Pressure slip casting of coarse grain oxide ceramics

This contribution investigates the pressure slip casting of large coarse grain oxide ceramic bodies with a water soluble organic additive system. This organic additive system allows the preparation of a stable and pumpable slip containing alumina rich magnesia aluminate spinel of a size of up to 3 mm and an easy demolding of crack free, dimensionally stable bodies with negligible gradients due to sedimentation. Cut out samples of fired bodies are examined on apparent porosity, dynamic elastic modulus, modulus of rupture, and pore size distribution. Computer tomography showed very homogenous and dense bodies. The effects of different maximum grain sizes as well as possible sedimentation and segregation of the slip on the mechanical properties and microstructure are evaluated by using the Student's t-test. The most promising results of this study indicate that it is possible to reproducible fabricate coarse grain ceramics for refractory and other high temperature applications by pressure slip casting.     

Cellular zirconia ceramics processed by direct emulsification

Highly porous zirconia ceramics for prospective use as separators in alkaline electrolysis cells have been processed by emulsification of paraffin in concentrated zirconia suspensions. Effects of processing parameters on porosity, cell size distribution, and pore interconnectivity are studied through Taguchi statistical design. The porosity of zirconia ceramics has shown a strong correlation with paraffin-to-suspension ratio employed in emulsification. High paraffin-to-suspension ratios combined with high gelatin content in the emulsion are beneficial for creating highly porous cellular materials with interconnected cell sizes. The attained microstructural features allow high percolation of ionic species in these highly porous ceramics after impregnation with aqueous electrolytes, facilitating charge transport within the liquid medium inside the ceramic bodies, as required for lower ohmic losses. This work provides guidelines for developing efficient ceramic separators for future use in alkaline electrolysis cells and other applications requiring thermally and chemically-stable ceramics with high and well-interconnected porosity.     

Aqueous slip casting of transparent yttrium aluminum garnet (YAG) ceramics

Transparent YAG ceramics were prepared by slip casting an aqueous dispersed mixture of commercial Al₂O₃ and Y₂O₃ powders. The powders were co-dispersed with poly(acrylic acid) and citric acid. Polyethylene glycol of 0.5 wt.% (PEG 4000) and 0.5 wt.% tetraethyl orthosilicate were added as binder and a sintering aid, respectively. Dried samples were vacuum sintered at 1800 °C for 16 h. In general, YAG ceramics cast from Newtonian suspensions were optically transparent and had optical transmittances >80% from 340 to 840 nm. Slightly flocculated dispersions, as evidenced by higher viscosity and non-Newtonian rheology, resulted in translucent samples with large pores and lower optical transmittances.     

Microstructure of TiO2 porous ceramics by freeze casting of nanoparticle suspensions

During freeze casting of TiO₂ porous ceramics, the porous architecture is strongly influenced by TiO₂ particle size, solids loading, and cooling temperature. This work investigates the influences of particle size, freezing substrate, and cooling temperature on the TiO₂ green bodies prepared by freeze casting. The results show that the lamellar channel width with 100 nm particles is larger than that of 25 nm particles, yet the ceramic wall thickness is noticeably decreased. The lamellar structure is more ordered when using a copper sheet than glass as its freezing substrate. A finer microstructure results when frozen at − 50 ℃ than − 30 ℃. Such porous materials have application potentials in a wide range of areas such as photocatalysis, solar cells, and pollutant removal and should be further studied.     

Fabrication of cordierite foam ceramics using direct foaming and slip casting method with plaster moulds

The cordierite foam ceramics were successfully fabricated using direct foaming and slip casting method with plaster moulds. Kaolin, attapulgite and magnesium oxide were used as starting materials with Arabic gum added as the dispersant. The samples were sintered at 1200°C, and then the microstructure, porosity, bulk density and thermal conductivity were characterised. The results show that the cordierite foam ceramics had a porous structure of open cells and the struts had abundant small pores. The maximum open porosity achieved 87·65% with a bulk density of 329 kg m^??3, and the thermal conductivity was as low as 0·095 W (m K)^??1. Therefore, these cordierite foam ceramics show promise for use as the thermal insulator.     

Cellular mullite materials processed by direct foaming and protein casting

In this paper, cellular mullite bodies were developed by thermal direct-consolidation of foamed aqueous mullite-bovine serum albumin (BSA) and mullite-BSA-methylcellulose suspensions, burning out (650 °C, 2 h) and sintering (1600 °C, 2 h). Some modifications to the shaping route conventionally used in protein casting were incorporated in the proposed processing to obtain bodies with controlled and homogeneous microstructures. The materials were characterized by porosity measurements, analysis of phases by XRD, and microstructural analysis by SEM and Hg-porosimetry. Characteristic parameters of cell size distributions, percentage of open and closed cells, window size and interstitial pore size distributions, and microstructural features of the mullite matrix were determined. Moreover, basic 2D cell size parameters and global 3D stereological parameters were analysed. The obtained results showed that mullite bodies with hierarchical porosity and different microstructural features were developed from the design and control of processing routes, which use BSA as a foaming and binder/consolidator agent.     

Cellular ceramics from emulsified suspensions of mixed particles

In this work concentrated alumina suspensions have been emulsified with decane using silica nanoparticles of a range of hydrophobicities as stabiliser giving rise to novel porous ceramic materials. The materials are characterized by open porosities between 49 and 61% and average cell sizes between 18 and 25???m. Comparison with surfactant-stabilized emulsified suspensions is given. In particular, high temperature strength is a key property of the particle-stabilized materials.     

Magnetic slip casting for dense and textured ceramics: A review of current achievements and issues

Ceramic materials are ubiquitous in technologies operating under high mechanical, thermal or chemical constrains. Research in ceramic processing aims at creating ceramics with properties that are still challenging to obtain, such as toughness, transparency, conductivity, among others. Magnetic slip casting is a process where an external magnetic field is used to createcontrolled texture in ceramics. Over the past 20 years of research on magnetic slip casting, dense and textured ceramics of multiple chemistry were found to exhibit enhanced properties. This paper reviews the progress in the field of magnetic slip casting, details the processing parameters and the textures obtained for a diverse range of compositions. The structural and functional properties of the magnetically textured slip casted and sintered ceramics are presented. This overview of the magnetic slip casting process allows to identify critical directions for future advancement in advanced technical ceramics.     

Investigation of affecting parameters on slip casting of yttria-magnesia IR-transparent bodies

Slip casting of MgOY₂O₃ suspensions has been investigated in this research. For this purpose relatively pure commercial raw materials were used. The Taguchi experiment method was used for design of experiments and selection of suitable solvents and dispersants and theirs optimum values. Preparation of suspensions was done in two stages, according to particle size distributions of powders after different milling time and Taguchi results. The best suspensions were selected based on viscosity and solid load of suspensions. Green bodies were slip casted using the optimized suspensions in order to obtain the green bodies with maximum density and a homogeneous structure in term of porosity size distribution. Results showed that using from ethanol as solvent, an optimum dispersant level of 3%wt. TEA +1%wt. DEA with a constant TEA/DEA ratio of 3/1 and mean particle size of 155 nm (milling time of 48 h) was the best choice for preparing of optimum suspensions. The highest relative green density of 65.91% was obtained for a suspension containing 35 vol % of solid particles casted under 3MPa. The dense Y₂O₃MgO submicron composite samples were sintered by SPS technique at 1250?°C under 70MPa for 6?min with a density about 99.5%.     

Aqueous colloidal processing of PLZT ceramics near the morphotropic phase boundary

Abstract

Abstract

Single phase PLZT powder was fabricated using the mixed oxide approach, with a composition (6/60/40) lying on the rhombohedral-tetragonal (morphotropic) phase boundary. Aqueous suspensions showed an IEP of pH?9·7 in the absence of a polyelectrolyte surfactant. Inductively coupled plasma optical emission spectrometry analysis demonstrated considerable leaching of Pb²⁺ and La³⁺ ions under acidic conditions. The addition of an ammonium salt of poly(methacrylate), or PMA-NH₄, decreased the IEP to pH?5-6. Suspensions were readily stabilised at basic pH using low PMA-NH₄ concentration (<0·1?wt-% of solids). Concentrated suspensions exhibited shear thinning behaviour under low shear rates, with a transition to shear thickening at modest rates (i.e. 100?s^?1). Excessive shear thickening imposed a maximum solids loading of 46?vol.-% on concentrated suspensions, which related to the somewhat ‘flake-like’ particle morphology. Slip casting resulted in green and sintered (1250°C for 30?min) densities of 56±1% and 98·3±0·6% of theoretical, respectively.     

Dense m-Li2ZrO3 formed by aqueous slip casting technique: Colloidal and rheological characterization

An aqueous colloidal processing method was proposed to prepare m-Li₂ZrO₃ bodies, with high green density and a homogenous microstructure, employing a pressureless forming technique. For this purpose, the preparation conditions of m-Li₂ZrO₃ aqueous suspensions were optimized considering colloidal processing variables, such as milling time (0–40 min), dispersant concentration (1–10 wt% of the solids content) and maximum solid loading (41–74 wt%). Particle size distributions and steady-state flow curves of the slips were analyzed, together with phase composition and microstructure of the ceramic bodies. The results show that after planetary ball milling for 30 min, the aqueous suspension with 63.1 wt% of m-Li₂ZrO₃, and a dispersant content equal to 10 wt% of the solid loading, yielded green bodies with bulk densities close to 66% TD by slip casting. Then, after the heat treatment process at 1100 °C for 12 h, bulk densities close to 88% TD were achieved. It was found that density values and microstructure of the green and sintered products bear a direct relationship to the particle size and the consistency of the slips.     

Direct coarse powder aqueous slip casting and pressureless sintering of highly transparent AlON ceramics

Direct coarse powder aqueous slip casting was proposed to prepare AlON green body for pressureless sintering of highly transparent AlON ceramics. It was reported that anti-hydrolysis treatment to AlON powder before aqueous slip casting was essential due to hydrolysis reaction between AlON and water. However, both XRD of hydrolyzed powder and pH value of aqueous slurry indicate that the hydrolyzate amount is positively correlating with hydrolysis reaction area, i.e., surface area of powder. Therefore, replacing fine particle with coarse one to reduce surface area of powder is an efficient hydrolyzates reduction scheme by lessening hydrolysis reaction sites. As a result, the hydrolyzate in green body of direct slip casting coarse AlON powder (91 vol% of particles have a size of >1.0 μm) was successfully reduced to an insignificant amount so that the sintered ceramics had a high transmittance (84.1% at ~3750 nm), which also verified the effectiveness of the proposed hydrolyzate-reduction scheme.     

Indirect solid freeform fabrication by binder assisted slip casting

A method has been developed for the consolidation of ceramic bodies in molds prepared by solid freeform fabrication. The method is based on slip casting with an added latex binder. The latex binder makes it possible to create internal structures, such as pore channels, without the cracking that usually takes place when shrinkage is obstructed by internal mold structures. The latex binder adds plasticity to the consolidating body in the wet state. This was observed by rheological measurements during slip casting. Measurements of mechanical properties showed that the latex binder also adds plasticity and strength in the dry state.     

Cellular silica-based ceramics prepared by direct foaming at high temperature

Cellular silica-based ceramics, including Si₃N₄/SiO₂ composite ceramics and monolithic silica ceramics, with dense shell and closed cells with dense and crack-free cell wall inside was prepared by the direct foaming of the green-compacts at 1310–1370 °C. The influences of the heat-treatment temperature on the relative density as well as the mechanism of the cell formation were investigated. The porosity of the obtained cellular silica and Si₃N₄/SiO₂ ceramics was within 60.0–84.0%, the cell size distribution was in the range of 10–120 μm, and the flexural strength was 9.7–16.3 MPa.     

YAG ceramic processed by slip casting via aqueous slurries

Stable YAG (Y₃Al₅O₁₂) aqueous slurry with ammonium polyacrylate (NH₄PAA) polyelectrolytes as dispersant was prepared by ball mill method. The effects of polyelectrolyte concentration and pH value on the stability of the suspension is described here, and the stability maps are constructed at different pH value and polymer concentration. The rheological behavior of YAG slips of different solid loading (60–70%) has been studied by measuring their viscosity and shear stress as a function of shear rate and pH of the slurry. An optimal amount of dispersant and pH value for the suspension was found. YAG suspension displays a maximum in zeta potential values and a minimum viscosity in pH range of 9–11. Slips behaved as near Newtonian at the pH value up to a solid loading of 60 wt% and as non-Newtonian with thixotropic behaviors above this solid loading value. The density and the green as well as sintered microstructure of the cast products bear a direct relationship to the state of this slips induced by the alternation in the pH and the concentration of the dispersant as well as solid loading.     

Porous alumina ceramics for slip casting molds

Although a slip cast process using a gypsum mold enables large and complicated dense green body to be made, the green body is contaminated with calcium and sulfur impurities from the gypsum mold. In the case of a resin mold, external pressure must be applied due to its poor water wettability. Porous alumina ceramics are used as a mold material for the first time to overcome these problems. The porous alumina ceramics show higher water wettability than gypsum or resin. The porous alumina molds have submicron pores, and provide larger penetration pressure and higher casting rate than gypsum molds.     

Texturing of hydrothermally synthesized BaTiO3 in a strong magnetic field by slip casting

Barium titanate powder was processed by slip casting in a rotating strong magnetic field of 9.4 T. The orientation factor of the sintered compact was analyzed by the X-ray diffraction technique and the microstructure (grain-size) was analyzed by scanning electron microscope. The hydrothermally prepared barium titanate was used as matrix material and the molten-salt synthesized barium titanate, with a larger particle-size, was used as template for the templated grain-growth process. Addition of large template particles was observed to increase the orientation factor of the sintered cast (5 vol% loading). Template particles acted as starting grains for the abnormal grain-growth process and the average grain-size was increased after sintering. Increasing the solid loading (15 vol%) resulted in a similar orientation factor with a decrease of the average grain-size by more than half. However, addition of templates to the 15 vol% cast had a negative effect on the orientation factor. The impingement of growing particles was stated as the primary cause of particle misorientation resulting in a low orientation factor after sintering. Different heating conditions were tested and it was determined that a slow heating rate gave the highest orientation factor, the smallest average grain-size and the highest relative density.     

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.     

Manufacture of carbon-bonded alumina based on a lactose-tannin binder system via slip casting

Binders play an important role in the manufacture of carbon-bonded alumina (Al₂O₃-C) refractory materials. To diminish the currently used coal tar pitch binder Carbores® P, which could release hazardous polycyclic aromatic hydrocarbons during operations, environmental-friendly lactose and tannin are applied as the main binders in this study. Compact cylinders of fine-grained Al₂O₃-C based on the lactose-tannin (16 wt%) binder system with significantly reduced pitch content (4 wt%) are successfully prepared via slip casting for the first time. The lactose to tannin mass ratio is systematically varied from 5:1 to 1:5 in the raw material formulation. The specimens based on the new binder system are less dense, more porous and contain less residual carbon than the pitch-based reference. Mechanical tests show that the specimens with lactose-tannin ratio of 5:1, 4:1 and 3:1 have considerable cold crushing strengths, although the splitting tensile strengths are low. In contrast to the well-established pressing route, the slip casting route for compact Al₂O₃-C is more comparable to the replica route for Al₂O₃-C foam filter due to the pressureless shaping process.