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Engineering Materials 2E VOLUME2 Episode 3

Tham khảo tài liệu 'engineering materials 2e volume2 episode 3', kỹ thuật - công nghệ, cơ khí - chế tạo máy phục vụ nhu cầu học tập, nghiên cứu và làm việc hiệu quả | Case studies in phase diagrams 41 Fig. 4.6. Schematic plot of eqn. 4.9 showing how small k and long l give the best zone refining performance. Co - kCL Co - kCo exp kx T 4.7 Substituting CS kCL in eqn. 4.7 produces Co - Cs Cq 1 - k exp kx l 4.8 This gives at last 4.9 I I kx II Cs Co h - 1 - k exp j - Y 4 which we have plotted schematically in Fig. 4.6. Figure 4.6 is interesting because it shows that for the best refining performance we need both a long zone and an impurity that is relatively insoluble in the solid low k . Unfortunately long liquid zones can be destabilised by convection and impurities with a low k do not come to order Commercial zone refining processes may therefore involve a large number of passes done one after the other Fig. 4.7 . This obviously adds a lot to the cost of the pure material but the process can be speeded up considerably by using the multi-heater arrangement shown in Fig. 4.8. Making bubble-free ice People who go in for expensive cocktails like to cool them with ice cubes which are crystal clear - it adds to the aura of bejewelled refreshing purity. Unfortunately ice cubes grown in an ordinary fridge are cloudy. So establishments which cater for upmarket clients install special machines to make clear ice. 42 Engineering Materials 2 Fig. 4 .7. If the bar is repeatedly zone refined from left to right then more and more of the impurity will be swept to the right-hand end of the bar. A large number of zone-refining passes may be needed to make the left-hand half of the bar as pure as we need. The right-hand half is cut off and recycled. Note that eqn. 4.9 can only be used to calculate the impurity distribution produced by the first pass. A computer program has to be written to handle each subsequent pass. - - - - - Fig. 4.8. A multi-heater arrangement gives much faster zone refining. The cloudiness of ordinary ice cubes is caused by thousands of tiny air bubbles. Air dissolves in water and tap water at 10 C can - and usually does - .