3 Glasses

Question 1

What is a glass? (5)

Answer 1

• amorphous solid
• transparent
• isotropic material
• region glass transformation material when cooled from the melt
• when cooled to rigidity w/o crystalizing

Question 2

What is an isotropic vs anisotropic material?

Answer 2

isotropic (glass): have properties all directions
anisotropic (wood): properties directionally dependent

Question 3

Glass notation/ structure (4)

Answer 3

• [SiO4] 4-
• tetrahedral
• pyramids

-[Si2 O7] 6-

Question 4

How to maintain charges?
what does that mean?

Answer 4

• add cations
  -means lacks long range order

Question 5

Change in V when reaches Tm (melt Temp) of Crystal vs glass

Answer 5

glass: gradual increase
xtal: sudden increase

so if cooling a glass will have gradual decrease in V once at Tm

Question 6

In glass what does Final V depend on

Answer 6

cooling rate

Question 7

what does high viscosity affect

Answer 7

stops atoms in glass to move w/ solid and become crystals

Question 8

what are the 2 time scales in glass/xtal formation

Answer 8

internal: determined by viscosity
external: determined by cooling rate

Question 9

Glasses are ______ ______ that are prevented from crystalizing by _____ _____ of the system

Answer 9

Supercooled liquids
high viscosity

Question 10

what 2 things do you need for crystal formation?

Answer 10

• nucleation
• crystal growth

Question 11

Homo vs Hetero nuc

Answer 11

Homo: nuclei formed spontaneously from melt
Hetero: nuclei from pre existing surface

Question 12

a glass is formed if (3)

Answer 12

• no nuclei
• barrier nuc is high
• no crystal growth even with nuc

Question 13

Thermodynamics of Nuc?

Answer 13

Gn=4/3pir^3Gv + 4pir^2y

• Gn:Free E nuc, neg for xtals
• Gv: Change V free E, interior, always neg cuz crystal free E less melt
• y: Cystal/melt interfacial E, always pos cuz surface atoms have high E

Question 14

why are surface atoms high E

Answer 14

cuz they have stained bond angles or not coordinatively saturated

Question 15

what is the r*?

Answer 15

• critical radius
• once past it xtal themo favorable
  r*= -2y/Gv

Question 16

how is r* dependent on T?

Answer 16

• r* dependent Gv
• Gv dependent on T, below Tm Gv is is small and r* is big
• as T decreases Gv increases and r* decreases

r*= -2y/Gv

Question 17

Material dependent place where themo xtal growth favorable

Answer 17

• Gv very large (very neg) and r* is be few tenths of nm
• can be fractions to several degree diff

Question 18

what is the downside to low T

Answer 18

as T decreases Viscosity increases so kinetic barrier

Question 19

Crystals can grow any temp ______ Tm
- why would a glass be formed

Answer 19

• below
• if large diff bw the max rate nuc and max rate crystal growth glass formed (viscosity)

Question 20

Types Ceramics/glasses (3)

Answer 20

• oxide type from mineralogical materials (top down)
• oxide ceramics from small molecular percussors (bottom up)
• non oxide ceramics prepared by high T processes

Question 21

what is a ceramic?

Answer 21

• opaque, inorganic, can be xtals, semi-xtals, amorphous

Question 22

most common type glass

Answer 22

silica, silicates, aluminosilicates

• balanced by Na, K, Al

Question 23

Are ceramics resistance thermal shock?

Answer 23

• yes
• 90% crystal, 10% silica microcrystals
• crystals give T resistance and microcrystals give in room

Question 24

how to make cloured glass

Answer 24

at metal oxides or carbonates to disrupt Si-O bonds