Manganese oxide formation by heat treatment of MnCO3 in air.

MnCO3 + ½ O2MnO2+CO2

<500 C Reaction 1

>500 C Reaction 2

2 MnCO3 + ½ O2Mn2O3 + 2 CO2

Note that in reaction 1, there is a net increaseOf ½ mole of gas for each mole of Mn, and for reaction 2 there is a net increase of ¾ of a mole of gas for each mole of Mn. What can you say about the entropy change in each reaction? How does this help explain the temperature dependence between the two reactions?

ab

c

xy

z

a

b

c

x

y

z

0.2 um

As the manganese oxide particles form from the carbonate salt, they begin to grow together, or ‘sinter’. The figure below is a TEM micrograph of neck formation during the

sintering of Mn2O3 particles.

Why do the necks get larger and the pores get smaller as the heat treat time and or temperature increases?

Pore

Neck

Neck

The idea of stress concentration at a ‘pore’

Powder Processing and the Problem with Porosity

Liquid Phase Sintering

T

A B

A powder + B powder

LTsint

When we heat a powder pack of A powder + B powder to Tsint at the overall composition shown on the phase diagram, liquid fills the pores between the A particles. Why does the liquid spread to fill the pores?

A+B

Hot Isostatic Pressing

Ductile metalenvelop subjectedto high gas pressure

Sintering Kinetics (shrinkage)

Sintering Kinetics (density)

rdr

rddrrd

Volumed

Aread 2

)3/4(

)4(

)(

)(3

2

ΔP

r

Curved surfaceFlat surface

P=Patmσ=Patm + 2γ/rΔP

Two sphere model

r

r1

r2

The neck has a negative curvature component (-1/ρ), acting to reduce the pressure relative to the spherical surface.

P 1

r

1

P 1

r 1

1

r 2

Sintering of Nickel powder. The time lapse photography illustrates Neck formation and coarsening.

How does the driving force for sintering of cylindrical particles differ from that for sintering of spherical particles?Why do the ends of the cylindrical particles blunt during sintering?