Onset coarsening-coalescence kinetics of ZrO2 and less refractory TiO2 nanoparticles: Effect of homologous temperature and phase transformation

Specific surface area change of ZrO₂ (predominant tetragonal - (t) symmetry, 30-50 nm) and less refractory TiO₂ anatase nanoparticles (20-50 nm) upon isothermal firing at 700-1000 °C in air was determined by N₂ adsorption-desorption hysteresis isotherm. The nanoparticles underwent onset coarsening-coalescence within minutes without appreciable phase transformation for TiO₂, but with extensive transformation into monoclinic (m-) symmetry for ZrO₂. The apparent activation energy of such a process being not much higher for ZrO₂ (77 ± 23 kJ/mol) than TiO₂ (56 ± 3 kJ/mol) nanoparticles can be attributed to transformation plasticity. The minimum temperature for coarsening/coalescence of the present ZrO₂ and TiO₂ nanoparticles was estimated as 710 and 641 °C, respectively.     

Novel self-reinforcing ZrO2–SiO2 aerogels with high mechanical strength and ultralow thermal conductivity

Novel self-reinforcing ZrO₂–SiO₂ aerogels with high mechanical strength and ultralow thermal conductivity are fabricated by impregnating hydrolyzed ZrO₂–SiO₂ sol into wet gel matrix and drying. The ZrO₂–SiO₂ sol fills the macropores and defects of ZrO₂–SiO₂ aerogel matrix generating during the gelation process, which contributes to the improvement of the mechanical properties of the ZrO₂–SiO₂ aerogel matrix. The mechanical and thermal properties of the as-prepared ZrO₂–SiO₂ aerogel are investigated and discussed. The results show that the mechanical strength of the self-reinforcing aerogels obviously increases from 0.51 to 3.11?MPa with the increase of impregnation times, while the thermal conductivity of the aerogels slightly increases from 0.0235 to 0.0306?W?m^?1 K^?1. The novel self-reinforcing ZrO₂–SiO₂ aerogel could have interesting applications in aerospace and energy because of its outstanding mechanical and thermal properties.     

Preparation and properties of transparent PMMA/ZrO2 nanocomposites using 2-hydroxyethyl methacrylate as a coupling agent

Transparent PMMA/ZrO₂ nanocomposites were prepared by in-situ bulk polymerization of methyl methacrylate (MMA)/ZrO₂ dispersions that were firstly synthesized using nonaqueous synthesized ZrO₂ nanocrystals and the function monomer, 2-hydroxyethyl methacrylate (HEMA), as the ligand. The dispersion behavior of ZrO₂ nanoparticles in MMA, structure, mechanical and thermal properties of the PMMA/ZrO₂ nanocomposites were investigated comprehensively. It was found that ZrO₂ nanoparticles were well dispersed in MMA with HEMA ligand, but the MMA/ZrO₂ dispersions easily destabilized in air as well as at elevated temperatures. The destabilization temperature of the dispersion is raised by increasing the molar ratio of HEMA/ZrO₂ to match the bulk polymerization temperature. The PMMA/ZrO₂ nanocomposites showed an interesting chemical structure (namely, highly cross-linked structure even at ZrO₂ content as low as 0.8 wt% and hydrogen bonding interaction between polymer matrix and ZrO₂ nanoparticles), with enhanced rigidity without loss of the toughness and improved thermal stability. The relationship between the structure and the properties of the PMMA/ZrO₂ nanocomposites based on the HEMA coupling agent was discussed.     

Thermal conductivity in mullite/ZrO2 composite coatings

Mullite-based multilayered structures have been suggested as promising environmental barrier coatings for Si₃N₄ and SiC ceramics. Mullite has been used as bottom layer because its thermal expansion coefficient closely matches those of the Si-based substrates, whereas Y–ZrO₂ has been tried as top layer due to its stability in combustion environments. In addition, mullite/ZrO₂ compositions may work as middle layers to reduce the thermal expansion coefficient mismatch between the ZrO₂ and mullite layers. Present work studies the thermal behaviour of a flame sprayed mullite/ZrO₂ (75/25, v/v) composite coating. The changes in crystallinity, microstructure and thermal conductivity of free-standing coatings heat treated at two different temperatures (1000 and 1300 °C) are comparatively discussed. The as-sprayed and 1000 °C treated coatings showed an almost constant thermal conductivity (K) of 1.5 W m⁻¹ K⁻¹. The K of the 1300 °C treated specimen increased up to twice due to the extensive mullite crystallization without any cracking.     

A rapid method for the preparation of silica-coated ZrO2 nanoparticles by microwave irradiation

A novel synthesis of silica-coated ZrO₂ nanoparticles is reported based on microwave irradiation (MW) method. The synthesis of silica-coated ZrO₂ nanoparticles was realized by a rapid uniform hydrolysis and subsequent copolymerization of the precursor tetraethoxysilane (TEOS) on ZrO₂ surface. One of the advantages of this MW irradiation method is the very short coating time and uniform heating in comparison to the conventional ones, allowing the synthesis of uniformly coated ZrO₂ nanoparticles with silica. The XPS analysis revealed the shifts in binding energies for Zr 3d5/2 and Zr 3d3/2 peaks after coating confirming the formation of silica layer on the surface of ZrO₂ nanoparticles. Characteristic silica peaks were observed in the FTIR spectra of coated nanoparticles. The shift in the isoelectric point measured by dynamic light scattering method was indicator of silica coverage of the ZrO₂ surface. The coatings formed at 70 °C were thin and uniform and extended up to 2 nm from the ZrO₂ surface as confirmed by the HR-TEM images.     

Matrix–filler interactions in polysilazane-derived ceramics with Al2O3 and ZrO2 fillers

Interactions between a poly(vinyl)silazane and Al₂O₃ or Y₂O₃-stabilised ZrO₂ fillers were studied during the fabrication of polysilazane-derived bulk ceramics in order to investigate the influence of oxide fillers on resulting properties. Specimens were produced by coating of the filler powders with the polysilazane, warm-pressing of the resulting composite powders, and pyrolytic conversion in flowing N₂ at various temperatures between 1000 °C and 1400 °C. Significant differences in densification were observed, depending on the filler used. Reactions between the polysilazane-derived matrix and Al₂O₃ or ZrO₂ at temperatures ≥1300 °C resulted in the formation of Si₅AlON₇ or ZrSiO₄, respectively. Reactivity in the polysilazane-derived component was a result of SiO₂ contamination caused primarily by adsorbed species on the filler particle surface. Knowledge of polysilazane/filler interface processes is found to be decisive for the prediction of properties such as shrinkage and porosity, which heavily influence performance of a material.     

CO2 hydrogenation to methanol over Cu/ZnO/ZrO2 catalysts prepared via a route of solid-state reaction

Cu/ZnO/ZrO₂ catalysts were prepared by a route of solid-state reaction and tested for the synthesis of methanol from CO₂ hydrogenation. The effects of calcination temperature on the physicochemical properties of as-prepared catalysts were investigated by N₂ adsorption, XRD, TEM, N₂O titration and H₂-TPR techniques. The results show that the dispersion of copper species decreases with the increase in calcination temperature. Meanwhile, the phase transformation of zirconia from tetragonal to monoclinic was observed. The highest activity was achieved over the catalyst calcined at 400 °C. This method is a promising alternative for the preparation of highly efficient Cu/ZnO/ZrO₂ catalysts.     

Reduction of NO by CO on Cu/ZrO2/Al2O3 catalysts: Characterization and catalytic activities

ZrO₂, γ-Al₂O₃ and ZrO₂/γ-Al₂O₃-supported copper catalysts have been prepared, each with three different copper loads (1, 2 and 5 wt%), by the impregnation method. The catalysts were characterized by nitrogen adsorption (BET), X-ray diffraction (XRD), temperature programmed reduction (TPR) with H₂, Raman spectroscopy and electronic paramagnetic resonance (EPR). The reduction of NO by CO was studied in a fixed-bed reactor packed with these catalysts and fed with a mixture of 1% CO and 1% NO in helium. The catalyst with 5 wt% copper supported on the ZrO₂/γ-Al₂O₃ matrix achieved 80% reduction of NO. Approximately the same rate of conversion was obtained on the catalyst with 2 wt% copper on ZrO₂. Characterization of these catalysts indicated that the active copper species for the reduction of NO are those in direct contact with the oxygen vacancies found in ZrO₂.     

Improvement of coke resistance of Ni/Al2O3 catalyst in CH4/CO2 reforming by ZrO2 addition

This work investigates the improvement of Ni/Al₂O₃ catalyst stability by ZrO₂ addition for H₂ gas production from CH₄/CO₂ reforming reactions. The initial effect of Ni addition was followed by the effect of increasing operating temperature to 500–700 °C as well as the effect of ZrO₂ loading and the promoted catalyst preparation methods by using a feed gas mixture at a CH₄:CO₂ ratio of 1:1.25. The experimental results showed that a high reaction temperature of 700 °C was favored by an endothermic dry reforming reaction. In this reaction the deactivation of Ni/Al₂O₃ was mainly due to coke deposition. This deactivation was evidently inhibited by ZrO₂, as it enhances dissociation of CO₂ forming oxygen intermediates near the contact between ZrO₂ and nickel where the deposited coke is gasified afterwards. The texture of the catalyst or BET surface area was affected by the catalyst preparation method. The change of the catalyst texture resulted from the formation of ZrO₂–Al₂O₃ composite and the plugging of Al₂O₃ pore by ZrO₂. The 15% Ni/10% ZrO₂/Al₂O₃ co-impregnated catalyst showed a higher BET surface area and catalytic activity than the sequentially impregnated catalyst whereas coke inhibition capability of the promoted catalysts prepared by either method was comparable. Further study on long-term catalyst stability should be made.     

Electrochemical behaviour of ZrO2 sol-gel pre-treatments on AA6060 aluminium alloy

ZrO₂ pre-treatments applied with the sol-gel technique are a possible replacement of chromium based pre-treatments on aluminium alloys. The thickness and homogeneity of the films deposited on AA6060 alloy are strongly related to the process parameters like preparation of the surface, number of dips and thermal treatment of the film.ZrO₂ films were prepared using the dip-coating technique in sol obtained from metal-organic precursors in an organic solvent (0.1 M Zr(OBuⁿ)₄ in anhydrous n-butanol with addition of acetic acid as complexing agent). Different layers were applied on AA6060 changing number of dips and thermal treatment (150 °C for 1 h or 250 °C for 4 min). The typical thickness of the deposited layers was in the range 70-180 nm depending on process parameters. The electrochemical behaviour of the pre-treated alloy in diluted Harrison solution (0.05 wt% NaCl + 0.35 wt% (NH₄)₂SO₄) was investigated by means of potentiodynamic polarization, open circuit potential measurements, and electrochemical impedance spectroscopy. In addition, the electrochemical behaviour of ZrO₂ sol-gel films was compared with that of chromatized AA6060 and fluotitanated/fluozirconated AA6060. In order to evaluate the adhesion properties of the films, ZrO₂ pre-treated AA6060, chromatized AA6060 and fluotitanated/fluozirconated AA6060 were painted with a polyester resin and subjected to thermal cycles in 0.05 wt% NaCl. Each thermal cycle consisted of heating the samples at 90 °C, permanence at 90 °C for 6 h, cooling at room temperature and permanence at room temperature for 18 h. Impedance measurements were performed at the end of each cycle.Potentiodynamic polarization curves and impedance spectra indicate that ZrO₂ pre-treatments have similar barrier properties to those of chromatized AA6060. However, no self-healing ability is observed for ZrO₂ films.The barrier properties of ZrO₂ films are strongly dependent on process parameters. In particular, the number of dips determines the amount of defects in the film and its homogeneity. The electrochemical behaviour strongly improves increasing the number of dips in the deposition bath. Thermal aging cycles evidence good adhesion properties for ZrO₂ pre-treatments.     

Laser stereolithography of ZrO2 toughened Al2O3

Ceramic laser stereolithography is a manufacturing process suitable candidate for the production of complex shape technical ceramics. The green ceramic is produced layer by layer through laser polymerisation of UV curable ceramic suspensions. A number of critical issues deserve attention: high solid loading and low viscosity of the suspensions, high UV reactivity, prevention of interlayer delamination in the green and in the sintered body, good mechanical performance. In this work, ZrO₂-reinforced Al₂O₃ components have been obtained from an acrylic modified zircon loaded with alumina powders. The zircon compound is effective as organic photoactivated resin and allows the dispersion of a high volume fraction of Al₂O₃ powder (up to 50 vol.%) while keeping viscosity at reasonable low values. The zircon compound also represents a liquid ceramic precursor that converts to oxide after burning out of the binder. Thank to the good dispersion of the alumina powder in the zircon acrylate, a uniform dispersion of ZrO₂ submicron particles is obtained after pyrolysis. These are located at the grain boundaries between alumina grains. Formation of both monoclinic and tetragonal ZrO₂ occurs as evidenced by XRD. No delamination occurs in bending tests as evidenced by SEM fractography, satisfactory modulus and strength values were concurrently found.     

Trimerization of isobutene over WOx/ZrO2 catalysts

Trimerization of isobutene to produce isobutene trimers has been investigated over WO_x/ZrO₂ catalysts that were obtained by wet-impregnation and successive calcination at high temperatures. Very stable isobutene conversion and high selectivity for trimers are attained over a WO_x/ZrO₂ catalyst obtained by calcination at 700 °C. From the XRD study it can be understood that tetragonal ZrO₂ is beneficial for stable performance; however, monoclinic ZrO₂ is not good for trimerization. Nitrogen adsorption and FTIR experiments suggest that amorphous WO_x/ZrO₂ is inefficient catalyst even though it has high surface area and high concentration of acid sites. The observed performance with the increased selectivity and stable conversion demonstrates that a WO_x/ZrO₂ having tetragonal zirconia, even with decreased porosity and acid sites, is one of the best catalysts to exhibit stable and high conversion, high selectivity for trimers and facile regeneration.     

Preparation of ZrO2/Gd2O3 composite ceramic materials by coprecipitation method

High burnup is a goal for further development of advanced nuclear power in the future. However, along with the increase of burnup, it becomes more diffidult to control reactor reactivity, which affects the operation safety of the nuclear reactor. Al₂O₃/B₄C burnable poison materials widely used in pressurized water reactor currently will not meet the requirements of burnable poison materials in high burnup nuclear power. Because of the better performance of ZrO₂/Gd₂O₃ burnable poison materials than that of Al₂O₃/B₄C, this paper studies the preparation of ZrO₂/Gd₂O₃ composite ceramic materials by the coprecipitation method. The experimental results show that at the sintering temperature of 1500–1650 °C, ZrO₂/Gd₂O₃ composite ceramic grains are small, compact and uniform with the generation of homogeneous solid solution. At 1600 °C, ZrO₂–10%Gd₂O₃ has the highest density and mechanical strength.     

Influence of carbon on phase stability of tetragonal ZrO2

In the present work, nanocrystallite ZrO₂ was synthesized by calcining zirconium (IV) acetylacetonate. X-ray diffraction results show the existence of tetragonal phase even when the nanocrystallite size reached ~47 nm. The presence of amorphous carbon was identified by Raman analysis while the Zr–C bonding was confirmed by X-ray photoelectron spectroscopy analysis. Transmission electron microscopy showed no hard-agglomeration in the samples treated at high temperatures. These results indicated that carbon has influence on the phase stability of t-ZrO₂ in two aspects: the amorphous carbon creates a reducing atmosphere, meanwhile carbon atoms bonded with zirconium atoms act as a stabilizer.     

Methane combustion over Pd/ZrO2/SiC,Pd/CeO2/SiC,and Pd/Zr0.5Ce0.5O2/SiC catalysts

The performances of different promoters (CeO₂, ZrO₂ and Ce_0.5Zr_0.5O₂ solid solution) modified Pd/SiC catalysts for methane combustion are studied. XRD and XPS results showed that Zr⁴⁺ could be incorporated into the CeO₂ lattice to form Zr_0.5Ce_0.5O₂ solid solution. The catalytic activities of Pd/CeO₂/SiC and Pd/ZrO₂/SiC are lower than that of Pd/Zr_0.5Ce_0.5O₂/SiC. The Pd/Zr_0.5Ce_0.5O₂/SiC catalyst can ignite the reaction at 240 °C and obtain a methane conversion of 100% at 340 °C, and keep 100% methane conversion after 10 reaction cycles. These results indicate that active metallic nanoparticles are well stabilized on the SiC surface while the promoters serve as oxygen reservoir and retain good redox properties.     

Crystallization behaviour and hardness of glass ceramics rich in nanocrystals of ZrO2

This research is mainly directed toward the development of hardness of glass ceramic by adding different amounts of ZrO₂ to the glass and by applying different heat-treatments. Differential thermal analysis (DTA), X-ray diffraction (XRD) and Scanning electron microscope (SEM) were used to study the crystallization behaviour of the glass samples. The only observed crystalline phases were tetragonal and monoclinic zirconia. Hardness was found to increase by increasing time and temperature of heat-treatment due to the formation of monoclinic phase as a result of the martensitic reaction. This transformation opposes crack opening.     

All-solid-state rechargeable thin film lithium batteries with LixMn2O4 and LixMn2O4-0.5ZrO2 cathodes

Li_xMn₂O₄ (LMO) and Li_xMn₂O₄-0.5ZrO₂ (LMZO) thin films have been fabricated by radio-frequency (rf) sputtering deposition combined with conventional annealing method. The structures and surface morphology of thin films are characterized by X-ray diffraction, transmission electron microscopy, selected area electron diffraction and scanning electron microscopy techniques. It is shown that the addition of mass quantity of ZrO₂ in LMZO can control the grain growth of nano-crystalline LMO. The electrochemical performances of all-solid-state thin film lithium batteries (TFBs) based on these thin films as cathodes are examined by cyclic voltammetry (CV), galvanostatic mode and alternate-current impedance measurements. Our results demonstrated that the addition of electrochemically inactive ZrO₂ could significantly overcome the disadvantage of two distinct plateaus in 3 and 4 V regions. LMZO is expected to become a promising cathode material for future TFBs.     

Synthesis of functional gradient BCP/ZrO2 bone substitutes using ZrO2 and BCP nanopowders

BCP/BCP-ZrO₂/ZrO₂ scaffold with a functionally gradient layered structure (FG BCP/ZrO₂) was fabricated by the polymeric sponge replica method and subsequent dipping process. To enhance the compressive strength and bioactive properties of monolithic ZrO₂ scaffold, ZrO₂ and BCP phases were selected as a main frame and surface layer, respectively. The formation of microcracks was significantly decreased by incorporating an intermediate layer consisting of BCP-ZrO₂ phase. The thicknesses of the monolithic ZrO₂, BCP-ZrO₂, and BCP layer were around 10-30 μm, 3-5 μm, and 2-3 μm, respectively. The FG BCP/ZrO₂ scaffold showed highly interconnected pores as well as good material properties, which were 68% porosity and 7.2 MPa of compressive strength. Average pore size of FG BCP/ZrO₂ scaffold was about 220 μm in diameter. From MTT assay and SEM observation of osteoblast-like MG-63 cells, FG BCP/ZrO₂ scaffold showed good cell viability and faster proliferation behavior.     

The role of Ce reduction in the segregation of metastable phases in the ZrO2-CeO2 system

The complexity of the ZrO₂-CeO₂ phase diagram arises from several factors: the low solubility of each compound into the other one, the slow kinetics of cation diffusion, the occurrence of Ce reduction at high temperatures, and the existence of several metastable phases whose appearance and evolution depend on synthesis method and thermal history of the sample. Identification of phase content is moreover complicated because the X-ray diffractograms of some ZrO₂-CeO₂ phases are very close or even indistinguishable, which imposes the use of other techniques more sensitive to small oxygen displacements. In this work we present a Raman spectroscopic study of phase segregation in the ZrO₂-CeO₂ system between 1300 and 1650 °C, focusing on the effect of Ce reduction at high temperatures and its relation with the appearance of metastable phases upon cooling. The nature of the high-temperature defective cubic phase is discussed.