The Different Precursors

Alkoxides Yttrium and barium alkoxides can be obtained by direct reaction of the metal with alcohols (10X10- They readily 

However, copper alkoxides with longer chains appear to be more soluble in their parent alcohol. S. Shibata et al. (20) have used the n-butoxides of Y, Ba and Cu dissolved in n-butanol and hydrolyzed with water. They obtain a precipitate of oxides that is composed of a very fine submicron powder that readily sinters starting above 250°C. However, the different reaction rates for the hydrolysis and the precipitation of the three different cations lead to cationic segregation. 

Another way to dissolve the ethoxides was described by Uchikawa et al. (21) and used a mixture of water, diethylenetriamine and acetic acid, in which cations are both hydrolyzed and complexed. Upon solvent evaporation, very viscous thermoplastic gels are obtained from which fibers may be prepared. The unfired fibers contain 45% by weight inorganic materials. They yield good superconducting transitions once heat-treated. 

The nonpolar copper ethoxide and methoxide may be substituted by more polar groups such as N,N-diethylethanolamine or butoxyethanol (16). The resulting alkoxides can be mixed with Y- and Ba-isopropoxides in tetrahydrofuran and hydrolyzed. Hydrolysis appears to be quite complete since the resulting precipitates are almost carbon-free. Thus, crystallization of the superconducting phase occurs at temperatures lower than 800°C. 

Other attempts have used the cations dissolved in a chelating solvent such as methoxyethanol that forms alkoxide species (22)(23). When water is added to a solution of these elements in a ratio between 1 and 4 of H20/alkoxide, gels are obtained that are suitable for thin film deposition. 

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Another point that one has to observe from analysis of Figure 10, is that despite the different precursor atmospheres, and consequently different N precursor partial pressures in the deposition, there is a coincidence of the deposition rate behavior upon nitrogen content (for mixtures other than C2H-N2). This points to a strong dependence of growth kinetics with nitrogen content. 

The object of this chapter will be to review all the different precursors that have been used in the literature and then to describe all the processing difficulties encountered in the different synthesis steps. 

It is important to note that conceptually it seems easy enough to mix the different precursors together, but chemically it s complicated. The individual silanes have different rates of hydrolysis and condensation that can lead to materials that are phase... 

A similar phenomenon was also studied when comparing the gelation process of bis(triethoxysilyl)ethene precursors 61-63140. As a consequence of the different precursors structure, cyclization of 61 and 63 apparently leads to the respective cyclic compound 69, 70 and 71 while no cyclization is observed for 62 having the tram geometry. Here again acidic conditions were found to promote the cyclization phenomenon while basic... 

Rate coefficients have been determined for the decomposition of (C8H8) ions from cyclooctatetraene and styrene to give (C6H6)t [773]. The ions from the different precursors exhibited the same k(E) vs. E curves, once the energy scales were shifted to permit comparisons, and it was suggested that the ions isomerise to a common structure or mixture of structures prior to decomposition after microseconds. 

Fig. 15.5. Schematic models of supramolecular fibrillar assemblies of Afi(l-40) fibrils. Variation in morphology can arise at the level of oligomeric species, protofilaments, or initial short fibrils. They associate together on the quartz surface, creating three types of supramolecular fibrillar assemblies Straight fibrils (Type I), spherulitic assemblies (Type II), and worm-like fibrils (Type III). A mixed architecture of type I and fibrils (Type I/II) was also observed when the internal density is coarse. It is to be noted that the different precursors are represented together in a box and that the relationships between amyloid precursors and final products remain unclear. Reproduced from [18] with permission...

Another consideration concerns the profiles obtained for the different precursor gases. We expect the same radicals to be present in the plasmas of different gases. For example, C2H5 will certainly be produced in an ethane plasma by simple dissociation, but it will also be produced in a methane discharge due to various gas phase reactions. Therefore, it seems reasonable to model the profiles from all cavity positions and for all precursor gases with superpositions of one common set of single-/ profiles. The question that arises... 

In Table 1 are listed the different precursors together with their acronyms that have been used for the CVD of nickel. Mainly two types of ligands and their derivatives are currently used in organometallic chemistry to achieve high volatility, and have been used for the synthesis of these compounds the cyclopentadienyl C H, and the b-diketone CH3COCH2COCH3. In most of these compounds, nickel is found in the oxidation state (+ II). 

The possibUity of multiple pathways in bile acid biosynthesis in man has been discussed by Vlahcevic et al. [180-182]. A number of labelled 7 -hydroxylated intermediates in bile acid biosynthesis were administered to bile fistula patients as well as patients with an intact enterohepatic circulation. In accordance with previous work with bile fistula rats, the spedfic activity of the isolated chohc add was in general considerably lower than that of chenodeoxychohc acid. On the basis of this finding, it was suggested that a portion of chohc acid was synthesized via a route not involving initial 7a-hydroxylation of cholesterol. It must then be assumed that the administered intermediate mixes with the endogenous pool of the same steroid. However, due to compartmentation, the metabolic fate of a precursor reaching the hepatocyte might be different from that of the the same compound formed within the cell. Normally, the different precursors are present in the cells in trace amounts... 

The yield of excited states in organic liquids, and in particular in benzene solutions, is critically compared using results from light emission measurements, light absorption in pulse radiolysis studies, and total chemical yields, in particular as determined by isomerization methods. It is shown that the total yield of excited states originating from solvent excited states and from solute excited states formed without previous solvent excitation is not less than G = 5. Quantitative data are obtained for the yields of the different precursors including singlet and triplet states. 

Fig. 40.2. Reduction of folate to tetrahydrofolate (FH4). The same enzyme, dihydrofolate reductase, catalyzes both reactions. Multiple glutamate residues are added within cells (n 5). Plants can synthesize folate, but humans cannot. Therefore, folate is a dietary requirement. R is the portion of the folate molecule shown to the right of N °. The different precursors of FH4 are indicated in the figure. PABA = para-aminobenzoic acid.

Scheme 1. The different precursors, schematically shown in their monomeric form.

The porous structures obtained by CO2 dissolution foaming from the different precursors previously described are reported in this subsection according to these main parameters. 

Polyene macrolides are synthesited (as are other, better known nonpolyene macrolides) from two- and three-carbon units activated as malcmyl-CoA and methylmakxiyl-CoA. A few polyene macrolides contain an aromatic moiety that derives from p-aminobenzoyl-CbA and is used as starter unit. The different precursor units are condensed by polyketide synthases. Aminosugars, for example, mycosamine or pero mine, are attached glycosidi-cally to the lactone ring. 

All these results confirm that both the onset temperature and the amount of hydrogen evolved foUow the same trend as the reactivity of the carbon precursor in oxygen-transfer carbon-gas reactions, as shown in Figure 1.33. This figure presents a thermogravimetric (TG) analysis of the different precursors done in air (60 mL/min), at S C/min. The TG curves follow the expected order for the reactivity of coals the most reactive is the lignite, whereas the anthracite is the... 

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