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.     

Fabrication of porous fibrous alumina ceramics by direct coagulation casting combined with 3D printing

A novel forming method for preparing porous alumina ceramics using alumina fibers as raw materials by direct coagulation casting (DCC) combined with 3D printing was proposed. Porous fibrous alumina ceramics were fabricated through temperature induced coagulation of aqueous-based DCC process using sodium tripolyphosphate (STPP) as dispersant and adding K₂SO₄ as removable sintering additives. The sacrificial coated sand molds was fabricated by 3D printing technology, followed by the infiltration of silica sol solution for the subsequent suspension casting. Stable alumina suspension of 40 vol% solid loading was obtained by adding 2.0 wt% STPP and 40 wt% K₂SO₄. The controlled coagulation of the suspension could be realized after heating at 90 °C for about 35 min. The ceramic sample sintered at 1450 °C for 2 h showed the highest compressive strength of 24.33 MPa with porosity of 57.38%. All samples sintered at 1300–1450 °C had uniform pore size distributions with average pore size of 7.2 µm, which indicated the good structure stability when sintered at high temperature.     

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.     

Hydrophobic coagulation of alumina slurries

This study reports a new wet forming method by hydrophobic modification of short-chain anionic dispersants. The influence of the content and type of hydrophobic chains on the rheology of alumina slurries, the drying shrinkage rates of wet bodies, the density and strength of the green bodies, and the density and microstructure of the resultant ceramics were intensively investigated. It is revealed that hydrophobic modification can endow short-chain dispersants with coagulation ability, when 0.3 wt% short-chain PIBM (a copolymer of isobutylene and maleic anhydride) was combined with 0.1 wt% TMAC (tetramethylammonium chloride), solid loading of the slurry was increased to 58 vol% and the slurry can coagulate to a wet green body in a short time. The resultant green body showed a uniform microstructure and was successfully sintered into translucent ceramics.     

Comparison of different methods to prepare MgO whiskers

Magnesia whiskers have been made by hydrolysis of MgCl₂–KCl melt and conversion of magnesium hydroxide whiskers. The morphology of MgO whiskers was examined by SEM and TEM. Composition of products was identified by XRD. Comparisons have been made on the morphology and quality of the products by different methods. Discussions have been made on the efficiency, product quality control and scale-up feasibility of these methods. Whiskers prepared by hydrolysis have uniform diameter but other shapes of magnesia are often found. Conversion of Mg(OH)₂ whiskers to MgO whiskers is simple and easy to scale-up, but the product often has many defects caused by decomposition. Nevertheless, the single crystal structure can be reserved as revealed by electron diffraction. XRD results show that the purity of products by hydrolysis is better than that by conversion. The conversion method is more hopeful for industrial production.     

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.     

Enhanced densification and dielectric properties of CaTiO3-0.3NdAlO3 ceramics fabricated by direct coagulation casting

Severe sintering conditions always cause defects that increase the dielectric loss of microwave ceramics. Direct coagulation casting (DCC) of submicron powder suspension, was applied to accelerate densification and enhance dielectric properties of 0.7CaTiO₃-0.3NdAlO₃ (CTNA) ceramics. The rapid in-situ coagulation of suspension was achieved by tailoring the pH value to the isoelectric point to weaken the repulsion of electric double layer and steric hindrance effect. Compared with the dry-pressed samples, the DCC-fabricated samples are more compacted, which provides a high initial densification value and a shortened diffusion path, and consequently promotes the material transportation and composition homogenization that inhibits the segregation of CaAl₁₂O₁₉. Additionally, the submicron powders accelerate the phase transition and domains growth, leading to a significant reduction of lattice defects in microstructure. Therefore, Q⋅f value of CTNA ceramics reaches more than 40,000 GHz, which is improved by 15 % compared with that of dry-pressed samples sintered under the same conditions.     

Rapid and uniform in-situ solidification of alumina suspension via a non-contamination DCC-HVCI method using MgO sintering additive as coagulating agent

A novel rapid, uniform and non-contamination in-situ solidification method for alumina suspension by DCC-HVCI method using MgO sintering additive as coagulating agent was reported. MgO was used to release Mg²⁺ in suspensions via reaction with acetic acid generated from glycerol diacetate (GDA) at elevated temperature as well as to improve density and suppress grain growth of alumina ceramics during sintering. Influence of adding 0.7 wt% MgO with 2.0 vol% GDA in alumina suspension on coagulation process and properties of green bodies and sintered samples were investigated. It was indicated that the controlled coagulation of the suspension could be achieved after treating at 70 °C for 10 min. Homogeneous composition distribution of Mg element in EDS result indicated the uniform solidification of suspensions. Compressive strength of wet-coagulated bodies is 2.09±0.25 MPa. Dense alumina ceramics with relative density of 99.2% and flexural strength of 354±16 MPa sintered at 1650 °C for 4 h present homogeneous microstructure. The result indicated that the novel DCC-HVCI method via a sintering additive reaction with no contamination, short coagulation time and uniform in-situ solidification is a promising colloidal forming method for preparing high-performance ceramic components with complex shape.     

Freeze casting of aqueous alumina slurries with glycerol for porous ceramics

Experiments have been performed to show that the mechanical properties of alumina porous ceramics may be improved by introducing glycerol into the raw slurries and then casting them under a constant cooling rate. The effects of glycerol on the freeze casting process and thereby on the microstructure and mechanical properties of porous ceramics obtained are investigated. It is shown that the addition of glycerol will increase both the slurry viscosity and sample sintered density. SEM images for microstructure of the final ceramics reveal that a good connection between ceramic lamellae has been promoted. This connection makes as-prepared porous ceramics obtain high mechanical properties. For the 30 vol.% alumina slurry with glycerol, the axial and radial compression strengths reach to, respectively, 255.1 MPa and 105.8 MPa.     

Rapid in-situ solidification of SiO2 suspension by direct coagulation casting via controlled release of high valence counter ions from calcium iodate and pH shift

Rapid in-situ solidification of SiO₂ suspension under the joint action of releasing calcium ions and shifting pH has been proposed. When the suspension was heated up to 60 ℃, decomposition of calcium iodate which released calcium ions, as well as hydrolysis of diacetate (GDA) which shifted the pH toward the isoelectric point, both contributed to the solidification of suspension. The controlled coagulation of SiO₂ suspension could be realized via controlled release of high valence counter ions and pH shift at 60 ℃ within 30 min, which could considerably shorten the coagulation time compared with present reported results (1–3 h). Green body prepared by heating the SiO₂ suspension with 6.5 g L⁻¹ calcium iodate and 2.0 vol% GDA at 60 ℃ for 30 min shows uniform microstructure with compressive strength of close to 0.3 MPa. SiO₂ ceramics sintered at 1275 ℃ for 3 h possess homogeneous microstructure with bulk density of 2.06 g cm⁻³ and flexural strength of 40.3 MPa.     

Electrospinning preparation and characterization of alumina nanofibers with high aspect ratio

Alumina nanofibers were successfully prepared via an electrospinning technique combined with a sol–gel method. The electrospinning solution was prepared by dissolving aluminum isopropoxide (AIP) in distilled water and then mixing with a polyvinyl alcohol (PVA) aqueous solution. The as-spun fibers were calcined at different temperatures and characterized by TG–DTA, XRD, SEM–EDS, TEM–SAED, and BET analysis. Results showed that the average fiber diameter decreases with increasing calcination temperature. The as-spun nanofibers were amorphous. After calcination at 1000 °C, the nanofibers formed were composed of α-Al₂O₃ and γ-Al₂O₃, showing an average diameter of 30–90 nm and an aspect ratio of greater than 1000. The pore size of the obtained fibers was approximately 5 nm, which implies that these fibers are mesoporous materials.     

Direct coagulation casting of yttria-stabilized zirconia using magnesium citrate and glycerol diacetate

Direct coagulation casting via controlled release of high valance counter ions (DCC-HVCI) has been reported in recent years. In this paper, concentrated yttria-stabilized zirconia (YSZ) suspensions were coagulated using DCC-HVCI method with magnesium citrate as coagulating agent assisted by pH shift in the presence of glycerol diacetate. The effect of ammonium polyacrylate (PAA-NH₄) on the dispersibility of YSZ powder was investigated. The influence of concentrations of glycerol diacetate and magnesium citrate on pH and viscosities of YSZ suspensions was studied. The results indicate that concentrated YSZ suspensions can be coagulated by adding 2 vol% glycerol diacetate and magnesium citrate above 0.5 wt% at room temperature for 2–5 h. The compressive strength of coagulated wet samples is above 2.0 MPa. YSZ ceramics sintered at 1450 °C show homogeneous microstructures with relative densities of 98.9–99.2%. Flexural strength of YSZ ceramics is 869±84 MPa.     

Flow properties of PLZTN aqueous suspensions for tape casting

Several aqueous suspensions of lead zirconate titanate powder doped with lanthanum and niobium (PLZTN) were prepared, tape cast and characterized by rheological measurements in steady shear flow. Experimental data were fitted by some viscous models available on literature and the model parameters compared in order to obtain information about the effects of the composition on suspension viscosity. Such approach represents a necessary step in the desirable optimization of tape casting process, which includes also the analysis by flow simulations. Five viscous models have been considered: from the classic plastic models of Bingham and Herschel–Bulkley, to the modified plastic model of Papanastasiou, up to the pseudoplastic models of Cross and Roberts–Barnes–Carew. The parameter-based comparison was carried on with the latter, resulting, with its eight parameters, the best-fit model, while for flow simulation purposes the ones with few parameters, as the Cross and Herschel–Bulkley models, resulted the more appropriate from some simple considerations.     

Tape casting of alumina/zirconia suspensions containing graphene oxide

The introduction of carbon derivatives (nanotubes, graphene, etc.) as a second phase in ceramic matrices has limitations arising from their difficult processing. This paper studies the colloidal stability and the rheological behaviour of concentrated suspensions of alumina with 5 vol.% Y-TZP (AZ) and the effect of the addition of 2 vol.% of graphene oxide (AZGO) on the suspension stability, rheological behaviour and tape casting performance. The colloidal stability was studied using zeta potential measurements in terms of concentration of deflocculants and pH and homogenisation was optimised adjusting the sonication mode and time. The best results were obtained for pulsed mode. The optimum rheological properties were obtained for solid loadings of 53 vol.% and 40 vol.% for AZ and AZGO. Homogeneous, flexible tapes with thickness of ∼120 μm were obtained reaching densities of >60% of theoretical density in which secondary phases are well dispersed.     

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.     

Effect of additives on porosity of alumina materials obtained by freeze casting

The influence of two different additives, glycerol and polyvinyl alcohol (PVA), on the rheological behaviour and freeze casting performance of 35?vol.-% aqueous alumina suspensions is studied. Suspensions with PVA were prepared by either mixing all the components together or adding the PVA in a second step on the dispersed slurry. Although the slight increase in viscosity suggests that competitive adsorption occurs, the microstructure seems not to change depending on the order of addition of additives. Considering its lower molecular weight, glycerol provides lower viscosity, and as a consequence of its cryoprotective character, sintering leads to smaller pore size, being the porosity 35% for an added content of 10?wt-% with respect to solids. In the case of PVA, contents of 2?wt-% are enough to obtain solid firm bodies with a porosity of 48%. The porosity and the size of the pores decrease with increasing concentration of PVA.     

Gel-tape casting as a novel method for the production of flexible fine-grained alumina sheets

The paper reports on the development of a new aqueous gel-tape casting method for processing nanosized powders into thin fine-grained sheets with a high relative density after sintering. The method was based on casting a low-viscosity slurry into a tape, using the doctor-blade method and gelling the slurry after the casting. The gelled tape was removed from the tape carrier without any solvent evaporation and the drying of the gelled tape was performed later in a climate box under controlled temperature and humidity. The epoxy-based ceramic slurry could be prepared with a solid loading of 42 vol.%. Ceramic sheets with flat and smooth surfaces were prepared in a thickness range from 0.08 to 0.4 mm after sintering. The sintered sheets reached a relative density of 99.4%, with an average grain size of about 1.1 μm. The strength and flexibility of alumina sheets were investigated and discussed.     

Effect of plasma treatment of polymeric tape carriers on wetting behaviour of aqueous ceramic tape casting slurry

Due to environmental and health aspects, aqueous ceramic slurries are preferred to traditional organic solvent systems in tape casting. An important obstacle associated with the high surface energy of water is poor wetting of aqueous ceramic slurries on polymeric tape carriers. Therefore, we measured the contact angles of an aqueous epoxy-based ceramic slurry on polyethylene terephtalate (PET), polypropylene (PP), polymethyl methacrylate (PMMA), and aluminium-coated polyethylene terephtalate (PET-Al) films and investigated approaches to improving their wetting. We evaluated the effect of plasma treatment of the tape carrier surface on the wetting behaviour and compared it with the effect of adding non-ionic amphiphilic surfactants to the ceramic slurry. The treatment of the tape carrier surface by low-temperature plasma substantially improved the wetting behaviour of aqueous ceramic slurry. The lowest contact angle of 31° was obtained on the PET film. Although the addition of non-ionic surfactants improved both the wetting behaviour of the slurry and the detachment of the polymeric carrier from the ceramic tape even better than the plasma treatment of the carrier surface did, the plasma-treated carriers still present a useful alternative to the surfactants. In the case of the plasma-treated PET carrier the surfactants could be fully eliminated and potential drawbacks related to the use of surfactants could be prevented.     

Aqueous tape casting of alumina using natural rubber latex binder

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.