Acid acting as redispersing agent to form stable colloids from photoactive crystalline aqueous sol–gel TiO<Subscript>2</Subscript> powder

In this work, the redispersion of three nanocrystalline TiO₂ colloids is studied: one pure and two Fe-doped titania. These three colloids are produced by an easy aqueous sol–gel synthesis using precipitation-acidic peptization of Ti precursor. For the two Fe-doped TiO₂, one is doped during synthesis (primary doping) and the other is doped after the synthesis (secondary doping). The initial colloids are composed of crystalline TiO₂ particles around 7?nm with good photocatalytic properties, tested on PNP degradation under visible light (wavelength >390?nm). The powders obtained by air drying of these three colloids are redispersed in water to produce colloids, which are compared to the initial colloid produced. For each colloid, five cycles of drying redispersion are achieved. The colloids are characterized by dynamic light scattering, zeta potential measurements, inductively coupled plasma–atomic emission spectroscopy, X-ray diffraction, nitrogen adsorption–desorption measurements, Mössbauer spectroscopy, diffuse reflectance spectroscopy, and photocatalytic tests. The results show that similar products are obtained between the cycles, maintaining homologous properties of colloids. This property of redispersion is mainly due to the acid (HNO₃, HCl, or H₂SO₄) which protonates the surface of the TiO₂ nanoparticle leading to high-surface charges and electrostatic repulsions between aggregates. This property can be very useful for industrial applications of this synthesis, especially as it allows the volume and weight to be reduced for transportation and storage. Moreover, results show that the pure TiO₂ powder can be doped during its redispersion step. The redispersion of the TiO₂ developed here is possible without surface functionalization or multiple step processes, contrary to commercial Degussa P25. A 2-year stability study of all the produced colloids has been performed by following the evolution of the macroscopic aspect and the physicochemical properties of these sols. This study showed high stability of the produced colloids.

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Aggregation phenomena in the suspension polymerization of VCM

In the suspension polymerization of VCM, insoluble polymer particles are formed inside the monomer droplets. The growth and aggregation of these particles are responsible for important polymer properties, such as porosity. It is well established that the most characteristic polymer particles, the primary particles, are of a narrow distribution with a size (diameter) ranging from 0.10–0.20 m. This work studied the formation of primary particles based on the aggregation phenomena that take place inside a monomer droplet. This was done by formulating a population balance equation, which was based on the following considerations: a) polymerization occurs in both the monomer and the polymer phases; b) there is continuous formation of the basic particles in the monomer phase; c) the growth of the polymer particles occurs as a result of both polymerization in the polymer phase and aggregation of the particles; d) the colloidal properties of the particles that are responsible for the aggregation phenomena were considered to be the net result of attraction and repulsion energies.It was shown that for particles carrying a constant charge it was not possible to predict the formation of primary particles of size 0.10–0.20 m. The particle size distribution had a mode diameter equal to the diameter of the basic particles. Consequently, the particle charge was allowed to vary in a way proportional to the particle radius raised to a power coefficient. For values of the coefficient greater than zero, i. e., when the particle charge increased during polymerization, the aggregation of the basic particles was efficient enough to result in the formation of large primary particles.     

Disperse Composition of Titanium–Magnesium Catalyst Particles and <Emphasis Type="Italic">trans</Emphasis>-1,4-Polyisoprene Granules in Short-Time Polymerization

The disperse composition of trans-1,4-polyisoprene granules and supported titatium–magnesium catalyst particles in the ultrarapid polymerization of isoprene within 0.1–0.7 s is studied. It is shown that within this period the alteration of external and internal fragmentations occurs between two fractions of polymer granules that are formed by 0.1 s of polymerization and already contain significantly fragmented catalyst particles. The correlation between these processes and molecular mass characteristics of trans-1,4-polyisoprene is investigated. It is found that the external fragmentation is accompanied by a decrease in the average molecular masses of the polymer, while the internal fragmentation leads to formation of a higher molecular mass trans-1,4-polyisoprene. As a result, the fraction of polymer granules with a diameter of 7.5 μm is formed by 0.7 s of polymerization and replication to high conversions is developed on their basis.     

Influence of nonionic emulsifier included inside carboxylated polymer particles on the formation of multihollow structure by the alkali/cooling method

The influence of nonionic emulsifier, included inside styrene-methacrylic acid copolymer [P(S-MAA)] particles during emulsion copolymerization, on the formation of multihollow structure inside the particles via the alkali/cooling method (proposed by the authors) was examined in comparison to emulsifier-free particles. It was clarified that the nonionic emulsifier included inside the P(S-MAA) particles eased the formation of multihollow structure.Part CCL of the series studies on suspension and emulsion     

Structure of Redispersible SnO2 Nanoparticles

Suspensions of undoped SnO₂ nanoparticles and containing Eu³⁺ ions were prepared by a sol-gel procedure. Using the classical synthesis method (precipitation), the particles tend to grow by a coarsening process in order to minimize the surface free energy. This effect can strongly be reduced by the addition of an amide and surfactant during the synthesis, which decreases the surface free energy of the colloidal particles. These additives promote the formation of powders composed of very small primary particles formed by a crystallite of 10 Å, and exhibit good redispersion properties. The local and long order structures of the redispersible powder were studied by X-rays absorption spectroscopy at Sn L_I edge and X-rays diffraction, respectively. The structure of the colloidal aggregates in suspension was investigated by small angle X-rays scattering (SAXS). SAXS results indicate the sol are composed by a polidisperse system of hard spheres resulting of agglomeration of the primary particles and their size increasing by agglomeration for progressively higher Eu³⁺ content.     

Colloidal stability of electrostatically stabilized sol particles. Part I: The role of hydration in coagulation and repeptization of ferric hydroxide sol

FeO(OH)·0.5 H₂O powders were prepared by drying portions of a FeO(OH)·0.5 H₂O sol at different relative vapor pressures, and the adsorption of water on the powders was determined. The magnitude of electrostatic potential barrier for the sol is of about 9 mJ·m^–2. The reduction in the immersion enthalpy and in the surface free-energy in a range of the relative vapor pressures of 0.0–0.9 is as high as 140 mJ·m^–2 and 130 mJ·m^–2, respectively.It follows from the foregoing that two potential barriers may form. The electrostatic barrier presumably regulates the rate of flocculation and the hydration barrier (at closer separations) regulates the rate of particle coalescence or sintering.Peptizability of the FeO(OH)·0.5 H₂O powders dried at relative vapor pressures between 0.4 and 0.9 was found to be fairly high, presumably because the adsorbed water prevented the formation of close contacts between the primer particles. Lowering the vapor pressure, however, resulted in a notable decrease in the peptizable amount, and also a considerable increase in the particle size of the peptized sol.     

The effects of preparation method on the characteristics of alumina-zirconia powders

Two series of Al₂O₃-ZrO₂ powders with various contents of ZrO₂ were synthesized by sol-gel (chemical polymerization) and coprecipitation methods. The effect of ZrO₂ content and preparation method on the structure and texture of the resultant powders were determined. The samples were characterized by nitrogen sorption, thermal gravimetric analysis, X-ray diffraction, scanning electron microscopy, and Fourier transform infrared spectroscopy. The sol-gel method yields more homogeneous powder and retards the aggregation of particles. It produces powders with a larger surface area, a smaller pore size, and a narrower pore size distribution than the coprecipitated one. The X-ray diffraction results show that the powders are amorphous at the calcination temperature below 700°C for both methods. Incorporation of zirconia into alumina greatly affects the surface properties of the powders upon heating, exerting a protective effect against sintering, and inhibiting both the crystallization of the -Al₂O₃ phase and the - to -phase transformation. Zirconia is dispersed in alumina matrix and does not form a new structure.     

Photocatalytic properties of ZnO powders synthesized by conventional and microwave-assisted solution combustion method

Zinc oxide (ZnO) powders have been prepared by solution combustion synthesis method using conventional and microwave ignition routes. The effects of starting solution acidity on the combustion behavior, phase evolution, microstructure, optical properties and photocatalytic performance were investigated by thermal analysis, X-ray diffractometry, electron microscopy and diffuse reflectance spectrometry techniques. The chelated species in dried gels were predicted by theoretical calculations and confirmed by Fourier transform infrared spectroscopy. The combustion reaction rate increased with the increase of pH values. Single phase and well-crystalline ZnO powders were achieved by both of ignition methods regardless of pH values. The hexagonal particles (200–80?nm) formed by microwave ignition were larger than the spherical particles (60–40?nm) in conventional heating. Conventionally combusted ZnO powders exhibited higher photocatalytic activity under ultraviolet irradiation, due to their narrower band gap and smaller particle size.

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C/C0 vs. irradiation time for photodegradation of MB dye under ultraviolet light irradiation by the as-combusted ZnO powders (filled symbols present conventional combusted powders and open symbols are for the microwave combusted powders)

     

Kinetics of Structurization Processes in Three-Phase Disperse Systems under the Dynamic Conditions (Vibration) during Mixing

Regularities of the formation kinetics of the three-phase disperse systems of the solid–liquid–gas type under dynamic conditions were considered. The mechanism of the action of continuous shear and vibration (with various frequencies and amplitudes) on the formation and breakage of dispersed structures in such systems was studied. The interrelation of contact interactions between the particles and particle aggregates with the bulk rheological properties of three-phase disperse systems during mixing of finely dispersed solid phases with liquid medium and consequent compaction of dispersions was established. The revealed regularities of the formation dynamics of three-phase disperse structures provide the basis for chemical technology of highly concentrated disperse systems and highly filled disperse composites.     

Solvent replacement (decane → hexane) in concentrated silver organosols stabilized with sodium bis(2-ethylhexyl) sulfosuccinate

The structure-related properties of silver nanoparticles synthesized in reverse micellar solutions of sodium bis(2-ethylhexyl) sulfosuccinate (AOT) have been studied in the processes of electrophoretic concentraton of nanoparticles, drying of concentrates, and redispersion in n-hexane and n-decane. It has been shown that the dispersity of nanoparticles and stability of organosols to aggregation remain preserved upon the replacement of a low-volatile solvent (decane) by a high-volatile solvent (hexane). The developed procedures enable one to easily regulate important technological characteristics of metal-based inkjet “nanoinks,” such as concentration and size of particles, as well as viscosity and drying rate of the inks.     

Modeling of a counterflow plasma reactor

Modeling of a counterflow plasma reactor is presented, using liquid injection for the synthesis of fine particles. An experimental reactor has been developed in this laboratory, and feasibility has been demonstrated for synthesizing advanced ceramic powders. The flow field calculations show two major recirculating regions which are of importance for increasing the particles' residence time inside of the reactor. In addition, the temperature within these recirculation zones remains relatively uniform. For simulation, water droplet trajectories have been calculated for droplets produced by an injection probe. It is shown that the droplets in a size range below 50 m in diameter will follow the streamlines and evaporate completely within a short traveling distance. This finding suggests that this reactor configuration provides a favorable environment for the synthesis of fine particles using liquid precursors.     

Preparation and Characterizations of ZnTiO<Subscript>3</Subscript> Powders by Sol–Gel Process

ZnTiO₃ powders were prepared by a sol–gel method. The gelation conditions and the gelation mechanism were investigated. The crystallization behavior and characteristics of the ZnTiO₃ were also investigated in detail. The experimental results show that homogenous and translucent gels can be prepared when the gelation conditions are appropriate and the gelation temperature remains constant. The gel structure can be described as a network of Ti and O, in which zinc ions are well distributed. The ZnTiO₃ phase can be detected by XRD in the powders calcined above 700 °C. The formation of ZnTiO₃ is a slow reaction process, which leads to the development of large ZnTiO₃ particles, with dimensions after calcination at 900 °C for 2 h in the range of 30–50 m.     

Thermostimulated transformations of highly disperse powders of platinum group metals in an argon atmosphere

The behavior of palladium, rhodium, iridium, and platinum powders during their heating in an argon atmosphere was studied by differential scanning calorimetry (DSC). The DSC curves showed the exothermal peaks corresponding to the enlargement of the platinum group metal powders under study. Electron microscopy and XRD studies revealed that the particle size of highly disperse powders changed during the heating. Chemisorbed oxygen was removed from the powder surface above 800 K. During the heating of the mechanical mixture of highly disperse platinum and palladium powders, a solid solution formed with an exothermal effect.     

甲基丙烯酸对聚合物乳胶粉再分散性影响机理

采用半连续种子乳液聚合法,以甲基丙烯酸(MAA)为壳层亲水功能单体,合成了丙烯酸酯原乳液,并通过喷雾干燥法制得具有可再分散性的聚合物乳胶粉.讨论了原乳液粒子粒径随pH值和MAA量的变化关系;重点研究了MAA量对乳胶粉水分散液稳定性、再分散乳液zeta电位、乳胶粒粒径分布及乳胶粉内部微观形貌的影响,并分析其作用机理.研究结果表明:原乳液粒子粒径随pH值的增大逐渐增大,且MAA含量越高,粒径增幅越大;随MAA量增加,再分散液稳定性增强,zeta电位绝对值增大,平均粒径逐渐变小,乳胶粉再分散性显著改善.透射电子显微镜(TEM)结果显示:当MAA含量较高时,乳胶粉内部出现较大孔径的中空微孔结构.中空微孔结构提供水分向乳胶粉内部扩散通道,因而优化其水分散性,再分散乳液的"绒毛结构"与较高的zeta电位赋予其优异的分散稳定性.     

Formation of “monodispersed” SnO2 powders of various morphologies

Several procedures for the preparation of colloidal SnO₂ powders, consisting of particles of narrow size distribution and of various morphologies, are described. The hydrolysis of SnCl₄ aerosols in the presence of gaseous ammonia produces poorly crystalline SnO₂ spheres of modal diameters <2 m. Depending on the preparation conditions, the forced hydrolysis of acidic (HCl) SnCl₄ solutions yields either prismatic particles or spherulites composed of tiny, strongly aggregated, elongated subunits. In both cases the particles are in the nanometer size range (<100 nm) having crystalline rutile structure. Rod-like SnO₂ particles (0.3 m) of good crystallinity can be obtained by hydrolysis of SnCl₄ solutions in the presence of formamide. It is concluded that the hydrolysis of dissolved SnCl₄ tends to produce elongated SnO₂ particles which, depending on the experimental conditions, may be at different aggregation states.     

Preparation and Characterization of Forsterite (Mg2SiO4) Xerogels

Mg2SiO4 gels were prepared from alkoxide precursors, and the formation of the forsterite crystal phase was studied after heat treatments up to 1200°C. Prehydrolyzed TEOS in solution with 2-methoxyethanol was mixed with Mg(OEt)2, and the solution was hydrolyzed using excess water. The resultant gels were dried at 100°C to form xerogels which were subsequently powdered. These powders were characterized using thermal analysis (DTA and TGA), surface area analysis (BET), X-ray diffraction (XRD) and transmission electron microscopy (TEM).DTA and XRD indicated that forsterite crystallized at 770°C, and by 1000°C the powders were predominantly crystalline. BET gave powder surface areas between 400 and 550 m2 g–1. TEM revealed angular particles with sizes between 0.2 and 2 m. The low temperature of crystallization of forsterite indicates a high degree of intimate mixing between the precursor alkoxides, although XRD indicated some degree of inhomogeneity.     

Effect of microcrystallite preparation conditions on the formation of colloid crystals of cellulose

Stable colloidal suspensions of cellulose microcrystallites may be prepared from filter paper by sulfuric acid hydrolysis. Above a critical concentration, the suspensions form a chiral nematic ordered phase, or colloid crystal. The preparation conditions govern the properties of the individual cellulose microcrystallites, and hence the liquid crystalline phase separation of the cellulose suspensions. The particle properties and the phase separation of the suspensions were strongly dependent on the hydrolysis temperature and time, and on the intensity of the ultrasonic irradiation used to disperse the particles. The particle size of the microcrystallites was characterized with transmission electron microscopy and photon correlation spectroscopy. The surface charge was determined by conductometric titration. It was possible to fractionate the microcrystallites by size using the partitioning between isotropic and liquid crystalline phases; the longer microcrystallites migrate to the liquid crystalline phase     

Mechanical activation of aluminum: 2. Size,shape, and structure of particles

The structure of active Al/C composite prepared by the mechanochemical method from aluminum and graphite powders (10–30 wt % C) is studied by scanning and transmission electron microscopies, atomic force microscopy, local elemental analysis, X-ray diffraction, as well as by adsorption and sedimentation measurements. It is established that the particles of chemically active Al/C composite represent porous plate-like aggregates with a mean size smaller than 2.5–5 m and composed of nanocrystalline aluminum blocks with a size of 15–60 nm distributed in a loose amorphous carbon. Single aluminum particles with a size of up to 10 nm are also observed; however, their weight fraction is small.Translated from Kolloidnyi Zhurnal, Vol. 66, No. 6, 2004, pp. 819–828.Original Russian Text Copyright © 2004 by Streletskii, Pivkina, Kolbanev, Borunova, Leipunskii, Pshechenkov, Lomaeva, Polunina, Frolov, Butyagin.     

Film Formation and Redispersion of Waterborne Latex Coatings

Poly(vinyl acetate-co-ethylene) latex dispersions are prepared and their films investigated with a focus on the effect of composition upon redispersion. Films of dispersions containing sufficient amounts of poly(vinyl alcohol) (PVA) can be redispersed in water. This property is lost in the presence of surfactant, a fact which suggests a procedure to control film formation. It is demonstrated that redispersion is due to a PVA-membrane which separates the particles. Loss of redispersibility in the presence of surfactant proceeds with the breakup of the membranes and a corresponding change of film properties. Experimental data is provided by light microscopy, mechanical testing, and TEM in conjunction with a staining method new to the field. The hypothesis is developed that interaction with surfactant leads to imperfect PVA-membranes that are no longer able to prevent latex polymer interdiffusion. Fluorescence correlation spectroscopy demonstrates the formation of surfactant micelles, as well as the simultaneous adsorption and aggregation of PVA onto the micelles. It is concluded that the competing surface of the surfactant micelles traps enough PVA to cause thinning and fragmentation of the membranes surrounding the particles, which enables interdiffusion of latex polymer. This effect can be used to convert the system from one forming a redispersible coating to one forming a nonredispersible (permanent) film. Copyright 2000 Academic Press.     

Ethanol dehydrogenation on copper catalysts with ytterbium stabilized tetragonal ZrO<Subscript>2</Subscript> support

The physicochemical and catalytic properties of Cu-containing crystalline zirconia, obtained via sol–gel synthesis in the presence of Yb³⁺ ions and polyvinylpyrrolidone, are studied. DTG/DSC, TEM, XRD and BET methods are used to analyze the crystallization, texture, phase uniformity, surface and porosity of ZrO₂ nanopowders. It is shown that increasing the copper content (1, 3, and 5 wt % from ZrO₂) raises the dehydrogenation activity in the temperature range of 100–400°C and lowers the activation energy of acetaldehyde formation. It is found that the activity of all Cu/t-ZrO₂ catalysts grows under the effects of the reaction medium, due to the migration and redispersion of copper.