Thierry Chartier

Laminar ceramic composites

Abstract The processing of laminar ceramic composites from stacking of layers obtained by tape casting is described. In the case of composites for structural applications, the reinforcement mechanisms are briefly reviewed. It is shown that both strength and toughness can be improved. The evaluation of residual stresses allows a strategy for tailoring the mechanical properties of such composites to be developed. As an example, results are given in the case of laminar composites with layers made of alumina with various zirconia contents.

Dynamics and stabilization of a high power side-pumped Yb- doped double-clad fiber laser

Abstract We present an experimental study of the dynamics of a high power Yb3+-doped double-clad fiber laser in various optical configurations operating in the 1.08 μm wavelength. The fiber is side-pumped with a high power laser diode using the V -groove technique. Different self-pulsing regimes are identified resulting from third-order nonlinear effects. The influence of the cavity losses on the dynamical behavior is also investigated. In addition, we show that the system is efficiently stabilized in a unidirectional ring cavity where Brillouin backscattering is suppressed.

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.

Processing of aqueous α-Al2O3, α-SiO2 and α-SiC suspensions with polyelectrolytes

Abstract The preparation of aqueous α-Al 2 O 3 , α-SiO 2 and α-SiC suspensions with polyelectrolytes, i.e. the NH 4 + salt of polymethacrylic acid (PMA-NH 4 + ) and the acidic form of polyethylene imine (PEI-H + ), is investigated. It is based on the adsorption of these polyelectrolytes which in turn depends mainly on the nature of the polyelectrolyte, and the charge density that develops on the powder surface in water. The efficiency of these dispersants is primarily attributed to the electrostatic interaction between -COO − groups of PMA-NH 4 + and positive sites on the alumina surface at pH 8–9 as well as between -NH 2 + -groups of PEI-H + and negative sites of the silica surface or of the oxidized surface of silicon carbide in acidic media. Good dispersion and stability of dispersions are then obtained through electrostatic and steric stabilization. As an application, the preparation of an Al 2 O 3 -SiC aqueous mixed slurry was investigated as a preliminary step in the processing of Al 2 O 3 /SiC nanocomposites.

Stereolithography of structural complex ceramic parts

A cost-effective method of complex ceramic parts manufacturing using stereolithography has been developed. The process consists in fabricating ceramic pieces by laser polymerization of an UV curable monomeric system, subsequent removal of organic components and sintering. Highly concentrated suspensions of well dispersed ceramic particles (up to 60 vol%) in a reactive acrylic monomer, with a suitable rheological behaviour for the spreading of thin layers (down to 25 μm) were defined. Adequate cured depth (higher than 200 μm) is obtained even at high scanning speeds. Nevertheless, a compromise has to be found between the cured depth and the cured width to improve the dimensional resolution. The dimensional resolution reached on alumina partterns is about 200 μm with an energy density of 0.05 J · cm−2.

Aqueous suspensions for tape-casting based on acrylic binders

Abstract The use of water-based systems represents an interesting alternative to the widespread non-aqueous tape-casting but the low strength of water-based binders generally limits their applicability. A tape-casting slurry is a complex system where each organic component has a substantial effect on the rheological behaviour. In this study the effect of the dispersant and binders in alumina aqueous tape-casting slurries were characterized with electrophoretic mobility and rheological measurements. In aqueous medium, a 4,5-dihydroxy-1,3-benzenedisul-fonic acid, was found to be a very effective dispersant for alumina. The amount of dispersant required to achieve a minimum of viscosity was equal to 0.2 wt%. Two aqueous emulsions of acrylic polymers were used as binders. These binders strongly affect the rheology of the suspensions. The best conditions to obtain a homogeneous stable slurry with a high powder loading suitable for tapecasting were determined in terms of order of component addition, rheological behaviour and ageing of the suspensions. Acrylic binders should act through a cohesive mechanism and lead to green tapes with good mechanical strength.

Casting method for producing low-loss chalcogenide microstructured optical fibers

We report significant advances in the fabrication of low loss chalcogenide microstructured optical fiber (MOF). This new method, consisting in molding the glass in a silica cast made of capillaries and capillary guides, allows the development of various designs of fibers, such as suspended core, large core or small core MOFs. After removing the cast in a hydrofluoric acid bath, the preform is drawn and the design is controlled using a system applying differential pressure in the holes. Fiber losses, which are the lowest recorded so far for selenium based MOFs, are equal to the material losses, meaning that the process has no effect on the glass quality.

Low loss microstructured chalcogenide fibers for large non linear effects at 1995 nm

Microstructured optical fibers (MOFs) are traditionally prepared using the stack and draw technique. In order to avoid the interfaces problems observed in chalcogenide glasses, we have developed a new casting method to prepare the chalcogenide preform. This method allows to reach optical losses around 0.4 dB/m at 1.55 µm and less than 0.05 dB/m in the mid IR. Various As(38)Se(62) chalcogenide microstructured fibers have been prepared in order to combine large non linear index of these glasses with the mode control offered by MOF structures. Small core fibers have been drawn to enhance the non linearities. In one of these, three Stokes order have been generated by Raman scattering in a suspended core MOF pumped at 1995 nm.

Stereolithography for manufacturing ceramic parts

Among the different rapid prototyping technologies, solid freeform fabrication (SFF) is the most suitable for ceramics. Here a stereolithographic technique is presented that allows the usage of pastes composed of ceramic particles dispersed in a photocurable resin for the fabrication of alumina pieces. They exhibit a similar flexural strength than alumina parts made by classical techniques like pressing.

Processing of aqueous tape-casting of alumina with acrylic emulsion binders

Abstract Two commercial acrylic emulsion binders (Tg= −40°C and Tg=10°C) have been investigated as potential binders for aqueous alumina suspensions for the tape casting process. Coupled with an aqueous low-molecular weight ammonium polyacrylate salt solution as dispersant, they lead to homogeneous systems which seem very compatible. The aqueous suspensions do not show apparent foaming. The suspensions exhibit a typical shear-thinning character with a very few time dependent rheological behaviour, whatever the binder compositions tested. Fabrication of homogeneous green tapes with very smooth surfaces and relative densities above 50% has proven to be relatively easy from these suspensions.