Sol–gel thermal barrier coatings: Optimization of the manufacturing route and durability under cyclic oxidation

A new promising and versatile process based on the sol–gel transformation has been developed to deposit yttria-stabilised thermal barrier coatings. The non-oriented microstructure with randomly structured pore network, resulting from the soft chemical process, is expected to show satisfactory thermo-mechanical behaviour when the TBC is cyclically oxidized. First stage of the research consists of optimizing the processing route to generate homogeneous microstructure and controlled surface roughness. The objective is to reduce, as much as possible, the size and depth of the surface cracks network inherent to the process. Indeed, the durability of the TBC when cyclically oxidized strongly depends on the sharpness of those cracks that concentrate thermo-mechanical stresses and generate detrimental propagation resulting in spallation. Cyclic oxidation tests are performed using a cyclic oxidation rig instrumented with CCD cameras to monitor in a real time basis the mechanism of crack propagation and spallation. The impact of various parameters either directly related to the processing route, e.g. the intimate microstructure of the TBC and the TBC thickness, or to the thermal loading, e.g. the oxidation temperature and the cumulated hot time, on the durability of the TBC is investigate

In situ CCVD synthesis of carbon nanotubes within a commercial ceramic foam

Consolidated nanocomposite foams containing a large quantity of carbon nanotubes (CNTs) within millimetre-sized pores are prepared for the first time. A commercial ceramic foam is impregnated by a 60 g L21 slurry of a (Mg(12x)(Co0.75Mo0.25)xO solid solution (x = 0.01, 0.05, 0.1 and 0.2) powder in ethanol. Three successive impregnations led to deposits several tens of mm thick, with a good coverage of the commercial-ceramic pore walls but without closing the pores. The materials were submitted to a CCVD treatment in H2–CH4 atmosphere in order to synthesise the CNTs. When using attrition-milled powders, the carbon is mostly in the form of nanofibres or disordered carbon rather than CNTs. Using non-milled powders produces a less-compact deposit of catalytic material with a higher adherence to the walls of the ceramic foam. After CCVD, the carbon is mostly in the form of high-quality CNTs, as when using powder beds, their quantity being 2.5 times higher. The so-obtained consolidated nanocomposite materials show a multi-scale pore structuration

Matemática, una actividad humana

Año tras año constatamos que los alumnos aspirantes a ingresar - tanto a la Universidad Nacional del Sur como a la Facultad Regional Bahía Blanca, UTN - se enfrentan con distintos obstáculos asociados con los objetos básicos del cálculo, los que se reiteran en forma sistemática. Desde nuestra experiencia creemos que algunas posibles causas que obstaculizan el aprendizaje y muy especialmente el de los contenidos matemáticos las podemos relacionar con una educación matemática basada durante mucho tiempo en ideas que provienen de un enfoque formalista de la disciplina, en métodos didácticos apoyados fuertemente en la memoria y la algoritmia, sobre valorando los procedimientos analíticos, otorgando excesiva prioridad al marco algebraico o al numérico. En cambio, si consideramos a la matemática como una construcción humana que surge como consecuencia de la necesidad del hombre para dar respuesta a cierta clase de problemas, problemas que se pueden referir al mundo natural y social o bien pueden ser internos a la propia disciplina, los objetos matemáticos (conceptos, procedimientos, teorías,…) surgen y evolucionan progresivamente, para dar respuesta o solución a estos problemas. Esta visión obliga a una reformulación epistemológica, la cual consiste en considerar al humano haciendo matemática y a diseñar situaciones donde el foco de atención no esté sólo en la adquisición del conocimiento, sino también en el desarrollo de actividades, para lo cual la formación docente es un factor primordial para iniciar el cambio

The sol–gel route: A versatile process for up-scaling the fabrication of gas-tight thin electrolyte layers

Sol–gel routes are often investigated and adapted to prepare, by suitable chemical modifications, submicronic powders and derived materials with controlled morphology, which cannot be obtained by conventional solid state chemistry paths. Wet chemistry methods provide attractive alternative routes because mixing of species occurs at the atomic scale. In this paper, ultrafine powders were prepared by a novel synthesis method based on the sol–gel process and were dispersed into suspensions before processing. This paper presents new developments for the preparation of functional materials like yttria-stabilized-zirconia (YSZ, 8% Y2O3) used as electrolyte for solid oxide fuel cells. YSZ thick films were coated onto porous Ni-YSZ substrates using a suspension with an optimized formulation deposited by either a dip-coating or a spin-coating process. The suspension composition is based on YSZ particles encapsulated by a zirconium alkoxide which was added with an alkoxide derived colloidal sol. The in situ growth of these colloids increases significantly the layer density after an appropriated heat treatment. The derived films were continuous, homogeneous and around 20 μm thick. The possible up-scaling of this process has been also considered and the suitable processing parameters were defined in order to obtain, at an industrial scale, homogeneous, crack-free, thick and adherent films after heat treatment at 1400 °C

An Ice-Structuring Mechanism for Zirconium Acetate

International audienceThe control of ice nucleation and growth is critical in many natural and engineering situations. However, very few compounds are able to interact directly with the surface of ice crystals. Ice-structuring proteins, found in certain fish, plants, and insects, bind to the surface of ice, thereby controlling their growth. We recently revealed the icestructuring properties of zirconium acetate, which are similar to those of ice-structuring proteins. Because zirconium acetate is a salt and therefore different from proteins having icestructuring properties, its ice-structuring mechanism remains unelucidated. Here we investigate this ice-structuring mechanism through the role of the concentration of zirconium acetate and the ice crystal growth velocity. We then explore other compounds presenting similar functional groups (acetate, hydroxyl, or carboxylic groups). On the basis of these results, we propose that zirconium acetate adopts a hydroxy-bridged polymer structure that can bind to the surface of the ice crystals through hydrogen bonding, thereby slowing down the ice crystal growth

Structural study of metastable tetragonal YSZ powders produced via a sol-gel route

Sol-gel yttria stabilized zirconia (YSZ) is investigated in this paper. The final aim is to process YSZ powders into stable slurries in order to prepare thick coatings for thermal barrier to be applied on hot turboengine components. In fact, this system is well-known for its excellent thermomechanical resistance at elevated temperatures but the relationship between these performances and the structural and microstructural characteristics of these materials is not fully understood. This paper reports a preliminary study concerning recent progress on the structural properties control of YSZ powders synthesized by solâ��gel process and the main advantages of this process compared to conventional methods. As a first step towards this understanding, structural investigations of ZrO2 doped with various xmol%YO1.5 coatings, have been performed using X-ray diffraction, structural Rietveld refinement, Raman spectra analysis and transmission electron microscopy. The evolution of the crystallographic structure of YSZ powders after air annealing at various temperatures 1100 °C, 1200 °C and 1400 °C was studied to well understand the conditions of the formation of desired metastable tetragonal phase (t). Then, this work should allow to correlate chemical and thermomechanical parameters as YSZ formulation and sol-gel elaboration conditions, temperature and t phase performances

La macana del Mukanda (Reseña casi imposible de un clásico)

Víctor Turner fue un antropólogo escocés (1920-1983) cuyos inicios en la disciplina fueron dentro del estructural-funcionalismo británico, pero su posterior paso por el Rhodes-Livingstone Institute en África, bajo la dirección de Max Gluckman, lo influenciará durante el período en el escribe su libro La Selva de los Símbolos y el capítulo que aquí comentaremos, “Mukanda: el rito de la circuncisión”

Morphology and structure of YSZ powders: Comparison between xerogel and aerogel

Yttria-stabilized zirconia (YSZ) fine powders were prepared via sol–gel route in order to shape thermal barrier coatings (TBCs) from these powders. The main objective is to develop new undirectional coatings to allow best thermo-mechanical accommodations compared to conventional processes. To reach this aim, powders have to be able first to be highly dispersed into a sol (non-agglomeration, high specific surface area, etc.) and second to crystallize in the required metastable phase t0. Two routes have been used to dry gels: the conventional way which consists of simple evaporation of the solvent is compared to drying in supercritical conditions. Both YSZ powders after calcination at 950 8C of xerogel (Ex-xero-YSZ powder) and aerogel (Ex-aero-YSZ powder) crystallize in the tetragonal form. N2 adsorption/desorption analysis of the Ex-xero-YSZ powder indicates an Sw of 2.8 m2/g. For the Ex-aero-YSZ powder, the Sw (26 m2/g) is much higher than of Ex-xero-YSZ, leading to a better sintering capability. This high Sw is correlated to the small crystallite size (26 nm) and the alveolar morphology of Ex-aero-YSZ powders compared to Exxero-powder (49 nm). By reducing particles size and increasing the Sw of the powders, supercritical drying appears as a promising way to prepare stable slurries or loaded sols from fine YSZ particles for TBC applications. Indeed, after preparing nanometric powders, they are dispersed into a sol before shaping on superalloys substrates. After thermal treatment at 950 8C for 2 h which corresponds to the working temperature of TBC, the final aim will be to prepare ceramic YSZ coatings

Cardiovascular and renal effects of hyperuricaemia and gout.

A number of epidemiological studies have reported an association between serum uric acid levels and a wide variety of high-risk conditions including hypertension, insulin resistance, and kidney and cerebro-cardiovascular disease. All things considered, serum uric acid may induce cardiovascular and kidney events both directly and indirectly by promoting other well-known mechanisms of damage. While asymptomatic hyperuricemia is currently not considered to be an indication for urate lowering therapy, there is growing evidence indicating a linear relationship between pharmacological reduction in serum uric acid and incidence of cardiovascular and renal events