Amorphous alloys structural characteristics

In the present work, the nanostructured Co-Cu powders with different Co/Cu ratio are prepared after 2-3 h of ball milling. As a precursor we use the Co/Cu composite powders with particles consisting of amorphous Co-P alloy core and Cu shell [3]. The structural and magnetic characteristics are analyzed for different Cu content and compared with the Co-Cu alloys produced by traditional mechanical alloying techniques. Also the aim of this work is to obtain additional information on Co-Cu alloy structure formed during the ball-milling process. 

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. 

The demands of practical applications led to attempts to overcome the high electric resistance of thin ribbons by a new technical solution of laser-induced surface vitrification (105, 106). First an amorphous alloy ribbon was adhered uniformly to a nickel plate by heat treatment. Subsequently, this surface alloy layer was transformed to the amorphous structure by laser surface melting and self-quenching (107). A sample consisting of Pd56Rh23P oSi9 adhered to bulk crystalline nickel exhibited anodic characteristics very similar to those of the melt-spun amorphous ribbon (102). Clearly, similar improvements forced by practical demands will be a part of the future use of amorphous alloys. 

Laser Raman spectroscopy has been used as a tool to elucidate the molecular structure of crystals, liquids, and amorphous alloys in the As-S-Se-Te system. Characteristic monomer and polymer structures have been identified, and their relative abundances have been estimated as a function of temperature and atomic composition. These spectroscopic estimates are supported by calculations based on the equilibrium polymerization theories of Tobolsky and Eisenberg (1,2) and of Tobolsky and Owen (3). Correlations between the molecular structure of the amorphous alloys and physicochemical properties such as the electron drift mobility and the glass transition temperature are presented. 

By courtesy of Dr. Colliex (1980) we report some peculiar aspects of magnetic domains in amorphous alloys which have been prepared either by e-gun or by thermal evaporation. Fig. 5 shows nearly aligned Bloch walls with cross-ties observed in YFe films after slow annealing before heating it appeared as a randomly rippled structure. Figs 6 to 9 show characteristic domains of YCo, TbCo, CeNi and DyNi films. Unfortunately their compositions were not determined. 

Pm is proportional to pspd of eq. (23), au(2kF) is the partial structure factor at P( is a parameter characteristic of the local magnetic order and Ci is the concentration of magnetic ions. Eq. (71) describes the contribution from magnetic ordering to Pmag which is positive or negative according to whether On > 1 or On < 1. The application of this expression to R-amorphous alloys will be discussed in section 5.2.5. 

Electrochemical corrosion characteristics of nickel were carried out by potentiodynamic polarization and impedance spectroscopy methods. Corrosion tests of nickel produced by electrocrystallization were ap>plied to its micrometric (Nim) and nanometric (Ni ) crystalline structures and for NiP amorphous alloy of nickel with phosphorus at content of 10.7% by weight (Eftekhari, 2008), (Kowalewska Trzaska, 2006). 

Wang et al. [140] have studied amorphous CoSn/C alloys decorated with Pt as highly efficient electrocatalysts for ethanol oxidation. The catalysts were prepared using a two-stage chemical synthesis (sol-gel preparation and Steady-state replacement method). XRD results evidenced that the CoSn-base was in the amorphous state, but the characteristic peaks of the Pt fee crystalline structure appeared after Pt deposition. The TEM images confirmed that Pt was deposited onto CoSn/C. The electrochemical measurements showed that the mass activity of the Pt-CoSn/C catalyst was 454.6mA mgp, , which was about 1.71 and 1.74 times those of Pt/C and PtSn/C. The authors attributed this behavior to the possible modification in the Pt electronic structure elicited by the amorphous CoSn alloy. Materials based on amorphous alloys can open new perspectives regarding ethanol electro-oxidation. 

Another characteristic point is the special attention that in intermetallic science, as in several fields of chemistry, needs to be dedicated to the structural aspects and to the description of the phases. The structure of intermetallic alloys in their different states, liquid, amorphous (glassy), quasi-crystalline and fully, three-dimensionally (3D) periodic crystalline are closely related to the different properties shown by these substances. Two chapters are therefore dedicated to selected aspects of intermetallic structural chemistry. Particular attention is dedicated to the solid state, in which a very large variety of properties and structures can be found. Solid intermetallic phases, generally non-molecular by nature, are characterized by their 3D crystal (or quasicrystal) structure. A great many crystal structures (often complex or very complex) have been elucidated, and intermetallic crystallochemistry is a fundamental topic of reference. A great number of papers have been published containing results obtained by powder and single crystal X-ray diffractometry and by neutron and electron diffraction methods. A characteristic nomenclature and several symbols and representations have been developed for the description, classification and identification of these phases. 

In this chapter, general aspects and structural properties of crystalline solid phases are described, and a short introduction is given to modulated and quasicrystal structures (quasi-periodic crystals). Elements of structure systematics with the description of a number of structure types are presented in the subsequent Chapter 7. Finally, both in this chapter and in Chapter 6, dedicated to preparation techniques, characteristic features of typical metastable phases are considered with attention to amorphous and glassy alloys. 

General characteristics of alloys such as those presented in Fig. 3.3 have been discussed by Fassler and Hoffmann (1999) in a paper dedicated to valence compounds at the border of intermetallics (alkali and alkaline earth metal stannides and plumbides) . Examples showing gradual transition from valence compounds to intermetallic phases and new possibilities for structural mechanisms and bonding for Sn and Pb have been discussed. Structural relationships with Zintl phases (see Chapter 4) containing discrete and linked polyhedra have been considered. See 3.12 for a few remarks on the relationships between liquid and amorphous glassy alloys. 

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