Nano Gear groups of molecules

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Nano Gear groups of molecules
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Expending Reading
1. The unique properties of Nano materials
2. Problems related to nano composites preparation
3. Illustrate several kinds nano materials and
their applications
Due by Week 11
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Solid State Sintering
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Study Objectives
1. To know about the processes of atomic diffusion
2. To know about the densification mechanisms
3. To know about the driving forces for the
densification
4. To review some fundamentals about
ceramics (vapor pressure, curvature, ionic
crystal, covalent crystal points defects and
their characterization)
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Just try
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Review
Grain growth during the firing step
Nucleated grains and normal growth
Primary Recrystallization
Excessive grain growth
Secondary Recrystallization
Consuming the uniform-grain-size-matrix harmful
to mechanical properties
Normally, fusion occurs during firing
Concerns on Changes in grain size
As for solid state sintering, what is the most
concern ????
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Solid-vapor Interface
Originally Porous Compact
25 - 60 porosity
sintering
Expected Dense Compact
Elimination of porosity by materials transfer
Progressive changes in grain shape, pore shape,
and pore size
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Driving Force for Densification
Decrease in surface area and lowering of the
surface free energy
Solid-Solid Interface
Fine Particle size
The sintering rate is roughly proportional to the
inverse of the particle size
Small Radius of curvature
Specific surface area
High Pressure
Helpful for the expending reading ?
High Reactive Activity
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Mechanisms of Mass Transfer
Surface atoms evaporation and sink to the neck
due to the pressure difference
Evaporation and Condensation
Viscous Flow
Liquid diffusion
Capillarity forces
Surface diffusion
surface atoms from one part to another
Grain-boundary or lattice diffusion
Plastic deformation
Hot pressing
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Alternate Paths for Matter Transport
1. Surface diffusion
2. Lattice diffusion from surface
3. Vapor transport
4. Boundary diffusion
5. Lattice diffusion from G.B.
6. Lattice diffusion from dislocations
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Review
Please indicate the types of points defects in an
ionic crystal
Frenkel disorder Schottky disorder
Problems in Class
How to characterize the points defects?
High Resolution Transmission Electronic
Microscope To show the atomic arrangement
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Real Grain Growth
1. Grain contact
2. Neck formation
3. Neck growth
moving of the interface
4. Resultant grown grains
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