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Uric acid, the major component in many kinds of kidney stones, as well as its sodium, ammonium, calcium, and barium salts were successfully prepared as uniform dispersions by precipitation in basic aqueous solutions. The effects of the reactant concentrations, pH, and the stabilizers were evaluated in detail. Except for the platelets of the pure acid, all prepared compounds appeared as needles or their aggregates. The electron micrographs showed that kidney stones consisted of such aggregates although less regular in size and morphology. All prepared urate salts had a 1:1 cation/uric acid ratio, regardless of the valence of the cation. The electrokinetic measurements showed all these particles to have negative ζ-potentials over the pH range 3-9. The precipitated salt particles were chemically and morphologically unstable at low pH values by decomposing into ill-defined aggregates of the pure uric acid.

Two methods for the preparation of uniform dispersions of dehydrocholic acid of different morphologies are described. In the first case, the drug was dissolved in acetone and then re-precipitated by adding a non-solvent (either water or an aqueous stabilizer solution), which yielded rod-like particles. In the second procedure, spheres, consisting of small elongated subunits, were obtained by acidification of basic aqueous solutions of the drug. The resulting particles were characterized in terms of their structure and surface charge characteristics.

Uniform stable nanosize nickel platelets were prepared by first coating Ni(OH)2 particles of the same shape with gelatin and then heating them at moderate temperatures in nitrogen without addition of any reducing agents. The resulting platelets consist of a mixture of Ni and NiO. Depending on the amount of gelatin coating and temperature, up to 73% of solids were converted to the metal. Subsequent heating of such powders at 190 degrees C in a stream of 5% H2 in N2 produced only Ni platelets with a modal length of approximately 60 nm and several nm thick.

Uniform dispersions of magnesium fluoride particles of different morphologies were prepared by precipitation in aqueous solutions. The resulting cubic, prismatic, and platelet-like nanosize solids had single crystal structure with X-ray pattern characteristic of the mineral sellaite. In contrast, two kinds of polycrystalline MgF2 spheres were obtained by aggregation of the nanosize subunits. The mechanisms of the formation of the resulting particles of different shapes are explained by the role of the pH and ionic strength. In addition, for prospective numerical modeling the surface tension of spherical and platelet particles of MgF2 was evaluated from the X-ray data by a lattice parameter change method.

Uniform cubic particles of neighborite (NaMgF3) were prepared by mixing solutions of magnesium chloride and sodium fluoride, followed by aging for extended periods of time (up to 3 h). Such particles could be obtained directly either by using sodium fluoride in sufficient excess, or by first producing spherical particles of magnesium fluoride and converting them into neighborite cubes by admixing sodium fluoride. It was shown that both MgF2 and NaMgF3 particles so prepared are polycrystalline and that in both procedures to form neighborite a two stage reaction takes place. In the first stage nanosize subunits of MgF2 are formed, which are subsequently converted in the presence of excess sodium fluoride to neighborite crystallites. The latter are then reorganized into larger subunits that constitute colloidal cubes.

It is shown that many monodispersed colloid particles, precipitated in homogeneous solutions, are formed by aggregation of nanosize subunits. A model is described that specifies conditions which may yield such spherical particles of narrow size distribution by interactions of precursor singlets. A good agreement was achieved for size selection of gold and cadmium sulfide dispersions. It is illustrated that particles of other shapes may also formed by the aggregation mechanism, and the challenges facing attempts to quantify such processes are pointed out. Finally, examples are given of consequences caused by particles being composed of nanosubunits.

Well-dispersed uniform spheres of crystalline CeO2 were prepared by calcining precursor particles obtained by heating cerium(III) acetate in 1,2 propane diol (propylene glycol, PG) at the boiling point of the polyol (∼187 °C) for several hours. The first precipitated amorphous spherical particles consist of a complex mixture of ceria, ceria hydrate, cerium hydroxide, cerium glycolate complex and residual PG. To convert these precursors to pure CeO2, the solids were calcined at 500 °C in air at a slow heating rate (≤0.1 °C/min). The purification of the precursor particles depended strongly on the solvent used. By washing with water, much of the organic material was leached out resulting in smaller spheres, while rinsing with methanol showed little effect. The TGA, FTIR, XRD, EDX analyses of the different purified particles revealed the stages in the formation of the final pure well-defined CeO2 powder.

Uniform, well-dispersed silver particles of various morphologies have been prepared by reducing highly acidic silver nitrate solutions with ascorbic acid in the presence of a sodium naphthalene sulfonate-formaldehyde copolymer as dispersing agent. By varying the temperature of the reaction, the free acid content, the addition rate of the reductant, and the aging time, both isometric and anisotropic silver particles could be obtained. It was found that the latter were formed by aggregation of nanosize subunits, which were identified by electron microscopy and X-ray diffractometry.

Nano- and micrometer size spherical bismuth particles were prepared by reducing bismuth salts in simple and mixed polyols at elevated temperatures. In general, the conversion to metallic bismuth was preceded by the formation, at intermediate temperatures, of bismuth-polyol compounds. By changing the type of bismuth salt, the composition of the polyol mixture, and the temperature of the process, these precursors were successfully converted into spherical bismuth particles. It was found that the reduction process proceeds only if the temperature of the polyol exceeds the values at which the intermediate bismuth-polyol are stable, which was determined to be around 220 °C.

This study explains the relative importance of the *OH radical in the presence of other additives in hydrogen peroxide based slurries during chemical−mechanical planarization of copper. For this purpose, we investigated the effects of three different amino acids, that is, glycine, serine, and cysteine, in the presence of H2O2 on the dissolution of copper coupons and on the polishing of copper disks with well-dispersed slurries of fumed silica particles over the pH range 2.0 and 8.0. While these amino acids produced different amounts of *OH, the polish rates did not depend on the concentration of this radical alone but were influenced by other parameters such as the pH of the slurries and the specific interactions of the amino acids with other components of the slurry.

Controlled precipitation of the diagnostic imaging agent ethyl 3,5-di(acetylamino)-2,4,6-triiodobenzoate has been used to produce fine particles of various sizes, morphologies, and degrees of crystallinity, which depended on experimental conditions. In addition, two distinct polymorphic forms of the drug have been fully characterized by single crystal X-ray diffraction studies, and evidence for a third polymorph was also observed. Some of the so prepared dry particles were coated with a thin layer of silica.

The preparation of colloidal particles of different morphologies, including spheres, of two drugs, loratadine and danazol, is described. In principle these particles were obtained by precipitation when nonsolvents (water or aqueous surfactant solutions) were added to ethanol solutions of the drug. In addition, procedures were developed that made it possible to use the drug particles thus obtained as cores to be then coated with either silica or aluminum (hydrous) oxide layers. The presence of these inorganic shells was confirmed by electron microscopy, energy dispersive spectroscopy, and electrophoresis.

Monodispersed spherical polycrystalline cadmium sulfide particles were synthesized by precipitation in acidic aqueous solutions during the thermally controlled reaction between cadmium nitrate and ...

The recently developed two-stage growth model of synthesis of monodispersed polycrystalline colloidal particles is utilized and improved to explain growth of uniform cadmium sulfide spheres. The model accounts for the coupled processes of nucleation, which yields nanocrystalline precursors, and aggregation of these subunits to form the final particles. The key parameters have been identified that control the size selection and uniformity of the CdS spheres, as well as the dynamics of the process. This approach can be used to generally describe the formation of monodispersed colloids by precipitation from homogeneous solutions., Comment: 20 pages in PDF

Thermal oxide covered silicon wafers were polished with slurries containing (i) only nano-sized particles of ceria, monodispersed colloidal spherical silica, or hematite of different shapes, (ii) a binary mixture of the same nano-sized and uniform colloidal particles, and (iii) the same colloids coated with nano-sized ceria. The procedures for the preparation of the coated particles are described in this article. The polish rates and surface qualities were in all cases higher with mixed slurries, and even more so with coated particles. The performance of composite systems also depended on the shape of the particles, cubic ones being the most and spheres least efficient. Experimental results indicated that ceria in mixtures was responsible for the enhanced polish process, while core materials enhanced a closer contact of nano-sized particles with the wafer. In general, the polish rates were higher with the larger contact area between the abrasives and the wafer. This mechanism was further verified by polishing oxide wafers on 3-M fixed abrasive pads, which have cylindrical structures with flat surfaces.

Uniform, spherical CuCl particles were obtained by mixing aqueous solutions of CuCl2 and ascorbic acid in the presence of polyvinylpyrrolidone (PVP) as dispersing agent. The size and the uniformity of the resulting particles depended on the volume ratio of the reactant solutions, their concentrations, the distribution of the stabilizers, and the mixing method. The single jet precipitation yielded large spheres of broad size distributions, while the particles obtained by the double jet technique were rather uniform in size. The final colloidal CuCl particles were formed by the aggregation of nanocrystals, initially generated in the system. Depending on the pH of the reaction mixture, these particles slowly change to large CuCl crystals on aging in the mother liquor.

Colloidal copper particles with a high degree of crystallinity were obtained by heating solutions of copper(II) acetate in ethylene glycol (EG) and tetraethylene glycol (TEG). The formation mechanism of copper particles involved first the formation of sparingly soluble nano-sized copper(I) oxide, which aggregated into uniform spheres. On prolonged heating the Cu2O particles were reduced to nano-sized copper crystallites followed by their sintering to colloidal metal. The conversion of the copper(I) oxide to metallic copper proceeds uniformly within the body of each particle.

Catalytic decomposition of urea by enzyme urease in aqueous strontium and barium chloride solutions was used to rapidly precipitate witherite (BaCO3) and strontianite (SrCO3) particles at room temperature. At the early stages of the process, uniform spheroidal particles were generated, which were shown to consist of nanosized subunits. On continuous aging of the same systems the additionally precipitated alkali earth carbonates grow as whiskers onto original core particles, which eventually fully transform into crystalline rodlike clusters.

Monodispersed spherical cadmium sulfide particles were used as a model system in order to explain the size selection in the formation of colloids by aggregation of nanosize subunits. Several procedures of mixing the reactants were employed to precipitate these solids and follow the kinetics of particle growth. Efficient numerical simulation techniques for the model rate equations were developed to fit the experimental results. Our results have confirmed the recently proposed mechanism of two-stage growth by nucleation of nanosize crystalline primary particles and their subsequent aggregation into polycrystalline secondary colloids., Comment: 18 pages (with 6 figures) in PDF

Uniform fluorescent silica-dye microspheres have been prepared by coating preformed monodispersed silica particles with silica layers containing rhodamine 6G or acridine orange. The resulting dispersions exhibit intense fluorescent emission between 500 and 600 nm, over a broad excitation wavelength range of 460 to 550 nm, even with exceedingly small amounts of dyes incorporated into the silica particles (10-30 ppm, expressed as weight of dye relative to weight of dry particles). The fluorescent particles can be prepared in micrometer diameters suitable for analyses using flow cytometry with 488-nm laser excitation.

Thermal oxide covered silicon wafers were polished with slurries containing either nano-sized ceria (CeO2) or newly prepared uniform colloidal silica particles coated with ceria. The polish rate of the latter was significantly higher than that of pure ceria. The experiments were carried out using different concentrations of the abrasives at pH 4 and 10. Little effect on the polishing rates was noted when the conditions of the slurries were varied, which was explained by the compensation of two opposite polishing mechanisms.

The analysis of early-stage heteroaggregation (or heterocoagulation) in binary colloidal systems composed of oppositely charged latex particles of different size in the submicrometer size range with multiangle static and dynamic light scattering is presented. The solution conditions were adjusted to exclude any significant homoaggregation. The apparent rate obtained from static light scattering mostly strongly decreases with increasing scattering angle regardless of the number fractions, while the rate from dynamic light scattering varies in a more complicated manner. The light scattering data could be interpreted quantitatively on the basis of the Rayleigh−Gans−Debye approximation. The values of absolute heteroaggregation rate constants were found to be the same within experimental error when evaluated from static or dynamic light scattering, and they were independent of the mixing ratio. The average hydrodynamic radius of the doublet obtained from the dynamic light scattering was in good agreement with theoretical estimates based on an exact hydrodynamic treatment at low Reynolds numbers. A simple formula is proposed to estimate the hydrodynamic radius of the asymmetric particle doublet, and this formula is shown to agree well with experimental data and with theory. The new conclusion from this study is that multiangle dynamic light scattering represents the method of choice for the determination of absolute heteroaggregation rate constants.

The retention performance of pigment whiteners was studied in an experimental papermaking process to illustrate the interactions between whitener particles and the cellulose fibers in the presence of retention aids (polyamine and alum). Special emphasis is paid to the effects of the electrokinetic properties of the reactants.

Ample experimental evidence has been accumulated demonstrating that the formation of monodispersed colloids proceeds through a more complex mechanism, than the generally excepted diffusional "burst nucleation" process. Instead, the synthesis of narrow-size-distribution colloidal dispersions involves several stages, i.e., nuclei produced in a supersaturated solution, grow to nanosize subunits, which then aggregate to form much larger uniform secondary particles. To explain the size selection in such a series of processes, a kinetic model was developed which combined two growth/aggregation stages. This earlier study has shown the burst-nucleation growth of the primary particles to depend strongly on the value of the effective surface tension entering the surface term in the free energy of the subcritical embryos. The aim of the present work has been to identify an appropriate control parameter in the process of secondary particle aggregation. Modifications of the aggregation rates to account for singlet size and aggregate diffusivity produced only small changes. However, introduction of a "bottle-neck" factor in the dimer formation rate had a profound effect on the final size distribution and suggested a possible size-control mechanism., 30 pages in PDF

The effects of nonionic, cationic, and anionic dextrans on the precipitation of calcium hydroxide and calcium carbonate particles in aqueous solutions were investigated by varying different experimental parameters. Nonionic dextrans had no observable effect on the resulting dispersions. The addition of ionic dextrans in basic solutions altered the morphology of calcium hydroxide. Precipitation of calcium carbonate particles, in the presence of urea, was influenced significantly only by the anionic dextran, which affected both the morphology and the crystal structure of the precipitated particles. In neutral or slightly acidic solutions, Ca 2+ ions reacted with anionic dextrans to form a coacervate, which was the result of neutralization of the sulfate groups of the polymer by this cation. The so formed ion pairs could be eliminated by the addition of an excess of nonbinding Na + ions, resulting in the dissolution of the separated phase.

Catalytic decomposition of urea by urease in aqueous calcium chloride solutions was used to rapidly prepare calcium carbonate polymorphs at room temperature. The nature of the resulting particles depended on the concentration of the enzyme and, in a strong manner, on the agitation of the reacting solutions. In an undisturbed system an amorphous precipitate is formed first, which readily crystallized to vaterite and upon aging changed to calcite. Under the influence of magnetic stirring, the amorphous phase could be not observed; instead smaller particles were initially obtained, which aggregated to vaterite and calcite. Similarly, the application of ultrasonic energy produced small vaterite particles at the early stages. It is apparent that enzyme macromolecules are important in the development of calcite faces and, as such, they exert significant influence on calcite morphology, without being present in detectable amounts in the resulting solids. Copyright 2001 Academic Press.

A new route for obtaining calcium-deficient apatites with a Ca/P ratio lower than 1.5 is described, in order to study their proton conduction at temperatures lower than 400 °C. The process is based on the hydrolysis of a mixed solution of Ca(NO3)2 and NH4H2PO4 in the presence of hexamethylenetetramine at a pH of approximately 5 and temperatures of 85–90 °C. The resulting spherical particles of 14 μm in average diameter were aggregates of smaller needles with approximate composition Ca8.5(HPO4)2(PO4)4OH · H2O. The effects of the reagent concentrations, pH, aging time, and temperature were studied, and the solids were characterized by x-ray diffraction, infrared absorption spectroscopy, and electron microscopy. The ionic conduction measured by alternating-current impedance spectroscopy yielded a value of 3 μSm−1 at 200 °C.

The use of the packed column technique for the study of deposition of colloidal particles from dispersions in liquids onto collector beads is described. The technique also allows to investigate the removal of adhered particles, by rinsing the loaded column with solutions of different compositions. A number of examples are described illustrating the effects of various experimental conditions, including the surface and intrinsic properties of reactants, the nature and the ionic strength of the liquid phase, the effects of various additives, and of superimposed external magnetic fields, etc. The results are presented in terms of breakthrough curves and interpreted by means of three phenomenological parameters, which are related to interaction forces between colloidal particles and the collector. This approach makes it possible to distinguish between monolayer and multilayer depositions.

Stable aqueous dispersions consisting of CdS nanoparticles having modal diameters, ranging between 2 and 8 nm, were prepared with amino-derivatized polysaccharides (aminodextrans, hence abbreviated as Amdex) as the stabilizing agents. The size, stability, and luminescence intensity of such dispersions were shown to be dependent on the types of the cadmium salts and aminodextrans used, as well as on the reactant concentrations. Specifically, it was demonstrated that the degree of substitution of amino groups in the aminodextran molecules greatly affected the properties of the dispersions; i.e., with higher degree of substitution, smaller CdS particles and higher luminescence intensity were achieved. It was also shown that the Amdex−CdS nanoparticle complexes could be activated and conjugated with antibody by conventional means. Molecular weight ranges of the Amdex and their complexes with CdS nanoparticles and the purity of antibody−Amdex−CdS nanoparticle conjugates were determined by polyacrylamide gel el...

The kinetics of deposition of monodispersed spherical silver particles stabilized by a surfactant, Daxad 19, on aluminum beads was studied as a function of the pH and ionic strength using the packed column technique. At low pH values, the attachment proceeded from mono- to multilayer with increased ionic strength, while at high pH the adsorbed surfactant was found to play a significant role in the attachment process, resulting in multilayer deposition only. The kinetics of removal was followed by rinsing the column loaded with silver with a solution of the same composition as that of the suspension used in the deposition process. The results suggest a possible bond formation between the beads and the stabilizer adsorbed on the particles. In order to interpret the adhesion data, a uniform dispersion of silver, 60±5 nm in particle diameter, was synthesized and its stability towards different electrolytes was determined. Furthermore, the surface charge characteristics of silver particles and aluminum beads were evaluated over a broad pH range at different ionic strength.

Monodispersed spherical gold particles ranging in modal diameter from 80 nm to 5 μm, were prepared by reducing tetrachloroauric(III) acid with iso-ascorbic acid in aqueous solutions at 20°C. The particle size was altered by changing the pH, which affected the composition of gold(III) solute complexes. The latter controlled the redox potential of the system, essential to the formation of the initial nanosize gold dispersions. Depending on the experimental conditions, the resulting primary particles remained either stable or they aggregated to form much larger uniform spheres. The mechanisms of the precipitation of the precursors (primary) particles and of their mutual interactions to yield the final dispersions are discussed.

Uniform colloidal polypyrrole particles ranging from 17 to 59 nm in diameter were prepared by the oxidation of pyrrole with sodium persulfate in the presence of the nonionic Rhodasurf TB970 polymeric stabilizer and 4-ethyl benzenesulfonic acid. The adhesion of these particles on glass beads was studied as a function of the pH using the packed column technique. Polypyrrole was found to deposit on glass only at pH values below its isoelectric point (i.e.p.), forming a monolayer. The entire amount of the adhered polypyrrole could be rapidly removed by rinsing the column with 1×10-2 mol dm-3 NaOH solution.

The synthesis of uniform colloidal rod-like In(OH)3 particles from relatively concentrated solutions of InCl3 (0.1 mol dm-3) in short reaction time (

A new procedure for the preparation of nanosized In(OH)3 particles in high concentrations without added surfactants is described. The process is based on the peptization of In(OH)3 dispersions consisting of micrometer colloids, which are obtained by the hydrolysis of InCl3 in ethylene glycol to which aqueous sodium hydroxide solutions are added. Depending on conditions, aging such systems at elevated temperatures (150−175 °C), yields colloidal particles of different morphologies (platelets, fibers), which are composed of nanometer subunits. The latter are liberated by repeated washing with water or 2-propanol, resulting in uniform spherical particles of ∼80 nm in diameter. It is suggested that an In−glycolate compound forms at high temperatures, which keeps the nanoparticles together, but it is decomposed on treating with water or 2-propanol, allowing for their release. The original micrometer-size precipitates retain their shape after calcination; hence, In2O3 particles of different morphologies can be o...

Uniform colloidal particles of thorium and cesium tungstophosphates of different morphologies have been prepared by controlled precipitation. The method consists of direct reaction of aqueous solutions of salts of the corresponding metals and tungstophosphoric acid, H3PW12O40, at elevated temperatures (90 °C) under certain conditions of the reactant concentrations and the pH. The resulting Cs compound was stoichiometric, whereas the thorium heteropoly particles contained excess thorium due to the incorporation of its hydrolysis products, generated under the employed experimental conditions. The Cs compound showed a secondary structure similar to that of the parent acid, while the Th compound was amorphous to X-rays. Both samples exhibited different electrokinetic behavior, the isoelectric point of Cs−tungstophosphate being at pH 1.5, and that of the Th compound at pH 5.4.

The kinetics of deposition of colloidal hematite particles on glass beads in the presence of a cationic surfactant, 1-dodecylpyridinium chloride (DPC), at pH 4.0 and 10.5 was studied using the packed column technique. At the high pH, the surfactant adsorbed both on particles and beads, which influenced the attachment process; depending on conditions either monolayer or multilayer deposition was observed. At pH 4.0, only a monolayer of hematite formed at all surfactant concentrations used. At this pH value, the DPC adsorbed little on particles but significantly on beads, making a part of the collector surface inaccessible for the particle attachment. A modification of the multilayer adhesion theory was proposed which allows one to account for the latter effect. The kinetics of particle release from the monolayer, by rinsing the loaded column with 10 −2 mol dm −3 NaOH solutions, was also investigated. It was shown that the detachment rate was insensitive to the amount of surfactant present in the dispersions used for the deposition.

This review article deals with the preparation, characterization and mechanisms of formation of uniform simple and composite metal particles of different modal diameters and shapes. In principle, such dispersions can be obtained by reduction of metal ions, in uncomplexed or complexed state, with suitable reducing agents and appropriate additives. The effect of the redox potential in a given oxidation/reduction system on the nature of the final particles is discussed in some detail. In another approach monodispersed particles of metal compounds are prepared first and then reduced to pure metals either in a liquid or a gaseous medium. In doing so, one can produce powders of a given morphology by selecting the precursor particles of the desired shape. A special case is represented by coated particles, consisting of cores and shells of different chemical composition. Depending on the materials, it is possible to reduce either one or both components by appropriate chemical reactions.

Aqueous dispersions of PdS particles were prepared by the controlled double-jet precipitation technique using PdCl 2 or Na 2 [PdCl 4 ] and Na 2 S solutions. Uniform spherical particles 20–30 nm in diameter were obtained in acidic media in the absence and in the presence of surfactants, whereas the reaction in alkaline media yielded particles with a diameter of 2–5 nm. Surfactants of the AVANEL S series enhanced the deposition of PdS particles on the epoxy circuit boards, resulting in a conductive layer, which also facilitated the electroplating by copper in holes. The electrokinetic mobilities of the chloride-containing dispersions reveal negative charges over the entire pH range, whereas chloride-free samples display isoelectric points at pH 3.0±0.2 or between pH 5 and 6, depending on the preparation conditions.

Uniform colloidal pigment dispersions of narrow size distribution were obtained by interacting aqueous solutions of D&C Red #6 dye and BaCl2. The original particles were elongated in shape but the anisometry could be altered by input of ultrasonic energy. The addition of an anionic surfactant caused a decrease of the average pigment size to ∼ 40 nm. The so prepared colored dispersions showed 100% transparency, which is considerable higher than the commercial Ba lake of the same dye.

Methods for the preparation of uniform dispersions of inorganic particles of simple and composite natures in varying shapes, ranging in size from several nanometers to several micrometers are described. The advantages of the use of such powders in different applications, including ceramics, catalysis, and pigments are illustrated.

Uniform cubic manganese carbonate, MnCO3, particles of two modal sizes (∼0.8 and 8 μm) were produced by homogeneous precipitation from aqueous solutions of manganese sulfate and urea at elevated temperatures under limited experimental conditions. The larger particles were converted into Mn2O3 and MnO on calcination at 700 °C either in the flow of air and or in nitrogen. The MnCO3 particles were also coated with NiCO3.Ni(OH)2·H2O by aging dispersions of the cores in aqueous solutions of nickel sulfate and urea at 80 °C, yielding a molar ratio [Ni]/[Mn] = 0.45 in the final solid. On calcination at 700 °C in air, the shell and core materials of the coated particles transformed independently into NiO and Mn2O3, which on further heating at 350 °C in hydrogen were reduced to Ni and Mn, respectively. The coating solutions aged at the same temperature in the absence of cores resulted in nearly spherical of NiCO3·Ni(OH)2·H2O particles of narrow size distribution. The latter solids transformed into NiO on calcinati...

A method for the preparation of nanosized zirconium (hydrous) oxide particles of narrow size distribution is described. The procedure yields stable dispersions at low temperatures and short reaction times in the absence of surfactants, using inorganic zirconium salts. Crystal structure, particle size distribution, electrokinetic properties, stability, and thermal behavior of the prepared particles were investigated. Colloidal dispersions were treated with ultrasound to study their effect on the crystal structure of the calcined samples.

We present evidence for long-range correlations C2(t) in temporal intensity fluctuations of multiply scattered visible light. The time autocorrelation function of the angular-averaged light intensity transmitted through a thin slab containing particles undergoing Brownian motion was determined for a series of different sample thicknesses, beam spot sizes at the sample surface, and optical transport mean free paths l*. The results for both the amplitude ~i.e., the inverse conductance 1/g! as well as the time ( t) dependence of the correlation function C2(t) are in good overall agreement with theory. Significant deviations are found, however, when correlations generated in a layer of 1 l* to 2l* adjacent to the incoming surface become important. In particular the predicted t 21/2 long-time tail of C2(t) was not observed. A physical explanation of these deviations is given based on the argument that at least one scattering event is required before long-range correlations C2(t) can develop. @S0163-1829~97!08341-0#

The hydrolysis of SbCl3 in hydrochloric acid solution (2.0 mol dm−3 HCl) at 0 °C yields an amorphous product consisting of uniform spherical particles (d∼0.5 µm), which on continuous aging at the same temperature transform to larger crystals, indicated by XRD to be Sb4O5Cl2. In contrast, in the same solution kept at 25 °C crystalline particles of the same composition form directly after an induction period and then grow with time. The final products, obtained at 0 °C and 25 °C consist of aggregated subunits. These powders on calcination in nitrogen are converted to Sb2O3 and in air to Sb2O4.