Showing papers in "Journal of The European Ceramic Society in 1998"

Processing of porous ceramics by ‘starch consolidation’

Abstract: A new consolidation method for forming porous ceramics was developed using starch as both consolidator/binder and pore former. Simple and complex-shaped components of porous alumina were shaped and demoulded in wet state. After drying, burn-out and sintering, materials with ultimate porosities between 23 and 70% were obtained. The overall pore structures were dominated by the large spherically shaped pores (10–80 μm) left by the starch partides. Furthermore, the average size of the small pores connecting the large pores, measured by Hg porosimetry, was controlled by the total solids loading and starch content in the originally prepared slips and varied between 0.5 and 9.5 μm. Chemically modified starch was found to give better dimensional control and regularity regarding average size of the connecting pores than native starch owing to more stable properties during water processing. This new forming technique can be used in the manufacture of various porous ceramics for thermal insulation applications, as filters/membranes and gas burners, or as bioceramics. Major advantages are the simplicity of the process, the possibilities of forming complex shapes using various mould materials and the lowcost processing equipment/materials needed.

Biomorphic Cellular Silicon Carbide Ceramics from Wood: I. Processing and Microstructure

Abstract: Processing of cellular ceramics with anisotropic pore structures by infiltration of liquid Silicon into carbonized wood and subsequent reaction to SiC was investigated. Natural wood of different pore size distribution and composition was carbonized at 800–1800°C in inert atmosphere resulting in a one-to-one reproduction of the original wood structure. The carbon template was converted to SiC by a rapid liquid infiltration-reaction process at 1600°C. Spontaneous infiltration was achieved by using the continous tracheidal cells in wood as a transportation path for liquid silicon. β-SiC formed by solid–liquid reaction at the pore surface exhibits a crystalline texture which may be related to the initial microfibril orientation in the cell walls of wood. Depending on the initial cellular microstructure of the various kinds of wood (ebony, beech, oak, maple, pine, balsa) ceramic materials of different anisotropic pore structures in the form of pseudomorphs of the original wood were obtained.

Electrical properties of new type high oxide ionic conductor RE10Si6O27 (RE = La, Pr, Nd, Sm, Gd, Dy)

Abstract: Electrical properties of the high oxide ionic conductive ceramic RE10Si6O27 (RE=La, Pr, Nd, Sm, Gd, Dy) sintered at 1700–1800 °C by the use of MgO-stabilized zirconia setter during a sintering have been investigated. The lowest activation energy and the highest conductivity at 200 °C were achieved for Pr10Si6O27 (55.4 kJmol−1) and La10Si6O27 (1.32 × 10−5 S cm−1), respectively. The conductivity of La10Si6O27 at 200 °C was higher by order of 1.5 × 101 times than that (8.82 × 10−7 S cm−1 at 200 °C) of (Bi2O3)0.75(Y2O3)0.25. The three-point bending strength of La10Si6O27 was 100 MPa.

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.

Ceramic foams by powder processing

Abstract: Ceramic foams show a significant potential of development and application, essentially due to the emergence of environmental preoccupations. A brief overview of the state of the art in cellular ceramic application, preparation and characterization is presented in order to introduce some new data concerning the elaboration of mullite and PZT foams by a replication and a bubble generation method, respectively. Some discrepancies between the theory, developed for describing the properties of open-cell foams, and the experimental mechanical behaviour of these semi-closed cell materials were also shown.

Biomorphic cellular silicon carbide ceramics from wood : II. Mechanical properties

Abstract: Silicon carbide ceramics with anisotropic pore microstructures pseudomorphous to wood were obtained by liquid Si infiltration of porous carbonized wood templates. Depending on the initial cellular microstructure of the various kinds of wood (ebony, beech, oak, maple, pine, balsa) ceramic materials of different density, pore structure and degree of anisotropy were obtained. Strength and elastic modulus of the pyrolyzed carbon preform and of the final silicon carbide ceramic were measured in different loading directions with respect to the initial cell orientation, e.g. axial, radial and tangential. Generally, the mechanical properties increase with fractional density. Strength and strain-to-failure in axial direction exhibit significantly higher values compared to loading in radial and tangential directions. The orientation dependence of microstructure-property relations may become important for the development of advanced anisotropic light weight structural materials.

Carbon nanotubes–Fe–Alumina nanocomposites. Part II: microstructure and mechanical properties of the hot-Pressed composites

Abstract: Carbon nanotubes-Fe-Al2O3 massive composites have been prepared by hot-pressing the corresponding composite powders, in which the carbon nanotubes are arranged in bundles smaller than 100 nm in diameter and several tens of micrometers long, forming a web-like network around the Fe-Al2O3 grains. In the powders, the quantity and the quality of the carbon nanotubes both depend on the Fe content (2, 5, 10, 15 and 20 wt%) and on the reduction temperature (900 or 1000°C) used for the preparation. Bundles of carbon nanotubes are present in the hot-pressed materials but with a decrease in quantity in comparison to the powders. This phenomenon appear to be less pronounced for the powders containing higher-quality carbon, i. e. a higher proportion of nanotubes with respect to the total carbon content. The presence of carbon as nanotubes and others species (Fe carbides, thick and short tubes, graphene layers) in the powders modifies the microstructure of the hot-pressed specimens in comparison to that of similar carbon-free nanocomposites : the densifications are lower, the matrix grains and the intergranular metal particles are smaller. The fracture strength of most carbon nanotubes-Fe-Al2O3 composites is only marginally higher than that of Al2O3 and are generally markedly lower than those of the carbon-free Fe-Al2O3 composites. The fracture toughness values are lower than or similar to that of Al2O3. However, SEM observations of composite fractures indicate that the nanotubes bundles, which are very flexible, could dissipate some fracture energy.

Synthesis of nanocrystalline silicon carbide powder by carbothermal reduction

Abstract: A solution of sugar in silica sol was used as parent material for the manufacture of nanocrystalline SiC powders. After mixing and freeze drying of the components the conversion was performed in two steps. First the sugar was converted to carbon particles and secondly the silica carbon mixture was heated up to the approved reaction temperatures between 1550 and 1800 °C under an argon pressure of 180 kPa. When the synthesis temperature was reached the pressure was reduced to 0.02 kPa what leads to a vigorous reaction resulting in extremely fine particles. The obtained powders were characterized with regard to the particle and crystallite size, the BET surface and their chemical composition. The oxygen content was < 0.5 wt% and the particles were < 0.5 μm with crystallites < 100 nm and specific surface areas in the range of 20–30 m2g−1. Conclusions concerning the synthesis parameters to the resulting powder properties are given in this paper.

Differentiation between hydroxyapatite and β-tricalcium phosphate by means of μ-Raman spectroscopy

Abstract: We present a comparative study of the Roman spectra of polycrystalline β-tricalcium phosphate (β-TCP) and hydroxyapatite (HA) over their whole optical frequency range. Due to their structural differences, Raman scattering permits to differentiate between these two calcium phosphates, not only through the detection of vibrational modes associated with the OH − group of hydroxyapatite, but also by comparing the bands arising from internal PO 4 3− modes. The widths of the Raman-scattering bands associated with PO 4 3− bond-bending modes and the frequency gap between them are characteristic features that can be used to distinguish between β-TCP and HA. The method is used to identify a HA layer a few microns thick which precipitates on the surface of TCP-wollastonite eutectic soaked in simulated body fluid.

Oxidation Protection Coatings for C/SiC based on Yttrium Silicate

Abstract: The factor which currently precludes the use of carbon fibre reinforced silicon carbide (C/SiC) in high temperature structural applications such as gas turbine engines is the oxidation of carbon fibres at temperatures greater than 400°C. It is, therefore, necessary to develop coatings capable of protecting C/SiC components from oxidation for extended periods at 1600°C. Conventional coatings consist of multilayers of different materials designed to seal cracks by forming glassy phases on exposure to oxygen. The objective of this work was to develop a coating which was inherently crack resistant and would, therefore, not require expensive sealing layers. Yttrium silicate has been shown to possess the required properties for use in oxidation protection coatings. These requirements can be summarised as being low Young’s modulus, low thermal expansion coefficient, good erosion resistance, and low oxygen permeability. The development of protective coatings based on a SiC bonding layer combined with an outer yttrium silicate erosion resistant layer and oxygen barrier is described. Thermodynamic computer calculations and finite element analysis have been used to design the coating. C/SiC samples have been coated using a combination of chemical vapour deposition and slip casting. The behaviour against oxidation of the coating has been evaluated.

Processing and mechanical properties of boron carbide sintered with TiC

Abstract: The suitability of TiC as a sintering aid for boron carbide is investigated. The in situ reaction of TiC with boron carbide generates elemental carbon and TiB 2 which both aid the sintering process and permit pressureless sintering at temperatures between 2150 and 2200 °C. Relative densities of as-sintered materials exceed 93% of theoretical, but can be increased to nearly full density by subsequent hot isostatic pressing. The grain size of both B 4 C and TiB 2 increases with sintering temperature and decreases with the amount of sintering aid. The presence of TiB 2 causes a slight increase in Young's modulus and a small decrease of hardness as compared to single phase B 4 C materials. The flexural strength of B 4 C-TiC is found to decline with increasing fracture toughness. This behavior is related to crack length dependent toughness, particularly to the slope and steady state value of the R -curve. The dependence of R -curve properties on the grain size of the B 4 C matrix is discussed.

Second phases in doped lanthanum gallate perovskites

Abstract: Synthesis of Lal − xSrxGal − yMgyO3 − δ with 0.098 ≤ x mol% ≤ 0.100 and 0.196 ≤ y mol% ≤ 0.200 was performed in three ways: according to the solid state route either from oxides, or from oxides and carbonates and from powders prepared by ultrasonic spray pyrolysis. The samples were heterogeneous with a ‘crust’ of a darker color. Impurity phases could be found in this crust and in the nucleus. Their nature and concentration depend on the method of synthesis, the sintering temperature, time and atmosphere and on strontium content. La synthese de Lal − xSrxGal − yMgyO3 − δ avec 0.098 ≤ x Mol% ≤ 0.100 and 0.196 ≤ y mol% ≤ 0.200 a ete effectuee suivant trois methodes: par voie solide, la premiere en partant des oxydes, la deuxieme en partant des oxydes et des carbonates et la troisieme voie en partant de poudres preparees par pyrolyse d'un aerosol genere par ultrasons. Les echantillons sont en general heterogenes avec la formation d'une croute de couleur plus foncee. Des phases secondaires ont ete mises en evidence dans la croute et au coeur du materiau. Leur nature et leur concentration dependent du mode de synthese, de la temperature, du temps et de l'atmosphĕre de frittage et du taux de strontium.

Fabrication and mechanical properties of 5 vol% copper dispersed alumina nanocomposite

Abstract: An optimum route to fabricate the Al 2 O 3 Cu nano-composite with sound microstructure and desired mechanical properties was investigated. Two methods for developing a uniform dispersion of Cu particles in Al 2 O 3 were compared on the basis of the resulting microstructures and mechanical properties. SEM and TEM analyses for the composites fabricated by reduction and sintering process using Al 2 O 3 CuO powder mixture showed that the nano-sized Cu particles were well distributed and situated on the grain boundaries of the Al 2 O 3 matrix. The composite, hot-pressed at 1450 °C, exhibited the maximum fracture strength and enhanced toughness compared with monolithic Al 2 O 3 . The strengthening was mainly attributed to the refinement of Al 2 O 3 matrix grains. The toughening mechanism in Al 2 O 3 Cu composite was discussed by the observed microstructural features and theoretical predictions based on crack bridging model and thermal residual stress effect.

Comparative study of fabrication of Si3N4/SiC composites by spark plasma sintering and hot isostatic pressing

Abstract: A Si3N4/SiC composite with additions of La2O3, Y2O3 and SiO2 was fabricated by spark plasma sintering (SPS). The porosity and phase formation resulting by this method are compared with those obtained by sinter/hot isostatic pressing (sinter/HIPing). The open porosity was completely eliminated by SPS in 10 min of heating to 1900°C and 5 min dwell time. The complete α to β transformation was achieved with the formation of elongated β Si3N4 grains. The same result was achieved by sinter/HIPing at 1750°C with a dwell time of 160 min. There were differences in the % β transformation from α, open porosity and the presence of minor phases between the two processes. The comparison of SPS with sinter/HIP showed the former to be a rapid fabrication process for Si3N4 which warrants further investigation to optimise the microstructure.

Microstructural evolution in self-forming spinel/calcium aluminate-bonded castable refractories

Abstract: Calcium aluminate bonded alumina-spinel castable refractories have been fabricated with in-situ spinel formation Spinel formation occurs between 1200 and 1400 °C with a net-like morphology interlinked with CaO-MgO-Al2O3-SiO2 phases Spinel generated at 1400 °C is nearly stoichiometric but at higher temperatures it progessively enriches in Al2O3 The calcium aluminate phases in the cement bond react to form platey CA6 crystals between 1200 and 1400 °C which coexist with the spinel and penetrate and bond to tabular alumina grains The potential effect of these morphologies on properties is discussed

Carbon nanotubes–Fe–alumina nanocomposites. Part I: influence of the Fe content on the synthesis of powders

Abstract: Oxides based on a-alumina and containing various amounts of Fe (2, 5, 10, 15 and 20 cat.%) were prepared by decomposition and calcination of the corresponding mixed-oxalates. Selective reduction of the oxides in a H2-CH4 atmosphere produces nanometric Fe particles which are active for the in-situ nucleation and growth of carbon nanotubes. These form bundles smaller than 100 nm in diameter and several tens of micrometers long. However, the carbon nanotubes-Fe-Al2O3 nanocomposite powders may also contain Fe carbide nanoparticles as well as undesirable thick, short carbon tubes and thick graphene layers covering the Fe/Fe carbide nanoparticles. The influence of the Fe content and the reduction temperature on the composition and micro/nanostructure of the nanocomposite powders have been investigated with the aim of improving both the quantity of nanotubes and the quality of carbon, i. e. a smaller average tube diameter and/or more carbon in tubular form. A higher quantity of carbon nanotubes is obtained using a-Al1.8Fe0.2O3 as starting compound, i. e. the maximum Fe concentration (10 cat.%) allowing to retain the monophase solid solution. A further increase in Fe content provokes a phase partitioning and the formation of a Fe2O3-rich phase which upon reduction produces too large Fe particles. The best carbon quality is obtained with only 5 cat.% Fe (a-Al1.9Fe0.1O3), probably because the surface Fe nanoparticles formed upon reduction are a bit smaller than those formed from a-Al1.8Fe0.2O3, thereby allowing the formation of carbon nanotubes of a smaller diameter. For a given Fe content (≤ 10 cat.%), increasing the reduction temperature favours the quantity of nanotubes because of a higher CH4 sursaturation level in the gas atmosphere, but also provokes a decrease in carbon quality.

Electrophoretic and electrolytic deposition of ceramic coatings on carbon fibers

Abstract: Alumina, zirconia, titania, lead zirconate titanate, hydroxyapatite and chemically bonded ceramics in the system CaO-SiO 2 -P 2 O 5 were deposited via electrophoretic or/and electrolytic deposition methods on individual carbon fibers, bundles and felts, which served as cathodic substrates. Experimental conditions were determined for formation of uniform deposits of controlled thickness. After burning out of carbon fibers the corresponding ceramic replicas can be obtained. The possibility to form multilayer alumina/zirconia hollow fibers with sharp interfaces between alumina and zirconia layers has been demonstrated. Obtained coatings and fibers were studied by SEM. Experimental results obtained by electrophoretic and electrolytic deposition methods were compared.

High temperature mechanical behaviour of liquid phase sintered silicon carbide

Abstract: Silicon carbide was liquid phase sintered using Y 2 O 3 and AlN as sintering additives. A globular grained microstructure was obtained from α-SiC powder whereas a mixture of β-SiC, with a small amount of α-SiC seeds, revealed platelet shaped grains with an aspect ratio of eight. Beside this difference in grain morphology of the α-SiC grains, the phase content of the grain boundary phase is different. The influence of both aspects onto the mechanical behaviour, i.e. four-point bending strength between room temperature and 1400°C and fracture toughness between room temperature and 1100°C, are investigated.

Optimising the dispersion on an alumina suspension using commercial polyvalent electrolyte dispersants

Abstract: This paper describes the process optimisation of certain alumina ceramic aqueous suspensions by the use of three commercial and widely used dispersants. Three different commercially available dispersants — ‘Darvan C’ an ammonium poly(methacrylate), (R. T. Vanderbilt Company, Inc., USA), ‘Aluminon’ (aurintricarboxylic acid ammonium salt, Fluka Chemicals, UK) and ‘Tiron’ (4-5-dihydroxy-1, 3-benzenedisulfonic acid disodium salt, Fluka Chemicals, UK) have been employed as aqueous ceramic stabilising agents for a commercial alumina. It is shown that there is a critical concentration for each dispersant, which gives the lowest viscosity and other rheological parameters such as, the complex, storage and loss moduli. This critical concentration, which provides the lowest viscosity (called here the optimum concentration), was found as 1, 0·25 and 0·10 wt% for ‘Darvan C’, ‘Aluminon’ and ‘Tiron’, respectively, for the current alumina. The adsorption isotherm data show that, up to the optimum concentration, the dispersant molecules adsorb onto the alumina particles with a relatively high affinity. Beyond this concentration, the additive is virtually unadsorbed and then remains in the supernatant liquid and consequently interferes with the stabilising influence of the adsorbed species. Electrophoresis experiments have also been performed to measure the electrical potential of the alumina particles and to further establish a link to the origin of the concentration dependent stability of these suspensions.

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.

Influence of the stabilising mechanism and solid loading on slip casting of alumina

Abstract: Two commercial surface active agents that act through different stabilisation mechanisms, were used to disperse aqueous alumina suspensions. Electrophoretic measurements were performed to determine the zeta potential curves of the bare and surface charge modified alumina particles and, hence, to determine the stabilising performance of the dispersants. Drying–shrinkage behaviour (Bigot’s curves) and Hg porosimetry were used to characterise the slip cast bodies prepared from the suspensions at different solid loading. The rheological behaviour of the starting alumina suspensions was also determined in order to relate the flow properties with the superficial nature of the suspended particles and the wet body microstructure. Rheological measurements have showed that the slips containing the dispersant that confers the highest zeta potential values to the alumina particles were more viscous than when using the less capable dispersant in terms of surface charge modifier. These findings were explained by a larger interaction size of the particles dispersed through a pure electrostatic mechanism which resulted in a higher effective volume solid fraction of these suspensions. For both dispersants used, the Bigot’s curves indicated that the shrinkage and the slopes of the straight lines corresponding to the first stage of the drying decrease with increasing solids concentration. The variations in the slope derive from a broadening pore size distribution as a result of the increasing difficulties in stabilising the concentrated alumina suspensions, in agreement with the rheological measurements. Good correlations were found between the green microstructure, the rheology of the suspensions and the particle interaction forces.

The wear characteristics of silicon nitride

Abstract: Friction and wear properties of silicon nitride were investigated using ball-on-disk tribometer under various relative humidity levels (RHL). Friction tests were conducted against various metals (copper, nickel, titanium, aluminium). The results show that the influence of humidity depends on the material of the couples. Tribological behaviour of silicon nitride sliding on very reactive metals such as titanium and aluminium is not influenced by RHL. In contrast, the friction coefficient and wear mechanism of nickel and copper are strongly affected by adsorbed films of water vapour. Tribological properties of Si 3 N 4 /Si 3 N 4 couple were also studied and the effect of humidity was analyzed.

Ultrafine titania by flame spray pyrolysis of a titanatrane complex

Abstract: Ultrafine titania particles can be produced by flame spray pyrolysis of a chelated metal alkoxide. The precursor can be made by reacting a titanium hydrosol with triethanolamine in ethylene glycol. The chelate, dissolved in ethanol, is misted as an aerosol into an oxidizing flame where it undergoes combustion. The combustion process generates particles, probably by a gas phase condensation process, that are discrete single crystals that exhibit some faceting. The powder is a mixture of anatase and rutile (<10%), with a 45±5 m2 g−1 BET surface area. The calculated equivalent spherical diameter (34+4 nm) is consistent with typical particle sizes found in TEM micrographs. The particles are briefly compared with commercial ultrafine particles produced by flame hydrolysis of TiCl4 and by Ti vapor condensation.

Influence of particle size distribution on colloidal processing of alumina

Abstract: Two commercial alumina powders with different particle size distribution (PSD) shapes were blended in different proportions, and the rheological behaviour of the aqueous slurries was analysed. Slip casting experiments were performed in order to correlate the flow properties of the alumina slips with the green density. It was observed that the coarser alumina powder component which exhibits a bimodal PSD near to that predicted by the Furnas model, leads to slips with lower viscosity and green compacts with higher density. In order to reduce particle segregation phenomena and further increase the packing density of green compacts, the solid volume fraction of the bimodal powder with the highest packing ability was increased and maximised. Suspensions with a total solids loading as high as 70 vol% could be achieved by using a polyelectrolyte dispersant and milling to deagglomerate the powders. The results obtained show that the green density in the slip cast body continuously increases with increased solids loading, reaching a maximum value of about 78% of the theoretical density.

Combustion synthesis of aluminium titanate

Abstract: Initial interest in aluminium titanate was due to its low thermal expansion coefficient and high thermal shock resistance, but further research was soon discouraged following the discovery of the expansion anisotropy and the instability of the compound over a specific range of temperatures. The development of a suitable active precursor powder could provide a possible solution to the fabrication difficulties (microcracking and decomposition). The scarce available thermodynamic data for the formation of Al2TiO5from its constituent oxides indicate that the reaction is endothermic and only possible at high temperature because of the titanate being entropy stabilised. The present work describes a straightforward combustion synthesis technique to prepare submicron Al2TiO5 powders, using the corresponding metal precursors-urea mixtures, at low temperature and short reaction times. A thermodynamic interpretation of the reaction is provided and the characteristics of the powder produced, like morphology, specific surface area and grain size, are discussed. The thermal behaviour of the combustion powder is compared with that of Al2TiO5 produced via the conventional ceramic solid state route.

Grain boundary mobility of BaTiO3 doped with aliovalent cations

Abstract: The effect of three aliovalent cations, Nb 5+ , La 3+ and Co 2+ , on the grain growth kinetics of nearly fully dense BaTiO 3 (Ba/Ti atomic ratio = 1.001) was measured in O 2 at 1300 °C and for dopant concentrations of up to 1.25 atomic per cent (at%). For the donor cation Nb 5+ , the boundary mobility initially increased with cation concentration but then decreased markedly above a doping threshold of 0.3–0.5 at%. The boundary mobility of the BaTiO 3 doped with the acceptor cation Co 2+ decreased monotonically with dopant concentration. At a cation concentration of 0.75 at%, the boundary mobility was reduced by a factor of approximately 25, 10 and 50 times by Nb 5+ , La 3+ and Co 2+ , respectively. A major role of the dopants is seen to be their ability to influence the boundary mobility. The effects of the dopants on the boundary mobility are discussed in terms of the defect chemistry and the space-charge concept.

Comparison of different binders for water-based tape casting of alumina

Abstract: Three different types of binders, latexes, polyvinyl alcohol and cellulose ether were studied for tape casting of alumina. The rheological properties of the different systems were compared by continuous shear and oscillatory measurements. The cast tapes were evaluated with regard to particle packing, binder burnout and sintering behaviour. The microstructure of the final materials was investigated as well. Owing to the different characteristics, mainly the viscosity of the binders, the solids loading of the suspensions had to be varied between 0·2 and 0·55. All compositions gave stable suspensions but the rheological behaviour varied from viscous to viscoelastic. It was found that the latexes were the easiest ones to work with as they gave low-viscosity suspensions and fewer foaming problems. They had the highest solids loading, which enables quick drying and resulting high green density. Significant differences in sintering behaviour and final microstructure were observed between the formulations with different binders. Upon sintering, anisotropic shrinkage was observed between thickness direction and the other directions, and all formulations had larger shrinkage in the thickness direction. Sintering anisotropy between the casting direction and the direction transverse to casting was also found, but there was a large variation between the binders. The anisotropic sintering shrinkage is believed to be caused by the orientation of the particles/polymer during casting and drying.

Characterization of SnO2-varistors with different additives

Abstract: Some aspects of the effects of additives on the sintering process and their influence on the microstructure and electrical properties of SnO 2 varistors are still not well understood. Aiming at a better interpretation of the involved phenomena, the effects on microstructures and electrical properties of adding Co 3 O 4 , CuO, MnO 2 , Bi 2 O 3 and Sb 2 O 3 to SnO 2 have been studied. We found that Co 3 O 4 and MnO 2 enhance the densification of these ceramics by increasing the number of oxygen vacancies and, as a consequence, the sintering rate increases. Sb 2 O 3 , on the other hand, reduces the oxygen vacancy concentration and then the sintering rate is decreased. CuO and Bi 2 O 3 form a liquid phase during the sintering process enhancing the sintering rate. We found that electrical properties are improved by the addition of all the studied dopants, due to the modifications in the microstructure and in the defect concentration.

Crack Deflection in Ceramic Laminates Using Porous Interlayers

Abstract: Ceramic laminates have been made from alternating layers of silicon carbide and silicon carbide containing a fugitive polymer, which can be pyrolysed to produce porous interlayers. It is shown that such interlayers can be used to deflect cracks and that the volume fraction of porosity, due to the added polymer, that is required to cause crack deflection is approximately 0·4. A simple model has been developed which describes the fracture behaviour of porous solids and also predicts the volume fraction of porosity required to give crack deflection in the laminate and which is in good agreement with experiment.

Hydroxyapatite precipitation: A case of nucleation-aggregation-agglomeration-growth mechanism

Abstract: The interpretation of the precipitation data of hydroxyapatite in MSMPR systems presents several problems due to the lack of consistency of the classical nucleation-growth-agglomeration mechanism in the analysis of the Crystal Size Distribution In the proposed model, the mechanism of formation of particles starts with a nucleation episode, followed by the aggregation of supercritical nuclei ruled by surface energy excess, which counterbalance the repulsive forces between particles The aggregates agglomerate, forming solid bridges, and the agglomerates continue to grow as distinct particles due to repulsive forces