Effect of Concentration of Ammonium Poly(acrylate) Dispersant and MgO on Coagulation Characteristics of Aqueous Alumina Direct Coagulation Casting Slurries

Coagulation of aqueous alumina slurries prepared using various concentrations (0.43–1.04 wt% based on alumina) of ammonium poly(acrylate) dispersant by MgO has been studied for direct coagulation casting (DCC) of alumina. The slurries prepared at dispersant concentration below 0.84 wt% are not suitable for DCC at room temperature (∼30 ° C) as they undergo premature coagulation. Mixing the slurry with MgO at a low temperature of nearly 5 ° C slows down the reactions leading to coagulation and keeps the slurry viscosity low for a sufficient period of time. Coagulation of slurries prepared at a dispersant concentration of 0.92 wt% and above at room temperature requires MgO concentrations much higher than the equivalent amount required for reaction with the dispersant. This anomalous behavior at higher dispersant concentration is explained such that the Mg-poly(acrylate) formed by the reaction between ammonium poly(acrylate) and MgO formed a sheath over the remaining MgO particles and prevented them from further dissolution at room temperature. Faster coagulation could be achieved by heating the slurries after casting in closed molds. The Mg-poly(acrylate) acts as a binder and stabilizes the coagulated bodies as their strength and stability against oscillatory stresses increase with an increase in dispersant concentration.     

Magnesia induced coagulation of aqueous PZT powder suspensions for direct coagulation casting

Coagulation characteristics of poly(acrylate) dispersed PZT powder suspensions by MgO coagulating agent have been studied. The PZT powder suspensions undergoes coagulation at MgO concentrations much lower than the equivalent amount to react with the dispersant indicating a major shift in the coagulation mechanism from the corresponding alumina powder suspensions. Unlike in alumina powder suspensions, the Mg²⁺ ions generated from the MgO reacts with the ammonium poly(acrylate) adsorbed on particle surface that result in cross-linking of PZT particles by Mg²⁺ through the ammonium poly(acrylate) molecules. The particle bridging induces faster coagulation of the slurry cast in a mould as required for an ideal DCC process. The compressive strength and stability against oscillatory stresses of the wet-coagulated bodies increased with increase in number of cross-links between the particles. The PZT green bodies prepared by the DCC process sintered to near theoretical density and the MgO added as coagulating agent (～0.1 wt%) had only minor influence on its piezoelectric characteristics.     

A new direct coagulation casting process for alumina slurries prepared using poly(acrylate) dispersant

Direct coagulation casting (DCC) of concentrated aqueous alumina slurries prepared using ammonium poly(acrylate) dispersant has been studied using MgO as coagulating agent. Addition of small amounts of MgO increased the viscosity of the concentrated alumina slurries with time and finally transformed it in to a stiff gel. Sufficient working time for degassing and casting could be achieved by cooling the slurries to a temperature of ∼5 °C after proper homogenization after the addition of MgO. The DCC slip with alumina loading in the range of 50–55 vol% showed relatively low viscosity (0.12–0.36 Pa s at shear rate of 93 s⁻¹) and yield stress (1.96–10.56 Pa) values. The wet coagulated bodies prepared from slurries of alumina loading in the range of 50–55 vol% had enough compressive strength (45–211 kPa) for handling during mould removal and further drying. The coagulated bodies prepared from slurries of alumina loading in the range of 50–55 vol% showed linear shrinkage in the range of 4.8–2.3 during drying and 17.1–16.2 during sintering respectively. Near-net-shape alumina components with density >98% TD could be prepared by the DCC process.     

Direct coagulation casting of YSZ powder suspensions using MgO as coagulating agent

Direct coagulation casting (DCC) of aqueous 8 wt% yttria stabilized zirconia (YSZ) powder suspensions prepared using ammonium poly(acrylate) dispersant has been studied using MgO as coagulating agent. Small amount (<0.1 wt% based on YSZ) of MgO powder dispersed in the YSZ powder suspension at ∼5 °C set the suspension in to stiff wet-coagulated body when exposed to room temperature (30 °C) due to the reaction between ammonium poly(acrylate) and MgO. MgO concentration equivalent to react with dispersant did not coagulate the YSZ powder suspension though it precipitate the whole ammonium poly(acrylate) dispersant as Mg-poly(acrylate). This is because of the ability of the YSZ powder to disperse in water at alkaline pH (∼9.5) without any dispersant by electrostatic mechanism. The YSZ powder suspensions form stiff coagulated bodies at MgO concentration double or more of the equivalent amount required for reacting with the dispersant. Setting of the YSZ powder suspension is due to the heterocoagulation of the YSZ particles and MgO particles added in excess of the equivalent amount to react with the dispersant, having opposite surface charges. The wet-coagulated body showed relatively high compressive yield strength (155 kPa) and Young’s modulus (3.1 MPa). The green bodies prepared by humidity controlled drying of the wet-coagulated bodies sintered to >98% TD at 1550 °C.     

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.     

Heterocoagulation moulding of alumina powder suspensions prepared using citrate dispersant

Coagulation characteristics of aqueous alumina suspensions prepared using di-ammonium hydrogen citrate (DHC) dispersant has been studied using MgO coagulating agent for direct coagulation casting (DCC). Mg–citrate complexes formed by the reaction between the DHC and MgO act as dispersant for alumina at pH near its iso-electric point. Setting of the alumina suspensions takes place, at MgO concentrations higher than the equivalent amount required to react with the DHC, by heterocoagulation of the alumina and MgO particles due to their opposite surface charges. The yield strength and Young's modulus of the wet-coagulated alumina bodies increased with aging time due to the hydration of the excess MgO. The minimum time required for mould removal decreases and yield strength and Young's modulus of the wet-coagulated bodies increases with MgO concentration. Alumina green bodies prepared at MgO concentrations in the range of 0.2–0.5 wt% sintered to ～97% theoretical density at 1550 °C and the sintered ceramics showed more or less similar microstructures irrespective of MgO concentrations.     

Cardanol: A New Dispersant for Alumina in Toluene

Cardanol, which is a naturally occurring C₁₅ unsaturated aliphatic chain-substituted phenol derived from cashew nut shell liquid, was used as a dispersant for alumina in toluene. Adsorption data, along with the results of sedimentation studies, green density, and slurry viscosity (as a function of dispersant concentration), suggest that cardanol adsorption initially proceeded through surface coverage by a phenolic group that lies flat on the surface and that the best powder dispersion occurred at surface saturation by more closely packed end-on adsorbed cardanol molecules. The alkyl-chain unsaturation significantly contributed to dispersion, such that the saturation by hydrogenation led to an increase in the slurry viscosity by a factor of ∼2.5. Concentrated slurries generally showed shear-thinning flow behavior, and the measured viscosity of a highly concentrated (53 vol%) slurry was <1 Pa·s at a shear rate of 93 s⁻¹.     

Dispersion and Setting of Powder Suspensions in Concentrated Aqueous Urea Solutions for the Preparation of Porous Alumina Ceramics with Aligned Pores

Highly concentrated alumina powder suspensions have been prepared in aqueous urea solutions of concentrations in the range 200–360 g/100 mL using an ammonium poly(acrylate) dispersant at 80°C. The dispersant concentration for the suspension viscosity minimum in the urea solutions is higher than that in water due to the higher processing temperature. The urea solutions having higher dielectric constant than that of water offer higher interparticle potential that resulted in better dispersion of the powder as evidenced from the lower viscosity and yield stress of the suspensions. The decrease in temperature increased the suspension viscosity and the suspension formed a strong gel when cooled to room temperature due to the crystallization of urea. The minimum urea solution concentration for a 55 vol% alumina suspension to form a dimensionally stable gel is 240 g/100 mL. The compressive strength and Young's modulus of the gels increased with the increase in urea solution concentration. The alumina ceramics prepared by the urea removal followed by sintering at 1500°C had porosity in the range 28–36 vol% with the rectangular rod‐shaped aligned pores.     

Effect of medium condition change on green body density during casting and drying

The present study examined whether the optimum slurry conditions in which the green body with the highest packing fraction was fabricated were the same regardless of whether tape or slip casting methods were used. Additionally, we investigated the optimum slurry evaluation method to predict the packing fraction of each green body. Specifically, aqueous dense alumina slurries, whose particle dispersion state was changed by varying pH and the amount of dispersant additive, were prepared. After changing the pH, the optimum slurry conditions were achieved regardless of the casting method. The most suitable amount of additive dispersant differed due to the changing solution conditions during the casting and drying process; the medium of the slurry permeated into the plaster during slip casting and dried on the substrate during tape casting. Thus, in slip casting, non-adsorbed polymers could also be permeated. Hence, it is crucial for slurry evaluation to predict the packing fraction of the slip cast body. A sedimentation test, which involved particle condensation as well as casting, was valid for the green sheet. Conversely, it was experimentally shown that the constant pressure filtration test, which involved the permeation of the medium in addition to particle condensation, was suitable for slip casting.     

Coagulation of electrosterically dispersed concentrated alumina suspensions for paste production

The effect of salt induced coagulation of concentrated alumina suspensions initially electrosterically dispersed with ammonium polyacrylate (NHPA) has been investigated. Addition of ammonium chloride caused enhanced adsorption of the NHPA onto the alumina surface. In terms of suspension characteristics minimum viscosity and minimum sediment height coincided with the attainment of complete surface coverage of the dispersing agent (3.7 mg g⁻¹). Coagulation resulted in a reduction in the degree of sample homogeneity in all samples. Coagulation of suspensions initially dispersed with NHPA levels below that needed for complete surface coverage under coagulated conditions (1.4 mg g⁻¹) produced viscous pastes with a yield stress suitable for processing by extrusion. At 70 wt% solids the green densities could not be distinguished from corresponding flocculated samples. Significantly, however, at 80 wt% solids a small degree of homogeneity was retained upon coagulation resulting in a higher green density than those prepared by a flocculated route. The approach of initial dispersion followed by coagulation is therefore of potential benefit in the processing of viscous, pastes such that higher degrees of homogeneity can be achieved compared to those prepared by a flocculated route.     

Experimental study on the time change in fluidity and particle dispersion state of alumina slurries with and without sintering aid

The time change of the fluidity and particle dispersion state of alumina slurries with and without a sintering aid were investigated. The apparent viscosity of the slurry was measured at certain intervals. The hydrostatic pressure of the slurry, which represents the particle dispersion state, was also measured. We showed that the pH value, adsorbed amount of dispersant, and Mg ion concentration of the slurry hardly changed with time even though the apparent viscosity of the slurry increased with time. We suggest that the time change in the apparent viscosity, that is, the behaviour in which the apparent viscosity of the slurry increased with time, occurs when the electrostatic repulsion force is insufficient to maintain the particle dispersion state for a long time, such as shortage in the dispersant to its saturated amount.     

Dispersion Properties of BaTiO3 Colloids with Amphoteric Polyelectrolytes

An amphoteric water-soluble copolymer, i.e., poly(acrylamide/(α- N,N -dimethyl- N -acryloyloxyethyl) ammonium ethanate) (PAAM/DAAE), was evaluated as a novel dispersant for aqueous BaTiO₃ (BT) slurries. The dispersing property of this copolymer was examined by means of rheology, sedimentation, particle size, green density, zeta potential, and leached Ba²⁺ concentration measurements. The results indicate that PAAM/DAAE could reduce the viscosity of slurries greatly, cause BT particle sizes to shift to smaller values, and make green compacts more consolidated. Compared with a commercial dispersant, ammonium salt of poly(methacrylic acid) (PMAA-NH₄), it is as effective or even better in preparing stabilized suspensions. More importantly, PAAM/DAAE could lessen the leached Ba²⁺ concentration. This is related to the adsorption behavior of this copolymer onto BT particles, and the interaction between the adsorbed dispersant and dissolved barium ions.     

Coagulation of mixed organic systems and alumina particles for paste production

Pastes of alumina with plastic properties were prepared by direct coagulation casting (DCC) of concentrated suspensions (≥50 vol%). Two polyelectrolytes were used to produce mixed systems of organic and mineral particles. An anionic polymer, bearing both sulfonate and carboxylate functions, was used to disperse efficiently alumina powder in water. A cationic polymer, the chitosan (CT), which complexes with the anionic one was added into the stable suspension to form an organic network dispersed into the ceramic matrix. The sequence of addition of the chitosan in the suspension appears to be a significant parameter in order to optimise the suspension formulation. After in situ coagulation, cylinders were extruded.     

凝胶注成形纳米碳化硅增韧氧化铝防弹陶瓷材料工程化研究及应用

本研究用单体(丙烯酰胺以后简称单体)、交联剂(N,N—亚甲基双丙烯酰胺以后简称交联剂)、氧化铝、纳米碳化硅等,使用分散剂丙烯酸和丙烯酸铵的共聚物,球磨后料浆颗粒D50(中位粒径)为2μm,体积固含量为55％的陶瓷料浆。在催化剂(N,N,N,,N,—四甲基乙二胺)、引发剂(硫代硫酸铵)条件下凝胶注成形氧化铝陶瓷坯体,干燥...     

State of the Dispersant and Particle Surface During Wet-Jet Milling for Preparation of a Stable Slurry

Wet-jet-milled alumina slurries exhibited distinctly different stability behavior compared with ball-milled ones in terms of reflocculent efficiency and rheological properties. The distinction was attributed to the different behavior of the same dispersant (NH₄⁺ salt of poly(acrylic acid); PAA–NH₄⁺) in ball milling and wet-jet milling. Alumina particles after the wet-jet milling retained the initial surface conditions, although ball-milled alumina particles yielded more hydroxyl groups on the surface. Furthermore, degradation of dispersant on milling was investigated qualitatively. Infra-red analysis and isoelectric point measurements suggested that degradation of the dispersant occurred during ball milling, indicating neutralization of the dispersant due to formation of monodentate COOX groups. On the other hand, in the case of wet-jet milling, COO⁻ groups of PAA–NH₄⁺ were not degraded at all, indicating maintenance of the structures of dispersant with electrostatic repulsion. Change in the polymer size of the dispersant by different milling methods was confirmed using high performance liquid chromatography (HPLC) measurements. The HPLC results of the wet-jet milled samples showed that a dispersant with a larger molecular size was generated. Force measurement on approach via the colloid probe method in the centrifuged supernatant of the milled slurries confirmed that the interaction distance between an α-alumina colloid probe and c-sapphire surface was about two times larger in the wet-jet-milled slurry supernatant compared with the ball milled one. Thus, it was found that wet-jet milling process led to a stable slurry because of the maintenance of not only steric repulsion but also electrostatic repulsion.     

Dispersion behavior of HfC-based nanopowders in ethanol

The dispersion behavior of two different types of ultrafine HfC-based powders in ethanol was investigated using polyethyleneimine (PEI) as a dispersant. The first type was synthesized by carbothermal reduction using a modified spark plasma sintering technique (d50: 125 nm). For the 10 wt% HfC suspensions, the highest zeta potential value (67.7 mV), the least sediment after sedimentation test for one day, and finest particle sizes were obtained when the concentration of PEI was 2.0 wt%. The concentrated HfC slurries with a solid loading of 40 vol% were achieved using 1.0 wt% PEI. The second type was mixing the HfC powder with HfSi2-C additives using a high-energy ball milling. The concentrated HfC slurry containing 20 wt% of HfSi2-C sintering additives was prepared up to 50 vol% solid loading using 0.50 wt% PEI. This is the first report for producing highly concentrated HfC-based nano slurries, which were highly suitable for the wet process of ultrahigh-temperature ceramic matrix composites (UTHCMCs).     

The effects of ball milling time on the rheological,optical, and microstructural properties of YAG transparent ceramics

Stable slurries dispersed mixture of commercial Al₂O₃ and Y₂O₃ powders, ammonium poly meta acrylate (Dolapix CE64) as the dispersant, and tetraethyl orthosilicate (TEOS) as the sintering aid were prepared by ball milling process. The effects of the milling time on the fabrication of transparent polycrystalline yttrium aluminum garnet (YAG) ceramics were investigated by slip casting and vacuum sintering. The results showed that the best milling time for the deagglomeration of the powder mixture was 16 hours and the slurry prepared during this time showed a near-Newtonian behavior due to the better deagglomeration and lower viscosity. X-ray patterns also showed that all samples were pure YAG phase. The results also revealed that the sample prepared by 16 hours ball milling time suspensions exhibited higher relative green and final density, as well as the maximum transmittance at 1064 nm (≈ 77%). These samples had a more uniform microstructure too.     

Rheology of Concentrated Suspensions Containing Weakly Attractive Alumina Nanoparticles

The use of nanoparticles for the fabrication of new functional ceramics and composites often requires the preparation of concentrated fluid suspensions. However, suspensions containing nanoparticles are limited in solids content because of the excluded volume formed by the dispersant adlayer around the particles. We investigated the effect of the adlayer thickness on the rheological behavior of suspensions containing model alumina nanoparticles, using dispersant molecules with deliberately tailored chain length. The apparent viscosity and yield stress of the particle suspensions were markedly decreased by increasing the dispersant length, mainly due to a reduction of the attractive forces among particles. Fluid suspensions with solids content up to 35 vol% were prepared in toluene using a dispersant length of 2.5 nm. Our experimental results and viscosity predictions based on a hard sphere model indicate that fluid suspensions with up to 43 vol% of 65 nm alumina particles could be prepared using an optimum dispersant length of about 3.6 nm.     

Fully sintered alumina with a higher Vickers hardness prepared using a gel-casting process

The dispersant (commercial ammonium polyacrylate), sintering additive (Mg²⁺), and chelating agent (EDTA) effects on the Zeta potential and rheological behavior of alumina slurry with high solid content were investigated. The alumina ceramic green microstructures and sintered microstructures prepared using slurries with different additives using the gel-casting process were also studied. It was observed that the dispersion deteriorated after adding Mg²⁺. Slurry simultaneously added with dispersant and EDTA-chelated Mg²⁺ produced higher absolute Zeta potential value and low viscosity due to EDTA chelating with Mg²⁺. The sample added with dispersant and EDTA-chelated Mg²⁺ exhibited a uniform green microstructure, high relative sintered density (99.5% theoretical density), and a nearly pore-free microstructure with an average grain size of about 1.5 μm. For the first time to our knowledge, the maximum Vickers hardness (22 GPa) was obtained for alumina simultaneously added with Mg²⁺ and EDTA, pressureless sintered at 1500°C in air.     

Preparation of alumina by aqueous gelcasting

The alumina ceramic was prepared by aqueous gelcasting. The effects of zeta potentials, solid loading, dispersant content and milling time on the alumina suspension were studied systematically. The dispersant content has remarkable effects on the viscosity of the suspension. The appropriate dispersant concentration for alumina aqueous slurry with the solid loading of 55 vol.% is 0.6 wt.%. It can be seen that all suspensions (50–56 vol.% solid loading) exhibited a shear-thinning behavior and relatively low viscosity, which was suitable for casting. The degree of shear thinning and the viscosity at high shear rates increased with increasing volume fraction of solid. As the milling time prolongs, viscosity of the suspension decreases first, then the plateau appears and the average diameter keeps changeless. When the milling time was shorter than 20 h, the viscosity of slurries decreased gradually as the time of milling increased. After 20 h milling, the viscosity of the slurry tended to be consistent. Therefore, the ball milling time should be equal to or more than 20 h to obtain a stable suspension at equilibrium. The time available for casting the slurry (idle time) can be controlled by the amounts of initiator and catalyst added to the slurry as well as by the processing temperature. Micrograph of the gelcast green body was homogeneous.