Precursors condensation

The Stober method can be used to form core-shell silica nanoparticles when a presynthesized core is suspended in a water-alcohol mixture. The core can be a silica nanoparticle or other types of nanomaterials [46, 47]. If the core is a silica nanoparticle, before adding silicon alkoxide precursors, the hydroxysilicates hydrolyzed from precursors condense by the hydroxide groups on the surface of the silica cores to form additional layers. If the core is a colloid, surface modification of the core might be necessary. For example, a gold colloid core was modified by poly (vinylpyrrolidone) prior to a silica layer coating [46]. 

Olefinic Precursor Condensable Species Condensable Species Vapor Pressure, mm Hg Minimal Precursor Concentration ... 

Grigg et al. (34) also conducted extensive studies of the thermal 1,2-prototropic generation of azomethine ylides and this can be exemplihed by the diastereofacially selective cycloaddition of 7-aminocephalosprin ylide precursors. Condensation of aryl aldehydes with 120, in refluxing toluene, furnished imines 121, which, in the presence of A -phenylmaleimide, furnished a mixture of cycloadducts 122 and 123 in essentially quantitative yield in a 2 1 ratio. The only observed products... 

Fig. 2 Scheme of a ALD processing window limited by b precursor condensation, c insufficient reactivity, d precursor decomposition and e precursor desorption. If the deposition rate is dependent on the number of available reactive sites as in f, no actual ALD window is observed... 

Only one synthesis leads to 2 alkyl-4-nitroisothiazolium salts (68) from open-chain precursors. Condensation of nitroketeneaminals with isothiocyanates followed by N—S bond formation using bromine leads to the isothiazoles 68 (20-25%) (Scheme 14).60... 

Another type of reactor using a simple injector is a reactor with a dome type cover.In this case, the injector is installed on the bottom plate of the reactor. Figure 11 shows the typical configuration of this type of reactor. In this case the dome is usually heated to a proper temperature by a separate heater to prevent precursor condensation on it. It is rather difficult to expect uniform deposition of a thin film from this asymmetrical gas injection geometry. However, it has actually been proved that good thickness and compositional uniformity are obtained over the 8 wafer surface for Ta O thin films. Figure 12 shows the typical variation in thickness of a Ta O thin film using this reactor. 

Macroporous VPO Phases. - The macroscale templating of bulk mixed metal oxide phases in the presence of colloidal sphere arrays typically consists of three steps shown in Figure 18. First, the interstitial voids of the monodisperse sphere arrays are filled with metal oxide precursors. In the second step, the precursors condense and form a solid framework around the spheres. Finally, the spheres are removed by either calcination or solvent extraction leading to the formation of 3D ordered macroporous structures [137]. 

Dialdehydes, obtained by periodate oxidation of appropriate cyclic polyhydroxy precursors, condensing with some nitrogen-containing reagents in basic solution has been developed into a useful synthesis of amino sugars. Thus, the dialdehyde 121, on treatment with nitromethane in the presence of sodium methoxide produces a mixture of aci-mim salt isomers 122, which mainly give 3-amino-3-deoxyhexopyranosides 123 and 124 after reduction (O Scheme 58) [96]. 

Well ordered mesoporous silicate films were prepared in supercritical carbon dioxide.[218] In the synthesis in aqueous or alcoholic solution, film morphology of preorganized surfactants on substrate cannot be fully prescribed before silica-framework formation, because structure evolution is coincident with precursor condensation. The rapid and efficient preparation of mesostructured metal oxides by the in situ condensation of metal oxides within preformed nonionic surfactants can be done in supercritical CCU- The synthesis procedure is as follows. A copolymer template is prepared by spin-coating from a solution containing a suitable acid catalyst. Upon drying and annealing to induce microphase separation and enhance order, the acid partitions into the hydrophilic domain of the template. The template is then exposed to a solution of metal alkoxide in humidified supercritical C02. The precursor diffuses into the template and condenses selectively within the acidic hydrophilic domain of the copolymer to form the incipient metal oxide network. The templates did not go into the C02 phase because their solubility is very low. The alcohol by-product of alkoxide condensation is extracted rapidly from the film into the C02 phase, which promotes rapid and extensive network condensation. Because the template and the metal oxide network form in discrete steps, it is possible to pattern the template via lithography or to orient the copolymer domains before the formation of the metal oxide network. 

A second major structural type, commonly known as benzylisoquinoline (BIQ) or tetrahydrobenzylisoquinoline (TBIQ), alkaloids constitute one of the two most numerous types of alkaloids. As is true for the simple isoquinoline alkaloids mentioned above, all are derived from a 3,4-dihy-droxytyramine (dopamine) precursor condensed with a carbonyl compound. In this case, the precursor of most, if not all, benzylisoquinoline alkaloids is 4-hydroxyphenylacetal-dehyde. Benzylisoquinoline alkaloids are found in a limited group of plant families some have great medical impor-... 

The reverse microemulsion method is based on the controlled hydrolysis of tetra-ethoxysilane (TEOS) molecules and their ammonia catalysed condensation like the Stober method, but the reaction milieu is in this case a stable and macroscopically isotropic dispersion of a surfactant and water in a hydrocarbon. In this system the hydrolysis is confined inside the aqueous nuclei where precursors condense to form the nanoparticles. Optimized synthetic protocols and experimental conditions allow one to obtain nanoparticle samples in the dimensional range of about 15-200 nm [70, 76] (Fig. 11). 

Gel formation in organoboron systems involves the partial hydrolysis of the borate precursor, condensation of monomers to form primary units, and linkage of primary units to form extended networks. The hydrolysis and condensation processes as well as the kinetic stability of the borate network are all influenced by the electrophilic behavior of the trigonal boron atom. In solution environments containing molecules that can dissociatlvely react... 

Nucleation corresponds to the first stages of precursor condensation and solid fonnation. What are the parameters that affect the size of nuclei and their rate of formation ... 

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