BONUS) Differential scanning calorimetry (DSC) is a technique used to investigat

Question

BONUS) Differential scanning calorimetry (DSC) is a technique used to investigate phase changes in materials. In this technique, a sample is heated and then cooled while the flow of energy into and out of the sample is precisely measured. Phase changes require the addition or removal of heat energy beyond what is needed to change the temperature of the sample, resulting in "peaks" on DSC plots of heat flow. Generally, heat flows into a material to cause melting (endothermic process) and heat flows out of a material upon solidification (exothermic process). Many materials can exhibit delayed solidification from a liquid phase due to a variety of factors, but transition from solid to liquid almost always occurs promptly at the melting temperature. Composition (at% Sn) 40 80 100 327 C -600 300 500 B+L 400 183 C 18.3 61.9 97.8 300 100 100 0 20 60 100 (Pb) Composition (wt% Sn) (Sn) Above is a phase diagram for the lead-tin alloy system. Using this phase diagram, estimate the alloy composition for each of the following DSC plots. Briefly explain your answers. (1 ea)

Explanation / Answer

Part1

Peak 1: endotherm ~ 180 C

Peak 2: exotherm ~ 150 C

Based on DSC the alloy of 61.9 % Sn + 38.1 % Pb melts at ~ 180 C

Mixture is 61.9 % Sn + 38.1 % Pb

Part 2

Peak 1: endotherm ~ 325 C

Peak 2: exotherm ~ 310 C

Based on DSC the alloy of pure Pb melts at ~ 327 C

Alloy is pure 100 % Pb

Part 3

Peak 1: endotherm ~ 180 C

Peak 2: endotherm ~ 295 C

Peak 3: exotherm ~ 260 C

Peak 4: exotherm ~ 150 C

Based on DSC the alloy of 61.9 % Sn + 38.1 % Pb melts at ~ 180 C

Any mixture with Pb between 38.1% and 81.7% would show 2 melting points, with some composition's 2nd m.p. near 300 C.

Alloy should be approximately 18.3 % Sn + 81.7 % Pb because it is the highest composition of Pb which show 2 m.p. with 1 being ~180 C and have the highest 2nd m.p.

Part 4

Peak 1: endotherm ~ 235 C

Peak 2: exotherm ~ 195 C

Based on DSC the alloy of pure Sn melts at ~ 232 C

Mixture is pure 100 % Sn

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