Ceramic processing: more on sintering

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What occurs during the initial stage of sintering?
Necks form between particles creating a micro structure with open porosity and ~60% relative density.
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What occurs during the intermediate stage of sintering?
Particles reshape to form grain boundaries and there is some formation of closed porosity with ~95% density.
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What occurs during the final stage of sintering?
There is only closed porosity present in the microstructure.
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Describe and explain the density versus temperature graph
There is a fast sintering rate initially as the temperature increases.However it is difficult to get rid of the last bit of porosity as it requires very high temperatures which is not feasible. The rate of sintering thus, slows down significantly.
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What is the macroscopic driving force for sintering?
The surface energy.
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How can the macroscopic driving force be reduced/ affected by?
The energy is reduced by: particle growth(increase in volume= decrease in surface area); Replacing free surface with solid-solid interfaces(adding grain boundaries)
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What does the reduction in surface energy produce and why is it not the main driving force?
Densification. BUT: the driving force is small that is why sintering needs high temps and pressure.
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What is the microscopic driving force for sintering?
Surface curvature
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Explain the significance of surface curvature
There is a pressure difference across curved surfaces --> atom diffusion from high to low pressure regions. For a grain boundary,direction of boundary motion is opposite to atom diffusion.
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How can the pressure difference be calculated?
It is calculted from the radius of the curvature and the surface energy. ΔP = 2γ/r
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What are the three origins of surface curvature?
1)Wetting angle;2
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Card 2

Front

What occurs during the intermediate stage of sintering?

Back

Particles reshape to form grain boundaries and there is some formation of closed porosity with ~95% density.

Card 3

Front

What occurs during the final stage of sintering?

Back

Preview of the front of card 3

Card 4

Front

Describe and explain the density versus temperature graph

Back

Preview of the front of card 4

Card 5

Front

What is the macroscopic driving force for sintering?

Back

Preview of the front of card 5
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