Biomedical Engineering Reference
In-Depth Information
nanosized powder. One explanation for the limited effect of grain growth
inhibitors has to do with the size of agglomerates. If the mixing and
distribution scale of grain growth inhibitors are larger than the original
nanoscaled grain sizes and closer to the size of the agglomerate, then the
grain growth inhibitors are effective only in that same dimensional scale.
13.5.3 Deagglomeration
Deagglomerating agglomerated particles prior to sintering is another critical
strategy for minimizing the initial dynamic grain growth. Based on the
understanding that a pore's stability is dependent upon its coordination
numbers, a powder compact that has uniformly distributed fine pores would
have the most efficient densification without relying on coarsening. An ideal
scenario for minimizing grain growth while, at the same time, achieving full
densification is the utilization of green powder compacts composed of
monosized, spherical nano particles without agglomerates. The pores within
such a compact would be evenly distributed and would have uniform size.
To achieve this type of green compact structure, Lange proposed a
methodology in which the powders would be treated prior to sintering
according to colloidal processing principles.
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First, in order to deagglome-
rate the particles, dry powders are dispersed in fluid containing a surfactant
that produces inter-particle repulsive forces. After removing large particles
that cannot be deagglomerated, the powder slurry is flocked by changing the
inter-particle forces from repulsion to attraction. As a result, the powder
becomes a weak, continuous network of touching primary particles.
Colloidal treated slurries could be used directly for consolidation. The
effects of deagglomeration on the density of green compacts and sintering of
nanosized particles are experimentally demonstrated in many reported
studies.
74,121-126
In particular, Ahn et al.
127
showed that the grain size of
sintered nano SnO
2
particles that have a dense green structure is
dramatically smaller than that of a compact with loose structure.
13.5.4 Pressure-assisted sintering
The use of pressure-assisted processes is another straightforward approach
for minimizing grain growth while achieving maximum densification. A
variety of pressure-assisted sintering processes has been used in sintering
nanosized powders, including hot pressing (HP), hot isostatic pressing
(HIP), spark plasma sintering (SPS) and sinter forging. It is generally
believed that the applied pressure is able not only to enhance densification
by increasing sintering driving force, assisting particle rearrangement and
promoting diffusion creep,
2,128-130
but also to inhibit grain growth by
decreasing the diffusivity and thus the grain boundary mobility.
128
The total
