Two different tungsten powders (theoretical density = 19.3 g/cm 3 ) are analyzed

Question

Two different tungsten powders (theoretical density = 19.3 g/cm3) are analyzed for particle size using a streaming technique and found to have an equivalent mean size of 5 micrometers. However other properties are quite different.

Powder A - specific area (m2/g)=0.26, apparent density (g/cm3) =2.3, tap density (g/cm3)=4.6

Powder B - specific area (m2/g)=0.12, apparent density (g/cm3) =4.5, tap density (g/cm3)=8.1

a) Explain why there might be a difference in surface areas.

b) What equivalent spherical diameter would give the same surface areas for each powder?

c) What differences might explain the packing properties?

d) What additional information would be useful

Explanation / Answer

a)Powder B has a higher apparent density and tap density and a lower surface area than powder A. These figures of surface area, apparent density and tap density shows that powder A has a spongy or agglomerated particle structure.

b) Equivalent spherical diameter is given by = 6/Sm

where S = specific surface area, and

m= theoretical density

m= 19.3 g/cm3

Specific Surface area for Powder A= 0.26 m2/g

Equivalent spherical diameter for powder A = 6/(0.26*19.3) = 1.2 m

Specific Surface area for Powder B= 0.12 m2/g

Equivalent spherical diameter for powder B = 6/(0.12*19.3) = 2.6 m

c) The packing characteristics are similar with respect to the ratio of tap density to apparent density (2). However, powder A has a much lower apparent density and tap density, as would be expected from an agglomerated powder. This is consistent with the higher surface area, but equal agglomerate size.

d) Most useful will be an optical or scanning electron micrograph.

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