Improvement in performance of MgO–CaO refractories by addition of nano-sized ZrO2

MgO–CaO refractories added with different sized ZrO₂ powders were sintered at 1600 °C, the effect of the ZrO₂ powders on performance of MgO–CaO refractories was investigated. The results showed that the densification of the MgO–CaO refractories was appreciably promoted when a small amount of ZrO₂ was added owing to the formation of small size CaZrO₃ facilitated to sintering, and the densification was promoted further with increasing the amount of ZrO₂ due to the volume expansion caused by the reaction of the added ZrO₂ and CaO to form CaZrO₃ in the refractories, and the addition of nano-sized ZrO₂ was more effective. The thermal shock resistance of the MgO–CaO refractories was improved by modification of the microstructure due to the formed CaZrO₃ particles that predominately located on the grain boundaries and triple points in the whole microstructure, and the addition of nano-sized ZrO₂ was more effective attributed to its well dispersion and the critical addition amount was effectively decreased to 6%. The slaking resistance of the MgO–CaO refractories was appreciably improved by addition of ZrO₂ due to its effect on decreasing the amount of free CaO in the refractories, promotion of densification as well as modification of microstructure, the nano-sized ZrO₂ addition was more effective due to its higher activity. The slag corrosion resistance of the MgO–CaO refractories was enhanced by addition of ZrO₂ due to the increase of the viscosity of the liquid phase and thus inhibited further penetration of slag at elevated temperatures.     

Synthesis and thermo-mechanical properties of mullite–alumina composite derived from sillimanite beach sand: effect of ZrO2

A mullite–alumina composite was developed by reaction sintering of sillimanite beach sand and calcined alumina. ZrO₂ (2–6 wt.%) was added as additive. The raw materials and additive were mixed, attrition milled and sintered in compacted form at 1400–1600°C with 2 h soaking. The effect of ZrO₂ on the densification behaviour, thermo-mechanical properties and microstructure was studied. It was found that addition of ZrO₂ slightly retards the densification process. All the samples achieved their highest bulk density at 1600°C. Thermo-mechanical properties of the sintered samples are not effectively altered by the presence of ZrO₂. ZrO₂ containing samples always show better resistance to thermal shock than the ZrO₂ free samples. Scanning electron micrography shows that ZrO₂ occupies both an intergranular and intragranular position in the mullite matrix. The mullite formed at 1600°C is mostly equiaxed in nature that suggests densification mainly occurs through solid state sintering.     

Homogeneous ZrO2–Al2O3 Composite Prepared by Nano-ZrO2 Particle Multilayer-Coated Al2O3 Particles

ZrO₂–Al₂O₃ nanocomposite particles were synthesized by coating nano-ZrO₂ particles on the surface of Al₂O₃ particles via the layer-by-layer (LBL) method. Polyacrylic acid (PAA) adsorption successfully modified the Al₂O₃ surface charge. Multilayer coating was successfully implemented, which was characterized by ξ potential, particle size. X-ray diffraction patterns showed that the content of ZrO₂ in the final powders could be well controlled by the LBL method. The powders coated with three layers of nano-ZrO₂ particles, which contained about 12 wt% ZrO₂, were compacted by dry press and cold isostatically pressed methods. After sintering the compact at 1450°C for 2 h under atmosphere, a sintered body with a low pore microstructure was obtained. Scanning electron microscopy micrographs of the sintered body indicated that ZrO₂ was well dispersed in the Al₂O₃ matrix.     

Y2O3-CeO2-doped ZrO2 ceramics from coprecipitated oxalate precursors

Y₂O₃-CeO₂-doped ZrO₂ ceramics were prepared from coprecipitated oxalate precursor material. The thermal decomposition behaviour of the oxalates was studied to find proper conditions for conversion to the oxides. The properties of the calcined oxide powders were measured and sintering conditions to prepare ceramic specimen were investigated. The mechanical properties of the sintered bodies are reported and their relations to composition and sintering conditions will be discussed.     

Sintering behavior of Al2TiO5 base ceramics and their thermal properties

Sintering behavior of Al₂TiO₅ without and with various additives and the thermal properties of the sintered material—thermal expansion and decomposition—were investigated. The precursors of Al₂TiO₅ powders were prepared by homogeneous precipitation and coprecipitation. Sintering of pure Al₂TiO₅ gave a fine grained-structure at 1300°C, but resulted in large-grained and cracked microstructures at 1400 and 1500°C. Addition of ZrO₂ or BaO gave fine-grained microstructures with a small increase in thermal expansion. Addition of ZrO₂, BaO or ZrSiO₄, especially ZrSiO₄, was effective in suppressing the thermal decomposition of Al₂TiO₅ at 1100°C. ©     

Exaggerated Grain Growth in ZrO2-Toughened Al2O3

Annealing of ZrO₂-toughened Al₂O₃ (ZTA) at elevated temperatures causes growth of both the intergranular ZrO₂ particles and the Al₂O₃"matrix" grains. Exaggerated ("breakaway") grain growth occurs in some, but not all, specimens. Analytical electron microscopy of two ZTA's, both of which contained a continuous amorphous (glassy) grain-boundary phase, but only one of which showed breakaway grain growth, revealed that the occurrence of breakaway grain growth could be correlated with the chemistry of the ubiquitous glassy grain-boundary phase.     

Microstructural development of sintered ZrO2---CeO2 ceramics

Microstructural development associated with diffusionless phase transformation was investigated in sintered ZrO₂-10–60 mol% CeO₂ ceramics cooled rapidly from a high temperature, using TEM and XRD techniques. The results show that (112) reflections appeared and a domain structure was found in ZrO₂-20–40 mol% CeO₂ samples, which is a result of c-t′ diffusionless transition, while the structure of the ZrO₂-60 mol% CeO₂ sample was fully stabilized zirconia, in which no forbidden reflections of c-ZrO₂ appeared. Finally, plate martensite and lath martensite structure were found in the ZrO₂-10 mol% CeO₂ sample; the former is the tetragonal phase with internal twins and the latter is the twinned monoclinic phase.     

Strength-Toughness Relations in Sintered and Isostatically Hot-Pressed ZrO2-Toughened Al2O3

The fracture toughness of fine-grained undoped ZrO₂-toughened Al₂O₃ (ZTA) was essentially unchanged by postsintering hot isostatic pressing and increased monotonically with ZrO₂ additions up to 25 wt%. The strength of ZTA with 5 to 15 wt% tetragonal ZrO₂, which depended monotonically on the amount of ZrO₂ present before hot isostatic pressing, was increased by pressing but became almost constant between 5 and 15 wt% ZrO₂ addition. The strength appeared to be controlled by pores before pressing and by surface flaws after pressing; the size of flaws after pressing increased with ZrO₂ content. The strength of ZTA containing mostly monoclinic ZrO₂ (20 to 25 wt%) remained almost constant despite the noticeable density increase upon hot isostatic pressing because the strength was controlled by preexisting microcracks whose extent did not change on postsintering pressing. These strength-toughness relations in sintered and isostatically hot-pressed ZTA are explained on the basis of R -curve behavior. The importance of the contribution of microcracks to the toughness of ZTA is emphasized.     

Autothermal reforming of methane over Ni catalysts supported over CaO–CeO2–ZrO2 solid solution

Ni catalysts supported on various solid solutions of ZrO₂ with alkaline earth oxide and/or rare earth oxide were synthesized. The catalytic activities were compared for partial oxidation of methane and autothermal reforming of methane. For partial oxidation of methane, the Ni catalyst supported on a CaO–ZrO₂ solid solution showed a high activity. Incorporation of CaO in the ZrO₂ matrix was effective for increasing the reduction rate of the NiO particles and for decreasing the coke formation. On the other hand, the Ni particles supported on the CaO–CeO₂–ZrO₂ solid solution had a strong interaction with the support, and the Ni particles showed high activity and stability for autothermal reforming of methane.     

Origin of Solid-State Activated Sintering in Bi2O3-Doped ZnO

Activated sintering in Bi₂O₃-doped ZnO has been studied with emphasis on the mechanistic role of intergranular amorphous films. The atomic-level microstructures and bismuth solute distributions in doped powders have been investigated using high-resolution electron microscopy and scanning transmission electron microscopy. Densification is observed to be significant below the bulk eutectic temperature in the presence of Bi₂O₃ concentrations as low as 0.58 mol%. Transmission electron microscopy of as-calcined and sintered powders shows that significant neck growth and particle coarsening occur in the solid state. Intergranular amorphous films of ∼1 nm thickness, terminating in wetting menisci at sinter-necks, are observed to form concurrently with the onset of activated sintering. In a few instances, amorphous films are also observed at surfaces of the ZnO particles. These films appear to be the free-surface counterpart to equilibrium-thickness intergranular films. Activated sintering in this binary system is attributed to rapid mass transport through subeutectic, equilibrium-thickness intergranular films, with the amorphous phase also providing capillary pressure.     

Ethylene dimerization over NiSO4 supported on Fe2O3-promoted ZrO2 catalyst

The NiSO₄ supported on Fe₂O₃-promoted ZrO₂ catalysts were prepared by the impregnation method. Fe₂O₃-promoted ZrO₂ was prepared by the coprecipitation 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 temperatures because of the interaction between nickel sulfate (or Fe₂O₃) and ZrO₂. 15-NiSO₄/5-Fe₂O₃–ZrO₂ containing 15 wt.% NiSO₄ and 5 mol% Fe₂O₃, and calcined at 500 °C exhibited a maximum catalytic activity for ethylene dimerization. NiSO₄/Fe₂O₃–ZrO₂ catalysts was very effective for ethylene dimerization even at room temperature, but Fe₂O₃–ZrO₂ without NiSO₄ did not exhibit any catalytic activity at all. The catalytic activities were correlated with the acidity of catalysts measured by the ammonia chemisorption method. The addition of Fe₂O₃ up to 5 mol% enhanced the acidity, surface area, thermal property, and catalytic activities of catalysts gradually, due to the interaction between Fe₂O₃ and ZrO₂ and due to consequent formation of Fe–O–Zr bond.     

ZrO2/聚甲基丙烯酸甲酯纳米复合物:制备及其在聚合物阵列中的分散（英文）

ZrO2/PMMA nanocomposite particles are synthesized through an in-situ free radical emulsion polymerization based on the silane coupling agent (Z-6030) modified ZrO2 nanoparticles, and the morphology, size and its distribution of nanocomposite particles are investigated. Scanning electron microscopy (SEM) images demonstrate that the methyl methacrylate (MMA) feeding rate has a significant effect on the particle size and morphology. When the MMA feeding rate decreases from 0.42ml·min-1 to 0.08ml·min-1 , large particles (about 200-550nm) will not form, and the size distribution become narrow (36-54nm). The average nanocomposite particles size increases from 34nm to 55nm, as the MMA/ZrO2 nanoparticles mass ratio increased from 4:1 to 16:1. Regular spherical ZrO2/PMMA nanocomposite particles are synthesized when the emulsifier OP-10 concentration is 2mg·ml-1. The nanocomposite particles could be mixed with VAc-VeoVa10 polymer matrix just by magnetic stirring to prepare the ZrO2 /PMMA/VAc-VeoVa10 hybrid coatings. SEM and atomic force microscopy (AFM) photos reveal that the distribution of the ZrO2 /PMMA nanocomposite particles in the VAc-VeoVa10 polymer matrix is homogenous and stable. Here, the grafted-PMMA polymer on ZrO2 nanoparticles plays as a bridge which effectively connects the ZrO2 nanoparticles and the VAc-VeoVa10 polymer matrix with improved comparability. In consequence, the hybrid coating with good dispersion stability is obtained.     

Processing and mechanical properties of ZrO2–TiB2 composites

In this paper, a strategy is described to develop high toughness yttria-stabilised tetragonal zirconia polycrystalline (Y-TZP) composites reinforced with hard TiB₂ particles. The experimental results revealed that fully dense Y-TZP composites with 30 vol.% TiB₂ can be obtained with a moderate hardness of 13 GPa, a high strength up to 1280 MPa and an excellent indentation toughness up to 10 MPa m^1/2 by hot pressing in vacuum at 1450 °C. The toughness of the composites can be tailored between 4 and 10 MPa m^1/2 by varying the yttria stabiliser content of the ZrO₂ matrix between 3 and 2 mol%. An optimum composite toughness was achieved for a ZrO₂ matrix with an overall yttria content of 2.5 mol%, obtained by mixing pure monoclinic and 3 mol% Y₂O₃ co-precipitated ZrO₂ starting powders. An important observation is that the thermal residual tensile stress in the ZrO₂ matrix due to the TiB₂ addition, needs to be taken into account when optimising the transformability of the ZrO₂ matrix in order to develop high toughness Y-TZP composites.     

Oxygen-Enhanced Crystallization of Solution-Derived 12CaO·7Al2O3

12CaO·7Al₂O₃ (C12A7) composed of nanosize cage structure can clathrate oxygen radicals (O⁻) and has a high potential to application of strong oxidizing catalysis. In the present report, we demonstrate a fabrication route to C12A7 fine powders by Chemical Solution Deposition method in order to enhance the catalytic reactivity. Aluminum sec-butoxide, calcium nitrate tetrahydrate, acetylacetone, 2-methoxyethanol, and nitric acid were used as raw materials. Precursor solution was dried and annealed at 800°–900°C in air or O₂ atmosphere. Crystalline C12A7 powders were obtained by annealing at 900°C in O₂ atmosphere. Scanning electron microscope and transmission electron microscope images of the obtained powders revealed C12A7 particles were sintered and formed several micrometer particles with many pores. BET specific surface area of the powders was 4.2 m²/g. Possibility for synthesizing C12A7 powder with higher specific surface area by the solution process was indicated.     

Modified Co3O4/ZrO2 catalysts for VOC emissions abatement

The catalytic activity study of cobalt oxides dispersed on different supports evidenced first the highest performances of zirconia based catalysts in the reaction of toluene oxidation. The influence of the presence of ethylenediamine (en) during the preparation of Co/ZrO₂ and the ZrO₂ support modification by Y₂O₃ were then studied and compared with reference catalyst prepared conventionally by impregnation of ZrO₂ with an aqueous solution of Co(NO₃)₂. Addition of an aqueous solution of ethylenediamine to a cobalt nitrate solution led to a strong increase on the catalytic activity of the activated solids in the toluene deep oxidation as compared with the reference catalyst. The best catalytic results were explained in terms of cobalt oxides dispersion but also in terms of Co–support interaction. The generated cobalt species were reducible at much lower temperatures and then were more active in the toluene total oxidation. Finally an efficient catalyst for VOC oxidation was produced combining the modifications of ZrO₂ by yttrium and of the precursor.     

Mechanical properties of Al2O3 + ZrO2 + nano-SiCp ceramic composites

The mechanical properties of Al₂O₃ matrix composites reinforced by ZrO₂(2 mol% Y₂O₃) and nanometre scale SiC dispersions have been investigated. It is shown that the Al₂O₃ matrix is simultaneously strengthened and toughened by both ZrO₂(2 mol% Y₂O₃) and nano-SiC particles. The maximum flexural strength and fracture toughness of the composites are 945 MPa and 7.3 MPam^1/2, respectively. The reinforcing effect of both t-m phase transformation of ZrO₂ (2 mol% Y₂O₃) and nano-SiC particles appears to be synergetic.     

Catalytic activity of PdO/ZrO2 catalyst for methane combustion

Catalytic activity of ZrO₂ supported PdO catalysts for methane combustion has been investigated in comparison with Al₂O₃ supported PdO catalysts. It was found that the drop of catalytic activity owing to decomposition of PdO at a high temperature region (600–900°C) was suppressed by using ZrO₂ support. Temperature-programmed reduction (TPR) measurements of the catalyst with hydrogen revealed that the PdO of PdO/Al₂O₃ catalyst was reduced at the temperature less than 100°C, whereas in PdO/ZrO₂ catalyst the consumption of hydrogen was also observed at 200–300°C. This result indicates that the stable PdO species were present in the PdO/ZrO₂ catalyst. In order to confirm the formation of the solid solution of PdO and ZrO₂, X-ray diffraction (XRD) analyses of the mixtures of ZrO₂ and PdO calcined at 700–900°C in air were carried out. The lattice volume of ZrO₂ in the mixture was larger than that of ZrO₂. Furthermore, the Pd thin film on ZrO₂ substrate was prepared as a model catalyst and the depth profile of the elements in the Pd thin film was measured by Auger electron spectroscopy (AES). It was confirmed that Zr and O as well as Pd were present in the Pd thin film heated at 900°C in air. It was considered that the PdO on ZrO₂ support might be stabilized by the formation of the solid solution of PdO and ZrO₂.     

Grain growth inhibition and mechanical property enhancement by adding ZrO2 to Al2O3 matrix

The critical amount and critical size of ZrO₂ for effective dragging of grain boundary migration occurred at 8 vol% addition for Al₂O₃ ceramic composite, pressureless-sintered at 1600°C for 2h. The fracture toughness was increased from 4·1 to 5·4 MPa m^1/2, and the flexural strength from 290 to 410 MPa at optimal conditions. The enhancement of mechanical properties is attributed to the stress-induced phase transformation toughening when ZrO₂ particles were located intergranularly. Al₂O₃ grain growth is inhibited by ZrO₂ particles pinning at the grain boundaries.     

Obtaining CeO2–ZrO2 mixed oxides by coprecipitation: role of preparation conditions

The preparation of CeO₂–ZrO₂ mixed oxides preparation was studied by evaluating the influence of several conditions. Coprecipitation was taken as the standard method and the effects brought about by the cerium salt precursor ((NH₄)₂Ce(NO₃)₆ or Ce(NO₃)₃), the introduction of drying and aging steps as well as pH controlling upon precipitation were analyzed. The samples were characterized by X-ray diffraction, Raman spectroscopy, temperature-programmed reduction, infrared spectroscopy, oxygen storage capacity and surface area. The use of Ce(NO₃)₃ leads to the formation of c-CeO₂ and t-ZrO₂ mixed oxide whereas a solid solution is achieved by using (NH₄)₂Ce(NO₃)₆. It was observed that the cerium precursor is the most significant parameter of preparation procedure since it defines the crystalline phases and consequently the reducibility behavior of the CeO₂–ZrO₂ system.     

Phase diagram of the system: Al2O3–ZrO2

The system Al₂O₃–ZrO₂ was studied by differential thermal analysis in inert atmosphere and in vacuum. The eutectic was located at 1866°C and 40% mass of ZrO₂. Zirconia solid solution at the eutectic temperature is up to 1.1±0.3% mass of Al₂O₃. Enthalpy of melting of this eutectic is 1080±90 J/g. Pure ZrO₂ transforms from monoclinic to tetragonal at 1162±7°C, but the saturated solid solution of ZrO₂, with 0.7±0.2% mass Al₂O₃ at this temperature, transforms at 1085±5°C. Inverse transitions occur with hysteresis correspondingly at 1055±5 and 995±5°C. Enthalpy of transformation of pure ZrO₂ from monoclinic to tetragonal phase is 42±5 J/g (5.2±0.6 J/mol) but only 30±5 J/g for a ZrO₂ saturated solid solution.