A.J. Millán

Aqueous colloidal processing of nickel powder

Abstract Nickel micronic powders are processed by a colloidal route using aqueous suspensions. The optimum amount of dispersant is selected by means of rheological tests. The slurries show a plastic behaviour that retards sedimentation. Dispersed slurries are mixed with a gelling agent (κ-carrageenan) and cooled in order to provide the sample with strength enough for handling. Dynamic and static sintering studies are performed on green samples at temperatures ranging from 700 to 1400°C in flowing argon atmosphere. Porous and dense materials fabricated by this route are characterized by SEM and mercury porosimetry.

Thermogelling polysaccharides for aqueous gelcasting: Part III: mechanical and microstructural characterization of green alumina bodies

Abstract The use of gelling additives, such as polysaccharides, in colloidal processing provides adequate mechanical properties to the green bodies to be handled. In this work, the green density and the mechanical behaviour (stress–strain relationships, elastic modulus, bend strength and fracture mechanism) at room temperature of gelcast alumina bodies are studied, in order to establish the influence of the type and the concentration of additive. Furthermore, the previous concentration of polysaccharide solutions is also taken into account as an important variable. Agar, agarose and carrageenan were used as gelling additives. Values of the bend strength up to 4 MPa are obtained, significantly higher than those corresponding to slip cast alumina without gelling additives, and they increase with the final concentration of polysaccharide, while Youngs modulus values are mainly influenced by the concentration of additive in the precursor solution. For bodies with a large final concentration of additive, extensive plastic deformation during fracture is observed.

Aqueous injection moulding of silicon nitride

Abstract The fabrication of pressureless sintered silicon nitride parts by low pressure injection moulding of aqueous slurries is described. A basic requirement for this process is the preparation of a stable suspension with high solids content and appropriate rheological properties. The temperature has also a determining role in the stability conditions of the slip. In this work the effect of temperature on the rheological behaviour of silicon nitride aqueous slips was studied taking into account slip parameters such as the type and concentration of deflocculant, pH, ageing, etc. Once the stability conditions were determined, the effect of a gelling agent (agarose) on the slip rheology was also studied in order to optimize the injection conditions. The rheological behaviour of slips containing 1 wt.% agarose was studied as a function of temperature as well as the influence of sintering aids. Injection moulding was performed at temperatures of 60–65°C and a pressure of 0.4 MPa. Final densities of about 90% theoretical are obtained after sintering at 1750°C in N 2 flow.

Improved consolidation of alumina by agarose gelation

A great effort has been devoted to develop low-cost effective forming methods for the production of complex-shaped parts. One of the most promising processes is low-pressure injection moulding in water in which a slurry gelates in the presence of a thermogelling binder. For this technique agarose has been found to be a suitable gelling additive. This work deals with the manufacturing of alumina by agarose gelation. For such purpose the rheological behaviour of aqueous alumina slips is studied as a function of the polyelectrolyte concentration, the solid loading and the temperature, in order to reproduce the injection conditions. The gelation behaviour of agarose solutions is studied from rheological measurements. Two different routes for preparing the injection slip are studied, the first by adding agarose as a solid powder and then heating the slurry, and the second by preparing independently the slurry and the agarose solution and further mixing at temperatures higher than 60°C. This second route demonstrates to be suitable for obtaining reliable alumina parts with a green density of 58% of theoretical and a sintered density of 98.7% of theoretical, significantly higher than the values reported in previous works.

Near-net shaping of aqueous alumina slurries using carrageenan

The production of alumina pieces by gelation of concentrated aqueous slurries using carrageenan is described. The potassium carrageenan forms a low viscosity solution when heated, that provides a strong gel on cooling. Aqueous suspensions of alumina are prepared at a solid loading of 50 vol.% at different polyelectrolyte concentrations. The optimum dispersing conditions are selected from zeta potential, particle size distribution and viscosity measurements. Afterwards, the effect of temperature on the rheological behaviour is studied. The rheological properties of carrageenan solutions are studied for different concentrations and the gelation hystheresis is determined. The evolution of viscosity of the alumina slurries containing 0.25 wt.% carrageenan is studied on cooling. Casting the hot blend in a refrigerated non-porous mould allows us to obtain pieces with high green strength in a few seconds. The green density of the casts is 58.2% of theoretical and after sintering at 1500°C/2 h a final density of 98% is reached.

Gel‐Forming of Nickel Powders from Aqueous Slurries

the use of H2 and Ar was critical; when these gases were substituted by helium, no SWNT rope growth occurred in our equipment. We suppose that a SWNT grows in an aopenendedo model, and that the maintenance of the aopen endo will be critical to growth. In our method, we argue that it is hydrogen that effectively prevents the closure of the tube tip, since the aC-o dangling bonds can be temporarily saturated by hydrogen. Thus, longer SWNTs can be synthesized by the hydrogen and argon arc discharge method. The addition of a suitable amount of Ar is to mitigate the vaporization of the anode, which may be quite intense under the atmosphere of pure H2, so that a suitable plasma flow rate can be achieved for the growth of SWNT ropes. In fact, few SWNT ropes were obtained when no Ar was added. Moreover, the two electrodes used in our apparatus are not vertically opposite each other but at an oblique angle. Thus, when the electric arc was operated, a gas flow, caused by the arc blowing force, formed in the reactor chamber. Carried by the gas flow, the SWNTs formed in the reaction zone (near the arc) move together along the flow direction in the chamber and tend to aggregate with each other into bundles. The orientations of the bundles are determined by the gas flow and therefore are roughly the same. When the SWNT bundles are combined by a van der Waals force or are mechanically twisted together, ropes of aligned SWNT bundles are formed. In short, macroscopically long ropes of aligned SWNTs were synthesized by the hydrogen and argon arc discharge method, these long ropes will be convenient for measurement of the properties (mechanical, electrical, optical, etc.) of SWNTs and should also be useful for practical applications of SWNTs. For example, we have directly measured the tensile strength of a SWNT rope composite, from which the tensile strength of a single-walled carbon nanotube was estimated to be 6.0 GPa, about one third of the theoretically predicted strength of SWNTs. Further investigations on SWNT property characterizations based on the ropes are in progress.

Aqueous injection moulding of alumina using agarose

Injection moulding in water is receiving great interest for the low-cost production of near-net-shaped ceramic parts. In this technique, aqueous suspensions are gelated in the presence of a low concentration of gelling additive. In this work, alumina bodies are injection moulded in water by adding 1 wt.% agarose. The agarose is incorporated to the previously dispersed slurry and the blend is then milled in a ball mill. Agarose dissolves upon heating, but the dispersed slurry maintains stability up to 70°C. Rheological behaviour is studied as a function of temperature for the ceramic slurry with and without agarose. The injected parts show a homogeneous microstructure without binder agglomerates.

Rheological characterisation of Al2O3 slips for aqueous low pressure injection moulding

Abstract Near net shaping processes are receiving increased attention for manufacturing low cost, reliable ceramics. Low pressure injection moulding (LPIM) is a suitable forming procedure that allows the production of complex shaped parts by using aqueous suspensions, with interesting economic and environmental benefits. In the present work, aqueous Al2O3 slips were prepared and characterised for LPIM, focusing on their rheological properties not only at room temperature, but also at the injection temperatures (up to 70°C), as well as on the influence of the homogenisation procedure. Agarose (1 wt-%) was added as gelating binder. Injection was performed at temperatures of 55, 60, and 70°C, and the resulting green pieces were characterised in terms of density, shrinkage, and microstructure.