Metals with amorphous structure

In this chapter we present a survey of our current understanding of interrelations between the electronic and ionic structure in late-transition-polyvalent-element metallic glasses. Evidence of a strong influence of conduction electrons on the ionic structure, and vice versa, of the ionic structure on the conduction electrons, is presented. We discuss as well the consequences to phase stability, the electronic density of states, dynamic properties, electronic transport, and magnetism. A scaling behaviour of many properties versus Z, the mean electron number per atom, is the most characteristic feature of these alloys. Crystalline alloys which are also strongly dominated by the conduction electrons are often called electron phases or Hume-Rothery phases. The amorphous alloys under consideration are consequently described as an Electron Phase or Hume-Rothery Phase with Amorphous Structure. Similar theoretical concepts as applied to crystalline Hume-Rothery alloys are used for the present amorphous samples. 

One of the major drawbacks to many promising copolymers is their unsatisfactory electrochemical stability. Carbonyl groups which feature in many of the back-bone/chain linking groups are likely to cause stability concerns. Likewise, urethane, alcohol, and siloxane functions are sensitive to lithium metal. With this in mind, a recent trend has been to find synthetic routes to amorphous structures with... 

In addition to Au and noble metals, Ni-Zn nanoclusters with an amorphous structure were successfully deposited on Ti02 nanoclusters. The state of Ni was metallic. The catalytic activity of Ni-Zn/Ti02 in olefin hydrogenation was ca. 10 times higher than unsupported Ni nanoclusters. Selective deposition onto Ti02 and the addition of Zn seemed to play an important role to stabilize Ni nanoclusters and to decrease the size of Ni nanoclusters, respectively. Also, clearly Zn promoted the hydrogenation activity of Ni and inhibit the growth of the size, but did not substantially affect Ni nature itself... 

SoUd ice forms a crystal of diamond structure, in which one water molecule is hydrogen-bonded with four adjacent water molecules. Most (85%) of the hydrogen bonds remain even after solid ice melts into liquid water. The structure of electron energy bands of liquid water (hydrogen oxide) is basically similar to that of metal oxides, 6dthough the band edges are indefinite due to its amorphous structure. 

Metastable amorphous materials can be produced by the rapid quenching of melts in the form of metallic alloys with glassy structures [149]. These materials have attracted the attention of metallurgists, physicists, and, recently, chemists because of their exceptional properties (easy magnetisation, superior corrosion resistance, high mechanical toughness, interesting electronic properties) [150]. The use of these materials in catalysis was reported some years ago [151]. 

Within a discussion of nanostructured catalysts, mention must be made of zeolitic systems. Zeolites are a broad family of natural and synthetic aluminosiU-cates that exhibit two important properties that makes them ideal for consideration as heterogeneous catalysts they are crystalline and porous. Crystallinity brings with it precise definition at the atomic scale that is absent with amorphous or polycrystaUine metal oxides. The combination of a well-defined structure and... 

The concept zeolites conventionally served as the synonym for aluminosilicates with microporous host lattice structures. Upon removal of the guest water, zeolites demonstrate adsorptive property at the molecular level as a result they are also referred to as molecular sieves. Crystalline zeosils, AlPO s, SAPO s, MAPO s (M=metal), expanded clay minerals and Werner compounds are also able to adsorb molecules vitally on reproval of any of the guest species they occlude and play an Important role in fields such as separation and catalysis (ref. 1). Inclusion compounds are another kind of crystalline materials with open framework structures. The guest molecules in an inclusion compound are believed to be indispensable to sustaining the framework structure their removal from the host lattice usually results in collapse of the host into a more compact crystal structure or even into an amorphous structure. 

Spherical particles of various metal phosphate particles can be prepared by precipitation using urea as a homogeneous precipitation agent. Surface-active agents, such as SDS and CTAC, are effective in preparation of uniform-size spherical particles. The formed spherical particles are amorphous and contain OH- and H20, except cobalt phosphate particles with layered structure. These panicles are agglomerates of primary particles, and have pores of different sizes ranging from ultramicropore to mesopore. 

Polymers don t behave like the atoms or compounds that have been described in the previous sections. We saw in Chapter 1 that their crystalline structure is different from that of metals and ceramics, and we know that they can, in many cases, form amorphous structures just as easily as they crystallize. In addition, unlike metals and ceramics, whose thermodynamics can be adequately described in most cases with theories of mixing and compound formation, the thermodynamics of polymers involves solution thermodynamics—that is, the behavior of the polymer molecules in a liquid solvent. These factors contribute to a thermodynamic approach to describing polymer systems that is necessarily different from that for simple mixtures of metals and compounds. Rest assured that free energy will play an important role in these discussions, just as it has in previous sections, but we are now dealing with highly inhomogeneous systems that will require some new parameters. 

A glass is an ionic solid with an amorphous structure resembling that of a liquid. Glass has a network structure based on a nonmetal oxide, usually silica, Si02, that has been melted together with metal oxides that act as network modifiers. ... 

The electrochemical properties of passive layers lead to the question of their structure on a mesoscopic scale and at atomic resolution. Their barrier character with respect to metal corrosion postulates a dense, poreless film their electronic properties, in some cases, crystalline structures. The change of their properties with film aging, as in e.g. film-breakdown phenomena, support the existence of many defects that may heal with time. In many cases an amorphous structure is assumed. Some ex situ... 

The investigation of anodic oxide on various metals shows that at first usually amorphous structures are formed with a dense coverage of the terraces with grains, which change to nano-crystallites with time. The extent and the rate of this change depend on the system under study. This crystallization occurs for Cr within hours [127], whereas Cu keeps the amorphous grain structure for a very few minutes only and develops a well-ordered, faceted, crystalline layer covering the whole electrode surface [128, 129], In the next section, the details of the structure of layers formed on Cu are discussed, followed by a summary of some other more reactive metals like Ni and Cr. 

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