Transport properties amorphous alloys

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. 

After presenting the sample preparation in Sect. 5.2, we give an introduction to the theoretical background in Sect. 5.3. In Sect. 5.4, we briefly review the electronic influence on structure and phase stability of crystalline Hume-Rothery phases. In Sect. 5.5, we discuss the properties of non-magnetic amorphous alloys of the type just mentioned. The electronic influence on structure (5.5.1) and consequences for the phase stability (5.5.2) are also discussed. Structural influences on the electronic density of states are shown in 5.5.3. Electronic transport properties versus composition indicate additionally the electron-structure interrelation (5.5.4), and those versus temperature, the influence of low-lying collective density excitations (5.5.5). An extension of the model of the electronic influence on structure and stability was proposed by Hdussler and Kay [5.21,22] whenever local moments are involved as, for example, in Fe-containing alloys. In Sect. 5.6, experimental indications for such an influence are presented, and additional consequences on phase stability and magnetic properties are briefly discussed. 

Friederich et al. (1979) have reported the first data obtained by EPR for non-s rare earth ions (Nd ), e.g. NdnAggs, which is paramagnetic down to 4.2 K. The experiments were done at 293,100 and 4.2 K in a magnetic field of 1000 to 5000 G, and showed the presence of some complicated structure. The results are discussed in terms of the g factor. The resonance observed above 100 K indicates the existence of sites having a non-axial crystal field. The authors think that EPR is promising for investigation of the electrical field gradients (or crystal fields) in amorphous rare earth alloys (see next section for transport properties). 

McGuire and Gambino (1980) have reported the transport properties of GdGe amorphous film. Compared to other Gd alloys, such a sample has a large resistivity (285 jufl cm) and a low Tc (70 K). 

A common feature of R-amorphous alloys and transition metal-metalloid alloys is that they show similar transport properties such as high resistivities with weak temperature dependence and often at low temperature a logarithmic variation with temperature (Kastner et al., 1980). The magnetic R-alloys often show additional transport behaviour. We will now discuss this in detail for several R-amorphous alloys. 

We have also described some of the existing data for the transport properties of amorphous rare earth alloys. We feel that these are parallel systems to the crystalline RI compounds since they can be made in the same concentration. Also a greater range of concentrations is available in RI amorphous alloys because they all have similar real space structure and are therefore not subject to the metallurgical constraints of crystalline RI compounds. These alloys should prove useful in helping to establish the variety of scattering mechanisms for electrons in RI intermetallic systems. 

The electrical transport properties of amorphous alloys differ considerably from those of crystalline materials. The resistivity of the latter is usually low in the liquid-He temperature range (a few iiii cm). At higher temperature it tends to increase linearly with temperature, leading to a room temperature value of several... 

Transport mechanisms in amorphous oxides therefore are more complicated than in crystalline solids, and only partially elucidated. For example, if the structure of an amorphous oxide varies with the film thickness, the transport properties would also depend on thickness. This could offer an explanation for the hypothesis (8.26) made to justify a direct logarithmic growth law. In the same fashion, alloying elements are able to modify the structure of the growing oxide film and thereby influence the transport mechanism and the oxidation rate. 

Attempts were made to describe theoretically the current flowing at microelectrodes under various conditions, including also migrational transport. Deposition of monomolecular Langmuir-Blodgett films at electrodes, their different properties, as well as electrodeposition of amorphous metallic alloys were studied. 

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