ZrO2 Matrix Research Articles

Retention behaviours of carbon-containing species in as-prepared, water-treated, and transition-metal-contaminated ZrO2 gels

Retention behaviours of carbon-containing species in ZrO2 gels derived from the Zr-n-propoxide-acetylacetone-water-isopropanol system have been studied by the Fourier transform infrared (FT-IR) pellet technique. The carbon retention patterns based on evolution of FT-IR spectra at different calcination temperatures for ZrO2 gels are examined in detail. It is found that the carbon retention behaviours for as-prepared gels and for thermal, water and transition metal (Ni2+) pre-treated ZrO2 gels differ. For most captured carbon species, for example, solvent isopropanol and additive acetylacetone, carbon can be depleted at a relatively low temperature of about 400°C. However, for lattice-included species, especially those incorporated into the lattice during the phase transition, a temperature around 900°C is needed to eliminate carbon in the gels. In the study of the effect of moisture on gel calcination, it is found that water treatment alone does not provide new routes for carbon retention in as-prepared ZrO2 gels. However, when gels are pre-treated with Ni2+ and water, new forms of carbon-containing species can be observed. These forms are attributed to an inter-diffusion of Ni2+ and ZrO2 matrix and the catalytic effect of the resultant solid solution, NiOZrO2, on the retained carbon species.

Monoclinic ZrO2and its supported materials Co/Ni/ZrO2for N2O decomposition

Monoclinic ZrO2and its supported materials Co/Ni/ZrO2(Co:Ni = 1:1) for catalytic decomposition of N2O have been studied with GC, FTIR, EDAX, XPS, and the evaluation of catalytic activity of the materials. It is found that monoclinic ZrO2alone has the catalytic effect for N2O decomposition, although higher activities are found for Co/Ni/ZrO2systems. XPS study shows that only Co exists in the surface region of ZrO2, which is attributed to the formation of NiO–ZrO2solid solution resulting from an interdiffusion between Ni2+and ZrO2matrix. The gas decomposition on Co/Ni/ZrO2can be described as first order with respect to partial pressure of N2O. Surface reactions on ZrO2and Co/Ni/ZrO2will also be addressed.

セラミック材料 ＣｅＯ２安定化ＺｒＯ２の微細組織と機械的性質に及ぼす酸化物ドーピングの影響

The purpose of this study is to examine the effects of some oxide additives on the grain growth and transformation behaviors of ceria doped tetragonal zirconia (Ce-TZP) to improve its mechanical properties. Monolithic Ce-TZP and some alkaline earth oxide (CaO, SrO and BaO) doped Ce-TZP were fabricated by the pressureless sintering technique at 1500 to 1700°C in air atmosphere. The grain growth of ZrO2 matrix was suppressed by doping alkaline earth oxide. BaO doping induced the transformation of Ce-TZP, whereas CaO doping was effective in stabilizing the tetragonal phase of CeTZP. CaO-doped Ce-TZP exhibited the high strength over 1000MPa. BaO doped Ce-TZP increased the fracture toughness of monolithic Ce-TZP. There was no effect of SrO-doping on transformation behavior and mechanical properties for Ce-TZP. The relationship between the microstructure and mechanical properties of oxide doped Ce-TZP were discussed.

Effect of NiO dissolution on the microstructural development of t′-ZrO2 with 5 mol% Y2O3

The ageing (1300 °C for 1–300 h) dependence of the microstructure of the metastable tetragonal (t′-) ZrO2 (plasma-sprayed Y-PSZ with 5 mol% Y2O3) in the sintered (1300 °C, 5 h) t′-ZrO2/NiO (10 mol%) composite was studied by transmission electron microscopy. Short-term (10 h) ageing caused incomplete decomposition of t′-ZrO2 into tweed assemblages containing t′-ZrO2 relics and tetragonal (t-) ZrO2 precipitates in the cubic (c-) ZrO2 matrix. The t′-ZrO2 relics retained anti-phase domain boundary (APB) and primary deformation twins, but no secondary deformation twins. Further ageing (e.g. up to 100 or 300 h) caused growth of the twin variants of t-ZrO2 with {101} habit plane. The large t-ZrO2 variants transformed directly into m-ZrO2 twins, but not into t-ZrO2 colonies. In addition to the relics in the aged samples, t′-ZrO2 with APB may also form locally from a solute-redistributed c-ZrO2 by slow cooling.

Oxidative Coupling of Methane over Alkali-Metal-Compound-Promoted Zirconia Catalysts

Upon promoting ZrO2 with alkali metal compounds (chlorides, nitrates, carbonates, or acetates), effective catalytic systems are obtained which are more active, selective, and stable with time-onstream than unpromoted ZrO2 or unsupported promoter compounds at 750°C, PCH4 = 243.2 Torr, CH4/O2 = 4, and a space velocity of 7500 cm3 g−1 h−1. The performance of the catalysts is found to strongly depend on the nature of the alkali salt, the promoter content, the method of catalyst preparation, and the reaction conditions (temperature, partial pressures. CH4/O2 ratio, and contact time). The most effective catalytic system is obtained with Na+-ZrO2-Cl− prepared via the sol-gel method, which leads to a methane conversion of 13.8 mol%, total C2 selectivity of 77 mol% (C2 yield 10.6%), and an ethylene-to-ethane ratio of 3.3 for at least 30 h under the aforementioned conditions and atmospheric pressure. Addition of carbon tetrachloride as a source of chlorine species to the feedstream improves the performance of the catalyst and its stability with time-on-stream. In the latter case, a CH4 conversion of 22.0% and a C2 yield of 17.5% with a C2H4/C2H6 molar ratio of 2.7 are obtained at a space velocity of 3750 cm3 g−1 h−1. In contrast, Na+-ZrO2-Cl− prepared via impregnation, gel-precipitation, physical mixing, or ion-exchange, and the Li-, K-, Rb-, or CsCl-promoted ZrO2 catalysts prepared by any of the above methods, were less effective, even upon the addition of carbon tetrachloride to the feedstream. The relatively high performance of Na+-ZrO2-Cl− (sol-gel) is attributed to the incorporation of Na+Cl− into the ZrO2 matrix, the exposure of an appreciable amount of Na+ and Cl− on the surface, and the increase in basicity and base strength. The relationship between the catalytic performance and the physico-chemical characteristics of the catalysts revealed by XRD, XPS, and basicity (stepwise thermal desorption of CO2) measurements is explored.

高強度ＺｒＯ２‐Ａｌ２Ｏ３複合体の微細構造

ZrO2-Al2O3 composites with the bending strength of 3000MPa were developed. To discuss the mechanism which gives such a high bending strength, microstractures and atomic diffusions of incoherent ZrO2-Al2O3 interfaces in the composite of ZrO2-2 mol% Y2O3 with Al2O3 addition of 20 wt% were investigated using high resolution analytical electron microscopy. The results obtained are summarized as follows.(1) In the composite Al2O3 grains were dispersed uniformly in ZrO2 matrix and no crack and no flaw were observed.(2) In many ZrO2 grains dislocations were brought about by tensile stress in ZrO2 grains generated by the thermal expansion mismatch between ZrO2 and Al2O3. They contributed to the division of tetragonal ZrO2 grain and the relaxation of residual tensile stress in ZrO2 grain.(3) ZrO2 and Al2O3 grains formed direct interfaces without any reacting phases.(4) At ZrO2-Al2O3 grain interfaces, the diffusions of Zr and Al to Al2O3 and ZrO2 grains were observed respectively. It was considered that these chemical effects strengthened ZrO2-Al2O3 grain interfaces.

Microindentation‐Induced Transformation in 3.5‐mol%‐Yttria‐Partially‐Stabilized Zirconia Single Crystals

The temperature dependence of the Vickers microhardness was studied in 3.4‐mol%‐Y2O3‐partially‐stabilized ZrO2 (Y‐PSZ) single crystals up to 1000°C; the samples had previously been annealed at 1600°C for 150 h to develop “colony” precipitates of tetragonal ZrO2 in the cubic ZrO2 matrix. Indentation caused extensive stress‐induced martensitic transformation of the colony precipitates to monoclinic symmetry in zones which extended in extreme cases up to several hundred micrometers from the indent. For indents made at 500°C and above, the Md and Mf temperatures are 450° and 310°C, respectively; As is ∼600°C (Md is the temperature of initial transformation (the “martensite start temperature”) in deformed samples; Mf is the temperature at which the final transformation occurs; As is the temperature at which the reverse (monoclinic → tetragonal) transformation begins). However, extensive transformation zones are also found for indents made at 200°, 300°, and 400°C. The dislocation density introduced during indentation is responsible for nucleating the transformation in a zone adjacent to the indent. However, the transformation zone extends further than the plastic zone around the indent, indicating extensive autocatalytic transformation. Transformation within the zone appeared to occur in individual plates with {110} habit planes. The plate dimensions (∼100 μm ×∼175 μm ×∼10 μm) are large compared to the size of the colony precipitates (∼2 μm in maximum dimension).

Characteristics of ZrO2-Dispersed Si3N4 without Additives Fabricated by Hot Isostatic Pressing

Dense, ZrO2-dispersed Si3N4 composites without additives were fabricated at 180 MPa and ∼1850° to 1900°C for l h by hot isostatic pressing using a glass-encapsulation method; the densities reached >96% of theoretical. The dispersion of 20 wt% of 2.5YZrO2 (2.5 mol% Y2O3) in Si3N4 was advantageous to increase the room-temperature fracture toughness (∼7.5 MPa˙m1/2) without degradation of hardness (∼15 GPa) because of the high retention of tetragonal ZrO2. The dependence of fracture toughness of Si3N4–2.5YZrO2 on ZrO2 content can be related to the formation of zirconium oxynitride because of the reaction between ZrO2 and Si3N4 matrix in hot isostatic pressing.

酸素雰囲気ＨＩＰ処理したＺｒＯ２セラミックスの破壊強度

Y-PSZ materials containing 2.5 to 5.0mol% Y2O3 prepared by post hot isostatic pressing under oxygen-containing atmosphere (white-Hipping) were found to retain their original strength even after ageing at temperatures higher than 600°C for 1000h.In this paper, the effects of heat-treatment at 1000°C to 1400°C on the bend strength and microstructure of Y-PSZ materials prepared by white-Hipping were reported.The bend strength of 5.0mol% Y-PSZ material after heat-treating at 1100°C for 1000h increased greatly from 720 to 1080MPa. The enhancement of strength was explained by the crack-precipitate interaction (CPI) toughening mechanism.The modulated, tweed or colony structures of fine strip crystals orthogonally crossing each other were observed within the cubic ZrO2 matrix. The 5.0mol% Y-PSZ material showed less decrease in strength at temperatures above 600°C, and higher strength at 1000°C to 1200°C than the 4.0mol% Y-PSZ material.

Microstructure of Y-PSZ after ageing at high temperature

Partially stabilized zirconia (PSZ) materials containing 2.5 and 5.0 mol % Y2O3 were prepared by pressureless sintering and aged at 1200° C for 1000 IS, and their microstructures were analysed by transmission electron microscopy and electron diffraction methods. Tetragonal zirconia polycrystal (TZP) containing 2.5 mol % Y2O3 before ageing showed nearly 100% tetragonal microstructure and 0.5 μm grain size, but after ageing the microstructure changed greatly, exhibiting no simple grain structure over wide areas. Repeated twin structures within the grains were observed. Y-PSZ material containing 5.0M01% Y2O3 before ageing showed a tetragonal (I structure within a cubic (c) stabilized ZrO2 matrix, After ageing, structures of fine strip crystals crossed each other orthogonally within the cubic matrix and typical diffuse scattering in the diffraction pattern was observed. Repeated twins were found on the plane of (100)m, and the orientational relationship between tetragonal (t) and monoclinic (m) crystal was determined to be (100)m [(100)t, [010]m \tT [001b]t.

In situ observation of the martensitic transformation in (Mg,Y)-PSZ

Partially-stabilized ZrO2 (PSZ) ceramics have considerable potential for advanced structural applications because of their high strength and toughness. These properties derive from small tetragonal ZrO2 (t-ZrO2) precipitates in a cubic (c) ZrO2 matrix, which transform martensitically to monoclinic (m) symmetry under applied stresses. The kinetics of the martensitic transformation is believed to be nucleation controlled and the nucleation is always stress induced. In situ observation of the martensitic transformation using transmission electron microscopy provides considerable information about the nucleation and growth aspects of the transformation.

Morphology of Tetragonal Precipitates in Partially Stabilized ZrO2

Tetragonal (t) ZrO2 precipitates in Mg‐, Ca‐, and Y‐partially‐stabilized ZrO2 (Mg‐PSZ, Ca‐PSZ, Y‐PSZ) have different habit planes and different morphologies. These differences arise because of differences in lattice parameters of precipitate and cubic (c) ZrO2 matrix in the three systems. The approximate {001} habit plane and oblate spheroid precipitate morphology observed in Mg‐PSZ are explained in terms of anisotropic elasticity using the theory of Khachaturyan. The aspect ratio of ∼5 of these particles is used to calculate a c/t interfacial energy of ∼0.15 J·m–2. The aligned equiaxed precipitates observed in Ca‐PSZ and the twinned colonies observed in Y‐PSZ can also be explained using this theory and arise from interactions between strain fields during coarsening; the aligned particles in Ca‐PSZ may actually represent an intermediate state before the formation of colonies in this system.

Determination of Magnesium and Calcium in Stabilized Zirconium Dioxide by Atomic Absorption Spectrometry

AbstractA procedure is described which permits the determination of magnesium (< 4% (w/w)) and calcium (< 1% (w/w)) in zirconium dioxide by atomic absorption spectrometry. Two different dissolution methods are used and evaluated. In the first method the ZrO2 is dissolved in a mixture of concentrated HF, HCl and HNO3 acids in a teflon bomb, while in the second method the ZrO2 matrix is brought into solution with a (NH4)2SO4, concentrated H2SO4 mixture. The optimalization of the analytical parameters is studied. For both elements the method of standard addition is employed. Powders of ZrO2, partially sintered (600‐1000°C) and completely sintered (1650°C) ZrO2 tubes are analyzed with both optimized methods for magnesium and with the first method for calcium.

Characterization of Interfacial Reactions Between β‐Al2O3 and Y2O3‐Partially‐Stabilized ZrO2

The interfacial reaction between Y2O3‐partially‐stabilized ZrO2 and α‐Al2O3 was studied. It was noted that α‐Al2O3 forms inside the periphery of the β‐Al2O3 grains; its formation suggests the loss of Na2O from the p‐Al2O3, either by evaporation or by dissolution in the ZrO2 matrix. The presence of Na2ZrO3 is suspected.

In situ study of the sintering process of yttria stabilized zirconia by impedance spectroscopy

In this paper the authors analyse the merits of the impedance spectroscopy technique for studying the sintering process of yttria stabilized zirconia. The great advantage of this technique with respect to classical ones is the possibility of separating the behavior of the grains from grain boundaries during thermal cycles. Because of large conductivities at high temperatures, measurements must be performed in the range 300–600°C after a rapid cooling of the sample down from firing temperature. In this temperature range both intragrain and grain boundary contributions are observable. The results give valuable informations on:•-the solid state reaction of the dopant dissolution into the ZrO2 matrix at the initial stage of the process•-the grain boundary formation and evolution for different firing conditions.

Directional Solidification of the ZrO2‐MgO Eutectic

The eutectic between ZrO2 and MgO was located at 27 wt% MgO. Directional solidification of this composition yields MgO rods in a cubic ZrO2 matrix. Plane front growth occurred at solidification rates of <2.0 cm/h. The interfiber spacing was proportional to the inverse square root of the solidification rate. In samples where cellular growth occurred, the colony size was not affected by the solidification rate. The room‐temperature fracture strength, ∼23,000 psi, which was not affected by the MgO fiber size, was controlled by the colony size. Annealing for 24 h at 1200°C increased the fracture strength by 50% as a result of a compressive stress in the matrix caused by the cubic‐to‐monoclinic transformation in the ZrO2.