What is the typical process of manufacturing ceramics?
Powder production –> mixing –> milling –> drying –> compaction –> sintering –> finishing
Synthesis of powders?
Chemical synthesis: -Gas phase reaction -Liquid phase reaction -Solid phase reaction Mechanical synthesis: -Comminution
What is the solid phase reaction?
Calcination, thermal decomposition, a reaction between solid chemicals, raw materials are mixed and heated
Describe comminution
Small particles are produced by reducing the size of the larger ones
What is the critical speed for ball mills?
The rotating speed which would cause media to be held against the mill lining by centripetal action.
What is the critical speed for dry milling?
65-80% of Nc
What is the critical speed for wet milling?
60-65% of Nc
Disadvantage of dry milling?
Agglomeration can occur BUT it doesn’t need to be separated from a solution
Advantage of wet milling?
Less agglomeration BUT some components may dissolve in solution, volume ratio must be carefully selected to optimise viscosity
What is the purpose of X-Ray Diffraction?
It uses grain boundary information to obtain the crystal structure of a ceramic.
What is packing density?
The volume occupied by the particles in an unit volume.
Packing different sized particles will make powder dense. Ideally ratio of coarse to fine of about 7
How does thermal conductivity change with grain size?
Thermal conductivity increases with grain size.
How does fracture toughness change with grain size?
Fracture toughness decreases with increasing grain size. Finer grains tend to obstruct dislocations in a material.
Name the various powder analysis methods in order of grain size µm.
TEM - 0.001 SEM - 0.01 OPTICAL MICROSCOPY - 1 SEDIMENTATION - 0.1 to 100 COULTER COUNTER - 0.5 to 400 SIEVING - 20 to 10,000
Equation for maximum packing density?
PF=PFc+(1-PFc)PFm+(1-PFc)(1-PFm)PFf
How do grain boundaries affect phonon movement?
They disturb phonon movement.
Define phonon.
Elastic vibration within the lattice which carries heat through the material.
What is the critical speed equation?
Nc=(1/pi)((g/2(D-d))^0.5)
At what percentage of the critical rotation speed for ball milling should your rotation speed be?
About 65 to 80%.
What is the purpose of a phase diagram?
To identify optimum conditions for suitable aiming phases. They save energy.
What is sintering?
Green compact is heated and particles merge.
- removal of pores btw particles
- formation of strong bonds btw particles
- shrinkage of compact
Equation for porosity
1-ρm/ρt
Powder size ratio ball mill
25:1 minimum media to feed
powder volume ratio ball mill
20-60% of container