Clusters interstitial

The third term in Eq. 7, K, is the contribution to the basal plane thermal resistance due to defect scattering. Neutron irradiation causes various types of defects to be produced depending on the irradiation temperature. These defects are very effective in scattering phonons, even at flux levels which would be considered modest for most nuclear applications, and quickly dominate the other terms in Eq. 7. Several types of in-adiation-induced defects have been identified in graphite. For irradiation temperatures lower than 650°C, simple point defects in the form of vacancies or interstitials, along with small interstitial clusters, are the predominant defects. Moreover, at an irradiation temperatui-e near 150°C [17] the defect which dominates the thermal resistance is the lattice vacancy. 

Cogoni, M., Uberuaga, B.P., Voter, A.F., Colombo, L. Diffusion of small self-interstitial clusters in silicon temperature-accelerated tight-binding molecular dynamics simulations. Phys. Rev. B 2005, 71(12), 121203-1-1. 

The effects of damage by ion implantation on the low-temperature diffusion of dopant can also be studied by implanting Si+ or Ge+ ions into predeposited layers in Si. Recently, Servidori et al. (58) studied the influence of lattice defects induced by Si+ implantation. Using triple crystal X-ray diffraction and TEM, they confirmed (1) that below the original amorphous surface-crystal interface, interstitial dislocation loops and interstitial clusters exist and (2) that epitaxial regrowth leaves a vacancy-rich region in the surface. 

The existence of free interstitial point defects forming the complements to the vacancy centers is generally not observed following irradiation at room temperature. At these temperatures the interstitials cluster together to form interstitial aggregates and dislocation loops. However, lattice disorder can slow down or prevent the aggregation process due to interstitial trapping. 

They showed that the reversible charge capacity of heat-treated cokes as a function of their degree of graphitization is in remarkable agreement with the expression (Equation 7.6) if xaa = 0.75, xaP = 0.20, and Xpp = 1. The lower value of xap was qualitatively confirmed by STM observations on pyrocarbons heat-treated at 2000°C.172 In the ap state, interstitial clusters of carbon atoms which commensurate with the graphite lattice hindered the intercalation of lithium and, thus, decreased the stoichiometric coefficient x with respect to a random distribution of interstitials. 

Fully bonded outer cluster with interstitial cluster... 

In an ideal situation dislocation lines would penetrate the whole crystal. In reality they mostly extend from one grain boundary to another one or they are pinned by impurities. If the lines form a closed circle inside the crystal, they are called loops. Summarizing, one may say that dislocations can arise from vacancy clusters as well as from interstitial clusters due to their pressure on the lattice. Very often they are the final products of an annealing procedure. Dislocations already existing interact with point defects and impurities acting as traps or sinks. 

The simulation model includes the coupled reaction-diffusion-migration equations for interstitial atoms, interstitial clusters and related defects. These equations have the general form for species i ... 

As mentioned before, the interstitial clusters gain a remarkable stabilization from the establishment of several interactions. The result, at least in the case... 

Figure 2. Skeletal structures of some interstitial clusters (a) [RusC(CO)i5] (b) [Rh6C(CO)i5] (c) [Co8C(CO)ig]2- (d) [Rhi2Sb(CO)27] -.

It is clear that in the past few years a broad and diverse chemistry of interstitial cluster compounds has developed. Synthetic routes have been established and some clues to the functions of the encapsulated atoms have emerged. The stereochemical... 

Fig. 2a-d. Vacancy-interstitial clusters in Fei- O. b, c and d are different forms of linking the basic 4 1 cluster shown in a... 

In the first part of this chapter, we provide an overview of the radiation damage processes that control the form of the irradiation-induced microstructure under the radiation conditions reactor pressure vessel (RPV) steels experience in service. We demonstrate that the irradiation damage in RPV steels can broadly be classified as matrix damage (for instance voids, interstitial clusters, dislocation loops, complexes trapped at sinks such as... 

The synthetic approaches taken for the preparation of interstitial clusters are based on the activation of E-E, E-H, E-C, E-Si, E-O, E-S and E-Q bonds of suitable molecules. The methods to obtain clusters having exposed maingroup elements or elemental organic fragments are either exactly the same or differ only in their use of a source molecule which contains two kinds of E-X bonds... 

In both metal lattices and closo polyhedral metal clusters there are cavities present, whose dimensions are a function of the number of vertices, the shape of the polyhedral moiety, and the interatomic separation. Partial occupation of these sites within the metal lattice by main group elements results in the formation of interstitial alloys such as, for example, metal hydrides, carbides, and nitrides. Occupation of the cavity within a molecular metal cluster gives rise to interstitial clusters. Although close topological relationships are sometimes found between interstitial alloys and interstitial clusters, the greater degree of freedom of a molecular assembly of metals, in comparison to a three dimensional infinite array of metals, increases the possible number of interstitially lodged elements... 

Core expansion of the /.ts-oxotriiron cluster 107 is reported (Scheme 8). Interaction with Mn and Re electrophiles leads to the formation of the heterotetranuclear /X4-0X0 clusters with the butterfly metal array 108 and the ji -oxo clusters with expanded metal frameworks 111-113, " which arise by Fe20 108 and Fes face capping 111-113, respectively. Interstitial clusters (e.g., 14- and 15-type complexes Scheme I) are rare for the 0x0 cluster compounds (see below) in contrast to the related carbido and nitrido clusters. 

The displacement process produces two types of crystalline point defects, vacant crystaUine positions (vacancies) and displaced atoms in interstitial crystalline positions (interstitials). Vacancy and interstitial clusters are also created directly from displacement cascades. Molecular dynamics (MD) simulations show that the fraction of point defects surviving after the displacement cascade completely cools down is eventually only 20—40% of that predicted by the Norgett Robinson and Torrens (NRT) model [4]... 

Interstitial clusters can evolve into dislocation loops and vacancy clusters can develop into vacancy loops or cavities. These clusters can contribute to changes in both mechanical properties and dimension. The types of defect clusters formed under radiation depend on the alloy crystal stmcture (e.g., body centered cubic (bcc) versus face centered cubic (fee)), alloy composition, and temperature, as discussed below. 

Figure 7.16 Cluster dynamics modeling of radiation damage in materials, (a) A typical interstitial cluster size distribution obtained from cluster dynamics modeUng [80]. (b) Comparison of the saturated interstitial cluster density at different temperatures in Fe-12Cr alloys and NF616 steels between cluster dynamics modeling and experiments [81].

Recendy, a spatially resolved model has also been applied to study the 1-MeV Kr irradiation in Fe-Cr F-M alloys and the commercial NF616 steels [81]. The in situ irradiation experiments showed that the interstitial cluster density saturates at around 10 dpa and the saturation is insensitive to temperature when the temperature is below 300°C [83,84]. In addition, interstitial clusters were found to hop for a few tens of nanometers and then remain immobile for some time when the irradiation beam is on. However, the clusters are immobile when the beam is off. It is likely that impurities trapped in the clusters make the clusters immobile, while the irradiation may detrap the impurities so that the clusters become mobile until they are trapped by impurities again [85]. 

Kohnert et al. [81] showed that using a conventional CD model in which defect and cluster diffusion are thermally activated, the interstitial cluster density increases with the inverse of temperature and the temperature-insensitive behavior below 300°C observed in experiments cannot be captured. However, if a nonthermal diffusion term is added to the cluster diffusivity to account for the beam-assisted athermal hops of clusters, the experimental observed trend can be well captured in CD models, as seen in Fig. 7.16(b). 

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