Antonio Javier Sanchez-Herencia

Functionally gradient ceramics by sequential slip casting

Abstract Sequential slip casting is proposed as a route for future developing of a new family of dense functionally gradient ceramics (FGCs) with complex shapes and tailored microarchitectures. Following this route an alumina/yttria tetragonal zircona polycrystal (Y-TZP) FGC with close to theoretical density, homogeneous layers and sharp layer interfaces has been obtained.

Aqueous electrophoretic deposition of AL2O3/ZrO2 layered ceramics

Abstract Layered ceramics are being prepared by different forming methods such as slip or tape casting, dip coating, etc. Microlaminate materials have been also prepared by electrophoretic deposition (EPD), but usually from non-aqueous suspensions. In this work, the preparation of Al 2 O 3 /ZrO 2 layered ceramics from aqueous suspensions is described. The growth of the deposit thickness can be controlled in order to design laminar ceramics either as coatings or as self-supported deposits, with a few thick layers, reducing warping effects, and avoiding environmental problems.

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.

Electrophoretic Forming of Al2O3/Y-TZP Layered Ceramics from Aqueous Suspensions

Electrophoretic forming is emerging as an economical and reliable method for the fabrication of coatings and laminate ceramics. The method is very simple, but a major problem is the toxicity and environmental problems associated with the organic solvents usually employed as vehicle media. This work aims to obtain Al2O3/Y-TZP laminates by electrophoretic deposition using water as the liquid vehicle. The slip characteristics have been studied in terms of viscosity, zeta potential, and conductivity for both starting powders. Electrophoretic deposition was performed at constant values of current density to obtain the monolithics and layered materials. Very small variations in the deflocculating conditions allow control over the thickness of each layer in the laminate, providing a powerful method for obtaining tailored complex microstructures.

Electrical transport properties in zirconia/alumina functionally graded materials

Functionally graded materials (FGMs) are promising candidates for the fabrication of technological components, not only as structural devices, but also in electrochemical ones, such as solid oxide fuel cells (SOFC), or high-efficiency hybrid direct energy conversion systems. In the present work FGMs were prepared by the sequential slip casting technique, starting with an yttria tetragonal zirconia polycrystalline layer and increasing subsequently the amount of Al2O3 in the following layers. Electrochemical impedance spectroscopy (EIS) analysis was used to evaluate the electrical characteristics of these materials and to compare with those of the monolithic compacts. In general, it was observed that the FGM conductivity is ruled by the conductivity of the layer which contains the highest amount of alumina blocking particles. By EIS no electrical interfaces between adjoining layers were detected and, accordingly, no specific electric ohmic losses were observed. The conductivity of the FGMs is close to that calculated using the normalized thicknesses and the alumina volume fractions of the layers after measuring the conductivity of the monolithic materials with the same composition to what correspond to that of the final layer in the FGM. These results suggest that the gradient structure can be used to control the oxygen vacancy motion, and then applied in electrochemical devices.

Fracture behaviour of alumina–calcium hexaluminate composites obtained by colloidal processing

Three diAerent alumina (90 vol.%)‐calcium hexaluminate (10 vol.%) composites have been prepared by colloidal processing of high purity starting powders in aqueous media and conventional sintering at 1500, 1550 and 1600C. Calcium hexaluminate has been synthesised by high temperature reaction of alumina and calcium carbonate powders. The optimum dispersion conditions for the mixture have been selected from data for alumina and calcium hexaluminate slips studied independently. The microstructure of the obtained materials has been characterised by scanning electron microscopy. All of them showed a highly dispersed distribution of small CA6 grains inside the alumina matrix. Grain size and shape of the alumina grains was highly dependent on the sintering temperature whereas the grain size of calcium hexaluminate remained almost constant. The fracture behaviour of the composites was analysed using Vickers indentation and optical and scanning microscopies. Higher loads could be applied to the composites without additional lateral cracking than to a monophase alumina material of the same average grain size. The fracture toughness values of the composites were highly dependent on the microstructure and for two of them values were larger than for the alumina material. Results have been discussed in terms of residual thermal stresses between the grains due to thermal expansion anisotropy. # 2000 Elsevier Science Ltd. All rights reserved.

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.

Water Based Colloidal Processing of Ceramic Laminates

Multilayered materials and coating are complex structures proposed among others to face the structural requirements of ceramics. The development of reinforcement mechanism by laminated structures can be due to deflection criteria or to the presence of residual stresses and requires of tailored laminates. These designs are characterized by the phases, thickness and distribution of the layers as well as the joining strength between them. In this sense water based colloidal processing techniques are used to fabricate layered structures by consolidating the layers from fluid dispersions of the powders in water. In these processing methods the phases presented in the final laminate are mainly given by the composition of the starting slurries while the changes in thickness and sharpness of the layers are controlled by acting on the processing parameters. The achievement of stable slurries is a shared step for all the colloidal processing techniques. In the water based slurries the stability will be dominated by the polar media, the surface behavior of the particles and the presence of dispersant additives to increase the repulsion between particles. The stable slurry ensures an effective milling and dispersion of the phases as well as high solid loadings, if required. Further processes associated to shaping and consolidation of the layers requires the incorporation of additives and-or water removal. The shaping methods based on aqueous slurries can be classified taking into account the process of solid-water separation. For each of those shaping methods, the nature and amount of the additives is different in order to get the optimum rheological behavior and green strength after drying. Depending on the thickness of layers and coatings as well as the shape and dimensions of the samples, the shaping method can be selected alone and combined with others.

Al2O3/Y-TZP continuous functionally graded ceramics by filtration-sedimentation

Abstract The filtration-sedimentation method is applied as an alternative route to obtain functionally graded ceramics (FGCs) without defined interfaces. Using this technique a fully dense Al2O3/Y-TZP continuous functionally graded ceramic has been obtained after sintering at 1550 °C. The concentration gradient shows a parabolic distribution of phases which depend on the starting slurry solid concentration.

Improvement of Ti Processing through Colloidal Techniques

The colloid-chemistry control of metallic powders in aqueous slurries is proposed as a way to prepare Ti powders with small particle size for a better pressing behavior through the spray dry process. The chemical-physic behavior of titanium powders with two different particle size distributions dispersed in water has been studied by measuring the zeta potential as a function of pH, and dispersant concentration. The employment of poly-acrylic dispersants allowed the fabrication of stable slurries with solid contents up to 50 vol% that have been sprayed under different conditions to form agglomerates ranging between 50 and 200 µm. Conditions were selected to achieve spherical agglomerates formed by a broad distribution of particle sizes that shown excellent flowability. Agglomerates were pressed in a uniaxial die to measure the compressibility, showing an improvement in pressing behavior with respect to powders with bigger particle size. The sintering behavior is also improved, as values of 96 % of the theoretical density were obtained for compacts sintered in vacuum at 1100 °C for 30 minutes.