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MT2%20partial%20solutions

# MT2%20partial%20solutions - What type of phase diagram is...

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What type of phase diagram is this? ___ Binary Eutectic ____________________ What is the melting point of pure gold, Au? _________°C What is the melting point of pure silicon, Si? _________°C Be sure you read the correct scale, Centigrade not Farenheit! If 2.4 grams of Au are mixed with 9.6 grams of Si,. what phase(s) will be present at 1300°C? Draw the state point on the phase diagram. 9.6 g / (2.4g + 9.6g) = 0.80 wt % Si. Be sure to draw this in the correct spot. At 1300 C, this is Liquid + solid. What are the compositions of each phase present? Draw a tie line and read the compositions off the end points. For the above alloy, how many grams of (Si) phase will be present at 1100°C? (Draw the state point) Drop down to the lower temperature and draw your new state point. Draw a tie line Measure the left and right portions of the lever and use the lever rule. What is the highest temperature that gold on Silicon can safely be used without a chance of melting? Just below the eutectic temperature.

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When a crystal of silicon grows in the Czochralski process, is energy absorbed or released ? Released. When bonds form, the energy is reduced. Experimentally, explain what can be done to increase the driving force for crystallization of silicon. Draw a free energy diagram (Free energy, G, vs temperature) to support your explanation. Experimentally, Reduce the temperature. Look in the textbook on page 291 for the appropriate Graph to use with your explanation of why this works. Explain how crystallization of silicon would differ if no seed crystal were used. a) Would the conditions to crystallize silicon need to be different? In what way? Why? b) Would the quality of the resulting silicon crystal be different? In what way? Why? Your answers can vary a bit, but should include the main ideas. In order to get crystallization without a seed, more undercooling is necessary. Crystallization may occur heterogeneously on the walls of the container. If this can be avoided (heat the Walls) then homogeneous nucleation can occur spontaneously at unpredictable locations in the undercooled Silicon melt. This will result in many crystals forming, not a single crystal. These crystals will have various Random orientations with respect to one another, and will grow together, forming a polycrystalline material. There are many
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