Sintering towards theoretical density

Question 1

What are the tools of sintering towards theoretical density?

Answer 1

~ time, temp, atmosphere, additives, green body conditions

Question 2

What is the effect of vapor pressure on sintering?

Answer 2

~ powders with high vapor pressures heat rapidly to sintering dwell temp

Question 3

What’s the difference between small and large particles in terms of vapor pressure?

Answer 3

~ smaller particles have a higher vapor pressure

~ evaporation from small onto large particles results in particle coarsening, reducing the surface area, driving sintering

~ rapid heating brings compact to temps where sintering rates are comparable to coarsening rate

Question 4

What happens at low temps?

Answer 4

~ evaporation, condensation, and surface diffusion dominate neck formation

~ sintering mechanisms (particle centers approach) aren’t yet viable

~ rapid heating permits more useful surface energy to contribute towards sintering

Question 5

Why do green bodies need uniform interparticle spacing?

Answer 5

~ if spray dried granules don’t fully merge during pressing, then initial sintering shrinkage within the granule causes the gaps between granules to widen; these wide gaps are difficult to eliminate with continued sintering heat-treatment

~ can result in sintering at two different length scales

Question 6

What is the effect of excessive sintering dwell temps?

Answer 6

~ can accelerate grain growth to the point at which pores are abandoned to the grain interiors

~ poor mass/vacancy diffusion to/from them make them difficult to eliminate with further heat treatment

Question 7

What is the effect of using finer starting particle sizes?

Answer 7

~ provides more surface area and shorter/more diffusion parths for sintering

~ 10-100nm scale powders can be more difficult to process = high tendency to agglomerate

~ high surface energy LOWERS the MELT TEMP of nanopowders: they can melt, agglomerate, and reform as coarse particles during sintering heat treatment

Question 8

What is two-step sintering?

Answer 8

~ high temp sintering for a short time to 75% relative density

~ followed by lowered temp dwell for a long time (12-24hr) can sometimes sustain sintering with minimal grain growth

Question 9

What are the effects of pores with insoluble gases?

Answer 9

~ they are difficult to eliminate

~ many oxide ceramics (ex: ZnO) become anion-deficient at elevated temps

~ easy oxygen diffusion makes pure oxygen a better sintering environment

~ vacuum sintering can be hepful if powders do not have a high vapor pressure

Question 10

What are the effects of the second phase being insoluble in the first form GB phases?

Answer 10

~ they inhibit grain growth

~ introducing the second phase as a water-soluble component of the spray-drying slurry permits intimate coating of particles with second phase precipitates

Question 11

What are the effects of small additions of the second phase that is soluble in the first phase, but with cations of different size valence?

Answer 11

~ inhibits grain growth

~ these ions at the GB can compensate for unsatisfied bonds, and thus lower the GB energy: this lowers the driving force for grain growth

~ by lowering GB energy, there is a thermodynamic driving force for migration of these soluble ions to the GB