Biomedical Engineering Reference
In-Depth Information
13.15
Relationship between grain growth and densification during
sintering of nano particles. I: early stage of grain growth; II: late stage
of grain growth.
phenomenon is observed in many different materials.
2,3,89,90
This relation-
ship implies that the grain growth during sintering consists of two stages: the
early stages of sintering, before the powder compact reaches 90% relative
density; and the late stages of sintering, when relative density is greater than
90%. It is believed that the late stages of grain growth can be viewed as
'normal' grain growth, similar to that found in bulk materials by boundary
migration, but incorporating the effect of pinning by closed pores. In
contrast, the early stage of grain growth during sintering is often referred to
as 'coarsening.'
13.4.2 Initial grain growth - coarsening - of nano particles
during early stages of sintering (rel. density
90%)
The above discussion provides evidence of an initial stage of grain growth.
This part of grain growth occurs in the beginning of the sintering, often
during heating up when the relative density is less than 90%. Thus, the
initial grain growth during sintering of nanosized particles can be treated as
non-isothermal grain growth. In conventional sintering of micron-sized
powders, the contribution of the initial grain growth to the final grain size is
relatively minor, compared to that of the normal grain growth during late
stages of sintering. For sintering of nano particles, however, the amount of
the initial grain growth is significant and sufficient in many cases to cause
the material to lose its nanocrystalline characteristics.
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