Biomedical Engineering Reference
In-Depth Information
sintering, the considerable densification obtained is primarily connected to
removal of small pores rather than shrinkage of larger ones.
From the perspective of achieving full densification and elimination of
pores, it is logically desirable to de-agglomerate powders or to avoid the
formation of agglomerates in the first place. Fundamentally, the formation
of agglomerates is attributed to balance of the inter-particle forces,
specifically the van der Waals force, which acts to bind particles, and the
electrostatic repulsion which opposes agglomeration. To de-agglomerate,
opposite measures must be taken in order to stabilize a colloidal solution. In
other words, the repulsive forces must be boosted to achieve a balance
between the attractive and repulsive forces such that the dispersion of
particles can be stabilized. Methods for nano particle dispersion include
electrostatic charge stabilization, steric stabilization, or a combination of the
two. Details of the principles of stabilizing colloidal solutions can be found
elsewhere.
73
These techniques can be implemented in the processes,
including mixing and milling of the powders that are necessary prior to
sintering of these materials.
To sinter nano particles for the fabrication of bulk engineering
components, a colloidal solution must be dried; agglomerates will inevitably
form. Ideally, the agglomerates are soft and the inter-agglomerate pores are
small. Lange provided a more extensive discussion of ceramic powder
processing techniques for avoiding agglomeration and achieving uniform
pore distributions within a powder compact.
74
Effect of pores on sintering of nano particles
A common thread for the effect of green density and agglomeration on
sintering is the effect of pores on densification.
63
A compact consists of
particles and pores, and each pore has a volume, shape and coordination
number. The pore coordination number is defined as the number of
touching particles surrounding and defining each void space. A pore's
surface morphology is determined by the dihedral angle and the pore's
coordination number. In general, for a given dihedral angle, a critical
coordination number, n
c
, exists that defines the transition of the pore surface
morphology from convex (n
n
c
). Kingery and
Francois
62
first recognized that only those pores with n
>
n
c
)
to concave (n
<
n
c
are able to
shrink during sintering because the concave surface morphology with
negative chemical potential is thermodynamically unstable. As a result,
atoms will diffuse to the pore surface and fill the void space. Figure 13.10
illustrates the stability of pores and their dependence on both the dihedral
angles and the coordination number. For any given dihedral angle, which is
dictated by the material, a critical coordination number exists below which
the pores will shrink and above which the pores will grow.
<
