Composites oxidation problems

Lead oxides Diverse changes of composition and of lattice structure occur during the several successive steps involved in the decompositions of the various lead oxides. Problems of characterization of the intermediates involved have slowed down progress in this topic, because a knowledge of the nature and properties of each intermediate phase is essential for the mechanistic interpretation of kinetic data. The literature includes many references to distorted lattices and non-specific formulae PbO to describe phases of indeterminate and variable composition. Some... 

In many cases it is of interest to have information about compositions in very thin layers of materials, such as the first-formed oxide on an alloy. A group of techniques, which are highly surface sensitive, are available for such determinations. Grabke et al have presented a good review of the application of these techniques in the analysis of oxidation problems. 

Internal pores in SiC/RBSN composites are unavoidable and reduce their oxidation resistance and thermal conductivity. Functionally graded oxidation resistant surface coatings appear to avoid internal oxidation problems for unstressed conditions. 

As described above, dense ceramic membranes are made of composite oxides with a large number of oxygen vacancies in the crystaUine lattice. Such materials are inherently catalytic to the oxidation and dehydrogenation reactions. Therefore, dense ceramic membrane may serve as both catalyst and separator, and catalyst is not required in the membrane reactor. As shown in Fig. 7.5a, the lattice oxygen directly takes part in the chemical reactions. Since the chemical reactions take place on the membrane surface, it is required to have a very porous membrane surface so as to contain a sufficient quantity of active sites. This can be achieved in the membrane preparation process, or by coating a porous membrane material after the preparation. The main potential problems for this are that the membrane may not have sufficient catalytic activity, and the catalytic selectivity cannot be modulated with respect to the considered reactions. 

Sample Preservation Without preservation, many solid samples are subject to changes in chemical composition due to the loss of volatile material, biodegradation, and chemical reactivity (particularly redox reactions). Samples stored at reduced temperatures are less prone to biodegradation and the loss of volatile material, but fracturing and phase separations may present problems. The loss of volatile material is minimized by ensuring that the sample completely fills its container without leaving a headspace where gases can collect. Samples collected from materials that have not been exposed to O2 are particularly susceptible to oxidation reactions. For example, the contact of air with anaerobic sediments must be prevented. 

Surface analysis has made enormous contributions to the field of adhesion science. It enabled investigators to probe fundamental aspects of adhesion such as the composition of anodic oxides on metals, the surface composition of polymers that have been pretreated by etching, the nature of reactions occurring at the interface between a primer and a substrate or between a primer and an adhesive, and the orientation of molecules adsorbed onto substrates. Surface analysis has also enabled adhesion scientists to determine the mechanisms responsible for failure of adhesive bonds, especially after exposure to aggressive environments. The objective of this chapter is to review the principals of surface analysis techniques including attenuated total reflection (ATR) and reflection-absorption (RAIR) infrared spectroscopy. X-ray photoelectron spectroscopy (XPS), Auger electron spectroscopy (AES), and secondary ion mass spectrometry (SIMS) and to present examples of the application of each technique to important problems in adhesion science. 

Quantifying the effect of surface roughness or morphology is difficult, however. Surface preparations that provide different degrees of surface roughness also usually produce surfaces that have different oxide thicknesses and mechanical properties, different compositions, or different contaminant levels. The problem of separation of these variables was circumvented in a recent study [52] by using a modified microtome as a micro milling machine to produce repeatable, well-characterized micron-sized patterns on clad 2024-T3 aluminum adherends. Fig. 2 shows the sawtooth profile created by this process. 

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