Gelation of water-based PZT slurries in the presence of ammonium polyacrylate using agarose

Gel casting is a suitable near net shape-forming technique based on the solidification of aqueous suspensions for the production of green parts with interesting economic and environmental benefits. In the present work, PZT-5 suspensions in aqueous media were successfully prepared using ammonium polyacrylate (PAA-NH₄) as the dispersing agent. The gelation of the 46–48 vol.% PZT suspensions in aqueous media based on agarose was studied by measuring the height of the samples before and after demoulding. The study found that the suitable amount condition for development of the green parts made was achieved at 0.082–0.19 wt.% agarose (based on dry solid). The density of the green body produced at the optimum content of agarose was determined. A very uniform part with green density of 52% and 55% before and after drying, respectively, was developed. Small shrinkage was measured after drying and heat treating at 550 °C. Sintered samples at 1250 and 1285 °C showed a theoretical density of 99% with an even smaller shrinkage of 16.2%. The dielectric constant and longitudinal piezoelectric charge coefficients, d₃₃, of the selected samples were 2550 and 680 pC/N, respectively.     

Preparation and properties of AlN ceramic suspension for non-aqueous gel casting

A non-aqueous gel casting process based on the mixed solvent (ethanol and polyethylene glycol) and low-toxicity N,N-dimethylacrylamide (DMAA) was developed for an aluminum nitride (AlN) ceramic. In the present work, rheological properties of non-aqueous concentrated AlN suspensions were investigated in the presence of mixed solvent, dispersant, milling time, monomer and solid loading, in order to screen for the most suitable experimental conditions to obtain a good rheological behavior for gel casting. The results showed that the 50 vol% slurry with 0.2 wt% dispersant concentration, 2 h milling time, 6 wt% -monomer content, and a solvent ratio of 3:1, can meet the requirements for the casting process of AlN ceramic slurries. After being dried at 100 °C for 1 h, the optimum bulk density and maximum flexural strength of the AlN green bodies were as high as 1.97 g/cm³ and 18.68 MPa, respectively. SEM photographs revealed that the green body had a relative uniform microstructure when the solid loading was 50 vol%. The shrinkage and deformation of shaped sintered bodies prepared through gel casting were small after sintering. The sintering shrinkage, apparent porosity, bulk density and flexural strength were 14.8%, 0.22%, 3.21 g/cm³ and 310 MPa, respectively.     

New laser machining technology of Al2O3 ceramic with complex shape

A new method was developed for the fabrication of complex-shaped Al₂O₃ ceramic parts by combining laser machining and gelcasting technique. The unwanted ceramic powders parts were selectively removed by laser machining specified by a computer program, and the gelcast Al₂O₃ green bodies were machined to a designed shape by a CO₂ laser at a wavelength of 10.6 μm. The influences of solid loading, laser output power, scanning speed and nitrogen purge on the machining of green Al₂O₃ ceramic bodies were studied. The experimental parameters were optimized, the green Al₂O₃ bodies with solid loading of 40 vol% or below were easier to be machined, while the green bodies with solid loading of 45 vol% or above were hard to be further machined due to the surface sintering. Better machining quality and deeper machining depth could be obtained when the laser power is 30 W. The green Al₂O₃ bodies with complex shape were obtained by the laser machining.     

A PCE-based rheology modifier allows machining of solid cast green bodies of alumina

The performance of a poly(carboxylate ether) (PCE)-based superplasticizer to enable the machining of green bodies that are solid cast from suspensions of alumina was investigated. An alumina loading of 35 vol% in the presence of 1.25 wt% superplasticizer was established to be suitable for lathing and removal of significant amount of material through drilling. A reduction of 77% in the diameter of green bodies that corresponds to a 59% reduction in volume was achieved. The lathed green bodies exhibited smooth terraces without visible cracks. All of the green bodies were sintered without a polymer burnout step.     

3D multiscale controlled micropatterning of lead-free piezoelectric electroceramics via Epoxy Gel Casting and lift-off

We report on the fabrication of multiscale three-dimensional (3D) micropatterning of 0.5Ba(Zr_0.2Ti_0.8)O₃ − 0.5(Ba_0.7Ca_0.3)TiO₃ (BZT − BCT) electroceramic material using soft lithographic PDMS moulds. The ceramic microcomponents were successfully consolidated by Epoxy Gel Casting from stable aqueous suspension of BZT-BCT material. A successful aqueous processing of electroceramic material was employed with surface protection of powder particles against hydrolysis as considerably an important parameter to achieve high solid loading suspension with pseudoplastic behaviour suitable for casting in micro scale moulds. The process to obtain micro sized pattern of material was divided in two steps: (i) production of high quality SU-8 master moulds and the respective negative replicas in PDMS (soft mould) by photolithography and soft mould replication, respectively; (ii) casting the soft moulds by stable high solid loading BZT-BCT suspensions in presence of gelation agents followed by de-moulding, drying and sintering. The resulting green micropattern structures show interesting features, including complex structures with a periodic variations through their length and, multiscale array of hexagonal shaped pillars of different aspect ratios (AR) (max AR ≈ 8 with approx ∼320 μm of height and side length ∼40 μm) with smooth side wall features along height. The green pillar arrays were further sintered to show the suitability of aqueous processing of material and proposed casting method.     

Preparation of concentrated barium titanate suspensions incorporating nano-sized powders

In order to reduce agglomeration and overcome the low packing density issues of working with nano-sized powders, a colloidal processing route has been chosen in this study. Commercial BaTiO₃ (BT) powders with a particle size in the range of 50 nm have been dispersed in the aqueous media. Rheological properties have been analyzed on suspensions with different solids loading, dispersant concentration, and pH conditions. Optimum dispersing conditions were obtained for suspensions prepared at basic pH (pH 10) with 0.646 wt% ammonium poly (acrylic acid) (NH₄PAA) as a dispersant. Suspensions have been centrifugally cast to obtain the green body, and the sintering conditions have been investigated by examining the phase evolution, microstructures and electrical properties of the sintered samples through XRD, SEM and dielectric measurements, respectively. The results show that for a 45 vol% suspension sintered at 1325 °C, the density of bulk ceramic can reach 5.85 g/cm³, nearly 97.0% of the theoretical density.     

The influence of solid loading in suspensions of a submicrometric alumina powder on green and sintered pressure filtrated samples

The present work deals with preparation of stable suspensions of a submicrometre alumina powder with different contents of solid for pressure filtration. The optimum dispersant content (2.2 wt.% of Darvan C-N) was determined by sedimentation tests and viscosity measurements. By modification of the solid loading and dispersant content two kinds of aggregation were observed. One type of aggregates is related to the use of excessive solid loading in suspension. In samples prepared from these suspensions only minor effect on sintered microstructure was observed, which increased with increasing volume fraction of hard aggregates. In case of excess dispersant addition weak aggregates formed as the result of depletion flocculation. Weak aggregates had stronger negative effect on green microstructure, with consequent negative impact on sinterability.     

Effects of solid content on the phase assemblages,mechanical and dielectric properties of porous α-SiAlON ceramics fabricated by freeze casting

Porous Y-α-SiAlON ceramics were prepared by freezing camphene-based suspensions at 0 °C and subsequently sintering at 1900 °C for 1 h. The effects of solid loading content in the suspensions on porosities and formation of α-SiAlON as well as mechanical and dielectric properties of the porous ceramics were investigated. An XRD analysis performed on sintered samples indicated that the α-SiAlON did not fully form in the sample with initial solid loading content of 10 vol%, due to the high porosity of 90 vol% and interconnected pore of the green body. With the increase of initial solid loading content from 10 vol% to 30 vol%, the porosity decreased from 62.3% to 23.1% and the average pore size decreased from 19 μm to 8 μm. As a result, the flexural strength increased significantly from 72.4 MPa to 190.2 MPa, fracture toughness increased from 1.20 MPa m^1/2 to 3.48 MPa m^1/2, as well as the dielectric constant increased from 3.3 to 6.3. The dielectric loss (tan δ) of obtained material varied between 1.4×10^?2 and 2.8×10^?2, which did not depend on the porosity of samples.     

Surface modification of α-Al2O3 with dicarboxylic acids for the preparation of UV-curable ceramic suspensions

Stereolithography of UV-curable ceramic suspensions can benefit from the preparation of stable, low viscosity and high solid loading ceramic suspensions without yield stress. Appropriately adding dispersants could optimize the rheological behavior to meet the requirements of stereolithography. In this work, short-chain dicarboxylic acids were utilized to modify the alumina particles and achieve well dispersed ceramic suspensions. The maximum adsorption capacities of dicarboxylic acids were determined by the method of High Performance Liquid Chromatography and the mechanism of surface modification and dispersion was also discussed. Dicarboxylic acids’ influence on the rheology behavior was systematically studied. When doses of dicarboxylic acids reach their maximum adsorption capacities, the alumina suspensions would achieve their lowest viscosities and yield stresses. 45 vol% alumina suspension with a viscosity ˂2 Pa s at shear rate 30 s⁻¹ was successfully formulated. A sintering density of 96.5% can be achieved for the sebacic acid-modified alumina UV-curable suspension.     

A viscoelastic approach from α-Al2O3 suspensions with high solids content

The flow properties of high solids concentration suspensions has been extensively studied and established, however, the rheological behavior for ceramics suspensions with very high solids loading, up to 60 vol.% is poorly known. This is due to both difficulties of preparation and behavior measurement. The theory shows ceramics suspensions with very high solids loading may present viscoelastic properties attributed to interparticle forces active. The oscillatory and creep-recovery rheological measurements can provide this knowledge when discretion used. Thus, rheological behavior of alumina suspensions with high solids loading (>43 vol.%), properly stabilized and the characterization of slip casting shaped samples has been analyzed. High solids loading suspensions, over 43 vol.%, present adequate flow and suspensions with solid content since 60 vol.% present viscoelastic properties attributed to interparticle forces active. The rheological parameters can provide subsidies to obtain after shaping process, such as extrusion, injection and plastic pressing, suitable products for the desired application.     

Direct coagulation casting of silicon nitride suspension via a dispersant reaction method

A direct coagulation casting method for silicon nitride suspension via dispersant reaction was reported. Tetramethylammonium hydroxide (TMAOH) was used as dispersant to prepare silicon nitride suspension with high solid loading and low viscosity. Influences of TMAOH and pH value on the dispersion of silicon nitride powder were investigated. Glycerol diacetate (GDA) was used to coagulate the silicon nitride suspension. Influences of the concentration of glycerol diacetate on the viscosity and pH value of the suspension were investigated. It was indicated that high viscosity sufficient to coagulate the suspension was achieved by adding 1.0–2.0 vol% glycerol diacetate at 40–70 °C. The coagulation mechanism was proposed that the silicon nitride suspension was destabilized by dispersant reacting with acetic acid which was hydrolyzed from glycerol diacetate at elevated temperature. Coagulated samples could be demolded without deformation by treating 50 vol% silicon nitride suspensions with 0.2 wt% tetramethylammonium hydroxide and 1.0–2.0 vol% glycerol diacetate at different temperatures. Dense silicon nitride ceramics with relative density above 98.8% had been prepared by this method using glycerol diacetate as coagulating agent sintered by different methods.     

Reliable high strength alumina fabricated by DCC-HVCI using submicron calcium citrate complex

Ceramics with high strength and reliability are highly demanded in engineering applications. In this paper, a modified direct coagulation casting via high valence counter ions (DCC-HVCI) method for alumina using calcium citrate complex assisted by glycerol diacetate was investigated. Calcium citrate complex suspensions were prepared by mixing tri-ammonium citrate and calcium chloride in water. Effect of reaction time on the chelating properties of the prepared suspensions was investigated. Concentrated alumina suspensions with a solid loading of 50 vol% were prepared by mixing the calcium citrate complex suspensions and alumina powder at pH of 10.5. Then the suspensions were coagulated by adding 3–6 vol% glycerol diacetate at temperatures of 40–70 °C for 2–6 h. The compressive strength of the coagulated wet samples is in the range 1.1–2.4 MPa. Alumina ceramics sintered at 1550 °C shows homogeneous microstructures with flexural strength and Weibull modulus of 455±17 MPa and 30, respectively.     

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.     

Aqueous gelcasting of ZrB2–SiC ultra high temperature ceramics

ZrB₂–SiC ultra high temperature ceramics containing B₄C and C as sintering additives were successfully prepared by aqueous gelcasting and pressureless sintering. Polyacrylic acid (PAA) was used as the dispersant throughout this research. The various effects of zeta potential, pH value, dispersant concentration, solid loading and ball-milling time on the rheology and fluidity behavior of ZrB₂–SiC suspension were investigated in detail. A well-dispersed suspension with 50 vol.% solid loading was prepared at pH 10 with 0.4 wt.% PAA after ball milling at 240 rpm for 24 h. Then crack-free green ZrB₂–SiC ceramics were obtained by gelcasting process and then pressureless sintered at 2100 °C to about 98% relative density. The microstructure and mechanical properties were examined, and the flexural strength and fracture toughness were 405 ± 27 MPa and 4.3 ± 0.3 MPa m^1/2, respectively.     

The effect of the surfactants on the formulation of UV-curable SLA alumina suspension

Stereolithography (SLA) has been regarded as the most promising rapid prototype (RP) production method for ceramic parts recently because of its better precision in size and site control. The rheological behavior and curing behavior of suspensions are controlling factors for ceramic stereolithography. In this work, oleic acid (OA), stearic acid (SA) and Poly(Acrylate Ammonium) (PAA-NH₄) were studied as the surfactant for HDDA-based suspension formulation. Modified alumina particles exhibit different wettability with HDDA monomer and thus the different rheology behavior and stability between the formulated suspensions. OA shows the best performance among all three dispersants. 40 vol% alumina suspensions with a viscosity ˂3 Pa s at 30 s⁻¹ shear rate were successfully formulated with OA and SA. A sintering density of 95% can be reached for the OA- and SA-modified alumina UV-curable resins.     

Selected sugar acids as highly effective deflocculants for concentrated nanoalumina suspensions

The possibility to fabricate nanoceramic materials by techniques based on colloidal processing requires the use of effectively working deflocculants (dispersing agents) which are able to stabilize and decrease the viscosity of aqueous nanopowders suspensions. Galacturonic and lactobionic acids have been used in the research as deffloculants for nanoalumina; they allowed to obtain ceramic suspensions of high solid loading (50 vol%) and relatively low viscosity (ca. 4 Pa s at a shear rate 10 s⁻¹). High solid loading could be achieved through combining three factors: the application of sugar acids as deflocculants, water leaching of the nanopowders from the surface impurities and precise defoaming of the slurries. Density of sintered bodies equalled 98% TD.     

Two alternative routes for starch consolidation of mullite green bodies

The starch consolidation forming method can be used in the manufacture of porous ceramics. In this method, based on swelling and gelatinization properties of starch in aqueous suspension at temperature (55–80 °C), the starch granules perform as both consolidator/binder of the green body and pore former at high-temperature.Commercially available powders of mullite and cassava starch were employed as raw materials. Mullite/starch aqueous suspensions (0.25 starch volume fraction of 40 vol.% total solid loading) were prepared by intensive mechanical mixing and homogenization in a ball mill.Two alternative forming routes of thermogelling mullite/starch aqueous suspensions—the Conventional Route (CR) and the Pre-Gelling Route (PGR)—were studied. With the CR, disks were formed by pouring the mullite/starch suspension at room temperature directly into metallic molds and heating at different temperatures (70 and 80 °C) and times (1 and 2 h). With the PGR, disks were shaped by pouring pre-gelled mullite/starch suspensions at 59 °C into heated molds and heating at the same experimental conditions. Once the consolidation process was finished, samples were removed of the mold and dried. Green bodies shaped by the two processing routes and obtained before (CR_bb and PGR_bb) and after (CR_ab and PGR_ab) burning out the starch, were characterized by bulk density and apparent porosity measurements and microstructural analysis by SEM/EDAX on the external and fracture surfaces. The homogeneity of the distribution of raw materials and pores, and the volume porosity were taken into account to establish the optimum consolidation conditions to be used in the preparation of mullite porous materials with homogeneous microstructures.     

Room-temperature injection molding of aqueous alumina-polyvinylpyrrolidone suspensions

Room-temperature injection molding, a novel, environmentally benign ceramic processing method, produced dense, near-net shape alumina rings by utilizing unique flow properties of aqueous, highly loaded (>50 vol.%) ceramic suspensions with ≤5 vol.% polyvinylpyrrolidone (PVP) dispersed using Darvan 821A. The rheological behavior of suspensions along with microstructural and mechanical properties of resulting specimens were evaluated by varying PVP content to determine the optimal composition for forming. Parallel-plate rheometry revealed that suspensions containing ≤5 vol.% PVP were yield pseudoplastic at room temperature, which facilitated processing without heating or complex chemical reactions. Alumina rings with high green densities (>60% true density (TD)) were machined before binder removal, and increasing PVP content was observed to enhance green machinability. After binder burnout and sintering, bulk densities were ∼98%TD with <16% linear shrinkage. Scanning electron microscopy revealed minimal pore formation within specimens. Ultimate strength of samples was determined using ASTM C1323-10, and a maximum C-strength of 261 ± 57.6 MPa was obtained.     

Additive manufacturing of zirconia parts by indirect selective laser sintering

Thermally induced phase separation (TIPS) was used to produce spherical polypropylene–zirconia composite powder for selective laser sintering (SLS). The influence of the composition of the composite starting powder and the SLS parameters on the density and strength of the composite SLS parts was investigated, allowing realizing SLS parts with a relative density of 36%. Pressure infiltration (PI) and warm isostatic pressing (WIPing) were applied to increase the green density of the ZrO₂–PP SLSed parts. Infiltrating the SLS parts with an aqueous 30 vol.% ZrO₂ suspension allowed to increase the sintered density from 32 to 54%. WIPing (135 °C and 64 MPa) of the SLS and SLS/infiltrated complex shape green polymer–ceramic composite parts prior to debinding and sintering allowed raising the sintered density of the 3 mol Y₂O₃ stabilized ZrO₂ parts to 92 and 85%, respectively.     

Effect of powder dispersion on sintering behavior and mechanical properties of nanostructured 3YSZ ceramics

The aim of this work was to investigate the feasibility of producing dense nanostructured ceramics from commercial zirconia nanopowder by colloidal processing. Our study has demonstrated the adverse effect of the presence of aggregates in the production of nanostructured components. Two techniques were employed to remove aggregates – centrifugation and high energy milling. The finer powder thus produced by centrifugation and/or high energy milling, with an average size of particles around 60–70 nm was consolidated into a green body by a relatively simple low pressure filtration technique (300 kPa). It was possible to prepare green bodies of 47% density with relatively fine and uniform size pores from suspensions containing these powders. The green compacts obtained from finer powder could be sintered to fully dense nanostructured products at a significantly lower temperature (1200 °C) with a soaking period of 0.5 h only with an average grain size of 90 nm only in the sintered compact. In contrast the as received aggregated powder could not be sintered at all to full density even at as high temperature as 1600 °C. The bulk nanostructured 3 mol% yttria stabilized zirconia (3YSZ) compact with an average grain size of 110 nm exhibited Vickers hardness of 12.8 GPa, which is comparable to the available results in the literature.