Ceramics- Controlling Sintering Flashcards
How does mean density change as percentage of theoretical density change with sintering temperature?
Gradually rises with temperature up to a peak then curves down. Starts and remains higher the smaller the crystals
Relationship between shrinkage, temperature and time
Shrinkage (ΔL/L0) increases with temperature
Shrinkage rate d(ΔL/L0)/dt decreases with time (concave graph of shrinkage Vs time gets higher with larger T).
After a certain amount of time density appears to reach a limiting value
Formula for shrinkage
Aka densification
Formulae for ΔL/L0 that are very long on page 8 of lecture 9
Which factors help shrinkage?
Having a longer sintering time
Having smaller grains
Curvature of grain boundaries
Each curved side of a boundary has a centre of curvature. The boundary will move towards this centre during sintering
Grain growth depending on number of sides
For grains with fewer than 6 sides they tend to have convex boundaries so centre of curvature towards centre of grain. These tend to shrink.
For grains with more than 6 sides they tend to be concave so centre of curvature away from grain centre. These tend to grow
Grain size over time formula
dt^n-d0^n=2AMbγbt Where dt is grain size at time t d0 is initial grain size n is constant 2 or 3 A is geometry constant M sub b is mobility constant of boundary γ sub b is interfacial tension of boundary t is time
Rate of grain growth depending on temperature
Log(rate) Vs 1/RT
Aka coarsening rate
Straight line with shallow negative gradient
Rate of sintering depending on temperature
Log(rate) Vs 1/RT
Aka densification rate
Straight line with steep negative gradient
Rapid sintering at high temperature
Increases the vacancy diffusivity. Faster densification rate. Means densification with a smaller grain size
Slow sintering at lower temperature
Slow diffusion causing grain coarsening. Slower densification rate. Means less densification
What densities can most metal oxides be sintered to?
70-95% density realtive to the theoretical density. Not easy to reach 0% porosity (100% density)
Gases trapped in closed pores
Gases can get trapped in closed pores during sintering and have to be removed by bulk transport through the crystalline lattice. N2 from air, SO2 and Cl2 from impurities hardly diffuse through the crystalline lattice of metal oxides. But H2 can easily diffuse through it and can also produce oxygen vacancies in the lattice (as is reducing agent) which enhances diffusion of pores.
Other considerations for sintering atmosphere
Oxygen pressure or partial pressure of substances, e.g PbO, should be controlled when sintering body includes such substances. Sintering atmosphere may oxidise or reduce cations
