Consecutive processes

The minienvironment approach to contamination control has been increasing in use. A minienvironment is a localized environment created by an enclosure that isolates the product wafer from contamination and people (48). Another approach is using integrated processing, where consecutive processes are linked in a controlled environment (32). Both requite in situ sensors (qv) to measure internal chamber temperatures, background contamination, gas flow rates, pressure changes, and particularly wafer temperature (4). 

Combinations of two or more of the above steps into consecutive processes, such as spin—draw or spin—draw—texture, reduces manufacturing costs. In addition to continuous-fHament yam, nylon is also offered in staple, tow, and flock forms. Staple is made by cutting crimped continuous-fHament yam into 3—20-cm lengths. In manufacturing, tow is made by combining many yam ends, either flat or crimped, to give a total tow size of 6—111 ktex. 

One of the possibilities is to study experimentally the coupled system as a whole, at a time when all the reactions concerned are taking place. On the basis of the data obtained it is possible to solve the system of differential equations (1) simultaneously and to determine numerical values of all the parameters unknown (constants). This approach can be refined in that the equations for the stoichiometrically simple reactions can be specified in view of the presumed mechanism and the elementary steps so that one obtains a very complex set of different reaction paths with many unidentifiable intermediates. A number of procedures have been suggested to solve such complicated systems. Some of them start from the assumption of steady-state rates of the individual steps and they were worked out also for stoichiometrically not simple reactions [see, e.g. (8, 9, 5a)]. A concise treatment of the properties of the systems of consecutive processes has been written by Noyes (10). The simplification of the treatment of some complex systems can be achieved by using isotopically labeled compounds (8, 11, 12, 12a, 12b). Even very complicated systems which involve non-... 

Berwick, A. The Study of Simple Consecutive Processes in Electrochemical Reactions 5... 

In general, the substrate temperature will remain unchanged, while pressure, power, and gas flow rates have to be adjusted so that the plasma chemistry is not affected significantly. Grill [117] conceptualizes plasma processing as two consecutive processes the formation of reactive species, and the mass transport of these species to surfaces to be processed. If the dissociation of precursor molecules can be described by a single electron collision process, the electron impact reaction rates depend only on the ratio of electric field to pressure, E/p, because the electron temperature is determined mainly by this ratio. 

More complicated reactions schemes, including first-order reversible consecutive processes and competitive consecutive reactions, are considered in a textbook by Irwin [89]. Professor Irwin s textbook also includes computer programs written in the BASIC language. These programs can be used to fit data to the models described. 

Figure 3.2 shows the flowsheet for the illustrative example. The data for the example is given in Table 3.1, where i, fc, and is are consecutive processing units, while d2,3 is a dedicated intermediate storage vessel between processing units and is. The time horizon of interest in this example is 9 h. 

The most powerful approach, at least in principle, is the measurement of the rate of the desired reaction as a function of potential and reagent concentration. In essence, any reaction can be written as a set of consecutive steps this is true even if the reaction is apparently a simple process such as the electrolyte deposition of a monovalent cation such as Ag +, since loss of water of hydration from the cation and the (possibly assisted) transport of atoms over the surface to appropriate lattice sites are clearly consecutive processes. 

A good example of a consecutive process in which the second reaction is much slower than the hrst is the reaction occurring in a car exhaust. The engine forms carbon monoxide (CO) as its initial product, and only at later times will CO(g) oxidize to form C02(g). In fact, the second reaction (CO(g) + 02(g) C02(g)) is... 

The kinetics of the thermally induced homogeneous decomposition of phosphine (PH3) have not yet been studied. The species PH2, PH and P2 are formed on flash photolysis of PH3 and could be identified by their absorption spectra63. There are proposals as to the mechanism of the consecutive process after the photochemical primary step, but nothing is known about the kinetic parameters of these reactions. With arsine and antimony hydride only the heterogeneous decomposition has been studied64,65. 

The overall kinetics of crystal precipitation has to consider that the process consists of a series of consecutive processes in simple cases, the slowest is the rate determining step. Assuming the volume diffusion is not the rate determining step, we still have at least the following reaction sequences ... 

Propargylamines could serve as a suitable allyl moiety in aza-Claisen rearrangements. The 3,3-sigmatropic bond reorganization led to allenes, which easily underwent consecutive processes like nucleophile addition and cycliza-tion in a tandem process. 

We come to the conclusion that in the radical cmions of the dinitrocompounds one of the nitro-groups is excluded from contributing to the spin density distribution, most probably by fast protonation by the solvent. In this way a reduction process is started. That such reactions may play a significant role in the consecutive processes of 3,5-dinitroanisole radical anion is substantiated by the formation of 3,3 -dimethoxy-5,5 -dinitroazoxybenzene upon electrolysis of 3,5-dinitroanisole in acetonitrile-water (1 1). 

A remarkable number of organic compounds luminesce when subjected to consecutive oxidation-reduction (or reduction-oxidation) in aprotic solvents1-17 under conditions where anion radicals are oxidized or cation radicals are reduced. In many instances, the emission is identical with that of the normal solution fluorescence of the compound employed. In these instances the redox process has served to produce neutral molecules in an excited electronic state. These consecutive processes which result in emission are not special examples of oxidative chemiluminescence, but are more properly classified as electron transfer luminescence in solution since the sequence oxidation-reduction can be as effective as reduction-oxidation.8,10,12 A simple molecular orbital diagram, although it is a zeroth-order approximation of what might be involved under some conditions, provides a useful starting... 

The simple second-order nature of the kinetics in this system leads to immediate conclusions of some consequence. The rate-determining step is clearly not the heterolytic breaking of a metal-sulfur bond to produce the free R-S group, which then might undergo reaction. Further, the fact that there is no evidence suggesting consecutive processes eliminated the possibility that any such scheme could enter into the total rate except essentially as a pre-equilibrium—e.g., Equations 14 and 15. 

They proposed a polymerization scheme closely related to other well-known chemical reactions of metal alkoxide with carbonyl compounds (20). In Scheme 2, complex [A] is converted to [B] by hydride ion transfer from the alkoxyl group to the carbon atom of aldehyde (Meerwein-Ponndorf reduction). Addition of one molecule of monomer to the growing chain requires transfer of the alkoxide anion to the carbonyl group to form a new alkoxide [C]. Repetition of these two consecutive processes, i.e., coordination of aldehyde and transfer of the alkoxide anion, constitutes the chain propagation step. 

Distillation of Ammonium Bicarbonate Solutions. Vapor-liquid equilibrium data for ammonium bicarbonate solutions at the boil are apparently not available in the literature. The data in the literature, however, do indicate that when the temperature of such a solution is increased, or the pressure on it decreased, the gas that is evolved is predominantly carbon dioxide. Thus, it appears that such a distillation would be two consecutive processes first, a steam stripping of the carbon dioxide in the solution, followed by a distillation of ammonia from an ammonia-water mixture containing perhaps some carbon dioxide. Possibly the ammonia, carbon dioxide, and water in the distillate product would recombine completely in the condenser to form an ammonium bicarbonate solution. Perhaps an absorption tower would be necessary to effect the recombination. 

A quantum chemistry study of the reaction of chloroprene with CI2 has revealed two transition states for substitutive chlorination, which is consistent with two consecutive processes chlorination to give a carbocation followed by abstraction of the originally allylic proton. On the other hand, a single transition state was observed for additive chlorination. The potential barriers for the former process lay below that for the latter102. 

The kinetic results agree with a mechanism involving an exciplex as a reaction intermediate, but it was not explicitly stated that the data are incompatible with a scheme in which exciplex formation and adduct formation are competing instead of consecutive processes. 

For a fast reaction, the time scales for mixing and reaction are of the same order (i.e., iV2m bar), so mixing and reaction are no longer consecutive processes but simultaneous ... 

Table VI lists various ways in which the elimination of small molecules has been used to produce silicon-transition-metal bonds most can be pictured as proceeding via consecutive processes of oxidative addition and reductive elimination. Dihydrogen may result from reaction between compounds with M-H and Si-H bonds (entries 1-10).

---