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Exam 3 Answers Key - Materials Science | ENSC 3313, Exams of Materials science

Oklahoma State University (OSU) - StillwaterMaterials science
Prof. James Smay

Material Type: Exam; Professor: Smay; Class: MATERIALS SCIENCE; Subject: Engineering Science; University: Oklahoma State University - Stillwater; Term: Fall 2012;

Typology: Exams

2012/2013

Uploaded on 10/22/2013

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ENSC 3313, Fall 2013 EXAM 3 - KEY Name __________________
Answer ALL questions. Show all work. 2 points each Grade:______/20
1
1. Match a name from the left column for each composition in the right column.
Name Composition (wt%)
(a) 69.92% Fe, 19% Cr, 9% Ni, 2% Mn, 0.08% C
d Brass (b) 90% Cu, 10% Al
e Cast iron (c) 99.8% Fe, 0.10% C
f Plain, high-carbon steel (d) 67% Cu, 29% Zn, 3% Pb, 1% Sn
_a_Stainless steel (e) 97% Fe, 3% C
_c_Low-carbon steel (f) 99% Fe, 1% C
b Bronze
2. In lab 4, you tested the hardenability of steel. Label the axes of the hardenability graph and explain why
you superimposed "hardenability bands" to identify possible matches to your sample in question 4 of that
lab.
!
pf3
pf4

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ENSC 3313, Fall 2013 EXAM 3 - KEY Name __________________ Answer ALL questions. Show all work. 2 points each Grade:______/ 1

  1. Match a name from the left column for each composition in the right column. Name Composition (wt%) (a) 69.92% Fe, 19% Cr, 9% Ni, 2% Mn, 0.08% C d Brass (b) 90% Cu, 10% Al e Cast iron (c) 99.8% Fe, 0.10% C f Plain, high-carbon steel (d) 67% Cu, 29% Zn, 3% Pb, 1% Sn _a_Stainless steel (e) 97% Fe, 3% C _c_Low-carbon steel (f) 99% Fe, 1% C b Bronze
  2. In lab 4, you tested the hardenability of steel. Label the axes of the hardenability graph and explain why you superimposed "hardenability bands" to identify possible matches to your sample in question 4 of that lab.
  1. Describe sintering of ceramics. In your description, use the terms: "surface area", "energy", "porosity", "green body", "high temperature" and "time". Ceramic " green bodies " are formed by powder processing (e.g., slip casting, pressing, tape casting). The green body is a loose assembly of particles (typ. 40-50% by volume), binder, and air. By heating to the " high temperature " required for sintering, the particle can fuse into a polycrystalline microstructure and thereby reduce " porosity " and strengthen the ceramic. The process is diffusion controlled and takes " time " to complete at the sintering temperature (i.e., an isothermal hold). The driving force for sintering is reduction of " energy " by reducing the internal " surface area ."
  2. Recollect the figure of specific volume vs. temperature for a polymer. There are two characteristic temperatures for most polymers. (a) Name and describe these two temperatures melt temperature (Tm) – a transition in properties associated with melting crystalline part of polymer glass transition temperature (Tg) – temperature below which fracture is brittle; chain sliding not possible (b) Why are both temperatures lower for low density polyethylene compared to high density polyethylene LDPE is branched and less crystalline than HDPE (c) Why does epoxy typically not have these characteristic temperatures? Epoxy is a network thermoset polymer. Upon heating, it decomposes rather than melting. Extensive cross linking causes brittle fracture behavior.
  3. Natural rubber made of cis-1,4 polyisoprene can be vulcanized by addition of S and lead carbonate at elevated temperature. How much S, in kg, is required to cross link 20% of the available sites in 10 kg of cis-1,4 polyisoprene if the S is only 80% efficient in forming cross links? Note: C=12 amu, H=1amu, S= amu. First, calculate the mer weight: MW=5⋅12+8=68kg/kmol. In 10kg of mer, there are 10/68 = 0.147 kmol. Also, 2⋅0.147=0.294 kmol of cross link sites. Each S consumes two cross link sites when it cross links, so need 0.8⋅NS=0.2⋅0. ∴ Ns=0.0368kmol→ms=32kg/kmol⋅0.0368→ ms=1.18kg

C C C

H CH 3

H

H H

H

C

  1. Silicon has 8 atoms per unit cell. What fraction of the atoms contribute an electron to the conduction band for silicon at 20oC? Recollect that the charge of an electron is 1.6 × 10 –^19 C. For an intrinsic semiconductor, conductivity equation is: σ = nq(μe+μh) Conduction electron density: Atom density: 5 × 10 -^4 = n(1.6 × 10 –^19 )(0.19 + 0.0425) N=8/(0.543× 10 -^9 )^3 =5× 1028 /m^3 n =1.34× 10 16 /m 3 at T= o C (308K) we have n 308 K = n 293 K exp − Eg 2 kB
  • →^ n^308 K^ =^1.^34 ×^10 16 exp −

2 ⋅ 8. 62 × 10

− 5

n 308 K = 2. 89 n 293 = 3. 89 × 1016 m −^3 n 308 K / N = 7. 74 × 10 − 13

  1. In class, we discussed how a reverse biased p-n junction can be used as a light detector. Draw a diagram of this device and explain (briefly) how it works. Is the photodiode forward or reverse bised? As discussed in class, the current in the reverse biased condition is due to the generation of “electron- hole pairs” (ehp's) in the depletion region of the p-n junction. The normal thermal generation rate is relatively small, however, if we illuminate the junction such that ehp's are generated by absorption of “photons” , we can increase the reverse bias current proportionally to the generation rate ( g ). Note, each ehp created in the junction results in the rapid “ drift” of the electron to the n side of the junction and hole to the p side due to the electric field in the junction, hence reverse biased photocurrent is directly proportional to the number of photons absorbed.