Serge Corbel

Ceramic suspensions suitable for stereolithography

Abstract Ceramic three dimensional parts have been fabricated by a Stereolithography (SL) process using a ceramic slurry containing alumina powder, UV curable monomer, diluent, photoinitiator and dispersant, subsequent removal of organic components and sintering. The SL process consists of fabricating parts with complex shapes layer by layer by laser polymerization of a ceramic/resin mixture. The effects of each component on the rheology of the ceramic suspension were investigated. Both, the addition of dispersant and diluent to the curable monomer and the increase in temperature decrease the viscosity down to suitable values for tape casting of the layers and for SL. The homogeneous and stable high ceramic concentration suspensions (53 vol%) exhibited a shear thinning behavior, which is favorable for casting the layers. Adequate cured depth (above 200 μm) and width were obtained even at high scanning speeds with an argon ionized laser.

Stereolithography for the fabrication of ceramic three‐ dimensional parts

Ceramic three‐dimensional parts can be produced by a stereolithography (SL) process using a ceramic suspension containing alumina powder, UV curable monomer, diluent, photoinitiator and dispersant. The monomer reacts to UV radiation (argon ionized laser) and is transformed into a solid polymer which is then removed by thermal treatment (debinding). Subsequent sintering of green parts leads to dense ceramic parts. The effect of each component on the rheology of the alumina suspensions has been studied first. Both the addition of dispersant and diluent and the increase in temperature allow a significant decrease of the viscosity of the suspensions. The highly loaded (more than 55 vol. per cent), homogeneous and stable suspensions have a shear thinning behaviour which is favourable for casting the layers. Adequate cured depth (above 200μm) and satisfactory transversal resolution have been obtained and these allow the production of ceramic parts, which demonstrates the feasibility of the process. Sintering at 1,580°C leads to dense ceramic parts with homogeneous microstructure. The process still needs to be optimized to improve even more the mechanical properties.

Stereolithography of PZT ceramic suspensions

The characteristics of PZT suspensions have been studied and fit to stereolithography restraints. On one hand, researches concern the influence of fillers contents, dispersant concentration, temperature and resins nature and amount on suspensions rheological behaviour. On the other hand, the influence of photoinitiator and PZT concentrations, density of energy and nature of the resin on suspension reactivity was investigated. These experiments have led to the choice of two photosensitive suspensions suitable for stereolithography purpose; which use depend on the fillers content. Furthermore, the stereolithography process has been modified owing to the balance between suspensions rheological and photochemical properties in order to shape piezoelectric ceramics. Thanks to these improvements, PZT ceramics/polymer composites dedicated to transducers and medical imaging applications have been fabricated.

UV Curable Systems for Tape Casting

Abstract The substitution of solvents by photopolymerizable binders in the tape casting process allows to achieve high ceramic loading and to eliminate the drying stage which is a critical step of the tape casting process. After the rapid UV polymerization of the resin, the high strength green tapes can be debinded and sintered. Ceramic suspensions containing alumina or zirconia powder, dispersant, UV curable binder and photoinitiator have been prepared. The use of a low viscosity organic vehicle allows to prepare low viscosity ceramic suspensions, containing about 50 vol% powder, which have a shear thinning behaviour. Because of the rapid attenuation of the incident light in UV curing systems containing ceramic particles it is important to estimate the thickness of the tape that can be treated. The effect of incident energy, of photoinitiator concentration and of powder volume fraction was studied. There is an optimal photoinitiator concentration which maximizes the cured depth and which depends on the volume fraction of solid. A theoretical model based on the Beer–Lambert’s law enables the prediction of cured depth for any volume fraction of solid. To prove the ability to manufacture ceramic sheets by tape casting, some suspensions were tape cast.

Rheological properties of PZT suspensions for stereolithography

The study of the rheological behaviour of PZT suspensions is essential to a better understanding of piezocomposites fabrication through stereolithography process. Research concerns the influence of fillers contents, surfactant doses, temperature and resin nature on suspensions rheological behaviour. They have led to the choice of two photosensitive suspensions suitable for stereolithography purpose; which use depends on the fillers content. Furthermore, the stereolithography process has been modified owing to the balance between suspensions rheological and photochemical properties in order to shape piezoelectric ceramics. Thanks to these improvements, PZT ceramics/polymer composites dedicated to transducers and medical imaging applications have been fabricated.

Industrial photochemistry XXIV. Relations between light flux and polymerized depth in laser stereophotolithography

The stereophotolithography process allows a three-dimensional object to be built by layer-by-layer, space-resolved, light-induced polymerization of a liquid monomer into a solid polymer. The relationship between the light penetration depth in the photoreacting medium, the scanning speed of the laser beam on the surface of the resin and the laser power has been studied. Experimental measurements have been performed and compared with a kinetic model. Empirical laws of evolution of the polymerized depth vs. macroscopic parameters have also been determined allowing the optimal operating conditions in stereophotolithography to be defined.

Impact of the design and the materials of rectangular microchannel reactors on the photocatalytic decomposition of organic pollutant

Abstract The objective of this article is to find the optimal design of rectangular microchannel reactors, in terms of reactor dimensions and materials, in order to increase the photocatalytic activity. Microchannel reactors with immobilized titanium dioxide (TiO2) as photocatalyst have been designed, fabricated, and tested. The photocatalytic degradation of salicylic acid is investigated as a function of microchannel size, materials that constitute the reactor, contaminant concentration and flow rate. All the reactors exhibit the same behavior. Higher degradation is observed for low pollutant concentrations and flow rates. The nature of the constituent element of the reactor has practically no influence on the photocatalytic process. The degradation performance is affected by the microchannel dimensions. The reactor with the largest channel width and length, and the lowest channel height displays the highest photocatalytic activity. For an optimal design of the rectangular microchannel reactors a relation between the degradation ratio X and the dimensions of the microchannel is reported. A linear relationship between X and wL/h2 (L: length, w: width, h: height of the channel) is found experimentally. The product wL emphasizes the uniform irradiance over the entire catalyst surface and confirms that the bottom of the channel covered with TiO2 is photoactivated. The term in 1/h2 is necessary to take into account the mass transfer limitation.

Use of surfactants to reduce the driving voltage of switchable optical elements based on electrowetting.

The advantage of using electrowetting as a novel principle for a reflective display has been previously demonstrated. The principle is based on the controlled two-dimensional movement of an oil/water interface across a hydrophobic fluoropolymer insulator. The main objective of this paper is to show experimentally the influence of surfactants on the electro-optic behavior of a single electrowetting pixel. The concentration and type of nonionic surfactant (Tween 80 and Span 20) have been varied. The experimental data are compared with calculations from the electro-optic model developed previously. The electro-optic performance is significantly affected by the nature and the concentration of surfactant. In the presence of Tween, at concentrations lower than the critical micelle concentration (CMC), and mixtures of Tween and Span the electro-optic behavior can be related to the interfacial tension. When decreasing the oil/water interfacial tension, the amplitude of the driving voltage required for obtaining a given oil displacement decreases and the switching curve becomes steeper. These effects can be accurately reproduced by means of the previously developed electro-optic model. Mixtures of Tween and Span produce a significant synergetic reduction of the driving voltage. For Tween concentrations higher than the CMC and Span, a strong disagreement is observed between the previously developed model and experimental data. Here a new physical model is reported that describes the electro-optic behavior of electrowetting-based optical elements in the presence of surfactants. The model takes into account the actual voltage used to control the liquid movement in electrowetting (lower than the applied voltage), the amount of surfactant adsorbed at the decane/water interface, and the dipole moment of the surfactant molecules. The calculated results are in very good agreement with experimental data without employing fitting parameters. The dipoles interact with the applied field and lower the actual applied field. This reduction of the effective electric field across the solid-liquid interface induces a decrease in the charge density at the solid-liquid interface and reduces the electrowetting force. For surfactant concentrations higher than the CMC, the electro-optic performance does not depend on the surfactant concentration. This demonstrates that the reduction of the electrowetting field due to the large dipole moment of the surfactant molecules occurs at the oil/water interface. A new method for the test cell fabrication is also presented.

Modelling and design of microchannel reactor for photocatalysis

Chemical reactions in a microreactor can offer new possibilities for many chemical engineering application fields and have been employed in photocatalysis studies. The purpose of this work is to investigate the photocatalytic degradation of salicylic acid (SA) as a pollutant model, in a continuous flow microchannel reactor in order to evaluate the influence of radial concentration profile on the photocatalytic efficiency. The study has been developed on both experimental and theoretical aspects. Microreactors have been manufactured by stereolithography in epoxy resin (Accura® SI 30) and are composed of a channel with a rectangular cross-section of about 1 mm2 with different aspect ratios defined as width/depth. Photocatalytic experiments have been performed at different flow rates. Results can be predicted by the Langmuir-Hinshelwood (LH) kinetic model coupled with surface diffusion. Computational fluid dynamics (CFD-Comsol Multiphysics) can model phenomena and confirmed the reliability of our experimental results with diffusion limitation at low flow rate.

ZnO nanoparticles sensitized by CuInZnxS2+x quantum dots as highly efficient solar light driven photocatalysts

Alloyed CuInZnxS2+ x (ZCIS) quantum dots (QDs) were successfully associated to ZnO nanoparticles by a thermal treatment at 400 °C for 15 min. The ZnO/ZCIS composite was characterized by TEM, SEM, XRD, XPS and UV–vis absorption spectroscopy. ZCIS QDs, with an average diameter of ≈4.5 nm, were found to be homogeneously distributed at the surface of ZnO nanoparticles. ZCIS-sensitized ZnO nanoparticles exhibit a high photocatalytic activity under simulated solar light irradiation for the degradation of Orange II dye (>95% degradation after 180 min of irradiation at an intensity of 5 mW/cm2). The heterojunction built between the ZnO nanoparticle and ZCIS QDs not only extends the light adsorption range by the photocatalyst but also acts to decrease electron/hole recombination. Interestingly, the ZnO/ZCIS composite was found to produce increased amounts of H2O2 and singlet oxygen 1O2 compared to ZnO, suggesting that these reactive oxygen species play a key role in the photodegradation mechanism. The activity of the ZnO/ZCIS composite is retained at over 90% of its original value after ten successive photocatalytic runs, indicating its high stability and its potential for practical photocatalytic applications.