9. Answer of the following: a) Describe what you would do to increase the grain
ID: 882427 • Letter: 9
Question
9. Answer of the following: a) Describe what you would do to increase the grain size of a crystalline material. What would you do if you wanted to decrease it? b) Describe the differences between brittle and ductile fracture with respect to temperature range, mechanism of consuming energy to relieve stress, and appearance of the fracture surface. c) SiC can be described as an fcc array of C atoms with Si in the tetrahedral sites. What fraction of these sites are filled? MgAl2O4 is an fcc of O^2- ions with Mg^2+ in tetrahedral sites and Al^3+ in octahedral sites. What fraction of each are filled? d) Name two processing variables that could be modified to increase the strength of a thermoplastic polymer formed by addition polymerization (excluding crosslinking). Explain why both are effective.Explanation / Answer
9. a) To increase the grain size of a crystalline material we would have to recrystallize the material with either redissolving it and recrystallizing with crystal nucleus slowly at low temperature or we could use the same grain and put it in a solution of material and let it grow at low temperature. Similarly to decrease the size of crystalline grain, we would dissolve it and recrystallize at high temperature or rapid crystallization would give low grain size. Alternatively, putting the grain in very dilute solution also would rduce the size of the crystalline grain.
b) The amount of plastic derformation separates the ductile material from brittle material. While ductile shows higher plastic deformation, brittle material show lower or negligible plastic deformation. With temperature the plastic deformation of ductile material would increase in greater ranges as opposed to brittle material. Since the plastic deformation required energy the ductile material would absorb or consume greater energy then brittle material and would undergo plastic deformation. The appearence of the fracture in ductile material is more uniform due to plastic deformation whereas, in brittle material due to rapid fracture this is more rough.
c) For SiC an FCC array of C atomsa nd Si in the tetrahedral sites, we have 50% of these sites filled.
In case of MgAl2O4 : For an fcc unit cell contains 4 “main” atoms, 4 octahedral holes and 8 tetrahedral holes. So, for every 4 O2-, there are 4 octahedral holes. If the two Al3+ are in two octahedral holes than 1/2 of them are full and two remain empty per unit cell. An FCC unit cell contains 4 “main” atoms, 4 octahedral holes and 8 tetrahedral holes. So, for every 4 O2-, there are 8 tetrahedral holes. If the one Mg2+ is in one of the tetrahedral holes than 1/8th of them are full and seven remain empty per unit cell. Each Al+3 has 6 Oxygen neighbours and 6 Mg2+.
d) Increase the steric or bulk character of the monomer increases the strength of the thermoplastic polymers. Making a mixture of polymers may also lead to an increase in the strength of the thermoplastic polymers.
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