ZrO2 Solid Solution Research Articles

Fabrication of novel ZrO2(Y2O3)–Al2O3 ceramics having high strength and toughness utilising pulsed electric current pressure sintering (PECPS)

Pulsed electric current pressure sintering (PECPS) makes it possible to produce novel monolithic and composite materials with well controlled microstructure or nanostructure, which has not been realised before. ZrO2 based ceramics containing 25 mol.-%Al2O3 and 0·90–1·125 mol.-%Y2O3, i.e. ZrO2(1·2–1·5 mol.-%Y2O3)–25 mol.-%Al2O3, revealing high bending strength σb≧1 GPa and high fracture toughness KIC ≧15·5 MPa m1/2 simultaneously, have been fabricated by PECPS of cubic ZrO2 solid solution (ss) powders containing both Al2O3 and Y2O3 prepared via a sol–gel process. Tetragonal ZrO2 (t-ZrO2) phase composite ceramics sintered at 1573 to 1623 K (1300–1350°C) for 10 min under 60 MPa in Ar were composed of ∼φ200 nm grains with high relative density ≧97·5%. Transmission electron microscopy observation cleared that these mechanical properties might originate from homogeneous distribution of α-Al2O3 particles in the dense t-ZrO2 matrix; the precipitation of α-Al2O3 could be achieved by adopting the ss powders and PECPS.

Structural stability and electrochemical properties of Gd-doped ZrO2 nanoparticles prepared by sol–gel

Cubic, tetragonal and monoclinic Gd-doped zirconia nanoparticles with nominal composition GdxZr1−xO2 in the range 0 ≤ x ≤ 0.2, were prepared by annealing dried gels of Gd-containing zirconia at temperatures over the range between 450 and 1,300 °C. The synthesized zirconia-based nanoparticles with increased gadolinium load were characterized by X-ray powder diffraction, infrared and Raman spectroscopies, and transmission electron microscopy. The stabilization of the crystalline forms of Gd-doped ZrO2 solid solutions depends on the amount of Gd dopant and the annealing temperature. For low Gd loads in GdxZr1−xO2 being x < 0.05, the tetragonal form is the single phase up to 1,100 °C, whereas the monoclinic is the crystalline form detected up to 1,300 °C. Within the range of compositions 0.05 ≤ x < 0.1, is the tetragonal the only stabilized zirconia crystalline structure over the whole range of temperature up to 1,300 °C. For higher Gd-contents, in the range 0.1 ≤ x ≤ 0.2, is the cubic zirconia form the only stable phase for the whole range of annealing temperatures. Solid-state electrochemistry of the gadolinium-doped zirconia performed by the voltammetry of microparticles approach allowed distinguishing different electrochemical answers of Gd cation associated with slightly different local coordination surrounding of cations. Enantioselective electrocatalytic effect of monoclinic Gd-doped ZrO2 on the oxidation of l-(+)-tartaric acid and d-(−)-tartaric was also studied.

Nanometric Colloidal Sols of CeO2–ZrO2 Solid Solution as Catalyst Modifiers. II. TWC Performance

Abstract A stable colloidal sol consisting of CeO2–ZrO2 solid solution (CZ) was studied as an efficient modifier for supported Rh catalysts. After coating CZ sol onto Al2O3 by simple impregnation, the catalytic activity for CO–NO–C3H6–O2 reaction increased to a different extent depending on an air-to-fuel weight ratio (A/F). A significant effect on the activity arises largely from water-gas shift reaction and C3H6 steam-reforming in a rich region (A/F &amp;lt; 14.6), and promoted C3H6–O2 reaction in a lean region (A/F &amp;gt; 14.6). Moreover, CZ coating yielded an oxygen storage capacity (OSC), the effect of which could be highlighted in the catalytic reaction under a dynamic lean–rich perturbation.

Nanometric Colloidal Sols of CeO2–ZrO2 Solid Solution as Catalyst Modifiers. I. Preparation and Structure

Abstract Stable nanometric colloidal sols consisting of fluorite-type metal oxides were prepared for use as chloride-free precursors for automotive catalysts. The aqueous solution containing metal carbonates and tetramethylammonium (TMA) ions was degassed and hydrothermally treated at 140–160 °C. Colloidal suspensions thus formed were purified by ultrafiltration to yield very stable and condensed sol solutions of CeO2 (CE), ZrO2 (ZR) and their solid solutions, CeO2–ZrO2 (CZ), which were stabilized by TMA. Each oxide sol contained 6–8 nm primary particles, but they formed aggregates larger than 40 nm. Local structural analysis by means of XAFS suggested that CZ sol exhibited two types of solid solution domains containing more or less Ce. The dried CZ sol exhibited higher thermal stability and oxygen storage capacity than CE and ZR sols.

Effect of lattice defects on thermal conductivity of Ti-doped, Y2O3-stabilized ZrO2

The evolution of lattice structure and thermal conductivity has been studied systematically for a range of Ti-doped, Y2O3-stabilized ZrO2 (YSZ) solid solutions. The mechanism of reducing the thermal conductivity by Ti doping has been determined. Ti4+ mainly substitutes for Zr4+ below a critical composition factor (x⩽0.08), above which the interstitial Ti4+ need to be considered separately. The effect of lattice defects caused by mass and radius differences between Ti4+ and Zr4+ ions on the phonon scattering coefficient was discussed quantitatively. And the reduction of oxygen vacancy by interstitial Ti4+ ions which increases the thermal conductivity at high Ti doping content was also determined. Concerning the integrated phase stability and thermo-mechanical properties, Ti-doped YSZ is believed to be a promising candidate for thermal barrier coatings at higher temperature.

Experimental Investigation and Thermodynamic Modeling of the ZrO2–MgO System

a b s t r a c t Solid state phase equilibria of the ZrO2–MnOx system as well as the equilibria including liquid were investigated at air and argon atmospheres in the whole compositional range using high temperature DTA, X-ray diffraction (XRD) and scanning electron microscopy combined with energy dispersive X-ray spectroscopy (SEM/EDX). The influence of the oxygen partial pressure on solubility of Mn in the c-ZrO2 phase, on the c-ZrO2 low temperature limit as well as on the L c-ZrO2 + MnOx eutectic composition was shown. The thermodynamic parameters of the liquid phase as well as of the ZrO2 solid solutions were assessed by CALPHAD method using the obtained experimental results. The calculated phase diagrams of the ZrO2–MnO system at air (0.21 bar) and Ar (1·10 −4 bar) conditions were compared with the experimental results obtained in this work and those available in the literature. The calculated results were found to be in reasonable agreement with the experimental data.

Effects of porous C/C density on the densification behavior and ablation property of C/C–ZrC–SiC composites

C/C–ZrC–SiC composites were prepared by precursor infiltration and pyrolysis process using a mixture solution of organic zirconium-containing polymer and polycarbosilane as precursors. Porous carbon/carbon (C/C) composites with density of 0.92, 1.21 and 1.40g/cm3 were used as preforms, and the effects of porous C/C density on the densification behavior and ablation resistance of C/C–ZrC–SiC composites were investigated. The results show that the C/C preforms with a lower density have a faster weight gain, and the obtained C/C–ZrC–SiC composites own higher bulk density and open porosity. The composites fabricated from the C/C preforms with a density of 1.21g/cm3 exhibit better ablation resistance with a surface temperature of over 2400°C during ablation. After ablation for 120s, the linear and mass ablation rates of the composites are as low as 1.02×10−3mm/s and −4.01×10−4g/s, respectively, and the formation of a dense and continuous coating of molten ZrO2 solid solution is the reason for their great ablation resistance.

中和共沈法で調製したZrO2(Y2O3)–Al2O3 系固溶体粉体のパルス通電加圧焼結法による 高強度・強靭性セラミックスの作製

ZrO2 composite ceramics containing 25mol%Al2O3 and a small amount of Y2O3, i.e., 75mol%ZrO2 (0.5～2.0mol%Y2O3: 0.5～2.0Y)–25mol%Al2O3 ceramics, have been fabricated by sintering cubic ZrO2 solid solution (ss) nano-powder prepared via the neutralization coprecipitation method. Their sintering was performed using a pulsed electric-current pressure sintering (PECPS) at 1300～1350˚C for 10 min under 60 MPa in Ar. Dense tetragonal-ZrO2 (ss) ceramics containing α–Al2O3 were composed of 135 − 150 nm grains with a high relative density ≥99.9%. They showed extreme high bending strength (σb≥1 GPa) and high fracture toughness (KIC≥15 MPa • m1/2) simultaneously. FE-SEM observation revealed that these mechanical properties were originated from homogeneous distribution of α–Al2O3 particles around the t–ZrO2 matrix; the precipitation of α–Al2O3 could be achieved by adopting both the (ss) powders containing α–Al2O3 and PECPS.

Combination of dry reforming and partial oxidation of methane on NiO–MgO–ZrO2 catalyst: Effect of nickel content

The effect of nickel content on NiO–MgO–ZrO2 catalysts was investigated. They were characterized by XRD, in-situ XRD, TPR and BET and tested in POM and DRM coupled reactions (1.5CH4:1CO2:0.25O2), to produce syngas (H2/CO), at 750°C for 7h. Among these catalysts the NiO-MgO–ZrO2 catalyst with 20wt.% nickel content showed the best activity and selectivity in the catalytic processes. The MgO–ZrO2 (MZ) solid solution and the interactions between Ni and MZ increased the rate of conversion of reactants (CH4 and CO2) into syngas on the catalysts. Although the Ni° active sites are required for high activity and selectivity, high nickel content favored the formation of large Ni° particles during the reforming reaction, thus lowering the catalytic performance. The combination of DRM and POM exhibited high conversion rates and selectivity on the NiMZ catalyst with 20wt.% Ni and 20mol% MgO, demonstrating that this coupled reaction can be performed efficiently and that problems arising from hot spots on the catalyst, due to POM, can be solved.

Electrical Conductivity and Phase Transition Studies in the ZrO&lt;sub&gt;2&lt;/sub&gt;-CdO System

The Solid Solution of ZrO2: Cdo Was Prepared by the Conventional Solid-State Reaction. Effect of Dopant Concentration on the Electrical Conductivity of ZrO2Was Studied for Different Compositions at Different Temperatures. the Conductivity Increases with the Addition of Cdo due to the Migration of Vacancies. the Conductivity Increases with Rise in Temperature up to 180°C and Thereby Decreases due to the Collapse of Fluorite Framework. A Second Rise in Conductivity at High Temperature beyond 460°C Is due to the Phase Transition of ZrO2from Monoclinic to Tetragonal. DSC, X Ray Powder Diffraction, Impedance Measurements and FTIR Spectral Studies Were Carried Out for Confirming the Doping Effect and Transitions in ZrO2. the Addition of Cdo to Zro2 Shifted the Phase Transition of ZrO2to Higher Temperatures as Confirmed by the DSC Results. the Occurrence of Single Semi Circle with a Low Frequency Inclined Spike Indicates that the Conductivity Is Mainly due to the Movement of Oxide Ions.

Lattice oxygen mobility and acidity improvements of NiO–CeO2–ZrO2 catalyst by sulfation for NOx reduction by ammonia

Different amounts of sulfates were introduced to NiO–CeO2–ZrO2 catalyst by a citric-aided sol–gel method for NOx reduction with ammonia. The catalysts were characterized by X-ray diffraction, Raman, H2 temperature programmed-reduction, NH3 temperature-programmed oxidation, DRIFTS of NH3/NO adsorption, oxygen storage capacity and activity measurements for selective catalytic reduction of NOx with ammonia. The results show that the interactions between sulfates and NiO–CeO2–ZrO2 solid solution lead to an enhanced Lewis acidity of the catalyst due to the electron withdrawing effect of sulfates mainly on Nin+. The addition of sulfates decreases the amount of readily available oxygen on the catalyst, but promotes the lattice oxygen mobility by the formation of Nin+OSn+ bond in CeO2–ZrO2 mixed oxides. The ammonia oxidation is inhibited by the reduced amount of surface active oxygen, while the adsorption of ammonia on the catalyst is promoted with the sulfate-derived acid sites. Meanwhile, the oxidation of nitrites to nitrates on the sulfated catalyst at the temperature interval for high NOx conversion is accelerated by the increased lattice oxygen mobility. These effects result in a high NH3-SCR activity and high selectivity to nitrogen over the sulfated NiO–CeO2–ZrO2 catalysts.

Ni catalyst on mixed support of CeO2–ZrO2 and Al2O3: Effect of composition of CeO2–ZrO2 solid solution on the methane steam reforming reaction

In this study, catalysts containing 5wt.% Ni deposited on a support composed of a CeO2–ZrO2 solid solution deposited on alumina were tested in the steam reforming of methane. The supports, with various ratios of Ce to Zr, were prepared by co-precipitation of the oxide precursors, followed by calcination in synthetic air. The catalysts were then prepared by Ni impregnation of the supports. The prepared solids were characterized by temperature-programmed reduction with H2 (TPR‐H2), in situ X-ray diffraction (XRD) and X-ray absorption near-edge structure (XANES) spectroscopy. The XRD analysis confirmed the formation of a solid solution between ZrO2 and CeO2. In the catalytic tests, it was found that catalysts with higher Ce content did not exhibit deactivation during 6h of reaction. The catalyst with highest Ce content, Ni(0.8Ce0.2Zr)Al, provided the best result, with the highest rate of conversion of methane and the lowest carbon deposition, which may be partly due to the smaller Ni0 crystallites in this sample and also the segregated CeO2 particles may have favored H2O adsorption which could lead to higher C gasification.

Temporal Evolution of Crystallization in MgO–Al2O3–SiO2–ZrO2 Glass Ceramics

From glasses with the composition 51.9 SiO2·21.2 MgO·21.2 Al2O3·5.7 ZrO2 (in mol %), high and low-quartz solid solution glass ceramics were formed upon heat treatment at 950 °C for different dwell times. A combination of X-ray diffractometry, transmission electron microscopy, and scanning transmission electron microscopy with annular dark field imaging and energy-dispersive X-ray analysis revealed the temporal evolution of the volume crystallization. It is found that the crystallization of high-quartz solid solution is nucleated either directly at star-shaped ZrO2 crystallites or within a zone of Zr-depleted glass surrounding the latter and proceeds isotropically into the sample volume, thereby expelling secondary, circular ZrO2 precipitates. With advancing annealing time, the quartz solid solution is depleted in Mg and Al since indialite (Mg2Al4Si5O18) and spinel (MgAl2O4) are formed as well, thus enabling a change from high-quartz solid solution to the low-quartz modification while sample cooling. Upon analyses of samples heat treated for different soaking times, different states of crystallization were observed. Using electron diffraction and nanoanalytical techniques, a growth model with special emphasis on the early stages of volume crystallization was developed.

Morphology and crystal structure of CeO2-modified mesoporous ZrO2 powders prepared by sol–gel method

Mesoporous ZrO2 powder modified by CeO2 has been prepared by sol–gel method combined with novel heat-treatment without any templates. The morphology and crystal structure were characterized by Fourier transform infrared spectrophotometer (FT-IR), thermal gravimetry/differential thermal analysis (TG/DTA), X-ray diffraction (XRD), Raman spectroscope, N2 absorption–desorption, scanning electron microscope (SEM) and high-resolution transmission electron microscope (HRTEM). It was demonstrated that the morphologies and crystal structures were varied with the CeO2 content and calcination temperatures. Some circular grooves with diameter about 300–500nm and thickness of 150–200nm were observed for 30mol% CeO2 modified samples. Tetragonal ZrO2 and cubic (Ce,Zr)O2 solid solution were in turn observed with CeO2 content increasing. The oxygen storage capacity (OSC) of the mesoporous powders was determined by thermalgravimetry (TG) under cyclic thermal treatments.

The Wear Mechanism Comparison between MgO-Based Chrome-Free Brick and MgO-Cr&lt;sub&gt;2&lt;/sub&gt;O&lt;sub&gt;3&lt;/sub&gt; Brick in the Lower Part of RH Vacuum Degasser

The damage and improvement mechanism of rebonded MgO-Cr2O3 brick and MgO-ZrO2 brick used in the lower part of RH vacuum degasser were studied. The results show that the main damage of rebonded MgO-Cr2O3 brick is structure spalling because of the penetration of slag. MgO-ZrO2 brick has characteristics of high direct-bonding degree, small air permeability and making the viscosity of penetrated slag increased by means of ZrO2 absorbing CaO in slag to form CaZrO3 or ZrO2 solid solution, which make the degree of slag penetration decrease and improve the structure spalling resistance. MgO-ZrO2 brick has the defect of poor thermal shock resistance in use, the incorporation of MgO-rich spinel into MgO-ZrO2 brick significantly improved both thermal shock resistance and hot modulus of rupture because of the formation of eutectoid structure of grain-refining spinel and zirconia, which also improved structure spalling resistance because air permeability was decreased to 5% of MgO-ZrO2 brick

Oxygen vacancy formation on the Ni/Ce0.75Zr0.25O2(111) surface. A DFT+U study

Ni/CeO2–ZrO2 solid solutions are well known by their promissory catalytic role in hydrogen production processes such as reforming and partial oxidation of hydrocarbons. CeO2–ZrO2 support can effectively disperse Ni particles and donate oxygen promoting Ni active phase resistance to poisoning by carbon deposition. Thus, in this work we study the influence of Ni on oxygen vacancy formation on the Ce0.75Zr0.25O2(111) surface, by using the density functional theory (DFT) approach with the Hubbard’s correction (U) for Ce(4f) electrons. We performed DFT+U energetic calculations on Ni/Ce0.75Zr0.25O2 (Ni is adsorbed over an O–O bridge site) and Ni–Ce0.75Zr0.25O2 (Ni is inserted into the slab) systems, with an O-vacancy. Moreover, we analyzed the electronic structures of these solids by drawing their spin polarized density of states (DOS) plots. Our results indicate that Ni insertion between two O subsurface layers facilitates Ce0.75Zr0.25O2 mixed oxide reduction and enhances its ionic mobility and oxygen donation properties.

Distribution of Additives in Single-Modified TiO2

ABSTRACTIn the present work the influence of different modifiers, calculated to B2O3, CeO2, Sb2O3, ZnO, and ZrO2, on their distribution in TiO2 is investigated. The phase composition and phase transformation of prepared rutile-TiO2 is determined by the selective leaching method, ICP-AES, XRD and FT-IR techniques. The addition of Sb2O3 to TiO2 has no influence on the anatase–rutile phase transformation, CeO2 and ZrO2 act as inhibitors of the TiO2 phase transformation and the addition of ZnO or B2O3 to TiO2 accelerates rutile formation. It is observed that boron is located in TiO2 in the form of soluble B2O3, zinc partly reacts with titanium forming co-phase TiZn2O4 and antimony addition to TiO2 presumably causing the formation of a co-phase of Sb with Ti. Cerium forms a separate phase, CeO2, and reacts partly with titanium, probably creating co-phase, CexTi(1-x)O2 (for example Ce0.8Ti0.2O2). Zirconium addition in TiO2 forms separate ZrO2 phase and solid solution of Zr with Ti.

Y2O3–MgO–ZrO2 Infrared Transparent Ceramic Nanocomposites

Increased strength and improved erosion resistance are required for future infrared windows working in harsher mechanical and thermal environments than those used today. In this study, Y2O3–MgO–ZrO2 composite nanopowders were synthesized using a sol–gel combustion process. The fully dense Y2O3–MgO–ZrO2 nanocomposites were prepared by pressureless sintering of the nanopowders at 1400°C for 2 h, followed by hot isostatic pressing at 1350°C for 1 h. The effects of ZrO2 concentration on the microstructure, infrared transmittance, and microhardness of the nanocomposites were studied. It is shown by X‐ray diffraction (XRD) and transmission electron microscopy (TEM) that ZrO2 is selectively soluble in Y2O3 with the concentration up to 13.2 mol%, forming supersaturated solid solution. The microhardness of the Y2O3–MgO–ZrO2 nanocomposites scales linearly with the square root of the ZrO2 concentration (from 10.6 GPa without ZrO2 to 13.5 GPa with 13.2 mol% ZrO2 addition). Good mid‐infrared transmittance (53%–70%) over 3–7 μm wavelength was maintained with ZrO2 concentration up to 8.8 mol%, compared with that of Y2O3–MgO nanocomposites (62%–80%) at the same wavelength range. This work shows ZrO2 solid solution strengthened Y2O3 (ZSSY) nanocomposites for their potential to be used as infrared transparent materials with enhanced mechanical properties.

Influence of CeO2 addition on crystal growth behavior of CeO2–Y2O3–ZrO2 solid solution

Nanopowders with cubic fluorite-type structure as well as uniform distribution in particle size were synthesized by hydrothermal method in the ternary oxide zirconia–yttria–ceria system with ceria content of 0–25mol%. X-ray diffraction (XRD), thermogravimetric analysis/differential scanning calorimeter (TG/DSC), Fourier transform infrared spectroscopy (FT-IR), Raman spectroscopy (Raman), specific surface area (SBET) and high resolution transmission electron microscopy (HRTEM) were applied to characterize the structure, thermal decomposition, morphological characteristic and crystal growth of the produced powders. Qualitative analyses indicate that the as-synthesized nanoparticles are single-phase crystallites with an average particle size of 4–9nm. The specific surface area, lattice parameter and microstrain are closely related to Ce4+ concentration. Moreover, activation energy of crystal growth is significantly dependent on the dopant (CeO2) concentration. It firstly increased and then decreased with increasing dopant concentration, and the maximum value was observed at the dopant concentration of 5mol%.

Direct Decomposition of NO into N2 and O2 on C-type Cubic Y2O3–ZrO2 and Y2O3–ZrO2–BaO

Abstract Catalytic activities for direct NO decomposition were investigated on C-type cubic Y2O3–ZrO2 and Y2O3–ZrO2–BaO prepared by coprecipitation. Introduction of excess oxide anions in the Y2O3 lattice was achieved by partial substitution of the Y3+ sites with Zr4+, and high NO decomposition activity was obtained for the (Y0.97Zr0.03)2O3.03 catalyst. In addition, the catalytic activity was further enhanced by partial substitution of the Y3+ sites in the Y2O3–ZrO2 solid solution with Ba2+, and the (Y0.89Zr0.07Ba0.04)2O3.03 catalyst exhibited the highest NO decomposition activity among the samples prepared; NO conversion to N2 reached 90% at 1173 K in the absence of O2 (NO/He atmosphere), and a relatively high conversion ratio was observed even in the presence of O2, H2O, or CO2, compared with the activities of conventional direct NO decomposition catalysts. These results indicate that the C-type cubic Y2O3–ZrO2–BaO catalyst is a new potential candidate for direct NO decomposition.