Buried metals, stability

The stability of buried metals largely depends on a combination of pH and redox (Edwards 1996). Under high redox values (oxidizing conditions) most metals will easily corrode, whereas under low redox values (reducing conditions) they will tend to remain as uncorroded metal. In addition, acidic conditions (low pH) will assist corrosion, whereas alkaline conditions will tend result in the formation of a stable corrosion matrix in most metals. Thus, in a well-drained, acidic sand or gravel site, all metals except the most inert (e.g., gold) will corrode rapidly and extensively. However, under most other burial conditions, most metal will be capable of recovery, albeit in a corroded state even after many centuries. 

There are, however, continuing difficulties for catalytic applications of ion implantation. One is possible corrosion of the substrate of the implanted or sputtered active layer this is the main factor in the long-term stability of the catalyst. Ion implanted metals may be buried below the surface layer of the substrate and hence show no activity. Preparation of catalysts with high surface areas present problems for ion beam techniques. Although it is apparent that ion implantation is not suitable for the production of catalysts in a porous form, the results indicate its strong potential for the production and study of catalytic surfaces that cannot be fabricated by more conventional methods. 

Abstract. We describe the state-of-the-art in the creation of ordered superlattices of adsorbed atoms, molecules, semiconductor quantum dots, and metallic islands, by means of self-assembly during atomic-beam growth on single crystal surfaces. These surfaces often have long-period reconstructions or strain relief patterns which are used as template for heterogeneous nucleation. However, repulsive adsorbate-adsorbate interactions may also stabilize ordered superlattices, and vertical correlations of growth sequences of buried islands will be discussed in the case of semiconductor quantum dots. We also present new template surfaces considered as particularly promising for the creation of novel island superlattices. 

After adsorption one side of the protein molecule is oriented towards the sorbent surface, turned away from the aqueous solution. As a consequence, hydrophobic parts of the protein that are buried in the interior of the dissolved molecule may become exposed to the sorbent surface where they are still shielded from contact with water. Because hydrophobic interaction between apolar amino acid residues in the protein s interior support the formation of secondary structures as a-helices and P-sheets, a reduction of this interaction destabilizes such structures. Breakdown of the a-helices and/or P-sheets content is, indeed, expected to occur if peptide units released from these ordered structures can form hydrogen bonds with the sorbent surface. This is the case for polar surfaces such as oxides, e.g. silica and metal oxides, and with sorbent retaining residual water at their surfaces. Then the decrease in ordered secondary structures leads to an increased conformational entropy of the protein. This may favour the protein adsorption process considerably.13 It may be understood that proteins having an intrinsically low structural stability are more prone to undergo adsorption-induced structural changes. 

The silicon surface can be stabilized using surface modification techniques which are divided into three categories (1) attachment of redox mediator which consumes the holes on the surface (2) attachment of electronically conducting polymer and (3) coating with thin metal or semiconducting films to create a buried semiconductor interface. Combinations of these approaches can also be used to stabilize the sihcon surface. ... 

In this coimection, a cryochemical solid-phase synthesis of metal-polymer systems is of special importance. As a result of such a synthesis, metal clusters and organometallic assemblies formed at low temperatures are buried in a polymer environment, which offers possibilities to stabilize and study these products over a large temperature range. This method was first offered and described in reference 10. The thermal rearrangement of the initial low-temperature system is governed by relaxation processes in polymer matrix. In particular, the aggregation of metal atom clusters to form metal nanocrystals in cryochemically produced metal-polymer systems yields new nanocomposite materials with valuable properties. The study of the mechanism of cluster aggregation, which depends on the characteristics of the polymer matrix, will allow the nanocomposite structure to proceed in the needed direction. Thus, it becomes possible to determine the methods of cryochemical synthesis of metal-polymer materials with predetermined properties. 

Hydraulic structures using scrap tires for bank protection include tire mats, revetment (retaining walls, seawalls, revet mattresses), and tire-concrete imits. In search for economical bank-protection structures, the use of scarp tires as a less-expensive alternative is desirable, considering the costs of the metal and concrete used in reinforced-concrete construction, especially in developing countries. Whole scrap tires can be utilized for surface erosion control, beach and slope protection, and stream bank stabilization. In these applications, scrap tires are banded together and partially or completely buried on imstable slopes. Tires can be used with other stabilization materials to reinforce an unstable highway shoulder or protect a channel slope remained stable and can provide economical and immediate solutions. In bank protection structures, tires are laced together by steel cables and used as a protective layer or mat over stream banks or soil embankments. The top, toe, upstream and downstream ends of the mattress are tied into the banks. Used tires with metal cords were shown to bean excellent construction material that can partially replace reinforced concrete for protection of river banks and canal walls [19]. 

Physical vapor deposition methods (PVD) offer the possibility of preparing catalysts in which no foreign ions or molecules are introduced as is the case in the conventional "wet" impregnation methods. In evaporation methods however, the contact between metal and substrate produced by the deposition of metallic vapors is too weak to favor strong interactions and to enhance the catalytic activity and stability. By contrast, when a high-energy method like ion implantation is used, the metal is buried too deeply in the substrate and only a limited number of sites are available for the catalytic reactions. So far, direct-current sputtering has been the only PVD method whereby reasonable amounts of active catalysts could be prepared [1]. 

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