Shrinkage and swelling of alkoxide silica gels on heating part II

Rabinovich, É. M.; Nassau, K.; Miller, Anne; Gallagher, P. K.

doi:10.1016/0022-3093(88)90188-3

Cited by 14 publications

(4 citation statements)

References 8 publications

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“…Ammonium hydroxide is perhaps the most common basic catalyst employed in the synthesis of alcogels, but in spite of its use, the gelation time in the single-step and two-step polymerization protocols is counted in hours or even days. The fluoride catalysts (NaF, NH 4 F, HF) are known for their ability for boosting gelation and reduce its time to minutes; − the mixed system comprising NH 4 F and NH 4 OH also exhibits the same effect . The shortening of gelation is of appreciable advantage for practical purposes, e.g.…”

Section: Introductionmentioning

confidence: 99%

“…However, the question of the influence of F - ions on the alcogel/aerogel morphology has as yet proved elusive. Indeed, extensive investigations carried out in the eighties by Rabinovich et al. , focused mainly on the mechanisms of fluoride ion operation in a silanol environment and less on xerogel texture. Only very recently several papers reported on the effect of reactant concentration on the porous texture of aerogels obtained from the process catalyzed by NH 4 F , or the two-component system of NH 4 F and NH 4 OH. , The mechanism of fluoride anion operation is still not entirely clear,14a and hence it can hardly facilitate prediction of the F - concentration influence on the resulting gels' morphology.…”

Section: Introductionmentioning

confidence: 99%

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Morphology of Silica Aerogels Obtained from the Process Catalyzed by NH₄F and NH₄OH

et al. 1997

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Twelve samples of silica aerogels were synthesized using different concentrations of catalysts (NH4F, NH4OH), and their morphology was investigated by means of small-angle X-ray scattering (SAXS), N2 adsorption, and transmission electron microscopy. Both SAXS and the nitrogen adsorption method consistently show a different influence of the catalysts; fluoride anions promote formation of a tenuous branched-polymeric structure, with mass fractal dimension Dm = 2.3-2.5, while addition of hydroxyl ions results in a sparser structure (Dm = 2.1-2.2) self-similar over a narrower range. The morphology of aerogels obtained from the process catalyzed by fluoride hydroxide resembles that of aerogels produced by the two-step method. This may suggest similar mechanisms of gel formation in both processes, and, consequently, a dramatic acceleration of the hydrolysis reaction caused by NH 4F.

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Section: Introductionmentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

Morphology of Silica Aerogels Obtained from the Process Catalyzed by NH₄F and NH₄OH

et al. 1997

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“…However, using NH 4 F as a acid catalyst, TEOS based alcogel sets easily by single step sol-gel route in less gelation time which is about few minutes. The rapid reduction in the gelation time of the alcogels is due to the use of fluoride catalyst which is known for the ability to boost the gelation mechanism [25,26]. When NH 4 F was added in the single step sol-gel process, the hydrolysis and condensation reactions occurred simultaneously, as shown below.…”

Section: Gelation Timementioning

confidence: 99%

Rapid reduction in gelation time and impregnation of hydrophobic property in the tetraethoxysilane (TEOS) based silica aerogels using NH4F catalyzed single step sol–gel process

Nadargi

Kalesh

Rao

2009

Journal of Alloys and Compounds

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“…It is well known that fluoride ions act as a nucleophilic catalyst and accelerates the hydrolysis and condensation rates of TEOS [116][117][118][119][120]. The increased hydrolysis comes from a nucleophilic attack, where the fluoride ion assists with the removal of an alcohol chain.…”

Section: The Effect Of Fluoride Ionsmentioning

confidence: 99%

Mesoporous Building Blocks: Synthesis and Characterization of Mesoporous Silica Particles and Films

Björk¹

2013

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On the cover, the structure of SBA-15 is illustrated. Inside the pores SEM micrographs have been inserted showing (clockwise from the top): rods, fibers, a film consisting of rods grown on a substrate, a film consisting of platelets grafted on a substrate, short rods, and platelets. In the center the sheet morphology is presented.© Emma Björk, unless otherwise stated.ISBN: 978-91-7519-516-2 ISSN: 0345-7524 Printed by LiU-tryck, Linköping, Sweden, 2013 Research is formalized curiosity. It is poking and prying with a purposeZora Neale Hurston (1903Hurston ( -1960 i ABSTRACTCatalyst supports, drug delivery systems, hosts for nanoparticles, and solar cells are just some examples of the wide range of exciting applications for mesoporous silica. In order to optimize the performance of a specific application, controlling the material's morphology and pore size is crucial. For example, short and separated particles are beneficial for drug delivery systems, while for molecular sieves, the pore size is the key parameter.In this thesis, mesoporous silica building blocks, crystallites, with hexagonally ordered cylindrical pores were synthesized, with the aim to understand how the synthesis parameters affect the particle morphology and pore size. The synthesis of the particles is performed using a sol-gel process, and in order to increase the pore size, a combination of low temperature, and additions of heptane and NH 4 F was used. By variations in the amounts of reagents, as well as other synthesis conditions, the particle morphology and pore size could be altered. Separated particles were also grown on or attached to substrates to form films. Also, a material with spherical pore structure was synthesized, for the first time using this method.It was found that a variation in the heptane concentration, in combination with a long stirring time, yields a transition between fiber and sheet morphologies. Both morphologies consist of crystallites, which for the fibers are joined end to end, while for the sheets they are attached side by side such that the pores are accessible from the sheet surface. The crystallites can be separated to a rod morphology by decreasing the stirring time and tuning the HCl concentration, and it was seen that these rods are formed within 5 min of static time, even though the pore size and unit cell parameters were evolving for another 30 min. Further studies of the effects of heptane showed that the shape and mesoscopic parameters of the rods are affected by the heptane concentration, up to a value where the micelles are fully saturated with heptane. It was also observed that the particle width increases with decreasing NH 4 F concentration, independent of heptane amount, and a platelet morphology can be formed. The formation time of the particles decrease with decreasing NH 4 F, and the growth mechanism for platelets was further studied. The pore sizes for various morphologies were altered by e.g. variations in the hydrothermal treatment conditions, or the method for removing the surfactants.ii T...

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Shrinkage and swelling of alkoxide silica gels on heating part II

Cited by 14 publications

References 8 publications

Morphology of Silica Aerogels Obtained from the Process Catalyzed by NH₄F and NH₄OH

Morphology of Silica Aerogels Obtained from the Process Catalyzed by NH₄F and NH₄OH

Rapid reduction in gelation time and impregnation of hydrophobic property in the tetraethoxysilane (TEOS) based silica aerogels using NH4F catalyzed single step sol–gel process

Mesoporous Building Blocks: Synthesis and Characterization of Mesoporous Silica Particles and Films

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Shrinkage and swelling of alkoxide silica gels on heating part II

Cited by 14 publications

References 8 publications

Morphology of Silica Aerogels Obtained from the Process Catalyzed by NH4F and NH4OH

Morphology of Silica Aerogels Obtained from the Process Catalyzed by NH4F and NH4OH

Rapid reduction in gelation time and impregnation of hydrophobic property in the tetraethoxysilane (TEOS) based silica aerogels using NH4F catalyzed single step sol–gel process

Mesoporous Building Blocks: Synthesis and Characterization of Mesoporous Silica Particles and Films

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Morphology of Silica Aerogels Obtained from the Process Catalyzed by NH₄F and NH₄OH

Morphology of Silica Aerogels Obtained from the Process Catalyzed by NH₄F and NH₄OH