A. A. Il’icheva

Preparation and Properties of Ceramic Composites with Oxygen Ionic Conductivity in the ZrO 2 -CeO 2 -Al 2 O 3 and ZrO 2 -Sc 2 O 3 -Al 2 O 3 Systems

Powder precursors in the ZrO2-CeO2-Al2O3 and ZrO2-Sc2O3-Al2O3 systems are prepared by the sol-gel synthesis. It is revealed that the electrical conductivity of the sample doped with scandium oxide is higher than the electrical conductivity of the sample doped with cerium oxide despite the higher content of the nonconducting phase Al2O3 (corundum). The thermal expansion coefficients are determined for all the ceramic samples under investigation. It is established that the Al2O3 dopant affects the thermal expansion coefficient. The ceramic materials studied can be used as solid-electrolyte sensors.

Microstructure and phase composition of ZrO2-CeO2-Al2O3 materials modified with MgO and Y2O3

This paper presents our findings on phase formation processes during heat treatment of sol-gel synthesis products with the composition (mol %) 65(88ZrO2 + 12CeO2) + 35Al2O3 modified with 1 mol % MgO or Y2O3. The composites modified with 1 mol % MgO have been shown to differ significantly in phase composition from the parent nanopowders. Sintering is accompanied by partial decomposition of the tetragonal zirconia (T-ZrO2) based solid solution and the formation of a monoclinic zirconia (M-ZrO2) based solid solution and two aluminum-containing phases: corundum and the mixed oxide MgAl11CeO19. The addition of 1 mol % Y2O3 leads to successive formation of T-ZrO2 and corundum and improvement of their structural perfection. The observed differences in phase formation during heat treatment result in different grain size compositions in the microstructure of the composites.

Effect of the precipitation sequence on phase formation in the ZrO2-CeO2-Al2O3 system

We have studied phase formation processes during heat treatment of precipitates in the ZrO2-Al2O3 and ZrO2-CeO2-Al2O3 systems. During heat treatment of powders prepared by coprecipitation of precursors to ZrO2, CeO2, and Al2O3, α-Al2O3 is formed at higher temperatures, which is due to the formation and decomposition of T-ZrO2 and metastable Al2O3 phases. The precipitation sequence in the ZrO2-CeO2-Al2O3 system influences the lattice parameters of the forming T-ZrO2-based solid solutions because of the different degrees of Ce4+ and Al3+ substitutions for Zr4+.

Phase transformations in t-ZrO2-Al2O3 nanoprecursors and formation of the microstructure of related ceramic materials

The phase transformations in nanosized precursors (precipitation products) prepared by the sol-gel processing and the formation of the microstructure of ceramic materials in the t-ZrO2-Al2O3 system are investigated. It is demonstrated that the phase transformation occurring during the precipitation depends on the precrystallization state that is governed by a sequence of precipitation of the components in the Al2O3-ZrO2-CeO2 system in the course of the synthesis of precursors. The nascent metastable phases of solid solutions based on t-ZrO2 and θ-Al2O3 subsequently have a substantial effect on the sizes of grains in the microstructure of the corresponding ceramics.

High-strength ceramic materials based on polycrystalline tetragonal ZrO2 stabilized by Lu2O3

The effect of Lu2O3 on low-temperature stabilization of the tetragonal modification of ZrO2 is considered. The subject of study is ZrO2- Lu2O3 binary systems with 1 –9 mole % Lu2O3 for specimens of ZrO2 powders prepared by the method invented by the authors. It is established that low-temperature stabilization oft- ZrO2 occurs in the region of 6 –8 mole % concentration of Lu2O3. The obtained ceramic material has a density of 99.8% of the theoretical value, an ultimate bending strength of at least 800 MPa, and a crack resistance of at least 13 MPa · m1/2 at a content of Lu2O3 equal to 6 –8 mole %

Phase Formation in Al 2 O 3 –ZrO 2 –CeO 2 Nanopowders Modified with Calcium Cations

We have studied phase formation in calcium-modified Al2O3–ZrO2–CeO2 nanopowders during sol–gel synthesis. The results demonstrate that heat treatment of the nanopowders first leads to the formation of a zirconium dioxide-based solid solution stabilized with cerium cations. Raising the heat treatment temperature helps the crystallization of corundum, a stable phase of aluminum oxide, to reach completion. In the temperature range 1400–1550°C, we observe the formation of a second aluminum-containing phase: calcium cerium hexaaluminate consisting of long prismatic grains.

Low-temperature aging of ceramic on the basis of tetragonal zirconium dioxide stabilized by cations of yttrium and ytterbium

The effect of low-temperature aging of ceramics Zr0.97Y0.03O2 (Y-TZP) and Zr0.98Y0.02O2 (Yb-TZP) by the methods of the high-grade diffraction analysis and atomic force microckopy is studied. Formation of the phase M-ZrO2 in ceramic (Y-TZP) in a large quantity after hydrothermal impact in subsurface layers of ceramic samples is shown. A positive effect of the stabilizing cation Yb+3 on the phase composition stability and strength properties of ceramic based on T-ZrO2 is registered. Adaptation of the structural ceramic based on T-ZrO2 according to medical requirements is the aim of this study.

Dispersion Hardening of Composites in the System Aluminum Oxide and Cerium Cation Stabilized Tetragonal Zirconium Dioxide

The production of composites based on nanopowders synthesized by the sol-gel method in the system aluminum oxide and tetragonal zirconium dioxide stabilized by cerium cations (Al2O3–[Ce–TZP]) is described. It is shown that modification of the compositions by calcium oxide promotes the formation of composites of a dispersion-hardening phase in the form of long-prismatic grains in the sintering process. The presence of this phase affects the increase of strength and resistance to brittle fracture of composites with a ZrO2 matrix and with an Al2O3 matrix. The strength in static bending of the composites reaches 1000 MPa and the cracking resistance K1c increases to 11.0 MPa·m1/2.

Modified composites of Al2O3–(Ce-TZP) system as materials for medical use

This article presents an investigation of the influence of modification of Al2O3–(Ce-TZP) with alkaline earth elements Ca+2 and Mg+2 on the phase composition, microstructure, and strength characteristics of composites. It is established that modification of Mg+2 compositions leads to a decrease in the strength parameters of composites, while introduction of Ca+2 contributes to an increase in the strength parameters of the respective composites owing to aggregation of effects of transformational and dispersive hardening. The obtained high-strength composites can be applied as constructional materials in engineering and medicine.

Electrtic transport properties and the size effect in ZrO2-based ceramic materials

Amorphous nanomaterials in the ZrO2-CeO2 system are synthesized. The aging and crystallization processes in a complex zirconium-cerium hydrogel are investigated. Powders with a crystal size of 45–100 nm are prepared. Samples having a porosity of less than 2.2% are produced from the powders. It is demonstrated that tetragonal zirconia-based solid solutions Zr1−xCexO2−δ sintered at a temperature of 1500°C possess ionic conductivity in air and in noble gasses. The assumptions are made regarding the mechanisms of ionic (surface anionic) and electronic (hopping) conduction in samples of the 88ZrO2 · 12CeO2 (mol %) composition. The solid solutions prepared can be used at a temperature of 720°C as high-strength inexpensive solid electrolytes.