Zirconia Crystals Research Articles

Microstructural Changes Induced by Heating at High Temperatures in the Single Crystal of Yttria-Stabilized Cubic Zirconia Implanted with Ag Ions

Ag ions were implanted into single crystals of yttria-stabilized cubic zirconia (YSZ) with largely different energies of 20 keV, 128 keV, and 1.5 MeV, and the cross sections of the implanted layer were examined. After implantation, thermal treatments were performed for the samples at temperatures up to 1000 °C in air. The surface of the sample implanted at 20 keV with a fluence of 3×1016 ions/cm2 was violet and showed an absorption peak at 508 nm. The samples implanted with 128 keV or 1.5 MeV with fluences of 3×1016 and 2×1016 ions/cm2, respectively, were light brown. However, after heating at 800–1000 °C, their color changed from light brown for the as-implanted state to violet. The absorption spectra of the samples implanted with markedly different energies showed different aspects of changes when the samples were heated at high temperatures. Scanning transmission electron microscopy (STEM) and high-angle annular dark field (HAADF) observations were conducted. Ag nanoparticles of approximately 6–12 nm were densely formed in the as-implanted state for both samples implanted with the energies of 20 and 128 keV, but they could not be observed for the sample implanted with 1.5 MeV. Ag nanoparticles were formed after heating at 1000 °C for the sample implanted at 1.5 MeV. While Ag nanoparticles remained stably even when heated at 900–1000 °C for the samples implanted at 128 keV or 1.5 MeV, those particles for the sample implanted at 20 keV gradually reduced in number when heated at high temperatures.

Nanoindentation of yttria-doped zirconia: Effect of crystallographic structure on deformation mechanisms

This article presents a nanoindentation study of polycrystalline and single crystals of yttria-doped zirconia with both tetragonal and cubic phases. Analysis of the deformation mechanisms is performed by both atomic force microscopy (AFM) and micro-Raman spectroscopy. Phase transformation from tetragonal to monoclinic phase is clearly distinguished on tetragonal crystals, whereas in cubic crystals the plastic deformation seems to be controlled by dislocation nucleation and interactions. AFM observations in tetragonal zirconia grains have shown that both grain size and autocatalytic transformation strongly influence the size of the transformed zone. Furthermore, the martensitic phase transformation seems to be also strongly dependent of the indenter shape. Experimental results suggest that a critical contact pressure is necessary to induce the phase transformation.

Investigation of the microstructure and nanostructure of partially stabilized zirconia crystals

This paper reports on the results of investigations into the dependence of the structure of partially stabilized zirconia crystals on the composition and technological conditions used for their growth. The zirconia crystals containing from 0 to 4 mol % Y2O3 are grown by directional crystallization in a cold container with the use of direct high-frequency heating. The microstructure and nanostructure of the grown crystals are examined using transmission and scanning electron microscopy. It is established that the formation of the structure depends directly on the composition of the crystals and the conditions of their growth.

Germanium-induced stabilization of a very high-k zirconia phase in ZrO2/GeO2 gate stacks

Electrical data on ZrO2/GeO2 stacks prepared by atomic oxygen beam deposition on Ge at 225 °C reveal a relatively weak dependence of the stack equivalent oxide thickness upon the ZrO2 thickness. This trend points to a very high zirconia dielectric permittivity (k) value which is estimated to be around 44. This is indicative of zirconia crystallization into a tetragonal phase which is also supported by x-ray diffraction data. X-ray photoelectron spectroscopy analysis is in line with the assumption that due to a finite GeO2 decomposition, Ge is incorporated into the growing ZrO2, thus, stabilizing the high-k tetragonal phase.

Cubic zirconia as a dynamic compression window

Symmetric impact experiments were used to characterize the elastic response of ⟨100⟩ cubic zirconia crystals under shock wave compression. Elastic response was determined from the apparent velocity structure upon free surface release and the preservation of light passing through the compressed sample. The Hugoniot and window correction were determined below 9GPa, and the estimated elastic limit was found to be near 10GPa. These results indicate that cubic zirconia may serve as a useful alternate to sapphire in dynamic compression experiments.

The method of assessment of the of friction contact fatigue parameters of technological ceramics and crystals

Two formulas were derived earlier using the theory of wear of I.V. Kraguelskii to calculate the wear resistance of friction pairs. To use them it was necessary to determine the friction contact fatigue parameters. The present paper deals with the method of experimental calculated assessment of these parameters and the results of its validation using wear resistant nanostructured alloyed zirconium dioxide crystals and engineering ceramics.

Synthesis of zirconia nanoparticles on carbon nanotubes and their potential for enhancing the fracture toughness of alumina ceramics

Nanoparticles of zirconia (ZrO 2) were in situ synthesized on the surface of carbon nanotubes by means of liquid phase reactions and a proper heat treatment process. The size of the nanoparticles could be controlled by the amount of zirconium source materials in a solution and its reaction times. In this study, the size of the nanoparticles ranged from several nanometers to twenty nanometers. It was particularly noted that the synthesized zirconia possessed a cubic structure (c-phase) which generally existed as a stable form of zirconia crystals at high temperatures (above 2370 °C) as well as a form of zirconia that could be used for enhancing the fracture toughness of alumina ceramics. Experimental results showed that the mechanical properties of alumina ceramics mixed with in situ synthesized nanoparticles on the surface of carbon nanotubes were much better than that of pristine nanotubes or zirconia nanoparticles alone. The existence of the nanoparticles on the surface of nanotubes results in improving the dispersion and bonding properties of the nanotubes in alumina matrix environment. The fracture toughness of CNT/ZrO 2 alumina ceramics was also improved by the mechanism of bridging effect.

On the Role of Calcination Temperature in Pt-SO 4 2− /ZrO2−Al2O3 Preparation and Catalytic Behaviors During the n-Hexane Hydroisomerization

Effects of calcination temperature for Pt-SO 4 2− /ZrO2−Al2O3 (PSZA) catalysts in n-hexane hydroisomerization were investigated by N2-adsorption, XRD, TG-DTA, FTIR, XPS and H2-TPR. An optimum calcination temperature is helpful to complete the crystallization process, resulting in better distribution of alumina into zirconia crystal and producing new acid centers responsible for higher catalytic activity.

Stability of Cubic Zirconia in a Granitic System Under High Pressure & Temperature

AbstractCubic zirconia is a well known, highly durable material with potential uses as an actinide host phase in ceramic waste forms and inert matrix fuels and in containers for very deep borehole disposal of some highly radioactive wastes. To investigate the behaviour of this material under the conditions of possible use, a cube of ∼ 2.5 mm edge was made from a single crystal of yttriastabilized cubic zirconia doped with 0.3 wt.% CeO2. The cube was enclosed in powdered granite within a gold capsule and a small amount of H2O added before sealing. The sealed capsule was held for 4 months in a cold-seal pressure vessel at a temperature of 780°C and a pressure 150 MPa, simulating both the conditions of a deep borehole disposal involving partial melting of the host rock and the conditions under which the actinide waste form might be encapsulated in granite prior to disposal. At the end of the experiment the quenched, largely glassy, sample was cut into thin slices and studied by optical microscopy, EMPA, SEM and cathodoluminescence methods. The results show that no corrosion of the zirconia crystal or reaction with the granite melt occurred and that no detectable diffusion of elements, including Ce, in or out of the zirconia took place on the timescale of the experiment. Consequently, it appears that cubic zirconia could perform most satisfactorily as both an actinide host waste form for encapsulation in solid granite for very deep disposal and as a container material for deep borehole disposal of highly radioactive wastes (HLW), including spent fuel.

Coloration Related to Nanostructure of Yttria-Stabilized Cubic Zirconia Single Crystal Implanted with Ag Ions

Ag ions were implanted into single crystals of yttria-stabilized cubic zirconia (YSZ) at 1.5 MeV to a fluence of 2×1016 ions/cm2. After implantation, heat treatments were performed at 500, 900, and 1000 °C in air. The surface color tone markedly changed from brown to a beautiful violet after heat treatments. Scanning transmission electron microscope (STEM) observations revealed that Ag nanoparticles were formed when samples were heated at temperatures higher than about 900 °C. The absorption spectrum in the visible region was measured. A large absorption peak appeared at 510 nm after heating at 900 °C. The peak could be attributed to the surface plasmon resonance of Ag nanoparticles.

Influence of zirconia crystal phase on the catalytic performance of Au/ZrO2 catalysts for low-temperature water gas shift reaction

Abstract The influence of crystal phase of zirconia on the performance of Au/ZrO2 catalysts for low temperature water gas shift reaction was investigated. Au/ZrO2 catalysts with pure tetragonal and monoclinic phases of ZrO2 were prepared by the deposition-precipitation method with similar gold loading and dispersion. It was found that the Au/m-ZrO2 catalyst showed much higher activity than that of the Au/t-ZrO2 catalyst, which could be attributed to the higher CO adsorption capacity of the Au/m-ZrO2 catalyst. The chemical state of gold that was strongly related to the pretreatment atmosphere also played an essential role in determining the catalytic activity for water gas shift reaction. Hydrogen and/or helium pretreated samples only contained Au0 species and exhibited higher activity than that of the sample pretreated with oxygen-containing atmosphere which resulted in the co-existence of Au0 and Au+ species. FTIR study further revealed that the formate species formed by the reaction of the adsorbed CO on gold nanoparticle with the hydroxyl groups on the surface of m-ZrO2 acted as the most important intermediates for the water gas shift reaction.

Thermal behaviour and catalytic properties towards propene combustion of zirconia modified by different first row transition metals

Zirconium oxyhydroxide was synthesized in the presence of different transition metals from the first row (Cr, Mn, Fe, Co, Ni, Cu) and calcined at two different temperatures 450 °C and 700 °C. Differential Thermal Analysis exhibits a delay in zirconia crystallization with high specific surface areas when the transition metal is associated with zirconium. Transition metals improved zirconia catalytic activity and selectivity in propene deep oxidation to CO 2 and H 2O. Catalytic activity enhancement was explained by the stabilization (total or partial) of zirconia tetragonal phase (evidenced by ambient and temperature programmed XRD studies) which leads to higher amount of anionic vacancies. The existence of reduction peaks (Temperature Programmed Reduction with hydrogen) in the temperature range of catalytic activity explained the better catalytic performances of Cr, Mn, Co and Cu samples compared to Fe- and Ni-based ones.

Myth busting — in the world of x‐rays

AbstractHave you seen the television show ‘MythBusters’? It was decided to carry out a radiographic version of ‘MythBusters’. The study involved the investigation of the urban myth about a female radiographer who accidentally x‐rayed her hand, only to discover that her diamond engagement ring was a fake. To prove or disprove the legend, a number of natural diamonds and cubic zirconia crystals were radiographed. Sure enough, there was a definite and consistent difference between the resultant images. The study confirmed part of the myth that radiography can differentiate between a diamond and a cubic zirconia fake diamond.

Twin nucleation mechanisms at a crack tip in an hcp material: Molecular simulation

Twinning is an important deformation mode in hexagonal close-packed (hcp) materials that can strongly affect fracture toughness. In order to clarify the early stages of twin nucleation, lattice statics simulations of zirconium crystals containing mixed-mode basal cracks with [ 1 ¯ 2 1 ¯ 0 ] and [ 1 1 ¯ 0 0 ] front orientations were carried out using an embedded-atom method potential. The simulations show that crack tip twin nucleation is a two-stage process: (I) initial plastic deformation occurs within a thin layer ahead of the crack, possibly involving basal slip, crack tip blunting by the formation of Frank partials or an hcp-face-centered cubic (fcc) transformation produced by Shockley partials emitted from the crack tip and (II) a twin forms in the surrounding hcp matrix. In this second stage, either a { 1 1 2 ¯ 1 } twin is nucleated homogeneously or a { 1 0 1 ¯ 1 } twin is nucleated heterogeneously by Shockley partials that nucleate inside the fcc region and penetrate the hcp matrix.

Temperature dependence of the damage induced by Cs ion implantation in zirconia

Single crystals of cubic zirconia were implanted at RT and 750°C with increasing fluences of low energy Cs2+ ions. In situ RBS/C and TEM experiments were performed to determine the amount of damage, the nature of radiation defects and the possible occurrence of Cs precipitates as a function of the Cs fluence at both temperatures. Results indicate that the damage build-up and the nature of defects depend on the implantation temperature. TEM micrographs do not reveal the formation of any secondary phase for both implantation temperatures up to 3at.%.

Double differential neutron scattering cross sections of materials for ultra high temperature reactors

The five materials with the highest melting point are hafnium, tantalum, niobium and zirconium (ZrC) carbides and graphite (that sublimes). Graphite is the material of choice for very high temperature reactors (VHTR); ultra high temperature reactors (UHTR), like the thermal nuclear propulsion reactor NERVA use a dispersion of ZrC and UC in graphite as the material in the reactor core. Presently there are neither inelastic nor elastic double differential scattering data available that describe the thermalization process in ZrC. We therefore, calculated coherent elastic and incoherent inelastic cross sections for the ZrC crystal which has a face centered cubic (fcc) lattice. The phonon spectrum for the ZrC lattice was calculated with the computer code PHONON using the Hellman–Feynman forces computed with ab-initio methods [Jochyn, P.T., Parlinski, K., 2000. Ab initio lattice dynamics and elastic constants of ZrC. Eur. Phys. J. B 15, 265–268]. This phonon spectrum was then used to compute the S( α, β, T) matrices for the inelastic scattering cross sections for C and Zr in the ZrC lattice using modified versions of the computer codes GASKET, HEXSCAT and NJOY. The results were applied to calculate, with the proper S( α, β, T), criticality and reactivity coefficients of temperate of reactor systems containing ZrC and UC. For comparisons, these parameters were also calculated with approximations of S( α, β, T), i.e. the gas or the graphite scattering kernels. Depending on the degree of thermalization, k eff is underestimated between 0.6% and 1%, and the values and the shape of the reactivity coefficients as a function of temperature change by substantial amounts.

Thermal Stability and Mechanical Properties of Plasma Sprayed Al<sub>2</sub>O<sub>3</sub>/ZrO<sub>2</sub> Nano-Composite Coating

Alumina/zirconia nano-composite coating was fabricated by plasma spraying using agglomerated feedstock from fine powders of about 100 nm. The coating was consisted of fine γ-alumina and zirconia crystals with size of several nano meter and some amorphous boundary layers. The amorphous phase was crystallized and disappeared after heat treatment at 930°C. However, the crystallite size was kept under 50 nm even after 1500°C-100hr heating, so the alumina-zirconia nano-composite showed good thermal stability against the grain growth.

Active laser media based on fianite crystals

The spectral and lasing parameters of Yb3+ ions in zirconium dioxide crystals with a stabilised cubic structure (fianites) are studied. It is found that the absorption and luminescence spectra of Yb3+ in these crystals are characterised by a considerable inhomogeneous broadening. The nature of this broadening is investigated and the model of optical centres consistent with the properties of the inhomogeneous spectra of Yb3+ in the cubic zirconium dioxide crystal is proposed. It is shown that fianites can be used in solid-state lasers. Lasing in Yb3+ ions in diode-pumped fianite crystals was obtained and studied.

Preparation and properties of toughened yellow zirconia ceramics

To obtain the high toughness yellow ceramic, using the Y2O3 (5%) stabilized ZrO2 as main starting material, Al2O3 as toughen reagent, NH4VO3, V2O5 as colorant, and mixed with ball mill, molded with dry press method and two times sintered. By testing the properties and structure of the ceramic, we have optimized the best technical conditions, and the mechanism of toughness and color present has been studied. The result shows that the NH4VO3 as colorant is better than V2O5, and the color is between X = 0.396–0.412 and Y = 0.402–0.390. The highest bending strength of toughened ZrO2 ceramics is 1138 MPa, and the fracture toughness is 14.8 MPa m1/2. The toughen mechanism is that phase transformation of ZrO2 and the dispersion of toughen reagents, and the color causing is by the colorful ion (V4+) entered into the zirconia crystals lattice of the ceramic.

‘In situ’ diagnostics of solid electrolyte sensors measuring oxygen activity in melts by a developed impedance method

An impedance method for the periodic ‘in situ’ diagnostics of the solid electrolyte/liquid-metal electrode interface during the lifespan of yttria-stabilized zirconia (YSZ) based sensors measuring oxygen partial pressure in melts was developed. Polarization effects on the YSZ, resulting from the corrosive measuring environment (molten alkaline metals), may be interpreted as a blocking reaction layer on the electrolyte/liquid-metal electrode interface. The proposed impedance method allows information to be obtained about the level of polarization of the YSZ/liquid-metal electrode interface that is characterized by a second semi-arc of a hodograph of impedance. The second semi-arc represents the parameters of polarization resistance (Rf) and capacitance of the double electrical layer on the interface YSZ/liquid metal (CD). Analysis of the impedance method on the single crystal zirconia sensor measuring dissolved oxygen in molten lead at temperatures of 380–480 °C revealed that this sensor, with a Bi–Bi2O3 reference electrode, showed a negligible level of polarization effects on the electrode/electrolyte interface at temperatures as low as 380 °C. The results of the present work may be applicable for the diagnostics of oxygen sensors with more complicated applications, such as in the measurement of oxygen activity in lead–bismuth, sodium or lithium heat carriers in liquid-metal nuclear facilities.