聚乙烯吡咯烷酮对PESA-H2O-SiC悬浮液流变性能的影响

研究了聚乙烯吡咯烷酮(PVP)对聚环氧琥珀酸(PESA)稳定的水基碳化硅(SiC)悬浮液流变性能的影响.结果表明,在pH值为9.0时,添加一定比例的少剂量的二元分散剂即可改善SiC悬浮液的分散性.当分散剂总添加量为0.4wt％,PESA与PVP的质量之比为3∶1时,悬浮液的流变性最佳.固含量为40vo1％时,PESA稳定的SiC悬浮液完全絮凝,而添加PVP作为第二分散剂可使体系较好的分散.此外,添加二元分散剂PESA/PVP显著提高了SiC悬浮液的抗电解质性能.     

蔗糖对PAA-H2O-Al2O3浆料流变性能的影响

研究了蔗糖(sucrose)对聚丙烯酸(PAA)-水基Al2O3悬浮液流变性能的影响.实验表明:在酸性与碱性条件下,蔗糖的添加都有助于提高PAA稳定的水基Al2O3悬浮液流变性.pH为9.0时,PAA-H2O-AlO2悬浮液粘度随着蔗糖含量的增加逐渐降低,当含量大于4wt％时其粘度趋于不变.其中蔗糖对低含量PAA(0.3wt％)稳定的浆料流变性提高尤为明显.对于固含量为25 ～ 40vol％的PAA-H2O-Al2O3悬浮液,4wt％的蔗糖的加入都能提高其稳定性,当固含量为40vol％时,效果最为明显.此外,蔗糖的添加能够较好的提高PAA-H2O-Al2O3悬浮液的抗电解质性能.     

Stabilizing Highly Loaded Silicon Nitride Aqueous Suspensions Using Comb Polymer Concrete Superplasticizers

The stabilization of highly loaded silicon nitride suspensions will afford the processing of complex and near‐net shaped parts using methods such as injection molding or direct write additive manufacturing. In this study, aqueous silicon nitride suspensions up to 45 vol% solids loading were dispersed using commercially available comb‐type copolymer. These copolymers are used as superplasticizers in the concrete industry and are referred to as water‐reducing admixtures (WRAs). Four different WRA dispersants were examined and chemical analysis determined that each was made up of a sodium salt of polyacrylic acid (PAA‐Na) backbone with neutral polyethylene oxide (PEO) side chains that afford steric stabilization. The general structures of the WRAs were compared to each other by measuring the relative areas of their prominent FTIR peaks and calculating a PAA‐Na/PEO peak ratio. Suspensions were made with as‐received silicon nitride powders with 5 wt% aluminum oxide and 5 wt% yttrium oxide added as sintering aids. Three of the four WRA dispersants studied were able to produce suspensions with 43 vol% solids loading and 5 vol% polymer dispersant, while exhibiting a yield‐pseudoplastic behavior up to 30 s^?1. At higher solids loading (45–47 vol%), a shift to shear‐thickening behavior was observed at a critical shear rate for these WRAs. Those WRAs with a lower PAA‐Na/PEO peak ratio displayed better stabilization and diminished shear‐thickening behavior. The vol% of the dispersant was optimized producing yield‐pseudoplastic suspensions containing 45 vol% solids loading with yield stresses less than 75 Pa, no shear‐thickening behavior, and viscosities less than 75 Pa·s for shear rates in the range of 1–30 s^?1.     

Effect of Milling Time on the Rheology of Highly Loaded Aqueous-Fused Silica Slurry

The effects of milling time on the rheological behavior of 70-vol% loaded fused silica slurries were studied. A transition from shear thickening to shear thinning behavior was observed with milling time. Slurry viscosity and thixotropy attained a minimum after 18 h of milling and remained almost unchanged with additional milling. Infrared spectroscopy and particle surface charge measurements confirmed the development of increasing amount of surface hydroxide and surface charge respectively with progress in milling time, leading to shear thinning behavior. An initial static aging treatment of 24 h aided the development of silanol groups, indicating that the milling time can be reduced to save energy and time.     

Rheology of Concentrated Suspensions Containing Weakly Attractive Alumina Nanoparticles

The use of nanoparticles for the fabrication of new functional ceramics and composites often requires the preparation of concentrated fluid suspensions. However, suspensions containing nanoparticles are limited in solids content because of the excluded volume formed by the dispersant adlayer around the particles. We investigated the effect of the adlayer thickness on the rheological behavior of suspensions containing model alumina nanoparticles, using dispersant molecules with deliberately tailored chain length. The apparent viscosity and yield stress of the particle suspensions were markedly decreased by increasing the dispersant length, mainly due to a reduction of the attractive forces among particles. Fluid suspensions with solids content up to 35 vol% were prepared in toluene using a dispersant length of 2.5 nm. Our experimental results and viscosity predictions based on a hard sphere model indicate that fluid suspensions with up to 43 vol% of 65 nm alumina particles could be prepared using an optimum dispersant length of about 3.6 nm.     

Effect of Particle Packing on the Filtration and Rheology Behavior of Extended Size Distribution Alumina Suspensions

The effect of particle packing on the rheology and casting behavior and cast cake characteristics of aqueous alumina suspensions (5O vol%) was investigated using the Andreasen approach. Varied packing was produced by blending two starting materials that differed in average size by a factor of 10. Formulations closest to ideal packing lowered viscosity. Specifically, the lowest viscosity suspension, 196 mPa s, was produced with the distribution closest to the ideal packing distribution. Typically, a well-dispersed suspension (characterized by low viscosity) casts slower than one of poorer dispersion given the same solids content. However, the suspension that provided the lowest viscosity cast at a rate of 0.41 mm²/s, which was the fastest rate of the compared formulations. Therefore, suspensions consisting of extended size distributions do not necessitate slow filtration rates. The colloidal properties of the individual starting materials, low shear rate rheology, and mercury porosimetry were used to explain the unexpected casting behavior. The dispersion and structure formation within the suspension were investigated using electrokinetic sonic amplitude measurements. Mercury porosimetry characterized the flow channels that developed in the casts. Low shear rate rheology confirmed the presence of flocculation that was first suggested by the porosimetry results.     

Curing of Highly Loaded Ceramic Suspensions in Acrylates

The objective of this study is to use differential scanning calorimetry (DSC) to characterize the curing kinetics of Al₂O₃ and hydroxyapatite suspensions in acrylates. The heat evolution of the acrylate premix and ceramic suspensions in dynamic and isothermal curing was measured. The calculated activation energies for the acrylate premix and the two ceramic suspensions were 133, 74, and 78 kJ/mol, indicating a catalytic effect of the ceramic fillers. The curing behavior of the two ceramic suspensions modeled with the autocatalytic polymerization model with satisfactory results.     

pH Valaue Effects in Shear Rheology of Concentrated Alumina Suspensions

1 INTRODUCTIONThe suspensions employed in ceramic processing arehighly concentrated.These suspensions are normallyknown as slip or slurry in materials science and engi-neering.It is of great interest that concentrated sus-pension exhibit many of the phenomena of very dilutecolloidal suspensions.This is probably because boththe suspension systems have charged particles,so thatthe principles of colloid chemistry can be applied forthem.     

pH值对氧化铝高浓度悬浊液剪切流变性能的影响

1 INTRODUCTION The suspensions employed in ceramic processing are highly concentrated. These snspensions are normally known as slip or slurry in materials science and engi-neering. It is of great interest that concentrated sus-pension exhibit many of the phenomena of very dilute colloidal suspensions[i]. This is probablybecause both the snspension systems have charged particles, so that the principles of colloid chemistry can be applied for them.     

Effect of Aging on Glaze Suspensions Rheology

The rheological behavior of glaze suspensions has to be closely controlled to obtain an adequate final product. In this work, we have studied glaze suspensions produced with a range of industrial frits. The main objective was to establish a correlation between changes in rheological behavior with aging and modifications in the ionic composition of the liquid phase, as a consequence of frit dissolution. Suspension rheology was followed through flow and dynamic oscillatory tests. It was found that frits releasing Ca²⁺, B³⁺, and Pb²⁺ can pose handling problems. To better understand the effect of cations' presence on glaze rheology, industrial glazes were compared with simulated glazes produced with inert silica and several cations. These suspensions reproduced the behavior of the industrial glazes and may be used in the future in stabilization tests.     

Adsorption Effects on the Rheological Properties of Aqueous Alumina Suspensions with Polyelectrolyte

The influence of adsorption coverage and free polymer on the rheological properties of aqueous alumina suspensions with polyelectrolyte was studied. The flow curves of the suspensions followed the Casson model very well in the observed range of shear rate. The Casson yield value, τ_c, was used to evaluate the stability. The coverage exerted a profound influence on the rheological properties by affecting the interaction between particles. The zeta potential increased and the value of τ_c decreased as the coverage increased. The free polymer could cause weak flocculation as its concentration was increased to a certain level. The opposite effects of the adsorbed polymer and free polymer on stability resulted in the presence of optimum coverage for stabilization. The concentration of free polymer increased rapidly as the adsorption coverage approached the saturation limit, and then the free polymer started to dominate the stability. Stabilization could be achieved at a condition of unsaturated adsorption and was related to the non-high-affinity adsorption at alkaline pH (9.2). The impact of adsorption affinity on stability was discussed.     

Plasticizing Aqueous Suspensions of Concentrated Alumina with Maltodextrin Sugar

Aqueous suspensions of submicrometer, 20 vol% Al₂O₃ powder exhibited a transition from strongly flocculated, thixotropic behavior to a low-viscosity, Newtonian-like state upon adding small amounts of maltodextrin (0.03 g of maltodextrin/(g of Al₂O₃)). These suspensions could be filter pressed to highly dense (57%) and extrudable pastes only when prepared with maltodextrin. We analyzed the interaction of maltodextrin with Al₂O₃ powder surfaces and quantitatively measured the resulting claylike consolidation, rheological, and extrusion behaviors. Benbow extrusion parameters were comparable to, but higher than, those of kaolin at approximately the same packing density of 57 vol%. In contrast, Al₂O₃ filter cakes without maltodextrin at 57 vol% density were too stiff to be extruded. Measurements of rheological properties, acoustophoresis, electrophoresis, sorption isotherms, and diffuse reflectance Fourier infrared spectroscopy supported the hypothesis that sorbate-mediated steric hindrance, rather than electrostatic, interparticle repulsion, is important to enhancing the consolidation and fluidity of maltodextrin–Al₂O₃ suspensions. Viscosity measurements on aqueous maltodextrin solutions indicated that free maltodextrin in solution does not improve suspension fluidity by decreasing the viscosity of the interparticle solution.     

Dispersion and Rheology of Aqueous Suspensions of Nanosized BaTiO3

The zeta potential and the rheological behavior of aqueous suspensions of a commercial nanosized BaTiO₃ powder were studied. The influence of the volume fraction of solids, the kind and concentration of dispersant, the homogenization technique, and times were investigated. Because the as-received powder contained ∼5 wt% organics, the powder was calcined to 350°C/0.5 h. After calcination, a good dispersion was achieved for suspensions with up to 80 wt% solids using a polyacrylic-based dispersant combined with tetramethylammonium hydroxide and 10 min of ultrasound. Green bodies obtained by slip casting had relatively low densities (2.6 g/cm³, 43% TD), but homogeneous, agglomerate-free microstructures.     

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.     

In Situ Rheological Investigation of the Coagulation in Aqueous Alumina Suspensions

Electrostatically stabilized alumina suspensions can be destabilized by the enzyme-catalyzed decomposition of urea (direct coagulation casting). Depending on the conditions, this reaction can shift the pH of a suspension to the buffer pH of the reaction products or increase the ionic strength at the buffer pH. The coagulation for both mechanisms was investigated using in situ rheological measurements. Using a vane tool in oscillation mode, the measuring conditions were optimized to find a reasonable method for time-dependent measurements. Constant parameters (stress or strain) proved to be unsuitable, because the linear viscoelastic region shifted considerably during the coagulation. Furthermore, the gel structure produced on coagulation via increase of ionic strength (Δ I ) was very sensitive to the oscillation. Therefore, for long-time experiments, a short continuous measurement with a low strain was followed by amplitude sweeps with increased intervals to determine the linear values of G ' and G ". In this way, the increase of the moduli G ' and G " could be followed for longer times, and it was possible to demonstrate two results. First, the final G ' of the network was about 10 times higher for Δ I -coagulated material than for suspensions coagulated via pH shift (ΔpH). Second, particle rearrangement processes took place in Δ I -coagulated networks even after the chemical changes were finished, whereas ΔpH-coagulated samples were "frozen-in" when approaching the isoelectric point and showed no further physical changes afterward.     

Effect of Glass Additions on the Indentation-Strength Behavior of Alumina

The effect of small calcium aluminosilicate (CAS) glass additions on the microstructure and flaw tolerance of alumina ceramics is investigated, and the results compared to a high-purity alumina. The high-purity alumina specimens were dense with microstructures consisting of a uniform grain size distribution and equiaxed grain morphology. Additions of only 1 wt% glass phase resulted in a bimodal grain size distribution containing large, elongated grains within a fine-grain matrix. Indentation-strength tests indicated enhanced flaw tolerance with the bimodal microstructure, even though both materials had nominally the same average grain size. The strength of unindented specimens was also observed to decrease with glass additions. Observations of crack paths show a greater propensity for bridging in the glass-containing alumina due to the presence of coarse, elongated grains and perhaps a lower grain boundary toughness. However, crack extension occurs transgranularly when the size of the coarsest grains becomes too large. This suggests that an optimum in flaw tolerance will be achieved with an elongated grain morphology and intermediate grain size.     

Rheology of Cross-Linked Chitosan–Alumina Suspensions Used for a New Gelcasting Process

The rheological behavior of a low-toxicity, aqueous, Al₂O₃–chitosan–2,5-dimethoxy-2,5-dihydrofuran (DHF) system was examined over a temperature range of 20°–98°C. At room temperature, a high-volume-fraction solids, shear-thinning suspension was produced. The suspension remained in a liquidlike state for ∼24 h. By contrast, at elevated temperatures, DHF became active as a cross-linking agent for chitosan, and a rapid transition from liquidlike to solidlike suspension behavior was apparent. The rate of gelation and the gel strength increased with increased temperature and increased DHF concentration. Gelation occurred at pH 1.4–4.5, which is a complex function of pH. At pH above ∼4, rapid gelation was not possible because of the low-pH solution conditions required for the cross-linking reaction. An examination of the strength and shape-forming properties of several Al₂O₃–chitosan–DHF systems showed them to be suitable for production of strong, reliable, complex-shaped ceramic components. In particular, small, dense Al₂O₃ pseudorotors were formed and found to be of sufficient strength for mold removal and subsequent handling. In all formulations examined, chitosan concentrations of <0.5 wt% (by weight of ceramic) were required; therefore, the need for a discrete binder burnout process was eliminated.     

Effect of Particle Volume Fraction on the Gelation Behavior of the Temperature Induced Forming (TIF) Aqueous Alumina Suspensions

Stable aqueous Al₂O₃ suspensions that can be flocculated by increasing temperature were prepared using tri-ammonium citrate and poly(acrylic acid) (PAA). Effects of particle volume fraction on the rheological properties of Al₂O₃ suspensions were investigated. The modified Dougherty–Krieger equations were used to describe the volume fraction dependence of relative viscosity for both the PAA-addition and PAA-free suspensions. An equation using percolation model was proposed to analyze the temperature and volume fraction dependence of the relative viscosity of the suspensions. The calculated results were in good agreement with the experimental data.     

Rheology of Glaze Suspensions

Rheological properties of suspensions and ceramic glaze slurries under steady flow conditions have been considered. Colloidal forces play an important role in the rheology of such ceramic slurries. Since the potential function characterizes the rheology of colloidal systems, a new dimensionless group, viz. potential number, is introduced within a dimensional analysis representing the relative significance of the potential to the Brownian energy. In order to relate the relative viscosity to other dimensionless groups, a new model is proposed by the inclusion of an extra term in addition to that of the hard‐sphere theory owing to the fact that the presence of colloidal forces always increases the fluid viscosity with respect to that predicted by the hard‐sphere. Steady viscosity measurements have been carried out on ceramic glaze suspensions at different volume fractions, particle diameters, and shear rates. Experimental results have been used to modify the model relating the relative viscosity to the Péclet number, potential number, volume fraction, and maximum packing fraction.     

Coagulation Method of Aqueous Concentrated Alumina Suspensions by Thermal Decomposition of Hydroxyaluminum Diacetate

Consolidation of aqueous concentrated suspensions was used to shape alumina green bodies because it enabled us to obtain complex-shape components with accurate sizes. A high state of alumina particle dispersion was achieved by using (HO)₂C₆H₂(SO₃Na)₂ (Tiron), which allowed us to obtain stable alumina suspensions at pH 9 with a powder concentration higher than 60 vol%. The addition to the suspension of hydroxyaluminum diacetate, (CH₃CO₂)₂AlOH, which decomposed as the temperature increased, permitted us to coagulate an alumina suspension dispersed with Tiron efficiently. Adsorption measurements, electrokinetic mobility, and the rheological behavior of the suspensions provided useful methods to characterize each processing stage. Dense green bodies with sufficient cohesion could be demolded and dried, demonstrating that the dispersant and the flocculant agent chosen permit one to optimize the direct coagulation casting processing of alumina components.