Electrokinetic Properties of Colloidal Zirconia Powders in Aqueous Suspension

Pure zirconia, yttria, and three yttria-doped zirconia powders of submicrometer size have been dispersed in various aqueous solutions. The zeta potential (zeta) of the zirconia powders is determined primarily via streaming-current (SC) detection and is confirmed using electrophoretic spectroscopy techniques. The results reveal that the isoelectric point (IEP) of these zirconia powders is in the pH range of 5.6-7.2 and zeta is controlled primarily by the yttrium content of the zirconia powders and the type of electrolyte. In addition, the yttria content strongly affects the potential and SC in zirconia suspensions only at high solids contents (>1 vol%). The electrokinetic data reveal that the surface of the yttria-doped tetragonal zirconia powder (TZP) can be modified via the adsorption of ionic molecules or polymeric species in the suspension. The adsorption of an anionic polymer can stabilize zirconia particles in a solution that is almost neutral or weakly basic (in the IEP range of pure ZrO₂). The interaction of the zirconia and yttria particles with the electrolytes in an aqueous suspension will be discussed to reveal the roles of hydrated oxide formation and zirconia surface interaction with polymeric dispersants.     

Selection of Suitable Dispersants for Aqueous Suspensions of Zirconia and Titania Powders using Acoustophoresis

Acoustophoresis was used to study the effect of adding various commercially available dispersants onto aqueous suspensions of two zirconia and two titania powders. These powders were characterised for elemental composition by X-ray fluorescence (XRF) spectroscopy and for surface area by BET single point nitrogen adsorption. From the maximum value of the zeta potential, it was possible to select the most promising dispersants. From the shape of the curve the minimum amount of dispersant required to stabilise the powder particles was noted. The iso electric point (i.e.p) of the powders was also identified. Several dispersants can be recommended for the first titania powder, whilst none can be recommended for the second titania as the final zeta potentials on addition of the dispersants were low. The two powders had different surface chemistries which was reflected in a large difference in their i.e.p; the first at pH 7·5 and the second at pH 6·1. This was due to different coatings on the powder surfaces; alumina and an organic material respectively. Removal of this organic coating by calcinatian then enabled the dispersants to fully adsorb. Similarly dispersants for the first zirconia powder could be identified and the i.e.p identified at pH 5·4. However, no dispersants can be recommended for the second zirconia powder as yttria dissolves out of the powder under the naturally occurring weakly acidic conditions. The i.e.p was estimated to be pH _i.e.p 7–7·5.     

Investigation on rheological behavior of 8 mol% yttria stabilized zirconia (8YSZ) powder using Tiron

In this research, 8 mol% yttria stabilized zirconia (8YSZ) powder were dispersed in de-ionized water by the use of different amounts of Tiron as dispersant. The results of rheological and sedimentation measurements of each suspension were evaluated and the optimum amount of Tiron was selected (0.8% Tiron). Also, various pH were investigated and the best stabilized suspension is achieved at pH 10. Furthermore, the zeta potential of suspension with and without adding dispersant was obtained. The isoelectric point (IEP) of as-received 8YSZ powder was about 8.5 and shifted obviously to acidic region after adding dispersant to the suspension.     

Electrostatic and electrosteric stabilization of aqueous slips of 3Y–ZrO2 powder

Yttria-doped zirconia powder (3Y-ZrO₂) was characterized and dispersed in distilled water. The state of dispersion was evaluated in terms of zeta potential, apparent viscosity and the mean particle size of solid phase in the slip. Zeta potential, apparent viscosity and the mean particle size as a function of pH indicated the pH range of electrostatic stabilization. These results showed that electrostatic stabilization of the slip can be accomplished in low acidic and high basic range of pH. Dissolution of yttria from the powder surface in acidic pH was found to be high and fast, risky to the stability of the slip as well as the microstructure of the sintered body. Electrosteric stabilisation by addition of an anionic polyelectrolyte (PMAA-NH₃) shifted the isoelectric point (IEP) to lower pH. The state of dispersion was further investigated by particle size measurements of the solid phase in the slip. The optimum amount of dispersant is discussed in terms of zeta potential and viscosity. The adsorption of polyelectrolyte is considered at pH 4, native pH of suspension and at pH 10, below and above the IEP of powder and correlated with the dissociation rate of polymer and the net surface charge of particles at given pH. Higher adsorption of dissociated polymer to the positively charged surfaces justifies the higher optimum amount of polyelectrolyte at pH 4.     

Yield Stress of Alumina-Zirconia Suspensions

The yield stress of concentrated suspensions of alumina, zirconia, and mixed alumina-zirconia powders was measured by the vane technique as a function of solids loading, relative amounts of alumina and zirconia, and pH. At the isoelectric point (IEP), the yield stress varied as the fourth power of the solids loading. The relative ratio of alumina and zirconia particles was important in determining the yield stress of the suspension at the IEP. The yield stress of single and mixed suspensions showed a marked variation with pH. The maximum value occurred at or near the IEP of the suspension. The effect of electrical double-layer forces on the yield stress can be described on the basis of the Derjaguin-Landau-Verwey-Overbeek (DLVO) theory. A normalized yield stress—that is, the ratio of the yield stress at a given pH to the yield stress at the IEP predicted by this model—showed good correlation with experimental data.     

Effects of Solids Loading and Dispersion Schedule on the State of Aqueous Alumina/Zirconia Dispersions

The effects of solids loading and dispersion order on the state of aqueous slips of either zirconia or alumina, and binary slips of these two powders, have been examined. Since these powders can acquire surface charge when they are dispersed in aqueous media, changes in the ionic composition of the slip can occur as a result of the addition of solids. At higher solids loading, a substantial fraction of the ionic activity within the slip may be attributed to the solid itself. As the solids loading within unary powder suspensions is increased, it is possible to drive the suspension pH toward the point of zero-charge for the powder, resulting in diminished electrostatic stabilization and possible powder agglomeration. An analytical model was constructed to account for the observed results. This embodied a simple surface site complexation model which was augmented to include material and charge balance constraints on ionized surface sites as well as solvated ions. For binary powder dispersions of alumina and zirconia, it was found that the order of dispersing the two components could have a pronounced effect on the overall suspension behavior. The variability in results is attributed to the solubility of the alumina in the basic processing media.     

Electrophoretic mobility of alumina,titania and their mixtures in aqueous dispersions

The electrophoretic mobility of commercial alumina and titania powders and their mixtures (ratio of alumina:titania mixtures (by weight) 1:2 and 4:1) dispersed in 10⁻³ mol dm⁻³ KNO₃ was determined as a function of pH in the range pH 3–11, using both microelectrophoresis and mass transport methods. The results are in good agreement with the previous experimental results. The value of the isoelectric point (IEP) for alumina and titania was found to be at pH9 and 5.5, respectively. It appears that the dispersion concentration has no significant effect on the electrophoretic mobility versus pH. The results also suggest that the addition of titania to alumina has lowered the IEP value from pH ∼9 to ∼8.4 and the addition of alumina to titania has increased the IEP value from pH ∼5.5 to 6. The electrophoretic mobility versus pH profiles for the mixtures of alumina and titania powders appear to represent the electrokinetic behavior of both alumina and titania powders; however, the shape of the electrophoretic mobility versus pH profiles and the position of the isoelectric point appear to be governed by the powder with a large total surface area in the mixture, but not necessarily the weight.     

Effects of washing and calcination–milling on ionic release and surface properties of yttria stabilized zirconia

Crystallographic features, physical properties and ionic release from yttria stabilized zirconia (YSZ) in suspension were studied by means of XRD, TEM, light-scattering particle size, BET, ICP and zeta potential analysis. It was found that Zr, Y, Na, and to a lesser extent Ca, Hf and Pd leach from 8 mol% YSZ powder. The impurities present increase the zeta potential of suspensions made from as-received YSZ. A trace amount of tetragonal phase observed in 8 mol% YSZ persists following washing and calcination–milling. Dislocations and crystallographic defects together with fractured crystals which form during milling of the calcined powder should lead to the formation of more broken bonds; as a result the surface of the particles can support higher surface charge density. Washing and calcination–milling lead to a shift of the isoelectric point of 8 mol% YSZ from pH 8.4 to pH 6.3 and 6.8, respectively. Due to higher chemical stability and previously shown positive impacts on microstructure and performance of fuel cells, use of calcined YSZ can be more advantageous than as received powder.     

Yield stress of oxide dispersions—intermolecular forces of adsorbed small ionic additives and particle surface roughness

The yield stress‐pH and zeta potential‐pH behaviour of α‐alumina and zirconia dispersions with adsorbed small ionic molecular additives such as phosphate and pyrophosphate were determined. The result for adsorbed citrate was included for comparison. Adsorbed phosphate at high surface coverage increased the maximum yield stress of low surface area α‐Al₂O₃ (AKP30 and AA07) dispersions slightly. This increase is attributed to the intermolecular hydrogen bonding between phosphates adsorbed on interacting particles. With high surface area ZrO₂ (Tosoh) dispersions, however, the adsorbed phosphate decreased the maximum yield stress. This is due to its very rough surface morphology limiting the extent of intermolecular hydrogen bonding between adsorbed phosphate layers. Unlike phosphate, pyrophosphate reduces the maximum yield stress of AKP30 α‐Al₂O₃. This is due to the presence of intramolecular hydrogen bonding, thereby impeding effective bridging. A similar result is observed with citrate. The adsorbed pyrophosphate acts as an effective steric barrier keeping interacting particles further apart, thereby weakening the van de Waals attraction. These dispersions with the presence of non‐DLVO forces, that is bridging and steric, did not affect the linear relationship between yield stress and the square of the zeta potential as predicted by the yield stress–DLVO force model. However the relative importance of these non‐DLVO forces affect the value of the critical zeta potential at the point of transition from flocculated to dispersed state. © 2011 Canadian Society for Chemical Engineering     

Effect of varied powder processing routes on the stabilizing performance and coordination type of polyacrylate in alumina suspensions

The influence of initial pH and energy input during suspension homogenization on the stabilizing performance and coordination type of commercial available polyacrylate dispersant were studied. Additionally to widely used rheology and electroacoustic measurement techniques the alumina suspensions were analysed with centrifugal separation and in situ ATR-FTIR to study the impact of varied powder processing in detail. In contrast to zeta potential analysis and viscosity measurements only the determination of sedimentation properties by centrifugal separation shows the effect of macroscopic changes in powder processing. A combination of positively charged alumina surface and a high shear homogenization leads to the most stable suspension. Accordingly ATR-FTIR results show a correlation between improved suspension stability and inner-sphere coordination of polyacrylate. Moreover it was possible to determine an optimal pH range for inner-sphere adsorption. It can be shown that macroscopic changes in powder processing influence the coordination of dispersant and thus the suspension stability.     

pH值对包覆改性SiC料浆分散特性和流变性的影响

通过zeta电位、沉降实验、流变特性、粘度等测试表征pH值对包覆改性SiC料浆分散特性和流变性的影响。研究表明:以偶联剂作为基础层,有机聚电解质作为分散功能层的有机包覆改性SiC粉体主要通过静电空间位阻效应实现稳定分散,调节pH值可以控制接枝聚合物水解产物的解离方式,从而改变了颗粒表面的电荷种类和电荷密度,包覆改性粉体的流动特性也发生变化。调整料浆pH值约11可制备出固相体积分数达56％、表观粘度为568 MPa·s的稳定料浆。     

Effect of Polyvinylpyrrolidone Additions on the Rheology of Aqueous,Highly Loaded Alumina Suspensions

The control of the rheological behavior of highly loaded ceramic/polymer suspensions affords the development of near‐net shape forming techniques. In this study, suspensions containing sub‐micrometer diameter alumina (up to 56 vol%) were fabricated using an anionic dispersant (≈4 vol%) and water‐soluble polyvinylpyrrolidone (PVP). The amount and ratio of molecular weights of PVP in the suspension were varied to influence flow behavior. The final pH of the system, ≈9.5, was higher than the isoelectric point (IEP) of alumina implying that the alumina powder possesses a negative surface charge. In the case of alumina at this pH, PVP does not adsorb onto the surface of the powder. The flow behavior of the PVP‐containing suspensions displayed yield‐pseudoplastic characteristics that closely agreed with the Herschel–Bulkley fluid model. The addition of PVP significantly changed the rheology of the system, increasing the shear yield stress and altering flow behavior. Interparticle interaction approximations of the suspensions were modeled to correlate with experimental observations.     

Fabrication of transparent neodymium-doped yttrium aluminum garnet ceramics by high solid loading suspensions

Dense neodymium-doped yttrium aluminum garnet (Nd:YAG) transparent ceramic was obtained by slip casting and solid-state reaction. The colloidal behavior of the aqueous suspensions of neodymia, yttria, and alumina mixed powders using Dispex A as dispersant was investigated. The variation in zeta potential due to pH alteration was studied. The isoelectric point (IEP) was at pH 4.5 and 4 for the specimens with and without Dispex A, respectively. The optimal dispersion conditions were achieved for the suspensions at pH 9.6 with 0.4 wt% Dispex A. The green body prepared by slip casting was vacuum sintered from 1200 °C to 1750 °C. The grain size of the sintered body increased, and the pore size decreased with increasing sintering temperature. Pore-free Nd:YAG transparent ceramic with a grain size of 5–10 μm was obtained by sintering at 1750 °C for 10 h. The in-line transmittance of the annealed specimen reached 80.8% at 1064 nm.     

Aqueous Degradation and Chemical Passivation of Yttria-Tetragonally-Stabilized Zirconia at 25°C

The objective of this study was to establish the mechanism(s) controlling degradation of yttria-tetragonally-stabilized zirconia (Y-TZP) powder in aqueous suspensions and determine the significance of this degradation to the aqueous physical chemistry of Y-TZP. Experiments were performed on commercially available Y-TZP powder placed in aqueous suspensions at 25°C. Experimental investigations included analysis of the aqueous chemistry of Y-TZP in water via ICP-MS, determination of the surface and bulk structure of the powder via XRD and solid-state NMR, and observation of changes in surface charges via zeta potential determinations. The goal of this study was to control the surface chemistry of Y-TZP in aqueous suspension to promote dispersion and permit aqueous processing of Y-TZP powders.     

ZTA复相陶瓷凝胶注模成形工艺的研究

本文研究了ＺｒＯ２（３Ｙ）－Ａｌ２Ｏ３复相陶瓷的凝胶注模成形工艺,着重研究了低粘度高固相体积分数浓悬浮体的制备。     

Development and mechanical characterization of novel ceramic foams fabricated by gel-casting

Porous ceramic materials are of considerable interest for a variety of chemical and industrial applications in extremely harsh conditions, particularly at very high temperatures for long time periods. A combined gel-casting-fugitive phase process employing agar as a natural gelling agent and polyethylene spheres as pore formers was exploited to produce porous ceramic bodies. Alumina and alumina–zirconia powders were used to prepare samples having a porosity of about 65–70–75 vol%. The composite powder was produced by a surface modification route, i.e. by coating a well-dispersed alpha-alumina powder with a zirconium chloride aqueous solution. On thermal treatment, ultra-fine tetragonal zirconia grains were formed on the surface of the alumina particles. SEM observations and image analysis were used to characterize the microstructure of porous samples and uniaxial compressive tests were carried out to measure their mechanical behavior.     

Influence of preparation method and alumina content on crystallization and morphology of porous yttria stabilized zirconia

Influence of addition of alumina and preparation methods (sol-gel synthesis and mechanochemical preparation) on crystallization and morphology of yttria stabilised zirconia was examined. Presence of alumina was found to delay crystallization of zirconia, the effect being more pronounced at higher alumina content. The two oxides form easily distinguished separate phases. Milling lowers the crystallization temperatures of the sol-gel derived powders since nuclei are formed during the milling and smaller particle size allows easier removal of residual organic components. The milling results in crystallization of some monoclinic zirconia, both in sol-gel derived powders and in case of mechanochemical processing. There are no significant differences between the preparation methods in pore size and relative density of sintered tablets: powders obtained by mechanochemical processing and milled sol-gel derived powders both give tablets with homogeneous morphology. The advantage of sol-gel process is preparation of pure tetragonal zirconia phase without traces of monoclinic phase.     

Citric Acid—A Dispersant for Aqueous Alumina Suspensions

The interaction between citric acid and alumina in aqueous solution is characterized. Adsorption isotherms of the dispersant on the alumina surface, electrophoretic mobility of the alumina particles as a function of the citric acid concentration, and attenuated total reflection Fourier transform infrared (ATR-FTIR) spectroscopy of the citratealumina surface complex have been used. The adsorption behavior of citric acid is dependent on the pH of the suspension and the concentration of the citric acid. The maximum amount of citric acid adsorbed on the alumina surface, 2.17 μ.mol/m² at pH 3, decreases to 1.17 μmol/m² at pH 8. The adsorption of citrate causes a highly negatively charged powder surface and a shift of the isoelectric point (IEP) to lower pH values. The IEP of alumina can be fixed at any pH value between 9 and 3 by proper adjustment of the citric acid concentration. In situ ATR-FTIR spectroscopy of the citrate-alumina surface complex gives evidence for a direct interaction between the carboxylate groups of the citrate and the surface aluminum(III) atoms. The rheological properties of alumina suspensions are studied as a function of the citric acid concentration. The data obtained from the viscosity and dynamic electrophoretic measurements correlate well and allow the construction of a stability map of alumina suspensions stabilized with citric acid. The influence of citric acid on the viscosity is discussed using the Derjaguin-Landau-Verwey-Overbeek (DLVO) theory. The interaction potential between the particles is determined by the citrate adsorbed on the surface, leading to a negative particle charge, and the citrate anions remaining in the solution, resulting in an increase of the ionic strength. The adsorption of citric acid also creates a steric barrier that inhibits the complete mutual approach of the individual alumina particles.     

Charging Behavior at the Alumina–Water Interface and Implications for Ceramic Processing

The interaction of water and the alumina surface is comprehensively reviewed. Water can be incorporated in the alumina crystal structure resulting in the formation of aluminum hydroxides such as gibbsite. Alumina dissolves into water to an extent that depends primarily upon the solution pH and temperature. The soluble Al (III)_aq species (hydrolysis products) likewise depend upon the solution pH, temperature, aluminum, and other salt concentrations. The development of charge on the surface of alumina is controlled by amphoteric surface ionization reactions. The charging behavior of both alumina powders and single crystal faces is compared. The differences can be explained by the reactivities of different types of surface hydroxyl groups. The substantial difference in surface charging behavior of single crystal sapphire and alumina powders indicates that experiments and modeling conducted on single crystals is of limited use in predicting suspension behavior. The atomic scale structure of the hydroxylated sapphire (0001) basal plane is nearly identical to the gibbsite (001) basal plane. The observed surface structures are consistent with the charging behavior of the surfaces. The role of surface charge on the adsorption of processing additives is briefly discussed. How surface charge and processing additives at the alumina aqueous solution interface influence surface forces between particles is reviewed. The influence of these forces on suspension properties such as rheological behavior is outlined. The importance of controlling these behaviors to improve colloidal ceramic powder processing is stressed.