3 Glasses Flashcards
What is a glass? (5)
- amorphous solid
- transparent
- isotropic material
- region glass transformation material when cooled from the melt
- when cooled to rigidity w/o crystalizing
What is an isotropic vs anisotropic material?
isotropic (glass): have properties all directions
anisotropic (wood): properties directionally dependent
Glass notation/ structure (4)
- [SiO4] 4-
- tetrahedral
- pyramids
-[Si2 O7] 6-
How to maintain charges?
what does that mean?
- add cations
-means lacks long range order
Change in V when reaches Tm (melt Temp) of Crystal vs glass
glass: gradual increase
xtal: sudden increase
so if cooling a glass will have gradual decrease in V once at Tm
In glass what does Final V depend on
cooling rate
what does high viscosity affect
stops atoms in glass to move w/ solid and become crystals
what are the 2 time scales in glass/xtal formation
internal: determined by viscosity
external: determined by cooling rate
Glasses are ______ ______ that are prevented from crystalizing by _____ _____ of the system
Supercooled liquids
high viscosity
what 2 things do you need for crystal formation?
- nucleation
- crystal growth
Homo vs Hetero nuc
Homo: nuclei formed spontaneously from melt
Hetero: nuclei from pre existing surface
a glass is formed if (3)
- no nuclei
- barrier nuc is high
- no crystal growth even with nuc
Thermodynamics of Nuc?
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
why are surface atoms high E
cuz they have stained bond angles or not coordinatively saturated
what is the r*?
- critical radius
- once past it xtal themo favorable
r*= -2y/Gv
how is r* dependent on T?
- 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
Material dependent place where themo xtal growth favorable
- Gv very large (very neg) and r* is be few tenths of nm
- can be fractions to several degree diff
what is the downside to low T
as T decreases Viscosity increases so kinetic barrier
Crystals can grow any temp ______ Tm
- why would a glass be formed
- below
- if large diff bw the max rate nuc and max rate crystal growth glass formed (viscosity)
Types Ceramics/glasses (3)
- oxide type from mineralogical materials (top down)
- oxide ceramics from small molecular percussors (bottom up)
- non oxide ceramics prepared by high T processes
what is a ceramic?
- opaque, inorganic, can be xtals, semi-xtals, amorphous
most common type glass
silica, silicates, aluminosilicates
- balanced by Na, K, Al
Are ceramics resistance thermal shock?
- yes
- 90% crystal, 10% silica microcrystals
- crystals give T resistance and microcrystals give in room
how to make cloured glass
at metal oxides or carbonates to disrupt Si-O bonds
