tailieunhanh - The Materials Science of Thin Films Episode 2

Tham khảo tài liệu 'the materials science of thin films episode 2', kỹ thuật - công nghệ, cơ khí - chế tạo máy phục vụ nhu cầu học tập, nghiên cứu và làm việc hiệu quả | 28 A Review of Materials Science Figure 1-12. Ge-Si equilibrium phase diagram. Reprinted with permission from M. Hansen Constitution of Binary Alloys McGraw-Hill Inc. 1958 . wise these diagrams hold at atmospheric pressure in which case the variance is reduced by 1. The Gibbs phase rule now states f n ỉ - ỳ or 3 -0. Thus at most three phases can coexist in equilibrium. To learn how to interpret binary phase diagrams let US first consider the Ge-Si system shown in Fig. 1-12. Such a system is interesting because of the possibility of creating semiconductors having properties intermediate to those of Ge and Si. On the horizontal axis the overall composition is indicated. Pure Ge and Si components are represented at the extreme left and right axes respectively and compositions of initial mixtures of Ge and Si are located in between. Such compositions are given in either weight or atomic percent. The following set of rules will enable a complete equilibrium phase analysis for an initial alloy composition xo heated to temperature To. 1. Draw a vertical line at composition xo. Any point on this line represents a state of this system at the temperature indicated on the left-hand scale. 2. The chemical compositions of the resulting phases depend on whether the point lies a in a one-phase field b in a two-phase field or c on a sloping or horizontal isothermal boundary between phase fields. a. For states within a single-phase field. . L liquid s solid or a compound the phase composition or chemical analysis is always the same as the initial composition. . Thermodynamics of Materials 29 b. In a two-phase region . L s a 3 etc. a horizontal tie line is first drawn through the state point extending from one end of the two-phase field to the other as shown in Fig. 1-12. On either side of the two-phase field are the indicated single-phase fields L and S . The compositions of the two phases in question are given by projecting the ends of the tie line vertically down and .