Radiation induced segregation

Even in the ab en(pe of preferential sputtering, i.e, SA=Sg, it is not necessarily the case that the concentration ratio of A and B is uniform with depth under steady state ion bombardment. The subsurface layer can have a different concentration ratio from the bulk or the top surface even if the latter two are identical. This can occur because of the subsurface effects of ion bombardment discussed below, which can produce an altered subsurface composition. As a result, there are two altered layers possible the top surface layer where composition at steady state is completely controlled by RgA and the subsurface layer where composition is controlled by segregation, radiation induced diffusion, radiation induced segregation, recoil implantation and cascade mixing. 

As mentioned in Section 5.3.2.3, depth resolution may also be affected by primary ion beam-induced modification of the substrate chemistry. This is typically noted for reactive primary ion beams. As an example, 02 primary ion beams can oxidize the outer surface of certain materials. This can then facilitate the movement of the elements of interest within the solid, i.e. induce segregation to/from the surface. This effect is referred to as oxidation-induced segregation. Radiation-induced segregation discussed in Section 3.2.3.1 may also occur. Concomitant secondary ion yield variations resulting from these surface chemical modifications will further complicate matters. 

At low temperatures dissimilar-defect segregation arises under permanent particle source due to local fluctuations of particle (defect) densities. Radiation-induced production by a change of two or more similar defects (say, A) nearby creates a germ of their aggregate which is more stable and has a greater chance to survive rather than two isolated defects A since the probability that two or more defects B will be created statistically in the same... 

In addition to thermal desorption, gas desorption has been found to result from electron, ion and photon bombardment of surfaces. Therefore, simultaneous particle and photon bombardments can be expected to alter desorption rates, as well as the nature and charge distribution of the desorbed species. Furthermore, simultaneous bombardment of a surface by neutrons and ions could affect diffusion processes, e.g., by radiation-induced segregation. In turn, desorption processes can be influenced by altering the diffusion of species from the bulk to the surface. The type, energy, and angular distribution of particles expected to strike neutral beam injector dump areas (such areas can represent 1/9 of total first wall area) can cause synergistic effects on gas desorption which can be quite different from those expected from the interaction of plasma radiations with the first wall. 

If a surface is sputtered at sufficiently low temperature to avoid bulk diffusion, atoms of the species preferentially sputtered can reach the surface by displacement mixing and radiation-induced segregation. This leads to a so-called altered layer, which has a composition different from that of the bulk and a thickness close to the penetration depth of the projectiles. Since surface diffusion has a much lower activation barrier than bulk diffusion, annealing a sputtered alloy surface first leads to a local equilibrium between the surface and the immediate subsurface layers, which still belong to the altered layer. Only after the onset of bulk diffusion is reached, usually around 60 - 70% of the melting temperature, the altered layer equilibrates with the bulk and true equilibrium segregation is observed [45]. For alloys of atoms with different size the existence and dissolution of an altered layer can be studied by STM because of the development of a misfit dislocation network between the altered layer and the bulk [46] (Fig. 7). 

The final effect to be considered is radiation induced segregation (RIS). The creation and annihilation of point defects can be spatially separated. This leads to defect fluxes or an equivalent atom flux that depends on which alloy element is involved. When this leads to a non-uniform distribution of elements within a previously homogeneous alloy it is called RIS. It is an active area of research, particularly for alloys used in nuclear power plants, which has been reviewed recently (39). The effect can be strong enough to produce phase separation and growth under ion bombardment. Depths affected are of the same order as RID because they both involve thermally activated processes which can produce effects well beyond the ion damage layer at elevated temperatures. 

Materials science (microelectronics analysis, surface layers, multilayers, PIXE channeling of dopants in crystals, depth profiling, binary alloys, impurities deposited in nuclear fusion devices, magnetic relaxation in nanocrystalline iron, insulating materials, radiation-induced segregation, superconductors, catalysts, and diffusion studies). 

Different secondary ions can also display different depth resolution values for the same substrate. As an example. Copper typically yields poor depth resolution because of its high diffusion coefficient, particularly when sputtered. This sputter-induced enhancement is otherwise referred to as radiation-enhanced diffusion. Radiation-induced segregation may also be initiated, with different primary ion/secondary ion combinations resulting in different trends. As a result, any emissions collected as a result of this form of sputtering will always emanate from what is termed an altered layer, as opposed to the initial intrinsic substrate layer. Exceptions are sometimes noted for large cluster ion impact, because, as mentioned in Section 4.1.1.3, these can remove sputter-induced damage. 

The effect of P on irradiation embrittlement was also attributed to segregation leading to temper embrittlement. However, there is direct physical evidence of radiation-induced P segregation at grain boundaries. 

The rest of the chapter is divided into three sections. A brief description of the radiation effects in materials is provided in Section 7.2. For a detailed description of the various radiation effects in stmctural and functional materials, readers can consult reference [3], in which a much greater level of details are provided in hve volumes. In addition. Section 7.2 discusses radiation-induced defects, solute segregation, and phase transformations in Generation IV reactor materials. Section 7.3 focuses on the techniques used for characterization of defects in irradiated materials and the advances made in the past 10 years. The discussion on mesoscale modeling of radiation damage is provided in Section 7.4. 

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