Rheology and pressure filtration of aqueous SiC suspensions of nanometer-sized bimodal particles

The consolidation behavior of mixed SiC particles (30 and 800 nm diameters) at pH 7 was examined using a developed pressure filtration apparatus at a constant crosshead speed of a piston. The addition of 30 nm SiC to 800 nm SiC changed the rheological properties of colloidal suspension from Newtonian to pseudoplastic flow through Bingham flow with increasing amount of 30 nm SiC. A phase transition from a well dispersed suspension to a flocculated suspension occurred when the applied pressure exceeded a critical pressure (ΔP_tc). The ΔP_tc value decreased with increasing amount of 30 nm SiC. The packing density of bimodal SiC particles was analyzed based on the partial molar volume of each powder. The larger particles were more densely packed than the smaller particles in the mixed powder compact.     

Rheology of organics-free aqueous ceramic suspensions for additive manufacturing of dense silicon nitride ceramics

A dense structure of silicon nitride ceramic was fabricated by direct ink writing using aqueous suspensions. The rheology of the suspensions was carefully tailored by the particle chemical state and the ion concentrations, without the use of any organics. The surface chemical states of the Si₃N₄ powder were modified by calcination at 600 °C at various times. The minimum absolute value of zeta potential, calculated by the DLVO theory, was 19 mV at which the interaction was attractive interaction. Suspensions with solid volume fraction of 25–40 vol% exhibited pseudoplastic behavior with yield stresses ≥55 Pa and equilibrium storage moduli ≥ 10⁴ Pa. These enabled suspensions to flow through the nozzle and retain the shape of the printed parts. The flexural strength of Si₃N₄ ceramics produced using a 40 vol% suspension was 348 MPa. This strategy provides a simple process for fabricating high-performance ceramics based on the DIW.     

Gel casting of aqueous suspensions of BaTiO3 nanopowders

Commercial nano-BaTiO₃ powders have been formed into green bodies using colloidal forming routes. A study of the rheological behaviour of the suspensions as a function of dispersant concentration and homogenisation time was made in order to prepare stable concentrated suspensions of the nanopowders. Bulk components were then manufactured using aqueous slip and gel casting involving polysaccharides that gel on cooling, i.e. agar. The performance of theses consolidation techniques for obtaining dense green bodies from the BaTiO₃ nanopowders was studied. It was possible to prepare relatively big gel cast samples with a similar density and microstructure and in a shorter time compared to those obtained by slip casting.     

Stabilization of highly-loaded boron carbide aqueous suspensions

Injection molding of boron carbide (B₄C) slurries affords the production of complex-shaped personal armor. To injection mold, however, requires preparation of a well dispersed, flowable suspension with >45 vol% B₄C loadings to reduce porosity that must be removed during sintering. In the present study, the preparation of highly-loaded B₄C suspensions is investigated using zeta potential and rheological measurements, varying dispersant type, molecular weight, and amount. Of those dispersants investigated, polyethylenimine (PEI) with a molecular weight of 25,000 g/mol was found to produce suspensions with up to 56 vol% B₄C and the requisite rheological properties suitable for injection molding. A PEI concentration of 1.83 mg/m² was established as the appropriate to produce highly-loaded B₄C suspensions. The effect of a prior B₄C powder treatment (ethanol washed or attrition milled) on rheological properties of the suspensions was also investigated. The PEI was completely burned out in argon, nitrogen, and air at 450 °C.     

Preparation of high solids content nano-titania suspensions to obtain spray-dried nanostructured powders for atmospheric plasma spraying

Nanosized TiO₂ powder with an average primary size of ∼20 nm and surface area of ∼50 m²/g (Aeroxide^® P25, Degussa-Evonik, Germany) was used as starting material. A colloidal titania suspension from the same supplier was also used (W740X). The dispersing conditions were studied as a function of pH, dispersant content, and solids loading. Well-dispersed TiO₂ nanosuspensions with solids contents up to 30 vol.% (62 wt%) were obtained by dispersing the powder with 4 wt% PAA. Suspensions with solids contents as high as 35 vol.% were prepared by adding the TiO₂ nanoparticles to the TiO₂ colloidal suspension under optimised dispersing conditions.TiO₂ powder reconstitution was performed by spray drying both types of nanosuspensions to obtain free-flowing micrometre-sized nanostructured granules. The spray-dried nanostructured TiO₂ granules were deposited on austenitic stainless steel coupons using atmospheric plasma spraying. Coating microstructure and phase composition were characterised using scanning electron microscopy and X-ray diffraction techniques.     

Flow behavior of polymerizable ceramic suspensions as function of ceramic volume fraction and temperature

The effect of volume fraction and temperature on flow behavior is reported for suspensions of coarse silica powders in two non-aqueous polymerizable solutions. The concentration dependence of the viscosity at temperatures 25-75 °C can be reduced to a single Krieger-Dougherty curve for all suspensions. The temperature dependence of viscosity for suspensions with 60 vol% silica could be fit to an Arrhenius equation. The suspensions had a larger apparent activation energy than the suspension medium. This could be explained in terms of thermal dilution, where the higher thermal expansion of the liquid reduces the solids loading for very concentrated suspensions.     

Rheology,Sedimentation, and Filtration of TiO2 Suspensions

It is shown that with aqueous suspensions of colloidal oxide and hydroxide particles simple rheometric and sedimentation experiments allow the determination of the optimum pH for filtration and washing. This approach is more practical than zeta potential measurements, as explained by a theoretical review and illustrated by experiments with industrial TiO₂ white pigments of varying surface chemistry. Besides, the solids content of the filter cake and its dependence on pH is correctly predicted from the rheology of the suspension, thus giving an idea of the mechanism of filter cake formation. This follows from a comparison to the results of filtration experiments. There, the influences of filter vacuum as well as of pH and solids content of the pigment feed suspension on filtration capacity and washing efficiency have been studied. The experiments also illustrate an inexpensive method of how to find the optimum set of parameters for an economical drum filter operation in the plant.     

Fabrication of alumina parts by electrophoretic deposition from ethanol and aqueous suspensions

This paper discusses the effect of the properties of alumina suspensions in ethanol and in water, on green and sintered ceramic parts formed by electrophoretic deposition. The results of the study demonstrate that a small amount of water present in ethanol suspensions as a hidden additive due to the hygroscopicity of alumina powder and ethanol can detrimentally affect the behaviour of the suspension, thus lowering the reliability of the process. Electrophoretic deposition from aqueous suspensions appears to be advantageous over ethanol, from a reliability standpoint, and due to higher achievable green and sintered densities of the deposits and higher deposition rates. Dolapix CE64 appears to be superior surfactant in water as it results in deposits with the lowest green and sintered porosities.     

Effect of dispersants on the rheology of aqueous silicon carbide suspensions

The effect of cationic and anionic dispersants on aqueous suspensions of as-received and surface-modified silicon carbide particles has been studied via observation of the rheological behaviour. Only the cationic dispersants were effective for the as-received SiC, with polyethyleneimine being superior to Hyamine 2389 probably as a result of a greater electrosteric interaction. SiC particles modified using Al(NO₃)₃ behaved like alumina and so could be dispersed using the anionic dispersants ammonium polyacrylate and polymethacrylate. Such dispersions displayed no heteroaggregation when alumina was added, although the order of mixing could significantly affect the rheological behaviour of the suspension. Nevertheless, the suspensions appeared robust to slight fluctuations in pH.     

Colloidal processing of low-concentrated zirconia nanosuspension using osmotic consolidation

Colloidal processing was applied to a water-based suspension with 5 vol% zirconia nanoparticles of 10–15 nm in size. The nanosuspension was concentrated by evaporation or by a newly developed method of osmotic dehydration. The viscosity and stability of concentrated nanosuspensions were investigated. Osmotic consolidation of both nanosuspensions, concentrated by evaporation and osmotically dehydrated, was performed in a solution of polyethylene oxide separated from the nanosuspensions using a permeable membrane. Osmotic pressure generated a water flow from the nanosuspension to the polymer solution, concentrating the nanosuspension and eventually consolidating the nanoparticle network. The pore size distribution in dried nanoparticle compacts and pore size evolution during sintering were evaluated and discussed. The nanozirconia compacts were densified by pressure-less sintering to a relative density of up to 99.7% while maintaining the nanocrystalline structure.     

Rheological behaviour of submicron mullite–carbon nanofiber suspensions for Atmospheric Plasma Spraying coatings

Mullite is widely used as a structural material for applications like thermal and environmental barriers coatings. For some of these applications, thermal spray is a suitable technique due to its fast production time and versatility. This makes mullite a very interesting coating material for thermal spray industry. In the present work, the viability to produce coatings by thermal spray using mullite–CNFs agglomerated powders is analyzed. The stability of aqueous mullite and mullite–CNFs suspensions was studied in terms of zeta potential and rheological behaviour of concentrated slurries. Slurries were optimized in terms of dispersant concentration and solid content. The optimized suspensions were used for the granules preparation by spray drying technology. The obtained granules were characterized through the determination of particle size distribution and shape factor by field emission scanning electron microscopy and laser scattering. These granules were used to form the coatings by Atmospheric Plasma Spraying which were characterized by evaluating the composition, structure, shape, and thickness.     

Effects of dispersant concentration and pH on properties of lead zirconate titanate aqueous suspension

Lead zirconate titanate (PZT) aqueous suspensions were prepared at 60 wt.% solids loading using a commercial ammonium polyacrylate (APA) dispersant. Effects of the dispersant concentration on rheological behavior, dispersion and stability of PZT aqueous suspensions were investigated by means of zeta potential, viscosity and sedimentation height measurements. The results showed that, under suitable conditions, APA dispersant promoted particle dispersion and stabilization in PZT aqueous suspensions. For 60 wt.% solids loading suspensions, the dispersant concentration yielding the lowest viscosity was 0.5 wt.% based on PZT powder dried weight basis. Effects of pH on particle dispersion in the suspensions prepared with APA were studied by laser light scattering technique and scanning electron microscopy. The results showed an improvement in particle dispersion for the alkaline condition, which led to relatively low viscosity and highly stable suspension. Possible particle stabilization mechanisms at various pHs were discussed based on dissociation of the dispersant in water, polymer conformation and adsorption behavior of the dispersant on the particle surface.     

Rheology of UV curable colloidal silica dispersions for rapid prototyping applications

Silica is a very promising material for micro-devices produced by rapid prototyping techniques due to its high transparency, thermal and chemical resistance. UV curable dispersions for rapid prototyping methods such as stereolithography should posses specific rheological properties which are crucial for such applications. We developed highly filled, low viscosity silica dispersions (up to 60 vol%) in UV curable, acrylate based systems. The influence of silica particle size, solid loading, temperature and shear rate on the viscosity of dispersions was investigated. The dispersions exhibited different types of shear thickening depending mainly on the size of particles. The critical shear rate defined as the onset of shear thickening, was found to be dependent on temperature, particle size, solid loading of the dispersions. The understanding of these rheological properties enables the design of new dispersions that meet rapid prototyping process requirements.     

Rheology of aqueous silicon nitride suspensions

Highly concentrated silicon nitride slips with solid-volume concentrations φ of up to 0·475 were prepared from preoxidized, submicron powders in aqueous solutions at pH>8. Viscosity measurements reveal a pseudo-plastic flow behaviour at φ>0·35, which could not be described by existing flow equations. On the basis of the elastic-floc model for highly concentrated suspensions, a modified flow equation was derived, which is only determined by the initial floc-volume ratio and the particle-packing density. The calculated viscosity fits very well with the experimental values from low to high particle-volume concentrations φ from 0·2 to 0·475.     

Gelcasting of aqueous mesocarbon microbead suspension

Gelcasting, an in situ consolidation colloid forming process, is promising for the manufacture of structural ceramics with various shapes. In this case, the gelcasting process was adopted to obtain carbon artifacts with complex shapes. The mesocarbon microbead (MCMB) that has self-sintering property was chosen as the starting material. In order to optimize the MCMB suspensions for gelcasting, the zeta potentials of MCMB particles dispersing in water and the rheological behaviors of MCMB suspensions were investigated. A stable MCMB suspension with solid loading up to 62.5 wt.% was prepared, from which uniform green body with complex shape was successfully gelcast. The performance of green bodies and sintered samples were measured and their microstructures were observed.     

Dispersion of mixtures of submicrometer and nanometre sized titanias to obtain porous bodies

The stability and rheological behaviour of bimodal titania suspensions was studied. Bimodal mixtures were prepared by mixing nanosized TiO₂ powders with an average primary size of ～20–40 nm and surface area of ～50 m² g^?1 and/or a colloidal titania suspension of the same nanopowders dispersed in water with a submicrometer sized titania. The dispersing conditions were studied as a function of pH, type and content of dispersant, and sonication time for a constant solids content of 30 vol% (62 wt%). The mixtures were slip cast and presintered at low temperatures (800–1000 °C) in order to obtain porous materials with anatase as the major phase. The pore size distribution, microstructure and phase composition were characterised using MIP, SEM and XRD techniques, respectively.     

Relating the molecular structure of comb-type superplasticizers to the compression rheology of MgO suspensions

We have investigated the effect of superplasticizers on the rheological properties of concentrated MgO suspensions. The comb-type anionic polymers with grafted polyethylene oxide chains adsorb onto the MgO surface and infer a steric repulsion where the range scales with the length of the PEO side chains. Consolidation experiments, where the volume fraction gradient of particle networks has been determined in response to a centrifugal force field, offer a simple, yet accurate, way of investigating flocculated, partly stabilized and stable suspensions under compression. The compression rheology behaviour could be related to the estimated thickness of the adsorbed superplasticizers and a scaling analysis was used to quantitatively assess the importance of the length of the grafted PEO-chains on the magnitude of the inter-particle bond strength.     

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.     

Low-temperature reaction-sintering of mullite ceramics with an Y2O3 addition

Mullite ceramics were fabricated at relatively low temperatures from powder mixtures of -Al₂O₃ and quartz, with an Y₂O₃ addition. The mullitization process was analyzed by X-ray diffraction. The densification behavior was investigated as a function of the Y₂O₃ content, sintering temperature and holding time as well as mullite seeds. It has been shown that mullitization occurs via a nucleation and growth mechanism within an yttrious aluminosilicate glass, but lattice and grain-boundary diffusion becomes important during the densification process. Moreover, the incorporation of mullite seeds was observed to enhance both mullitization and densification. At 1400°C for 5 h or 1450°C for 2 h, 15 mol% Y₂O₃-doped and 5 mol% mullite-seeded specimens can be sintered to almost full density.