Point Defect Interactions

Charged particles lose their energy in target materials by two processes electronic interaction and nuclear interaction. In the electronic interaction, kinetic energy of the incident particles is transferred to electrons of the target and finally dissipated as heat. In the nuclear interaction, point defects... 

As can be seen, the degree of association falls off very steeply with increasing temperature for low concentrations of dopant. As well as directly neighbouring defects which form complexes or associates, we must also consider excited states of defect associates in which the interacting point defects are more widely separated. must then be modified to Zg exp A (t3i (e)/R T, The summation over e indicates a summation over the possible... 

It can be noticed that a linear behaviour of both thermodynamic p>arameters AH" and AS" versus 5, can be related to a random distribution of non-interacting point defects as components of ideal solution. Generally, when the nonstoichiometiy increases, noticeable deviation from linearity can be observed. In the case of Ndi.95Ni04+6 oxide, ideal-solutionlike state means that the interaction among defect species is nearly constant regardless of the defect concentration. 

Some good papers have been published recently. Unfortunately the corresponding experimental data are most often lacking. The point-defect properties calculated from the electronic structure will have to be integrated in a proper thermodynamic theory. Such knowledge will also allow study in important fields that are practically unexplored up to now in intermetallic compounds point defect-impurity interaction, point defect-dislocation interaction, and consequences on the mechanical properties, etc. Considerable work is still required. 

Two German physical chemists, W. Sehottky and C. Wagner, founded this branch of materials seience. The story is very clearly set out in a biographical memoir of Carl Wagner (1901 1977) by another pioneer solid-state chemist, Hermann Schmalzried (1991), and also in Wagner s own survey of point defects and their interaction (Wagner 1977) - his last publieation. Sehottky we have already briefly met in connection with the Pohl school s study of colour centres... 

For the deformation of NiAl in a soft orientation our calculations give by far the lowest Peierls barriers for the (100) 011 glide system. This glide system is also found in many experimental observations and generally accepted as the primary slip system in NiAl [18], Compared to previous atomistic modelling [6], we obtain Peierls stresses which are markedly lower. The calculated Peierls stresses (see table 1) are in the range of 40-150 MPa which is clearly at the lower end of the experimental low temperature deformation data [18]. This may either be attributed to an insufficiency of the interaction model used here or one may speculate that the low temperature deformation of NiAl is not limited by the Peierls stresses but by the interaction of the dislocations with other obstacles (possibly point defects and impurities). 

The nucleation behavior of transition metal particles is determined by the ratio between the thermal energy of the diffusing atoms and the interaction of the metal atoms at the various nucleation sites. To create very small particles or even single atoms, low temperatures and metal exposures have to be used. The metal was deposited as metal atoms impinging on the surface. The metal exposure is given as the thickness (in monolayer ML) of a hypothetical, uniform, close-packed metal layer. The interaction strength of the metals discussed here was found to rise in the series from Pd < Rh < Co ( Ir) < V [17,32]. Whereas Pd and Rh nucleate preferentially at line defects at 300 K and decorate the point defects at 90 K, point defects are the predominant nucleation center for Co and V at 300 K. At 60 K, Rh nucleates at surface sites between point defects [16,33]. 

These effects can all be enhanced if the point defects interact to form defect clusters or similar structures, as in Fej xO above or U02, (Section 4.4). Such clusters can suppress phase changes at low temperatures. Under circumstances in which the clusters dissociate, such as those found in solid oxide fuel cells, the volume change can be considerable, leading to failure of the component. 

Photochromic behavior depends critically upon the interaction of two point defect types with light Frenkel defects in the silver halide together with substitutional Cu+ impurity point defects in the silver halide matrix. It is these two defects together that constitute the photochromic phase. 

The treatment assumes that the point defects do not interact with each other. This is not a very good assumption because point defect interactions are important, and it is possible to take such interactions into account in more general formulas. For example, high-purity silicon carbide, SiC, appears to have important populations of carbon and silicon vacancies, and Vsj, which are equivalent to Schottky defects, together with a large population of divacancy pairs. 

Although this estimate of the interaction energy between defects is simplistic, it demonstrates that a fair number of defects may cluster together rather than remain as isolated point defects, provided, of course, that they can diffuse through the crystal. It is difficult, experimentally, to determine the absolute numbers of point defects present in a crystal, and doubly so to determine the percentage that might be associated rather than separate. It is in both of these areas that theoretical calculations are able to bear fruit. 

The vast subject of dislocations, particularly with respect to mechanical properties, will not be considered in this book, and only a few aspects of dislocations, especially interactions with point defects, will be explored. 

INTERACTION OF DISLOCATIONS AND POINT DEFECTS 3.7.1 Dislocation Loops... 

Edge dislocations play an important role in the strength of a metal, and screw dislocations are important in crystal growth. Dislocations also interact strongly with other defects in the crystal and can act as sources and sinks of point defects. 

The interaction between a charged point defect and neighboring magnetic ions in magnetically doped thin films has been described in terms of a defect cluster called a bound magnetic polaron (Fig. 9.5a). The radius of a bound magnetic polaron due to an electron located on the defect, r, is given by... 

Analyses of the defect chemistry and thermodynamics of non-stoichiometric phases that are predominately ionic in nature (i.e. halides and oxides) are most often made using quasi-chemical reactions. The concentrations of the point defects are considered to be low, and defect-defect interactions as such are most often disregarded, although defect clusters often are incorporated. The resulting mass action equations give the relationship between the concentrations of point defects and partial pressure or chemical activity of the species involved in the defect reactions. 

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