Athena Tsetsekou

Optimization of the rheological properties of alumina slurries for ceramic processing applications Part I: Slip-casting

Aqueous powder slurries are widely employed in ceramic manufacturing processes like slip-casting and spray-drying. The aim of the present work was to identify the conditions for the preparation of stable alumina slurries with high solids content for the production of slip-cast objects with improved properties, as well as to correlate the slurry properties to the final object properties. For slurry stabilization, three commercial dispersants were compared. It was found that for each dispersant there exists an optimum concentration range within which low viscosity is achieved for a slurry of high solids content. In addition to the slurry solids content, the choice of a particular dispersant also affects the slurry viscosity and through that the casting rate. The combination of high slurry solids content and slower casting rate results in objects with higher densities both in the green and fired state.

Deposition of meso-porous γ-alumina coatings on ceramic honeycombs by sol-gel methods

Abstract The deposition of meso-porous γ-alumina coatings on multi-channeled cordierite honeycombs via sol–gel methods was investigated with the aim to correlate the deposition characteristics such as loading percentage, thickness and integrity of the coating to the support pore structure properties. Even though the mean pore size of the honeycomb supports was much higher than the size of the deposited particles, proper adjustment of sol viscosity prevented penetration of the sol into the support and led to the formation of a smooth coating of uniform, adjustable thickness. Sol viscosity was adjusted with the addition of poly-vinyl-alcohol (PVA) and with sol concentration by controlled evaporation, and fine-tuned in order to control loading percentage from 2 to 8 wt.% per impregnation, corresponding to coating thickness from 2 to approximately 10 μm respectively. The mean pore diameter of the support was found to affect the loading percentage. However, scanning electron microscopy observations have revealed that a very high loading percentage almost inevitably induces cracks on the coatings surface. The combination of sols and slurries of powders as coating media seems to be the optimum technical solution that can provide for satisfactory loading percentage per impregnation together with structural integrity of the coating.

Thermal expansion and microstructural analysis of experimental metal-ceramic titanium alloys

UNLABELLED Statement of problem Low-fusing porcelains for titanium veneering have demonstrated inferior color stability and metal-ceramic longevity compared to conventional porcelains. PURPOSE This study evaluated the microstructure and thermal expansion coefficients of some experimental titanium alloys as alternative metallic substrates for low-fusing conventional porcelain. MATERIALS AND METHODS Commercially pure titanium (CP Ti) and various metallic elements (Al, Co, Sn, Ga, In, Mn) were used to prepare 8 titanium alloys using a commercial 2-chamber electric-arc vacuum/inert gas dental casting machine (Cyclarc). The nominal compositions of these alloys were the following (wt%): I: 80Ti-18Sn-1.5In-0.5Mn; II: 76Ti-12Ga-7Sn-4Al-1Co; III: 87Ti-13Ga; IV: 79Ti-13Ga-7Al-1Co; V: 82Ti-18In; VI: 75.5Ti-18In-5Al-1Co-0.5Mn; VII: 85Ti-10Sn-5Al; VIII: 78Ti-12Co-7Ga-3Sn. Six rectangular wax patterns for each test material (l = 25 mm, w = 3 mm, h = 1 mm) were invested with magnesia-based material and cast with grade II CP Ti (control) and the 8 experimental alloys. The porosity of each casting was evaluated radiographically, and defective specimens were discarded. Two cast specimens from CP Ti and alloys I-VIII were embedded in epoxy resin and, after metallographic grinding and polishing, were studied by means of scanning electron microscopy and wavelength dispersive electron probe microanalysis. One specimen of each material was utilized for the determination of coefficient of thermal expansion (CTE) with a dilatometer operating from room temperature up to 650 degrees C at a heating rate of 5 degrees C/minute. RESULTS Secondary electron images (SEI) and compositional backscattered electron images (BEI-COMPO) revealed that all cast specimens consisted of a homogeneous matrix except Alloy VIII, which contained a second phase (possibly Ti(2)Co) along with the titanium matrix. The results showed that the coefficient of thermal expansion (CTE) varied from 10.1 to 13.1 x 10(-6)/ degrees C (25 degrees -500 degrees C), depending on the elemental composition. CONCLUSION The CTE of titanium can be considerably changed by alloying. Two-phase alloys were developed when alloying elements were added in concentrations greater than the maximum solubility limit in alpha-titanium phase.

The effect of powder characteristics on washcoat quality. Part II: Zirconia, titania washcoats — multilayered structures

Abstract Novel automotive catalyst designs based on the use of washcoat systems such as yttria-stabilized zirconia (YSZ) and titania, in combination with the traditionally used γ-alumina have been recently proposed. For the development of an integrated, robust multi-layered system, good adhesion of the inner layer on the support as well as among the various layers has to be ensured. In the present work, the adhesion of zirconia and titania washcoats on cordierite honeycombs was investigated. Irrespective of the nature of the powder used, reduction of the agglomerates’ size down to the order of few (2–5) microns is necessary in order to ensure firm adhesion of the washcoat to the support, comparable to that of commercial catalysts. In the deposition of multi-layered structures, particle size compatibility of the powders of the various washcoat layers can enhance the adhesion among them and induce better coherence of the overall washcoat.

Evaluation of sol-gel methods for the synthesis of doped-ceria environmental catalysis systems. Part I: preparation of coatings

Sol-gel technology can be employed for the synthesis of multi-component nanophase environmental catalysts with enhanced catalytic activity, controlled composition and tailor-made pore structure together with their simultaneous deposition upon porous supports. The conditions for effective deposition of nano-phase doped-ceria systems onto cordierite honeycombs through sol-gel routes were investigated. For the direct casting from the sol phase, the sol rheological characteristics were adjusted and optimized so that homogeneous smooth washcoats were obtained. In this way, integrated support-catalyst assemblies suitable for high-temperature gas-solid catalysis were prepared. To overcome the inherently low loading percentage achieved per impregnation when depositing directly the sols, the synthesis of sol-gel powders and their deposition from aqueous slurries, as well as hybrid deposition methods using sol-gel powders dispersed in sols were tested as alternatives. The hybrid sol/powder systems proved to be the most attractive by combining an effective loading procedure with desirable and fine-tuned coating characteristics.

Microstructure effect on the properties of a commercial low-fusing dental porcelain

The aim of this study was to investigate the effect of firing cycle on a dental porcelain microstructure in order to correlate microstructure changes with mechanical and thermal properties. A commercial low-fusing dental porcelain powder (Omega 900, Vita) was investigated for this purpose. The powder was treated at different temperatures in the range 750–1000 °C. The fired samples were characterized in terms of their morphology and microstructure, and their mechanical and thermal properties were evaluated. The results showed that firing temperature affects porcelain microstructure influencing significantly in this way both the mechanical properties and the thermal expansion coefficient of the fired objects. Firing at 800 °C led to a homogeneous structure. After treatment at this temperature, the leucite crystals exhibit their maximum concentration and they are well dispersed into the glassy phase. As a consequence the optimum mechanical strength and the maximum thermal expansion coefficient are observed in these samples.

The effect of YBa2Cu3O7-x powder characteristics on thick coatings prepared by atmospheric plasma spraying

The development of superconducting YBa2Cu3O7-x plasma sprayed coatings on metal substrates can be very useful for applications such as targets for thin-film deposition techniques (sputtering, laser ablation, ion assisted deposition) or magnetic shielding, due to the brittle nature of bulk superconductors. The plasma spraying technique is very flexible and can be used for manufacturing components with a large variety of geometries. This technique requires the use of powders with good rheological characteristics. In this study, YBa2Cu3O7-x powders were produced by using the conventional solid-state reaction route and also by spray drying a solution of nitrate precursors. Both powders, as well as mixtures of them, were plasma sprayed to develop coatings on stainless-steel substrates, with the aim of studying the effect of the feedstock powder characteristics on the coating properties. It was found that by optimizing the plasma spraying conditions, good quality coatings could be obtained. However, the powder morphology and homogeneity significantly affect the coating quality. More homogeneous powders lead to better results, the spray-dried powder being the best because of its enhanced rheological properties and good morphology.

Development of YBCO coatings by atmospheric plasma spraying

Abstract Superconducting Y–Ba–Cu–O thick films were produced by the atmospheric plasma spraying method. The effect of processing parameters (powder characteristics, spraying parameters) on the coatings properties was studied. X-ray diffraction analysis, SEM studies combined with EDS microanalysis and scratch test experiments were carried out in order to characterize the adhesion of the coatings to the substrate, the coatings morphology the thickness and crystalline structure as well as the powder phase transformations during spraying. For restoring the superconducting phase after deposition, the coatings were heated in oxygen in the temperature range 750–930°C. It was shown that the quality of the coatings and the adhesion to the substrate are greatly dependent on the deposition conditions. By calcining in oxygen under the appropriate conditions coatings consisting of the pure superconducting phase can be obtained.

Processing effect on microstructure and superconducting properties of sintered ReBa2Cu3Oy ceramics — the role of ionic radius

Abstract A comparison study of the processing parameters for the production of single phase sintered ReBa 2 Cu 3 O y samples (Re=Nd, Eu, Gd, Dy, Ho, Er), with good superconducting properties was carried out. The relationships between the microstructure of sintered ReBa 2 Cu 3 O y superconductors and processing variables (sintering time, sintering temperature) were examined. It was found that the superconducting properties and microstructural parameters such as density, mean grain size and the presence of secondary phases are strongly dependent upon the processing conditions and the Re-ion size of the ReBa 2 Cu 3 O y systems.

Development of superconducting YBa2Cu3O7−x powders by spray drying

Spray drying is a technique used for the production of powders with high chemical homogeneity, consisting of spherical shaped grains with well-controlled particle size distribution, which show very good rheological characteristics. In the present study, the spray drying technique was employed for the production of a homogeneous precursor powder for the synthesis of the superconducting YBa2Cu3O7−x powder. The study, which is concerned with the spray drying of either a solution of precursor nitrate salts or a slurry of precursor oxides, showed that both the powder characteristics and the process yield depend significantly on the drying parameters and that the maximum yield can be obtained within narrow condition limits. A comparison study between the drying of the nitrate salts solution and that of the oxides slurry showed that in the second case the efficiency achieved was much higher.