Zirconia-based nanocrystals in the ZrO2-In2O3 system

Zirconia-based ZrO₂-In₂O₃ nanocrystals are prepared by hydrothermal treatment of coprecipitated zirconium oxyhydroxide and indium hydroxide. Indium oxide is shown to dissolve predominantly in cubic zirconia nanocrystals. Its solubility in nanocrystalline zirconia notably exceeds the equilibrium In₂O₃ solubility in ZrO₂ single crystals.     

Phase Composition of Heat-Treatment Products in the ZrO(OH)2–Y(OH)3–FeOOH System

The phase composition of the heat-treatment products in the ZrO(OH)₂–Y(OH)₃–FeOOH system is determined as a function of the precipitation procedure and calcination temperature (620–1570 K) for the compositions 0.97ZrO₂· xY₂O₃· yFe₂O₃(x+ y= 0.03; x= 0, 0.01, 0.015, 0.02, 0.03) and (1 – x– y)ZrO₂· xY₂O₃· yFe₂O₃(x= y= 0.02, 0.025, 0.03, 0.04). At a given ZrO₂: stabilizer ratio, partial substitution of Fe³⁺for Y³⁺increases the degree of ZrO₂stabilization and retards the low-temperature degradation of the material.     

Effect of combined doping (y<Superscript>3 +</Superscript> + fe<Superscript>3 +</Superscript>) on structural features of nanodispersed zirconium oxide

Fully stabilized zirconium dioxide is widely used. One of the basic requirements to this material is the thermal stability of the structure. The most effective stabilizer for zirconium oxide is yttrium oxide. However, the structure of Y-ZrO₂ degraded at low temperature. Partial substitution of Fe^{3 +} for Y³⁺ decreases both the crystallization and sintering temperature of zirconia ceramic. The aim of present work is the investigation of structural peculiarities of zirconium oxide stabilized by combined dopant depending on chemical composition, synthesis conditions and heat treatment. The polymorphic composition of a ZrO₂-based materials has been determined in series of samples that correspond to the formula [1−(x+y)]ZrO₂⋅xY₂O₃⋅yFe₂O₃ in the temperature range 620–1570 K. It has been found that at the same molar ratio ZrO₂ : doping oxides, the degree of ZrO₂ stabilization increases, and the low-temperature degradation process is retarded by the partial substitution of Fe^{3 +} for Y³⁺. Nonequivalent sites of Fe^{3 +} ions have been identified: two with octahedral coordination for CPH and three with octa-, penta- and tetrahedral coordination for SPH. The possibility of cluster distribution of Fe³⁺ ions and the dependence of the number of vacancies on synthesis conditions have been shown.     

Growth of whiskered ZrO2 crystals by hydrothermal decomposition of zirconium oxide sulphate pseudo-crystals

Several kinds of metastable compounds, pseudo zirconium oxide sulphates (PZOS) with a chemical composition of Zr₃O₅SO₄·nH₂O, were previously synthesized by the thermal hydrolysis of solutions containing zirconium sulphate at 200 or 240°C. The obtained PZOS samples were again hydrothermally treated in different sulphuric acid solutions (<1.0 mol l^-1) at 240°C, and their hydrothermal decomposition behaviour was investigated by TEM observation. The PZOS samples mostly crystallized to plate-like zirconium oxide sulphate (ZOS) in the concentrated sulphuric acid solution (>0.5 mol l^-1), but long-whiskered monoclinic ZrO₂ crystals grew with decomposition of the PZOS samples obtained from the starting mixtures with added Zr(OH)₄ when rapidly heated to the hydrothermal treatment temperatures. It was found that many ultrafine monoclinic ZrO2 crystals were simultaneously formed during the hydrothermal preparation of the PZOS samples, and during the following hydrothermal decomposition of the PZOS samples, the whiskered crystals of monoclinic ZrO2 grew with the consumption of PZOS from the coexisting ultrafine monoclinic ZrO2 particles which act as seed crystals. This revised version was published online in November 2006 with corrections to the Cover Date.     

Mössbauer Study of Tetragonal ZrO2–Y2O3–Fe2O3 Solid Solutions

Tetragonal Zr_0.886Y_0.057Fe_0.057O_{2 –} solid solutions prepared by calcining coprecipitated and successively precipitated hydroxide mixtures were studied by Mössbauer spectroscopy immediately after calcination and after long-term storage. The results indicate that the solid solutions prepared via coprecipitation and successive precipitation contain Fe³⁺ in two (octahedral coordination) and three (octahedral, fivefold, and tetrahedral coordinations) inequivalent sites, respectively. Partial Fe³⁺ substitution for Y³⁺ is shown to prevent or substantially slow down the low-temperature structural degradation of stabilized zirconia.     

Preparation of new zirconium phosphates by solvothermal reaction

Two types of new zirconium phosphates, [enH₂]Zr(OH)(PO₄)(HPO₄) (en; ethylene diamine) and (NH₄)₅[Zr₃(OH)₉(PO₄)₂(HPO₄)] were prepared under solvothermal condition using diethylene glycol as a solvent and their crystal structures were determined by using single crystal X-ray diffraction data. The former compound has the layer structure similar to that of γ-Zr(PO₄)(H₂PO₄) · 2H₂O, and protonated ethylene diamines were located in the interlayer space. At elevated temperatures, this compound decomposed by releasing protonated ethylene diamines and finally changed to ZrP₂O₇. The interlayer space was soft-chemically inactive unlike α-Zr(HPO₄)₂ · H₂O and γ-Zr(PO₄)(H₂PO₄) · 2H₂O. The later compound has the tunnel structure built up by corner-sharing ZrO₆ octahedra and PO₄ tetrahedra, and NH₄ ⁺ ion was located in the tunnel.     

On the stabilizing behavior of zirconia: A Combined experimental and theoretical study

Reactive zirconia powder was synthesized by the complexation of zirconium metal from zirconium hydroxide using a solution of 8-hydroxiquinoline. The kinetics of zirconia crystallization was followed by X-ray diffraction, scanning electron microscopy and surface area measured by the nitrogen adsorption/desorption technique. The results indicated that zirconia with a surface area as high as 100 m²/g can be obtained by this method after calcination at 500°C. Zirconia presents three polymorphic phases (monoclinic, tetragonal and cubic), which are reversibly interconversible. The cluster model Zr₄O₈ and Zr₄O₇ ⁺² was used for a theoretical study of the stabilization process. The ab initio RHF method was employed with the Gaussian94 program and the total energies and the energy gap of the different phases were calculated and compared with the experimental energy gap. The theoretical results show good reproducibility of the energy gap for zirconia.     

Investigation of phases and stability of solid electrolytes in the NASICON system

In order to study the phase change and stability of the NASICON structure, sodium, lithium and magnesium ions were chosen to substitute the zirconium ion at octahedral sites in the NASICON network. It was found that the zirconium ion can not be replaced by these ions. All the synthesized products are Na_1+xZr₂Si_xP_3?xO₁₂ and phosphate salts. NASICON immersed in liquid sodium at 300°C also results phosphate salt and ZrO₂. It was found that an appropriate excess of sodium phosphate in NASICON will improve the chemical stability, corrosion against sodium and mechanical properties.     

Crystallization of CaHf1? x Zr x Ti2O7 (0?≤ x ≤?1) zirconolite in SiO2–Al2O3–CaO–Na2O–TiO2–HfO2–ZrO2–Nd2O3 glasses

Glass-ceramics containing (Hf,Zr)-zirconolite crystals (nominally CaHf_1−x Zr _x Ti₂O₇ with 0 ≤ x ≤ 1) were envisaged to immobilize minor actinides and plutonium. Such materials were prepared in this study by controlled crystallization of glasses belonging to the SiO₂–Al₂O₃–CaO–Na₂O–TiO₂–HfO₂–ZrO₂–Nd₂O₃ system. Neodymium was used as trivalent actinides surrogate. The effect of total or partial substitution of ZrO₂ by HfO₂ (neutron poison for fission reactions) on glass crystallization in the bulk and near the surface is presented. It appeared that Hf/Zr substitution had not significant effect on nature, structure, and composition of crystals formed both on glass surface (titanite + anorthite) and in the bulk (zirconolite). This result can be explained by the close properties of Zr⁴⁺ and Hf⁴⁺ ions and by their similar structural role in glass structure. However, strong differences were observed between the nucleation rate I_Z of zirconolite crystals in glasses containing only HfO₂ and in glasses containing only ZrO₂. Hf-zirconolite (CaHfTi₂O₇) crystals were shown to nucleate only very slowly in comparison with Zr-zirconolite (CaZrTi₂O₇) crystals. Composition changes - by increasing either HfO₂ or Al₂O₃ concentration or by introducing ZrO₂ in parent glass - were performed to increase I_Z in hafnium-rich glasses. The proportion of Nd³⁺ ions incorporated in the zirconolite phase was estimated using ESR.     

Combustion synthesis and properties of nanostructured ceria-zirconia solid solutions

Nanostructured ceria-zirconia solid solutions (Ce_{1 − x}Zr_xO₂, x = 0 to 0.9) have been synthesized by a single step solution combustion process using cerous nitrate, zirconyl nitrate and oxalyl dihydrazide (ODH) / carbohydrazide (CH). The as-synthesized powders show extensive XRD line broadening and the crystallite sizes calculated from the XRD line broadening are in the nanometer range (6–11 nm). The combustion derived ceria zirconia solid solutions have high surface area in the range of 36–120 m²/g. Calcination of Ce_{1 −x}Zr_xO₂ at 1350 °C showed three distinct solid solution regions: single phase cubic (x ≤ 0.2), biphasic cubic-tetragonal (0.2 < x 0&#x030C;.8) and tetragonal (x > 0.8). When x ≥ 0.9, the metastable tetragonal phase formed transforms to monoclinic phase on cooling after calcination above 1100 °C. The homogeneity of Ce_{1 − x}Zr_xO₂ has been confirmed by EDAX analysis. The Temperature Programmed Reduction (TPR) measurement of Ce_0.5Zr_0.5O₂ was carried out with H₂ and the TPR profile showed two water formation peaks corresponding to the utilization of surface and bulk oxygen.     

Influence of ZrO<Subscript>2</Subscript> addition on the structure,thermal stability,and dielectric properties of ZnTiO<Subscript>3</Subscript> ceramics

The zinc titanates doped with zirconium were synthesized by conventional solid-state reaction using metal oxides. X-ray diffractometry and differential scanning calorimetry analysis results indicated that the stable region of the hexagonal Zn(Zr _x Ti_1−x )O₃ phase extended to a high temperature (above 945 °C). The c/a value decreased as the Zr concentrations increased, which may be caused by the Zr⁴⁺ addition resulting in a shorter distance between the center ion and its nearest neighbors of the octahedron, and the bonding force between the B-site ion and oxygen ion of ABO₃ perovskite-like structure becoming stronger. The dielectric properties exhibited a significant dependence on the sintering temperatures and the amount of ZrO₂ addition. The dielectric constant decreased and Curie temperature (T _c) increased slightly with the increasing amounts of Zr ions. This is caused by the second phase of ZnZrO₃ which was deposited at the grain boundaries and inhibited the grain growth. Furthermore, diffuse phase transition with a maximum permittivity at a transition temperature that is close to room temperature in Zn(Zr _x Ti_1−x )O₃ was observed.     

Structure, composition and electrical properties of YSZ films deposited by ultrasonic spray pyrolysis

Yttria-stabilized zirconia (YSZ) films with different yttria concentrations were prepared by ultrasonic spray pyrolysis on Si substrates at 525 °C, using solutions of zirconium and yttrium acetylacetonates in methanol. The chemical composition, structure and electrical properties of the films were studied by X-ray photoelectron spectroscopy (XPS), X-ray diffraction (XRD), and electrochemical impedance spectroscopy (EIS). XPS measurements show that the Y content in the films increases as the Y precursor in the solution increases. Carbon incorporation was also found in the films, although the concentration of this impurity was reduced as the incorporation of Y increased. XRD spectra show that the Zr_1−x Y _x O_2−x/2 polycrystalline films have the cubic phase of ZrO₂ and fully stabilized 8YSZ (8 at.% Y₂O₃ + 92 at.% ZrO₂), and that their lattice constant increases slightly as the Y content increases. The conductivity of all the as-deposited films as a function of temperature, showed an Arrhenius behavior, and with the exception of the film with the maximum Y content, the activation energies were in the range of 0.98–1.11 eV. The ionic conductivity of one of these films was similar to that measured for a pellet made of the 8YSZ standard powder.     

Characterization of sol-gel derived TiO2/ZrO2 films and powders by Raman spectroscopy

The effect of zirconium dioxide addition on crystal structure of sol-gel TiO₂ mesoporous films and powders has been investigated by means of Raman spectroscopy, X-Ray diffraction, and Atomic force microscopy. Zirconium incorporation (up to 30 mol%) into TiO₂ lattice resulted in the formation of Ti_1 − xZr_xO₂ solid solution with anatase structure for the binary powders has been proved. Appearance of tetragonal ZrO₂ phase was observed for the samples with high zirconium content.     

Effect of high-energy milling on hydrogen desorption and the structure of mixtures of Zr-containing materials

Mass-spectrometric investigations of hydrogen liberation from the mixture of powders 3ZrO_0.2H_x + ZrFe₂, the overall composition of which corresponds to Zr₄Fe₂O_0.6H_x, after its treatment by ball milling for 5 and 50 h show a substantial dependence of hydrogen desorption on the time of mechanical treatment. X-ray phase analysis demonstrates a significant disordering of the structure of components after high-energy ball milling. The combination of mechanical treatment and hydrogen desorption from the prepared mixtures does not lead to the formation of the Zr₄Fe₂O_0.6 intermetallic phase as was observed during desorption-recombination in the HDDR process. __________ Translated from Fizyko-Khimichna Mekhanika Materialiv, Vol. 41, No. 6, pp. 55–59, November–December, 2005.     

Catalytic properties of NiSO4/ZrO2 promoted with Fe2O3 for acid catalysis

A series of catalysts, NiSO₄/Fe₂O₃–ZrO₂, for acid catalysis were prepared by the impregnation method, where support, Fe₂O₃–ZrO₂ was prepared by the co-precipitation method using a mixed aqueous solution of zirconium oxychloride and iron nitrate solution followed by adding an aqueous ammonia solution. No diffraction line of nickel sulfate was observed up to 20 wt.%, indicating good dispersion of nickel sulfate on the surface of Fe₂O₃–ZrO₂. The addition of nickel sulfate (or Fe₂O₃) to ZrO₂ shifted the phase transition of ZrO₂ from amorphous to tetragonal to higher temperature because of the interaction between nickel sulfate (or Fe₂O₃) and ZrO₂. 15-NiSO₄/Fe₂O₃–ZrO₂ containing 15 wt.% NiSO₄ and 5 mol% Fe₂O₃, and calcined at 700 °C exhibited maximum catalytic activities for both reactions, 2-propanol dehydration and cumene dealkylation. The catalytic activities for both reactions were correlated with the acidity of catalysts measured by the ammonia chemisorption method. The addition of Fe₂O₃ up to 5 mol% enhanced the acidity, thermal property, and catalytic activities of NiSO₄/Fe₂O₃–ZrO₂ gradually due to the interaction between Fe₂O₃ and ZrO₂ and consequent formation of FeOZr bond.     

Iron-stabilized nanocrystalline ZrO2 solid solutions: Synthesis by combustion and thermal stability

The synthesis of Fe³⁺-stabilized zirconia by the nitrate/urea combustion route was investigated. Using several characterization techniques, including X-ray diffraction, field-emission-gun scanning electron microscopy and notably Mössbauer spectroscopy, it was possible to determine the appropriate amount of urea that allows to obtain a totally stabilized Zr_0.9Fe_0.1O_1.95 solid solution. The nanocrystalline zirconia solid solution is mostly tetragonal, but the presence of the cubic phase could not be ruled out. An in-depth study of the thermal stability in air showed that the Fe³⁺ solubility in the stabilized solid solution starts to decrease at about 875 °C which results in the formation of hematite (possibly containing some Zr⁴⁺) at the surface of the zirconia grains and further provokes the progressive transformation into the monoclinic zirconia phase.     

Influence of some precipitation variables on thermal behavior of ZrO2-Y2O3 and ZrO2-CeO2 precipitated gels

The scope of this work consisted of producing pure zirconia powders, yttria stabilized zirconia powders and ceria stabilized zirconia powders from zirconium oxychloride and yttrium and cerium chlorides using urea as precipitating agent and polyacrylic acid as dispersing agent. A factorial analysis was designed to study the effects of some precipitation variables (precipitation temperature, precipitation time, urea concentration, yttria concentration, ceria concentration and polyacrylic acid concentration) on the thermal behavior of zirconia gels using differential scanning calorimeter (DSC). It was observed that the addition of yttrium and cerium in the solution raised the crystallization temperature of the gels, due to changes on the characteristics of the Zr–OH bonds caused by the substitution of zirconium by yttrium or cerium. For the ZrO₂-Y₂O₃ system, the presence of polyacrylic acid increased the crystallization temperature. It was suggested that polyacrylic acid promoted the formation of a random polymeric structure of zirconia gels that required higher temperatures to crystallize.     

The influence of mechanochemical treatment of the Bi2O3–ZrO2 system on the structural and dielectric properties of the sintered ceramics

A powder mixture of -Bi₂O₃ and ZrO₂, both monoclinic, in the molar ratio 2 : 3, was mechanochemically treated in a planetary ball mill in an air atmosphere for up to 20 h, using steel vial and hardened-steel balls as the milling medium. Mechanochemical reaction leads to the gradual formation of an amorphous phase. After 5 h of milling the starting -Bi₂O₃ and ZrO₂ were transformed fully into a non-crystalline phase. After milling for various times the powders were compacted by pressing and isothermal sintering. The pressed and sintered densities depended on the milling time. Depending on the duration of the mechanochemical treatment and sintering temperature, the phases: -Bi₁₂(Zr _x Fe_1–x )O₂₀; Bi(Zr _x Fe_1–x )O₃ and Bi₂(Zr _x Fe_1–x )₄O₉ were obtained by reactive sintering, whereby the Fe originates from vial and ball debris. The dielectric permittivity of the sintered samples significantly depends on the milling time. Samples milled for 10 and 15 h and subsequently sintered at 800 °C for 24 h exhibit a hysteresis dependence of the dielectric shift (in altering electric fields higher than 10 kV/cm at room temperature), confirming that the synthesized materials possess ferroelectric properties.     

Magnetic and Electronic Properties of Point Defects in ZrO2

Using the Korringa–Kohn–Rostoker Coherent Potential Approximation (KKR-CPA) method in connection with the Generalized Gradient Approximation (GGA), we study the magnetic and electronic properties of different point defects in cubic ZrO₂. In particular, we discuss the zirconium interstitial (Zr_i), zirconium antisite (Zr_O), zirconium vacancy (V_Zr), oxygen interstitial (O_i), oxygen antisite (O_Zr), and oxygen vacancy (V_O) defects. It has been shown that oxygen vacancy and zirconium interstitial (V_O, Zr_i) are n-type, while the other point defects are p-type. The magnetic moments are observed only in the oxygen interstitial and antisite (O_i, O_Zr) cases. The corresponding ferromagnetic states are more stable than the spin–glass states. It has been found that the mechanism responsible of such stabilities is the double exchange. Based on Mean Field Approximation (MFA), the Curie temperature (T _C ) is estimated. Moreover, it has been found that the O_i and O_Zr defects provide half-metallic properties being the responsible for ferromagnetism.     

Synthesis and characterization of low density calcia stabilized zirconia ceramic for high temperature furnace application

Five weight percent calcia stabilized zirconia (CSZ) low density ceramic material was synthesized from zirconia and calcia powder by solid state reaction. Powder specimens were sintered at various temperatures for different time durations in air and argon atmosphere. Physico-chemical properties such as thermal stability, purity, crystalline phases identification, particle size, particle size distribution, specific surface area and porosity of CSZ were measured by thermogravimetry and differential thermal analysis (TG/DTA), X-ray diffraction (XRD), scanning electron microscopy (SEM), Brunauer–Emmett–Taller (BET) and particle size analyzer (PSA). Total relative shrinkage and coefficient of thermal expansion were estimated by dilatometry. The density and Vicker's microhardness were measured in order to evaluate the material performance. A combination of cubic zirconia and calcium zirconium oxide (Ca_0.15Zr_0.85O_1.85) phases were found when specimen sintered at 1500 °C in air while under argon atmosphere at 1750 °C and 1850 °C, the ZrO and Ca_0.15Zr_0.85O_1.85 phases were established. At higher sintering temperature (1950 °C) for 4 h in argon medium three phases ZrO, CaZrO₃ and Ca_0.15Zr_0.85O_1.85 were observed. It was found that with the increase of sintering time these phases convert into a single cubic phase Ca_0.15Zr_0.85O_1.85. Pellets sintered at different temperatures and duration were also studied by scanning electron microscopy showing that the grain structure becoming fine with increase of temperature and duration.