Article

Nanostructural study of sol-gel-derived zirconium oxides

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Abstract

Two sol-gel derived zirconia powders were prepared at pH = 0.5 and pH = 5.5. They were investigated as a function of temperature using mainly perturbed angular correlation spectroscopy. The aim was to elucidate the relationship between the nanoscopic configurations around Zr4+ ions and the morphology and structure of the powders. The highly porous material resulting from the solution at higher pH could be described mainly by defective and disordered, very hydrolyzed tetragonal arrays. As temperature increased, the amount of these arrays decreased while they became increasingly asymmetric, thus suggesting their superficial localization. The easy removal of hydroxyls led to the early appearance of the monoclinic phase. The gel obtained from the precursor at pH = 0.5 was entirely described by configurations still involving organic residues. After their calcination, the powder underwent a well-defined two-step hydroxyl removal thermal process leading to the crystallization of the tetragonal and the monoclinic phases. The thermal stability of the metastable tetragonal phase in the investigated powders seems to be controlled by their different capability to absorb oxygen.

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... Thus, the solutions P 1 and P 3 were prepared using a quantity of water greater than the stoichiometric one, which enables the hydrolysis of all propoxide groups of the metallic precursor. On the other hand, although the hydrolysis rate of alkoxides of transition metals is high, the nitric acid presence in the solgel solutions provides a high concentration of hydrogen ions H þ that reduces the nucleophilic character of water and consequently the hydrolysis rate [12]. For this reason, the precursor solution P 1 (without HNO 3 ) undergoes a faster hydrolysis than the solutions P 3 and P 4 , which results in the formation of zirconium preferentially linked to OH-groups instead of propoxide groups thus favoring the fact that condensation occurs through the oxolation mechanism. ...
... It is important to explain the reasons for the complete retention of tetragonal ZrO 2 in the powders P 1 and P 4 . It has been proposed that this can be due to different causes [12,13], one of which is the crystallite size [14]. The crystallite sizes calculated for the powders annealed at 1250 1C were quite similar, that is, 163, 96, 131, and 156 nm for P 1 , P 2 , P 3 and P 4 , respectively. ...
Article
Y2O3-stabilized ZrO2 (YSZ) powders have been prepared by the sol–gel method using different synthesis parameters. Specifically, zirconium n-propoxide was dissolved in propanol at pH 0.5 or 5 (provided by HNO3), with or without acetic acid in the hydrolysis medium. Subsequently, the YSZ powders obtained by gelation and drying of these solutions was characterized using scanning and transmission electron microscopies, X-ray diffractometry, and N2-adsorption. Compacts made from these YSZ powders which were then sintered were also analyzed. It was found that the pH of the hydrolysis medium has a notable influence on the microstructure, morphology, color, crystallinity, and sintering behavior process of these YSZ sol–gel powders. It was also found that the use of acetic acid also affects the YSZ powder features, and results in compacts with higher residual porosity after sintering. Finally, the compacts prepared from the YSZ powders obtained at pH 5 and without acetic acid exhibit the greatest sinterability.
... The films were thermal treated at 370 C for 1 h in a normal atmosphere. The burnt films were 100 nm thick [1][2][3]. ...
... In contrast to the called structural vacancies that are generated to compensate the dopant, the nonstructural vacancies are residual vacancies of the synthesis and they are annihilated when the diffusion mechanism of oxygen becomes operative at higher temperatures (700 C) than the ones used in the burning [2]. The presence of non-structural vacancies produces two main effects [3]. First, it stabilizes the tetragonal or cubic phases in zirconia; this occurs, although transiently, even in zirconia where the concentration of ''structural vacancies'' is not sufficient for stabilization of the tetragonal or cubic phases. ...
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In this work preliminary results of amorphous zirconium crystallization using ion beam pulses are presented. Energetic argon- and oxygen-ion beams generated by a plasma focus device were used to promote crystallization on amorphous ZrO2–2.5 mol% Y2O3 film deposited by chemical solution deposition onto silica glass substrate. The films were burnt at 370 °C for 1 h in normal atmosphere previous to plasma irradiation. The irradiation was performed by means of successive pulses of ion beams. The evolution of the surface morphology and crystallization was followed by AFM and X-rays diffraction in a grazing incidence asymmetric Bragg geometry (GIAB), respectively. Argon-irradiated films showed highly nucleated cubic zirconia after 10 pulses. On the other hand, oxygen-irradiated films showed a delayed and less extensive cubic nucleation, but a more ordered structure and well-defined grains.
... [12,25] This is, probably due to that the larger crystallographic distortion derives in an increase of free energy and reduction in bonding energy, and thus in bigger atom diffusivity. [41,42] The authors believe that the unpredicted high density value gotten for the h 12 sample, sintered at 1200 ºC, it is resulted from the biggest reactivity of h 12 powder particles regarding P 1 . So, these results reveal that it is possible to increase the sinterability of the Y-TZP powders, by adjusting to different H 2 O/Zr molar ratio in the sol-gel synthesis. ...
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In the present work, ZrO 2 -3 mol% Y 2 O 3 (Y-TZP) nanoparticles prepared by sol-gel method are incorporated in a matrix and uniaxially compacted applying different pressures. Sol-gel powders are of high reactivity, but very often the aim to obtain full densification at low sintering temperature, is not achieved. Thus to obtain a dense pieces with these powders require a higher sintering temperature and this has a negative effect on their mechanical properties. However, if one obtains higher density of green compact favours the subsequent densification process of the final piece. In this work, to obtain the compact ones three kinds of zirconia powders are prepared in different ways. Their compaction to high pressures and sintering behaviour are studied and correlated with the densification tests. The obtained green compacts show elevated densification, small grain size and improving mechanical properties after the thermal treatment at 1200 ºC in air.
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Room temperature mechanical properties measured for natural fibrous jade materials with random fibre orientations were similar to strengths (e.g. 100 MPa) and toughnesses,K IC, (e.g. 3M Pa m1/2) in other studies. However, nearly three- and five-fold higher values were found respectively for strength and fracture toughness of jade with highly aligned fibres for crack propagation perpendicular to the fibres. Further, the results indicate significantly higher strength and toughness with decreasing fibre diameter and increasing aspect ratio, and an accompanying increase in intrafibre fracture. However, failure was predominantly catastrophic in character for all fibre orientations, indicating some material (i.e. matrix) is necessary for non-catastrophic failure as found in fibre composites.
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Zirconia aerogels have been prepared by the sol–gel method using tetra-n-butoxy zirconium(IV) and acid catalysis. The nitric acid-to-alkoxide ratio and the alcoholic solvent, as important sol–gel parameters, were varied. After aging, the solvent entrapped in the sol–gel products was removed applying two different methods: supercritical drying at high-temperature and low-temperature extraction with supercritical CO2. Finally, for gel stabilization and removing of organic residues the aerogels were calcined at 573 and 773 K, respectively. The effects of the varied sol–gel parameters, different supercritical drying methods, and calcination temperature on the structural and textural properties were investigated. The aerogels were characterized by means of N2 physisorption, X-ray diffraction, thermal analysis, scanning and transmission electron microscopy. Depending on the kind of alcoholic solvent, nitric acid-to-alkoxide ratio and drying method, the specific surface areas of the mesoporous aerogels varied from 55 to 205 m2 g−1 after calcination in air at 773 K. The aerogels supercritically dried at high-temperature possessed larger pores (17–65 nm) and BET surface areas (143–205 m2 g−1) compared to the aerogels dried by low temperature extraction (<20 nm and 55–112 m2 g−1, respectively). The width of the pore size distribution of all high-temperature supercritically dried gels became smaller with increasing amount of acid. All aerogels dried by low temperature extraction with supercritical CO2 were X-ray amorphous. Upon calcination in air at 773 K, the amorphous aerogels crystallized to tetragonal zirconia. The high-temperature supercritically dried aerogels contained predominantly tetragonal zirconia. The fraction of monoclinic ZrO2 shares increased with increasing nitric acid-to-alkoxide ratio, the use of a long-chained branched alcoholic solvent, such as t-butanol, and increasing calcination temperature.
Article
The chemical reactivity of metal alkoxides offers a broad range of possibilities for chemical modification of these molecular precursors. The whole hydrolysis-condensation process may then be completely different leading to new products. An analysis is presented concerning some of the most common chemical additives used in the sol-gel process. Their role is explained in terms of chemical reactivity. The most important parameters appear to be the reactivity of the new ligand towards hydrolysis, the charge distribution in the new molecular precursor and the coordination numbers of the metal atom.
Article
Relaxation of **1**8**1Ta nuclei due to diffusion of oxygen vacancies in cubic zirconia/yttria has been measured by perturbed angular correlation spectroscopy. The activation energy of the vacancy jump rate is found in a temperature range above 750 degree C and is in reasonable agreement with the activation energy of ionic conduction. Preliminary results of a **1**1**1In/**1**1**1Cd perturbed angular correlation investigation of high-temperature monoclinic and tetragonal zirconia are also reported.
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