表面与界面课件.ppt

Adsorption isotherms for tin in iron interfaces 从上图可以看出，液态铁表面对锡的吸附存在饱和现象，即以被吸附的锡形成单原子层为达到最大吸附量的标志，而固态铁表面与固态铁内界面对锡的吸附都没有饱和现象，暗示它们可以形成多层吸附。 Grain-boundary energy and segregation isotherms for phosphorus in iron This can be shown to be related to the slope of the surface tension isotherm as: field-ion micrograph of boron-decorated boundary in Ni3Al. Bright spots on boundary were identified as individual boron atoms using single atom identification technique of the atom probe. 平衡偏聚与非平衡偏聚 表面、界面上的平衡偏聚起源于溶质、杂质与表面、界面之间的结合能，即溶质、杂质位于表面、界面与其位于完整晶格内两种情况下，体系自由能不同，由此导致表面、界面处溶质浓度不同于晶内。 表面、界面上的非平衡偏聚与过程有关，当体系保持在同一状态的时间趋于无穷时，非平衡偏聚趋于消失，非平衡偏聚常与空位相关。 Free energy diagram for solutes segregated at surfaces and grain boundaries. In the dilute limit of element A segregating in a matrix of element B, one may consider the relative concentrations C1(A) and C2(A), at different sites, 1 and 2, to obey the Maxwell-Boltzmann relation: Mclean proposed a model of P solute atoms distributed at random amongst N lattice sites and p solute atoms distributed at random amongst n independent grain-boundary sites. The total free energy due to the solute atoms is then where E and e are the energies of the solute atom in the lattice and in the grain boundary, respectively, and the final term is the configurational entropy for the arrangement of the solute atoms in the bulk and grain boundary. The equilibrium state of the system occurs at the minimum value of G. This may be determined by differentiating G with respect to p, noting that the sum of p and P remains constant. Corresponding to this minimum, the grain-boundary analogue of Langmuir adsorption at free surfaces is obtained: where K is the solute bulk modulus, G is the solvent or matrix shear modulus and N is Avogadros number. The final column shows the experimental values. 偏聚动力学 The kinetics of the segregation are described by: Qualitative shape of a chemisorption potential φ as a function of the distance