Role of impurities

Repeatedly in this book, the important functions of dopants , intentional additives made in small amounts to materials, have been highlighted the use of minor additives to the tungsten used to make lamp filaments is one major example. The role of impurities, both intentional and unintentional, in matters such as phase transformations, mechanical properties and diffusion, was critically reviewed in one of the early seminar volumes published by the American Society for Metals (Marzke 1955). But extreme purity was not considered that came a little later. 

The authors concluded that the side reactions normally observed in amine-initiated NCA polymerizations are simply a consequence of impurities. Since the main side reactions in these polymerizations do not involve reaction with adventitious impurities such as water, but instead reactions with monomer, solvent, or polymer (i.e., termination by reaction of the amine-end with an ester side chain, attack of DMF by the amine-end, or chain transfer to monomer) [11, 12], this conclusion does not seem to be well justified. It is likely that the role of impurities (e.g., water) in these polymerizations is very complex. A possible explanation for the polymerization control observed under high vacuum is that the impurities act to catalyze side reactions with monomer, polymer, or solvent. In this scenario, it is reasonable to speculate that polar species such as water can bind to monomers or the propagating chain-end and thus influence their reactivity. 

PURITY PROBLEMS, THE ROLE OF IMPURITIES 6.3.1 Introductory remarks... 

Because of the very important role of impurities in determining semiconductor properties, it is desirable to know their concentrations, at least of the electrically active ones. Of course, the techniques we have discussed in this chapter never make a positive identification of a particular impurity without confirmation by one of the established analytical techniques, such as spark-source mass spectroscopy (SSMS) or secondary-ion mass spectroscopy (SIMS). Once such confirmation is established, however, then a particular technique can be considered as somewhat of a secondary standard for analysis of the impurity that has been confirmed. It must be remembered here that an analytical method such as SSMS will see the total amount of the impurity in question, no matter what the form in the lattice, whereas an electrical technique will see only that fraction that is electrically active. 

A new mechanism, called the methane-formaldehyde mechanism, has been put forward for the transformation of the equilibrium mixture of methanol and dimethyl ether, that is, for the formation of the first C-C bond.643 This, actually, is a modification of the carbocation mechanism that suggested the formation of ethanol by methanol attaching to the incipient carbocation CH3+ from surface methoxy.460,462 This mechanism (Scheme 3.3) is consistent with experimental observations and indicates that methane is not a byproduct and ethanol is the initial product in the first C-C bond formation. Trimethyloxonium ion, proposed to be an intermediate in the formation of ethyl methyl ether,447 was proposed to be excluded as an intermediate for the C-C bond formation.641 The suggested role of impurities in methanol as the reason for ethylene formation is highly speculative and unsubstantiated. 

The Role of Impurities in Tokamaks 3.1 Effect of Impurities on Plasma Characteristics... 

Before expanding on the role of impurities in defining polymerization rates and yields, it must be acknowledged that there is a dose-rate effect which may be a contributing factor to the discrepancies mentioned. Reports of such an effect have been made, but they are at variance as to the nature of the change in kinetics associated with changes in dose rates (1, 6, 15). In the current work the dose rates have been kept constant. 

More work remains to be done to better understand the role of impurities on water tree growth and we feel that micro-PIXE is a very powerful technique for such measurements since it has the required sensitivity and spatial resolution to provide detailed contour maps of the impurity concentrations, which can then be correlated with the visual tree. Our present micro-PIXE equipment with its 20 micron diameter beamspot is ideally suited for such measurements as it is very easy to use and provides online data. However, the use of only a few point measurements could miss essential components of the tree, and raster scans of the whole tree area would provide more complete information, and at the same time reduce beam induced damage. 

Sometimes the isolation of individual members of the series is not an issue, as in polymer synthesis. Pure neat propellane polymerizes spontaneously in a matter of hours at room temperature. The process can be suppressed by dilution with a solvent or addition of a small amount of a radical inhibitor. Although a possible catalytic role of impurities and Teflon-coated container walls has not been ruled out completely rigorously, it appears likely that this may be a genuine example of a process in which two closed-shell molecules react to produce a biradical which then triggers oligomerization and polymerization. A SINDOl computational study has led to the proposal that the reaction proceeds through a [2]staffane-3,3 -diyl triplet formed by the interaction of two monomers followed by intersystem crossing . ... 

The role of impurities in the defect creation is also controversial. There is no doubt that the density of metastable defects increases when the concentration of oxygen or nitrogen is above about 1 at% (Stutzmann, Jackson and Tsai 1985). However, the likely explanation is that alloying changes the network disorder to allow easier defect creation, rather than the impurity being associated directly with the light-induced defect. Samples of a-Si H still show the effect even when the impurity density is greatly reduced. 

H. C. GATOS (M. LT.) hi connection with the role of impurities in maintaining undersaturation at a source, some recent work that we have performed may be of interest. For indium antimonide, and other HI-V intermetallic compounds, a... 

Insulators for which the forbidden region often has values exceeding 200 kcal. mole i, will prove the most efficient ones, for the transfer by electronic excited states. Intrinsic semiconductors with a much smaller forbidden zone, of the order of 15-30 kcal., can give rise to this kind of phenomena only for a limited number of reactions. Extrinsic semiconductors require an examination in each particular case since the value of the forbidden region varies within large limits. Therefore, the nature of the solid plays the leading part because it fixes the value of the energy gap. The role of impurities is less important, and this constitutes an important difference with respect to activation phenomena. 

It is, in fact, hard to describe carbon as a particular type of catalyst, since it can promote many reactions. A typical cross-section of the relevant literature (Table 2) shows the diversity of some of these reactions. In some cases, the range of reactions is such that the role of impurities as possible catalysts has been investigated. As a result, it is preferable to discuss the catalytic reactions under headings that are fairly individual to carbons, and that reflect one or more aspects considered to be important in each system. 

Elements of Group IV clusters. Investigation of C0-02 mixtures also revealed reactions between Ol and CO. The role of impurity reactions involving H20 is considered in detail and the implications of all data to the vapour-phase radiolysis of C02 are discussed.202 A wide range of heteromolecular clusters containing CO and/or C02 together with S02, NO, or H20 has been found in isentropically expanding jets 203 the observed clusters and their formation conditions are summarized in Table 12. These clusters, particularly the hydrates, are of importance in atmospheric chemistry since favourable conditions for their formation are known to be present in jet-aircraft exhausts.203... 

It is also necessary, however, to recognize that the validity of the underlying assumptions, particularly for processes occurring within the reaction interface, has to be confirmed. The model will need to be developed to account for complicating factors, such as the formation of reaction intermediates, the occurrence of melting, the role of impurities, and the observed topotactic nature of some decompositions. 

In developing a process the chemist may encounter water in the roles of impurity, beneficial additive, or solvent. Some examples of water as solvent and cosolvent were discussed in Chapter 4. Water may also be necessary in the crystallization of a desired hydrate (see Chapters 11 and 12). This chapter will examine some of the more subtle effects of water on processing. 

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