Aqueous Alumina Slurries Research Articles

Impact of additives on the quality of oxide/oxide tow-pregs obtained by continuous fibre impregnation

This paper focuses on the formulation of aqueous alumina slurries compatible with manufacturing alumina/alumina ceramic matrix composite (CMC) by a continuous tow impregnation process. Two hygroscopic organic plasticizers, sorbitol and glycerol, along with a gelling powder, boehmite, are evaluated for their impact on both slurry behaviour and the quality of impregnated tows. The interest of using glycerol instead of sorbitol is demonstrated, as glycerol exhibits smaller impact on slurry viscosity and stability, while being more hygroscopic and plasticizing, enabling a reduction in organic content. Combining glycerol with boehmite powder improves the surface and the tack of impregnated tows, although boehmite increases slurry viscosity and shear-thinning behaviour. Furthermore, thermal degradations of sorbitol and glycerol are studied through thermogravimetric analysis, to define an autoclave curing cycle for each plasticizer removing, to achieve optimal compaction of tow-pregs and hence reduce the porosity of the resulting CMCs.

Hidden correlation between rheological dynamics and crack formation in water-based slurry

Cracks generated in sheet coating of water-based slurries are an increasingly important concern in the development of diverse material science, such as fine ceramics, Pt catalysts, and Li transition metal oxides, from an environmental safety perspective. However, a comprehensive understanding of crack formation remains elusive due to the intricately entangled interplay among various characteristics of slurries. Here, we systematically ascertain the causes at work in crack initiation by analyzing the physical parameters of a series of aqueous alumina slurries using machine learning. The analysis reveals a strong correlation between rheological hysteresis and sheet cracking. As a verification of the correlation, the crack suppression by controlling the rheological hysteresis is experimentally confirmed. Furthermore, we visualize the microstructure in the slurry that induces cracks with a liquid-phase scanning electron microscope, clarifying the rheological dynamics. These results disentangle that rheological hysteresis is a good indicator directly reflecting microstructural durability against drying stress.

Study on particle dispersion changes over time in aqueous Al2O3 slurries containing ammonium polyacrylate

In this study, we investigated the effects of additive amounts of polyelectrolyte and the amounts of unadsorbed polyelectrolyte on the long-term changes in the amount of polyelectrolyte adsorbed and the particle dispersion state of aqueous alumina slurries containing polyacrylic acid. Slurries were evaluated on the basis of viscosity, amount of adsorbed polymer, and elution amount of Mg and Al ions. At 80 days after the preparation of the sample slurry, the amounts of ammonium polycarboxylic acid adsorbed in the slurries had stabilized. The slurries were classified into two types: one in which particles agglomerate as the adsorption increases, and one in which dispersion occurs instead with increasing adsorption. In a slurry in which the amount of polymer electrolyte added is less than the saturated adsorption amount of the polymer electrolyte, polyvalent metal ions eluted from the powder bind the adsorption layers on the particle surface to each other, and the particles aggregate over time. In a slurry in which the amount of polymer electrolyte added is larger than the saturated adsorption amount of the polymer electrolyte, the eluted polyvalent metal ions bind to the unadsorbed polymer electrolyte; hence, the particles do not aggregate with each other and disperse with increase in the amount of polymer electrolyte adsorbed. Thus, when adding a polymer electrolyte to a slurry in which polyvalent metal ions are eluted, the amount of added polymer electrolyte should be greater than the amount of saturated polymer electrolyte adsorbed on the particles to maintain the dispersed state for a long period.

Impact of bio-based binders on rheological properties of aqueous alumina slurries for tape casting

This work describes the preparation of concentrated alumina suspensions based on bio-sourced additives with a low environmental impact and with the rheological properties adapted to the tape casting process. Natural polysaccharides extracted from plants and fruits (pectin, psyllium…) were characterized (molar weight) and identified as promising binders for substituting the classical organic binders from petrochemical industry. The rheological behavior of the additives and of the suspensions containing these bio-sourced additives was studied in flow and in oscillations. The mechanical properties of the green tapes obtained with the different bio-sourced binders were determined from tensile experiment. This paper studies the relations between the nature of the binder, the rheological behavior of the suspension, the ability to tape cast the suspensions and the final properties of the green and sintered tapes.

Soft lithographic fabrication of free-standing ceramic microcomponents using poly(N-isopropylacrylamide) brushes grafted poly(dimethylsiloxane) micromolds

In order to fabricate intact and precise ceramic microcomponents applied in microelectromechanical systems (MEMS), thermo-responsive poly(N-isopropylacrylamide) (PNIPAAm) brushes are grafted onto poly(dimethylsiloxane) (PDMS) micromolds by visible light-induced polymerization. The application of PDMS-Br@PNIPAAm micromolds in soft lithographic fabrication of ceramic microcomponents is demonstrated by producing alumina green microcomponents. The PDMS-Br@PNIPAAm micromolds are hydrophobic with a water contact angle of 129.7 ± 0.3° at 15 °C. During the molding process, the micromolds are changed to hydrophilic with a water contact angle of 81.3 ± 0.4° at 4 °C, which could improve the flowability of aqueous alumina slurry by forming intermolecular hydrogen bonding with PNIPAAm chains. During the demolding process, the micromolds are hydrophobic as the intermolecular hydrogen bonding is broken after drying. Consequently, free-standing alumina green microcomponents with microchannels of 73.7 ± 0.8 μm wide are fabricated by microtransfer molding, a maximum aspect ratio of 3.6 could be achieved. The lateral and longitudinal shrinkages of the microchannels are 9.8 ± 0.6% and 11.5 ± 0.4% respectively. The PDMS-Br@PNIPAAm micromolds can be reused more than 13 times. It is believed that moisture-sensitive PDMS-Br@PNIPAAm micromolds can be effectively used to fabricate ceramic microcomponents with high aspect ratios for MEMS.

Aqueous tape casting of alumina using natural rubber latex binder

Abstract Eco-friendly and sustainable tape casting of alumina powder suspensions using concentrated natural rubber (poly isoprene) latex binder has been studied. The high negative zeta potential values of the aqueous alumina slurry (−53 to −72 mV) and rubber latex (−67 to −84 mV) at pH in the range of 9–11.5 enables their co-dispersion to produce tape casting slurries of solids (alumina + rubber) concentration >60 vol% with adequate flow characteristics. Drying of the slurry tape-cast on Mylar substrate is achieved within 15 min at 70 °C due to its high solid concentration. The green tapes containing 14.2 to 18.1 wt% of rubber shows tensile strength and strain at failure in the ranges of 1.85 to 1.61 and 41–254%, respectively. The flexible green tapes turn rigid by annealing at 200 °C due to the self-cross-linking of rubber chains induced by the Lewis acid sites of alumina. Thickness reduction to the extent of 20% by rolling of the green tape before annealing improves the green microstructure which results in an enhancement in sintered density from 93 to 98% of the theoretical value. However, the additional rolling and annealing steps consume extra time and energy compared to the tape casting processes using other reported binders.

Hierarchical‐Porous Ceramic Foams by a Combination of Replica and Freeze Technique

Open‐porous alumina foams with additional strut porosity are fabricated by a two‐step sponge‐replication based manufacturing process. As the first step, open cellular ceramic foams are prepared following the Schwarzwalder sponge replication technique. Therefore, organic foam templates are coated with different aqueous alumina slurries with a solid load between 20 and 40 vol% In a second step, an additional porosity is generated inside the foam struts by freezing of the foams at temperatures between −196 and −20 °C and subsequent sublimation drying. The hierarchical structure of both, cell pores and strut pores in the freeze‐dried material remains intact after drying, template removal, and sintering. Size, connectivity, and morphology of the strut pores strongly depend on the solid load of the alumina slurry and on the freezing temperature. Cellular structures with a strut porosity between 50% and 60% and a total porosity exceeding 90% are prepared, which show a compressive strength in between 0.4 to 0.6 MPa. Due to the additional strut porosity, the specific surface area of freeze‐dried replica foams increases from 70 cm2 g−1for conventionally prepared foams to 200 cm2 g−1for freeze‐dried foams, respectively.

MgO calcination for easy direct coagulation casting of aqueous alumina slurries

The reactivity of MgO with ammonium poly(acrylate) and diammonium hydrogen citrate dispersants was decreased by high-temperature calcination which enabled easy preparation of direct coagulation casting slurries without cooling. The decrease in reactivity of MgO with an increase of calcination temperature (30–1200 °C) was due to the decrease of surface area (52.7–0.7 m2/g) as a result of an increase of average particle size (285–2075 nm) as well as a change of particle morphology from flaky to near spherical. The MgO calcined at a temperature of 1000 °C and above provided sufficient time for mixing with aqueous alumina slurries by ball milling at room temperature (~30 °C) without producing an adverse increase in viscosity before casting. The setting time of 55 vol% alumina slurries was in the ranges of 260–1070 and 10–50 min at room temperature and at 70 °C, respectively, at MgO concentrations in the range of 0.1123–1.2 wt%. The faster setting at 70 °C was due to a combination of faster dispersant-MgO reaction, faster hydration of MgO and high valance counter ion effect.

Freeze-cast alumina pore networks: Effects of processing parameters in steady-state solidification regimes of aqueous slurries

Aqueous alumina slurries with varying solids loading and particle size were freeze cast under seven freezing conditions to investigate the influence of these on pore network characteristics including pore size and geometric specific surface area. Slurry temperatures were recorded in situ to determine freezing front position and velocity during solidification, which were then analyzed via regression and modeled using solidification theory. Classic mathematical models for the time dependence of freezing front position and velocity were found to hold for freeze-cast slurries. Building on these, a one-phase Stefan problem was used to describe freezing kinetics. Models for freezing front velocity were combined with solidification theory to obtain predictions of microstructural feature size from freezing kinetics. Results showed that, while there may be a dependence on solids loading, the combination of mathematical modeling of solidification and classic solidification theory is applicable to freeze-cast ceramics and accurately describes pore network characteristics from processing parameters.

Fabrication of lamellar porous alumina with axisymmetric structure by directional solidification with applied electric and magnetic fields

A novel directional solidification method was used to fabricate lamellar porous alumina with an axisymmetrical structure by combining multiple cold sources, electrostatic field, and magnetic field during the freezing of aqueous alumina slurries. Multiple cold sources with mutually perpendicular directions formed by the bottom and parallel sides of rectangular copper molds induced a specific growth direction for ice crystals. The pore channels of porous ceramic are arranged parallel to the direction of electric field lines, and the axisymmetrical structure was obtained after applying the electrostatic field. The applied magnetic field can reduce pore channel size; however, the field has no effect on pore channel direction and distribution, and leads to cracking in the central region of porous ceramics. The compressive strength along the electrostatic field direction is higher than that achieved by conventional freeze casting.

Fabrication of alumina micromixer with two‐dimensional serpentine microchannels by centrifuge‐assisted micromoulding

A two-dimensional (2D) serpentine mixer was made of alumina using centrifuge-assisted micromoulding. Concentrated aqueous alumina slurry was filled into microstructured polydimethylsiloxane micromoulds with the aid of centrifugation to form green micromixers, which were then sintered to obtain the final products. In contrast to the micromixer fabricated by micromoulding (filling without the aid of centrifugation), it is found that using centrifugation significantly improved the structural integrity of the green micromixer. Straight and serpentine microchannels with rectangular cross-sections were precisely fabricated. After sintering at 1600(o)C, defect-free alumina micromixer with a relative density of 98.7%, linear shrinkage of about 18%, hardness of 20.9 GPa, flexural strength of 328.2 MPa and roughness of 143.9 nm was attained. The results show that ceramic microdevices with complex-shaped 2D microchannels (with diameters 100 m and aspect ratios 2) can be reliably fabricated by centrifuge-assisted micromoulding.

Novel freeze-casting fabrication of aligned lamellar porous alumina with a centrosymmetric structure

Abstract A novel freeze-casting method is used to fabricate aligned lamellar porous alumina with a centrosymmetric structure from aqueous alumina slurries. Two cold fronts oriented perpendicularly to each other, originating from the bottom and side of the cylindrical copper mold, induce the growth of ice crystals in specific directions along the radius of the cylindrical mold. Lamellar channels of porous alumina are arranged centrosymmetrically along the radial axis. The pore distribution of the currently prepared porous ceramics is more regular when compared with that of porous ceramics prepared by conventional freeze casting. This affords porous ceramics with improved mechanical properties and stability. The current method addresses the issue of partial failure as induced by the randomly distributed channels in lamellar porous ceramics.

Novel Direct Coagulation Casting of Alumina Suspensions Using Y<sup>3+</sup>-Ion Releasing Substances as Coagulant

Direct coagulation casting (DCC) is a relatively new ceramic near-net-shape forming process which can form homogeneous ceramic green body with complex shape and high density. Direct coagulation casting of aqueous alumina slurries by adding Y3+ions, which have been frequently used as additive for the sintering of alumina ceramics, were studied. Two different kinds of Y3+-ion releasing substances, Y2O3powder and Y(NO3)3, were used as coagulants and were introduced into stabilized alumina suspensions directly after dispersing. Compared with that of Y2O3powder, both the coagulation time and the added amount required for a reasonable shaping were significantly reduced for using Y(NO3)3as coagulant. The effects of the two coagulants on the properties of the consolidated green bodies and sintered ceramics were studied. The mechanisms of coagulation induced by the addition of Y3+-ions were discussed on the basis of interactions between Y3+ions and the added polyelectrolyte dispersant.

Gelcasting of alumina using the hexamethylenediamine–paraformaldehyde monomer system

A new aqueous alumina gelcasting system using hexamethylenediamine (HMDA) and paraformaldehyde monomers has been studied. The 500vol% aqueous alumina slurries ‘A’ and ‘B’ containing paraformaldehyde and HMDA, respectively, undergo gelation after thorough mixing of the two due to the polymerization of HMDA and formaldehyde. The gelation time of the slurries cast in a mold is in the range of 7–2.4min at HMDA to formaldehyde mole ratio in the range of 1.1–1.5. The faster reaction between HMDA and formaldehyde prevents the formaldehyde emission during the processing. The minimum HMDA to formaldehyde mole ratio required for the formation of a mechanically stable gel is 1.1. The compressive strengths and Young's modulus of the wet and dry alumina bodies increased with an increase in HMDA to formaldehyde mole ratio. Though the wet gelcast alumina bodies had low compressive strength (11.2–88.7kPa) and Young's modulus (0.17–5.9MPa) the dried ones showed high strength (6–11.7MPa) and Young's modulus (209–364MPa). The binder removal by slow heating to a temperature below 500°C followed by sintering at 1600°C produced alumina ceramics with ~97% of theoretical density.

Poly(maleic acid) – A novel dispersant for aqueous alumina slurry

Poly(maleic acid) (PMA) was investigated as an anionic long chain dispersant for the preparation of highly loaded aqueous alumina slurry. Suspensions with 1 wt% alumina powder were prepared through variation of PMA concentrations at pH 9 to assess its dispersion efficacy in aqueous medium. Addition of PMA increased the stability of 1 wt% alumina suspensions as evidenced by higher zeta potential and RSH (relative suspension height). 55 vol% alumina slurries were prepared with different PMA concentrations at pH 9. Based on the rheological studies, 3.8 mg of PMA per gram of alumina was obtained as the optimized dispersant amount at pH 9. All the slurries had shear thinning behavior and the slurry with optimum dispersant amount had insignificant thixotropy. Further, adsorption of dispersant was evident by Fourier transform infrared spectroscopy (FTIR) analysis and thermogravimetric analysis (TGA). The slurry at pH 9 with optimum dispersant amount had lowest viscosity and easy flowability. Microstructure of bisque fired and sintered body by SEM microscopy revealed homogeneous particle distribution and uniform grain growth with sintered density of 3.94 g/cm3. The mechanical property evaluation revealed that the samples prepared with optimum dispersant amount had the flexural strength of 483 ± 22 MPa.

A urea crystal templating method for the preparation of porous alumina ceramics with the aligned pores

Abstract A novel process for the preparation of porous alumina ceramics with an aligned pore structure has been reported. The urea dissolved in the aqueous alumina slurry at higher temperature formed rod shaped crystals aligned in the direction of gravity when cooled to room temperature. The gelatin used to set the slurry, controls the crystallization of urea such that the urea crystals produced in the suspension containing the gelatin had much lower width and thickness compared to that produced in the suspension without the gelatin. The alumina powder catalyzed the thermal decomposition of urea that enabled the removal of the majority of the urea crystals from the alumina green body by isothermal heating at temperature much lower than the melting point of urea. The void space created by the removal of the urea crystals remained as pores in the sintered ceramics.

Structure and properties of flax/polylactide/alumina nanocomposites

Composites composed of polylactide (PLA), woven flax fiber textiles (weave style of 2 × 2 twill and 4 × 4 hopsack) and boehmite alumina were produced by hot press. The spraying technique served for the pre-dispersion of the alumina nanoparticles. The aqueous alumina slurry was produced by mixing the water with water dispersible alumina. The dispersion of the flax structures and alumina particles in the composites was studied by scanning electron microscopy. The polylactide composites were subjected to water absorption and instrumented falling weight impact tests. The creep and thermomechanical properties of the composites were determined in short-time creep tests (performed at various temperatures), thermogravimetric analysis and dynamic-mechanical thermal analysis, respectively. It was found that the incorporation of alumina particles reduced the water uptake compared to the polylactide/flax blends. The impact energy and stiffness value of polylactide/flax blends was markedly higher than that of PLA but reflected the effects of composite structures. Incorporation of alumina particles enhanced storage modulus and the creep resistance compared to the polylactide/flax blends but slightly incremented thermal resistance at high temperature. No clear trend in the flax weave style-effect was found in the thermal behavior. The creep master curves were constructed by applying the time–temperature superposition principle. The Findley power law could satisfactorily describe the creep compliance versus time traces for all systems studied.

Silica containing highly porous alumina ceramic

Porous alumina ceramic were produced by slip casting of aqueous alumina slurry with added small amount of metallic aluminium powder. Pores form in result of chemical reaction of aluminum with water by hydrogen gas evolution reaction and solidification of suspension. Porosity of such materials sintered at a temperature of 1600 – 1750°C varies from 60 to 90%. Pore size distribution and mechanical strength of these materials depend largely on the grain size of used raw materials. The major part of pores in the materials produced without additive of silica are larger than 10 ±m, but with 5 – 10 wt.% additive of silica in the raw mix pore size decreases considerably. The sintering shrinkage decreases to 2.5%. Coefficient of thermal expansion equally decreases from 8.9-10−6 K−1 to 7.1 10−6 K−1 and classification temperature increases to 1600°C, while deformation at high temperature decreases considerably.

Shape forming alumina slurries via Colloidal IsoPressing

The relative density, deformation behavior, and dynamic rheology of bodies consolidated from aqueous alumina slurries, formulated at pH 4 and containing NH 4Cl, were studied as a function of the salt concentration (0.1–0.75 M). The smallest salt concentration produced the highest relative density (0.596), which was independent of consolidation pressure. Nearly identical relative densities were obtained for the other salt concentrations, ranging from 0.57 to 0.59 for consolidation pressures of 5–100 MPa, respectively. The consolidated bodies exhibited either a plastic or an elastic behavior when tested in compression. It was observed that the transition from a plastic to an elastic behavior occurred at a critical consolidation pressure. This critical consolidation pressure was directly dependent on the salt concentration. The storage (elastic) modulus for the fluidized, plastic bodies increased with salt concentration. The storage modulus was also observed to be dependent on the time the bodies were allowed to rest after they were subjected to a high shear rate. A pre-consolidated and fluidized slurry formulated to contain 0.5 NH 4Cl was extruded into a rubber mold cavity and further consolidated by isopressing at 200 MPa for 2 min. The body not only increased its relative density (0.58–0.62), but it also became elastic to enable its removal from the rubber cavity without shape distortion. No linear shrinkage was detected after the isopressed bodies were dried. Because of the lack of shrinkage, rapid drying was possible. For example, bar specimens (6.4 mm × 12.7 mm × 60 mm) would survive a 34 °C/min heating rate to 300 °C; the specimen was completely dried within 20 min. Because shrinkage does not occur during drying, bodies could be placed directly into a furnace and heated to 1500 °C for 0.5 h to achieve a relative density of >0.99 and an average grain size of 1.6 μm.

Preparation of Gel-casting Slurry to Fabricate Large Scale Green Body

In this study, a gel-casting has been optimized to fabricate larger scale ceramic products. An aqueous alumina slurries with different powder mixing ratio of larger (18 μm) and smaller (0.4 μm) alumina powder, and the initiator which generates the common free radical (Ammonium persulfate) or thermal radical (2, 2' -Azobis [2-(2-imidazolin-2-yl)-propane] dihydrochloride) were prepared to investigate the effect of the gelation time on the structure of the resulting green body. Results showed that the common radical generator (Ammonium persulfate) took longer gelation time, resulting in the distorted green body due to the different shrinkage behavior in upper and lower part of green body. While thermal radical generator (2, 2'-Azobis [2-(2-imidazolin-2-yl)-propane] dihydrochloride) accelerated the polymerizing rate to exhibit uniform shrinkage behavior without cracking in the green bodies because of the uniform density especially for the case of large size powders. As a result, large-scale ceramic green body with uniform packing density was successfully prepared by optimizing the gel-casting conditions, showing the advantage of the gel-casting using starting reagent of thermal radical generator and the raw powders of the mixture of larger and smaller size powders.