Alloyed cluster formation

In some other cases, the intermetal electron transfer does not occur even during hour-long irradiations. The initial reduction reactions (1) are followed by mixed coalescence and association of atoms and clusters with ions as in (33, 34). Besides dimerization of atoms of the same metal into Mj and M coalescence of both types of atoms occurs twice more frequently  

Then alternate association (37) and reduction reactions (38) progressively build bimetallic alloyed clusters according to the statistics of encounters, therefore to the relative initial ion abundance.  

The possible formation of an alloyed or a core-shell cluster depends on the kinetic competition between, on one hand, the irreversible release of the metal ions displaced by the excess ions of the more noble metal after electron transfer (reactions (33, 34) and, on the other hand, the radiation-induced reduction of both metal ions (reactions (1, 7, 8, 38)) which depends on the dose rate (Table 5). Once the reduction of both metal ions is complete, any further intermetallic electron transfer is unlikely at room temperature. A pulse-radiolysis study of a mixed system suggested that a very fast and total reduction by the means of a powerful and short irradiation delivered for instance by an electron beam (EB) should produce alloyed clusters. Indeed such a decisive effect of the dose rate has been demonstrated. However, the competition imposed by the intermetal electron transfer is more or less serious, since, depending on the couple of metals, the process may not occur, or on the contrary lasts hours, only minutes or even seconds.  

The alloyed or layered character of a small bimetallic cluster structure is generally quite difficult to conclude experimentally. Even if the surface plasmon transitions of both pure metals are specific (with one possibly in the UV), the unknown spectra of alloyed or bilayered clusters are both expected in the same intermediate region. The structure of the composite cluster is derived indeed from the evolution observation of the absorption spectrum at increasing dose. Because the spectrum is due to surface phenomena, it changes with dose when electron transfer occurs. The composition of the bilayered cluster 

due to a slow electron transfer (within seconds) from Au atoms of the alloy to the unreduced ions Ag , a supplementary formation of Ag correlates with the complete dissolution of Au . By y-radiolysis at increasing dose, the spectrum of pure silver clusters, more noble due to the CN ligand, is seen first at 400 nm. Then the spectrum is red-shifted when gold is reduced at the surface of silver clusters in a bilayered structure, as when the cluster is formed in a two-step operation (Table 5). However, when the same system is irradiated at high dose rate with an electron beam, allowing the sudden (out of redox thermodynamics equilibrium) and complete reduction of all the ions prior to the metal displacement, the band maximum of the alloyed clusters is at 420 nm.  

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Actually, when a mixed solution of two ionic precursors M and M is irradiated or chemically reduced, both situations of alloyed or bilayered cluster formation may be encountered without clear prediction [102,173]. Moreover, an unambiguous characterization of the intimate structure of nanometric mixed clusters is quite difficult and requires appropriate methods, applied at different steps (or different doses) of the mixed cluster construction. 

This book is the first attempt to summarize, probably from our subjective point of view, the state of the art in a very rapidly developing theory of many-particle effects in bimolecular reactions in condensed matter, which up to now was a subject of several review papers only [1—10]. We have focused mainly on several basic bimolecular reactions trying not to cover all possible cases (e.g., more complicated reactions, cooperative processes in alloys under irradiation [11] or initial macroscopic separation of reactants, etc.) but to compare critically results and advantages/limitations of numerous approaches developed in the last years. We focused on processes induced by point particles (defects) only the effects of dislocation self-organization are discussed in [12-16] whereas diffusion-limited particle aggregation with a special attention to fractal cluster formation has extensive literature [17-21],... 

It also should be noted that we have herein, in effect, turned the charging problem around and made it an auxiliary morphological tool. In order to demonstrate the versatility of this tool, we have employed the aforementioned negative shift with cluster formation not only to study metals dispersions, but also to assist in the study of the integrity of thin film interfaces (16), and metals segregation during alloying corrosion (H). 

Bigger clusters have been formed, for instance, by the expansion of laser evaporated material in a gas still under vacuum. For metal-carbon cluster systems (including M C + of Ti, Zr and V), their formation and the origin of delayed atomic ions were studied in a laser vaporization source coupled to a time-of-flight mass spectrometer. The mass spectrum of metal-carbon cluster ions (TiC2 and Zr C j+ cluster ions) obtained by using a titanium-zirconium (50 50) mixed alloy rod produced in a laser vaporization source (Nd YAG, X = 532 nm) and subsequently ionized by a XeCl excimer laser (308 nm) is shown in Figure 9.61. For cluster formation, methane ( 15% seeded in helium) is pulsed over the rod and the produced clusters are supersonically expanded in the vacuum. The mass spectrum shows the production of many zirconium-carbon clusters. Under these conditions only the titanium monomer, titanium dioxide and titanium dicarbide ions are formed. 

Polymer alloy, containing two or more polymers in a co-continuous network form, each physically crosslinked. The crosslinking originates in crystallinity, ion cluster formation, presence of hard blocks in copolymers, etc. 

Because the dynamics observed by means of pulse radiolysis indicated that the displacement process was not instantaneous, it was suggested that very short, intense irradiation, with a dose sufficient to achieve the complete reduction of all the ions, could efficiently prevent the segregation, due to electron transfer between the metals. Therefore, the method could enable the formation of alloyed clusters, of major interest for various applications, particularly catalysis. The positive influence of high dose rates, which quench the atoms in an alloyed cluster, has been demonstrated a bilayered cluster would be obtained from the same system by irradiation at a lower dose rate. " Moreover, as for monometallic clusters (Section 3.13.4.3), the high dose rate favors nucleation rather than growth, and the final sizes of the alloyed clusters are particularly small. " " ... 

Formation of bimetallic alloy clusters from M, Na-Y (M=Pt, Ir, Rh, Ru) after partial ion exchange with CUSO4 solution, oxygen treatment and subsequent reduction in flowing hydrogen was evidenced by the shift of the band position of adsorbed CO from 2090 cm" by about 50 cm" to lower wavenumbers [632]. Similar experiments were carried out with silicalite-1 or ZSM-5 as supports. 

Fe or Ni ion irradiation with energies of a few MeV is mostly used for studies of radiation damage of RPV steels. Previous studies have provided information on the effects of Cu, Mn and other elements, carbides, dose rate and tensile stress on hardness, matrix damage evolution and solute cluster formation in model alloys and commercial steels (e.g. Fujii and Fukuya, 2005 Murakami et al., 2009). These data provide clear evidence of the effects of various metallurgical parameters on hardening and microstructural evolution in Fe-based alloys and RPV steels, although these data cannot be directly or quantitatively correlated to data from neutron-irradiated materials. 

DAP in Fe-(0.1-0.3) Cu alloys after heavy ion irradiation (Pareige et al., 2001,2005). In these studies, needle-like tips with lOOnm in thickness were irradiated with 150, 180 or 300 keV Fe + ions at 573 K so that the peak damage depth, 50 nm, corresponds to the centre of the tip. The early stage of Cu-rich cluster formation was identihed. 

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