Monolith Formation Research Articles

In Situ Fluorescence Probing of Molecular Mobility and Chemical Changes during Formation of Dip-Coated Sol−Gel Silica Thin Films

Pyranine (trisodium 8-hydroxy-1,3,6-pyrenetrisulfonate) and PRODAN (6-propionyl-2-(dimethylamino)naphthalene) are used as in situ fluorescence probes to monitor the molecular mobility and chemical evolution during sol−gel silica thin film deposition by the dip-coating process. Measurements are made on the evolving film as it emerges from the sol at thicknesses changing from <4 μm for the starting film to 300 nm for the final film. The thickness is monitored by interferometry. Results of the spatially resolved fluorescence depolarization for films withdrawn from a fresh sol show that the tumbling motion of both probes is retained until the final deposition stage when an extensive cross-linking silica framework forms. Intermediate probe mobility occurs at an earlier time in films pulled from an aged sol. A comparison between thin film deposition and monolith formation is made.

Nanoparticle synthesis of titanium silicalite for fiber, film, and monolith formation

Zeolite fiber, film, and monolith were prepared by using nanosize titanium silicalite (TS-1) crystals. Films and fibers of TS-1 zeolite were optically transparent, which might be important for advanced optozeolitic materials. A permeation result of gases on monolith indicates that the gas permeation is governed by the Knudsen mechanism. All types of TS-I zeolites manufactured in this work are observed to be MFI structure with an orthorhombic symmetry. These results suggest that nanosize zeolites have potential to allow morphological design with applications for advanced functional materials.

Carbonate hydroxyapatite gel monolith formation and drying

The effect of carbonate in reducing the crystal size of precipitated hydroxyapatite by approximately an order of magnitude has not been used previously in the preparation of gel monoliths for the fabrication of carbonate hydroxyapatite ceramics. The aim of this study was to devise a method whereby gel monoliths of carbonate hydroxyapatite could be repeatably produced without cracking. A precipitation reaction was used for the preparation of carbonate hydroxyapatities with carbonate contents of 5.8 and 7.8 wt%. Biaxial vacuum filtration was used to form disc shaped monoliths. The rate of filtration of a 7.8 wt% carbonate hydroxyapatite sol was measured throughout the gelation process. Gel monoliths were dried slowly in air and the mass and dimensions of the gel were recorded once approximately every 24 hours. Using this data, the permeability, water volume fraction with time, rate of water loss, gelation point and gel density were determined. The pore size distribution was measured using mercury porosimetry for a carbonate apatite gel and a pressed powder pellet of a commercial hydroxyapatite. In tact monoliths were formed with masses up to 9.9 g. It was found that gelation behaviour was independent of monolith size and carbonate content and the final green density of all monoliths was 37%. Gelation was found to occur at 50-55 vol% water. Gel monoliths were found to have a monomodal pore size distribution with a mean pore size of 9.1 nm, whereas a pressed pellet of hydroxyapatite had a bimodal pore size distribution.

ChemInform Abstract: Low‐Temperature Preparation of Ultrafine Rare‐Earth Iron Garnets.

Ultrafine rare-earth iron garnets, (R3Fe5O12 where R = Sm, Tb, Dy, Ho, Er, Yb, (YGd), and (YNd)) have been prepared by thermal decomposition of a citrate precursor, R3Fe5(cit)25· (36 +n)H2O. The precursors decompose at lower temperatures, below 450°C, and are characterized using DTA, DSC, TG, and IR spectroscopy. Ultrafine amorphous garnets having particle size 10 to 35 nm and surface area 30 to 75 m2/g have been obtained and characterized by XRD, TEM, Mossbauer spectra, particle size analysis, and magnetic and surface area measurements. Superparamagnetism indicates the ultrafine characteristics of the garnet materials. The nature of crystallite aggregates and agglomerates is of special interest because it represents finite clusters. An intercrystallite bond exists between crystallites having 1.0- to 1.5-nm size. The rupture of intercrystallite bonds during crystallization leads to monolith formation.

Low‐Temperature Preparation of Ultrafine Rare‐Earth Iron Garnets

Ultrafine rare‐earth iron garnets, (R3Fe5O12 where R = Sm, Tb, Dy, Ho, Er, Yb, (YGd), and (YNd)) have been prepared by thermal decomposition of a citrate precursor, R3Fe5(cit)25· (36 +n)H2O. The precursors decompose at lower temperatures, below 450°C, and are characterized using DTA, DSC, TG, and IR spectroscopy. Ultrafine amorphous garnets having particle size 10 to 35 nm and surface area 30 to 75 m2/g have been obtained and characterized by XRD, TEM, Mössbauer spectra, particle size analysis, and magnetic and surface area measurements. Superparamagnetism indicates the ultrafine characteristics of the garnet materials. The nature of crystallite aggregates and agglomerates is of special interest because it represents finite clusters. An intercrystallite bond exists between crystallites having 1.0‐ to 1.5‐nm size. The rupture of intercrystallite bonds during crystallization leads to monolith formation.

Alumina Monolith Formation by Flocculation of Boehmite Sols

Flocculating a boehmite sol with ammonia results in drying and shrinkage uniformity which contributes to the formation of a dry monolithic gel. It is demonstrated that flocculation immobilizes the randomly dispersed platelet‐shaped boehmite particles, inducing uniform shrinkage and density in the gel structure. Flocculation also increases the rate of gelation and an increased gel integrity is attributed to the cumulative effects of both van der Waals attractive forces and increased hydrogen bond formation (gelation). Flocculation does not adversely effect either phase transformation or sintering of the seeded boehmite gels.