Strain localization in compressed ZrO<Subscript>2</Subscript>(Y<Subscript>2</Subscript>O<Subscript>3</Subscript>) ceramics

Spatiotemporal distributions of local components of the distortion tensor of a nonplastic material—yttria partially stabilized tetragonal zirconia (YTZ) ceramics—have been studied under active compressive straining conditions using double-exposure speckle photography techniques. The strain localization patterns are presented and the features of macroscopic strain inhomogeneity in the elastic state of YTZ ceramics are considered.     

Preparation and properties of Y<Subscript>2</Subscript>O<Subscript>3</Subscript> partially stabilized ZrO<Subscript>2</Subscript> crystals

We have studied the effect of composition and growth conditions on the structure and properties of 2.5–5 mol % Y₂O₃ partially stabilized ZrO₂ crystals grown by directional solidification in a cold-wall crucible. The phase composition and density of the crystals have been determined. The crystals are shown to be uniform in composition, with local changes in Y₂O₃ content within ±0.5 mol %. The dimensions and quality of the single crystals are influenced by the growth conditions.     

Effect of Al<Subscript>2</Subscript>O<Subscript>3</Subscript> on the properties of nanocrystalline ZrO<Subscript>2</Subscript> + 3 mol % Y<Subscript>2</Subscript>O<Subscript>3</Subscript> powder

We have studied the properties of nanocrystalline ZrO₂〈3 mol % Y₂O₃〉 and 90 wt % ZrO₂〈3 mol % Y₂O₃〉-10 wt % Al₂O₃ powders prepared via hydrothermal treatment of coprecipitated hydroxides at 210°C. The results demonstrate that Al₂O₃ doping raises the phase transition temperatures of the metastable low-temperature ZrO₂ polymorphs and that the structural transformations of the ZrO₂ and Al₂O₃ in the doped material inhibit each other.     

Effect of Y<Subscript>2</Subscript>O<Subscript>3</Subscript> stabilizer content and annealing on the structural transformations of ZrO<Subscript>2</Subscript>

The structure of partially stabilized zirconia crystals has been studied by transmission electron microscopy before and after annealing. Structural characterization of Y₂O₃-doped (2.8 to 4 mol %) zirconia before annealing showed that all of the samples consisted of twin domains whose size was dependent on the stabilizer content. Annealing at 2100°C increased the domain size in the composition range 2.8–3.7 mol % Y₂O₃ and reduced it at 4 mol % Y₂O₃. These structural changes allowed us to determine the position of the representative point relative to the phase boundary in the equilibrium phase diagram of the system.     

Impurity and vacancy segregation at symmetric tilt grain boundaries in Y<Subscript>2</Subscript>O<Subscript>3</Subscript>-doped ZrO<Subscript>2</Subscript>

Segregation energies of impurity ions and oxygen vacancies at grain boundaries in Y₂O₃-doped ZrO₂ as calculated from atomistic simulations using energy minimization and Monte Carlo methods are reported. Based on these energies, local defect equilibrium concentrations have been estimated. It is found that it is more energetically favorable for an yttrium ion to be accompanied by an oxygen vacancy at grain boundaries, although decrease in energy when associated with an oxygen vacancy differs from boundary to boundary. The segregation energy for a neutral defect complex consisting of a two yttrium ions and an oxygen vacancy at infinitely dilute concentration is highly correlated with the coordination environment of each site in the vicinity of the grain boundary (GB), and, in turn, GB energy. Although the estimated local equilibrium concentrations of these defects are similar, detailed analysis of the atomic coordination and defect distributions in the vicinity of a GB reveal that defect distributions, especially of oxygen vacancies, are dependent on the characteristics of the particular GB and that segregation in effect reduces lattice strains at the GB. Equilibrium concentration distributions of yttrium at grain boundaries are also given as a function of spatial resolution, and are useful for interpretation of experimental results.     

Microstructural evaluation of thermally fatigued SiC-reinforced Al<Subscript>2</Subscript>O<Subscript>3</Subscript>/ZrO<Subscript>2</Subscript> matrix composites

Using the water-quenching technique, the thermal fatigue behaviour of an alumina/zirconia and platelets- and particulates-SiC reinforced alumina/zirconia matrix composites hot-pressed at 1500 and 1700^∘C has been studied. The addition of 10 wt% SiC either in the form of platelets or particulates can obviously improve the thermal fatigue resistance of alumina/zirconia composites. The damages present after 40 thermal fatigue cycles in the composites was illustrated by the microstructure examinations and EDX.     

Crack propagation in layered Al<Subscript>2</Subscript>O<Subscript>3</Subscript>/ZrO<Subscript>2</Subscript> composites prepared by electrophoretic deposition

Crack propagation through layered Al₂O₃/ZrO₂ composites was studied. The specimens were prepared via electrophoretic deposition of alumina and zirconia powders from suspensions with monochloroacetic acid and isopropanol. The kinetics of electrophoretic deposition could be described fully if the electrophoretic mobility and conductivity of suspensions were known. The conductivity of suspensions increased in the course of deposition. Adjusting to properly controlled kinetics of deposition and sintering, deposits were prepared with strongly bonded layers of different pre-defined thicknesses and, consequently, with different magnitudes of residual stress. Cracks, produced by an indentation technique, propagated askew towards layer interfaces deflected towards the interface in the Al₂O₃ layers and away from the interface in the ZrO₂ layers. Changes in the direction of crack propagation were described for the whole range of angles of incidence (0°–90°). The biggest change in the crack propagation was observed for the angle of incidence 45° and was ca. 15°, irrespective of the magnitude of residual stress in the layers.     

Properties of nanocrystalline ZrO<Subscript>2</Subscript>-Y<Subscript>2</Subscript>O<Subscript>3</Subscript>-CeO<Subscript>2</Subscript>-CoO-Al<Subscript>2</Subscript>O<Subscript>3</Subscript> powders

We have studied the properties of nanocrystalline ZrO₂-Y₂O₃-CeO₂-CoO-Al₂O₃ powders prepared via hydrothermal treatment of a mixture of coprecipitated hydroxides at 210°C. A number of general trends are identified in the variation of the properties of the synthesized powders during heat treatment at temperatures from 500 to 1200°C. Our results demonstrate that the addition of 0.3 mol % CoO to nanocrystalline ZrO₂-based powders containing 1 to 5 mol % Al₂O₃ allows one to obtain composites with good sinterability at a reduced temperature (1200°C).     

Transparent Y<Subscript>2</Subscript>O<Subscript>3</Subscript>:Nd<Superscript>3+</Superscript> ceramics produced from nanopowders by magnetic pulse compaction and sintering

Transparent Nd-doped cubic Y₂O₃ ceramics have been prepared by magnetic pulse compaction of nanopowders, followed by sintering. Analysis of the transport of nonequilibrium thermal phonons at liquid helium temperatures has been used to assess the effect of preparation conditions on the microstructure, average thickness, and stability of the grain boundaries in the ceramics. The average grain-boundary thickness in the transparent ceramics is shown to be comparable to the lattice parameter of Y₂O₃.     

Change in the mechanism of conductivity in ZrO<Subscript>2</Subscript>-based crystals depending on the content of stabilizing Y<Subscript>2</Subscript>O<Subscript>3</Subscript> additive

The interrelationship between the structure, phase composition, and transport characteristics of solid electrolytes based on ZrO₂ has been studied as dependent on the content of stabilizing Y₂O₃ additive. It is established that twin boundaries do not lead to the appearance of additional mechanism of ionic conductivity acceleration in ZrO₂–Y₂O₃ crystals. The maximum conductivity has been observed in ZrO₂–(8–10) mol % Y₂O₃ crystals containing a t” phase, in which oxygen atoms are displaced from high-symmetry positions characteristic of the cubic phase.     

Effect of Y<Subscript>2</Subscript>O<Subscript>3</Subscript> on the crystallization behavior of SiO<Subscript>2</Subscript>–MgO–B<Subscript>2</Subscript>O<Subscript>3</Subscript>–Al<Subscript>2</Subscript>O<Subscript>3</Subscript> glasses

A series of glass comprising of SiO₂–MgO–B₂O₃–Y₂O₃–Al₂O₃ in different mole ratio has been synthesized. The crystallization kinetics of these glasses was investigated using various characterization techniques such as differential thermal analysis (DTA), thermo gravimetric analysis (TGA), X-ray diffraction (XRD), and scanning electron microscopy (SEM). Crystallization behavior of these glasses was markedly influenced by the addition of Y₂O₃ instead of Al₂O₃. Addition of Y₂O₃ increases the transition temperature, T _g, crystallization temperature, T _c and stability of the glasses. Also, it suppresses the formation of cordierite phase, which is very prominent and detrimental in MgO-based glasses. The results are discussed on the basis of the structural and chemical role of Y³⁺ and Al³⁺ ions in the present glasses.     

Preparation of magnetic composites through SrO-Fe<Subscript>2</Subscript>O<Subscript>3</Subscript>-Al<Subscript>2</Subscript>O<Subscript>3</Subscript>-B<Subscript>2</Subscript>O<Subscript>3</Subscript> glass crystallization

Glasses with nominal compositions 11SrO · 5.5Fe₂O₃ · 4.5Al₂O₃ · 4B₂O₃ (1) and 15SrO · 5.5Fe₂O₃ · 4.5Al₂O₃ · 4B₂O₃ (2) were prepared by rapidly quenching oxide melts between counterrotating steel rollers. The glasses were then heat-treated in the range 650–950°C to produce glass-ceramic samples. The samples were characterized by X-ray diffraction, electron microscopy, and magnetic measurements. The phase composition of the glass-ceramics was determined, and their microstructure and magnetic properties were studied. The annealing temperature was shown to have a strong effect on the coercivity of the materials, which reaches 650 and 570 kA/m for compositions 1 and 2, respectively.     

Multifunctional anti-stokes Y<Subscript>2</Subscript>O<Subscript>2</Subscript>S-based white phosphors

We have established the key trends in the variation of the intensity of the visible and IR luminescence of (Y_{1 − x − y − z} Yb _x Tm _y Er _z )₂O₂S solid solutions in relation to their composition under laser excitation at λ = 0.940 μm. The results obtained have been used to develop multifunctional anti-Stokes white phosphors with various tinges and a predetermined relative intensities of visible and IR emission bands.     

Microstructure and high-temperature strength of Al<Subscript>2</Subscript>O<Subscript>3</Subscript>/Er<Subscript>3</Subscript>Al<Subscript>5</Subscript>O<Subscript>12</Subscript>/ZrO<Subscript>2</Subscript> ternary melt growth composite

A new Al₂O₃/Er₃Al₅O₁₂(EAG)/ZrO₂ ternary MGC (Melt Growth Composite) with a novel microstructure has been fabricated by unidirectional solidification. This ternary MGC has a microstructure consisting of continuous networks of single-crystal Al₂O₃, single-crystal EAG and fine cubic-ZrO₂ phases without grain boundaries. The ternary MGC has also characteristic dimensions of the microstructure of around 2–4 m for EAG phases, around 2–4 m for Al₂O₃ phases reinforced with around 0.4–0.8 m cubic-ZrO₂ phases. No amorphous phases are formed at interfaces between phases in the ternary MGC. The ternary MGCs flexural strength at 1873 K is approximately 700 MPa, more than twice the 330 MPa of the Al₂O₃/EAG binary MGC. The fracture manner of the Al₂O₃/EAG/ZrO₂ ternary MGC at 1873 K shows the same intergranular fracture as the Al₂O₃/EAG binary MGC, but is significantly different from the transgranular fracture of the sintered ceramic.     

Electrical properties of some Y<Subscript>2</Subscript>O<Subscript>3</Subscript> and/or Fe<Subscript>2</Subscript>O<Subscript>3</Subscript>-containing lithium silicate glasses and glass-ceramics

The ac electrical properties of some lithium silicate glasses and glass-ceramics containing varying proportions of Y₂O₃ and/or Fe₂O₃ were measured to investigate their electronic hopping mechanism. There is a clear variation of these properties with composition. The obtained results were related to the concentration and role of Y₂O₃ and/or Fe₂O₃ in the lithium silicate glass structure. In crystalline solids the electrical properties data obtained were correlated to the type and content of the mineral phases formed as indicated by X-ray diffraction analysis (XRD). The conductivity, dielectric constant and dielectric loss of the studied glasses were studied using the frequency response in the interval 30 Hz–100 KHz and the effect of compositional changes on the measured properties was investigated. The measurements revealed that the electrical responses of the samples were different and complex. The addition of Y₂O₃ generally, decreased the ac conductivity, dielectric constant and dielectric losses of the lithium silicate glasses. The addition of Fe₂O₃ in Y₂O₃-containing glasses increases the conductivity, while, the dielectric constant and dielectric losses were found to be decreased. However, the addition of Fe₂O₃ instead of Y₂O₃ led to decrease the ac conductivity and increased their dielectric constant and dielectric losses. The obtained data were argued to the internal structure of the lithium silicate glass and the nature or role-played by weakness or rigidity of the structure of the sample. Lithium disilicate-Li₂Si₂O₅, lithium metasilicate-Li₂SiO₃, two forms of yttrium silicate Y₂Si₂O₇ & Y₂SiO₅, iron yttrium oxide-YFeO₃, lithium iron silicate-LiFeSi₂O₆ and α-quartz phases were mostly developed in the crystallized glasses. The conductivity of the crystalline materials was found to be relatively lower than those of the glass. At low frequency, as the Y₂O₃ content increased the ac conductivity, dielectric constant and dielectric loss data of the glass-ceramics decreased. However, the addition of Fe₂O₃ to the Y₂O₃ containing glass-ceramic led to increase the conductivity. The addition of high content of Fe₂O₃ instead of Y₂O₃ in the glass ceramic led to increase the ac conductivity.     

Inducing multiferroic behaviour in the diamagnetic Y<Subscript>2</Subscript>O<Subscript>3</Subscript> system

The synthesis and characterization of Y_2−xFe_xO₃ (where x = 0–0.3) compounds has been carried out for their importance in the field of multiferroic materials. The powder X-ray diffraction reveal that the compounds Y_1.95Fe_0.05O₃, Y_1.9Fe_0.1O₃, Y_1.85Fe_0.15O₃ and Y_1.8Fe_0.2O₃ crystallize in tetragonal structure whereas Y_1.75Fe_0.25O₃ and Y_1.7Fe_0.3O₃ compounds crystallize in orthorhombic structure. The change in crystal system with respect to the concentration of Fe may be attributed to the variation in occupancy position of Fe³⁺ into the Y³⁺ site of Y₂O₃ system. Variation in crystal structure, surface morphology and composition was studied by micro-Raman analysis, SEM and EDX analysis. The shift in intense Raman signals from 426 to 385 cm⁻¹ confirms the change in the crystal structure of the prepared compounds. Further it is also identified that the E_g mode of vibration is the dominant in the Fe substituted compounds. The substitution of Fe in the Y₂O₃ system leads to the increase in the intensity of resonance band, which indicates a large polarisability variation in the Y_2−xFe_xO₃ compounds. Diffused reflectance studies show a red shift in energy gap values while increasing the concentration of Fe. The room temperature magnetization and electron paramagnetic resonance studies reveal that the incorporation of Fe in the Y₂O₃ system leads to magnetic phase change from diamagnetic to ferromagnetic. The electric polarization studies imply that the substitution of lower ionic radii element Fe³⁺ in the Y³⁺ site leads to distortion in the lattice and show the way to spontaneous dipole moment and it was found that the Y_1.8Fe_0.2O₃ compound exhibits the possibility of multiferroic behaviour. Therefore this paper explores the possibility of inducing ferromagnetic and ferroelectric behaviour in the Fe substituted yttrium oxide system.     

Preparation of porous ZrO<Subscript>2</Subscript>/Al<Subscript>2</Subscript>O<Subscript>3</Subscript> macrobeads from ion-exchange resin templates

In this work, we will report a method to prepare porous ZrO₂ and ZrO₂/Al₂O₃ macrobeads using cation-exchange resins with sulfonate groups as templates. The preparation process involves metal ion-loading, ammonia-precipitation, and calcination at an appropriate temperature. Several characterization methods, such as TGA, XRD, SEM with EDX, TEM and N₂ adsorption and desorption, were used to characterize the ZrO₂ and ZrO₂/Al₂O₃ macrobeads. The results showed that the porous structures of the resin templates were negatively duplicated in the two kinds of macrobeads. We found the following interesting results: (1) The ZrO₂/Al₂O₃ macrobeads are composed of tetragonal zirconia nanocrystals that are more technologically important, while the pure ZrO₂ macrobeads consist of a mixture of tetragonal and monoclinic zirconia. (2) In the ZrO₂/Al₂O₃ macrobeads, the size of ZrO₂ nanocrystals is about 5 nm smaller than that (about 19 nm) found in the pure ZrO₂ macrobeads. (3) The ZrO₂/Al₂O₃ macrobeads have more mesopores and, therefore, have a larger surface area than the pure ZrO₂ macrobeads. These oxide macrobeads will have potential applications in catalysis by taking advantage of their macrobeads shape and pores structure.     

The influence of activation of Y<Subscript>2</Subscript>O<Subscript>3</Subscript> polycrystalline matrices by Bi<Superscript>3+</Superscript> ions on the luminescence of Y<Subscript>2</Subscript>O<Subscript>3</Subscript>:Eu<Superscript>3+</Superscript>

The influence of activation of the Y₂O₃ matrix of the Y₂O₃:Eu³⁺ phosphor by Bi³⁺ ions on the luminescence of Eu³⁺ and Bi³⁺ ions in it and on conditions of the excitation energy transfer to luminescence centers is studied. It is shown that the presence of Bi³⁺ ions leads to the appearance of recombination luminescence with participation of bismuth ions at low concentrations (up to 6–8 at %) of the dominant activator europium and to an increase in the threshold of intrinsic concentration quenching of its luminescence.     

Nanocoating of Al<Subscript>2</Subscript>O<Subscript>3</Subscript> additive on ZrO<Subscript>2</Subscript> powder and its effect on the sintering behaviour in ZrO<Subscript>2</Subscript> ceramic

ZrO₂ powder was coated with Al₂O₃ precursor generated by a polymeric precursor method in aqueous solution. The system of nanocoated particles formed a core shell-like structure in which the particle is the core and the nanocoating (additive) is the shell. A new approach is reported in order to control the superficial mass transport and the exaggerated grain growth during the sintering of zirconia powder. Transmission electron microscopy (TEM) observations clearly showed the formation of an alumina layer on the surface of the zirconia particles. This layer modifies the sintering process and retards the maximum shrinkage temperature of the pure zirconia.     

Study of wear resistance and nanostructure of tertiary Al<Subscript>2</Subscript>O<Subscript>3</Subscript>/Y<Subscript>2</Subscript>O<Subscript>3</Subscript>/CNT pulsed electrodeposited ni-based nanocomposite

We study the electrodeposition of tertiary Alumina/Yitria/carbon nanotube (Al₂O₃/Y2O₃/CNT) nanocomposite by using pulsed currents. The process of coating is performed in a nickel-sulfate bath and the nanostructure of the obtained compound layer is examined with the help of high-precision figure analysis of SEM nanographs. The effects of process variables, i.e., the Y₂O₃ concentration, treatment time, current density, and the temperature of electrolyte are experimentally investigated. Statistical methods are used to achieve the minimum wear rate and average size of nanoparticles. Finally, the percentage contributions of different effective factors are revealed, and the confirmation run showed the validity of the obtained results. It is also revealed that the wear properties of the coatings undergo significant changes if the sizes of nanoparticles change. The atomic-force microscopy (AFM) and transmissionelectron microscopy (TEM) analyses confirm the smooth surfaces and average sizes of nanoparticles in the optimal coating.