Particulate processes in freshly prepared silver iodide hydrosols Part I. Radiometric and electron-microscopic investigations

Ageing of negative, stable AgI sols prepared in statu nascendi was investigated utilizing radiometric (heterogeneous exchange) and optical (electron microscopy) methods. AgI sols with different molar concentrations C_o, of AgI have shown that the examined sols age by two processes of different speed. The duration interval Δt_A, of the faster ageing process is approximately inversely proportional to the molar concentration of the sols. Results of a comparative analysis of the heterogeneous exchange process obtained by two different exchange procedures in identically prepared and aged sols revealed that primary particles of the sols approximately double their volume during the faster ageing process. This result is confirmed by analysing electron micrographs of differently aged stable sols. On the basis of data obtained by two different exchange procedures and the results of electron-microscopy analysis respectively, a new structural form of silver iodide is predicted. Such an up to now unknown form of silver iodide transforms spontaneously and gives a mixture of well-known cubic and hexagonal modifications of silver iodide during the aggregation of primary particles into secondary ones. This transformation is markedly accelerated during electrolytic coagulation of the sols.     

Monte Carlo simulations of colloidal particle coagulation and breakup under turbulent shear

Both a Monte Carlo model and an algorithm were presented to simulate the particle coagulation and breakup phenomena taking place in a colloidal solution under turbulent fluid shear. The model is represented by the probability density functions that describe the stochastic coagulation and breakup phenomena taking place among numerous particles. From a dimensional analysis of the model two dimensionless groups,K _c andK _b , were derived that represent the relative intensity of the coagulation and breakup phenomena. In order to overcome the memory problem in saving the sizes of a large number of particles, the model was converted to a form suitable for carrying out a sectional mass balance. Detailed simulation steps were presented and applied to acrylonitrile-butadiene-styrene (ABS) latex coagulation. Numerical simulations revealed that the steady state particle size distribution does not depend on the initial distributions but on theK _c /K _b ratio. Setting the operation variables to increase the ratio was found to shift the particle size distribution toward larger particles.     

乙醇对溶胶-凝胶法制备SiO_2溶胶性能的影响

以HNO3为催化剂,用正硅酸乙酯(TEOS)溶胶-凝胶工艺制备纳米多孔二氧化硅薄膜。较详细地研究了工艺参数乙醇/TEOS摩尔比对溶胶性能的影响.结果表明,随着乙醇/TEOS摩尔比的增大,溶胶粘度减小,凝胶时间变长,胶体粒径有变小趋势。     

Colloidal Sol–Gel Synthesis and Photocatalytic Activity of Nanoparticulate Nb2O5 Sols

A detailed study of a novel synthesis via colloidal sol–gel route for obtaining nanoparticulate Nb₂O₅ was performed. Parameters such as temperature and H⁺:Nb⁵⁺ and Nb⁵⁺:H₂O molar ratios were controlled in order to determine the best conditions of synthesis. Moreover, particle size distribution, zeta potential, structure by X‐ray diffraction, and the photocatalytic activity of the particulate sols were also evaluated. The obtained results indicate that the colloidal sol–gel synthesis is a good alternative for obtaining Nb₂O₅ either as stable nanoparticulate sol or as a nanosized powder. Nb₂O₅ amorphous nanoparticles with an average size of 20 nm were obtained by controlling the synthesis variables. The heat‐treatment process allowed the formation of Nb₂O₅ with orthorhombic structure that transforms at higher temperatures to monoclinic phase. The highest photocatalytic activity was observed under λ = 365 nm, the smallest UV energy used in the experimental tests.     

Formation of cobalt nanoparticle dispersions in the presence of polysiloxane block copolymers

Stable suspensions of superparamagnetic cobalt nanoparticles have been prepared in poly(dimethysiloxane) (PDMS) carrier fluids in the presence of poly[dimethylsiloxane-b-(3-cyanopropyl)methylsiloxane-b-dimethylsiloxane] (PDMS-PCPMS-PDMS) triblock copolymers as steric stabilizers. A series of the polysiloxane triblock copolymers with systematically varied molecular weights were prepared via anionic polymerization using LiOH as an initiator. These copolymers formed micelles in toluene and served as ‘nanoreactors’ for thermal decomposition of the Co₂(CO)₈ precursor. The nitrile groups on the PCPMS central blocks are thought to adsorb onto the particle surface, while the PDMS endblocks protrude into the reaction medium to provide steric stability. The particle size can be controlled by adjusting the cobalt to copolymer ratio. TEM shows non-aggregated cobalt nanoparticles with narrow size distributions which are evenly surrounded with copolymer sheaths. However, some degree of surface oxidation was observed over time, resulting in a decrease in magnetic susceptibility.     

Derivation and verification of an aerosol dynamics expression for the below-cloud scavenging process using the moment method

An aerosol dynamics equation for the below-cloud scavenging process considering phoretic and electric charging effects in addition to the conventional mechanisms (the Brownian diffusion, interception, and impaction) is developed by using the moment method. Then, the dynamics of particle size distribution by the below-cloud scavenging process is calculated by using the developed equation and verified with the measurement data. The calculated particle size distribution changes are quite small compared to the measured changes. The calculated removal rate is smaller by 10^?2–10^?3 than the measured data when only the conventional mechanisms are considered. With the extended mechanisms, the scavenging coefficient increases upto 20 times, mainly for the particle size range of 0.1 μm<d_p<3.0 μm. However, the difference between the calculated and measured scavenging coefficient is still large, especially, for d_p<0.1 μm. Other possible scavenging mechanisms that might affect the below-cloud scavenging process such as coagulation and condensational growth of hygroscopic particles, turbulence, and updraft into cloud are discussed. It is recommended that further studies on wet scavenging process are needed.     

An experimental study on gasification of Colombian coal in fluidised bed

The main results of an experimental work on gasification of Colombian coal in a fluidised bed are reported in this paper. Experiments were carried out at different steam/coal (F_s/F_c) and air/coal (F_a/F_c) ratios and temperatures of gasifying agent. In addition, the influence of bed temperature on coal conversion was analysed. Results show a maximum value in the curve of high heating value versus F_a/F_c. From the environmental standpoint, low concentrations of sulphur compounds were obtained but more work should be done in order to decrease particulate matter.     

The log-normal size distribution theory of brownian aerosol coagulation for the entire particle size range: part II—analytical solution using Dahneke’s coagulation kernel

In this study an analytical solution of the Brownian coagulation equation utilizing the method given in Park et al. [(1999) J. Aerosol Sci. 30, 3–16] but using a more detailed coagulation coefficient instead of the simple approach—the harmonic mean—Park et al. [(1999) J. Aerosol Sci. 30, 3–16] is derived. Comparisons are made with the analytical solution given in Park et al. [(1999) J. Aerosol Sci. 30, 3–16], with a method of moments model, with a log-normal reference model as well as with a sectional model (Landgrebe and Pratsinis, (1990) J. Colloid Interface Sci. 139, 63–86).In order to make a fair comparison between the models, all of them should use the same coagulation coefficients except Park et al. [(1999) J. Aerosol Sci. 30, 3–16] which is already fixed to the harmonic mean method. The most frequently used coagulation coefficients to describe the process of Brownian coagulation in the entire size regime (Fuchs, Dahneke, harmonic mean) were compared and a universal enhancement function to the near-continuum coagulation kernel is given. Furthermore, Fuchs’ coagulation kernel is reviewed and modified for the use with different sized particles. It was shown that Dahneke’s coagulation kernel is very close to those of Fuchs and Wright with a relative error of less than 1%. Based on this finding, Dahneke’s coagulation kernel is used in the models developed as well as during the comparison.In order to develop fast and easy-to-use coagulation models we used the moment method assuming a log-normal size distribution function [Lee et al., (1997), J. Colloid Interface Sci. 188, 486–492]. In the first step the integrals over the particle size distribution are solved analytically and DGEAR (IMSL, 1980) is used to solve for the time dependence. In the second step we solved also the time dependence of the parameters of the size distribution analytically. The models developed are compared over a wide parameter range with a reference model based on the moment method of log-normal size distribution functions as well as a sectional reference model. For a worst case, the moment method shows a relative error of less than 4% for the number concentration; the mean geometric standard deviation and the volume square concentration and for the geometric standard deviation the relative error is less than 8% in the self-preserving state. The analytical solution developed shows a maximum relative error of less than 10% if the dimensionless time is limited to K_coN₀t=1.     

Colloidal stability of gadolinium-doped ceria powder in aqueous and non-aqueous media

In this work the colloidal behaviour of Ce_0.9Gd_0.1O_2?δ powder in aqueous and non-aqueous media (ethanol) is studied. Commercial powder was characterised by particle size distribution, specific surface area measurements, X-ray diffraction and scanning electron microscopy. Diluted suspensions were characterised by particle size distribution and zeta potential, using dynamic light scattering and laser Doppler velocimetry principles, respectively. The solubility of the powder in water as a function of pH was analysed by inductively coupled plasma atomic emission spectrometry. Colloidal stability was studied as a function of pH, type and concentration of dispersants (polyacrylic-based deflocculant in water and a phosphate ester in ethanol). The time stability of the suspensions was analysed by multiple light scattering. The most stable suspension was obtained with a phosphate ester content of 2.0 wt% in ethanol. Finally, preliminary coatings have been obtained by dip coating using concentrated suspensions, which could be accessible to industrial scale so that they can be used as interlayers in solid oxide fuel cells.     

Polymerization with zeolite catalysts: I. Polymerization of n-butylvinylether by H-mordenite

n-Butylvinylether sorbed from the vapor phase onto hydrogen mordenite powder has been found to polymerize readily near room temperature. In a polymer sample from this reaction the polymer molecules were found to contain a number average of ten monomer units. The reaction over a considerable time interval followed the relations Q_t = k √t + γk = (ap + b)F(H₂O) where Q_t is the mass uptake (mainly as polymer) at pressure p in time t; k, γ, a and b are coefficients and F(H₂O) is a function of the amount of water in the zeolite. The kinetics have been taken to indicate partial control by a diffusion process in which the monomer penetrates a growing layer of polymer formed around each catalyst particle. The monomer migrates to the catalyst-polymer interface and polymerizes at or near the interface. In small quantities water functioned as a cocatalyst, but around 5% weight uptake F(H₂O) passed through a maximum. The rate of the reaction appeared to reach a maximum between 28 and 30 °C. A carbonium ion mechanism of polymerization is proposed.     

Effect of blend ratio on rheological properties of aqueous SiC suspensions

The relationship between blend ratio and rheological properties of concentrated suspensions is of great importance since it is the key to get high solid suspensions. The rheological properties of bidisperse aqueous suspensions made of two SiC powders with different particle size [d_(0.5)=1.63 and 18.43 μm, respectively] has been studied as a function of blend ratio ξ (the volume fraction of larger particle size). The results showed that the value of critical blend ratio ξ, at which the viscosity is minimized, is in close relation to the shear rate applied. At shear rates below 10 S⁻¹, the critical ξ was greater than 70%. But at shear rates from 10 to 500 S⁻¹, ξ turned to be 50%. The change of shear region from shear-thinning behavior to shear-thickening behavior may be used to account for the variation of critical ξ. Dynamic oscillatory tests showed that the moduli and the linear viscoelastic region of suspensions with higher ξ are smaller than those with low ξ and the increment of blend ratio ξ leads to the change of suspension from nearly an elastic response to a liquid like response.     

Size-controllable gold nanoparticles stabilized by PDEAEMA-based double hydrophilic graft copolymer

Poly(N-isopropylacrylamide)-b-[poly(ethyl acrylate)-g-poly(2-(diethylamino)ethyl methacrylate)] (PNIPAM-b-(PEA-g-PDEAEMA)) double hydrophilic graft copolymers were employed to prepare stable colloidal gold nanoparticles in situ with controllable size in aqueous media without any external reducing agent. PDEAEMA side chains served as reduction agent and stabilizer and PNIPAM segment acted as a hydrated layer to enhance the stability of gold nanoparticles. These gold nanoparticles showed a remarkable colloidal stability without any observable flocculation or aggregation for at least 2 months and they were characterized by UV-vis, XRD, TEM and AFM in detail. The size of gold nanoparticles can be tuned by adjusting the length of PDEAEMA side chains and the molar ratio of [DEAEMA]/[AuCl₄⁻]. Both the increasing of the length of PDEAEMA side chains and the decreasing of [DEAEMA]/[AuCl₄⁻] molar ratio resulted in the fall of size.     

Carbon nanotubes prevent the coagulation at high shear rates of aqueous suspensions of equiaxed ceramic nanoparticles

Equiaxed ceramic nanoparticles and their mixtures are expected to exhibit shear-thinning behaviour when dispersed colloidally in aqueous media, whereas shear-thickening is the expectation for large aspect ratio phases such as, for example, carbon nanotubes (CNTs). Here, contrary experimental evidence is presented demonstrating the occurrence of severe coagulation at high shear rates in colloidally stable, semi-concentrated, aqueous suspensions of equiaxed SiC nanoparticles (major phase) mixed with equiaxed Y₃Al₅O₁₂ nanoparticles (liquid-phase sintering additive), and how CNT addition prevents this coagulation if sufficient sonication is applied. It is also shown that although shear-thinning is the natural behaviour of the ceramic suspension up to moderate shear rates, coagulation is eventually a phenomenon inherent to the aqueous colloidal processing of these suspensions, with the critical shear rate for coagulation increasing and the rheopexy decreasing the better is the initial dispersion state achieved with the sonication. It is also shown that the critical shear rate for coagulation depends on the exact condition of shear rate increase, and that the re-sheared suspensions coagulate more significantly and at lower shear rates than the fresh suspensions. The mechanisms by which this coagulation occurs and is impeded by the CNTs are discussed, together with broader implications of these phenomena for the environmentally friendly processing of nanostructured ceramics and ceramic composites.     

Dynamic light scattering applied to the synthesis of colloidal zeolite

The use of dynamic light scattering as an analysis method within the field of zeolite synthesis has proved to be a powerful and robust tool with which the particle size and the corresponding particle size distribution can be determined. The method has been employed in the evaluation of the crystallization of several types of colloidal zeolite from clear homogeneous solutions. Examples of such zeolites are zeolite N-Y, hydroxysodalite, and TPA-silicalite-1. The fact that the particle size can be determined in as-synthesized zeolite sols as a function of synthesis time enables one to follow, for example, the crystallization process in terms of particle size increase, the process of particle size tailoring as well as to obtain information on the growth mechanism in zeolite synthesis. The colloidal nature of sols following redispersion of zeolite powders and colloidal zeolite organosols has been assessed using dynamic light scattering. The advantages as well as problems associated with the use of dynamic light scattering for characterization of colloidal zeolites are discussed.     

Rheological behavior and microstructure of bimodal suspensions of core‐shell structured swollen particles

The rheological behavior and microstructure of bimodal suspensions of core‐shell structured swollen particles have been examined with changing volume ratio of two different sized particles. As the volume fraction of large particles increases, the viscosity, degree of shear‐thinning, and the critical shear stress σ_c decreases, while the interparticle distance ξ of the microstructure increases. The suspensions exhibit single mode rheological behavior and have a single diffraction peak in the SAXS profiles. These results suggest that the bimodal suspensions of the core‐shell structured swollen particles behave likely to unimodal suspensions of hard spheres with alloy like single mode microstructure composed of hypothetical intermediate size particle. The relationship between σ_c and ξ can be represented as σ_c = 3kT/4πξ³, which corresponds to the dynamics of the Brownian hard sphere model with ξ being the particle diameter. These findings indicate that the shear‐thinning of the suspensions can be attributed to dynamical competition between the thermal motion and the hydrodynamic motion under shear flow and that the mechanism can be applied to bimodal suspensions of the swollen particles as well as unimodal suspensions of hard spheres. © 2006 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 102: 2212–2217, 2006     

Generation of TiO2 Aerosols from Liquid Suspensions: Influence of Colloid Characteristics

The influence of the colloidal characteristics of aqueous TiO₂ nanoparticle suspensions and of the operating conditions on the total particle concentration and the particle size distribution of aerosols generated by nebulization has been studied. A commercial nebulization unit coupled to a diffusion dryer was used to generate aerosols using two different sources of titanium dioxide nanoparticles. Stable, concentration-tunable aerosols could be obtained for both types of nanoparticle suspensions. The effect of operating conditions during nebulization (air flow rate, purity of water source, nanoparticle concentration, and pH of the precursor suspension) was studied. The results obtained indicate that the degree of agglomeration in the liquid phase previous to aerosol formation has a direct influence both on the total nanoparticle count and on the particle size distribution of the generated aerosols.

Copyright 2013 American Association for Aerosol Research     

Production of rare-earth orthoferrite ceramic fibres by aqueous sol-gel blow spinning process

Rare-earth orthoferrite, LnFeO₃ (Ln=La, Sm, Gd, Dy, Er and Yb) ceramic fibres were produced by aqueous sol-gel blow spinning process at low-temperatures. Stable, charge stabilised, colloidal precursor sols of orthoferrites were prepared by room temperature processing of inexpensive and commercially available metal salts. The average diameter (Z_av) of the colloidal sol particles was in the range 4–7 nm and had a narrow size distribution. The sols were concentrated, combined with spinning aids, and processed further to a viscous ‘spinning solution’. The gel fibres of about 6 μm diameter were blow spun, collected as random fibres, dried and heated to increasingly higher temperatures at a rate of 50 °C/h. The gel fibres converted to flexible ceramic fibres, and single-phase perovskite structure crystallised directly for all the LnFeO₃ (Ln=La, Sm, Gd, Dy, Er and Yb ) fibres on heating them to 700 °C. The ceramic fibres had mean diameter of about 3–4 μm, and consisted of randomly oriented submicron sized grains.     

Formulation and preparation of low-concentrated yttria colloidal dispersions

High-temperature creep resistant steels for nuclear applications consist of a steel matrix reinforced with a dispersion of nano-sized refractory ceramic oxide particles, e.g. yttria (Y₂O₃). In this study, the formulation and preparation of low-loaded (2, 3 and 4 vol.%) Y₂O₃ colloidal dispersions for its possible application as suspension precursor in the production of high-temperature creep resistant steels is discussed. The final end product is formulated as a low-concentrated and low-viscous colloidal suspension consisting of non-agglomerated Y₂O₃ nano-particles in the (sub)-100 nm range.The spherical as-received and submicron agglomerated powder necessitates ball-mill processing in order to reduce the as-received size distribution down to nano-level. A comparative study on the desagglomeration of Y₂O₃ nano-powder using conventional planetary ball-milling or nano-milling technology has been done. Formulation of stable colloidal suspensions by screening suitable dispersants has led to the selection of one specific dispersant for this application, Calgon N. Suspension characterization by the determination of particle size distribution (PSD), ζ-potential measurements, colloidal suspension visualization using transmission electron microscopy (TEM), rheology and suspension life-time is discussed in this paper.     

Investigation of processes of obtaining cerium dioxide sol with polyvinyl alcohol having bioactive properties

Cerium dioxide sols are widely used in photocatalysis, catalysis, medicine and cosmetics. It is believed that sols with a CeO₂ size of up to 10 nm have the most active biological properties. The method of sols preparation affects their functional properties. They are usually obtained through the stage of separating the oxide in the solid phase from the solution, which can lead to particle agglomeration, and then transferring the oxide to a colloidal solution under the action of various stabilizers. In this study, we propose the method of CeO₂ sol obtaining without step of solid phase oxide separation. The stabilizer (polyvinyl alcohol (PVA)) is present at the stage of formation of weakly aggregated CeO₂ nanoparticles from cerium(III) salt with an ammonia and hydrogen peroxide solution. Using IR, UV spectroscopy, viscometry and titration it was found that H₂O₂ eventually oxidizes Ce(III) in solution to Ce(IV) and does not oxidize PVA. The intermediate formed after the addition of ammonia solution to cerium(IV) salt has the composition Ce(ООН)₃OН?nH₂O, which is confirmed by the data of proton magnetic resonance, weight and thermal analyses. The process of CeO₂ sol formation stabilized by PVA goes through the stages of Ce(ООН)₃ОН dissolution accompanying with an endothermic effect, followed by decomposition. Peroxide compounds in the solid phase are stable in air up to 90–100 °C. The prepared sol has the composition CeO₂–PVA–ammonium nitrate, exhibit antioxidant properties, is not a nutrient medium for E. Coli and S. Aureus and is capable to form a calcium-phosphate layer on its surface from a model solution of SBF.