Particle packing Flashcards
What are the effects of optimizing the green (unfired) density of a body?
~ minimizes sintering shrinkage and potential associated warpage and distortion
What is the relative density of perfectly-packed (12-fold coordination FCC or HCP) uniformly sized spheres? What is the relative density for loose (imperfect) packing?
~ 74.05%
~ 60% (interparticle bridging can occur)
What are the characteristics of real particles?
~ irregular in shape and can retain surface roughness/texture
What does a high angle of repose indicate?
~ considerable interparticle friction, low packing density and low packing coordination
What happens when the mean particle size is below 100μm?
~ the increased surface area enhances the influence of electrostatic attraction and moisture adsorption, which happens preferentially at the contact points between particles
~ result is the particle agglomerates are inefficiently mutually assembled
What does vibration do? What is the result of excessive vibration?
~ significantly improves tap density (un-compressed powder assembly)
~ excessive vibration causes segregation, in which fine particles fall through the interstices of coarse particles
What is the effect of mixing spherical particles of two sizes (bimodal)? What is the large/small diameter ratio?
~ increase the packing density
~ especially when the particles are small enough to fall into and occupy the interstices of coarse ones
~ large/small diameter ratio 7/1
What do you do for optimum packing?
~ use only enough fines to occupy the interstices of the coarse spheres
~ 60vol% coarse, 40vol% fine
What happens if fines are in excess?
~ coarse spheres are pushed apart
What is the relationship between packing density and distribution spread for continuous particle size distributions?
~ packing density improves with increasing distribution spread up to a limit (σ=~1)
What happens when mixing two log normal powders?
~ packing density improves with increasing ratio of mean particle size (wide spread)
