Open-volume defects

As most experimenters want to know not the absolute value of the linewidth but how it changes as a function of physical parameters, these ratios have been taken up as the simplest way of describing the linewidth. C/T is called the S (sharpness) parameter, and (A+E)/T is W, the wing parameter. As we can induce from Figure 3.7, S and W should be sensitive to changes in the momentum density of lower- and higher-momentum electrons, respectively. Positron annihilation in open volume defects thus typically leads to an increase in 5 and a decrease in W. 

Bamford, D., Jones, M., Latham, J., Hughes, R. J., Alam, M. A., Stejny, J., and Dlubek, G., Anisotropic nature of open volume defects in highly crystalline polymers. Macromolecules, 34, 8156-8159 (2001b). 

Diffusion of thermalised positron and subsequent trapping at an open volume defect ( 10 s)... 

In particular, the most powerful method for studing lattice defects, due to the high sensitivity of positrons to open volume defects such as vacancies, vacancy clusters, voids, dislocations, grain boundaries, etc., is positron annihilation spectroscopy (PAS). A diagram illustrating the applicability of PAS and other techniques as a function of defect size and density versus depth in material is shown in Figure 4.25. Thus, PAS represents a non-local experimental technique that is sensitive to microstructural defects at the atomic scale. A well-established theory of positron annihilation phenomena is currently available. Especially for metallic materials, it is possible to perform ab initio calculations of positron parameters for various defects and atomic arrangements [72,73]. 

The trapping of positrons in defects is based on the formation of an attractive potential at open-volume defects, such as vacancies, vacancy agglomerates, and dislocations. The main reason for this potential is the lack of a repulsive positively charged nucleus in such a defect. The sensitivity range for vacancy detection in metals starts at about one vacancy per 10 atoms. This extremely high sensitivity is caused by the fact that the positron diffuses about 100 nm through the lattice and... 

The potential sensed by the positron at open-volume defects (e.g., monovacancies or vacancy clusters) is lowered because the positive ion cores reduce the repulsion. The transition from the delocalised state to the localised one is called positron trapping. 

PAS Data Treatment. The lifetime spectra are resolved into two components, a long lifetime due to positrons trapped at defects (Td) and a short lifetime that mainly comes from free positrons in the matrix. For general interpretation, it can be considered that the positron lifetime (PL) technique is a well-established method for studying open-volume-type atomic defects and defect impurity interactions in metals and alloys. The lifetime of positrons trapped at vacancies, vacancy-impurity pairs, dislocations, microvoids, etc., is longer than that of free positrons in the perfect region of the same material. As a result of the presence of open-volume defects, the average positron lifetime observed in structural materials is found to increase with damage [127,128],... 

Most perfect ceramic crystals have more unoccupied volume than there is in metals, so they tend to be less dense. There is thus more open volume through which point defects can move. 

Lower apparent densities can be obtained by allowing air intrusions into the binder (dotted line in Fig. 1). If the concentration of filler exceeds a critical value (67 vol %), the quantity of binder falls below that of the free volume between the microspheres. The binder then no longer covers all microspheres, the homogeneity of the system is disturbed, and defects (cavities) occur. This can formally be regarded as the appearance of open pores in the structure, the consequence of which is the deterioration of all macroscopic properties of the material. 

The free spaces where Ps can form and o-Ps can have a reasonably long lifetime may be extrinsic defects, as just illustrated, or intrinsic defects, such as created when heating a pure solid compound. More generally, they may correspond to the natural voids present in any solid matrix (e.g., "free volume" in polymers, treated elsewhere in this book). Ps can be formed not only in molecular solids, including frozen liquids, but also in a number of ionic solids, even when the open spaces are rather small. For example, Ps is formed in such a highly packed lattice as KC1 [44, 45] where the largest space available corresponds to the tetrahedral sites circumscribed by 4 Cf anions, with a radius of only 0.0845 nm, resulting in an o-Ps lifetime of about 0.65 ns. 

These primary defects would relax rapidly and be transformed into secondary defects the volume of which is related with the local volume difference between the DIPS and their open-merocyanines. Assuming a non-uniform distribution of distances between the merocyanine molecules and these secondary defects, they found indeed a time-dependent concentration of the merocyanine given by the following equation... 

Open-pore microcellular aluminium foams can be produced by a process known as replication . This consists in infiltration of NaCl powder preforms by a melt, which is then solidified to form a composite. The NaCl is subsequently leached out with water, to leave a network of open pores, of volume fraction roughly varying between 65 and 90% [15], The foams can be produced to feature good microstructural homogeneity over a comparatively wide range of metal alloy compositions, pore size and component shape. They furthermore serve as attractive model materials for the investigation of microstructure/property relations in metal foams because of their macroscopically uniform and fine-scale microstructure, and because the metal making the foam can be varied with relatively wide latitude and produced free of internal defects. 

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