Experimental Evaluation of a New Flocculation-Filtration Model for Ceramic Shape Forming Processes

The effect of suspension rheology on the casting mechanics of 37 vol% aqueous silicon carbide slips was examined. Slip viscosities of 180, 120, 90, and 58 mPa.s were evaluated. The 58 mPa.s slip was considered to be fully deflocculated; the others were underdeflocculated to varying degrees. The fully deflocculated slip cast according to parabolic rate behavior, while the others cast at a faster than parabolic rate. Deviations from parabolic rate behavior were correlated with a nonuniform cake structure observed through ultrasound and mercury porosimetry measurements. The observed casting behavior was explained on the basis of a suspension flocculation-filtration model. From rheological measurements, it was found that flocculation of the SiC particles during casting in the underdeflocculated suspensions caused the local void volume within the forming cake to increase as a function of cake thickness. The fully deflocculated slip was rheologically stable, and therefore cast with a uniform microstructure.     

Aqueous and Nonaqueous Colloidal Processing of Difficult‐to‐Densify Ceramics: Suspension Rheology and Particle Packing

Aqueous and nonaqueous colloidal processing of zirconium diboride (ZrB₂) and boron carbide (B₄C) has been investigated. The aqueous and nonaqueous ZrB₂ and B₄C suspension formulations have been optimized. The suspensions were cast into green bodies using slip casting. The correlation between the state of dispersion with the rheological properties of the suspensions and the resulting packing density was observed in both aqueous and nonaqueous processing. The attractive interactions between powder particles in water were difficult to overcome with electrical double layer or electrosteric repulsion. Reasonably low viscosity aqueous ZrB₂ suspensions up to 45 vol% solids could be prepared. It was not possible to produce low viscosity (viscosity below 1 Pa·s at shear rate of 100 s^?1) aqueous B₄C suspensions with solid content above 30 vol%. Slip casting of the weakly aggregated ZrB₂ suspensions resulted in low packing densities (~55% relative density) of the green bodies. On the other hand, dispersion of powder particles in nonaqueous media (cyclohexane and dodecane) enabled suspensions with lower viscosities and a higher maximum solid concentration (up to 50 vol%) to be prepared. The well‐dispersed nonaqueous suspensions promoted an efficient particle packing, resulting in higher green densities (64% and 62% relative density for ZrB₂ and B₄C, respectively) compared to aqueous processing. The significantly high green densities are promising to allow densification of the materials at lower sintering temperature.     

The properties of water suspensions of commercial alumina and rational conditions for slip casting

Conclusions The properties of water suspensions of finely ground commercial alumina, precalcined at 1450°C, over a wide pH range and concentration, were investigated. Curves were drawn to determine the viscosity of the limiting structural breakdown.The presence in the suspensions of two pH ranges, in which maximum zeta-potential values are reached corresponding to minimum viscosity, was confirmed and also for viscosity of the limiting structural breakdown.It is shown that the slip in acid conditions compared with alkaline, belongs to a more liquid system characterized by a sharp reduction in shear force and thixotropic reinforcement for the minimum viscosities, and also by a greatly reduced viscosity of the limiting structural breakdown. In acid conditions there is an increase in the absolute values of zeta-potential and the cast specimens become quite dense.Confirmation was obtained of the desirability of casting products from commercial alumina in acid conditions and of getting the optimum pH interval.To increase the density of the products they should be cast from vacuumized slips and preferably under vacuum.Heating the slip to 30–40°C reduces its viscosity, increases the casting rate and in conjunction with the vacuumizing even slightly increases the density of the casts. As a result it becomes possible to increase the density of the dried casts and to reduce the shrinkage during firing.To increase the strength of the wet and dried casts saccharose appears to be the best bond.     

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.     

Microstructural Characterization of Alumina and Silicon Carbide Slip-Cast Cakes

The effect of solids loading, particle-size distribution, and suspension viscosity on the resultant microstructure of slipcast monolithic ceramics prepared from aqueous suspensions of alumina and silicon carbide was studied. Unimodal alumina suspensions (average particle size = 0.6 μm) were prepared at 35, 37, and 42 vol%. Silicon carbide suspensions (average particle size = 0.7 μm) were produced with different quantities of dispersant at 37 vol%. Similarly, aqueous alumina suspensions of 42 and 50 vol% were produced with a bimodal particle-size distribution. The slip-cast microstructures were characterized by mercury porosimetry and small-angle neutron scattering, which provided pore size (distribution), pore fraction, and pore morphology. Essentially, the combination of these techniques deciphered packing differences obtained in the cake microstructures. For the alumina cakes produced from the 35,37, and 42 vol% suspensions, the individual characterization techniques, mercury intrusion, and the neutron scattering measurements showed that the cake microstructures were similar in pore size and quantity. However, comparison of the techniques and their assumptions showed differences in the pore shape. Mercury porosimetry and neutron scattering showed bimodal porosity for the cake produced from a mixture of 85% 6-μm particles and 15% 0.6-μm particles. Pore volume fraction and pore size increases were correlated with increased viscosity in the silicon carbide suspensions. In addition, the silicon carbide cake microstructures were measured, and homogeneity was evaluated as a function of position in the cast.     

Effect of dispersant on the rheological properties and slip casting of concentrated sialon precursor suspensions

The ability of Hypermer KD1 to disperse high solids loading reaction sialon suspensions for slip casting has been characterised. It has been found to be a very effective dispersant in organic media of 60-vol.% MEK and 40-vol.% Ethanol, yielding fluid and highly homogeneous suspensions. The effects of added amounts of KD1 have been observed through adsorption data, sedimentation tests and rheology measurements. KD1 imparts low viscosity and stability to the suspension. It has been found that 3-wt.% addition of KD1, based on the weight of reaction sialon powders, results in a very stable and high flowable suspension with near Newtonian flow behavior. Less amounts of dispersant lead to unstable suspensions with obvious shear thinning flow behaviors, while adding excessive dispersant leads to high viscosities, especially at high solids loading. Measuring the pore size distribution of green bodies from different suspensions has proved the effects of dispersant amounts on dispersing the slurries and on slip casting performance.     

单分散氧化硅浓缩悬浮液浸渍Kevlar复合材料的防刺性能

以St(O)ber和离心交换法制备了不同浓度的单分散SiO_2/PEG浓缩悬浮液,并研究了其流变性能和用其浸渍Kevlar编织布所得复合材料的防刺性能.研究发现,所制备浓缩悬浮液在质量浓度低于50%时,具有微弱的剪切增稠效应,增稠所能达到的粘度值随浓度增加缓慢升高,而临界剪切速率降低;当浓度在55%左右时,先是剪切增稠,随后出现轻微的剪切减稀现象;当浓度达到65%时,出现较为显著的剪切增稠效应.所制备复合材料的防刺性能比纯Kevlar编织布有一定的提高,当SiO_2质量百分比浓度在40%～45%区间时,抗刺力可提高到1.66倍左右.     

Effect of dispersion states (electrostatic/electrosteric stabilization) on particles arrangement in the yttria-stabilized zirconia sediments

In the present work, particle arrangement and their packing in the sediment layer of zirconia suspension were studied. To evaluate the particle settling, aqueous suspensions of zirconia nanoparticles were prepared in different dispersion states. In one state, Dolapix CE64 was used as a dispersant to provide electrosteric mechanism. In another state, pH of the suspension was adjusted at 4 to provide electrostatic mechanism. The other state was the combination of dispersant and pH adjustment which resulted in the most stable suspension. First of all, the stability of all dispersion states was evaluated by zeta potential, sediment volume (SV) and height, viscosity, and packing density (PD). Then, the sediment layers of all suspensions were characterized. Incorporation of electrostatic mechanism was resulted in a main decrease in viscosity with high surface charges, while electrosteric mechanism caused lower sedimentation of particles. Fall velocities of particles/agglomerates were estimated, and the influences of dispersion states on the particles fall velocities were characterized. The microstructural observation revealed homogeneous packing of particles in the sediment layer of the stable suspension demonstrating the proper dispersion of particles. Dolapix CE64 and pH adjustment resulted in a uniform arrangement of particles without agglomeration and spherical and regular granules with a uniform shape.     

Investigation of Crystal Morphology and Shock Sensitivity of Cyclotrimethylenetrinitramine Suspension by Rheology

Crystal morphology and shock sensitivity of a series of cyclotrimethylenetrinitramine (RDX) particles suspended from ethylene glycol were investigated. Flow rheology was employed to measure the rheological properties of the suspensions at constant temperature; it was observed that the stress‐shear rate and viscosity behavior of the suspensions were controlled by the particle morphology. The viscosity of the RDX suspensions changed with the roundness/smoothness of RDX crystals at all applied shear rates. The suspensions containing crystals with smoother morphology showed reduced viscosity. When the viscosity data was compared to the shock sensitivity results from the RS‐RDX Round Robin study, a good correlation was obtained. This study has validated the use of flow rheology to indicate the morphology and shock sensitivity of crystalline particles.     

Effects on the melt rheology of systems of Nd‐Fe‐B particles suspended in a poly(phenylene sulfide) and liquid crystalline polymer blend

The melt rheology of blends of a liquid crystalline polymer (LCP) and poly(phenylene sulfide) (PPS) and their composites with ferromagnetic Nd‐Fe‐B particles (MQP) was studied. We investigated the effects of LCP concentration, Nd‐Fe‐B particle volume fraction and size, distribution, and shear rate on the rheological properties of these composites. Enthalpy of fusion changes that were observed resulted from the addition of the LCP and Nd‐Fe‐B particles to the polymer blends/composites. The shear rate and frequency dependencies of the materials revealed a viscosity reduction at low (1–3 wt%) and moderate (10–15 wt%) LCP concentrations, and strong effects on the shear‐thinning characteristics of the melt. The suspensions of polydispersed Nd‐Fe‐B particle configurations in PPS that were of lower size ratios gave better processability, which is contradictory to previously reported behavior of suspensions containing spherical particles. Specifically, the compositions with unimodal and a bimodal distribution of Nd‐Fe‐B particles gave the lowest viscosities. The experimental data were correlated with semi‐empirical viscosity model equations of Maron‐Pierce, Krieger‐Dougherty, Eilers, and Thomas and were found to be consistent with the data. The maximum packing fraction, ϕ_m, of the MQP particles was estimated to be within the range of 0.78 ϕ ≤_m ≤ 1.0 through graphical and parametric evaluation methods.     

Particle Crowding Analysis of Slip Casting

The particle crowding index and interparticle spacing terms were calculated for seven alumina suspensions. The particle crowding index was used to interpret the casting rate for the tested alumina suspensions; the index was successfully correlated with the casting rate for cakes that produced the same modes of porosity. Unfortunately, this index could not be correlated with the casting rate for the particle system that produced varied porosity as a function of composition. The interparticle spacing term was correlated with viscosity for particle size distributions between 31 and 0.1 μm. For particle size distributions extended to 44 μm, the viscosity could not be correlated with interparticle spacing, because the quantity of fine particles, rather than the particle packing, controlled the viscosity.     

A review on the rheological behavior and formulations of ceramic suspensions for vat photopolymerization

Vat photopolymerization is an additive manufacturing process that produces high-performance ceramic parts. A critical step in the process is the preparation of a suspension that meets the requirements of high ceramic loading and proper rheological behavior, since an increase in solid loading might compromise the suspension rheology, resulting in non-uniform layer recoating. This review examines the rheological behavior of ceramic suspensions for vat photopolymerization, discussing the influence of the suspension formulation (solid loading, ceramic particle size and size distribution, monomers, diluents, and dispersants) on rheological aspects such as viscosity, shear-thinning/thickening behavior, critical shear rate, yield stress, and thixotropy. It provides a summary of the best formulations, which achieved low viscosity (<3 Pa.s) and high solid loading (>40 vol%), and reports the main trends and challenges of ceramic vat photopolymerization, suggesting general guidelines for the preparation of highly loaded photocurable ceramic suspensions with low viscosity.     

Rheological and Morphological Evaluation of Natural Rubber Latex-Based PSAs Containing Waterborne Aliphatic Hydrocarbon Tackifier Dispersions

The effect of three waterborne aliphatic hydrocarbon tackifier dispersions each with different softening points (70°C, 85°C, and 95°C) were evaluated with natural rubber latex at two addition levels (25% and 50%) for pressure-sensitive adhesive (PSA) applications. No other additives were incorporated into the PSA formulations so that rheological effects of waterborne aliphatic hydrocarbon tackifier resin dispersions in natural rubber-based PSAs could be clearly understood. Application of these waterborne PSAs was evaluated, in terms of rheology, since flow parameters have a very important influence on the convertibility (coating ability) of such adhesives. Morphological correlations with wet rheology for these waterborne PSA formulations and starting materials revealed that the interaction between the latex particle and tackifier dispersion particle has a major influence in determining the viscosity characteristics at low to medium shear rate, where stirring, pumping, and filtration processes occur. A shear-thinning effect was also predominant in formulations with lower tackifier dispersion levels. The extent of shear-thinning can be correlated well to morphology. Interestingly, all the PSA formulations tend to follow Newtonian behavior above a shear rate of 1000 s^?1 and no shear-thinning or shear-thickening at higher shear rates was observed. The minimal change in viscosity at higher shear rates is a key parameter for high-speed coating techniques such as curtain coating and reverse gravure, since any change in viscosity can affect coating uniformity and the wetting of the substrate surface during coating.     

SiO_2/PEG分散体系的流变学性能研究

随着剪切增稠液体在防护领域的应用,其流变学性能的研究显得尤为重要。本实验制备了一系列SiO2/PEG分散体系,并解释了制备过程中所出现的实验现象;采用高压毛细管流变仪测试了SiO2/PEG分散体系的流变性能。研究结果表明,随着剪切速率的变化,分散体系的表观粘度先减小后增大,并且剪切增稠现象具有可逆性;分散介质相对分子量的增大,不利于分散体系的剪切增稠;SiO2固含量越大,剪切增稠现象越明显。     

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.     

The importance of surface hydration and particle shape on the rheological property of silica-based suspensions

Two types of ball milling methods, wet planetary ball milling and dry tumbling ball milling, were used to grind fused silica powders for the preparation of silica-based suspensions in this experiment. The effect of surface hydration and particle shape caused by the two milling methods on the slurry rheology was investigated. The results showed that, with similar particle size distribution, the viscosity of the suspensions prepared from the powders milled by wet ball milling ranged from 275 mPa s to 311 mPa s within 20 min and the suspension exhibited a continuous shear thickening behaviour whereas the viscosity of the suspensions prepared from the powders milled by dry ball milling ranged from 69 mPa s to 74 mPa s and the suspension exhibited a shear thinning behaviour first, and then exhibited a slightly shear thickening behaviour. The reasons were attributed to the differences in surface hydration and particle shape. It was postulated that the two factors affected the slurry rheology through the modification of particle interactions during the flow of high concentration suspensions.     

Rheological properties of alcohol-coal suspensions based on Ukrainian coals of different ranks

The rheological behavior of alcohol-coal suspensions based on coals of different ranks prepared with the use of lower alcohols from the methanol-propanol series as a dispersion medium was studied. It was found that an increase in the length of an alkyl radical leads to an increase in the shear stress and viscosity of the alcohol-coal suspensions. In this case, the suspensions obtained with the use of isoalcohols are characterized by lower viscosity than the suspensions obtained with the use of normal alcohols. From the dependence of the shear stress and viscosity of alcohol-coal suspensions on the physical properties of lower alcohols, it follows that an increase in the dynamic viscosity and dipole moment, as well as a decrease in the dielectric constant (with increasing the length of the hydrocarbon radicals of alcohols), leads to an increase in the above rheological parameters.     

THE FLOW BEHAVIOR OF SUBMICRON α-Al2O3 AQUEOUS SUSPENSIONS

The viscosity of aqueous, charged a-Al₂O₃ suspensions with and without added polyelectrolytes (polyacrylic acid (PAA 1800)) is investigated over a wide range of volume fractions and shear rates. The Carreau-Yasuda model is used for both cases for examining the shear-rate dependence; for the electrostatically-stabilized suspensions the Krieger-Dougherty and Quemada models are used to determine the maximum packing fraction. The latter analysis shows that the suspension behavior changes from liquid-like to solid-like at a volume fraction of 0.71 in the low-shear limit and at 0.89 at the high-shear limit (because of the polydispersity of the suspension). The results with polyelectrolytes indicate that the dosage of the polymer plays an important role in the viscosity of suspensions and that there is an optimum dosage of polyelectrolyte that must be added to reduce the viscosity at high volume fractions of solids.     

聚烯烃弹性体的流变性能

研究了加工温度、辛烯含量和炭黑填充量对用茂金属催化剂合成的聚烯烃弹性体（ Ｐ Ｏ Ｅｓ） 流变性能的影响。结果表明, Ｐ Ｏ Ｅｓ 的流变指数小于１ ,属于切力变稀的假塑性流体,提高剪切速率,切敏性增强。 Ｐ Ｏ Ｅｓ 的粘度对温度变化不敏感,但随着剪切速率增加,粘度显著下降,表现出良好的切敏性。辛烯含量高时,切敏性强。     

凝聚剂对氧化铁悬浮液流变性的影响

应用LVDV-Ⅲ+型可编程流变仪测定了凝聚剂对氧化铁悬浮液的流变特性的影响。实验结果表明,高价凝聚剂对于悬浮液颗粒的凝聚能力要比低价凝聚剂更加明显;随着凝聚剂浓度的增加,悬浮液颗粒的凝聚程度也相应增加;凝聚剂种类的不同,对于悬浮液流变性能的影响也不同,高价凝聚剂对于悬浮液流变性能的影响比低价凝聚剂大。在相同剪切速率下,添加高价凝聚剂悬浮液的表观粘度和剪切应力都大于添加低价凝聚剂悬浮液的表观粘度和剪切应力;凝聚剂浓度的不同,对于悬浮液流变性能的影响也不同。在相同剪切速率下,添加高浓度凝聚剂悬浮液的表观粘度和剪切应力都大于添加低浓度凝聚剂悬浮液的表观粘度和剪切应力,增加凝聚剂浓度对提高悬浮液粘着性能具有积极意义。