材料科学基础英文课件3

1

What will it happen with temperature Nanjing University of Technology

(a)(b)(c)(d)(e)Nanjing University of Technology

Crystal

Glass

arranged in different patterns

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V & Q

T m

V & Q changes associated with heating and cooling in systems susceptible to glass formation.

: melting temperature

It shows linear relation

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melt

Cooling

down

=

TΔ

T

Nucleation

f(x,T) = exp(-x/T) function

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W(r

Metastable zone of undercooling: no stable nuclei form at these

T m shown above.

Kinetic Considerations

forth across the liquid/solid interface:

Time –Temperature –Transformation Curves (TTT)

Most favorable conditions for crystallization (and so least

Note: for smaller transformation volumes, t

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Structural Approach to Glass Formation Hence, local structure is disordered, but there

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form loose network. structures.

Pauling's packing rule: satisfies Zachariasen's

•

All Rules are Satisfied: SiO

Pauling's packing rule:•violates Zachariasen's rule #2.•

Rules are Not Satisfied: Na

modifying ions

Nonbridging Oxygens

glass forming ions Al O does not form a glass.

octahedral CN preferred in Al2O3 violates Zachariasen's rule #1.

aluminophosphate

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Glasses

Alkalis are structural modifiers:

Every alkali ion creates one

Every alkali oxide 'molecule'

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Effects of Alkaline earth oxides

Y↓, O

b ↓, O

nb

↑, structure “open”, ion easy to transfer,

TEC ↑, σ↑, η↓

•T

g (B

2

O

3

) ~260ºC (2D-network)

•T

g (SiO

2

) ~1200ºC (3D-network)

What happens when alkali oxide is added to a borate glass?

added Fewer network bridging