Other Surface Analytical Methods

Recently, it has been shown by a combination of XPS with other surface-analytical methods (UPS, ISS) that Ae Na+ distribution in partially exchanged NaH zeolites can give rise to another difference between surface and bulk properties [2]. After the typical thermal treatments used to obtain acidic protons (calcination of the NH4 form, reduction of reducible cations cf. also [47]), Na+ is depleted in the external surface layer. The effect was interpreted as resulting from an exchange of Na+ at near-surface sites by protons from deeper layers due to a difference in Madelung stabilization for Na ions and the less-charged protons between surface and bulk sites. 

Still falling prices for the instruments described above will support even wider use. Combination with other surface-analytical methods will help in more careful interpretation of results. 

In this chapter we introduce high resolution diffraction studies of materials, beginning from the response of a perfect crystal to a plane wave, namely the Bragg law and rocking curves. We compare X-rays with electrons and neutrons for materials characterisation, and we compare X-rays with other surface analytic techniques. We discuss the definition and purpose of high resolution X-ray diffraction and topographic methods. We also give the basic theory required for initial use of the techniques. 

Appropriate methods for the study of the materials surface have been treated by the keyword Surface Analytical Methods. Especially XPS is a powerful tool even for practical corrosion problems and other problems which may occur at surfaces or surface layers. In many cases one may give solutions to practical problems caused by contaminations, wrong treatment of the surfaces and unexpected surface attack due to changes in the environment. This method has been applied to many practical problems in the macroscopic and microscopic environments ranging from large metal constructions to the micro- and nanoworlds of electronics. 

A thin-layer electrophoretic method for separation of pilocarpine from drug sources has been developed. Other recent analytical methods used for the determination of pilocarpine include t.l.c., potentiometry with ion-selective membrane electrodes,and photodensitometry. Histamine (7) has been found not to be active at the water-air interface by electric-potential and surface-tension measurements of aqueous solutions of the alkaloid. ... 

As in the case of other stuface analytic methods, in order to examine well-defined stufaces, the sample surface must be prepared by cleaning and subsequent treatments such as adsorption and/or annealing processes in an ultrahigh vacuum (UHV). After that, the surface species must be desorbed from the surface by a specific excitation method and are analyzed by a specific detector. To induce desorption of the adsorbed species or the surface atoms, the surface is thermally excited by increasing the temperature (as in most cases) or electronically excited by irradiation... 

Generally, passive layers are not simple homogeneous oxide or hydroxide films. Usually, they have at least a bilayer or a multilayer structure, even for pure metal substrates. As already mentioned in Sec. 1.4.2 on surface analytical methods, oxides are located at the metal surface followed by an overlayer of hydroxide. Lower valency species are located inside, whereas higher valency cations are found in the outer parts of the films. In the case of alloys, very specific depth profiles are found which are related to the specific passive properties of these metals (Strehblow, 1997). The accumulation of one component relative to the other is a consequence of the thermodynamics of its anodic oxidation or its dissolution characteristics. As a consequence, one metal component may be accumulated at the metal surface, e.g., copper for AI/Cu (Strehblow et al., 1978) (Fig. 1-28) and copper for Cu/Ni alloys (Druska et al., 1996 Druska and Strehblow, 1996). The preferential dissolution of iron and the accumulation of Cr(III) within the film is another example (Fig. 1-27). 

The above comments are seen to be reinforced by observations on the failure path in joints before and after environmental attack. The locus of joint failure of adhesive joints when initially prepared is usually by cohesive fracture in the adhesive layer, or possibly in the substrate materials. However, a classic symptom of environmental attack is that, after such attack, the joints exhibit some degree of apparently interfacial failure between the substrate (or primer) and the substrate. The extent of such apparently interfacial attack increases with time of exposure to the hostile environment. In many instances environmental attack is not accompanied by gross corrosion and the substrates appear clean and in a pristine condition, whilst in other instances the substrates may be heavily corroded. However, as will be shown later, first appearances may be deceptive. For example, to determine whether the failure path is truly at the interface, or whether it is in the oxide layer, or in a boundary layer of the adhesive or primer (if present), requires the use of modern surface analytical methods one cannot rely simply upon a visual assessment. Also, the presence of corrosion on the failed surfaces does not necessarily imply that it was a key aspect in the mechanism of environmental attack. In many instances, corrosion only occurs once the intrinsic adhesion forces at the adhesive/substrate interface, or the oxide layer itself, have failed due the ingressing liquid the substrate surface is now exposed and a liquid electrolyte is present so that post-failure corrosion of the substrate may now result. 

Over the years, other national and international bodies have developed procedures which apply to surface analytical methods. ISO documentary standards take a considerable time to write, and frequently it can take seven years from the initial proposal to the draft international standard stage [5], Hence, to avoid replication and to increase the rate of development of the ISO standards, ISO TC201 has been particularly active in setting up links with many of these associations. and the present Category A liaisons are with the following bodies ... 

Hydrogen Hydrogen is difficult to detect with many surface analytical methods. In HREELS, the adsorption of hydrogen and deuterium is relatively easy, both experimentally, and in first instance interpretation of the spectral features. In the case of hydrocarbons and other molecules, the CH, NH or OH modes are often studied. The CH modes in aromatic molecules are observed to be partially impact modes. Figure 7 shows several OH modes in particular it is possible to observe natural line width broadening for the 8(0-H) mode on Ag(llO) with the new high resolution spectrometers. 

The growth may be examined by the evaluation of electrochemical currents and charges taking into account the corrosion current density as described above. Other methods are based on the application of in situ ellipsometry or electrochemical quartz crystal micro-balance or of surface analytical methods working in UHV like XPS, AES, ISS, and RBS, sometimes in combination with sputter depth profiling. Examples are given in the following section. 

This section presents a detailed discussion of the passivity of iron. It is intended as an example of the procedure and strategy in which the results of several electrochemical and surface analytical methods can be combined to get to a clear view and a sound model of the nature of passivity. For other pure metals and alloys, the same methods have been applied. However, the results for the other systems will be presented in a more condensed form to avoid redundancies and to keep the chapter sufficiently short. References to a more detailed description in literature are given. 

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