Download Fang

Materials Subject Exam Summary: First Half with Dan Steingart Alta Fang 1/20/14 Examiners: Craig Arnold, Dan Steingart Others present: Mikko Haataja (my advisor), Julia Mikhailova (chair), Barrie Royce Courses: • MSE501 Introduction to Materials • MSE 452 Phase Transformations and Evolving Microstructures in Hard and Soft Matter Systems • MSE 504 Monte Carlo and Molecular Dynamics Simulation in Statistical Physics & Materials Science • MAE 534 Energy Storage Systems At first they said they would ask questions together but it ended up mostly being Dan for the first half and Craig for the second half, although both of them were there and listening the whole time. Both of them brought mystery materials but we only ended up talking about the one brought by Dan. Dan gave me a piece of aluminum and asked me about it. • How can we identify what material this thing is made of? • Why are metals shiny/reflective? • What is the difference between a metal and a semiconductor and an insulator? Draw the band diagrams. • How do photons interact with these materials? • How can we tell whether a material is a metal or a semiconductor? o Resistivity of a metal increases with temperature, while resistivity of a semiconductor decreases with temperature. • What is the difference between an insulator and a semiconductor? o I struggled with this a bit, since I just said the insulator has a bigger band gap than a semiconductor, but he wanted more. He asked me what happens when you apply a voltage? Both Dan and Craig looked pleased that I said the words “breakdown voltage”. However, Dan then probed me more about breakdown and I struggled. Eventually he got me to say that breakdown occurs locally rather than throughout the insulating material, and then he asked me what happens in the region very close to the breakdown region (~3 atoms away). Despite his trying to give me hints, I was unable to say what he wanted to hear, which is that the temperature increases and the material heats up, so it can expand/melt. Also he asked me what happens to the material after it breaks down, is it the same afterwards? • Draw the I vs. V curve for a metal vs. a semiconductor vs. an insulator. • What are the different types of defects in a material? •
•
•
•
•
•
•
•
•
•
•
•
What other properties could we use to identify the material, besides electrical and optical? o I started to talk about mechanical properties but he stopped me and wanted me to talk about magnetic properties first, so I said iron is ferromagnetic but copper is not. It seemed like he just wanted me to say that metals could be differentiated by their magnetic properties, so then we moved on to mechanical properties. o I said we could measure the elastic modulus, so he asked me what ratio do people look at, with regards to elastic modulus or to yield stress? I said the ratio of elastic modulus to density, and he said what do we call that? I said strength-­‐to-­‐weight ratio. This seemed to be what he wanted. What other experimental techniques can we use to identify the material? Why use X-­‐rays for diffraction? Talk about X-­‐ray diffraction. What information can you get from X-­‐ray diffraction? How would you determine the lattice parameter from X-­‐ray diffraction? He tried to ask me a question to get me to say “structure factor” and mention the selection rules for various crystal structures (e.g. for fcc, h, k, l have to be all odd or all even). I said these things, which were what he wanted, but I can’t remember exactly how he phrased the question. Why does a material with smaller grains have a broader diffraction peak? How can we control grain size? Does the history of a material affect its structure? What is the word for cooling something down very quickly? What might happen when we quench a liquid? What is nucleation? In a beaker of liquid that you are cooling to get a solid, where would nucleation occur? Why? Craig jumped in and started to ask me to derive the barrier to nucleation but then it was time for a break.