Dispersion of Zirconium Diboride in an Aqueous, High-Solids Paste

The dispersion of ZrB₂ particles was investigated. In aqueous systems, the surface of ZrB₂ consists of a thin layer of ZrO₂ that controls the surface chemistry and surface charge. Measurements showed that the ZrB₂ had an isoelectric point of pH=4.7 and a maximum ζ potential of −50 mV at pH=9. The addition of a dispersant, either an ionic ammonium polyacrylate or a nonionic alkoxylated polyether, increased the ζ potential of ZrB₂ by as much as 60 mV to −110 mV. Viscosity measurements were used to optimize dispersant concentrations. High-solids loading (∼45 vol.% ZrB₂) aqueous pastes were prepared with two different dispersants. The pastes had viscosities of 40–50 Pa·s which was acceptable for extrusion, and were used to fabricate three-dimensional components from ZrB₂.     

Effect of Polyethylenimine on Hydrolysis and Dispersion Properties of Aqueous Si3N4 Suspensions

The roles of polyethylenimine (PEI) in the hydrolysis and dispersion properties of aqueous Si₃N₄ suspensions were studied in terms of the hydrolysis, adsorption, electrokinetic, and rheological measurements. It was found that the pH change of the suspensions in the acidic environment could be minimized in the presence of ≥0.5 dwb% PEI. The ammonia and oxygen measurements suggest that this phenomenon is primarily attributed to the buffer mechanism generated by the ionized PEI, instead of the protection mechanism. The constant pH enables the suspensions to retain a better stability with time at acidic pH. The adsorption of PEI on Si₃N₄ is a high-affinity type at highly basic pH, but is a low-affinity type at acidic pH. As the PEI amount increases, the adsorption shifts the isoelectric point (IEP) of Si₃N₄ from pH 5.9 to pH ∼11 until complete coverage is attained. The stability of Si₃N₄ suspensions is found to depend strongly on the saturated adsorption of PEI, which is as a function of the pH and PEI amount. Once the saturated adsorption limit is reached, the excess free PEI molecules become more detrimental to the stability with increased solid loading. The stabilization mechanisms of Si₃N₄ suspensions by PEI were discussed in detail.     

Aqueous Processing of Titanium Carbide Green Sheets

TiC sheets were prepared by an aqueous tape-casting process. The zeta potential measurement showed that the isoelectric point for TiC powders in the absence of dispersant had a pH value of ∼3.3. According to the surface properties of TiC powders, a cationic polymer PEI was selected as dispersant. In the presence of dispersant, the isoelectric point increased to a pH value of ∼10.4. The slip stability was determined by visual observation of the fluidity of the slip as well as the settling of the powders. Results showed that the amount of dispersant required to achieve a minimum of viscosity for 50 vol% suspensions was equal to 1.2 wt%. In the absence and presence of dispersant, stable slips could be obtained in the pH ranges 7–9 and 11–12, respectively. The rheological measurements showed that with PEI as dispersant, TiC suspensions exhibited a small time dependent behavior. With polyvinyl alcohol as binder and glycerol as plasticizer, suspensions showed a thixotropic feature. As-cast tapes were dried in air at room temperature. The results showed that it was possible to fabricate homogeneous green tapes with smooth surfaces from these suspensions.     

Dispersion of SiC in Aqueous Media with Al2O3 and Y2O3 as Sintering Additives

It has been well accepted that polyethylene imine (PEI) is an effective dispersant for silicon carbide (SiC) in aqueous media. However, after the addition of sintering additives (Al₂O₃ and Y₂O₃), this dispersing effect is reduced significantly. In this work, a second dispersant, citric acid, was used to resolve this problem. It was found that citric acid could decrease the slurry viscosity (without sintering additives) and enhance the PEI adsorption on SiC particle surface. The optimal amount of citric acid required to achieve a minimum viscosity for 55 vol% SiC suspensions was equal to ∼0.87 wt% (at pH ∼6.8). With the aid of citric acid, well-stabilized SiC suspensions (containing sintering additives) were realized, which exhibited slight shear thinning rheologies. After tape casting and SPS sintering, dense SiC samples were obtained with a homogeneous fine-crystalline microstructure. Results showed that citric acid was an effective dispersant for improving the dispersion of SiC particles containing sintering additives.     

Hydrolysis and Dispersion Properties of Aqueous Y2Si2O7 Suspensions

The dispersion of aqueous γ-Y₂Si₂O₇ suspensions, which contain only one component but have a complex ion environment, was studied by the introduction of two different polymer dispersants, polyethylenimine (PEI) and polyacrylic acid (PAA). The suspension without any dispersant remains stable in the pH range of 9–11.5 because of electrostatic repulsion, while it is flocculated upon stirring due to the readsorption of hydrolyzed ions on the colloid surface. However, suspensions with 1 dwb% PEI exhibit greater stability in the pH range of 4–11.5. The addition of PEI shifts the isoelectric point (IEP) of the suspensions from pH 5.8 to 10.8. Near the IEP (pH_IEP=10.8), the stability of the suspensions with PEI is dominated by the steric effect. When the pH is decreased to acid direction, the stabilization mechanism is changed from steric hindrance to an electrosteric effect little by little. PAA also has the effect of reducing the hydrolysis speed via a "buffer effect" in the basic pH range, but the lack of adsorption between the highly ionized anionic polymer molecules and the negative colloid particle surfaces shows no positive effect on hydrolysis of colloids and on the stabilization of Y₂Si₂O₇ suspensions.     

Effect of Polyelectrolyte Dispersants on the Preparation of Silica-Coated Zinc Oxide Particles in Aqueous Media

Sodium silicate was utilized to obtain a SiO₂ coating on ZnO particles to prevent a photocatalytic reaction between ZnO and phenol. During the coating process, pH control is important to avoid dissolution of the ZnO as well as to obtain a good dispersion. Two kinds of polyelectrolyte dispersants were used to control the surface charge of the ZnO particles in aqueous media. As a result, poly(ethylenimine) (PEI) shifted the isoelectric point of ZnO from pH 9 to pH 10, whereas poly(ammonium acrylate) (PAA) made the surface charge of ZnO negative between pH 6 and pH 11. The change in the ZnO surface charge produced by adding polyelectrolyte dispersants makes it possible to obtain uniform silica–coated ZnO particle in aqueous media. UV–irradiation experiments showed that PEI, which can make the surface charge opposite to that of SiO₂, is more effective in obtaining a thick silica coating on ZnO.     

Effects of polyethylenimine on the dispersibility of hollow silica nanoparticles

In this study, two different methods were applied to disperse hollow silica nanoparticles (HSNP); one employed polyethylenimine (PEI) as the dispersant during the synthesis processes for preparing HSNP, while the other added PEI into suspensions of the prepared HSNP and used milling treatment to achieve the desired dispersion. It was found that adding PEI during the synthesis process of HSNP had no noticeable improvement in the dispersion, while adding PEI into suspensions of the prepared HSNP and utilizing milling treatment resulted in remarkable dispersion improvement. Therefore, the latter was chosen as the method in dispersing HSNP suspensions. The adsorption of PEI on the surface of HSNP and the stability of the aqueous suspensions was investigated. The results indicated that the adsorption of PEI on the surface of HSNP would increase the repulsive energy among particles, hence reducing the agglomeration of HSNP and improving the stability of the aqueous suspensions. The change of HSNP’s ζ potential after adding PEI and the relationship between the adsorbed amount of PEI and pH were also investigated. Translated from Journal of Chemical Engineering of Chinese Universities, 2006, 20(5): 752–758 [译自: 高校化学工程学报]     

Dispersion of Cu2O particles in aqueous suspensions containing 4,5-dihydroxy-1,3-benzenedisulfonic acid disodium salt

Effect of 4,5-dihydroxy-1,3-benzenedisulfonic acid disodium salt's addition upon Cu₂O dispersion properties is studied to clarify dispersant's role in colloidal properties change and its underlying stabilization mechanism, and to quantify processing conditions for the oxide. Dispersing effectiveness was studied through adsorption, rheological and electrophoretic measurements, using as-received and surface charge modified Cu₂O particles. Maximum solid loading attained without dispersant was 73 wt.% (31 vol.%), with corresponding viscosity of 152.5 ± 7.3 Pa s. Addition of dispersant resulted in viscosity between 21.0 Pa s and 5.4 Pa s. No isoelectric point was found for as-received particles’ suspensions nor for dispersed suspensions, with particles presenting negative surface charge in all studied pH range, from pH 4 to 10. Adsorption of the organic molecule caused an absolute downshift of 8–25 mV of the electrophoresis curve. Dispersant/Cu₂O interaction was assessed through FTIR analysis. Attained results suggest that, at the natural suspensions pH, dispersant-modified Cu₂O suspensions are stabilized through inner-sphere complexation mechanism, resulting in high dispersion ability.     

氧化铁-水悬浮液的流变性研究

应用DV-Ⅲ+型可编程流变仪测定了三氧化二铁-水悬浮液的流变特性。考察因素包括颗粒浓度、pH值以及分散剂用量。结果表明,悬浮液在测定范围内表现出宾汉流体的特征。表观粘度随颗粒浓度的增大显著增加,其模型可用本文提出的三阶多项式模型表示。悬浮液的表观粘度受pH值的影响,pH在等电点处时,溶液的流动性最差;pH低于等电点时的粘度比pH高于等电点时要明显降低。这些现象可用颗粒的空间结构化理论及凝聚原理予以解释。改善悬浮液流动性的分散剂添加量存在最佳点。     

Cement Composition Effects on the Rheological Property Evolution in Concentrated Cement–Polyelectrolyte Suspensions

We have studied the rheological property evolution and hydration behavior of white and ordinary portland cement (type I) pastes and concentrated cement–polyelectrolyte suspensions. Cement composition had a marked effect on the elastic property evolution ( G '( t )) and hydration behavior of these suspensions in the presence of poly(acrylic acid)/poly(ethylene oxide) copolymer (PAA/PEO), even though their affinity to adsorb such species was nearly identical. Both white and ordinary portland cement pastes exhibited G '₀ values of ∼10⁴ Pa and fully reversible G '( t ) behavior until the onset of the acceleratory period ( t = 2 h), where the pastes stiffened irreversibly. In contrast, cement–PAA/PEO suspensions exhibited G '₀ values of ∼1 Pa and G '( t ) behavior comprised of both reversible and irreversible features. Interestingly, ordinary portland cement–PAA/PEO suspensions experienced a gel-to-fluid transition on high shear mixing at short hydration times (<1 h), and the particle network did not rebuild until ∼24 h of hydration. In sharp contrast, white portland cement–PAA/PEO suspensions remained weakly gelled throughout the initial stage of hydration even after high shear mixing. At longer hydration times (>1 h), both cement–PAA/PEO suspensions exhibited G ' _i ( t ) ∼ exp( t /τ_c) with τ_c values of 5.6 and 1.3 h for ordinary and white portland cement, respectively. Our observations suggest that hydration phenomena impact interparticle forces during early stage hydration and, ultimately, lead to initial setting through the formation of solid bridges at the contact points between particles within the gelled network.     

Rheological Property Evolution in Concentrated Cement-Polyelectrolyte Suspensions

We have studied the rheological property evolution and hydration behavior of pure white portland cement (Type I) pastes and concentrated cement-polyelectrolyte suspensions. Polyelectrolyte species have a marked effect on the initial stability, elastic property evolution ( G '( t )), and hydration behavior of this cement system. Pure cement pastes exhibited an initial G ' value of ∼10⁴ Pa and fully reversible G '( t ) behavior until the onset of the acceleratory period ( t ∼ 2 h), where the pastes stiffened irreversibly. In contrast, cement-polyelectrolyte suspensions exhibited initial G ' values of ∼1 Pa and G '( t ) behavior comprised of both reversible and irreversible features. Their initial G ' values, measured after disrupting the particle network under high shear conditions, grew exponentially with hydration time, where G _i'= G _i,0' exp( t /τ_c) and τ_c corresponds to the characteristic hydration time determined from calorimetry measurements. Our observations of these cement-polyelectrolyte systems suggest that hydration phenomena impact interparticle forces during early stage hydration and, ultimately, lead to initial setting through the formation of solid bridges at the contact points between particles within the gelled network.     

Preparation of SiO2 Glass from Model Powder Compacts: I, Formation and Characterization of Powders, Suspensions, and Green Compacts

Dense SiO₂ glass was produced at ∼1000°C by using highly ordered compacts of spherical, nearly monosized, amorphous SiO₂ particles. In Part I of this study, the formation and characterization of powders, suspensions, and green bodies are described. Thermogravimetry and DTA revealed that substantial loss of bound water occurs in powders calcined at temperatures as low as 200°C. Surface area and density measurements were used to show that the water loss occurs without micropore formation. FTIR spectroscopy revealed that residual silanol groups persist to the highest temperatures (1050°C) studied. The state of particulate dispersion in suspensions was modified by pH adjustment and monitored by rheological measurements. Flocculated suspensions (low pH) produce inhomogeneous, low-density powder compacts with highly bimodal pore-size distributions. Uniform green bodies (with higher packing densities) were prepared using well-dispersed suspensions (high pH). Two-dimensional, close-packed hexagonal arryas of particles were observed in these compacts. Pore-size distributions were narrower, but still bimodal due to the presence of three-particle and four-particle pore channels. The sintering behavior of these compacts is described in part II.     

Role of the Initial Degree of Ionization of Polyethylenimine in the Dispersion of Silicon Carbide Nanoparticles

The role of the initial degree of ionization (α_i) of polyethylenimine (PEI) in the dispersion of SiC nanopartilces in water was investigated by sedimentation and rheological measurements. The ionization of PEI was characterized by potentiometric titration, which indicated a pH-dependent conformational transition. The dispersion of SiC particles in the presence of PEI was found to strongly depend on the α_i. In the presence of 0.4% by weight PEI, the 23.8% SiC by volume (Φ_SiC= 23.8) slurries showed a Newtonian behavior for α_i= 0.12–0.4 values, whereas a shear-thinning behavior was observed for α_i > 0.4 in the optimal pH range of around pH 4. The rheological behavior of the slurries exhibited a strong dependence on the concentration of PEI of α_i= 0.12–0.4 and the slurry showed a Newtonian behavior at an optimal concentration of 0.4% by weight. The stabilization may be dominated by an electrosteric effect arising from the adsorbed PEI of α_i= 0.12–0.4. The flocculation mechanism of the slurries with PEI of α_i > 0.4 is also discussed.     

Surface Chemistry, Dispersion Behavior, and Slip Casting of Ti3AlC2 Suspensions

The surface chemistry and dispersion properties of aqueous Ti₃AlC₂ suspension were studied in terms of hydrolysis, adsorption, electrokinetic, and rheological measurements. The Ti₃AlC₂ particle had complex surface hydroxyl groups, such as ≡Ti–OH,=Al–OH, and −OTi–(OH)₂, etc. The surface charging of the Ti₃AlC₂ particle and the ion environment of suspensions were governed by these surface groups, which thus strongly influenced the stability of Ti₃AlC₂ suspensions. PAA dispersant was added into the Ti₃AlC₂ suspension to depress the hydrolysis of the surface groups by the adsorption protection mechanism and to increase the stability of the suspension by the steric effect. Ti₃AlC₂ suspensions with 2.0 dwb% PAA had an excellent stability at pH=∼5 and presented the characteristics of Newtonian fluid. Based on the well-dispersed suspension, dense Ti₃AlC₂ materials were obtained by slip casting and after pressureless sintering. This work provides a feasible forming method for the engineering applications of MAX-phase ceramics, wherein complex shapes, large dimensions, or controlled microstructures are needed.     

Photocatalytic degradation of short-chain organic diacids

The heterogeneous photocatalytic oxidation of fumaric, maleic and oxalic acids over TiO₂ has been investigated. For aqueous suspensions at pH lower than the point of zero charge (pzc) of TiO₂, the photocatalytic degradation of the three studied diacids follows the Langmuir–Hinshelwood kinetic model, with the rate constant of the process decreasing in the order oxalic acid>maleic acidfumaric acid. At these low pH media, the adsorption of the organic diacids onto TiO₂ particles is a key feature for their degradation, which is initiated by a photo-Kolbe process. For fumaric and maleic acids, a cis–trans isomerisation induced by the interaction between adsorbed molecule and semiconductor surface occurs. At pH’s higher than the pzc of TiO₂ the rate of oxalic acid oxidation decreases noticeably, while fumaric and maleic acids are both efficiently degraded in homogeneous phase by reacting with OH√ radicals photochemically generated on the TiO₂ surface, giving rise to a significant increment of both isomers degradation rate with increasing pH. At these pH’s higher than the pzc of the TiO₂, the three studied diacids show a very low degree of adsorption onto the semiconductor surface and no evidence of cis–trans isomerisation for both maleic and fumaric acids is detected. In accordance with the observed pH effects on degradation rate and over detected intermediates, a different mineralisation pathway is proposed as function of initial pH.     

Preparation of high solid loading,low viscosity ZrB2–SiC aqueous suspensions using PEI as dispersant

PEI was used as dispersant for ZrB₂ and SiC powders in water. The dispersion behavior of ZrB₂ and SiC in water was studied by zeta potential measurements, particle size distribution measurements and interparticle interaction calculations. Well-dispersed ZrB₂ and SiC aqueous suspensions were obtained using 0.6 wt% PEI at pH 6. The rheological behavior of ZrB₂–SiC aqueous suspensions was also investigated. Finally, a high solid loading (52 vol%), low viscosity (980 mPa s at 100 s⁻¹) ZrB₂–SiC aqueous suspension was successfully prepared.     

Dispersion Behavior of ZrB2 Powder in Aqueous Solution

Dispersion conditions of ZrB₂ powder in water were investigated using poly(ethyleneimine) (PEI) as a dispersant. Pulverization of ZrB₂ powder to submicrometer size was difficult and a substantial amount of large particles remained after an intensive planatery milling for 72 h. The isoelectric point (IEP) of ZrB₂ powder was measured to be pH 5.8 by electrophoresis, which shifted to pH 6.2 after milling. The application of PEI changed the IEP of the boride slurry to ∼pH 11. Well-dispersed aqueous ZrB₂ slurries with a high solid loading (up to 45 vol%) were fabricated at pH 6.5–7.5 by the application of 1.5 wt% PEI.     

Interparticle action and rheology of kaolinite-amorphous iron hydroxide (ferrihydrite) complexes

The complexes of kaolinite and amorphous iron hydroxides (ferrihydrite) initially prepared at pH 3.0 and 9.5 were brought to equilibrium at different pH values at pH 3–10. Bingham yield stress for the suspensions of the complexes have been examined as a function of pH. The complexes prepared at pH 3.0 and 9.5 both gave higher Bingham yield stress than the kaolinite in higher pH range, and the amorphous iron hydroxides in the complexes prepared at 9.5 were more effective in enhancing yield stress. The results are discussed in terms of the charge characteristics of the amorphous iron hydroxide and its association with the kaolinite surfaces by means of electron microscopy and zeta potential and surface area determinations.     

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.     

Preparation of High Solids Content Nanozirconia Suspensions

A new colloidal route leading to the production of ∼99% dense 3-mol% yttria-stabilized zirconia nanostructured ceramics, while retaining a final average grain size of ∼75 nm, has been developed. The process was based on the production of stable, homogeneous nanosuspensions with solids contents of up to 28 vol% (70 wt%), but viscosities <0.05 Pa·s at any shear rate in the range of study were obtained. The suspensions were formed by the concentration and optimization of precursor, dilute (5.0 vol%) commercial nanosuspensions, the approach requiring a change of pH, from the 2.4 of the as-received suspension to 11.5, and the use of an appropriate anionic dispersant. Exposure of the nanosuspensions to ultrasound also helped to reduce the viscosity further, although it only worked when the dispersant was optimized. The nanosuspension was slip cast to form homogenous green bodies with densities of ∼55% of the theoretical without agglomeration in the nanostructure; these were subsequently densified using a two-step sintering technique.