Compression and Reswelling of Nanometric Zirconia Dispersions: Effect of Surface Complexants

Dispersions of bare zirconia nanoparticles are not redispersable after a strong osmotic compression. In order to obtain a spontaneous redispersion from concentrated pastes, the surfaces of particles have been complexed by organic molecules (acetylacetone, acetic acid, citric acid, and diethanolamine) prior to compression. Controlled cycles of deswelling-reswelling have been performed using the osmotic stress method in order to test the reversibility of the compression. Osmotic pressure and small-angle X-ray scattering measurements indicate that the chemical nature of a complexant is of the highest importance to control the interaction potential between particles.     

Adhesion of Fine Particles in Dispersions

When a dispersion of fine particles is concentrated, the product can contain clumps which arise from the aggregation of the particles. There are several drivers: sedimentating, drying, filtration, forcing the particles together to produce agglomerated structures which are much larger than the primary particles. The problem of understanding this phenomenon is twofold: on the one hand, it is difficult to measure aggregates in a concentrated slurry; on the other, there is no theory to predict when aggregates should form in an apparently-stable dispersion. This paper describes a new experimental method for measuring aggregates in concentrated suspensions, showing that the aggregation phenomenon can be followed over a wide range of experimental conditions. In particular, the results show that the aggregates exist at small concentrations in ostensibly stable dispersions even before concentrating takes place. Colloids based on polymers, ceramics, biological cells and emulsions all showed this aggregation effect. We have called these aggregated structures “multiplets” to distinguish them from the more normal flocs produced by destabilising the colloid. A theory of aggregation is proposed to fit the experimental results. This theory is based on the idea that multiplets form as a consequence of small adhesion forces between particles immersed in liquid; a molecular dynamics simulation using this concept of adhesion forces is used to demonstrate the formation of multiplet material at low concentrations. The theory seeks to show how the size of multiplets should vary with adhesion and with particle concentration.     

Ti-ZrO2系梯度材料的制备与显微结构研究

采用粉末冶金法热压合成了Ｔｉ－ＺｒＯ２系梯度功能材料,用有限元法模拟了设计材料的残余热应力分布。利用扫描电镜、电子探针,透射电镜和Ｘ射线衍射等手段,对Ｔｉ－ＺｒＯ２系梯度材料组成相的分布形态、相组成以及组元之间的界面结构状态等进行了详细的研究,结果表明：合成的材料具有宏观组织不均匀性与微观组织连续性的特征,Ｔｉ－ＺｒＯ２系梯度材料由α－Ｔｉ四方相ＺｒＯ２和单斜相ＺｒＯ２组成,合成条件下组元Ｔｉ和Ｚ     

Examination of rheological properties of fine particles as carriers for controlled drug release

Development of lipid-based fine particles as potential drug carriers requires detailed investigation of possible effects of these carriers on rheological properties of blood. In this study, we have investigated the influence of dynamic conditions on aggregate formation and stability in dispersions of lipid-based fine particles in whole blood under in vitro conditions. Rheological parameters of two concentrations of liposome dispersion and two concentrations of lipid emulsion in blood were studied by assessing shear stress/shear rate relationships. The magnitude of attractive interactions between aggregates and/or particles, A, and the effective-to-real volume fraction of particles, ? f /? p , were estimated for rheological quantification of lipid-based fine particles-blood interactions and aggregate stability. Addition of lipid-based particles induced aggregate formation in blood, which was more pronounced at higher concentrations of lipid-based fine particles. Furthermore, larger and more stable aggregates were formed in liposome dispersions as compared to lipid emulsions in blood.     

Creep of MgO Containing a Dispersion of Carbon

Creep experiments on polycrystalline MgO containing a fine dispersion of carbon are reported. The samples were prepared by hot-pressing and creep was measured in four-point bending under stresses of 1000 to 5000 psi at 1000° to 1300° C. Although results for MgO creep without carbon addition are in good agreement with published reports, introducing dispersions of 0.5 to 2 vol% C into MgO modifies its creep behavior. Comparing specimens of the same grain size, the samples containing the carbon dispersion had lower creep rates than those of MgO without addition. This effect increased with increasing carbon additions. The stress exponent remained small (1 to 2) in all cases. The apparent activation energy for creep increased dramatically with the carbon additions. Possible mechanisms for the influence of this dispersion on creep are discussed briefly.     

EXAMINATION OF RHEOLOGICAL PROPERTIES OF FINE PARTICLES AS CARRIERS FOR CONTROLLED DRUG RELEASE

Development of lipid-based fine particles as potential drug carriers requires detailed investigation of possible effects of these carriers on rheological properties of blood. In this study, we have investigated the influence of dynamic conditions on aggregate formation and stability in dispersions of lipid-based fine particles in whole blood under in vitro conditions. Rheological parameters of two concentrations of liposome dispersion and two concentrations of lipid emulsion in blood were studied by assessing shear stress/shear rate relationships. The magnitude of attractive interactions between aggregates and/or particles, A, and the effective-to-real volume fraction of particles, ϕ f /ϕ p , were estimated for rheological quantification of lipid-based fine particles-blood interactions and aggregate stability. Addition of lipid-based particles induced aggregate formation in blood, which was more pronounced at higher concentrations of lipid-based fine particles. Furthermore, larger and more stable aggregates were formed in liposome dispersions as compared to lipid emulsions in blood.     

Effects of concentration of dispersions on particle sizing during production of fine particles in wet grinding process

Stirred media milling is a prospective technology for producing colloidal dispersions by means of wet grinding process. In the past, many researchers have studied the effects of different operating parameters such as size, shape, nature and quantity of grinding medium, the speed of agitator in grinding chamber, the feed rate of dispersions, etc. in stirred media mills. However, it is still less known how particle sizing which generates valuable information of particle size of the product to interpret, control and optimize the grinding process, is influenced by the concentration of the dispersion during stirred media milling where particles change their size from micron to colloidal range rapidly. One of the reasons of this lack had been our incapability in the past to study the particle size distribution of dispersions without dilution. The recent advent of acoustic attenuation spectroscopy is known to be capable of studying dispersions without dilution, under real process conditions and on line. The study employs acoustic attenuation spectroscopy to investigate the effects of concentration of dispersions of CaCO₃ on its particle sizing during size reduction process in a stirred media mill (LabStar manufactured by NETZSCH). The dispersions of CaCO₃ at 5%, 10%, 20% and 30% (m/m) were studied about six hours under a selected set of operating conditions. Contrary to the existing knowledge obtained through other techniques of particle sizing that are based on the principle of dilution, acoustic attenuation spectroscopy shows that, under certain grinding time at given operating conditions, increase in concentration of dispersion results in better grinding results yielding smaller particles. The causes behind the differences in results of acoustic attenuation spectroscopy and dynamic light scattering have been thoroughly investigated. We find certain limitations of acoustic attenuation spectroscopy in particle sizing. A typical phenomenon which causes misleading trends in particle sizing is multiple scattering in acoustic measurements. Multiple scattering, particularly, influences acoustic results when particles approach to fine size range during size reduction process.     

The Presence of Adsorbed Proteins on Particles Increases Aggregated Particle Sedimentation,as Measured by a Light Scattering Technique

Protein conformational changes are often induced when bound to surfaces and can modulate colloidal stability of protein coated particles in dispersion. We evaluated bovine serum albumin (BSA) adsorbed onto polystyrene particles at room temperature. A z-axis translating laser light scattering device (ZATLLS) measured the sedimentation velocity of protein-coated particles tracking aggregation characteristics compared with non-coated ones. Sedimentation velocities of particles moving in the dispersion, and the resulting viscosity and density of the residual solution following sedimentation determined aggregate size in the dispersion using Stoke's law. Our experiments objectively show that albumin-coated polystyrene forms aggregates. Interestingly, coating particles with protein slows the sedimentation velocity which should correspond to a more dispersed system, but it leads to higher aggregate sizes due to the larger influence of proteins in solution raising solution viscosity. Protein-bound particles were observed to fall out of solution in a more controlled and steady manner compared with uncoated particles.     

Reinforcement of model filled elastomers: synthesis and characterization of the dispersion state by SANS measurements

This work is the first part of a study devoted to the understanding and the determination of the molecular mechanisms that are at the origin of the specific properties shown by reinforced elastomers. Different model filled elastomers composed of cross-linked polyethylacrylate chains reinforced with grafted silica nanoparticles were prepared varying the reactivity of the coupling agent with the ethylacrylate monomers. They were synthetized applying and adapting the method developed by Ford et al. [11] which consists to polymerize a colloidal suspension of grafted silica particles in acrylate monomers. In this paper we will present how filled elastomers having different dispersion states can be prepared whilst keeping the same interactions between the particles and the polymer chains. The dispersion states were characterized by Small Angle Neutron Scattering. We found that there are two opposite effects which control the final dispersion state of these filled elastomers during the polymerization. The first one is a depletion mechanism favoring the formation of aggregates. The second one is a repulsive steric interaction due to the growth of polymer chains from the particle surfaces avoiding contacts between the silica inclusions. Using these results we can prepare sets of samples having the same particle/matrix interface but different dispersions states. By comparing their mechanical properties we should to able to estimate the relative weight of the dispersion state quality and the one of the particle/matrix interface on the mechanical behavior of these filled elastomers.     

Effects of mixing state of composite powders on sintering behavior of cathode for solid oxide fuel cells

For efficient development of high-performance composite electrodes for solid oxide fuel cells (SOFCs), it is crucial to precisely tailor the microstructural features of the electrodes, such as their grain size, phase connectivity, and pore structure. Herein, we report the effects of the mixing state of component powders of a composite cathode composed of Sr-doped LaMnO₃ (LSM) and yttria-stabilized zirconia (YSZ) on its sintering behavior. LSM-YSZ composite powders were synthesized by a particle-dispersed glycine-nitrate process using YSZ particles as inclusions in the LSM precursor solution. The dispersion state of the YSZ particles in the solution was varied from a well-dispersed state to a highly flocculated state through adjustment of the amount of adsorbed polyethylene glycol. The dispersion state of the component powders was found to strongly impact the densification behavior of the composite, which was explained by the formation of a continuous network of the “slow-sintering” inclusion particles. A highly porous structure with phase connectivity and sufficient triple phase boundaries could be achieved by enhancing the mixing homogeneity and optimizing the mixing scale. The proposed concept provides new insights into the microstructural evolution of composites in constrained sintering, and it could potentially enable development of the ideal electrode structure for SOFCs.     

Inline rheological behavior of dispersed water in a polyester matrix with a twin screw extruder

Solvent‐free extrusion emulsification (SFEE) is a complex process using twin‐screw extrusion to prepare solid‐liquid dispersions of high viscosity polymers and has received little study to date on its inherent mechanisms. To gain rheological insights into the earliest stage of SFEE as the interfacial boundary between water and polymer grows, prior to phase inversion, an inline orifice‐plate type viscometer is introduced to monitor transient behavior over a wide range of viscosities. The presented work examines rheological changes of a polyester‐water system produced by varying two factors thought to significantly control the final state of the dispersion, specifically polar group contributions to surface energy and viscosity. A processing modifier was combined with the polyester to study the influence of these two factors. The inline viscometer revealed an abrupt transition in viscosity occurred with the developed state of water dispersion, confirming observations of a prior batch study. Analysis of the rheological response indicated that a higher polar surface energy contribution had the greatest influence on the state of this transition, and that a steeper transition was related to greater incorporation of water within the polyester matrix. POLYM. ENG. SCI., 2017. © 2017 Society of Plastics Engineers     

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.     

Rheological study of aqueous dispersions of in situ gelable thermosensitive polymer nanogels

Thermosensitive poly(N‐isopropylacrylamide‐co‐acrylamide) nanogels with varied monomer compositions were prepared by precipitation polymerization. The aqueous dispersions of these thermosensitive nanogels (9 wt% of nanogel particles in phosphate‐buffered saline solution) exhibited in situ gelable characteristics. A steady‐state and dynamic rheological analysis demonstrated that the nanogel dispersions were typical pseudoplastic liquids at room temperature. The viscosity of the nanogel dispersions at a constant shear rate increased with an increase of the acrylamide content. When the temperature was increased to 37°C, the pseudoplastic liquid turned into an elastic solid, as the result of a sol–gel phase transition of the nanogel aqueous dispersion during the raise of the temperature. Dynamic temperature ramp circle curves suggest that the sol–gel transition was reversible but with a clear hysteresis. POLYM. ENG. SCI., 2009. © 2008 Society of Plastics Engineers     

倒锥型液固流化床减小液相返混型的研究

It is found analytically that the parabolic radial profile of liquid velocity in cylindrical liquid-solid fluidized bed (LSFB) causes particles to circulate around some radial position. This is the main reason for liquid-phase axial dispersions. The liquid-phase axial dispersion is depressed as the liquid velocity presents a flatter Bessel radial profile in a converging taper LSFB. The void fraction increases with axial distance in converging taper LSFB.The behavior produces less liquid-phase axial dispersion. Exverimental results show good coincidence.     

A new discretization of space for the solution of multi-dimensional population balance equations: Simultaneous breakup and aggregation of particles

In this work, we show that straight forward extensions of the existing techniques to solve 2-d population balance equations (PBEs) for particle breakup result in very high numerical dispersion, particularly in directions perpendicular to the direction of evolution of population. These extensions also fail to predict formation of particles of uniform composition at steady state for simultaneous breakup and aggregation of particles, starting with particles of both uniform and non-uniform compositions. The straight forward extensions of 1-d techniques preserve 2ⁿ properties of non-pivot particles, which are taken to be number, two masses, and product of masses for the solution of 2-d PBEs. Chakraborty and Kumar [2007. A new framework for solution of multidimensional population balance equations. Chemical Engineering Science 62, 4112-4125] have recently proposed a new framework of minimal internal consistency of discretization which requires preservation of only (n+1) properties. In this work, we combine a new radial grid [proposed in 2008. part I, Chemical Engineering Science 63, 2198] with the above framework to solve 2-d PBEs consisting of terms representing breakup of particles. Numerical dispersion with the use of straight forward extensions arises on account of formation of daughter particles of compositions different from that of the parent particles. The proposed technique eliminates numerical dispersion completely with a radial distribution of grid points and preservation of only three properties: number and two masses. The same features also enable it to correctly capture mixing brought about by aggregation of particles. The proposed technique thus emerges as a powerful and flexible technique, naturally suited to simulate PBE based models incorporating simultaneous breakup and aggregation of particles.     

Osmotic Consolidation of Suspensions and Gels

An osmotic method for the consolidation of suspensions of ceramic particles is demonstrated. Concentrated solutions of poly(ethylene oxide) are separated from a suspension of ceramic particles by a semipermeable membrane, creating a gradient in solvent chemical potential. Solvent passes from the suspension into the polymer solution, lowering its free energy and consolidating the suspension. Dispersions of stable 8-nm hydrous zirconia particles were consolidated to over 47% by volume. Suspensions of α–alumina in three states of aggregation (dispersed, weakly flocculated, and strongly flocculated) were consolidated to densities greater than or equal to those produced in conventional pressure filtration. Moreover, the as–consolidated alumina bodies were partially drained of fluid during the osmotic consolidation process, producing cohesive partially dried bodies with improved handling characteristics.     

不同工艺制备ZrO2—Al2O3复合陶瓷超细粉体的研究

采用正滴定工艺，反滴定工艺和水解工艺来制备ＺｒＯ２－Ａｌ２Ｏ３系复合陶瓷超细粉体。研究了制备工艺对水合氧化锆凝胶的包裹状态，煅烧后粉体中ＺｒＯ２颗粒的弥散状态以及烧结体显微结构的影响。结果表明采用水解工艺，ＺｒＯ２颗粒能均匀弥散在Ａｌ２Ｏ３颗粒周围，最终获得均匀细晶的陶瓷烧结体；在反滴定工艺中，虽然水合氧化锆凝胶能较好包裹Ａｌ２Ｏ３颗粒，但由于Ａｌ２Ｏ３颗粒本身得不到有效分散，因此在烧结体中出现了     

Axial and lateral dispersion of fine particles in a binary-solid riser

Axial and lateral mixing of fine particles in a binary-solid riser have been investigated using a phosphor tracer method. The measured bimodal residence time distribution (RTD) demonstrated two types of axial dispersions of the fines: the dispersion of discrete particles and that of clusters. A proposed one-dimensional, bimodal dispersion model describes the bimodal RTDs very well. The axial Peclet number of the fines is not sensitive to the fraction of coarse particles, gas velocity and solids circulation rate. Lateral solids dispersion was determined by measuring the solids RTD at different radii using a point source tracer injection. A two-dimensional dispersion model describes the measured RTDs satisfactorily. Lateral solids mixing decreased as coarse particles were added into the riser. Correlations of the axial and lateral Peclet numbers obtained fit the experimental data well.     

Separation of buoyant particles from an aqueous dispersion of corn germ particles using a bubble column

Experiments were conducted to study the use of a bubble column to separate oil from dispersions of corn germ particles in buffered aqueous solution. Particles and aggregates as large as 1 mm which contained oil droplets about in size were suspended in the dispersions. The dispersion was subsequently heated, dosed with enzyme and pumped into a bubble column. Buoyant fine particles and aggregates were lifted through the 2.9 L of the dispersion in a column by nitrogen bubbles and were incorporated into a foam layer at the top of the liquid. The foam drained from the column through a port a few cm above the top of the dispersion and was collected and subsequently centrifuged to separate a free oil layer. The oil yields were comparable to those obtained by centrifuging entire dispersions churned in an incubator/shaker without the bubbling and foaming. With only endogenous surfactant present in the dispersion, the collected foam comprised a quarter of the dispersion mass and about 3/4 of the dispersion's oil (half as a separate oil layer after centrifugation). The rate of free oil collection was the same whether or not the dispersion was bubbled for several hours prior to foam collection.     

Dispersion polymerization of aniline using hydroxypropylcellulose as stabilizer: role of rate of polymerization

Hydroxypropylcellulose (HPC) has been used as a steric stabilizer for preparing polyaniline dispersions using the route of oxidative dispersion polymerization of aniline. Using strongly acidic conditions (1 mol l^?1 HCl), low temperature of about 2 °C and a concentration of aniline as low as 0.5%, ammonium peroxodisulfate at 1.25% and hydroxypropylcellulose concentrations at 0.5–1 g d l^?1, unstable dispersions were obtained not only in water but also in aqueous alcohols (ethanol and methanol) up to at least 70 vol% alcohol. In contrast, dispersions that remained stable for at least 72 h were obtained when the alcohol concentration of the medium was as high as about 80 vol%. Kinetic studies of the polymerization systems suggested that success in the latter case was due to a lowering of the rate of polymerization. Transmission electron microscopy studies showed that dispersion particles prepared in 80 vol% alcohol media are spherical in shape and their diameter decreases with increasing stabilizer concentration. However, a change of morphology from spherical to aggregated needle‐shaped was observed when the rate was increased by increasing the aniline concentration from 0.5% to 0.75% g d l^?1 in the above recipe. The aggregated particles were found to be broken down to spherical nanoparticles when the as‐prepared dispersions were sonicated for about 30 min. The sonicated dispersion on drying showed the presence of fractal clusters of polyaniline particles in the dried film. The fractal dimension was determined to be 1.77 which agreed well with the theoretical value determined by computer simulation based on a diffusion limited cluster–cluster aggregation model in three dimensions. © 2001 Society of Chemical Industry