Amorphous alloys stability

Amorphous Silicon. Amorphous alloys made of thin films of hydrogenated siUcon (a-Si H) are an alternative to crystalline siUcon devices. Amorphous siUcon ahoy devices have demonstrated smah-area laboratory device efficiencies above 13%, but a-Si H materials exhibit an inherent dynamic effect cahed the Staebler-Wronski effect in which electron—hole recombination, via photogeneration or junction currents, creates electricahy active defects that reduce the light-to-electricity efficiency of a-Si H devices. Quasi-steady-state efficiencies are typicahy reached outdoors after a few weeks of exposure as photoinduced defect generation is balanced by thermally activated defect annihilation. Commercial single-junction devices have initial efficiencies of ca 7.5%, photoinduced losses of ca 20 rel %, and stabilized efficiencies of ca 6%. These stabilized efficiencies are approximately half those of commercial crystalline shicon PV modules. In the future, initial module efficiencies up to 12.5% and photoinduced losses of ca 10 rel % are projected, suggesting stabilized module aperture-area efficiencies above 11%. 

Several materials have been investigated as cathode activators. Among the most studied systems we find CuTi, CuZr, NiTi, NiZr, FeCo, NiCo. A variety of methods are available to prepare amorphous alloys [562] and, as expected, the resulting activity is largely dependent on them. Normally, amorphous phases are obtained by rapidly quenching a melt. The material can thus be obtained in the form of ribbons, but mechanical alloying by compaction is also possible [572]. The metallic components are usually alloyed with non-metallic components such as B, Si and P which stabilize the metastable non-crystalline structures. Electrodeposition is thus also a viable preparation route [573, 574],... 

The total BOs for each structure explain the structural stabilities in each system as well. The original structure models are more stable than replaced models. In the previous studies [15], the glass-formation abilities of the amorphous alloys were estimated by the calculation of the small representative clusters. However the glass-formation abilities depend not only on the stabilities of the amorphous alloys but also those of crystals. Moreover, the small clusters cannot cover the varieties of the bonds (the combinations and the distances of the pair). 

Very little is known about the influence of grain growth, or crystallization if the membrane is composed of an amorphous alloy, on membrane durability. The as-fabricated permselective metal membrane will be polycrystalline or amorphous, depending on the alloy composition and fabrication method. Amorphous, or metallic glass, structures are far less common than are polycrystalline structures. Both amorphous and polycrystalline structures are quasi-stable, meaning that structures are kinetically stabilized and slow to rearrange to the thermodynamically favored structure. In both cases, this would be a single crystal of the metal. 

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. 

A. Inoue, Stabilization of metallic supercooled liquid and bulk amorphous alloys // Acta mater. 48 (2000), P. 279-306. 

Copper-based amorphous alloys also proved to be active in the oxidation of formaldehyde (108,109). As it was reported earlier in connection with the hydrogen evolution reaction (62) (see Section III,A,1), HF treatment leads to the formation of a copper-rich porous surface layer. As a result, electrodes with very high electrocatalytic activity for anodic formaldehyde oxidation could be prepared. It was found that the rate-determining step is a one-electron transfer and the oxidation proceeds via the hydroxymethanolate ion HOCH2O". However, it is not clear whether the catalytically active copper species is Cu° or Cu+. It would be interesting if either Cu° or Cu+ could be stabilized in amorphous alloys. 

Models exist that describe the thermal stability Tx in terms of transformation rates (kinetic approach), where Tx is proportional to AE. In these cases AE is estimated on the basis of a weighted mean sublimation energy of the constituent elements (Davies 1976) or the heat of compound formation (Kiibler et al. 1981) or the formation enthalpy AHh of a hole of size equivalent to the smaller type of atom (Buschow 1982). Of these possibilities the latter appears to be the most promising for predicting the thermal stability of a large variety of amorphous alloys. 

Recent years there have been a considerable interest in studying of binary catalytic systems based on stabilized nanocomposites and amorphous alloys of copper with other metals. The reason is that the catalytic activity of such systems in many cases is sufficiently higher than that of individual metals. The most convenient model for theoretical description of binary systems characterized by the absence of far order is a cluster model. However, quantum-chemical study of binary clusters comprises the significantly more c omplicated problem than that o f individual metals, b ecause a correct theoretical description of metal-another metal cluster systems requires that the used method should be in a position to provide good results of calculations of geometrical, electron stmctures and energetic characteristics of both of individual metals. 

As we will see in the next section, a relative low melting temperature is indeed favourable for glass formation. Since the Nagel and Tauc model has mainly been used to describe the stability of amorphous alloys we will treat this model in more detail in section 4. 

In the literature several models have been described in which amorphous alloys are, considered to be relatively stable if certain requirements are fulfilled. In many of the alloys Aj that can be obtained in the amorphous state there is a substantial difference in size between the metallic radii of the components (r > rg). In addition, the composition of many amorphous alloys made by means of liquid quenching is close to X = 0.2. This led Polk (1970) to propose a stability criterion for amorphous alloys in which the size difference in atomic radii and the asymmetry in composition is of prime importance. This stability criterion is based on the possibility of obtaining a higher packing density when the holes available in the dense random packing of the larger A atoms are filled by the smaller B atoms. In recent years it has... 

In both the above-mentioned stability criteria the stability of the amorphous alloys is measured as the energy difference between the crystalline and the amorphous state. Undoubtedly, this energy difference can be regarded as a measure of the driving force for the transformation of amorphous into crystalline material. It will be shown, however, that this energy difference is actually small compared with the activation energy AE) that appears in the rate equations of the transformation reaction of the amorphous alloy into crystalline material discussed in the previous section. 

From these results it can be concluded that any favourable (2kp= Q ) or unfavourable (2kp Q ) influence on E, even if it were of the order of AH, would have little effect on the magnitude of AE. This means that the Nagel and Tauc criterion is less suited for describing thermal stability of amorphous alloys, i.e. their resistance against crystallization. As briefly indicated above, it is mainly the transformation kinetics that governs the thermal stability of amorphous alloys. A description of the kinetic approach to thermal stability will be presented in the following section. 

In the kinetic approach to thermal stability of amorphous alloys the rate of transformation is assumed to be diffusion-controlled (Uhlman, 1972 Davies, 1976 Takayama, 1976). Atomic motion is expected in all alloys to set in at a temperature where the viscosity ij reaches a critical value (about 10 P). In terms of entropy theory of viscous flow as proposed by Adam and Gibbs (1965) this latter quantity can be given by... 

In a first approximation one may assume that the atomic arrangement in all amorphous alloys is similar and corresponds to a statistical distribution of atoms. In that case the values of can be taken to be the same in all alloys. The thermal stability of the glass, when expressed in terms of (or its upper limit T ), is therefore mainly determined by AE. In fact one would derive from eq. (14) that (or Tjj) in all alloys is proportional to AE, i.e. 

The particular position taken by alloys for which 2kp = was already mentioned in connection with the Nagel and Tauc criterion discussed in sections 3.1 and 4.2. In the latter section we showed that the Nagel and Tauc criterion is less suited for describing the thermal stability of amorphous alloys. In the former section the possibility was discussed that amorphous alloys for which the criterion 2kp is satisfied may be amenable to easy glass formation owing to the presence of an enhancement of the deepness of the eutectic. 

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