Zirconia as a support for catalysts: influence of additives on the thermal stability of the porous texture of monoclinic zirconia

A single-phase monoclinic zirconia (the thermodynamically stable modification up to a temperature of 1170°C), having a specific surface area of 67 m2g¿1 and a well-developed mesoporous texture, has been prepared by gel-precipitation followed by calcination at 450°C. A commercially available high-surface area monoclinic zirconia powder (SBET=71 m2g¿1) has also been studied. It was found that the specific surface area and pore volume of monoclinic zirconia both decreased markedly on increasing the calcination temperature; despite the fact that the crystal structure was that of the stable modification, this did not seem to impart any substantial resistance to thermal sintering. The thermal stability of monoclinic zirconia could however be improved significantly by addition (by an impregnation technique) of various oxides: CaO, Y2O3, La2O3 all led to an improvement in the thermal stability up to 900°C while MgO exhibited stabilizing properties only up to 700°C; the best results were obtained with La2O3. All the additives investigated other than MgO were found to bring about a partial transition of the monoclinic to a fluorite-like phase of zirconia upon heat treatment; this phase has been shown in the case of the CaO-doped sample to be cubic zirconia and in the cases of the Y2O3- and La2O3-doped samples to be tetragonal zirconia. As little as 20¿50% of a theoretical monolayer quantity of La2O3 was sufficient to give satisfactory thermal stability. The results can be explained by a model involving mass transport by a surface diffusion mechanism

Fracture Resistance of Zirconia Oral Implants In Vitro: A Systematic Review and Meta-Analysis

Various protocols are available to preclinically assess the fracture resistance of zirconia oral implants. The objective of the present review was to determine the impact of different treatments (dynamic loading, hydrothermal aging) and implant features (e.g., material, design or manufacturing) on the fracture resistance of zirconia implants. An electronic screening of two databases (MEDLINE/Pubmed, Embase) was performed. Investigations including > 5 screw-shaped implants providing information to calculate the bending moment at the time point of static loading to fracture were considered. Data was extracted and meta-analyses were conducted using multilevel mixed-effects generalized linear models (GLMs). The Šidák method was used to correct for multiple testing. The initial search resulted in 1864 articles, and finally 19 investigations loading 731 zirconia implants to fracture were analyzed. In general, fracture resistance was affected by the implant design (1-piece > 2-piece, p = 0.004), material (alumina-toughened zirconia/ATZ > yttria-stabilized tetragonal zirconia polycrystal/Y-TZP, p = 0.002) and abutment preparation (untouched > modified/grinded, p < 0.001). In case of 2-piece implants, the amount of dynamic loading cycles prior to static loading (p < 0.001) or anatomical crown supply (p < 0.001) negatively affected the outcome. No impact was found for hydrothermal aging. Heterogeneous findings of the present review highlight the importance of thoroughly and individually evaluating the fracture resistance of every zirconia implant system prior to market release

Electrochemical characterisation of 3Y-TPZ-Fe2O3 composites

The influence of the addition of ferric oxide to 3Y-TZP on the conductivity and microstructure of sintered Y-stabilised tetragonal zirconia ceramics (3Y-TZP) was investigated. A comparison was made between two different dense 3Y-TZP¿¿-Fe2O3 composites. Compacts were made by pressureless sintering at 1150 °C or by sinterforging at 1000 °C and 100 MPa. The sinterforging process resulted in smaller zirconia and hematite grains and a higher monoclinic zirconia content as compared to the compact that was sintered pressureless. The high monoclinic content led to loss of ionic conductivity. The addition of ferric oxide caused electronic conductivity. The sinterforging resulted in a high concentration of metastable defects in the zirconia¿hematite composite, leading to a relatively high electronic conductivity. Heating above 380 °C caused irreversible loss of these defects and a large decrease in electronic conductivity

Intergranular metal phase increases thermal shock resistance of ceramic coating

Dispersed copper phase increases the thermal shock resistance of a plasma-arc-sprayed coating of zirconia used as a heat barrier on a metal substrate. A small amount of copper is deposited on the granules of the zirconia powder before arc-spraying the resultant powder composite onto the substrate

Durability of zirconia thermal-barrier ceramic coatings on air-cooled turbine blades in cyclic jet engine operation

Thermal barrier ceramic coatings of stabilized zirconia over a bond coat of Ni Cr Al Y were tested for durability on air cooled turbine rotor blades in a research turbojet engine. Zirconia stabilized with either yttria, magnesia, or calcia was investigated. On the basis of durability and processing cost, the yttria stabilized zirconia was considered the best of the three coatings investigated

Overview of zirconia with respect to gas turbine applications

Phase relationships and the mechanical properties of zirconia are examined as well as the thermal conductivity, deformation, diffusion, and chemical reactivity of this refractory material. Observations from the literature particular to plasma-sprayed material and implications for gas turbine engine applications are discussed. The literature review indicates that Mg-PSZ (partially stabilized zirconia) and Ca-PSZ are unsuitable for advanced gas turbine applications; a thorough characterization of the microstructure of plasma-sprayed zirconia is needed. Transformation-toughened zirconia may be suitable for use in monolithic components

X-ray reflectivity, diffraction and grazing incidence small angle X-ray scattering as complementary methods in the microstructural study of sol–gel zirconia thin films

X-ray reflectometry, X-ray diffraction and grazing incidence small angle X-ray scattering have been complementary used to fully characterize zirconia (ZrO2) thin films obtained by the sol–gel route. The films were synthesized on various sapphire (Al2O3), silicon (Si) and glass mirrorpolished wafers by a dip-coating process in a zirconia precursor sol. Versus the synthesis parameters as alkoxide sol concentration, withdrawal speed and annealing temperature, the microstructure of the layer is managed and its different microstructural parameters such as thickness, mass density, crystalline phase, grain size and spatial arrangement have been determined. The as prepared layers are amorphous. During a thermal treatment at low temperature (<1000 -C), the layers thickness decreases while their mass density increases. Simultaneously the zirconia precursor crystallises in the zirconia tetragonal form and the coating is made of randomly oriented nanocrystals which self organise in a dense close-packed microstructure. At low temperature, this microstructural evolution is similar whatever the substrate. Moreover, the layer evolves as the corresponding bulk xerogel showing that the presence of the interface does not modify the thermal microstructure evolution of the layer which is controlled by a normal grain growth leading to relatively dense nanocrystalline thin films

Catalytic coatings on steel for low-temperature propane prereforming to solid oxide fuel cell (SOFC) application

Catalyst layers (4–20 lm) of rhodium (1 wt%) supported on alumina, titania, and ceria–zirconia (Ce0.5Zr0.5O2) were coated on stainless-steel corrugated sheets by dip-coating in very stable colloidal dispersions of nanoparticles in water. Catalytic performances were studied for low-temperature (6500 C) steam reforming of propane at a steam to carbon ratio equal to 3 and low contact time (0.01 s). The best catalytic activity for propane steam reforming was observed for titania and ceria–zirconia supports for which propane conversion started at 250 C and was more than three times better at 350 C than conversion measured on alumina catalyst. For all catalysts a first-order kinetics was found with respect to propane at 500 C. Addition of PEG 2000 in titania and ceria–zirconia sols eliminated the film cracking observed without additive with these supports. Besides, the PEG addition strongly expanded the porosity of the layers, so that full catalytic efficiency was maintained when the thickness of the ceria–zirconia and titania films was increased

Electrochemical blackening of yttria-stabilized zirconia : morphological instability of the moving reaction front

Electrochemical reduction was performed on both polycrystals of calcia-stabilized zirconia (CSY) and single crystals of yttria-stabilized zirconia (YSZ) at 500°C. A glass-encapsulation was used to prevent access of molecular oxygen from the gas phase and to realize a virtually two-dimensional geometry for the reacting crystal. The resulting blackening process which occurs by the advancement of a morphologically unstable reaction front was observed in-situ in a heating-stage by the use of an optical microscope