Biomedical Engineering Reference
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to micron-sized powders, raises fundamental issues that deserve detailed
studies. It is notable that the driving force for sintering nano particles is
significantly higher than for micron-sized particles. The linear approxima-
tion used in conventional theories of sintering, with regard to modeling the
driving force and the diffusion equations, is no longer valid. The rate of
sintering predicted by the nonlinear diffusion model is much higher than
that predicted by the conventional linear diffusion model.
With regard to the complex mechanisms of sintering of nano particles,
several possible mechanisms discussed in this chapter appear to contribute
to the initial densification, including the rapid diffusion rate due to non-
equilibrium defect concentrations in nanosized powders, the indirect role of
mass transport by surface diffusion, and the possible surface melting of
nano particles, all of which contribute to the densification as well as
coarsening of the nano particles.
From a technology perspective, proof that the sintering temperature
drastically decreases as particle size decreases to nanoscale represents an
actionable knowledge that can be exploited in the production of engineering
materials from nanosized powders.
The greatest challenge for sintering nanosized powders is the ability to
retain nanoscale grain sizes while achieving full densification. The grain
growth of nanosized powders is characterized by two parts of grain growth:
the initial dynamic growth and the normal grain growth, which is
reminiscent of that in bulk materials. The initial grain growth is the result
of the coarsening of particles via the inter-particle mass transport. For
nanosized powders, the initial grain growth causes the material to lose
nanocrystalline characteristics. Therefore, if at least part of the goal of
sintering is the retention of nanoscaled grain sizes, the initial grain growth
must be controlled and minimized. On the other hand, in the absence of the
need to retain nanoscaled grain size, the initial grain growth or coarsening is
one of the mechanisms that can be exploited to aid densification.
The methods for retaining grain growth include the use of grain growth
inhibitors, various high-pressure hot consolidation processes, and decou-
pling grain growth from densification by manipulating different diffusion
mechanisms at different temperatures. The popular use of SPS for
consolidation of nanosized powders combines the advantages of rapid
heating rate and pressure. However, regardless of the sintering technique
used, powder processing and green compact fabrication techniques are
crucial for controlling grain growth and densification. In general, it is
desirable to have minimum agglomeration of particles, minimum pore size
and uniformly distributed pores.
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