超细氧化铝水悬浮液分散稳定性研究

阴离子聚电解质分散剂(PAA)使α-Al2O3粉体的等电点降低;超声分散作用改善了分散剂在粒子表面的分布状态,因而使浆料的流变性有明显的改善;离子强度低于0.08mol/L时,浆料的粘度随离子强度提高而增加,稳定性降低.但在0.08～0.12mol/L的范围内,离子强度对浆料粘度的影响不明显;温度对浆料的粘度有显著的影响.分散剂添加量低于饱和吸附添加量时,温度升高,浆料粘度增加,稳定性下降.而在分散剂添加量高于饱和吸附添加量时,温度升高,粘度则降低.     

Preparation of alumina by aqueous gelcasting

The alumina ceramic was prepared by aqueous gelcasting. The effects of zeta potentials, solid loading, dispersant content and milling time on the alumina suspension were studied systematically. The dispersant content has remarkable effects on the viscosity of the suspension. The appropriate dispersant concentration for alumina aqueous slurry with the solid loading of 55 vol.% is 0.6 wt.%. It can be seen that all suspensions (50–56 vol.% solid loading) exhibited a shear-thinning behavior and relatively low viscosity, which was suitable for casting. The degree of shear thinning and the viscosity at high shear rates increased with increasing volume fraction of solid. As the milling time prolongs, viscosity of the suspension decreases first, then the plateau appears and the average diameter keeps changeless. When the milling time was shorter than 20 h, the viscosity of slurries decreased gradually as the time of milling increased. After 20 h milling, the viscosity of the slurry tended to be consistent. Therefore, the ball milling time should be equal to or more than 20 h to obtain a stable suspension at equilibrium. The time available for casting the slurry (idle time) can be controlled by the amounts of initiator and catalyst added to the slurry as well as by the processing temperature. Micrograph of the gelcast green body was homogeneous.     

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.     

Direct Coagulation Casting of Alumina Suspension from Calcium Citrate Assisted by pH Shift

Direct coagulation casting of alumina suspension via controlled release of high valence counter ions (DCC–HVCI) using calcium citrate as coagulating agent was reported. Hydrolysis of glycerol diacetate shifts the pH of suspension to weakly acidic region which helps to decompose calcium citrate and release calcium ions. The effect of concentration of glycerol diacetate and calcium citrate on the pH and viscosity of alumina suspension was investigated at 25°C and 60°C, respectively. The pH of suspensions with glycerol diacetate and calcium citrate decreases to 8.6 and 7.5 treated at 25°C and 60°C, respectively. It is indicated that high viscosity is achieved by adding 2 vol% glycerol diacetate and 0.5 wt% calcium citrate which is enough to coagulate the suspension. Green body with compressive strength of 1.0 MPa is obtained by treating the alumina suspension with 2 vol% glycerol diacetate and 0.5 wt% calcium citrate at 60°C for 1 h. The alumina ceramics sintered at 1550°C have homogeneous microstructure with relative density above 99.0%.     

DNP/HMX熔铸炸药的流变性能

为了解3,4-二硝基吡唑(DNP)/HMX悬浮液在不同影响因素下的流变行为,采用Brookfield R/S Plus流变仪对其流变性能进行测试,分析了HMX含量、粒度、颗粒级配、体系温度以及不同添加剂对悬浮液流变性能的影响。结果表明,DNP单质为牛顿流体,表观黏度约为16.4mPa·s,比TNT高82%,比DNAN高140%;同一剪切速率下,DNP/HMX悬浮液表观黏度随固含量的增加而增加,当HMX质量分数为30%时,悬浮液近似牛顿流体;HMX质量分数高于30%时,表观黏度随剪切速率的增加呈指数型下降的趋势愈发明显;悬浮液表观黏度随颗粒粒径的增大和温度的增加而降低,当温度从95℃升到105℃时,黏流活化能(E)从29211J/mol增至38458J/mol;固含量为60%时,平均粒径(d50)分别为16.6μm和575.6μm的HMX颗粒的最佳质量比为1∶5,此时悬浮液表观黏度最小。N-甲基-4-硝基苯胺(MNA)降低了悬浮液的表观黏度,乙酸丁酸纤维素(CAB)和微晶蜡-80(MV80)增加了悬浮液的表观黏度。     

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.     

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。     

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.     

Aqueous Colloidal Characterization and Forming of Multimodal Barium Titanate Powders

Aqueous suspensions of two barium titanate (BaTiO₃) powders, with 100 and 500 nm nominal size, were found to be stabilized by a predominantly steric mechanism when using a polyelectrolyte surfactant, an ammonium salt of poly(methacylate) (PMA-NH₄). Considerable amounts of barium ions were shown to readily dissolve from BaTiO₃ powders in an aqueous environment, even without the addition of acid or base. This barium dissolution obscured the accurate measurement of the isoelectric point of the two powders. Suspension stabilization was observed to occur at basic pH when using PMA-NH₄. Concentrated suspensions of each individual powder exhibited shear thinning behavior, with the onset of shear thickening occurring at relatively high shear rates. Suspensions containing 85 vol% coarse/15 vol% fine powders demonstrated the lowest apparent viscosity for a given solids loading, while the highest sintered density was obtained with a mixture of 70 vol% coarse/30 vol% fine, when sintering at 1300°C for 2 h (97.4% of theoretical, slip cast from a suspension containing 50 vol% solids).     

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.     

BaTiO_3悬浮液的稳定性研究

研究了pH值及PAA对BaTiO3 水悬浮液稳定性的影响。结果表明 ,由于Ba2 + 在低pH值条件下的溶解 ,使BaTiO3 粉体的等电点比化学计量的BaTiO3 等电点小 ,为pH =6 .4。当PAA的加入量一定时 ,悬浮液的稳定性随pH值的增加而增加。而在一定的pH值条件下 ,存在一个PAA的最佳加入量 ,此加入量随pH值的增加而减小     

YAG ceramic processed by slip casting via aqueous slurries

Stable YAG (Y₃Al₅O₁₂) aqueous slurry with ammonium polyacrylate (NH₄PAA) polyelectrolytes as dispersant was prepared by ball mill method. The effects of polyelectrolyte concentration and pH value on the stability of the suspension is described here, and the stability maps are constructed at different pH value and polymer concentration. The rheological behavior of YAG slips of different solid loading (60–70%) has been studied by measuring their viscosity and shear stress as a function of shear rate and pH of the slurry. An optimal amount of dispersant and pH value for the suspension was found. YAG suspension displays a maximum in zeta potential values and a minimum viscosity in pH range of 9–11. Slips behaved as near Newtonian at the pH value up to a solid loading of 60 wt% and as non-Newtonian with thixotropic behaviors above this solid loading value. The density and the green as well as sintered microstructure of the cast products bear a direct relationship to the state of this slips induced by the alternation in the pH and the concentration of the dispersant as well as solid loading.     

Direct coagulation casting of yttria-stabilized zirconia using magnesium citrate and glycerol diacetate

Direct coagulation casting via controlled release of high valance counter ions (DCC-HVCI) has been reported in recent years. In this paper, concentrated yttria-stabilized zirconia (YSZ) suspensions were coagulated using DCC-HVCI method with magnesium citrate as coagulating agent assisted by pH shift in the presence of glycerol diacetate. The effect of ammonium polyacrylate (PAA-NH₄) on the dispersibility of YSZ powder was investigated. The influence of concentrations of glycerol diacetate and magnesium citrate on pH and viscosities of YSZ suspensions was studied. The results indicate that concentrated YSZ suspensions can be coagulated by adding 2 vol% glycerol diacetate and magnesium citrate above 0.5 wt% at room temperature for 2–5 h. The compressive strength of coagulated wet samples is above 2.0 MPa. YSZ ceramics sintered at 1450 °C show homogeneous microstructures with relative densities of 98.9–99.2%. Flexural strength of YSZ ceramics is 869±84 MPa.     

Role of Sodium Citrate in Aqueous Milling of Aluminum Oxide

A study was initiated to identify the mechanism(s) by which chemical additives affect aqueous milling of oxide suspensions. Properties of milled Al2O3 suspensions containing sodium citrate were correlated with ground-powder characteristics. Maximum grinding efficiency was obtained at the sodium citrate concentration which resulted in the maximum (absolute value) zeta potential and the minimum suspension viscosity. The results indicate that sodium citrate indirectly affects comminution by altering the particle arrangements (state of dispersion) in suspension. Fracture strength measurements, determined on porous bulk Al2O3 samples pretreated in distilled water and aqueous sodium citrate solutions, did not support chemomechanical mechanisms.     

α-Al2O3-H2O-聚丙烯酸悬浮液流变性的研究

研究了聚丙烯酸(polyacrylic acid,PAA)添加量、固含量和吸附途径对α—Al2O3浆料流变性的影响。实验表明：浆料具有最小粘度时,分散剂添加量存在最佳值。随着固含量提高,浆料最低粘度值提高,分散剂有效添加量范围变窄。吸附途径对浆料的流变性存在一定的影响,在其它条件相同的情况下,低pH值浆料经历非亲合力状态下的吸附,浆料流变性明显改善。     

Temperature induced forming of zirconia from aqueous slurries: mechanism and rheology

Several near-net shape-forming techniques via colloidal process were received a great attention for manufacturing low cost and reliable advanced ceramics. Temperature induced forming is known as a suitable procedure that allows the solidification of aqueous suspension, which produce green parts, with interesting economics and environmental benefits. In the present work, well-dispersed zirconia suspensions in aqueous media were prepared using tri-ammonium citrate (TAC) as dispersant agent. Results of electrokinetics phenomena and rheological properties of the investigated system were used to optimize the content of TAC and to maximize the solid loading. Krieger–Dougherty's model was applied to describe the relation between relative viscosity of the dispersion and volume fraction of the solids in this system. Consolidation mechanism of the 40-vol.% zirconia suspensions in aqueous media was studied using temperature induced forming technique. The consolidation conditions were evaluated via measuring of viscoelastic functions of the sample, storage modulus (G′) and loss modulus (G″) as a function of temperature at frequency of 10 s⁻¹. To confirm the gel formation, the heights of the moulded samples were measured at different temperatures after keeping the mold in the oven for 14 h at the desired temperature. The obtained green body by this technique was characterized in terms of density and microstructure.     

Slip casting of highly concentrated ZnO suspensions: Rheological studies,two-step sintering and resistivity measurements

The paper presents research on elaboration of well dispersed and stable aqueous suspensions of ZnO fine powder. Within the work the influence of the type and concentration (0.2 wt% - 1.2 wt%) of selected dispersing agents (i.a. poly(acrylic acid)-based polyelectrolyte and tetramethylammonium hydroxide), solid loading (30 - 50 vol%) and milling time (1–3 h) on the rheological properties of the slurries was investigated. Two-step sintering (970/920 °C, 2 h) was applied to sinter the green bodies obtained by slip casting.The lowest viscosity of ZnO suspensions was obtained for the addition of 0.4 wt% of poly(acrylic acid)-based polyelectrolyte (PAA) and TMAH. ZnO suspension containing PAA had negative zeta potential in the whole pH range. The highest solid loading obtained in the study was 50 vol%. The applied two-step sintering allowed to obtain samples of high density (above 96% of TD) and homogeneous microstructure of average grain size of 640 nm. ZnO sintered bodies were characterized by different electric properties at the core part and outer part of the sample which was caused by the differences in concentration of oxygen vacancies.     

Fabrication of piezoelectric nano-ceramics via stereolithography of low viscous and non-aqueous suspensions

A high-performance piezoelectric nano-ceramic was fabricated through stereolithography of low viscosity and high solid loading ceramic/polymer composite suspensions. Through the proper fitting calculation of experimental data, the maximum theoretical solid loading, rheological and curing behaviors of the suspension system were evaluated and lucubrated. The suspensions with a 40 vol% solid loading of the BaTiO₃ nanoparticles displayed shear thinning behavior and a relatively low viscosity of 232 mPa·s at 46.5 s⁻¹ shear rate. After post-process, the 3D printed ceramic specimens showed a nanometer-level grain size with a density of 5.69 g/cm³, which corresponds to about 95% of the theoretical density. The printed ceramics exhibit a piezoelectric constant of 163 pC/N and relative permittivity of 2762 respectively. The results achieved in this research indicate that the stereolithography process is a promising 3D printing technology to fabricate piezoelectric materials with complex geometries and exquisite features for the applications of ceramic components.     

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.     

Effect of Magnesium Oxide and the Testing Rate on the Viscosity and Adhesion Properties of Epoxidized Natural Rubber/Acrylonitrile–Butadiene Rubber Blend Based Adhesives

The effect of magnesium oxide loading on the adhesion properties of epoxidized natural rubber (ENR 50)/acrylonitrile–butadiene rubber (NBR)-based pressure-sensitive adhesives was systematically investigated using 40 parts per hundred parts of rubber (phr) of coumarone–indene resin as the tackifier. The concentration range of magnesium oxide was from 10–50 phr. Toluene and polyethylene terephthalate (PET) films were selected as the solvent and the substrate, respectively, throughout the experiment. A Sheen hand coater was used to coat the adhesive onto the PET substrate at various coating thicknesses. The viscosity of the adhesive was measured using a Brookfield viscometer, whereas the loop tack, peel strength, and shear strength were determined using an adhesion tester operating at 10–60 cm/min. The results indicate that the viscosity increases with magnesium oxide loading, an observation which is attributed to the concentration effect of the filler. However, loop tack, peel strength, and shear strength increase with magnesium oxide loading up to 30 phr before decreasing upon further addition of the filler. This observation is ascribed to the effect of a varying degree of wettability of the adhesive, which culminates at 30 phr of magnesium oxide loading. At a fixed loading of magnesium oxide, all the adhesion properties of adhesives increase upon increasing the coating thickness and rate of testing.