Modification Of ZrO2 Research Articles

Kinetics of the nanocrystal formation in the ZrO2-In2O3-H2O system under hydrothermal conditions

The hydrothermal synthesis of nanocrystals in the ZrO2-In2O3-H2O system is investigated kinetically. The presence of indium hydroxide results in an increase in the dehydration temperature of this system. The size of nanocrystals is equal to 20–30 nm and virtually does not depend on the temperature and time of hydrothermal treatment or on the type of crystalline modification of ZrO2. An increase in the synthesis temperature leads to an increase in the fraction of the monoclinic zirconia modification.

Oxidation Processes for Alloys in the Ni – Zr System. Part 2. Phase Composition of Scale Formed on Ni7Zr2

We have used x-ray and metallographic layer-by-layer phase analysis to study the structure and composition of scale formed on the alloy Ni7Zr2 during its oxidation in air over a period of 1 h and 10 h in the temperature range 500-1200°C. In the scale we find NiO, the cubic and monoclinic modifications of ZrO2, and also Ni and Ni5Zr. The phase components are nonuniformly distributed over the thickness of the scale. The outer scale consists of the oxides NiO and ZrO2, while the composition of the inner scale includes Ni and Ni5Zr in addition to monoclinic ZrO2. Cubic ZrO2 is formed on the surface of the specimen in the initial stages of its oxidation at 500-700°C. For T ≥ 900°C, on the surface of the scale we find both modifications of ZrO2, while the nickel phase is itself a solid solution Ni(Zr). We note that the mechanisms for the formation of low-temperature (T ≤ 800°C) and high-temperature (T ≥ 900°C) scales are different. It is hypothesized that these differences are determined mainly by the fact that at high temperatures, diffusion of zirconium ions toward the outer boundary of the scale is superimposed on diffusion of oxygen toward the scale – alloy boundary.

Electron microscopy investigations on structures of ZrO2‐rich phases in the quasibinary system ZrO2‐Zr3N4

AbstractSlowly cooled nitrided zirconia samples were investigated and characterized by means of transmission electron microscopy (TEM). Besides the well‐known different modifications of ZrO2 some ZrO2‐rich oxynitride phases could be further found, called β‐type phases. An overview regarding the formation of such different structural modifications could be gained, supported by previously performed measurements using powder X‐ray diffractometry (XRD). Similar to fast cooled nitrided zirconia samples Zr(N,O), the slowly cooled ones also contain different kinds of ZrO2 precipitates, which can be obviously emphasized by using the methods of diffraction contrast. A super cell was successively built, derived from the unit cells of the β and β′ phase, in order to explain the structure of the observed modulated β″ phase. Agreements from the comparisons between experimental high resolution images and the simulated ones of such modulated structures confirm the suggested starting points. (© 2004 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Structure of the Active Sites on H3PO4/ZrO2 Catalysts for Dimethyl Carbonate Synthesis from Methanol and Carbon Dioxide

The catalytic properties of H3PO4/ZrO2 catalysts for dimethyl carbonate (DMC) synthesis from CH3OH and CO2 were investigated. The modification of ZrO2 with H3PO4 promoted the activity for selective DMC formation. The characterization of H3PO4/ZrO2 catalysts was performed by means of BET, XRD, 31P MAS NMR, LRS, DRIFT, and NH3-TPD. From the characterization results, it was determined that ZrO2 had Lewis acid sites and did not have Bronsted acid sites. The presence of Bronsted acid sites as well as Lewis acid sites was observed on H3PO4/ZrO2 (P/Zr = 0.05). The acidity of the Bronsted sites is weak, and the site's structure is a bridged OH. Furthermore, this Bronsted site is formed by the interaction between tetragonal Zr and P. In the reaction scheme of DMC formation, the Bronsted acid site can contribute to CH3OH activation and to the enhancement of the catalytic activity.

Optical Properties of ZrO2–Containing Anodic Coatings on Aluminum

Anodic oxide coatings (AOC) formed at the potentials of spark breakdowns in aqueous solutions of potassium hexafluorozirconate (K2ZrF6) contain up to 36 wt.% of Zr. The modification of ZrO2 in AOC is tetragonal. The reflection coefficients of AOC have been measured in the range 430–670 nm. After UV irradiation for a few hours the reflection coefficient decreases by several percent. The treatment of AOC in a solution containing F− and [Fe(CN)]3− ions leads to a change in the aluminum and zirconium concentrations in the surface layer and to an increase of the AOC reflectivity.

Approximating the Liquidus Surface of the ZrO2 ― Y2O3 ― La2O3 Phase Equilibrium Diagram with Reduced Polynomials

A mathematical model of the liquidus surface of the ZrO2 ― Y2O3 ― La2O3 phase equilibrium diagram was constructed by the use of reduced polynomials. It was established that the liquidus surface of the ternary system includes five primary solidification fields: solid solutions based on the cubic (C) and hexagonal (H) modifications of Y2O3, cubic modification of ZrO2 with the fluorite structure (F), high-temperature cubic modification of La2O3 (X), and ordered phase La2Zr2O7 with a pyrochlor structure (Py). Three four-phase invariant equilibria of the eutectic and peritectic types exist in the ternary system.

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 %

Ferroelasticity of t‘‐Zirconia: I, High‐Voltage Electron Microscopy Studies of the Microstructure in Polydomain Tetragonal Zirconia

The microstructure of polydomain tetragonal zirconia (t‐ZrO2), i.e., a ZrO2 modification exhibiting ferroelastic behavior, is studied by high‐voltage electron microscopy. This material consists of three domain variants of the tetragonal phase with their c‐axes nearly orthogonal to each other. Always two variants of these platelike domains are alternately arranged, forming elongated regular colonies. Hence, in both variants the common habit plane of the domains is a {110} twin plane. The colonies are of columnar shape with a longitudinal axis. They are bound by {110} planes, too, which are twin planes for the domains in the contiguous colonies. Owing to their particular structure and the helical arrangement of the adjoining colonies, the material remains coherent and pseudocubic over large macroscopic regions, although it is formed by different tetragonal domains.

Refractory articles based on the ZrO2-Y2O3(Nd2O3)-Cr2O3 systems

The nature of interactions in compositions of stabilized ZrO2—rare-earth chromite at 1750°C and the technical properties of specimens of zirconia-chromite refractories are investigated. It is shown that the use of induction melting for stabilizing the cubic modification of ZrO2 is very effective if the material is intended for the production of conducting ceramics with high thermomechanical characteristics.

Formation of the phases and structure in mullite-ZrO2 and aluminomagnesia spinel-ZrO2 system materials hardened from the molten state

The influence of cooling method in crystallization from the molten state on the phase relationship and features of the microstructure in mullite-ZrO2 and spinel-ZrO2 systems was studies by x-ray diffraction and differential thermal analysis methods and by investigations of the microstructure in reflected light. It was shown that an increase in hardening rate promotes formation of a glassy phase especially in the mullite-ZrO2 system and formation of the tetragonal and cubic modifications of ZrO2 even without addition of a stabilizer.

Surface oxidic phases of binary and ternary zirconia-supported metal catalysts investigated by Raman spectroscopy

The prevailing zirconia modification in a series of metal/ZrO2 catalysts has been investigated by Raman spectroscopy. Activation of an amorphous Cu70Zr30 alloy, or coprecipitation of zirconium dioxide with either copper, nickel or silver oxides at constant pH results in amorphous solids containing varying fractions of monoclinic or tetragonal zirconia. The cubic ZrO2 modification is not present in significant concentrations according to the Raman results. For Cu/ZrO2, strong Raman signals of Cu2O are detected, which indicate an enrichment of this compound on the catalyst surface. Copper and nickel ions apparently stabilize the tetragonal zirconia modification, whereas in the presence of palladium or silver, the formation of monoclinic zirconia domains is favoured. In ternary systems, the addition of 5 at.% of either Ag or Pd results in pronounced changes in the structure of Cu/ZrO2: Raman signals of monoclinic zirconia are observed for Ag(5%)/Cu(45%)/ZrO2 and Pd(5%)/Cu(45%)/ZrO2, and at the most only traces of surface Cu2O are detected.

Zirconium dioxide based hollow granules

Zirconium-based hollow spherical granules are promising materials that can be used as fillers when producing refractory thermal insulation products. The investigated granules (that were obtained according to the method of blowing through a melt) are formed mainly by the monoclinic modification of ZrO2. Addition of Y2O3 as a stabilizer of ZrO2 in the form of a mechanical mixture did not give the desirable effect.

Shock synthesis of the cubic modification of ZrO2

Shock synthesis of the cubic modification of ZrO2

Crystal structure of oxides in internally oxidized alloys based on niobium and molybdenum

The paper presents the results of investigations on the crystal structure of zirconium and hafnium oxides in internally oxidized niobium and molybdenum alloys. It has been shown that the crystal structure of the oxides in the low-temperature range depends on the particle size. Particles of a size 2r>20 nm have a monoclinlc lattice, in accordance with the phase diagram. When the particle size is 2r<4 nm hct modifications of ZrO2 and HfO2 are observed, which is at variance with the phase diagram. In the range 4<2r<20 nm previously unknown orthorhombic modifications of the oxides were discovered. The change in the crystal lattice of the oxides as the particle size decreases is due exclusively to scale effects and can be explained by taking account of the contribution of the surface energy to the free energy of the particles.