Presence Of Tetragonal Zirconia Research Articles

Effect of calcination temperature on the application of sodium zirconate solid base catalyst for biodiesel production from Jatropha curcas oil

ABSTRACTInfluence of catalyst calcination temperature on the catalyst characteristics and catalytic transesterification of Jatropha curcas oil for biodiesel production was studied by using sodium zirconate (Na2ZrO3) solid base catalyst. Na2ZrO3 catalysts were prepared by impregnation method followed by calcination at temperatures of 700, 800, and 900°C. The prepared catalysts were characterized by X-ray diffraction analysis, Fourier transform infrared spectroscopy, and field emission scanning electron microscopy. Important parameters influencing the catalytic activity and fatty acid methyl ester yield were investigated. It was found that the increase in calcination temperature showed marked increase in activity due to the increased porosity and presence of tetragonal zirconia. Investigation of the reusability of the catalysts showed that the catalytic activity was retained even after five cycles of reaction.

La-, Mn- and Fe-Doped Zirconia Washcoats Deposited on Monolithic Reactors via Sol-Gel Method: Characterization and Evaluation of their Mass Transfer Phenomena and Kinetics in Trichloroethylene Combustion

Abstract La-, Mn- and Fe-doped ZrO2 was synthesized using the sol-gel method, washcoated on cordierite monoliths and used in trichloroethylene (TCE) combustion. A sol from sol-gel synthesis method was used to obtain the zirconia washcoatings. The washcoatings deposited on the cordierite monolith materials were characterized by scanning electron microscopy (SEM), X-ray diffraction (XRD), Fourier transform infrared (FT-IR) spectroscopy and N2 physisorption measurements, being the crystalline phase determination the most correlate with the catalytic activity. The washcoat properties were correlated to their performance in trichloroethylene combustion. The catalytic washcoatings contained a mixture of crystalline tetragonal ZrO2 and monoclinic ZrO2, and the monoclinic phase exhibited strong interactions with the α-cordierite crystalline phase. For the kinetic tests, the mass transfer into the washcoated monolith channels was evaluated using a recently developed model for the internal mass transfer coefficients for diffusion and reactions in catalytic monoliths. This model involves three resistances. In a plot of the overall resistance as function of the Thiele modulus was determinate that he reaction occurred in the kinetically controlled regime. The kinetic data were fit to two Langmuir–Hinshelwood (LH) models, and the reaction rate was fitted as a function of the trichloroethylene inlet concentration. The adsorption parameters obtained with both LH models were validated based on thermodynamic criteria for the changes in the standard enthalpy of adsorption(${\Delta}H_{ads}^0$) and standard total entropy of adsorption (${\Delta}S_{ads}^0$). It was found that adsorbed TCE and oxygen atoms should be more mobile over the catalyst, which had a higher proportion of tetragonal phase than monoclinic phase of zirconia. The La, Fe-doped catalyst exhibited the highest activity, mainly due the presence of tetragonal zirconia.

Alumina- zirconia coatings produced by Plasma Electrolytic Oxidation on Al alloy for corrosion resistance improvement

Compact alumina-zirconia nano composites with corrosion protection potential were coated on 7075 Al alloy through the Plasma Electrolytic Oxidation (PEO) method in DC galvanostatic mode. The layers were coated at constant current density of 20 A/dm2 and 100–350 s growth time in an alkaline K2ZrF6 containing electrolyte. The characteristics of the coatings were investigated as a function of PEO processing time. Electrochemical properties of the layers were studied by conducting potentiodynamic polarization experiments in 3.5% NaCl solution. The results showed that under the present PEO experimental conditions, alumina-zirconia nanostructured coatings can be produced with 10–30 μm thickness and 0.4–2.35 μm roughness depending on the processing time. Phase analysis showed that the nanostructured coatings contained alumina and zirconia high temperature phases (tetragonal zirconia and α-alumina). Processing for 300 s was found to produce the most compact layer with low surface porosity (0.69%) and 26 μm thickness. This particular PEO treatment was found to reduce the corrosion rate by 2.5 orders of magnitude compared to the uncoated substrate. This significant improvement in corrosion resistance is attributed to the barrier effect of the dense layer and the presence of tetragonal zirconia.

Internal stresses at the crystalline scale in textured ZrO2 films before lateral cracking

Zirconium oxide layers are submitted to internal stresses that play a role in damage of the layer. Lateral cracking is often observed during Zr alloys oxidation. In this paper, we investigated the influence of the microstresses at the crystalline scale on the lateral cracking within a growing oxide on a plane substrate. A parametric study was carried out taking into account the crystallographic texture of the oxide and the presence of a tetragonal zirconia at the metal–oxide interface. Macroscopic computations and polycrystalline aggregate computations were performed. The result indicating the (106¯) fiber texture as the most favorable was recovered. It was found that under macroscopic compressive stresses parallel to the plane metal–oxide interface, positive microstresses perpendicular to the interface develops. They can trigger the lateral cracking and the phenomenon is promoted by the presence of tetragonal zirconia at the metal–oxide interface.

Microstresses in ZrO<sub>2</sub> Layer and Lateral Cracking

Micromechanical simulations of polycrystalline zirconia using the finite element method are performed in order to obtain the stresses at the grain scale of a zirconium oxide layer, since these microstresses are important for damage prediction of the layer and then oxidation kinetics. The crystallographic texture of the layer of monoclinic zirconia is taken into account. The results show that even under high compressive macroscopic stresses, the microstresses can contribute to lateral cracking promoted by the presence of tetragonal zirconia.

Densification and Microstructure Development in Zirconia Toughened Hardmetals

Different process methods and parameters together with different amount of additives were used to fabricate WC-Ni-ZrO2 hardmetals with mechanical properties aiming at improved performance under erosive wear. XRD observation showed the presence of tetragonal zirconia in the cermet matrix after processing. The best erosion resistance with erosion rate of about 0.7 mm3/kg was demonstrated by the specimen produced either by vacuum sintering or SPS and added by 0.2 wt% of free carbon. This cermet has also demonstrated the highest hardness of 17.7 GPa.

Variable incidence angle – X-ray absorption spectroscopy for the study of Zircaloy corrosion layers

Variable incidence angle X-ray absorption fine structure (VIAXAFS) spectroscopy has been used for its non-destructive ability to probe and investigate nano-structures in zirconia films. The theory of X-ray absorption–detection in fluorescence mode was revisited and applied to the variable incidence angles of the photon beam for studying a corrosion film. The expression derived for the depth profile was used to evaluate nano-structures and atom environments in the film. This technique was applied, spanning angles from a grazing towards normal incidence on a sample layer obtained by zirconium alloy corrosion. XAFS analysis on the Zr K edge suggests a lower number of next neighbour Zr atoms in the corroded samples. This may be due to the relative density for given average atom distances suggesting a high density of dislocations and defects. The discussion underlines that the technique reveals nano-pores, dislocations, vacancies or defect features. The presence of tetragonal zirconia is not observed for zirconium alloy corrosion layers.

N-Butane Isomerization Catalyzed by Sulfated Zirconia Nanocrystals Supported on Silica or γ-Alumina

Supported sulfated zirconia catalysts with zirconia contents of 10, 20 and 50 wt% were prepared by impregnation of SiO2 and γ-Al2O3 supports with H2SO4/ Zr(SO4)2 solutions followed by calcination at 923 K. The catalysts were characterized by X-ray diffraction, extended X-ray absorption fine structure measurements, thermal analysis, UV–vis spectroscopy, and electron microscopy. Tetragonal zirconia was detected in all silica-supported samples but only in the 50 wt% zirconia-containing alumina-supported sample, indicating high dispersion of zirconia on alumina. Alumina-supported samples retained additional sulfate, at least in part as Al2(SO4)3. All samples were active in n-butane isomerization (1 kPa n-butane, 378 K). There was no relation between the presence of tetragonal zirconia in these samples and the catalytic performance.

Effect of preparation method on sinterability and properties of nanocrystalline MgAl2O4and ZrO2-MgAl2O4materials

Nanocrystalline MgAl2O4 and ZrO2-MgAl2O4 powders were synthesised by combustion and conventional solid state reaction routes. The synthesised powders were processed, dry pressed, and sintered for 3 h at temperatures ranging from 1550 to 1625°C. The sintered pellets were then characterised in terms of phase (XRD), microstructure (SEM), relative density, apparent porosity, water absorption, hardness, three point bend strength, and fracture toughness. The XRD studies revealed that ZrO2 was present in tetragonal form in the case of combustion synthesised powders (CSP), whereas in powders obtained by solid state reaction (SSP) it was present in the monoclinic form. This study also revealed that the addition of ZrO2 improved the mechanical properties of sintered MgAl2O4 samples: 20 wt-%ZrO2-MgAl2O4 composites prepared from CSPs and conventional SSPs and sintered at 1625°C for 3 h had fracture toughness of 5·96 and 4·33 MPa m1/2 and three point bend strength of 269 and 98 MPa respectively. Higher sintered density, the presence of tetragonal zirconia as a major phase, and the finer microstructure are probably responsible for the superior mechanical properties exhibited by sintered CSP materials as compared with the sintered SSPs.

Analysis of silica fume produced by zircon desilication

Novel chemical methods have been developed to allow for the determination of the components of silica fume produced by zircon desilication. Hitherto, no methods have been described for the analysis of this material. The amorphous silica is first removed by treatment with sodium hydroxide. The residue from the hydroxide treatment may then be subjected to a suite of reagents to determine the zircon, the total zirconia, the monoclinic zirconia and the tetragonal zirconia content of the fume. The zircon content of the fume is determined by treatment of the hydroxide residue with concentrated hydrofluoric acid (HF). The total zirconia content of the fume is determined by digestion of the hydroxide residue with fuming sulphuric acid (oleum), while the relative amounts of monoclinic and tetragonal zirconia may be found by treatment of the hydroxide residue with 10%w/v HF, which attacks the less stable tetragonal phase. Both X-ray diffraction and particle size analysis were used to validate the steps in the analytical procedure. An explanation of the presence of tetragonal zirconia in the fume is proposed. A greater understanding of the composition of the fume led to the installation of a separator in the company's production line to remove the zircon. Australian Fused Materials (AFM) now produces a vastly superior grade of fume marketed under the code SF-98.