Question

Material Science Help

1. Select the element in the periodic table that is commonly used as an n-type dopant in a silicon semiconductor:

a. C

b. B

c. P

d. O

2. The compound with the smallest cation is:

a. AuCl

b. CaF2

c. ScCl3

d. LiBr

3. The percentage ionic character of MgO is:

a. 73

b. 68

c. 81

d. 27

4. The expression for the volume of a unit cell in a BCC crystalline structure is:

a. 64R3/[3(3)1/2]

b. 16R3(2)1/2

c. 24(2)1/2R3

d. N[4/3]?R3

5. In the BeF2 crystal structure you may expect to have:

a. the fluoride ion surrounded by beryllium ions due to the smaller size of the anion.

b. the beryllium ion surrounded by fluoride ions due to the smaller size of the cation.

c. the structure exhibit directional bonding due to the percent ionic character of the bonds.

d. the fluoride and beryllium ions not equally distributed in the crystalline network.

6. A condition that will make a polymer totally non-crystalline is:

a. it is atactic

b. it is semicrystalline

c. it has a low glass transition temperature

d. the melting temperature is high

7. The following point defects cannot be present in metal crystalline structures:

a. vacancy

b. self interstitial

c. screw dislocation

d. Frenkel defect

8. Fick�s Second Law does not predict.

a. The concentration profile and time scale in diffusion.

b. Diffusion at steady state conditions follows a linear profile.

c. Diffusion at non-steady state conditions has curvature in the concentration profile.

d. The natural timescale in diffusion is temperature dependent.

9. The stress-strain curve of a ceramic is best described as:

a. large engineering tensile stress at low engineering tensile strain

b. small engineering tensile stress at low engineering tensile strain

c. large engineering tensile stress at large engineering tensile strain

d. small engineering tensile stress at large engineering tensile strain

10. The reason why grains grow during heat treatment is:

a. the overall energy of the system is decreased

b. the ductility is increased

c. the system has an entropy increase

d. the tensile strength is increased

11. What material is best suited to build a cylinder that will hold a noncorrosive gas at high pressure? a. plastic b. concrete c. metal d. rubber

12. During processing to make a fiber it is common to stretch a polymer while solidifying, this causes:

a. an increase in tensile strength

b. alignment of amorphous regions

c. alignment of crystalline regions

d. all the above

13. The maximum stress found on an S-N curve is:

a. tensile

b. yield

c. modulus

d. mean

14. Use the rule of mixtures to find the maximum modulus possible for a material that has 30 vol% glass fibers with a modulus of 89 GPa dispersed in an epoxy resin with a modulus of 3.5 GPa.

a. 63.4 GPa

b. 29.2 GPa

c. 4.9 GPa

d. 44.6 GPa

Answer 1

1.c.   P

2.d. LiBr

3.b. 68

4. a.    64R³/(3sqrt(3))   

Answer 2

n-type Semiconductor: when a silicon crystal is doped with atoms of group-15 elements then only four of the five valance electrons of each impurity atom participate in forming covalent bonds and fifth electron is almost free to conduct electricity. Such type semiconductors are called n-type as increases in conductivity is due to negatively charged electrons Si Si-P 1sth group elements are N,P, As, Sb, and Bi. Therefore, the correct option is "c"