Ceramic processing: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) Thermal grooving;3)Grain boundary angle
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Explain/Describe the wetting angle
A drop of liquid on a solid surface can either stay as a blob or spread out;the wetting angle quantifies how spread out the drop is on the surface.
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Explain/Describe thermal grooving
A.K.A - thermal etching:creates contrast and dips on the surface of the material. Applying heat to the solid to minimise the energy of the boundaries.
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Explain/Describe the grain boundary angle
When 3 grains meet at a triple junction their surface energies are almost equal. The eqm contact angle is ~120
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What is the Young-Dupre equation and how does help us deduce the surface curvature?
cosθ = (γ_SV -γ_SL) /γ_LV ] where V is vapour . The system will minimise the boundary with the highest surface energy
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Give an equation that quantifies thermal grooving
cos(φ/2) =γ_GB /2γ_SV ] where GB: grain boundary
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How is the equilibrium angle achieved for grains with less than six edges?
They have convex grain boundaries (outwardly curving).
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How is the equilibrium angle achieved for grains with more than six edges?
They have concave grain boundaries (inward curving).
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During diffusion where do grain boundaries move towards.
They move towards the centre of curvature.
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Why is thermal grooving used?
It is useful for imaging the microstructures of ceramics because ceramics are resistant to chemical etching. It changes the surface as one cannot see any contrast using optical microscopes if samples are flat.
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What does temperature promote?
Coarsening (grain/particle growth)
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What does pressure promote?
Densification
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Define coarsening
Particle growth without densification.
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Define densification
Movement of particle centres towards each other; the elimination of porosity by particle reshaping.
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What happens during sintering?
Both coarsening and densification occurs simultaneously. The average pore size decreases.
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State the categories of sintering
1)Vitrification;2)Liquid phase sintering;3)Solid state sintering
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How are pores classified?
Open,closed,transport and blind pores.
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What is continuous porosity?
self -connected in 3 dimensions, continuous path for gas atoms.
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What is closed porosity?
isolated pores
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What is inter-granular porosity?
Between grains, shape determined by curvature of grain surfaces,
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What is intra-granular porosity?
Within grains, shape usually close to spherical.
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What do inter-granular pores do?
They slow down grain growth
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What does densification rely on?
The diffusion of gas atoms from the pore to the outer surface of the ceramic.
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How does diffusion work during sintering?
Grain boundaries provide a fast diffusion path, diffusion is slower in intra-granular pores.
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Give the properties of ceramic whiteware
Good strength but brittle.
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Explain transformation toughening (e.g. Zirconia)
Dopants stabilise high temperature cubic/tetragonal forms. Thus, under tensile stress zirconia transforms to monoclinic form (increase in volume = alternating in crack propagation)
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Describe fibre toughnening in ceramics
Fine-scale discontinuous fibres provide a useful toughening mechanism in ceramics.
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What are the effects of porosity on ceramics?
It is detrimental to mechanical and functional properties. Beneficial: making foam ceramics,improve thermal shock resistance and thermal conductivity.
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Explain Vitrification
When 1 phase of in a ceramic has a low T_m and melts to fill in the gaps between the solid particles.
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Explain Liquid phase sintering
When 1 phase melts but not enough to fill the gaps but it joins the particles together and the surface tension closes the gaps. Partial dissolution of the particles occurs and re-precipitation at the edge of the neck region is seen.
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Explain solid-state sintering
All particles remain solid and pressure and temperature are the driving force. Solid state diffusion moves particles across.
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Why does sintering in oxygen atmosphere produce less porosity than in air?
Nitrogen gas trapped in pores is not soluble in oxide ceramics and further densification is prevented by the formation of intra-granilar pores. Thus less porosity would be a result if the sintering was done in oxygen atmosphere.
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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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