Cécile Pagnoux

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.

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.

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.

Catechol derivatives and anion adsorption onto alumina surfaces in aqueous media: influence on the electrokinetic properties

Abstract A high state of dispersion of alumina particles can be achieved by using molecules derived from Catechol 1,2(OH) 2 C 6 H 4 , such as Tiron (OH) 2 C 6 H 2 (SO 3 Na) 2 which allows to obtain stable alumina suspensions. In the field of understanding the dispersion mechanism of Tiron and the contribution of each functional group grafted onto the benzene ring of the Tiron molecule, adsorption experiments and surface charge measurements were conducted by using several compounds, containing a benzene ring, and by varying the nature and the number of substituents. The influence of the size and of the charge of the counter ion, electrostatically attracted by a positive charged alumina surface, onto the suspension stability was also studied. Electrokinetic properties of the suspensions versus pH appear to be very useful to characterize the ion adsorption sequence for an oxide surface. With a high immersion heat, an alumina surface better adsorbs ions with a high structuring power for water molecules. This influences the charge developed onto the oxide surface.

Mechanical and lamination properties of alumina green tapes obtained by aqueous tape-casting

Abstract Mechanical characterisation and lamination were carried out on alumina green tapes prepared by aqueous tape casting using two acrylic emulsions having different glass transition temperatures (Tg) as binders. The tensile strength and strain were strongly dependent on the binder nature and content. Namely, the mechanical properties of the green tapes reflected those of the binders at room temperature: the green tapes obtained with the higher Tg binder showed a brittle behaviour, whereas those obtained with the lower Tg binder showed an elastoplastic behaviour. The mechanical properties of the green tapes prepared by mixing the two acrylic binders lies in between, giving the possibility of tailoring the flexibility and strength in the range of the values obtained for pure binders. Lamination gave rise to an increase of both green and sintered densities, compared with monolayer specimens, whatever the composition of the binder system. Such improvements significantly depended on lamination pressure, but were insensitive to lamination temperature for the two temperatures tested higher than the Tg of the two binders. ©

Tape casting using UV curable binders

Abstract High-density tapes were obtained by tape casting of a slurry composed of alumina powder, dispersant, UV curable binder and photoinitiator, and by subsequent photopolymerization. The use of a photopolymerizable binder offers the advantage of eliminating the drying stage, which is a critical step of the tape casting process. The effects of the solid loading, of the dispersant and photoinitiator concentrations and of the temperature on the rheological behaviour of the slurries were investigated. Both addition of dispersant and photoinitiator had a positive effect on the rheology of the slurries. The highly loaded alumina suspensions (82 wt%, i.e. 57 vol%) exhibited a shear-thickening behaviour, which was not detrimental in this solvent-free process because of the possibility of hardening the sheet instantly, by polymerization, just after casting. Nearly theoretical densities (98.9%) were obtained after sintering at 1540 °C of the irradiated alumina tapes.

Effect of copolymer dispersant structure on the properties of alumina suspensions

The effects of the molecular structure of sulfonic copolymer dispersants on the interaction with alumina particle surfaces in water and on the rheological properties of alumina suspensions are studied. Sulfonic copolymers with various contents of sulfonate groups were synthetised. The polyelectrolyte-solid surface interactions were investigated through electrokinetic properties and adsorption measurements. The optimum conditions to ensure the stability of the alumina suspension, estimated through particle size distribution and rheology behavior, were obtained for the maximum electrostatic repulsive force with the highest SO3− group fraction. Changes in the configuration of the adsorbed polymer versus the fraction of SO3− group was also discussed.

Influence of the temperature on the stability of aqueous alumina suspensions

Abstract Ceramic process need the preparation of very stable suspensions with high solid loading and the temperature is a fundamental parameter in surface chemistry and at various steps in the production process. The influence of the temperature on the stability of aqueous alumina suspensions dispersed either with the polymethacrylic acid sodium salt either with the Tiron has been studied. The strong adsorption of the two dispersants onto alumina surface is not affected by the variation of the temperature but temperature higher than 40°C strongly influences the amplitude of the repulsive potential created between the particules. By using the potentimetric data fitting software FITEQL 3·2, the electrokinetic properties of the suspensions at 20°C are interpreted and viscosity measurements show that a temperature of 60°C improves the stability of the suspensions and permits to concentrate it.

Self-assembly of oppositely charged particles in dilute ceramic suspensions: predictive role of simulations

Ceramic suspensions composed of oppositely charged alumina and silica particles are studied experimentally and by means of Brownian dynamics simulations. Alumina and silica particles have quite similar sizes, the former having an average diameter larger by a factor 1.6. The suspension behavior is studied as a function of composition. The aggregation state, the aggregate composition, structural aspects at several length scales and the aggregate growth kinetics are analysed. A good agreement between numerical and experimental results is obtained. The simulations allow us to describe in detail the aggregation process and mechanisms. Simulations appear as an important tool to predict and control the particle assembly in such binary suspensions, whose behaviour depends on several parameters.

Influence of the electrostatic interactions in a Pickering emulsion polymerization for the synthesis of silica-polystyrene hybrid nanoparticles.

HYPOTHESISnSilica-polystyrene hybrid nanoparticles were synthesized by Pickering emulsion polymerization. The coupling effect of initiator type and silica surface charge was studied to exhibit the predominant role of electrostatic interactions in the synthesis mechanisms.nnnEXPERIMENTSnNon-ionic hydrophobic initiator (2,2-azobis(2-methylpropionitrile), AIBN) or anionic hydrophilic initiator (sodium persulfate, NaPS), and positively or negatively charged silica were used as reactants with styrene for Pickering emulsion polymerization. Their interactions were evaluated by Zeta potential measurements. The droplet size and the stability of the Pickering emulsions, and the hybrid particle morphology, surface coverage, size and agglomeration were evaluated by laser granulometry and microscopy.nnnFINDINGSnSimilar surface charge between negatively charged silica particles and an anionic initiator led to strong repulsions and thus to non-covered polystyrene nanoparticles. With positively charged silica, a high decoration was obtained due to attractive interactions between the inorganic and the organic phases, but a strong agglomeration was also observed. The use of a non-ionic initiator led to a homogeneous coverage with negatively charged silica. With positively charged silica micronic sizes were formed by following two different mechanisms. These data, by enriching the existing literature, led to a more complete and robust description of the emulsion polymerization synthesis for hybrid nanostructures.