Effect of Particle Packing on the Filtration and Rheology Behavior of Extended Size Distribution Alumina Suspensions

The effect of particle packing on the rheology and casting behavior and cast cake characteristics of aqueous alumina suspensions (5O vol%) was investigated using the Andreasen approach. Varied packing was produced by blending two starting materials that differed in average size by a factor of 10. Formulations closest to ideal packing lowered viscosity. Specifically, the lowest viscosity suspension, 196 mPa s, was produced with the distribution closest to the ideal packing distribution. Typically, a well-dispersed suspension (characterized by low viscosity) casts slower than one of poorer dispersion given the same solids content. However, the suspension that provided the lowest viscosity cast at a rate of 0.41 mm²/s, which was the fastest rate of the compared formulations. Therefore, suspensions consisting of extended size distributions do not necessitate slow filtration rates. The colloidal properties of the individual starting materials, low shear rate rheology, and mercury porosimetry were used to explain the unexpected casting behavior. The dispersion and structure formation within the suspension were investigated using electrokinetic sonic amplitude measurements. Mercury porosimetry characterized the flow channels that developed in the casts. Low shear rate rheology confirmed the presence of flocculation that was first suggested by the porosimetry results.     

Ultrasonic Real-Time Monitoring of Cake Structure during Slip Casting

An ultrasonic monitoring system was developed which utilizes two longitudinal transducers configured in a pulse-echo mode to monitor the structure of a growing slip cast cake. By applying appropriate volume conservation equations to the planar, uniaxial casting apparatus, it is shown that the cake permeability and local volume fraction of voids in a cake can be measured while casting is taking place. Values obtained from the ultrasound measurements were compared to traditional methods of measuring cake permeability and cake porosity. Reasonable agreement was found. Advantages of the ultrasound technique over traditional methods, as well as limitations of the technique, are discussed.     

Compression Permeability of Al2O3 Cakes Formed by Pressure Slip Casting

The pressure gradient naturally effected across the cake on pressure casting can lead to pore-character gradients and dimensional distortion if the cake is significantly compressible. The compressibilities of cakes formed on pressure slip casting of aqueous Al₂O₃ slips defiocculated with an organic polyelectrolyte were determined for applied pressures ≤500 psi. In general the compressibility decreased with increasing pressure. Improved deflocculation of slips from which cakes were formed drastically reduced the level and pressure dependence of compressibility; cakes cast from well-deflocculated slips were insignificantly compressible. Specific resistances determined concurrently agreed well with results obtained previously in kinetic studies. Compression-permeability behavior can serve as a quality control test for the slip casting of technical ceramics.     

Experimental Analysis and Modeling of Slip Casting

The filtration mechanics of slip casting is extended to account for the filtrate transporting the finer particles to the bottom of the cake. Scanning electron micrographs of alumina (Al₂O₃) green microstructures illustrate that a higher concentration of fine particles can accumulate at the bottom section of a cake. The rheological behavior of alumina suspensions with different solids loadings, particle-size distributions, and amounts of deflocculant is discussed. Slip-casting experiments demonstrate that the rheology of a suspension greatly affects the green density and growth rate of the cake.     

The slip casting rate for quartz ceramics

Conclusions In an investigation of the influence of the principal factors on the rate of slip casting quartz ceramics it was established that the density (porosity) of the deposited cast body is the main determining factor in the casting rate. The density of the body depends on the degree of stabilization, the pH, and the granularity of the slip suspension, and on the use of vibration. The casting rate may vary tenfold with the porosity of the casting. The factors which increase the density of the casting reduce the casting rate to the same extent, and vice versa.The casting rate is influenced, moreover, also by the water/plaster ratio of the mold and by the moisture content (density) of the slip suspension.Translated from Ogneupory, No. 8, pp. 48–52, August, 1973.     

Mechanics of Pressure Slip Casting

The quantitative mechanics of pressure slip casting an aqueous Al₂O₃ slip deflocculated with an organic polyelectrolyte were investigated at 50 to 500 psi. The specific resistance, casting rate, and microstructural characteristics of the resultant cakes were especially sensitive to the electrophoretic mobility of the slip. The mechanochemical action of the polyelectrolyte in effecting deflocculation was somewhat different from that of simple ionic electrolytes.     

Application of a Two-Dimensional Computer Simulation to Cake Growth in Slip Casting with Complicated-Shape Gypsum Molds

A two-dimensional computer simulation method, developed by the authors using the method of finite differences, was applied to estimate the cake growth in slip casting of alumina with a triangular gypsum mold and a box-type gypsum mold with a convex bottom. The cake growth patterns, water penetration patterns, water flow rate distributions, and pressure distributions were simulated in the molds and/or cakes. The simulated cake growth patterns were in good agreement with those observed experimentally in both molds. Moreover, the cake growths could be well understood from the results of the water flow rate distributions in each case. The present method is applicable to cake growth simulation in slip casting with complicated-shape gypsum molds.     

Two-Dimensional Simulation of Cake Growth in Slip Casting

A two-dimensional computer simulation of cake growth was studied for the slip casting of alumina with a gypsum mold. Based on the Adcock and McDowall model, the method of finite differences was applied to the simulation, and numerical calculations were performed for the cake growth on the inside wall of a rectangular, box-type mold by using the cake-growth controlling parameters that were obtained experimentally. Good agreement was observed between the experimental and calculated cake growth in the vicinity of the boundary between the gypsum mold and the silicon rubber and at the rectangular corner of the gypsum mold. The present simulation method is expected to be useful for estimating cake growth on molds with complicated shapes in slip casting.     

Effect of dispersant on the rheological properties and slip casting of concentrated sialon precursor suspensions

The ability of Hypermer KD1 to disperse high solids loading reaction sialon suspensions for slip casting has been characterised. It has been found to be a very effective dispersant in organic media of 60-vol.% MEK and 40-vol.% Ethanol, yielding fluid and highly homogeneous suspensions. The effects of added amounts of KD1 have been observed through adsorption data, sedimentation tests and rheology measurements. KD1 imparts low viscosity and stability to the suspension. It has been found that 3-wt.% addition of KD1, based on the weight of reaction sialon powders, results in a very stable and high flowable suspension with near Newtonian flow behavior. Less amounts of dispersant lead to unstable suspensions with obvious shear thinning flow behaviors, while adding excessive dispersant leads to high viscosities, especially at high solids loading. Measuring the pore size distribution of green bodies from different suspensions has proved the effects of dispersant amounts on dispersing the slurries and on slip casting performance.     

Mechanics of Constant-Rate Filter Pressing of Highly Flocculated Slurries

The growth of a powder compact from a highly flocculated slurry has been simulated by a computer program that models non-steady-state Darcian flow. Computer simulations have been compared with filter-pressing experiments. Constant-rate filter-pressing experiments are divided into two regimes of piston stress—displacement behavior: an initial, almost-linear, but concave-up, regime during cake growth, followed by a second region of rapidly increasing piston stress when the piston comes into contact with the cake. Linear piston stress—displacement behavior is expected, from theory based on a uniform cake model. Highly flocculated slurries show highly nonlinear behavior. Nonlinear behavior is shown to be consistent with nonuniform growth of the cake. The permeability and consolidation behavior of the cake has been determined by a consolidometer experiment. Computer simulations indicate that the particle-packing density profiles during cake build-up are surprisingly similar during cake growth. Conditions for uniform consolidation can be determined from a general equation for non-steady-state Darcian flow. Results are directly applicable to constant-flow-rate pressure casting or slip casting.     

Thermally enhanced slip casting of alumina ceramics

Slip casting is a process which is very suitable for the production of large components and thin-walled bodies of complex shape but which is also time and energy intensive. One method of increasing the casting rate is by heating the slip. This has been examined via the use of both microwave and convectional heat energy for 72 wt% nitric acid dispersed aqueous alumina slips. It has been found to provide an increase in the casting rate over ambient temperature processing, with a gain of 55% when using microwave energy and 20% using a convection oven at 45°C. The enhanced rate is apparently achieved via two mechanisms. At temperatures less than approximately 40°C it is probably due to a reduction in the slip viscosity enabling easier water movement through the cast layer and mould. At higher temperatures there is a change in the dispersion state of the slip. It appears that as the slip temperature increases above ambient the slip becomes increasingly flocculated and that above about 45°C this influences the casting rate. By 65°C the slip is fully flocculated. A further enhancement in the casting rate is achieved with microwave energy over convectional heat energy. This is attributed to the rate of movement of water through the mould being increased more than the rate of water uptake by the mould. This in turn is believed to be due to the plaster of paris being essentially transparent to microwave radiation so that the only significant heating that occurs in the mould is due to the presence of the water. Since the water in the mould cannot move around as freely as it can in the slip, local temperature gradients can rapidly develop. This causes the water front within the mould to move faster than with convectional heating. No evidence has been found that this accelerated rate has any detrimental effect on the cast body produced.     

PROPERTIES OF CLAY CASTING SLIPS*

Nine casting slips were prepared, using bodies of the sanitary porcelain type. The electrolyte was a combination of 60%“N” brand sodium silicate and 40% sodium carbonate. Its amount was adjusted in each body slip to produce a cast having the right characteristics according to the “feel” test; the specific gravity of each slip was constant. The grain-size distribution of each slip was determined as well as the casting rate, consistency, and pH when the casting slips were first prepared and again after 24-, 48-, and 96-hour aging periods. There is a reasonably close relation between castability and the grain-size distribution. The pH of the body compositions differed, but there was little or no change in its value with the age of the casting slip up to 96 hours. It is concluded that the determination of grain size and pH should be of considerable aid in the control of casting slips.     

Colloidal stability-slip casting behavior relationship in slurry of mullite synthesized by the USP method

This study presents the outcome of a research concerning the relationship between the colloidal stability of mullite powders synthesized by the USP (ultrasonic spray pyrolysis) method and its slip casting behavior. The colloidal stability of mullite slurry has been investigated under three different pH conditions (4.5, 8.9 and 10.9) derived from pH-dependent zeta potential (ZP) curves. Employing these pH values, mullite slurries with 50 wt.% solid content were prepared and slip cast. The microstructures of dried and sintered specimens were examined using SEM. It is concluded that the pH significantly influences the stability and in turn the slip casting behavior of the mullite slurry. In order to prepare homogeneous and stable mullite slurry for efficient slip casting it is preferable to utilize a basic rather than an acidic medium. High pH (i.e. 10.9) tends to leads to more closely packed mullite particles resulting in a homogeneous microstructure and greater structural integrity.     

ZrB_2-SiC复相陶瓷的注浆成形与无压烧结

研究了ZrB2-SiC复相陶瓷的注浆成形技术,着重讨论了pH值和固相体积含量对料浆粘度的影响;分析了注浆成形后,生坯断面的显微结构以及相对应的烧结体的显微结构。结果表明:ZrB2原始粉料的研磨处理有利于复相陶瓷制品的烧结致密化。当pH值为11时,可制得固相体积含量为50%,粘度为660mPa.s的ZrB2-SiC浆料。     

Influence of the aging time of yttria stabilized zirconia slips on the cracking behavior during drying and green properties of cast tapes

Tape-casting process was used to produce yttria stabilized zirconia (YSZ) substrates in an aqueous system using a low amount of an acrylic latex binder. Concentrated suspensions with different aging times were cast, and the influence of the slip aging time on the drying kinetics and cracking behavior of the tapes were studied. In addition, the effect of the slip aging time on the properties of the resultant green tapes was investigated. The latex particles consolidated by coalescence during the aging time of the slips and resulted in an increase in the smaller pore size of the cast tapes. The pore radius increased with increasing the slip aging time up to 14 days thereby decreasing the capillary pressure in the liquid. Aging times over 14 days did not change the pore radius and consequently the capillary pressure. The capillary tension drove the consolidation; the tapes produced from slips with lower aging times which had higher capillary pressure shrank more, had lower pore volume and consequently higher green density. Cracking was found in tapes prepared from slips with aging times shorter than 14 days; the crack area decreased with increasing the slip aging time. For slip aging time ≥14 days cracking was not observed. Aging before casting up to 14 days reduced cracking in tapes prepared with low amounts of latex; however, the lower capillary pressure resulted in low green density of the cast tapes.     

Restraining of calcium contamination in near‐net shape alumina ceramics during slip casting

The traditional method for shaping ceramics is by slip casting on gypsum molds; however, its application for near‐net shaping of ceramic components is limited due to contamination by calcium ions. The focus of this study is the modification of the mold to limit Ca²⁺ contamination and to maintain favorable sucking properties. Cement was added to a standard gypsum mold to suppress its erosion, and a decrease in the sucking rate was observed due to its reduced macroporosity. The highest values of green densities were obtained at gypsum/cement weight ratios of 90/10 and 70/30. The microstructure analysis showed that alumina blocks prepared from the molds containing higher quantities of cement (30 or 50 wt%) were resistant to abnormal grain growth caused by Ca²⁺ contamination from the gypsum. The gypsum/cement mixtures for making molds for slip casting significantly limit mold erosion due to a lower sucking rate and abnormal grain growth of the slip cast samples because of the decreased diffusivity of Ca²⁺ ions. Therefore, the present modification of the mold renders the slip casting method more suitable for the near‐net shaping of ceramics.     

Aqueous processing of carbothermally prepared Ca-α-SiAlON and β-SiAlON Powders: powder and suspension characterisation

Properties of carbothermally prepared Ca-α-SiAlON and β-SiAlON powders and aqueous suspensions thereof were determined. The isoelectric points of Ca-α-SiAlON and β-SiAlON were 3·4 and 4·6. After addition of deflocculant, Dolapix CE64, the behaviour of both suspensions is nearly identical. The isoelectric points become 5·5 and 5·3, respectively. Despite differences in bulk composition, grain size distribution, grain size and shape, both SiAlON suspensions show a similar dependence of a zeta potential on pH. Optimum slip casting properties, i.e. lowest viscosity values (below 10 mPa s), the highest absolute zeta potential values, the smallest floc size and sediment volume were found between pH 10–11 for both powders. The potentials of the different suspension characterisation techniques were compared and zeta potential and viscosity measurements were found the most convenient.     

The effect of slip casting parameters on the green density of MgAl2O4 spinel

The aim of this paper was to investigate the effect of slip casting parameters on the green density of MgAl₂O₄ spinel. In order to obtain samples with suitable mechanical and optical properties, it is necessary to prepare bulk samples with a fine grain size along with a low level of impurity and high density. Slip casting is widely used in the processing of optical ceramics to achieve a body with high green density and low sintering temperature. In the present study, several spinel suspensions with similar solid content but different viscosities and particle sizes (90, 150, 300 and 500 nm) were prepared and shaped into a dense body. Viscosity of suspension depended on dispersant content, such that the addition of dispersant firstly caused viscosity to decrease, but it was increased by further dispersant addition, irrespective of the suspension particle size. The green density range of samples was 36–67% of the theoretical value. Rheological behaviour and green density measurements showed that powder particles smaller than 90 nm were unsuitable for slip casting because agglomeration of powder particles led to high viscosity and hence, low green density. The optimal particle size for slip casting was found to be 150 nm.     

Cellular ceramics by slip casting of emulsified suspensions

Cellular ceramics were processed by slip casting of alumina suspensions emulsified with sunflower oil. A model behavior was derived on assuming that the templating dispersed phase and its droplet size distribution are retained as porosity in the resulting cellular ceramics, whereas the continuous ceramic suspension evolves to dense struts and thin inter-pore walls. Representative values of solid load and oil to suspension ratio were selected to seek close packed spatial distribution of nearly spherical. Stirring rate and additions of surfactant were also varied for greater flexibility in adjusting droplet size and rheology of the corresponding emulsified suspensions; this also contributes to minimize undue losses of porosity, relative to the model behavior, and determines microstructural features such as size distributions and average cell size.     

Theory of Filtration of Ceramics: I, Slip Casting

The theory of colloidal filtration in slip casting of ceramics as developed by Aksay and Schilling for incompressible cakes has been extended to compressible materials. The rate of cake deposition in the mold depends upon the capillary pressure and permeability of the mold. Assuming that the capillary pressure is inversely proportional to an average diameter typifying the mold and that the permeability is directly proportional to the square of the diameter, an optimum diameter exists for production of maximum pressure drop across the cake and maximum rate of deposition.