Gels, amorphous

The fact that the initial setting process for magnesium oxychloride cements takes place without observable formation of either the 5 1 8 or the 3 1 8 phase is important. It indicates that formation of an amorphous gel structure occurs as the first step, and that crystallization is a secondary event which takes place from what is effectively a supersaturated solution (Urwongse Sorrell, 1980a). This implies that crystallization is likely to be extremely dependent upon the precise conditions of cementition, including temperature, MgO reactivity, heat build-up during reaction and purity of the components in the original cement mixture. 

The 11 nm-sized Ti02 were crystallized using either hydrothermal or thermal methods from 100 nm, amorphous gel spheres. The Ti02 crystal and agglomerate sizes were determined by X-ray diffraction (Philip 1080) and transmission electron microscopy (JEOL JEM 2010), respectively. The surface area and chemistry of the nanostructured Ti02 were analyzed by nitrogen physisorption (Coulter SA 3100) and Fourier transform infrared spectroscopy (FTIR, Perkin-Elmer GX 2000). Metal catalyst was deposited by incipient... 

Evans D. L. Adams J. B. (1980) Amorphous gels as possible analogs to martian weathering products. Proc. ll Lunar Planet. Sci. 

Figure 8 illustrates the transformations induced on the surface structure of HT materials after 10 days of corrosion. The amorphous gel layer that appears during corrosion is observed on most HT samples and reveals pits, holes, and corrosion paths from case to case. For many samples, crystalline secondary phases in the 1-10 pm range cover the altered surface, and spallation/cxfoliation of the gel layer is observed or at least suggested in several instances, indicating that corrosion is a discontinuous process, even under static conditions. 

The forms of SiC found in sediments and sedimentary rocks are quite varied but those which could be suspected of near surface origin are generally as follows quartz, chalcedony, opal, amorphous gels and ionic forms in solution. Natural occurrences indicate that the solid forms of silica precipitate which has crystallized after the time of initial deposition (Siever, 1962). 

Zeolite crystallization can be interpreted in terms of a ripening mechanism. The initially formed gel consists of amorphous dispersed particles of the order of 100-300 A in size. Growth of these particles to approximately 1000 A occurs during the induction period after which zeolite crystals appear imbedded in the amorphous gel matrix. This is especially evident in electron microscopic studies of gel solids (66,88). Ciric comments on the observation of growing crystals imbedded in gel particles which, as the crystals grow, tend to shrink together, resulting in coalescence (74)-... 

Some materials (for example, certain muscle proteins) form fibers spontaneously or are naturally found in fibrous form. Many other polymeric substances, like DNA, can be induced into fibers by pulling them from an amorphous gel with tweezers or a glass rod. For data collection, the fiber is simply suspended between a well-collimated X-ray source and a detector, such as film (Fig. 9.1). 

As previously described, the hydrothermal synthesis of aluminosilicate zeolites is carried out with highly reactive aluminosilicate gels in autogenous conditions. This kind of synthesis may involve the structure-directing role of alkaline cations in solution-mediated crystallization of the amorphous gel [55],... 

Collagen is a major structural element in connective tissues skin, tendons, muscle, and internal organs. It combines with inorganic compounds in bones and teeth. Cartilage is collagen mixed in an amorphous gel. Dentine, which makes up the bulk of a tooth, is a mixture of collagen and hydroxyapatite (a mineral), and water. 

Carnivorous an organism that eats or lives on the tissue of animals (meat). Cartilage fibrous collagen in an amorphous gel. 

The use of a law typical of the solid state to describe the growth of zeolites was probably suggested by the presence of an amorphous gel in the synthesis medium. The rearrangement of this phase to form a zeolite network has been a favored theory until researchers showed that zeolites can nucleate and grow directly from solutions free of suspended solids (9). 

Cuprous oxide is produced in the form of an orange-yellow, amorphous gel containing water by the reduction of an alkaline cupric solution with sodium hyposulphite 7... 

The aluminum in the zinc phosphate cements was considered very important, van Dalen [21] recognized its importance first. The reaction of zinc oxide and phosphoric acid was greatly moderated by aluminum. This effect was attributed to formation of an aluminum phosphate gelatinous coating on zinc oxide particles. In fact, Wilson and Nicholson believe that the gelatinous substance may even be zinc aluminophosphate phase [3], which subsequently crystallizes into hopeite and aluminophosphate amorphous gel (AlP04-nH20). 

Titanium and zirconium hydroxides. It is difficult to obtain these hydroxides in crystalline form, therefore, the effect of mechanical activation on dehydration and crystallization of amorphous gels at room temperature was studied in [12]. 

Figure 2.53. The crystal structure of tobermorite, viewed along the be plane. The amorphous gel produced during cement formation likely contains defects such as missing/disordered silicate tetrahedra and/or water sites. Reprinted from Chem. Geol. 2000, 167,129, Copyright 2000, with permission from Elsevier.

If ten times the amount of water is added to the undried 5% Si02 content gel than required to fill the pore voids a much more dense silica-alumina jAiase is formed than in the previous example. After drying at 120 and calcination at 500 a material of 283 m2/g surface area and a pore volume of 0.9 oc/g is formed. The average pore diameter of this material is 8 nm. There is an unmistakable parallel in the recrystallization of the silica-free gel to the 5% silica-content silica described here. Addition of limited water to the amorphous gel produces large pores and a hi pore volume. Addition of water so that a slurry is formed with the amorphous gel results in smaller pores and a relatively dense phase i n recrystallization. 

Nanocrystalline niobium(V) oxide fihns are accessible by an extended sol-gel method (Nb(OEt)5 + 2acacH + H2O in ethanol solution) as reported by Brooks and coworkers . Autoclaving leads to amorphous gels, from which homogeneous nanocrystalline, porous and crack-free Nb205 fihns can be obtained. The orthorhombic T-phase is formed on annealing at 600 °C. 

Solvothermal reactions in alcohols are sometimes called alcohothermal reactions this word is derived from alcoholysis based on the similarity between hydrothermal and hydrolysis. Alcohols are the most common solvents for sol-gel synthesis. Primary alcohols are fairly stable at higher temperatures (up to 360°C) and therefore are widely used for the solvothermal reactions." For example, amorphous gel derived by hydrolysis of metal alkoxides can be crystallized by solvothermal treatment in alcohols. Since lower alcohols (methanol, ethanol, and 1-propanol) are completely miscible with water, water molecules present in the precursor gel may be replaced with the solvent alcohols. Therefore the precursor gel is easily dispersed in the solvent, where crystallization takes place. Detailed mechanisms for the formation of crystals are not yet fully elucidated. Crystallization of metal oxides is usually reported to take place by dissolution-recrystallization mechanisms, but the mechanism seems to depend on the gel structure. Moreover, water molecules dissolved from the gel in the reaction medium may facilitate crystallization of the product. More discussion is given in Section III.D of this chapter. 

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