Phase boundary segregation

When a hot face of a refractory is exposed to a liquid, various microstruc-tural features of the refractory interact with the hot liquid. The features that could be present in a refractory are refractory crystal, orientation boundary, phase boundary, segregated impurity, unbonded boundary, matrix, liquid boundary film, connected porosity, and open joint. 

In the strong segregation phase boundaries were calculated for xN 100 and compared to the experimental results for Pl-arm-PS stars. The results are shown in Fig. 36. 

Similar calculations were also performed in the strong segregation limit. In this case, the two-phase and disordered homogeneous regions were found to be smaller, and the phase boundaries were more vertical (see Fig. 6.49) (Shi and Noolandi 1995). This phase diagram was interpreted on the basis of interfacial curvature. If the diblocks are completely segregated, the phase boundaries are determined by the total composition / (=

phase boundaries are parallel to this line (dashed line sloping to the left in Fig. 6.49) ( one-component approximation ). This explains the approximately parallel... 

Several parameters can influence strongly the superplastic behaviour of ceramics, i.e. the strain rate at which the material can be superplastically deformed. Between them can be mentioned the grain size, second phases and segregation of impurities at the grain boundaries, etc. 

The process of spontaneous formation of ordered structures that occur as reaction, adsorption, and organization of alkanethiols (X-(CH2) -SH) on gold is a good example of molecular self-assembly [1,31]. Their construction is driven by the thermodynamically favored segregation of molecules to the phase boundary between solid gold and solution (or vapor) of alkanethiols. The chemical bonding between... 

A more sophisticated approach for determination of the grain boundary segregation is similar to the determination of the surface tension of silicon melt. The novel approach of surface tension simulation has been successfully implemented in the thermochemical database. Hence, the assessment of the parameters for impurity segregation in solid silicon phase may greatly extend the application of the thermochemical database. The calculation results for C and O segregation are shown as dashed lines in Fig. 13.28. The McLean segregation isotherm can be reproduced using the approach similar to the surface tension simulation. 

Surfaces always differ in behaviour from the bulk of a material because of the abrupt changes that occur at and near phase boundaries. Surface atoms and molecules are not in equilibrium states, since they are neither in one phase nor in the other. Unsaturated bonds abound. This leads to an excess energy associated with the surface, the so-called surface free energy which has different values for different crystallographic orientations. There are different ways to minimize surface energy. A simple way would be to reduce the surface area under the influence of surface tension. But this is not realistic with solid materials. Surface free energy, however, can also be lowered by adsorption and segregation phenomena. 

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