Fabrication of engineering ceramics by injection moulding a suspension with optimum powder properties

The mechanical properties of alumina test bars fabricated using injection moulding as the shaping operation, are described. The modulus of rupture, hardness, fracture toughness and wear behaviour of the ceramic specimens are reported. Test bars were injection moulded using a polypropylene-based high molecular weight organic vehicle. The processing and microstructure of the test bars were discussed in Part I. The alumina powder was selected in a previous investigation, which considered the compounding, compression moulding, removal of organic vehicle and sintering of several formulations containing different alumina powders.     

Interfacial factors affecting the incidence of defects in ceramic mouldings

Ceramic injection moulding suspensions consisting of two alumina powders of surface area 9.5 m² g^–1 and 0.3 m² g^–1 were prepared at 60 vol % ceramic loading in the same organic composition. Cylinders of various diameters of each suspension were companion fired on an identical temperature ramp in nitrogen. For the fine powder, bloating occurred for all diameters above 8 mm. The coarse powder cylinders of 25 mm diameter could be prepared free from internal voids. The problems presented by fine powders during binder removal are discussed in terms of adsorbed layers, water emission and catalytic degradation.     

On the fabrication of ceramic windings

Components in the form of springs and windings were fabricated from technical ceramics by the extrusion of a polymer-ceramic suspension. Control of thermomechanical properties of the suspension allowed the ceramic to be extruded as a wire and to be plastically worked under gentle heating. The organic component of the blend was removed by pyrolysis before sintering the ceramic powder. This method may be particularly suited to the production of hightemperature ceramic springs, heating elements and ceramic superconductor windings for electrical power applications. Defects produced during removal of the polymer and the methods used to avoid them, shed light on the complex processes taking place in crowded suspensions during extraction of the organic vehicle.     

Silica moulds built by stereolithography

In this work the production of ceramic moulds for aluminium casting using a stereolithographic apparatus is presented. Suspensions of silica powders in a photoreactive resin were used in standard STL equipment in order to build green parts. UV curable pre-ceramic suspensions have been studied using photocalorimetric and rheological characterizations. Thermogravimetry was used to analyse the behaviour of the pre-ceramic green during thermal treatment. Stereolithgraphy apparatus (SLA) was modified in order to fabricate ceramic green. Silica objects were obtained by pyrolysis of the organic binder and subsequent sintering at high temperature. A characterization of mechanical properties of the green and sintered ceramic materials was performed. Finally moulds for investment casting of aluminium were built by stereolithography and used for casting as a green as well as after sintering.     

Ceramic green tape extrusion for laminated object manufacturing

Al₂O₃ and SiC green tapes with 0.2 mm thickness used for LOM (laminated object manufacturing) were prepared from a 75 wt.% ceramic suspension in a LDPE based organic vehicle followed by extrusion. Rheological characterization of the alumina and SiC suspensions were studied. TG-DTA analysis was carried out in air and microstructures were examined by SEM.     

Stereolitography of ceramic suspensions

The need of fast production of prototypes of complex shapes in very short time lead to the development in the last years of many additive rapid prototyping (RP) technologies for the production of single objects or of very limited series. The new fabrication concept allowed the construction of complex parts, starting from a 3D-CAD model, without using a mould. However, most of these additive processes produce polymeric objects and only recently the laser sintering of metal powders has been commercially introduced. In this work the production of ceramic objects by stereolithography is presented starting from the development of UV curable pre-ceramic suspensions for free form fabrication of alumino-silicate parts. The suspensions are characterized by 40%–50% by volume of powder content and by a reactivity and a viscosity compatible with their application in stereolithography. The ceramic green is built in a stereolithographic system operating with a He-Cd laser (325 nm). Then, the ceramic objects are obtained by pyrolisis of the organic binder and subsequent sintering of the green at 1600 °C. Finally, a characterisation of the mechanical properties and of the microstructure of the samples is presented.     

Fabrication of engineering ceramics by injection moulding a suspension with optimum powder properties

A suspension containing a polypropylene-based high-molecular weight organic vehicle and an alumina powder (which best satisfied criteria for successful compounding, compression moulding, removal of organic vehicle and sintering stages, each necessary for the fabrication of the final ceramic shape) was further assessed in this investigation. Test bars were injection moulded from this suspension under various conditions. After removing the organic vehicle, test bars were sintered and the resulting microstructure studied. The mechanical properties of the test bars will be discussed in Part II.     

Properties of tape-cast Y-substituted strontium titanate for planar anode substrates in SOFC applications

This work evaluates the use of fine-grained yttrium-substituted strontium titanate powders for the preparation of planar anode supported solid oxide fuel cells. Starting from a submicron-sized powder of Y-substituted strontium titanate Sr_0.895Y_0.07TiO₃ (SYT), which was synthesised via spray pyrolysis followed by a grinding process, suspensions of high solid concentration were prepared by steric stabilisation. From these suspensions, tape casting slurries of up to 25 vol% were produced and further processed to ceramic green tapes using the doctor blade technique. The rheological behaviour of the slurries was investigated in dependence on the content of solids and organic additives. Furthermore, the binder burnout and sintering behaviour of the green sheets were characterised. After firing, crack-free substrates of high planarity were obtained. The achieved properties of the sintered tapes such as density, porosity, warping, mechanical strength, and electrical conductivity were determined in dependence on sintering temperature.     

Colloidal processing of lead lanthanum zirconate titanate ceramics

Well-dispersed aqueous slurries of fine ceramic powders with high solids loading are often required for various shape forming techniques such as slip and tape casting in order to fabricate advanced ceramics with a dense and uniform microstructure. Colloidal processing of lead lanthanum zirconate titanate (PLZT) powders was conducted at various pH using ammonium polymethacrylate as a dispersant. Suspensions were characterized by viscosity and zeta-potential measurements. The effect of pH on polymer adsorption and the rheological behavior of the slurries were investigated and stabilization mechanisms discussed. Through optimization of the dispersant concentration and pH, solids loadings of the suspensions up to 50 vol.% with a relatively low viscosity were obtained.     

Investigation of the electrophoretic deposition behaviour of non-aqueous ceramic suspensions

The topic of the investigations was the electrophoretic shaping of AlN, SiC and Al₂O₃, respectively, using ethanolic suspensions of the powders. Suitable organic additives were chosen for achieving an effective suspension stabilisation and a sufficiently high electrophoretic mobility of the particles, which are the basic requirements for a successful electrophoretic deposition. Most of the EPD experiments were carried out in the direction of the gravitational force. Either the voltage or the current was kept constant during the deposition. In the case of the EPD of AlN, the dependence of the deposit mass on the experimental parameters has been investigated in detail and explained on the basis of changes in the effective electric field strength. Several factors influencing the effective field strength are discussed in this paper. By EPD from the different suspensions, green bodies with monomodal pore size distributions reflecting a high homogeneity of the particle packing were produced. It was shown that by the simultaneous deposition of non-oxide powders and sintering additives from suspensions with high solid contents deposits with a homogeneous distribution of the sintering additive can be obtained, whereas in diluted suspensions different electrophoretic mobilities cause a separation of the components.     

Metal-coated colloidal particles

Procedures are described for coating of submicrometre ceramic powders with copper and nickel. The process required precoating of the core particles with a palladium catalyst. A precoating procedure was developed in which palladium chloride is reduced by sodium hypophosphite or stannous chloride on the powder surface in aqueous suspensions. Commercial electroless solutions were used to deposit copper and nickel on the catalysed powders. The effect of various experimental parameters on the effectiveness of the surface conditioning of the particles was also investigated.     

Preparation of highly concentrated and stable suspensions of silver nanoparticles by an organic base catalyzed reduction reaction

Highly concentrated and stable suspensions of silver nanoparticles have been synthesized by chemical reduction from silver nitrate in a formaldehyde reductant using an organic base, triethylamine, as the reaction promoter. In this reaction, a low molecular weight organic compound, thiosalicylic acid (TSA), was used as the protecting agent. The average size of the silver nanoparticles prepared from this method was less than 10 nm, which allowed low-temperature sintering of the metal. The suspensions were further stabilized by the addition of excessive triethylamine, which forms an amine salt with TSA. A 50 wt%, stable suspension has been prepared. The suspensions of silver nanoparticles prepared by this method are free from any metal ion contamination, and are suitable for use in semiconductor industries.     

Capillary Flow Behaviour of Microcrystalline Wax and Silicon Carbide Suspension

Suspensions of ceramic particles in low or high molecular weight polymers are shaped into various three-dimensional parts using various moulding and extrusion technologies. Such bodies are subsequently fired-up and sintered to remove the binder. The utilities of such three-dimensional ceramic bodies depend on the restrictions related to the shapeability of the ceramic suspension, hence to the flow and deformation behaviour of the suspension. In this study, factors affecting the flow and deformation behaviour of a 50% by volume of silicon carbide in a wax binder was investigated. Consistent with the previously observed behaviour of other highly filled materials, the ceramic suspension exhibited viscoplasticity, plug flow and wall slip. Furthermore, flow instabilities associated with the axial migration of the low viscosity binder under the imposed pressure gradient were observed. These results pinpoint to the various difficulties associated with the collection of rheological data and emphasize the relevance of various flow mechanisms, including wall slip and mat formation and filtration based flow instabilities, which would also occur in processing/shaping flows of such ceramic suspensions including extrusion and moulding.     

纳米3Y—TZP粉体的表面改性研究

用月桂酸对纳米3Y—TZP粉体进行表面改性,实验结果表明,改性最佳工艺为：月硅酸用量10％、改性时间75min、改性温度60％。经表面改性的粉体,由引起团聚的强极性多羟基表面结构变为非极性有机表层结构,粉体间作用力减小,硬团聚消失,大幅度提高了陶瓷体的烧结性能。     

Polymer Derived Engineering Ceramics

Engineering ceramics in the system Si–O–C–N–B manufactured from preceramic silicon containing polymers gain in significance with increasing availability of advanced precursor materials such as poly(carbosilane), ‐(silazane), ‐(siloxane) or ‐(borosilazane). While high temperature resistant Si–C and Si–N ceramic fibers are already used to reinforce ceramic matrix composites (CMC) in air‐ and spacecraft structures, novel products such as coatings, tapes, foams, and complex shaped components for medium and low temperature applications in the fields of energy, environmental, transportation, and communication technologies become more important in the future. Preceramic polymers offer the possibility of using versatile plastic shaping technologies as well as advanced laminated object manufacturing techniques. Properties can be varied in a wide range by tailoring the molecular structure and composition of the precursor polymer and by loading the polymer with intert or reactive filler powders. Partial conversion of the organic polymer into organic/inorganic hybrid materials yields novel materials which exhibit properties between polymers and ceramics.     

Electrokinetic behaviour and dispersion characteristics of ceramic powders with cationic and anionic polyelectrolytes

Three different ceramic powders, viz. alumina, zirconia and silicon nitride were dispersed using two polyelectrolytes, one cationic (Betz 1190) and the other anionic (Darvan-C). All powders examined during the study could be well dispersed only under conditions of polymer dosage and pH such that the working pH is at least 2 pH units away from the pH_IEP of the powder-dispersant combination. The shift in the isoelectric point (IEP) of the powders were determined through electro-acoustic measurements on 1% volume suspensions. Specific free energy of interaction were also computed using a model based on the electrical double layer theory of surfactant absorption. Certain trade names and company products are mentioned in the text or identified in illustrations in order to adequately specify the experimental procedure and equipment used. In no case does such identification imply recommendation or endorsement by the authors and their respective institutes nor does it imply that the products are necessarily the best available for the purpose.     

Si3N4粉体表面分析及偶联剂作用量的选择

在Si3N4的粉体的生产和存储过程中，氧化和水解是两个不利的因素，但在偶联剂的作用下，它们在提高陶瓷粉体与有机载体的相容性方面起到了促进作用。本文测定了未经处理的Si3N4粉及通过氧化和水解改性的Si3N4粉的表面性质，并分析了它们对偶联剂选择作用量的影响。     

Photocatalytic decolouration of Orange II by ZnO active layers screen-printed on ceramic tiles

In this work ZnO layers have been deposited by screen-printing in common ceramic tiles. These layers were characterized and tested for the photocatalytic degradation of the organic dye Orange II in aqueous solutions, using a batch photoreactor either under visible light provided by a Philips ML-160 W lamp or under direct exposure to sunlight. For sake of comparison, ZnO suspensions have also been evaluated for similar reacting conditions. The influence of experimental parameters such as (i) firing temperature of the printed layer; (ii) layer thickness; and (iii) operation time have been investigated. Screen-printed ZnO layers obtained in optimal processing conditions showed photocatalytic activity comparable to aqueous ZnO suspensions. The maximal attenuation degree is over 70% and decolourisation rate, assuming that reaction kinetics follows a pseudo-first order rate law, is over 0.015 min(-1). Thus these ZnO-layered ceramic tiles can be regarded as an alternative to photocatalytic suspensions of the same material with the advantage of avoiding the removal of the photocatalyst.     

Si3N4粉体表面分析及偶联剂作用量的选择

在Si₃N₄粉体的生产和存储过程中,氧化和水解是两个不利的因素,但在偶联剂的作用下,它们在提高陶瓷粉体与有机载体的相容性方面起到了促进作用.本文测定了未经处理的Si₃N₄。粉及通过氧化和水解改性的Si₃N₄粉的表面性质,并分析了它们对偶联剂选择作用量的影响.     

SPINNING FIBERS FROM POWDER SUSPENSIONS

Green ceramic fibers from Al₂O₃, Si₃N₄, Ce-ZrO₂, SiC, and other ceramics can be prepared by dry spinning of powder-loaded suspensions. The green fibers contain 54 to 64 vol. % ceramic powder in an ethyl methacrylate polymer base. Continuous fibers with an average diameter of 60 to 175 µm were spun at rates of 10 to 40 meters/minute. Spinnable dope compositions are defined for simple ternary powder + polymer + solvent systems, and are compared for several powders in MEK-based solvents. Spinnability is related to extrusion pressure, and drying conditions, and the shear rheological behavior of the dope. Fiber preparation by suspension dry spinning is compared with melt spinning of powder-loaded thermoplastics.