studies showed that ZrO2 stabilization with concentrates of yttrium-subgroup REM oxides was promising for the microstructural design of TBC ceramic topcoats.

The nanosized 90 mol.% ZrO 2 –2 mol.% Y 2 O 3 –8 mol.% CeO 2 powder was produced by hydrothermal synthesis in an alkaline environment and heat-treated in the range 400–1300°C. The powder properties were examined by X-ray diffraction (XRD), SEM and TEM, petrography, and BET. According to the XRD data, a low-temperature metastable cubic ZrO 2 (F-ZrO 2 ) solid solution formed after hydrothermal synthesis. According to the petrography and electron microscopy data, TZrO 2 began to form already in the hydrothermal synthesis process. The F-ZrO 2 → T-ZrO 2 phase transformation was completed in the range 700–850°C. Some T-ZrO 2 particles were characterized by a twin substructure. The T-ZrO 2 unit cell volume monotonically increased from 133.58 · 10 –3 nm 3 to 137.09 · 10 –3 nm 3 and the degree of tetragonality from 1.0033 to 1.0140. No M-ZrO 2 was found to form. The powder specific surface area decreased from 94 to 2 m 2 /g in the heat treatment process. The sizes of primary powder particles (5–10 nm) remained almost unchanged in heat treatment up to 1150°C. The Vickers hardness of the ceramics produced from the powder treated at 850°C was 3.1 GPa and critical fracture toughness factor K Ic was 8.4 MPa · m 1/2 . The preservation of the tetragonal structure (T-ZrO 2 ), which is capable of the martensitic T-ZrO 2 → M-ZrO 2 transformation, and the strength characteristics determined open ways for microstructural design of smart materials, including shape memory ones, in the ZrO 2 –Y 2 O 3 –CeO 2 system.

The standard material of the ceramic layer in thermal barrier coatings (TBCs)—a solid solution of ZrO 2 stabilized with (6–8 wt.%) Y 2 O 3 (YSZ)—approaches the temperature limit of its application (<1200°C) because the ZrO 2 t′ phase sinters and undergoes t′-ZrO 2 → T-ZrO 2 + F-ZrO 2 phase transformations to form M-ZrO 2 at elevated temperatures. Ceramic materials for a new generation of TBCs need to be developed to increase the operating temperature (up to 1600°C), efficiency, and productivity of gas-turbine engines. The overview paper analyzes research efforts focusing on the development of TBCs using solid solutions of ZrO 2 with rare-earth metal and titanium oxides. When Y 2 O 3 in YSZ is partially substituted by CeO 2 , TiO 2 , La 2 O 3 , Sc 2 O 3 , Gd 2 O 3 , Nd 2 O 3 , Yb 2 O 3 , Er 2 O 3 , and Ta 2 O 5 , ceramics with high phase stability (ZrO 2 t′ phase being retained in the coating) up to 1500°C, lower thermal conductivity, and required fracture toughness and sintering resistance but shorter thermal fatigue life than that of standard YSZ are produced. The concepts of greater tetragonality of the ZrO 2 t′ phase (ceramics in the ZrO 2 –CeO 2 –TiO 2 system) and a ‘multicomponent defective cluster’ (ceramics in the ZrO 2 –Y 2 O 3 –Nd 2 O 3 (Gd 2 O 3 , Sm 2 O 3 )–Yb 2 O 3 (Sc 2 O 3 ) system) explain how the operating temperature of the TBC ceramic layer increases to 1350°C and 1600°C, respectively. The thermal conductivity of TBC ceramics in the binary ZrO 2 –CeO 2 , ZrO 2 –Er 2 O 3 , ZrO 2 –Sm 2 O 3 , ZrO 2 –Nd 2 O 3 , ZrO 2 –Gd 2 O 3 , ZrO 2 –Dy 2 O 3 , and ZrO 2 –Yb 2 O 3 systems is lower than that of YSZ. Ceramics with high phase stability and low thermal conductivity have been produced in the ternary ZrO 2 –Sc 2 O 3 –Gd 2 O 3 , ZrO 2 –CeO 2 –Gd 2 O 3 , ZrO 2 –YbO 1.5 –TaO 2.5 , and ZrO 2 –Yb 2 O 3 –TiO 2 systems. An integrated approach is needed to choose the composition of the ceramic layer based on the ZrO 2 solid solution, select the coating technique, and improve the coating architecture to design effective TBCs with balanced properties.

Variations in the phase composition, specific surface area, and morphology of structural components in the ultrafine powder of composition (wt.%) 70 (90 ZrO 2 (3 Y 2 O 3 , 2 CeO 2 )–10 Al 2 O 3 )–30 CoAl 2 O 4 (70ZA30CoA), produced by hydrothermal synthesis combined with mechanical mixing, were studied in the heat treatment process up to 1300°C. The study employed Xray diffraction, scanning and transmission electron microscopy, petrography, and BET. The formation of CoAl 2 O 4 in the 70ZA30CoA powder in the heat treatment process was accompanied by reversible phase transformations: T-ZrO 2 → M-ZrO 2 → T-ZrO 2 . The M-ZrO 2 content increased from 15% to 46% in the temperature range 850–1000°C and decreased to 13% after heat treatment to 1150°C. The process involved slight coarsening of the primary T-ZrO 2 particles, while the size of the primary M-ZrO 2 particles remained practically unchanged. The phase transformation was due to a decrease in the free energy of the ultrafine 70ZA30CoA powder, representing a thermodynamically nonequilibrium system. The phase composition changed color of the 70ZA30CoA powder in the following sequence: gray → gray blue → dark cyan → bright blue. Morphological analysis of the structural components showed that the CoAl 2 O 4 formation and reversible T-ZrO 2 → M-ZrO 2 phase transformation were accompanied by shape change, loosening, and subsequent sintering of the agglomerates. The chain-like agglomerates of various shapes and sizes indicate that the 70ZA30CoA powder sinters actively at 1300°C. The decrease in the specific surface area from 46 to 1 m 2 /g depending on the heat treatment temperature was determined by the development of three structural transformation processes: formation of CoAl 2 O 4 , phase transition of the ZrO 2 solid solution, and sintering of the 70ZA30CoA powder. The established regularities are of fundamental importance for the microstructural design of ZrO 2 composites such as ZrO 2 –Y 2 O 3 –CeO 2 –Al 2 O 3 –CoO materials of blue and other colors for various applied purposes.

Nanocrystalline powders in the ZrO 2 −Y 2 O 3 −CeO 2 system were produced by hydrothermal synthesis in an alkaline environment. The powder properties were studied by differential thermal analysis, Xray diffraction, electron microscopy, petrography, and BET. A low-temperature ZrO 2 -based cubic solid solution crystallized in the powders in hydrothermal conditions. The specific surface area of the powders was 81−110 m 2 /g. The lattice parameters of the ZrO 2 -based solid solution increased monotonically with higher CeO 2 amount. The research results are needed for the microstructural design of composites in the ZrO 2 −Y 2 O 3 −CeO 2 system with high resistance to low-temperature ageing.

Transformation toughening, based on the martensitic T-ZrO2 → M-ZrO2 phase transformation, causes the high strength of ZrO2-based composites. The reversible martensitic phase transformation of fine-grained composites ZrO2–12 mol% CeO2 occurs below ambient temperature. Bulk material 12Се-TZP is characterized by “shape memory”, high fracture toughness (К1с up to 35 МПа·м 0,5) and tolerance to aging. Properties of composites depend on the properties of the starting powders. Hydrothermal synthesis in an alkaline medium is perspective for producing nanocrystalline powders of ZrO2-based solid solutions . Variations of physico-chemical properties of hydrothermal nanocrystalline 88 ZrO2–12CeO2 (mol%) powder after synthesis and thermal treatment in the 400–1300 °C range were investigated. It was found that after hydrothermal synthesis the thermodynamically non-equilibrium system consisting of a low temperature metastable cubic solid solution based on ZrO2 (F-ZrO2) and an X-ray amorphous phase were formed. The primary particles size was ≈ 10 nm. The powder specific surface area was 107 m2/g. Low-temperature F-ZrO2 remained up to 700 °C. Temperature increase up to 850 °C was accompanied by the F-ZrO2→T-ZrO2 phase transformation. This transformation is completed at 1000 °C. The powder specific surface area decreases from 107 m2/g to 0.27 m2/g during thermal treatment. Variation of powder specific surface area depends on both the ZrO2- based solid solution phase transformations and the sintering of freely poured powders. The primary particles size of the ZrO2-based solid solution increased to 20 nm. Morphology of powder varies topologically continuously during thermal treatment. The unit cell volume of T-ZrO2 phase decreased from 135.32 to 135.20 A after thermal treatment in the 1150–1300 °C range. Under these conditions the T-ZrO2 tetragonality increases from 1.0137 to 1.0139. Powder was characterized by high activity to sinter ing. The investigation results will be used for microstructural design of high-performance ZrO2-based composites.