Applications Laves phases

As a conclusive comment to the Miedema model, especially with reference to the enthalpy evaluation, we have to underline that while it may be useful in order to define a reference behaviour, however, its approximate (in a way qualitative) character cannot be forgotten. A critical discussion on the application and limits of this model has been published for instance by Chen et al. (2004) see the comments on the thermochemistry of the Laves phases in 3.9.3, see also a few more remarks on this subject in 4.4.7.1. 

Miedema s theory and structural information. The Miedema model for energy effects in alloys, presented in 2.2.1.3 has been very useful in an evaluation, albeit approximate, of the formation enthalpies and in the prediction of compound formation capability. For an example of the application and limits of this model, see the comments on the thermochemistry of the Laves phases reported in 3.9.3. However notice that the general usefulness of the Miedema approaches has diminished with time, both for its inherent approximation and for... 

In essence, metal/intermetallic hydrides of AB, AB (Laves phases), AB, and A B have not been improved to any remarkable extent since the end of the 1990s. Nevertheless, there have been some efforts directed to either improvement of synthesis or properties of metal/intermetallic hydrides particularly by application of ball milling associated with nanostructuring. Some of these more recent efforts will be briefly discussed in the following sections. 

Inspection of the experimental data for the systems with fast localized hydrogen motion suggests that the applicability of Eq. (26.29) is not restricted to hydrogen in Laves phases. In particular, this equation appears to describe the behavior of rfi(T) for the trapped hydrogen in Nb(OH)QQ3j [H8] in the range 100-300 K and for the mobile H atoms in a-ScHo.i6 [133] in the range 125-300 K. The values of Tq estimated from these data are 130 K for Nb(OH)o on and 146 K for a-ScHo ig. 

The An-An spacing in the Laves phases of the light actinides, with exception of AnAl2 compounds, is far below the Hill limit. Therefore the overlap of the 5f wave functions must be regarded as the main delocalizing mechanism. This provides the justification for the application of the band approach, at least in a first approximation. 

Various Laves phases have been regarded as promising for both functional and structural applications (Livingston, 1992), and examples are given in the following sections. 

The present monograph was first written as a chapter for Volume 8 of the series Materials Sdence and Technology A Comprehensive Treatment , edited by Robert W. Cahn, Peter Haasen, and Edward J. Kramer (Volume Editor Dr. Karl Heinz Matucha). Its aim is to give an overview of intermetallics, which is both detailed and comprehensive and which includes the fundamentals as well as applications. The result is an extended, critical review of the whole field of intermetallics with an emphasis on those intermetallic phases which have already been applied as functional or structural materials or which are currently the subject of materials developments. A historical introduction and a discussion of the relationship between atomic bonding, crystal structure, phase stability and properties is followed by a discussion of the major classes of intermetallics. The titanium aluminides, nickel aluminides, iron aluminides, copper phases, A15 phases. Laves phases, beryllides, rare earth phases, and siliddes are reviewed. In particular, the crystal structures, phase diagrams, and physical properties as well as the mechanical and corrosion behavior are treated. The state of developments as well as prospects and problems are discussed in view of present and future applications. The publisher has decided to publish the review as a separate monograph in order to make it accessible to a wider audience. 

This section presents a brief survey of metallic systems in which ESR of lanthanide ions have been studied. Only those host materials are included which may be classed as metals, alloys, or intermetallic compounds of the rare earths, in parallel with the survey of applications of ESR in insulators in section 4.1. The following subsections treat, respectively, rare-earth metals and alloys (especially those based on La, Y, Sc, and Lu), Laves phase intermetallic compounds, and the so-called Van Vleck para-magnetic compounds, of which many fall in the category of the rare earth mono-pnictides. The ESR of lanthanide ions in non-lanthanide host materials such as the noble metals is not included. Such systems are discussed in the extensive reviews by Orbach et al. (1974) and by Taylor (1975). 

Fig. 31. Dependencies of Curie temperature, hyperfine field (T- 0) and isomer shift (T = 4.2K.) on the relative change of Np-Np separation (Ad p) as induced by application of pressure for the cubic Laves phases NpOsj and NpAlj. Note the extremely small compressibility of NpOsj. 10 kbar = 1 GPa.

A traditional strength, and still a major application, of INS is the study of hydrogen-in-metal systems. In these systems hydrogen occupies a weU-defined lattice site, but is often free to move between sites. This proton mobility has led to numerous applications in battery technology, particularly for LaNisHx and the AB2 Laves phase compounds. The simplest model is to consider that the jumps are all of the same length (1), that jumps in any direction are equally probable, and that the jump time is negligibly small compared to the residence time (r). With these assumptions Equation [6] becomes ... 

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