Sol-gel Microspheres Research Articles

An improvement of microfluidic‐assisted internal gelation in the preparation of millimeter‐sized ceramic microspheres

AbstractThe microspheres prepared by the microfluidic‐assisted internal gelation process are still challenging to reach the millimeter size due to the large gravity. The gravity would affect the sphericity and size uniformity of microspheres. The improved microfluidic‐assisted internal gelation process is designed and optimized to produce large‐sized monodisperse ceramic microspheres with good sphericity. The movement mechanism of millimeter‐sized sol droplets and gel microspheres in the microchannel is summarized and the state of the dispersed phase entering the measuring cylinder is controlled. The friction between the gel microspheres and the tubing wall is reduced as much as possible, and ZrO2 ceramic microspheres of 500 ± 5 μm with good sphericity are prepared with the broth at room temperature. The mechanism of sol droplets entering the oil surface is explored to demonstrate the superiority of the improved microfluidic‐assisted internal gelation process. The improved microfluidic‐assisted internal gelation process could be directly extended to preparing other millimeter‐sized monodisperse ceramic microspheres.

Effects of a pressurized water treatment on internal gelation sol–gel microspheres

Based on previous observations that a pressurized water treatment (PWT) prevented cracking of sol–gel microspheres, we investigated the effects of a PWT on microsphere crystallinity, density, and specific surface area (SSA). Results were used to determine how a PWT alters the properties of microspheres upon drying and heating. Microspheres with diameters near 100–200 µm were prepared with and without a PWT and measured using x-ray diffraction (XRD), transmission electron microscopy (TEM), nitrogen adsorption (BET), and pycnometry. Properties of air-dried microspheres processed with and without a PWT are compared. In addition, the properties of microspheres processed using a PWT are reported after heating to 150, 450, 750, 1050, and 1350 °C. XRD measurements indicate that the PWT step improves the crystallinity of air-dried microspheres. XRD data were also used to calculate crystallite size, which increases with higher heat-treatment temperatures. TEM images support crystallite size calculations from XRD data and provide an indication of the range of crystallite sizes, particularly for samples processed at higher temperatures where crystallite sizes are too large for estimation using the Scherrer formula. Density and SSA measurements performed as a function of heat-treatment temperature indicate that a PWT increases the density of air-dried microspheres, creates a pore network, and that significant densification occurs between 450 and 750 °C. These results may be used to inform decisions on internal gelation flowsheet parameters to optimize the microsphere formation and gelation step, prevent microsphere cracking, and produce microspheres suitable for subsequent coating operations or pressing into pellets.

Use of a pressurized water treatment to prevent cracking of internal gelation sol-gel microspheres

Pressurized water treatments were investigated as a means to prevent cracking of internal gelation sol-gel microspheres upon drying and heating. A batch of gelled cerium oxide microspheres was prepared, split into two samples that were processed with and without a pressurized water treatment, and dried in air at room temperature. Thermogravimetric analysis was used to compare the mass loss profiles between the two samples and to understand volatile emissions from cerium oxide microspheres during heat treatments up to 600 °C. Microspheres that were processed without a pressurized water treatment lost ~4.6 times more mass upon heating than microspheres that were subjected to a pressurized water treatment. Comparison of the cracking behavior of microspheres prepared with and without a pressurized water treatment indicated that microspheres subjected to a pressurized water treatment were virtually crack-free after heat treatments. Conversely, microspheres processed without a pressurized water treatment, which is typical of traditional washing processes, exhibited extensive cracking. Analysis of residues dried from pressurized water treatment effluents indicated that hexamethylenetetramine and urea, which are excess reactants from the internal gelation process, were removed from gelled microspheres during the pressurized water treatment step. Traditional processing methods for preparing internal gelation microspheres use specific feed solution and gelation parameters to reduce subsequent cracking upon drying and heating. Results from this study indicate that more aggressive gel-forming conditions may be used to produce crack-free microspheres, if a pressurized water treatment step is included after washing and before air-drying.

Impurity levels in cerium oxide microspheres prepared by internal gelation sol–gel methods

Impurity levels were measured in cerium oxide microspheres fabricated by modified internal gelation sol–gel methods. A combination of glow discharge mass spectrometry, electron impact mass spectrometry, combustion gas analysis, and instrumental gas analysis were used to assess a wide range of potential elemental impurities. Low concentrations of carbon, nitrogen, and hydrogen impurities in microspheres showed a dependence on how microspheres were washed. Silicon impurities are believed to derive from silicone oil used during sol–gel processing. Spheres washed by the preferred approach had impurity levels below 100 ppm for all elements tested. For applications such as nuclear fuels, sol–gel methods could meet purity specifications as long as metal nitrate feed solutions of sufficient purity are used and microspheres are washed appropriately. In this study, cerium was used as a surrogate for plutonium-238, which is used in radioisotope power systems as a heat source, to determine whether carbon or other impurities were concentrated during internal gelation processing and remained after heat treatments. Analyses indicated low concentrations of impurities in cerium oxide microspheres after sintering steps that were well below documented limits for plutonium-238 oxide fuels. Modified washing methods, combined with a pressurized water treatment, resulted in sintered cerium oxide sol–gel microspheres with low impurity levels.

Production of monodisperse cerium oxide microspheres with diameters near 100 µm by internal-gelation sol–gel methods

Internal-gelation sol–gel methods have used a variety of sphere-forming methods in the past to produce metal oxide microspheres, but typically with poor control over the size uniformity at diameters near 100 µm. This work describes efforts to make and measure internal-gelation, sol–gel microspheres with very uniform diameters in the 100–200-µm size range using a two-fluid nozzle. A custom apparatus was used to form aqueous droplets of sol–gel feed solutions in silicone oil and heat them to cause gelation of the spheres. Gelled spheres were washed, dried, and sintered prior to mounting them on glass slides for optical imaging and analysis. Microsphere diameters and shape factors were determined as a function of silicone oil flow rate in a two-fluid nozzle and the size of a needle dispensing the aqueous sol–gel solution. Nine batches of microspheres were analyzed and had diameters ranging from 65.5 ± 2.4 µm for the smallest needle and the fastest silicone oil flow rate to 211 ± 4.7 µm for the largest needle and the slowest silicone oil flow rate. Standard deviations for measured diameters were less than 8% for all samples and most of them were less than 4%. Microspheres had excellent circularity with measured shape factors of 0.9–1. However, processing of optical images was complicated by shadow effects in the photoresist layer on glass slides and by overlapping microspheres. Based on the calculated flow parameters, microspheres were produced in a simple dripping mode in the two-fluid nozzle. Using flow rates consistent with a simple dripping mode in a two-fluid nozzle configuration allows for very uniform oxide microspheres to be produced using the internal-gelation sol–gel method.

Cover Picture: Sol-Gel Microspheres Doped with Glycerol: A Structural Insight in Light of Forthcoming Applications in the Polyurethane Foam Industry (ChemistryOpen 2/2015)

The cover picture shows a GreenCaps microcapsule breaking and releasing encapsulated glycerol after the organosilica microspheres are sprayed from a pressurized polyurethane foam can. This shows how glycerol acts as a solid curing agent, promoting crosslinking of partially polymerized diphenylmethane diisocyanate. For more details, see the Full Paper on p. 120 ff.

Study on Preparation and First-Stage Sintering Kinetics of ThO2-UO2Pellets Made by Sol-Gel Microspheres Technique

Study on Preparation and First-Stage Sintering Kinetics of ThO₂-UO₂Pellets Made by Sol-Gel Microspheres Technique

Sol-gel microspheres doped with glycerol: a structural insight in light of forthcoming applications in the polyurethane foam industry.

Invited for this months cover are the groups of Professor Mario Pagliaro at the Istituto per lo Studio dei Materiali Nanostrutturati in Palermo and Professor Laura Ilharco at the Instituto Superior Técnico in Lisboa. The cover picture shows a­ GreenCaps­ microcapsule breaking and releasing encapsulated glycerol after the organosilica microspheres are sprayed from a pressurized polyurethane foam can. This shows how glycerol acts as a solid curing agent, promoting crosslinking of partially polymerized diphenylmethane diisocyanate. For more details, see the Full Paper on p. 120 ff.

Sol-gel microspheres doped with glycerol: a structural insight in light of forthcoming applications in the polyurethane foam industry.

Porous silica-based microspheres encapsulating aqueous glycerol can be potential curing agents for one-component foams (OCFs). Such agents have the advantage of an enhanced sustainability profile on top of being environmentally friendly materials. A synthetically convenient and scalable sol-gel process was used to make silica and organosilica microspheres doped with aqueous glycerol. These methyl-modified silica microspheres, named “GreenCaps”, exhibit remarkable physical and chemical stability. The microspheres were characterized by scanning electron microscopy, transmission electron microscopy at reduced pressure, and cryogenic nitrogen adsorption—desorption analysis. The structure of the materials was also analyzed at the molecular level by diffuse reflectance infrared Fourier transform (DRIFT) spectroscopy. As expected, the degree of methylation affects the degree of encapsulation and pore structure. Microspheres similarly methylated, however, can differ considerably in surface area and pore size due to the templating effect of glycerol on the organosilica structure. The results of the structure analysis reveal that glycerol is efficiently encapsulated, acts as a template, barely leaches over time, but is released by depressurization. A proper application of these microspheres can later on enhance both the environmental and health profile, as well as the technical performance (curing speed, foam quality, and froth thixotropy) of spray polyurethane foams.

Thermogravimetric investigation of sol–gel microspheres doped with aqueous glycerol

Abstract Background Silica-based microspheres encapsulating aqueous glycerol are promising curing agents affording the formation of better one-component polyurethane foams, namely thermoset polymers cured by atmospheric moisture that are widely and increasingly utilized in the construction industry. The use of renewable, non toxic glycerol from biodiesel and oleochemicals industry to cure PU foams in place of traditionally employed oil-derived mono and diethylene glycols is both technically and environmentally beneficial. The higher amount of hydroxyl groups in glycerol compared to both mono- and diethylene glycol results in considerably lower percentage of free monomeric methylene diphenyl diisocyanate (MDI) and higher crosslinking density of the cured foam. Results We investigate by thermogravimetric analysis the nature and amount of the microencapsulated species in sol–gel silica and organosilica microspheres encapsulating aqueous glycerol. Along with a percentage of glycerol in weight up to 35%, the microspheres contain about 5 wt% water and 4 wt% entrapped surfactant. Conclusions The investigation shows the efficacy of the surfactant-assisted sol–gel microencapsulation aqueous glycerol in silica as well as in methyl-silica particles at least until 10% degree of methylation. The results are relevant to the practical development of functional materials that can be used to cure better and greener polyurethane foams largely employed as coatings, adhesives and sealants in many industrial sectors.

Enhanced One-Component Spray Polyurethane Foams via Sol–Gel Microspheres Doped with Aqueous Glycerol

The sol–gel microencapsulation of aqueous glycerol in silica-based microspheres affords functional materials that can be used to cure one-component polyurethane foams (OCF) formulations affording better and greener foam formation. These findings are important and may open the route to more sustainable materials, such as foams, coatings, adhesives, and sealants that are widely utilized in many industrial sectors.

Fabrication of Ceramic Microspheres by Diffusion-Induced Sol–Gel Reaction in Double Emulsions

We demonstrate an approach to prepare zirconium dioxide (ZrO2) microspheres by carrying out a diffusion-induced sol-gel reaction inside double emulsion droplets. A glass capillary microfluidic device is introduced to generate monodisperse water-in-oil-in-water (W/O/W) double emulsions with a zirconium precursor as the inner phase. By adding ammonia to the continuous aqueous phase, the zirconium precursor solution is triggered to gel inside the emulsions. The double emulsion structure enhances the uniformity in the rate of the sol-gel reaction, resulting in sol-gel microspheres with improved size uniformity and sphericity. ZrO2 ceramic microspheres are formed following subsequent drying and sintering steps. Our approach, which combines double-emulsion-templating and sol-gel synthesis, has great potential for fabricating versatile ceramic microspheres for applications under high temperature and pressure.

The controlled release of drugs from emulsified, sol gel processed silica microspheres

Controlled release silica sol gels are room temperature processed, porous, resorbable materials with generally good compatibility. Many molecules including drugs, proteins and growth factors can be released from sol gels and the quantity and duration of the release can vary widely. Processing parameters render these release properties exquisitely versatile. The synthesis of controlled release sol gels typically includes acid catalyzed hydrolysis to form a sol with the molecules included. This is then followed by casting, aging and drying. Additional steps such as grinding and sieving are required to produce sol gel granules of a desirable size. In this study, we focus on the synthesis of sol gel microspheres by using a novel process with only two steps. The novelty is related to acid–base catalysis of the sol prior to emulsification. Sol gel microspheres containing either vancomycin (antibiotic) or bupivacaine (analgesic) were successfully synthesized using this method. Both drugs showed controlled, load dependent and time dependent release from the microspheres. The in vitro release properties of sol gel microspheres were remarkably different from those of sol gel granules produced by grinding and sieving. In contrast to a fast, short-term release from granules, the release from microspheres was slower and of longer duration. In addition, the degradation rate of microspheres was significantly slower than that of the granules. Using various mathematical models, the data reveal that the release from sol gel powder is governed by two distinct phases of release. In addition, the release from emulsified microspheres is delayed, a finding that can be attributed to differences in surface properties of the particles produced by emulsification and those produced by casting and grinding. The presented results represent an excellent data set for designing and implementing preclinical studies.

Emulsified Sol-Gel Microspheres for Controlled Drug Delivery

Controlled release silica sol-gels are room temperature processed, porous, resorbable, and biocompatible materials. Many molecules including drugs, proteins, and growth factors can be released from sol-gels, and the quantity and duration of the release can vary widely. Processing parameters render these release properties exquisitely versatile [1]. The synthesis of controlled release sol-gels involves several steps: an acid-catalyzed hydrolysis to form a sol with the molecules included, followed by casting, aging, and drying. Additional steps such as grinding and sieving are required to produce sol-gel granules of a desirable size. In this study, we focus on the synthesis of controlled release sol-gel microspheres by using a novel process, which involves only two steps:sol formation followed by emulsification. Sol-gel microspheres containing either vancomycin (antibiotic) or bupivacaine (analgesic) were successfully synthesized via this synthesis route. Both drugs showed controlled, load-dependent and time-dependent release from the microspheres. The in vitro release properties of sol-gel microspheres were different from those of sol-gel granules produced by grinding and sieving. In comparison to a fast, short-term release from the granules, the release from the microspheres was slower and of longer duration. In addition, the degradation rate of microspheres was significantly slower than that of the granules. These data enable the use of sol-gel powders for controlled long-term release.

Carbon aerogel spheres prepared via alcohol supercritical drying

We have developed a method that would allow for the fabrication of carbon aerogel (CA) spheres. The inverse phase suspension polymerization of resorcinol and formaldehyde monomers with Na 2CO 3 as a catalyst followed by supercritical drying was explored. The effects of the chemical formulation and processing procedures and the conditions of the structures of organic and related carbon aerogels were studied. The experimental results indicated that it was easy to avoid the accumulation of polymerization heat during gelation, and easy to take out the products from the reaction container, through this fabrication method. Sol–gel microspheres with diameters ranging from about 30–1000μm could be obtained. After drying the sol–gel spheres under alcohol supercritical drying conditions, aerogel spheres with a bulk density of 0.8–1.0 g/cm 3were prepared, and by subsequently pyrolyzing them, CA spheres with surface areas of 250–650 m 2/g were obtained. The resultant CA spheres could be used as the electrode materials of supercapacitors. The specific capacitance of the CA spheres was as high as 215 F/g, and the equivalent series resistance at 48 Hz was about 1 Ω.

In Vitro Behavior of Yttrium Silica Sol-Gel Microspheres

Biodegradable silica microspheres containing neutron activable yttrium cations were investigated as a potential material for in situ radiotherapy of cancers. The sol-gel method used for preparing the microspheres allows obtaining at relative low temperatures materials of high purity and homogeneity, with controlled rate of biodegradability. The influence of yttrium on the biodegradability of the silica microspheres was studied in a simulated body fluid (SBF). The chemical durability of the microspheres was investigated under in vitro conditions and the concentration of yttrium released from the microspheres was determined.

Effect of Colloidal Silica on the Porous Structure Parameters of Microspheres Prepared from Urea Formaldehyde Resin and Silica Gel

The paper concerns studies of the porous structure of spherical microparticles resulting from the polycondensation of urea and formaldehyde in the presence of silica sol (UFR–SG) and silica gel microspheres prepared from the latter. It is disclosed that, when the content of colloidal silica increases, the specific surface area of UFR–SG system rises and the porous structure (pore size distribution) of the obtained silica gel microspheres becomes more uniform.

Effect of heating atmosphere on densification of sol-gel (Th, U)O 2 microspheres

Densification behavior of sol-gel (Th, U)O 2 microspheres in Ar-4%H 2, air or steam is studied and compared. In all the atmospheres studied, they are sintered up to above 99% TD at 1300°C. Below 1300°C, the densification is affected by atmosphere and by U content. In Ar-4%H 2, the densification proceeds by maintaining the most homogeneous microstructure among the three atmospheres and is completed at the lowest temperature; both sinterability and microstructural homogeneity during densification are in the order: Ar-4% H 2 > air > steam. However, once the microstructure has been degraded and becomes inhomogeneous, further densification in Ar-4%H 2 becomes more difficult than in the other atmospheres; the sinterability in this case is in the order: steam > air > Ar-4% H 2. It is indicated that the highest sinterability in Ar-4%H 2 is attributed to very fine (nm order) particles in the sol-gel microspheres, and that the enhancement of oxide fuel sintering in steam as has been reported should be attributed not only to its high oxygen potential but also to some other factors.

Sintering Behaviour of Ceramic Nuclear Fuel Pellets Prepared from Sol Gel Microspheres

Sintering Behaviour of Ceramic Nuclear Fuel Pellets Prepared from Sol Gel Microspheres

A Method for Preparing Dense Spherical Particles of Thorium and Thorium-Uranium Dicarbides

A process, developed for preparing dense, spherical particles of ThC2 and (Th,U)C2, consists of heating dense sol-gel oxide microspheres with lampblack in a rotating crucible under a flow of argon ...