The Role of Impurities

Among the effects of impurities on selective etching, two deserve particular attention. Impurities present in an etching solution facilitate the revelation of etch pits at dislocation and often change the pit morphology. The actual process of impurity adsorption may be quite complicated for different crystal-etchant systems, but certain features are common for different systems. 

In the case of etching of the (100) face of NaCI in ethanol containing CdClj impurity, it may be seen from Fig. 7 that with an increase in impurity concentration C p, Vp regularly decreases, but both v, and v first slowly increase and then, at high c p values after attaining a maximum value, 

It is well known that dislocation etch pits on the surfaces of metals are produced in solutions of salts of other metals as a result of contact displacement reactions (53)(54). The size of pits formed by these solutions depends on the concentration of a salt and the time of etching. However, prolonged etching often leads to the precipitation of mono- or polycrystalline displaced metal at relatively more active sites where dissolution is faster than that at the rest of the surface. Subsequent etching can yield etch hillocks, as observed In the case of etching white tin in acidic solutions of CuSO. Whether etch hillocks or etch pits will be formed at dislocation sites Is determined by the exchange kinetics at the electric double layer and by the diffusion kinetics. 

If A is the metal being etched and B is in the form of the salt, the condition for the displacement reaction to occur is that the electrode potential of B in the solution of B + ions is higher than that of the crystal A in the solution of A M.e.,E(B VB) E(A VA),or, 

Taking displacement reactions as the basis and assuming that the adsorption potential at the site of a dislocation (D), double kink, i.e., a pair of neighboring kinks of opposite sign in a ledge (DK), ledge (L), and surface terrace (T) changes in the sequence D DK K L T, one finds 

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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... 

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. 

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. 

To test the role of impurities, a series of a aluminas, having different contents of mineral impurities, yet at ppm levels, was investigated (Table 1). 

The role of impurities in a liquid during film boiling is not known. Because surface tension appears to be unimportant, at least for tubes of intermediate size, a first guess is that impurities should be of small importance also. Experimental study is indicated. 

The chemical aspect of the Hecker effect was analyzed by Melrose et al. [17]. Kuhn and Argoul [18,19] found that the growth morphology is very sensitive to small chemical perturbations, for example, pH changes, the presence of small amounts of oxygen, and minute quantities of alkali metal ions. Fleury et al. [20] studied the role of impurities and their dynamic behavior. 

The marked effects of disorder in pseudo-ID systems have been clarified by both experiment and theory. These include (a) transport in the absence of collective effects (INV 1, INV 13), (b) the role of impurities in pinning incommensurate CDW s and the effect on charge transport (F3), and (c) the relative effect of impurities on the Peierls and superconducting transition temperature. 

Impurities. The role of impurities cannot be overemphasized. The key to many successful commercial products has been the identification of the right pinch of magic dust. The role of impurities has been extensively studied, and to date their effect can be summarized as follows ... 

Heat capacities as function of temp can be determined directly from the output serial from weighed specimens without the need of a separate calibration (see Sect 5.2.3). Enthalpies are obtained from the integrated areas under the DSC curve, as can the fraction of sample reacted which is proportional to the fraction of the area generated with time or temp. Likewise, reaction temps, initiation temps and the role of catalysts on mixts can be observed directly. The use of DSC for kinetic studies of reactive materials is limited by the tendency of such materials to undergo sublimation or melting. Suitable corrections can be applied to the data by the simultaneous use of TG. The role of impurities and of decompn products in promoting autocatalysis can be investigated by the use of reactive atms or reduced pressures as illustrated by the use of open (perforated) and closed sample pans (Ref 51)... 

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