Processing alumina spheres by a colloidal route using silica-polystyrene hybrid nanoparticles

Colloidal granulation of alumina is an original shaping process leading to spherical granules directly in suspension from a diluted system. It is based on a heterocoagulation mechanism where silica added in small amount acts as a binding agent. The role of a submicronic latex of polystyrene was investigated, free, or as a hybrid object covered by silica nanoparticles synthesized by a Pickering emulsion polymerization method. Latex particles were not able to form primary agglomerates despite a high negative zêta potential. An addition of latex in combination with silica improved the state and homogeneity of the final microstructure which could be attributed to a decrease of the internal strains during the processing. Hybrid nanoparticles, by combining the binding agent (silica) and the organic phase, were able to form primary agglomerates, and to finally obtain alumina spheres exhibiting both a spherical morphology and a greater homogeneous microstructure.     

The viscosity of colloidal spheres in deionized suspension The importance of charge density and monodispersity of polystyrene spheres

Summary Reduced viscosities (specific viscosity divided by concentration) of polystyrene spheres (diam. ca. 120 nm) show the quite different concentration dependencies when their charge densities (c.d.) and monodispersities (m.d.) are different. A sharp peak, which corresponds to the transition between liquid-like and crystal-like structures, is observed for spheres having low c.d. and high m.d. The rigidities and yield stresses of crystal-like structures, which are evaluated from the steady-flow viscometry, are sensitive to c.d. and m.d. The way the viscometric properties are influenced by c.d. and m.d. of spheres demonstrates that the electrostatic intersphere repulsion and the elongated Debye-screening length around sphere are both essential for the appearance of the extraordinary crystal-like and liquid-like structures in deionized colloidal spheres.     

Analysis of the action mechanism of polymer dispersant on dense ethanol alumina suspension using colloidal probe AFM

The action mechanism of a polymer dispersant on dense Al₂O₃ ethanol suspension was investigated using a colloidal probe AFM and branched and linear polyethyleneimines (PEIs). To obtain the minimum viscosity and Newtonian flow property of the dense ethanol suspension, the optimum molecular weights of the branched PEIs were determined over the range from 10,000 to 70,000. The linear PEI with Mw 1400 did not reduce the suspension viscosity compared to the branched PEI with the same molecular weight. The amount of adsorbed PEI did not significantly change regardless of the molecular structure and weight of the PEIs. However, the surface interaction between α-Al₂O₃ solids depended on the molecular structure and weight of the PEIs. The branched chain of the PEI adsorbed on the Al₂O₃ surface facilitated the short-range steric repulsion between particles. Based on the results, the increase in steric repulsive force and the disappearance of the adhesion force by the adsorption of the polymer prompted the dispersion of aggregates in suspension and reduced the viscosity of ethanol dense suspension.     

Adsorption of synthetic polyelectrolytes on colloidal spheres

Summary The adsorption of synthetic polyelectrolytes on the surfaces of monodisperse polystyrene spheres and colloidal silica spheres is studied by electrophoretic mobility measurements. Electrolytes used are NaCl, CaCl₂, LaCl₃, Na₂SO₄, sodium poly(ethylenesulfonate) (NaPES), sodium poly(styrenesulfonate) (NaPSS), polybrene^? (PB), poly-4-vinyl-N-ethylpyridinium bromide (C2PVP), poly-4-vinyl-N-benzylpyridinium chloride (BzPVP), and copolymer of 4-vinyl-N-benzylpyridinium chloride (95%) and 4-vinyl-N-n-hexadecylpyridinium bromide (5%) (C16BzPVP). Electrophoretic velocity (u) and the effective charge number (α) of a colloidal sphere increase in the presence of PB, C2PVP, BzPVP, and C16BzPVP, and turn to the positive from the negative values in their absence. Addition of NaPES and NaPSS further decreases u and α values. Adsorption of the polymers on the colloidal spheres are explained by the hydrophobic and/or dipoledipole interactions in addition to the electrostatic forces. Weak adsorption of simple electrolytes on the colloidal spheres is deduced from the electrophoretic measurements.     

有机添加剂对氧化铝造粒粉性能的影响

讨论了有机添加剂对氧化铝造粒粉性能的影响,实验结果表明：S分散剂对氧化铝陶瓷料浆的稳定分散作用最好,适宜的添加量为0.3％-0.5％;PVA添加量为1.2％-1.5％时,料浆的粘度适中,适合喷雾造粒;消泡剂BYK-603的适宜添加量为0.2％-0.4％。另外,PH值对料浆的稳定性也起到非常重要的作用,料浆偏弱酸性时稳定...     

Influence of fibers on the microstructure and compressive response of directional ice-templated alumina ceramics

Directional ice-templated ceramics have unique lamellar porous channels between ceramic walls which run from bottom to top of the samples. This highly oriented morphology results in the anisotropy in mechanical properties both parallel and perpendicular to freezing direction. In this research, fiber-enhanced ice-templated porous ceramics were fabricated by introducing fibers into alumina slurry for freezing. The trans-lamellar fiber bridges connecting adjacent ceramic walls were formed by adjusting the aspect ratio of fibers and freezing velocity, which greatly enhanced the compressive response in two directions and weakened the anisotropy in mechanical properties of the directional ice-templated ceramics. The results showed that the of fiber bridges increased with the increase of fiber aspect ratio and the longitudinal and transverse mechanical properties increased by 73.5% and 232.6%, respectively.     

胶体黏度和纤维化工艺条件对80多晶氧化铝纤维性能的影响

采用溶胶-凝胶法制备80多晶氧化铝纤维。以结晶氯化铝、铝粉和硅溶胶为主要原料制备出黏度分别为8、14、20、26和32Pa.s的甩丝胶体;采用甩丝法进行纤维化试验,控制甩丝盘转速分别为4000、5000、6000、7000和8000r.min-1;热风温度分别为60、70、80、90和100℃;并将甩丝后的纤维坯体于1250℃烧成,检测其平均直径和平均单丝拉伸强度。结果表明:在胶体黏度为20Pa.s、甩丝盘转速为6000r.min-1、热风温度为80℃时,纤维的平均直径为3.8μm,平均单丝拉伸强度超过1000MPa,是一种性能较优异的氧化铝纤维。     

Influence of co-doped alumina on the microstructure and radioluminescence of SrHfO3:Ce ceramics

SrHfO₃:Ce and SrHfO₃:Ce,Al powders were prepared by the solid-state synthesis using the commercial HfO₂, SrCO₃, CeO₂ and α-Al₂O₃ as the starting materials. All the SrHfO₃:Ce and SrHfO₃:Ce,Al powders calcined at different temperatures within 1100 °C–1300 °C for 8 h in air exhibited a pure SrHfO₃ phase. Meanwhile, the effects of different calcination temperatures and co-doped alumina on the microstructure of SrHfO₃:Ce,Al powders were discussed. SrHfO₃:Ce and SrHfO₃:Ce,Al ceramics were successfully fabricated via the vacuum sintering at 1800 °C for 20 h. The alumina acted as the sintering aid, which promoted the densification and eliminated the intragranular pores of SrHfO₃:Ce,Al ceramics. The charge disbalance was effectively compensated by co-doped alumina. SrHfO₃:Ce,Al ceramics exhibited a higher light yield of 5700 ph/MeV compared with 4600 ph/MeV obtained in SrHfO₃:Ce ceramics. The charge traps in ceramics and their effects on scintillation properties were also investigated by thermoluminescence measurement.     

Effect of additives on the microstructure of porous alumina

Protein forming, a direct consolidation technique for shaping of powders into a rigid body, use globular protein as gelling agent, which makes the process environment friendly. The present paper combines foaming and gelling capabilities of globular protein (albumin) along with starch to consolidate the ceramic bodies to develop porous structure based on alumina. The effect of albumin and starch on the gel strength, rheological properties of alumina slurry as well as the microstructure of final structure were studied in detail. It has been observed, that albumin which gels by forming strong polymer network dominates the gel properties, and thereby the strength of the consolidated green bodies, whereas starch, being insoluble in water at room temperature, increases the viscosity of slurry, but increases stability of foamed slurry. Also starch acts as pore former to introduce connectivity between pores and hence increases open porosity. Hence, by controlling albumin and starch ratio in the slurry one can control the microstructural developments in the cast body to obtain the desired micro-structure.     

Influence of surfactant addition sequence on the suspension properties and electrophoretic deposition behaviour of alumina and zirconia

A well-known polyelectrolyte salt, ammonium polymethacrylate (Darvan-C) is used to stabilise ethanol-based Al₂O₃ and Ce-ZrO₂ suspensions with butylamine addition. The sequence in which n-butylamine and Darvan-C are added to the suspension greatly affects the properties of the wet deposit obtained by electrophoretic deposition. To investigate this effect, electrical conductivity of the suspension and the shear rate dependence of its viscosity are investigated. When n-butylamine is added first, the equilibrium in the suspension is almost immediately reached and a plastically deformable wet deposit is obtained over a large n-butylamine/Darvan-C ratio. The suspension has a shear-thinning viscosity and the deformable deposit is characterised by a high solvent content, which allows the rearrangement of particles during drying. When Darvan-C is added before the n-butylamine, the wet deposit is smooth and rigid. The suspension has a lower viscosity and a near-Newtonian behaviour is observed. A similar behaviour is observed for Al₂O₃ and Ce-ZrO₂ suspensions. The green density of the dried deposits is not influenced by the addition sequence and higher green densities are obtained for Al₂O₃ when compared to Ce-ZrO₂.     

Experiments on the suspension of spheres in inclined tubes—II. Suspension by air

Data are presented on the suspension of single spheres by air, in inclined tubes ranging in diameter from 0·97 to 5·13 cm. The angle of inclination of the tubes to the horizontal axis varied from 0 to 90°. The data, which relate the pressure drop and suspending velocity to the geometric and physical parameters of the system, are compared with data generated previously using water as the suspending fluid. Dimensionless expressions are presented which correlate both sets of data for pressure drop and suspending velocity in terms of sphere/tube diameter ratio, tube diameter, density of solid and fluid and angle of inclination of the tube.     

热处理气氛对添加铝粉的铝碳材料性能和结构的影响

为提高铝碳材料的性能,在碳结合的基础上增加陶瓷结合相,将添加铝粉的铝碳试样在不同气氛 (Ar、埋炭和N2 )中经 1300℃热处理 5h,测定试样热处理前后质量变化率、抗折强度、体积密度、显气孔率和抗水化性,并用扫描电镜和XRD分析试样的显微结构和物相组成。研究表明: 埋炭保护热处理后材料中有大量亚微米级柱状和类柱状物的晶须;Ar保护热处理后材料中有带状、锥状的氧化铝和少量晶须生成;N2保护热处理后材料中有较多粒状AlN产生;热处理气氛不同,材料显微结构明显不同,但不能避免Al4C3的生成;埋炭和N2保护热处理后,材料强度提高,显气孔率降低;N2保护热处理最有助于提高材料的抗水化性。     

Direct foaming of Al2O3-based macroporous ceramics containing pre-foamed colloidal alumina,calcite and CAC suspension

Adding pre-foamed colloidal alumina to ultrastable Al₂O₃-stabilised foams can be a path towards partially counteracting the firing shrinkage of these materials and producing macroporous ceramics with smaller pores. Nevertheless, this system still presents a long setting time and high sintering-induced shrinkage, which hinders the production of larger samples and reduces its porosity. In the present work, it was observed that adding calcium aluminate cement suspension (CAC_S) and CaCO₃ (calcite) to the aforementioned system can speed up its solidification kinetics, improve its mechanical strength and reduce its shrinkage after firing, maintaining high porosity and smaller pore sizes. By using these raw materials, samples with an average pore size below 60 μm, total porosity above 70%, and a narrower pore size distribution were attained after thermal treatment at 1600 °C for 5h. Moreover, due to the in situ formation of calcium hexaluminate, their shrinkage after sintering was almost halved (from ~20% to 11%).     

Synthesis and characterization of millimetric gamma alumina spherical particles by oil drop granulation method

An oil drop granulation process was developed to prepare porous meso-structured gamma (??) alumina (Al₂O₃) granules. The ??-Al₂O₃ granules prepared were 2?mm diameter spherical particles with low sphericity factor (0.02?±?0.01). The XRD results validated that the synthesized granule possesses crystallinity of ??-Al₂O₃ phase. The N₂ adsorption?Cdesorption isotherm and BJH pore size distribution of the synthesized granule elucidated the existence of mesoporous characteristic with unimodal pore in the mesopore region. The synthesized and characterized granules can be used as a potential support for catalyst in either moving or fluidized bed reactor.     

Effect of zeta potential on properties of foamed colloidal suspension

We reveal that zeta potential has a notable effect on properties of foamed suspension, which remains poorly explored. It is demonstrated low zeta potential is beneficial for foam stability, and the experimental results show that absolute value of zeta potential of 30–40 mV is the boundary between stable foams and unstable foams. High zeta potential above 40 mV gives rise to instability of foamed colloidal suspension, owing to strong interparticle repulsion force that prevents particles from forming a closely packed particle network at liquid/air interfaces. Besides, it is found that high zeta potential leads to low foamability of foamed colloidal suspension. It is also demonstrated herein that stable and uniform foams with thin and homogeneous-thickness wall without agglomerates can be prepared at the isoelectric point (IEP) using long-chain surfactant, which generates weak agglomerated particles that can be re-separated to monodisperse particles due to the steric hindrance effect of surfactant.     

油田采出水悬浮液微观形态研究

采用动态法测定了固体颗粒-液体的接触角及固体颗粒的润湿性,创新性地采用显微镜法观察了油-水-固体颗粒体系的微观形态。结果表明,固体颗粒的极性力为22.04 mN/m,稍强于非极性力;模拟水和原油模拟油在固体颗粒上的表观接触角分别为64.65°和74.09°,固体颗粒对模拟水和模拟油均有一定的润湿性;油-水-固体颗粒体系中存在多种形态,即:固体颗粒粘附在油珠表面、油珠附着在固体颗粒表面、固体颗粒的聚集体以及单独的油珠,且原油中含有微生物。     

The effect of the surfactants on the formulation of UV-curable SLA alumina suspension

Stereolithography (SLA) has been regarded as the most promising rapid prototype (RP) production method for ceramic parts recently because of its better precision in size and site control. The rheological behavior and curing behavior of suspensions are controlling factors for ceramic stereolithography. In this work, oleic acid (OA), stearic acid (SA) and Poly(Acrylate Ammonium) (PAA-NH₄) were studied as the surfactant for HDDA-based suspension formulation. Modified alumina particles exhibit different wettability with HDDA monomer and thus the different rheology behavior and stability between the formulated suspensions. OA shows the best performance among all three dispersants. 40 vol% alumina suspensions with a viscosity ˂3 Pa s at 30 s⁻¹ shear rate were successfully formulated with OA and SA. A sintering density of 95% can be reached for the OA- and SA-modified alumina UV-curable resins.     

Influence of CeO2 addition on the microstructure and mechanical properties of Zirconia-toughened alumina (ZTA) composite prepared by spark plasma sintering

In this study, zirconia-toughened alumina (ZTA) samples with different amounts of CeO₂ were prepared by the spark plasma sintering method. The phase composition and microstructure of the samples were examined by X-ray diffraction, scanning electron microscopy, and energy-dispersive X-ray spectroscopy. The addition of CeO₂ results in grain refinement and density increase; moreover, CeO₂ stabilises the high-temperature metastable phase. As the amount of CeO₂ reaches 7 wt%, a new CeAl₁₁O₁₈ phase appears. The Vickers hardness, modulus, and fracture toughness of the samples depend to a large extent on the grain size, relative density, and existence of the second phase. Among the composites, that with 5 wt% CeO₂ shows the best performance with the highest values of relative density, Vickers hardness, and fracture toughness: 96.51%, 1688 HV, and 9.91 MPa.√m, respectively.     

Influences of polyacrylic acid adsorption and temperature on the alumina suspension stability

The temperature influence on the alumina (Al₂O₃) suspension stability in the absence and presence of polyacrylic acid (PAA) was studied. The investigated temperature range was 15-35 °C. Stability measurements were carried out by the use of Turbiscan Lab^Expert with a thermostatic system. Additionally, at 25 °C the suspension stability was determined from the spectrophotometric data.The obtained data and calculated values of Turbiscan Stability Indexes (TSI) suggest that for the alumina suspensions without polymer, the values of TSI and the rates of solid particles settling as well as the thickness of sediment were the lowest at all investigated temperatures. Moreover the PAA adsorption causes pronounced decrease of system stability at 15 and 35 °C while at 25 °C polyacrylic acid adsorption slightly improves the alumina stability. The conformational changes of adsorbed polymer macromolecules are responsible for such behaviour of the investigated systems. At all investigated temperatures the solid suspension is not stable in the presence of PAA. In the case of PAA 2 000 the coagulation by charge neutralization takes place, whereas in the case of PAA 240 000 the bridging flocculation is the most probable process. The destabilization of systems by the adsorbed polymer occurs with the lowest effectiveness at 25 °C.     

Creep behaviour of alumina/YAG nanocomposites obtained by a colloidal processing route

The high temperature creep behaviour (1200–1400 °C and 30–250 MPa) of high-purity alumina (A) and an alumina/YAG nanocomposite (AY) prepared by using a colloidal processing route has been studied. Creep parameters were correlated with microstructural features in order to determine the dominant creep mechanisms in both materials.It was found that the creep rate value of AY was 1 order of magnitude lower than the one of undoped alumina A. The creep mechanism for AY was found to be lattice diffusion (Nabarro–Herring) compared to a combination of grain boundary (Coble) and lattice diffusion for A. When the slow crack growth region of both materials was compared, a significant improvement was observed, i.e. the slow crack growth region of alumina shifted to nearly 2.5 times the stresses applied for AY at the temperatures of 1200, 1300 and 1400 °C.