Fluidity and dispersion of alumina suspension at the limit of thickening by ammonium polyacrylates

The limit of thickening of an alumina suspension by ammonium polyacrylates (PAA) and its molecular weight dependence of the limit were determined from the lowering of the flow point to be a measure of simultaneous promotion of the thickening and the dispersion. PAA of a smaller molecular weight gave a lower flow point minimum and a thicker alumina suspension retaining fluidity up to 85 wt % alumina for PAA of molecular weight 2500. The suspension thickened to the limit has the smallest gap between the flow point and the wet point, supporting Daniel's statement on good dispersion. The average water layer thickness, calculated by dividing the amount of water of suspension at the limit of thickening by the particle numbers, indicated no linearity with the chain length of the PAA. The thickening for PAA with molecular weights smaller than 21 000 resulted in a limit in the average water layer thickness of 30 nm being accompanied by dilatant flow. The suspension at high solid loadings showed various extension features on the glass plate with changes in the PAA concentration around the flow point minimum. The flow behaviour of the alumina suspension around the limit of thickening was characterized by the Bingham model with two parameters of the yield stress, ₀, and the Bingham viscosity, . Increase in the fluidity on PAA addition was strongly attributed to a greater lowering of ₀ than of . A balanced ratio between the two parameters in the apparent viscosity under a suitable shear rate was suggested to be necessary for the flow of the castable thick suspension.     

Extensions of polyacrylic acid ammonium salts in the adsorption layer to fluidize alumina slurries

An average shell volume occupied in the adsorption layer on alumina by a polyacrylic acid ammonium salts molecule (PAA) defined as the average area occupied by an adsorbed PAA on alumina multiplied by the average thickness of the water layer at the limit of thickening, has been calculated from the adsorbed amount of PAA and from the flow points of alumina in the presence of PAA of different molecular weights. A steric effect of the PAA dispersant on the dispersion of alumina resulted in a flow with no yield stress. This was due to the change of the extension of PAA in the adsorption shell from a sphere to an ellipsoid with a long axis which exceeded the effective distance that the van der Waals attraction force reaches at a molecular weight for the PAA of between 10 000–20 000.     

Preparation, Characterization, and Gelation of Temperature-Induced Forming (TIF) Alumina Slurries

Temperature-induced forming is a novel direct-casting method that has potential for ceramic industry applications. Less than 0.5 wt.% TAC and PAA were used to prepare TIF alumina slurries. These slurries were gelled to form alumina parts by controlling temperature only. The viscosity of the PAA addition slurries increases with temperature, while the viscosity of PAA-free slurry has no evident change in the experimental temperature range. The temperature dependence of modulus showed that the slurries had "fluidlike" to "solidlike" transition that lead to gelation of TIF slurries. The composition-gelation diagram under the experimental conditions is also reported. By tailoring the slurry solid loading and PAA concentration, complex shape alumina ceramic parts with good surface finish and green density of about 65% theoretical density were successfully fabricated by TIF direct casting.     

MOLECULAR WEIGHT EFFECTS OF OLIGOMERIC DISPERSANTS ON AQUEOUS ALUMINA SUSPENSIONS

The objectives of this study are to synthesize an oligomer (75% acrylic acid, 15% acrylamide, and 10% methyl acrylate) with different molecular weights and determine the effects of molecular weight on the dispersing ability of the oligomers. Molecular weights were calculated using Proton NMR spectroscopy for end-group analysis. Rheology and adsorption measurements of aqueous alumina slurries at various polymer dosages were acquired at pH 9.5. Based on the results of these studies, it was concluded that the molecular weight of an oligomer does have a significant effect on alumina suspensions. Moreover, there is an optimum molecular weight at which suspension viscosity is the lowest due to the amount of oligomer adsorbed onto alumina particles. In this study, oligomer C (1970 g/mol) with 2 mg/g of alumina dosage in 0.50 volume fraction alumina suspensions yielded the lowest viscosity and highest adsorption. Viscosity and adsorption values increased above and below 1970 g/mol molecular weight. In summary, efficiency of a dispersant is essentially related to the conditions under which it is used. Therefore, it is important to optimize all conditions, molecular weight and charge interactions being most fundamental.     

Effect of hydrolysed aluminium treatment on rheological characteristics of α-alumina slurries

This paper describes the process for the hydrolysed aluminium treatment (HA) on alumina surface and its influence on the rheological characteristics of alumina slurries. Three different commercial grade aluminas were provided with the surface treatment with hydrolysed aluminium using aluminium nitrate and hexamethylenetetramine (HMTA) under controlled conditions. The HA treatment increased the concentration of Al-OH surface sites resulting in higher H⁺ adsorption on the alumina surface. A highly concentrated (>55 vol%) electrostatically stabilized alumina slurry was prepared from HA treated alumina powders. The rheology of such slurry was studied and the results on the viscosity and yield stress are presented. The alumina slurries followed the Casson Model flow behaviour. The wet and flow behaviour of the alumina with and without HA treatment was also studied and the results are compared. The surface treatment showed the advantage of maintaining low viscosity and yield stress of alumina slurries even at higher solids loading (>55 vol%) that are prepared in the acidic aqueous medium. The results on viscosity and yield stress were compared with that of the polyelectrolyte dispersed system.     

Rheological properties of aqueous alumina suspensions

The aim of the present work was to investigate rheological behaviour of alumina suspensions, considering different amounts of polyvinyl alcohol (PVAL) as a binder. Three different aqueous suspensions were prepared, containing 60, 70 and 80 wt.% of alumina powder. Spinel was added as a sintering agent and Darvan 821‐A as a dispersant, in an amount of 0.08 and 0.4 wt.% of dry powder weight, respectively. The alumina suspensions flow curves were recorded and fitted satisfactorily to the power law, Herschel‐Bulkley and Bingham models. Obtained results indicate that apparent viscosity of alumina suspensions increases with increasing Al₂O₃ and polyvinyl alcohol amount.     

Dependence of the rheological behaviour of electrostatically stabilized alumina slurries on pH and solid loading

Alumina slurries with different solid loadings were prepared using aqueous solutions of hydrochloric acid the concentrations of which were predetermined as suitable for dispersing media; their rheological behaviour was investigated with respect to the pH of the slurry. In regions of low pH (below 3.5), the viscosity of the slurries gradually increased with decreasing pH of the slurry, irrespective of the amount of solid loading. Near the isoelectric point of alumina (about pH 8–8.5), the viscosities of the slurries were high and dependent on the solid loading. The pH range which exhibits the regions with low viscosity narrows on increasing the solid loading. These results were interpreted using an interparticle potential energy diagram for the slurry system. © 1998 Kluwer Academic Publishers     

HEMA-TBA凝胶体系制备多孔氧化铝陶瓷

采用HEMA-TBA凝胶体系制备具有高气孔率, 高强度的多孔氧化铝陶瓷, 研究多孔氧化铝陶瓷的浆料和坯体制备工艺, 并系统研究了分散剂含量和固相体积分数对浆料粘度和悬浮稳定性的影响、干燥和单体含量对生坯性能的影响以及固相体积分数和烧结温度对烧结体微观结构的影响。结果表明: 加入柠檬酸可以使浆料粘度降低, 稳定性提高, 柠檬酸加入量达到2wt%后浆料粘度和稳定性趋于稳定; 固相体积分数的增加会导致浆料粘度和稳定性的增加; 生坯在干燥过程中的收缩比水基体系小很多, 干燥时间也相对更短; 单体含量对生坯强度影响较大, 当单体含量为25wt%时, 生坯强度较高; 通过选择不同的固相体积分数和烧结温度, 可以有效地控制烧结体的微观结构, 气孔率的变化范围在40%~65%, 同时烧结体强度也会随之发生变化, 变化范围在5.7~91.2 MPa。     

D-山梨醇对聚丙烯酸-水基氧化铝悬浮液流变性能的影响

研究了D-山梨醇(D-sorbitol)对聚丙烯酸(poly(acrylic acid))(PAA)稳定的水基氧化铝(Al₂O₃)悬浮液流变性能的影响. 实验发现: 在pH为9.0时, 对于固含量为30vol% ~ 40vol%的Al₂O₃悬浮液, 少量D-山梨醇的加入皆能明显提高其分散稳定性. 当D-山梨醇的添加量占分散剂总量(0.5wt%)的20%时, 悬浮液粘度最低. 且添加二元分散剂PAA/D-山梨醇的悬浮液具有更好的抗电解质性能. 分别对单一PAA分散的Al₂O₃及二元分散剂分散的Al₂O₃进行了红外光谱表征. 结合流变与红外实验结果, 分析了山梨醇的作用机理: 部分山梨醇吸附在Al₂O₃颗粒表面, 部分山梨醇与PAA以氢键形式结合, 增大了颗粒间的空间位阻, 提高了悬浮液的流变性能.     

硅氧烷改性环氧树脂复合水分散液的分散稳定性和流变性能

通过苯丙类单体与环氧树脂进行接枝聚合反应,制备了大分子链中含有羧基的水性自乳化环氧-苯丙共聚物的复合树脂,然后加入三乙醇胺中和,再进行硅氧烷改性制成硅氧烷改性的环氧树脂复合水分散液。对复合水分散液的分散稳定性和流变性能进行研究,结果表明,随着亲水单体(MAA)含量的增加,水分散稳定性提高,表观黏度增大;中和度降低,表观黏度减小,中和度在90%~110%时,水分散稳定性最好,表观黏度基本不变;在分散液稳定的前提下,硅氧烷含量增大,表观黏度逐渐减小。     

Powder dispersion and aqueous tape casting of YSZ-NiO composite

Powders of YSZ and NiO were prepared by precipitation of hydroxides, calcination and wet grinding. The dispersion conditions were optimized from zeta-potential and viscosity studies. Aqueous suspensions of both the powders exhibited maximum zeta-potential values in two ranges of pH, 3 to 4 (positive) and 9 to 11 (negative), establishing the feasibility of a dispersion of both oxides together. Viscosity measurements indicated presence of effective dispersion of a 1:1.5 mixture of YSZ:NiO below a solid concentration of 25 volume percent. Aqueous tape cast slurries were formulated using PVA solution as binder and glycerol as plasticizer. The rheological behavior of these slurries (viscosity and visco-elastic parameters) was strongly influenced by slight variation in the solid content. Visco-elastic measurements revealed all the slurries to be predominantly viscous and become more elastic with increasing solid content. Defect free flat tapes could be obtained by casting the slurry with optimum flow characteristics (i.e., pseudoplastic but non-thixotropic) that could be sintered to flat sheets with 80% T.D. (theoretical density) and 20% O.P. (open porosity) at 1450°C. Upon reduction in hydrogen at 1000°C, it formed in to YSZ-Ni composite that retained the same micro structural features of YSZ-NiO.     

氧化铝黑瓷浆料流变性能的研究

通过对氧化铝黑瓷粉料进行预烧技术处理,增大颗粒尺寸、降低粉体比表面能、改善其团聚现象,制备出固相体积分数达55%的氧化铝黑瓷稳定料浆,并研究了分散剂用量、体系固相体积分数、R值(塑性剂/粘结剂)对料浆流变性的影响.结果表明:浆料的粘度随剪切速率增加而下降,呈剪切稀化的流变学特性,随着固相体积分数的增加,浆料的最佳分散剂用量也相应增加;浆料的粘度随着R值的增加而急剧降低,流变性能得到明显改善.     

Dispersion and Rheological Properties of Alumina/Zirconia Slurries with Methyl Isobutyl Ketone/Ethanol Solvents

The dispersion and rheological behavior of alumina, zirconia, and alumina/zirconia mixed slurries were investigated using various solvent ratios of methyl isobutyl ketone (MIBK)/ethanol (EtOH), by measuring sedimentation bulk density, particle size distribution, and viscosity. Well-dispersed suspensions were obtained in MIBK-rich solvents with additional dispersant and in EtOH-rich without dispersant. The shear viscosity of the slurries was dependent on both the Al₂O₃/ZrO₂ ratio and MIBK/EtOH ratio. At a constant solvent ratio, however, similar rheological behavior was shown regardless of the relative amounts of the two solids. At low shear rate, a Newtonian plateau was absent in the Al₂O₃/ZrO₂ slurries. With increasing shear rate (>600 s^–1), Al₂O₃ slurries exhibited a Newtonian plateau while ZrO₂ demonstrated continuous shear thinning.     

SiC/纳米TiN水基复合料浆特性及喷雾干燥

以四甲基氢氧化铵(TMAH)为分散剂, 纳米TiN、亚微米SiC粉体为原料, 分别制备单组分水基料浆及复合料浆, 分析了SiC/纳米TiN复合料浆的分散稳定机制以及喷雾干燥行为. 研究结果表明, TMAH在纳米TiN、SiC颗粒表面形成特征吸附, 通过静电作用和空间位阻协同作用提高颗粒的分散性; 当pH=8、TMAH加入量为0.75wt%时, 复合浆料分散稳定, 喷雾干燥制备的SiC/纳米TiN复合粉体流动性好, 且分布均匀.     

Flow characteristics of a coal-oil-water mixture prepared by disintegration of de-ashed coal agglomerates

The flow characteristics of a coal-oil-water mixture (COW) were investigated. COW was prepared by disintegrating de-ashed coal agglomerates that had been obtained by oil agglomeration of coal. When the volume fraction of dispersed waterdrops in COW was smaller than 0.2, the dispersed waterdrops had a similar effect to that of the dispersed coal particles on the flow characteristics of COW. COW behaved as a Bingham fluid. Both the yield value and Bingham viscosity increased with increases in the total disperse-phase volume fraction in COW and with decreases in temperature. The relative viscosity of COW was well-correlated by an equation derived from Mooney's equation.     

Compression moulding of Carbon/PEEK Randomly-Oriented Strands composites: A 2D Finite Element model to predict the squeeze flow behaviour

Compression moulding of Randomly-Oriented Strands (ROS) of pre-impregnated thermoplastic composites is a process that enables the forming of complex parts with features such as ribs, thickness variations and holes in one single moulding step. This paper focuses on the macroscopic squeeze flow behaviour that occurs during forming. This mechanism rules the filling of intricate features of the mould initially empty. A 2D Finite Element model was developed to predict the squeeze flow of ROS composites. The material was modelled as a Bingham fluid and the equivalent viscosity and yield stress of three different strand sizes were determined using an inverse method. The viscosity and yield stress were found to increase with strand length. Experimental validation of the model was performed using Carbon/PEEK ROS flat samples and the average difference between experimentally measured and predicted final strain was below 5% for all cases.     

超高性能混凝土流变特性及其对纤维分散性的影响

优异的分散性能是纤维充分发挥增强增韧作用的关键。为了明确高掺量钢纤维在超高性能混凝土(UHPC)中的分散特征并提高纤维的分散性,采用抗压强度、抗弯强度等力学性能试验、混凝土流变仪以及图像分析技术,分别研究了降粘掺合料、钢纤维掺量对UHPC力学性能、流变性能以及纤维分散性能的影响。结果表明:降粘掺合料对UHPC力学性能无明显提升作用,但可显著降低UHPC基体的屈服应力和塑性粘度,同时可降低钢纤维导致的屈服应力和塑性粘度增加幅度;随着纤维掺量的增加,纤维轴向取向系数和有效利用率降低,而降粘掺合料可提高纤维轴向取向系数和有效利用率;UHPC基体的流变性能、纤维分散性能以及力学性能三者密切相关,基体流变参数越小,纤维轴向取向系数越高、纤维有效利用率越高,则UHPC力学性能越好。     

氧化铝悬浮液的分散行为及其影响因素研究

详细研究了氧化铝悬浮液的分散行为及固含量、pH值和聚丙烯酸(PAA)添加量对分散行为的影响。实验结果表明,固含量、pH值和PAA添加量对氧化铝悬浮液的分散行为有显著影响。粘度法及Zeta电位测试法所反映的悬浮液分散性随pH值和分散剂添加量的变化规律基本一致。当pH=9～10、分散剂添加量的质量分数为2.0%～2.5%时粘度最低、Zeta电位绝对值最大、分散效果最好。分散剂添加量存在最佳范围,且最佳范围不随固含量的变化而改变。随固含量的增加,粘度开始缓慢上升,到达一定程度后迅速增大。     

Effect of adsorption behaviour of polyelectrolytes on fluidity and packing ability of aqueous graphite slurries

The effect of surface hydrophobicity on the adsorption behaviour of polyelectrolytes is investigated using graphite and alumina powder slurries. Graphite slurries containing carboxymethylcellulose (CMC) have a relatively low apparent viscosity and afford a sediment with a relatively high packing fraction as compared to that obtained when using sodium polyacrylate (Na-PAA) as a dispersant, although both have the same functional group. As a greater amount of CMC is adsorbed, it is concluded that its adsorption mechanism involves hydrophobic interaction, thus making it a better dispersant for hydrophobic powders in aqueous media. In contrast, Na-PAA is more effective in dispersing relatively hydrophilic powders such as alumina, as it adsorbs mainly through electrostatic interactions.     

纳米Y-TZP形成稳定浆料的流变性质

本文研究了纳米Y－TZP超细粉体分别添加聚丙烯酸钠（NaPAA）及聚丙烯酸铵（NH₄PAA）所制备的浆料的稳定性.综合考虑Zeta电位和分散剂的解离条件,pH8～12为浆料的稳定范围.通过对浆料流变性质的测定,给出固含量与其相应的最佳分散剂用量关系,并比较了NaPAA与NH₄PAA在稳定高固含量浆料中的分散作用.结果表明,使用NH₄PAA可获得较NaPAA低的粘度,且由于它不引入杂质而有更大的优越性.对不同纳米尺寸的Y－TZP,如果吸附等量的分散剂,颗粒越大粘度越低.在一定范围内达到相同的粘度,颗粒越小,所需分散剂用量越多.