Impurity segregation

Uniformity characterization of luminescent materials (e.g., mapping of defects and measurement of their densities, and impurity segregation studies)... 

Here, we show two cases for impurity segregation between melt and crystal as it grows in time. Note that an initial purification occurs in both cases but the distribution coefficient for the case on the right is such that the amount of impurity actually incorporated into the crystal, ki Cq. 

This illustrates the fact that impurity segregation and purification processes are dependent upon the type of impurity involved and its individual segregation coeffleient. As we illustrated above in 6.8.1., the problem is that the impurity is initially rejected from the solid, but its concentration builds up in front of the growing OTStal. The segregation coefficient, k,, then operates on that increased concentration and the product, ki Co. increases. 

These authors produced TEM samples of Bi-doped, Sb-doped and Ag-doped copper foils, thinned to electron transparency using conventional preparation procedures. In all cases the presence of impurity segregation was confirmed using conventional X-ray energy-dispersive spectrometry. The EELS measurements were carried out with a STEM operating at 100 keV, with a nominal probe size of 1 nm (full width at half maximum) with a current of about 0.5 nA. The conditions required to optimize detection sensitivity for interface analysis require the highest current density and are not consistent with achieving the smallest probes. 

The mechanical properties of ceramics maybe increased by minimising impurity segregation, decreasing grain size and increasing sintered density. 

For instance, dislocations have been shown to play a key role in the accommodation process in YTZP, justifying the threshold stress in YTZP, in contrast with the hypothesis that this threshold stress is due to the electric field created by impurity segregation. However, dislocations are not systematically observed in YTZP furthermore it was shown that in yttria-stabilized tetragonal zirconia single crystals, the stress necessary to activate dislocations at 1400°C was over 400 MPa, one order of magnitude higher than the stresses used during superplastic deformation of YTZP at the same temperature. It will be necessary to conduct a systematic study of the microstructure of the monolithic ceramics such as YTZP before and after deformation and to correlate their relationship with the superplastic features. 

Tlic results also suggest that substitutional formation is most favorable on the (110) surface. This supports the view that the (110) surface will be more catalytically active than the (111) surface, as impurities segregate preferentially to this surface. Sayle et al. note that the segregation energies (i.e. the differences between bulk and surface energies) are larger for the Af cations than for cations due to elec-... 

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