Volatile precursors

Gallium compounds that incorporate amidinate ligands have been studied because of their potential use as volatile precursors to nitride materials.The two most common classes of Ga amidinate compounds, however, are [RC(NR )2]GaX2 species, X = Cl, alkyl I in Scheme 32), in which one amidinate... 

Of course, commercially available transition metal complexes are stable at room temperature because they have achieved an 18-electron noble gas-like electronic configuration. Thus, molecules like iron pentacarbonyl [Fe(CO)s], ferrocene [Fe(C5H5)2], as well as piano-stool complexes such as C5H5Co(CO)2 are chemically quite inert. In order to study bimolecular reactions, it is necessary to first prepare unsaturated complexes. For studies using molecular beams, one approach is through photolysis of a stable volatile precursor in a supersonic nozzle. 

Silver(I) /3-diketonate derivatives have received significant attention due to the ease with which they can be converted to the elemental metal by thermal decomposition techniques such as metal organic chemical vapor deposition (MOCVD).59 The larger cationic radius of silver(I) with respect to copper(I) has caused problems in achieving both good volatility and adequate stability of silver(I) complexes for the use in CVD apparatus. These problems have been overcome with the new techniques such as super critical fluid transport CVD (SFTCVD), aerosol-assisted CVD (AACVD), and spray pyrolysis, where the requirements for volatile precursors are less stringent. 

The apparatus and techniques of ion cyclotron resonance spectroscopy have been described in detail elsewhere. Ions are formed, either by electron impact from a volatile precursor, or by laser evaporation and ionization of a solid metal target (14), and allowed to interact with neutral reactants. Freiser and co-workers have refined this experimental methodology with the use of elegant collision induced dissociation experiments for reactant preparation and the selective introduction of neutral reactants using pulsed gas valves (15). Irradiation of the ions with either lasers or conventional light sources during selected portions of the trapped ion cycle makes it possible to study ion photochemical processes... 

In chemical vapor deposition (CVD or MOCVD), a film of the desired material is prepared by evaporation of volatile precursor molecules which then decompose to give a film deposited on the substrate. The ordering in the film as it grows is dictated by the surface ordering of the substrate, hence the deposition is epitaxial . The necessary decomposition of the precursor molecules can take place either on the surface of the substrate or in the gas phase close to it. 

Model results in seawater are in good agreement with observational data of PFOA. Most differences can be attpageributed to deficiencies of the emission scenario. Despite this fact, the difference between model results and observational data are due to the limited horizontal and process resolution and the fact that the physical parameters of the model (temperature, surface pressure, vorticity or divergence of the wind velocity field) were not relaxed to observational data. Regarding these limitations, in particular individual vertical profiles compare quite well with observations. This study underlines the importance of the ocean as a transport medium of PFOA. The contribution of volatile precursor substances to long-range transport needs to be assessed. 

In the absence of suitable volatile precursors, aerosol-assisted CVD (AACVD) has been used, whereby a precursor solution is atomized and the micron-sized particles deposited onto a heated substrate to grow into thin films [98], LSM thin layers (4 to 5 4m) have been deposited in this way [99-100],... 

In this example, the volatile precursor compounds were heated to obtain the product. Other energy sources are also used, notably electromagnetic radiation. An example of vapour phase deposition involving photo-decomposition is given in the next section on vapour phase epitaxy. 

The curves show the general shape of the 235U fission mass-yield curve. The yields at masses 89, 91, and 140 are down because of their gaseous and volatile precursors (see text). The yields at masses 103 and 129 are also low owing to the volatile prop-ties of the elements Ru and Te... 

Figure 1. Correlation of the ratio 140Ba/"Mo with the ratio 156Eu/"Mo in a set of airborne-debris samples from a nuclear event. 156Eu has only refractory species for precursors the mass-99 chain exhibits volatile behavior, perhaps owing to the volatility of MoOs. The strong negative correlation indicates that 140Ba has at least some volatile precursors.

Carbon films and graphite films have been prepared by vacuum evaporation [28,42,76,77], pyrolysis [29,36,78-83], screen printing [46,62,63,65,66,84], and laser photoactivation of sites on a graphite or glassy carbon substrate [85]. Various pyrolytic processes have been successful, most based on the deposition of volatile precursor compounds. For example, methane can be pyrolyzed while... 

Concerning the application of gallium alkoxides, one can mention the selective catalytic activity of Ga(OPh)3in the condensation reactions of isobutene with phenols. In(OR)3 is used for the preparation of solutions for production of ln203 and In2Oj-related conduction films [1618] and also in the synthesis of volatile precursors for MOCVD deposition of In2Oj [830]. 

At distances of about 100 km from ground zero of the Nevada tests, the particle size of fallout ranged from a few jum to a few 100 jum. Fission products with volatile precursors were enhanced by about a factor two compared with refractory fission products. The fractionation was greater the smaller the particles (Hicks, 1982). 

These examples illustrate the important general point that the metal silicon ratio of the silicide film is always the same as that in the volatile precursor. 

These reactions appear to involve a rapid stripping process, reminiscent of that occurring in a mass spectrometer (Section IV,D). Hydrogen and CO are lost before the system is able to reach equilibrium the products are therefore different from those formed in sealed-tube reactions and favored thermodynamically. Provided that it is possible to synthesize sufficiently volatile precursors with the desired metal sili-... 

Volatile Precursors. Investigations involving model systems for meat aromas have been revieved (31-34). More than 600 volatiles have been identified from meat or simulated meat precursors. 

Precursor transport Although related to the volatility described earlier, transport is mentioned separately here because of potential impact on this area. Liquid delivery methods have demonstrated advantages over conventional delivery methods for thermally unstable and low-volatility precursors, as illustrated by some of the examples described in this review. The development of improved transport techniques might alleviate the necessity for better volatilities and thermal stabilities required by traditional delivery methods of these precursors. However, whether liquid delivery methods will be adopted into Si-based manufacturing processes remains to be seen. It seems more likely that these transport methods will be used in applications where less stringent control over purity is acceptable. 

The most important applications of organolanthanides are as Ziegler-Nutta catalysts in polymerization of olefins and as reagents in stoichiometric organic synthesis. Organolan-thanide compounds are also useful as volatile precursors for MOCVD applications in the... 

Chemical Vapor Deposition CVD is a crystal growth process whereby a solid material is deposited from the gas phase onto a controlled substrate using a suitable mixture of volatile precursor materials which react to produce the desired deposit on the substrate surface (Table 4).Types of films and structures that can be produced include the following ... 

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